sfo filtration 1

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Filtration

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Filtration

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FiltrationFiltrationIt is the separation of solids from a suspension in a liquid by means of porous medium or screen which retains the solids and allows the liquid to pass.

Classification of filtration(A) By driving forceGravityUnder pressureCentrifugal force across the screen

Classification of filtration(B) By Filtration mechanismCake filtrationFilter medium filtrationClassification of filtration(C) By FunctionCake is desired productFiltrate is desired productBothClassification of filtration(D) By operation cycle1. Batch2. ContinuousClassification of filtration(E) By Nature of solidsCake filtrationCompressibleIncompressibleFilter medium filtrationDeformableRigidThe factors to be considered when selecting equipment are:The properties of the fluid, particularly its viscosity, density and corrosive propertiesThe nature of solidParticle sizeShapeSize distributionPacking characteristicsThe factors to be considered when selecting equipment are:3. The concentration of solids in suspension4. The quantity of material to be handled and its value5. Whether the product of importance is solid, liquid or both6. Whether it is necessary to wash the filtered solids

The factors to be considered when selecting equipment are:7. Whether feed liquor may be heated or any other pretreatment is necessaryThe rate of filtration depends on:The pressure drop from the feed side to the far side of the filter mediumThe area of filtering surfaceViscosity of filtrateResistance of filter cakeResistance of filter medium and initial layers of the cakeFilter MediaFunction: Generally to act as a support for the filter cake.Manufactured fromCottonSynthetic polymersGlassAsbestosMetalCarbon

Selection of filter mediaAbility to build the solids on the pores quickly after the feed is startedMinimum resistance to flow of filtrateResistance to chemical attackSufficient strength to withstand the filtering pressureAcceptable resistance to mechanical wear

Selection of filter media6. Ability to discharge cake easily and cleanly7. Ability to conform mechanically to the type of filtering equipment8. Minimum cost9. Long life Filter aidIt is a solid material, hard, strong particles that are incompressible.They are forming permeable filter cake.Used for sludges that are difficult to filter.Useful in slurries containing very finely divided solids or slimy material.Filter aidThey should be porous and capable of forming a porous cake.They must be chemically inert to the filtrate.Materials used as filter aid:Diatomaceous earthCellulosic fiberCarbon fullers earthMagnesiaGypsum

Filter aids can be used in 2 waysPre-coatingA thin layer of material laid on the filter before the sludge is pumped to the equipment.This pre-coat prevents the colloidal particles of the sludge from entangling with the filter cloth.Hence, prevents the resistance of the cloth to become very high.

Filter aids can be used in 2 waysAlso facilitates the removal of cake at the end of filtration.Here, the filtering medium is really the pre-coat material rather than the filter cloth itself.It is useful when the filtration is merely a finishing operation.

Filter aids can be used in 2 waysUsing filter aid to incorporate it with the sludge before sending it to the equipment.The presence of filter aid increases the porosity of the sludge, decreases its compressibility and reduces the resistance of the cake during filtration Constant rate filtrationHere, constant rate of filtrate is maintained by starting at low pressure and continuously increasing the pressure to overcome the increasing resistance of the cake until maximum pressure is reached at the end of the runConstant rate filtrationDisadvantage: The maximum pressure is reached only at the end of the run and hence the whole cycle is less than maximum capacity

Constant pressure filtrationHere, full pressure is applied at the start of filtration and this pressure is maintained constant throughout the run.Constant pressure filtrationDisadvantage:If the initial pressure is high, the first particles caught will be compacted into a tight mass that largely fills the pores of the cloth and results in a low rate of filtration throughout the rest of the cycleRemedy:Constant pressure filtration is to operate at constant rate during the first part of filtration and as soon as the cloth is coated with solids ,the pressure is increased to the maximum and filtration is completed at constant pressure.