Filtration,equipments used for filtration,Calrification

1. CLARIFICATION
AND FILTRATION
Prepared by
Mrs. Ashwini P. Shewale
M.Pharm
(PDEA’s Shankarrao Ursal College of
Pharmacy, Kharadi)
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2. Clarification and Filtration
Filtration is the process, whereby solid particles are separated
from the liquid or gas by passing it through a porous medium
which retains the solid but allows the fluid to pass.
When solids are present in a very small proportion i.e. not
exceeding 1.0 per cent the process of its separation from liquid
is called 'clarification'.
Ultrafiltration may be defined as the separation of intermicellar
liquid from solids by the use of pressure on a semi permeable
membrane .
The porous medium used to retain the solids is known as filter
medium. The solid which gets collected on the filter is called
filter cake and the clear liquid passing through the filter is known
as filtrate.
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4. When the object of filtration is to remove large visible particles by
using coarse filtering medium such as muslin and cotton wool, the
process is known as "straining".
Theory of Filtration The theory of filtration gives an idea about
the factors influencing the rate of filtration through the filtering
medium,rather than the mechanism by which the particles are retained.
The factors affecting the rate of filtration were studied by a scientist
named. Darcy and he expressed it in the form of an equation, which is
known as "Darcy's law". The equation is:
where V = Volume of filtrate
K = Permeability coefficient and is dependent on the nature of the precipitate to be
filtered and the filter medium
A = Area of filter bed
ΔP = Pressure difference on the liquid and below
the filter medium
ŋ= Viscosity of the fluid
l = Thickness of filter cake
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5. Factors Affecting the Rate of Filtration
1. Pressure : The rate of filtration of liquid is directly proportional
to the pressure difference between the 'filter medium' and 'filter cake'.
Thus, the rate of filtration can be increased by applying pressure on the
liquid being filtered or by decreasing the pressure beneath the filter.
2. Viscosity : The rate of filtration is inversely proportional to the
viscosity of the liquid undergoing filtration. Liquids which are very
viscous get filtered slowly in comparison to liquids with low viscosity.
Reduction of viscosity of a liquid by raising the temperature is frequently
done in order to accelerate filtration e.g. syrups are more quickly filtered
when hot than cold.
3. Surface area of filter media : The rate of filtration is directly
proportional to the surface area of filter media. Pleating the filter paper
or using a fluted funnel increases the effective surface area of filter paper
for filtration. Filter press also works on the same principle.
4. Temperature of liquid to be filtered : Temperature plays an
important role in the rate of filtration. Viscosity is reduced by a rise in
temperature and the filtration of viscous oils, syrups etc. is often
accelerated by filtering them while they are still hot. 5

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7. 5. Particle size : The rate of filtration is directly proportional to the
particle size of the solid to be removed. It is easier to filter a liquid
having coarse particles than that having finely divided particles because
coarse filtering medium can be used to filter liquid having coarse particles
and hence it increases the rate of filtration. Therefore before filtration, some
method should be adopted to agglomerate the finely divided particles into
coarse particles or to increase the particle size by precipitation.
6. Pore size of filter media : The rate of filtration is directly
proportional to the pore size of the filter media. The liquid having coarse
particles requires a coarse filtering media to remove them. So, the rate of
filtration is increased when a coarse filter medium is used for filtration.
7. Thickness of cake : The rate of filtration is inversely proportional
to the thickness of the filter cake formed during the process of
filtration. As the filtration process proceeds, the solid particles start
depositing on the filter medium, and thus, it increases the thickness of
the cake and decreases the rate of filtration.
8. Nature of the solid material : The rate of filtration is directly
proportional to the porosity of the filter cake. The porosity of the filter
cake depends on the nature of the solid particles 10 be removed from the
liquid. Filter aids are sometimes added to the filtering liquid to make a
porous cake.
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8. To sum up, the rate of filtration which depends on various factors can
be written as:
Rate of Filtration = area of filter x pressure difference
Viscosity x resistance of cake and filter
Filter Media
The surface upon, which solids are retained in the process of filtration is known as
'filter medium'. A good filter media should possess the following characteristics:
1. It should not absorb the constituents of the liquid preparation undergoing
filtration.
2. It should be resistant to corrosive action of the liquid.
3. It should have smooth surface for easy discharge of cake.
4. It should allow free flow of liquid.
5. It should be resistant to chemical reaction with the contents of the liquid to be
filtered.
6. It should have sufficient mechanical strength to resist the damage that occurs
during filtration of liquid under pressure.
The selection of filter media depends on:
1. The quantity of the material to be filtered.
2. The nature of the product to be filtered i.e. its viscosity etc.
3. The size of the solid particles to be retained on the filter
4. Purpose of filtration. 8

9. The various filtering media employed for filtration are:
1. Filter paper : Filter paper is most commonly used as filter media for
general filtration. The filter papers are classified as coarse,
medium and fine according to their pore size. For the filtration of
pharmaceutical preparations, a fine grade of filter paper is used. The
main drawback of a filter paper is that it has an absorbing property and also
it sheds fine particles into the filtrate.
2. Cotton wool : Cotton wool is generally used for filtering of moderately
coarse particles. The disadvantage of cotton is that, it sheds fine cotton hair
in the filtrate. This problem can be solved to a large extent by using cotton
wool of long staple and wetting the cotton wool plug with liquid before its use.
3. Glass wool : It consists of fine fibres of glass which are resistant to
corrosive liquids. So it is used for filtering corrosive liquids, such as
strong acids, alkalies and oxidising agents e.g. solutions of chromic acid
and potassium permanganate.
4. Asbestos : It is used for the same purpose as glass wool. It is commonly
used in gravimetric analysis for collecting fine precipitates
which are to be subsequently ignited.
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10. 5. Fine muslin : It is used for removing very coarse particles. This
is done generally before passing the liquid through a fine filter media, in
order to avoid choking of the fine filter with coarse particles.
6. Filter cloth : Both synthetic and cotton cloth is used as filter
media for large scale filtration e.g. in filter press synthetic cloth has
more strength than cotton cloth. Moreover, synthetic cloth has less
absorbing property than cotton cloth. Hence, synthetic clothes are
preferred nowadays.
7. Membrane filters : These are made of cellulose acetate and
other cellulose derivatives, nylon and polyvinyl chloride. These are
mainly used to remove microorganism and very fine particles from
ophthalmic solution and parenteral products.
8. Sintered glass filters : These are made from borosilicate glass.
The glass is powdered and sifted to produce uniform particles. These are
heated to sintered point and molded into a flat or convex plate, which is
fitted into the glass funnel of suitable shape and size. These filters are
mainly used for the filtration of liquids meant for parenteral use.
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13. Filter Aids-
These are the substance which reduce the resistance of the filtrate to flow.
These are added to the preparation in concentration from 0.1 to 0.5 per
cent before filtration. An ideal filter aid should possess the following
qualities:
1. It should be able to remain suspended in the liquid.
2. It should be free from impurities.
3. It should be inert to the liquid being filtered.
4. It should have a particle size distribution suitable for the
retention of solids as required.
5. It should have a structure that permits formation of porous
cake.
The object of the filter aid is to prevent the filter medium from
becoming blocked and to form an open, porous cake, so reducing the
resistance to flow of the filtrate. Hence filter aid must be light, porous
and inert. Filter aids are mainly used in clarification process where the
solids are discarded. The commonly used filter aids in pharmacy are
diatomite (dialomaceous earth), perlite, asbestos, cellulose, activated
charcoal, talc and kaolin etc.
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15. FILTERING DEVICES USED IN PHARMACEUTICAL INDUSTRY
Various filtering devices used in pharmaceutical industry are:
1. Filter press 2. Meta filters
3. Filter leaf 4. Membrane filters
5. Filter candles 6. Sintered filters
Filter Press
There are two basic forms of the filter press but only the Plate and
Frame Press has a wide application in pharmaceutical industry.
PLATE AND FRAME FILTER PRESS
Construction It consists of plates and frames. The frame is open
and is used as an inlet for the material to be filtered. Plate has a grooved
surface which gives support to the filter cloth. The plate and frame may
be made of various metals which provide resistance to corrosion or
prevent metallic contamination of the filtrate. Filter cloth is fitted on each
side of the plate. The plates and frames are placed alternatively and
fitted in the outer frame of the press. Each plate acts as a single filtration
unit. The outlet of each plate is connected to a common outlet pipe.
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18. Working As shown in the diagram (C) the filtering liquid enters the frame
under pressure from the feed channel. The filtrate passes through the
filter medium on to the surface of the plate.
The filtrate is collected in the plates from where it is collected through
common outlet pipe. The cake is deposited in the frames. The process of
filtration is continued until the frame is filled with filter cake.
When the process is stopped, the frame is emptied and the cycle is
restarted.
The thickness of the cake can be varied by using frames of different
thickness. The thickness of filter cake depends mainly on the solid
content present in the filtering liquid and the resistance of filter cake.
Advantages of filter press
1. The filtering media can be used repeatedly.
2. Operation and maintenance is simple.
3. It requires less space.
4. It provides a large surface area for filtration.
5. Efficient washing of cake is possible.
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19. Disadvantages of filter press
1. It is not a continuous process.
2. It is used where the proportion of solid in the liquid to be filtered is about
5% or less.
3. The process becomes costlier because a lot of labour is required for
washing and replacement of the filter cloth.
4. Leakage between the plates may take place through faulty assembly.
Meta Filters
Construction It consists of a grooved, drainage rod on which, are
packed a number of metallic rings. These rings are usually of stainless
steel and have 0.8 mm thickness, 15 mm inside diameter and 22 mm
outer diameter. These rings have a number of semicircular projections
on one surface and when they are packed on the rod, the opening between
the rings is about 0.2 mm.
Working The entire assembly is placed inside a pressure vessel,
containing the liquid to be filtered. When vacuum is applied, liquid will
flow from outside to inside. In this form a metafilter can only be used as
strainer for coarse particles, j But for separation of finer particles, a bed
of suitable materials such as Kieselguhr is used. In this way the pack of
rings acts as a base on which the true filter medium is supported. 19

20. Uses The meta filter are mostly used for clarification of syrups, elixirs and
parenteral solutions.
Advantages of meta filter
1. It possess considerable strength and hence can withstand high pressure.
2. It is an economical filter.
3. The corrosive liquids can be filtered without any problem.
4. Cake can be removed effectively without any difficulty.
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22. Filter Leaf
Construction It is the simplest form of filter. It consists of a frame
enclosing a drainage screen or a grooved plate. Filter cloth is used as the
filtering medium, the whole unit being covered with filter cloth. The frame
used may be of any shape i.e. square, rectangular or of a circular shape.
Working The filter leaf is placed in a vessel containing the liquid to be
filtered. When vacuum is applied, the pressure inside the leaf is decreased.
Due to the difference in external pressure and pressure inside the leaf, the
liquid moves inside the filter through filter cloth. The filtrate is collected in
the receiver, whereas the cake gets collected on outside of the cloth.
This method has the advantage that the liquid can be filtered from
any vessel and cake can be washed simply by immersing it in a vessel
containing water. The cake can be removed just by flowing air in the
reverse direction.
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25. Membrane Filters
Membrane filters arc made of thin and flat membranes of cellulose;
derivatives, such as, cellulose acetate and cellulose nitrate. These fillers
are brittle when in dry condition and can be stored for an indefinite period.
The filters are between 50 and 150 jj thick and are available in sizes upto
60 cm2.
A membrane filter has 400 to 500 million pores per square centimetre
of filter surface. The pores are absolutely uniform in size and occupy
about 80% of filter volume. To avoid rapid clogging of a membrnne.
pre-filtration is often required. The selection of a membrane filter for
particular application depends on the particles to be removed- Table 8-2
shows the approximate pore size required to remove specified particles.
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26. A membrane filter is fixed in a metallic holder before its use. These
filters are mainly used for sterilization of both aqueous and oily liquids.
The membrane filters cannot be used for filtration of organic solvents,
such as alcohols, ketones, esters and chloroform.
Filter Candles
These are ceramic filters and are made of porcelain or Kieselguhr.
Kiesel-guhr filters are usually softer than the porcelain variety. These
are cylindrical candles with an opening which is connected to, vacuum
pump for reducing the pressure under it, during the filtration process.
The candles are available in a range of different pore size which is
designated by a number. The candle is placed in the solution to be filtered.
When vacuum is applied, the liquid will pass through the thick wall of the
candle and gets collected inside the candle from where
it is removed. The filter candle gets blocked with continuous use. This can
be cleaned by scratching the external surface with a nail brush and
passing water through it in the reverse direction. Fig 8-4 Filter Candle
These candles are used for sterilization of solutions. It has the tendency to
absorb materials from aqueous solutions. Therefore, it is not used
commonly. 26

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28. Sintered Filters
These are made of borosilicate glass. Borosilicate glass is finely
Powdered sieved and particle of desired size are separated. It is then
packed into a disc mould and heated to a temperature at which adhesion
takes place between the particles. The disc is then fused to a funnel of
suitable shape and size.
The sintered glass filters are available in different pore size. Hence
the funnel with a sintered filter is numbered according to the pore size.
The filtration is carried out under reduced pressure. These funnels are
used for bacterial filtration.
Sintered filters are also available in stainless steel which has a greater
mechanical strength. However these are very much liable to attack by
the solutions passing through them.
Pre-filtration are given to increases the rate of filtration These treatment is
mainly given to slurry and hence as pre-filtration treatment.
They include following treatments :
1. Sedimentation or setting 2. Viscocity reduction by heating
3. PH effects : Seperation of proteins 4. Agglomeration for removal of oil
globules with talc 5. Removal of proteins and tannins
6. Addition of filter aids.
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