Clarifying Filter

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CLARIFYING FILTERS

An Assignment Submitted for the partial fulfillment of the

course of

Food Engineering- I

By

VASU CHOPRA(Roll no- 011,V semester)

To

Ms. Renu Khedkar

AMITY INSTITUTE OF FOOD TECHNOLOGY

AMITY UNIVERSITY, NOIDA

INTRODUCTION


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Clarifying filters remove small amounts of solids orliquid droplets from either liquids or gases. Theparticles are trapped inside the filter medium or onits surfaces. Clarification differs from screening, inthat pores in the filter medium are larger-sometimes much larger- than the particles to beremoved. The particles are caught by surfaceforces and immobilized o the surfaces or within theflow channels, where they reduce the effectivediameter of the channels but usually do not blockthem completely.

The usual objective of clarifying filtration is toseparate solids at a very low concentration from aliquid stream. The liquid may be drinking (potable)water, wine, beer, 04 etc. and it is usually theliquid which is the valuable product. Thetechniques used in clarification processes include:deep-bed, precoat, candle and cartridge filtrationall of which involve capture of particles inside theporous mass of the filter. Such techniques produceclearer filtrates than those obtained in clarificationby sedimentation.

CAPTURE MECHANISMS

All clarifying filtration techniques include at least

one of the following mechanisms:a) Strainingb) Sedimentationc) Interceptiond) Inertial impactione) Diffusionf) Hydro dynamic interactiong) Electrostatic interaction

h) Electrical double layer repulsion


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i) van der Waals forcesIt is assumed that once a particle has touched thesurface of the target grain it will remain attachedto that surface, in the absence of unbalancedrepulsive forces, and is effectively captured fromthe liquid flow. The particle may be made todetach later, during filter cleaning, but only aftersubstantially altering the operating conditions.

Straining. This is the simplest of the collectionmechanisms and occurs when the particlediameter is larger than the constriction throughwhich the fluid flow streamlines pass. The grainsize plays a important role in this mechanism asnarrower passages are found with smaller grainedcollection media.

Sedimentation. When the fluid flow is directeddownwards through a filter, gravitationalsedimentation effects will cause particles to settlevertically through the flow streamlines, as thelatter distort around the collector.

Interception. If the suspended particle radius isgreater than the distance between the flowstreamline which contains the particle and thecollecting media grain, then the suspended particlew ill contact the target, in the absence of anyrepulsive mechanisms.

Inertial Impaction and Bounce. Whenever fluidflow streamlines change direction it is possible that


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particles may be less able to alter directionbecause of their greater inertia. If a collectionsurface is nearby the particles may make contactwith the target before becoming trapped inanother flow streamline.

Diffusion. All particles suspended within a fluidare subject to bombardment by the surroundingfluid molecules. Small solid particles may acquiresufficient momentum from the fluid molecules tocause the solid particle to move. It will continue todo so until more bombardment causes the particleto change direction. This Brownian motion of thesuspended particle may cause it to approach thesurface of a target.

TECHNIQUES USED

1) Deep-Bed Filtration:These have been in common use for well over onehundred years. The principle involved is used innature to filter duly water through porous rock, inpotable water wells and springs. Commercial deep-bed filters are of simple construction consisting ofan enclosed cylindrical metal tank and a pressurevessel or open rectangular concrete tank

containing a packed bed of solids through whichthe process fluid to be filtered is passed. Thecommon types of deep bed filter are:

Slow Sand Filter - Fine grains of sand are usedwhich leads to efficient suspended particleremoval, but slow filtration rates. Relativelyshallow beds are employed and biological activity

on the surface is common. The initial sand layers


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become clogged first and cleaning is by removaland replacement of these layers. The depositformed on the d a c e is sometimes calledschmutzdecke.

Rapid filter - A coarser grain size, and sometimes anarrower size distribution than slow filters areused; often pressurized flow is employed. A varietyof rapid filters exist, for example, downflow,upflow, mixed-media and continuous.

Direct Filtration ( Contact Flocculation) - This is amixture of filtration and flocculation in which thesuspension entering the bed has been flocculated,but the flocs have not fully formed prior to bedentry. The technique is applied to suspensions oflow concentration of colloidal matter.

2) Precoat Filtration:Precoat filtration is the application of a filter aid ona mainly conventional filter under clean conditions;i.e. in the absence of suspended solids other thanthe filter aid, to form a highly porous and uniformfilter cake. The suspension to be filtered is thenintroduced onto the filter. Clarification by filtrationresults by the action of the filter aid trapping thesuspended solids within the filter aid cake. If thefiltration equipment is continuous by nature, e.g. arotary vacuum filter, a thick filter aid cake may beemployed, alternatively, if the filtration isconducted in a batch vessel thinner filter aid cakesare usually employed.

3) Cartridge Filtration:


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Cartridge filters are a very important means ofeffecting a clarification and have found use in awide variety of industries. Advances in membranemanufacture have enabled cartridge suppliers touse filtration media capable of retaining allparticulates down to 0.1 m in diameter, and withsignificant removal of colloidal material below thissize. Cartridge filters can be classified into threegroups: depth, surface and edge, with additionalsubdivision.

4) Filter Candles:Filter candles for clarification generally rely on theuse of a precoat to prevent media blinding. Inmany respects their operation is similar to thatdescribed for batch precoat pressure filtration.Cylindrical candle diameters of 10 mm or more areused, with many candles inside a pressure vessel.


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They usually operate vertically with the dirtyprecoat back-flushed off the support candle.Bottom discharge of the solids is then followed bycoating the candles with precoat beforerecommencing clarification.

REFERENCE:

1)UNIT OPERATIONS OF CHEMCAL ENGINEERINGWarren L. McCabe, Julian C. Smith, Peter

Harriot

2)SOLID-LIQUID FILTERATION AND SEPERATIONTECHNOLOGYA. Rushton, A. S. Ward, R. G. Holdich

Clarifying Filter

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