Table Of ContentButterworths 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
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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
/
