What is the principle of Filtration?

Filtration is defined as a physical process that removes the particulates from a liquid or gas through a porous filtering media. The main purpose of filtration is to improve the purity of the filtered material. The filtration are of three types i.e. Reverse Osmosis deals with the particles of size 0.0003 to 0.03 microns, Ultra filtration of 0.003 to 1 microns and the Conventional filtration deals with the particles of size more than 1 micron.

Filtaration

Conventionally filtration has been divided into two areas wet filtration (solid-liquid separation) and dry filtration (solid-gas separation) both having the same objectives i.e.

•         To eliminate the contaminant particles so as to recover dispersing fluid.
•          To recover solid particles eliminating the dispersing fluid.

The obvious difference between the two areas is the enormous difference in density between water and air and in fact that in one case the filtered solids are sludge and in the other case a dry powder.

Wet filtration is again subdivided into vacuum and pressure filtration depending upon the means employed to force the liquid through the porous media. In vacuum filter the solids are separated at atmospheric pressure and in the case of pressure filters, the solids that are removed from the liquid are in a region considerably above atmospheric pressure.

Principle of Filtration

Filtration is essentially a mechanical operation and is less demanding in energy than evaporation or drying where the high latent heat of the liquid, which is usually water, has to be provided. The filtration operation is illustrated in fig.1, which shows the filter medium, in this case a cloth, its support and the layer of solids or filter cake, which is already formed by continuous accumulation on the surface of cloth during filtration. 

Filtration Process

In the typical operation shown in the above figure, the cake gradually builds up on the medium and the resistance to flow progressively increases. During the initial period of flow, particles are deposited in the surface layers of the cloth to form a true filtering medium i.e. filter cake.

The most important factors on which the rate of filtration depends will be:

•          The drop in pressure from the feed to the far side of the filter medium.
•          The area of the filtering surface.
•          The viscosity of the filtrate.
•          The resistance of the filter cake.
•          The resistance of the filter medium and initial layers of cake.

Tapping effect : Where the shape of the pore is different from that of a particles, the latter will be separated even it is smaller than the pore. Hence the particles remain of surface.

Theory of Filtration Mechanism

Filtration takes place at different planes in the fabric. Therefore, there is a high probability that the fibres due to physical bonding forces between particles and fibres capture the particles in the fluid stream.

The combination of mechanism responsible for filtration is :

Sieve effect or direct interception : Where the particles bigger than the pores are separated.

Inertia : Due to when, particle because of it momentum crosses the fluid streamliners and strikes the fibre.

Diffusion effect : This mechanism can be neglected because face velocities are so high with filter cloth that the diffusion effect rarely takes place. Particle size would be below 0.2-0.05 microns.

Electrostatic effect : Where depending on the electrostatic charge, the particles are either attracted or repelled by the filter media.

Gravitational forces : Under the influence of gravity, a particle that is sinking, may collide with the fibre and get caught.

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Textile Points is a textile education blog. Its provide information about Textile Fiber, Yarn,Spinning, Fabric, Technical Fabric, Wet Processing of Textile, Finishing and Technical applications of Textile.