Terms used in Filtration
- FILTRATION may be defined as the “Separation of solids from a fluid by means of a porous medium that retains the solids but allows the fluid to pass.
- CLARIFICATION: When solid does not exceed 1.0%, filtration is termed as clarification & filtrate is the primary product.
- STRAINING: The object of filtration is to remove large visible particles using a coarse filtering medium (muslin/cotton wool).
- ULTRA-FILTRATION: Separation of inter-micellar liquid from solid by use of high pressure on a semi-permeable membrane
- SLURRY/FEED: Suspension of solid in liquid to be filtered
- FILTER MEDIUM: Porous medium used to retain/entrap the solids
- FILTER CAKE/RESIDUE: Accumulation of solids on the filter medium
- FILTRATE/EFFLUENT: Clear liquid passing through the filter medium
- CAKE FILTRATION: If the recovery of the solid is desired, the process is called cake filtration.

Darcy’s Theory (Rate of Filtration)
V= Volume of filtrate
K= Permeability coff.
A=Area of filter bed
∆P= Pressure difference on liquid & below the filter medium
η= Viscosity of liquid
l= thickness of filter cake
Factor affecting the rate of filtration
- Factor affecting the rate of filtration
- Filtration
- Viscosity
- Surface area of filter media
- Temp.
- Particle size
- Pore size of filter media
- Thickness of cake
- Nature of solid materials (porosity of filter cake)
TYPES OF FILTER MEDIA
- TYPES OF FILTER MEDIA
- Filter paper (Grade-Coarse, medium & fine)
- Glass wool (Corrosive liquids)
- Cotton wool
- Asbestos(Al. silicate): Corrosive liquids, Alkalinity of filtrate)
- Sintered glass: borosilicate glass, Parenteral, corrosive liquid & Oxidising agents)
- Membrane filter
- Filter cloths
FILTER AIDS:
Substances that reduce the resistance of the filter cake & increase the filtration. (Added to the liquid slurry to be filtered)
Filter aids Should form a porous cake & Insoluble in liquid
Filter aids forms a fine surface deposit- prevent solid from contacting & plugging the supporting filter medium.
Filter aids added by 2 methods:
- Pre-coating &
- Body mix
Materials | Composition | Advantage | Disadvantage |
---|---|---|---|
Carbon | Carbon | Filtering strong alkaline solutions | Coarse grade only & Expensive |
Cellulose | Cellulose | Higy purity & Excellent chemical resistance | Expensive |
Diatomaceous earth | Silica | Wide range; Used for very fine filtration | Slightly soluble than diatomites in acid & alkali |
Asbestos | Aluminosilicate | Good retention on coarse screens | More soluble than diatomites in acid & alkali |
Perlite | Silica & Aluminosilicate | Wide range; not capable of finest retention | More soluble than diatomites in acid & alkali |
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