Chemical Synthesis Using Supercritical Fluids

Chemical Synthesis Using Supercritical Fluids

Edited by Philip G. Jessop and Walter Leitner

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner

Weinheim - New York * Chichester Brisbane - Singapore - Toronto

Dr. Walter Leitner Max-Planck-Institut fur Kohlenforschung Kaiser-Wilhelm-Platz 1 D-45470 Mulheim an der Ruhr Germany

Prof. Philip G. Jessop Department of Chemistry University of California Davis, California 95616 USA

This book was carefully produced. Nevertheless, the authors, editors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No. applied for. A catalogue record for this book is available from the British Library. Deutsche Bibliothek Cataloguing-in-Publication Data: Chemical synthesis using supercritical fluids I ed. by Philip G. Jessop and Walter Leitner. Weinheim ; New York ; Chichester ; Brisbane ; Singapore ; Toronto : Wiley-VCH, 1999 ISBN 3-527-29605-0 0 WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999 Printed on acid-free and chlorine-free paper. All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Composition: Hagedorn Kommunikation, D-685 19 Viernheim Printing: Strauss Offsetdruck GmbH, D-69503 Morlenbach Bookbinding: J. Schaffer GmbH & Co. KG, D-67269 Grunstadt Printed in the Federal Republic of Germany.

Preface

Reactions under supercritical conditions have been used for large scale industrial production for most of the 20th century, but the application of supercritical fluids (SCFs) in the chemical synthesis of complex organic molecules or specialized materials is only just emerging. Research in this field has been particularly active in the last decade of this century. This book is intended to introduce the reader to the wide range of opportunities provided by the various synthetic methodologies developed so far. Supercritical fluids may be alternatives to liquid solvents, but they are neither simple nor simply replacements of solvents. The experimental chemist could not modify a written synthetic method by simply crossing out the word “benzene” and replacing it with the words “supercritical carbon dioxide”. Many other modifications to the procedure would be necessary, not only because of the need for pressurized equipment but also because of the inferior solvent strength of many SCFs. On the other hand, additional degrees of freedom in the reaction parameters emerge from the high compressibility of SCFs, allowing density to be introduced as an important variable. This is only one of the reasons why the result of a chemical synthesis can sometimes be dramatically changed, often for the better, by this solvent switch. It is only fair to say that we are still far away from a detailed understanding of supercritical solvent effects. More basic research will be needed before we learn how to exploit these benefits in the most efficient way, In the meantime, it is our hope that the chemist considering using a SCF as a medium for a reaction will turn to this volume to find out both what has been done and, more importantly, how to do it. Previous monographs concerning SCFs have emphasized their use for extractions and separations, usually with a single chapter dedicated to reaction chemistry. Now we turn the tables. We have selected the chapter topics to guide the reader through the process of planning and carrying out chemical syntheses in SCFs. The subjects include equipment, safety concerns, in-situ monitoring techniques, reaction methods, and purification using SCFs. The largest part of the book is then devoted to various types of chemical reactions involving SCFs as solvents and/or reactants. The emphasis is on synthetic reactions,

VI

Preface

rather than reactions tested for the purpose of investigating near-critical phenomena or reactions involved in destructive or combustion chemistry. For coverage of reactions in SCFs with less of an emphasis on chemical synthesis and related experimental techniques, the reader will be well served by the Chemical Reviews special issue and by a book just published in Japanese (Principles and Developments of Supercritical Fluids Reactions, T. Ikariya, Ed., CMC, Tokyo, 1998). Although the present book provides the newcomer with a summary of the physico-chemical background and purification techniques using SCFs, the excellent introduction to these topics by McHugh and Krukonis (M. McHugh, V. J. Krukonis, Supercritical Fluid Extraction, 2nd edition, Butterworth-Heinemann, Boston, 1994) is highly recommended for further reading. At this point, we must offer a safety warning and disclaimer. Supercritical fluids are used at high pressures and in some cases at elevated temperatures. The chemist contemplating their use must become acquainted with the safety precautions appropriate for experiments with high pressures and temperatures. Some SCFs also have reactivity hazards. The safety considerations mentioned in chapters 1.1 and 2.1 are meant neither to be comprehensive nor to substitute for a proper investigation by every researcher of the risks and appropriate precautions for a planned experiment. The contributors to the present volume, all leading experts in the field, have given us a wide view of the types and methods of research being performed in supercritical fluid reaction chemistry. Many of the techniques that the reader will find described in these pages have been laboriously developed by these contributors and their colleagues. We gratefully thank all of the contributors for agreeing to take time out from their research schedules to write chapters for this volume, and for agreeing to share with the reader the detailed experimental methods and “tricks of the trade”. The editors also gratefully acknowledge fruitful discussions with and advice from a number of colleagues and friends, especially Prof. M. Augustine (University of California, Davis, UCD), Prof. J. Brennecke (University of Notre Dame), Prof. C. Eckert (Georgia Institute of Technology), Prof. A. Fisher, (UCD), Prof. T. Ikariya (Tokyo Institute of Technology), Dr. A. Levelt Sengers (NIST), Prof. R. Noyori (Nagoya University), Prof. T. Patten (UCD), Dr. W.-J. Richter (MPI fur Kohlenforschung) and Prof. S. Tucker (UCD). We also thank the following people and institutions for providing us with information or photographic material on the historical aspects and the industrial use of SCFs: Dr. J. Abeln (Forschungszentrum Karlsruhe), Dr. U. Budde (Schering AG), Dr. H.-E. Gasche (Bayer AG), Dr. P. Mgller (Poul Mgller Consultancy), Dr. T. Muto (Idemitsu Petrochemical), Prof. G. Ourisson and representatives of the Acadkmie des Sciences, Dr. A. Rehren (Degussa AG), M.-C. Thooris (Ecole Polytechnique Palaiseau) and representatives of Eco Waste Technology and General Atomics. Special thanks are due to Drs. Nicole Kindler and Anette Eckerle at WileyVCH for their competent help and collaboration in producing this book. Furthermore, we wish to express our sincere thanks to the members of our

Preface

VII

research groups, for their talents and their enthusiasm, which make our research efforts devoted to SCFs so much fun. Finally, and most importantly, we dedicate our own contribution to this book to our wives and families, for all their love and understanding throughout the years and especially during the preparation of this volume. October 1998

Philip Jessop and Walter Leitner

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

V

Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IX

List of Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . .

XVII

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1

1 1

1.1 Supercritical Fluids as Media for Chemical Reactions . . . . . 1.1.1 What is a Supercritical Fluid (SCF)? . . . . . . . . . . . . . . Practical Aspects of Reactions in Supercritical Fluids (SFRs) . 1.1.2 Motivation for Use of SCFs in Modern Chemical Synthesis . . 1.1.3 1.1.4 A Brief History of Chemical Synthesis in SCFs . . . . . . . . 1.1.4.1 Discovery of SCFs and their Use as Solvents . . . . . . . . . 1.1.4.2 Early Examples of Chemical Reactions in SCFs . . . . . . . . 1.1.4.3 Industrial Use of SCFs as Reaction Media . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 9 13 13 20 25 30

1.2 Phase Behavior and Solubility . . . . . . . . . . . . . . . . . . 1.2.1 Basic Physical Properties of Supercritical Fluids . . . . . . . . 1.2.2 Phase Behavior in High Pressure Systems . . . . . . . . . . . 1.2.2.1 Types of Binary Phase Diagrams . . . . . . . . . . . . . . . . 1.2.2.2 Asymmetric Binary Mixtures . . . . . . . . . . . . . . . . . . 1.2.3 Factors Affecting Solubility in Supercritical Fluids . . . . . . . 1.2.3.1 SCF Solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3.2 Chemical Functionality of the Solute . . . . . . . . . . . . . . 1.2.3.3 Temperature and Pressure Effects . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37 37 41 41 43 47 48 49 51 53

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1.3 Physical Properties as Related to Chemical Reactions . . . . . 1.3.1 Behavior of Diffusion Coefficients . . . . . . . . . . . . . . . 1.3.2 Diffusional Effects on Reactions . . . . . . . . . . . . . . . . 1.3.3 Transition-state Theory Applied to SCFs . . . . . . . . . . . . 1.3.4 Density Dependence of Two Competing Reactions . . . . . . . 1.3.5 Solvation Effects on Reactions . . . . . . . . . . . . . . . . . 1.3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Experimental Techniques . . . . . . . . . . . . . . . . . . . .

54 55 56 58 62 63 65 65 67

2.1 High-pressure Reaction Equipment Design . . . . . . . . . . . 2.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Basic Equipment and Components . . . . . . . . . . . . . . . 2.1.2.1 Design of Thick-Walled Vessels . . . . . . . . . . . . . . . . . 2.1.2.2 Closures and Connectors . . . . . . . . . . . . . . . . . . . . . 2.1.2.3 Tubing and Fittings . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2.4 Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2.5 Compressors and Pumps . . . . . . . . . . . . . . . . . . . . . 2.1.2.6 Stirring and Mixing . . . . . . . . . . . . . . . . . . . . . . . 2.1.2.7 Optical Windows . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3 High Pressure Systems . . . . . . . . . . . . . . . . . . . . . . 2.1.3.1 Single-batch High-pressure Reactors . . . . . . . . . . . . . . 2.1.3.2 View Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3.3 Systems for Continuous Processing . . . . . . . . . . . . . . . 2.1.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67 67 69 69 71 73 74 76 78 78 78 80 83 84 86 86

Extraction and Related Separation Techniques . . . . . . . . . General Aspects of Supercritical Fluids as Mass Separating Agents . . . . . . . . . . . . . . . . . . . . . . . . Extraction from Solids . . . . . . . . . . . . . . . . . . . . . . Basic Process Design . . . . . . . . . . . . . . . . . . . . . . Process Parameters . . . . . . . . . . . . . . . . . . . . . . . . Modeling the Extraction . . . . . . . . . . . . . . . . . . . . . Solids in Multiple Stages and Countercurrent Operation in SFE Continuous Extraction of Contaminated Soil with Supercritical Water . . . . . . . . . . . . . . . . . . . . . . . . Countercurrent Multistage Extraction . . . . . . . . . . . . . . Basic Process Design . . . . . . . . . . . . . . . . . . . . . . Phase Equilibria . . . . . . . . . . . . . . . . . . . . . . . . . Separation Analysis with Respect to Theoretical Stages . . . . Multicomponent Process Simulation . . . . . . . . . . . . . . . Determination of the Height (Length) of a Theoretical Stage . Determination of Column Diameter . . . . . . . . . . . . . . . Chromatographic Separation with Supercritical Fluids . . . . .

88

2.2 2.2.1 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.2.4 2.2.2.5 2.2.3 2.2.3.1 2.2.3.2 2.2.3.3 2.2.3.4 2.2.3.5 2.2.3.6 2.2.4

88 90 91 92 93 95 95 97 98 99 101 102 102 103 104

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2.2.4.1 Design of SFC Apparatus . . . . . . . . . . . . . . . . . . . . 2.2.4.2 Methods for Scale-up of Chromatography . . . . . . . . . . . 2.2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

105 106 106 106

2.3 Precipitation and Crystallization Techniques . . . . . . . . . . 2.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Thermodynamics and Phase Equilibria . . . . . . . . . . . . . 2.3.2.1 CSS, PGSS and RESS . . . . . . . . . . . . . . . . . . . . . . 2.3.2.2 GASP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Process Basics and Reference Schemes . . . . . . . . . . . . . 2.3.3.1 Crystallization from a Supercritical Solution (CSS) . . . . . . . 2.3.3.2 Formation of Particles from Gas Saturated Solution (PGSS) . . 2.3.3.3 Rapid Expansion of a Supercritical Solution (RESS) . . . . . . 2.3.3.4 Precipitation by a Gas or a Supercritical Antisolvent (GASP) . 2.3.4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

108 108 109 109 112 115 115 115 117 120 123 125 125

2.4 Microemulsions, Emulsions and Latexes . . . . . . . . . . . . 127 2.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 2.4.2 Interfacial Tension . . . . . . . . . . . . . . . . . . . . . . . . 128 130 2.4.3 Microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . 131 2.4.3.1 Phase Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . 132 2.4.3.2 UV-Vis Indicators . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3.3 Electron Paramagnetic Resonance Techniques . . . . . . . . . . 133 2.4.3.4 Neutron Scattering . . . . . . . . . . . . . . . . . . . . . . . . 134 134 2.4.3.5 Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3.6 Organic-CO2 Microemulsions . . . . . . . . . . . . . . . . . . 135 135 2.4.4 Emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4.1 Organic-CO2 Emulsions . . . . . . . . . . . . . . . . . . . . . 135 2.4.4.2 Water-COZ Emulsions and COz-Water Emulsions . . . . . . . 137 2.4.5 Reactions in SCF Emulsions, Microemulsions and Latexes . . 140 2.4.5.1 Phase-Transfer Reactions Between Water and C 0 2 . . . . . . . 140 2.4.5.2 In Situ Studies of Dispersion Polymerization . . . . . . . . . . 142 143 2.4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 144 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 3

Spectroscopy of SCF Solutions . . . . . . . . . . . . . . . . .

147

3.1 3.1.1 3.1.2 3.1.2.1 3.1.2.2

Vibrational Spectroscopy . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Dilute Solutions . . . . . . . . . . . . . . . . . . . . . . . . . Infrared Spectroscopy . . . . . . . . . . . . . . . . . . . . . . Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . .

147 147 148 148 150

XI1

Contents

3.1.2.3 Polymer Modification . . . . . . . . . . . . . . . . . . . . . . 3.1.2.4 Vibrational Spectroscopic Studies of Aqueous Microemulsions in SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 Concentrated Solutions. . . . . . . . . . . . . . . . . . . . . . 3.1.4 Monitoring of Fast Reactions in SCFs using Time-resolved Vibrational Spectroscopy . . . . . . . . . . . . . 3.1.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5

NMR Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . Some Toroid Probe Designs . . . . . . . . . . . . . . . . . . General Properties of Supercritical Media . . . . . . . . . . . Measurement of Dynamic and Equilibrium Processes in C 0 2 Rate and Selectivity Measurements Associated with Propylene Hydroformylation in C 0 2 . . . . . . . . . . . . . 3.2.6 Diffusion and Relaxation Time Measurements . . . . . . . . . 3.2.7 NMR of Quadrupolar Nuclei in Supercritical C 0 2 . . . . . . 3.2.8 Future Research . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.9 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3 UV, EPR, X-ray and Related Spectroscopic Techniques . . . 3.3.1 UV-Vis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Electron Paramagnetic Resonance . . . . . . . . . . . . . . . 3.3.4 X-ray Absorption Fine Structure. . . . . . . . . . . . . . . . 3.3.5 X-ray and Neutron Scattering and Diffraction . . . . . . . . 3.3.5.1 Small Angle Scattering . . . . . . . . . . . . . . . . . . . . . 3.3.5.2 Wide Angle Scattering . . . . . . . . . . . . . . . . . . . . . . 3.3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.7 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . .

151 153 154 156 161 161 161 165 165 167 172 177

. . .

184 187 188 191 192 193

.

195 195 198 199 200 206 207 208 209 209 209

. . .

4

Reactions in SCF . . . . . . . . . . . . . . . . . . . . . . . .

213

4.1 4.1.1 4.1.2 4.1.3 4.1.3.1 4.1.3.2 4.1.3.3 4.1.3.4 4.1.4

Synthesis of Inorganic Solids . . . . . . . . . . . . . . . . . . Historical Summary . . . . . . . . . . . . . . . . . . . . . . . Experimental Techniques . . . . . . . . . . . . . . . . . . . . . Hydrothermal Chemistry . . . . . . . . . . . . . . . . . . . . . Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphates and Silicates . . . . . . . . . . . . . . . . . . . . . Metal Sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . Other Hydrothermal Syntheses . . . . . . . . . . . . . . . . . . Supercritical Amines . . . . . . . . . . . . . . . . . . . . . . .

213 214 216 226 226 229 232 233 234

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Contents 4.1.5 Other Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

237 238 239

4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.5.1 4.2.5.2 4.2.5.3

243 243 244 245 248 252 253 255

Synthesis of Coordination Compounds . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . Detailed Syntheses . . . . . . . . . . . . . . . . . . . . . . . . Preparation of C T ( C O ) ~ ( C ~.H.~ .) . . . . . . . . . . . . . . . Preparation of C P M ~ ( C O ) ~ ( ~ ~. -.H. ~. ) . . . . . . . . . . . Preparation of C P * M ~ ( C O ) ~ ( ~ from ~-H~) C P * M ~ ( C O )-HSiEt3) ~(~~ . . . . . . . . . . . . . . . . . . . . . 4.2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

256 257 257 257

4.3 Stoichiometric Organic Reactions . . . . . . . . . . . . . . . 4.3.1 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Supercritical Carbon Dioxide . . . . . . . . . . . . . . . . . 4.3.2.1 Diels-Alder Reactions . . . . . . . . . . . . . . . . . . . . . . 4.3.2.2 Reduction and Coupling . . . . . . . . . . . . . . . . . . . . . 4.3.2.3 Esterification . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2.4 Cracking and Rearrangements . . . . . . . . . . . . . . . . . 4.3.3 Superheated and Supercritical Water . . . . . . . . . . . . . 4.3.3.1 Oxidation of Methane . . . . . . . . . . . . . . . . . . . . . . 4.3.3.2 Cleavage/Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . 4.3.3.3 Elimination Reactions . . . . . . . . . . . . . . . . . . . . . . 4.3.3.4 Diels-Alder Reactions . . . . . . . . . . . . . . . . . . . . . . 4.3.3.5 Rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4 Other Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

259 260 260 262 265 266 266 267 267 268 271 272 273 275 276

4.4 4.4.1 4.4.1.1 4.4.1.2 4.4.1.3 4.4.1.4 4.4.2 4.4.2.1 4.4.2.2 4.4.2.3 4.4.3 4.4.3.1

. .

. .

Photochemical and Photo-induced Reactions . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Definition of a Supercritical Fluid . . . . . . . . . . . . . . . . Solvent Properties of SCFs . . . . . . . . . . . . . . . . . . . Scope of this Chapter . . . . . . . . . . . . . . . . . . . . . . Experimental Considerations . . . . . . . . . . . . . . . . . . . Photochemical Reactions in Supercritical Fluid Solvents . . . . Geometric Isomerization . . . . . . . . . . . . . . . . . . . . . Photodimerization . . . . . . . . . . . . . . . . . . . . . . . . Carbonyl Photochemistry . . . . . . . . . . . . . . . . . . . . Photo-induced Reactions in Supercritical Fluid Solvents . . . . Free Radical Brominations of Alkyl Aromatics in Supercritical Carbon Dioxide . . . . . . . . . . . . . . . . . . . . . . . . .

280 280 280 280 281 282 285 285 285 287 289 289

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4.4.3.2 Free Radical Chlorination of Alkanes in Supercritical Fluid Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

290 293 294 294

4.5 Polymerizations in Dense Carbon Dioxide . . . . . . . . . . . 4.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Homogeneous Solution Polymerizations . . . . . . . . . . . . . 4.5.2.1 Free-Radical Chain Growth . . . . . . . . . . . . . . . . . . . 4.5.2.2 Cationic Chain Growth . . . . . . . . . . . . . . . . . . . . . 4.5.3 Heterogeneous Polymerizations . . . . . . . . . . . . . . . . . 4.5.3.1 Free-Radical Chain Growth . . . . . . . . . . . . . . . . . . . 4.5.3.2 Cationic Chain Growth . . . . . . . . . . . . . . . . . . . . . 4.5.4 Metal-catalyzed Polymerizations . . . . . . . . . . . . . . . . . 4.5.4.1 Ring-opening Metathesis Polymerization . . . . . . . . . . . . 4.5.4.2 Epoxide-C02 Copolymers . . . . . . . . . . . . . . . . . . . . 4.5.4.3 Oxidative Coupling Polymerizations . . . . . . . . . . . . . . . 4.5.5 Step-growth Polymerizations . . . . . . . . . . . . . . . . . . . 4.5.6 Hybrid Systems . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.8 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

297 297 300 300 303 305 305 317 319 319 319 320 320 321 321 322 322

4.6 4.6.1 4.6.2 4.6.2.1 4.6.2.2 4.6.2.3 4.6.3 4.6.3.1 4.6.3.2 4.6.3.3 4.6.4 4.6.4.1

326 326 321 327 329 33 1 333 334 336 338 341

Free-Radical Polymerization in Reactive Supercritical Fluids . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Methods and Techniques . . . . . . . . . . . . . On-line Spectroscopy . . . . . . . . . . . . . . . . . . . . . . Kinetic Coefficients from Laser-Assisted Techniques . . . . . . Continuously Operated High-pressure Polymerization Reactors Ethene Homopolymerization . . . . . . . . . . . . . . . . . . . Propagation and Termination . . . . . . . . . . . . . . . . . . Chain-Transfer to Monomer . . . . . . . . . . . . . . . . . . . Chain-Transfer to Polymer and p-Scission . . . . . . . . . . . Ethene Copolymerizations . . . . . . . . . . . . . . . . . . . . Reactivity Ratios for Copolymerizations of Ethene with Acrylic Acid Esters . . . . . . . . . . . . . . . . . . . . . 4.6.4.2 Homopropagation and Homotermination Kinetics of the Comonomers . . . . . . . . . . . . . . . . . . . . . . . 4.6.4.3 Modeling of High-pressure Ethene Copolymerizations . . . . . 4.6.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

341 345 347 348 348

4.7 Metal-Complex-Catalyzed Reactions . . . . . . . . . . . . . . . 351 4.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 4.7.1.1 Potential Benefits of Using Supercritical Fluids in Metal-Complex-Catalyzed Reactions . . . . . . . . . . . . . 351

Contents

XV

4.7.1.2 Practical Considerations . . . . . . . . . . . . . . . . . . . . . 4.7.1.3 Solubility of Metal Complexes in SCFs . . . . . . . . . . . . . 4.7.2 Hydrogenation and Related Reactions . . . . . . . . . . . . . . 4.7.2.1 Hydrogenation of C 0 2 under Supercritical Conditions . . . . . 4.7.2.2 Hydrogenation of C = C Double Bonds . . . . . . . . . . . . . 4.7.2.3 Hydrogenation of Imines . . . . . . . . . . . . . . . . . . . . . 4.7.3 Hydroformylation . . . . . . . . . . . . . . . . . . . . . . . . 4.7.4 C-C Coupling Reactions . . . . . . . . . . . . . . . . . . . . 4.7.4.1 Cobalt-catalyzed Cyclization Reactions . . . . . . . . . . . . . 4.7.4.2 Coupling Reactions Involving scC02 as Solvent and Substrate 4.7.4.3 Olefin Metathesis . . . . . . . . . . . . . . . . . . . . . . . . 4.7.4.4 Palladium-catalyzed Coupling Reactions of Aryl Halides . . . . 4.7.4.5 Cyclopropanation . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.5 Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.5.1 Peroxides as Oxidants . . . . . . . . . . . . . . . . . . . . . . 4.7.5.2 Molecular Oxygen as Oxidant . . . . . . . . . . . . . . . . . . 4.7.6 Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.6.1 Polymerization of Alkenes under Supercritical Conditions . . . 4.7.6.2 Polymerization Utilizing Compressed C 0 2 . . . . . . . . . . . 4.7.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

352 356 358 358 360 363 36.5 369 369 370 371 373 375 377 377 378 380 380 381 384 384

4.8 4.8.1 4.8.1.1 4.8.1.2 4.8.1.3 4.8.1.4 4.8.1.5

388 389 390 391 393 393

Heterogeneous Catalysis . . . . . . . . . . . . . . . . . . . . . Fischer-Tropsch Synthesis . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . Reaction Performance in Three Reaction Phases . . . . . . . . Diffusion Behavior of Synthesis Gas . . . . . . . . . . . . . . Diffusion and Reaction of the Products . . . . . . . . . . . . . Wax Production: Addition of Heavy Alkene to the Supercritical Phase . . . . . 4.8.2 Isomerization Reactions . . . . . . . . . . . . . . . . . . . . . 4.8.3 t-Butyl Alcohol Synthesis by Air Oxidation of Supercritical Isobutane . . . . . . . . . . . . . . . . . . . . 4.8.4 Supercritical Phase Alkylation Reactions over Solid Acid Catalysts 4.8.5 Synthesis of Fine Chemicals and Other Products . . . . . . . . 4.8.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.9 4.9.1 4.9.2 4.9.3 4.9.3.1 4.9.3.2 4.9.3.3 4.9.3.4 4.9.3.5

Enzymatic Catalysis . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzyme Reactors . . . . . . . . . . . . . . . . . . . . . . . . . Batch Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . Recirculating Batch Reactor . . . . . . . . . . . . . . . . . . . Extractive Batch Reactor . . . . . . . . . . . . . . . . . . . . . Semicontinuous Flow Reactors . . . . . . . . . . . . . . . . . Continuous Reactors . . . . . . . . . . . . . . . . . . . . . . .

395 398 399 403 407 411 411 414 414 415 416 416 419 419 420 420

XVI

Contents

4.9.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . 4.9.4.1 A Compilation of Published Experiments . . . . . . . . . . . 4.9.4.2 Enzyme Stability in Supercritical Fluids . . . . . . . . . . . 4.9.4.3 The Role of Water . . . . . . . . . . . . . . . . . . . . . . . . 4.9.4.4 Pressure Effects . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.4.5 Mass Transfer Effects . . . . . . . . . . . . . . . . . . . . . . 4.9.4.6 Effects on Selectivity . . . . . . . . . . . . . . . . . . . . . . 4.9.5 Downstream Processing and Costs . . . . . . . . . . . . . . 4.9.5.1 Downstream Processing Schemes . . . . . . . . . . . . . . . 4.9.5.2 Prosessing Cost Estimate . . . . . . . . . . . . . . . . . . . . 4.9.6 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

421

. 421 . 425

428 431 434 436 . 439 . 439 441 441 443

4.10 Phase Transfer and Ammonium Salt Catalyzed Reactions . . . 446 4.10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 446 446 4.10.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449 4.10.3 Published Work . . . . . . . . . . . . . . . . . . . . . . . . . 451 4.10.4 Experimental Procedure . . . . . . . . . . . . . . . . . . . . . 4.10.5 Related Catalytic Reactions . . . . . . . . . . . . . . . . . . . 452 453 4.10.6 Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . 453 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A: Conversion Factors . . . . . . . . . . . . . . . . . . . . . Appendix B: Abbreviations and Symbols . . . . . . . . . . . . . . . .

455 455 457

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

465

List of Contributors

Olli Aaltonen VTT Chemical Technology Biologinkuja 7 Otaniemi, PL 1401 02044 VTT Finland

Sabine Beuermann Institut fur Physikalische Chemie Universitat Gottingen Tammannstr. 6 37077 Gottingen Germany

Masahiko Arai Institute for Chemical Reaction Science Tohoku University Katahira, Aoba-ku Sendai 980 - 8577 Japan

Gerd Brunner Arbeitsbereich Verfahrenstechnik 11 Technische Universitat HamburgHarburg 2 1071 Hamburg Germany

Eric Beckman University of Pittsburgh 1243 Benedum Hall Pittsburgh, PA 15261 USA

Michael Buback Institut fur Physikalische Chemie Universitat Gottingen Tammannstr. 6 37077 Gottingen Germany

Alberto Bertucco Istituto di Impianti Chimicj Universiti di Padova via Marzolo, 9 35131 Padova PD Italy

Markus Busch Institut fur Physikalische Chemie Universitat Gottingen Tammannstr. 6 37077 Gottingen Germany

XVIII

List of Contributors

Anthony A. Clifford School of Chemistry University of Leeds Leeds LS29JT UK Christy W. Culp School of Chemical Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0100 USA Tammy A. Davidson Department of Chemistry East Tennessee State University Box 70695 Johnson City, TN 37614-0695 USA Janet DeGracia Department of Chemical Engineering University of Colorado Boulder, CO 80309 USA Joseph M. DeSimone Department of Chemical Engineering CB #3290 - Venable Hall University of North Carolina Chapel Hill, NC 27599 USA Charles A. Eckert School of Chemical Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0100 USA Cornelis J. Elsevier J. H. Van’t Hoff Research Institute Universiteit van Amsterdam Nieuwe Achtergracht 166 1018 WV Amsterdam The Netherlands

Li Fan Dept. of Applied Chemistry, School of Engineering University of Tokyo 7-3 -1, Hongo, Bunkyo-ku Tokyo 113-8656 Japan Ralf Fink BASF AG ZKD/B 1 67056 Ludwigshafen Germany David E. Fremgen Chemical Technology Division Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 USA Neil Foster School of Chemical Engineering and Industrial Chemistry The University of New South Wales Sidney, NSW 2052 Australia Kaoru Fujimoto Dept. of Applied Chemistry, School of Engineering University of Tokyo 7-3-1, Hongo, Bunkyo-ku Tokyo 113-8656 Japan John L. Fulton Pacific Northwest National Laboratory PO BOX999, MS P8-19 Richland, WA 99352 USA

List of Contributors

XIX

Sander Gaemers J. H. Van’t Hoff Research Institute Universiteit van Amsterdam Nieuwe Achtergracht 166 1018 WV Amsterdam The Netherlands

Keith P. Johnston Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA

Rex E. Gerald I1 Chemical Technology Division Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 USA

Jenny L. King Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD UK

Michael W. George Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD UK

Robert J. Klingler Chemical Technology Division Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 USA

Steven M. Howdle Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD UK

Joseph W. Kolis Department of Chemistry Clemson University Clemson, SC 29634 USA

Yutaka Ikushima National Industrial Research Institute of Tohoku 4-2-1 Nigatake, Miyagino-ku Sendai 983-8551 Japan Gunilla B. Jacobson Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA Philip G. Jessop Department of Chemistry University of California Davis, CA 95616 USA

Michael B. Korzenski Department of Chemistry Clemson University Clemson, SC 29634 USA David R. Lamb School of Chemistry Georgia Institute of Technology Atlanta, GE 30332-0400 USA C. Theodore Lee Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 7871211062 USA

XX

List of Contributors

Walter Leitner Max-Planck-Institut fur Kohlenforschung Kaiser-Wilhelm-Platz 1 45470 Mulheim an der Ruhr Germany

Jerome W. Rathke Chemical Technology Division Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 USA

John C. Linehan Pacific Northwest National Laboratory PO BOX999, MS P8-19 Richland, WA 99352 USA

Sandro R. P. Da Rocha Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA

Charles L. Liotta School of Chemistry Georgia Institute of Technology Atlanta, GE 30332-0400 USA

James M. Tanko Department of Chemistry Virginia Polytechnic Institute and State University Blacksburg, VA 24061-0212 USA

Frank P. Lucien School of Chemical Engineering and Industrial Chemistry The University of New South Wales Sidney, NSW 2052 Australia Carson Meredith Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA Martyn Poliakoff Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD UK Theodore W. Randolph Department of Chemical Engineering University of Colorado Boulder, CO 80309 USA

Klaus Woelk Institut fur Physikalische Chemie Universitat Bonn WegelerstralSe 12 53115 Bonn Germany Matt Z. Yates Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA Clement R. Yonker Pacific Northwest National Laboratory P. 0. BOX999, MSIN K2-44 Richland, WA 99352 USA

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

1 Introduction 1.1 Supercritical Fluids as Media for Chemical Reactions PHILIPG. JESSOPand WALTER LEITNER

1.1.1 What is a Supercritical Fluid (SCF)? What is a supercritical fluid? The definition used in this book, modified from that of IUPAC, refers to a SCF as the defined state of a compound, mixture or element above its critical pressure @), and critical temperature (T,) but below the pressure required to condense it into a solid (Figure 1.1-1). It is noteworthy that the definitions by IUPAC [l], the Oxford English Dictionary [2], and other dictionaries [3] omit the clause concerning condensation into a solid. That the melting curve extends over the supercritical region [4-61 is often forgotten even though the pressure at this curve is not always impractically high. For example, the pressure required to solidify C02 at its critical temperature is only 5 700 bar [7], but that for water is an enormous 140 000bar [8]. Some dictionaries [9, 101 omit all restrictions on pressure and only require that the temperature be greater than T,. Technically, a SCF is a gas but not a vapor. The term “gas” refers to any phase which “will conform in volume to the space available” [9]. A “vapor” is defined as “a gas whose temperature is less than the critical temperature” [9]. This was the definition proposed by Thomas Andrews [12] in 1869 and is still generally, but not universally [lo], accepted today. In practice, however, the terms gas and vapor are often used interchangeably. The conditions under which a compound is investigated are often described in terms of redttced temperature (T,) and reduced pressure @,), defined as the actual values of T and p divided by T, and p,, respectively (eqs 1.1-1 and 1.1-2). The “law of corresponding states” as introduced by van der Waals [14] implies that compounds behave similarly under the same values of the reduced variables. This allows valuable comparison of different compounds under various conditions, but deviations can be substantial in close proximity to the critical point.

T, = TIT,

(1.1-1)

Pr = PIP,

(1.1-2)

2

I Introduction

10,m

-

1OOO-

P [bar]

10 -

100

10.1

-100

I -50

I

I

0

50

100

T Wl

Figure 1.1-1The phase diagram of C 0 2 [7,11]. The critical and triple points are shown as filled circles. The inset (with a linear pressure scale) shows an expanded view of the area around the critical point; the tear-shaped contour indicates the compressible region.

The properties of SCFs are frequently described as being intermediate between those of a gas and a liquid. This Janus-faced nature of SCFs arises from the fact that the gaseous and liquid phases merge together and become indistinguishable at the critical point. Figure 1.1-2 shows how the meniscus between the phases disappears upon reaching the critical point for COz. Not all properties of SCFs are intermediate between those of gases and liquids; compressibility and heat capacity, for example, are significantly higher near the critical point than they are in liquids or gases (or even in the supercritical state further from the critical point). Although the properties of a compound may change drastically with pressure near the critical point, most of them show no discontinuity. The changes start gradually, rather than with a sudden onset, when the conditions approach the critical point. It is quite common to refer to the somewhat ill-defined region where such changes are noticable as the “near-critical” region. The synonym “para-critical” may be occasionally found in older literature. Technically, the near-critical region extends all around the critical point, but the expression is commonly used to refer to the nonsupercritical section only. The very similar expression “compressible region” refers to the area around the critical point in which the

I . 1 Supercritical Fluids as Media for Chemical Reactions

3

Figure 1.1-2 The meniscus separating liquid and gaseous COz disappears when the critical point is reached by heating liquid COz in a closed vessel. A small amount of a highly COz-soluble and brightly coloured metal complex [13] was added for better contrast.

compressibility is significantly greater than would be predicted from the ideal gas law. Although a significant portion of the compressible region lies inside the SCF section of the phase diagram, there is overlap with the liquid and vapor regions as well (inset of Figure 1.1-1). Thus, even liquids do have significant compressibility near the critical point, although they are virtually incompressible at T, << 1. Liquid phases at temperatures below but not too far below T, are called “subcritical liquids”, whereas “subcritical gases” are those at pressures below p,.

I Introduction

4 1,2

1

1

=.

I

I

I

/”

cyclohexane I

o,o! 0

. . .

..

i

loo

. .

.

1

200

.

. .

,

. . .

300

n-pentane

I

400

. . .

I

..

.L 0

500

pressure [bar]

Figure 1.1-3 The density and the solvent power (as expressed by the Hildebrand parameter) of scCOz as a function of temperature and pressure [15,16].

When working with a SCF, it is valuable to refer to a plot of the dependence of density (6)on pressure and temperature, as presented for supercritical C02 (scCO2) in Figure 1.1-3. Note that the density changes sharply but continuously with pressure in the compressible region. At higher pressures, the density changes occur more gradually. The critical density d, (i.e. d at T, and p,) is the mean value of the densities of the gas phase and the liquid phase and amounts to 0.466 g/mL for C02. The reduced density is defined in analogy to the other reduced variables (eq 1.1-3). The density data shown in Figure 1.1-3 correspond to the bulk density of the medium, but density fluctuations lead to microscopic areas of decreased and increased local densities in SCFs (local density augmentation). Because of the very large compressibility, these density fluctuations are most pronounced very near to the critical point. If the fluctations are of the same order of magnitude as the wavelength of visible light, scattering of the light leads to critical opalescence, which may be apparent as a clouding or coloration of the SCF and can be used to determine the critical point. d, = did,

(1.1-3)

Many solvent properties are directly related to bulk density and will therefore have a pressure dependence similar to the one shown in Figure 1.1-3. The best known example is the continuous variation in “solvent power” over a fairly wide range, which provides the basis for the technical use of SCFs in highly selective natural product extractions. The solvent power is a rather ill-defined property, but there have been experimental approaches to devise scales for liquid solvents. One of the most successful attempts was put fore-

I . 1 Supercritical Fluids as Media for Chemical Reactions

5

20 18-

b)

161412E

0

I 100

I 200

P [bar]

I 300

I 400

501 0

10-

0

0.5

1.0

1.5

2.0

25

dr

Figure 1.1-4 The dielectric constant of H20(4OO0C, T, = 1.04) [20,21], COz (40°C, T, = 1.03) [17], and CHF3 (30°C, T, = 1.01) [18,19], as functions of (a) pressure and (b) reduced density.

ward by Hildebrand [ 161; the so-called Hildebrand parameter for solvent power was found to be directly proportional to the density of SCFs [15], as shown in Figure 1.1-3 for COz. Some typical organic solvents are marked on the Hildebrand scale for comparison to give some indication of the tunability of the solvent power of C02. It should be apparent from the diagram that in order for a SCF to have significant solvating ability, it must usually have d, of greater than 1. The dielectric constant (E) also increases sharply with pressure in the compressible region (Figure 1.1-4a), and this behavior parallels to some extent the change in density. Thus E is more logically plotted against density (Figure 1.1-4b). The magnitude of the increase in E depends on the nature of the SCF. For COz at T, - 1, E changes from ca. 1.3 at d, = 1 to just above 1.6 at d, = 2, i.e. scCOz remains a very non-polar solvent regardless of the actual conditions [17]. In contrast, the dielectric constants of fluoroform [18, 191 and water [20,21] increase, with the same density change, from approximately 3 to 6 and from 5.4 to 13.5, respectively. The use of the tunable dielectric constant of polar supercritical fluids is a topic that has not yet received much attention, but could assist in studies of solvent effects and the polarity of transition states. The possibility of using SCFs as “tunable solvents” not only for extraction (SFEi) but also for chemical reactions (SFR) is one of the many interesting features associated with their application in modern synthesis. Before we discuss the many potential benefits in detail (Section 1.1.3), it seems appropriate to give a brief introduction to some practical aspects of the use of SCFs on a laboratory scale.

6

I Introduction

1.1.2 Practical Aspects of Reactions in Supercritical Fluids (SFRs) Inorganic and organic compounds which are frequently used as SCFs are listed, together with leading references for their volumetric behavior, in Tables 1.1-1 and 1.1-2. The listed SCFs include those that are most commonly employed for syntheses and chemical reactions and they are compared in terms of critical parameters, physical properties and approximate cost. For critical parameters and volumetric data of other SCFs, the reader is referred to several reviews [22-261 and the series of contributions by the IUPAC Commission 1.2 on Thermodynamics, Subcommittee on Thermodynamic Data [27-331. Even more detailed volumetric data for SCFs can be found in two excellent series of monographs: the IUPAC Thermodynamic Tables Project [ l l , 34-43] and the Thermodynamic Properties series by the National Standard Reference Data Service of the USSR [44-501. The purity of the SCF is an important consideration in the planning of a synthesis. Low concentrations of impurities can have noticeable effects on the volumetric and phase behavior of SCFs. For example, helium can be present in commercial C 0 2 because it is sometimes added as a "head-gas" to ensure nearly-complete delivery of the cylinder contents and this has been found to affect the use of scC02 as a solvent for analytical and preparative purposes [51-531. The He head-gas is unnecessary if a cooled pump is used for C02 delivery. Purity can also have an effect on the cost of the SCF. The prices in Table 1.1.-1 are typical for research quantities with the purity closest to 99.99 % as of early Table 1.1-1 Selected inorganic supercritical fluids.

SCF Name

Ar COz HCl HBr HI H20 NH3 N20 Kr SF6 Xe

TC

V-3

Pc (bar)

argon -122.5 48.6 carbon dioxide 31.1 73.8 82.6 hydrogen chloride 51.5 85.5 hydrogen bromide 90.0 hydrogen iodide 150.7 83 water 374.0 220.6 ammonia 132.4 113.2 nitrous oxide 36.4 72.5 krypton -63.76 54.9 sulfur hexafluoride 45.5 37.6 xenon 16.6 58.3

dc MW WmL)

p

We-

Costb References ($k)

bye)"

0.531 0.466 0.42 n.a. n.a. 0.322 0.235 0.453 0.912 0.737 1.099

39.95 44.01 36.46 80.91 127.9 18.02 17.03 44.01 83.80 146.1 131.3

0 6 0 3 20 1.08 0.82 50 0.44 n.a. n.a. 1.85 3 1.47 0.167 50 0 3000 0 50 0 4000

[46, 1921 [ l l , 1931 [24, 194, 1951 [24] [24] [196-1981 [26, 199, 2001 [201] [46] [202-2051 [46, 2061

Reference 207. 'Cost: $ per kg assuming 99.99% purity, prices from common distributors. All of these prices fluctuate, especially those of the noble gases. n.a. = not available.

a

I . I Supercritical Fluids as Media f o r Chemical Reactions

7

Table 1.1-2 Selected organic supercritical fluids.

CHF3 CH2F2 CH4 CH40 C2H4 C2H6 C2H60 C2H& C3H6 C3H8 C4H,o C4H10 C5H12 C6H6 C6H14

fluoroform difluoromethane methane methanol ethene ethane dimethyl ether ethylenediamine propene propane n-butane isobutane n-pentane benzene n-hexane

25.8 78.1 -82.6 239.5 9.2 32.2 126.9 320 91.8 96.7 152.0 134.7 196.6 289.5 234.5

48.2 57.8 46.0 80.8 50.4 48.7 54 62.8 46.0 42.5 38.0 36.4 33.7 49.2 30.3

0.525 0.424 0.163 0.273 0.214 0.207 0.242 0.29 0.228 0.220 0.228 0.224 0.232 0.300 0.234

70.01 1.65 125 52.02 1.97 n.a. 16.04 0 80 32.04 1.70 18' 16 28.05 0 30.07 0 100 46.07 1.30 15 60.10 1.99 2OOd 42.08 0.366 9 44.10 0.084 10 58.12 <0.05 15 58.12 0.132 15 72.15 n.a. 30 78.11 0 30' 86.18 n.a. 300

[45, 2081 1209, 2101 [43, 2'111 [30, 40, 2121 [27, 48, 2131 [31, 47, 2141 [231 1251 127, 381 [31, 44, 2151 131, 2151 12151 r311 [216, 2171 1311

Reference 207. bCost: $ per kg assuming 99.99% purity except for higher hydrocarbons (ethene and C3 or greater), which are 99.5 %. Prices from common distributors as of early 1998. All of these prices fluctuate, especially those of the halocarbons. n.a. = not available. 99.9 % purity. 99.5 % purity.

a

1998. However, for some materials, especially the higher hydrocarbons, the price is highly dependent on the purity and very high purities are prohibitively expensive. For these compounds the price is listed for 99.5 % purity; the major contaminants are usually other hydrocarbons of similar molecular weight. Specialized equipment is required for experiments with supercritical fluids, as described in more detail in Chapter 2.1. Figure 1.1-5 shows a photograph of typical laboratory scale equipment, which allows the investigation and visual inspection of common SCFs at temperatures up to 150"C and pressures up to 250-300 bar. A wide variety of modifications to this standard set-up is possible allowing addition of reagents or catalysts, sampling or online spectroscopic investigations. With this arsenal of techniques at hand, chemical reactions in SCFs can be studied in as much detail as in conventional solvents and many examples will be discussed throughout this book. The physical and chemical properties of the SCFs can sometimes present hazards to the experimentalist [54]. All researchers in the field should search the literature for information concerning the hazards of the materials with which they are working. The following information is presented as a brief overview only, and should not be considered a comprehensive review on the subject.

8

I Introduction

Figure 1.1-5 n p i c a l laboratory scale reactors as used at the Max-Planck-Institut fur Kohlenforschung for the investigation of chemical synthesis in SCFs. The reactors are basically window-equipped modifications of standard high pressure vessels of 225 mL and 25 mL volume, respectively. In addition to the monitoring devices for pressure and temperature, various fittings and connections for addition, sampling or online analysis can be installed on the lid of the larger design on the left.

All SCFs are compressed gases, and therefore contain a great deal of potential energy which can be released upon catastrophic failure of the equipment. As safety regulations vary from country to country and also depend on the size of the reactor and the maximum applied pressure, we can only give some general advice here. All new equipment should be pretested by filling with an incompressible liquid (water, oil, hexane) under pressures of approximately 1.5-2 times the maximum operating pressure. One of the simplest and most effective safety rules when working with SCFs is to avoid direct exposure of the operator to the pressurized vessel, for example by using strong polycarbonate or Lexan shields or similar optically transparent safety equipment. The use of angled mirrors or video equipment also allows visual inspection without direct exposure. Other than the large potential energy, most of the hazards of SCFs are related to the chemical reactivity of the gas itself. Several SCFs, such as scH20, scHCl and the other acids, corrode standard stainless steel reaction equipment, which could result in catastrophic failure. Explosive deflagration or decomposition is common with acetylene even at subcritical pressures [55]. Perfluoroethylene (scC2F4) will explode at pressures above 2.7 bar unless inhibitor is added [56]. Even seemingly stable SCFs can explosively decompose at sufficiently high temperatures (e.g. 500 "C for hexane) [57]. Runaway polymerization can be a concern when polymerizable SCFs like scC2H4 are used [58]. The

1.1 Supercritical Fluids as Media for Chemical Reactions

9

polymerization can be initiated by free radicals, 02,or metal catalysts including even stainless steel components. SCFs, like most other compounds, can be incompatible with some other materials; examples of incompatible combinations that have exploded or violently reacted in the past include NH3/ethylene oxide, HBr/Fe203, HCVAl, C2H2/Cu and many others [56]. Flammable SCFs include d l of the hydrocarbons plus others like scCH30H, scCH30CH3, scNH3, and ethylendiamine. Silane (scSi&) is particularly dangerous because it could autoignite upon leaking from the vessel, independent of any spark source [59]. The only commonly used oxidizing SCF is scN20. Mixing significant quantities of combustible material with scNzO has led to at least two explosions in the past. One of these occured when ethanol (9 vol %) was used as a cosolvent in scN20 [60,61]. while the other occured when 1 g of roasted coffee was exposed to scN20 in a 2.5 mL extraction vessel [62]. Most of the SCFs listed in Tables 1.1-1 and 1.1-2 have a comparatively low level of acute toxicity [63], but the high local concentrations which may result from use under high pressures require appropriate safety considerations, such as sufficient ventilation. Irritant poisons include ethylenediamine, NH3 and the acidic SCFs (HCl, HBr, and HI) [56]. SCFs such as scCHF3 and scN20 (laughing gas) are known to act as narcotics when at high concentrations [64]. Carcinogenic SCFs such as benzene should be replaced by other SCFs with similar properties wherever possible. Toxic compounds dissolved in SCFs can be spread throughout the laboratory if the pressurized solutions are vented outside of a fume hood. The same action can result in contamination of stock chemicals in the laboratory [65]. Those SCFs with particularly high T, values (eg. scH20) could cause thermal burns to operators if a leak were to occur. Chemical bums could result from leaks if irritants or acids are used as SCFs or are dissolved therein. These chemical risks, and the procedures for avoiding them, should be familiar to the practicing chemist utilizing SCFs on a laboratory or technical scale.

1.1.3 Motivation for Use of SCFs in Modern Chemical Synthesis Why use SCFs as solvents for chemical reactions? There are numerous advantages associated with the use of SCFs in chemical synthesis, all of which are based on the unique combination of properties of either the materials themselves or the supercitical state. Different types of reactions may benefit particularly from a specific property, and these sometimes spectacular effects will be discussed in detail in the individual chapters of the book. Here, we try to summarize briefly the various potential improvementsthat can be expected if SCFs are employed as solvents for synthetically useful chemical reactions. The advantages fall into four general categories: environmental benefits, health and safety benefits, process benefits, and chemical benefits (Table 1.1-3).

10

I Introduction

Table 1.1-3 Advantages of using SCFs rather than liquids as reaction media. Category

Advantage

Which SCFsa

Environment

do do no no

most mest CO2, H2O CO; and other volatile SCFs

Health and safety

noncarcinogenic nontoxic nonflammable

most (but not C6H6) most (but not HC1, HBr, HI, en, NH3) C02, N20, H20, Xe, Kr, CHF3

Process

no solvent residues : facile separation of products high diffusion rates low viscosity adjustable solvent power adjustable density inexpensive

C 0 2 and other volatile SCFs C 0 2 and other volatile SCFs all all all all COz, H20, NH3, Ar, hydrocarbons

Chemical

high miscibility with gases variable dielectric constant high compressibility local density augmentation high diffusion rates altered cage strength

all the polar SCFs all all all all

a

not contribute to smog not damage ozone layer acute ecotoxicity liquid wastes

Of those which appear in Tables 1.1-1 and 1.1-2.

Environmental benefits are most often cited for processes with scC02 or scH20 as the solvents. Although C02 and many of the other SCFs are greenhouse gases, the use of C02 as an industrial solvent would still be of benefit to the environment because it would allow the replacement of environmentally far more damaging liquid organic solvents. Processes involving C 0 2 as a solvent would not increase C 0 2 emissions, but rather provide an opportunity for the recycling of waste COz. Also, reactions that result in the fixation of the scCO2 would consume a small amount of waste C02 and decrease our dependence on fossil fuels as sources of carbon-containing molecules. At present, most recovered C 0 2 is generated as a byproduct of ammonia and hydrogen production, but efforts to sequester C 0 2 from flue gases of power plants are rapidly increasing [66]. Health and safety benefits include the fact that the most important SCFs, scCO2 and scH20, are noncarcinogenic, nontoxic, nonmutagenic, nonflammable and thermodynamically stable. Very few traditional liquid solvents fit this description. However, supercritical water may not be appropriate as a solvent for some organic syntheses because of its extremely high critical temperature. The foremost application for which scH20 is being tested and used is oxidative destruction of toxic wastes, which is beneficial to the environment.

I . I Supercritical Fluids as Media for Chemical Reactions

11

A detailed analysis of the environmental impact of a chemical process must also include the energy consumption during operation, which is of course also a major concern for economical reasons. This balance is not always straightforward; for example energy would be saved during removal of the solvent by releasing C02 instead of distillation of an organic solvent. On the other hand, the compression of C 0 2 is energy costly. However, it is often forgotten in these comparisons that a process operating with compressed C02 would not alternate between the pressure required in the supercritical mixture and a full expansion to ambient pressure; reducing the pressure to any value that allows separation of the product is sufficient and the required pressures may well be close to or even above the pc of pure C02. Recompression is usually achieved by cooling the :gas to a liquid and then repumping the liquid, a process which is considerably less expensive than recompressing the gas directly. Furthermore, temperature rather than pressure may be the variable of choice for many separation processes. Therefore, such comparisons require sufficiently detailed information on potential technical solutions for the process under scrutiny in order to avoid decisions based on prejudice. Process benefits derived from the physical properties of SCFs, such as high diffusivity, low viscosity, and intermediate density, make SCFs particularly suitable for continuous-flow processes. The high flow rates and fast reactions often encountered with SCFs allow the design of high-throughput reactions in relatively small scale reactors. The engineering solution for up-scaling of SFRs that have been tested in batch or semibatch reactors on laboratory scale will in most cases not involve an increase in the size of the reaction vessel, but rather the design of a continous-flow system with high space-time yields. In other words, the actual technical solution of a successful synthesis in SCFs will most likely bear more resemblance to a gas phase process, whereas the exploratory test phase can be widely similar to the screening of liquid phase reactions. One of the most obvious advantages of SCFs for chemical synthesis is their adjustable solvating power. The considerable body of knowledge concerning extraction and solubility in SCFs can be brought to bear to solve engineering problems in the separation and purification steps of industrial chemical processes in SCFs. For example, the extractive properties of SCFs may be exploited to separate products from byproducts or to recover homogeneous catalysts. The design of integrated SCF-based processes replacing wasteful and time-consuming work-up and separation schemes seems highly attractive, but has been met piecemeal at best. An elegant application of this concept is the in situ regeneration of heterogeneous catalysts by SCF extraction of wax or tar byproducts that would block pores and active sites in gas phase reactions. Some control of the molecular weights of growing polymer chains is also possible by control of the precipitation. It might also be possible to isolate intermediate reaction products by selective precipitation or extraction and prevent them from further reacting or decomposing. The volatility of many SCFs allows complete removal from the product without the need for costly or energy consuming drying processes. Thus, solvent residues

12

I Introduction

in products can be avoided; this is particularly valuable in the preparation of cosmetics, pharmaceuticals, food additives, and materials for use in electronics. In addition to making processes cleaner and more efficient, the use of SCFs as solvents can also have beneficial effects directly on chemical reactions. Many reactions that can be performed in SCFs-occur also in liquid solvents, but there is a considerable number of examples -where the use of an SCF as the solvent increase the rate of the reaction over that which would be observed in a liquid medium. Several of the unique properties of SCFs can cause such a change in rate, and it is not always easy to identify the most important contribution. For example, extremely rapid reactions that are diffusion controlled or altered by solvent cage effects can be more rapid in SCFs because of the higher diffusivity and weaker cage effects. Local solute-solute “clustering” can lead to high local concentrations of reagents, increasing the rates of reactions which are performed near the mixture critical point. Another advantageous property of SCFs is their miscibility with other gases, which can lead to high rates of reactions if the kinetics are first order or higher in the concentration of the dissolved gas. The presence of a single homogeneous phase is especially important for catalytic reactions that would be operating under mass transport limitations under two-phase liquid-gas conditions. Similarly, reactions that involve mass transfer between a liquid phase and a solid phase such as a polymer or zeolite are often mass transport limited. The rates of such reactions can be greatly enhanced under supercritical conditions. The same arguments predict that diffusionally limited reactions involving suspended enzymes in liquid organic solvents would be faster in an SCF. Optimization of the rate of a SFR can be achieved by varying the choice and concentration of cosolvent, a technique known as “cosolvent tuning”. Finally, the SCF may take part directly in the chemical reaction, for example as one of the reagents (hydrolysis, C02 fixation) or by modifying substrates or catalytically active sites. Other chemical benefits can be related to selectivity changes. Any of the above factors known to affect rates could affect selectivity by altering the rates of competing reactions. In addition, the pressure dependence of typical solvent parameters like dielectric constant of some SCFs may cause a considerable tuning effect on the selectivity of enzymatic and homogeneous catalysis. Owing to the high compressibility of the supercritical phase, the chemical potential of reactants and catalysts can also be varied widely, without changing the reaction volume. All these factors may affect the chemo-, regio-, and stereoselectivity of chemical reactions, and open additional degrees of freedom for the optimization of synthetic processes. It is possible to become too caught up in the excitement over environmentally benign processes, rate increases and other potential benefits of SFRs. If the reaction can be performed anywhere close to adequately in a relatively benign liquid solvent, then there is little motivation for the switch to SCFs, because their use is still considered to be expensive. However, in cases where chemical, process, or environmental benefits can be obtained, industrial use of supercritical conditions is economically feasible and often already a reality

I . 1 Supercritical Fluids as Media for Chemical Reactions

13

(Section 1.1.4.3). Although the costs for the implementation of high pressure equipment and for operating a SFR process are arguably higher than using an existing reaction vessel, a more detailed analysis must include the costs for all steps of the process including work-up and waste treatment and may well lead to different results. As mentioned above, it is also important to consider possible engineering solution; which are quite different from the intial screening procedure and may lead t o much lower equipment and operating costs than anticipated. Furthermore, one should bear in mind that small-scale operating units for SFR can be highly flexible and may allow the equipment to be switched from one process to another without long down times. Nevertheless, it seems fair to conclude that new applications of SFR will be most likely in the synthesis of high-value fine chemicals that are given directly to the customer, rather than for commodities which are used as intermediates for further downstream processing. One key aspect which is often neglected in this context is marketing: it might still be cheaper to produce decaffeinated coffee using CH2C12 as a solvent, but it is very unlikely that the customer would accept this product. We suspect that the market would also react favorably to food preservatives, pharmaceuticals or cosmetics which were produced using natural “carbonic acid” (scC02) instead of organic solvents.

1.1.4 A Brief History of Chemical Synthesis in SCFs 1.1.4.1 Discovery of SCFs and their Use as Solvents The interest in SFRs has seen a tremendous increase over the last few years, because the special properties of SCFs make them particularly attractive solvents for modern synthetic chemistry. We should be aware of the fact, however, that the idea of using SCFs as reaction media has been emerging ever since the discovery of this “peculiar state of matter” early in the nineteenth century by Baron Charles Cagniard de LaTour, an experimental physicist in France [67]. The experiments which led to the discovery of the critical point were prompted by the research of Denys Papin in England in 1680. He designed a high pressure vessel, his “digester” (Figure 1.1-6), and used it to prove that the boiling of water could be suppressed by the action of pressure. He demonstrated a practical application for the raised boiling temperature of water by cooking a meal in his digester for King Charles I1 [68]. In France, Baron Cagniard de LaTour (Figure 1.1-7) speculated that this suppression of boiling must have a limit, and his risky experiments to test this theory in 1822 proved the existence of the critical point [67] (see box on p. 15).

14

I Introduction

Figure 1.1-6 The “digester” made by Denys Papin in 1680 [72]. The first vessel of Cagniard de LaTour was based on this design (Oxford University Press).

Figure 1.1-7 No portraits of Baron Cagniard de LaTour were found by the authors, but we do know, from this registration form, that he had chestnut hair, grey eyes, a small mouth, long chin and was 1.73 m tall (photo courtesy of Ecole Polytechnique).

1.1 Supercritical Fluids as Media for Chemical Reactions

15

I introduced into a small Papin’s digester, built from the end of a thickwalled gun barrel, a certain quantity of alcohol at 36 degrees and a marble or sphere of flint; the liqui pied nearly a third of the interior capacity of the apparatus. Havin rved the kind of noise that the marble produced upon m y making it roll in the barrel at first cold, and then heated little by little over a fire, I arrived at a point where the marble seemed to bounce at each collision, as if the liquid no longer existed inside the barrel [69].

The same effect was observed with “l’ether sulfurique” (diethyl ether) and petroleum ether, but not with water because of the high critical temperature of water. He called this new state of matter “1’Ctat particulier”. With the use of sealed glass tubes (Figure 1.1-8) he was able to observe the transition, describing it in the following manner [67]:

n Figure 1.1-8 The glass tube design used by Cagniard de LaTour to observe the transition from a liquid past the critical point to a supercritical fluid. The internal diameter was l mm over most of the length, but the section def had a diameter of 4 mm. Mercury was introduced into section bcde and ether was put into ef. The ends a and f were then sealed and the tube heated over a fire. At the moment when the ether was transformed into a vapor, the level b of the mercury had climbed to point g. The pressure was calculated from the distances to be 37-38 bar [67].

16

I Introduction

The liquid, after approaching double its original volume, completely disappeared, and was converted into a vapor so transparent that the tube appeared entirely empty [70].

He refined his methods to allow for the determination of critical temperatures and pressures 1711. His values for diethyl ether and carbon disulfide are within 15°C and 4 bar of the values accepted today. Slowly at first, other researchers continued his work. The nature of the supercritical state and the significance of the critical point were debated by Michael Faraday, Dimitrii Mendeleev, Thomas Andrews, and others [73]. Faraday referred to the critical point as the “disliquefying point” and the supercritical state as “that beautiful condition which Cagniard de la Tour has made known” 1741 which he later shortened to “Cagniard de la Tour’s state”. Andrews (Figure 1.1-9) introduced the term “critical point” 1121 and described the true nature of the supercritical state in his thorough study of carbon dioxide (Figure 1.1-10) 175,761.

Carbonic acid at 35.5” and under 108 atmospheres of pressure, stands nearly midway between the gas and the liquid; and we have no valid grounds for assigning it to the one form of matter any more than to the other. . . . The gaseous and liquid states are only distant stages of the same condition of matter, and are capable of passing into one another by a process of continuous change. [I21

Figure 1.1-9 Thomas Andrews (1813-1885) [77].

I . I Supercritical Fluids as Media for Chemical Reactions

17

Only 4 years after these remarkable experiments, van der Waals wrote his PhD thesis [ 141 at the University of Leiden entitled Die Kontinuitut des Piissigen und gasfiirmigen Zustands (on the continuity of the liquid and gaseous f

0fr

Figure 1.1-10 The equipment used by Thomas Andrews to explore the phase behaviour of carbon dioxide. The glass tube had a diameter of 1.25 mm except in the centre section where the diameter was 2.5 mm. The tube was flushed through with C 0 2 and then a was sealed. The lower end m was put into a mercury tube. Heating or applying external vacuum caused some C02 to be expelled and then some mercury was drawn up into the tube. The glass tube was placed within a copper tube containing water, which was compressed with a steel screw to pressures up to 400 bar [12].

-

0

-

a

18

1 Introduction

states) where he developed his equation of state for nonideal gases. His theory also provided a first qualitative explanation for the critical phenomena of gases at temperatures around or above T,, but it failed for T < T,. In 1875, Maxwell introduced the equal area construction which allowed the inclusion of subcritical temperatures as well [78]. Our current Understanding of the behavior of gases is still mainly based on their work. The sometimes quite spectacular phenomenon of critical opalescence also attracted‘the interest of many physicists at the beginning of this century. The relationship between density fluctuation and light scattering formed the basis of theoretical work by von Smoluchowski [79] and Einstein [80,8 11, with important contributions devoted directly to critical opalescence from Ornstein and Zernike [82]. Following Andrews’ pioneering work, the number of experimental investigations devoted to SCFs increased rapidly in the last quarter of the nineteenth century, including for example the studies of Sajotschewsky who located the critical points for eight compounds in 1879 [83]. A journal covering the field of compressed gases, Zeitschrijit fur Komprimierte und Flussige Gase, started publication in 1897. The early phase behaviour and critical point studies were mainly devoted to pure gases [22-24,841, but the possibility of making gaseous solutions by dissolving solids in compressed gases soon became of interest, too. Experiments by Gore in 1861 had already shown that liquid C02 had the ability to dissolve compounds such as camphor, naphthalene and iodine [85]. However, the failure of other solids, including charcoal, aluminium metal and iron sulfate, to dissolve led Gore to remark rather unfairly that liquid C02 was “a very feeble solvent of substances in general.” Hannay and Hogarth undertook the first systematic solubility studies on SCFs in 1879-1880 [86,87], demonstrating that chlorophyll, KI and other solids were soluble in supercritical alcohol. They extended the work to sulfur and other reagents in supercritical carbon disulfide. Their astute observations included the comments that “a liquid has its critical point raised by the solution in it of a solid,” [87] and that a very finely divided solid could be produced by rapidly reducing the pressure of a supercritical solution. The latter phenomenon is the basis for the modern-day RESS process (Chapter 2.3). Paul Villard (Figure 1.1-11) a physicist better known for his later discovery of gamma rays [MI, was working on phase behavior in 1896 when he found that iodine, alkanes, stearic acid, and camphor could be dissolved in supercritical ethylene (scCZH4) [89,90]. He noticed that the action of the S C C ~ H ~ caused the camphor to melt before it dissolved, even though the temperature was not being raised (see the PGSS process, Chapter 2.3). Studies of solubilities and phase behavior continued through the twentieth century, and the results are described in a number of reviews [91-961. More details on the current understanding of solubility data and phase behaviour can be found in Chapter 1.2 of this book. Starting from the early 1920s, practical applications of compressed gases were recognized for extraction and purification. E. B. Auerbach from Berlin filed several international patents on a process utilizing liquid C02 in the

I . I Supercritical Fluids as Media for Chemical Reactions

19

Figure 1.1-11 Paul Villard (18601934) [97] (photo used with permission from the AcadCmie des Sciences).

years between 1926 and 1928 [98,99]. SCFs were investigated especially in the rapidly growing petrochemical industry. Shell Oil patented various processes utilizing para-critical (i.e. near-critical) hydrocarbons and other gases including COz [loo, 1011. In the mid 1950s, several technical solutions for SFE processes for coal, mineral oil and wool were proposed by Zhuze [102]. In 1955, Todd and Elgin pointed out the analogy of SCF/solid and SCFAiquid separation with conventional extraction [99]. A major technical breakthrough came with the development of natural product extraction using scC02 by Kurt Zosel (Figure 1.1-12) at the MaxPlanck-Institute for Coal Research in the early 1960s [103]. His interest in SCFs originated from accidental observations made during the preparation of higher olefins from ethene in the presence of aluminium trialkyls (Aufbau reaction), when experiments were carried out under conditions beyond T, and pc of ethene. His enthusiasm for scC02 was initially seen with some scepticism, but it soon became apparent that extraction with toxicologically benign carbonic acid was highly attractive for the food industry. The first applications of “destraction” - as the process was named by Zosel - to the extraction of caffeine from green coffee beans and of hops aroma were already implemented on a technical scale between 1975 and 1985. Today, more than 100000 t of decaffeinated coffee are produced per year world-wide using this technology. Some recent developments in this area, which is now commonly referred to as supercritical fluid extraction (SFE), and aspects related to chemical synthesis are found in Chapter 2.2 of this book.

20

I Introduction

Figure 1.1-12 Kurt Zosel (1913-1989) (photograph used with permission from the Photoarchiv MPI fur Kohlenforschung, MulheimRuhr).

1.1.4.2 Early Examples of Chemical Reactions in SCFs When was the first use of a SCF as a medium for a reaction? If we exclude the fact that Nature has been doing chemistry in scH20 in the Earth’s crust for a very long time [104], the first reactions observed in SCFs appear in the nineteenth century. It was again Baron Cagniard de LaTour who noticed as early as 1822 that near-critical water seemed particularly reactive. He observed that it was “capable of decomposing glass by dissolving its alkali” and speculated that “one could possibly obtain some other interesting results for chemistry, in multiplying the applications of this process of decomposition” [ 1051. Indeed, most of the studies of SFR during the next few decades primarily concerned H 2 0 at temperatures above T,. Pioneering studies were carried out by Gabriel-Auguste Daubrk, a geologist who held the Chair of Mineralogy and Geology at Strasbourg [106]. In 1857 he reported tests of the reactions of scH20 with glass and with minerals at 400 “C. Daubrke put the water and mineral in a glass tube, and put the sealed tube in water in an iron outer tube. The whole assembly was then heated. Thus, the glass would not break because it would experience roughly equal pressures from both inside and out. The pressures were, in his words, “enormous”, but as they were not measured, we know only that the water was supercritical in terms of temperature (ignoring the effect of dissolved minerals on the T,). Hot spring water containing potassium silicate, when heated in this manner, deposited quartz. Reaction of this water with kaolin produced crystals of feldspar. Wood was‘transformed to a material resembling anthracite under similar conditions [107].

1.1 Supercritical Fluids as Media for Chemical Reactions

21

Early research on hydrothermal reactions was also carried out by Charles Friedel, an instructor at the Ecole Normale SupCrieure [108], the same institution where Paul Villard received his education. Over many years Charles, and later his son Georges Friedel, published a series of papers on the reaction of minerals in H20 at temperatures and presumably also pressures far above critical, starting with the preparation of quartz in 1879 [log, 1101. Charles Friedel is more famous for his discovery of the Friedel-Crafts alkylation of aromatics. The works of de LaTour, DaubrCe and the Friedels together ensured that the first experimental reactions of and in SCFs were those of H20, as reviewed in great detail by Morey [l 111. Reactions in SCFs other than water started, so far as we have been able to determine, with Hannay and Hogarth’s [86] solubility experiments in 1879. They found that a blue liquid ammonia solution of elemental sodium underwent a reaction when heated past the critical temperature of the ammonia, producing a white solid and hydrogen gas. With hindsight, we would formulate the reaction as shown in eq 1.1-4. Just as unexpected was their observation that solid zinc oxide reacted with scCCb at 300°C without dissolving [87]. The products were zinc chloride and an unidentified gas. Elemental arsenic seemed to dissolve in scCS2 but when the pressure was released, reddish yellow crystals were precipitated. Hannay and Hogarth believed these to be arsenic sulfide arising from a reaction with the SCF. Villard also observed reactions during his measurements of the solubility of solids in SCFs in 1898 [89,90]. When he tried to dissolve iodine in scC2H4 at 17°C and 300 bar he found that, once dissolved, the iodine slowly reacted with the ethylene (eq 1.1-5). Over a period of 1-2 h the deep violet colour of the solution faded. The solid white product was isolated in crystalline form by pressure reduction.

(1.1-5) 300 bar A wide variety of high pressure reactions were described by Vladimir Ipatiev (Figure 1.1-13) [108], a professor at the Chemistry Lab of the Michail Artillery Academy in St. Petersburg, in articles from 1904 to 1914 (Scheme 1.1-1). Many of these reactions were at supercritical conditions, although not all of his reports accurately specified the pressures at which the reactions were performed. We now take for granted the high pressure “bomb” or autoclave, which he was the first [ 1121 to develop (Figure 1.1-14). Among his significant discoveries was the first observation that heating and pressurizing S C C ~ H caused ~ it to be non-catalytically oligomerized into liquid alkanes and cycloalkanes of a range

22

1 Introduction

Figure 1.1-13 Vladimir N. Ipatiev (18671952) in 1897, 6 years before he started his high pressure work [log].

70 bar

Ni

(CHd2C=0 (CH3)2CHOH ~ ( C H ~ ~ C H O H )

+

H2 + CH, etc.

200-325"C

0;i,2*p I

+otherproducts

l10bar

Scheme 1.1-1 Some of the reactions of supercritical fluids or mixtures studied by Ipatiev.

1.1 Supercritical Fluids as Media for Chemical Reactions

23

Figure 1.1-14 A, model of Ipatiev’s high’ pressure “bomb” [ 1081.

of molecular weights up to cyclotetradecane [113-1151. He also noticed that with ZnC12 or A1Cl3 as presumably heterogeneous and homogeneous catalysts, respectively, oligomerization occurred at much lower temperatures [ 1151. Other heterogeneously catalyzed supercritical phase reactions reported by Ipatiev included the dehydrogenation of 2-propanol [ 1161, the dehydration of 2-butanol [ 1171, the dehydrogenative coupling of benzene to diphenyl [ 1181, and the isomerization of cyclohexane to methylcyclopentane and other products [57]. Most of his work was dedicated to the development of heterogeneously catalyzed hydrogenation reactions such as the Ni-catalyzed hydrogenation of benzene in what was probably a supercritical mixture of benzene and hydrogen [ 1181. He revolutionized heterogeneous catalysis [ 1121, introducing not only the use of high pressures but also multicomponent catalysts such as Ni203/A1203. In all, 550 articles and patents were written or cowritten by Ipatiev [112]. Several other chemists were active in the field of supercritical fluid reactions at the beginning of the twentieth century. For example, Briner studied the decomposition and reactivity of supercritical fluids such as scNO and scC0 (e.g., eq 1.1-6) [119-1221. He also investigated the system N2-H2 and reported that N2 and H2 do not combine at room temperature and 900 bar [122]. At the Chemical Institute of the University of Berlin, Arthur Stiihler studied the reactions of alkyl halides such as chloroethane with scNH3 (eq 1.1-7) [123]. One of the earliest attempts to utilize SCFs for the selective synthesis of low molecular weight organic products dates to the early 194Os, when Patat at the University of Innsbruck studied the hydrolysis of aniline under supercritical conditions (eq 1.1-8) [124]. 2N0

+ 2HC1

scNO RT 300 bar

NOCl

+ H20 + hC12 + hN2

(1.1-6)

24

-

1 Introduction EtCl

+ NH3

SCNH~ 220 "C - 220 bar

[EtNH3]Cl+ [Et2NHZ]C1+ [Et,NH]Cl (1.1-7) 80 % 15 % 5%

(1.1-8) 440 "C Early mechanistic work on reactions in SCFs includes a kinetic study of the bimolecular decomposition of supercritical HI in 1928 [125]. In the following two decades, a number of researchers tried to address the question of the effect of near-critical temperatures on reaction rates, although not all of them took into account pressure and density effects [126-1291. In one of the more careful studies, Holder and Maass [I271 in 1938 described tests of the rate of the reaction between HCl and propene in liquid and supercritical mixtures at different temperatures. The density within a series of experiments was kept constant by adjusting the pressure. The rates of reaction went through a minimum at the critical temperature for three of the four chosen densities with d, > 1. The minimum was most marked, in percentage terms, when d, was close to 1. In 1946, Toriumi et al. [I291 reported that the rate of uncatalyzed oxidation of NH3 by oxygen reached a maximum at the critical temperature. Similarly, the rate of the Pt-catalyzed oxidation of SO2 by oxygen was found to be highest at the T, of SO2. Rapid diffusion at the critical temperature was offered as one possible explanation. The themes of rate changes near the critical point and the effectiveness of SCFs as solvents for heterogeneous catalysis (Chapter 4.8) have fascinated researchers ever since and have been the subject of many more recent studies. Chemical fixation of C02 under supercritical conditions was attempted for the first time in the middle of the twentieth century in the polymerization field. In a 1949 patent, Sargent suggested that C02 could be incorporated into polyethylene to give a more waxy product. This was achieved by copolymerizing S C C ~ and H ~ scCO2 at 72-88°C and 1000 bar in the presence of a free-radical initiator such as benzoyl peroxide. The resulting polymers were reported to incorporate one COz for every 29 ethylene units [130]. This contrasts with more recent studies which have demonstrated the use of scCO2 as an "innocent" solvent for free radical polymerizations (see Chapter 4.5). In 1951, Buckley and Ray [I311 patented the preparation of polymeric ureas by the treatment of carbamates with scCO2 at 200°C and 500 bar, but it is not clear to what extent any of the carbamates dissolved in the supercritical phase. Stevens patented in 1966 the copolymerization of C02 and ethylene oxide to form polycarbonates; in several of his examples the reaction mixtures were clearly at supercritical conditions [ 1321. There are many more reports and patents of reactions in SCFs than the selected examples mentioned here, especially in the period 1910-1945. Since 1945, the literature has become so extensive that it is not possible to review

25

1.1 Supercritical Fluids as Media for Chemical Reactions

it in this short chapter, but detailed coverage of individual aspects is given in other chapters of this book. Further information on the historical background of SFR is also given in the excellent reviews written by Subramaniam and McHugh (covering the years 1945 to 1985) [133], Savage et al. (covering 1986-1994) [ 1341 and Scholsky (covering polymerization from 1940 to 1990) [135]. There are also many symposia on supercritical fluids [94,136-1421.

1.1.4.3 Industrial Use of SCFs as Reaction Media Of the many industrial applications of SFR (Table 1.1-4), the first and the most famous is arguably the synthesis of ammonia, although some might not include this as an example of SFR because the reduced density is less than 1. Highpressure studies of the reaction between H2 and N2 to form ammonia started in 1901 with Le Chatelier's patent on the uncatalyzed process at up to 100 bar [143]. Fritz Haber later found heterogeneous catalysts such as osmium for this process, but using pressures as high as 177 bar [144]. The chemical and technical problems associated with the ammonia formation and the drastic conditions required were solved by Haber and Bosch. These studies led to the commercialization of the iron-catalyzed process by BASF (eq 1.1-9), with operations of the first plant starting in September 1913 at Oppau, Germany [143,145,146]. Table 1.1-4 Some industrial processes involving reactions in or with SCFs. Reaction

Processlproduct

0xidation Polymerization Hydrogenation Hydrogenation Hydration Transfer hydrogenation Hydrogenation Nitration

LDPE Ammonia Methanol Alcohols Estradione various Energetics

N2

+ 3H2

scwo

Fe 100-350 bar 400-530 "C

2NH2

SCF

Status Production Production Production Production Production Pilot plant Pilot plant Pilot plant in preparation

(1.1-9)

Modern plants for ammonia production by the Haber-Bosch process [145] operate at 100-350 bar and 40O-53O0C, well above both Tc and p c for the reaction mixture. Most notably, the largest plants use the highest pressures because this allows a greater throughput per unit volume of vessel. The actual ammonia synthesis step may not be the only part of the plant that operates at supercritical conditions. The production of H2 in the Shell and Texaco ammo-

26

I Introduction

nia processes is performed by the partial oxidation of fuels by 0 2 at 1250°C and pressures up to 80 bar, followed by the water gas shift reaction. The conditions of this partial oxidation step may well be supercritical depending on the choice of fuel and the f ~ e l - 0ratio. ~ In 1913, the same year as the first commercial ammonia process was launched, BASF began initial tests of a synthesis of methanol from pressurized H2/C0 mixtures, leading to the first plant for the production of synthetic methanol (eq 1.1-10) at Leuna in Germany in 1923 [147,148]. Patart in France later claimed prior invention [149, 1501. CO

+ 2H2

Zn/Cr oxide > CH30H 250-350 bar 320-450 "C

(1.1-10)

'

The high pressure of the process was required because of the poor activity of the catalyst, but this problem was alleviated by the introduction of better catalysts by ICI and Lurgi in the 1960s and 1970s [148]. The current process for the synthesis of methanol, as operated by ICI in England [148], requires a CO/CO2/H2 feed mixture and a pressure of between 50 and 100 bar, which may or may not be supercritical, depending on the composition and pressure. Some other industrial processes are or have been carried out under supercritical conditions even though the fact may not have been generally recognized. For example, McKee and Parker pointed out in 1928 that some oil cracking processes performed industrially at the time occurred above the critical temperature of the reaction mixtures [151]. These processes have since been replaced by lower temperature catalytic cracking methods. Other historical industrial processes which probably involved supercritical phases include the synthesis of melamine [ 1521 from dicyanodiamide, N2 and NH3, and the alkylation of aniline by methanol [ 1521. The oxidation of light alkanes by air or O2 at supercritical temperatures and pressures was explored by Standard Oil in the mid-1920s [153]. Experiments were performed at the laboratory and then semicommercial plant level. The primary products were alcohols. For example, the oxidation of pentane was performed at supercritical conditions (240"C, around 200 bar and a few mole per cent 02)and produced primarily C2-C3 alcohols and acids. However, the oxidation of heptane was performed at subcritical temperatures (225 "C) and produced primarily C6-C7 alcohols. The change in selectivity was attributed to either the difference in phase or more likely the difference in temperature. Other commercial processes for the formation of alcohol denaturants or formaldehyde were reported in the same decade [154,155], but it is unclear whether those reactions were operated at supercritical pressures. Modern processes involving alkane oxidation are heterogeneously catalyzed and operated at subcritical pressures [156]. The polymerization of ethene attracted a great deal of commercial interest two decades after Ipatiev's pioneering studies. Fawcett and co-workers, in a 1937 patent for ICI, reported a dramatic improvement in the method

1.1 Supercritical Fluids as Media for Chemical Reactions

27

Figure 1.1-15 A 9 L high pressure reactor for the production of high temperature and high pressure polyethylene at ICI at 900 bar and 200 "C [ 1601.

[157-1591. If a higher pressure of around 2000 ,ar and rigorous temperature control (-170°C) were used, solid products of molecular weights around 3500 could be obtained. Failure to control the temperature resulted in explosive production of carbon and hydrogen. Further, they found that if oxygen were present in the scC2H4, then solid products could be obtained at only 500 bar. The fortuitous introduction of oxygen in their experiment was due to a leaky vessel! Their 9 L vessel is shown in Figure 1.1-15. ICI built a plant in Cheshire (UK) which produced polyethylene, primarily as an insulator for cables which were badly needed for the wartime radar system. To increase production, the secrets of the method were passed on to American companies [161]. Immediately after the war, DuPont patented refinements to the process. Krase, the investigator at DuPont, found that stepwise reduction of the pressure of the scC2H4allowed the fractionation of the polyethylene by selective precipitation [162,163]. The system was further developed and the relevant phase behavior published by Ehrlich in a series of papers in the 1960s and 1970s [164-1671, including a major review on the subject [168]. The high pressure synthesis of low density polyethylene (LDPE) as currently practiced [ 1691 involves the polymerization of S C C ~ at H ~1000-3000 bar and at 80-300°C. Oxygen is still one of the initiators employed, others being benzoyl peroxide and azodi-iso-

28

1 Introduction

butyronitrile (AIBN). Approximately 3.5 X lo7 metric tons per annum of LDPE is produced in the USA alone, and the amount is still increasing [170]. Supercritical or - near-critical water has found technical applications for hydrothermal syntheses, as discussed in detail in Chapter 4.1. Another more recent industrial application of chemical reactiqns in SCFs is the oxidative destruction of chemical wastes in scH20 (SCWO, supercritical water oxidation). A detailed coverage of the large and prolific field of SCWO is outside the scope of this book on chemical synthesis. The extensive pilot plant activity, primarily by MODAR (now General Atomics) and Eco Waste Technologies, has recently been summarized by Schmieder [171]. The first commercial plant was opened by Huntsman Chemical in collaboration with Eco Waste. The hydration of light alkenes has often been performed with the alkene in the supercritical state. For example, the hydration of ethene to ethanol occurs when a mixture of ethene and water is fed through the reactor at 300°C and 70 bar. The conditions are kept above the dew point of the ethene-water mixture because condensed water would deactivate the catalyst, typically phosphoric acid on a silica support. This process is carried out by Shell, BP, Erdol-Chemie and Hibernia-Chemie [172,173]. Isopropanol is made from propene at ICI with tungsten oxide catalysts at 270°C and 250 bar [172] and at VEBA with a supported phosphoric acid catalyst at 180-260°C and up to 650 bar [174]. The synthesis of 2-butanol for eventual dehydrogenation into 2 -butanone (methyl ethyl ketone, MEK) is performed in an Idemitsu Petrochemical Co. plant (Figure 1.1-16) in Tokoyama, Japan by a two-phase process. Supercritical butene is bubbled through liquid water at low pH, 200°C and 200 bar. The hydration of the butene (a mixture of 1- and 2-butene) takes place in the aqueous phase, following which the 2-butanol dissolves preferentially in the SCC4Hs. The supercritical phase then exits the reactor from the top, is cooled to the liquid state, and is then separated into 2-butanol and unreacted butene. The butene is recycled. The extracting ability of the SCC4H8 is helpful in isolating the product and in preventing the build-up of polymeric residues in the reactor. Annual production of 2-butanone is 40000 metric tons, and the plant has been in production since 1985 [175,176]. The highly successful applications of SFR in the production of polymers and bulk chemicals illustrate that the technical challenges associated with reactions under high pressure and high temperature can be overcome even on a very large scale. Despite this encouraging precedence, the industrial application of SFR to the production of fine chemicals, pharmaceuticals or other more specialized products is still in its infancy. The following examples illustrate the current industrial interest in fine chemical SFR The reduction of 1,4-androstadiene-3,17-dione(AAD) as an industrial route to estradione using supercritical tetralin as both the solvent and the hydrogen donor was investigated at Schering AG in cooperation with the University of Gottingen [ 177,1781. The reaction kinetics were studied at between 350-600 "C and 50-300 bar. A pilot plant with a possible throughput of up to 30 L h-' was studied successfully over 1 year with continuous processes run up to 4 days at 575°C and pressures up to 100 bar at contact times of less than 1 s.

1.1 Supercritical Fluids as Media for Chemical Reactions

29

Figure 1.1-16 The Idemitsu Petrochemical plant for the hydration of supercritical butene to butanol, for eventual conversion to 2-butanone (MEK). Of the three tallest towers, that on the right (with the framework around it) is the hydration reactor, whereas that on the left is the MEK fractionation column (photo courtesy of Idemitsu Petrochemical).

Heterogeneously catalyzed hydrogenation processes involving gaseous hydrogen have been studied for industrial applications most extensively. Hoffman La Roche has investigated the technical feasibility of a variety of such processes [ 1791, including the heterogeneously catalyzed hydrogenation of vitamin precursors in a continuous-flow process in scC02 on a pilot plant scale. The reactor vessel has an internal volume of 40 L and an output of 800 t per annum [180,181]. Flowcharts comparing batch and continuousflow processes have been published [ 1821, but apparently no full production facility is planned for the near future. Longstanding research efforts at Degussa have revealed that the hardening (hydrogenation of the C-C double bonds) of edible oils and fatty acids and their esters can be achieved with high efficiency and selectivity in scCO2 using commercially available supported fixed-bed Pd catalysts [ 183-1861. Up to 15 times higher space-time yields were achieved and 3 times higher catalyst productivities resulted from the extended lifetime of the catalyst in scC02 compared to reactions in a trickle-bed process. Based on this technology, Thomas Swan Ltd in cooperation with the University of Nottingham has developed a miniature flow-through system that allows the hydrogenation of large amounts of unsaturated substrates in surprisingly small reactors [ 1871.

30

1 Introduction

In cooperation with the University in Grothenburg, Sweden, the Danish company Poul Mdler Consulting has developed various processes including hydrogenation of fatty acid methyl esters (FAME) to fatty alcohols and the synthesis of hydrogen peroxide using SCF technology [188,189]. The reactions benefit mostly from increased reaction rates owing to enhanced availibility of the gaseous substrates, especially H2.Three such processes are expected to enter the pilot plant stage in 1998 and 1999. Enzymatic reactions have been tested in SCFs on a pilot plant scale, and a detailed analysis of potential technical solutions and costs is given in Chapter 4.9. Finally, the synthesis of energetic materials has been suggested to be performed in liquid or supercritical COz. The Indian Head Division of the US Naval Surface Warfare Center is nbw constructing a pilot plant with a 100 L vessel for the synthesis of such energetics as MTV (magnesium/Teflon/ Viton) and poly-3-nitratomethyl-3-methyl oxetane [190, 1911. Some of the advantages associated with the use of SCFs for chemical reactions have been known for a very long time, ever since chemists started investigating these media. Nevertheless, it is only now that we are starting to fully appreciate all of the benefits, with the development of new synthetic methodologies. In many cases, the basic principles of carrying out such processes in SCFs have still to be explored and synthetic chemists as well as spectroscopists will find ample opportunity for ground-breaking studies. The challenge that lies ahead is to combine these efforts with new engineering skills based on the knowledge available from extraction and purification processes. If these innovations are given a chance, it seems likely that they can provide new solutions for economically efficient and ecologically benign synthetic chemistry.

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1.1 Supercritical Fluids as Media for Chemical Reactions

31

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32

I Introduction

[45] V. V. Altunin, V. Z. Geller, E. K. Petrov, D. C. Rasskazov, G. A. Spiridinov, T. B. Selover Jr. (Eds.), Thermophysical Properties of Freons. Methane Series, Part 1, Hemisphere Publishing-Corp., Washington, DC, 1987. [46] V. A. Rabinovich, T. B. Selover (Eds.), Thermophysical Properties of Neon, Argon, Krypton, and Xenon, Hemisphere Publishing Corp., Washington, DC, 1988. [47] V. V. Sychev, A. A. Vasserman, A. D. Kozlov, V. A, Zagoruchenko, G. A. Spiridinov, V. A. Tsymarny, T. B. Selover Jr. (Eds.), Thermodynamic Properties of Ethane, Hemisphere Publishing Corp., Washington, DC, 1987.‘ [48] V. V. Sychev, A. A. Vasserman, E. A. Golovsky, A. D. Kozlov, G. A. Spiridinov, V. A. Tsymarny, T. B. Selover Jr. (Eds.), Thermodynamic Properties of Ethylene, Hemisphere Publishing Corp., Washington, DC, 1987. [49] V. V. Sychev, T. B. Selover (Eds.), Thermodynamic Properties of Nitrogen, Hemisphere Publishing Corp., Washington, DC, 1987. [50] V. V. Sychev, T. B. Selover (Eds.), Thermodynamic Properties of Methane, Hemisphere Publishing Corp., Washington, DC, 1987. [51] A. Kordikowski, D. G. Robertson, M. Poliakoff, Anal. Chem. 1996, 68, 4436-4440. [52] M. Roth, Anal. Chem. 1998, 70, 2104-2109. [53] N. L. Porter, B. E. Richter, D. J. Bornhop, D. W.Later, F. H. Beyerlein, J. High Resolut. Chromatogr. Chromatog,: Commun. 1987, 10, 477. [54] J. Y. Clavier, M. Penut, in High Pressure Chemical Engineering: Proceedings of the 3rd International Symposium on High Pressure Chemical Engineering, Zurich, Switzerland, 7-9 October, 1996, P. R. von Rohr, C. Trepp (Eds.), Elsevier, Amsterdam, 1996, p. 627-63 1. [55] P. G. Urben (Ed.), Bretherick’s Handbook of Reactive Chemical Hazards, Butterworth/ Heinemann, 1995. [56] R. J. Lewis Sr., Sax’s Dangerous Properties of Industrial Materials, Van Nostrand, New York, 1992. [57] V. Ipatiev, N. Dovgelevich, J. Russ. Phys. Chem. SOC. 1911, 43, 1431-1436. [58] J. Albert, G. Luft, Chem. Eng. Processing 1998, 37, 55-59. [59] Encyclopedie des Gaz, L‘ Air LiquideElsevier, Amsterdam, 1976. [60]B. N. Hansen, B. M. Hybertson, R. M. Barkley, R. E. Severs, Chem. Muter. 1992, 4, 749-752. [61] R. E. Severs, B. Hansen, Chem. Eng. News 1991, 69(29), 2. [62] D. E. Raynie, Anal. Chem. 1993, 65, 3127-3128. [63] Merkblatter gefahrliche Arbeitsstoffe (Kuhn-Birett), Vof. 8, Landsberghch, 1990. [64] S. Budavari, M. J. O’Neil, A. Smith, P. E. Heckelman, J. F. Kinneary (Eds.), Merck Index, Merck & Co., Inc., Whitehouse Station, NJ, 1996. [65] Y. Fang, Y. K. Chau, Appl. Organomet. Chem. 1995, 9, 365. [66] R. Pierantozzi, in Encyclopedia of Chemical Technology, Vol. 5, John Wiley & Sons, J. I. Kroschwitz, M. Howe-Grant (Eds.), New York, 1993, p. 35-53. [67] C. Cagniard de LaTour, Ann. Chim. Phys. 1822, 21, 127-132, 178-182. [68] I. Asimov, Asimov’s Biographical Encyclopedia of Science and Technology, Doubleday, Garden City, NJ, 1982. [69] “J’ai introduit dans une petite marmite h Papin, construite avec un bout de canon de fusil trbs Cpais, une certaine quantitt d’alcool h 36 degrts et une bille ou sphbre de silex; le liquide occupait h-peu-prts le tiers de la capacit6 inttrieure de l’appareil. Ayant observt l’espbce de bruit que la bille produisait en la faisant rouler dans le canon d’abord froid, et ensuite 6chaufft peu il peu sur un brasier, je suis arrive h un point oh la bille semblait bondir h chaque percussion, comme s’il n’avait plus exist6 de liquide dans le canon.“ [671 [70] “Le liquide, aprbs &treparvenu 8-peu-prbs au double de son volume primitif, a disparu complbtement, et s’est converti en une vapeur tellement transparente que le tube semblaif &tretout-&-fait vide...‘‘ [67] [71] C. Cagniard de LaTour, Ann. Chim. Phys. 1823, 22, 410-415.

I . I Supercritical Fluids as Media for Chemical Reactions

33

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34

I Introduction

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35

[149] C. Lormand, Ind. Eng. Chem. 1925, 17, 430-432. [150]G. Patart, Ind. Eng. Chem. 1925, 17, 859. [151] R. H. McKee, H. H. Parker, Ind. Eng. Chem. 1928, 20, 1169-1172. [152] G. Bandel, E. Bocker, D. Henninger, H. Petri, in Chemische Technologie, Band 4, Organische Technologie II, K. Winnacker, L. Kuchler (Eds.), Carl Hanser Verlag, Munchen, 1972, p. 124-233. [153] P. J. Wiezevich, P. K. Frolich, Ind. Eng. Chem. 1934, 26, 267-276. [154] F. J. Carman, T. H. Chilton,-U. S . 1,697,106 1929. [155] W. P. Bitler, J. H. James, Chem. Met. Eng. 1928, 35, 156. [156] G. Franz, R. A. Sheldon, in Ullmann’s Encyclopedia of Industrial Chemistry, Vol. A18, B. Elvers, S. Hawkins, G. Schulz (Eds.), VCH, Weinheim, 1991, p. 261. [157] E. W. Fawcett, R. 0. Gibson, M. W. Perrin, J. G. Paton, E. G. Williams, Brit. 471,590 1937. [158] E. W. Fawcett, R. 0. Gibson, M. W. Perrin, U. S. 2,153,553 1939. [159] M. W. Perrin, J. G. Paton, E. G. Williams, U. S. 2,188,465 1940. [160] G. D. Wilson, in The Development of Plastics, S . T. I. Mossman, P. J. T. Moms (Eds.), Royal Society of Chemistry, Cambridge, 1994, p. 70-86. [161] M. Kaufman, The First Century of Plastics, the Plastics Institute, London, 1963. [162] N. W. Krase, U. S. 2,388,160 1945. [163] N. W. Krase, A. E. Lawrence, U. S. 2,396,791 1946. [164] P. Ehrlich, J. Polymer Sci., A 1965, 3, 131-136. [165] P. Ehrlich, R. H. Fariss, J. Phys. Chem. 1969, 73, 1164-1167. [166] P. Ehrlich, J. Macromol. Sci.-Chem. 1971, A5, 1259-1263. [167] T. Takahashi, P. Ehrlich, Macromolecules 1982, 15, 714-719. [168] P. Ehrlich, G. A. Mortimer, Adv. Polymer Sci. 1970, 7, 386-448. [ 1691 J. A. Brydson, Plastics Materials, Buttenvorth Heinemann, Oxford, 1995. [170] Chem. & Eng. News 1998, 76(26), 40-81. [171] H. Schmieder, N. Dahmen, J. Schon, G. Wiegand, in Chemistry Under Extreme or Non-Classical Conditions, R. van Eldik, C. D. Hubbard (Eds.), Wiley, New York, 1997, p. 273-316. [172] H.Bestian, H.-J. Friedrich, 0. Horn, in Chemische Technologie, Band 4, Urganische Technologie 11, K. Winnacker, L. Kuchler (Eds.), Carl Hanser Verlag, Munchen, 1972, p. 1-123. [173] N. Kosaric, A. Farkas, H. Sahm, S. Bringer-Meyer, 0. Goebel, D. Mayer, in Ullmann’s Encyclopedia of Industrial Chemistry, Vol. A9, W. Gerhartz, Y. S . Yamamoto, L. Kaudy, J. F. Rounsaville, G. Schulz (Eds.), VCH, Weinheim, 1987, p. 587. [174] A. J. Papa, in U l l m n n k Encyclopedia of Industrial Chemistry, Vol. A22, B. Elvers, S . Hawkins, W. Russey, G. Schulz (Eds.), VCH, Weinheim, 1993, p. 173-183. [175] T. Yamada, T. Muto, Sekiyu Gakkaishi 1991, 34, 201-209. [176] T. Yamada, T. Muto, K. Yamaguchi, AIChE 1987 Summer National Meeting, 1987. [ 1771 M. Buback, in Supercritical Fluids Fundamentals for Application, NATO ASI Series E, Vol. 273, E. Kiran, J. M. H. L. Levelt Sengers (Eds.), Kluwer Acad. Publ., Dordrecht, 1994, p. 481-498. [1781 A. Hanke, Anwendungsperspektiven von Oberkritischen Medien, DECHEMA, Frankfurt a. Main, 1995. [179] M. Jansen, C. Rehren, EP 0841 314 A1 1998. [180] M. Poliakoff, S . Howdle, Chemistry in Britain 1995, 118-121. [181] Roche Magazin 1992, (41, May), 2-12. [ 1821 K. H. Pickel, K. Steiner, 3rd International Symposium on Supercritical Fluids, Strasbourg, 1994, pp. 25-29. [183] T. Tacke, S. Wieland, P. Panster, Process Technol. Proc. 1996, 12, 17-21. [184] T. Tacke, Chem. Anlagen Verfahren. 1995, 18. [ 1851 T.Tacke, S. Wieland, P. Panster, 4th International Symposium on Supercritical Fluids, Sendai, Japan, 1997, pp. 511.

36

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[186] T. Tacke, S. Wieland, P. Panster, M. Bankmann, R. Brand, H. Magerlein, U. S. 5,734,070 1998. [187] M. Poliakoff, T, M. Swan, T. Tacke, M. G. Hitzler, S. K. Ross, S. Wieland, WO 97138955 1997. [188] M. H h o d , P. M~ller,WO 96/01304 1996. [189] M. H h o d , M.-B. Macher, J. Hogberg, P. Meller, 4rh Italian Conference on Supercritical Fluids and their Applications, Capri, Italy, 1997, pp. 319. [190] R. E. Farncomb, G.W. Nauflett, Waste Management 1997, 17, 123-127. [191] R. E. Farncomb, G.W. Nauflett, International ZCT Conference, Karlsruhe, 30 June 3 July, 1998, 1997 . [192] R. B. Stewart, R. T. Jacobsen, J. Phys. Chem. Re$ Data 1989, 18, 639. [193] R. Span, W. Wagner, J. Phys. Chem. Re$ Data 1996, 25, 1509-1596. [194] A. L. Horvath, Process Technol. Znt. 1973, 18, 67-69. [195] K. Mangold, E. U. Franck, Ber. Bunsenges. Phys. Chem. 1962, 66, 260. [196] G.S. Kell, in Watel; a Comprehensive Treatise, Vol. 1 , F. Franks (Ed.), Plenum Press, New York, NY, 1972, p. 363. [197] J. M. H. Levelt Sengers, B. Kamgar-Parsi, F. W. Balfour, J. V. Sengers, J. Phys. Chem. Ref. Data 1983, 12, 1. [198] H. Sato, K. Watanabe, J. Sengers, J. S. Gallagher, P. G. Hill, J. Straub, W. Wagner, J. Phys. Chem. Re$ Data 1991, 20, 1023-1044. [199]A. Harlow, G. Wiegand, E. U. Franck, Ber: Bunsenges. Phys. Chem. 1997, 101, 1461- 1465. [200] L. Haar, J. S. Gallagher, J. Phys. Chem. Ref. Data 1978, 7, 635-677. [201] E. J. Couch, K. A. Kobe, L. J. Hirth, J. Chem. Eng. Data 1961, 6, 229-237. [202] J. Berg, Z . Wagner, Fluid Phase Equilib. 1990, 54, 35-45. [203] H . Pohler, E. Kiran, J. Chem. Eng. Data 1997, 42, 389-394. [204] Z . Gokmenoglu, Y. Xiong, E. Kiran, J. Chem. Eng. Data 1996, 41, 354-360. [205] W. H. Mears, E. Rosenthal, J. V. Sinka, J. Phys. Chem. 1969, 73, 2254. [206] 0. Sifner, J. Klomfar, J. Phys. Chem. Re$ Data 1994, 23, 63. [207] R. C. Weast (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida, 1982. [208] A. G. Aizpiri, A. Rey, J. Dlvila, R. G. Rubio, J. A. Zollweg, W. B. Streett, J. Phys. Chem. 1991, 95, 3351-3357. [209] R. Tillner-Roth, A. Yokozeki, J. Phys. Chem. Re$ Data 1997, 26, 1273-1328. [210] P. F. Malbrunot, P. A. Meunier, G.M. Scatena, W. H. Mears, K. P. Murphy, J. V. Sinka, J. Chem. Eng. Data 1968, 13, 16-21. [211] U. Setzmann, W. Wagner, J. Phys. Chem. Re$ Data 1991, 20, 1061. [212] R. D. Goodwin, J. Phys. Chem. Re$ Data 1987, 16, 799. [213] M . Jahangiri, R. T. Jacobsen, R. B. Stewart, R. D. McCarty, J. Phys. Chem. Re$ Data 1986, 15, 593-650. [214]D. G.Friend, H. Ingham, J. F. Ely, J. Phys. Chem. Re$ Data 1991, 20, 275. [215] B. A. Younglove, J. F. Ely, J. Phys. Chem. Re$ Data 1987, 16, 577. [216] P. N. Cheremisinoff, A. C. Morresi, Benzene. Basic and Hazardous Properties, Marcel Dekker, Inc., New York, 1979. [217] R. Deul, S. Rosenzweig, E. U. Franck, Ber: Bunsenges. Phys. Chem. 1991, 95, 5 15-519.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

1.2 Phase Behavior and Solubility FRANK P. LUCIENand NEIL R. FOSTER

The ability of supercritical fluids (SCFs) to act as solvents has been known for well over a century [l] yet the most significant developments in the application of this technology have taken place in the last few decades. Among their most recent applications, SCFs have been employed as media for chemical reactions. As a reaction medium, the SCF may either participate directly in the reaction or simply act as a solvent for the various chemical species. The physical properties of SCFs are highly dependent on pressure and temperature which makes it possible to fine-tune the reaction environment. These characteristics are unique to SCFs and provide the potential to tune the reaction environment in order to optimize reaction rate and selectivity. In view of their importance to process design, fundamental aspects of SCF technology are described in this section. Basic physical properties of SCFs are presented initially, followed by a consideration of phase behavior in high pressure systems. Lastly, the factors affecting the solubility of components in SCFs are described.

1.2.1 Basic Physical Properties of Supercritical Fluids A fluid is defined as being supercritical if it is maintained at conditions above its critical temperature and pressure (T, and pc). The critical point, as defined by T, and p c , marks the end of the liquid-vapor coexistence curve in the phase diagram for a pure substance, as shown in Figure 1.2-1. A supercritical region originating from the critical point can also be identified in this phase diagram. In general, only a single homogeneous phase can exist in the supercritical region, irrespective of the temperature and pressure. However, solidification may occur at very high pressure. Changes in phase from liquid to vapor are accompanied by abrupt changes in physical properties but, in the supercritical region, properties may be varied continuously by manipulating the temperature and pressure. It is possible to construct the path shown in Figure 1.2-1, where

I Introduction

38

I

I

I I

s

E

n Region

Figure 1.2-1 Phase diagram for a pure substance. Temperature

Tc

the progression from a liquid state (A) to a gas state (B), and vice versa, proceeds without any visual signs of boiling or condensation. The properties of SCFs vary over a wide range depending on the temperature and pressure but are generally intermediate to those of gases and liquids. Selected physical properties of SCFs are shown in Table 1.2-1 together with typical values for gases and liquids. The properties of SCFs are very sensitive to small changes in temperature and pressure in the vicinity of the critical point. This applies especially to density as shown in Figure 1.2-2. This figure represents the projection of the liquid-vapor coexistence curve in the densitypressure plane. Pressure and temperature are represented in terms of their reduced values which are simply the actual values divided by the critical values br = p/p,, Tr = T/T,). The left portion of the diagram reveals the two-phase envelope and the equilibrium between liquid and vapor phases is indicated by a vertical tie-line. The supercritical region in this view of the phase diagram is defined by the region to the right of the T, = 1.0 isotherm and for pr > 1.0. In the immediate vicinity of the critical point, the density of supercritical C 0 2 is around 0.4 g/mL,. For reduced pressures greater than 2, it can be seen that the density of supercritical C02 is comparable to that for liquid C02. It is this liquid-like density that enables many materials to be solubilized Table 1.2-1 Comparison of the physical properties of gases, liquids and SCFs [22].

Property Density (g/mL) Viscosity (Pa s) Diffusivity (cm2/s)

Gas

SCF

Liquid

10-3

0.3 lo4

10-3

10-5 0.1

10-3

1 5 x 10"

1.2 Phase Behavior and Solubility 1.2

39

Reduced

Figure 1.2-2 Variation of the density of CO2 in the vicinity of its critical point. D Reduced Pressure (plpc)

to a level which is several orders of magnitude greater than that predicted by ideal gas considerations. As density is a measure of the solvating power of an SCF, temperature and pressure can be used as variables to control the solubility and separation of a solute. For example, at T, = 1.1 in Figure 1.2-2, decreasing the reduced pressure from 3 to 1 lowers the density by around 80%. The conditions of temperature and pressure of most interest in SCF processes are usually bounded by reduced temperatures between 1.0 and 1.2 and reduced pressures greater than 1. The diffusion coefficient (or diffusivity) and viscosity represent transport properties which affect rates of mass transfer. In general, these properties are at least an order of magnitude higher and lower, respectively, compared with liquid solvents. This means that the diffusion of a species through an SCF medium will occur at a faster rate than that obtained in a liquid solvent, which implies that a solid will dissolve more rapidly in an SCF. In addition, an SCF will be more efficient at penetrating a microporous solid structure. However, this does not necessarily mean that mass transfer limitations will always be absent in an SCF process. For example, in the extraction of a solute from a liquid to an SCF phase, the resistance to diffusion in the liquid phase will probably control the overall rate of mass transfer. Stirring will therefore continue to be an important factor in such systems. The diffusion coefficient varies with both temperature and pressure and is strongly influenced by density and viscosity [2]. Density and viscosity both increase with pressure with a corresponding decrease in the diffusion coefficient. The effect is less pronounced at higher pressure because density becomes less sensitive to pressure. The diffusion coefficient generally increases with temperature at constant pressure. However, at constant density, temperature appears to have a minimal effect.

40

I Introduction

Supercritical fluids have a number of distinct advantages over conventional liquid solvents. The adjustable solvent strength and favorable transport properties have already been mentioned and it is these features which really differentiate SCFs from liquid solvents. Most SCFs are low-molecular-weight gases which have relatively low critical temperatuTes. Operations may therefore be carried out at moderate temperatures which is desirable in the recovery of thermally labile materials. Perhaps the most important advantage offered by SCFs is that after the release of pressure, components are left virtually free of residual supercritical solvent. The majority of studies involving SCFs has focused on four fluids: CO,, ethene, ethane and water. The first three fluids all have critical temperatures below 35 "C. Carbon dioxide is by far the most widely used SCF and has several advantages over the hydrocarbons in that it is nontoxic, nonflammable and readily available in high purity. Carbon dioxide also exhibits low miscibility with water and is a moderately good solvent for many low-to-medium molecular weight organics. Reported solubilities for nonpolar solids and liquids in CO;! range from 0.1 to 10 mol%. Marginally higher solubilities are obtained in ethane and ethene. Supercritical water (SCW) has a very high critical temperature and pressure in comparison with the other commonly used SCFs. Nonetheless, SCW has been investigated for more than a decade as a medium for the oxidation of organic wastes [3]. The properties of SCW are quite different to those of ordinary water. In the vicinity of the critical point, SCW behaves like a moderately polar organic liquid. The dielectric constant is reduced to the point where organic materials are readily soluble while the solubility of inorganic species is greatly reduced. These solvation characteristics make SCW an ideal medium for the oxidation of organics. The oxidation process leads to the formation of simple and nontoxic compounds such as water and carbon dioxide. The addition of a caustic solution to the process promotes the conversion of heteroatoms such as C1, F, P, and S into simple salts which precipitate from the reactor effluent. A list of other commonly used SCFs and their critical properties is given in Table 1.2-2. It is evident from this table that SCFs usually require the use of pressure in excess of at least 40 bar. It is interesting to note that linear hydrocarbons generally have a critical pressure below 50 bar and a critical temperature that increases with molecular weight. In addition, substances capable of hydrogen bonding require relatively high critical temperatures and pressures. In some applications, the pressure required for an SCF process may result in prohibitively high capital investment for process equipment. However, it is possible to increase the solvent power of a primary SCF with the addition of small amounts of cosolvents, such as methanol or acetone. The significance of this is that lower operating pressures (and temperatures) are made possible. The selectivity of the SCF for a particular component may also be improved with the addition of a cosolvent.

1.2 Phase Behavior and Solubility

41

Table 1.2-2 Critical data of supercritical fluids [23]. Solvent

Critical Temperature ("C)

Critical Pressure (bar)

COZ N20 Ethane Ethene Propane Propene Ammonia Methano1 Water CHF3 CClF3 SF6

31.0 36.4 - 32.3 . 9.2 96.7 91.7 132.4 239.5 374.0 26.2 28.9 45.5

73.8 72.5 48.8 50.5 42.5 46.0 113.5 81.0 220.6 48.5 38.7 37.7

1.2.2 Phase Behavior in High Pressure Systems The development of SCF processes involves a consideration of the phase behavior of the system under supercritical conditions. The influence of pressure and temperature on phase behavior in such systems is complex. For example, it is possible to have multiple phases, such as liquid-liquid-vapor or solidliquid-vapor equilibria, present in the system. In many cases, the operation of an SCF process under multiphase conditions may be undesirable and so phase behavior should first be investigated. The limiting case of equilibrium between two components (binary systems) provides a convenient starting point in the understanding of multicomponent phase behavior.

1.2.2.1

Types of Binary Phase Diagrams

The phase behavior of most binary systems can be described by nine types of phase diagrams that can be predicted qualitatively with the van der Waals equation of state [4]. The nine types of phase diagrams may be further grouped into five major classes designated I to V. A sixth class exists for some aqueous systems but it is much less common than the other classes and is not considered here. The phase diagrams are based on the projection of three-phase lines and mixture critical lines from pressure-temperature-composition (p-T-x) space onto the p-T plane. A detailed discussion on the three-dimensional features of the various classes of phase diagrams is given by McHugh and Krukonis [5] and by Streett [6]. It should be noted that some differences exist between the classification systems of these two authors. The system referred to here is that described by Streett [6].

42

1 Introduction

component 2

-Pure component vaporlzation curve -.----

fIn t a

Mixture critical line

c2

Two-phase LV

Figure 1.2-3 Class I binary phase diagram: (a) three-dimensional representation in p-T-x Space; (b) p-T projection. (b)

Temperature

The simplest class of binary phase diagram is class I as shown in Figure 1.2-3. The component with the lower critical temperature is designated as component 1. The solid lines in Figure 1.2-3(b) represent the pure component liquid-vapor coexistence curves which terminate at the pure component critical points (C, and C,). The feature of importance in this phase diagram is that the mixture critical line (dashed line in Figure 1.2-3(b)) is continuous between the two critical points. The mixture critical line represents the locus of critical points for all mixtures of the two components. The area bounded by the solid and dashed lines represents the two-phase, liquid-vapor (LV) region. The mixture-critical

1.2 Phase Behavior and Solubility

43

Table 1.2-3 Examples of the five classes of phase diagrams 161. Class

System

I I1

Ad&, N2/02 COz/n-octane, C02/2-hexanol ethanelmethanol, C02/H20 C02/nitrobenzene ethane/ethanol, ethaneln-propanol

111 IV V

.

line is denoted as L = V as it represents the merging of liquid and vapor phases. The mixture critical line shown in Figure 1.2-3 does not necessarily exhibit a maximum and can take on a variety of shapes. The distinctions between the various classes of phase diagrams are based mainly on the behavior of the mixture critical line. The class I1 phase diagram, like class I, also has a continuous mixture critical line between the pure component critical points. In classes IV and V, the mixture critical line originating from C2 exhibits a maximum in pressure and then intersects an additional three-phase, liquid-liquid-vapor (LLV) line. In the class I11 phase diagram, the mixture critical line which originates from C2 rises to very high pressures and can take on a number of different shapes. Examples of the different shapes of mixture critical lines in class I11 systems are given by Schneider [7]. Some typical examples for each of the five classes of phase diagrams are given in Table 1.2-3.

1.2.2.2 Asymmetric Binary Mixtures Phase diagrams for binary systems consisting of components with widely separated critical temperatures are considered to be a subset of class I11 systems [6]. This situation often arises in supercritical extraction operations in which relatively nonvolatile components are extracted. The general features of the class I11 phase diagram are shown in three dimensions in Figure 1.2-4(a) and as the two-dimensional p-T projection in Figure 1.2-4(b). The following discussion refers to Figure 1.2-4(b). The mixture critical line originating from C2 (L = V) may pass through an inflection point before rising to higher pressures. These critical lines usually end at an intersection with a three-phase, solidliquid-vapor (SLV) line (not shown). The dashed line represents a threephase, LLV line which terminates at a critical end point (open triangle). The critical end point also marks the termination of another mixture critical line (L = V) originating from C,. The points MI, M2 and M3 are located on the respective mixture critical lines or three-phase LLV line for the given temperature T I . Critical end points represent the conditions at which two of three coexisting phases merge and become identical. Both lower and upper critical end points are possible, depending on whether the end point occurs at a high temperature

44

I Introduction

component 2

\

L=L

mebphaseline

\ L=V

\

Figure 1.2-4 Class I11 binary phase diagram: (a) three-dimensional representation in p-T-x Space; (b) p-T projection. Temperature

branch of a three-phase line (UCEP) or a low temperature branch (LCEP). At the UCEP in Figure 1.2-4,a liquid and vapor phase critically merge into a single vapor phase in the presence of another liquid phase. The presence of the LLV line in Figure 1.2-4 causes the upper mixture critical line to pass continuously from L = V to L = L in the vicinity of the UCEP. At a temperature TI, between C1 and the UCEP, the p-x diagram has the characteristics shown in Figure 1.2-5. As pressure is increased fromp, to p2, a single vapor phase splits into a two-phase LV region and eventually a three-phase LLV point is reached. The horizontal line at p2 is a tie line which connects the three coexisting phases 'at a fixed temperature and pressure. It is the p-T projection of the

1.2 Phase Behavior and Solubiliry Mixture critical points

I

45

\

Three-phase tie-line

\

u \

LV region

Figure 1.2-5 p-x diagram for a class I11 binary mixture at a temperature which intersects the three-phase LLV line.

P1

pure component 1

pure component 2

Composition

tie-line which appears as one point (M3) on the three-phase LLV line in Figure 1.2-4(b). At pressures higher than p2 there are two distinct two-phase regions in the form of closed domes. The stationary point on each closed dome represents the critical point of the two-phase mixture. The critical point of the LL envelope at the right (M2) represents a point on the mixture critical line which originates from C2. The other critical point (MI) appears on the mixture critical line from C , .

Figure 1.2-6 Class III phase diagram for an asymmetric binary mixture consisting of a solid and an SCF. Temperature

46

1 Introduction

In a binary mixture consisting of a solid and an SCF, the critical temperatures of the pure components are normally far apart. The critical temperature of the supercritical solvent, in particular, lies below the triple point of the solid and there is no common range of temperature where the pure components both exist as liquids. As a consequence, the three-phase LLV line in Figure 1.2-4 becomes an SLV line which terminates at a LCEP as shown in Figure 1.2-6. A second, higher temperature branch of 'a SLV line originates from the triple point of the solid. This SLV line, which was not shown in Figure 1.2-4, intersects the L = V mixture critical line from C2 at an UCEP. At both the LCEP and UCEP, a liquid and vapor phase critically merge into a single fluid phase in the presence of a solid phase. An interesting feature of Figure 1.2-6 is that the higher temperature branch of the SLV line lies at lower temperatures than the solid-liquid coexistence curve of the pure solid. This indicates that melting point depression of the solid can occur in the presence of an SCF. As an example of melting point depression, consider the three-phase SLV line for the binary octacosane-ethane system [8]. The normal melting point of octacosane occurs at 64.5 "C whereas the UCEP occurs at 38.9 "C and 87.5 bar. For temperatures between the UCEP and the normal melting point of the solid, the phase behavior is modified by the presence of the SLV line as shown in Figure 1.2-7. As pressure is increased from p 1 to p 2 , a single vapor phase splits into a twophase SV region until the SLV line is reached as indicated by the horizontal tie-line at pressure p 2 . For pressures greater than p2 two regions of twophase behavior appear. When the overall composition of the system is less than xL (to the right of xL in Figure 1.2-7), a region of solid-vapor equilibria exists up to very high pressures. If the overall composition of the system is greater than xL (to the left), LV equilibria are established up to the stationary point at pressure p 3 which represents the critical point of the two-phase mix-

SV region

4

Mixture critical point

Threephase tie-line (SLV)

SV region

:

Vapor region

I

PureSCF

:

Pure solid

Figure 1.2-7 p - x diagram for an asymmetric binary mixture at a temperature which intersects the three-phase SLV line.

1.2 Phase Behavior and Solubility

47

ture. This point lies on the mixture critical line connecting the UCEP and C2 (Figure 1.2-6). The solids of interest in SCF processes usually have relatively low vapor pressures which results in an LCEP which is located close to the critical point of the SCF. The region between the critical point of the SCF and the UCEP generally defines t h e temperature range where unconstrained solidvapor equilibria exist, i.e. liquid phase formation is absent. However, operating at temperatures below the UCEP does not always ensure unconstrained solidSCF equilibria because the SLV line can exhibit a temperature minimum. At low pressures the SLV line begins with a negative slope, passes through a temperature minimum and then continues with a positive slope to the UCEP. Octacosane exhibits:this type of behavior under the influence of C 0 2 [8]. The complexity of the phase diagrams presented clearly demonstrates that phase behavior under high pressure conditions can vary markedly. This is particularly important for chemical synthesis in SCFs because the number and types of phases has a direct impact on the progress of reaction. A knowledge of phase behavior is therefore essential for the proper interpretation of experimental data.

1.2.3 Factors Affecting Solubility in Supercritical Fluids The solubility of compounds in SCFs has perhaps been the most extensively investigated area of SCF research. Solubility data indicate how well the SCF performs as a solvent for a particular solute and are an important starting point in the consideration of potential process applications. Although the solubility of a single solute in an SCF is not necessarily the same as that obtained in a multicomponent system [9],binary solubility data are nonetheless useful for estimating the selectivity of an SCF for a particular solute. The solubility of a component is mainly influenced by its chemical functionality, the nature of the SCF solvent, and the operating conditions. The discussion that follows is centered on the solubility of solids, although similar principles apply for the case of liquids. Referring back to Figure 1.2-6, the region between the LCEP and the UCEP defines the temperature range where unconstrained solid-vapor equilibria exist with respect to pressure. Solid solubility data available in the literature are normally measured in this range of temperature. Furthermore, a check is usually made to determine the maximum temperature at which data can be measured before the onset of melting. As stated previously, the high-temperature branch of the SLV line may exhibit a minimum with respect to temperature and this has the effect of reducing the temperature range for unconstrained solid-vapor equilibria.

48

I Introduction

1.2.3.1

SCF Solvent

The solubility (y) of a material in an SCF is usually expressed in terms of the overall mole fraction of the solute in the SCF phase. The ability of SCFs to dissolve many substances arises from the highly nonideal behavior of pure SCFs. The solubility of a component, as predicted by the ideal gas law, decreases asymptotically with increasing pressure because the solubility is simply the ratio of the vapor pressure (p""') to the total pressure (p). Under supercritical conditions, however, the solubility is enhanced by several orders of magnitude above that predicted by the ideal gas law. The solubility enhancement of a component, particularly in the vicinity of the critical point, is driven primahly by the augmentation in the density of the SCF. The extent of solubility enhancement which occurs in the SCF phase is usually expressed in terms of an enhancement factor (E) which is defined as the actual solubility divided by the solubility predicted from the vapor pressure of the solute and ideal gas considerations:

E = -Y P

(1.2-1)

PSat

The enhancement factor measures the extent of solubility in excess of that generated from the vapor pressure of the pure solid. This provides a convenient way of comparing the solubilities of solids with different vapor pressures. As the actual solubility of a solid is heavily influenced by the SCF density, the enhancement factor displays a similar dependence on density. Values of the enhancement factor ty ically vary between lo3 and lo6, although enhancement factors as high as lotg have been reported for some systems [lo]. The solubility of a given solute also depends on the type of SCF as shown in Table 1.2-4. Fluoroform has the highest mass density under the conditions shown but displays the lowest affinity for naphthalene. The variation in the solubility of naphthalene in different SCFs therefore suggests that there are varying degrees of intermolecular interaction between the solid and SCF. The different levels of intermolecular interaction can be explained in terms of solvent polarity. The overall effect of solvent polarity on the solubility of naphthalene follows the same general solubility rule in liquid extractions that "like dissolves like". Naphthalene is a nonpolar solid and is most soluble in supercritical ethane. Carbon dioxide behaves as a nonpolar solvent but less so because of its quadrupole moment [ 111. Fluoroform is the most polar solvent because of the elecTable 1.2-4 Solubility of naphthalene in various SCFs at 200 bar and 45°C [16].

Solvent

Solubility (mole fraction)

Density (g/mL)

Ethane Carbon dioxide Fluoroform

4 . 7 5 lo-' ~ 2.42X10-' 1.17xlO-'

0.39 0.8 1 0.94

1.2 Phase Behavior and Solubility

49

tron-withdrawing capability of the fluorine atoms. In general, nonpolar solvents such as ethene and ethane are the preferred solvents for aromatic hydrocarbons. For polar solids, the effect of solvent polarity is not as straightforward. Although it is true that nonpolar SCFs exhibit lower affinities for polar solutes, the maximum solubility of a polar solute does not necessarily occur in the most polar SCF. For example, C 0 2 is a better solvent for benzoic acid than fluoroform [12]. Polar SCFs may exhibit greater potential for polar molecules when they contain functional groups that increase the level of intermolecular interaction with the solvent. Fluoroform, for example, is as good a solvent as COz for 2-aminofluorene probably as a result of hydrogen bonding between the amino group and the acidic proton in fluoroform [12]. In the case of 2-cyanonaphthalene, intermolecular interactions between the solute and fluoroform lead to much higher solubilities than those obtained with C 0 2 [13].

1.2.3.2 Chemical Functionality of the Solute The difference between solid solubilities in a given SCF depends mainly on the solid vapor pressure and intermolecular interactions between the solvent and solute. The individual solubilities of solids can vary greatly although most values are well below 10 mol %. The differences between enhancement factors are less pronounced [14,15], however, which suggests that the vapor pressure of the solid exerts the primary influence on solubility. Intermolecular interactions between the solvent and solute depend on the types of functional groups present in their chemical structures. In general, the intermolecular interactions are dominated by dispersion forces, which accounts for the similar values of the enhancement factor for many solids. It was previously described how the degree of intermolecular interaction can increase when a polar SCF is used for polar solutes. As another example, consider the solubilities of naphthalene and 1,4-naphthoquinone in fluoroform shown in Table 1.2-5. The solubility of naphthalene is much larger than that for 1,4-naphthoquinone under similar conditions mainly because of the difference in their vapor pressures. With the effect of vapor pressure removed, the enhancement factor for 1,4-naphthoquinone is substantially larger than that for naphthalene. This suggests that a higher degree of intermolecular interaction occurs between fluoroform and 1,4-naphthoquinone. The additional interaction between solvent and solute may be associated with hydrogen bonding Table 1.2-5 Comparison of the enhancement factors for naphthalene and 1,4-naphthoquinone in supercritical fluoroform at 200 bar and 45 "C [16]. Solute

Solubility (mole fraction)

Enhancement factor

Naphthalene 1,6Naphthoquinone

1.17 x lo-' 3.1 1x 10-3

3 380 92 500

50

I Introduction

Y

.0 E3

-

Figure 1.2-8 Dampening effect of substituents on the solubility of indole derivatives at 35 "C I

.

I

.

I

[24,25,26].

.

i0

between the acidic proton in fluoroform and the carbonyl groups in 1,4naphthoquinone [ 161. The vapor pressure of a solid and the intermolecular interactions it is capable of are ultimately determined by its chemical structure. Differences between the solubilities of solids can also be explained in terms of structural features which limit or enhance solubility. Beginning with a parent compound, the addition of a functional group generally has the effect of reducing solubility. This applies especially to hydroxy (-OH) and carboxy (-COOH) functional groups as described by Stahl et al. [ 171. The carboxy functional group has a greater dampening effect on solubility than the hydroxy group, as shown in Figure 1.2-8. Beginning with indole as the parent compound, the solubility decreases by around 1.5 orders of magnitude with the addition of the hydroxy group (5 -hydroxyindole). The addition of the carboxy group to indole (indole-3 -carboxylic acid) decreases the solubility by a much larger amount of around 2.5 orders of magnitude. For comparison, the addition of a methyl group to indole (skatole) causes a relatively minor solubility reduction of less than 0.5 orders of magnitude. A detailed study on the effect of structural features on solubility is given by Dange el al. [18]. This study considers structural features such as chain length, branching and the number of rings in addition to the position and types of substituents. Structure-solubility relationships using C 0 2 as a solvent are illustrated for several classes of compounds including hydrocarbons, alcohols, phenols, aldehydes, ethers, esters, amines, and nitro compounds. Normal alkanes, for example, are completely miscible with COz when the carbon number is below 12. Solubility decreases rapidly beyond a carbon number of 12. For a given carbon number, the solubility of acyclic hydrocarbons is favored by unsaturation; for example, 1-octadecene is more soluble than n-octadecane.

1.2 Phase Behavior and Solubility

51

Branching leads to more favorable solubilities compared with normal alkanes. The maximum limit of complete miscibility for branched alkanes also occurs at a much higher carbon number which is between 19 and 30. Although the degree of intermolecular interaction can increase when a polar SCF is used for polar solutes, the overall solubility of polar solutes is normally much lower than that for nonpolar solutes. Organic salts will exhibit a wide range of solubility values in SCFs depending primarily on vapor pressure. Inorganic salts exhibit very low solubility in SCFs and in most cases may be considered as insoluble. Metal ions are insoluble in SCFs because of the charge neutralization requirement but, if they are bound to organic ligands, they may become soluble to an appreciable extent. A good example of this is the complexation of metal ions with tri-n-butyl phosphate (TBP) and di(2 -ethylhexyl) phosphoric acid [ 191. The choice of the organic chelating agent impacts significantly on solubility. For example, diethyl dithiocarbamate (DDC) forms stable complexes with a wide range of metals and such complexes are soluble in supercritical CO;! to varying degrees. Fluorination of the ligands, as in bis(trifluoroethy1) dithiocarbamate (FDDC), can increase the solubility of the metal complexes by several orders of magnitude [20]. It is interesting to note that fluorination of an organic structure is a useful way of increasing its solubility in supercritical C02. As another example, the technique has been used as a means of increasing the solubility of hydrocarbon polymers [21]. 1.2.3.3 Temperature and Pressure Effects Solubility data are typically presented in the form of solubility isotherms as shown for the naphthalene-C02 system in Figure 1.2-9. The characteristic feature to note in this diagram is that solubility increases rapidly at lower pres-

Retrograde

328 K

Solute vapor pressure dominant

Figure 1.2-9 Solubility of naphthalene

I

I Pressure (bar)

52

I Introduction

sures, generally near the critical pressure of the SCF, whereas at higher pressures the increase in solubility is less pronounced. The behavior of the solubility isotherm in this way simply reflects the density changes which are occurring in the SCF. The effect of temperature on solubility is more complex and involves both a consideration of the solute vapor pressure as wdl as the density of the SCF. The solubility isotherms shown in Figure 1.2-9- are typical of most solidSCF systems in that they intersect within a narrow range of pressure. For any two isotherms, the point of intersection, or crossover pressure, represents a change in the temperature dependence of solubility. The region of pressure below the crossover pressure is known as the retrograde region. In this range of pressure, solubility decreases with an increase in temperature because the density of the SCF falls sharply. The decrease in density is sufficient to overcome any increases in solute vapor pressure that would normally lead to an increase in solubility. Above the crossover pressure, the decrease in solvent density is less sensitive to temperature and so solubility increases with temperature because the vapor pressure effect becomes dominant. Solubility data provide important information on achieving separation between the solute and the SCF. The solubility data shown in Figure 1.2-9 suggest that there are two ways in which this separation might be achieved. The first separation scheme, which is perhaps the most conventional procedure, involves an isothermal decrease in pressure. This would preferably be accomplished in the vicinity of the critical pressure of the SCF where solubility is sensitive to changes in pressure. The reduction in pressure need not occur completely to atmospheric pressure because an order of magnitude reduction in solubility is easily accomplished in the steeply rising portion of the solubility isotherm. The second separation scheme involves an increase in temperature under isobaric conditions. This can be achieved in the retrograde region, and again an order of magnitude reduction in solubility can occur with a modest increase in temperature. This separation method is probably more favorable than the first method in terms of energy consumption because the first method involves a significant recompression step after separation. However, a separation based on a temperature increase requires much more specific information on the solubility behavior of the solute. In contrast, a separation based on a pressure reduction mainly involves consideration of the critical pressure of the solvent. A near 100% separation can be achieved once the pressure is reduced to below the critical pressure. Solubility data also suggest potential operating conditions for an extraction process. Referring again to Figure 1.2-9, an extraction at 200 bar requires a lower solvent-to-feed ratio than extraction at 100 bar. This advantage may be offset by the higher capital costs and a larger recompression duty. A separation at 200 bar based on a temperature decrease involves a significant reduction in solubility but the overall separation efficiency remains low in comparison with a temperature increase at 100 bar in the retrograde region. This

1.2 Phase Behavior and Solubility

53

example demonstrates that higher pressures favor the extraction step whereas lower pressures, particularly in the retrograde region, favor the separation step. The actual choice for the operating pressure is therefore a compromise between process yield (or solvent-to-feed ratio) and operating cost.

References [ l ] J. B. Hannay, J. Hogarth, Proc. R. SOC. London Sex A 1879, 29, 324. [2] K. K. Liong, P. A. Wells, N. R. Foster, Znd. Eng. Chem. Res. 1991, 30, 1329. 131 R. W. Shaw, E B. Brill, A. A. Clifford, C. A. Eckert, E. U. Franck, Chemical and Engineering News 1991, Dec. 23, 26. [4] P. H. van Konynenburg, R. L. Scott, Phil. Trans. Roy. SOC. 1980, 298, 495. [5] M. A. McHugh, V. J. Krukonis, Supercritical Fluid Extraction: Principles and Practice, Butterworths, Boston, 1986, Chapter 3. [6] W. B. Streett, in Chemical Engineering at Supercritical Fluid Conditions, M.E. Paulaitis, J. M. L. Penninger, R. D. Gray, P. Davidson (Eds.), Ann Arbor Science, Ann Arbor, MI, 1983, Chapter 1. [7] G. M. Schneider, Adv. Chem. Phys. 1970, 17, 1. [8] M. A. McHugh, T. J. Yogan, J. Chem. Eng. Data 1984, 29, 112. [9] F. P. Lucien, N. R. Foster, Znd. Eng. Chem. Res. 1996, 35, 4686. [lo] K. G. Liphard, G. M. Schneider, J. Chem. Thermodyn. 1975, 7, 805. [ll] J. M. Prausnitz, R. N. Lichtenthaler, E. G.de Azevedo, Molecular Thermodynamics of Fluid-Phase Equilibria, Prentice-Hall, Englewood Cliffs, NJ, 1986, Chapter 4. [12] W. J. Schmitt, R. C. Reid, J. Chem. Eng. Data 1986, 31, 204. [13] T. Nakatani, K. Ohgaki, T. Katayama, Znd. Eng. Chem. Res. 1991, 30, 1362. [14] J. F. Brennecke, C. A. Eckert, AZChE J. 1989, 35, 1409. [15] J. M. Dobbs, K. P. Johnston, Znd. Eng. Chem. Res. 1987, 26, 1476. [16] W. J. Schmitt, R. C. Reid, in Supercritical Fluid Technology, J. M. L. Penninger, M. Radosz, M. A. McHugh, V. J. Krukonis (Eds.), Elsevier Science Publishers B.V., Amsterdam, The Netherlands, 1985, pp 123-147. [17] E. Stahl, W. Schilz, E. Schutz, E. Willing, Angew. Chem. Znt. Ed. Engl. 1978, 17, 731. [18] D. K. Dange, J. P. Heller, K. V. Wilson, Znd. Eng. Chem. Prod. Res. Dev. 1985, 24, 162. [19] F. Dehghani, T. Wells, N.J . Cotton, N. R. Foster, J. Supercrit. Fluids 1996, 9, 263. [20] K. E. Laintz, C. M. Wai, C. R. Yonker, R. D. Smith, J. Supercrit. Fluids 1991, 4, 194. [21] D. Betts, T. Johnson, C. Anderson, J. M. DeSimone, ACS Polym. Prepx, Div. Polym. Chem. 1997, 38, 760. [22] K. Johnston, in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., M. Grayson, D. Eckroth (Eds.), John Wiley & Sons, New York, 1984, Supplement volume. [23] CRC Handbook of Chemistry and Physics, 75th ed., D. R. Lide, H. P. R. Frederikse (Eds.), CRC Press, Inc., Boca Raton, FL, 1994, Section 6. [24] T. Nakatani, K. Ohgaki, T. Katayama, J. Supercrit. Fluids 1989, 2 , 9. [25] S . Sako, K. Ohgaki, T. Katayama, J. Supercrit. Fluids 1988, 1 , 1. [26] S . Sako, K. Shibata, K. Ohgaki, T. Katayama, J. Supercrit. Fluids 1989, 2, 3. [27] Y. V. Tsekhanskaya, M. B. Iomtev, E. V. Mushkina, Russ. J. Phys. Chem. 1964, 38, 1173.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

1.3 Physical Properties as Related to Chemical Reactions ANTHONY CLIFFORD

The physical properties that are important for reactions in supercritical fluids (SCFs) are (a) phase behavior, (b) diffusion rates and (c) interactions between reagents, products and transition states with the fluid. First, phase behavior is important in bringing reagents into the same phase to allow them to react more effectively. For example light gases, such as hydrogen, are often miscible with SCF substances. Second, phase behavior can be important in separating the product of a reaction, possibly also stopping the reaction at an intermediate stage or pushing a reversible reaction to completion. The phase behavior of the reacting system must therefore be understood. At the minimum it will be necessary to know whether the reaction is in a single phase, as is usually required. This will often be a matter of being at a sufficiently high pressure. It will also be important to estimate the critical temperature, T,, of the multicomponent reacting mixture in order to know whether the system is a liquid (below T,) or an SCF (above T,). The difference in behavior in passing through T, is not dramatic, but as the temperature moves below T, the medium becomes less compressible and loses the control by pressure characteristic of a supercritical fluid [l]. Because it has been covered in chapter 1.2, phase behavior will not be further discussed in this section. Diffusion coefficients are typically higher in SCFs than in liquids. This is partly because the substances used as the solvent, such as carbon dioxide, have typically lighter and smaller molecules than organic liquid solvents and partly because the density of an SCF is typically less than a liquid. Consequently, reactions controlled by diffusion may be faster than in a liquid, giving the advantage of smaller process plant size. However, in the region of the critical point, diffusion coefficients can show an anomalous lowering, which can effect reaction rates. The behavior of diffusion coefficients is therefore discussed in Section 1.3.1 and its effect on reactions in Section 1.3.2. For reactions controlled by activation, solvation effects on the reagents and the transition state can affect the equilibrium coefficient for the formation of the transition state and therefore the reaction rate. Equilibria can also be affected by solvation effects on reagents and products. In a supercritical fluid, solvation effects can be controlled by density, and therefore at constant

1.3 Physical Properties as Related to Chemical Reactions

55

temperature by pressure, and so in Section 1.3.3 transition-state theory is adapted to the context of SCFs. The important application to the density dependence for two competing reactions is then discussed in Section 1.3.4. The effect of solvation and transition-state effects on chemical equilibria is finally treated in Section 1.3.5.

1.3.1 Behavior of Diffusion Coefficients For dilute solutions, the diffusion coefficient of a solute in an SCF has an inverse density relationship [2]. Figure 1.3-1 shows some experimental measurements [3] for the diffusion coefficient of naphthalene in carbon dioxide at 313K. The open circles represent values of D12multiplied by density, d, and then plotted against density. The solid point near zero density is the value calculated for 1 bar from rigorous kinetic theory, using an experimental measurement of the diffusion coefficient for naphthalene in air [4] to obtain the distance parameter in the intermolecular potential for naphthalene. The dotted line emanating from this point was calculated from Thorne-Enskog theory [5]. It can be seen that the product d D I 2for both the experimental and calculated values stays within a 25% band over the density range and the dashed curve shows the general trend, without having any theoretical basis. Other published measurements also fall near the same curve. A possible explanation of the rising part of the curve can be made using the concept of edgewise diffusion. This concept is used to explain that flat molecules, such as benzene, diffuse more rapidly in viscous fluids than would be predicted by hydrodynamic theories based on their behavior in more mobile fluids [6]. It is argued that when rotational diffusion of molecules becomes slow, as the density rises, translational movements persist more strongly along particular molecular axes.

Kinetic 0'

8

Figure 1.3-1 The product of the density and diffusion coefficient for naphthalene in C 0 2 at 313K, plotted versus density.

.\.. .

0

a

---

Critlcal density 600

300 Density (kg m4 )

900

56

I Introduction

Whether this explanation is correct or not, the discussion illustrates the difficulty in predicting diffusion coefficients in dense media. At constant higher density, there is undramatic behavior of the diffusion coefficient as the temperature is lowered from supercritical conditions, through the critical temperature, to liquid conditions [ 7 ] . In the region of the critical point, diffusion coefficients show a lowering effect, to an extent dependent on concentration 181. As the critical point is approached closely, the diffusion coefficient tends to zero for finite concentrations. A physical explanation of this is that, as the temperature is lowered towards the critical temperature at the critical density, a situation is being approached where two phases with two different concentrations of the solute coexist in equilibrium, and where there is no tendency to reduce the concentration difference. Some experimental observations of this decrease of the diffusion coefficient towards a critical region have been made [9].

1.3.2 Diffusional Effects on Reactions Some rapid homogeneous reactions are controlled by the rate of diffusion of reagents towards each other. In some cases, as pressure increases in an SCF, a reaction may pass from activation control to diffusion control [lo]. In the simplest analysis of diffusion control, the second-order rate coefficient in terms of mole fractions, k,., is given by the Smoluschowski equation,

where DA and DB are the binary diffusion coefficients of the reagents in the fluid, r is the distance of approach necessary for reaction and q5 is a statistical or steric parameter of less than unity which takes into account, for example, the necessary relative orientation of the two molecules for the reaction to take place. The diffusivity of a dilute solute in an SCF, somewhat removed from the critical point, is typically an order of magnitude greater than in liquid solvents at comparable temperatures. The Stokes-Einstein equation, relating the diffusion coefficient to the fluid viscosity, is used to rewrite eq (1.3-1) in terms of viscosity as

k, = 8RTBq

( 1.3-2)

where q is the coefficient of viscosity of the medium. Good agreement between experiment and the predictions of eq (1.3-2) have been obtained in SCFs [ 1 I]. According to eq (1.3-2), all diffusion-controlled second-order rate coefficients are the same in a particular solvent. Diffusion is also important for unimolecular fission. Thus, radical initiators under SCF conditions are able to escape more readily from solvent cages, and the rate coefficient for the initiation process is markedly increased. Pro-

1.3 Physical Properties as Related to Chemical Reactions

57

cesses propagated by free radicals, such as polymerization, are also rate enhanced. For unimolecular decompositions, initiated thermally or by light, decreasing the diffusion coefficient by increasing density can decrease the decomposition rate. This is ascribed to slower diffusion out of a cage of solvent molecules, giving rise to an increased rate of the geminate recombination of two molecular fragments Tormed by decomposition. In a complex reaction scheme, where only some of the steps are diffusion controlled, the course of the reaction can be changed by controlling the diffusion coefficients. The archetypal example here is polymerization. The effect of diffusion control on overall rate and the rates of the different steps is first discussed. A simplified equation, sufficient for a qualitative discussion, for the instantaneous rate of polymer formation is (1.3-3) where E is the initiator efficiency, xI and xMare the mole fractions of initiator and monomer, and ki, kp, and kt are the rate coefficients for the initiation, propagation and termination processes respectively. The factor kdkt“2, which gives the pressure dependence of the reaction if its initiation is pressure independent, is found to increase with increasing pressure. This is readily explained by termination steps becoming diffusion controlled at much lower pressures than propagation steps because the former, in general, involve two large polymer radicals, whereas the latter involve a smaller monomer molecule as one of the reacting species. However, initiation may also be affected if it is suggested that diffusion control is associated with the formation of solvent cages, as indicated in eq (1.3-4) by curly brackets around reagent molecules. A B e (Aa-B}+ A * + * B

(1.3-4)

When the molecule concerned is a radical initiator this has implications for the efficiency of radical production, related to cage escape. Under supercritical conditions, geminate recombination can be controlled by density, as described earlier. In the region of the critical point, diffusion coefficients can fall for finite concentrations, as described in Section 1.3.1. The behavior of reactions in the critical region can therefore be discussed qualitatively using this effect. However, in a more integrated approach, the methods of nonequilibrium thermodynamics [12] can be used as a basis for discussion of what effects can be expected on both the rates, including diffusion-controlled rates, and equilibrium positions of chemical reactions due to the proximity of a critical point. These arguments have been reviewed and applied to the discussion of a number of experimental studies by Greer [13]. Critical region behavior of the diffusion coefficient can have effects on product ratios. This is probably the explanation of the increase in yield of the photo-Fries enolization products near the critical density in the photochemical

58

I Introduction

-

O

IRH Scheme 1.3-1

A

reaction of 1-naphthyl acetate with 2-propanol in supercritical carbon dioxide [14], as shown in Scheme 1.3-1. The photo-Fries enolization products are formed within the solvent cage, and diffusion out of the cage is necessary to give the other product, 1-naphthol. Outside the critical region the ratio of photo-Fries enolization products to naphthol is around four, but this increases to more than 12 near the critical density. Very often catalytic heterogeneous reactions are controlled by the rate of diffusion to the catalyst surface and enhanced diffusion rates in supercritical fluids can be an advantage. An important category of such reactions are enzymatic conversions, as enzymes are insoluble in most normal organic solvents and the use of nontoxic carbon dioxide can be an advantage. Diffusion is not only enhanced in the bulk fluid, but also diffusion within the pores of particles containing enzyme molecules or other heterogenous catalysts is more rapid. Enhanced swelling of such particles, compared with that in liquid solvents can be beneficial as well.

1.3.3 Transition-state Theory Applied to SCFs During a reaction process, the reacting species, A and B, are considered to pass through a potential energy maximum in their transition from reagents to products. The minimum energy pathway on the relevant potential energy surface is termed the reaction coordinate, and the molecular configuration coyesponding to its zenith is the activated complex or transition state species, C . Although the activated complex does not have any stable existence, it is regarded as a species and treated thermodynamically. Transition-state theory assumes that the activated complex is formed in a rapid pre-equilibrium with the reagents, as indicated in eq (1.3-5) for the common case of a bimolecular reaction. A

+B

Cf

+ products

(1.3 -5)

1.3 Physical Properties as Related to Chemical Reactions

59

The theory then considers that this species breaks up by a first-order process and describes the rate of formation of one of the immediate products, P, in terms of the amoynt of the activated complex and an appropriately defined rate coefficient, k : dnp/dt = k'

(1.3-6)

nCt,

where niare numbers of moles. It is assumed throughout, unless stated otherwise, that pressure and temperature are kept constant, to simplify the notation in derivatives such as dnpldt = (&zp/at)p,T. Although transition-state theory is usually presented in terms of concentrations, in the context of SCFs it is more convenient to use mole fractions, xi. This is because changes in concentration are often dominated by volume changes, which are not of direct interest, and which cause many of the expressions to have compressibility terms. As can be easily shown by differentiation, if s is the total number of product molecules formed from the activated complex, (1.3-7) dxpldt = (lln) ( 1 - (s-l)xp} dnp/dt and with eq (1.3-6) this becomes dxpldt = k

* XC:

(1

-

(S-l)Xp}

( 1.3 - 8)

Thus, if only one product molecule is formed from the activated complex, it becomes true that drpldt = k' xct

(1.3-9)

and eq (1.3-9) is also approximately true in other cases, provided the mole fraction of any product is small, and dilute conditions of reagents and products will now be assumed throughout. The theory then assumes that the equilibrium between the reagents, A and B, and the activated complex, C , is governed by an equilibrium constant. In a near-critical fluid, the most convenient approach is to define the equilibrium in terms of fugacities, Alpe, referred to the standard state of p*, typically 1 atmosphere: ( 1.3 - 10)

The quantity KjO is an equilibrium constant for the ideal-gas state and is thus equal to that used for dilute-gas reactions. The fugacities are given in terms of the mole fractions, xi, the fugacity coefficients, &, and the pressure, P , by (1.3-11) A = 4i P xi

I Introduction

60

The substitution of eq (1.3-10) and (1.3-11) into equation (1.3-9) gives the following expression for the rate of increase of the product P: ( 1.3- 12)

In transition-state theory, the first two factors on-the right-hand side of equation (1.3-12) are modified (leaving the product of the factors unchanged in value), using the arguments of statistical mechanics given in many textbooks in physical chemistry, to produce the following more useful equation: (1.3-13) where kB and h are Boltzmann’s and Planck’s constants, respectively. The --te bar above K p indicates the modification of the equilibrium constant: it is called a pseudo equilibrium constant, but treated like a normal equilibrium constant. The rate equation in terms of mole fractions and the measured rate constant, k,, is dxpldt = k,

(1.3-14)

XA xB,

Comparison of eq (1.3 -13) and (1.3 -14) gives the following expression for the rate coefficient: (1.3 -15) Equations (1.3-14) and (1.3-15) thus give the prediction from transition-state theory for the rate of a reaction in terms appropriate for an SCF. The rate is seen to depend on: (i) the pressure, the temperature and some universal constants; (ii) the equilibrium constant for the activated-complex formation in an ideal gas; and (iii) a ratio of fugacity coefficients, which express the effect of the supercritical medium. Equation (1.3-15) can therefore be used to calcu--te late the rate coefficient, if K p is known from the gas-phase reaction or calculated from statistical mechanics, and the ratio ~ j ~ / 4estimated ~t) from an equation of state. Such calculations are rare; an early example is the modeling of the dimerization of pure chlorotrifluoroethene (T, = 105.8 “C) to 1’2-dichlorohexafluorocyclobutane (Scheme 1.3-2) and comparison with experimental results at 120°C, 135°C and 150°C and at pressures up to 100 bar [15].

>=
F 2x

F

F

-

F F#

CI

Scheme 1.3-2

1.3 Physical Properties as Related to Chemical Reactions

61

Fugacities were calculated from the Redlich-Kwong equation of state and there were differences of 30 % between experiment and prediction, which reflects the uncertainties in calculating fugacities. After taking the natural logarithm of eq (1.3-15) and using

--te

--te -

AGp +RTlnK,

( 1.3- 16)

=O

--te

where AGp is the modified standard Gibbs function change for the formation of the activated complex in the ideal gas, we obtain

[kiT)

--te ( 1.3- 17)

Ink, =In - - A G ~ +In RT

In order to make calculations from this equation, two 0: the terms are combined to give a Gibbs function change for the reaction, AG , which is still standard in terms of mole fractions, but which applies to the particular SCF conditions used, where AGt= AEiO -ln[z]4 ~ 4 ~ RT

( 1.3- 18)

In terms of AG', eq (1.3-17) becomes ( 1.3- 19)

Using the general thermodynamic relationships ( d G / d ~ =) ~V~and RT

[7 ) v i - q - RT

=

T,x

(1.3 -20)

P

are partial molar volumes under general and ideal-gas conwhere & and ditions, respectively, and the subscript x indicates that composition is kept constant for the derivative, equation (1.3-17) is transformed, after differentiation with respect to pressure at constant temperature, to

RT

[7 )

--te

=-AV

T,,

-(V,-~-VA-VB)

+ (G-q-T) (1.3-21)

p e terms on the right-hand side of equation (1.3-21) are now considered. AV is the volume of activation in an ideal gas, as normally used. (Vet V, - V,) is the rate of change of the volume of the reaction mixture with respect to the amount of activated complex formed in moles at constant pressure and temperature. In view of the large partial molar volumes possible in SCFs, this is often important, especially in the critical region. It can appropriately be given the symbol AVt. is the volume change for the

(G-e-e)

62

1 Introduction

formation of the activated complex in the ideal gas, arising from the voluyes --8 of the three species themselves. It is almost identical to the first term AV , the difference arising from the modification of the equilibrium constant in transition-state theory, as described in the last section, and indicated by the bar. If this difference between the first and third terms is ignored and they are cancelled, this yields the equation (1.3-22) Equation (1.3 -22) can be used to calculate a volume, of activation from ratecoefficient data in a supercritical fluid. It gives an activation volume which is the difference between the partial molar volume of the activated complex and those of the reagents and includes both intrinsic and solvent effects. Partial molar volumes have been discussed in Chapter 1.2 and shown to be very large and negative in the critical region. Similarly, in many cases activation volumes in SCFs in the critical region are found to be large and negative, both experimentally and in modeling studies. As an example of the latter, modeling of the dimerization of chlorotrifluoroethene discussed earlier [15] gave a value for AVt of -3290 cm3 mol-', close to the critical density and 14°C above the critical temperature. These extreme negative values are a reflection of the fact that the density of the medium is changing rapidly with pressure, and as a consequence the rate coefficient is increasing rapidly from a gas-like to a liquid-like value. They are not an indication of important effects on reaction behavior due to the proximity of the critical point.

1.3.4 Density Dependence of Two Competing Reactions The large effects due to rapid changes in density with pressure can be eliminated if two reactions are considered that have the same reagents, but produce two different products. In this case there will be two rate coefficients, ki and k,: and also other thermodynamic quantities which will be indicated by the single and double primes. The ratio of the rate coefficients will be of importance as this will indicate the ratio of products that will be obtained. By subtracting two equations of the form of eq (1.3-22), the following expression for this ratio is obtained: (1.3 -23) Because the reagents are the same in both cases eq (1.3 -23) will simplify to (1.3 -24)

1.3 Physical Properties as Related to Chemical Reactions

63

The following relationship is true for partial molar volumes at infinite dilution:

vi = v - ( a v / a p ) , ~ ( ~ p / a x i ) T , V

(1.3 -25)

Therefore, assuming that all reagents, transition states and products are at infinite dilution, the following expression is obtained from eq (1.3 -24), after the molar volume terms cancel:

The term involving the compressibility, ( ~ V I ~ Pcan ) ~now ~ , be divided out of equation (1.3-26) to give R T [ aln(ki/k:)

av

-[%I

) T,x

axc,

T,v

(1.3-27) T,V

Thus the density dependence of the ratio of rate coefficients is related to the difference in the derivatives of pressure with respect to the mole fractions of the transition states. These quantities arise from interactions between the transition states and the solvent molecules [l] and can be described as the tuning functions for the transition states.

1.3.5 Solvation Effects on Reactions Solvation can affect the equilibrium constant of a reaction in an SCF, expressed here in terms of mole fractions, Kx. The sensitivity to pressure is given by the standard thermodynamic relationship

[y) T,x

= - AV

(1.3 -28)

where AV is the difference between the partial molar volumes of the products minus those of the reagents, multiplied by their stoichiometries. Thus the dramatic values of partial molar volumes in SCFs may cause interesting behavior. If there is the same number of product molecules as reagent molecules, the molar volume terms in eq (1.3 -25) will cancel, and the rate of change of equilibrium constant with density becomes equal to differences in tuning functions. A number of studies have been made and one example is described here: the tautomerization between 2 -hydroxypypiridine and 2-pyridone in 1,l-difluoroethane and propane (Scheme 1.3-3) [16]. The reaction was monitored in situ spectroscopically, at reagent concentrations causing negligible self-association of 2 -pyridone. The equilibrium con-

I Introduction

64

O

O

H

-

H

H

0-

Scheme 1.3-3

stant was shown to be a strong function of pressure for both solvents in their respective critical regions, with stronger variation'in polar 1,l -difluoroethane. Solvation of reagents and transition states also affects reaction rates, for which the theory was provided in Section 1.3-3.A number of studies of single reactions have been made and these are characterized by rapid changes in rate with pressure near the critical density, especially when the reaction is just above the critical temperature. These rapid changes correspond to large (usually negative) values of the activation volume, on the basis of eq (1.322). An example was given in Section 1.3-3 as the equations were discussed. These effects overwhelmingly reflect the fact that, near the critical density, the density changes rapidly with pressure and the solvent is rapidly changing from gas-like to liquid-like. For two competing reactions, however, the effect of the large compressibility cancels, as shown in Section 1.3.4, and a more subtle effect of solvation, expressed by tuning functions can be observed. An experimental study on the Diels-Alder reaction between cyclopenta-1,3-diene and methyl acrylate was carried out [ 11, which produces two stereoisomeric products, the endo and exo forms (Scheme 1.3-4).

OMe

C0,Me C0,Me endo addition

ex0 addition

Scheme 1.3-4

Experiments were carried out at low concentrations to ensure that conditions were supercritical and that the limit of infinite dilution was closely approached. Results for the ratio of endo to ex0 product, plotted against density at 313K, are shown in Figure 1.3-2.The product ratio is seen to go through a maximum, although the range of values of the ratio is not different than can be obtained by changing the solvent in the liquid phase reaction [17]. However, by controlling the density, this range can be obtained in a single solvent. The curve shown is derived from Eq (1.3-27)using the van der Waals equation to obtain the difference in tuning functions and fitting to the experimental values. The maximum does not occur at the critical density and thus is not associated with clustering in the critical region. Instead, it appears that the maximum is associated with tuning of the average distance between the solute molecules and the transition states as the pressure and density vary.

1.3 Physical Properties as Related to Chemical Reactions

65

4.0

Figure 1.3-2 Experimental data, shown as points, for the ratio of endo to ex0 product from the reaction of methyl acrylate and cyclopentadiene in carbon dioxide at 3 13K, with the curves shows predictions derived from eq (1.3-27).

1.3.6 Conclusions

.

Thus there are a number of characteristics of the physical properties in supercritical fluids which may lead to advantages in carrying out reactions in the medium. The first has to do with phase behavior, which may, on the one hand, facilitate reaction by bringing reagents and catalysts into one phase and, on the other, bring about required separation of products. The second is that diffusion is typically faster than in liquids, which may speed up both homogeneous and heterogeneous reactions. Finally, because both pressure and temperature can be used to alter the compressible medium, there may be greater control of reaction pathways through both diffusion and transitionstate mechanisms.

References [ l ] A. A. Clifford, K. Pople, W. J. Gaskill, K. D. Bartle, C. M. Rayner, J. Chem. SOC.,Faraday Trans. 1998, 94,1451. [2] A. A. Clifford, Fundamentals of Supercritical Fluids, Oxford, 1998. [3] R. Feist, G . M. Schneider, Separation Science and Technology 1982, 17, 261. [4] E. Mack, J. Am. Chem. SOC. 1925, 47, 2468. [5] G. C. Maitland, M. Rigby, E. B. Smith, W. A. Wakeham, Intermolecular Forces, Clarendon Press, Oxford, 1981. [6] H. J. V. Tyrrell, P. J. Watkiss, DifSusion in Liquids, Butterworths, London, 1979. [7] H. H. Lauer, D. McManigill, R. D. Board, Anal. Chem. 1983, 55, 1370. [8] A. A. Clifford, S. E. Coleby, Proc. R. SOC.London, Ser. A 1991, 433, 63. [9] H. A. Saad, E. Gulari, Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 834.

66

I Introduction

01 H. Hippler, V. Schubert, J. Troe, J. Chem. Phys. 1984, 81, 3931. 11 J. F. Brennecke, in Supercritical Fluid Engineering Science, ACS Symp. Ser. 514, E. Kiran, J. F. Brennecke (Eds.), American Chemical Society, Washington, DC, 1993, Chapter 16. [12] S. R. De Groot, P. Mazur, Non-equilibrium Thermodynamics, North Holland, Amsterdam, 1962. 1131 S. C. Greer, Phys. Rev. 1985, A31, 3240. [14] D. Andrew, B. T. Des Islet, A. Margaritis, A. C. Weedon, J. Am. Chem. SOC. 1995, 117, 6132. [15] G. M. Simmons, D. M. Mason, Chem. Eng. Sci. 1972, 27, 89. [16] D. G. Peck, A. J. Mehta, K. P. Johnston, J. Phys. Chem. 1989, 93, 4297. [17] J. A. Berson, Z. Hamlet, J. Mueller, J. Am. Chem. SOC. 1962, 84, 297.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

2 Experimental Techniques 2.1 High-pressure Reaction Equipment Design RALFFINKand ERICJ. BECKMAN

High-pressure processes and equipment have been important to the chemical industry ever since the development of ammonia synthesis by Haber and Bosch. Today a wide range of industrial processes make use of high pressure technology, including the liquefaction of gases, the synthesis of low density polyethylene, and the synthesis of diamond and boron nitride. In particular, the use of supercritical fluid (SCF) solvents in separation and extractions [ 1-61, and chemical reaction processes [7-91 are described in numerous examples in the literature, and similar design and safety principles for both industrial processes and benchtop reactors (either for scientific or pilot scale purposes) can be applied. The wide variety in application and scale of high-pressure technology makes it necessary to be selective when considering the design and safety of such systems. Nonetheless, because of the numerous design examples it seems impossible to give a complete (or even partial !) description of high-pressure reaction equipment. Consequently, this chapter focuses on small-scale (benchtop) high-pressure equipment, their related safety questions, and their application to chemical reactions and extractionheparation as shown in the literature. It needs to be stressed that in-depth information is given in the corresponding literature and, and in particular, by the manufacturers of commercially available components, thus recommending the study of the specific catalogues. Most of the manufacturers mentioned in this chapter maintain pages on the World Wide Web, and further information can be obtained there.

2.1.1 Introduction The internationally accepted (SI) unit of pressure is the Pascal (Pa) and consequently for high pressure processes the megapascal. In synthetic applications it is more common to use the bar as the unit of pressure (1 MPa = 10 bar). Therefore, the unit used in this book is also the bar. Because various other units are still in use it is important to be familiar with the most important conversions.

68

2 Experimental Techniques

Table 2.1-1 Conversion factors of commonly used pressure units.

MPa 1 MPa

1

1 bar

bar

atm

10

9.8692

0.1

1

0.9869

1 atm

0.1013

1.0133

1

1 kg/cm2

9.8067X10-2 9.8067X10-' 0.9678

kg/cm2 10.197 1.0197

lb/in2 (psi)

mm Hg (tom)

145.04

7500.6

14.504

750.06

1.0332

14.696

760.00

1

14.223

735.56

1 lb/in2 6.8947~ 6.8947~ 6.8046X 7.0306X 1 51.716 (psi) 1 mm Hg 1.3332x1O4 1 . 3 3 3 2 ~ 1 0 - ~1 . 3 1 5 8 ~ 1 0 - ~1 . 3 5 9 5 ~ 1 0 - ~ 1.9337X10-2 1 (tom)

Table 2.1-1 gives conversion factors for commonly used pressure units in the literature. A list of additional useful conversions can be found in Appendix A. The literature is not entirely consistent as to the definition of high and low pressure. However, at pressures above 6000 bar the question of material choice (particularly if corrosive fluids are to be employed in the system) becomes critical and a specialized reactor design must be chosen [17,18].As such, this discussion focuses on systems designed for use at pressures below 6000 bar. The design of high pressure equipment makes it necessary to consider first some essential questions: (1) the maximum pressure at which the reactor will be operated; (2) volume and shape of the inner chamber (stirrer type and shape); (3) whether a batch process will be sufficient, or continuous feeding or extraction of products will be required; (4)temperature range of the process (control, rate of change, removal of exothermic reaction heat); (5) whether corrosive materials will be used; (6) miscellaneous factors such as fast opening, easy cleaning. In all cases conservative safety factors of at least 2:l (yield strength) or 4:1 (ultimate strength) for allowable stresses should be applied. Additionally, it should be kept in mind that repeated application of high pressure to components, such as in repetitive loading or generation of pressure, or as a result of mechanical vibration, may cause fatigue cracks that can propagate through the wall of the component so that leakage can occur. However, if the material of construction lacks toughness, the crack may lead to fast fracture and the component fails catastrophically. The numerous possible high pressure applications and their associated requirements can be satisfied by a variety of different assemblies of reaction vessels, closures, tubing, valves, pumps or compressors. First, an overview of these different components is presented, after which several complete high-pressure systems found in the literature are discussed. At this point it should be emphasized that in showing equipment from one manufacturer or another, no preference for a particular product is being expressed. Further details on equipment employed for spectroscopy at high pressure are provided in Chapter 3.

2. I High-pressure Reaction Equipment Design

69

2.1.2 Basic Equipment and Components 2.1.2.1

Design of Thick-Walled Vessels

Much information is available on the deformation and fatigue behavior of simple thick-walled cylinders [ 10-171, but it must be remembered that most process reactors will not be a simple hollow cylinder. Components such as connectors, threads and sleeves, windows, and removable closures make a complete analytical solution for a high-pressure system design problem quite involved. Useful design criteria for thick-walled vessels can be derived, however, under the,assumption that the material of which the vessel is made is isotropic and that the cylinder is long (more than five diameters) and initially free from stress. The radial and tangential stresses in the walls are then only functions of the radius coordinate ( r ) and the internal pressure. Given the outer-toinner wall radius ratio as o/i = w, and the yield point (Yo) of the material, the yield pressure (py)is p y = Yo (w-1)/(3

(2.1-1)

The yield pressure is hereby defined as the point where the material (e.g. AISI 316 steel) reaches its elastic limits, namely where any pressure increase irreversibly deforms the material. As seen in eq (2.1-1), as the wall thickness increases, the inherent strength of the material is employed less efficiently. For example, increasing the wall thickness ratio w beyond 2 does not increase the yield pressures significantly, and p y asymptotically approaches Yd3'" as w approaches infinity. One solution in order to achieve higher yield pressures is to utilize steels with higher yield points. The yield point, the typical composition, and other characteristic data of high-strength austenitic stainless steels are listed in Table 2.1-2. In particular, these steels are resistant to atmospheric corrosion and exhibit excellent resistance to stress-corrosion cracking (e.g. AISI 316 stainless steel is frequently used for high-pressure tubing and reactors). However, due to the nature of steel, acidic conditions lead to an increased rate of corrosion [17,18]. This may become a problem in COz-water mixtures, where the pH can easily drop to ca. 3.0. Clearly, use of supercritical water presents an extreme challenge in material selection. An alternative strengthening mechanism is prestressing, called autofrettage, which ensures a more uniform stress distribution under load [16-201. This intentional over-pressurizing of the vessel leads to a plastic-elastic interface that moves outward as the pressure increases. When the pressure is released, the residual stresses left as a result of radial expansion allow the vessel to be used up to the pressure at which it was subjected to autofrettage, without exceeding the yield point of the material. Easier to achieve than the autofrettage in order to increase the yield pressure is the construction of a compound cylinder. Here a second cylinder (or jacket)

70

2 Experimental Techniques

Table 2.1-2 Typical high-strength austenitic stainless steels [ 171.

Steel

Composition (wt. %)

AISI 202

AISI 301

AISI 316

Micromach (Washington Steel Co.)

USS Tenelon

0.15 max. C, 7.5-10 Mn, 17-19 Cr, 4.6 Ni 0.1 c, 2.0 max. Mn, 1.0 max. Si, 17.0 Cr, 7.0 Ni 0.08 C, 2.0 Mn, 1.0 Si, 16-18 Cr, 10-14 Ni, 2.0-3.0 MO 0.11 C, 13.5 Mn, 16.2 Cr, 4.6 Ni, 0.92 V, 0.36 N 0.08-0.12 C, 2.0 max. Mn, 1.0 max. Si, 17.3 Cr, 6.2 Ni 0.10 C, 14.5 Mn, 17.0 Cr, 0.4 N

Maximum yield strength (bar)

Maximum tensile strength (bar) ~

Elongation

Hardness

(“/.I

(Rb)

3900

7000

55

90

2800

7000

60

85

2600

6000

65

78

18 000

19 000

2

12700

14 100

14800-16900 15800-17900

3-9

is heated and then contracted around the inner cooled cylinder, such that the yield pressure becomes:

p y = 2Yo ( ~ o ~ c * - ( c ~ + ~ ~ x ~ ) / ( ~ w ~ ) ~ / ~

(2.1-2)

where o = outer radius, i = inner radius, and c = intermediate wall radius. When c = (oi)’l2

p y = 2Yo ( 0 - 1 ) / ( 3 ~ ~ ) ~ / ~

(2.1-3)

The temperature limit for the outer heat-treated cylinder, as well as for the autofrettaged vessel, is limited to 500°C; otherwise the temper is lost (Figure

2.1-1). Both high and low temperatures have a tremendous effect on the physical properties of materials used to construct high pressure vessels. With regard to very low temperatures, the toughness and impact strength of most alloy steels decrease as the temperature is reduced and one has to choose the applied materials carefully to avoid embrittlement. At high temperatures, however, the

2.1 High-pressure Reaction Equipment Design

71

Figure 2.1-1 Thick walled vessels: (a) double walled cylinder with interference fit d; (b) single walled, autofrettaged. (0 = outer radius, i = inner radius, c = interference

diameter).

yield and tensile strength of steel decreases. For most alloy steels used in construction of high-pressure equipment, at temperatures above 350 "C the stresses to the wall of the vessel are no longer independent of time. Additionally, any chemical reaction performed in such vessels that produces exothermic heat can produce a temperature gradient, possibly exceeding the temperature limitation of the vessel at the inner wall, thereby leading to creeping loss of temper and possible vessel failure. 2.1.2.2

Closures and Connectors

In general, the vessel will be tight if the seal pressure (p,) is higher than the inner pressure @i). Various closure designs have been made to provide the needed seal pressure [21-251. Closures usually consist of three basic elements: the cover, a coupling device keeping the cover in position to the vessel body, and a seal between the cover and the vessel. The sealing ring or gasket is always made of a softer material (such as copper or an elastomer such as Buna N). The seal deforms under pressure, smoothing and/or filling any irregularities on the connecting surfaces. The O-ring is a typical example (Figure 2.1-2(a)). Initial sealing is afforded by the diametric squeeze on the O-ring; then, after the internal pressure is applied, the O-ring is forced to the side of the groove. The maximum pressure, which can be applied to such seals is about 300 bar. Similar seal designs with an optimized support for the O-ring (or a soft metal flat gasket) allow significantly higher pressure ratings of up to ca. 15 x lo3 bar (Figure 2.1-2(b)). Additionally, O-rings made of Buna N, for example, tend to swell in a carbon dioxide environment, thus increasing the seal pressure but preventing the use of highly corrosive chemicals and temperatures above 90 "C. Two other draw-

72.

2 Experimental Techniques

Figure 2.1-2 Different seal designs: (a) O-ring; (b) soft gasket; (c) unsupported-area seal (Bridgeman seal).

backs to the use of elastomeric O-rings are that non-covalently bound additives can be extracted during use at high pressure, interfering with spectroscopic studies, and the swelling of O-rings can lead to their explosive failure if depressurized too quickly. The latter problem can be dealt with, however, by the use of elastomers with a higher crosslink density (thus decreasing swelling), and their low cost makes them attractive seals. Although polytetrafluoroethylene (PTFE) gaskets are popular in chemistry, as they are resistant to most chemical environments, they are known to swell in carbon dioxide (see Chapter 2.5 and 4.7). This could cause problems if PTFE is used to seal against moving parts where tolerances are tight. Pressure-energized seals (Bridgeman seals) have pressure ratings up to 20 X lo3 bar and can also be used for leak-prone gases such as helium and hydrogen. Support rings made of Teflon or a soft metal (such as copper) prevent the extrusion of the gasket during use at high pressure. These seals use the principle of unsupported area (a) in order to provide the condition ps>pi (Figure 2.1-2(c)). While the inner pressure acts over the total area ( A ) of the plug head (sometimes referred to as a mushroom plug), the seal pressure is constrained over the reduced area A-a, thus increasing the stresses in the seal ring higher than the working pressure in the vessel: (2.1-4) However, the seal pressure cannot be increased without limit. In addition to extrusion of the seal ring between the plug and the wall at pressures higher than 2000 bar, the seal is also pushed to the wall and the plug, which can lead to the deformation or failure of the wall or stem material. For corrosive materials, or when the inner fluid should not be contaminated with any kind of seal lubricants, metal-to-metal seals have been successfully employed. In this case, for example, two conical segments impact along a line, making easy opening of the vessel possible. Coupling devices to keep the closure in place are often bolted flanges. These flanges have to withstand not only the internal pressure, but also the stress, which is induced by (repeated) tightening. Flanges can be integrally forged

2. I High-pressure Reaction Equipment Design Gland

Sleeve

73 Collar

Figure 2.1-3 Tubingvessel connections for pressures up to 10000 bar makes use of threaded tubing (High Pressure Industries Co.).

to the reactor body or alternatively screwed (welded) to it. As a rule, the diameter of the bolt circle should be as close as possible to the gasket so that the coupling takes place nearby, thus reducing the thickness of the end closure. Another solution is the use of screwed closures, which are inexpensive and easy to open and close. It should be kept in mind that in case of leakage the coupling devices as well as any closure should never be loosened or tightened under pressure, because enormous stresses are developed in the flanges, especially when the pressure changes during operation. In order to perform intertubing or tubing-vessel connections, sleeve or collar type connectors are used. For pressures of up to 1000 bar and tubing diameters smaller than 3/8", taper seal connections are suitable. Here a gland nut is tightened onto the connector, ensuring the metal-to-metal seal of the sleeve. For pressure applications of up to 10000 bar the tubing is additionally threaded into the sleeve (collar), improving the tubing-sleeve connection (Figure 2.1-3).

2.1.2.3

lhbing and Fittings

In general, tubing can be seamless, welded or drawn. For the highest pressure ratings (above 1000 bar) cold-drawn, seamless tubing is preferred. Preferred materials include AISI 304 and 3 16 stainless steel which exhibit, especially at small diameters, very high pressure ratings [26-281 and are stress-corrosion resistant as long as acidic conditions are avoided. Some sizes and related ratings of commercially available tubing are given in Table 2.1-3. Despite these high (and naturally conservative) ratings, the tubing for a system should be chosen with great care. In particular, the pressure in the tubing of a reciprocating pump cycles much more than that of the system, and thus it is subject to more repetitive stressing than other parts of the system. Furthermore, any scratches or nicks derived from bending or installation under stress must be avoided. The tubing can be cut by hand or machine, although with small tubing 'care should be taken to avoid crushing [10,26-281. Tubing should always be secured to the benchtop or the frame of the apparatus.

74

2 Experimental Techniques

Table 2.1-3 Pressure ratings of typical high pressure tubinga ~

Outer diameter (in)

Inner diameter (in)

odlid ratiob

Material'

1/16 1I4 318 1I4 318 314 1I4 318

0.03 0.125 0.25 0.109 0.203 0.438 0.063 0.063

2.08 2 1.5 2.29 1.85 1.71 4 6

316 S.S. 316 S.S. 304 S.S. 316 S.S. 316 S.S. 316 S.S. 316 S.S. 304 S.S.

~~

Pressure rating (bar)

~~~~~

100 66 66 200 200 200 6600 10000

All values are approximate, based on typical data published by manufacturers. Ratings for specific tubing may differ. Temperature, use of corrosive materials, and fatigue factors should be considered sperately. od = outer diameter, id = inner diameter. ' In these cases all tubings are cold drawn, seamless, and not annealed. a

2.1.2.4

Valves

As with all other parts of a high pressure system, an initial decision has to be made as to the working conditions (pressure range, temperature, corrosive materials, size, etc.) and purpose (odoff-valve, flow rate adjustment, flow restrictions when open) a valve should fulfill [29-301 prior to selection. In some applications, such as measuring the swelling of polymers via the differential pressure method, or when employing expensive reagents, valves with low dead volume are also desirable. A great variety of commercially available designs are able to match almost any requirement. For many applications a simple ball valve (on/off) is suitable and, although these valves are usually operated manually in lab-scale systems, use of pneumatic or hydraulic actuators allows remote control. Needle valves can also be used in high-pressure applications of up to 15 x lo3 bar, particularly where greater sensitivity in flow control is desired. When closing the valve, the needle moves vertically into the valve seat. The stem tip is made of steel but soft packing can also be used. Several stem designs are available, such as from High Pressure Equipment Co. (HIP), where the bottom part of the stem is attached to the body with an internal screw thread, or in case of the rolling design where the stem floats within the outer threaded enclosure (Figure 2.1-4). The top part of the stem does not move vertically, but rather rotates and thereby moves the bottom part up or down. These designs, as is the case with most modern valves, avoid the rotating of the tip on the seat, which could lead to grinding and galling of the stem and seat parts. Check valves ensure flow in one direction only [31,32]. Their design consists of a spring which holds a piston, a disc, or a ball in place to open or close the connection at a predetermined pressure, set by the tension in the

2. I High-pressure Reaction Equipment Design

75

Figure 2.1-4 Example of two-way odoff-valve designs: (a) positive guide stem, (b) rolled style stem (taper seal, rated for ca. 650 bar, High Pressure Equipment Company).

spring, in the direction of flow only. Pressure ratings up to 10000 bar are available for ball type check valves, which renders them particularly useful in compressors (Figures 2.1-5 and 2.1-6). At considerably higher spring forces, the check valve finds its use as a safety feature - the pressure relief valve [33]. As the internal pressure increases above the preset release pressure the valve opens and closes automatically, protecting the vessel and its components from overpressure (Figure 2.1-5). Relief valves with pressure ratings of up to 2000 bar are available [10,26,34]. A relatively simple method to prevent the system from failing catastrophically due to overpressure is the use of a rupture disc. Whereas the relief valve opens and closes, releasing excess material (and thus lowering pressure) while maintaining the vessel operational, the rupture disc bursts, venting the entire contents of the vessel [35]. For further use the disc must then be replaced. The rupture disc should be used at significantly lower working pressures than the burst pressure (ca. 90% of the burst pressure). Ratings for the design shown in Figure 2.1-5 are up to 400 bar. Filters should be used when a system part (mostly a recirculating pump) needs to be protected from solid impurities. Line filters such as shown in Figure 2.1-5, with customized filter sizes and pressure ratings, should only be used where small amounts of impurities are expected, otherwise clogging will occur. Filters designed to withhold higher amounts of solids, e.g. during venting of a batch reactor, usually have a higher filter volume [26]. The design of back-pressure regulators uses a similar principle as that employed in check valves. Here the state of the valve (openklosed) is set by three factors: the spring force, and the inner and outer pressures. For example, at a given inner pressure the opening (and closing) of the valve is determined by the spring force and the outer pressure, maintaining a constant ratio between outer and inner pressure and thus allowing, for example, a controlled release under changing working conditions. Back-pressure regulators are commonly used in gas tank regulators, keeping outlet pressure constant during changing inlet pressure conditions. The pressure difference between inlet and outlet for a back-pressure regulator can either be set manually, or one can

76

2 Experimental Techniques

Figure 2.1-5 Scheme of (from the top): (a) check valve, (b) line filter, (c) rupture disc, (d) pressure relief valve (High Pressure Equipment Co.).

employ an automated version such as that manufactured by JASCo (Japan Spectroscopy Co), although the latter is significantly more expensive. Sometimes the injection of an accurately defined amount of a fluid to a highpressure reactor is necessary. Sample injection valves as used in high-performance liquid chromatography (HPLC) fulfill such needs [36]. In the loading mode a fluid fills a defined inner volume, moving into the reactor upon adjusting the valve to discharge mode. Such systems are often combined with displacement pumps of the syringe type (see Figure 2.1-6).

2.1.2.5 Compressors and Pumps The definitions of “compressor” and “pump” given in the literature are not entirely consistent [37-391. Here, compressors are referred to as units for compressing and liquefying gases, whereas pumps are used for recirculating and compression of fluids. However, both classes can be further distinguished by the basic mode of ‘operation: either a rotating blade or a reciprocating piston.

2.1 High-pressure Reaction Equipment Design

77

Piston Handle

Figure 2.1-6 (a) Small-volume reciprocating pump head. Single ball check valves are used for controlling the airlfluid flow (Superpressure Inc.). (b) Laboratory scale positive displacement pump (syringe pump; High Pressure Equipment Co.).

Reciprocating compressors make use of pistons similar to those in an automobile engine. In general, a gas or fluid is drawn into the inner chamber through an inlet check valve, compressed and expelled through an outlet check valve, all by moving a piston. As these pistons need lubrication, reciprocating compressors may have the disadvantage of the contamination of the compressed gas or fluid with such lubricants. Today the use of a diaphragm avoids this effectively [32]. Typical compression ratios of 10-2O:l are obtained for each working stage. Further important design factors are the operating pressure and the flow rate (m3/s, at standard condition). The compressor itself can be driven by hand, compressed air or a motor, yielding outlet pressures for benchtop systems of up to 2000 bar. However, because of the nature of such pistonbased systems, the fluctuating pressures during the different operational stages may be a disadvantage, especially when used for recirculation at constant pressures. Rotary type compressors give a more pulse-free output. In such a unit the fluid is sucked into rotating blades, from which it gains kinetic energy. This energy can be converted into flow or compression, although such systems are typically used as pumps for recirculating purposes. Rotary systems can be further divided into kinetic or positive displacement type. Although the design of the kinetic type is similar to that of a turbine, the positive displacement pump consists of a piston or a screw which displaces the fluid in a single con-

78

2 Experimental Techniques

tinuous working step (reciprocating systems are therefore sometimes also referred to be a positive displacement pump). Such displacement pumps can be driven manually (syringe type, Figure 2.1-6) or electrically (for example, those manufactured by ISCo), and are ideal for the metered addition of fluids to a reaction vessel. For highest accuracy of sych addition, sample injection valves as already described above can be used. 2.1.2.6 Stirring and Mixing As austenitic stainless steel is not magnetic, it is sufficient in some cases (for example in small reactors in which only low-molecular-weight liquid reagents are employed), to use a simple magnetic stirring bar. For higher volumes, very low (and very high) viscosities, and in particular for reactions containing a solid phase, more vigorous stirring is often necessary. In principle, there are two different types of stirrers: packless magnetic drives and stirrers with self-sealing packing glands. The magnetic drive offers, because the inner rotor is completely enclosed within a non-rotating housing with fixed seals, higher achievable working pressures. The upper pressure limit of such stirring assemblies is limited primarily by the type of seal used (e.g. Buna N O-ring, Figure 2.1-7). As the use of most type of seals also limits the upper reaction temperature, the stirring unit should be equipped with a cooling jacket to protect the seal from excessive temperatures [40]. 2.1.2.7 Optical Windows

Optically transparent windows for high-pressure applications are typically made of synthetic sapphire [41]. Their form is usually of a cylinder which can be sealed for high pressure applications by using, for example, two opposed O-ring seals, rated for ca. 500 bar, (Figure 2.1-8). More detail on the use of high-pressure windows to perform spectroscopy in high-pressure environments is presented in Chapter 3.1, devoted to vibrational spectroscopy. A variety of materials have been used to construct high pressure windows, and information is available in the literature regarding the relationships between material specification and the chemical environment/application, the transmittance for a specific wavelength [42], and the cell design itself [43].

2.1.3 High Pressure Systems In general, high-pressure systems can be distinguished as either batch or continuous process reactors. Each type can then be subdivided by their use for chemical reactions or for extraction/fractionation purposes. Continuous reaction processes often consist of a reaction unit followed by an extraction process in order to recover at least one of the reactants.

2.1 High-pressure Reaction Equipment Design

Figure 2.1-7 Packless magnetic drive with attached water cooling sleeve (ratec for ca. 300 bar, Parr Instrument Co.).

Figure 2.1-8 Window assembly with two opposite O-ring seals (similar to that used in Figure 2.1-9).

79

2 Experimental Techniques

80

2.1.3.1 Single-batch High-pressure Reactors The simplest type of batch high-pressure reactor is essentially a hollow cylinder with one or two end caps, often referred to as a pressure bomb. Such vessels can be purchased from a variety of manufacturers, including Pam, Autoclave Engineers, and Pressure Products Industies, to name just a few. These reactors typically are equipped with a stirring system, such as described in an earlier section, a heating jacket, and ports for introduction of a thermocouple, as well as addition or removal of samples. Sizes range typically from 50 mL up to several liters, and pressure and temperature ratings vary from one manufacturer to another. Although the reactors described above are suitable for a number of situations, it is often found beneficial in the course of performing research in supercritical fluid technology to design and construct a reactor for a specific purpose/project, completing the system through the purchase of commercially available auxiliary components (stirring assembly, pressure transducers, thermocouples, and controller). One example of such a system is shown in Figure 2.1-9,where this system has been used primarily for emulsion and dispersion polymerization in carbon dioxide [44,45]. It was felt that this approach provided more flexibility in system design at a lower cost. The high pressure reactor shown in Figure 2.1-9is made of AISI 316 stainless steel with an internal cylindrical cavity of ca. 55 mL, and has been used at pressures and temperatures up to ca. 400 bar and 80°C.The inner diameter is approximately 3 cm while, as suggested by eq (2.1-1),the wall thickness is twice as large. The reactor is equipped with two opposed sapphire windows (1.5 cm thickness) which are held in place by screwed closures as shown in Figure 2.1-8,with high-hardness Buna N O-rings used as seals. Although the elastomeric O-rings must be replaced after every polymerization experiment, their low

T

Avent

P

tt

C02-tank

-

I

Rupture disc

Variable Speed Drive

11

L7-1

II

U

- ~

L S u..w-i Valve ~

55 mi High Pressure Reactor

wlth View Wlndows

Figure 2.1-9 Scheme of a single batch high pressure view reactor.

2. I High-pressure Reaction Equipment Design

81

cost renders the economic impact of this choice negligible. The use of windows in the reactor allows visual monitoring of the reaction and also the performance of photochemistry. Three ports provide access to the inner chamber, allowing C 0 2 supply, venting, and monitoring of the temperature. A thermocouple is introduced into the_ inner chamber and sealed using a high-pressure fitting which had been enlarged to allow passage of the thermocouple. Heating of the reactor is accomplished using cylindrical elements (4 X 200 W, 240 V) pushed into holes drilled into the outer reactor walls. The reactor cap is fitted with an adapter machined to allow connection of a commercial magnetic stirrer, as high intensity mixing is vital to a successful emulsion or dispersion polymerization. Other auxiliary components (temperature controller, pressure transducers, valves, fittings, tubing) are obtained from commercial suppliers, as described previously. Various reactor outputs (temperature, pressure, torque on the stirrer) can be recorded on a PC using a standard A/D board. Although in Figure 2.1-9 the rupture disk is shown connected to the reactor via tubing, in most cases it should be connected directly to the reactor for maximum safety. A typical continuous reactor would again be a hollow cylinder with end caps, where material would be introduced at one end and removed from the other. The end caps should be fitted with filters (as in those manufactured by Thar designs, for example) to retain catalyst or other solids in the reactor, preventing contamination of the pump or compressor. It is important to note that a major stumbling block in continuous processing at high pressure, either in the lab or on a commercial scale, is that robust technology for the continuous introduction (and removal) of solids to high-pressure environments is not currently available commercially. Most fundamental kinetic quantities needed for scale-up can be obtained in batch systems, particularly if real-time spectroscopic analysis is used to measure product formation (see Chapter 3). This may often be preferred as batch systems use much smaller quantities of material. The problem of the defined addition of fluids or gases into a laboratory scale high pressure reactor can be overcome, for example, by the use of a syringetype positive displacement pump [46]. Furthermore, for small and highly accurate volumes a sample addition valve should be used. A reactor in which both have been utilized is shown in Figure 2.1-10. In this system the kinetics of a heterogeneous palladium-catalyzed hydrogenation of a C02-soluble anthraquinone derivative, the first step of a C02-based synthesis of hydrogen peroxide, have been studied. As usual, aromatic redox processes of this kind can be easily monitored using UV spectroscopy.

82

2 Experimental Techniques

6

Nltank

ve [47].

per stroke, ca. 300 bar) and the UV-cycle are filled with liquid carbon dio while T-valve 5 remains dosed, thus leaving the UV-spectrometer cycle uncontaminated from the anthraquinon by revolving the sample injection and opening valve 4;the syringe pump is used (T1 open) to pump the hydrogen through the loop into t by the changes in th none is consumed. I is equipped with an internal stain front of the spectrometer prevents any possible clogging or contamination of the UV cell with itoring of changes in almost pulse-free an pressure gear pump (Micropump, Concord, CA) was used.

v4

T1

Figure 2.1-11 Sample addition valve in loading position (Rheodyne Inc., rated for ca. 450 bar).

2. I High-pressure Reaction Equipment Design

2.1.3.2

83

View Cells

The successful implementation of processes employing near-critical or supercritical fluids as solvents depends to a great extent upon knowledge of the phase behavior of reactants and products as a function of temperature, pressure, and composition. For example, use of a process simulator such as that developed by Aspen Technology, Inc. (Aspen Plus Process Simulator, Version 10.0) to design a high pressure separation process is relatively straightforward if one has the relevant equilibrium thermodynamic information (choice of modeVequation of state, pure component and mixture parameters) and transport properties (flow rates, densities, viscosities) close at hand. Equilibrium thermodynamic infom'ation is most easily obtained through use of a variable volume, high pressure view cell, which allows one to find the equilibrium pressure or temperature of phase separation at constant composition, and also the compositions of the phases in equilibrium if samples can be withdrawn from the view cell. For the case of extractions, one usually requires equilibrium partition coefficients, i.e. the extent to which a particular substance will partition between two phases (for example, water and COs) vesus temperature, pressure, and perhaps composition. This requirement is no different than that for the design of a traditional extraction column. It should be noted that it is difficult to scale up (and analyze for economic viability) a process from purely supercritical fluid extraction (SFE) data, as such data incorporates both equilibrium and transport influences, whose relative contributions may not be easy to separate. Thus, SFE laboratory-scale systems are not examined in this chapter, although some information is given in Chapter 2.2. The simplified scheme of a variable volume view cell is shown in Figure 2.1-12 (D.B. Robinson and Associates, Edmonton, Alberta, Canada) [47,49]. The cell is rated to 200°C and 700 bar. Mixing is accomplished by mechanically rocking the entire cell. The main component is the thick-walled, hollow Oil Rerewoir

Figure 2.1-12 Scheme of a high pressure view cell (Robinson Cell, D.B. Robinson and Associates) [47].

84

2 Experimental Techniques

cylinder (Pyrex) into which a movable piston is introduced. The piston is equipped with a Bridgeman-type seal, which maintains its integrity as the piston moves vertically within the cylinder. The cylinder itself is completely surrounded by silicone oil, eliminating any pressure gradient across the cylinder wall. Besides the piston, the sample vdume is isolated from the silicone oil on the top of the glass cylinder by supported O-rings. As the glass wall and the oil are transparent, the contents of'the sample area can be viewed under high pressures through borosilicate windows on each side of the high-pressure cell. Again, the pressure and temperature can be monitored using a pressure transducer and a thermocouple respectively. In addition to the design described above, a highly successful variablevolume view cell introduced by McHugh [49] has been widely applied, where essentially a view cell is affixed to the end of a manual syringe pump, creating a variable-volume view cell. A device similar to that designed by McHugh is available commercially from Supercritical Fluid Technology (Newark, DE). In addition, a recent design by Franck and co-workers [50] differs from the preceding two in its use of water as a hydraulic fluid, which can be useful for applications where small or even minute silicone contamination needs to be avoided. Finally, visual inspection of the phase behavior in a high-pressure cell is not desirable from a safety standpoint, and thus some commercially available view cells (such as that from Thar Designs) come equipped with a video recorder with remote camera. An important feature to a high pressure view cell, particularly if polymeric samples are to be examined, is the presence of a high intensity mixer. Although any of the designs mentioned above will provide the location of phase boundaries (versus temperature and pressure), it is also important to know the compositions of the two phases in equilibrium. Note that while tie lines (lines connecting phases in equilibrium on T-x or p-x diagrams) are horizontal for simple binary mixtures, this is not true for phase separation in multicomponent systems (most notably polymer-fluid systems where the polymer sample contains chains of various lengths). Consequently, ports which allow withdrawal of samples following phase separation and equilibration are an important feature of view cells. Such ports also allow for the measurement of partition coefficients of solutes between, for example, aqueous and CO2 phases. 2.1.3.3 Systems for Continuous Processing Batch or semicontinuous processing is appropriate for many laboratory scale operations. However, continuous processes are typically used by industry to allow higher throughput with smaller equipment size. To gain scale-up information, the importance of relatively inexpensive but fully operable pilot plants (or miniplants) cannot be overestimated. Many types of continuous reactors (fixed bed, fluidized bed, continuous stirred tank CSTR) can be adapted for high pressure service.

2. I High-pressure Reaction Equipment Design

85

Krukonis et al. [511 developed a continuous-countercurrent process using scCO2 for the production of high purity esters (eicosapentanoic acid (EPA)/ docosahexanoic acid (DHA)) from urea-adducted ethyl esters derived from menhaden oil (Figure 2.1-13), able to produce ca. 4.5 kg per day EPA (at 90% yield and 90% purity). The separation from the feed is based on solubility differences of the esters in C02, mainly caused by their different chain lengths (c16, C18, C20, and C22).Consequently, the first step in the design of such a continuous-countercurrent process is the acquisition of the equilibrium partition coefficient data for the major components of the anticipated feed material. Based on these partition coefficients a decision has to be made at which pressures and temperatures it is useful (efficiency, cost) to operate. Krukonis et al. selected a temperature of 60°C and a pressure of ca. 150 bar. In Figure 2.1-13 the scheme of this pilot plant is shown. Ester-containing streams are shown with bold lines. The ester feed is introduced in the side of the first of the two columns (C-1). At the chosen reaction conditions the ester density exceeds that of the scCOz and the esters tend to move down the column, contacting the C 0 2 which is fed in continuously from the bottom. The C02 strips the shorter-chain-length materials present in the feed, whereas the esters with the higher molecular weight remain at the column bottom. At the top of C-1, the c16 and c18 esters are condensed by reducing the pressure (e.g. back-pressure regulator) or by increasing the temperature, while the depleted C02 is recycled to the column. In addition, a portion of the liquid extract is returned to the head of the column in order to provide a constant

0XhCl

COT+ makeup

I Figure 2.1-13 Scheme of the continuous extraction of esters (eicosapentanoic acid (EPA)/docosahexanoic acid (DHA)) [5 11.

86

2 Experimental Techniques

downward-flowing reflux stream in a countercurrent manner (as is the case for any distillation or liquid-liquid extraction). The C20-rich and C22-ri~hliquid phase collected at the column bottom is then fed into C-2, which operates in the same way as the first column, thus separating the fraction into C20-rich and C22-ri~hphases.

2.1.4 Summary This chapter provides an introduction to the construction of high-pressure equipment for the laboratory. Clearly, it is not possible to cover all the necessary design materiaVoptions in such’ a short chapter, yet sufficient information should be available to allow researchers to explore chemistry in high-pressure fluids, such as C02, a field which continues to show great promise.

References [ l ] E. Karmana, B. Eiler, D. Maniero, M. Bedner, R. M. Enick, Res. Cons. Recycl. 1997, 20, 143. [2] P. Peterson, S . L. Reese, G. Yi, H. Yun, A. Malik, J. S. Bradshaw, B. E. Rossiter, M. L. Lee, K. E. Markidas, J. Chrom. A 1994, 684, 297. [3] M. A. McHugh, V. J. Krukonis, Supercritical Fluid Extraction, 2nd ed., ButtenvorthHeinemann, Boston, 1994. [4] J. W. King, G. R. List, Supercritical Fluid Technology in Oil and Lipid Chemistry, AOCS-Press, Champaign (Illinois), 1996. [5] (a) M. Gottesmann, R. Prasad, R. A. Scarella, US Patent 4341804; (b) K. Zosel, Angew. Chemie 1978, 90, 748; Angew. Chemie Int. Ed. 1978, 17, 702. [6] M. L. Cygnarowicz, W. D. Seider, in Supercritical Fluid Technology, T. J. Bruno, J. F. Ely (Eds.), CRC Press, Boca Raton (Florida), 1991. [7] B. Subramaniam, M. A. McHugh, Ind. Eng. Chem. Process Des. Dev. 1986, 25, 1. [8] P. E. Savage, S. Gopalan, T. I. Mizan, C. J. Martino, E. E. Brock, Reac. Kinet. Cat. 1995, 41, 1723. [9] C. B. Wu, S. C. Paspek, M. T. Klein, C. LaMarca, in Supercritical Fluid Technology, T. J. Bruno, J. F. Ely (Eds.), CRC-Press, Boca Raton (Florida), 1991. [lo] I. L. Spain, J. Paauwe, High Pressure Technology, Vol. I , Marcel Dekker, New York, 1977.

[ l l ] J. F. Harvey, Theory and Design of Pressure Vessels, Van Nostrand Reinhold Co. Inc., New York, 1985. [12] H. H. Bednar, Pressure Vessel Design Handbook, 2nd ed., Van Nostrand Reinhold Co. Inc., New York. [ 131 W. F. Sherman, A. A. Stadtmuller, Experimental Techniques in High-pressure Research, John Wiley & Sons Ltd., Chicester, 1987. [14] W. Cross, History of the ASME Boiler and Pressure Vessel Code, ASME, New York, 1989.

[15] V. C. D. Dawson, Computer Program for a Monobloc Hollow, Closed-End Cylinder Subjected to Internal Pressure, NOLTR (US.Naval Ordnance Laboratory), White Oak (Maryland), 1970, p. 70-41. [16] V. C. D. Dawsonc in High Pressure Technology, Volume I , I. L. Spain, J. Paauwe (Eds.), Marcel Dekker, New York, 1977.

2. I High-pressure Reaction Equipment Design

87

[17] (a) D. M. Fryer, J. F. Harvey, High Pressure Vessels, Chapman & Hall, New York, 1998; (b) H. H. Bednar, Pressure Vessel Design Handbook, Van Nostrand Reinhold Co. Inc., New York, NY, 1986. [18] T. Lyman, Metals Handbook, 8th ed., American Society of Metals, 1961. [19] V. C. D. Dawson, A. E. Seigel, Reversed Yielding of Fully Autofrettaged Tube of Large Wall Ratio, NOLTR (US. Naval Ordnance Laboratory), White Oak (Maryland), 1963, p. 63- 123. [20] K. V. Raghavan, Chem. and Proc. Eng, 1970. [21] D. F. Fredrick, Technical reprint 2751, Autoclave Engineers, Inc., Erie (PA). [22] W. S. Goree, B. McDowell, G.A. Scott, Rev. Sci. Instr. 1965, 36, 99. [23] Machine Design, Reference Issue, Seals, 9th ed., Penton Publishing Co., Cleveland (Ohio), 1969. [24] B. A. Niemeier, Trans. A. S. M. E 1953, 369. [25] R. A. Rothman, in Chemical Engineering/Deskbook, McGraw-Hill, New York, NY, USA, 1973. [26] High Pressure Equipment Co. Catalogue, 12/97, Erie, PA, USA. [27] S. Crocker, Piping Handbook, McGraw-Hill Book Company, New York. [28] W. C. Mack, Chemical Engineering 1976, 83 (June 7), 145. [29] A. Brodgesell, in Chemical Engineering/Deskbook, McGraw-Hill, New York, NY, USA, 1971, p. 218. [30] Fundamentals of Valves, Petroleum Managment, The Engineer, General Section 1963. [31] A. Fleming, Instruments of Control Systems 1967, 40. [32] Superpressure Inc. Co. Catalogue, Newport Scientific, Inc./Superpressure Div., Jessop, MD, USA. [33] R. Kern, Chemical Engineering 1977, 84, 187. [34] Nupro Co. Catalogue, Nupro Company, Willoughby, OH, USA. [35] V. Ganapathy, Chemical Engineering 1976, 83 (Sep. 13), 199. [36] J. W. Hutchison, ISA Handbook of Control Valves, 2nd ed., Instrument Society of America, Pittsburgh (PA). [37] I. J. Karassik, W. C. Krutzsch, W. H. Fraser, J. P. Messina, Pump Handbook, McGrawHill Book Co., Cleveland, 1976. [38] American Petroleum Institute Publications, (a) #822-61700; Standard 61 7, Centrifugal Compressors for General Refinery Services, 2nd ed., 1973; (b) #822-61800; Standard 618, Centrifugal Compressors for General Refinery Services, 3rd ed., 1974; (c) #822-6 1900; Standard 619, Rotary Type, Positive Replacement Compressors for General Refinery Services, American Petroleum Institute, Washington, DC. [39] D. H. Newhall, Industrial and Engineering Chem. 1957, 49, 1949. [40] Parr Instruments Co. Catalogue, Parr Instrument Co., Moline, IL, USA. [41] Optical Materials for Infrared Instrumentation, U.S. Department of Commerce State of the Art Report, PB 181087. [42] W. Paul, W. M. DeMeiss, J.M. Besson, Rev. Sci. lnstr. 1968, 39, 928. [43] S . J. Gill, W.D. Rummel, Rev. Sci. lnstr. 1961, 32, 752. [44] F. A. Adamsky, E. J. Beckman, Macromolecules 1994, 27, 312. [45] (a) C. Lepilleur, E. J. Beckman, Macromolecules 1997, 30, 745; (b) R. Fink, E. J. Beckman, Macromolecules, submitted for publication. [46] A. Bertucco, P. Canu, L. Devetta, Ind. Eng. Chem. Res. 1997, 36, 2626. [47] M. S. Super, R. M. Enick, E. J. Beckman, J. Chem. Eng. Data 1997, 42, 664. [48] Y-H. Li, K. H. Dillard, R. L. Robinson, J. Chem. Eng. Data 1981, 26, 53. [49] M. A. McHugh, A. J. Seckner, T. J. Yogan, Ind. Eng. Chem. Fund 1984, 23, 493. [50] R. Diguet, R. Deul, E. U. Franck, Ber: Bunsenges. Phys. Chem. 1987, 91, 551. [51] V. J. Krukonis, J. E. Vivian, C. J. Bambara, W. B. Nilsson, R. E. Martin, in Seafood and Biochemistry: Composition and Quality, G.Flick, R. E. Martin (Eds.), Technomic Publishing Co., Lancaster (PA), 1992, 169.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

2.2 Extraction and Related Separation Techniques GERDBRUNNER

2.2.1 General Aspects of Supercritical Fluids as Mass Separating Agents Separation processes with supercritical gases, called supercritical fluid extraction (SFE),is a group of separation processes that applies supercritical fluids (SCFs) as separating agents in the same way as other separation processes, such as liquid-liquid extraction or absorption, make use of liquid solvents. In these processes, the solvent is a supercritical component or a supercritical mixture of components [ 1-31. SCFs may be used in the same way as other ordinary solvents taking into account their different properties and behaviors. Supercritical fluids can replace liquids solvents in many processes, such as extractions from solids (leaching), countercurrent multistage separations, chromatographic separations, and others, provided the solvent properties of the SCFs are adequate. SFE must be seen in comparison to separation processes which are well known, widely used, and sufficiently well understood. These processes comprise the various forms of distillation (rectification), evaporation, crystallization, liquid-liquid extraction, liquid-solid extraction, and stripping. SFE is used only if there are major advantages apparent at an early point of decision. In synthetic chemistry, SFE can be attractive as an alternative to conventional methods for purification of reaction products such as vitamins, pharmaceuticals and many other high-value products [4]. Its technical use is currently mainly restricted to applications in food industry for extraction from natural products and in some cases for the fractionation of the products [ 5 ] . This chapter therefore focuses on these types of production and purification processes, but all the methodologies discussed may be readily adapted to synthetic applications. Equipment for carrying out separation processes of various sizes is available from commercial suppliers, and is described in more detail in Chapter 2.1 and elsewhere [l]. Situations where SCFs may be useful emerge from the effects that supercritical fluids have on pure compounds and mixtures. SCFs can change com-

89

2.2 Extraction and Related Separation Techniques Solvent + A

r

Feed

+

Regeneration

Separation

Product P1

precipitation

Solvent Product P2

I

Make-up solvent

(compressedgas)

Figure 2.2-1 Generalized scheme of a process for applying a solvent, such as an SCF.

pound properties, phase equilibria, chemical equilibria, and therefore often the rate of related processes. A generalized scheme for a process applying a mass separating agent, or solvent, is shown in Figure 2.2-1. The characteristic features are solvent flow; cycling of solvent or release, if quantities are low; recovery of products from the solvent; removal of remaining solvent from the products (extract and raffinate). The basis for a separation process by means of SCFs is the solvent power of the dense gaseous fluid and the different solubility of compounds in this solvent. SFE processes can be carried out in different modes: single stage, multiple stage, multiple stage and countercurrently, as a chromatographic process, and with the aid of chemical reactions. The various process modes are applied primarily on the basis of the properties of the feed mixture. Solids are processed mainly in single stage or multiple stage operational mode. Fluid feed mixtures containing compounds of similar solubility in the solvent are best treated in a multiple stage countercurrent process. The separation of isomers falls within the field of chromatography or selectively catalyzed reactions. Chemical reactions, e.g. esterification of free fatty acids, may be part of the separation process. In the following, the different types of processing modes will be discussed. It is the intention to review methods for basic design. This basic design enables understanding, planning of experiments, feasibility studies, the design of pilot plant and demonstration units, and creates a database for comparison of economics.

90

2 Experimental Techniques

2.2.2 Extraction from Solids SFE from solids is carried out by continuously contacting the solid substrate with the supercritical solvent. In most cases the solid substrate forms a fixed bed. During extraction, the supercritical solvent flows through a fixed bed of solid particles and dissolves the extractable components of the solid. The solvent is fed to the extractor and evenly distributed to the inlet of the fixed bed. The loaded solvent is removed from the extractor and fed to the precipitator. The solid material will be depleted from the extractable material in the direction of flow. Concentrations of extract components increase in the direction of flow in the supercritical solvent and in the solid material. The shape of the concentration curve depends on the operating conditions, the kinetic extraction properties of the solid material, and the solvent power of the supercritical solvent, as discussed below. The transport of substances can occur within a solid and across the borders of a structure, for example plant material structure, in the extraction of natural products. Such structures may differ from substrate to substrate. Furthermore, the solid material can consist of particles of different size and form and the size distribution of the particles may vary. The bed of particles may form different geometries and can even change its geometry during the process. In some cases it may be stirred or even fluidized. The substances to be extracted may be adsorbed on the outer surface, on the surface of pores, or evenly distributed within the solid or the plant cells. Each of these different distributions has some influence on the course of the extraction. However, the integral extraction curve is usually of a relative simple form. The extraction of soluble compounds from solid plant material proceeds in several steps: 1. The plant matrix absorbs the supercritical solvent and other fluids that are deliberately added to influence the extraction process. 2. In parallel to step 1, the extracted compounds are dissolved by the solvent. A chemical reaction may occur prior to solvation. 3. The dissolved compounds are transported to the outer surface of the solid. Diffusion is the most important transport mechanism. 4. The dissolved compounds pass through the outer surface. 5 . The compounds are transported from the surface layer into the bulk of the supercritical solvent and are subsequently removed with the solvent from the bulk of the solid material. Similar sequences apply to the SFE of other solid materials.

2.2 Extraction and Related Separation Techniques

91

2.2.2.1 Basic Process Design For a laboratory experiment the goal is to achieve an extract containing the required substances. With a more technical approach, the goal is to determine the optimum process parameters, the size of the extracting equipment, and the quantity of solvent needed €or a certain extraction result. As the technical approach includes the necessary knowledge of laboratory experiments, the technical approach is now considered. This approach is partly empirical. Rigorous models, based on physico-chemical properties, thermodynamics, and related basic physical laws, are fitted to experimental results by introducing coefficients as adjustable parameters. An investigation of the extraction is carried out, determining the major influences of the most important parameters on the product specifications. These parameters include the process temperature T, process pressure p , extraction time t, amount of solvent ms, solvent ratio (amount of solvent per unit of time and amount of solid) S, conditions of extract removal (precipitation) from solvent Tp, p p , and eventual pretreatment of solid feed material. In addition, the solubility of the extracted compounds in the solvent as a function of pressure and temperature may be investigated. The course of the extraction is an unsteady process for the solid as well as for the solvent. The course of the process can be followed by determining the amount of extract against time of extraction. From these data, more information on the process can be deduced, as discussed below. The amount of extract accumulating during the course of the extraction will, in principle, follow the curve of Figure 2.2-2(a). The first part of the curve may be a straight line, corresponding to a constant extraction rate. The second part is nonlinear, approaching a limiting value which is given by the total amount of extractable substances. The gradient of the first part of the graph can be given by the equilibrium solubility of the extract compounds in the supercritical solvent. From this gradient the equilibrium solubility may be determined. However, a straight line can be caused by constant mass transfer resistance and is no proof that equilibrium solubility is obtained during the extraction. In the second part of the extraction, two effects cause a declining medium concentration of extract in the outflowing solvent: 1. The extract in the solid substrate near the solid-gas interface is depleted for most of the solid substrate. Transport of the extract within the solid to the interface then adds an additional transport resistance. 2. The length of the fixed bed containing the initial content of extract is not long enough to enable the maximum loading of the solvent.

The maximum value of extract concentration in the supercritical solvent is given by the equilibrium solubility of extractable components. Due to different transport resistances and equilibrium distribution coefficients for different compounds, they may be successively extracted at different rates. The maxi-

92

2 Experimental Techniques

Figure 2.2-2 (a) Integral extraction curve showing the total amount of extract against time of extraction. (b) Extraction rate curve showing the amount of extract in a given time interval, plotted against time of extraction or amount of solvent. time

mum concentration of extract in the supercritical solvent may then be a function of extraction time. 2.2.2.2 Process Parameters

Temperature A higher temperature often causes a higher extraction rate, if pressure is not too low. One reason for this is the dependence of solvent power on temperature. At relatively low pressures, the decrease in the density of the SCF with increasing temperature prevails, resulting in a decrease of the solvent power of the SCF and the solubility. At higher pressures, the increase in the vapor pressure of the solute with temperature prevails, resulting in higher solubility. The absolute values for low pressure and higher pressure vary for different solutes, but the general behavior is the same. Another reason for a higher amount of extract per unit of time is enhanced mass transfer rates with higher temperature. Pressure At the process conditions of SFE, the solvent capacity in general increases with pressure at constant temperature. Therefore, the amount of extract after a certain time of extraction will increase with pressure. Note that increasing pressure increases solvent power. For mixtures of extract compounds this will result in different extracted compositions at different pressures.

2.2 Extraction and Related Separation Techniques

93

Density With increasing density, the extraction rate increases at constant temperature. Density is responsible for the capacity and solvent power of a solvent, as the solubility of a compound rises with increasing density. In the extraction process, mass transfer is also of importance. Therefore, the extraction results will be different for the same density at different temperatures. Solvent Ratio The solvent ratio is the most important parameter for technical applications of SFE, once approximate values of pressure and temperature have been selected. With increasing solvent ratio the extraction rate can be enhanced more than by changing process parameters within relatively narrow limits. The influence of the solvent ratio must be discussed while also considering the economic consequences [ l ] which are beyond the scope of this chapter. Size of Solid Particles Mass transfer in extraction from solid substrates in most cases depends heavily on the transport rate in the solid phase. The length of the transport path determines mass transport in the solid phase. In general the extraction rate increases with decreasing particle size. However, mass transfer into the fluid phase has to be achieved. If the smaller particles hinder the flow of the fluid in the fixed bed, then the mass transfer rate and the amount of extract decrease with smaller particles. 2.2.2.3 Modeling the Extraction The extraction of substances from solid substrates with supercritical solvents can be analyzed and modeled in a simple way by considering only the medium values and by determination of unknown coefficients by fitting to the extraction curve and a mass balance [l]. This approach results in simple equations that can represent parts of the extraction curve sufficiently, but fail for others, especially during the first part of the extraction. If the process is to be modeled more accurately, the analysis is far more complex and beyond the scope of this chapter. Nevertheless, some parameters determining the extraction and influencing the result are listed below together with the description of a simplified model that may provide some insight into the applied methods.

Basic Considerations The extraction system involves a bulk solid phase and a fluid phase. The fluid phase comprises the supercritical solvent and the dissolved extract. The solid phase remains within the extraction vessel and the fluid phase is passed through the extraction vessel. Mass transport occurs between the two phases. Any of the following parameters that can influence the process may be of interest. 1. Fluid phase (extract phase) (a) Concentration of the extract downstream of the extraction vessel: accumulated quantity of extract;

94

2 Experimental Techniques quantity of extract per unit of time; composition of the extract as a function of time.

(b) Concentration af the extract in the extraction vessel: medium concentration over the total volume; concentration throughout the extraction vessel for plug flow; local concentrations considering radial distribution (no backmixing, but no plug flow); local concentrations considering radial and axial distributions (backmixing).

2. Solid phase (raffinate phase) (a) Concentration of the extractable substances in the bulk solid: accumulated depletion of the solid (mean value for the extraction vessel); depletion of the solid related to the remaining content of extractable substances (mean value for the extraction vessel); remaining concentration of extractable substances: radial and axial distribution. (b) Concentration of the extractable substances in single particles: mass transport by diffusion; mass transport resistance by chemical reactions and/or phase transitions; simple geometric particles, complex shape of particles; size and size distribution (polydispersity) of the solid particles.

3. Operating parameters Pressure, temperature, and density of the fluid, quantity of solvent per unit of time and mass of solid (solvent ratio); chemical composition of the extracting solvent. 4. Pretreatment of the solid size reduction and enlargement of surface; destruction of the plant cells; adjustment of the water content; chemical reactions to liberate the extracted compounds.

Simplified Model for SFE from Solids Realistically, this large number of parameters cannot be treated within a practical model. The goal of the modeling procedure is therefore to obtain a quantitative representation of the process with a simple system of equations and just a few physically meaningful parameters [l]. In such a model, the following parameters are usually considered sufficient to calculate the course and result of an extraction:

2.2 Extraction and Related Separation Techniques

95

equilibrium distribution between solid and supercritical solvent (adsorption isotherm); diffusion in the solid (effective diffusion coefficient or effective transport coefficient as defined by the transport model); mass transfer from the su-rface of the solid to the bulk of the fluid phase (supercritical solvent). Experimental extraction curves can be represented by this type of model, by fitting the kinetic coefficients (mass transfer coefficient to the fluid, effective transport coefficient in the solid, effective axial dispersion coefficient representing deviations from plug flow) to the experimental curves obtained from laboratory experiments. With optimized parameters, it is possible to model the whole extraction curve with reasonable accuracy. These parameters can be used to model the extraction curve for extractions in larger vessels, such as from a pilot plant. Therefore, the model can be used to determine the kinetic parameters from a laboratory experiment and they can be used for scaling up the extraction. 2.2.2.4 Solids in Multiple Stages and Countercurrent Operation in SFE

Multistage countercurrent contacting is the most effective mode for carrying out separation processes. It reduces the amount of solvent and makes possible the continuous production of extract. Real countercurrent contact is not easily established for solids, because special effort is necessary for moving the solid, with increased difficulties at elevated pressure. Therefore, it is easier not to move the solid material and to achieve countercurrent contact by other measures. For SFE from solids, a well known and often used configuration is to have several fixed beds in countercurrent contact with the solvent. 2.2.2.5 Continuous Extraction of Contaminated Soil with Supercritical Water

Continuous extraction of solids is not easily achieved in a high pressure environment, but can be achieved in certain cases. These favourable cases are met when the solids can be ground to a small size (below 1 mm) and a diluted suspension of the solid material in the solvent can be used. As this possibility is of fundamental importance, an experimentally verified example is treated here in some detail. The suitability of supercritical water for decontamination of soil material can be proved by several semibatch extraction experiments [6]. At 380°C and 250 bar, the extraction results of hydrocarbons from soil material are excellent, even if it has been weathered for over 20 years. This kind of soil material, often the highly contaminated effluent of a soil washing process, can not be further decontaminated by biological treatment because the hydrocarbon contamination has been exposed to microorganisms throughout the weathering

96

2 Experimental Techniques )

Id v2

exhaust

t

1

v1

8.I

extraction

preheate

I

'

C

t lb

---Pa

piping

---_-

compressed air exhaust

Figure 2.2-3 Flow diagram of apparatus for continuous extraction from solids [7]. magnetic stirrer, b feed vessel, c membrane pump, d buffer vessel, e, f preheater (28 mL), g extraction pipe (38 mL), h cooler, i flask, j COz meter, k evacuated glas

a

vessel.

process. The effluent must be incinerated or deposited. With semibatch extraction all of the initial contamination could be removed from the soil material in an extraction lasting 6 h. Extraction results are excellent, but extraction times are long. In a new apparatus suitable for continuous extraction (Figure 2.2-3), experiments with contaminated soil material and supercritical water were carried out. Figure 2.2-4 compares the results of a semibatch and a continuous extraction as a function of solvent to soil ratio. The semibatch extraction residence times can be reduced remarkably by continuous operation. For extraction under parallel (co-current) flow, solvent-to-soil-ratios must be high for sufficient dilution of contamination in the fluid phase, leading to a high concentration gradient. At low solvent-to-soil-ratios, the degree of extraction is not sufficiently high, and the solid material must be recharged. Recharging of the solid material may also be needed if the concentration of the solid increases. Experiments with a suspension of 2wt % soil material showed that after three runs of extraction the clean-up result could be improved by up to 94%. The total time of extraction was no longer than 90s.

2.2 Extraction and Related Separation Techniques

Figure 2.2-4 Extraction results from semibatch and continuous extraction from solids (1 MPa = 10 bar) 171.

0

100

200

300

solvent to soil ratio [kg,20/kg,r,

97

400 sDi,

1

This example reveals that SFE from solids is applicable to systems other than natural materials. Recent applications have been directed to dyeing fabrics, cleaning complicated and delicate mechanical structures, cleaning catalysts and stationary phases for chromatography, and desorption of compounds from adsorbents.

2.2.3 Countercurrent Multistage Extraction Countercurrent multistage extraction using SCFs may be compared to fractional distillation. Distillation uses thermal energy to transfer the components from the liquid to the gaseous state at their various boiling temperatures. SFE uses a supercritical solvent for that process. The transfer from the liquid to the gaseous state can now take place at any temperature higher than the critical temperature of the solvent. Both processes apply the approach to equilibrium as the driving force. Realizing an equilibrium between two flows results in one equilibrium stage which changes composition of the mixtures to some extent but not sufficiently. Therefore, several such equilibrium stages are added, forming a multistage separation (Figure 2.2-5). With the appropriate number of equilibrium stages such a procedure can separate a binary mixture or two fractions to a given specification. Countercurrent flow is achieved between the liquid and the gaseous phase, thus transporting the liquid and gas to the next stage. At the end of the cascades countercurrent flow must be initiated which is achieved at the bottom by the gaseous supercritical solvent and at the top by reintroducing some of the top product. Countercurrent multistage extraction is the application of SFE to the fractionation of liquid mixtures and the purification of liquid substances. SFE in the modification of countercurrent multistage operation extends the possibilities of separation processes, such as fractional distillation, absorption and liquidliquid extraction to the isolation and purification of components of low volatility. The separation of volatile liquid components is also possible at tempera-

2 Experimental Techniques

98

Enriching Section Supercdtical

Solvent

e=+ 5

4

6P

Cycle .'7 Pump or Compressor

Stripping Section

, & = 7-w I

Bottom Product

(Low P)

Figure 2.2-5 Flow diagram of continuous countercurrent

multistage extraction.

tures below their normal boiling points. Furthermore, SFE enables the separation of components with very similar properties if used in the countercurrent mode. Process temperatures are determined by the critical temperature of the solvent and not, as in the case of distillation, by the liquid-vapor transition of the feed mixture. Compared to liquid-liquid extraction, gas extraction makes it possible to operate a two-cascade separation column, applying a stripping and an enriching section. Combined, these possibilities allow SFE to be operated at very moderate temperatures and as a separation process for difficult separations. 2.2.3.1 Basic Process Design Multistage countercurrent separation is usually considered for laboratory applications. As the principles apply to laboratory size equipment as well as to large-scale installations, the procedure given below should also be used for laboratory experiments.

2.2 Extraction and Related Separation Techniques

99

The goal of a design procedure for a countercurrent multistage separation is to determine the number of theoretical stages needed for a separation, the height of a separation column, the capacity of the mass transfer equipment, and the diameter of a separation column. For technical purposes, these considerations on the separation equipment are completed by the design of product recovery and gas cycle [l]. The design procedure is outlined in the following discussion. Knowledge on the phase equilibrium is needed first and can be obtained from laboratory experiments. The phase equilibrium yields the extent of the two-phase region, the mutual solubilities - in particular the solubility in the gaseous phase - the separation factor and, if possible, its concentration dependence. The separation factor is the ratio of the distribution coefficients of compounds between the gaseous and the liquid phase. A separation factor greater than 1 indicates that a separation is possible. In SFE, separation factors as low as 1.3 can lead to effective separations, but separation factors are usually higher. With the equilibrium and the separation factor known, an initial simplified determination of the number of theoretical stages can be carried out using standard chemical engineering methods [8]. This analysis is carried out on a quasi binary system (key components) or with pseudo components and provides essential insight into the separation process. Limiting values for the number of theoretical stages and the amount of top product to be reintroduced to the separation are determined at this point. Neglecting these limiting values will lead to failure of the separation. Now, separation experiments are carried out in a laboratory column, ensuring the accuracy of the separation factor as determined by phase equilibrium measurements and providing initial data on the height (length) of a theoretical stage. In addition, some material of different composition to the feed is produced for further phase equilibrium measurements in order to determine the concentration dependence of the separation factor. A multicomponent simulation of the separation process can be carried out using a commercial process simulator. The simulation for technical purposes can include different methods and conditions of product recovery and gas cycles. Experiments in a column of higher capacity (3070 mm diameter) than normal laboratory installations (15-25 mm diameter) can be carried out. Material is produced for further evaluation, as samples for customers, or for product development. In parallel, simulation experiments serve to define the plant design and the optimal operation of a column. An economical evaluation will conclude the developmental stage of the process design.

2.2.3.2

Phase Equilibria

Information on phase equilibria can be obtained from measurements of mutual solubilities which reveal the extent of the two-phase region with respect to pressure (Figure 2.2-6). Solubilities over a certain range of pressure and temperature have to be determined. Most important for the separation process are

2 Experimental Techniques

100

,

Liquid phase

Vapor phase

,,

I

Figure 2.2-6 Phase equilibria of orange peel oil - carbon dioxide (1 MPa = 10 bar) [9]. 94

92 w

o

96.98

l o (D

co,

the distribution coefficients for the individual compounds and the separation factors determined from that data. For example, the separation factor of squalene for a separation from a tocopherol-sterol mixture is shown (Figure 2.2-7). It is also necessary to determine the dependence of the separation factor on the concentrations of the key components because these vary substantially along the separation column. It is not always easy to acquire data on the concentration dependence of the separation factor because only one feed mixture is available for equilibrium measurements. Therefore, mixtures of other composition must be produced by mixing or from the products of experimental separations (which are also neeeded to determine the height of theoretical stages). Experimental determination of all the properties and their dependences on the operating conditions is tedious and expensive. Computational thermodySqualene-a-Tocopherol 5 -

0

&

26.0MPa

;o

8

'I h, -

j 3c3 2l

T

29.1MPa

\

23.0MPa

A

20.0 MPa

p-655ks4nS

\ '\

2-

A

'---. --.--&\

1-

I

I

I

I

I

Figure 2.2-7 Separation factor for squalene from a tocopherolsterol mixture (1 MPa = 10 bar) [lo].

2.2 Extraction and Related Separation Techniques

10 1

namics provides methods for correlating phase equilibria using equations of state. These models are powerful tools and make it possible to correlate binary and multicomponent phase equilibria by adjusting the interaction parameters to fit the measured data. However, for complex mixtures of similar compounds of low volatility, their applicability is somewhat limited. The extent of the twophase area can be correlated, well away from the critical region of the system, with sufficient accuracy for technical purposes. To date, no method is able to correlate the distribution coefficients and the separation factors with sufficient accuracy for design purposes. Solubilities and separation factors provide the necessary basic data for carrying out an analysis of the separation process on the basis of theoretical stages. This analysis i s independent of the type of separation equipment used.

2.2.3.3 Separation Analysis with Respect to Theoretical Stages The basic model for a countercurrent separation process needs to consider equilibrium and kinetic relationships, as given in chemical engineering textbooks. For SFEi processes, equilibrium relationships include phase equilibria, mass balances, and energy balances, whereas kinetic relationships refer to mass transfer. The number of theoretical stages is first determined by well-known shortcut methods and then by a multicomponent simulation of the separation process. Both parts can and will be used for analyzing the separation process with respect to the effect of separation factor, reflux ratio (the amount of top product reintroduced to the separation), degree of separation etc. on the number of theoretical stages. There, computational methods provide insight into the separation process without actually carrying out the separation. According to the McCabe-Thiele method, the system for separation is considered on a quasi-binary basis. In this approach, it must be possible to neglect the influence of the solvent, which is acceptable if the phase boundary lines (solubilities) do not change much with concentration during separation. In this case, the number of theoretical stages, the minimal reflux (ratio), the minimum number of theoretical stages, and their mutual dependence can be determined. If the mutual solubility changes significantly with concentration, a stage to stage calculation can be carried out according to the method of PonchonSavarit by including the solvent in the calculation and by using a quasi-ternary system. The graphical representation is displayed on a Janecke diagram, which is similar to the enthalpy-concentration diagram in distillation with enthalpy being replaced by the solvent ratio for SFE. These short-cut methods provide a good model of the separation process without any experimentation. The difference in the theoretical stages for the two methods can be appreciable: it is small if the number of theoretical stages is high, and vice versa.

102

2 Experimental Techniques Figure 2.2-8 Simulation of concentration profiles with ASPEN+ in a multistage countercurrent separation (13.5MPa = 135 bar) 1111. - A set of equilibrium data, . determined experimentally, in

13.5 MPa

O

2.2.3.4

2o

30

40

+ Composition [wt.-%.]

6o

70

combination with mass and energy balances enables the calculation concentration profiles along a separation device operated in the countercurrent mode for multicomponent mixtures. ASPEN+ is a commercially available process simulation program.

Multicomponent Process Simulation

A better insight into composition of phases along the separation process is provided by multicomponent process simulation as it can be carried out with commercial process simulating programs, such as ASPEN+. As usual, the process is separated into theoretical stages. Normally, ASPEN+ provides thermodynamic models and calculates thermodynamic properties such as the distribution coefficients and separation factors. As the accuracy of these results is not sufficient for a design analysis in many cases, distribution coefficients (and if necessary solubilities) can be provided by a user-defined module which uses empirical correlations for these values. The simulation procedure is then as follows: The starting value for the number of theoretical stages is taken from short-cut methods. Distribution coefficients are provided from external correlations. Then, concentrations for the individual (pseudo) components are calculated with algorithms known from multicomponent multistage processes (Figure 2.2 - 8). In order to determine the height of a column, the height (length) of a theoretical stage must be known. 2.2.3.5

Determination of the Height (Length) of a Theoretical Stage

The height of a theoretical stage is the size of an equipment in which just one theoretical stage, i.e. equilibrium, can be achieved. For a countercurrent column this size is given by the length (height) of the column in which one equilibrium stage can be obtained. The height of a theoretical stage (HETS) can be calculated from the number of theoretical stages, determined from a separation experiment, and the height of the separation column, used for the experiment:

2.2 Extraction and Related Separation Techniques HETS = h/nth

103 (2.2-1)

Most literature data on the HETS are determined this way. For fatty acid ethyl esters, the average HETS was found to be 0.22m in a laboratory column (17.5 mm diameter) and 0.27-m for the pilot plant column (70 mm diameter) using wire mesh packing (Sulzer EX and Sulzer CY, respectively). Therefore, the laboratory column can be taken as the first guess for the HETS, at least for this system. The HETS is strongly dependent on the type of system (and of the type of separation equipment). According to our experiments, the HETS for the squalene-tocopherol-sterol system is in the range of 2.5-0.7 m. The HETS decreases with increasing liquid loading of the column, and consequently increases with increasing gas loading. The main reason for the different HETS values for fatty acid ethyl esters and squalene-tocopherol-sterol mixtures is, most probably, the very different viscosities of the gas-saturated liquid phase. The viscosity of the liquid phase of the ethyl ester system is 60 X 10-5Pas (at 140 bar, lOOOC), and for the squalene-tocopherol system is 400 x 10-5Pas (300 bar, l0OOC). ''

2.2.3.6 Determination of Column Diameter

The column diameter for a countercurrent separation for a given amount of feed depends essentially on the maximum allowable gas loading of the mass transfer equipment. For general orientation, an operating area can be defined. It is limited at high densities by the density difference between phases being too low; 0.1 g/mL is selected as the lower limit. At low densities, solubility in the supercritical solvent is not high enough. The lower temperature limit is set by the onset of solidification, low transport coefficients, and low concentration in the gaseous phase. The higher temperature limit is set by the stability of the compounds. These limiting values are somewhat arbitrary. For operating countercurrent columns, the pressure drop and flooding point are important values. In gas extraction, pressure drop is not as important as in other separation processes, but from the pressure drop, the mass transfer regime can be concluded. Furthermore, it characterizes the type of mass transfer equipment for flooding point calculations. The flooding point in countercurrent flow driven by gravity (the liquid phase flows downward due to its higher density) is the limiting amount of gas that can flow through a column. No countercurrent two-phase flow is possible at the flooding point. An example is given in Figure 2.2-9. Flooding points, as well as pressure drop curves, may be determined in the separation column, but it is better to exclude mass transfer effects and use a special experimental set-up. Countercurrent multistage separation using supercritical gases is a powerful tool for separating mixtures that are currently separated by high vacuum processes. Furthermore, separations that are otherwise impossible can be achieved

104

2 Experimental Techniques

.. .. .. .. .. .. ... .

... .. . .. . .. ... . . . . .

Packing: Sulzer CY 35 mm

Llnlen+Symbol

MellaPak 250 Y RaluPak 250 CY

B)

b 8

V

v

0,Ol

W

MontrE41-300 MonlrE41-200 Montz81-100

. . . . ... ... ... ...

..

..

1

091

Flow parameter

mo

Figure 2.2-9 Flooding point diagram for countercurrent gas-liquid flow [ 121 after Billet [13]. The lines mark the upper limit of gravity driven countercurrent flow. Under usually operating conditions, 80% of that limit can be used.

because the process temperatures are low, degrading of components is negligible, and the number of stages is not limited. Applying a separation column with two separation cascades (enriching and stripping section) makes it possible to obtain high purity and high yield at the same time.

2.2.4 Chromatographic Separation with Supercritical Fluids In a chromatographic separation, a mixture of substances is transported by a carrier, the mobile phase, over a surface, the stationary phase. Between the two phases mass transfer processes take place, which lead to different transport velocities along the surface of the stationary phase for different components of the mixture. The components reach the end of the stationary phase at different times and can be detected and collected separately. Supercritical fluid chromatography (SFC) uses supercritical gases as a mobile phase [14-171. The solvent power of supercritical gases is determined by density (see above) which can be easily modified by pressure [18]. Furthermore, separation of the eluted compounds is easily achieved by pressure release. Liquid solvents may be only applied in minor quantities as modifiers. Therefore, SFC has good potential for scale-up to preparative and production scale chromatography [ 15-17,19,20].

2.2 Extraction and Related Separation Techniques 2.2.4.1

105

Design of SFC Apparatus

An apparatus for chromatographic separations (Figure 2.2-10) with a supercritical gas consists of (1) the separation column as the central part in a temperature controlled environment; (2) the reservoir for the mobile phase; (3) a unit for establishing, maintaining, and controlling pressure; (4) an optional unit for adding a modifier; - ( 5 ) the injection part for introducing the feed mixture; (6) a measuring device (detector) for determining concentration of the eluted substances; (7) a sample collection unit; (8) a unit for processing the mobile phase; and (9) a unit for processing data and controlling the overall apparatus. The flow of. the SCF under pressure is maintained by long-stroke piston pumps, reciprocating piston pumps, or membrane pumps which deliver the mobile phase in liquefied form. The fluid is then heated to supercritical conditions before entering the column. Pressure and flow rate must be kept as constant as possible in order to maintain constant conditions for separation and to achieve a stable baseline in the chromatogram. Oscillating pumps can therefore have three heads, which deliver at different times, or a pulsation dampener in order to minimize pulsation. In preparative and more so in productive SFC, the quantity of the mobile phase is not small and it must be recycled. To avoid backmixing, the recycled mobile phase must be totally free from any dissolved substances. Concentrations of the dissolved substances are low. In most cases they will be of the order of 0.1 % or even 0.01 %. Then, separation methods for the dissolved substances from the mobile phase become important.

Gas

Treatment of Mobile Phase (8)

Detector (6)

---I

I

- - - - - - - - - - - - - - - - --- - - - -- - - - - - --- Sample

I

Expansion

-

Figure 2.2-10 Flow diagram of apparatus for supercritical fluid chromatography (SFC).

106

2 Experimental Techniques

2.2.4.2 Methods for Scale-up of Chromatography

Chromatography can be scaled up by several means: increasing size of the chromatographic separation column, employing more than one (many) chromatographic separation columns in parallel, and @creasing the amount of substance for one injection. Increasing the size of a tube is a straightforward process, but maintaining chromatographic conditions is not. The larger the diameter of the column, the more difficult it becomes to maintain plug flow. Pressure drop becomes a problem mainly in liquid chromatography, as the forces in large diameter columns are high. Therefore, particle diameters for larger columns are about a factor of 10 greater than for analytical packing, about 50ym as compared to 3-5 pm in analytical applications. Large size packing of small diameter particles is difficult to produce without inhomogeneities.

2.2.5 Conclusion SCFs provide an excellent tool for carrying out separations. Although to date mainly the extraction from solids has been applied in commercial processes, some countercurrent separations are also in operation and more will come soon. Chromatographic separation with supercritical gases is being investigated intensively for scale-up. Given the advantages of easy solvent handling, SFC should appear in commercial processes in a short time. For scientists working in laboratories, all the methods above described are available on a small scale. Particularly interesting applications will emerge if conventional methods are combined with SCF techniques.

References [ I ] G. Brunner, Gas Extraction. An Introduction to Fundamentals of Supercritical Fluids and their Application to Separation Processes, Steinkopff Darmstadt, Springer New York, 1994. [2] M. McHugh, V. Krukonis, Supercritical Fluid Extraction, Butterworth-Heinemann, Boston, 2nd ed., 1994. [3] E. Kiran, J. F. Brennecke (Eds.), Supercritical Fluid Engineering Science: