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64
SolidLiquid Separation
64.1 Unit Operations in SolidLiquid Separation
Screening Sedimentation Centrifugation Hydrocycloning
Flotation
64.2 Equipment
64.3 Fundamental Concept
64.4 Design Principles
Cake Filtration Ultrafiltration
64.5 Economics
Solidliquid 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 solidliquid separation.
Technically, solidliquid 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 solidliquid
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 SolidLiquid Separation
A description of each unit operation in solidliquid 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
solidliquid 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
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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
solidliquid 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 solidliquidsuspension. The centrifugal
force used in such equipment ranges up to 10,000 times the
gravitationalacceleration. Liquidsolid 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
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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 asolidliquid 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
RakeFeed Suspension + Flocculant
Transition ZoneCompression Zone
Settling Solids
Ove
rflow
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
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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
solidliquid 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 obstructionof 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 solidliquid 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 itssolidliquid interface by the
solidvapor 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 particlebubble
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 particlebubble 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
AirParticle-Laden Froth
Pulp
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64.2 Equipment
The most important criterion for the selection of equipment for
a given application of solidliquidseparation 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 solidliquid separation based on particle
size.
Wash Water
Feed Particle-Laden Froth
Froth ZoneInterface
ColumnDiameter
GasBubbles
Collection Zone
Gas Sparger
Tailings
molecules
104 103 102 101 1.0Mean Particle Size, micrometer
10 102 103 104
colloids ultrafine fine medium coarse
Bacteria
FineSand
GravelCoarseSand
Virus Clay Silt
FlotationDeep-bed Filtration
ScreeningCycloning
Sedimentation/Thickening
Cake Filtration
MicrofiltrationUltrafiltration
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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
solidliquid separation.
The most commonlyused filtration equipment is given in Table
64.1.
A detailed procedure for equipment selection for a given
requirement in solidliquid separation canbe found in
Perrys 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
solidliquid 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 solidliquid interfaces 5. Impingement 6. Attachment due to
electrokinetic forces
Cake filtration is the most commonly used industrial process for
separating fine particles from asolidliquid 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
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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
solidliquid 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= - = +
a
m
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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
Upst
ream
face
of c
ake
Direction of flowof suspension
Pa
PbP
Lc
dt
dV A p
cRm=
-
+
m
a
( )D
V
A
t
V
KV
qc
o
=
+2
1
Kc
A p q
R
A pc o
m= =
m a
m
2
1
D
D
and
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(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 solidliquid 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 solidliquid separation equipment
can be found in PerrysHandbook [Perry and Green, 1997].
a a= 0( )Dpa
J J J Jf i fav = + -0 33. ( )
V V Vf r p= +
A V V Jf r= -( )/ av
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Defining Terms
Cake filtration The separation of particles is effected by
contacting the solidliquid 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
truefiltering 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.
291330.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.
30:87104.McCabe, W. L., Smith, J. C., and Harriott, P. 2001.
Unit Operations of Chemical Engineering, 6th ed.
McGraw-Hill, New York. pp. 9861056.Muralidhara, H. S. (Ed.)
1989. Solid/Liquid Separation. Battelle Press, Columbus, OH.Perry,
R. H. and Green, D. W. (Ed.) 1997. Perrys Chemical Engineers
Handbook, 7th ed., McGraw-Hill,
New York. Chapters 18 and 22.
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64-12 The Engineering Handbook, Second Edition
Rushton, A., Ward, A. S., and Holdich, R. G. 2000. SolidLiquid
Filtration and Separation Technology, 2nded., Wiley-VCH, Weinheim,
Germany. Chapters 2 and 3.
Svarovsky, L. 1985. SolidLiquid Separation Processes and
Technology, Elsevier, Amsterdam, pp. 6871. 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):6167.Zeitsch, K. 1990. Centrifugal
filtration. In Svarovsky, L. Ed. SolidLiquid Separation, 3rd ed.,
Butterworths,
London. pp. 476532.Zettlemoyer A. C. 1969. Hydrophobic surfaces.
In Fowkes, F. M. Ed. Hydrophobic Surfaces, Academic
Press, New York. pp. 127.
Further Information
An excellent in-depth discussion on the theory and practice of
solidliquid separation is presented inSolidLiquid 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 solidliquid separation.
Four major journals cover the field of solidliquid
separation:
SolidLiquid 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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Chapter 64Solid - Liquid Separation64.1 Unit Operations in
SolidLiquid
SeparationScreeningSedimentationCentrifugationHydrocycloningFlotation
64.2 Equipment64.3 Fundamental Concept64.4 Design PrinciplesCake
FiltrationUltrafiltration
64.5 EconomicsDefining TermsReferencesFurther Information
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