Applications of Fluidization to Food Processing

Transcription

1 Applications of Fluidization to Food Processing

2 Applications of Fluidization to Food Processing P. G. Smith University of Lincoln UK Blackwell Science

3 2007 by Peter G. Smith Blackwell Science, a Blackwell Publishing company Editorial offices: Blackwell Science Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK Tel: +44 (0) Blackwell Publishing Professional, 2121 State Avenue, Ames, Iowa , USA Tel: Blackwell Publishing Asia Pty Ltd, 550 Swanston Street, Carlton, Victoria 3053, Australia Tel: +61 (0) The right of the Author to be identified as the Author of this Work has been asserted in accordance with the Copyright, Designs and Patents Act All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. First published 2007 ISBN: Library of Congress Cataloging-in-Publication Data Smith, P.G. Applications of fluidization to food processing / P.G. Smith. 1st ed. p. cm. Includes bibliographical references and index. ISBN-13: (hardback : alk. paper) ISBN-10: (hardback : alk. paper) 1. Fluidization. 2. Fluid dynamics. 3. Food industry and trade. I. Title. TP156.F65S dc A catalogue record for this title is available from the British Library Set in 10 on 13 pt Palatino by SNP Best-set Typesetter Ltd., Hong Kong Printed and bound in Singapore by Fabulous Printers Pte Ltd The publisher s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp processed using acid-free and elementary chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. For further information on Blackwell Publishing, visit our website:

4 For Liz, Alexis, Imogen and Verity

5 Contents Preface Glossary xii xv Part One: Fundamentals of Fluidization 1 1 A Description of Fluidized Bed Behaviour 3 An introduction to fluidization 3 Industrial applications of fluidization 7 Applications of fluidization in the food industry 8 Gas-solid fluidized bed behaviour 9 Influence of gas velocity 9 Geldart s classification 11 Bubbles and particle movement 13 Bubble formation at the distributor 13 Bubble growth and bubble shape 13 Minimum bubbling velocity 16 Bubble rise velocity 17 Particle movement due to bubble motion 18 Distributor plate design 20 Characterisation of particulate solids 22 Particle size distribution 22 Mean particle size 24 Particle shape 26 Bulk particle properties 27 Terminal falling velocity and particle drag coefficient 28 Minimum fluidizing velocity in aggregative fluidization 31 Voidage and pressure drop at incipient fluidization 32 Carman-Kozeny equation 32 Ergun equation 35 Minimum fluidizing velocity as a function of terminal falling velocity 37 Semi-empirical correlations 39 Experimental measurement 40

6 viii Contents Fluidized bed behaviour at high gas velocities 40 Slugging 40 Turbulent fluidization and fast fluidization 42 Elutriation and entrainment 42 Other types of fluidization 45 Spouted beds 45 Centrifugal fluidization 47 Particulate fluidization 48 Nomenclature 50 References 52 2 Characteristics of Aggregative Fluidization 55 Heat transfer 55 Correlations for heat transfer coefficients 55 Bed-surface heat transfer 56 Gas-particle heat transfer 57 Gas-particle heat transfer coefficient 58 Mass transfer 61 Correlations for mass transfer coefficients 61 Gas-particle mass transfer 62 Mixing 64 Introduction 64 Mechanisms of solids mixing 65 Mixing in fluidized beds 66 Vertical mixing of solids: the dispersion model 66 Rate of mixing 68 Mixing and segregation of dissimilar particles 68 Mechanisms 68 Patterns of particle segregation 70 Examples of fluidized bed segregation 73 Nomenclature 73 References 75 Part Two: Applications 77 3 Freezing 79 Low-temperature preservation of foods 79 Introduction 79 Industrial freezing equipment 80 Fluidized bed freezing 81 Capacity of fluidized bed freezers 84 Freezing rate and freezing point of foods 87 Prediction of freezing time 89

7 Contents ix Design of fluidized bed freezers 92 Introduction 92 Heat transfer in fluidized bed freezers 92 Mixing, dispersion and residence time 103 Applications of fluidized bed freezing 105 Nomenclature 108 References Drying 113 Introduction 113 Principles of drying 115 Water activity 115 Effect of water activity on microbial growth 116 Effect of drying on food structure 117 Isotherms and equilibrium 117 Drying kinetics 119 Classification of driers 121 Fluidized bed drying 122 Material and energy balances 122 The well-mixed drier 124 The plug flow drier 127 Variations in fluidized bed drier design 129 Other fluidized bed drying techniques 130 Vibro-fluidization 130 Mechanical agitation 131 Centrifugal fluidization 132 Spouted bed drying 132 Microwave drying 133 Nomenclature 134 References Granulation 139 Granulation and particle growth 139 Particle-particle bonding 142 Bonding mechanisms 142 Growth mechanisms in granulation 144 Fluidized bed granulation 145 Introduction 145 Principles of operation of fluidized bed granulation 146 Material and energy balances 149 Batch and continuous operation: population balance 150 Bed quenching 151 Effect of variables on growth 153 Rate and volume of feed 153

8 x Contents Nozzle position and atomising air rate 154 Bed temperature 155 Fluidizing gas velocity 155 Particle size 157 Binder properties 157 Fluidized bed granulation growth models 159 Layered growth model 159 Agglomeration model 161 A theory of fluidized bed granulation 163 Particle growth mechanisms in fluidized bed granulation 163 Fluidizing gas velocity and particle mixing 164 Binder properties 164 The balance between granulation and fluidization 165 Factors leading to bed quenching 166 An overall mechanism 166 Food applications of fluidized bed granulation 169 Instantising 169 Encapsulation and coating 171 Other applications 174 Spouted bed granulation 176 Nomenclature 177 References Gas-Solid Fluidized Bed Fermentation 185 Principles of fluidized bed fermentation 185 Fermentation of glucose by Saccharomyces cerevisiae 187 Metabolism 187 Production of cell mass and ethanol yield 188 Factors affecting ethanol production 189 Glucose concentration 189 Gaseous environment 190 Ethanol inhibition 191 Temperature 191 ph 192 Moisture content 192 Mass transport limitations 192 Fluidized bed fermentation systems 193 Basic fluidization considerations 193 Anaerobic ethanol production 194 Aerobic ethanol production 195 Agglomeration, quenching and the glucose sink 196 The work of Hayes (1998) 198 A description of the experimental system 198

9 Contents xi Dry quenching experiments 201 Ethanol production with grated pellets 202 Ethanol production with extruded pellets 203 A model for fluidized bed fermentation 206 Material and energy balances 206 Ethanol-water vapour-liquid equilibria 210 The model of Beck and Bauer (1989) 210 The model of Hayes (1998) 212 Modelling the time course of ethanol production 213 Comparison of models with experimental data 215 Conclusion 216 Nomenclature 219 References Other Applications of Fluidization 224 Introduction 224 Gas-solid fluidization 224 Blanching 224 Roasting 226 Explosion puffing 227 Sterilisation 228 Disinfestation of wheat 229 Freeze drying 230 Liquid-solid fluidization 231 Bioreactions 231 Sterilisation 233 Ultrafiltration and reverse osmosis 234 Other applications 235 References 235 Index 239

10 Preface Fluidization is a versatile technique for bringing about intimate contact between finely divided solids and a fluid. Although its origins are a little obscure, the first significant industrial application of fluidization was the catalytic cracking of heavy hydrocarbons using beds of catalyst particles in the 1940s. The extension to a wide range of other chemical, especially exothermic, reactions came about because heat can be transferred relatively easily to and from the bed and bed temperatures are uniform and can be held constant. The application of fluidization to a number of physical unit operations followed this early exploitation in reaction engineering. Very considerable research was undertaken in universities and research institutes, and in industry, between the 1960s and 1980s when much of the fundamental behaviour of gas-solid fluidized beds in particular was established and there followed a vast literature dealing with bubbles, mixing, heat and mass transfer, and so on. Very many fluidization research groups have been active in chemical engineering departments throughout the UK, USA, Australia, Canada, the then USSR and most European countries. In the UK this work was headed by, amongst many others, Rowe at UCL, Davidson at Cambridge, Richardson at Swansea and Geldart at Bradford. The lead in understanding the phenomenon of spouted beds was taken by Epstein and Mathur in Vancouver. Although the relevance of fluidization to food processing was recognised at an early stage, there is little in the literature which is specific to the fluidization of food particulates. On the one hand, the subject is poorly treated in almost all food technology and food engineering textbooks and, on the other, much of the specialised fluidization literature can be a little unapproachable to those with a specific interest in food applications. Topics such as fluidized bed freezing are simply ignored in the chemical engineering literature. This book is an attempt to bridge this gap. Much of what is written about foods and fluidization appears to have the food element tacked on and if this appears also to be true of the present work then it is because of the general paucity of relevant literature. It is difficult, but not impossible, to make experimental measurements on food systems in fluidized beds but it seems

11 Preface xiii that few researchers endeavour to do so. Equally, it must be said that the fundamentals of fluidization are often misunderstood; fluidization is a very powerful processing technique but it cannot solve every solids processing problem and it is not appropriate in every situation. This lack of relevant food-related fluidization literature has made it necessary to rely upon relevant work in other fields. For example, in Chapter 5 there is much to be gained in understanding the mechanisms of fluidized bed granulation by examining work with materials as diverse as nuclear waste and fertiliser. It has not been possible to cover all aspects of the principles of fluidization. A number of comprehensive texts on fluidized bed behaviour are available and inevitably I have drawn heavily on these. The reader who wishes to go into greater depth about the fundamental mechanisms at work in fluidized beds should consult those works by Davidson and Harrison (1971), Botterill (1975), Davidson, Clift and Harrison (1985), Kunii and Levenspiel (1991) and more recently Gibilaro (2001). Full references can be found at the end of Chapter 1. In addition, I have concentrated on gas-solid fluidized beds somewhat to the exclusion of liquid-solid fluidization although an indication of how particulate fluidization can be applied to biochemical reactors is given in Chapter 7. Part One of the book is a description of fluidized bed behaviour and covers the theory necessary to understand the applications which follow. Chapter 1 is a description of the characteristics of both gas-solid and liquid-solid fluidized beds, concentrating especially upon the predication of minimum fluidizing velocity. Chapter 2 covers the allimportant topics of heat transfer, mass transfer and particle mixing which underpin almost all the applications of fluidization, which are treated in Part Two. Over the last 40 years freezing has grown in importance because the quality of the frozen product is often significantly better than that produced using more traditional thermal preservation methods. Fluidized bed freezing (Chapter 3) finds particular application in the processing of large-volume products such as prepared vegetables and a variety of soft fruits. Despite the popularity of frozen food, however, drying remains a very widespread operation in the food industry. Fluidized bed driers (Chapter 4) exploit the rapid particle mixing and high heat transfer coefficients in aggregative fluidization and are one of the most versatile and widely used kinds of drier. The manufacture of particles with specific properties is of increasing importance in food processing; Chapter 5 covers the mechanisms of fluidized bed granulation and its application to agglomeration, layering, coating, instantising and encapsulation. Chapter 6 describes recent work on the use of gas-solid fluidized beds as novel bioreactors for fermentation reactions

12 xiv Preface and the culture of micro-organisms. This technique has considerable advantages over conventional fermentation systems and has the potential to manufacture a range of volatile products, including for example flavour compounds, using a variety of micro-organisms. Chapter 7 covers, in rather less detail, a wide range of other uses for gas-solid fluidization, notably blanching, roasting, explosion puffing, sterilisation and atmospheric pressure freeze drying. The use of liquid-solid fluidization as a turbulence promoter in ultrafiltration and in sterilisation is outlined, as well as its use as a bioreactor where the liquid phase is the substrate and the solid phase usually takes the form of immobilised enzyme beads. Finally, a brief note on spelling and terminology: the word fluidization may also be spelt fluidisation. The former has been used throughout this book except in the references to published works where the original published spelling has been retained. The term fluid bed is also in common usage and in most cases is taken to mean the same as fluidized bed. Again, I have used this where it appears in the titles of books and papers but not otherwise. I must express my sincere thanks to Dr Bill Hayes for permission to use material, including a number of diagrams, from his PhD thesis, which was written under my supervision. This appears in Chapter 6. My thanks also to Nigel Balmforth of Blackwell for his patience and encouragement over far too great a period. P.G. Smith Lincoln October 2006

13 Glossary Aggregative fluidization Bubble phase Bubbling fluidization Carry over Channelling Cloud Dense phase Dilute phase Disperse phase Distributor Fluidization characterised by the formation of bubbles of fluidizing fluid. This usually occurs where the fluidizing medium is a gas. The discontinuous phase of an aggregative fluidized bed made up of bubbles of fluid (almost always gas). See aggregative fluidization. Entrained particles which leave the fluidized bed column entirely. Above the nominal minimum fluidizing velocity, the passage of gas through narrow channels in a bed of solids; the bed may remain defluidized. At bubble rise velocities greater than minimum fluidizing velocity, the approximately spherical shell of gas which surrounds a bubble and circulates through it as it rises through the bed. Continuous (i.e. the non-bubble) phase in an aggregative fluidized bed. Fluidization at high gas velocities where all particles are carried in the gas stream. This corresponds to pneumatic conveying. The region of decreasing solids concentration occupying the freeboard. The porous plate or grid through which the fluidizing fluid passes into the bed.

14 xvi Glossary Elutriation Emulsion phase Entrainment Fast fluidization Fixed bed Fluid bed Fluidized bed Freeboard Geldart classification Heterogeneous fluidization Homogeneous fluidization Incipient fluidization Lean phase Minimum fluidization Minimum fluidizing velocity Packed bed Particulate fluidization The fractionation or preferential separation of entrained particles which changes with distance up through the column above the bed surface. See dense phase. The transport of particles into the exhaust gas stream above the bed surface. Fluidization of a bed of moderate solids concentration formed at extreme gas velocities by the recycling to the fluidized column of transported particles. See packed bed. An alternative term for fluidized bed. A bed of particulate solids through which a fluid passes, thus imparting liquid-like properties to the solids. The space between the surface of the dense phase and the gas exit. A system of classifying particles into four groups (A, B, C and D) according to their fluidization behaviour, first proposed by Geldart. See aggregative fluidization. See particulate fluidization. See minimum fluidization. See bubble phase. The state forming the boundary between a fixed bed and a fluidized bed. The point at which the drag force on a particle is equal to its net weight. The superficial gas velocity at and beyond which the bed is said to be fluidized. A stationary bed of particles up through which a gas or a liquid passes at low fluid velocities. Fluidization characterised by continuous and uniform bed expansion. In general, this occurs

15 Glossary xvii Slugging Spout Spouted bed Superficial velocity Turbulent fluidization Two-phase theory Voidage Wake when the fluidizing medium is a liquid. A state (axial slugging), at high superficial gas velocities, beyond the bubbling bed, where bubbles grow to the size of the column and are known as slugs. In an alternative type of slugging the bed is divided into alternate rising regions of dense phase and disperse phase. The trail of particles drawn up by a bubble as it rises through a fluidized bed. A bed of relatively large particles (usually over 1 mm in diameter), with a conical gas inlet, resulting in a regular circulatory pattern with a central high-velocity spout and an annulus of slow downward-moving particles. The volumetric flow rate of fluidizing fluid divided by the crosssectional area of the bed. At very high superficial gas velocities, discrete bubbles or slugs are no longer present and the bed is characterised by significant pressure fluctuations. The assumption that all gas over and above that required for minimum fluidization flows up through the bed in the form of bubbles. Properly, interparticle voidage; the fraction of a bed of particles occupied by free space. The space between the indented base of a bubble and the bubble sphere. It is occupied by particles as a bubble rises in the bed.