Ind2, Lec 6 Filtration

1. Industrial Pharmacy 2
Lecture 6,Filtration (1)

2. Filtration:
It is the separation of a solid from a liquid or gas by means of porous
medium that retain the solid but allow the fluid to pass.
2

3. Purposes:
1. A need for liquid, free of colloidal hazes, or insoluble liquid drops.
2. A need for amorphous or crystalline precipitates
3. Sterility specifications as removal of microorganisms, using Millipore
filter
4. Removal of dust from air, using Bag filters
5. Filtration of air using Hepa filter
3

4. Principles:
1. Filtration:
It is a unit operation in which a mixture of solid and liquid, the “feed” or
“slurry” is forced through a porous medium in which the solids are
deposited or entrapped. The solids form a “Cake” and liquid is “filtrate”.
4

5. 2. Cake filtration:
If recovery of the solids is desired, the process is called cake filtration.
In case of suspension with 3 to 20 %, the septum acts only as a support in
this operation. The actual filtration being carried out by the solids
deposited as a cake. In such cases, solids may completely penetrate the
septum until the deposition of an effective cake occurs. Until, this time,
cloudy filtrate may be recycled.
5

6. 3. Clarification (decantation):
When the solids do not exceed 1.0 % and the filtrate is the primary product.
4. Surface or Absolute Filtration:
By which pores or holes in the medium prevent the passage of the solids.
The life of surface or absolute filters depends on the number of pores
available for the passage of fluid.
6

7. 5. Depth filtration:
Filtration may be carried out by the use of thick media, in which particles
perhaps a hundred times smaller than the dimensions of the passage
through the medium are removed. Because of this, such filters are with
sufficient depth so that the probability of the passage of the smallest
particle is extremely small.
7

8. Factors affecting filteration:
The equation combines all factors that affect the rate of filtration is known as
“Darcy Law” and may be expressed as:
dv K. A. P
q = =
dt m. L
Where:
q = volumetric rate of flow,
v = volume of filtrate,
t = time of filtration,
K = permeability coefficient,
A = area of filter medium,
P = pressure drop across the filter medium and filter cake,
m = Viscosity of the filtrate, and
L = thickness of cake. 8

9. “Kozeny” defines the permeability coefficient, K:
e 3
K = 5 (1 - e) 2 So
2
Where:
e = is the porosity of the bed and
So = is the specific surface of particles making the cake cm2/cm3.
K = Permeability Coefficient
9

10. Factors affecting the rate of filtration
1. Permeability coefficient or specific resistance:
The principle properties of fine solid offering the resistance of
the cake are:
a) The surface area of the particles,
b) The porosity of the cake, and
c) Rigidity or compressibility of the particles,
2. Area of filter medium:
It will be obvious that the total volume of filtrate flowing from
the filter will be proportional to the area of the filter.
10

11. 3- Pressure Drop:
The rate of filtration is proportional to the overall pressure drop that is across
both the filter medium and filter cake.
The pressure drop can be achieved by:
A- Gravity: Head of slurry above the filter medium.
B- Vacuum:
The pressure below the filter medium may be reduced below atmospheric
pressure by connecting the filtrate receiver to a vacuum pump and creating
a pressure differential across the filter.
C- Pressure:
Probably the commonest method in practice is to obtain a suitable
difference by applying pressure to the surface of the slurry, the simplest
method being to pump the slurry into the filter under pressure.
D- Centrifugal Forces:
In the same way that gravitational force could be replaced by centrifugal
force in particle separation, so, use can be made of centrifugal force in
filtration processes
11

12. 4. Viscosity of filtrate
Increase the viscosity of the filtrate will increase the resistance of
flow, and subsequently decrease the rate of filtration.
The rate of filtration may be increased by:
1) Raising the temperature of the liquid, this may not be
practicable if thermolabile material is involved or if the filtrate is
volatile.
2) Dilution is another alternative, if the volume is doubled by
diluting the filtrate with an equal volume of the liquid, the rate
must be more than doubled to show any advantage.
12

13. 5. Thickness of Filter Cake:
Cake resistance is a function of a cake thickness, therefore, when the
thickness of cake increases the rate of filtration will decreased.
13

14. FILTER MEDIA
A filter medium is a porous material which holds back the suspended
solids and allows the fluid to cross through it.
Examples of filter media: Sheets of woven material such as filter paper,
synthetic fabrics such as nylon. Unwoven fibrous materials such as cotton,
wool, linen fibers are also used. Granular or powdered material such as
gravel and sand are frequently used.
A good filter medium should last long, withstand pressure difference,
readily removable with the filter cake from the filtration equipment. It
should be able to withstand the temperature of the slurry and the action
of acid, alkali or other chemicals the slurry may contain
14

15. FILTER AIDS
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 impervious
cake.
The objective 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.
Thus, the ideal filter aid performs its functions physically or
chemically; must be light, porous, inert, insoluble,
incompressible, and irregularly shaped
15

16. Methods of addition of filter aids:
Filter aids may be used in either or both of two ways:
1. Pre-coating Technique:
By forming a pre-coat over the filter medium by filtering a suspension of
the filter aid sufficient to give a coating up to 0.5 Kg/m2
2. Body mixture technique:
A small proportion of the filter aid (0.1 to 0.5 %) is added to the slurry to be
filtered. This slurry is recirculated through the filter until a clear filter is
obtained; filtration then proceeds to completion.
The body-mix method is more common in batch pharmaceutical operation
because it minimizes equipment requirements and cross contamination
potentials.
16

17. Examples of filter aids:
Filter aid include:
talc,
fuller's earth (native hydrated aluminum silicate),
pumice stone,
asbestos fibers,
pulped filter paper,
cellulose,
diatomite (Diatomaceous Earth, Kieselguhr ), and
carbonates (e.g. magnesium and calcium),
17

18. CLASSIFICATION OF FILTERS
VACUUM FILTERS: The rotary vacuum filter
18

19. 19
The rotary vacuum filter

20. The rotary vacuum filter
The rotary filters are continuous in operation and have a system
for removing the cake that is formed so that it can be run for
long periods handling concentrated slurries.
Construction:
1- A rotary drum filter can be visualized as two concentric
cylinders with the annular space between them divided into a
number of septum by radial partitions.
2- The outer cylinder is perforated and covered with a filter cloth.
Each septum has a radial connection to a complicating rotary
valve whose function is to perform the operation
20

21. Advantages of rotary vacuum filter
These can be summarized as follows:
1- The rotary filter is automatic and continuous in operation, so that labor
costs are very low.
2- The filter has large capacity.
3- Variation of the speed of rotation enables the cake thickness to be
controlled, and for solids that form an impenetrable cakes the thickness
may be limited to less than 5 mm.
Uses:
1- Pharmaceutical applications include the collection of CaCO3, MgCO3 and
starch.
2- The separation of mycelia (the vegetative part of the fungus) from the
fermentation liquor in the manufacture of antibiotics.
21

22. Disadvantages of rotary vacuum filter :
These can be summarized as follows
1- The rotatory filter is complex piece of equipment, with many moving
parts, and is very expensive. In addition to the filter itself auxillary
equipment such as vacuum, pumps and agitators are required.
2- The cake tend to crack due to the air drawn through, by the vacuum
system, so that washing and drying are not efficient.
3- Being a vacuum filter the pressure difference is limited and hot filtrate
may boil.
4- The rotatory drum filter is suitable only for straight forward slurries, being
less satisfactory if the solids form an impermeable cake or will not separate
cleanly from the cloth.
22