Friday, February 25, 2011

Centrifugation

It refers to the process of sedimentation by using centrifuge machine.

Basic idea behind centrifugation:
Centrifugation is based on the widely known idea of sedimentation by the use of centrifugal force, which represents a force that apparently moves a spinning or rotating object away from the axis of rotation in a curved path.

Centrifugal effect:
The processes using centrifugal force (F) can be described by the equations involving the gravitational constant (G). In this case, it is easy to determine the centrifugal force in the terms of the ratio of the centrifugal force to the gravitational force. In addition, this ratio represents the centrifugal effect (C).

Centrifugal effect (C) shows that how many times the centrifugal force is larger than gravitational force.

C = 2.013 dn^2

Where
d = diameter of rotation
n = speed of rotation

Here in this equation, “n” has the value in “s-1” and “d” has the value in “m”.


This equation shows that centrifugal effect is directly proportional to the diameter and to the square of the speed of the rotation i.e. greater will be the diameter of the tube or container more will be centrifugation and similarly for the speed of rotation.

Factors affecting centrifugation:

Centrifugation is basically affected by centrifugal effect. Moreover, nature of the liquid medium in which the particles are placed also affects the centrifugation.

Apparatus for centrifugation (Centrifuges):
Container is the most important part of centrifugation apparatus i.e. centrifuges. This container is used for the placement of a mixture or solution of solid and liquid or of a solution of two liquids.

This container is then rotated at greater speed resulting in the separation of the ingredients of the mixture takes place by the use of centrifugal force.

Mechanism for the separation in the apparatus of centrifugation:
A mixture of liquid or solid in a liquid of low density can be separated as the material of larger density is thrown in the outward direction to the bottom of the tube or container with a larger force. This results in the separation of pure, low-density liquid as a transparent or purified supernatant liquid which forms upper layer.

Types of centrifuges:

There are two basic types of centrifuges:

1. Sedimentation

2. Filtration

Sedimentation centrifuges:

The basic principle, in the sedimentation type of centrifuges, is difference in the densities of the ingredients of the mixture. In these types of centrifuges, the particles are settled to the wall by the action of the centrifugal force.

These types of centrifuges are used for the separation of ingredients of the mixture of solid in liquid as well as liquid in liquid.

Two types of centrifuges are based on the principle of sedimentation:

1. Bottle centrifuge

2. Disc type centrifuge

Bottle centrifuge:
It is mostly used centrifuge machine in the laboratories. It consists of a vertical rotating rod that causes the “bottle-type” containers or test tubes, which are fixed symmetrically, to be rotated in a horizontal plane resulting in the separation of the materials of varying densities. The vertical rod is rotated usually by means of electric motor. Sometimes, gas turbines can also be used for the rotation.

Disk type centrifuge:
It consists of vertical pile of thin conical disks, which are arranged in the manner of one on the top of another. The sedimentation of the particles takes place in the space between neighbouring cones. In this way, settling distance is greatly reduced, thereby increasing the rate at which the particles in the material are separated. The cones are adjusted in such a manner that heavier material moves down the surface easily upon reaching the inner surface of the cone.
Filtration centrifuges:
These types of centrifuges are used for the separation of the mixture of solid in liquid only performing the operation similar to the filtration process. These are also sometimes referred to as clarifiers.

It is same in the basic operation to the sedimentation types of centrifuges but instead of solid containers or tubes, it contains a porous wall or perforated containers or baskets, which causes the liquid phase to pass through it but keeps the solid phase on it.

Centrifuge based on the principle of filtration is “Basket centrifuge”.

Basket centrifuge:
Basket centrifuge consists of a porous wall and rotor which is cylindrical and tubular in structure. The porous wall is some times replaced by one or more of the fine mesh screens. The fluid go through the screen where as the particles larger in size are left on the screen.
Application of centrifugation:

Centrifugation is used for the separation of ingredients of a mixture of solid in liquid or liquid in liquid as the degree of separation achieved by centrifugation is of greater amplitude than the action due to gravity.

It is important specifically when the separation by normal filtration methods is difficult such as in the separation of fluids of highly viscous nature.

In the pharmaceutical research, it is considered as an important tool in determining the stability of emulsions.

Bottle centrifuge can be used for:

1. Finding the sediments present in crude vegetable and mineral oils
2. Determination of the butterfat content in the milk
3. Various clinical trials and tests

Disk type centrifuge can be used for refining of vegetable oils by removing soap stock

Basket centrifuge can be used for:

1. Drying and washing of several different kinds of crystals and fibrous materials
2. The preparation of cane sugar.
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