solid-liquid filtration and separation technology

Sedimentation and Flotation Washing and Deliquoring Forms of Cake Filtration Equation Constant Pressure Filtration Constant Rate Filtration Variable Pressure and Rate Filtration Effect o

Solid-Liquid Filtration and Separation Technology

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ISBN 3-527-28613-6

Solid-Liquid Filtration

and Separation Technology

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Chemical Engineering Loughborough University

of Technology Loughborough LEll3TU Great Britain

T h i s book was carefully produced Nevertheless, authors and publisher do not warrant the information contained therein to be free of errors Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate

1st edition 1996

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VCH Verlagsgesellschaft mbH, Weinheim (Federal Republic of Germany)

VCH Publishers, Inc., New York, NY (USA)

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Rushton, Albert:

Solid liquid filtration and separation technology / A Rushton ; A S Ward ; R G Holdich -

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ISBN 3-527-28613-6

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generate a somewhat bewildering array of particlehid separation problems Separation by filtration is achieved by placing a permeable filter m the path of the flowing suspension The barrier, ie a filter screen, medium or membrane m some cases is selected with a view to

retaining the suspended solids on the filter surfice, whilst permitting passage of the clarified liquid Other systems, e.g deep-bed or candle filters, operate m a diBerent mode, in

promoting deposition of the particles within the interstices of the medium Further purXcation

of the clarified liquid may proceed m u s e of adsorbents to remove dissolved solutes Alternatively, the two phases may be separated by sedimentation processes, m the presence of gravitational or cent&& force fields

Serious operational problems centre on the interaction between the particles and the filter medium Plugging of the latter, or collapse of the collected solids under the stress caused by flow through the filter, can result m low productivity Such effects are often related to the size

of particles being processed; enhanced effective particle size can be accomplished by pre treatment with coagulants or flocculants These techniques are discussed m detail m the text, which also reports recent improvements m the machinev of separatioq e.g the variable chamber presses, the cross-flow processes, ceramic dewatering filters, etc

Several of these newer modifications m filtration plant have followed trends m the developing science of solid-fluid separation and the growing understanding of the processes involved Fortunately, filtration processes have attracted the attention of mcreasing numbers

of scientists and engineers A large output of literature has resulted m a copious flow of design and operational informaton suflicient to place filtration on a much sounder scientific basis

Nevertheless, the random nature of most particulate dispersions has resulted in a wide range of machines m this unit operation Selection of the best available separation technique

is, therefore, a di&cult process problem It is the author’s viewpoint that m y existing separation problems would have been avoided by the application of available scientific data This text is aimed at the provision of theoretical and practical information which can be used

to improve the possi%ddy of selecting the best equipment for a particular separation It is relevant to record the recent increased commercial awareness of the need for this informaton

m the selection of plant used in environmental control

The material presented m the text has been used by the authors m short-course presentations over several years These courses are illustrated by a large number of practical problems m the SLS field; some of these problems have been used to illustrate the book Basic theoretical relationdqs are repeated m those chapters dealing with process calculations This feature minimises the need for back-referencing when using the book

A Rushton

AS Ward

Sedimentation and Flotation

Washing and Deliquoring

Forms of Cake Filtration Equation

Constant Pressure Filtration

Constant Rate Filtration

Variable Pressure and Rate Filtration

Effect of Pressure on Cake Filtration

Constant Pressure Filtration

Constant Rate Filtration

Analysis of Flow Inside a Cake

Variable Rate and Pressure Filtration for Compressible Cakes

Other Modes of Filtration

Filtration with Non-Newtonian Fluids

Vacuum Filter Leaf

Compression Permeability Cell

Capillary Suction Time

Other Laboratory Tests and Procedures

Operational Aspects of Woven Media in Filters

Loading of Yarns with Solids

Effect of Cloth Underdrainage

Aspects of the Cloth Selection and Performance

Cloth Shrinkage

Cloth stretching

Filter Cake Release

Cloth Structural Effects

Cloth Cleaning Process

Nonwoven Filter Media

Mathematical Models of Flow Through Filter Media

Permeability of Clean Media

Nonwoven, Random Fibre Media

Woven Media

M&if&ment Cloth Permeability

Monofhment Cloth Permeab@

Filter Pore Cloth Bridging

Bridge Failure and Particle Bleeding

How Resistance of Used Media

Angular Velocity and Acceleration

Particle Velocity in a Centrifugal Field Force

Centrifugal Sedimentation

Designs

Simple Sigma Theory

Particle Collection Efficiency

Hindered Settling in a Centrifuge

Decanter Scroll Discharge Machine

Applications

Sigma Theory for Scroll Discharge Decanters

Power and Efficiency

Disc Stack Machine

Modified Sigma Theory

Design Calculation Examples

Hydrocyclones

Cut Point and Fractionation

Reduced Grade Efficiency

Velocities

Tangential Velocity

Radial Velocity

Axial Velocity

Locus of Zero Vertical Velocity and Mantle

Equilibrium Orbit Theory

Residence Time Model

Dimensionless Group Model

Numerical Solutions of Continuity and Flow

BatchDischarge Centd3ge Capacity

Continuous Discharge Machines

Selection of Filtering Centd%ges

Centd3ge ProductiVities

Filtration and Permeation in Centfigation

Wash Time

spin Dry

Practical Equilibrium Saturation Studies

Theoretical Filtration Rates in Centfiges

Centrifkges Cake Thickness Dynamics

Centrifkgal Filter Developments

Equipment Scale-up and Modelling

Models of Flux Decline

Equilibrium Flux Models

Diafiltration

Permeate Flux Maintenance and Regeneration

Applications and Investigations

Filter Productivity Fundamentals

Filter Cake Dewaterhg and Washing

Continuous, Largescale Filters

Rotating Drum

Drum Filter Productivity

Cake Discharge and Tbiclrness

Theory vs Practice: Rotary Vacuum Drum Filter

Dewatering and Washhg on the RVF

Rotary Disc Filters

Horizontal Filters

Tihing Pan Filter

Rotating Table Filter

Horizontal Belt Filter

Batch-Operated, Large-scale Vacuum Filters

Nutsche Filters

Pressure Filters

Filter Presses

Opthum Filtration Time Cycle

Practical Deviations fiom Theoretical Predictions

Fatration of Highly Compressible Materials

Variable Chamber Filters

Filter Cake Compression

11.4.3 Continuous Filtration and Expression

11.4.4 Pressure Leafand Candle Filters

11.5 References

11.6 Nomenclature

Appendix A

Particle Size, Shape and Size Distributions

Particle Size Analysis

Statistical Diameters

Particle Size Distributions

Calculus of Size Distributions

1 Solid Liquid Separation Technology

1.1 Introduction

It is di&xlt to identi@ a largescale industrial process which does not involve some form

of solid-hid separation In its entirety, the latter activity involves a vast array of techniques

and machines This book is concerned only with those parts of this technological diversity which relate to solid-liquid separation (SLS)

Attempts have been made [Svarovsky, 19811 to catalogue the variety of processes and machines used m SLS systems; these are usually based on two principal modes of separation:

1 Filtration, m which the solid-liquid mixture is directed towards a “medium” (screen, paper, woven cloth, membrane, etc.) The liquid phase or filtrate flows through the latter whilst solids are retained, either on the surface, or within the medium

2 Separation by Sedimentation or Settling m a force field (gravitational, centrifbgal) Wherein advantage is taken of differences m phase densities between the solid and the liquid The

solid is allowed to sink m the &id, under controlled conditions In the reverse process of flotatiq the particles rise through the liquid, by virtue of a natural or induced low “solids” density

The large range of machinery shown m Figure 1.1 reflects the uncertainty which attaches to the processing of solids, particulaxly those m small particle size ranges

The filterability and sedimentation velocity of such mixtures depend on the state of dispersion of the suspension; m turn, the latter is strongly influenced by solid-liquid surface conditions which govern the stability of the mixture and the overall result of particleparticle contact The properties of such systems may also be time dependent, with Hterability and settling rate being a function ofthe history ofthe suspension [Tiller, 19741

The dispersive and agglomerative forces present m these systems are functions of temperature, agitation, pumping conditions, etc all of which complicate the situation and produce the result that suspension properties cannot be explained m hydrodynamic terms alone Despite these formidable problems, modern filtration and separation technology continues to produce separations m seemingly mtractable situations, and to eliminate the

‘00ttle neck” characteristic of the SLS stage m many processes

A first step m the rationalisation of such problems is to choose the most appropriate technology fiom filtration, sedimentation or a combmation of these two operations In

general, sedimentation techniques are cheaper than those m v o h g filtration; the use of

gravity settling would be considered fist, particularly Where large, continuous liquid flows are

involved person, 19811

A small density di@aence between the solid and fluid phases would probably eliminate

sedimentation as a posaiility, unless the density difference can be enhanced, or the force field

of gravity mcreased by centrifbgal action Such techniques for enhancing sedimentation would

copyright 0 VCH Verlagsgesellschaft mbH, 1996

be retained as a possiiitity in those circumstances where gravity separation proves to be impossible, and the nature of the partidates was such as to make Wation “dif3icdt” The latter condition would ensue when dealing with small, sub-micron material, or soft,

compressible solids of the type encountered in waste water and other eilluents Some separations require combmations of the processes of sedimentation and iiltration; preconcentration of the solids will reduce the quantity of liquid to be filtered and, therefore, the size of filter needed for the separation

Having decided upon the general separation method, the next stage is to consider the various separational techniques available within the two fields These operational modes may

be listed as:

A Sedimentation: gravity; centrifhgal, electrostatic; magnetic

B Filtration: gravity; vacuum; pressure; centrihgal

Another serious consideration, also mdicated m Figure 1.1, is whether the separation is to

be effected continuously or discontinuously; the latter method is known as ‘0atch” processing In this case, the separator acts intermittently between filling and discharge stages The concentration of solids m the feed mixture and the quantities to be separated per unit time are also factors which afEect the selection procedure

This activity is made more complicated by the fact that the separation stage rarely stands

alone Figure 1.2 [Tiller, 19741 includes various pre- and post-treatment stages which may be

required m the overall SLS process Thus, the settling rate of a suspension, or its filterabdity, may require improvement by pretreatment using chemical or physical methods ARer

Wation, wet solids are produced, and these may require M e r processing to deliquor (dry)

the filter cake; m some cases, the latter, being the principal product, requires purification by washing with clean liquid

It will be apparent that in the development of a typical process for: (a) increasing the solids concentration of a d h t e feed, @) pretreatment to enhance separation characteristics, (c) solids separation, (d) deliquoring and washing, many combmations of machine and technique are possiile Some of these combmations may result m an adequate, ifnot optimal, solution to the problem Full OptimiSation would inevitably be time consuming and expensive, if not impossiile m an industrial hation Certain aspects of filter selection are considered at the

end of this chapter and in Chapter 11, on pressure filter process calculations

As stated above, a typical medium for the Wation of coarse materials is a woven wire

mesh which will retain certain particulates on the surface of the screen As the size of the particulates decreases, other “screens” are required, e.g woven cloths, membranes, etc ; these are constructed with smaller and smaller openings or pores Flow through such a system is shorn m Figure 1.3 Where the p d d a t e s are extremely small and m low concentration, deposition may o c w m the depths of the medium, such as m water clarification by sand filters

Filtration

Discharge

Discontinuous

Continuous Discontinuous

- Vacuum

Continuous Discontinuous

Grids: Sieve Bends Rotary Screen Vibratory Screen Nutsche Filter Candle and Cartridge

Table or Pan Filter Rotary Drum or Disc Filter Horizontal Belt Pressure Nutsche Plate and Frame Filter Tube, Candle & Leaf Filter Cartridge Filter

Belt Press

Screw Press Basket: 3 Column Centrifuge

Sedimentation

Stationary Wall Hydrocyclone

Centrifugal

Disc Centrrfuge Batch or Continuous (scroll)

Rotary Wall ~ ~ ~ ~ ~ ~ e n t r f i u g e

Figure 1.1 General classiftcation of SLS ecpipment

Washing, Drying

(Tests: Concentration vs number of

displacements: moisture content

vs time during suction; volume of

air during suction)

CHEMICAL

Coagulation, Flocculation

(Tests: Sedimentation Buchner

Funnel, Capillary-Suction Time,

Cake moisture content)

Displacement Repulping Increased Filter pressure

(Test: Moisture content vs Mechanical Squeezing (Test: Cake Thickness vs time)

(Test: Concentration vs number of Displacements) pressure)

Pretreatment I

I

Solids Concentration I

PHYSICAL Crystallisation Ageing Freezing Filter Aid (Tests:Bomb Filter Filtrate Clarity)

Pressure, vacuum, gravity filters

CLARIFICATION

Granular-bed cartridge

(Tests: pressure drop vs

time: rate vs: time: clarity vs time)

Precoat-drum filter

(tests: clarity as a function of knife Media plus precoat

(Tests: blinding, cake discharge, clarity)

(C) Continuous Batch (Tests: rate of cake-thickness buildup: filtration rate vs

pressure, cake-moisture content vs pressure)

I Post-Treatment I

t

4 l Filtering

The filtration medium may be fitted to various forms of equipment which, m turn, can be operated m several modes Thus, plant is available which creates flow by raising the fluid pressure by pumping, or some equivalent device Such “pressure” filters operate at pressure levels above atmospheric; the pressure differential created across the medium causes flow of fluid through the equipment Plant of this type can be operated at constant pressure difEerential or at constant flow rate In the latter case, the pressure differential will mcrease with time whilst at constant pressure, liquid flow rate decreases with time

Figure 1.3 Particle deposition in filtration

Subatmospheric operation or vacuum is used m a wide number of applications In these filters, the absolute pressure downstream of the filter medium is maintained it a low, controlled level by vacuum pumps The suspension supplied to the filter is delivered at essentdly atmospheric pressure; this process is an example of “constant-pressure differential”

filtration Vacuum operation is, of course, a special case of pressure filtration Here a

relatively small pressure differential is available, and attention should be given to pressure losses, e.g m the f5ctional effects caused by &ation flow m associated pipes and fittings

Low-pressure conditions m a flowing h i d may lead to the phenomenon of “cavitation” m

which dissolved gases, or vapor bubbles, are released mto the liquid Passage of such mixtures

through a downstream higher pressure zone, e.g at the outlet of a pump, causes bubble

collapse and material damage to the pump

Vacuum filters h d applications in many areas of industry and are widely used m

laboratory tests; the latter are required in order to assess the filterability of the suspension and

the suitabdity of a filter medium Whilst a complete quantitative description of the SLS

process will be descnied later, at this stage it is d c i e n t to record that the rate of flow of fluid, at a particutar pressure different@ will depend on the resistance to fluid flow of the

particles and the filter medim

Flow can also be created by spjnning the suspension, thereby creating a centdhgal force m

the system Thus, centdkgal filters, fitted with suitable filter media are found m many applications m the food, beverage, and pharmaceutical industries

flow rate and viscosity7 particle size and concentration, filter medium pore rating and their

effects on the tihation process

As mentioned m the previous section, the separation stage is rarely required m isolation and is often followed by drying or dewatering of the porous deposits andor purilication of

the recovered solids by washing Re and post-treatment processes such as flocculation, coagulation and liquid expression, may have equhalent process importance in determining,

sometimes controlling, the overall separation process time It is vital to identi@ process time requirements of the various phases invoked in a separation, m order to identify possiile bottlenecks Many examples could be quoted of installed filters with which it is impossible to meet process specifications, and this particukuly applies to systems with stringent dewatering and washing requirements This points to the need for well-designed pilot mer trials,

preferably on a small-scale version of the machine of interest, before plant selection It is, perhaps, unfortunate that such hzformaton is not h a y s available, and selection has to proceed with relatively meagre data

A successll selection procedure is closely linked to the proper choice of the medium to be used m the separation A large proportion of industrial-scale process di0iculties relate to the interaction between the imphghg particles and the pores m the fdter medium, as depicted in

Figure 1.3 The ideal circumstance, where all separated particles are retained on the surface of

a medium is often not realised; particle penetration mto cloth or membrane pores leads to an increase m the resistance of the medium to the flow of tihate This process can ensue to the level of total blockage of the systm Such difficulties can be avoided, if the pores m the medium are all smaller than the smallest particulate m the mixture processed, as discussed below

The theoretical and practical considerations required for effective SLS are expounded m detail m the various chapters of this book Of particular importance are the fundamental aspects presented m Chapter 2 Here, the “surface deposition” mode depicted in Figure 1.3 B

is descriied by two series resistances to f i d flow:

a) The resistance of the filter medium&,

b) The resistance of the particulate layer or “cake” R

The fdtrate velocity v, through the clean fdter medium is proportional to the pressure differential AP imposed over the me- the velocity is inversely proportional to the viscosity of the flowing h i d p and the resistance of the medium These relationships may be expressed mathematically as:

v, = M I p R ,

Under the same overall pressure m e r e n t 4 the filtrate velocity, &er the deposition of

particles, decreases to v,where:

detail m Chapters 6 and 10 on membrane technology

Filter cake resistances vary over a wide range, fiom fiee filtering sand-like particulates to

hi& resistance sewage sludges Generally, the d e r the particle, the higher will be the cake resistance The latter is sensitive to process changes m shmy concentration, fluid velocity, fluid pressure, temperature, etc These effects have received much attention [Shirato and

Tiller, 19871 in the development of physical and mathematical models of the SLS process

As mentioned above, post-treatment deliquoring and washing of filter cakes are subjects of

great importance m SLS operations These subjects are filly discussed m Chapter 9 A

principal mterest in deliquoring wet cakes lies m the economic merence between solids drying by thermal and mechanical methods The& drying costs can be much hiaer (20-30 times) than costs incurred by mechanical dewatering Dewatered solids are more eady handled than wet sludges; this is of particular importance in waste water treatment processes

A high solidity in a dewatered filter cake can reduce handling costs and improve the possiijlity of continuous processing

In filter cake washing, an important aspect is the time (or wash vohune) required to

remove residual impurities Quite often, washing may be the controlling step m overall

fdtration cycles, as discussed in Chapter 9

In certain circumstances, process conditions preclude the possibility of using direct filtration as a means of separating a solid-liquid mixture IFtgb dilution or extreme fineness of particle lead to uneconomic sizes of filter or, m some cases, makexormal filtration imposaile without pretreatment or concentration of the feed

Obviously, any device whicb, at relatively small cost, reduces the absolute amount of liquid

m the feed finds application m processes involving large tonnages Thus the ubiquitous thickener, used for increasing the solids concentration in dilute feeds, is found m almost every section of process industry

Here, gravity sedimentation, involving a difference m density between solids and liquids, is used to produce an essentially clear overflow of liquid and a concentrated underflow of the mixture m e latter may now be more r e a m filterable, or be m a condition suitable for disposal, e.g m sewage handling Thickeners are used extensively in hydrometallurgical

applications, singly or m series, e.g m counter-current decantation (washing) plants Where

gravity forces lead to inordinately long settling times, the latter may be reduced by chemical treatment, ie flocculation and coagulation, as discussed below

Quantification of the sedimentation process starts with the well-known Stokes relationshy

for the setting velocity of a single particle m an infinite expanse of fluid

where u, is the Stokesian gravitational settling velocity, d s ; x is the particle size, m; g is the

acceleration of gravity, m/s2 ; pJ, p are the densities of the solid and flui4 respectively, kglm3 ;

p is the viscosity, Pas This fundamental relationship must be modified m applications to practical designs of equipment, as show in Figure 1.4, to allow for the effects of particle concentration on the settling velocity of the suspension U,

Figure 1.4 High-capacity thickener (Eimco Process Eqmpment Co Ltd, United Kingdom)

In general, mcreases in Sludge concentration C lead to decreases m U,; it follows that the calculation of thickening processes, mvohring large mcreases m solids concentration, requires information on the U, - C relationship The overd mtention m these processes is to produce

an overflow of clarified water and an underflow of concentrated sludge

Suspensions which possess unique U,- C relationships are termed "ideal"; other mixtures, e.g., biosuspensions, may exhibit %on ideal" settling characteristics where settling rate may

be affected by suspensions height, sedimentation column diameter, intensity of mixing before settling, etc Such suspensions are often descriied by equations of the type:

where k relates to the settling velocity at low concentrations Both k and rn vary widely fiom

suspension to suspension Equation 1.4 mdicates that the settling velocity of a suspension is

inversely proportional to the solids concentration

Fundamental aspects of the above processes are considered m detail m Chapter 3

Appropriate tests are required to measure the effect of changes m concentration on U, It may

be observed that since the downward fhvr of solids m a settling suspension equates to the product of U, and C, the possiiility of a minimum flux presents itsel€ Identihation of such

minima is required m the specification of process plant used for sedimentation and thickening Modem sedimentors are sometimes Wed \;vith mclined plates, spaced at mtervals, as shown m Figure 1.5 Theoretical aspects of settlement under mclined surfaces are presented m

Chapter 3; practical details of the design of such equipment are dealt with m Chapter 7 Gravity sedimentors compete with devices such as sedimenting cenages, hydrocyclones, flotation cells, m the process area of ftuid cMcation and solids concentration

Figure 1.5 Lamella cladiedthickener (Svedala, Pumps & Process AB, Sala, Sweden)

Along with the selection of filtration machinery, attempts have been made to provide a basis for the selection of sedimentation equipment wake- 19941 Table 1.1 contains some of this informatioq m abridged form which points to the principal factors inDuencing such selections

Table 1.1 Selection of sedimentation machinery [wakeman, 19941

Gravity Sedimentation centrifuges Hydrocyclones Flotation sedimentation

Tubular Disc Scroll

efficiency in some cases, attempts to use inadequate filter media will incur certain fidure It

medium in SLS processes

A wide variety of media is available to the filter user; the medium of particah interest wdl,

of course, be of a type which is readily installed m the filter to be used m the process Thus woven and nonwoven fabrics, (Figure 1.6) constructed fiom natural or synthetic fibres, are often used m pressure, vacuum and centrifbgal filters Ag@ these units can be fitted with woven metallic cloths, particularly in those circumstances where mer aids will be used in the process The same materials, and also rigid porous media (porous ceramics, sintered metals, woven wires, etc.), can be incorporated into cartridge and candle filters In these applications, the rigid medium will usually be fashioned into a cylinder, although other shapes exist

Random porous media (sand, anthracite, filter aids) will be used m clarification processes These generahations can also be extended to flexiile and rigid membranes descriied in

Chapter 10

The filtxation mechanisms invoked in separations using such media will depend mainly on

the mode of separation Thus m “cake” filtration, ideally, impinging particles should be larger than the pores m the mediutu Expyence shows that less processing &culty is experienced

m circumstances where media pores are much smaller than the particles

Figure 1.6 Woven and n o n w e n filter media

Despite the obviously higher Ihid flow resistance of tighter media, m practice, the eventual, used medium resistance will be more acceptable In clarification systems m v o h g depth filtration, loose media are used which are often associated with pores thousands of

times larger than the particles requiring filtration However, deposition of the movjng solids

onto the medium does occur, and clarified liquids are obtained Such separations depend on

the surfice condition and area of the media used m deep-bed systems

Chapter 4 explores some of the features of filter media, particularly those of the woven fabric variety The chapter is aimed at developing an understanding of the steps required m

media applications to attain process features such as:

(a) clear filtrates

(b) easily discharged filter cakes

(c) economic &ration times

(d) absence of media ‘%linding”

(e) adequate cloth lifetime

Filter media behaviour is also reported m other sections of the text, e.g Chapter 10 deals with the crucially important area of membrane separations Again, Chapter 6 deals with depth filtration systems such as deep sand filters and cartridges, and descriies the media used m

such equipment

Some aspects of media selection are also covered m the sections of Chapter 2 which hi&ght laboratory test procedures Certainly much can be gained fkom well-designed laboratory tests m filter media selection It will also be realised that a vast reservoir of

experience and information is available from filter media manufacturers who, fomately, continue to report their knowledge in the filtration literature

In this respect, it is interesting to note the newer developments m this subject Thus m

systems where the SLS process calls for filtration and deliquoring of the filtered solids, modem media are available (m woven and ceramic form) which prevent the leakage of gases through the system Thus Figure 1.7 shows the different behaviour of a modern “capillary

control” me- which allows the fkee flow of liquid durhg filtration and dewatering, but prevents the flow of air used m the last step This leads to considerable economies m vacuum production [Wuf and Miller, 19901 Chapter 11 deals with the influence of the filter

1.6 Pretreatment Techniques

In sedimentation systems dealing with very small particles, the action of gravity alone may

lead to inordinately long settling times, as reported (Table 1.2)

Table 1.2 Effect of particle size in gravity settling in water at 2OoC

Type of suspension Particle size range Time required to settle

1 S-13 s

13 S-20 h

20 h-20 a

* Assuming quiescent conditions and a solids density of 2650 kg/m3

The p e m e n t suspension of colloidal materials follows fiom the presence of electrically charged ions on the d c e of the particles These ions may be adsorbed fiom solution, or produced by part-solubilisation of the particle d c e Colloids, with particles less than 1 pn

m size, possess large surhce area to mass (or volume) ratios This property enhances the mutual repulsion of charged particles and prevents sedimentation In these systems, forces of

attraction also exist on the surface of the solids These are of an electrostatic nature, which act over very short distances fiom the surhce of the particle Stabilisation follows fiom the action of the repulsive forces in preventing the particles coming close enough for the

attractive forces to be operable, despite their random Brownian motion resulting fiom bombardment by water molecules

Elimination of the repulsive forces, and a “destabilisation” of the colloidal suspension may

be achieved by the addition of certain chemicals Such processes are of enormous importance

in municipal and industrial water clarification plants which contain sedimentation equipment

of the type shown in Figure 1.4

In addition to hster settling, a destabilised system will generally possess enhanced filtration characteristics These improvements follow fiom the increased possiility of particulate collisions, with the production of “clusters” or “floes" of particles The clusters wdl have a greater effective diameter Thus fiomthe Stokes relation+:

possessing poor filtration characteristics The process of coagulation may be enhanced by the action of synthetic long-chained, high molecular weight polyelectrolytes, of the type shown m Figure 1.8 These act to increase the size of the coagulated clusters, thereby increasing

sedimentation velocities and filtration rates of the suspension Similar action may be effected

by the use of naturally occurring polyelectrolytes, such as starch, gums, etc

Anionic Partially Hydrolysed Polyacrylamide:

Cationic Polyacrymide, Partially Substituted

Quaternary Ammonium Group:

Figure 1.8 Synthetic organic polyelectrolytes

Cluster growth proceeds firstly by “perikinetic” flocculation This mvohes quite small

species, submicrometre m diameter, and interception by Brownian motion Later

“orthoh&c” collisions caused by fluid motion, induced by the action of mixing, become more significant The importance of induced shear, m practical flocculation devices, for the production of stable clusters is discussed m Chapter 5, along with the different molecular mechanisms mvohed m coagulation and flocculation [Akers, 19861

Briefly, m coagulation, charge neutralisation and compression of the effective range of repulsive forces are related to fktors such as p e silt valency and ionic composition In

flocculation, it is believed that physical attachment of one part of the long-chained polyelectrolyte, followed by fiuther attachments to other particles, causes a bridging action fiom particdate to particulate To be effective, the polymer has to be extended m length during the flocculation step After bridging, the polymer coils together, thereby producing a large particle fiom several sma.Uer units

The reduction m &-ation resistance, referred to above, may also be achieved by adding a certain amount of fiesfiltering particles to the feed suspension Filter aids are naturally

occurring materials such as diatomaceous earths (kieselguhr), perlites (volcanic ash), cellulose, and carbon which are haracterised by a low resistance to the flow of fluids and, when deposited as filter cakes, 9 e relatively mcompressible The filter cakes produced fiom such substances are open, porous structures of hrgh permeabihty Various grades of filter aids are available, with the finest grades required for the retention of sub-micrometre particles [Akers, 19861

Perhaps the most common application of filter aids is m the cladcation of valuable liquid products, in circumstances where the latter are contaminated with small quantities of

suspended impurites Thus in beverage products, sheglife and product appearance will

require stringent control of product clarity Here the filter aid must be suitable, chemically and physically, for use m food products

Filter aids are applied m two ways: (a) as a pre-coat, where a thin (approximately 2 m)

layer of aid is deposited on the filter medium, before the commencement of filtration, or (b) as

a ‘%body feed” where a certain quantity of aid is added to the feed s€urry The latter procedure

is adopted m cases where the suspended solids have poor filterabilities and would form an jmpervious layer on the suzface of the filter medium, or even the filter aid pre-coat The combmed deposit of filter aid and suspended impurity possesses a much reduced filtration resistance Usually an optimum dosage of body feed is sought, m overall cost terms, as

dismssed m Chapter 5

Clarification of liquids often calls for the use of depth filtration equipment The process techniques used and the filtration mechanisms mvohed m depth filters are outlined m Chapter

6, which mcludes information on sand filters and cartridges

The effectiveness of the simple sand filter can be improved by superimposing a layer of anthracite on the surface of the sand layer This is shorn in Figure 1.9 where a dual layer of anthracitelsand is contained m a pressure vessel The anthracite particles, being coarser than

the sand, serve to prevent the formation of surface deposits on the sand suzface In low- pressure systems, avoidance of these deposits leads to longer operating cycles In some cases, dense solids, e.g., alumina will be used as a third layer, situated beneath the sand A general guide to the performance of these filters is provided m Table 1.3

Cartridge and diatomaceous earth pre-coat filters are often used as second-stage polishing fihers downstream of sandgravel units The information m Table 1.3 should be taken as a general guide only, since specific separations may involve quite difGerent r e d s ; the data below pertain to the cleaning of seawater In some cases, the above clarifiers are supported by upstream screens or strainers which are designed to remove larger detritus fiom the feeds Some idormation on screening operations is mcluded m Chapter 6

The relatively high efficiency of deep-bed filters in removing h e particles present m low concentration has generated mterest m industrial processes where water reclamation or polishing fiee of suspended morganics is a princjpal concern For exiunple, the removal of suspended salts fiom brine prior to electrolysis is an essential feature m chlorine production; the reduction m suspended solids from factory efltuents reduces sewage costs and in some

cases, provides recycle water to the process In the latter case, a reduction in supplied water

to the factory produces further economies The high capacity available m the modem sand

filter, particularly when polyelectrolytes are used to enhance capture, offers the possiihty of use m cases where conventional equipment could not produce a separation, or would be too

‘ Range of surface forces less

than thickness of this line

I Backwash

Filtrate

Filtration Stage Backwashing Stage

Figure 1.9 Dual layer deep bed filter [rves, 19821

Table 1.3 Clari.tjring filter performance data on seawater

Filter Design Maximum Average suspended Retained

flow rate, suspended solids solids, after particles

Water is passed through a granular bed of sand and anthracite, (Fig 1.9) in which the

diameter of the pores is 100-10 000 times larger than the diameter of some of the suspended particles 4 a r i fiom mechanical interception of the particles, determined by their size relative

to the pore size of the bed, interception due to electrostatic and Londodvan der Waals attractive forces also takes place The latter mechanism dominates for finer m a t e d Flow

may be directed downwards or upwards, with typical liquid velocities of 40 and 15 m/h,

respectively

During the percolation, the filter bed is contaminated with the substances filtered of€ At fixed times, the filter has to be rinsed clean in counterflow with water (back-wash); the filter bed is fluidised by the latter, quickly releasing accumulated particles deposited during the mation stage

The size of particles which are mechanic@ intercepted by granular bed filters can be

estimated The size depends on the smallest size of the filter bed material and the pore size of the filter cake formed During the initial stages of fi€tration, when no filter cake is formed, h

may be expected that particles with median diameters larger than 1/3 of the median pore

diameter of the finest filter bed will bridge and form an external cake on the surface of the

filter Particles smaller than 1/3 but larger than 1/7 of the filter bed pore size will form an internal cake, while the smaller particles will pass through the filter, or be retained by surface forces

Clarification of water in deep layers of granular material has been studied extensively and methods are available for the design and optimisation of these high-capacity units The enormous capacity of the deep-bed filter is related to the large surface available between the solids (sand, anthracite, gravel) and the flowing fluid The removed partidtes (sand, algae, clay, etc.) cause an increased resistance to flow; m systems where a limited fluid pressure is

available, this clogging manifests itselfin the gradual drop in flow rate In the cases where fluid is pumped under a substantial pressure, deposition causes a gradual buildup of pressure over the systq this m tum can cause solids to be scoured forward into the int&or ofthe sand Optimisation design procedures have been based on the model that the breakthrough of contaminants into the &ate should coincide with the attainment of an upper, limiting available pressure merential This involves simultaneous caldtion of the changing concentration and pressure prosles in the liquid; these topics are developed in Chapter 6

Designs vay, and must be related to the particular fihration problem in hand

Cartridge filters are used widely throughout process industries in the clarification of liquids The media used include: yarns, papers, felts, binder-fiee and resin-bonded fires, synthetic fibres, woven wire, sintered metal powders and fires, ceramics, etc The inclusion

of membraneous materials, Chapter 10, in cartridge constructions has extended the range of

application of these ubiquitous elements so that particles fiom approximately 500 pm down

to 0.1 pm are separated

These filter elements are usually constructed in qhdrical form, as shown in Figure 1.10

Filter housings, which may contain single or multiple cartridges, have to be designed with

particular attention to sealing of the media, and to the avoidance of “dead spots” in the filter Accumulated solids in the latter may be spontaneously discharged by sudden flow or pressure changes

Great care is taken in cartridge preparation and testing, in order to guarantee the performance of the medium in its capabdity of removing particles of a declared size fiom the filtered liquid Cartridges are rated in terms of their capacity to remove particles; these ratings

may be “absohte” or ‘hominal”, as discussed m Chapter 6 The passage of particles through the cartridge mto the filtrate is known as ‘0leeding”; cartridges are guaranteed to prevent the bleeding of solids greater than a certain size In fibrous media, an equivalent interest is given

to the prevention of fibre shedding In poorly prepared media, fibres may be torn off the cartridge under the S e n c e of fluid drag during filtration It follows that a poorly sealed assembly will present similar problems with bleeds

Figure 1.10 Cartridge filter elements (Courtesy: Vessel SRZ, Buccinasco, (MI), Italy)

Some of the thinner media, e.g paper, woven cloths, etc may be designed to separate solids on the &ce of the medim Such filters are often pleated, m order to mcrease the available filter area per cartridge, and, therefore, to mcrease the “solids holding capacity“ of the filter Depth cartridges can be constd’aed with radial variations m fire density; so that filtration ensues throughout the depth of the filter Particle removal efficiency wiU depend upon, inter alia, the relative sizes of particles and fibres, as discussed m Chapter 6 W e often, mixed fibres are used, of varying diameter, m order to improve particle capture Activated carbon which adds an adsorptive capability to the cartridge, finds widespread use for colour removal and odour control

filter, thereby increasing filter resistance Solids deposition continues up to an acceptable AP,

after which the cartridge is disposed of and replaced The solids deposited at this pomt, e.g

20 g per 40 cm long cartridge, is an important design feature of such filters High solids loadings will require the use of le parallel units The frequency of cartridge replacement

wt%) m the feed, with a corresponding small volume (100 1) of filtrate Filter testing, therefore, centres upon: (1) iiker integrity; (2) pressure diBerential-time relationships (solids loadinghapacity): and (3) particle removal rating

Filter integrity specifications refer to the availability of effecting sealing and the absence of

&re shedding The ability of the media to withstand large pressure difFerentials and high temperature is also of importance m some industries, e.g polymer processing Here, molten polymer of very high viscosity and temperature must be processed at extremely high AP It follows, in these cases, that rugged constructions are reqked, e.g the sintered metal fibre or powder media referred to above Cartridges of this type are quite expensive, and cannot be considered as disposable Such units are recycled frequently7 after rigorous cleaning

New, or recycled filters, are subjected to physical nondestructive testing, e.g permeability and bubble-pomt tests In the latter7 the gauge pressure required to cause the passage of air

bubbles through the iiker is measured whilst the cartridge is submerged in a “wetting fluid”’,

e.g isopropyl alcohoL This measurement may be used to calculate the diameter of the filter pore d, fiom the measured pressure diEerence dp fiom the equation:

is controlled by limiting the appli 7? ation of these units to low concentrations of solids (<0.01

where y is the surfkce tension of the ‘betting" liquid and k is a constant which depends on

the shape of the pore; see Equations 6.114.14, Chapter 6 This measurement and that of the

S d flow through the filter at a prescribed dp (used to calculate the “penneabw of the cartridge) can be compared with acceptable standards of the filter m question

As desmied m Chapter 6, particle capture efficiencies are determined by “destructive7’ flow tests involving the removal of standard particles fiom flowing fhids These tests may accompany others to measure bacterial retention, release of materials (extractables), and chemical compatibihty

Chapter 7 returns to the separation of solids by sedimentation methods, dealing with the practical details of settling tests required m dilute and concentrated suspensions Sedimentation processes can be clasaed into four types:

Type I Clarification of dilute solids

Type II Clarification of dilute flocculated solids

Type III Mass settling of high concentrations of flocculant solids

Type IV Compression of settled solids

Type 1 settling systems contain a t e suspensions of discrete particles which are stable, dqersed and characterised by the absence of flocs These systems may be designed using the

process equations presented in Chapter3 Type 11 suspensions require experimental measurements of the clarification rate; this follows fiom the fact that the removal rate depends

on the depth of the settler Larger flocs possess higher settling velocities than smaller flocs; collision of flocs tends to increase floc size, with a corresponding increase in settling velocity This leads to the use of long-tube tests to simulate the action of cMers, as described in

Chapter 7 Short-tube tests can be used m circumstances where the separation is governed by

the settling rate of the solids only In these tests, small batches of the suspension are contained

m vertical glass cylinders The height of the interfice between the settling solids (type m) and the clear supernatant liquid is measured as a fhction ofthe, as shown m Figure 1.11

Most of the difEculties encountered in the development of a sound design basis for sedimentation units handling type III and IV systems, centre on the problem of applymg batch test information to continuous separation Chapter 7 presents a review of these methods and their application to the specification of continuous sedimentation equipment Obviously, the use of continuous, small-scale clarifier-thickeners provides a sounder technical base for process scaleup to industrial size w a t i i and Howell, 19791

A detailed description is also presented on the reverse operation of flotation Some solid materials may be present in the feed suspension which are less dense than the mounding fhid Thus waste water is oRen contaminated with quantities of oil and grease These substances will have a natural tendency to float on the surface of the feed water Advantage may be taken of this effect by providing scrapers to remove the floating phase The flotation process may be accelerated by the introduction of air bubbles into the feed; attachment of air

bubbles to the contaminants will cause the latter to rise more quickly In mineral processes, solids which would normally sink in water can be caused to float by bubble attachment, after chemical modification of the solid surfaces

The efficiency of the attachment process depends upon the size of the gas bubbles Process equations are available for predicting the course of such separations; these design equations require the support of laboratory tests to provide relationships between flotation velocities, the volume of air per unit mass of solids, etc This chapter also deals with the design of inclined ‘lamella” separators which tend to intensify the sedimentation effect, thus requiring smaller vessel volumes when compared with conventional equipment

It is of interest to record the modem trend to the use of process systems containing two process functions m one vessel Thus flotation vessels are now available which are combined with settlers, as show in Chapter 7, or deep-bed iilters, as depicted in Figure 1.12 The use

of centrifbgal separation, in augmenting the sedimentation of small particles and in providing another means of separation by filtration, is outlined in Chapter 8 This section divides naturally into two: (a) centrifbgal sedimentation and (b) centrifbgal filtration

In the specification of sedimentation machinery idomation is required on the volumetric throughput of the feed suspension Q (m3/s), the settling characteristics of the particles ut (m/s), as descriied by the Stokes relationship, and the separating ‘bower’’ of the sedimentor The latter is provided by a so called, sigma fictor C (m’) [Ambler, 19521, which, in tum,

contains variables such as rotational speed o ( Us), machine dimensions, etc Typical solid-

bowl units, with a scroll discharge device rotating coaxially with the bowl are shown in

Figures 1.13a and b These units are sometimes called decanters

An elementary approach to deriving the process relationship between Q, u, and C starts

the centdkgal acceleration a r e d i n g fiom rotation at a speed o and radius r Thus a = w2r

machines relates to the high v h e s of Z which are avahble, either &om the high rotational

speed of such devices, or fromthe mclusion of settling plates or lamellae, m the high-powered

figure 1.12 Combined flotation cell/ deep bed lilter (Courtesy: Krofta Eng Corporatio~ USA)

A) Raw water inlet; I) spiral scoop; P) Sludge outlet;

B) Hydraulic joint; J) Flotation tank; Q) Chemical addition;

C) Inlet distributor;

F) Static hydraulic f l d a t o r , N) Air Compressor;

G) Air dissolving tube, 0) Centre sludge collector;

K) Dissolved air addtion; R) Sand lilter beds;

D)Rapidmixing; L) Bottom carriage; S) Indivictual clear wells;

LJ) Clear efluent outlet, V) Travelling hood H) B x k - m h pump^;

Several varieties of cenbifbgal filter are described in Chapter 8 Both batch-operated and continuous filters are available 9 the range of machines listed m Table 1.4, the latter also refers to the use of solid-bowl sedunentors for high-resistance mer cakes, e.g sludges

Figure 1.13 Sohd-bowl (a) and screen bowl (b) decanter centrifuges (Courtesy Thomas Broadbent and Sons

Ltd., Hudderdield, LJK)

Table 1.4 Centxifugal filter and sedimentors

B: batch, C: continuous filterability

In the table the ''specific'' filterability of the cake a, is related to the overall cake resistance

R (Eq 1.2) as discussed m Chapter 2 Some of these machines are also suitable for lilter cake dewatering and washing processes

1.9 Washing and Deliquoring

Post-treatment of mer cakes, or settled dudges, is of great importance m process industries and often occupies much process time The semoval of residual liquid fiom packed assemblies of particles, or the reduction in solute contamination m the residual liquid phase can be a slow, inefficient processes The main d i f E d e s arise when the material to be removed is trapped m inaccessiile parts of the deposit

the bed will be of a random nature, thereby creating the p o s a i i of bypassing “residual”

liquor in the cake Chapter 9 details such processes and presents process equations for the calculation of the amount of liquid remaining m the filter cake dusiag air displacement processes

The residual liquor is reported in terms of the “saturation” of the filter cake Saturation S is defined as:

Volume of liquid in filter cake

Volume of pores in filter cake

bed Such liquid cannot be removed by gas flow; M e r decrease below S, will proceed by

drying, which involves slow, difbsional mass transfer into the gas phase In large-scale,

continuous filtration systems, use of a heating medium (superheated steam) is sometimes proposed as a means of improving the dewatering (drying) kinetics Thus m horizontal belt filters (Chapter 1 l), the moving filter cake is surrounded by a blanket of steam, prior to cake

The latter chapter also reports machines which are designed to squeeze the filter cake,

aRer or instead of gas dewatering In these circumstances, the actual volume or porosity of the filter cake is reduced, thereby expelling residual liquor Thus m batch operated, variable chamber filters, the process of squeezing and blowing can re& m material reductions m S,

levels Combmed vacuum and compression devices are available for continuous vacuum

The process of ater cake washing is depicted m Figure 1.15, where the concentration of

solute m the liquor emanating downstream of the filter has been recorded as a hction of time The volume of the initial liquor will be determined by the porosity of the filter cake Delivery of the same vohme of wash fluid equates to “one void volume” of wash In an ideal

would be recorded on the washing w e shown m Figure 1.15 In practice, breakthrough of the flowing wash liquor leads to void vohme requirements of 2 or 3, even in fivourable circumstances Where sohte is trapped, even larger void vohunes of wash may be necessary; this leads to long wash times and lower plant productivity

wash volume

Irigure 1.15 Concentdoetime mhmg curve: Solute concentration vs time

Fundamental considerations pomt to the importance of dispersion m its effect on wash time and wash vohme requirements The inhence of system parameters such as cake thickness, particle size and Wiutioon, sohte &sVity, etc combme to produce either the optimal step function m the washing w e , or an inefficient “tad“ This information is of great importance, in view of the dominant role played by washing operations, where the latter are necessary

As would be expected, the conditions at the particle surfacefluid interface have important effects m dewatering and washing operations Thus the deliquoring rate is known to be a hction of the pH level m the system The presence of surfactants can also alter these processes quite seriously, as discussed m Chapter 11

1.10 Membrane Filtration

Membrane technology has expanded at an incredible rate over the past twenty years, and now embraces a multi-billion dollar industry Applications can be broadly divided into processes for (a) suspended particles and (b) dissohred solids Thus the filtration of particles in the size range 0.1-10 p q using relatively open membranes is descriied as microfiltration

Membranes for the removal of dissolved species are necessarily tighter, in pore Size terms,

than the microporous variety Table 1.5 contains information on membranes used for ultrafiltration (UF) (0.001-0.02 pm) and reverse osmosis (RO) (1-10 A)

(W)

Table 1.5 Membrane filtration media

Microfiltration Suspended particles 0.02- 10 200-lo5

Ultrafiltration Colloids macromolecules 0.001-0.02 10-200

“spherical proteins” Reverse osmosis Dissolved salts 0.000 1-0.00 1 1-10

In RO processes, the pumping system has to overcome the osmotic pressure of the salt in water This leads to the necessity for large pressure drops (25-70 bar) across RO membranes

in order to achieve acceptable filtrate rates In contrast, MF and UF processes operate at relatively low pressures (0.07-7 bar)

Chapter 10 descnies the various module coniigurations into which membranes are Wed: flat sheet, tubular and capiUary type The latter, in hollow fibre form are fistrated in Figure 1.16

Conventional filtration involves shury flow “dead-end” into the filter, ie the flow of fluid

is perpendicular to the &ce of the medium The subsequent accumulation of colloidal or submicrometresized particles are particularly diflicuh to filter and retention of such particles often culminates in plugging of the filter medium-

Dead-end filtration in UF and RO processes, results in an accumulation of dissolved sohtes on the d c e of the membrane, thus producing an effect called “concentration polarisation” The overall effect is to produce a gradual decline in filtrate flow with time The increased concentxation of dissolved species or colloids at the membrane d c e can

be ameliorated by causing the feed to move acrms the filter d c e , rather than normally

towards it The accompanying fluid shear at the membrane surkice results m a sweeping away

of accumulated deposits Unfortupately, even at a high cross-flow velocity (3-5 d s ) , some deposit persists on the surkice