Laboratory continuous settling

Laboratory batch settling data are representative of the conditions inside a continuously operating thickener. 

Settle, F.A., Laboratory automation and LIMS The continuing drama of Pit-... 

Its laboratory method of ptepn, given in Ref 8, p 192 and in PATR 1448, p 9, consists of two steps prepn of Pb hydroxide and of LDNR, Basic. To a soln of 18.96g of pb acetate in 67 ml of warm distd w add gradually, with stirring, 4.0 g of NaOH dissolved in 67 ml of w and continue stirring for 5 mins. After allowing to settle, wash the white ppt of Pb hydroxide, by decantation three times with 100ml of distd w, and use immediately for the next operation... 

Most of the available information on settling concerns small laboratory units. In practice, settling basins and thickeners are of large size and are kept in continuous operation. Comings (1940) presented an excellent practical discussion of these and other factors affecting their operation. However, it remained for Kammermeyer (1941) to evaluate the importance of these factors quantitatively. 

Semicontinuous Dench-scale tests Laboratory pumps are used which pump feed slurry and chemicals into settling cylinders from which overflow liquor and underflow slurry are continuously collected. 

Continuous crystallizers frequently employ draft tubes to enforce mixing up and down the vertical axis. Even with draft tubes, however, the solids will not be uniformly distributed in general. Figure 8.3 shows solids distribution in a laboratory model of a draft tube crystallizer (Green and Robertson 1993). Notice that there are variations with vertical position inside the crystallizer. In addition, the average concentration inside the draft tube is different from that outside because the net particle velocity and therefore particle hold-up is different where the particle settling velocity is opposite the net flow, i.e., in up flow than where they settle in the net direction of the flow, i.e., down flow. This is discussed in more detail in Mullin (1994), Jones and Mullin (1973), and the experimental modeling section below. 

Likewise, in continuous polymerizations, the end-use and polymer properties variables may be measured very infrequently, and most often, off-line. In addition, the laboratory analysis time adds a large delay to the measurement Under these circumstances, the measurements are substantially uncorrelated in time, and again, statistical process control may be an appropriate approach to control of polymer quality or end-use properties. If the samples are sufficiently infrequent that the process settles between samples, conventional SPC can be applied. This amounts to manual steady-state ccxitrol with the need for control identified by statistical techniques. If the system does not "settle between samples, more specialized techniques of analysis are available [63]. Good reviews of the applicability of SPC to chemical processes are given by MacGregor [64,65]. 

A considerable saving in the number of operations is clearly made by mechanization of mixing, settling and separating. This, however, is only done in the case of certain specialized laboratory applications. Instead, it is more usual to allow the two phases to flow continuously rather than in discrete portions, and a large variety of plant and equipment is now available for carrying out these processes of continuous counter-current solvent extraction on an industrial scale. 

---