Soli Liquid Separation

64

Solid–Liquid Separation

64.1 Unit Operations in Solid–Liquid Separation

Screening • Sedimentation • Centrifugation • Hydrocycloning • Flotation

64.2 Equipment

64.3 Fundamental Concept

64.4 Design Principles

Cake Filtration • Ultrafiltration

64.5 Economics

Solid–liquid separation plays a key role in nearly all manufacturing industries, including chemical,mineral, paper, electronics, food, beverage, pharmaceutical, and biochemical industries, as well as inenergy production, pollution abatement, and environmental control. It also serves to fulfill vital needsof our daily life, since we must have cartridge oil/fuel filters for operating an automobile, a paper filterfor the coffee machine, a sand filter bed for the municipal water treatment plant, and so on. In fact,modern society cannot function properly without the benefit of the solid–liquid separation.

Technically, solid–liquid separation involves the removal and collection of a discrete phase of matter(particles) existing in a dispersed or colloidal state in suspension. This separation is most often performedin the presence of a complex medium structure in which physical, physicochemical and/or electrokineticforces interact. Their analysis requires combined knowledge of fluid mechanics, particle dynamics, solu-tion chemistry, and surface/interface sciences.

Although the industrial equipment classified as solid–liquid separation devices are too numerous tobe cited individually, it is generally accepted that these may be grouped into six categories of unitoperations: (1)

screening

, (2)

sedimentation

, (3)

centrifugation

, (4)

hydrocycloning

, (5)

flotation

, and(6)

filtration

.

64.1 Unit Operations in Solid–Liquid Separation

A description of each unit operation in solid–liquid separation is presented in this section with theexception of filtration. Liquid filtration, one of the most commonly used industrial operations, is dis-cussed separately in subsequent sections to illustrate the fundamental concept and design considerationsfor solid–liquid separation.

Screening

Screening is the simplest mechanical operation to separate solid particles based on their sizes. Whensolids are placed on a screen, particles smaller than the screen opening pass through while the largerparticles are retained by the screen. In this manner, feed solids can be separated into two different parts,

Shiao-Hung Chiang

University of Pittsburgh

1586_book.fm Monday, May 10, 2004 12:54 PM

64

-2

The Engineering Handbook, Second Edition

namely, the undersized and the oversized portions. Often two or more screens of graded openings areused in series to separate a material into different size fractions. In many instances, screening is used asan analytical tool to determine particle size distribution in a sample of solid material. The particle sizedistribution can be used as a basis for equipment selection in solid–liquid separation (see Section 64.2below). In addition to size separation, an important usage of screening is to perform mechanical dewa-tering (often combined with washing) of solid materials [Svarovsky, 1985]. For industrial applications,screens are made of various metals in the forms of wire mesh and slotted or perforated plates. Theopenings of standard screens range from 10 cm (4 in.) down to as small as a few micrometers. In screeningoperations, mechanical vibration and shaking are often applied to the screen surface to enhance theeffectiveness of separation [Perry and Green, 1997].

Sedimentation

Sedimentation is a unit operation designed to separate suspended solid from a liquid stream by particlesettling under the influence of a body force, most commonly gravity. From an operation standpoint,gravity sedimentation can be divided into two basic types: clarification and thickening. The objective ofclarification is to remove small quantities of suspended particulates from the liquid stream to producea clarified effluent or overflow stream. On the other hand, thickening is to concentrate dilute suspensionsfor their subsequent treatment in filters or centrifuges.

The settling behavior of suspended particulates in a gravitational field is affected by three factors: theparticle size, the solid concentration, and the aggregation status of particles. In a dilute suspension, thesettling solid behaves as individual particles and the process is regarded as

particulate

or free settlingregime.

Most clarifier operations fall into this regime. As the solid concentration increases, the suspendedparticles have more chances to approach each other closely and to form aggregates. Once the concentra-tion reaches a level at which the suspended particles settle as a mass, the corresponding sedimentationis known as

hindered

or

zone settling.

In this regime, the settling behavior is related more to the solidconcentration than to the particle size. As the solid concentration increases further, a settled bed ofsediment mass is compressed by the overburden of sediment on top of it. Liquid is expressed from thelower sediment layers and flows upward through the sediment. This regime is termed

compression regime.

Sedimentation with the addition of chemical flocculant usually falls into this regime. A feed suspensionin a thickener (or clarifier) can be operated in any regime. Therefore, the design of sedimentationequipment must consider all three regimes.

Figure 64.1 shows a schematic diagram of a thickener that exhibits three distinct zones: a clean liquid(or clarification) zone at the top, a compression zone at the bottom, and a transition zone in between.The thickener consists of several basic components: a tank to contain the slurry, a feed well for feedsupply (with or without flocculant), a rotating rake mechanism, an underflow solids-withdrawal and anoverflow launder. In addition, an underflow recirculation system (not shown in the diagram) is oftenused. The physical size of a conventional thickener can vary from a few meters to more than 100 metersin diameter. For the operation of large vessels, careful consideration must be given to the design of thesupporting structure for the rotating rake mechanism and the control scheme for liquid levels and flowrates. Detailed descriptions of major components and instrumentations used for different types ofthickeners can be found in the literature [Perry and Green, 1997; Schweitzer, 1997].

Centrifugation

Centrifuges are equipment that employs centrifugal force for the effective separation of solid–liquidsuspension. The centrifugal force used in such equipment ranges up to 10,000 times the gravitationalacceleration. Liquid–solid separation centrifuges can be broadly divided into two types:

sedimentationcentrifuges

and

filtering centrifuges.

Due to its much stronger force field,

sedimentation centrifuges

can be used to separate very fineparticles as well as emulsions, which might normally be stable in a gravitational field. These centrifuges



64

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are not usually sensitive to feed solid concentration because theliquid does not have to move through the solids or a medium. Inorder for a particle of a given size to be removed from the liquid, asufficient time should be allowed for the particle to settle and reachthe wall of the separator bowl. For example, in a simple tubularcentrifuge, as shown in Figure 64.2, the bowl consists of a verticaltube with a large height to diameter ratio, which rotates at a highspeed about its vertical axis [McCabe et al., 2001]. The feed point isat the bottom and the liquid discharge is at the top. The incomingsuspension starts to rotate with the bowl, and its angular velocitywill soon become identical with that of the bowl. There is thereforeno tangential flow in the bowl. The rotating liquid moves upwardthrough the bowl at a constant velocity, carrying solid particles withit. In the meantime, under the influence of high centrifugal forcesthe solid particles begin to settle toward the wall. The total settlingtime is limited by the residence time of the liquid in the bowl. Atthe end of this time if the particle does not reach the wall, it leavesthe centrifuge with the liquid. Only those particles that reach thewall within the residence time are removed from the liquid.

Filtering centrifuges

separate solid particles and liquid from asolid–liquid suspension by employing pressure resulting from thecentrifugal action to force the liquor through the filter medium,leaving the solid particles behind [Zeitsch, 1990]. The density dif-ference between the solids and the liquid, which governs the sepa-

FIGURE 64.1

Schematic diagram of thickener operation.

OverflowLaunder

Solid-Withdrawal

Underflow

Clean Liquid Zone(Clarification)

Feedwell

Rake

Feed Suspension + Flocculant

Transition Zone

Compression ZoneSettling Solids

Overflow

Tank

FIGURE 64.2

Tubular centrifuge.(

Source:

McCabe, W. L., Smith, J. C.,and Harriott, P. 2001.

Unit Opera-tions of Chemical Engineering,

6thed., Figure 29.36, p. 1049. McGraw-Hill, New York.)

Feed

Solids

Liquid Liquid

Motor

Rotating Bowl


64

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ration in the

sedimentation centrifuges,

is no longer a prerequisite. The common feature of all filteringcentrifuges is a rotating basket having a slotted or perforated wall covered with a filter medium, such ascanvas or metal cloth, on which the deposited solid particles form a filter cake. Thus, the centrifugalfiltration can be viewed as a cake filtration process under constant pressure (due to centrifugal acceler-ation). The rate of filtration can be calculated using the two-resistance model for cake filtration (seeSection 64.4 below).

Hydrocycloning

The hydrocyclone is another device using centrifugal force to separate solids from liquid based ondifferences in density and particle size. A typical hydrocyclone consists of a cylindrical section and aconical section (with no internal rotating parts) as shown in Figure 64.3. An external pump is usedto transport the liquid suspension to the hydrocyclone through a tangential inlet at high velocity,which in turn generates the liquid rotation and the necessary centrifugal force. The outlet for the bulkof the liquid is connected to a vortex finder located on the axis of upper cylindrical section of thevessel. The underflow, which carries most of the solids, leaves through an adjustable opening (apex)at the bottom of the conical section. It should be noted that the solid–liquid separation in hydrocy-clones is never complete because there is always a significant amount of liquid discharging with thesolids through the underflow. This feature limits the applications of hydrocyclone to clarification andthickening. In some cases, the hydrocyclone is also used as a classifier to separate suspended particlesinto different size fractions.

The internal working of a hydrocyclone is best described in terms of a double spiral liquid flow patternwithin its body [Kelsall, 1952; Rushton et al., 2000; and Svarovsky, 1985]. A schematic view of the spiralflow inside a hydrocyclone is shown in Figure 64.3(b). Liquid on entry commences downward flow inthe outer regions of the hydrocyclone body. This combined with the rotational motion to which it isconstrained creates the outer spiral. At the same time, some of the downward-moving liquid begins tofeed across towards the center. The amount of inward motion of liquid increases as it approaches thecone apex. Liquid in this inward stream ultimately reverses its direction and flows upwards to the cycloneoverflow outlet via the vortex finder. The reversal applies only to the vertical component of velocity, andthe inner spiral rotates in the same circular direction as the outer one. Wall friction causing “obstruction”of tangential velocity results in a nontangential motion. Consequently, a strong axially directed currentoccurs near the wall, which carries solid particles to the apex opening and out of the hydrocyclone. Thus,it achieves the desired solid–liquid separation.

FIGURE 64.3

Schematic diagram of hydrocyclone: (a) principal features and (b) flow pattern.

FeedInlet

CylindricalSection

ConeSection

Apex Underflow

Inner Vortexwith Air Core

OuterVortex

Feed

OverflowVortex Finder

(a) (b)



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Flotation

The use of bubbles to float fine particles in a liquid is commonly known as

flotation process.

Such aprocess consists of attaching gas bubbles to the suspended solid particles to alter their apparent densityfor selective levitation of particles to be separated. The flotation operation involves not only the adhesionof small particles to gas bubbles, but also the collection of the gassy particles in the form of froth. Thus,the bubble flotation is also named as

froth flotation.

The flotation process is fundamentally different from other mechanical separation techniques in thatflotation is a surface property-driven process, which depends upon complex phenomena occurring atthe interface of solid particles and gas bubbles [Fuerstenau et al., 1985; Jaycock and Parfitt, 1981; andZettlemoyer, 1969]. In a bubble-particle attachment process, the tendency of the particle to replace itssolid–liquid interface by the solid–vapor interface is termed hydrophobicity or floatability. If a surface iscompletely wetted by water, it would be denoted as high-energy surface (i.e., hydrophilic). Most metalsand minerals exhibit high-energy surfaces. On the other hand, hydrocarbon surfaces are low energetic(i.e., hydrophobic). The particles with a low free energy have a high floatability.

The contact between particles and gas bubbles in a suspension is considered as a two-step process: (1)the collision between the particle and the bubble and (2) the attachment of the particle onto the gasbubble. Each step can be modeled as a stochastic event. Thus, the overall probability of particle collectionby gas bubble is defined as the product of the probability of particle–bubble collision and the probabilityof adhesion after the collision. The collision probability depends mainly on the hydrodynamic charac-teristics of the flotation cell while the adhesion probability is related to the hydrophobicity of the particle.

Traditionally, flotation is carried out in an open cell equipped with a gas-inducing agitator (turbineor impeller). As shown in Figure 64.4, air is induced through the air passage in the agitator shaft bysuction. The rotational motion of the agitator disperses air bubbles into the suspension. These bubblesattach to suspended particles to form

aggregates.

The particle–bubble aggregates float upward to the frothlayer, which is mechanically skimmed off or flows over a weir into the discharge launder as a frothproduct. The nonfloatable particles are withdrawn from the bottom of the cell as tailings. A more recentdevelopment in flotation is the use of bubble column as a flotation device [Finch et al., 1995]. In Figure64.5, it shows that the space in a flotation column can be divided into two parts: the collection zone andthe froth zone. The feed enters the column via a feed port at the middle and flows downward to the baseof the column. The gas bubbles are generated either by an internal sparger near the bottom of the columnor an external gas bubble generator. To minimize the effect of unexpected particle entrainment, a washwater device is added near the top of the column just below the overflow weir for cleaning the froth. Theoperating performance of a flotation column is generally superior to that of open cell flotation.

FIGURE 64.4

Conventional open-cell flotation.

Agitator

Bubble

Particles

Air

Particle-Laden Froth

Pulp


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64.2 Equipment

The most important criterion for the selection of equipment for a given application of solid–liquidseparation is the particle size of the system. Figure 64.6 shows the general range of applicability of majortypes of equipment in terms of the particle size and representative materials involved. Of course, thisrepresentation is an oversimplification of the selection process, as many other factors are not considered.For example, the solid concentration in the feed mixture (suspension) can influence the choice ofequipment type. In general, deep-bed filtration is best for treating dilute slurry with solid concentrationless than 1%, whereas cake filtration is the method of choice for slurries having solid concentration muchgreater than 1%.

FIGURE 64.5

Column flotation.

FIGURE 64.6

Equipment selection for solid–liquid separation based on particle size.

Wash Water

Feed Particle-Laden Froth

Froth Zone

Interface

ColumnDiameter

GasBubbles

Collection Zone

Gas Sparger

Tailings

molecules

10−4 10−3 10−2 10−1 1.0

Mean Particle Size, micrometer

10 102 103 104

colloids ultrafine fine medium coarse

Bacteria

FineSand

GravelCoarseSand

Virus Clay Silt

Flotation

Deep-bed Filtration

Screening

Cycloning

Sedimentation/Thickening

Cake Filtration

Microfiltration

Ultrafiltration



64

-7

It should also be pointed out that the various filtration processes (deep-bed filtration, cake filtration,microfiltration, and ultrafiltration) cover nearly the entire range of particle size. Therefore, the term

filtration

is often used as a synonym to represent the field of

solid–liquid separation.

The most commonlyused filtration equipment is given in Table 64.1.

A detailed procedure for equipment selection for a given requirement in solid–liquid separation canbe found in

Perry’s Chemical Engineers’ Handbook

[Perry and Green, 1997].

64.3 Fundamental Concept

There are two general types of operations for separating solid particulate matter from a liquid phase. Inthe first type, the separation is accomplished by moving the particles through a constrained liquid phase.The particle movement is induced by a body force, such as gravity or centrifugal acceleration. Forexample, in sedimentation, the solid particles settle due to a difference in density between solid andliquid under the influence of gravity. In centrifugation and hydrocycloning, the separation is effectedby centrifugal acceleration.

In the second type of operation, exemplified by the filtration process, the separation is accomplishedby contacting the solid–liquid suspension with a porous medium (see Figure 64.7). The porous mediumacts as a semipermeable barrier that allows the liquid to flow through its capillary channels and retainsthe solid particles on its surfaces. Depending on the mechanism for arrest and accumulation of particles,this type of separation can be further divided into two classes [Perry and Green, 1997]: deep-bed filtrationand cake filtration.

Deep-bed filtration is also known by terms such as

blocking filtration, surface filtration,

and

clarification

[see Figure 64.7(b)]. This type of filtration is preferred when the solid content of the suspension is lessthan 1%. In such an operation, a deep bed of packing material (e.g., sand, diatomite, or synthetic fibers)is used to capture the fine solid particles from a dilute suspension. The particles to be removed are severalorders of magnitude smaller than the size of the packing material, and they will penetrate a considerabledepth into the bed before being captured. The particles can be captured by several mechanisms [Tien,1989]:

1. The direct-sieving action at the constrictions in the pore structure2. Gravity settling 3. Brownian diffusion 4. Interception at the solid–liquid interfaces 5. Impingement 6. Attachment due to electrokinetic forces

Cake filtration is the most commonly used industrial process for separating fine particles from asolid–liquid suspension. In cake filtration, the filtered particles are stopped by the surface of a filtermedium (a porous barrier) and then piled upon one another to form a cake of increasing thickness [seeFigure 64.7(a)]. This cake of solid particles forms the “true” filtering medium. In the case of liquidfiltration, a filter cake with filtrate (the liquid) trapped in the void spaces among the particles is obtainedat the end of the operation. In many instances where the recovery of the solids is the ultimate objective,it is necessary that the liquid content in the cake be as low as possible. In order to reduce the liquid

TABLE 64.1

Filtration Equipment

Discontinuous Filters Semicontinuous Filters Continuous Filters

Plate and frame filter press Rotary pan filter Drum filterLeaf filter Semicontinuous belt filter Rotary disk filterTray filter Automatic filter press Vacuum belt filter

Electrical precipitator Rotary disk cross-flow filterRotating cylinder cross-flow filter


64

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content, the cake is subjected to desaturating forces. These forces can be mechanical, hydrodynamic,electrical, or acoustic in nature [Muralidhara, 1989].

When the mean particle size is less than a few micrometers, the conventional cake filtration operationbecomes ineffective, primarily due to the formation of high-resistance filter cake. To overcome thisobstacle, cross-flow filtration (often coupled with ultrafiltration) is used to limit the cake growth. In thecross-flow configuration (e.g., in continuous ultrafiltration), the solid–liquid suspension flows tangen-tially to the filter medium rather than perpendicularly to the medium as in conventional filtration. Theshear forces of the flow in the boundary layer adjacent to the surface of the medium continuously removea part of the cake and thus prevent the accumulation of solid particles on the medium surface. In thismanner, the rate of filtration can be maintained at a high level to ensure a cost-effective operation.

64.4 Design Principles

Cake Filtration

In the design of a cake filtration process, the pressure drop,

D

p

, the surface area of the cake,

A

, and thefiltration time,

t

, are important parameters to be determined. As the filtration proceeds, particles retainedon the filter medium form a filter cake (see Figure 64.8). For an incompressible cake the pressure drop,

D

p

, across the filter cake and filter medium can be expressed as:

(64.1)

where

m

is the viscosity of the filtrate,

u

is the velocity of the filtrate,

m

c

is the total mass of solids in thecake,

R

m

is the filter-medium resistance, and

a

is defined as the specific cake resistance. The specific cake

FIGURE 64.7

Mechanisms of filtration: (a) cake filtration, (b) deep-bed filtration, (c) cross-flow filtration (ultra-filtration). (

Source:

McCabe, W. L., Smith, J. C., and Harriott, P. 2001.

Unit Operations of Chemical Engineering,

6thed., Figure 29.3, p. 992. McGraw-Hill, New York.)

Filtrate

Filtrate(Permeate)

Suspension Concentrated suspension(Retentate)

Membrane(Medium)

Filtrate

(a)

(c)

(b)

Fluid-ParticleSuspension

Cake

Medium

Suspension

Filter medium

Dp p pm

AR ua b

cm= - = +Ê

ËÁˆ¯̃

am



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-9

resistance depends on particle size, shape, and distribution. It is also a function of porosity of filter cakeand pressure drop. For incompressible cakes,

a

is independent of the pressure drop and the position inthe filter cake.

For data analysis, Equation (64.1) is usually rewritten as follows: [McCabe et al., 2001]

(64.2)

where

t

is the filtration time,

V

is the volume of filtrate, and

c

is the mass of solid per unit filtrate volume.In order to use this equation for design of a cake filtration operation, the specific cake resistance and

filter-medium resistance must first be determined by performing experimental tests. Equation (64.2) canbe further integrated under constant pressure to give:

(64.3)

where

(64.4)

A plot of

t

/

V

against

V

yields a straight line with a slope of (

K

c

/2) and an intercept of (1/

q

o

). UsingEquation (64.4), the values of

a

and

R

m

can be calculated. For slightly compressible cake,

a

becomes afunction of pressure drop and can be represented by the following correlation:

FIGURE 64.8

Pressure gradient in filter medium and cake.

Filter cake

FiltrateM

ediu

m

Ups

trea

m fa

ce o

f cak

e

Direction of flowof suspension

Pa

Pb

P′

Lc

dt

dV A p

cRm=

-+È

ÎÍ˘˚̇

m a( )D

V

A

t

V

KV

qc

o

= ÊËÁ

ˆ¯̃

+2

1

Kc

A p q

R

A pco

m= =m a m2

1

D D and


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-10


(64.5)

where

a

0

and

a

are empirical constants.Equation (64.2) through Equation (64.5) can be used for design calculations for a filtration operation

with incompressible and slightly compressible filter cakes. For the case of highly compressible cakes theeffect of variations in cake porosity on specific cake resistance must be considered [Tiller and Shirato, 1964].

Ultrafiltration

Ultrafiltration is a membrane process (see Chapter 63 on membrane separation) capable of separatingor collecting submicrometer-size particles from a suspension or solution. It has been widely used toconcentrate or fractionate a solution containing macromolecules, colloids, salts, or sugars. The ultrafil-tration membrane can be described as a sieve with pore size ranging from molecular dimension to a fewmicrometers. It is usually polymeric and asymmetric, designed for high productivity (permeation flux)and resistance to plugging. Ultrafiltration membranes are made commercially in sheet, capillary, andtubular forms.

In the design of the ultrafiltration process, either batch operation or continuous operation (employinga cross-flow configuration) can be used. In the batch operation, the retentate is returned to the feed tankfor recycling through the filter unit. It is the fastest method of concentrating a given amount of materialand it also requires the minimum membrane area. In order to determine the membrane surface area,

A

,for the ultrafiltration process, the following three parameters are required: flux,

J

, which is a measure ofthe membrane productivity; permeate,

V

p

, which is the amount of material that has passed through themembrane; and retentate,

V

R

, which is the amount of material that has been retained by the membrane.During the batch ultrafiltration operation, flux decreases because of an increase in concentration in therecycled stream. Furthermore, the phenomenon of concentration polarization tends to cause a higherconcentration at the membrane surface than that in the bulk. Therefore, an average flux should be usedin the design. The average flux,

J

av

, can be estimated by the following equation:

(64.6)

where

J

f

is the final flux at the highest concentration and

J

i

is the initial flux. The material balance gives

(64.7)

where

V

f

,

V

r

, and Vp are volume of feed, retentate, and permeate, respectively.The membrane area can be expressed as

(64.8)

Equation (64.6) to Equation (64.8) can be used to estimate the membrane surface area required for agiven ultrafiltration operation [Cheryan, 1986].

64.5 Economics

The cost for a given solid–liquid separation process varies widely. For example, the cost for purchasingindustrial filtration equipment can vary from several hundred dollars to over ten thousand dollars persquare meter of filter area. Such a large variation in cost is due to a wide variety of individual featuresand materials of construction required by specific applications. A good source of information on the costof common industrial filtration and other solid–liquid separation equipment can be found in Perry’sHandbook [Perry and Green, 1997].

a a= 0( )Dp a

J J J Jf i fav = + -0 33. ( )

V V Vf r p= +

A V V Jf r= -( )/ av


Solid–Liquid Separation 64-11

Defining Terms

Cake filtration — The separation of particles is effected by contacting the solid–liquid suspension witha porous filter medium (made of cloth, synthetic fibers, or metals). The filter medium allowsthe liquid to flow through its pores while it retains the particles on its surface to form a cake.As filtration proceeds, the cake of solid particles grows in thickness and becomes the “true”filtering medium.

Centrifugation — Centrifugation is a separation process based on the centrifugal force either to holdthe material in it or to let the material pass through it. Separation is achieved due to thedifference in density.

Hydrocycloning — Hydrocycloning is a centrifugal separation process. The feed is introduced tangen-tially into the cylindrical portion of a cyclone, causing it to flow in a tight conical vortex. Thebulk of the liquid leaves upward through a pipe located at the center of the vortex. Solid particlesare thrown to the wall and discharged with a small portion of the liquid through the bottomapex of the cyclone.

Deep-bed filtration — In this type of filtration a deep bed of packing materials, such as sand, diatomite,or synthetic fibers, is used as the filter medium. The particles are captured within the packedbed while the liquid passes through it.

Flotation — Flotation is a gravity separation process based either on the use of a dense medium in whichthe desired particles will float or on the attachment of gas bubbles to particles, which are thencarried to the liquid surface to be separated.

Membrane filtration — In membrane filtration a thin permeable film of inert polymeric material is usedas the filter medium. The pore size of the membrane ranges from molecular dimension to afew micrometers. It is widely used to collect or fractionate macromolecules or colloidal sus-pensions. It is also applied to beverage filtration and preparation of ultrapure water.

Screening — Screening is an operation by which particles are introduced onto a screen of a given aperturesize to separate particles of different sizes.

Specific cake resistance — Specific cake resistance is the resistance of a filter cake having unit weight ofdry solids per unit area of filtration surface.

Thickening/sedimentation — Thickening/sedimentation is a gravity-settling process that removes themaximum quantity of liquid from a slurry and leaves a sludge for further processing.

Ultrafiltration — Ultrafiltration is a special type of membrane filtration. It is used for concentrationand purification of macromolecular solutes and colloids in which the solution is caused to flowunder pressure parallel to a membrane surface (in a cross-flow configuration). Solutes (orsubmicrometer particles) are rejected at the semipermeable membrane while the solvents andsmall solute molecules pass through the membrane.

References

Cheryan, M., 1986. Ultrafiltration Handbook. Technomic, Lancaster, PA.Finch, J. A., Uribe-Salas, A. and Xu, M. 1995. Column flotation, In: Flotation Science and Engineering,

Matis, K. A., Ed. Marcel Dekker, New York. pp. 291–330.Fuerstenau, M. C., Miller, J. D., and Kuhn, M. C. 1985. Chemistry of Flotation, Society of Mining Engineers,

New York, p. 2.Jaycock, M. J. and Parfitt, G. D. 1981. Chemistry of Interfaces, Ellis Horwood Limited, New York.Kelsall, D. F. 1952. A study of the motion of solid particles in a hydraulic cyclone. Trans. Inst. Chem. Eng.

30:87–104.McCabe, W. L., Smith, J. C., and Harriott, P. 2001. Unit Operations of Chemical Engineering, 6th ed.

McGraw-Hill, New York. pp. 986–1056.Muralidhara, H. S. (Ed.) 1989. Solid/Liquid Separation. Battelle Press, Columbus, OH.Perry, R. H. and Green, D. W. (Ed.) 1997. Perry’s Chemical Engineers’ Handbook, 7th ed., McGraw-Hill,

New York. Chapters 18 and 22.


64-12 The Engineering Handbook, Second Edition

Rushton, A., Ward, A. S., and Holdich, R. G. 2000. Solid–Liquid Filtration and Separation Technology, 2nded., Wiley-VCH, Weinheim, Germany. Chapters 2 and 3.

Svarovsky, L. 1985. Solid–Liquid Separation Processes and Technology, Elsevier, Amsterdam, pp. 68–71. Schweitzer, P. A. 1997. Handbook of Separation Techniques for Chemical Engineers, 3rd ed., McGraw-Hill,

New York. pp. 4.140-4.156.Tien, C. 1989. Granular Filtration of Aerosols and Hydrosols. Butterworths, Stoneham, MA.Tiller, F. M. and Shirato, M. 1964. The role of porosity in filtration: VI. new definition of filtration

resistance. AlChE J. 10(1):61–67.Zeitsch, K. 1990. Centrifugal filtration. In Svarovsky, L. Ed. Solid–Liquid Separation, 3rd ed., Butterworths,

London. pp. 476–532.Zettlemoyer A. C. 1969. Hydrophobic surfaces. In Fowkes, F. M. Ed. Hydrophobic Surfaces, Academic

Press, New York. pp. 1–27.

Further Information

An excellent in-depth discussion on the theory and practice of solid–liquid separation is presented inSolid–Liquid Separation, 3rd ed., by Ladislav Svarovsky, Butterworths, London, 1990.

The proceedings of the annual American Filtration and Separation Society meeting and the WorldFiltration Congress document new developments in all aspects of solid–liquid separation.

Four major journals cover the field of solid–liquid separation:

Solid–Liquid Separation Journal. Published by the American Filtration and Separation Society, Hous-ton, TX.

Particulate Science and Technology: An International Journal. Published by Taylor and Francis, Wash-ington, DC.

Transactions of Filtration Society. Published by the Filtration Society, Leics. LE67 8PP, UK.Separations Technology. Published by Butterworth-Heinemann, Stoneham, MA.


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