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filtration.
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BITS Pilani Hyderabad Campus
Separation processes –II CHE F313
BITS Pilani Hyderabad Campus
Filtration
BITS Pilani, Hyderabad Campus
Introduction Filtration may be defined as the separation of solids from liquids by passing a suspension through a permeable medium which retains the particles.
Figure 1. Schematic diagram of filtration system
BITS Pilani, Hyderabad Campus
The fine apertures necessary for filtration are provided
by fabric filter cloths,
by meshes and screens of plastics or metals, by beds of solid particles.
In some cases, a thin preliminary coat of cake, or of other fine particles, is put on the cloth prior to the main filtration process.
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Classification
Based on pressure Based on mode of operation
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1.Cake filters
used for cake filtration in which the solids are deposited in the form of a cake on the up-stream side of a relatively thin filter medium.
Figure2. Mechanism of cake filtration
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2.Clarifying filters used for deep bed filtration in which particle deposition takes place inside the medium and cake deposition on the surface is undesirable.
Figure 3. Mechanism of deep bed filtration
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3.Crossflow filters
Filtering aids
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The fluid passes through the filter medium, which offers resistance to its passage, under the influence of a force which is the pressure differential across the filter.
rate of filtration = driving force/resistance
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•The filter-cake resistance is obtained by multiplying the specific resistance of the filter cake, that is its resistance per unit thickness, by the thickness of the cake.
•The resistances of the filter material and pre-coat are combined into a single resistance called the filter resistance.
•It is convenient to express the filter resistance in terms of a fictitious thickness of filter cake.
•This thickness is multiplied by the specific resistance of the filter cake to give the filter resistance.
BITS Pilani, Hyderabad Campus
Factor affected on filtration
• Pressure drop ( ∆P ) • Area of filtering surface ( A )
• Viscosity of filtrate (µ)
• Resistance of filter cake ( α ) • Resistance of filter medium ( Rm )
• Properties of slurry ( ρ , density )
BITS Pilani, Hyderabad Campus
rate of filtration = driving force/resistance
-(P) or
Pressure
drop
Filter cake ()
Filter medium (Rm)
Viscosity ()
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