1. Department of Pharmaceutics
FILTRATION
Department of Pharmaceutics

2. Introduction:
Filtration is a unit operation, it may be defined as the process of
separation of solids from a fluid by passing the same porous
medium that retains the solid, but allow the fluid to pass through.
Filtration is also termed as CLARIFICATION.
Clarification is a termed used to describe process that involve the
removal or separation of a solid from a fluid, or a fluid from
another liquid

3. TYPES OF FILTRATION
Solid/fluid filtration
Solid/liquid filtration
Solid/gas filtration
Fluid/fluid filtration

4. Based on mechanism, three types of filtration are broadly identified:
Surface filtration
Depth filtration
Cake filtration

5. Surface filtration:
It is a screening action by which pores or holes of the medium prevent the passage of solids.
The mechanism of straining and impingement are responsible for surface filtration for this purpose plates with holes or woven sieves are used.

6. Depth filtration:
In this process, slurry penetrates to a point where the diameter of solid particles is greater than that of the tortuous void.
Depth filtration is aided by the mechanism entanglement .the solids are retained with a gradient with a gradient density structure by physical restriction or absorbing of the physical properties of the medium

7. Cake filtration: e.g. filter cake made from diatomite.
A filter consists of a coarse woven cloth through which a concentrated suspension of rigid particles is passed so that they bridge the holes and form a bed.
e.g. filter cake made from diatomite.

8. Mechanisms of filtration:
The mechanisms of filtration where by a filter is significant only in the initial stages of filtration. Some of the mechanisms:
a. Straining
b. impingement
c.entanglenment
d.attractive forces

9. straining:
It is similar to sieving, i.e., the particles of larger size cannot pass through the smaller pore sizes of the filter medium.

10. Impingement:
Solids having the momentum move along the path of streamline flow and strike the filter medium, so that solids are retained on the filter medium

11. Entanglement: Attractive forces:
Particles become entwined in the mass of fibres due to smaller size of particles than the pore size. Thus the solids are retained on the filter medium.
Attractive forces:
solids are retained on the filter medium as a result of attractive forces between particles and filter medium, as in case of electrostatic precipitation.

12. Factors affecting the rate of filtration:
Properties of liquids:
Density
Viscosity
Corrosiveness
Properties of solid:
Particle shape
Particle size distribution
Particle charge
Particle size
Rigidity
C. Proportion of solids in the slurry-rate at which the filter cake is formed, especially in large stages of filtration
d. Temperature
e. Area of the filter surface
f. Resistance of filter cake and filter
g. viscosity of the filtrate
h. The pressure difference across the filter

13. Theories of filtration:
The flow of a liquid through a filter follows the basic rules that govern the flow of any liquid through the medium offering resistance. The rate of flow can be expressed as
Driving force
Rate =
Resistance

14. Poiseuille’s equation:
Poiseuille considered that filtration is similar to the streamline flow of a liquid under pressure through capillaries
∏▲Pr4 V=
8Lŋ
Where V= Rate of flow, i.e., volume of liquid flowing in unit time,m3/s
▲P= Pressure difference across the filter,pa
r = radius of the capillary in the filter bed, m
L= thickness of the filter cake, m
Ŋ=viscosity of the filtrate, Pascal’s

15. Darcy’s equation:
Poiseille’s law assumes that the capillaries found in the filter are highly irregular and no uniform .therefore, if the length of a capillary is taken as a thickness of the bed, a correction factor for radius is applied so that the rate equation is closely approximated and simplified.
KA▲P V= ŊL
Where, K=permeability coefficient of the cake,m2
A=surface area of the porous bed (filter medium),m2
The term k depends on the characteristics of the cake such as porosity, specific area and compressibility.
Permeability may be defined quantitatively as the flow rate of a liquid of unit viscosity across a unit area of cake having unit thickness under a pressure gradient of unity.

16. Kozeny-carman equation:
Poiseuille’s equation is made applicable to porous bed, based on a capillary type structure by including additional parameters. Thus this equation is widely used for the filtration
A ▲pΣ3 V=
Ŋs2 kL (1-Σ) 2
Where Σ= porosity of the cake
S= specific surface area of the particle comprising the cake
K= Kozeny equation

17. Classification of filtration equipment:
Equipments are classified based on the application of external force:
Pressure filters- plate and frame filter press and meta filters
Vacuum filters- filter leaf
Centrifugal filters

18. Classification based on the operation of filtration:
Continuous filtration- discharge and filtrate are separated
steadily and uninterrupted
Discontinuous filtration- discharge of filtered solids is
intermittent. Filtrate is removed
continuously .the operation should
be stopped to collect the solids.

19. Classification based on nature of filtration:
Cake filters- remove large amounts of solid
Clarifying filters- remove small amount of solids
Cross-flow filters- feed of suspension flows under
pressure at a fairly high velocity
across the filter medium

20. Plate and frame filter press:

24. Principle:
The mechanism is surface filtration. The slurry enters the frame by pressure and flows through the filter medium. The filtrate is collected on the plates and sent to the outlet. A number of plates and frames are used so that surface area increases and consequently large volume of slurry can be processed simultaneously with or without washing.

25. working Filtration operation:
Slurry enters the frame from the feed channel and passes through the filter medium on to the surface of the plate.
The thickness of the cake is half of the frame thickness because on each side of the frame filtration occurs.
In general there will be optimum thickness of filter cake for any slurry depending on the solid content in the slurry and the resistance of the filter cake.
The filtrate drains between the projections on the surface of the plate and escapes from the outlet.
As filtration proceeds the resistance of the cake increases and the filtration rate decreases. The press is emptied and the cycle is restarted.

26. Washing operation:
Two cakes are built up in the frame meeting eventually in the middle .
This means that flow is brought virtually to a stand still.
A modification of the plate and frame press is used for this purpose an additional channel is included.
In half the wash plate there is a connection from the wash water channel to the surface of the plate.
The sequence of arrangement of plates and frames can be represented by dots as so on

27. Applications
1. Foods industry: Vinegar, Honey, Water, Fruit juice, Soft drinks, Edible oil
2. Chemicals: Paints, Varnish, Polishes, Pigments, Lubrication oils, Ink
3. Pharmaceuticals and Cosmetics
Syrups
Hand lotion
Shampoo
Soap
Proteins
Perfume

28. Advantages:
Construction of filter press is very simple and a variety of materials can be used like Cast iron, bronze, stainless steel, hard rubber or plastics wood
It provides the large filtering area in a relatively small floor space
Versatile
The sturdy construction permits the use of considerable pressure difference
Efficient washing of cake is possible
It produces dry cake in the form of slab

29. Disadvantages:
It is a down filter, so there is a good deal of down
down- time , which is non productive
Expensive filter
Emptying time, the labour involved and the wear and
tear of the cloth resulting in high costs.
Operation is critical.

30. Drum filters:

31. Drum filter Principle: Construction:
It functions on the principle of filtering the slurry through sieve like mechanism on a rotating drum surface under the conditions of vaccum. In addition compression drying and removing filter cake are possible.
Construction:
It consists of a perforated plate with curved surface which supports a filter cloth.
The drum is radially partitioned dividing the annular space into separate compartments. Each of it is connected by an internal pipe to the centre of the drum through a rotating valve.

33. Working: The drum is rotated at a speed less than 1 revolution/minute
As it dips vaccum is applied in this segment so that the solid is build up on the surface. The liquid passes through the filter cloth in to an internal pipe n valve.
In the drainage section excess liquid is drawn. Compression rollers consolidate cake for improving washing n drying process.
In washing zone water is sprayed on cake. A separate vaccum is applied on the panel in order to suck the wash liquid and air through the cake of solids.
Hot air in drying zone removes moisture.

34. Uses:
Utilised to filter slurries containing high proportion of solids up to 15 to 30%.
In the production of pencillins the extract is separated from mycellium by drum filters.
These are used for collectiong caco3, starch n magnesium carbonate

35. Advantages: Disadvantages:
Suitable for concentrated slurries.
Speed of rotation can be varied n cake thickness can be controlled
Disadvantages:
Expensive n contains moving parts like vaccum pumps,traps etc,.
Cake tends to crack.