Separation by coalescence

Filtration with coalescence by upflow filters can be contemplated when there is no interference by an overly-high SS concentration and when the HC present do not have too high a viscosity (hot water) or too high a pour point (paraffin risk). The required conditions are fulfilled by many types of produced water and all heating condensates, but less often by oily or accidentally oily refinery water. 

The coalescer (Fig. 32) has a lower percolation zone with sand that has been made oleophilic, and an upper zone for oil droplet separation by differential settling. 

In the lower zone, the aim is to promote coalescence of precoagulated and destabilized oils on an oU film coating the grains and becoming gradually thicker toward the top of the medium. At the top of the medium, the joint forces of gravity and entrainment cause shear and discontinuous breakaway of the film which frees large droplets (0.5 to 5 mm). 

The droplets move at a rising velocity of 50 to 100 m h and can be readily separated from the treated fluid which circulates cocurrently at a velocity of 20 to 25 m h k In this way separation is complete in the upper zone. 

This process is mainly designed to separate oil and not SS or floes. 

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After the motion of the drops, their separation by coalescence follows. During this process, the mass transfer is negligibly small compared with that where the drops are in motion, if the continuous liquid is not fed directly into the coalescence region. However, such a technique should be avoided so as not to disturb the settling process. 

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