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Centrifugation |
Centrifugation is a process that involves the use of the centrifugal force for the separation of mixtures, used in industry and in laboratory settings. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components of the mixture migrate towards the axis. Chemists and biologists may increase the effective gravitational force on a test tube so as to more rapidly and completely cause the precipitate ("pellet") to gather on the bottom of the tube. The remaining solution is properly called the "supernate" or "supernatant liquid". The supernatant liquid is then either quickly decanted from the tube without disturbing the precipitate, or withdrawn with a Pasteur pipette.
The rate of centrifugation is specified by the acceleration applied to the sample, typically measured in revolutions per minute (RPM) or g. The particles' settling velocity in centrifugation is a function of their size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particle and the liquid, and the viscosity.
A simple example of a centrifuge is a household washing machine which separates liquids (water) from solids (fabric/clothing) during the spin cycle.
In the chemical and food industries, special centrifuges can process a continuous stream of particle-laden liquid.
It is worth noting that centrifugation is the most common method used for uranium enrichment, relying on the slight mass difference between atoms of U238 and U235 in uranium hexafluoride gas.
Contents |
Sigma Analysis is a useful tool determining centrifuge properties. It is similar to the equation that relation volumetric flow rate Q, fluid velocity u, and flow path cross-sectional Area A:
In the case of sigma analysis, u is replaced by vg,the settling velocity at centripetal acceleration of g (9.81 m/s2), Σ replaces area, and is a property of the type of centrifuge, and Q is the input fluid flow rate. Σ has the same units as area.
Harrison, Roger G., Todd, Paul, Rudge, Scott R., Petrides D.P. Bioseparations Science and Engineering. Oxford University Press, 2003.
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