Pharmaceutical engineering (sem-3) unit 4-1

Filtration
Ms. Payal Pilaji
Assistant Professor
UNIT-4-1

Filtration: It may be define as a process of separation of
solids from a fluid by passing the same through a porous
medium that retains the solids but allows the fluid to pass
through.
Clarification: When solid are present in very low
concentration, i.e., not exceeding 1.0% w/v, the process of its
separation from liquid is called clarification.

Terms used in Filtration
 Slurry - Suspension to be filtered
 Filter medium - Porous medium used to retain solid
 Filter cake - Accumulated solids on the filter
 Filtrate - Clear liquid passing through the filter

Process of filtration
 Pores of filter medium are smaller than size of particles to be
separate.
 Filter medium (filter paper) is placed on a support (mesh)
 Slurry is placed over the filter medium
 Due to pressure difference across the filter, fluid flows through
the filter medium
 Gravity is acting over the liquid medium
 So solids are trapped on the surface of the filter medium

Applications of filtration
 Production of sterile products
 Production of bulk drugs
 Production of liquid dosage formulation
De-waxing of oils
Removing suspended oils from aqueous solutions
Removing of undesirable solids
Clarifying the potable water
 Effluents and waste water treatment

Mechanism of filtration
The mechanism whereby particles are retained by a filter is significant only in
initial stages of filtration.
1. Straining - Similar to sieving, i.e., particles of larger size can’t pass through
smaller pore size of filter medium.
2. Impingement - Solids having the momentum move along the path of
streaming flow and strike (impinge) the filter medium. Thus the solids are
retained on the filter medium.
3. Entanglement - Particles become entwined (entangled) in the masses of
fibres (of cloths with fine hairy surface or porous felt) due to smaller size of
particles than the pore size. Thus solids are retained within filter medium.
4. Attractive forces - Solids are retained on the filter medium as a result of
attractive force between particles and filter medium, as in case of electrostatic
filtration.

Types of filtration
Surface/ screen filtration –
 It is a screening action by which pores or holes of medium prevent
the passage of solids.
 Mechanism involved: straining and impingement
 For this, plates with holes or woven sieves are used.
Depth filtration -
 In this slurry penetrates to a point where the diameter of solid
particles is greater than that of the tortuous void or channel.
 Mechanism: Entanglement
 The solids are retained with a gradient density structure by physical
restriction or by adsorption properties of medium.

Theories of filtration
 The flow of liquid through a filter follows the basic rules that govern the flow of
any liquid through the medium offering resistance.
 The rate of flow may be expressed as- Rate = driving force / resistance
 The rate of filtration may be expressed as volume (litres) per unit time (dv/dt).
 Driving force = pressure upstream – pressure downstream
 Resistance is not constant.
 It increases with an increase in the deposition of solids on the filter medium.
 Therefore filtration is not a steady state.
 The rate of flow will be greatest at the beginning of filtration process, since the
resistance is minimum.
 After forming of filter cake, its surface acts as filter medium and solids

continuously deposit adding to thickness of the cake.
Resistance to movement= {pressure upstream- pressure downstream}/
length of capillaries

Poiseullie’s Equation
 Poiseullie considered that filtration is similar to the streamline
flow of liquid under pressure through capillaries.
 Poiseullie’s Equation is- V= ∆ /
𝝅 𝑷𝒓𝟒 𝟖𝑳ƞ
 Where, V = rate of flow, m3 /s (l/s)
 ΔP= Pressure difference across the filter, Pa
 r = radius of capillary in the filter bed, m
 L = thickness of filter cake (capillary length), m
 𝝶 = viscosity of filtrate, Pa.s
 If the cake is composed of bulky mass of particles and the
liquid flows through theinterstice, then flow of liquids
through these may be expressed by this equation.

Darcy’s Equation
 Poiseullie's law assumes that the capillaries found in the filter are highly irregular and
nonuniform.
 Therefore, if the length of capillary is taken as the thickness of bed, a correction factor
for radius is applied so that the rate is closely approximated and simplified.
 The factors influencing the rate of filtration has been incorporated into an equation by
 Darcy, which is: V= KA∆𝑷/ L
ƞ
 Where, K = permeability coefficient of cake, m2
 A = surface area of porous bed (filter medium), m2
 K depends on characteristics of cake, such as porosity, specific surface area and
compressibility.
 This equation is valid for liquids flowing through sand, glass beds and various
porous media.
 This model is applied to filter beds or cakes and other types of depth filter.
 This equation is further modified by including characteristics of K by Kozeny-
Carman.

Kozeny-Carman (K-C) equation
 Kozeny-Carman equation is widely used for filtration.
V= A/ S
Ƞ 2
*∆P/KL*€3
/(1-€)2
Where,
 𝝴 = porosity of cake (bed)
 S = specific surface area of particles comprising the cake m2 / m3
K = Kozeny constant (usually taken as 5)
Limitations:
 It does not consider the fact that depth of granular bed is lesser than the
actual path traversed by the fluid.
 The actual path is not same throughout the bed, but it is sinuous or
tortuous.

The following are the factors that affecting the rate of
filtration:
Permeability Coefficient: According to Darcy’s equation
permeability coefficient (K) represents the resistance of both
the filter medium and the filter cake. As the thickness of the
cake increase, the rate of filtration decreases. Also, the
surface area of the particles, the porosity of the cake, and the
rigidity or compressibility of the particles could affect the
permeability of the cake.

Area of Filter Medium: The total volume of filtrate flowing
through the filter is proportional to the area of the filter. The
area can be increased by using larger filters. For example, in the
rotary drum filter, the continuous removal of the filter cake
gives an infinite area for filtration.
Viscosity of Filtrate: It is considered that an increase in the
viscosity of the filtrate increases the resistance of flow so that
the rate of filtration is inversely proportional to the viscosity of
the fluid.

Pressure Drop: The rate of filtration is proportional to the pressure
difference across both the filter medium and filter cake. The pressure
drop can be achieved in several ways:
Gravity: A pressure difference could be obtained by maintaining a
sufficient head of slurry above the filter medium.
pressure developed will depend on the density of the slurry.
Vacuum: The pressure below the filter medium may be reduced below
atmospheric pressure by connecting the filtrate receiver to a vacuum
pump and creating a pressure difference across the filter.
Pressure: The simplest method is to pump the slurry onto the filter under
pressure.
Centrifugal force: The application of higher centrifugal force on slurry
increases the rate of filtration.

Thickness of Filter Cake: The rate of flow of the filtrate
through the filter cake is inversely proportional to the
thickness of the cake. Preliminary decantation may be
useful to decrease the number of solids that ultimately help
to increase the filtration rate.
Particle Size of Solids: Generally the larger the particle
size the higher the filtration rate (Kg/m2 /h). Small
average particle size and a narrow distribution range have
a high filtration rate.

Filter Media
 The surface upon which solids are deposited in a filter is called the “Filter

medium”
 Properties of ideal filter medium are as follows
• Be capable of delivering a clear filtrate at a suitable production rate.
• Have sufficient mechanical strength.
• Be inert.
• Retain the solids without plugging at the start of filtration.
• Not absorb dissolve material.
• Sterile filtration imposes a special requirement since the pore size must not
exceed the dimension of bacteria or spores.

 Material used as filter media
Woven material
 Made up of wool, silk, metal or synthetic fibres (rayon, nylon etc.).
 These include a- wire screening and b- fabrics of cotton, wool, nylon.
 Wire screening e.g. stainless steel is durable, resistance to plugging and
easily cleaned.
 Cotton is a common filter, however, Nylon is superior for pharmaceutical
use, since it is unaffected by mold, fungus or bacteria and has
negligible absorption properties.
 The choice of fibre depends on chemical reactivity with the slurry.
Perforated sheet metal
 Stainless steel plates have pores which act as channels as in case of
meta filters.

Bed of granular solid built up on supporting medium
 In some processes, a bed of graded solids may be formed to reduce resistance of flow.
 Ex. Of granular solids are gravel, sand, asbestos, paper pulp and keiselgur.
 Choice of solids depends on size of solids in process.
Membrane filter media •
 These are cartridge units and are economical and available in pore size of 100 μm to
even less than 0.2 μm. •
 Can be either surface cartridges or depth type cartridges. •
Surface cartridges –
 These are corrugated and resin treated papers and used in hydraulic lines.
 Ceramic cartridges and porcelain filter candles are examples.
 Can be reuse after cleaning.
 Depth type cartridges:
 Made up of cotton, asbestos or cellulose.
 These are disposable items, since cleaning is not feasible.

Filter Aids
 The objective of filter aid is to prevent the medium from becoming
blocked and to form an open, porous cake, hence, reducing the
resistance to flow of the filtrate.
 Filter aid forms a surface deposit which screens out the solids and also
prevents the plugging of supporting filter medium.
 Characteristics of filter aids:
• Chemically inert and free from impurities.
• Low specific gravity, so remain suspended in liquids.
• Porous rather than dense, so that pervious cake can be formed.
• Recoverable

Classification of filtration equipments
Based on application of external force:
 Pressure filter – Plate and frame filter press, metafilter
 Vacuum filters – Filter leaf
 Centrifugal filters
Based on operation of filtration
 Continuous filtration - Discharge and filtrate are separated steadily and
uninterrupted
 Discontinuous filtration - Discharge of filtered solid is intermittent. Filtrate is
removed continuously. Operation should be stopped to collect solid.

Based on nature of filtration
 Cake filters - Remove large amount of solids
 Clarifying filters - Remove small amounts of solids
 Cross-flow filters - Feed of suspension flows under pressure at fairly high
velocity across the filter medium

Membrane filters
Membrane filters act as a barrier to separate contaminants from water, or they
remove the particles contaminating the water. Reverse osmosis, ultrafiltration, and
nano-filtration all use a membrane in their different filtration processes.
Construction
• Membrane filters are made of thin and flat membranes of cellulose derivatives,
such
as, cellulose acetate and cellulose nitrate.
• These filters are brittle when in dry condition and can be stored for an indefinite
period.
• The filters are between 50 and 150 µ thick and are available in sizes upto 60 cm2

Working
• A membrane filter has 400 to 500 million pores per square centimetre of filter
surface.
• The pores are absolutely uniform in size and occupy about 80% of filter volume.
• To avoid rapid clogging of a membrane, pre-filtration is often required.
• The selection of a membrane filter for a particular application depends on the
particles to be removed.
Uses
• These filters are mainly used for sterilization of both aqueous and oily liquids.
• The membrane filters cannot be used for filtration of organic solvents, such as
alcohols, ketones, esters and chloroform.

SEITZ FILTER
Principle
• It is based on filtration of asbestos pad filter disc
Construction
• It consists of two parts. Lower part fitted with a perforated plate over which
compressed asbestos pad is placed.
• Upper part has a value through which pressure can be applied.
• Both parts joined together by winged nuts.
• A valve is present on the upper part through which vacuum is applied
• The asbestos pads may yield alkali and cause precipitation of alkaloids
• It may shed fibres into the filtrate and absorb drug from solution.

Advantages:
• No risk of contaminating the filtrate.
• Apparatus is very simple to use.
• For viscous solution they are more suitable.
Disadvantages:
• Asbestos may shed loose fibers.
• Pad may absorb sufficient amount of medicament

Rotary drum filter
Principle:
• Slurry filtered through sieve like mechanism on the rotation drum surface, under the
condition of vacuum.
• In addition compression, drying (using hot air), and removing the filter cake (using
knife) are possible.
Construction:
• It consists of a metal cylinder mounted horizontally.
• The drum may be up to 3 meters in diameter and 3.5 meters in length and gives
surface area of 20 meter square.
• The curved surface being a perforated plate, supporting a filter cloth.
• Internally, it is divided into several sectors and a separate connection is made between
each sector and a special rotary valve.

Working
• The drum is dipped into the slurry and vacuum applied to the outlet, which is
connected to the filtrate receiver.
• When the cake has formed, the cake drained or partially dried by vacuum.
• The drum is sprayed with water to wash the cake.
• Retaining the vacuum connection drains the cake and produces partial dryness then,
removed by a doctor knife.
• When the solids of the slurry are too much that the filter cloth becomes blocked with
the particles, a pre-coat filter may be used.
• A pre-coat of filter aid is deposited on the drum prior to the filtration process

Uses
• The rotary filter for continuous operation on large quantities of slurry.
• Suitable for slurry contains considerable amounts of solids in the range 15-30%.
• Examples of pharmaceutical application includes the collection of calcium
carbonate,
magnesium carbonate, and starch.
• The separation of the mycelium from the fermentation liquor in the manufacture
of

Advantages
• The rotary filter is automatic and is continuous in operation, so that the labour costs
are very low.
• The filter has a large capacity, so it is suitable for the filtration of highly concentrated
solutions.
• Variation of the speed of rotation enables the cake thickness to be controlled.
• Pre-coat of filter aid could used to accelerate the filtration rate.
• Filter has large surface area.
Disadvantages
• The rotary filter is a complex piece of equipment, with many moving parts and is very
expensive.
• In addition to the filter itself, some accessories are connected, e.g., a vacuum pump,
vacuum receivers, slurry pumps and agitators are required.
• The cake tends to crack due to the air drawn through by the vacuum system, so that
washing and drying are not efficient.
• Being a vacuum filter, the pressure difference is limited to 1 bar and hot filtrates may
boil.
• It is suitable only for straight- forward slurries

Cartridge filter
Principle:
 It is a thin porous membrane in which pre filter and membrane filter
are combined in a single unit.
 The filtration action is mainly sieve like and particles are retained on
the surface.

Construction:
 It has cylindrical configuration made with disposable or changeable filter
media.
 Made up of either plastic or metal.
 Consist of two membrane filters (sieve like) made of polypropylene: prefilter
and actual filter for filtration.
 A protective layer surrounds them.
 The cartridge is housed in a holder and a number of cartridges can be placed
in a same housing.
 The housing is closed with the lid.
 Housing has provisions for slurry inlet and outlets.

Working:
 Slurry is pumped into cartridge holder
 It passes through cartridge filter unit by straining
 The clear liquid passes through the centre
 Moves up to collect through outlet
Uses:
 Particularly useful for preparation of particulate free solutions for
parenterals and ophthalmic uses.
 This filter holder will process 1000 – 15000 litres of sterile solution
per hour.

Metafilter
Principle:
 Mechanism is surface filtration.
 In this, metal rings contain semicircular projections, which are arranged as a
nest to form channels on the edges.
 This channel offers resistance (strainer) to the flow of solids (coarse particles).
 The clear liquid is collected into receiver from the top.

Construction
 Metafilter consists of a series of metal rings.
 These are threaded so that a channel is formed on the edges.
 It contains a grooved drainage column on which a series of metal rings are packed.
 These rings are usually made up of stainless steel and have dimensions of about 15.0
mm internal diameter and 22.0 mm external diameter.
 Each metal ring has a number of semicircular projections (0.8 mm in thickness) on
one side of surface.
 The projections are arranged as a nest to form channels on the edges.
 These rings are tightened on the drainage column with a nut.
 Metafilters are also known as edge filters.

Working
 Filters are placed in a vessel
 Slurry is pumped under pressure or occasionally by applying reduced pressure to the outlet
side
 Slurry passes through the channels formed on the edges between the rings
 The clear liquid rises up and collected from the outlet into receiver
 For separation of fine particles, a bed of suitable materials such kieselguhr is first built up.
 The pack of rings serves essentially as a base on which the true filter medium is supported.
Uses
 Metafilters can be used for-
 Clarification of syrups
 Filtration of injection solutions
 Clarification of insulin liquors
 Filtration of viscous liquids can be achieved by applying pressure.

Filter leaf
Principle:
 It is an apparatus consisting of a longitudinal drainage screen covered with a filter cloth.
 The mechanism is surface filtration and acts as sieve or strainer.
 Vacuum or pressure can be applied to increase the rate of filtration.
Construction:
 The leaf filter is consisting of a frame enclosing a drainage screen or grooved plate.
 The frame may be any shape circular, square or rectangular.
 The whole unite being covered with filter cloth.
 The outlet for the filtrate connects to the inside of the frame through suction.

Working
 The filter leaf is immersed in the slurry
 Vacuum system is connected to the outlet
 The slurry passes through the filter cloth
 Finally filtrate enters the drainage canal and goes through the outlet into receiver
 Air is passed to flow in reverse direction which facilitates removal of cake.
Use:
 Use for the filtration of slurry which do not contain high solid content, about 5%, i.e.
dilute suspensions.

Sweetland filter (variant of filter leaf)
 An alternative method is to enclose the filter leaf in a special vessel into which the
slurry is pumped under pressure.
 A number of leaves are connected to a common outlet, to provide a large area for
filtration.

Plate and frame filter press
Principle:
 Mechanism is surface filtration.
 The slurry enters the frame by pressure and flows through filter medium.
 The filtrate is collected on the plates and send to outlet.
 A number of frames and plates are used so that surface area increases and
consequently large volumes of slurry can be processed simultaneously with or
without washing.

Construction
 The Filter press is made of two types of units, plate and frames.
 Usually made of aluminium alloy.
 Sometimes, these are also lacquered for protection against corrosive
chemicals and made suitable for steam sterilization.
Frame-
 It contains a open space inside wherein the slurry reservoir is maintained
for filtration and an inlet to receive the slurry.
 It is indicated by two dots in description.
 Frames of different thickness are available.
 It is selected based on the thickness of cake formed during filtration.
 Optimum thickness of frame should be chosen

 Plate
 The plate has a studded or grooved surface to support the filter cloth and an outlet.
 It is indicated by one dot in description.
 Plate supports the filter medium, receiving the filtrate and outlet.
 The filter medium usually cloth is interposed between plate and frame.
 Plate, filter medium, frame, filter medium and plate are arranged in sequence and clamed
to a supporting structure.
 It is normally described by dots as 1.2.1.2.1 so on.
 A number of plates and frames are employed so that the filtration area is as large as
necessary.
 Number of filtration units are operated in parallel.
 Channels for slurry inlet and filtrate outlet can be arranged by fitting eyes to the plates and
frames, these join together to form a channel.
 In some types only one inlet channel is formed, while each plate is having individual
outlets controlled by valves.

Working
 Working can be divided into two steps
1. Filtration operation
 Frame- marked by 2 dots
 Plate – marked by 1 dot
 Slurry enters the frame from the feed channel and passes through the filter medium on
the surface of the plate
 The solid forms a filter cake and remain in the frame
 The thickness of the cake is half of the frame thickness, because on each side of frame
filtration occurs
 Thus two filter cakes are formed , which meet eventually in the centre of the frame
 The filtrate drains between the projections of the surface of the plate and escape from the
outlet.
 As filtration proceeds, the resistance of the cake increases and filtration rate decrease
 At a certain point process is stopped and press is emptied and cycle is restarted.

Washing of cake (if desirable)
 When washing of cake is also required modified plate and frame filter is used.
 For this purpose an additional channel is included called as washing plate and
are identified by 3 dots.
 In the half of the washing plate, there is a connection from wash water cannel
to the surface of plate.
 The sequence of arrangement of plates and frames can be represented by dots
as 1.2.3.2.1.2.3.2.1 so on (between 1 and 1, 2.3.2 must be arranged.

Uses:
 Sterile filtrate can by obtain by using asbestos and cellulose filter sheet (for this,
whole filter press and filter medium have been sterilized previously).
 Filtration of viscous liquid can also be done by incorporating heating/cooling
coils in the press.
Disadvantages
 It is a batch filter, so it is a time consuming.
 The filter press is an expensive filter, the emptying time, the labour involved, and
the wear and tear on the cloths resulting in high costs.
 Operation is critical, as the frames should be full, otherwise washing is inefficient
and the cake is difficult to remove.
 The filter press is used for slurries containing less about 5 % solids In view of the
high labour costs , it is most suitable for expensive materials e.g. the removal of
precipitated proteins from insulin liquors.

Pharmaceutical engineering (sem-3) unit 4-1

More Related Content

What's hot

Similar to Pharmaceutical engineering (sem-3) unit 4-1

More from payalpilaji

Recently uploaded

Pharmaceutical engineering (sem-3) unit 4-1