Factors Influencing Filtration:
1. Area of filter surface 2. Permeability coefficient (k) 3. Pressure drop 4. Viscosity of Filtrate 5. Thickness of the filter cake
1. Area of filter surface:
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- According to Darcy’s equation, the rate of filtration is directly proportional to the surface area of the filter medium while according to the Kozeny-Carman equation rate of flow of filtrate is inversely proportional to the specific surface of the filter bed. Therefore the rate of flow can be enhanced by using large filters. For example in a rotary drum filter, the continuous removal of filter cake provides an infinite area of filtration.
2. Permeability coefficient (k):
- As the thickness of the cake increase, the filtration will decrease. The permeability coefficient depends on the characteristic of the cake such as porosity, compressibility, and specific surface area.
3. Pressure drop:
- The rate of filtration is proportional to the pressure difference across both the filter medium and filter cake. The pressure drop can be achieved by
- Maintaining ahead of slurry above the filter medium. The pressure developed will depend on the density of the slurry.
- The pressure below the filter medium can be reduced below atmospheric pressure by connecting the filter receiver to a vacuum pump and developing a pressure difference across the filter.
- Pumping the slurry into the filter under pressure.
4. Viscosity of Filtrate:
- An increase in the viscosity of the filtrate will increase the flow resistance. The filtration rate is inversely proportional to the viscosity of the fluid. This problem can be overcome by two methods:
- The filtration rate can be increased by raising the temperature of the slurry. However, it is not practical for thermolabile materials and also for volatile filtrate.
- Dilution is another approach, but the rate must be doubled.
5. Thickness of the filter cake:
- The rate of flow of the filtrate through the filter cake is inversely proportional to the thickness of the cake. The preliminary decanting is used to reduce the number of solids.
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