Filtration

DEFINITION
Process where by a SOLID
is separated from a
LIQUID or gas by means
of a POROUS MEDIUM.
POROUS MEDIUM which
retain the solid but allows the
fluid to pass.
SLURRY the suspension of
solid and liquid to be filtered
FILTER CAKE the
accumulation of solids on the
filter
FILTRATE the clear liquid
passing through the filter
Dr. O. Alkarim
Note: Clarification : Solid
present in small proportion

Factors affecting process:
1. Particle size.
2. Physical state.
3. Quantity of suspended matter.
4. The speed of the operation.
5. Characteristic of the fluid medium.
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PRINCIPLE
Note: if the viscosity of the
liquid is high, the waiting
period may increase
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 Generally liquid
allows to stand in a
suitable container until
the suspended
particles either
SETTLED or RISEN.

Failure!!!!
 If the particle in
colloidal size!!!!
 Approach densities
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Acceleration of process
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a. Centrifuge
b. Filtration
c. Adsorption
d. Absorption

Filter media
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 Selection depend on purpose of filtration and
quantities.
 General properties of filters:
1. Inert.
2. Maximum passage of liquid.
3. Mechanical strength.
4. Filter even negligible amount
5. Delivering a clear filtrate at a suitable production rate.

Classifications of media
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Woven material
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 these include:
a. wire screening e.g. stainless steel is durable, resistance to
plugging and easily cleaned.
b. fabrics of cotton, wool, and nylon.
Cotton is a common filter
Nylon is superior for pharmaceutical use, Because it is
unaffected by mold, fungus or bacteria and has negligible
absorption properties

Porous plates
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These include perforated metal or rubber plates,
natural porous materials such as stone,
porcelain or ceramics, and sintered glass

Membrane filter
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 These are basic tools for micro-filtration,
useful in the preparation of sterile solutions.
These filters are made by casting of various
esters of cellulose, or from nylon, Teflon,
polyvinyl chloride. The filter is a thin
membrane with millions of pores per square
centimeter of filter surface.

Un woven material
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 filter paper is a common filter medium since
it offers controlled porosity, limited
absorption characteristic, and low cost.

Granular material with suitable support
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 Include sand, gravels, diatomaceous, and charcoal

Through porous media
Mechanism of filtration
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Rate of Filtration
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Factors affecting rate of filtration
 Resistance
 Viscosity
 Pressure drop
 Size of the pore
 Blocking potential of
particle.
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FILTER AID
 Usually, the resistance to flow due to the filter
medium itself is very low, but will increase as a layer
of solids builds up , blocking the pores of the medium
and forming a solid cake.
 The object of the filter aid is to prevent the medium
from becoming blocked and to form an open, porous
cake, so reducing the resistance to flow of the filtrate.
The particles must be inert, insoluble, incompressible,
and irregular shaped.
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 Filter aids may be used in either or both two ways:
1. Pre- coating technique: by forming a pre-coat
over the filter medium by filtering a
suspension of the filter aid .
2. Body- mix technique: A small proportion of
the filter aid (0.1-0.5 %) is added to the slurry
to be filtered. This slurry is re-circulated
through the filter until a clear filtrate is
obtained, filtration then proceeds to
completion. Dr. O. Alkarim

 The following filter aids may be used:
1. DIATOMITE ( Kieselguhr ): obtained from
natural siliceous deposits.
2. PERLITE , it is an aluminum silicate.
3. CELLULOSE and ASBESTOS.
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FILTRATION EQUIPMENT
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Four groups may be listed:
1. Gravity filters.
2. Vacuum filters
3. Pressure filters.
4. Centrifugal filters.

Gravity filter
 Employing thick
granular beds
 widely used in water
filtration e.g. Sand
Filter
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VACUUM FILTERS
Vacuum filters operate practically at
higher pressure differentials than
gravity filters.
1. Leaf filter
2. Rotary vacuum filter
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LEAF FILTER
 consisting of a frame enclosing a
drainage screen or grooved plate
, the whole unite being covered
with filter cloth.
 The outlet for the filtrate
connects to the inside of the
frame,. The frame may be
circular, square or rectangular
shapes.
 The operation: The leaf filter is
immersed in the slurry and a
receiver and a vacuum system
connected to the filtrate outlet.
Dr. O. Alkarim

ADVANTAGES
1. The slurry can be filtered from any vessel.
2. The cake can be washed simply by immersing the filter in a
vessel of water.
3. Removal of the cake is facilitated by the use of reverse air
flow.
4. The filter can be modified by employing a suitable number of
unites.
5. The leaf filter is most satisfactory if the solids content of the
slurry is not too high 5 % being a suitable maximum.
6. Labor costs for operating the filter are comparatively
moderate. Dr. O. Alkarim

ROTARY FILTER
 In large –scale operation, continuous operation is sometimes desirable and
it may be necessary to filter slurries containing a high proportion of solids.
 The rotary filter is continuous in operation and has a system for removing
the cake that is formed , so, it is suitable for use with concentrated slurries.
 It is a metal cylinder mounted horizontally, the curved surface being a
perforated plate, supporting a filter cloth. Internally, it is divided into
several sectors and a separate connection is made between each sector and
a special rotary valve.
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Operation…
 The drum is immersed to the required depth in the slurry, which is agitated
to prevent settling of the solids, and vacuum is applied to those sectors of
the drum which is submerged.
 A cake of the desired thickness is produced by adjusting the speed of
rotation of the drum. Each sector is immersed in turn in the slurry and the
cake is then washed and partially dried by means of a current of air. Finally,
pressure is applied under the cloth to aid the removal of the cake. Removal
of the washed and partially dried cake is affected by means of a doctor
knife.
Dr. O. Alkarim

 Summary of the process:
The drum is dipped into the slurry and vacuum applied to the outlet, which is
connected to the filtrate receiver. When the cake has formed, the cake drained
or partially dried by vacuum.
The drum is sprayed with water to wash the cake. Retaining the vacuum
connection drains the cake and produces partial dryness then, removed by a
doctor knife.
When the solids of the slurry are too much that the filter cloth becomes
blocked with the particles, a pre-coat filter may be used. A pre-coat of filter
aid is deposited on the drum prior to the filtration process.
Dr. O. Alkarim

ADVANTAGES
1. The rotary filter is automatic and is continuous in
operation, so that the labor costs are very low.
2. The filter has a large capacity , so it is suitable for
the filtration of highly concentrated solutions.
3. Variation of the speed of rotation enables the cake
thickness to be controlled.
4. Pre-coat of filter aid could used to accelerate the
filtration rate.
Dr. O. Alkarim

DISADVANTAGES
1. The rotary filter is a complex piece of equipment , with
many moving parts and is very expensive.
2. In addition to the filter itself, some accessories are connected
,e.g. a vacuum pump, vacuum receivers , slurry pumps and
agitators are required.
3. The cake tends to crack due to the air drawn through by the
vacuum system, so that washing and drying are not efficient.
4. Being a vacuum filter, the pressure difference is limited to 1
bar and hot filtrates may boil.
5. It is suitable only for straightforward slurries
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USES
1. The rotary filter for continuous operation on large
quantities of slurry.
2. Suitable for slurry contains considerable amounts of
solids in the range 15-30%.
Examples of pharmaceutical application include the
collection of calcium carbonate, magnesium carbonate,
and starch, and the separation of the mycelium from the
fermentation liquor in the manufacture of antibiotics
Dr. O. Alkarim

PRESSURE FILTERS
 Due to the formation of cakes of low permeability, many types
of slurry require higher pressure difference for effective
filtration than can be applied by vacuum techniques.
 Pressure filters are used for such operations.
 However, high operational pressures, may prohibit
continuous operation because of the difficulty of discharging
the cake whilst the filter is under pressure.
 Examples are the sweet-land filter, plate and frame filter press.
Dr. O. Alkarim

PLATE AND FRAME FILTER PRESS
 This press is made up of two units, known
respectively as plates and frames, with a filter
medium, usually filter cloth, between the two.
 The frame is open, with an inlet for the slurry, while
the plate has grooved surface to support the filter
cloth, and with an outlet for the filtrate
Dr. O. Alkarim

Dr. O. Alkarim
Plate and Frame Press

Operation…
The slurry enters the frame from the feed channel,
 The filtrate passes through the filter medium on to
the surface of the plate while the solids form a filter
cake in the frame.
 The filtrate then drained down the surface of the plate
, between the projections on the surface and escapes
from the outlet.
 Filtration is continued until the frame is filled with
filter cake, when the process is stopped , the frame
emptied, and the cycle re-started.
Dr. O. Alkarim

Dr. O. Alkarim
 Channels for the slurry inlet and the filtrate outlet can be arranged by fitting
eyes to the plates and frames. This has the advantages that the filtrate from
each plate can be seen and, in the event of a broken cloth, the faulty plate can
be isolated and the filtration continued with one plate less.
 The thickness of the cake can be varied by using frames of different
thickness and, in general, there will be an optimum thickness of filter cake
for any slurry, depending on the solids content of the slurry and the resistance
of the filter cake.
 As filtration proceeds, the resistance of the cake increases and the filtration
rate will decrease. At a certain point it will be preferable in terms of the
overall output of the process, to stop and empty the press rather than to
continue filtration at a very low flow rate.
 Plates and frames may be made in various metals to provide resistance to
corrosion or prevent metallic contamination of the product. Non-metals e.g.
plastics is lighter, also varieties of wood are satisfactory materials of
construction.
 Plates and frames may be of considerable size, of about 1m square

ADVANTAGES
1. Construction is very simple and a wide variety of materials can be used.
2. It provides a large filtering area in a relatively small floor space.
3. It is versatile, the capacity being variable according to the thickness of the
frames and the number used.
4. The construction permits the use of considerable pressure difference.
5. Efficient washing of the cake is possible.
6. Operation and maintenance is straightforward , because there no moving
parts, filter cloths are easily renewable and, because all joints are
external, any leaks are visible and do not contaminate the filtrate.
Dr. O. Alkarim

DISADVANTAGES
1. It is a batch filter, so it is a time consuming.
2. The filter press is an expensive filter, the emptying time, the labour
involved, and the wear and tear on the cloths resulting in high
costs.
3. Operation is critical, as the frames should be full, otherwise
washing is inefficient and the cake is difficult to remove.
4. The filter press is used for slurries containing less about 5 % solids
5. In view of the high labor costs , it is most suitable for expensive
materials .e.g. the removal of precipitated proteins from insulin
liquors.

Filtration

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