Introduction
Filtration may be defined as the separation of
solids from liquids by passing a suspension
through a permeable medium which retains
the particles.
________
_______ _______
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Konsep
dasar
Filtrat
Residu
Feed
1.Surface filters
used for cake filtration in which the solids are
deposited in the form of a cake on the up-stream
side of a relatively thin filter medium.
Figure2. Mechanism of cake filtration
2.Depth filters used for deep bed filtration in which particle
deposition takes place inside the medium and
cake deposition on the surface is undesirable.
Figure 3. Mechanism of deep bed filtration
The fluid passes through the filter medium,
which offers resistance to its passage,
under the influence of a force which is the
pressure differential across the filter.
rate of filtration = driving force/resistance
•The filter-cake resistance is obtained by multiplying the specific resistance of the filter cake, that is its resistance per unit thickness, by the thickness of the cake.
•The resistances of the filter material and pre-coat are combined into a single resistance called the filter resistance.
•It is convenient to express the filter resistance in terms of a fictitious thickness of filter cake.
•This thickness is multiplied by the specific resistance of the filter cake to give the filter resistance.
Factor affected on filtration
• Pressure drop ( ∆P )
• Area of filtering surface ( A )
• Velocity of filtrate ( v )
• Resistance of filter cake ( α )
• Resistance of filter medium ( Rm )
• Properties of slurry ( μ)
rate of filtration = driving force/resistance
-(P) or
Pressure
drop
Filter cake ()
Filter medium (Rm)
Viscosity ()
Filter medium resistance; R
Normally be constant but may vary with
time (as a result of some penetration of
solid into the medium) and sometimes may
also change with applied pressure
(because of the compression of fiber in the
medium).
•As the overall pressure drop across an
installed filter include losses not only in the
medium but also in the associated piping
and in the inlet and outlet ports
•It is convenient in practice to include all
these extra resistances in the value of the
medium resistance R.
Constant pressure filtration
• Equation is useful because it covers a situation
that is frequently found in a practical filtration
plant.
• We could predict the performance of filtration
plant on the basis of experimental results.
• If a test is carried out using constant pressure,
collecting and measuring the filtrate at measured
time intervals
Filtration Equipment
The basic requirements for filtration equipment
are:
• mechanical support for the filter medium
• flow accesses to and from the filter medium
• 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.
1.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.
• 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.
• 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.
• The plate and frame filter press is cheap but it is difficult to mechanize to any great extent.
• Filtration can be done under pressure or vacuum. – 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
2.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.
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.
3.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.
4. Clarrifying filtration
• Filtration that use gravitational force only as the driving for
• It has low energy requirement
• Simple and low cost
• Need large amount of processing time
21/02/2017
EQUATIONS:
CONSTANT
PRESSURE: P
Rm
A
V
P
Cs
AV
t
2/
t
V
A
Rmt
t
V
A
CsP
.
2
2
CONSTANT
RATE:
21/02/2017
Excercise 1
A slurry containing 25 kg dry solids/m3 of filtrate
across the filter medium area 2 m2 at a constant
pressure (60 Pa). The viscosity of filtrate was
0.001 Pa.s. filtrate volume was changed by the
time as theese data:
Determine the specific cake resistance and filter
medium resistance.
Time (s) Filtrate volume
10 25 20 29 40 34 60 56
21/02/2017
Solution
Time
(min)
Filtrate
volume(m3) tA/V V/A
10 0.8 12.5 0.8
20 1.37931 14.5 1.37931
40 2.352941 17 2.352941
60 2.142857 28 2.142857
21/02/2017
mRm
xRm
P
Rm
/1_25092
604182.0)001.0(
4182.0
kgm
x
P
Cs
/69.2400
)60)(2(5002.0)25()001.0(
5002.02
21/02/2017
Excercise 2
A slurry containing 25 kg dry solids/m3 of filtrate
across the filter medium area 2 m2 at a constant rate
of 0.01 m3/s. The viscosity of filtrate was 0.001 Pa.s.
pressure drop was changed by the time as theese
data:
Determine the specific cake resistance and filter
medium resistance.
Time (min) Pressure drop
10 225
20 400
40 468
60 560
21/02/2017
mxRm
xRm
t
V
A
Rm
/1109.2
2145)01.0()001.0(
145
7
kgmx
x
t
V
A
Cs
/107168.1
)4(3.10701.0)25()001.0(
3.107
8
2
2
2
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