Gravity settling Vertical settlers

For very small particles or low density solids, the terminal velocity may be too low to enable separation by gravity settling in a reasonably sized tank. However, the separation can possibly be carried out in a centrifuge, which operates on the same principle as the gravity settler but employs the (radial) acceleration in a rotating system (o r) in place of the vertical gravitational... 

It follows from (13.55) and (13.56) that A), < Ay if r. is the same for horizontal and vertical settlers. The higher efficiency of emulsion separation in a horizontal separator as compared to a vertical one is explained by of the shape of drop distributions at the settler exit (Fig. 13.17). In a vertical settler, the part of the initial distribution with V > Va- gets cut off at the entrance, since drops of such volume cannot move against gravity. In a horizontal settler, drops of all sizes settle continuously along the whole length, and not only drops with V > Va-, but also a part of drops with V < Va get settled out, which does not happen in a vertical settler. 

Gravity Settlers Decanters These are tanks in which a liqmd-liquid dispersion is continuously settled and coalesced and from wriich the settled liquids are continuously withdrawn. They can be either horizontal or vertical. Figure 15-24 shows some typical horizontal decanters. For an uninstrumented decanter the height of the heavy-phase-liquid leg above the interface is balanced against the height of the hght-hquid phase above the interface, Eq. 15-50. 

Solid particles can be removed from a dilute suspension by passing the suspension through a vessel that is large enough that the vertical component of the fluid velocity is lower than the terminal velocity of the particles and the residence time is sufficiently long to allow the particles to settle out. A typical gravity settler is illustrated in Fig. 12-2. If the upward velocity of the liquid (Q/A) is less than the terminal velocity of the particles (Ft), the particles will settle to the bottom otherwise, they will be carried out with the overflow. If Stokes flow is applicable (i.e., NRe < 1), the diameter of the smallest particle that will settle out is... 

Lamella settlers are gravity settlers that use a large number of inclined flat plates, closely packed, so that the distance between the plates is small. The gravitational action makes the cells move in the direction of the lower surface of the individual separating space between two plates. Once settled, the particles slide down in a layer towards the plate periphery and then into the sludge hopper. For the same required sedimentation area, lamella settlers are much more compact than vertical ones. Theoretically, their total sedimentation area is the sum of the horizontally projected areas of all plates. In practice, however, only 50% of this total area is effective [137]. The main problem with inclined settlers is that the cells tend to adhere to the plates. Special coating of the plate surface and vibration of the whole lamella pack may alleviate this problem. 

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