centrifugation

1. CENTRIFUGATION

2. INTRODUCTION
• Centrifuging or Centrifugation is a process which involves the use of
the centrifugal force for the separation of heterogeneous mixtures with a centrifuge.
• It is considered to be a unit operation in pharmacy.
• It is either a batch process or a continuous process.
• Also, centrifugation is widely applied as research techniques in Pharmacy, Medicine
Biochemistry, Cellular and Molecular Biology.
• Centrifugal force replaces gravitation forces in separation by sedimentation.
• It is the driving force for the rapid separation of two immiscible liquids or a solid
from a liquid in centrifuges.
• Centrifugation technique is particularly useful when separation by ordinary
filtration is difficult.
• For example separating highly viscous mixtures and colloidal dispersions (particle
size less than 5 µm), or where the difference in the densities of components to be
separated is less.
• The clear liquid that lies above the solid residue after centrifugation is called as
supernatant.
• The supernatant liquid is either quickly decanted from the centrifuge tubes without
disturbing the precipitate, or withdrawn with a pipette..

3. OBJECTIVES
Rapid separation of solids from liquids
The rate of separation of solids from liquids can be controlled by controlling various
parameters of centrifuge. For example, increase in speed of rotation of the centrifuge,
increases rate of separation.
 Separation of immiscible liquids
To separate immiscible liquids from a system like emulsion, centrifuges can be employed.
 Use as a research tool
In biochemistry, drug delivery systems it is used as a research tool. It is used to isolate
nucleic acids, plasma proteins, liposomes, niosome, etc.
.

4. PRINCIPLES OF CENTRIFUGATION
• Larger the particle size, greater will be the rate of sedimentation due to gravitational
force and simple will be the filtration technique.
• If the size of particles is less (less than 5µm) they undergo Brownian motion and no
sedimentation takes place.
• In such cases a stronger centrifugal force is to be applied to separate the particles.
Also, if the difference in densities of the particles of two phases is less, centrifugal
force is required to increase their rate of separation.

7. APPLICATIONS OF CENTRIFUGATION
• 1. Production of bulk drugs After crystallization, the crystalline drugs are separated from the
mother liquor by centrifugation. For example traces of mother liquor is removed from aspirin
crystals by centrifugation method. This results in free flowing product with better stability.
• 2. Production of biological products Majority of the biological products are either
proteinaceous or macromolecules. During their preparation, they remain in colloidal dispersion
in water. It is difficult to separate the colloid particles by normal methods of filtration. In such
cases centrifugal methods are used. Examples of biological products separated by centrifugal
methods are: − Insulin is purified from other precipitates of protein materials by centrifugation.
− Blood cells are separated from plasma by centrifugal method. − Bacterial enzymes are
separated from bacterial culture medium by sedimenting the bacterial cells by centrifugation. −
Dirt and water are separated from olive oil and fish-liver oils by centrifugation.
• 3. Evaluation of suspensions and emulsions To evaluate the suspensions and emulsions,
centrifugation tool is used as a rapid evaluation parameter. In case of emulsion, creaming and
phase separation are slow processes. These processes can be accelerated by using a centrifuge. A
stable emulsion does not show any sign of creaming or phase separation even after centrifuging
at 2000-3000 rpm at room temperature.

8. • Molecular weight determination The analytical ultracentrifuge is used to determine
the molecular weights of solutes in the native state and as they exist in solution. This
method is applicable to molecules with molecular weights ranging from several
hundred up to many millions. Examples are sucrose, proteins, nucleic acids,
carbohydrates, virus particles and organelles. This method is applicable to any
substance whose absorbance (or refractive index) differs from that of the solvent.
• Biopharmaceutical analysis of drugs Drugs present in biological fluids such as
blood, tissue fluid, urine, CSF, etc. are normally present in the form of colloidal
dispersion. Centrifugation is used to separate the drugs from biofluids and then the
drugs are analysed by a suitable method. The centrifugation method is an essential
tool for the evaluation of pharmacokinetic parameters and bioequivalence studies

9. EQUIPMENTS
.

10. Perforated Basket (Bowl) CentrifugE
The following types of perforated basket centrifuge are available:
1. Batch type: On the basis of installed motor, they are: (a) Top –
driven (b) Under – driven
2. Semi continuous
3. Continuous

11. Batch Type
In this centrifuge, a basket is suspended from a shaft. This arrangement is described as top-
driven. Conversely, if the basket is mounted on the shaft, it is described as under-driven. These
are used for batch processes.
Principle:
 It is a filtration type centrifuge.
 The separation of solids and liquids is through a perforation wall based on the difference in
densities of the two phases.
 The basket (bowl) contains a perforated side wall.
 During centrifugation the liquid passes through the perforated side wall, while solid is
retained in the basket.
 The deposited solid is cut from the side of the basket and collected.

12. Top-Driven Centrifuge
CONSTRUCTION
• It consists of a rotating basket made of steel
or copper or monel or any other suitable
material that is resistant to corrosion.
• The basket is suspended on a vertical shaft
and is driven by a motor from top.
• The sides of the basket are perforated and
are also covered with a screen on the inside.
• A stationary casing surrounds the basket.
• This basket collects the filtrate and
discharges it at the outlet.

13. Under-driven Centrifuge
CONSTRUCTION
• It consists of a rotating basket that is
mounted on a vertical shaft and
driven by a motor from bottom.
• Rest of the construction is same as
that of the top-driven batch
centrifuge.

14. Working
Working of top-driven centrifuge and under-driven centrifuge is near about same .
• These machines are batch-type machines.
• The material (suspension) is put into the basket.
• The amount of material should be optimum to avoid excess of strain on the basket.
• The power is applied.
• The basket accelerates to its maximum speed in short time.
• During centrifugation, the particles and liquid are thrown by centrifugal force to the wall of the
basket.
• The liquid passes out through the screen and the solid particles retained on the screen as deposit.
• The liquid is collected at the outlet.
• After a definite time the power is turned off.
• By applying a brake the centrifuge is brought to rest.
• The discharge valve at the bottom of the basket is raised, and the deposited solid is cut from the side
of the basket into the opening

15. Uses
• Crystal can be separated from mother-liquor. For example
separation of aspirin from mother liquor.
• It is used to remove unwanted solids from liquids. Example,
precipitated proteins are removed from insulin.

16. Advantages
1. It is able to handle slurries with a high
proportion of solids and even those having paste
like consistency.
2. The final product obtained is very low in
moisture content.
3. The centrifuge is very compact and it requires
very little floor space..
4. The process is rapid..
Disadvantages
1. It is a batch process.
2. The operation is complicated and it involves
considerable labour costs.
3. If the material put into the basket is not
optimum, there are chances of breaking of
basket.
4. Prolonged operation results in considerable
wear and tear of the equipment.
5. On prolonged operation, the cake may
become too difficult to remove

17. Non-perforated Basket Centrifuge
Principle:
 The non-perforated centrifuge does not contain perforations or
screen to separate solids and liquids.
 It is a sedimentation centrifuge.

18. Construction
 This is a under-driven centrifuge
machine.
 It consists of a basket which is mounted
on a vertical shaft and is driven by a
motor using a suitable power system.
 The material of basket is stainless steel
or other suitable material that is resistant
to corrosion.
 liquid is removed by a skimming tube.

19. Working
 The basket is filled with the suspension continuously.
 During centrifugation, the solid is retained on the sides of the basket and the
liquid remains at the top.
 The liquid is removed by a skimming tube.
 The operation is stopped when the suitable depth of solids is deposited on the
walls of the basket.
 The solid is then removed by using a scraper blade.

20. Uses
• This is useful for the separation of solids and liquids from a
suspension containing high proportion of solids i.e. the case where
solids offer high resistance to the flow of liquid.

21. Semi Continuous Centrifuge
Principle
 It is a short cycle automatic, filtration type batch centrifuge.
 The separation of solids and liquids is through a perforation wall based on
the difference in densities of the two phases.

22. Construction
• It consists of a vertical perforated basket.
• The basket is supported from a horizontal shaft
driven by a motor.
• Feed pipe and wash pipe are introduced at the
center of the open side of the basket through
horizontal tubes.
• The thickness of the feed is controlled by a feeler
which rides over the feed, connected to
diaphragm valve through air supply.
• Hydraulic cylinder is attached in such a way
that the discharge chute enters from the sides of
basket, when discharge of crystals is required

23. Working
• The basket is rotated horizontally by a motor.
• The slurry is introduced through the slurry pipe.
• During centrifugation, the solid particles and liquid are thrown by centrifugal force to the
wall of the basket.
• The liquid passes out of the basket and collected at the outlet.
• The solid particles are retained in the basket where they form a cake.
• The cake is washed with water and the wash escapes from the basket through the filtrate
outlet.
• After the slurry reaches the desired thickness, the feeler cuts off the air supply to a
diaphragm valve that automatically blocks the entry of slurry.
• The hydraulic cylinder is actuated and it lifts the knife along with the discharge chute.
• The knife cut the cake to a level but not completely.
• The remaining level of the cake acts as a filter medium for further separation in the next
cycle. Diaphragm valve controls all steps through a timer and air supply mechanism.
• So the entire cycle is semiautomatic.

24. • Use: 1. Crystals can be separated from mother-liquor.
2. Liquids can be clarified by removing unwanted solids dirt from oils.
• Advantages: 1. As the process is semiautomatic, it saves time and energy.
2. It is used when solids can be drained fast from the basket.
• Disadvantages: 1. This centrifuge involves many moving parts which makes the
construction and working complicated.
2. During discharge, considerable amount of crystals may break.

25. Super Centrifuge
Principle
 It is a solid bowl type continuous, sedimentation centrifuge.
 It is used for separating two immiscible liquid phases.
 The separation is based on the density difference between the two
immiscible liquids.
 During centrifugation, the heavier liquid is thrown against the wall of the
bowl while the lighter liquid remains as an inner layer.
 The two layers are simultaneously separated.

26. Construction
• It consists of a long, hollow, cylindrical bowl of
small diameter.
• The bowl is suspended from a flexible spindle at
the top and the bottom is fitted loosely in a bush.
• It is rotated on its vertical axis.
• Feed inlet is provided at the bottom with a
pressure system.
• Two liquid outlets are provided at different
heights at the top of the bowl.
• Inside the bowl there are three baffles to catch the
liquid and force it to travel at the same speed of
rotation as the bowl wall

27. Working
• The centrifuge is allowed to rotate on its vertical axis at about 2000 rpm with the help
of drive assembly.
• The feed is introduced at the bottom through a nozzle under pressure.
• During centrifugation, two liquid phases separate based on the density difference
between them.
• The heavier liquid moves towards boundary and the lighter liquid forms an inner
layer.
• Both liquid climbs to the top of the vertical bowl.
• These two layers are separately removed at the same time from different heights
through modified outlets.
• The super centrifuge works continuously.

28. • Uses: It is used to separate liquid phases of emulsions in foods and
pharmaceuticals.