Filtration - Pharmaceutical Engineering B.Pharmacy 3rd Semester 2022 (2021-2022) Previous Year's Question Papers/Notes Download - HK Technical PGIMS

BP304T Pharmaceutical Engineering

Filtration

 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 concentration. This means not exceeding 1.0%
separation from liquid very low w/v, the process of its is called clarification.
Terms used in filtration
 Slurry: Suspension to be filtered.
 Filter medium: Porous medium used to retain solids.
 Filter cake: Accumulated solids on the filter.
 Filtrate: Clear liquid passing through the filter.

Process of filtration

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Applications of filtration
 Production of sterile products:
 HEPA filters or laminar air flow bench
 Membrane filters.
 Production of bulk drugs
 Production of liquid dosage
 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.
 Straining: Similar to sieving. This means particles of larger size can’t pass through
smaller pore size of filter medium.
 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.
 Entanglement: Particles become entwined (entangled) in the masses of fibers (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.
 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 Depth 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.
 Efficiency: defined in terms of
mean or maximum pore size.
 The size of particles retained is
 Slightly higher than the mean
pore size of medium.
 Mechanical strength of filter
medium is less, unless it is
made of stainless steel.
 It has low capacity.

 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.
 The size of particles retained is much
smaller than the mean pore size of
medium.
 Mechanical strength of filter medium
is high.
 It has high capacity.
 The size of particles retained is less
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 The size of particles retained is
more predictable.
 Equipment is expensive because
ancillary equipment such as
edge clamps is required.
 Example: Cellulose
membrane filter.

predictable.
 Equipment is cheaper because is not
sintered ancillary equipment required.
 Example: Ceramic filters and filters.

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:

 The rate of filtration may be expressed as volume (liters) per unit time (dv/dt).

 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.
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Poiseuille’s Equation
 Poiseullie considered that filtration is similar to the streamline flow of liquid under pressure
through capillaries.
 Poiseullie’s Equation is-

◼ 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 the interstice,
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 non-
uniform.

 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:

Where:
 K = permeability coefficient of cake (m2
)
 A = surface area of porous bed (filter medium) (m2
)
 Other terms are same as previous equation
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 K depends on characteristics of cake, such as porosity, specific Surface area and
compressibility.
 Permeability may be defined quantitatively as the flow rate of a liquid of unit viscosity
across a unit area of cake having unit thickness under a pressure gradient of unity.
 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

Where:
 ε = porosity of cake (bed)
 S = specific surface area of particles comprising the cake m2
/ m3

 K = Kozeny constant (usually taken as 5)
Other terms are same as previous equations
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.

Factors influencing filtration
Surface area of filter medium

 Rate can be increased either using large filter or connecting a number of small units in
parallel.
 Filter press works on principle of connecting units in parallel.
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Pressure drop across the filter medium
 According to K-C equation the rate of filtration is proportional to the overall pressure
drop across both the filter medium and filter cake.
 The pressure drop can be achieved in a number of ways:
i. Gravity
 A pressure difference could be obtained by maintaining a head of slurry above the
filter medium.
 The pressure developed will depend on the density of the slurry
ii. Vacuum (Reducing pressure): 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.
iii. Pressure
 The simplest method being to pump the slurry into the filter under pressure.
iv. Centrifugal force
 The gravitational force could be replaced by centrifugal force in particle separation.

Viscosity of filtrate

 According to K-C equation rate of filtration is inversely proportional to the viscosity of
the fluid.
 Reason behind this is an increase in the viscosity of the filtrate will increase the
resistance of flow.
 This problem can be overcome by two methods:

 The rate of filtration may be increased by raising the temperature of the liquid,
which lowers its viscosity. However, it is not practicable if thermolabile
materials are involved or if the filtrate is volatile.
 Dilution is another alternative but the rate must be doubled.

Filter Media
 The surface upon which solids are deposited in a filter is called the “Filter medium”.
 Properties of ideal filter medium:
It should:
 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.
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 Sterile filtration imposes a special requirement since the pore size must not
exceed the dimension of bacteria or spores

Material used as filter media
1. 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.
2. Perforated sheet metal
 Stainless steel plates have pores which act as channels as in case of meta filters.
3. 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.
Example: Of granular solids are gravel, sand, asbestos, paper pulp and keiselgur.
 Choice of solids depends on size of solids in process
4. Prefabricated porous solid units
 Used for its convenience and effectiveness.
 Sintered glass, sintered metal, earthenware and porous plastics are used for fabrication.
5. 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.
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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.
Disadvantages:
 Remove the coloured substances by absorbing them.
 Sometimes active principles such as alkaloids are absorbed on filter aid.
 Rarely, filters are source of contamination such as soluble iron salts, which can provoke
degradation of sensitive ingredient.
Handling of filter aids
 Filter aids may be used in either or both two ways:
1) Pre- coating technique: by forming a pre-coat over the filter medium by filtering a
suspension of the filter aid.
2) Body- mix technique: A small proportion of the filter aid is added to the
slurry to be filtered. This slurry is recirculated through the filter until a clear filtrate is
obtained, filtration then proceeds to completion.
 Different flow rates can be achieved depending on grade of aid:
1. Low flow rate: fine grade filter aids- mainly used for clarity
2. Fast flow rate: coarse grade filter aids- acceptable filtrate.
Examples of filter aids
 Diatomite (Keiselgur) obtained from natural siliceous deposits.
 Perlite it is an aluminium silicate. Cellulose, charcoal, talc, bentonite , fullers earth etc.
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Plate and frame filter press
Principle:
 Surface filtration.
 Type of pressure filters
 The slurry enters the frame by pressure and flows through filter medium.
 The filtrate is collected on the plates and sends 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
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 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
2. Washing of cake (if desirable)
 Filtration operation
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 Frame marked by 2 dots
 Plate  marked by 1 dot
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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.
Advantages

 Construction of filter press is very simple and a variety of materials can be used.

 Cast iron for handling common substances.
 Bronze  for smaller units.
 Stainless steel – contamination can be avoided.
 Hard rubber and plastic- used where metals must be avoided.
 Wood for lightness though it must be kept wet.

 Provide large filtration area in relatively small floor space. The capacity being variable
according to thickness of frames and number used.
 Sturdy construction permits the use of considerable pressure difference. (2000
Kilopascals normally used).
 Efficient washing of cake is possible.
 Operation and maintenance is easy.
 It produce dry cake in form of slab.
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.
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.
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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

Use:

 Use for the filtration of slurry which do not contain high solid content, about 5%, i.e.
dilute suspensions.

Advantages

 Simplest form of filter used for batch process.
 A number of units can be connected in parallel to increase the surface area of filtration.
 Pressure difference can be obtained either with vacuum or using pressure up to the order
of 800 kilopascals.
 Labour costs for operating the filter leaf are fairly moderate.
 The efficiency of washing is high.
 The slurry can be filtered from any vessel.
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 The cake can be washed simply by immersing the filter in a vessel of Water.

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 consist 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.
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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 include:

 The collection of calcium carbonate, magnesium carbonate, and starch.
 The separation of the mycelium from the fermentation liquor in the
manufacture of antibiotics.

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.
 Example: 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
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Meta filter
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.

Working

 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.
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Advantages
 Can be used under high pressures, without any danger of bursting the filter medium.
 Running cost are low, as separate filter medium is not used.
 Can be constructed from a material that can provide excellent resistance to corrosion and
avoid contamination of sensitive products.
 It is extremely versatile filter because fine as well as large both type of particles can be
separated.
 Removal of cake can be carried out by simply back- flushing with water.
 Change over from one batch to another or one product to another is easy.
 Sterile products can be handled
Uses
 Metafilters can be used for:
 Clarification of syrups
 Filtration of injection solutions
 Clarification of insulin liquors
 Filtration of viscous liquids pressure.

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: pre filter and actual
filter for filtration.
 A protective layer surrounds them.
 The cartridge are 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.
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Working:

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
Advantages and Disadvantages
Advantages:
 Autoclaving can be done for sterile operations due to stainless steel construction.
 Cartridge with self-cleaning devices are advantageous.
 Rapid disassembling as well as reusing of filter medium is possible.
 Cartridge are not brittle, when they are dry.
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 Used as in-line continuous filtration, which reduces handling of solutions. It minimize
chances of contaminations.
Disadvantages:
 A number of manufactures provide the components, which are generally not
interchangeable between suppliers.
 Cost of disposable elements offsets the labour saving in terms of assembly and cleaning
of cartridge clarifiers.

Membrane Filter

Seitz Filter