In the case of cake filtration, the choice of a filter medium is often the most important consideration in assuring the satisfactory operation of a filter. The filter medium in any filter must meet the following requirements.
Filter aid and filter medium
1. It should retain the solids to be filtered, giving a reasonably clear filtrate.
2. It should not be plugged or blind.
3. It should be Mechanically strong enough to withstand the process conditions.
4. It should be resistant to the corrosive action of fluids.
5. It should offer as little resistance as possible to the flow of the filtrate.
6. It should possess the ability to discharge cake easily and cleanly.
7. It should have acceptable resistance to mechanical wear.
8. It should be cheap.
9. It should have a long life.
In cake filtration, the medium is frequently a textile fabric.
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Canvas cloth, wooden cloth, metal cloth of monel or stainless steel, glass cloth, and synthetic fiber cloth, nylon cloth, polypropylene, etc. are commonly used as filter media in industrial filtration particles depending on the process conditions.
For an alkaline slurry, nylon cloth is used while for an acidic slurry, polypropylene cloth is used as a filter medium.
Filter Medium Resistance
A filter medium resistance can be defined by the equation
∆Pm/Rm = uμÂ
Rm = ∆Pm/uμ ------- 1.12
Where ∆Pm is the pressure drop over the filter medium. In filter medium resistance units, SI system, Rm has the unit of m¯¹ The dimension of Rm is L¯¹.
Filter medium resistance formula
dV/Adt = ∆P/μ [(c V α/A) + Rm] -----1.13
Equation 1.13 expresses the differential or instantaneous rate of filtration per unit area of the filtering surface as the ratio of the pressure drop to the product of the viscosity of the filtrate and the sum of the cake resistance and the filter medium resistance.
Filter aid in Filtration
Filtration of slurry containing very finely divided solids or slimy deformable flocs is very difficult due to the formation of a dense, impermeable cake that quickly plugs the filter media.Â
In such cases, practical Filtration demands the porosity of the cake be increased to permit passage of the filtrate at reasonable rates. This is achieved by adding a filter aid to the slurry before filtration. A filter aid is a granular or fibrous material that packs from a bed of very high voltage and therefore is capable of increasing the porosity of the filter cake.
A filter aid should be of low bulk density should be porous should be capable of forming a porous cake and must be chemically inert to the filtrate. The most popular commercial filter aids are diatomaceous earth, almost pure silica prepared from deposits of diatom skeletons, expanded pearlite, and asbestos fibers.Â
The filter aids are used for sludges that are difficult to filter and the use of filter aids is normally restricted to filtration methods in which the filtrate is valuable and the cake is the waste product. In some circumstances, however, the filter aid must be readily separable from the filter cake by physical or chemical means.
Filter Aids are Used
(i) Adding a filter aid to the slurry before filtration, andÂ
(ii) Precoating i.e. by depositing a layer of filters aid on the filter medium before filtration.
Precoats prevent gelatinous solids from plugging the filter medium and give a clear filtrate.
The pre-coats are a part of the medium rather than of cake.
When a filter aid is directly added to the slurry before filtration, the presence of it increases the porosity of the sludge decreases its compressibility, and reduces the resistance of the cake during the filtration operation.Â
Take these Notes is, Orginal Sources:Â Unit Operations-II, KA Gavhane