FILTRATION AND CENTRIFUGATION – Pharmaceutical Engineering B. Pharm Third Semester PDF Notes


of this session

Introduction to filtration

Mechanisms of filtration

Theories of filtration

Factors affecting filtration

Filter Aids

Classification of filtration

Filter leaf, candle filter and Meta

Plate and frame filter press

Centrifugation – Introduction and

Principle of centrifugation

Classification of centrifuges

Perforated basket, tubular bowl
centrifuge and conical disc centrifuge


At the end of this lecture student will
be able to:

Define filtration and various terms
associated with it

Explain the theories of filtration with

State various mechanisms of filtration

Enumerate the factors influencing

Compare filtration with other unit

Define filter aids with examples

Discuss handling of filter aids

List the various filtration equipments
based on different techniques

Describe filtration equipments like
filter leaf, candle filter and Meta filter

Describe the working of filter press

Draw the plate and frame filter press
with labelling

Define centrifugation

Enlist the pharmaceutical applications
of centrifugation

Explain the principle of centrifugation

Classify centrifugation equipments

Describe the working of perforated
basket centrifuge, tubular bowl centrifuge and conical disc



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

Removal of solid particles from a fluid
by passing the fluid through a filtering medium, or septum


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

Mechanism of filtration

The mechanism whereby particles are
retained by a filter is significant only in initial stages of


• Similar to sieving, i.e., particles
of larger size can’t pass through smaller pore size of filter


• 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


• 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

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

between surface and depth filtration

Surface filtration

• 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

• The size of particles retained is
more predictable

• Equipment is expensive because
ancillary equipment such as edge clamps is required. Ex. Cellulose
membrane filter

Depth filtration

• 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 predictable

• Equipment is cheaper because
ancillary equipment is not required. Ex. Ceramic filters and
sintered filters

of filtration

Surface/ screen

It is a screening action by which
pores or holes of medium prevent the passage of solids

Mechanism involved: straining and

For this, plates with holes or woven
sieves are used

Efficacy is defined in terms of mean or
maximum pore size.

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

of filtration

• Production of sterile products:

HEPA filters or laminar air bench

Membrane filters

• Production of bulk drugs

• Production of liquid dosage

• Effluents and waste water treatment

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

Rate = driving force /

• 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

• 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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