Palm Oil Engineering Bulletin No.79 (Apr - June 2006) p13-19
Clarification tank simulation

The clarifier is an industrial plant in which the concentration of a suspension is increased purely by sedimentation with the formation of a clear liquid. In palm oil mills, the clarification tanks serve the purpose of separating from the sludge the free oil and the coarse oil droplets, which rise to the surface fairly quickly. The most important task in designing a sedimentation process is recognizing the form of settling taking place in the suspension under examination. Dilute sedimentation is unpractical in the mills due to the large settling area needed. When the concentration of the particles become so high or the settling area is inadequate such that the particles no longer settle as individuals, the settling behaviour changes with increasing concentration, rapidly going through a transition stage in which clusters of particles develop and settle as clouds leading to hindered settling. Shearing force has to be introduced into the crude oil to reduce the apparent viscosity with minimum turbulence. Consolidation and compression occurs to some extent as solids reach the base of the vessel and the layers of particles build up, with the underlying ones constrained to pack more closely together thus achieving a higher concentration. No general solution to describe this phenomenon is available presently due to difficulties encountered in the process model development. The presence of an inclined surface in a sedimentation system has the effect of increasing the settling rate of particulates. Thus, the bottom of the clarification tank should be conical in shape. Tank height will increase with increased retention time, while the settling surface area needed is proportional to the feed rate. The length of the feeding pipe is determined based on the velocity of the rise of 30 µm oil droplets. Generally, the clarification of 1:1 diluted crude palm oil at 85°C — 90°C with a stirring rate of 5 rpm 3 hr of retention time.

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