Contents
of this session
Introduction to filtration
Mechanisms of filtration
Theories of filtration
Factors affecting filtration
Filter Aids
Classification of filtration
equipment’s
Filter leaf, candle filter and Meta
filter
Plate and frame filter press
Centrifugation – Introduction and
applications
Principle of centrifugation
Classification of centrifuges
Perforated basket, tubular bowl
centrifuge and conical disc centrifuge
Learning
objectives
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
equations
State various mechanisms of filtration
Enumerate the factors influencing
filtration
Compare filtration with other unit
operations
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
centrifuge
Definition
• 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
Removal of solid particles from a fluid
by passing the fluid through a filtering medium, or septum
• 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
Mechanism of filtration
The mechanism whereby particles are
retained by a filter is significant only in initial stages of
filtration.
Straining
• Similar to sieving, i.e., 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 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
Difference
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
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
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
Applications
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
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.
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