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Renin Angiotensin Mechanism.

Filtration: Introduction.

 

Definition

  • Filtration may be defined as the separation of a solid from a fluid by means of a porous medium that retains the solid but allows the fluid to pass.

Mechanism of filtration

  • Slurry: The suspension of solid and liquid to be filtered.

  • Filter medium: The porous medium used to retain the solids.

  • Filter cake: The accumulation of solids on the filter medium.

  • Filtrate: The clear liquid passing through the filter and collected in the receptor.


  • In the early stages of liquid filtration particles are retained on the fibers of filter medium by the following mechanisms:

  • (i) Straining, (ii) Impingement, (iii) Entanglement and 
    (iv) Attractive forces

  • After a preliminary layer of particles are deposited on the filter-medium, the filtration occurs through the filter cake. This time filtration obeys Kozeney’s equation.

  • Straining: The particles larger than the pore size of filter medium will be retained on the latter.


  • Impingement: When a dilute suspension approaches a fiber the fluid passes along the side of the fiber will pass with the fluid but the particles in between A – B region will hit directly on the fiber.

  • Entanglement: If the filter medium consists of a cloth or is a porous felt, then particles become entangled in the mass of fibers. Usually the particles are smaller than the pores.

  • Attractive forces: In some cases, particles may collect on a filter medium as a result of attractive forces. Gas flowing through a filter medium causes generation of charges on the filter surface. The particles containing charge get attracted to the surface.

FACTORS AFFECTING THE RATE OF FILTRATION.

  1. Properties of the filter medium and filter cake:

  • The resistance of the filter medium and filter cake is denoted by R. 

  • The rate of filtration decreases as the thickness of the cake increases. 

  • The resistance also depends on the properties of the solids, e.g. particle size, particle size distribution, particle shape, and compressibility of the solid. 

  • In case of compressible cake the porosity decreases with increasing pressure drop, so filter aids are incorporated to increase the filtration rate.

  1. Area of filter

  • The rate of filtration can be increased by increasing the area of filtration. 

  • This area can be increased by using larger filters or by using a number of small units in combination. 

  • In rotary filters, the filter cake is continuously removed, providing an infinite area of filtration.

  1. Pressure drop

  • Rate of filtration can be increased by increasing the pressure drop across the filter medium. 

  • Pressure drop can be achieved by (i) gravity, (ii) negative pressure (reduced pressure or under vacuum), (iii) positive pressure and (iv) centrifugal force.

  1. Viscosity of liquid

  • An increase in the viscosity of the liquid will decrease the flow rate. 

  • The viscosity of the liquid can be decreased by raising the temperature of the slurry or by dilution with a miscible liquid.

  1. Thickness of the filter cake

  • Thickness of the filter cake increases as the filtration progresses.  

  • Which decreases the rate of filtration.
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