Chapter 7 Filtration
Contents
1
Introduction
2
Fundamentals of Filtration
3
Filtration at Constant Pressure Drop
4
Filtration at Constant Volumetric Flow
5
Equipment
2
1. Introduction
1. Introduction
Filtration is a unit operation whose purpose is
the separation of an insoluble solid present in
solid–liquid suspension by passing the suspension
through a porous membrane that retains the
solid particles.
The porous membrane is called the filtering
medium, while the retained particles in the
membrane form a layer known as cake and the
liquid that passes through the porous membrane
and does not contain solids is called a filtrate.
4
1. Introduction
The desired phase in filtration can occur in the
filtrate, cake, or both.
When the desired solid part is obtained, the cake
should be washed to eliminate the impurities it
may contain.
Filtration can operate under simple gravity to
obtain the flow of filtrate that crosses the
filtrating medium, or by applying a pressure
higher than the atmospheric pressure to the front
part of the filtering medium or vacuum to the
back side.
5
1. Introduction
These processes are known as pressure filtration
and vacuum filtration, respectively.
Filtration is an operation widely used in the
industry in general; in the particular case of the
food industry, three characteristic types of
filtration can be distinguished.
The first includes suspensions containing
significant amounts of insoluble solids that, when
filtered, form a cake on the filtrating medium;
thus, obtaining the solid, filtrate, or both could be
interesting.
6
1. Introduction
Another type of filtration includes suspensions
with few insoluble solids that usually are
undesired, in which case the filtration is called
clarification.
Finally, microfiltration should be mentioned.
Microfiltration occurs when the size of the solid
particles to be separated is around 0.1 mm.
7
2. Fundamentals of Filtration
2. Fundamentals of Filtration
2.1 Pressure drop of fluid through filter cake
The most important components in the filtration
system are filters in which cakes are formed on
the filtering medium.
Among these types of filters, the most used are
the plate-and-frame press filters.
Initially, the feed or slurry passes through the
filtering medium, but, as it filtrates, a cake is
formed that increases its thickness, so the feed
should pass through not only the filtering medium
but also the cake.
9
10
2. Fundamentals of Filtration
Hence, the pressure drop along the filter is
supposed to be greater with time, resulting in the
filtrate flow being smaller with time.
This indicates that filtration can be done at
constant pressure drop or at constant filtrate flow.
In the first case, when maintaining a constant
pressure drop, the filtrate flow will decrease along
the filtration time.
On the other hand, when a constant filtrate flow is
desired, the pressure drop will be greater over
time.
11
2. Fundamentals of Filtration
As filtration proceeds, the thickness of the
deposited solids increases, becoming a nonstationary circulation of fluids through variable
filling height case.
Two zones can be distinguished in the filter: the
filtering cake and the filtering medium.
The former constitutes a filling that can change
its characteristics (specific surface, porosity, etc.),
while the filtering medium has fixed
characteristics.
12
2. Fundamentals of Filtration
In general, for a filter in which a cake is formed,
the total pressure drop experienced by the fluid
(-Δp) is the summation of the pressure drop
experienced through the filtering medium (-Δpm)
plus the pressure drop experienced through the
cake (-Δpc):
(-Δp) = -(Δpc + Δpm ) = (-Δpc )+ (-Δpm )
13
2. Fundamentals of Filtration
At this time the thickness of the cake is L m.
The filter cross-sectional area is A m2, and the linear
velocity of the filtrate in the L direction is v m/s based on
the filter area A m2.
FIG. Section through a filter cake
14
2. Fundamentals of Filtration
Darcy’s law—relate the flow rate through a porous bed of
solids to the pressure drop causing that flow.
k P
v

v = velocity of the liquid
P = pressure drop across the bed of thickness ℓ
P/ ℓ = pressure gradient
 = viscosity of the liquid
k = permeability of the bed, a proportionality
constant (dimension: L2)
* Like Ohm’s law, ℓ/k is the resistance of filtration.
2. Fundamentals of Filtration
Darcy’s law: v 
k P

Strictly speaking, Darcy’s law holds only when
 vd
 (1   )
5
where d is the particle size of the filter cake, ρ is the liquid
density, and ε is the void fraction in the cake.
* Biological separations almost always obey this inequality.
For a batch filtration,
1 dV
v
A dt

1 dV kP

A dt

where V is the total volume of filtrate, A is the filter
area, and t is the time.
2. Fundamentals of Filtration
Two contributions to the filtration resistance:

 RM  RC
k
where RM is the resistance of the filter medium (constant),
and RC is the resistance of the cake (varies with V).
The basic differential equation for filtration at constant
pressure drop can thus be obtained as:
1 dV kP


A dt

1 dV
P

A dt
 ( RM  RC )
2. Fundamentals of Filtration
Incompressible Cakes
V 
RC   0  
 A
 = specific cake resistance, cm/g
0 = mass of cake solids per volume of filtrate
1 dV
P

A dt  ( R M  RC )
1 dV
P
 A dt   [ (V / A)  R ] (I.C.: t = 0, V = 0)
0
M

At  0  V  R M
V 

 K   B
 
V
2 P  A  P
 A
2. Fundamentals of Filtration
At  0  V  RM
V 

 K   B
 
V
2 P  A  P
 A
 At 
Plot   versus
V 
V 
 0
   Slope = K 
 A
2 P
Known , 0, P   can be determined.
* Often, the medium resistance RM is insignificant, B = 0.
t
 0  V 
 
2 P  A 
2
2. Fundamentals of Filtration
The average velocity vx,av can be calculated by the
Hagen-Poiseuille equation, which relates the pressure
drop and the average velocity for laminar flow in a
horizontal pipe.
The Hagen-Poiseuille equation for laminar flow of a
liquid in circular tubes can be written as
20
2. Fundamentals of Filtration
The flow of the filtrate through the packed bed of cake
can be described by an equation similar to Poiseuille’s
law, assuming laminar flow occurs in the filter channels.
Poiseuille’s equation can be written
(7-1)
Δp: pressure drop (N/m2, lbf/ft2); v: open tube velocity
(m/s, ft/s); D: diameter (m, ft); μ: viscosity (Pa∙s,
lbm/ft∙s); gc: 32.174 lbm∙ft/lbf∙s2.
21
2. Fundamentals of Filtration
For laminar flow in a packed bed of particles, the
following relation has been shown to apply to filtration:
(7-2)
k1: a constant (4.17 for random particles of definite size
and shape); μ: viscosity of filtrate (Pa∙s, lbm/ft∙s); v: linear
velocity based on filter area (m/s, ft/s); L: thickness of
cake (m, ft); S0: specific surface area of particle in m2(ft2)
of particle area per m3(ft3) volume of solid particle; Δpc:
pressure drop in the cake (N/m2, lbf/ft2); ε: void fraction.
Void fractions, ε: the ratio of the void volume to the
total volume of the bed
22
2. Fundamentals of Filtration
For English units, the right hand side of Equation
(7-2) is divided by gc.
The linear velocity (superficial velocity) is based
on the empty cross-sectional area and is
(7-3)
A: filter area (m2,ft2); V: total m3(ft3) of filtrate
collected up to time t s.
The cake thickness L may be related to the
volume of filtrate V by a material balance.
23
2. Fundamentals of Filtration
If Cs is kg solids/m3 (lbm/ft3) of filtrate, a material
balance gives
(7-4)
ρp: density of solid particles in the cake in kg/m3
(lbm/ft3)
The final term of Eq. (7-4) is the volume of filtrate
held in the cake. This is usually small and will be
neglected.
Substituting Eq. (7-3) into (7-2) and using (7-4) to
eliminate L, we obtain the final equation (7-5).
24
2. Fundamentals of Filtration
(7-5)
α is the specific cake resistance in m/kg (ft/lbm),
defined as
(7-6)
25
2. Fundamentals of Filtration
For the filter medium resistance, we can write, by
analogy with (7-5),
(7-7)
Rm: the resistance of the filter medium to filtrate
flow (m-1, ft-1); Δpf: the pressure drop.
26
2. Fundamentals of Filtration
When Rm is treated as an empirical constant, it
includes the resistance to flow of the piping leads
to and from the filter and the filter medium
resistance.
Since the resistance of the cake and the filter
medium are in series, Eq. (7-5) and (7-7) can be
combined, and become
(7-8)
where
.
27
2. Fundamentals of Filtration
Sometimes Eq. (7-8) is modified as follows
(7-9)
where Ve is a volume of filtrate necessary to build
up a fictitious filter cake whose resistance is
equal to Rm.
28
2. Fundamentals of Filtration
The volume of filtrate V can also be related to W,
the kg of accumulated dry cake solids, as follows:
(7-10)
cx: mass fraction of solids in the slurry; m: mass
ratio of wet cake to dry cake; ρ: density of filtrate
(kg/m3, lbm/ft3).
29
2. Fundamentals of Filtration
2.2 Specific cake resistance
From Eq. (7-6)
we see that the specific cake resistance is a
function of void fraction ε and So.
It is also a function of pressure, since pressure can
affect ε.
By conducting constant-pressure experiments at
various pressure drops, the variation of α with Δp
can be determined.
30
2. Fundamentals of Filtration
If α is independent of -Δp, the sludge is
incompressible.
Usually, α increases with -Δp, since most cakes
are somewhat compressible.
An empirical equation often used is
(7-11)
where α0 and s are empirical constants.
The compressibility constant s is zero for
incompressible sludges or cakes.
The constant s usually falls between 0.1 and 0.8.
31
2. Fundamentals of Filtration
Sometimes the following is used:
(7-12)
where α’0, β, and s’ are empirical constants.
32
3. Filtration at Constant
Pressure Drop
3. Filtration at Constant Pressure Drop
3.1 Basic equations for filtration rate in batch
process
Often a filtration is done under conditions of
constant pressure.
Equation (7-8)
can be inverted and rearranged to give
(7-13)
where Kp is in s/m6 (s/ft6) and B in s/m3 (s/ft3):
34
3. Filtration at Constant Pressure Drop
(7-14)
(7-15)
35
3. Filtration at Constant Pressure Drop
For constant pressure, constant α, and
incompressible cake, V and t are the only
variables in Eq. (7-13).
Integrating to obtain the time of filtration in t s,
(7-16)
(7-17)
Dividing by V,
(7-18)
36
3. Filtration at Constant Pressure Drop
where V is total volume of filtrate in m3 (ft3)
collected to t s.
To evaluate Eq. (7-17)
it is necessary to know α and Rm.
This can be done by using Eq. (7-18).
Data for V collected at different times t are
obtained.
37
3. Filtration at Constant Pressure Drop
Then the experimental data are plotted as t/V
versus V, as in Fig. 7-1.
Fig. 7-1. Determination of constants in a
constant-pressure filtration run.
38
3. Filtration at Constant Pressure Drop
Often the first point on the graph does not fall on
the line and is omitted.
The slope of the line is Kp/2 and the intercept B.
Then, using Eqns. (7-14) and (7-15),
values of α and Rm can be determined.
39
3. Filtration at Constant Pressure Drop
Example 7-1. Evaluation of Filtration Constants
for Constant-Pressure Filtration
Data for the laboratory filtration of CaCO3 slurry
in water at 298.2 K(25⁰C) are reported as follows
at a constant pressure (-Δp) of 338 kN/m2. The
filter area of the plate-and-frame press was A =
0.0439 m2 and the slurry concentration was Cs =
23.47 kg/m3. Calculate the constants α and Rm
from the experimental data given, where t is time
in s and V in filtrate collected in m3.
40
3. Filtration at Constant Pressure Drop
Solution:
First, the data are calculated as t/V and the
tabulated in Table 7-1.
The data are plotted as t/V versus V in Fig. 7-2
and the intercept is determined as B = 6400 s/m3
and the slope as Kp/2 = 3.00 x 106 s/m6.
41
3. Filtration at Constant Pressure Drop
Table 7-1 Values of t/V for Example 7-1
(t = s, V = m3)
42
3. Filtration at Constant Pressure Drop
Fig. 7-2. Determination of constants for Example 7-1.
43
3. Filtration at Constant Pressure Drop
At 298.2 K the viscosity of water is 8.937 x 10-4
Pa∙s = 8.937 x 10-4 kg/m∙s.
Substituting known values into Eq. (7-14) and
solving,
Substituting into Eq. (7-15) and solving,
44
3. Filtration at Constant Pressure Drop
Example 7-2. Time Required to Perform a
Filtration
The same slurry used in Example 7-1 is to be
filtered in a plate-and-frame press having 20
frames and 0.873 m2 area per frame. The same
pressure will be used in constant-pressure
filtration. Assuming the same filter-cake
properties and filter cloth, calculate the time to
recover 3.37 m3 filtrate.
45
3. Filtration at Constant Pressure Drop
Solution:
In Example 7-1, the area A = 0.0439 m2, Kp = 6.0 x
106 s/m6, and B = 6400 s/m3.
Since the α and Rm will be the same as before, Kp
can be corrected.
From Eq. (7-14),
Kp is proportional to 1/A2.
The new area is A = 0.873(20) = 17.46 m2.
The new Kp is
Kp = 6.0 x 106(0.0439/17.46)2 = 37.93 s/m6
46
3. Filtration at Constant Pressure Drop
The new B is proportional to 1/A from Eq. (7-15):
Substituting into Eq. (7-17),
47
3. Filtration at Constant Pressure Drop
3.2 Equations for washing of filter cakes and total
cycle time
The washing of a cake after the filtration cycle has
been completed takes place by displacement of
the filtrate and by diffusion.
The amount of wash liquid should be sufficient to
give the desired washing effect.
To calculate washing rates, it is assumed that the
conditions during washing are the same as those
that existed at the end of the filtration.
48
3. Filtration at Constant Pressure Drop
It is assumed that the cake structure is not
affected when wash liquid replaces the slurry
liquid in the cake.
In filters where the wash liquid follows a flow
path similar to that during filtration, as in leaf
filters, the final filtering rate gives the predicted
washing rate.
For constant-pressure filtration, using the same
pressure in washing as in filtering, the final
filtering rate is the reciprocal of Eq. (7-13):
49
3. Filtration at Constant Pressure Drop
(7-19)
where (dV/dt)f = rate of washing in m3/s and Vf is
the total volume of filtrate for the entire period
at the end of filtration in m3.
For plate-and-frame filter presses, the wash liquid
travels through a cake twice as thick and an area
only half as large as in filtering, so the predicted
washing rate is one-fourth of the final filtration
rate:
(7-20)
50
3. Filtration at Constant Pressure Drop
In actual experience the washing rate may be less
than predicted because of cake consolidation,
channeling, and formation of cracks.
Washing rates in a small plate-and-frame filter were
found to be from 70 to 92% of that predicted.
After washing is completed, additional time is needed
to remove the cake, clean the filter, and reassemble
the filter.
The total filter-cycle time is the sum of the filtration
time, plus the washing time, plus the cleaning time.
51
3. Filtration at Constant Pressure Drop
Example 7-3. Rate of Washing and Total FilterCycle Time
At the end of filtration cycle in Example 7-2, a
total filtrate volume of 3.37 m3 is collected in a
total time of 269.7 s. The cake is to be washed by
through-washing in the plate-and-frame press
using a volume of wash water equal to 10% of the
filtrate volume. Calculate the time of washing and
the total filter-cycle time if cleaning the filter
takes 20 min.
52
3. Filtration at Constant Pressure Drop
Solution:
For this filter, Eq. (7-20) holds.
Substituting Kp = 37.93 s/m6, B = 16.1 s/m3, and Vf
= 3.37 m3, the washing rate is as follows:
The time of washing is then as follows for
0.10(3.37), or 0.337 m3 of wash water:
53
3. Filtration at Constant Pressure Drop
The total filtration cycle is
54
3. Filtration at Constant Pressure Drop
3.3 Equations for continuous filtration
In a filter that is continuous in operation, such as a
rotary-drum vacuum type, the feed, filtrate, and
cake move a steady, continuous rates.
In a rotary drum the pressure drop is held
constant for the filtration.
55
3. Filtration at Constant Pressure Drop
The cake formation involves a continual change in
conditions.
In continuous filtration, the resistance of the filter
medium is generally negligible compared with the
cake resistance.
So in Eq. (7-13),
B = 0.
56
3. Filtration at Constant Pressure Drop
Integrating Eq. (7-13), with B = 0,
(7-21)
(7-22)
where t is the time required for formation of the
cake.
In a rotary drum filter, the filter time t is less
than the total cycle time tc by
(7-23)
57
3. Filtration at Constant Pressure Drop
(7-23)
where f is the fraction of the cycle used for
cake formation.
In the rotary drum, f is the fraction
submergence of the drum surface in the
slurry.
58
3. Filtration at Constant Pressure Drop
Substituting Eq. (7-14) and Eq. (7-23)
into (7-22)
and rearranging,
(7-24)
59
3. Filtration at Constant Pressure Drop
If the specific cake resistance varies with
pressure, the constants in Eq. (7-11) are needed
to predict the value of α to be used in Eq. (7-24).
Experimental verification of Eq. (7-24) shows that
the flow rate varies inversely with the square root
of the viscosity and the cycle time.
60
3. Filtration at Constant Pressure Drop
Compressible Cakes
“Almost all cakes formed of biological materials are
compressible. As these cakes compress, filtration
rates drop.”
To estimate the effects of compressibility, we assume that
the cake resistance  is a function of the pressure drop.
   ' (P)
s
V 
Recall: RC   0  
 A
where ’ = a constant related largely to the size and shape
of the particles forming the cake
s = the cake compressibility
   ' (P) s  log   log  ' s log P
 Plot log versus logP, slope = s, intercept = log’.
   ' (P)
s
For incompressible cake
V 
Recall: RC   0  
 A
 For a rigid, incompressible cake, s = 0.
 For a highly compressible cake, s  1.
 In practice, s ranges from 0.10.8.
 When values of s are high, one should consider pretreating
the feed with filter aids.
3. Filtration at Constant Pressure Drop
When short cycle times are used in continuous
filtration and/or the filter medium resistance is
relatively large, the filter resistance term B must
be included, and Eq. (7-18) becomes
(7-25)
Then Eq. (7-25) becomes
(7-26)
64
3. Filtration at Constant Pressure Drop
Example 7-4. Filtration in a Continuous RotaryDrum Filter
A rotary vacuum-drum filter having a 33%
submergence of the drum in the slurry is to be
used to filter a CaCO3 slurry as given in Example
7-1 using a pressure drop of 67.0 kPa. The solids
concentration in the slurry is cx = 0.191 kg
solid/kg slurry and the filter cake is such that the
kg wet cake/kg dry cake = m = 2.0. The density
and viscosity of the filtrate can be assumed as
those of water at 298.2 K.
65
3. Filtration at Constant Pressure Drop
Calculate the filter area needed to filter 0.778 kg
slurry/s. The filter-cycle time is 250 s. The specific
cake resistance can be represented by α = (4.37 x
109)(-Δp)0.3, where –Δp is in Pa and α in m/kg.
Solution:
From references for water, ρ = 996.9 kg/m3, μ =
0.8937 x 10-3 Pa∙s.
From Eq. (7-10),
66
3. Filtration at Constant Pressure Drop
Solving for α, α = (4.37 x 109)(67.0 x 103)0.3 =
1.225 x 1011 m/kg.
To calculate the flow rate of the filtrate,
67
3. Filtration at Constant Pressure Drop
Substituting into Eq. (7-24),
neglecting the resistance of the filter medium and
setting B = 0, and solving,
Hence, A = 6.60 m2.
68
4. Filtration at Constant
Volumetric Flow
4. Filtration at Constant Volumetric Flow
In some cases filtration runs are made under
conditions of constant rate rather than constant
pressure. This occurs if the slurry is fed to the filter
by a positive-displacement pump.
Equation (7-8)
can be rearranged to give the following for a
constant rate (dV/dt) m3/s:
(7-27)
70
4. Filtration at Constant Volumetric Flow
where
(7-28)
(7-29)
Kv is in N/m5 (lbf/ft5) and C is in N/m2 (lbf/ft2).
71
4. Filtration at Constant Volumetric Flow
Assuming that the cake is incompressible, Kv and
C are constants characteristic of the slurry, cake,
rate of filtrate flow, and so on.
Hence, a plot of pressure, -Δp, versus the total
volume of filtrate collected, V, gives a straight
line for a constant rate dV/dt.
The slope of the line is Kv and the intercept is C.
The pressure increases as the cake thickness
increases and the volume of filtrate collected
increases.
72
4. Filtration at Constant Volumetric Flow
The equation can also be rearranged in terms of
-Δp and time t as variables.
At any moment during the filtration, the total
volume V is related to the rate and total time t as
follows:
(7-30)
Substituting Eq. (7-30) into Eq. (7-27),
(7-31)
73
4. Filtration at Constant Volumetric Flow
For the case where the specific cake resistance α
is not constant but varies as in Eq. (7-11),
(7-11)
this can be substituted for α in Eq. (7-27) to
obtain a final equation.
(7-27)
74
5. Analysis of Continuous
Rotary Vacuum Filters
5. Analysis of Continuous Rotary Vacuum Filters
There are three
stages involved in the
operation:
(1) cake formation
(2) cake washing
(3) cake discharge
(not affecting
the filter size
and the cycle
time)
5. Analysis of Continuous Rotary Vacuum Filters
Cake Formation
For compressible cake and negligible medium resistance,
t
 '  0  V 
1 s
2P
2
 '  0  V f 
2
 
  or t f 
1 s 
2P  A 
 A
where tf = cake formation time
Vf = volume of filtrate collected during the period
of tf
A = filtration area (submerged area of filter)
5. Analysis of Continuous Rotary Vacuum Filters
tf 
 '  0  V f 
1 s
2P
2
 
 A
Let tf = tc and A = AT

' 0  V f 
 tc 


1 s 
2P   AT 
2
where tc = cycle time
AT = total filter area
 = fraction of the drum submerged
5. Analysis of Continuous Rotary Vacuum Filters
Cake Washing
Two factors involved in the stage of cake washing:
(1) The fraction of soluble material remained after the wash
 Governing the volume of wash liquid required.
(2) The rate of wash liquid passes through the cake
 Controlling the fraction of cycle time for cake
washing.
5. Analysis of Continuous Rotary Vacuum Filters
Two factors involved in the stage of cake washing:
(1) The fraction of soluble material remained after the wash
 Governing the volume of wash liquid required.
An empirical equation for the fraction of soluble material
remained:
n
r  (1   )
where r = ratio of soluble material remained after the
wash to that originally present in the cake
n = volume of wash liquid divided by the volume of
retained liquid
 = washing efficiency of the cake
5. Analysis of Continuous Rotary Vacuum Filters
Two factors involved in the stage of cake washing:
(2) The rate of wash liquid passes through the cake
 Controlling the fraction of cycle time for cake washing.
The wash liquid contains no additional solids.
(1) The cake thickness is constant.
 The flow of wash liquid is constant.
(2) Wash rate
= filtration rate at the end of cake formation
5. Analysis of Continuous Rotary Vacuum Filters
1 dVw Vw
Wash rate 

A dt
At w
where Vw = volume of wash water required,
tw = time required for washing.
5. Analysis of Continuous Rotary Vacuum Filters
1 dV
Filtration rate at the end of cake formation =
A dt t t f
t
 '  0  V 
2P
1 s
 
 A
2
V  2( P) t 
or


A   '  0 
1 s
1/ 2
 ( P)

1 dV
d V 
Wash rate 
  


A dt t t f dt  A  t t f  2 '  0t f 
1 s

Vw  ( P)

Washing rate =

Atw  2 '  0t f 
1 s
1/ 2
1/ 2
5. Analysis of Continuous Rotary Vacuum Filters
A useful expression:
1/ 2
Vw  ( P ) 


Atw  2  '  0t f 
1 s
1/ 2
 2( P ) 
and


At f   '  0t f 
1 s
Vf
Vw  2  '  0 t f 
 tw  
A  ( P )1 s 
1/ 2
V f   '  0 t f 
and t f 
A  2( P )1 s 
1/ 2
 t w  2 V w  2 V w Vr  2nf
tf
Vf
Vr V f
where Vr = volume of liquid retained
f = ratio of the volume of retained liquid (Vr) to
the volume of filtrate (Vf)
6. Equipments
6. Equipment
Filtration Equipment
The basic requirements for filtration equipment are:
mechanical support for the filter medium,
flow accesses to and from the filter medium and
provision for removing excess filter cake.
In some instances, washing of the filter cake to remove
traces of the solution may be necessary.
Pressure can be provided on the upstream side of the
filter, or a vacuum can be drawn downstream, or both
can be used to drive the wash fluid through.
86
6. Equipment
Plate and frame filter press
In the plate and frame filter press, a cloth or mesh is
spread out over plates which support the cloth along
ridges but at the same time leave a free area, as large as
possible, below the cloth for flow of the filtrate.
This is illustrated in Fig. 7.3(a).
The plates with their filter cloths may be horizontal, but
they are more usually hung vertically with a number of
plates operated in parallel to give sufficient area.
87
6. Equipment
(a)
FIG. 7.3 Filtration equipment: (a) plate and frame press
88
6. Equipment
Filter cake builds up on the upstream side of the cloth,
that is the side away from the plate.
In the early stages of the filtration cycle, the pressure
drop across the cloth is small and filtration proceeds at
more or less a constant rate.
As the cake increases, the process becomes more and
more a constant-pressure one and this is the case
throughout most of the cycle.
When the available space between successive frames is
filled with cake, the press has to be dismantled and the
cake scraped off and cleaned, after which a further cycle
can be initiated.
89
6. Equipment
Fig. 7.3(a’) Filtration operation in a plate-and-frame press filter
90
6. Equipment
Fig. 7(a") Washing operation of the cake.
91
6. Equipment
The plate and frame filter press is cheap but it is difficult
to mechanize to any great extent.
Variants of the plate and frame press have been
developed which allow easier discharging of the filter
cake.
For example, the plates, which may be rectangular or
circular, are supported on a central hollow shaft for the
filtrate and the whole assembly enclosed in a pressure
tank containing the slurry.
Filtration can be done under pressure or vacuum.
92
6. Equipment
The advantage of vacuum filtration is that the pressure
drop can be maintained whilst the cake is still under
atmospheric pressure and so can be removed easily.
The disadvantages are the greater costs of maintaining a
given pressure drop by applying a vacuum and the
limitation on the vacuum to about 80 kPa maximum.
In pressure filtration, the pressure driving force is limited
only by the economics of attaining the pressure and by
the mechanical strength of the equipment.
93
6. Equipment
Rotary filters
In rotary filters, the flow passes through a rotating
cylindrical cloth from which the filter cake can be
continuously scraped.
Either pressure or vacuum can provide the driving force,
but a particularly useful form is the rotary vacuum filter.
In this, the cloth is supported on the periphery of a
horizontal cylindrical drum that dips into a bath of the
slurry.
Vacuum is drawn in those segments of the drum surface
on which the cake is building up.
94
6. Equipment
A suitable bearing applies the vacuum at the stage
where the actual filtration commences and breaks the
vacuum at the stage where the cake is being scraped off
after filtration.
Filtrate is removed through trunnion bearings.
Rotary vacuum filters are expensive, but they do provide
a considerable degree of mechanization and
convenience.
A rotary vacuum filter is illustrated diagrammatically in
Fig. 7.3(b).
95
6. Equipment
(b)
FIG. 7.3 Filtration equipment: (b) rotary vacuum filter
96
6. Equipment
Fig. 7.3(b’) Rotary vacuum filter
97
6. Equipment
Centrifugal filters
Centrifugal force is used to provide the driving force in
some filters.
These machines are really centrifuges fitted with a
perforated bowl that may also have filter cloth on it.
Liquid is fed into the interior of the bowl and under the
centrifugal forces, it passes out through the filter
material.
This is illustrated in Fig. 7.3(c).
98
6. Equipment
(c)
FIG. 7.3 (c) centrifugal filter
99
6. Equipment
Air filters
Filters are used quite extensively to remove suspended
dust or particles from air streams.
The air or gas moves through a fabric and the dust is left
behind.
These filters are particularly useful for the removal of
fine particles.
One type of bag filter consists of a number of vertical
cylindrical cloth bags 15-30 cm in diameter, the air
passing through the bags in parallel.
100
6. Equipment
Air bearing the dust enters the bags, usually at the
bottom and the air passes out through the cloth.
A familiar example of a bag filter for dust is to be found
in the domestic vacuum cleaner.
Some designs of bag filters provide for the mechanical
removal of the accumulated dust.
For removal of particles less than 5 mm diameter in
modern air sterilization units, paper filters and packed
tubular filters are used.
These cover the range of sizes of bacterial cells and
spores.
101