Filtration & Centrifugation

Topics: Filtration, Viscosity, Liquid Pages: 17 (5217 words) Published: January 2, 2012
“Filtration may be defined as a process of separation of solids from a fluid by passing the same through a porous medium that retains the solids, but allows the fluid to pass through.” The suspension to be filtered is known as slurry. The porous medium used to retain the solids is known as filter medium. The accumulated solids on the filter are referred to as filter cake, while the clear liquid passing through the filter is filtrate. When solids are present in a very low concentration i.e., not exceeding 1.0% w/v, the process of its separation from liquid is called ‘clarification’. Process of filtration:

The filtration operation is shown below in the figure
* The pores of the filter medium are smaller than the size of the particles to be separated. * Filter medium (for eg: filter paper or muslin cloth) is placed on a support (a sieve). * When slurry (feed) is passed over the filter medium, the fluid flows through the filter medium by virtue of a pressure differential across the filter. * Gravity is acting on the liquid column. Therefore, solids are trapped on the surface of the filter medium Figure 1: filtration

* Once the preliminary layer of particles is deposited, further filtration is brought about wherein the filter medium serves only as a support. * The filter will work efficiently only after an initial deposit. * After a particular point of time, the resistance offered by the filter cake is high that virtually filtration is stopped. For this reason, a positive pressure is applied on the filter cake (upstream) or negative pressure (suction) is applied below the filter medium (downstream). Factors affecting the rate of filtration:

The rate of filtration which depends on various factors can be written as: Rate of filtration = Area of filter X Pressure difference
Viscosity X Resistance of cake and filter The rate of filtration depends on the following factors:
1. Pressure:
* The rate of filtration of liquid is directly proportional to the pressure difference between the ‘filter medium’ and ‘filter cake’. * Thus, the rate of filtration can be increased by applying pressure on the liquid being filtered or by decreasing the pressure beneath the filter. 2. Viscosity:

* The rate of filtration is inversely proportional to the viscosity of the liquid undergoing filtration. * Liquids which are very viscous get filtered slowly in comparison to liquids with low viscosity. * Reduction of viscosity of a liquid by raising the temperature is frequently done in order to accelerate syrups are more quickly filtered when hot and cold. 3. Surface area of filter media:

* The rate of filtration is directly proportional to the surface area of filter media. * Pleating the filter paper or using a fluted funnel increases the effective surface area of filter paper for filtration. Filter press also works on the same principle. 4. Temperature of liquid to be filtered:

* Temperature plays an important role in the rate of filtration. * Viscosity is reduced by a rise in temperature and the filtration of viscous oils, syrups etc is often accelerated by filtering them while they are still hot. 5. Particle size:

* The rate of filtration is directly proportional to the particle size of the solid to be removed. * It is easier to filter a liquid having coarse particles than that having finely divided particles because coarse filtering medium can be used to filter liquid having coarse and hence it increases the rate of filtration. * Therefore before filtration, some method should be adopted to agglomerate the finely divided particles into coarse particles or to increase the particle size by precipitation. 6. Pore size of filter media:

* The rate of filtration is directly proportional to the pore size of the filter media. * The liquid having coarse particles requires a coarse filtering media to remove them....
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