Tài liệu Coulson & richardson's chemical engineering volume 3

Coulson & Richardson's CHEMICAL ENGINEERING VOLUME 3 THIRD EDITION Chemical & Biochemical Reactors & Process Control EDITORS OF VOLUME THREE J. F. RICHARDSON Department of Chemical Engineering University of Wales Swansea and D. G. PEACOCK The School of Pharmacy, London I E I N E M A N N Butterworth-Heinemann is an imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP, UK 30 Corporate Drive, Suite 400, Burlington, M A 01803, USA First edition 197 1 Reprinted 1975 Second edition I979 Reprinted with corrections 1982, 1987, I99 I Third edition 1994 Reprinted 2001, 2003,2005, 2006, 2007 Copyright 0 1991, J. M. Coulson, J. F. Richardson, J. R. Backhurst and J. H. Harker. Published by Elsevier Ltd. All rights reserved The right of J. M. Coulson, J. F. Richardson, J. R. Backhurst and J. H. Harker to be identified as the author of this work has been asserted in accordance with the Copyright. Designs and Patents Act 1988 No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier's Science &Technology Rights Department in Oxford UK: phone: (+a) (0) I865 843830; fax: (+44) (0) 1865 853333: email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-08-04 1003-6 For information on all Butterworth-Heinemann publications visit our website at books.elsevier.com Transferred to digital printing 2009 Working together to grow libraries in developing countries www.elscvier.com I www.bookaid.org I www.sahre.org Preface to the First Edition Chemical engineering, as we know it today, developed as a major engineering discipline in the United Kingdom in the interwar years and has grown rapidly since that time. The unique contribution of the subject to the industrial scale development of processes in the chemical and allied industries was initially attributable to the improved understanding it gave to the transport processes-fluid flow, heat transfer and mass transfer-and to the development of design principles for the unit operations, nearly all of which are concerned with the physical separation of complex mixtures, both homogeneous and heterogeneous, into their components. In this context the chemical engineer was concerned much more closely with the separation and purification of the products from a chemical reactor than with the design of the reactor itself. The situation is now completely changed. With a fair degree of success achieved in the physical separation processes, interest has moved very much towards the design of the reactor, and here too the processes of fluid flow, heat transfer and mass transfer can be just as important. Furthermore, many difficult separation problems can be obviated by correct choice of conditions in the reactor. Chemical manufacture has become more demanding with a high proportion of the economic rewards to be obtained in the production of sophisticated chemicals, pharmaceuticals, antibiotics and polymers, to name a few, which only a few years earlier were unknown even in the laboratory. Profit margins have narrowed too, giving a far greater economic incentive to obtain the highest possible yield from raw materials. Reactor design has therefore become a vital ingredient of the work of the chemical engineer. Volumes 1 and 2, though no less relevant now, reflected the main areas of interest of the chemical engineer in the early 1950s. In Volume 3 the coverage of chemical engineering is brought up to date with an emphasis on the design of systems in which chemical and even biochemical reactions occur. It includes chapters on adsorption, on the general principles of the design of reactors, on the design and operation of reactors employing heterogeneous catalysts, and on the special features of systems exploiting biochemical and microbiological processes. Many of the materials which are processed in chemical and bio-chemical reactors are complex in physical structure and the flow properties of non-Newtonian materials are therefore considered worthy of special treatment. With the widespread use of computers, many of the design problems which are too complex to solve analytically or graphically are now capable of numerical solution, and their application to chemical xvi PREFACE TO THE FIRST EDITION xvii engineering problems forms the subject of a chapter. Parallel with the growth in complexity of chemical plants has developed the need for much closer control of their operation, and a chapter on process control is therefore included. Each chapter of Volume 3 is the work of a specialist in the particular field, and the authors are present or past members of the staff of the Chemical Engineering Department of the University College of Swansea. W.J. Thomas is now at the Bath University of Technology and J. M. Smith is at the Technische Hogeschool. Delft. J. M.C. J. F. R. D. G. P. Preface to Second Edition Apart from general updating and correction, the main alterations in the second edition of Volume 3 are additions to Chapter I on Reactor Design and the inclusion of a Table of Error Functions in the Appendix. In Chapter 1 two new sections have been added. In the first of these is a discussion of non-ideal flow conditions in reactors and their effect on residence time distribution and reactor performance. In the second section an important class of chemical reactions-that in which a solid and a gas react non-catalytically-is treated. Together, these two additions to the chapter considerably increase the value of the book in this area. All quantities are expressed in SI units, as in the second impression, and references to earlier volumes of the series take account of the modifications which have recently been made in the presentation of material in the third editions of these volumes. xv Preface to Third Edition The publication of the Third Edition of Chemical Engineering Volume 3 marks the completion of the re-orientation of the basic material contained in the first three volumes of the series. Volume 1 now covers the fundamentals of Momentum, Heat and Mass Transfer, Volume 2 deals with Particle Technology and Separation Processes, and Volume 3 is devoted to Reaction Engineering (both chemical and biochemical), together with Measurement and Process Control. Volume 3 has now lost both Non-Newtonian Technology, which appears in abridged form in Volume 1, and the Chapter on Sorption Processes, which is now more logically located with the other Separation Processes in Volume 2. The Chapter on Computation has been removed. When Volume 3 was first published in 1972 computers were, by today’s standards, little more than in their infancy and students entering chemical engineering courses were not well versed in computational techniques. This situation has now completely changed and there is no longer a strong case for the inclusion of this topic in an engineering text book. With some reluctance the material on numerical solution of equations has also been dropped as it is more appropriate to a mathematics text. In the new edition, the material on Chemical Reactor Design has been re-arranged into four chapters. The first covers General Principles (as in the earlier editions) and the second deals with Flow Characteristics and Modelling in Reactors. Chapter 3 now includes material on Catalytic Reactions (from the former Chapter 2) together with non-catalytic gas-solids reactions, and Chapter 4 covers other multiphase reactor systems. Dr J. C. Lee has contributed the material in Chapters 1, 2 and 4 and that on non-catalytic reactions in Chapter 3, and Professor W. J. Thomas has covered catalytic reactions in that Chapter. Chapter 5 , on Biochemical Engineering, has been completely rewritten in two sections by Dr R. L. Lovitt and D r M. G. Jones with guidance from the previous author, Professor B. Atkinson. The earlier part deals with the nature of reaction processes controlled by micro-organisms and enzymes and is prefaced by background material on the relevant microbiology and biochemistry. In the latter part, the process engineering principles of biochemical reactors are discussed, and emphasis is given to those features which differentiate them from the chemical reactors described previously. The concluding two chapters by Dr A. P. Wardle deal, respectively, with Measurement, and Process Control. The former is a completely new chapter describing the xiii xiv PREFACE TO THIRD EDITION various in-line techniques for measurement of the process variables which constitute the essential inputs to the control system of the plant. The last chapter gives an updated treatment of the principles and applications of process control and concludes with a discussion of computer control of process plant. January 1994 J F RICHARDSON Department of Chemical Engineering University of Wales Swansea Swansea SA2 8 P P UK D G PEACOCK School of Pharmacy London WCl N 1 A X UK Contents PREFACE TO THIRD EDITION xiii PREFACE TO SECOND EDITION xv PREFACE TO FIRST EDITION xvi ACKNOWLEDGEMENTS xviii LISTOF CONTRIBUTORS xix 1. Reactor Design-General 1.1 1.2 1.3 1.4 1.5 1.6 I .7 Principles Basic objectives in design of a reactor 1.1.1 Byproducts and their economic importance 1.1.2 Preliminary appraisal of a reactor project Classification of reactors and choice of reactor type 1.2.1 Homogeneous and heterogeneous reactors I .2.2 Batch reactors and continuous reactors 1.2.3 Variations in contacting pattern-semi-batch operation 1.2.4 Influence of heat of reaction on reactor type Choice of process conditions 1.3.1 Chemical equilibria and chemical kinetics I .3.2 Calculation of equilibrium conversion 1.3.3 Ultimate choice of reactor conditions Chemical kinetics and rate equations 1.4.1 Definition of reaction rate, order of reaction and rate constant 1.4.2 Influence of temperature. Activation energy I .4.3 Rate equations and reaction mechanism 1.4.4 Reversible reactions 1.4.5 Rate equations for constant-volume batch reactors 1.4.6 Experimental determination of kinetic constants General material and thermal balances Batch reactors 1.6.1 Calculation of reaction time; basic design equation 1.6.2 Reaction time-isothermal operation I .6.3 Maximum production rate 1.6.4 Reaction time-non-isothermal operation 1.6.5 Adiabatic operation Tubular-flow reactors 1.7.1 Basic design equations for a tubular reactor 1.7.2 Tubular reactors-non-isothermal operation 1.7.3 Pressure drop in tubular reactors 1.7.4 Kinetic data from tubular reactors V 1 1 2 2 3 3 3 5 6 10 10 11 14 15 16 17 18 20 21 24 24 27 27 28 30 31 32 34 36 40 41 42 vi CONTENTS 1.8 Continuous stirred-tank reactors 1.8.1 Assumption of ideal mixing. Residence time 1.8.2 Design equations for continuous stirred-tank reactors 1.8.3 Graphical methods 1.8.4 Autothermal operation 1.8.5 Kinetic data from continuous stirred-tank reactors 1.9 Comparison of batch, tubular and stirred-tank reactors for a single reaction. Reactor output 1.9.1 Batch reactor and tubular plug-flow reactor 1.9.2 Continuous stirred-tank reactor 1.9.3 Comparison of reactors 1.10 Comparison of batch, tubular and stirred-tank reactors for multiple reactions. Reactor yield 1.10.1 Types of multiple reactions 1.10.2 Yield and selectivity 1.10.3 Reactor type and backmixing 1.10.4 Reactions in parallel 1.10.5 Reactions in parallel-two reactants 1.10.6 Reactions in series 1.10.7 Reactions in series-two reactants 1.1 1 Further reading I . 12 References 1.13 Nomenclature 2. Flow Characteristics of Reactors-Flow 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Modelling Non-ideal flow and mixing in chemical reactors 2.1.1 Types of non-ideal flow patterns 2.1.2 Experimental tracer methods 2.1.3 Age distribution of a stream leaving a vessel-E-curves 2.1.4 Application of tracer information to reactors Tanks-in-series model Dispersed plug-flow model 2.3.1 Axial dispersion and model development 2.3.2 Basic differential equation 2.3.3 Response to an ideal pulse input of tracer 2.3.4 Experimental determination of dispersion coefficient from a pulse input 2.3.5 Further development of tracer injection theory 2.3.6 Values of dispersion coefficients from theory and experiment 2.3.7 Dispersed plug-flow model with first-order chemical reaction 2.3.8 Applications and limitations of the dispersed plug-flow model Models involving combinations of the basic flow elements Further reading References Nomenclature 3. Gas-Solid Reactions and Reactors 3.1 Introduction 3.2 Mass transfer within porous solids 3.2.1 The effective diffusivity 3.3 Chemical reaction in porous catalyst pellets 3.3.1 Isothermal reactions in porous catalyst pellets 3.3.2 Effect of intraparticle diffusion on experimental parameters 3.3.3 Non-isothermal reactions in Dorous catalvst < Dellets . 3.3.4 Criteria for diffusion control' 43 43 44 47 49 50 51 52 52 54 55 56 57 57 58 61 63 67 68 68 68 71 71 71 71 73 75 78 80 80 83 84 88 93 96 98 102 104 105 105 106 108 108 111 112 115 116 122 124 I28 CONTENTS Selectivity in catalytic reactions influenced by mass and heat transfer effects 3.3.6 Catalyst de-activation and poisoning Mass transfer from a fluid stream to a solid surface Chemical kinetics of heterogeneous catalytic reactions 3.5.1 Adsorption of a reactant as the rate determining step 3.5.2 Surface reaction as the rate determining step 3.5.3 Desorption of a product as the rate determining step 3.5.4 Rate determining steps for other mechanisms 3.5.5 Examples of rate equations for industrially important reactions Design calculations 3.6.1 Packed tubular reactors 3.6.2 Thermal characteristics of packed reactors 3.6.3 Fluidised bed reactors Gas-solid non-catalytic reactors 3.7.1 Modelling and design of gas-solid reactors 3.7.2 Single particle unreacted core models 3.7.3 Types of equipment and contacting patterns Further reading References Nomenclature vii 3.3.5 3.4 3.5 3.6 3.7 3.8 3.9 3.10 4. Gas-Liquid and Gas-Liquid-Solid Reactors 4.1 Gas-liquid reactors 4.1.1 Gas-liquid reactions 4.1.2 Types of reactors 4.1.3 Equations for mass transfer with chemical reaction 4. I .4 Choice of a suitable reactor 4.1.5 Information required for gas-liquid reactor design 4.1.6 Examples of gas-liquid reactors 4.1.7 High aspect-ratio bubble columns and multiple-impeller agitated tanks 4.1.8 Axial dispersion in bubble columns 4.1.9 Laboratory reactors for investigating the kinetics of gas-liquid reactions 4.2 Gas-liquid-solid reactors Gas-liquid-solid reactions Mass transfer and reaction steps Gas-liquid-solid reactor types: choosing a reactor Combination of mass transfer and reaction steps Further reading References Nomenclature 4.2. I 4.2.2 4.2.3 4.2.4 4.3 4.4 4.5 5. Biochemical Reaction Engineering 5.1 5.2 Introduction 5. I . 1 Cells as reactors 5.1.2 The biological world and ecology 5. I .3 Biological products and production systems 5.1.4 Scales of operation Cellular diversity and the classification of living systems 5.2.1 Classification 5.2.2 Prokaryotic organisms 5.2.3 Eukaryotic organisms 5.2.4 General physical properties of cells 5.2.5 Tolerance to environmental conditions 129 139 143 144 146 148 148 148 150 151 151 172 180 181 182 183 186 190 190 192 196 196 196 196 197 202 204 205 216 218 223 229 229 230 23 1 235 248 248 249 252 252 254 255 256 257 259 260 262 265 269 270 viti CONTENTS Chemical composition of cells 5.3.1 Elemental composition 5.3.2 Proteins 5.3.3 Physical properties of proteins 5.3.4 Protein purification and separation 5.3.5 Stability of proteins 5.3.6 Nucleic acids 5.3.7 Lipids and membranes 5.3.8 Carbohydrates 5.3.9 Cell walls 5.4 Enzymes 5.4.1 Biological versus chemical reaction processes 5.4.2 Properties of enzymes 5.4.3 Enzyme kinetics 5.4.4 Derivation of the Michaelis-Menten equation 5.4.5 The significance of kinetic constants 5.4.6 The Haldane relationship 5.4.7 Transformations of the Michaelis-Menten equation 5.4.8 Enzyme inhibition 5.4.9 The kinetics of two-substrate reactions 5.4.10 The effects of temperature and pH on enzyme kinetics and enzyme de-activation. 5.4.1 1 Enzyme de-activation 5.5 Metabolism 5.5.1 The roles of metabolism 5.5.2 Types of reactions in metabolism 5.5.3 Energetic aspects of biological processes 5.5.4 Energy generation 5.5.5 Substrate level phosphorylation 5.5.6 Aerobic respiration and oxidative phosphorylation 5.5.7 Photosynthesis 5.6 Strain improvement methods 5.6.1 Mutation and mutagenesis 5.6.2 Genetic recombination in bacteria 5.6.3 Genetic engineering 5.6.4 Recombinant DNA technology 5.6.5 Genetically engineered products 5.7 Cellular control mechanisms and their manipulation 5.7. I The control of enzyme activity 5.7.2 The control of metabolic pathways 5.7.3 The control of protein synthesis 5.8 Stoichiometric aspects of biological processes 5.8.1 Yield 5.9 Microbial growth 5.9.1 Phases of growth of a microbial culture 5.9.2 Microbial growth kinetics 5.9.3 Product formation 5.10 Immobilised biocatalysts 5.10.1 Effect of external diffusion limitation 5.10.2 Effect of internal diffusion limitation 5.1 1 Reactor configurations 5.1 I . 1 Enzyme reactors 5.11.2 Batch growth of micro-organisms 5.11.3 Continuous culture of micro-organisms 5.12 Estimation of kinetic parameters 5.12.1 Use of batch culture experiments 5.12.2 Use of continuous culture experiments 5.3 27 I 27 1 273 275 277 277 27 8 278 278 278 279 279 279 28 1 282 285 286 28 7 289 29 I 294 29 5 298 298 298 302 304 304 309 315 315 316 318 320 320 325 326 326 327 334 337 339 342 342 345 352 354 356 360 364 364 365 367 386 386 393 CONTENTS 5.13 Non-steady state microbial systems 5.13. I Predator-prey relationships 5.13.2 Structured models 5.14 Further design considerations 5.14.1 Aseptic operation 5.14.2 Aeration 5.14.3 Special aspects of biological reactors 5.15 Appendices Appendix 5.1 Proteins Appendix 5.2 Nucleic acids Appendix 5.3 Derivation of Michaelis-Menten equation using the rapid-equilibrium assumption Appendix 5.4 The Haldane relationship Appendix 5.5 Enzyme inhibition Appendix 5.6 Information storage and retrieval in the cell 5.16 Further reading 5.17 References 5.18 Nomenclature 6. Sensors for Measurement and Control 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 Introduction The measurement of flow 6.2.1 Methods dependent on relationship between pressure drop and flowrate 6.2.2 Further methods of measuring volumetric flow 6.2.3 The measurement of mass flow 6.2.4 The measurement of low flowrates 6.2.5 Open channel flow 6.2.6 Flow profile distortion The measurement of pressure 6.3.1 Classification of pressure sensors 6.3.2 Elastic elements 6.3.3 Electric transducers for pressure measurement 6.3.4 Differential pressure cells 6.3.5 Vacuum sensing devices The measurement of temperature 6.4.1 Thermoelectric sensors 6.4.2 Thermal radiation detection The measurement of level 6.5.1 Simple float systems 6.5.2 Techniques using hydrostatic head 6.5.3 Capacitive sensing elements 6.5.4 Radioactive methods (nucleonic level sensing) 6.5.5 Other methods of level measurement The measurement of density (specific gravity) 6.6. I Liquids 6.6.2 Gases The measurement of viscosity 6.7. I Off-line measurement of viscosity 6.7.2 Continuous on-line measurement of viscosity The measurement of composition 6.8.1 Photometric analysers 6.8.2 Electrometric analysers 6.8.3 The chromatograph as an on-line process analyser 6.8.4 The mass spectrometer 6.8.5 Thermal conductivity sensors for gases ix 396 396 398 402 405 405 409 410 410 416 418 419 42 1 42 5 43 1 43 1 43 3 437 437 438 438 439 445 448 448 449 452 452 454 454 463 465 466 468 473 478 479 480 48 1 482 484 484 484 488 489 489 493 495 497 503 51 1 515 516 CONTENTS X 6.9 6.10 6. I 1 6.12 6.13 6.14 6.15 6.8.6 The detection of water 6.8.7 Other methods of gas composition measurement Process sampling systems 6.9.1 The sampling of single-phase systems 6.9.2 The sampling of multiphase systems (isokinetic sampling) The static characteristics of sensors 6.10.1 Definitions Signal conditioning 6.11. I Bridge circuits 6.1 1.2 Amplifiers 6.11.3 Signals and noise 6. 11.4 Filters 6. 11.5 Converters 6.1 1.6 Loading effects Signal transmission (telemetry) 6.12. I Multiplexers (time division multiplexing) 6.12.2 Serial digital signals 6.12.3 The transmission of analog signals 6.12.4 Non-electrical signal transmission 6.12.5 Smart transmitters and associated protocols-intelligent hardware Further reading References Nomenclature 7. Process Control 7. I 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 Introduction Feedback control 7.2.1 The block diagram 7.2.2 Fixed parameter feedback control action 7.2.3 Characteristics of different control modes-offset Qualitative approaches to simple feedback control system design 7.3.1 The heuristic approach 7.3.2 The degrees of freedom approach The transfer function 7.4.1 Linear systems and the principle of superposition 7.4.2 Block diagram algebra 7.4.3 The poles and zeros of a transfer function Transfer functions of capacity systems 7.5.1 Order of a system 7.5.2 First-order systems 7.5.3 First-order systems in series 7.5.4 Second-order systems Distance-velocity lag (dead time) Transfer functions of fixed parameter controllers 7.7.1 Ideal controllers 7.7.2 Industrial three term controllers Response of control loop components to forcing functions 7.8. I Common types of forcing function 7.8.2 Response to step function 7.8.3 Initial and final value theorems 7.8.4 Response to sinusoidal function 7.8.5 Response to pulse function 7.8.6 Response of more complex systems to forcing functions Transfer functions of feedback control systems 7.9.1 Closed-loop transfer function between C and R 519 523 523 523 528 528 528 535 536 536 537 539 539 542 546 547 547 549 549 552 552 553 555 560 560 560 562 564 566 570 57 1 573 575 576 577 579 579 579 579 583 589 592 593 593 594 594 594 597 600 600 603 605 608 608 CONTENTS Closed-loop transfer function between C and V Calculation of offset from the closed-loop transfer function The equivalent unity feedback system System stability and the characteristic equation 7.10.1 The characteristic equation 7.10.2 The Routh-Hurwitz criterion 7.10.3 Destablising a stable process with a feedback loop 7.10.4 The Bode stability criterion 7.10.5 The Nyquist stability criterion 7.10.6 The log modulus (Nichols) plot Common procedures for setting feedback controller parameters 7.1 1.1 Frequency response methods 7.1 1.2 Process reaction curve methods 7.1 I .3 Direct search methods System compensation 7.12.1 Dead time compensation 7.12.2 Series compensation Cascade control Feed-forward and ratio control 7.14.1 Feed-forward control 7.14.2 Ratio control MIMO systems-interaction and decoupling 7.15.1 Interaction between control loops 7.15.2 Decouplers and their design 7.15.3 Estimating the degree of interaction between control loops Non-linear systems 7.16.1 Linearisation using Taylor’s series 7.16.2 The describing function technique Discrete time control systems 7.17.1 Sampled data (discrete time) systems 7.17.2 Block diagram algebra for sampled data systems 7.17.3 Sampled data feedback control systems 7.17.4 Hold elements (filters) 7.17.5 The stability of sampled data systems 7.17.6 Discrete time (digital) fixed parameter feedback controllers 7.17.7 Tuning discrete time controllers 7.17.8 Response specification algorithms Adaptive control 7.18.1 Scheduled (programmed) adaptive control 7.18.2 Model reference adaptive control (MRAC) 7.18.3 The self-tuning regulator (STR) Computer control of a simple plant-the operator interface 7.19.1 Direct digital control (DDC) and supervisory control 7.19.2 Real time computer control 7.19.3 System interrupts 7.19.4 The operator/controller interface Distributed computer control systems (DCCS) 7.20.1 Hierarchical systems 7.20.2 Design of distributed computer control systems 7.20.3 DCCS hierarchy 7.20.4 Data highway (DH) configurations 7.20.5 The DCCS operator station 7.20.6 System integrity and security 7.20.7 SCADA (Supervisory control and data acquisition) The programmable controller 7.21.1 Programmable controller design 7.21.2 Programming the PLC 7.9.2 7.9.3 7.9.4 7.10 7.1 1 7.12 7.13 7.14 7.15 7.16 7.17 7.18 7.19 7.20 7.2 1 xi 609 609 61 1 612 613 614 617 619 625 632 632 634 635 638 638 638 640 645 646 646 65 1 653 653 654 658 660 66 1 664 672 672 675 677 679 68 1 684 686 686 688 689 690 69 1 692 692 694 696 696 698 698 698 700 703 703 708 708 709 709 71 1 xii CONTENTS 7.22 Regulators and actuators (controllers and control valves) 7.22.1 Electronic controllers 7.22.2 Pneumatic controllers 7.22.3 The control valve 7.22.4 Intelligent control valves 7.23 Appendices Appendix 7.1 Table of Laplace and z-transforms Appendix 7.2 Determination of the step response of a second-order system from its transfer function 7.24 Further reading 7.25 References 7.26 Nomenclature 712 712 715 719 724 726 726 726 729 729 73 1 Problems 737 Conversion Factors for Some Common SI Units 750 Index 753 Important: Before you read go to http://tiny.cc/CoulsonAndRichardsons important information needed to understand the books. CHAPTER 1 Reactor Design-General Principles 1.1. BASIC OBJECTIVES IN DESIGN OF A REACTOR In chemical engineering physical operations such as fluid flow, heat transfer, mass transfer and separation processes play a very large part; these have been discussed in Volumes 1 and 2. In any manufacturing process where there is a chemical change taking place, however, the chemical reactor is at the heart of the plant. In size and appearance it may often seem to be one of the least impressive items of equipment, but its demands and performance are usually the most important factors in the design of the whole plant. When a new chemical process is being developed, at least some indication of the performance of the reactor is needed before any economic assessment of the project as a whole can be made. As the project develops and its economic viability becomes established, so further work is carried out on the various chemical engineering operations involved. Thus, when the stage of actually designing the reactor in detail has been reached, the project as a whole will already have acquired a fairly definite form. Among the major decisions which will have been taken is the rate of production of the desired product. This will have been determined from a market forecast of the demand for the product in relation to its estimated selling price. The reactants to be used to make the product and their chemical purity will have been established. The basic chemistry of the process will almost certainly have been investigated, and information about the composition of the products from the reaction, including any byproducts, should be available. On the other hand, a reactor may have to be designed as part of a modification to an existing process. Because the new reactor has then to tie in with existing units, its duties can be even more clearly specified than when the whole process is new. Naturally, in practice, detailed knowledge about the performance of the existing reactor would be incorporated in the design of the new one. As a general statement of the basic objectives in designing a reactor, we can say therefore that the aim is to produce a specified product at a given rate from known reacfanfs.In proceeding further however a number of important decisions must be made and there may be scope for considerable ingenuity in order to achieve the best result. At the outset the two most important questions to be settled are: (a) The type of reactor to be used and its method of operation. Will the reaction be carried out as a batch process, a continuous flow process, or possibly as a hybrid of the two? Will the reactor operate isothermally, adiabatically or in some intermediate manner? 1 2 CHEMICAL ENGINEERING (b) The physical condition of the reactants a t the inlet to the reactor. Thus, the basic processing conditions in terms of pressure, temperature and compositions of the reactants on entry to the reactor have to be decided, if not already specified as part of the original process design. Subsequently, the aim is to reach logical conclusions concerning the following principal features of the reactor: (a) The overall size of the reactor, its general configuration and the more important dimensions of any internal structures. (b) The exact composition and physical condition of the products emerging from the reactor. The composition of the products must of course lie within any limits set in the original specification of the process. (c) The temperatures prevailing within the reactor and any provision which must be made for heat transfer. (d) The operating pressure within the reactor and any pressure drop associated with the flow of the reaction mixture. 1.1.1. Byproducts and their Economic Importance Before taking u p the design of reactors in detail, let us first consider the very important question of whether any byproducts are formed in the reaction. Obviously, consumption of reactants to give unwanted, and perhaps unsaleable, byproducts is wasteful and will directly affect the operating costs of the process. Apart from this, however, the nature of any byproducts formed and their amounts must be known so that plant for separating and purifying the products from the reaction may be correctly designed. The appearance of unforeseen byproducts on start-up of a full-scale plant can be utterly disastrous. Economically, although the cost of the reactor may sometimes not appear to be great compared with that of the associated separation equipment such as distillation columns, etc., it is the composition of the mixture of products issuing from the reactor which determines the capital and operating costs of the separation processes. For example, in producing ethylene‘” together with several other valuable hydrocarbons like butadiene from the thermal cracking of naphtha, the design of the whole complex plant is determined by the composition of the mixture formed in a tubular reactor in which the conditions are very carefully controlled. As we shall see later, the design of a reactor itself can affect the amount of byproducts formed and therefore the size of the separation equipment required. The design of a reactor and its mode of operation can thus have profound repercussions on the remainder of the plant. 1.1.2. Preliminary Appraisal of a Reactor Project In the following pages we shall see that reactor design involves all the basic principles of chemical engineering with the addition of chemical kinetics. Mass transfer, heat transfer and fluid flow are all concerned and complications arise when, as so often is the case, interaction occurs between these transfer processes and the reaction itself. In designing a reactor it is essential to weigh up all the REACTOR DESIGN-GENERAL PRINCIPLES 3 various factors involved and, by an exercise of judgement, to place them in their proper order of importance. Often the basic design of the reactor is determined by what is seen t o be the most troublesome step. It may be the chemical kinetics; it may be mass transfer between phases; it may be heat transfer; or it may even be the need to ensure safe operation. For example, in oxidising naphthalene or o-xylene to phthalic anhydride with air, the reactor must be designed so that ignitions, which are not infrequent, may be rendered harmless. The theory of reactor design is being extended rapidly and more precise methods for detailed design and optimisation are being evolved. However, if the final design is to be successful, the major decisions taken at the outset must be correct. Initially, a careful appraisal of the basic role and functioning of the reactor is required and a t this stage the application of a little chemical engineering common sense may be invaluable. 1.2. CLASSIFICATION OF REACTORS AND CHOICE OF REACTOR TYPE 1.2.1. Homogeneous and HeterogeneousReactors Chemical reactors may be divided into two main categories, homogeneous and heterogeneous. In homogeneous reactors only one phase, usually a gas or a liquid, is present. If more than one reactant is involved, provision must of course be made for mixing them together to form a homogenous whole. Often, mixing the reactants is the way of starting off the reaction, although sometimes the reactants are mixed and then brought to the required temperature. In heterogeneous reactors two, or possibly three, phases are present, common examples being gas-liquid, gas-solid, liquid-solid and liquid-liquid systems. In cases where one of the phases is a solid, it is quite often present as a catalyst; gas-solid catalytic reactors particularly form an important class of heterogeneous chemical reaction systems. It is worth noting that, in a heterogeneous reactor, the chemical reaction itself may be truly heterogeneous, but this is not necessarily so. In a gas-solid catalytic reactor, the reaction takes place on the surface of the solid and is thus heterogeneous. However, bubbling a gas through a liquid may serve just to dissolve the gas in the liquid where it then reacts homogeneously; the reaction is thus homogeneous but the reactor is heterogeneous in that it is required to effect contact between two phases-gas and liquid. Generally, heterogeneous reactors exhibit a greater variety of configuration and contacting pattern than homogeneous reactors. Initially, therefore, we shall be concerned mainly with the simpler homogeneous reactors, although parts of the treatment that follows can be extended to heterogeneous reactors with little modification. 1.2.2. Batch Reactors and Continuous Reactors Another kind of classification which cuts across the homogeneous-heterogeneous division is the mode of operation-batchwise or continuous. Batchwise operation, shown in Fig. ].la, is familiar to anybody who has carried out small-scale preparative reactions in the laboratory. There are many situations, however, CHEMICAL ENGINEERING 4 especially in large-scale operation, where considerable advantages accrue by carrying out a chemical reaction continuously in a flow reactor. Figure 1.1 illustrates the two basic types of flow reactor which may be employed. In the tubular-flow reactor (b) the aim is to pass the reactants along a tube so that there is as little intermingling as possible between the reactants entering the tube and the products leaving at the far end. In the continuous stirred-tank reactor (C.S.T.R.) (c) an agitator is deliberately introduced to disperse the reactants thoroughly into the reaction mixture immediately they enter the tank. The product stream is drawn off continuously and, in the ideal state of perfect mixing, will have the same composition as the contents of the tank. In some ways, using a C.S.T.R., or backmix reactor as it is sometimes called, seems a curious method of conducting a reaction because as soon as the reactants enter the tank they are mixed and a portion leaves in the product stream flowing out. To reduce this effect, it is often advantageous to employ a number of stirred tanks connected in series as shown in Fig. 1. Id. The stirred-tank reactor is by its nature well suited to liquid-phase reactions. The tubular reactor, although sometimes used for liquid-phase reactions, is the natural choice for gas-phase reactions, even on a small scale. Usually the temperature or catalyst is chosen so that the rate of reaction is high, in which case a comparatively small tubular reactor is sufficient to handle a high volumetric flowrate of gas. A few gas-phase reactions, examples being partial combustion and certain chlorinations, are carried out in reactors which resemble the stirred-tank reactor; rapid mixing is usually brought about by arranging for the gases to enter with a vigorous swirling motion instead of by mechanical means. Reactants chargd II b.ginning of reaction Products FIG. 1 . 1 . Basic types of chemical reactors (a) Batch reactor (b) Tubular-flow reactor (c) Continuous stirred-tank reactor (C.S.T.R.) or “backmix reactor” ( d ) C.S.T.R.s in series as frequently used