Coulson and Richardson's Chemical Engineering Volume 2 - Particle Technology and Separation Processes

836 Pages·2002·41.72 MB·English

Coulson and Richardson's
CHEMICAL ENGINEERING
VOLUME 2
FIFTH EDITION
Particle Technology and
Separation Processes
J. F. RICHARDSON
University of Wales Swansea
and
J. H. HARKER
University of Newcastle upon Tyne
with
J. R. BACKHURST
Univetsity of Newcastle upon Tyne
Butterworth-Heinemann is an imprint of Elsevier
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First edition 1955
Reprinted (with revisions) 1956, 1959, 1960
Reprinted 1962
Second edition 1968
Reprinted 1976
Third edition (SI units) 1978
Fourth edition 1991
Reprinted (with revisions) 1993, 1996, 1997, 1998, 1999,2001
Fifth edition 2002
Reprinted 2003,2005,2006,2007
Copyright 0 1991,2002, J. F. Richardson and J. H. Harker Published by Elsevier Ltd.
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INTRODUCTION
The understanding of the design and construction of chemical plant is frequently regarded
as the essence of chemical engineering and it is this area which is covered in Volume 6
of this series. Starting from the original conception of the process by the chemist, it is
necessary to appreciate the chemical, physical and many of the engineering features in
order to develop the laboratory process to an industrial scale. This volume is concerned
mainly with the physical nature of the processes that take place in industrial units, and,
in particular, with determining the factors that influence the rate of transfer of material.
The basic principles underlying these operations, namely fluid dynamics, and heat and
mass transfer, are discussed in Volume 1, and it is the application of these principles that
forms the main part of Volume 2.
Throughout what are conveniently regarded as the process industries, there are many
physical operations that are common to a number of the individual industries, and may
be regarded as unit operations. Some of these operations involve particulate solids and
many of them are aimed at achieving a separation of the components of a mixture.
Thus, the separation of solids from a suspension by filtration, the separation of liquids
by distillation, and the removal of water by evaporation and drying are typical of such
operations. The problem of designing a distillation unit for the fermentation industry, the
petroleum industry or the organic chemical industry is, in principle, the same, and it is
mainly in the details of construction that the differences will occur. The concentration
of solutions by evaporation is again a typical operation that is basically similar in the
handling of sugar, or salt, or fruit juices, though there will be differences in the most
suitable arrangement. This form of classification has been used here, but the operations
involved have been grouped according to the mechanism of the transfer operation, so that
the operations involving solids in fluids are considered together and then the diffusion
processes of distillation, absorption and liquid-liquid extraction are taken in successive
chapters. In examining many of these unit operations, it is found that the rate of heat
transfer or the nature of the fluid flow is the governing feature. The transportation of a
solid or a fluid stream between processing units is another instance of the importance of
understanding fluid dynamics.
One of the difficult problems of design is that of maintaining conditions of similarity
between laboratory units and the larger-scale industrial plants. Thus, if a mixture is to be
maintained at a certain temperature during the course of an exothermic reaction, then on
the laboratory scale there is rarely any real difficulty in maintaining isothermal conditions.
-
On the other hand, in a large reactor the ratio of the external surface to the volume which
is inversely proportional to the linear dimension of the unit -i s in most cases of a different
order, and the problem of removing the heat of reaction becomes a major item in design.
Some of the general problems associated with scaling-up are considered as they arise in
many of the chapters. Again, the introduction and removal of the reactants may present
difficultp roblems on the large scale, especially if they contain corrosive liquids or abrasive
xxxi
xxxii INTRODUCTION
solids. The general tendency with many industrial units is to provide a continuous process,
frequently involving a series of stages. Thus, exothermic reactions may be carried out in
a series of reactors with interstage cooling.
The planning of a process plant will involve determining the most economic method,
and later the most economic arrangement of the individual operations used in the process.
This amounts to designing a process so as to provide the best combination of capital
and operating costs. In this volume the question of costs has not been considered in
any detail, but the aim has been to indicate the conditions under which various types
of units will operate in the most economical manner. Without a thorough knowledge of
the physical principles involved in the various operations, it is not possible to select the
most suitable one for a given process. This aspect of the design can be considered by
taking one or two simple illustrations of separation processes. The particles in a solid-solid
system may be separated, first according to size, and secondly according to the material.
Generally, sieving is the most satisfactory method of classifying relatively coarse materials
according to size, but the method is impracticable for very fine particles and a form of
settling process is generally used. In the first of these processes, the size of the particle is
used directly as the basis for the separation, and the second depends on the variation with
size of the behaviour of particles in a fluid. A mixed material can also be separated into
its components by means of settling methods, because the shape and density of particles
also affect their behaviour in a fluid. Other methods of separation depend on differences
in surface properties (froth flotation), magnetic properties (magnetic separation), and on
differences in solubility in a solvent (leaching). For the separation of miscible liquids,
three commonly used methods are:
1. Distillation -depending on difference in volatility.
2. Liquid-liquid extraction -d epending on difference in solubility in a liquid solvent.
3. Freezing -d epending on difference in melting point.
The problem of selecting the most appropriate operation will be further complicated
by such factors as the concentration of liquid solution at which crystals start to form.
Thus, in the separation of a mixture of ortho-, meta-, and para-mononitrotoluenes, the
decision must be made as to whether it is better to cany out the separation by distillation
followed by crystallisation, or in the reverse order. The same kind of consideration will
arise when concentrating a solution of a solid; then it must be decided whether to stop
the evaporation process when a certain concentration of solid has been reached and then
to proceed with filtration followed by drying, or whether to continue to concentration by
evaporation to such an extent that the filtration stage can be omitted before moving on to
drying.
In many operations, for instance in a distillation column, it is necessary to understand
the fluid dynamics of the unit, as well as the heat and mass transfer relationships. These
factors are frequently interdependent in a complex manner, and it is essential to consider
the individual contributions of each of the mechanisms. Again, in a chemical reaction
the final rate of the process may be governed either by a heat transfer process or by the
chemical kinetics, and it is essential to decide which is the controlling factor; this problem
is discussed in Volume 3, which deals with both chemical and biochemical reactions and
their control.
INTRODUCTION xxxiii
Two factors of overriding importance have not so far been mentioned. Firstly, the
plant must be operated in such a way that it does not present an unacceptable hazard
to the workforce or to the surrounding population. Safety considerations must be in the
forefront in the selection of the most appropriate process route and design, and must also
be reflected in all the aspects of plant operation and maintenance. An inherently safe plant
is to be preferred to one with inherent hazards, but designed to minimise the risk of the
hazard being released. Safety considerations must be taken into account at an early stage
of design; they are not an add-on at the end. Similarly control systems, the integrity of
which play a major part in safe operation of plant, must be designed into the plant, not
built on after the design is complete.
The second consideration relates to the environment. The engineer has the responsi-
bility for conserving natural resources, including raw materials and energy sources, and
at the same time ensuring that effluents (solids, liquids and gases) do not give rise to
unacceptable environmental effects. As with safety, effluent control must feature as a
major factor in the design of every plant.
The topics discussed in this volume form an important part of any chemical engineering
project. They must not, however, be considered in isolation because, for example, a
difficult separation problem may often be better solved by adjustment of conditions in the
preceding reactor, rather than by the use of highly sophisticated separation techniques.
Preface to the fifth Edition
It is now 47 years since Volume 2 was first published in 1955, and during the inter-
vening time the profession of chemical engineering has grown to maturity in the UK, and
worldwide; the Institution of Chemical Engineers, for instance, has moved on from its 33rd
to its 80th year of existence. No longer are the heavy chemical and petroleum-based indus-
tries the main fields of industrial applications of the discipline, but chemical engineering
has now penetrated into areas, such as pharmaceuticals, health care, foodstuffs, and
biotechnology, where the general level of sophistication of the products is much greater,
and the scale of production often much smaller, though the unit value of the products is
generally much higher. This change has led to a move away from large-scale continuous
plants to smaller-scale batch processing, often in multipurpose plants. Furthermore, there
is an increased emphasis on product purity, and the need for more refined separation
technology, especially in the pharmaceutical industry where it is often necessary to carry
out the difficult separation of stereo-isomers, one of which may have the desired thera-
peutic properties while the other is extremely malignant. Many of these large molecules
are fragile and are liable to be broken down by the harsh solvents commonly used in
the manufacture of bulk chemicals. The general principles involved in processing these
more specialised materials are essentially the same as in bulk chemical manufacture, but
special care must often be taken to ensure that processing conditions are mild.
One big change over the years in the chemical and processing industries is the emphasis
on designing products with properties that are specified, often in precise detail, by the
customer. Chemical composition is often of relatively little importance provided that the
product has the desired attributes. Hence product design, a multidisciplinary activity, has
become a necessary precursor to process design.
Although undergraduate courses now generally take into account these new require-
ments, the basic principles of chemical engineering remain largely unchanged and this
is particularly the case with the two main topics of Volume 2, Particle Mechanics and
Separation Processes. In preparing this new edition, the authors have faced a typical
engineering situation where a compromise has to be reached on size. The knowledge-
base has increased to such an extent that many of the individual chapters appear to merit
expansion into separate books. At the same time, as far as students and those from other
disciplines are concerned, there is still a need for a an integrated concise treatment in
which there is a consistency of approach across the board and, most importantly, a degree
of uniformity in the use of symbols. It has to be remembered that the learning capacity
of students is certainly no greater than it was in the past, and a book of manageable
proportions is still needed.
The advice that academic staffs worldwide have given in relation to revising the book
has been that the layout should be retained substantially unchanged -b etter the devil
we know, with all hisfuults! With this in mind the basic structure has been maintained.
However, the old Chapter 8 on Gas Cleaning, which probably did not merit a chapter
xvii
xviii PREFACE TO THE FIFTH EDITION
on its own, has been incorporated into Chapter 1, where it sits comfortably beside other
topics involving the separation of solid particles from fluids. This has left Chapter 8 free
to accommodate Membrane Separarions (formerly Chapter 20) which then follows on
logically from Filtration in Chapter 7. The new Chapter 20 then provides an opportunity
to look to the future, and to introduce the topics of Product Design and the Use of
Zntens$ed Fields (particularly centrifugal in place of gravitational) and rniniaturisation,
with all the advantages of reduced hold-up, leading to a reduction in the amount of
out-of-specification material produced during the changeover between products in the
case multipurpose plants, and in improved safety where the materials have potentially
hazardous properties.
Other significant changes are the replacement of the existing chapter on Crystallisation
by an entirely new chapter written with expert guidance from Professor J. W. Mullin, the
author of the standard textbook on that topic. The other chapters have all been updated and
additional Examples and Solutions incorporated in the text. Several additional Problems
have been added at the end, and solutions are available in the Solutions Manual, and now
on the Butterworth-Heinemann website.
We are, as usual, indebted to both reviewers and readers for their suggestions and for
pointing out errors in earlier editions. These have all been taken into account. Please keep
it up in future! We aim to be error-free but are not always as successful as we would like
to be! Unfortunately, the new edition is somewhat longer than the previous one, almost
inevitably so with the great expansion in the amount of information available. Whenever
in the past we have cut out material which we have regarded as being out-of-date, there is
inevitably somebody who writes to say that he now has to keep both the old and the new
editions because he finds that something which he had always found particularly useful
in the past no longer appears in the revised edition. It seems that you cannot win, but we
keep trying!
J. F. RICHARDSON
J. H. HARKER
Cont ents
xvii
PREFACTEO THE FIFI’H EDITION
xix
PREFACTEO FOURTH EDITION
xxi
PREFACE TO THE 1983 REPRINT OF THE THIRDE DITION
xxiii
PREFACTEO THIRDE DITION
xxv
PREFACETO SECOND EDITION
xxvii
PREFACTEO FIRSTE DITION
xxix
xxxi
INTRODUCTION
1
1. Particulate Solids
1
1.1 Introduction
2
1.2 Particle characterisation
2
1.2.1 Single particles
3
1.2.2 Measurement of particle size
10
1.2.3 Particle size distribution
11
1.2.4 Mean particle size
1.2.5 Efficiency of separation and grade efficiency 17
22
1.3 Particulate solids in bulk
22
1.3.1 General characteristics
22
1.3.2 Agglomeration
1.3.3 Resistance to shear and tensile forces 23
1.3.4 Angles of repose and of friction 23
25
1.3.5 Flow of solids in hoppers
27
1.3.6 Flow of solids through orifices
1.3.7 Measurement and control of solids flowrate- 27
29
1.3.8 Conveying of solids
30
1.4 Blending of solid particles
30
1.4.1 The degree of mixing
33
1.4.2 The rate of mixkg
37
1.5 Classification of solid particles
37
1.5.1 Introduction
1.5.2 Gravity settling 40
46
1.5.3 Centrifugal separators
1.5.4 The hydrocyclone or liquid cyclone 48
1.5.5 Sieves or screens 55
1.5.6 Magnetic separators 58
61
1.5.7 Electrostatic separators
62
1.5.8 Flotation
1.6 Separation of suspended solid particles from fluids 67
67
1.6.1 Introduction
72
1.6.2 Gas cleaning equipment
87
1.6.3 Liquid washing
V
vi CONTENTS
91
1.7 Further reading
92
1.8 References
93
1.9 Nomenclature
2. Particle size reduction and enlargement 95
95
2.1 Introduction
95
2.2 Size reduction of solids
2.2.1 Introduction 95
2.2.2 Mechanism of size reduction 96
100
2.2.3 Energy for size reduction
2.2.4 Methods of operating crushers 103
2.2.5 Nature of the material to be crushed 105
2.3 Types of crushing equipment 106
2.3.1 Coarse crushers 106
2.3.2 Intermediate crushers 110
2.3.3 Fine crushers 117
2.3.4 Specialised applications 137
2.4 Size enlargement of particles 137
2.4.1 Agglomeration and granulation 137
2.4.2 Growth mechanisms 138
2.4.3 Size enlargement processes 140
143
2.5 Further reading
143
2.6 References
144
2.7 Nomenclature
3. Motion of particles in a fluid 146
3.1 Introduction 146
3.2 Flow past a cylinder and a sphere 146
3.3 The drag force on a spherical particle 149
3.3.1 Drag coefficients 149
3.3.2 Total force on a particle 153
3.3.3 Terminal falling velocities 155
3.3.4 Rising velocities of light particles 161
3.3.5 Effect of boundaries 161
3.3.6 Behaviour of very fine particles 162
3.3.7 Effect of turbulence in the fluid 163
3.3.8 Effect of motion of the fluid 163
3.4 Non-spherical particles 164
3.4.1 Effect of particle shape and orientation on drag 164
3.4.2 Terminal falling velocities 166
3.5 Motion of bubbles and drops 168
3.6 Drag forces and settling velocities for particles in non-Newtonian fluids 169
3.6.1 Power-law fluids 169
3.6.2 Fluids with a yield stress 172
3.7 Accelerating motion of a particle in the gravitational field 173
3.7.1 General equations of motion 173
3.7.2 Motion of a sphere in the Stokes’ law region 176
3.7.3 Vertical motion (general case) 178
3.8 Motion of particles in a centrifugal field 185
3.9 Further reading 187
3.10 References 188
3.11 Nomenclature 189
4. Flow of fluids through granular beds and packed columns 191
4.1 Introduction 191
4.2 Flow of a single fluid through a granular bed 191
vii
CONTENTS
4.2.1 Darcy’s law and permeability 191
4.2.2 Specific surface and voidage 192
4.2.3 General expressions for flow through beds in terms
of Carman-Kozeny equations 194
4.2.4 Non-Newtonian fluids 204
4.2.5 Molecular flow 205
4.3 Dispersion 205
4.4 Heat transfer in packed beds 21 1
4.5 Packed columns 212
4.5.1 General description 213
4.5.2 Packings 216
4.5.3 Fluid flow in packed columns 222
4.6 Further reading 232
4.7 References 232
4.8 Nomenclature 234
5. Sedimentation 237
5.1 Introduction 231
5.2 Sedimentation of fine particles 237
5.2.1 Experimental studies 231
5.2.2 Flocculation 245
5.2.3 The Kynch theory of sedimentation 25 1
5.2.4 The thickener 255
5.3 Sedimentation of coarse particles 261
5.3.1 Introduction 261
5.3.2 Suspensions of uniform particles 268
5.3.3 Solids flux in batch sedimentation 274
5.3.4 Comparison of sedimentation with flow through fixed beds 217
5.3.5 Model experiments 280
5.3.6 Sedimentation of two-component mixtures 282
5.4 Further reading 286
5.5 References 286
5.6 Nomenclature 288
6. Fluidisation 291
6.1 Characteristics of fluidised systems 29 1
6.1.1 General behaviour of gas-solids and liquid-solids system 29 1
6.1.2 Effect of fluid velocity on pressure gradient and pressure drop 293
6.1.3 Minimum fluidising velocity 296
6.1.4 Minimum fluidising velocity in terms of terminal failing velocity 300
6.2 Liquid-solids systems 302
6.2.1 Bed expansion 302
6.2.2 Non-uniform fluidisation 306
6.2.3 Segregation in beds of particles of mixed sizes 308
6.2.4 Liquid and solids mixing 312
6.3 Gas-solids systems 315
6.3.1 General behaviour 315
6.3.2 Particulate fluidisation 315
6.3.3 Bubbling fluidisation 316
6.3.4 The effect of stirring 320
6.3.5 Properties of bubbles in the bed 320
6.3.6 Turbulent fluidisation 324
6.3.7 Gas and solids mixing 326
6.3.8 Transfer between continuous and bubble phases 328
6.3.9 Beds of particles of mixed sizes 330
6.3.10 The centrifugal fluidised bed 33 1
6.3.11 The spouted bed 332
6.4 Gas-liquid-solids fluidised beds 333
6.5 Heat transfer to a boundary surface 334