FiltrationFiltration

• FILTRATIONFILTRATION defined as the separation of a solid defined as the separation of a solid

from a fluid (The term fluid includes liquids and from a fluid (The term fluid includes liquids and

gases) by means of a porous medium that gases) by means of a porous medium that

retains the solid but allows the fluid to pass.retains the solid but allows the fluid to pass.

• The principal pharmaceutical application of gas The principal pharmaceutical application of gas

filtration is the treatment of air to produce filtration is the treatment of air to produce

sterile atmospheres.sterile atmospheres.

Feed

Permeate

Feed Retentate

Permeate

Dead-end or Conventional Filtration Crossflow Filtration

The following terms must be The following terms must be concernedconcerned

• The slurryThe slurry

is the suspension of solid and liquid to be filtered.is the suspension of solid and liquid to be filtered.

• The filter mediumThe filter medium is the porous medium used to retain the solids. is the porous medium used to retain the solids.

• The filter cakeThe filter cake is the accumulation of solids on the filter.is the accumulation of solids on the filter.

• The filtrateThe filtrate is the clear liquid passing through the filter.is the clear liquid passing through the filter.

FACTORS INFLUENCE THE CHOICE OF FACTORS INFLUENCE THE CHOICE OF

METHOD OF FILTRATION AND THE RATE METHOD OF FILTRATION AND THE RATE

OF FILTRATIONOF FILTRATION The unit operation of filtration is affected by the The unit operation of filtration is affected by the

characteristics of the slurry, including:characteristics of the slurry, including:

•The properties of the liquid:The properties of the liquid:

such as density, viscosity, and corrosiveness.such as density, viscosity, and corrosiveness.

•The properties of the solid:The properties of the solid:

such as, particle shape, particle size, particle size such as, particle shape, particle size, particle size

distribution, and the rigidity or compressibility of the distribution, and the rigidity or compressibility of the

solid.solid.

•Whether the objective is to collect the solid, the liquid, or Whether the objective is to collect the solid, the liquid, or

both. both.

RATE OF FILTRATIONRATE OF FILTRATION

• Darcy's lawDarcy's law correlating the rate factors as following:correlating the rate factors as following:

Where:Where:

• V = volume of filtrate.V = volume of filtrate.

• t = time of filtration.t = time of filtration.

• K = constant for the filter medium and the filter cake.K = constant for the filter medium and the filter cake.

• A = area of filter medium.A = area of filter medium.

• ∆∆P = pressure drop across the filter medium and filter cake.P = pressure drop across the filter medium and filter cake.

• ηη = viscosity of the filtrate. = viscosity of the filtrate.

• l = thickness of cake.l = thickness of cake.

=dt

dVl

PKA

Properties of the Filter Properties of the Filter Medium and Filter CakeMedium and Filter Cake

The constant (K) represents the resistance of the The constant (K) represents the resistance of the

filter medium and filter cake.filter medium and filter cake.

The resistance of the filter medium is of The resistance of the filter medium is of

significance on the laboratory scale, it is relatively significance on the laboratory scale, it is relatively

less important on the large scale and can usually less important on the large scale and can usually

be neglected. be neglected.

The magnitude of the resistance of the filter The magnitude of the resistance of the filter

medium will change due to the early layers of medium will change due to the early layers of

solids which may block the pores or may form solids which may block the pores or may form

bridges over the entrances to the channels.bridges over the entrances to the channels.

For this reason, the pressure should be kept low at For this reason, the pressure should be kept low at

the start, to avoid the pores `plugging', and the start, to avoid the pores `plugging', and

increased as the cake builds up.increased as the cake builds up.

The usual procedure is to filter at constant rate & The usual procedure is to filter at constant rate &

increasing the pressure as necessary. When the increasing the pressure as necessary. When the

normal working pressure is reached it is maintained, normal working pressure is reached it is maintained,

hence the rate of filtration decreases as the hence the rate of filtration decreases as the

thickness of the cake increases.thickness of the cake increases.

When the rate is uneconomically low, filtration is When the rate is uneconomically low, filtration is

stopped, the filter cake removed, and filtration stopped, the filter cake removed, and filtration

restarted.restarted.

The principal properties of the solid affecting the The principal properties of the solid affecting the

resistance of the cake are:resistance of the cake are:

The surface area of the particles.The surface area of the particles.

The porosity of the cake.The porosity of the cake.

Rigidity or compressibility of the particles.Rigidity or compressibility of the particles.

Thus, any decrease in the resistance of the cake Thus, any decrease in the resistance of the cake

will show as an increased value of K, so will show as an increased value of K, so

increasing the rate of filtration. This can be done, increasing the rate of filtration. This can be done,

for example, by using for example, by using filter aids.filter aids.

Area of FilterArea of Filter The total volume of filtrate flowing from the The total volume of filtrate flowing from the

filter will be proportional to the area of the filter will be proportional to the area of the

filter. filter. Hence, this rate can be Hence, this rate can be

increased by:increased by:

using larger filtersusing larger filters

using a number of small using a number of small

units in parallel.units in parallel.

In the rotary filter, the filter cake is In the rotary filter, the filter cake is

removed continuously, giving, in removed continuously, giving, in

effect, an infinite area for filtration.effect, an infinite area for filtration.

Pressure DropPressure Drop The rate of filtration is proportional to the overall The rate of filtration is proportional to the overall

pressure drop across both the filter medium and pressure drop across both the filter medium and

filter cake.filter cake.

The pressure drop can be achieved in a number of The pressure drop can be achieved in a number of

ways:ways:

Reduced pressureReduced pressurePressurePressure

GravityGravityCentrifugal forceCentrifugal force

1.1. REDUCED PRESSURE REDUCED PRESSUREThe pressure below the filter medium may be reduced The pressure below the filter medium may be reduced

below atmospheric pressure by connecting the filtrate below atmospheric pressure by connecting the filtrate

receiver to a vacuum pump and creating a pressure receiver to a vacuum pump and creating a pressure

differential across the filter. differential across the filter.

Advantage Advantage of reduced pressure is the safety, since, if of reduced pressure is the safety, since, if

part of the equipment fails, it will collapse and not part of the equipment fails, it will collapse and not

explode. explode.

DisadvantageDisadvantage is that reduction of pressure lowers the is that reduction of pressure lowers the

boiling point of liquids so that it is possible for the boiling point of liquids so that it is possible for the

filtrate to boil in the receiver causing loss of liquid, and filtrate to boil in the receiver causing loss of liquid, and

the vapour may be damaging to the vacuum pump.the vapour may be damaging to the vacuum pump.

2.2. PRESSURE PRESSURE The commonest method is to obtain a pressure The commonest method is to obtain a pressure

difference by applying pressure to the surface of difference by applying pressure to the surface of

the slurry by pumping the slurry into the filter the slurry by pumping the slurry into the filter

under pressure.under pressure.

3. 3. GRAVITY GRAVITY A simple method of obtaining a pressure A simple method of obtaining a pressure

difference is by maintaining a head of difference is by maintaining a head of

slurry above the filter medium.slurry above the filter medium.

4.4. CENTRIFUGAL FORCE CENTRIFUGAL FORCE

The gravitational force can be used successfully in The gravitational force can be used successfully in

filtration processes.filtration processes.

Viscosity of Viscosity of FiltrateFiltrate

An increase in the viscosity of the filtrate will An increase in the viscosity of the filtrate will

increase the resistance to flow, so that the rate of increase the resistance to flow, so that the rate of

filtration is inversely proportional to the viscosity of filtration is inversely proportional to the viscosity of

the fluid.the fluid.

The rate of filtration may be increased by raising the The rate of filtration may be increased by raising the

temperature of the liquid, which lowers its viscosity.temperature of the liquid, which lowers its viscosity.

(This may not be practicable if thermolabile (This may not be practicable if thermolabile

materials are involved or if the filtrate is volatile).materials are involved or if the filtrate is volatile).

Dilution is another alternative, but may not always Dilution is another alternative, but may not always

be acceptable. be acceptable.

Thickness of Filter Thickness of Filter CakeCake

The filtrate must flow through the filter cake The filtrate must flow through the filter cake

formed on the filter medium as filtration formed on the filter medium as filtration

progresses; hence the rate of flow will be inversely progresses; hence the rate of flow will be inversely

proportional to the thickness of the cake.proportional to the thickness of the cake.

Cake thickness will be affected by area of filtration, Cake thickness will be affected by area of filtration,

so that for a given amount of slurry, increase in so that for a given amount of slurry, increase in

area will decrease the cake thickness. Also, in the area will decrease the cake thickness. Also, in the

rotary filter the filter cake is removed continuously, rotary filter the filter cake is removed continuously,

so that the cake thickness is minimized.so that the cake thickness is minimized.

Mechanisms of Filtration

STRAINING IMPINGEMENT ENTANGLEMENTENTANGLEMENT

ATTRACTIVE FORCESATTRACTIVE FORCES

1)1) STRAININGSTRAINING

► The simplest filtration procedure is The simplest filtration procedure is straining, straining,

in which, like sieving, the pores are smaller in which, like sieving, the pores are smaller

than the particles, which are retained on the than the particles, which are retained on the

filter medium.filter medium.

2)2) IMPINGEMENTIMPINGEMENT

► As a flowing fluid approaches an As a flowing fluid approaches an

object such as a cylinder, the flow object such as a cylinder, the flow

pattern is displaced, as shown pattern is displaced, as shown

diagrammatically by the diagrammatically by the

streamlines streamlines

► Solids having force would cross the streamlines Solids having force would cross the streamlines

and strike the cylinder, but, in practice, some will and strike the cylinder, but, in practice, some will

follow the streamlines and will not be collected or follow the streamlines and will not be collected or

will be re-entrained. Thus, particles between will be re-entrained. Thus, particles between

streamlines streamlines A A and and B B will be collected, and the will be collected, and the

accumulation of solids will form a ridge, roughly accumulation of solids will form a ridge, roughly

triangular in section, as indicated in the figure.triangular in section, as indicated in the figure.

3) ENTANGLEMENT3) ENTANGLEMENT

► If the filter medium consists of a cloth If the filter medium consists of a cloth

with a nap or is a porous felt, particles with a nap or is a porous felt, particles

become entangled in the mass of fibers. become entangled in the mass of fibers.

Usually the particles are smaller than the Usually the particles are smaller than the

pores, so that it is possible that pores, so that it is possible that

impingement is involved.impingement is involved.

4) ATTRACTIVE FORCES4) ATTRACTIVE FORCES

► In certain cases, particles may collect on a In certain cases, particles may collect on a filter medium as a result of attractive filter medium as a result of attractive forces due to the electrostatic forces due to the electrostatic precipitation, where large potential precipitation, where large potential differences are used to remove particles differences are used to remove particles from air streams.from air streams.

► In practice, the process may combine the In practice, the process may combine the various mechanisms, but the solids removal various mechanisms, but the solids removal is mainly by a straining mechanism, once is mainly by a straining mechanism, once the first complete layer of solids has begun the first complete layer of solids has begun to form the cake on the filter medium.to form the cake on the filter medium.

Filter Media

The filter medium may be responsible for the

collection of the solids, or a support for the filter cake.

The filter medium must be strong, have a low

resistance to flow, and be unaffected by the

substances in the slurry.

Materials used for filter media include:Materials used for filter media include:

Woven materials, such as felts or cloths in wool, Woven materials, such as felts or cloths in wool, cotton, silk, glass, metal or synthetic fibers (rayon, cotton, silk, glass, metal or synthetic fibers (rayon, nylon, etc.).nylon, etc.).

Perforated sheet metal.Perforated sheet metal.

Beds of granular solids built up on a supporting Beds of granular solids built up on a supporting medium.medium.

The solids may vary in size according to the needs The solids may vary in size according to the needs of the process. of the process.

Typical examples of granular solids used for Typical examples of granular solids used for

this purpose include gravel, sand, asbestos, this purpose include gravel, sand, asbestos,

paper pulp, and kieselguhr.paper pulp, and kieselguhr.

The final choice of filter medium will The final choice of filter medium will

depend on the chemical nature of the slurry.depend on the chemical nature of the slurry.

Filter Aids

The resistance to flow due to the filter medium itself is very low, but will increase as a layer of solids builds up, blocking the pores of the medium and forming a solid, impermeable cake.

The object of the filter aid is:– To prevent the medium from becoming blocked.– To form an open, porous cake, so reducing the

resistance to flow of the filtrate.

The filter aid must be a light, porous, The filter aid must be a light, porous,

inert solid and may be used in either or inert solid and may be used in either or

both of two ways.both of two ways.

Firstly, by forming a pre-coat over the Firstly, by forming a pre-coat over the

medium by filtering a suspension of the medium by filtering a suspension of the

filter aid sufficient to give a coating up filter aid sufficient to give a coating up

to 0.5 kg/m2.to 0.5 kg/m2.

Secondly, a small proportion of the Secondly, a small proportion of the

filter aid (0.1 to 0.5 percent) is added filter aid (0.1 to 0.5 percent) is added

to the slurry, ensuring that the filter to the slurry, ensuring that the filter

cake has a porous structure.cake has a porous structure.

Care must be taken in the choice of the filter aid, Care must be taken in the choice of the filter aid,

as materials with adsorptive properties (such as as materials with adsorptive properties (such as

kaolin) could remove solutes from the filtrate; kaolin) could remove solutes from the filtrate;

this may be done intentionally, as sugar solutions this may be done intentionally, as sugar solutions

may be decolorised by means of charcoal.may be decolorised by means of charcoal.

Kieselguhr is a very successful filter aid that 0.1 Kieselguhr is a very successful filter aid that 0.1

% added to slurry with 20 % solids, resulted in an % added to slurry with 20 % solids, resulted in an

increase in the rate of filtration of 5 times or increase in the rate of filtration of 5 times or

more.more.

Filter aids are limited to clarification processes Filter aids are limited to clarification processes

when the solids are discarded.when the solids are discarded.

Industrial FiltersIndustrial Filters

► The filter leaf consist of a The filter leaf consist of a

frame enclosing a drainage frame enclosing a drainage

screen or grooved plate, screen or grooved plate,

the whole unit is covered the whole unit is covered

with filter cloth. with filter cloth.

► The outlet for the filtrate The outlet for the filtrate

connects to the inside of connects to the inside of

the frame.the frame.

► The frame may be of any The frame may be of any

shape, circular, square or shape, circular, square or

rectangular shapes.rectangular shapes.

Filter LeafFilter Leaf

Principle of workPrinciple of workThe filter leaf is immersed in the slurry The filter leaf is immersed in the slurry

A receiver and vacuum screen system connected to A receiver and vacuum screen system connected to

the filtrate outlet. the filtrate outlet.

The slurry may be pumped under pressure.The slurry may be pumped under pressure.

The leaf is constructed with ribs on both sides to The leaf is constructed with ribs on both sides to

allow free flow of filtrate towards the neck and is allow free flow of filtrate towards the neck and is

covered with coarse mesh screens that support the covered with coarse mesh screens that support the

finer woven metal screens or filter cloth that retain finer woven metal screens or filter cloth that retain

the cake. the cake.

•Each leaf has a centrally located Each leaf has a centrally located

neck at its bottom which is inserted neck at its bottom which is inserted

into a manifold that collects the into a manifold that collects the

filtrate.filtrate.

A typical example is the A typical example is the Sweetland filterSweetland filter, ,

where it will be seen that the vessel is cylindrical where it will be seen that the vessel is cylindrical

and arranged so that it is supported by the upper and arranged so that it is supported by the upper

part, and the lower part can be swung away. part, and the lower part can be swung away.

This permits the cake to be discharged by

compressed air without removing the filter

leaves from the vessel.

Advantages:Advantages:1.The slurry can be filtered from any vessel and the 1.The slurry can be filtered from any vessel and the

cake can be washed simply by immersing the filter cake can be washed simply by immersing the filter

in a vessel of water. Removal of the cake is in a vessel of water. Removal of the cake is

facilitated by the use of reverse air flow.facilitated by the use of reverse air flow.

2. The pressure difference may be obtained with 2. The pressure difference may be obtained with

vacuum or by using pressures up to the order of 8 vacuum or by using pressures up to the order of 8

bars.bars.

3. The area can be 3. The area can be

variedvaried

by employing a by employing a

suitable suitable

number of units.number of units.

The leaf filter is most suitable if the solids The leaf filter is most suitable if the solids

content of the slurry is not too high, about 5 content of the slurry is not too high, about 5

% maximum.% maximum.

A higher proportion results in excessive non-A higher proportion results in excessive non-

productive time while the filter is being productive time while the filter is being

emptied and, provided this is observed, with emptied and, provided this is observed, with

cost operating the filter.cost operating the filter.

Application

This press is made up of two types This press is made up of two types

of units, Plates and frames, with a of units, Plates and frames, with a

filter medium, usually filter cloth, filter medium, usually filter cloth,

between the two.between the two.

The frame is open, with an inlet for The frame is open, with an inlet for

the slurry, while the plate has a the slurry, while the plate has a

grooved surface to support the grooved surface to support the

filter cloth, and with an outlet for filter cloth, and with an outlet for

the filtrate.the filtrate.

Filter PressFilter Press (PLATE AND FRAME PRESS)(PLATE AND FRAME PRESS)

The filtrate then drains down The filtrate then drains down

the surface of the plate, the surface of the plate,

between the projections on between the projections on

the surface and escapes the surface and escapes

from the outlet.from the outlet.

Principle of operation The slurry enters the frame (marked by 2 dots) from The slurry enters the frame (marked by 2 dots) from

the feed channel the feed channel

the filtrate passes through the filter medium on to the filtrate passes through the filter medium on to

the surface of the plate (marked by 1 dot), while the the surface of the plate (marked by 1 dot), while the

solids form a filter cake in the frame. solids form a filter cake in the frame.

The thickness of the The thickness of the

cake can be varied by cake can be varied by

using frames of using frames of

different thicknesses.different thicknesses.

Filtration is continued until the frame is filled with Filtration is continued until the frame is filled with

filter cake, when the process is stopped, the frame filter cake, when the process is stopped, the frame

emptied, and the cycle restarted.emptied, and the cycle restarted.

In practice, a large number of plates and frames In practice, a large number of plates and frames

are arranged alternately and clamped in a are arranged alternately and clamped in a

supporting structure. This gives a number of supporting structure. This gives a number of

filtration units operating in parallel and a filtration filtration units operating in parallel and a filtration

area as large as necessary.area as large as necessary.

Plates and frames may be made in various metals to provide Plates and frames may be made in various metals to provide

resistance to corrosion or prevent metallic contamination of resistance to corrosion or prevent metallic contamination of

the product.the product.

Non-metals avoid this problem and many have the advantage Non-metals avoid this problem and many have the advantage

of lightness as reinforced plasticsof lightness as reinforced plastics

Certain varieties of wood are materials of construction, Certain varieties of wood are materials of construction,

provided the press is kept wet at all times to avoid shrinkage.provided the press is kept wet at all times to avoid shrinkage.

AdvantagesAdvantages (a) Construction is very simple and a wide variety of (a) Construction is very simple and a wide variety of

materials can be used, as cast iron, stainless steel, hard materials can be used, as cast iron, stainless steel, hard

rubber or plastics.rubber or plastics.

(b) It provides a large filtering area in a relatively small (b) It provides a large filtering area in a relatively small

floor space.floor space.

(c) The capacity being variable according to the (c) The capacity being variable according to the

thickness of the frames and the number used.thickness of the frames and the number used.

(d) The strong construction permits the use of considerable (d) The strong construction permits the use of considerable

pressure difference up to about 20 bars.pressure difference up to about 20 bars.

(e) (e) Efficient washing of the cake is possible.Efficient washing of the cake is possible.

(f) Operation and maintenance is simple, because there are (f) Operation and maintenance is simple, because there are

no moving parts, filter cloths are easily renewableno moving parts, filter cloths are easily renewable

(g) Because all joints are external, any leaks are visible and (g) Because all joints are external, any leaks are visible and

do not contaminate the filtrate.do not contaminate the filtrate.

DisadvantagesDisadvantages (a) It is a batch filter.(a) It is a batch filter.

(b) The filter press is an expensive filter, the emptying time, (b) The filter press is an expensive filter, the emptying time,

the effort involved, and the wear and tear on the cloths the effort involved, and the wear and tear on the cloths

resulting in high costs.resulting in high costs.

(c) Operation is critical, as the frames should be full; (c) Operation is critical, as the frames should be full;

otherwise washing is inefficient and the cake is difficult to otherwise washing is inefficient and the cake is difficult to

remove.remove.

(d) The filter press is used for slurries (d) The filter press is used for slurries

containing less than about 5 % solids; containing less than about 5 % solids;

In view of the high work costs, it is most In view of the high work costs, it is most

suitable for expensive materials, as the suitable for expensive materials, as the

collection of bismuth salts, the collection of collection of bismuth salts, the collection of

precipitated antitoxins, and the removal of precipitated antitoxins, and the removal of

precipitated proteins from insulin liquors.precipitated proteins from insulin liquors.

Rotary FilterRotary Filter

The surface of the drum The surface of the drum

is covered with the is covered with the

filter medium and a filter medium and a

vacuum is vacuum is

maintained below the maintained below the

medium on the inside medium on the inside

of the drumof the drum

In operation, The drum rotates slowly about its In operation, The drum rotates slowly about its

horizontal axis and is partially submerged in a horizontal axis and is partially submerged in a

slurry reservoir, so that each unit passes through slurry reservoir, so that each unit passes through

the various zones. the various zones.

Principle of workPrinciple of work

Filtration is followed by cake washing and dewateringFiltration is followed by cake washing and dewatering

The outlet of the leaf is

connected to compressed

air, which detaches the

cake from the filter

medium.

If the cake is not

completely removed, this

can be effected by

scraping with a knife.

Liquid is sucked through the filter medium and Liquid is sucked through the filter medium and

solids in the feed are retained on the mediumsolids in the feed are retained on the medium

Rotary Filter Operation ZonePositionServiceConnected to

Pick-upSlurry trough

VacuumFiltrate receiver

Drainage-------VacuumFiltrate receiver

WashingWash spraysVacuumWash water receiver

Drying-------VacuumWash receiver

Cakeremoval

Scraper knifeCompressedair

Filter cakeconveyor

Cake Formation ZoneCake Formation Zone Cake Predrying ZoneCake Predrying Zone Cake washing ZoneCake washing Zone

Cake Final Drying ZoneCake Final Drying Zone Cake Discharge ZoneCake Discharge Zone Cake Dead ZoneCake Dead Zone

Rotary filters

The commonest form in use in the

pharmaceutical industry is the rotary drum

filter

Each filter unit is rectangular Each filter unit is rectangular

in shape with a curved profile so in shape with a curved profile so

that a number can be joined up that a number can be joined up

to form a drum.to form a drum. Each unit has a perforated Each unit has a perforated

metal surface to the outer part metal surface to the outer part

of the drum and is covered with of the drum and is covered with

filter cloth. filter cloth. Appropriate connections are Appropriate connections are

again made from each unit again made from each unit

through a rotating valve at the through a rotating valve at the

centre of the drum. centre of the drum.

The filter units have the shape of longitudinal The filter units have the shape of longitudinal

segments of the periphery of a cylinder. segments of the periphery of a cylinder.

Special attachments may be included for particular

purposes;

Cake compression rollers may be fitted if the

cake shrinks and cracks as it dries out.

It compress the cake to a homogeneous mass to

improve the efficiency of washing as the cake passes

through the washing zone, or to aid drainage of

wash water as the cake passes through the drying

zone.

A pre-coat filter may be used Where the

solids of the slurry block the filter cloth.

The pre-coat of filter aid is deposited on the

drum prior to the filtration process. Thus the

scraper knife removes the solid filtered from the

slurry together with a small amount of the pre-

coat, the knife advancing slowly as the pre-coat is

removed.

The string discharge rotary filter is

especially useful if removal of the cake presents

problems for certain pharmaceutical applications,

particularly for filtering the fermentation liquor in

the manufacture of antibiotics where the mould is

difficult to filter by ordinary methods because it

forms a felt-like `cake'.

The string discharge filter is operated by means

of a number of loops of string which pass round

the drum, and cause the cake to form over the

strings.

The strings are in contact

with the surface of the drum

up to the cake removal zone,

where they leave the surface

and pass over additional

small rollers before returning

to again contact the drum.

In operation, the strings

lift the filter cake off the filter

medium, and the cake is

broken by the sharp bend

over the rollers so that it is

easily collected while the

strings return to the drum.

AdvantagesAdvantages (a) The rotary filter is automatic and is continuous in operation, so (a) The rotary filter is automatic and is continuous in operation, so

that costs are very low.that costs are very low.

(b) The filter has a large capacity, the area of the filter as (b) The filter has a large capacity, the area of the filter as represented by A of Darcy's law is infinity.represented by A of Darcy's law is infinity.

(C) Suitable for use with concentrated slurries since the system (C) Suitable for use with concentrated slurries since the system removes the cake continuously. removes the cake continuously.

(d) Variation of the speed of rotation enables the cake thickness to (d) Variation of the speed of rotation enables the cake thickness to be controlled, and for solids that form an impenetrable cake the be controlled, and for solids that form an impenetrable cake the thickness may be limited to less than 5 mm. thickness may be limited to less than 5 mm.

If the solids are coarse, forming a porous cake, the thickness may be If the solids are coarse, forming a porous cake, the thickness may be 100 mm or more.100 mm or more.

DisadvantagesDisadvantages (a) (a) The rotary filter is a complex equipment, with many moving The rotary filter is a complex equipment, with many moving

parts, and is very expensive.parts, and is very expensive.

(b) (b) The cake tends to crack due to the air drawn through by the The cake tends to crack due to the air drawn through by the

vacuum system, so that washing and drying are not efficient.vacuum system, so that washing and drying are not efficient.

(c) Being a vacuum filter, the pressure difference is limited to 1 bar (c) Being a vacuum filter, the pressure difference is limited to 1 bar

and hot filtrates may boil.and hot filtrates may boil.

(d) The rotary filter is suitable only for straight(d) The rotary filter is suitable only for straight forward slurries, forward slurries,

but if the solids form an impermeable cake or will not separate but if the solids form an impermeable cake or will not separate

cleanly from the cloth it will be less satisfactory.cleanly from the cloth it will be less satisfactory.

USES OF THE ROTARY FILTERUSES OF THE ROTARY FILTER The rotary filter is most suitable for continuous operation on large The rotary filter is most suitable for continuous operation on large

quantities of slurry, especially if the slurry contains considerable quantities of slurry, especially if the slurry contains considerable

amounts of solids, that is, in the range 15 to 30 per cent.amounts of solids, that is, in the range 15 to 30 per cent.

Examples of pharmaceutical applications include the collection of Examples of pharmaceutical applications include the collection of

calcium carbonate, magnesium carbonate, and starch, and the calcium carbonate, magnesium carbonate, and starch, and the

separation of the mycelium from the fermentation liquor in the separation of the mycelium from the fermentation liquor in the

manufacture of antibiotics.manufacture of antibiotics.

Edge FiltersEdge Filters

Filters such as the leaf or press act by Filters such as the leaf or press act by

presenting a surface of the filter medium to presenting a surface of the filter medium to

the slurry. the slurry.

Edge filters use a pack of the filter medium, Edge filters use a pack of the filter medium,

so that filtration occurs on the edges. so that filtration occurs on the edges. Forms using packs of media such as filter Forms using packs of media such as filter

paper can be used, but in the paper can be used, but in the

pharmaceutical industry greatest use is pharmaceutical industry greatest use is

made of the made of the Metafilter.Metafilter.

METAFILTERMETAFILTER The metafilter consists of a The metafilter consists of a

grooved drainage rod on grooved drainage rod on

which is packed a series of which is packed a series of

metal rings. metal rings.

These rings, usually of These rings, usually of

stainless steel, are about stainless steel, are about

15 mm inside diameter, 22 15 mm inside diameter, 22

mm outside diameter, and mm outside diameter, and

0.8 mm in thickness, with 0.8 mm in thickness, with

a number of semicircular a number of semicircular

projections on one surface.projections on one surface. (a) surface view of ring )b (section through filter

When the rings are packed When the rings are packed

together, all the same way up, together, all the same way up,

and tightened on the drainage and tightened on the drainage

rod with a nut, channels are rod with a nut, channels are

formed that taper from about formed that taper from about

250250µµm down to m down to 2525µµm, m,

One or more of these packs is One or more of these packs is

mounted in a vessel, and the mounted in a vessel, and the

filter may be operated by filter may be operated by

pumping in the slurry under pumping in the slurry under

pressure or, by the pressure or, by the

application of reduced application of reduced

pressure to the outlet side.pressure to the outlet side.(a) surface view of ring

)b (section through filter

AdvantagesAdvantages (a) (a) The metafilter possesses strength that high The metafilter possesses strength that high

pressures can be used, with no danger of bursting pressures can be used, with no danger of bursting

the filter medium.the filter medium.

(b) (b) As there is no filter medium, the running costs As there is no filter medium, the running costs

are low, and it is a very economical filter.are low, and it is a very economical filter.

(c) The metafilter can be made from materials that (c) The metafilter can be made from materials that

can provide excellent resistance to corrosion and can provide excellent resistance to corrosion and

avoid contamination of the most sensitive product.avoid contamination of the most sensitive product.

(d) (d) Removal of the cake is effectively carried out by Removal of the cake is effectively carried out by

back-flushing with water. back-flushing with water.

USES OF THE USES OF THE METAFILTERMETAFILTER

The small surface area of the metafilter restricts the The small surface area of the metafilter restricts the

amount of solid that can be collected. amount of solid that can be collected.

This, together with the ability to separate very fine This, together with the ability to separate very fine

particles, means that the metafilter is used almost for particles, means that the metafilter is used almost for

clarification purposes.clarification purposes.

The strength of the metafilter permits the use of high The strength of the metafilter permits the use of high

pressures (up to 15 bars) making the method suitable pressures (up to 15 bars) making the method suitable

for viscous liquids. for viscous liquids.

It can be constructed in materials appropriate for It can be constructed in materials appropriate for

corrosive substances.corrosive substances.

Specific examples of pharmaceutical uses include the Specific examples of pharmaceutical uses include the

clarification of syrups, of injection solutions, and of clarification of syrups, of injection solutions, and of

products such as insulin liquors.products such as insulin liquors.