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
- Surface area of filter media
- 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
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
|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|