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

FILTRATION AND CENTRIFUGATION

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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