Solid/ Liquid Separation: Principles of Industrial Filtration

352 Pages·2005·36.508 MB·English

by  Richard Wakeman, Stephen Tarleton

Solid/Liquid
Separation:
Principles of
Industrial
Filtration
R. J. Wakeman
Professor, Department of Chemical
Engineering, University of Loughborough, UK
E. S. Tarleton
Senior Lecturer, Department of Chemical
Engineering, University of Loughborough, UK
UK Elsevier Ltd, The Boulevard, Langford Lane, Kidlington, Oxford 0X5 1GB, UK
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© 2005 Elsevier Ltd.
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First edition 2005
British Library Cataloguing in Publication Data
Wakeman, Richard J.
Solid/liquid separation : principles of industrial
filtration
1. Filters and filtration
I. Title II. Tarleton, E. S.
660/2'84'84245
ISBN 1 8561 74190
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.
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Transferred to Digital Printing, 2010
Contents
Preface ix
1 Introduction 1
1.1 The separation process 1
1.1.1 Solid/liquid separation in the process flowsheet 4
1.1.2 Filter media 6
1.1.3 Cake and depth filtration 10
1.1.4 "Laws" of filtration 10
1.2 Particle and liquid properties in filtration 13
1.2.1 Particle shape 13
1.2.2 Particle size 14
1.2.3 The solution environment 15
1.2.4 The nature of the fluid 18
1.3 Conclusions - Perfection in separation! 18
2 Fluid dynamic background to filtration processes 19
2.1 Motion of particles in a fluid 19
2.1.1 Gravity settling at low concentrations 22
2.1.2 Settling in a centrifugal field 24
2.1.3 Suspensions with high concentrations of solids -
hindered settling 25
2.2 Darcy's Law 26
2.2.1 Darcy's Law in filtration 28
2.3 Permeability and specific resistance 28
2.3.1 The Kozeny-Carman equation 29
2.3.2 Permeabilities calculated from cell models 33
2.3.3 Properties of porous media 34
2.3.4 The friction factor for particulate beds 36
2.4 Depth filtration (clarification) 38
2.4.1 Capture mechanisms 38
2.4.2 Modelling the capture process 42
2.4.3 Deposition and scour 43
2.5 Flow through fibrous and woven media 44
2.5.1 Flow through nonwoven, random fibre media 44
2.5.2 Monofilament woven media 45
2.5.3 Multifilament woven media 50
2.6 Cake deliquoring - multiphase flow in porous media 51
2.6.1 Capillary pressure and threshold pressure 52
2.6.2 Relative permeability and capillary pressure 53
2.6.3 Application to cake deliquoring 57
2.6.4 Other deliquoring models 58
2.7 Cake washing - Hydrodynamic dispersion 60
2.7.1 The equations of hydrodynamic dispersion 62
2.7.2 Washing deliquored filter cakes 65
2.7.3 Other models for cake washing 67
2.8 Expression - deliquoring by compression 68
2.9 Conclusions 71
Gravity clarification and thickening 72
3.1 Basics of gravity settling 72
3.1.1 Batch sedimentation 73
3.1.2 Types of sedimentation 74
3.2 Clarification 76
3.3 Thickening 79
3.3.1 Relating batch test data to thickeners 80
3.3.2 Thickener area 8 5
3.3.3 Thickener depth 88
3.3.4 Zone sedimentation models 92
3.3.5 Compression subsidence models 92
3.3.6 Zone settling vs. compression subsidence
models 93
3.4 Conclusions 95
Filtration: cake formation 96
4.1 Relationships between process variables 100
4.2 Compressible cake filtration 104
4.2.1 Pressure conditions in a compressible filter cake 105
4.2.2 Working relationships for compressible filtration 107
4.2.3 Effects of pressure on filter cake properties 108
4.2.4 Other factors affecting filter cake properties 109
4.2.5 Expressions for the solids mass deposited 116
4.2.6 Expressions for suspension concentration 116
4.2.7 Relationships between cake thickness and
filtrate volume 117
4.3 Basis for filtration calculations 118
4.3.1 Constant pressure operation 119
4.3.2 Constant rate operation 126
4.3.3 Variable pressure-variable rate operation 128
4.4 Filtration tests to determine cake formation data 129
4.5 Filtration onto cylindrical elements 133
4.5.1 Constant pressure filtration 134
4.5.2 Constant rate filtration 135
4.6 Filtration in centrifuges 137
4.6.1 Centrifugation rate and filtrate flow rate 138
4.6.2 Cake thickness dynamics 142
4.7 Filtration from suspensions of Non-Newtonian liquids 143
4.8 Process calculations 144
4.8.1 Optimisation of cycle times 152
4.9 Advanced models of cake filtration 156
4.9.1 Constitutive properties 158
4.9.2 Models incorporating interparticle interactions 160
4.10 Conclusions 161
Cake deliquoring 162
5.1 Threshold pressure and irreducible saturation 163
5.1.1 Irreducible saturation of vacuum or pressure
deliquored cakes 166
5.1.2 Irreducible saturation of centrifuge cakes 166
5.2 Design charts for deliquoring 168
5.2.1 Vacuum and pressure deliquoring 171
5.2.2 Centrifugal deliquoring 172
5.2.3 Gravitational drainage 173
5.2.4 Experimental validation of the design charts 173
5.3 Example calculations for deliquoring 177
5.3.1 Summary of deliquoring calculations for
vacuum and pressure filters 179
5.3.2 Effects of particle size, cake thickness and
pressure difference 187
5.3.3 Summary of deliquoring calculations for
centrifugal filters 187
5.4 Cake cracking 192
5.4.1 Stresses in partially saturated filter cakes 192
5.5 Conclusions 195
Cake washing 196
6.1 The wash liquid and washing curves 198
6.1.1 Measurement of washing curves 199
6.1.2 Representation of washing data 202
6.1.3 "Washing efficiency" 203
6.2 Cake washing models for design 205
6.3 Continuous washing processes 212
6.3.1 Correction factors for full scale filters 215
6.3.2 Washing deliquored filter cakes 220
6.3.3 Cake washability 221
6.4 Reslurry washing 222
6.4.1 Multiple reslurry systems 223
6.4.2 Countercurrent multiple reslurry systems 226
6.5 Multiple washing filter installations 228
6.5.1 Rotary vacuum filter flow sheets 229
6.5.2 Countercurrent washing on belt filters 236
6.6 Conclusions 245
Expression (compression deliquoring) 247
7.1 Experimental evaluation of expression 249
7.1.1 General treatment of experimental expression
data 250
7.2 Interpretation of expression data 252
7.2.1 Analysis of the first stage - cake formation 252
7.2.2 Analysis of the second stage - cake consolidation 254
7.2.3 Expression on curved (2-dimensional) filter
media 261
7.3 Factors affecting expression characteristics 261
7.3.1 Expression rate increase by inserted strings 263
7.3.2 "Torsional" expression 264
7.4 Conclusions 265
Membrane filtration 266
8.1 Membrane filtration processes 268
8.1.1 Crossflow filtration vs. normal (cake) filtration 270
8.1.2 How much permeate could be produced? 270
8.1.3 Membrane fouling and concentration
polarisation 271
8.1.4 Fouling by proteins and other biological
materials 274
8.1.5 Fouling by inorganic particulates - effects on MF
performance 276
8.2 Ultrafiltration 280
8.2.1 Rejection 280
8.2.2 Mass transfer analysis 280
8.2.3 Determining the mass transfer coefficient 282
8.2.4 The diffusion coefficient 283
8.2.5 Limiting flux 286
8.2.6 Limiting concentration 287
8.3 Microfiltration 287
8.3.1 Filtration rate equations 287
8.3.2 Deadend filtration 288
8.3.3 Crossflow filtration 290
8.4 Critical flux operation 291
8.4.1 Deposit characteristics 292
Conclusions 294
List of notattiioonn 296
y and additional bibliography 307
Index
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Preface
Other texts have dealt with the theory and equations of filtration,
sometimes with guiding examples. The link of theory to the design
process is not developed in these texts, limiting their usefulness from a
practical viewpoint. This seems to have been a contributory factor in
promulgating the view amongst many that theoretical development and
modelling of filtration processes is inappropriate and cannot be utilised
in practice. These notions must be dispelled. A total theoretical
description of filtration is not currently possible, although scientifically
based data are available for many of the processes which can be
modelled. The research literature is made confusing by researchers
claims that some new models are better than older ones - and indeed
this is in some senses the case when a model is developed that takes
account of previously ignored parameters. Unfortunately, in the
engineering context a more complex model that takes account of basic
variables in the process such as the properties of the particles to be
separated is not necessarily better than an earlier model where those
properties do not appear explicitly but are grouped into a lumped para-
meter. Many of those more complex models have unknown parameters
in them which can only be evaluated by comparison with experimental
data, making them little if any better than earlier models that are more
easily applied. Sometimes, however, the complex model has a role in
providing an understanding in a general sense of what would happen if
a variable were changed - exact values that come from models still
cannot be relied upon without the backing of experimental data.
The focus of this book is the presentation of models and calculation
methods relevant to industrial design and simulation, linking practical
aspects of filter design to models that have been proven by industrial
practice. The current state of knowledge is used to inter-relate the
various processes that may be operated during a filter cycle (i.e. cake