FILTRATION

Contentso Introductiono Type of Filtrationo Filter mediao Design Equation for Batch Filtrationo Specific Cases of Filtrationo Industrial Filter Equipmento Selection Criteria of various type of filter

FILTERATION

The 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

1. Draining the liquor 2. Filtration3. Filling with wash water4. Washing5. Draining the wash water6. Opening, dumping and reassembling7. Filling 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 filters

Note:• 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)Filtration

• Depth 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 process

Direction of Flow in Deep-Bed Filters• Up flow• Down flow (most common)

Examples of Deep-Bed Filters• Granular-bed filters• Deep-bed up flow filter• Pulsed-bed filter• Traveling-bridge filter

Backwashing• During backwashing water is pumped

upward, i.e., in the opposite direction of the suspension during normal operation

• The backwashing flow expands the bed to dislodge all the particles removed during filtration• In order for backwashing to be effective the filter medium must be fluidized

Backwashing

Type of Filter

• Cake Filter

• Clarifying Filter

• Cross Flow

• Ultra Filter cake filter

Cake Filter• A filter cake is 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 increases• After a certain time of use the filter cake has to be removed from the filter, e.g. by back flushing.

Filter cake

Clarifying Filter• Any 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 gas

Cross flow Filters

Cross 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 up

• Medium is ceramic, metal, or polymer with pores small enough to exclude most of suspended particles

• Some liquid passes through as clear filtrate, leaving more concentrated suspension behind

Ultrafiltration (UF)

• Ultrafiltration (UF) is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained, while water and low

molecular weight solutes pass through the membrane.

inclined ultrafiltration downward ultrafiltration

Design Equation for Batch FiltrationCake Filter

Pressure Drop During Cake Filtration

mc

baba

pp

ppppppp

)'()'(

Direction of flow of slurry

Filtrate

Lc

LdL

Ups

trea

m f

ace

of c

ake

p’pb

pa

p

Where 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

Kozeny’s equations can be used to compute the pressure drop across the cake (-P).

The velocity of fluid through the bed is:

Where, K = Kozeny’s 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 cake

Formed when cake is not made up of individual rigid particles

, sp/vp vary from layer to layer

varies with distance from filter medium

Cake 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 time

Design…

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,

Design…

Filter Media…When 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 to 325° 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

<- Nylon Monofilament(60x zoom

Polyester Multifilament60x zoom

Design…

Pressure drop across the filter medium is:

Rm is to be determined experimentally.

Therefore, the total resistance of filtration is

Then ultimate filtration equation is:

Rm may vary with P, as solid particles can be forced into the filter medium

Can also vary with age and cleanliness of filter medium

But, since it is important only during early stages of filtration, can usually be assumed as constant

Sperry correlation

Where s is the compressibility coefficient of the cake 0 for incompressible sludges + for compressible ones Usually 0.2 < < 0.8

It has a inherent limitation that it predicts zero resistance when the (P) is zero.

Ruth correlation

Both these correlations are the functions of (-P) only. Therefore, for compressible sludge also, is constant if the filtration is being conducted at constant pressure.

sp)(0

Design…

Empirical Equation for Resistance of Compressible Cakes

Donald and Hunneman correlation

0 < n < 20

Generalized correlation

Empirical Equation for Resistance of Compressible Cakes

Final filtration equation

Constant pressure and incompressible sludge (=const.)

where ,

For practice purpose,

Slope of line = KP which gives

Y-Intercept of line = B which gives Rm

Specific cases of filtration

Constant rate and incompressible sludge (=const.)

Where, and

Also

where,

-P

V or t

Slope of line = Kr or Kr’ which gives

Y-Intercept of line = Br which gives Rm

Specific cases of filtration

Constant pressure and compressible sludge

IfThen,

Assuming v to be constant (v is constant for small values of and for most of filter cakes particularly compressible cakes, is small).

Where,

and

sp)(0

-P1

-P2

-P3

Slopes of lines = Kp’ values

Y-Intercept of line = B which gives average value of Rm

Specific cases of filtration

Constant pressure and compressible sludge

-P

Kp’

log-log plot

Slope of line = (1-s) which gives value of s

Y-Intercept of line = (f 0v/A2) which gives value of0

Specific cases of filtration

Constant rate and compressible sludge

If = 0 (-Pc)s Then

since

If

Then

Specific cases of filtration

Continuous FiltrationContinuous removal of the deposited cake.

The cake thickness is thus not allowed to increase to large values and therefore the filtration process can be conducted at a constant rate employing a constant pressure difference.

• Filling

• Cake formation

• Dewatering

• Washing

• Dewatering

• Cake removal

Rotary drum filter (vacuum filter)

A full rotation of the drum is equivalent to a complete batch cycle. Thus, the equation developed for batch filtration can be used for continuous filtration as well, keeping in mind that the equation stands for one full rotation of drum.

The filtration equations

Where, t is the time for cake formation. If tc is the time for one full rotation of the drum,

t = f tc where f is fraction of the cycle available for cake formation.

f = fractional submergence of the drum in the slurry.

V = volume of filtrate collected during one rotation of the

drum and (V/tc) stands for the rate of filtration.

For compressible cake, sp)(0

Industrial Filtration Equipment

1. Discontinuous Pressure Filters: Apply large P across septum to give economically rapid filtration with viscous liquids or fine solids.• Plate and Frame Filter Press• Shell and leaf Filters

2. Continuous Vacuum Filter:Vacuum filters are simple and reliable machines and therefore have gained wide acceptance in the chemical, food and pharmaceutical industries.

• Rotary Drum filter• Horizontal Belt filter

1. Plate and Frame Filter Press

• Filter presses work in a "batch" manner.• The plates are clamped together, then a pump starts feeding

the slurry into the filter press to complete a filtering cycle and produce a batch of solid filtered material, called the filter cake.

• A filter press uses increased pump pressure to maximize the rate of filtration

Plate and Frame filter parts…

A medium scale frame and filter press..

Shell and leaf filter:Description:In these horizontal pressure leaf filter Jacketting and cloth enveloped filters are optional. There are 2 hydraulic cylinders to open and close the special bayonet wedge lock closure, which is provided at the lid. The retractable filtered shell is mounted on four external wheels and all nozzle connections are mounted on fixed head of filter vessel. Appropriate inter-locking (preventing opening under pressure) is also a key feature of this type of horizontal pressure leaf filter.Range: Available in 5 sq. m to 250 sq. m.Advantages:Horizontal Leaf Filter is also a multi utility device that has the following advantages: -» Nil spillage, as a result of close and compact operation.» No use of filtered cloth reduces operational expenses.» High productivity due to high rate of filtration.» Cheap and economical operational costs.» Highly user friendly.

Shell and leaf filter

Internal Structure

Rotary Drum Filter• Horizontal drum that turns at 0.1-2 r/min in an agitated slurry

trough• Filter medium covers face of drum, which is partially submerged • Vacuum and air are alternately applied as the drum rotates• As panel leaves slurry zone, a wash liquid is drawn through filter,

then cake is sucked dry with air, and finally cake is scraped off• From 30% up to 60-70% of filter area can be submerged• Cakes usually 3-40 mm thick• Drum sizes range from 0.3 m in diameter to 3 m in diameter

Rotary Drum Filter

• The Drum:The drum is supported by a large diameter trunion on the valve end and a bearing on the drive end. The drum face is divided into circumferential sectors each forming a separate vacuum cell. The internal piping that is connected to each sector passes through the trunion and ends up with a wear plate having ports that correspond to the number of sectors.

Parts of a Rotary Drum filter

Parts of Rotary Drum Filter

• The Valve: A valve with a bridge setting controls the sequence of the cycle so that each sector is subjected to vacuum, blow and a dead zone. When a sector enters submergence vacuum commences and continues through washing, if required, to a point that it is cut-off and blow takes place to assist in discharging the cake.

• The valve has on certain filters adjustable blocks and on others a fixed bridge ring. Adjustable bridge blocks enable the optimization of form to dry ratio within the filtration cycle as well as the "effective submergence" of the drum when the slurry level in the tank is at the maximum.

a. cake formation b. cake washing & drying c. cake blow discharge

a

b

c

Parts of Rotary Drum Filter

The Internal piping:

The Filter cloth:The filter cloth retains the cake and is fastened to the drum face by inserting special caulking ropes into the grooved division strips. Nowadays, with some exceptions, they are made from synthetic materials such as polypropylene or polyester with monofilament or multifilament yarns and with sophisticated weaves and layers. The image on the right shows the method of joining the cloth ends with clippers and to retain the fines from passing through to the filtrate multifilament strings are threaded across the entire cloth width. Another option quite often used on belt discharge filters is to join the ends with a special sewing machine.

The Horizontal Bed Filter

The Horizontal belt FilterOperation:The feed sludge to be dewatered is introduced from a hopper between two filter cloths (supported by perforated belts) which pass through a convoluted arrangement of rollers. As the belts are fed through the rollers, water is squeezed out of the sludge. When the belts pass through the final pair of rollers in the process, the filter cloths are separated and the filter cake is scraped off into a suitable container.[1] A belt filter is generally used in phosphate fertiliser plants to separate the solid from slurry. It comprises washing to different zone to minimise the product losses. Belt filters use a vacuum system to minimise off gas and effluent during operations. It has applications in many other areas of industry.

Horizontal Belt FilterAdvantages • Continuous operation (except for a Nutsche filter) • Intensive soluble recovery or removal of contaminants from the cake by

counter-current washing (specially on Horizontal Belt, Tilting Pan and Table Filters)

• Producing relatively clean filtrates by using a cloudy port or a sedimentation basin (on Horizontal Belt, Tilting Pan and Table Filters)

• Polishing of solutions (on a Precoat Filter) • Convenient access to the cake for sampling or operator's activities • Easy control of operating parameters such as cake thickness or wash

ratios • Wide variety of materials of construction Disadvantages • Higher residual moisture in the cake • Untight construction so it is difficult to contain gases • Difficult to clean (mainly as required for food grade applications) • High power consumption by the vacuum pump

Tray FilterDescription:• The Tray Filter, as opposed to the Horizontal Belt Filter.• It belongs to the group of top feed filters and is primarily applied to

the finer chemical compounds handling thin cakes although in recent years large machines suitable for thick cakes may be seen on the bulkier processes.

• Sizes may vary from 0.25 to 3.0 meter wide units and lengths of 2 meters for pilot units and up to 25 meters for industrial machines.

• The cloth moves continuously over reciprocating trays which move forwards with "vacuum on" in the forward stroke and retract with "vacuum off" in the backwards stroke.

• The cloth moves intermittently over fixed trays and stops with "vacuum on" in the filtration phase and "vacuum off" to enable its movement forwards.

Tray FilterThe Reciprocating Trays Type

• This filter consists of a series of trays which are allocated for cake formation, washing and drying.

• The trays are each connected through a flexible hose to a manifold that runs along the filter and collects the filtrate to vacuum receivers.

• To meet such requirements the manifold is separated by blind flanges which can be set at different positions

• The number of receivers is determined by the process requirements with one unit collecting the mother filtrate and others the wash filtrate and the cake drying.

The Reciprocating Trays Type

• The reciprocating trays are designed so that, while under vacuum, they can move freely in the forward direction together with the cloth and at the same velocity .

• When they reach the end of stroke point the vacuum is cut-off, the trays are purged by an atmospheric release and pulled back by an air driven pneumatic cylinder.

• During the backstroke the filter cloth keeps moving forwards since there is no vacuum between the retracting trays and the backside of the cloth which holds them together. two co-current washing stages->

Tray FilterThe Fixed try type

• The trays of this filter are fixed to the frame and the cloth moves forwards in short increments during "vacuum off" and remains stationary at the "vacuum on" mode.

• The vacuum modes are controlled by solenoid actuated valves arranged in a similar method described in the section on the Reciprocating

trays filter.

Cake Discharge Roll

Cloth Locking Roll

Cloth Tensioning Roll

Fixed Trays

3-Way Valves

Cloth Wash Manifold

Pneumatic Cylinder

Fixed roll with "on-off" motor actuator:• In this system the discharge roll is mounted on the frame in a fixed

position and a timer controlled pulley moves the cloth at "vacuum off" to allow cake discharge and "vacuum on" during filtration. A special tilting feed box pours the slurry onto the filter deck when the cloth moves at "vacuum off".

Selection Criteria for Tray Filter• They may be built from synthetic materials of construction which makes them suitable

to withstand highly corrosive applications without the use of exotic and expensive alloys.

• The sealing against loss of vacuum is simple as opposed to the rubber belt filters which use sacrificial moving belts to seal between the underside of the main belt and the top flanges that run along the vacuum box.

• They are built in a modular construction which enables expansion when circumstances so require.

• The separation of mother and wash filtrates is sharp and accurate since, as opposed to partitions in a vacuum box of a rubber belt filter, the filtrate is contained in a tray.

• They lend themselves better than rubber belt filters when two vacuum zones are required such as high vacuum for the feed and wash zones and low vacuum with high air rates for the drying zone.

• The power consumption is lower since the rubber belt filters require special arrangements to support the heavy belt and reduce friction.

• Features are found more often on tray filters such as gas tight enclosures, compression rolls and blankets, thermal drying, vibrating trays to seal cracks in the cake and ultrasonic or chemical cloth cleaning.

• They are however less in use for very thick and heavy cakes since, contrary to rubber belt filters, the friction during indexing due to the cake weight between the supporting grids that cover the trays and the backside of the cloth may cause extensive wear.

The Disc Filter• A disk filter is a type of water filter used primarily in irrigation,

similar to a screen filter , except that the filter cartridge is made of a number of disks stacked on top of each other like a pile of poker chips. The water passes through the small grooves in between and the impurities are trapped behind. Some types of disk filters can be backflushed in such a way that the disks are able to separate and spin during the cleaning cycle.

Single Disk Filter Multi Disk Filter

Disk FilterMAIN ELEMENTS:• Filtering Cylinders: made totally out of Stainless Steel, quality Aisi-

304 or 316, this cylinder is built in a specially designed machine and this element has two side lids which support the filtering material together with a metallic framework, which is easy to dismount or adjust.

• Filtering mesh of 10 to 1.000 microns made in polyester. Also available in Stainless Steel AISI 316L ( from 10 to 1.000 microns).

• Modular design.Easy access to the filtering cylinder from the exterior of the filter, quickly and easily.

• Shell or frame: In strong meccano construction welded in Stainless Steel, quality Aisi-304 or 316, supplied with entrance and exit connections, anchoring legs, water-tightness elements, etc…

System for elimination of residue: This is carried out by means of a combination of water sprinklers, the rotation of the filtering cylinder and the residue conveying hopper found inside the filtering cylinder.

Disk FilterCOMMON APPLICATIONS:• Tertiary treatment of municipal wastewater.• Pre-treatment of drinking water.• Treatment of storm water.• Treatment of water from industrial processes.CHARACTERISTICS:• Adequate for filtering small particles. • deal system for applications which recycle municipal water. • Equipment available with different diameters 0.5, 0.8, 1.2, 1.6, 2.0, 2.4 and 2.8

metres. • Automatic cleaning system with filtered pressurized water, activated with a

level sensor. • Different versions and standard models which can be adapted to multiple

needs. • Manufactured in Stainless Steel (AISI 304 or AISI 316L) and available in other

special materials depending on the application.

Disk FilterADVANTAGES & BENEFITS:• Wide range in solids concentration in the water to be treated. • Filtering mesh from 10 to 1.000 microns, according to

application. • Equipment with production flows from 10 to 5.400 m3/h. • Compact design which requires up to 75% less implantation

surface, compared with a conventional sand filter. • Low investment thanks to its compact design.• Reduced energy cost as the filtering is through gravity.• Continuous working , including in washing phase. • Very easy access to all filter elements. • Easy operating and maintenance.

Disk filterSelection Criteria• The main considerations in selecting a Disc Filter are: • When they suit an application that meets the following

requirements: • The form to dry time ratio is approximately ½ to 1. • No cake washing is required. • The cake parts easily from the cloth. • The cloth does not clog. • When a cloth on one of the sectors tears the entire sector may be

replaced within a very short downtime. • The filtration area may be expanded by adding more discs to a

barrel that has unused discs. • The Disc Filter provides for maximum area at minimum cost and

floor space

Disk filterOperational Sequence• The operation sequence of a Disc Filter is, except for washing, similar

to a Drum Filter.• Vacuum commences when the sector is fully submerged in the slurry

and the port of the rotating barrel passes the dead zone bridge. • The cake forms until the leading edge of the sector emerges from the

slurry and drying commences. • The sector continues to dry the cake under vacuum until the port in

the rotating barrel fully covers the bridge in the valve that separates the vacuum from the blow compartments.

• The port in the barrel passes the bridge and opens to constant low pressure air blow or snap blow and the cake falls off to the discharge chute.

• Once the barrel port passes the blow opening of the valve the sector enters a dead zone that continues until the port opens to vacuum with the sector fully submerged.

The Table FilterDescription:• The Table Filters belong to the top feed group, introduced in the early 40's and

were rather small and of a simple design. • limitation was at the discharge zone ,since the cake was contained in a fixed rim

and special sealing arrangements had to be provided in order to avoid the spillage of brine at the table's circumference.

• Another problem was that the thin heel left between the scroll and the surface of the table was dislodged by applying a back blow but not removed from the surface of the passing cell. So, as it reached the feed zone it was mixed with the incoming slurry without the cloth being washed.

• This has caused progressive media blinding which effected filtration rate and required frequent stoppage of the operation for cloth washing.

Table filterThe major subassemblies of the Table Filter are:• A series of fixed trapezoidal cells that form a rotating table and each

connected to a stationary valve in the centre of the filter. The cell is designed with steep sloped bottom for fast evacuation of the filtrate.

• A valve that may be raised from the top and has a bridge setting and compartments to control the various zones.

• An internal rim fixed to the table at the inner circumference and a continuous rubber belt that surrounds the table at the periphery and confine the slurry, wash liquids and the cake during the filtration cycle.

• Rollers that support the vertical loads, centering thrust and others that move the rim away from the table in the discharge zone and maintain it under tension.

• Radial rubber dams that separate between the feed, wash stages, cake discharge and cloth wash, and cloth drying zones to prevent the mixing of filtrates.

• A variable pitch screw that transports the cake radially towards the point of discharge.

Table filterFeed+Cloudy Port

Zone

Mother Filtrate Zone

1st Wash Filtrate Zone

2nd Wash Filtrate Zone

3rd Wash Filtrate +

Drying Zone

Cake Discharge

Zone

Cloth Washing Zone

Cloth Drying Zone

Cake Discharge Scroll

Cake Retaining Rubber Rim

 Central Filtrate Valve

Table filterSelection Criteria:• When the process downstream requires a de-lumped cake since the

screw disintegrates the solid lumps while conveying them to the periphery.

• When the solids are fast settling and cannot be kept as a homogenous slurry in bottom or side feed filters such as Drum or Disc Filters.

• When very short cycle times are required for fast dewatering cakes such as phosphate slurry.

• When a clear filtrate is required right from the start it is good practice to form a thin heel that serves as a filter medium over the exposed cloth. This is done by either a "cloudy port outlet" that is recirculated or, if solids are settling fast, by allocating a portion of the table after the cloth drying dam and prior to entering the vacuum zone to act as a "sedimentation pool".

• When intensive cake washing is required. • When a large filtration area is required but a Horizontal Belt Filter

does not fit into the layout. • When cakes tend to crack under vacuum measures such as a flapper

or pressure roll may assist in sealing the cracks thus avoiding loss of vacuum.

Table filterOperational Sequence• The cycle of a Table Filter that includes three counter-current washing stages

consists of the following zones:• With cells under vacuum during filtration:

– Cloudy Port Recycle or Sedimentation Pool (before applying vacuum).– Cake Formation.– Cake Predrying.– First Washing.– First Predrying.– Second Washing.– Second Predrying.– Third Washing.– Final Drying.

With cells purged to atmosphere:– Cake discharged dry and conveyed by the screw to the cake hopper.– Cloth wash and sluicing for the removal of the heel.

With cells under low vacuum:– Evacuation of cloth wash water.– Cloth drying (for such applications that dilution of mother liquor must be avoided)

The candle filter

Description:The Candle Filters are, as all pressure filters, operating on a batch cycle and may be seen in process lines handling titanium dioxide, flue gas, brine clarification, red mud, china clay, fine chemicals and many other applications that require efficient low moisture cake filtration or high degree of polishing.

Candle filterThe Candle Filter consists of three major components:• The vessel • The filtering elements • The cake discharge mechanism

1. The Vessel:There are two types of vessel configuration: • Vessels with conical bottom for cake filtration and polishing. • Vessels with dished bottom for slurry thickening.

Candle filter

.

The Cake Discharge Mechanism:There are two methods to discharge the cake at the end of the cycle:• Snap blow• Vibrating mechanismFor cakes that discharge readily a snap blow from the backside of the medium is sufficient to release the cake but cakes that are difficult to discharge require a mechanism that assists release by vibrating the entire pack of candles. In this instance it is good practice to incorporate special headers with high impact sprays in the upper part of the vessel to clean the candles and dislodge entrained particles.

Candle filterCandle Filters are best selected in the following instances:• When minimum floor space for large filtration areas is required.• When the liquids are volatile and may not be subjected to vacuum.• When there is a risk of environmental hazard from toxic, flammable or

volatile cakes specially secured discharge mechanisms may be incorporated.

• When high filtrate clarity is required for polishing applications.• When handling saturated brines that require elevated temperatures the

tank may be steam jacketed.• When the cake may be discharged either dry or as a thickened slurry.

They should be selected with care:• When the cake is thick and heavy and the pressure is not sufficient to

hold it on the candle.• When coarse mesh screens are used the filtration step must be

preceded with a precoat to retain cakes with fine particles. Precoating with a thin layer of diatomite or perlite is not a simple operation and should be avoided whenever possible.

Candle filterAdvantages:• Excellent cake discharge.• Adapts readily to slurry thickening.• Minimum floor space.• Mechanically simple since there are no complex sealing

glands or bearings.Disadvantages:• High headroom is required for dismantling the filtering

elements.• The emptying of the vessel in between cake filtration,

washing and drying requires close monitoring of the pressure inside the vessel to ensure that the cake holds on to the candles.

Centrifugal filtrationThe removal of a liquid from a slurry by introducing the slurry into a rapidly rotating basket, where the solids are retained on a porous screen and the liquid is forced out of the cake by the centrifugal action.

A highly accelerated form of sedimentation, centrifugation is a process used to separate or concentrate materials suspended in a liquid medium. Centrifugation uses gravity and centrifugal force to separate particles heavier than the liquid medium. Centrifuges spin the material at high rotation speeds and separate the particulate from the liquid. Centrifugal force can reach many thousand times that of gravity, quickly separating the liquid/solid material, sometimes even to the nano-particle level.

Centrifugal filtrationConsider a centrifugal of radius R and height b rotating at an angular speed ω as shown figure. Let us assume that the liquid surface inside the basket is vertical and the cake is also deposited parallel to the wall of the basket, as shown in the figure.

Since the centrifugal forces are many-fold larger in magnitude than the gravitational forces, the effectively pressure difference driving force for filtration may be assumed to be due to centrifugal action only. Thus

Centrifugal filtrationWe can therefore use an equation for constant pressure provided the inherent assumptions such as the flow of filtrate through the cake is laminar and the voids of the cake are completely filled with the filtrate ( no channeling ) are kept alive. Thus,

Where (- Δ P) is obtained from previous (above ) equation. It must be noted that the cross-sectional area of the cake Ac perpendicular to the direction of flow of filtrate changes as the thickness of the cake increases. A specific value of Ac cannot be therefore used in the above equation since Ac is a function of time.

Where, = arithmetic mean cake area,

Centrifugal filtration = logarithmic mean cake area,

(R-Rc) = final thickness of cake formed. Am is the area of the filter medium which can be assumed to be roughly equal to the inside surface surface area of the basket. Thus, Am = 2πRb.

Centrifugal filtrationselection criteria

• The properties of the fluid, particularly its viscosity, density and corrosive properties.

• The nature of the solid—its particle size and shape, size distribution, and packing characteristics.

• The concentration of solids in suspension.• The quantity of material to be handled, and its value.• Whether the valuable product is the solid, the fluid, or both.• Whether it is necessary to wash the filtered solids.• Whether very slight contamination caused by contact of the suspension

or filtrate with the various components of the equipment is detrimental to the product.

• Whether the feed liquor may be heated.• Whether any form of pre treatment might be helpful.

Filter aid

1. Diatomaceous earth:A naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 1 micrometre to more than 1 millimetre, but typically 10 to 200 micrometres. This powder has an abrasive feel, similar to pumice powder, and is very light as a result of its high porosity. The typical chemical composition of oven-dried diatomaceous earth is 80 to 90% silica, with 2 to 4% alumina (attributed mostly to clay minerals) and 0.5 to 2% iron oxide.

Examples…Question : A leaf filter with 1.0 m2 of filtering surface operated at

constant pressure of 1.8 bar(gage) gave the following results:

The original slurry contained 10% by weight of solid calcium carbonate (specific gravity = 2.72) in water and the cake formed is essentially incompressible.a)Determine the time required to wash the cake formed at the end of 70 minutes of filtering at the same pressure using 3.0 m3 of wash water.b)If the time for dumping the cake and reassembling the press is 60 minutes, what is optimum cycle time and what is the volume of filtrate collected per cycle? Assume wash water is used in the same proportion to the final filtrate as in (a).

Filtrate volume(m3) 3.99 6.09 7.65 9.63 11.33

Time(min) 10 20 30 45 60

Solution: %inputfs=1;%filtering surface=1 m^3p=1.8; %constant pressure= 1.8 barx=0.1; % weight % of calcium cabonate sp=2.72; % specific gravity =2.72tf=70; %70 mins of filteringtf=tf*60;vw=3; %volume of wash water m^3tetc=3600; t=[10,20,30,45,60];%t time in minsfv=[3.99,6.09,7.65,9.63,11.33]; %fv= filterate volume (m^3)n=5;% n=no. of values of filterate volumedt=size(n-1);dfv=size(n-1);dtf=size(n-1);

av=size(n-1);

for i=1:n t(i)=t(i)*60;end for i=1:4 dt(i)=t(i+1)-t(i); dfv(i)=fv(i+1)-fv(i); dtf(i)=dt(i)/dfv(i); av(i)=fv(i)+.5*dfv(i);end disp('time in secs=');disp(t);disp('Filtrate volume (m^3)=');disp(fv);disp('delta t=');disp(dt);disp('delta V=');disp(dfv);disp('delta t/delta V=');disp(dtf);disp('V avg=');disp(av); u=polyfit(av,dtf,1);b=u(2); %interceptkp=u(1); %slope disp('slope (sec m^-6)=');disp(s);disp('intercept (sec m^-3)=');disp(b); vf=b*b+4*.5*tf*kp;vf=sqrt(vf);vf=vf-b;vf=vf/kp;disp('Volume final (m^3)=');disp(vf); rw=vf*kp+b;rw=1/rw;disp('rate of washing (m^3/s)=');disp(rw);

Continue…tw=vw/rw;disp('t wash=');disp(tw);disp('sec');disp(' ');disp((tw/60));disp('min'); r=vw/vf;disp(r); vo=tetc/(r*kp+.5*kp);vo=sqrt(vo);disp('Volume final optimum (m^3)=');disp(vo); tc=(r*kp+.5*kp)*vo*vo+(b+r*b)*vo+tetc;disp('optimum cycle time (sec)=');disp(tc);disp(tc/3600);disp('hr');

Output:Output:time in secs= 600 1200 1800 2700 3600 Filtrate volume (m^3)= 3.9900 6.0900 7.6500 9.6300 11.3300 delta t= 600 600 900 900 delta V= 2.1000 1.5600 1.9800 1.7000 delta t/delta V= 285.7143 384.6154 454.5455 529.4118

Output: V avg= 5.0400 6.8700 8.6400 10.4800 slope (sec m^-6)= 44.2873 intercept (sec m^-3)= 70.0130 Volume final (m^3)= 12.2817 rate of washing (m^3/s)= 0.0016 t wash= 1.8418e+003sec 30.6967 min ratio= 0.2443 Volume final optimum (m^3)= 10.4507 optimum cycle time (sec)= 8.1104e+003 2.2529 hr