Fundamentals of Fluidized Bed Chemical Processes PDF
Preview Fundamentals of Fluidized Bed Chemical Processes
Butterworths Monographs in Chemical Engineering Butterworths Monographs in Chemical Engineering is a series of occasional texts by internationally acknowledged specialists, providing authoritative treatment of topics of current significance in chemical engineering. Series Editor JWMuIIin Professor of Chemical Engineering, University College, London Published titles: Solid-liquid separation Liquids and liquid mixtures—3rd edn Forthcoming titles: Enlargement and compaction of particulate solids Mixing in the process industries Diffusion and heat flow in liquids Introduction to electrode materials Butterworths Monographs in Chemical Engineering Fundamentals of Fluidized-bed Chemical Processes J G Yates, PhD, DIC, CChem, FRSC Department of Chemical and Biochemical Engineering, University College, London Butterworths London Boston Durban Singapore Sydney Toronto Wellington All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright holder, application for which should be addressed to the publishers. Such written permission must also be obtained before any part of this book is stored in a retrieval system of any nature. This book is sold subject to the Standard Conditions of Sale of Net Books and may not be resold in the UK below the net price given by the publishers in their current price list. First published 1983 ©J.G. Yatesl983 British Library Cataloguing in Publication Data YatesJ.G. Fundamentals of fluidized-bed chemical processes. —(Butterworths monographs in chemistry) 1. Fluidization I. Title 660.2'842 TP156.F65 ISBN 0-4Ü8-709Ü9-X Typeset by Scribe Design, Gillingham, Kent Printed by The Thetford Press Ltd, Thetford, Norfolk Preface Fluidized-bed reactors have been used on an industrial scale for over fifty years and during this time an enormous amount of work has been carried out on all aspects of their design and operation. This work has generated a correspondingly large body of literature which is being augmented at a steady rate. An author of a review of fluidized-bed chemical processes is therefore faced with the dilemma of either overloading his readers with a comprehensive coverage of the world literature on the subject or providing a more manageable survey which runs the risk of being superficial. I have tried in this book to steer a course somewhere between these two obstacles and to present the fundamentals of the subject in a suitably digestible form, but I acknowledge that in so doing I have been obliged to omit reference to much of the work that has been done in the field of fluidization in many industrial and university laboratories around the world. My hope is that this 'biased model' will not prove too unacceptable to those whose work I have treated too lightly or not at all. Following a short introduction the book is divided into five main chapters. The first deals with the basic physics of the subject while the second shows how the physics may be combined with chemical kinetics to generate models of fluidized-bed reactors. The next two chapters deal, again in a fundamental way, with two major applications of the technique, one the well-established Fluidized Catalytic Cracking process, the other a newer development concerned with the combustion and gasification of coal and heavy oils. The last chapter examines a number of miscellaneous processes that have been and are currently used for the production of chemicals such as phthalic anhydride, acrylonitrile and compounds of uranium. Although the prime aim of the book is to present a concise, up-to-date picture of the chemically-orientated aspects of gas-solid fluidization suit able for final-year undergraduate and postgraduate students of chemical engineering, it is hoped that practising engineers and scientists involved with gas-solid systems will also find something of value within its pages. It would not be fitting to end this Preface without expressing gratitude to all those in the fluidization field whom I have come to know and respect during the last fifteen years and on whose work and ideas I have so freely v vi Preface drawn. If I were to select one name for special thanks from those liberally distributed throughout the text it would be that of Peter Rowe who has led the fluidization research group at UCL since 1965 and who has never failed to stimulate and inspire all those who have worked with him. A final word of thanks goes to John Mullin at whose suggestion I wrote this book and without whose encouragement and help it would not have seen the light of day. J.G. Yates University College London The poet, the painter, the scientist, each superimposes his more or less ephemeral vision on the universe, each constructs his own biased model of reality by selecting and highlighting those aspects of experience which he considers significant and ignoring those which he considers irrelevant. Arthur Koestler Vll Nomenclature The list of symbols given below is not exhaustive, some having been omitted because they are only used once or twice in specialist sections of the book. All symbols are however defined in the text and so no confusion should arise from the omissions. SI units have been used wherever possible although in many cases temperatures are given in degrees Celsius and pressures in bar. A cross sectional area of bed m2 A area of bed occupied by bubble-cloud phase m2 c A area of bed occupied by emulsion phase m2 e a catalyst activity C concentration of reactant A mol/m3 A C initial concentration of reactant A mol/m3 Ao C drag coefficient D C specific heat capacity of solid kJ/kg K ps C carbon burning rate mol/s D diameter of bed m D molecular diffusion coefficient m2/s G d diameter of sieve aperture m a d bubble diameter m b d cloud diameter m c d initial diameter of particle m x d particle diameter m p E diffusion coefficient m2/s F mass flux of solids kg/m2s F solids flowrate at bed surface kg/m2s 0 Foo solids elutriation rate kg/m2s / fraction of bed occupied by bubbles b f mass fraction of feed deposited on catalyst as coke - c / fraction of bubble sphere occupied by wake w / fraction of bed occupied by slugs s r ri K 4pg(Ps " Pg)g Gn a Galileo number, μ2 (££>G)'/2 Ha Hatta number, XI xii Nomenclature bed height at minimum fluidization m h bed height above distributor m bed-to-surface heat transfer coefficient W/m2K ^bc gas exchange rate between bubble and cloud per unit s_1 bubble volume gas exchange rate between bubble-cloud and ^bce emulsion per unit bubble volume gas exchange rate between bubble and emulsion ^be per unit bubble volume gas exchange rate between cloud and emulsion per unit bubble volume elutriation rate coefficient kg/m s reaction rate coefficient s-1 bubble-emulsion mass transfer coefficient m/s &be rate coefficient of catalyst deactivation s-1 k cloud-emulsion mass transfer coefficient m/s sc K mass transfer coefficient m/s K surface reaction rate coefficient m/s K velocity coefficient L length m M mixing index number of transfer units Net number of mixing units NE number of reaction units n slug frequency s P pressure kN/m2 Q volumetric gas flowrate m3/s ßb bubble flowrate m3/s ßmf gas flowrate at minimum fluidization m3/s ßs slug flowrate m3/s total volumetric gas flowrate m3/s ÖT Rb bubble radius m Rr cloud radius m Ud p p g Re Reynolds number, μ ^mfrfpPg Re t Reynolds number at minimum fluidization, m Re Reynolds number under terminal fall conditions, t Ud p t p g r radial distance m S surface area m2 Sc Schmidt number, Pg^G Nomenclature xiii Sh Sherwood number, g p - T temperature K t time s t mean residence time s burn-out time of batch s 'c u velocity m/s u bubble belocity m/s b u cloud velocity m/s c u emulsion phase gas velocity m/s e u fluid velocity m/s f u minimum bubbling velocity m/s mb u superficial velocity at minimum fluidization m/s mf v particle velocity m/s P U minimum slugging velocity m/s s i/sl slip velocity m/s u particle terminal fall velocity m/s t U take-over velocity m/s TO V volume m3 v bubble volume m3 h v volume of gas in bubble plus cloud m3 bc v cloud volume ™3 c m X interphase mass transfer term - X mass fraction - z vertical coordinate m a ratio of bubble velocity to interstitial gas velocity - ß calcium-to-sulphur mole ratio - δ film thickness m ε voidage - emf voidage at minimum fluidization - θ angle degrees μ fluid viscosity Ns/m2 υ kinematic viscosity m2/s Ρ density kg/m3 Pf fluid density kg/m3 gas density kg/m3 Pg Ps solids density kg/m3 φ sphericity factor - fluid stream function - ΨΓ Introduction The technique of gas-solid fluidization was first used industrially in the Winkler process for the gasification of coal in the early 1930's, but for various reasons the process did not find widespread use and the technique was not developed further until the beginning of the Second World War. Then a group of companies, which included Standard Oil New Jersey, M.W. Kellogg, Shell and Universal Oil Products, in an effort designed primarily to exploit the catalytic cracking method for gasoline manufacture discovered by the French engineer Eugene Houdry but without using his fixed-bed reactors, designed a fluidized solids process that has formed the foundation of all subsequent developments in the field1. The Fluidized Catalytic Cracking (FCC) process was a spectacularly successful example of engineering innovation and it remains today one of the cornerstones of petroleum refining technology. Following the success of the FCC process a number of other processes using fluidized solids were put into commercial operation and although some of these were successful others were not and never operated to their full design specifications. It became clear that there were certain features of the FCC and similar processes that made them particularly suited to application in fluidized beds. That other processes were not so well suited was found to be a result of their having one or more of a number of unfavourable characteristics associated with, for example, the chemistry of the reactions, the properties of the solids or the pattern of gas-solid contact, and as a result of certain plant failures fluidized beds acquired a reputation for unreliability. The fluidized bed is only one of the many types of reactor employed in industry for carrying out gas-solid reactions but it has a number of advantages over its competitors that are worth noting at the outset. Its most important advantages stem from the fact that the solid particles it contains are in continuous motion and are normally very well mixed; the result is that 'hot spots' are rapidly dissipated and the bed operates in an essentially isothermal manner. Furthermore, because of the very high bed-to-surface heat transfer that can be achieved, again as a result of particle motion, temperature control is seldom a problem. Thirdly, the fluid-like properties of the gas-solid mixture enable the solid to be transferred without difficulty from one /
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