Cross-flow filtration

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Crossflow filtration is a type of filtration used when a liquid carries a high level of solid material.

Used in Membrane Filtration

[edit] Concept

Crossflow filtration is different from dead end filtration in which the feed is passed through a membrane or bed, the solids being trapped in the filter and the filtrate being released at the other end.

[edit] Operation

In crossflow filtration, the feed is passed across the filter membrane (tangentially to the filter membrane) at some pressure difference. Material which is smaller than the membrane pore size passes through the membrane as permeate or filtrate, and everything else is retained on the feed side of the membrane as retentate.

This mode of operation is used for high solids feeds because of the risk of blinding. With dead end filtration, solids material can quickly block (blind) the filter surface.

With crossflow filtration the tangential motion of the bulk of the fluid across the membrane causes trapped particles on the filter surface to be rubbed off. This means that a crossflow filter can operate continuously at relatively high solids loads without blinding.

Cross flow membrane filtration technology has been used widely in industry globally. It includes Reverse Osmosis, Nanofiltration, Ultrafiltration and Microfiltration. It provides a very cost effective alternative than the traditional evaporation method.

[edit] Flow Rate

The flux or flow rate in cross flow system is given by the equation:

  j=   ε.r².dp 
       g.η.dx   

j- liquid flux ε- membrane porosity r- radius or the pore size dp- transmembrane pressure η- liquid viscosity dx- pore length