CHEMICAL REACTOR DEVELOPMENT CHEMICAL REACTOR DEVELOPMENT from Laboratory Synthesis to Industrial Production by Dirk Thoenes Eindhoven University 0/ Technology, The Netherlands Springer-Science+Business Media, B.V. A C.I.P. Catalogue record for this book is available from the Library of Congress. ISBN 978-90-481-4446-4 ISBN 978-94-015-8382-4 (eBook) DOI 10.1007/978-94-015-8382-4 Printed on acid-free paper Reprinted with corrections 1998 1994 Springer Science+Business Media Dordrecht and copyright holders as specified on appropriate pages within. All Rights Reserved © 1994 Springer Science+Business Media Dordrecht and copyright holders as specified on appropriate pages within. Originally pub1ished by K1uwer Academic Pub1ishers in 1994. Softcover reprint of the hardcover 1st edition 1994 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. Preface This book is written primarily for chemists and chemical engineers who are concemed with the development of a chemical synthesis from the laboratory bench scale, where the fIrst successful experiments are performed, to the design desk, where the fIrst commercial reactor is conceived. It is also written for those chemists and chemical engineers who are concemed with the further development of a chemical process with the objective of enhancing the performance of an existing industrial plant. And, of course, this book is written for students of chemistry and chemical engineering. The term development may need some explanation. Institutes where this type of work is done are often called departments for research and development. and this type of development is then further divided into process- and product-development. The word development refers to the development of knowledge. By experimental research and by calculations, knowledge about a process is not only extended, but developed in such a sense that it can be used for designing a technical installation that is to be built for commercial purposes. A similar type of development work is aimed at the improvement of the processes that take place in chemie al plants that are already in existence. In both cases, the research and development work is aimed at predicting the behaviour of a chemical plant, with the object of optimizing its performance. One may well ask: What additional knowledge does one need, after one has carried out a chemical synthesis in the laboratory, to be able to predict what will happen in a technical installation where the same synthesis is to be carried out? Do molecules not react with one another in the same manner, irrespective of the scale 0/ operation. be it in a 100 ml flask in the laboratory, in a 10 m3 vessel in an industrial plant? Or, indeed, in volumes of many cubic kilometers in the the oceans or in the atmosphere, or even in celestial bodies of huge dimensions in far away galaxies? The answer is yes, if the molecules are distributed in space in the same manner, and if temperature and pressure, which also determine the rate of chemical reactions, have the same spatial profIles. But these conditions are seldom met. When larger portions of matter, containing species that may react chemically with each other, are brought together, the distances that individual molecules will have to travel before they can react, increase, so that greater local concentration differences may occur during the reaction. If heat is evolved in the chemical reaction, the distance between a point where heat is generated and a wall where cooling may take place is increased accordingly, resulting in larger temperature differences within the reacting mixture. In cases where the reacting mixture flows through a reactor vessel, local pressures may again be dependent on the scale of operation. Since the results of a chemical reaction are generally determined by local concentrations, and often by pressure and temperature, the outcome of a chemical reaction is as a rule dependent on the scale on which the reaction is carried out. The consequence is that a different type of equipment may be necessary to control reaction conditions on a larger scale, but even so it may not always be possible to imitate the conditions of the small scale reactor exact1y. The altered conditions will generally result in a different composition of the product mixture coming out of the larger scale reactor, and this may have several far reaching consequences. The most important are the following: - The yield of the desired product may be different, often lower, wh ich makes the process economically less attractive. v vi PREFACE - The selectivity of the process may be different, often resulting in more of an undesired byproduct. In many cases, this is the origin of environmental pollution. - The product quality may be different, for various reasons: - if the product is a mixture of chemical compounds (such as, e.g., motor petrol) the composition may be altered. - if the product is a polymer, which also is a mixture, the molecular mass or mass distribution of the product will be influenced. - if the product is asolid, the size, shape and structure of the product may be different. These consequences may determine the eventual success of the technical application of a new chemical synthesis. This fact is not always completely understood by laboratory chemists, and it is the aim of this book to elucidate these effects, and to present the necessary background. In general, chemical reactor development has to be aimed at increasing yields and selectivities, reducing pollution and increasing product quality. It is important to note at this stage, that the chemical interaction between molecules can be studied experimentally at any scale that is sufficiently larger than the molecular dimensions. A consequence is, for example, that unexpected side reactions, that are found when the reaction is carried out on a large sc ale (in a plant, or in the environment), may be studied in detail in the laboratory under well defmed conditions. The phenomena of which the rates are essentially scale dependent are all physical in nature, and in this context they can be summarized as physical transport phenomena. These phenomena can be studied separately or in combination with chemical reactions. This book focusses on the interaction between chemical reactions and physical transport phenomena. This area of science is called "Chemical Reaction Engineering", a term coined by Van Krevelen at the First European Symposium on Chemical Reaction Engineering (1957). The principles of this new area of science had shortly before been presented in three pioneering books, written by Frank-Kamenetskii (1955), Smith (1956), and Damköhler (1957). In the early sixties several textbooks appeared that since have been used widely in the teaching of this new science to chemical engineering students, namely those by Levenspiel (1962), Kramers and Westerterp (1963) and Denbigh (1965). Most of these books have since been published in revised editions and are still widely used. Since then the volume of literature in this area has grown enormously, and one can say that chemical reaction engineering has become a branch of science in its own right. This is most clearly demonstrated in the bi-annual International Symposia on Chemical Reaction Engineering (ISCRE), that are held alternately in Europe and in North America, and similar symposia held in Asia, and by the growing number of textbooks in this field. But as often happens, when a branch of science grows and diversifies, its practitioners become more specialized, and they grow apart in their interests, not only from each other, but also from the users of their branch of science. It seems almost unavoidable that fast growing branches of science gradually become less accessible to other scientists and particularly to engineers. This book is an attempt to bridge the gap that has grown between the chemie al reaction engineering science, and the area of applied chemistry and chemical process development where this science can be implemented. PREFACE vii This book can be read in different ways. For those who are not familiar with chemical reaction engineering, it may be advisable to read flrst the elaborate Introduction presented in Part I (Chapters 1 and 2). In these chapters the scene is set and various important aspects of chemical reactions are reviewed in a qualitative sense, with their consequences for reactor development. This part contains several examples that are meant to illustrate what is going to be treated in Part IL In the end of Chapter 2 the organization of this book is presented. Part 11, General Principles, is written for those who are interested in the application of chemical reaction engineering, and wish 10 get acquainted with the most important principles. The chapters in Part 11 are organized according to physical and chemical principles. Chapter 3 is a necessary introduction about ideal reactor models, it deals with the relation between conversion and time, both in batch and in continuous reactors. Chapters 4, 5 and 6 are about physical phenomena acting on the volume element scale in chemical reactors, and their influence on the rates of chemical reactions. Studying these phenomena in small scale experiments can give important information for reactor scale-up. Chapters 7, 8 and 9 deal with integral reactor models and selection of reactor types. In Part III some Applications of chemical reaction engineering are presented. Chapters 10 - 14 are arranged according to areas of application. Several examples are given in some detail. The more experienced reader may start here, and occasionally look back at the General Principles of Part 11. This book is written with the following question in mind: "I haw a chemical reaction; how do 1 fmd the best chemical reactor?".