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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
(ii) Semicontinuous centrifuge
(iii) Continuous horizontal centrifuge
(iv) Vertical solid bowl super centrifuge