Electrochemical Process Engineering A Guide to the Design of Electrolytic Plant
Electrochemical Process Engineering A Guide to the Design of Electrolytic Plant F. Goodridge and K. Scott University of Newcastle upon Tyne Newcastle upon Tyne, England Springer Science+ Business Media, LLC
Library of Congress Cataloging-In-Publication Data GoodrIdge. F., 1924- Electrochemlcal process engineering : a guide to the design of electrolytic plant I F. Goodridge and K. Scott. p. c. Includes bibliographical references and Index. ISBN 978-1-4899-0226-9 ISBN 978-1-4899-0224-5 (ebook) DOI 10.1007/978-1-4899-0224-5 1. ElectrocheMistry, Industrial. I. Scott, K. 1Kennethl, 1951- II. Title. TP255.G66 1994 660'. 297--dc20 94-42922 CIP ISBN 978-1-4899-0226-9 1995 Springer Science+ Business Media New York Originally published by Plenum Press, New York in 1995 Softcover reprint of the hardcover 1st edition 1995 10987654321 All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher
Our sincere thanks are due to our colleague Dr. R. E. Plimley for his considerable help during the preparation of this book.
Preface As the subtitle indicates, the overriding intention of the authors has been to provide a practical guide to the design of electrolytic plant. We wanted to show that the procedures for the design and optimization of such a plant are essentially simple and can be performed by readers comparatively new to the electrochemical field. It was important to realize that electrochemical engineering should not be confused with applied electrochemistry but had to be based on the principles of chemical engineering. For this reason, reference is often made to standard chemical engineering texts. Since this is a practical guide rather than a textbook, we have included a large number of worked examples on the principle that a good worked example is worth many paragraphs of text. In some examples we have quoted costs, e.g., of chemicals, plant or services. These costs are merely illustrative; current values will have to be obtained from manufacturers or journals. If this is not possible, approximate methods are available for updating costs to present-day values (see Refs. 1 and 3, Chapter 6). At the end of the book you will realize that there has been no recommendation or discussion of particular software packages for costing or design. There are two reasons for this omission. First, we wanted to emphasize the basic principles of design so that readers would be in a position to question in an informed way the basic assumptions of any design package. Second, changes and updates in design software are so frequent that any recommendations would soon be out of date. Although design software packages including electrochemical stages are still not numerous, any final design today will be computer-based. This has been no handicap vii
viii Preface to the present book, which is concerned mainly with preliminary cost estimation and design. We have emphasized the use of reaction, reactor, and process models. We have tried to show-and this is a message close to our hearts-that even simple models can lead to significant savings. For example, minimizing pilot plant experimentation can reduce development cost considerably. A reaction model can tell the development scientist or engineer which parameters under investigation are particularly important, in this way shortening experimentation time. One might ask, "If design methods are based on chemical engineering, is it worthwhile to present a text specializing in electrochemical plant?" The answer is that, in our experience of running short courses in electrochemical engineering, scientists and engineers have found it useful to be presented with detailed examples of the application of chemical engineering principles to an electrochemical situation. The reason can be found in university training, where electrochemistry is still the Cinderella of reaction techniques, particularly in chemical engineering courses. The treatment to be found below is loosely based on the short courses we have been running for many years in the Department of Chemical and Process Engineering at the University of Newcastle upon Tyne, England. Enthusiastic response from participants all over the world has led us to hope that this book will be considered as useful as, apparently, the courses have been. The book has been written for undergraduate and graduate students of science and engineering, as well as for the practicing chemist and chemical engineer who have had little previous experience of designing and scaling up electrochemical processes. Our sincere thanks are due to our colleague Dr. R. E. Plimley for his considerable help during the preparation of this book. University of Newcastle upon Tyne F. Goodridge K. Scott
Contents Chapter 1 Introduction to Electrochemical Engineering 1.1. What is Electrochemical Engineering?............. 1 1.2. Aims and Intentions of this Book................ 3 1.3. The Industrial Importance of Electrolytic Processes 3 1.4. Some Basic Concepts and Definitions............. 5 1.4.1. Electrolytic Cells and Faraday's Law........ 5 1.4.1.1. Basic Terminology................. 5 1.4.1.2. Faraday's Law................... 6 1.4.2. Current Density, Electrode Potential, and Current Efficiency..................... 7 1.4.3. Exchange Current Density................. 9 1.4.4. The Double Layer........................ 11 1.4.5. Cell Voltage............................. 11 1.5. Criteria for Reactor Performance................ 14 1.6. Electrochemical and Catalytic Reactions, and Reactors.................................. 15 References........................................ 16 Chapter 2 Aspects of Mass and Heat Transfer and the Energetics of Electrolytic Cell Systems 2.1. Introduction.................................. 17 2.2. Some Basic Aspects of Fluid Dynamics........... 18 2.3. Mass Transfer................................. 20 2.3.1. Mass Flux in a Fully Developed Turbulent Regime................................. 20 ix
X Contents 2.3020 Entrance and Exit Effects 00000 000000 000000 23 203030 Obtaining Numerical Values of kl by Calculation 0000000 0000000 00000 00000 000000 24 2.3.301. Rectangular Flow Channels 000000 00 24 2.3.3020 The Annulus 00000 000000 00000 000000 28 2.3.3030 Rotating Cylinder Electrodes 000000 0 29 20303.40 Rotating Disk Electrodes 00000 000000 36 2.3040 Obtaining Numerical Values of kl by Experiment 000000 0000000 000000 000000 00000 37 203.4.10 The Chemical System 00000 00000 0000 37 20304020 The Experimental Arrangement 00000 38 2.3050 Turbulent Flow Promoters 00000 00000 000000 43 2.3.501. Inert Promoters 000000 00000 00000 000 44 2.305020 Electroactive Promoters 00000 000000 0 50 2.3060 Mass Transfer in Two-Phase Flow 00000 0000 51 2040 Energetics 000000 0000000 000000 0000000 00000 00000 52 2.401. Voltage Requirements 000000 000000 00000 000 52 204020 Open-Circuit Voltage 00000 000000 00000 00000 52 2.40201. The Effect of Temperature on Open-Circuit Voltage 0000000 000000 0 54 20402020 The Effect of Activity or Concentration on Open-Circuit Voltage 00000 00 61 2.4.30 Parallel Reactions 00000 000000 000000 000000 0 66 2.4.40 Ohmic Voltage Losses 00000 000000 00000 00000 69 2.40401. Electrolyte Resistance 000000 00000 000 69 2.4.4020 Gas Evolution 000000 00000 00000 0000 72 2.4.4030 Diaphragm Resistance 00000 000000 00 73 2.4.4.40 Resistance of Solid Conductors 00000 74 204.4.50 Total Cell Voltage 00000 000000 000000 74 2.50 Energy Balances 0000000 000000 00000 000000 000000 0 76 2.501. Thermodynamic Relationships 00000 000000 00 76 205020 Reactor Energy Balances 00000 000000 00000 00 77 2.5o2ol. Continuous Operation 000000 000000 0 78 2.502020 Batch Operation 00000 000000 000000 0 79 20502030 Energy Balance for Aluminum Production 0000000 00000 000000 00000 82 2060 Concluding Remarks 00000 00000 00000 000000 000000 88 References 000000 00000 00000 000000 00000 000000 000000 0 89 Chapter 3 Rate Processes and Reaction Models 301. Rate Processes 000000 00000 00000 00000 00000 000000 91 3.1.1. Elementary and Overall Reactions 00000 00000 92
Contents xl Chapter 4 3.1.2. Kinetics of Elementary Reactions.......... 92 3.1.2.1. Velocity Constant................. 92 3.1.2.2. Reversibility and Irreversibility...... 93 3.1.2.3. The Effect of Temperature on Reaction Rate.................... 94 3.1.3. Reaction Mechanisms and Rate Laws....... 96 3.1.3.1. Steady State Approximation and the Development of Rate Laws......... 97 3.1.3.2. The Rate-Determining Step......... 100 3.1.4. Transition State Theory................... 101 3.1.5. Derivation of Some Kinetic Relationships... 103 3.1.5.1. Steps in an Electrode Process....... 104 3.1.5.2. Charge Transfer, Activation, or Kinetic Control................... 105 3.1.5.3. Diffusion or Mass Transport Control.......................... 110 3.1.5.4. Charge Transfer and Diffusion Control Combined................ 113 3.1.5.5. Charge Transfer with Adsorption on the Electrode..................... 117 3.1.6. Electrocatalysis........................... 121 3.2. Reaction Models.............................. 121 3.2.1. General Considerations................... 122 3.2.2. Experimental Methods of Obtaining Model Constants............................... 130 3.2.2.1. Methodology..................... 130 3.2.2.2. Experimental Equipment........... 131 3.2.2.3. Ohmic Correction to the Electrode Potential......................... 131 3.2.2.4. Methods of Determining the Ohmic Correction........................ 133 3.2.3. Examples of Reaction Models............. 137 3.2.3.1. The Reduction of U(VI) to U(IV)... 137 3.2.3.2. The Production of p-anisidine...... 143 3.3. Concluding Remarks........................... 151 References........................................ 151 Reactor Models 4.1. General Considerations......................... 153 4.2. The Batch Reactor............................. 154 4.2.1. Batch Reactors without Electrolyte Recycle.. 155 4.2.2. Batch Reactors with Electrolyte Recycle..... 161
xli Contents 4.2.2.1. Recirculation through a Plug-Flow Batch Reactor.................... 161 4.2.2.2. Recirculation through a Stirred-Tank Reactor.......................... 165 4.3. Modeling Batch Reactors....................... 167 4.4. The Continuous Reactor........................ 172 4.5. Concluding Remarks........................... 175 References........................................ 176 Chapter 5 Electrolytic Reactor Design, Selection, and Scale-up 5.1. Electrolytic Reactor Designs.................... 177 5.1.1. Classifications of Reactors................. 177 5.1.1.1. Scheme Based on Reactor Engineering Principles............. 177 5.1.1.2. Scheme Based on Electrochemical Modes of Operation............... 178 5.1.2. Electrical Connections to Cells............. 180 5.1.3. Hydraulic Connections.................... 180 5.1.4. General Purpose Flow Electrolyzers........ 183 5.1.5. Other Cell Designs....................... 186 5.2. Electrolytic Reactor Selection................... 190 5.3. Scale-up of Electrolytic Reactors................. 193 5.3.1. General View of Scale-up Procedures....... 194 5.3.2. Design Scheme for the Scale-up of Electrochemical Reactors.................. 194 5.3.3. Effect of Scale-up on Reactor Performance.. 197 5.3.4. Scale-up Methods and Similarity........... 199 5.3.4.1. Reactor Size and Geometric Similarity........................ 199 5.3.4.2. Fluid Mechanics and Kinematic Similarity........................ 200 5.3.4.3. Concentration Distribution and Chemical Similarity................ 200 5.3.4.4. Current Distribution and Electrical Similarity........................ 200 5.3.4.5. Heat Transfer and Thermal Similarity 201 5.3.5. The Effect of Scale-up on Mass Transfer.... 201 5.3.6. The Effect of Scale-up on Current Distribution 205 5.3.6.1. Primary Current Distribution....... 206 5.3.6.2. Secondary Current Distribution..... 213 5.3.6.3. Tertiary Current Distribution....... 217 5.3.6.4. The Effect of Gas Evolution on Current Distribution............... 217
Contents xiii 5.306050 The Effect of Finite Electrode Conductivity on Current Distribution 223 5.306060 Current Distribution in Three-Dimensional Electrodes 0 0 0 0 0 0 230 50306070 Concluding Remarks 0 0 0 0 0 0 0 0 0 0 0 0 0 0 240 503070 Multiple Electrode Modules 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 240 503080 Time Factors 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 241 References 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 242 Chapter 6 Cost Estimation, Profit Appraisal, Process Modeling, and Optimization 6.10 Cost Estimation and Profit Appraisal 000000 000000 245 6ol.l. Costing Procedures 00000 00000 00000 00000 000 245 601.1.1. Capital and Capital-Related Costs 000 246 601.1.20 Production and Production-Related Costs 00000 00000 00000 00000 00000 000 246 601.20 Example of a Predesign Cost Estimate for Producing Glyoxylic Acid by the Electrolytic Reduction of Oxalic Acid 00000 00 246 6ol.2ol. Basis for Cost Estimation 000000 000 246 601.2.20 Process Conditions 00000 00000 00000 247 6.102030 Capital Costs 00000 00000 00000 00000 247 601.2.40 Production Costs 00000 000000 000000 252 601.2050 Capital Depreciation 00000 00000 0000 252 6ol.2o6o Added Value per kg of Glyoxylic Acid 252 601.2070 Costing Figures for a Product Throughput of 1000 kg/annum 00000 253 601.2080 Maximum Amount of Glyoxylic Acid that Can Be Produced per Annum with the Two-Module Cell 000000 00 253 6ol.2o9o Costing Figures for a Product Throughput of 4000 kg/ Annum 0 0 0 0 253 601.20100 Comments on the Above Cost Calculations 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 256 6020 Profitability Criteria for Optimization 0 0 0 0 0 0 0 0 0 0 0 0 257 6030 Process Modeling and Optimization 0 0 0 0 0 0 0 0 0 0 0 0 0 261 60301. Example of the Optimization of Current Density in a Batch Reactor 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 262 603020 Interactions Between an Electrochemical Reactor and Associated Unit Processes 0 0 0 0 0 269 603.30 Example of a Process Model to Demonstrate Interaction 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 270 6030301. The Electrochemical Reactions 0 0 0 0 0 0 270
xlv Contents 6.3.3.2. Mass Balance..................... 271 6.3.3.3. The Reactor...................... 274 6.3.3.4. The Distillation Column........... 276 6.3.3.5. Interactive Behavior............... 282 6.3.4. Example of Optimization and a Choice between Alternatives...................... 286 6.4. Concluding Remarks........................... 289 6.5. Afterword.................................... 289 6.5.1. Costs Specific to Electrolytic Processes...... 289 6.5.2. Duty of Heat Exchangers................. 291 6.5.2.1. Continuous Operation............. 292 6.5.2.2. Batch Operation.................. 293 References........................................ 293 Appendix: Notation of Variables.................... 295 Index............................................ 309