FILTRATION

FILTRATIONContentsIntroductionType of FiltrationFilter mediaDesign Equation for Batch FiltrationSpecific Cases of FiltrationIndustrial Filter EquipmentSelection Criteria of various type of filter

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

Filtration

In the laboratory, the suspension is poured into a conical funnel fitted with a filter paper.

In the industrial equivalent, difficulties are encountered in the mechanical handling of much larger quantities of suspension and solids. A thicker layer of solids has to form and, in order to achieve a high rate of passage of liquid through the solids, higher pressures are needed, and a far greater area has to be provided.

Steps involved in filtration

Draining the liquor FiltrationFilling with wash waterWashingDraining the wash waterOpening, dumping and reassemblingFilling with slurry.

Principle of filtration

Principle of FiltrationSince the filter medium is permeable only to the fluid, it retains the solid particles and permits only the fluid to pass through which is collected as the filtrate. The volume of filtrate collected per unit time (dV/dt) is termed as the rate of filtration. As the filtration proceeds, solid particle accumulate on the filter medium forming a packed bed of solids, called filter cake.

As the thickness of the cake increases resistance to flow of filtrate increases rate of filtration gradually decreases.If rate is maintained to be constant then pressure difference driving force (-P) will increase.

Therefore, a batch filter is operated either at constant pressure or at constant rate.Constant rate and Pressure Filtration

Cake Filtration Cake filtration consists of passing a solid suspension (slurry) through a porous medium or septum (e.g., a woven wire). The solids in the slurry are retained on the surface of the medium where they build up, forming an increasing thicker cake. As more slurry is filtered the solids retained on the medium provide most of filtering action. In cake filtration the cake is the real filtering element.Cake Filtration (continued)As time goes by the thickness of the cake increases, as more solids are filtered. This results in a corresponding increase of the pressure resistance across the cake.

If the cake is incompressible (i.e., it does not change its volume as pressure builds up) the pressure resistance increases proportionally to the cake thickness.

However, since most cakes are compressible the pressure across the cake typically increases even faster than the cake build-up.Examples of Cake-Forming Filters Filter presses Belt filters Vacuum filters: - Rotary vacuum belt filters - Rotary vacuum precoat filters - Vacuum disk filtersNote:Cake filtration is intrinsically a batch process. Hence, it can be expected that as filtration proceeds the cake will build up and the pressure drop across the cake will increase.Mathematical modelling of batch cake filtration is based on the determination of the rate of formation of the cake and the calculation of pressure drop at any given time.Continuous filtration is often modelled as a succession of batch processes carried out over infinitesimally small time intervals.Depth (or Deep-Bed)FiltrationDepth filtration consists of passing a liquid, typically containing only a small amount of solids, through a porous bed where the solids become trapped.Solid entrapment occurs within the entire filter bed or a significant part of it. Depth filtration is typically a batch processDirection of Flow in Deep-Bed FiltersUp flowDown flow (most common) Examples of Deep-Bed FiltersGranular-bed filtersDeep-bed up flow filterPulsed-bed filterTraveling-bridge filterBackwashing During backwashing water is pumped upward, i.e., in the opposite direction of the suspension during normal operationThe backwashing flow expands the bed to dislodge all the particles removed during filtrationIn order for backwashing to be effective the filter medium must be fluidizedBackwashing

Type of FilterCake Filter

Clarifying Filter

Cross Flow

Ultra Filter cake filter

Cake FilterAfilter cakeis formed by the substances that are retained on a filter .The filter cake grows in the course of filtration, becomes "thicker" as particulate matter is being retained.With increasing layer thickness the flow resistance of the filter cake increasesAfter a certain time of use the filter cake has to be removed from the filter, e.g. by back flushing.

Filter cakeClarifying FilterAny filter, such as a sand filter or a cartridge filter, used to purify liquids with a low solid-liquid ratio; in some instances colour may be removed as well.

Disk-and-plate clarifying filter.N-pin series of clarifying filter for electrolytic aluminium flue gasCross flow FiltersCross flow filters feed suspension flows under pressure at high velocity across filter medium

Thin layer of solids may form on surface ,but high velocity keeps layer from building upMedium is ceramic, metal, or polymer with pores small enough to exclude most of suspended particlesSome liquid passes through as clear filtrate, leaving more concentrated suspension behind

Ultrafiltration(UF) Ultrafiltration(UF) is a variety ofmembrane filtrationin whichhydrostatic pressureforces a liquid against asemipermeable membrane.Suspended solidsandsolutesof highmolecular weightare retained, while water and low molecular weight solutes pass through the membrane.

inclined ultrafiltrationdownward ultrafiltrationSchematic diagram of flowing filter cake: (a) inclined ultrafiltration, and (b) downward ultrafiltration.22Design Equation for Batch FiltrationCake FilterPressure Drop During Cake Filtration

Direction of flow of slurryFiltrateLcLdLUpstream face of cakeppbpapWhere p = overall pressure drop pc = pressure drop over cake pm = pressure drop over medium

Design Equation for Batch Filtration Since the cake forms a porous bed over the filter medium, the flow of filtrate through the accumulated cake is analogous to fluid flow through a packed bed of granular solids.If the particles in the cake are uniformly wet by the filtrate then Kozenys equations can be used to compute the pressure drop across the cake (-P). The velocity of fluid through the bed is:

Where, K = Kozenys constant = 25/6, for random packed particles of definite size and shape RH = Hydraulic radius = flow area/wetted perimeter

Assumptions:

Flow of filtrate through the cake is laminar.Particles in the cake are uniformly wet by the filtrate.There is no channeling of the liquid through the cake. RH = Void volume/Total surface area of particle = / sp (1- ) Where, = Void fraction of the bed = Void volume/total volume sp = Specific surface (surface area per unit volume) of the particles Using value of RH we have ,

.The superficial velocity of the liquid Usup is defined as the volumetric flow rate of the liquid divided by the total (or empty) cross-sectional area and can be related U as, Usup/U = Void area / Total area = Void volume / Total volume =

Where,(-Pc) = pressure drop across the cake Lc = thickness of the cake

The factor is called the specific cake resistance and is a measure of the resistance offered by the cake to the flow of filtrate. The average value of is determined experimentally for each sludge.

Filter cake Incompressible Compressible is independent of pressure drop and position in the cakeFormed when cake is not made up of individual rigid particles, sp/vp vary from layer to layer varies with distance from filter mediumCake nearest the surface of the media is subjected to the greatest compressive force and has the lowest Pressure gradient is non-linear and local value of may vary with timeDesign

For the computation of Lc

Lc is expressed in terms of the volume of filtrate V, cake voidage and concentration of feed slurry. If filtrate is the solid-free liquid, thenMass of solids in the cake = Mass of solids in the feed slurryIf x is the mass fraction of solids in the feed slurry, then,Mass of solids in the feed slurry

Mass of solids in the cake

v is the volume of cake deposited by passage of unit volume of filtrate.Then,

DesignFilter MediaWhen choosing a filter material we must consider cost, particle size, operating temperature, and chemical resistance.1. PO (PONG)-Polypropylene FeltThis non-woven material is our popular workhorse. Polypropylene offers excellent chemical resistance. Its cost effectiveness makes it ideal for applications up to 200 F.This felt material can come plain (untreated felt) or glazed (high heat applied to exterior surface fibers).2. PMO-Polypropylene MeshSimilar to nylon mono filament mesh, polypropylene mono filament mesh has better acid resistance than nylon and is more cost effective for temperatures up to 200 F.3.PEMF-Polyester Microfiber FeltThis material is grown from raw microscopic fibers. Its long life and ultra-fine micron rating make it possible to use bag filters for applications that once required expensive high maintenance cartridges. It has a higher melting point than polypropylene microfiber making it ideal for hot oil and other applications up to325 F.4. NMO-Nylon MeshThis monofilament mesh is a woven material in which each thread is a single filament. This material's strength enables it to be used in a variety of different applications