Centrifugation
Syllabus:
Principles of centrifugation, industrial centrifugal filters
and centrifugal sedimenters.
APPLICATIONS OF CENTRIFUGATION IN PHARMACEUTICAL INDUSTRIES
1. Production of bulk drugs
After crystallization the drugs are separated from the
mother liquor by centrifugation. For example traces of mother liquor is
separated from aspirin crystals by centrifugation method.
2. Production of biological products
Most of the biological products are either proteinaceous
or macromolecules. During manufacturing they remain in colloidal dispersion in
water. By normal methods of filtration it is difficult to separate the colloid
particles. In those cases centrifugal methods are used. Insulin is purified
from other precipitates of protein materials by centrifugation. Blood cells are
separated from plasma by centrifugal method. Bacterial enzymes are separate
from bacterial culture medium by sedimenting the bacterial cells by
centrifugation. Dirt and water are separated from olive oil and fish-liver
oils.
3. Evaluation of suspensions and emulsions
One of the problems of suspensions is sedimentation and
one of the problems of emulsions is creaming. Immediately after the preparation
of a suspension or emulsion this problems may not appear quickly. To enhance
the rate of sedimentation and creaming the suspension or emulsion is introduced
in a centrifuge and rotate at an rpm of 200 to 3000. If still the problems do
not appear then the suspension or emulsion can be taken as stable formulation.
4. Determination of molecular weight of colloids
Polymers, proteins and such macromolecules often forms
colloidal dispersions. The molecular weights of those molecule can be
determined by ultracentrifugation. The larger molecules will be arranged at
periphery and the lighter molecules near the center.
PRINCIPLES OF CENTRIFUGATION
Particles having a size above 5mm sediment at the bottom
due to gravitation force. Such a suspension can be separated by simple
filtration techniques. If the size of particles are less than 5mm
they undergo Brownian motion. In such suspension a stronger centrifugal force
is applied to separate the particles.
Principle
of centrifugation
Let us consider a body of mass m rotating in a circular path of radius r at a velocity of v. The force acting on the body in a radial direction is given by:
where F = centrifugal force
m
= mass of body
v
= velocity of the body
r
= radius of circle of rotation
The gravitational force acting upon
the same body G = mg
Where, G = gravitational force
g
= acceleration due to gravity
The centrifugal effect is the ratio
of the centrifugal force and gravitational forces so that
Since, v = 2p r n where n = speed of rotation (r.p.m.)
where, k = = constant
D = maximum diameter of the
centrifuge
D can be measured from the center
of the centrifuge to the free surface of the liquid or to the tip of the
centrifuge tube.
From the equation C = kDn2
it can be concluded that
Centrifugal
effect µ diameter of centrifuge
Centrifugal
effect µ (speed of rotation)2
Applications of this principle
1.
If the particles of suspensions are very small then
high centrifugal effect will be required to separate the particles. To separate
such suspensions the size of the centrifuge is kept smaller but it is rotated
at very high speed (rpm).
2.
If a large amount of material is to be separated and a
low centrifugal effect is sufficient to separate the suspension then the
diameter (D) of the centrifuge is increased and speed (n) is kept low.
Classification of industrial centrifuges
A. Perforated bowl or filter
types
1.
Batch type
(a)
Top-driven
(b)
Under-driven
2.
Semicontinuous
3.
Continuous
B. Solid-bowl or sedimentation
types
1.
Vertical
(a)
Simple bowl
(b)
Bowl with plates
2.
Horizontal
(a) Continuous
decanters
BATCH TYPE TOP DRIVEN
CENTRIFUGE
Construction
It consists of a rotating basket suspended on a vertical
shaft and driven by a motor from top. The sides of the basket are perforated
and are also covered with a screen on the inside. Surrounding the basket is a
stationary casing that collects the filtrate.
Working
This machine is a batch-type
machine. The material (suspension) is put into the basket. Then power is
applied. The basket accelerates to its maximum speed. 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. After a definite time the power is turned off, a brake applied, and
the basket 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.
Use:
1.
Crystals can be separated from mother-liquor.
2.
Liquids can be clarified by removing unwanted solids
dirt from oils.
BATCH TYPE UNDER DRIVEN
CENTRIFUGE
Construction
It consists of a rotating basket placed on a vertical
shaft and driven by a motor from bottom. The sides of the basket are perforated
and are also covered with a screen on the inside. Surrounding the basket is a
stationary casing that collects the filtrate.
Working
This machine is a batch-type machine. The material
(suspension) is put into the basket. Then power is applied. The basket
accelerates to its maximum speed. 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. After a
definite time the power is turned off, a brake applied, and the basket brought
to rest. The cover at the top of the basket is raised, and the deposited solid
is cut from the side of the basket and collected.
Use:
1.
Crystals can be separated from mother-liquor.
2.
Liquids can be clarified by removing unwanted solids
dirt from oils.
3.
In cloth industries after washing the liquid is
strained and the cloths are taken out from the top cover.
SEMI-CONTINUOUS CENTRIFUGE
Construction
It consists of a rotating basket placed on a horizontal
shaft and driven by a motor from side. The side of the basket is perforated.
Surrounding the basket is a stationary casing that collects the filtrate.
Slurry is introduced through a pipe that enters the basket through the center.
To wash the crystal the wash-pipe is also introduced through the center of the
basket. The layer of cake is removed by a chute fitted with a knife. The knife
cuts down the cake within the basket. The knife-chute assembly is raised with
the help of a hydraulic apparatus.
Working
The basket is rotated horizontally by a motor. The slurry
in introduced through the slurry entry pipe. The liquid passes out through the
perforated side. The crystals remain within the basket. When the cake height is
about 2-3 inches the slurry entry is stopped by a “feeler-diaphragm valve
assembly”. The basket rotates a predetermined time then the cake is washed with
water. The basket is rotated for another predetermined time. After that the
hydraulic apparatus raise the knife-chute assembly to cut the cake. The cake is
collected through the chute.
Use
This is a semi-continuous type of centrifuge.
1.
Crystals can be separated from mother-liquor.
2.
Liquids can be clarified by removing unwanted solids
dirt from oils.
CONTINUOUS HORIZONTAL CENTRIFUGE
Construction:
This is a sedimentation
centrifuge. It consists of a cylindrical or conical bowl mounted horizontally
and rotates at 50 to 60rpm. The bowl has a diameter of 0.5 meters. Within the
bowl a screw-conveyor is placed that rotates in the same direction of
the bowl but at a slightly slower speed than the bowl. Slurry can enter the
bowl through the shaft of the screw-conveyor.
Working
The bowl and the screw-conveyor
both rotate in the same direction by suitable motors. The slurry is introduced
through the shaft of screw conveyor. The liquid moves into the wider portion of
the bowl and the solid is sedimented on the wall of the bowl. The solid cake is
continuously lifted and moved to towards the narrower side of the bowl by the
screw conveyor and is discharged continuously.
Uses
Horizontal continuous centrifuge
is used when the slurry contains high proportion of solids.
Advantages:
1.
It can filter very small particles from slurries even
as small as few microns.
2.
It can be used for slurries with concentrations ranging
from 0.5 to 50%.
VERTICAL SOLID BOWL CENTRIFUGE
(SUPER CENTRIFUGE)
Principle:
It is a solid bowl type
continuous centrifuge used for separating two immiscible liquid phases. It is a
sedimentation type centrifuge. 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.
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 is
introduced through the bottom through a nozzle. Two liquid outlets are provided
at different heights. Inside the bowl there are three baffles (not shown in the
figure) to catch the liquid and force it to travel at the same speed of
rotation as the bowl wall.
Working:
The centrifuge is allowed to rotate on its vertical axis
at about 2000rpm. The feed is introduced at the bottom through a nozzle under
pressure. During centrifugation, two liquid phases separated based on their
densities. The heavier liquid moves towards periphery and the lighter liquid
forms an inner layer. Both liquid climbs to the top of the vertical bowl. These
two layers are simultaneously separately removed from different heights through modified outlets.
Uses:
Supercentrifuge is used for
separating liquid phases of emulsions in foods and pharmaceuticals.
CONICAL DISK CENTRIFUGE (DE LAVAL
CLARIFIER)
Principle:
It is a sedimentation centrifuge.
The separation is based on the difference in the densities between the phases
under the influence of centrifugal force.
Construction:
It consists of a shallow form of
bowl containing a series of conical discs attached to the central shaft at
different heights. The discs are made up of a thin sheet of metal or plastic
separated by narrow spaces. Holes are there on the conical discs half-way in
between the axis and the wall of the bowl so that the holes form channels
through which the liquid passes.
Working:
The centrifuge is
rotated at low speed. The feed is introduced through the concentric tube
surrounding the shaft. The feed flows down and enters into the spaces between
the discs. During centrifugation, the liquid flows into the channels (through
the holes at the center of the discs) and upwards. The heavy liquids moves
towards periphery of the bowl and the lighter liquids towards the center. The
solids, if any, are deposited at the bottom of the conical bowl, which is
removed from time to time. The heavier and lighter liquids are taken out of the
bowl through separate outlets.
Uses:
1.
Two immiscible liquid phases are separated out after a
liquid / liquid extraction.
2.
Suspensions containing low concentration of solids can
be separated quickly.
3.
Gelatinous solids can be separated by disc-cone
centrifuge. Such materials are difficult to filter because they clog the filter
medium.
4.
In the manufacture of insulin, the liquor is clarified
to remove the precipitated proteins.
5.
Separation of cream from milk, concentration of rubber
latex, removing solids from lubricating oils, inks and beverages are possible.
Advantages:
1.
They are compact and occupy very small space.
2.
It has very high separating efficiency.
Disadvantages:
1.
Construction is complicated and difficult to wash also.
2.
Its capacity is limited.
3.
It is not suitable for cake forming solids.
What is the principle of
centrifugation?
Draw a neat sketch of the
following centrifuges and write the working principle of the following
centrifuges:
(i) Batch type top-driven centrifuge
(i) Batch type top-driven centrifuge
(ii) Semicontinuous centrifuge
(iii) Continuous horizontal
centrifuge
(iv) Vertical solid bowl super
centrifuge