ChE 3203/4221: Particle Technology

Filtration

Safat Anam
Lecturer
Department of ChE, RUET

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 The separation of solids from a suspension in
a liquid by means of a porous medium or screen
which retains the solids and allows the liquid to
pass is termed filtration.

 In general, the pores of the medium are larger

than the particles which are to be removed, and
the filter works efficiently only after an initial
deposit has been trapped in the medium.

 Filtration is essentially a mechanical operation

and is less demanding in energy than
evaporation or drying.

 The cake gradually builds up on the medium

and the resistance to flow progressively
increases.
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Filter Media ?
Filter Aid ?

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The main factors to be considered when selecting Filtration equipment and
operating conditions are:
(a) The properties of the fluid, particularly its viscosity, density and corrosive properties.
(b) The nature of the solid—particle size and shape, size distribution, and packing
characteristics.
(c) The concentration of solids in suspension.
(d) The quantity of material to be handled, and its value.
(e) Whether the valuable product is the solid, the fluid, or both.
(f) Whether it is necessary to wash the filtered solids.
(g) Whether very slight contamination caused by contact of the suspension or filtrate with the
various components of the equipment is detrimental to the product.
(h) Whether the feed liquor may be heated.
(i) Whether any form of pretreatment might be helpful.
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Self Study:
Principle
Applications
Advantage/Disadvantage

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Read : Chapter 30 (From Page 1002),
Unit Operations (McCabe/5th ed)

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Cake Filtration: the particles from the suspension, which usually has a high proportion of
solids, are deposited on the surface of a porous septum which should ideally offer only a small
resistance to flow. As the solids build up on the septum, the initial layers form the effective
filter medium, preventing the particles from embedding themselves in the filter cloth, and
ensuring that a particle-free filtrate is obtained.

Depth Or Deep-bed Filtration: the particles penetrate into the pores of the filter medium,
where impacts between the particles and the surface of the medium are largely responsible for
their removal and retention. This configuration is commonly used for the removal of fine
particles from very dilute suspensions, where the recovery of the particles is not of primary
importance.

There are two principal modes under which deep bed filtration may be carried out. In the first,
dead-end filtration the slurry is filtered in such a way that it is fed perpendicularly to the filter
medium and there is little flow parallel to the surface of the medium. In the second, termed
cross-flow filtration which is used particularly for very dilute suspensions, the slurry is
continuously recirculated so that it flows essentially across the surface of the filter medium at
a rate considerably in excess of the flow rate through the filter cake.
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Vertical Granular Bed Filter Description: Foust 9
Plate and Frame Filter Press
Description: McCabe 10
Description: Foust 11
Filter Press Description: Foust 12
 Horizontal
Plate Filter
Description: Foust

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Vertical Leaf Filter
Description: Foust
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Rotary Horizontal Vacuum Filter
Description: Foust

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Horizontal-tank Pressure Leaf Filter
Description: McCabe
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Continuous Rotary Vacuum Filter
Description: McCabe
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Flow Sheet For Continuous Vacuum Filtration
Description: McCabe 18
Horizontal Belt Filter
Description: McCabe

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 H2O2 Plant, SCCL

Secondary Filter (Capture Pd catalyst from reactor) Polish Filter (Capture very fine catalyst particles) 20
Polish filter cleaning and regenerated filter cartridge installation
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Regenerated Filter Cartridge

H2O2 Plant, SCCL 22

 Micro filter used in Lube Oil Filtration
(Pump/Turbine/compressor)
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Micro Filter of 83-BFW Feed Pump, Ammonia Plant, AFCCL
Bed Filter: Bed filters applies the principles of deep bed filtration in which the particles
penetrate into the interstices of the filter bed where they are trapped following impingement on
the surfaces of the material of the bed. For the purification of water supplies and for waste
water treatment where the solid content is about 10 g/m3 or less granular bed filters have
largely replaced the former very slow sand filters. The beds are formed from granular material
of grain size 0.6–1.2 mm in beds 0.6–1.8 m deep. The very fine particles of solids are removed
by mechanical action although the particles finally adhere as a result of surface electric forces
or adsorption.

Bag Filter: Bag filters have now been almost entirely superseded for liquid filtration by other
types of filter. A number of long thin bags are attached to a horizontal feed tray and the liquid
flows under the action of gravity so that the rate of filtration per unit area is very low. The filter
is usually arranged in two sections so that each may be inspected separately without
interrupting the operation. Bag filters are still extensively used for the removal of dust particles
from gases and can be operated either as pressure filters or as suction filters.

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Specific Surface Area (So)

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The factors on which the rate of filtration depends:
(a) The drop in pressure from the feed to the far side of the filter medium.
(b) The area of the filtering surface.
(c) The viscosity of the filtrate.
(d) The resistance of the filter cake.
(e) The resistance of the filter medium and initial layers of cake.

The initial stages in the formation of the filter cake are of special importance for
the following reasons:
(a) For any filtration pressure, the rate of flow is greatest at the beginning of the process
since the resistance is then a minimum.
(b) High initial rates of filtration may result in plugging of the pores of the filter cloth
and cause a very high resistance to flow.
(c) The orientation of the particle in the initial layers may appreciably influence the
structure of the whole filter cake.
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Filtration Calculation
The flow of filtrate through the filter cake is given by the Carman-Kozeny equation,

Ref: Foust 27
The cake thickness ( L) may be related to the volume of filtrate by a material balance,

(Basic Filtration Equation)

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 The flow rate of the filtrate is given by the following equation:

where V is the volume of filtrate which has passed in time t , A is the total cross-sectional area
of the filter cake, uc is the superficial velocity of the filtrate, l is the cake thickness, S is the
specific surface of the particles, e is the voidage, μ is the viscosity of the filtrate, and ΔP is the
applied pressure difference.
For incompressible cakes e in the equation may be taken as constant and the quantity
e3/[5(1 − e)2S2] is then a property of the particles forming the cake and should be constant for
a given material.

r is termed the specific resistance which depends on e and S. For incompressible cakes, r is
taken as constant, although it depends on rate of deposition, the nature of the particles, and on
the forces between the particles. r has the dimensions of L−2 and the units m−2 in the SI system.
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Inclusion of Filter- Medium Resistance and Integration of the Basic Filtration
Equation:
 In the basic filtration equation, both
driving force and resistance apply to the
filter cake alone. Practically, pressure
drop across the filter medium must be
considered.

 The resistance of filter cloth and flow

channel is usually expressed in terms of
an equivalent volume of filtrate.

The time θ necessary to pass any volume V of filtrate.

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 The bar- graph technique is used to indicate that
Constant pressure (-ΔP) Filtration: the value (ΔV/Δθ ), represents the average rate
during the interval between V and V + ΔV .

Slope =

Intercept =

(Determine α and Ve from the pilot

filtration data)

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Constant Rate (dV/dθ) Filtration:

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Filtration Calculations for Compressible Cakes:

At moderate pressure,

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ΔP vs t plot:

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Optimum Filtration Rate:

• The time θ necessary to pass any volume V of filtrate

• During constant pressure filtration the rate of flow starts at a maximum and progressively
decreases.
• Filter cake needs to be cleaned out, (dismantled etc) for continuation of operation. So there is
an optimum cycle time.
• The average volumetric flow rate is:

Where td = downtime, time required for washing, dismantling, and reassembling

At optimum filtration rate, dQ/dV = 0

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Illustration 22.12. Ruth and Kempe (35) report the results of laboratory filtration tests on a
precipitate of CaCO3 suspended in water. A specially designed plate and frame press with a single
frame was used. The frame had a filtering area of 0.283 sq ft and a thickness of 1.18 in . All tests
were conducted at 66 ° F and with a slurry containing 0.0723 weight fraction CaCO3. The density of
the dried cake was 100 lb/cu ft. Test results for one run are given below : P = 40 psi = constant

Determine the filtrate volume equivalent in resistance to the filter medium and piping (Ve), the
specific cake resistance (α) , the cake porosity ( ε) , and the cake specific surface ( So ). 36
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Illustration 22.13. A 30 by 30 in . plate -and -frame filter press with twenty frames 2.50 in .
thick is to be used to filter the CaCO3 slurry which was used in the test of Illustration 22.12. The
effective filtering area per frame is 9.4 sq ft, and Ve may be assumed to be the same as that
found in the test 60 run . If filtration is carried out at constant pressure with 40 psi , determine
the volume of slurry that will be handled until the frames are full, and the time required for this
filtration .

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 Foust Exercise Problem:
22.33, 22.34, 22.37, 22.39, 22.41, 22.42, 22.43

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