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Separation processes –II CHE F313

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Filtration

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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

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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.

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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 )

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rate of filtration = driving force/resistance

-(P) or

Pressure

drop

Filter cake ()

Filter medium (Rm)

Viscosity ()

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