Lamella settlers

Thus the eross-seetional area of the settler is most important, while the separation distanee should be kept small. This has led to the introduetion of designs having numerous baffles staeked within the settler as in the Lamella settler. 

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. 

Knaack et al. [77] developed an ingenious reactor design that incorporates a conical lamella settler within a conical reactor vessel. Unfortunately, this new design presents a serious scale-up limitation, since the maximum perfusion rate attainable decreases hyperbolically with the reactor working volume. For... 

Thompson and Wilson [78] operated a 21-L air-lift perfusion reactor coupled to an external lamella settler. The sedimentation device bearing an angle of inclination of 30° to the vertical was maintained at 37°C. They found a reasonable agreement between the theoretical and experimental values of breakthrough for viable and nonviable cells in the harvest stream. As expected (see Eq. 8), the maximum perfusion rates increased with an increasing settling area. 

The system used by Stevens et al. [79] consisted of an air-lift reactor with an external lamella settler. They did not need to pump the cell suspension through the settler, since free flow convection was achieved by cooling the cell suspension (20 °C) before entering the sedimentation device. As also found by other authors, they could achieve selective retention of viable cells by varying the perfusion rate. 

A process for phase separation based on density differences. A commercial lamella settler for suspensions or emulsions comprises a stack of parallel plates spaced apart from each other and inclined from the horizontal. The space between each set of plates forms a separate settling zone. The feed is pumped into these spaces, at a point near the longitudinal middle of the plates. The less dense phases rise to the underside of the upper plates... 

Settlers usually employed in cell separations (A) vertical settler (B) lamella settler (adapted from Henzler etal., 2003). 

Lamella Settler Up to 3.0 Up to 7.5 Inclined tubes or parallel plates to increase sedimentation Various... 

It is of interest that in the continuous operation of inclined settlers, termed lamella settlers, where the sediment is withdrawn continuously from the bottom of the channel and clarified product continuously from the top of the channel, that two modes of operation have been shown to exist for the same rate of clarified flow (Probstein et al. 1981, Leung Probstein 1983). In one mode the feed suspension layer expands down the channel subcritical mode), as in Fig. 5.4.8, whereas in the other mode supercritical mode) the layer contracts. The appearance of one mode or the other depends on the geometry and the manner in which the suspension is fed into the settler and the clarified liquid withdrawn. We would suggest that under appropriate conditions there exist two types of steady solutions for all continuous sedimentation processes. 

PROBSTEIN, R.F., YUNG, D. HICKS, R.E. 1981. A model for lamella settlers. In Theory, Practice and Process Principles for Physical Separations (ed. M.P. Freeman C J.A. FitzPatrick), pp. 53-92. New York Engineering Foundation. 

In both centrifuge and lamella settler operations highly emulsified samples which impair the separation processes have frequently been... 

Figure 7.3.20. Inclined lamella settler with continuous flow for particle classification at steady state.

When a suspension is introduced into the inclined lamella settler, the feed suspension may be characterized by means of its solids volume fraction and its particle size density function fjO p)- The corresponding quantities for the overflow and underflow streams are ji,/i(rp) (rp). Often such problems are analyzed instead using the solids volume fraction and the particle settling (terminal) velocity density function /[t/pzt), where the particle settling velocity Upzt in the Stokes law range is related to the particle radius tp by relation (6.3.1) ... 

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