Preface

Slurries are unusual mixtures. They adopt characteristics quite different from the bulk of solid particles carried with them or the liquids in which they are suspended or partially suspended. They range from mixtures with very fine particles to those with coarse particles such as sand and gravel.

This book is an important collection of technical information on slurry technology. Chapters cover such topics as high-concentration conveying, vertical transport of fine particle slurries, rheology of sludges, pipe wear, and wastewater applications. It begins with an introductory chapter by the editor that summarizes important advances in slurry technology and ends with a chapter on fluidisation.

Slurries are fascinating mixtures that have at least two major manifestations: settling and non-settling. Both types suffer from holdup, a measure of delayed and retained solids in a pipeflow, but the process of modelling it for the two is different. Chapter 2 by the editor models the phenomenon of holdup of settling media in horizontal ducts. It describes the important distinction between in situ concentration and delivered or efflux concentration and its implications. In situ concentration is always greater than efflux concentration because of settling, so there must be a gradation in axial velocity to satisfy the demands of the continuity equation. The two-layer model by Professor K.C. Wilson, invented more than 50 years ago at the time of this writing, simplifies this gradation into two layers: a suspension layer supported by hydrodynamic forces only, and a settling layer gaining additional support from pipe walls and hindered settling.

Vertical ducting is at least as important as horizontal ducting. In Chapter 3, Professor Bartosik presents a study of solid–liquid turbulent pipeflow in a vertical conduit and the mathematical model that it supports. The model centres around an empirical equation for the standard turbulence damping function, which is a factor in the computation of the turbulent viscosity. In its original form, it is a function of turbulent Reynolds number only. The author proposes a modification of the factor to account for variations in particle diameter and volumetric concentration, supported very clearly by experimental evidence. The author is at pains to state that the results apply to specific particle populations and vertical flow but leaves us in no doubt that particle diameter plays an important part in predicting frictional head loss as does solid concentration in a secondary role.

In Chapter 4, Dr. Pullum explains that the upper limit of concentrations for slurries is conventionally limited to 40% (volume/volume) to minimise risks of pipe blockage and restarting difficulties. In pneumatic transport, no such limit applies, and this underlies the motive, perhaps the credo, to extend the concentration range for slurries. The journey is by no means simple because slurries can be very complex mixtures, but the author explains in careful detail how high-concentration hydraulic conveying of coarse particles can be achieved. Inevitable increases in operating costs

and specialised equipment to separate coarse and fine are set against lower pumping power requirements and reduced wear.

have a great influence on the life of vulnerable components. Professor Wood opines that minor melioration in this area could be influential in increasing component

Sludges and slurries must be dewatered for good practical reasons. In some locations,

In warm countries, lagoons can be employed to evaporate the water content. If this natural advantage is limited or unavailable, large plate presses can be employed to reduce the moisture content of sludge to cakes of concentrations of about 20% volume/volume. Plate press installations are costly to build and equip, but the running costs are relatively low. An alternative to plate presses are belt filter presses, which are less expensive to install but with relatively high running costs due to the additives that are needed. Professor Fester and her co-author Rössie Werner explain

Finally, Chapter 10 covers an important application of slurry flow principles. The use of slurry-bed reactors can be traced back to the Second World War when they were used in the synthesis of oil. Professor Fanhui Meng and his co-author Muhammad Asif Nawaz compare three types of reactors: the mechanically stirred slurry reactor,

> **Dr. Trevor Frank Jones** TFJ Consulting Ltd., Derby, United Kingdom

the bubble column reactor and, the three-phase fluidised bed reactor.

reclamation of the water they contain is very important to a local community. Municipal sludge must be reduced in volume before transportation and dewatering is a good way to achieve this. The removal of water is also important in improving the calorific value if the waste is to be incinerated. The discharges of wet sludges and slurries can damage the quality of inland waterways. Dewatering can also prevent the

lifetimes.

release of harmful leachates.

the issues in Chapter 9.

There is a resonance between Dr. Pullum's chapter and that of Dr. Arno Talmon and his co-author Ebi Meshtaki in Chapter 5 on wall slip in harbour muds. Dr. Talmon covers the very interesting phenomenon of wall slip in rheometry applied to slurries and sludges. Rheometry measures the viscosity of a medium using a variety of techniques. If a vane and cup technique is applied, no wall slip occurs. If a bob-in-cup method is used, shear stress at the wall is sensibly constant for an initial application of shear strain before rising as further strain is applied. The concept has wide implications beyond the laboratory and Dr. Talmon presents results applied to harbour walls and sea defences.

Particle size and the grading of particle sizes are important factors in transporting slurry media. Broadly graded mixtures reduce energy loss due to friction in both horizontal and inclined flows. Ascending and descending flows show predictably different results for hydraulic grading (the rate of pressure loss with downstream distance). Stratification (the selective classification of particles based on size) is an important factor in this difference. In Chapter 6, Professor Matoušek and his co-authors Zdenĕk Chára and Jiři Konfršt present a wealth of data on pipe inclination and particle size.

Visualisation of the contents of a slurry duct has informed much of the research into flows of varying concentration distribution. At its simplest level, flow can be passed through transparent sections of pipe and photographed laterally. Penetration of the cross-section can be achieved by a variety of techniques including gamma-ray interrogation, particle image velocimetry, laser Doppler anemometry, and electrical impedance tomography. For some investigations computed tomography, including magnetic resonance tomography, has been borrowed from medical colleagues. A particularly useful technique has been electrical resistance tomography (ERT). Electrodes are positioned around the duct and the resistance experienced by an injected current is used to construct a grid of concentration measurements. ERT can be deployed even when the liquid phase is opaque. In Chapter 7, Dr. Lachlan Graham presents details of the basic theory of ERT and describes the difficult problem of image reconstruction.

Pipe wear is an inevitable consequence of transporting particle-laden liquids. Wear presents a huge cost to industries worldwide and has attracted research to measure, model, and hopefully reduce it. In Chapter 8, Professor Wood and his co-author Alexander Cook explain the research efforts directed at the problem. Computational Fluid Dynamics (CFD) is used to model the flow field with a mesh and to solve the conservation equations (mass, momentum, and energy) at the nodes and spaces. On paper, this technique holds great promise for the future of wear studies, but despite improved particle impact modelling, particle tracking, and dynamically deforming geometry, erosion predictions have so far been disappointing. The Moving Deforming Mesh technique has demonstrated great promise, however. The idea is to update the computational mesh at regular intervals based on local erosion rates and the geometry of the containing walls. In this way, erosive deformations are integrated into the flow field and secondary erosion patterns can be predicted. Pipe layout and internal finish

have a great influence on the life of vulnerable components. Professor Wood opines that minor melioration in this area could be influential in increasing component lifetimes.

Sludges and slurries must be dewatered for good practical reasons. In some locations, reclamation of the water they contain is very important to a local community. Municipal sludge must be reduced in volume before transportation and dewatering is a good way to achieve this. The removal of water is also important in improving the calorific value if the waste is to be incinerated. The discharges of wet sludges and slurries can damage the quality of inland waterways. Dewatering can also prevent the release of harmful leachates.

In warm countries, lagoons can be employed to evaporate the water content. If this natural advantage is limited or unavailable, large plate presses can be employed to reduce the moisture content of sludge to cakes of concentrations of about 20% volume/volume. Plate press installations are costly to build and equip, but the running costs are relatively low. An alternative to plate presses are belt filter presses, which are less expensive to install but with relatively high running costs due to the additives that are needed. Professor Fester and her co-author Rössie Werner explain the issues in Chapter 9.

Finally, Chapter 10 covers an important application of slurry flow principles. The use of slurry-bed reactors can be traced back to the Second World War when they were used in the synthesis of oil. Professor Fanhui Meng and his co-author Muhammad Asif Nawaz compare three types of reactors: the mechanically stirred slurry reactor, the bubble column reactor and, the three-phase fluidised bed reactor.

> **Dr. Trevor Frank Jones** TFJ Consulting Ltd., Derby, United Kingdom

**1**

Section 1

Introduction

Section 1 Introduction
