Crystallizers fines-destruction

If the solubility of the product in the solvent increases with temperature (normal solubility), heating the solution containing fines or the slurry will dissolve crystals. When heating, the supersaturation is relieved first, followed by dissolving the finer crystals. Fines destruction can also be accomplished by adding a solvent or unsaturated solution or steam. The temperature of the slurry must not be allowed to drop in these cases. 

Draft-Tuhe (DT) Crystallizer This ciystaUizer may be employed in systems in which fines destruction is not needed or wanted. In such cases the baffle is omitted, and the internal circulator is sized to have the minimum nucleating influence on the suspension. 

One method for improving the size distribution still further is to employ fines destruction (Jones etal., 1984 Jones and Chianese, 1988). In this technique, a classified stream of suspension, i.e. containing only fine not coarse crystals, is removed from the crystallizer in an elutriation leg (Figure 7.6(a)). 

Figure 7.6 Fines destruction during hatch crystallization after Jones etal., 1984)...

Figure 7.10 Fines destruction and classified product removal during continuous mixed suspension crystallization...

Jones, A.G., Chianese, A., 1988. Fines destruction during batch crystallization. Chemical Engineering Communications, 62, 5-16. 

Jones, A.G., Chianese, A. and Mullin, J.W., 1984. Effect of fines destruction on batch cooling crystallization of potassium sulphate solutions. In Industrial Crystallization 84. Eds. S.J. Jancic and E.J. de Jong, Amsterdam Elsevier, pp. 191-194. 

Industrial crystallizers are often equiped with an annular zone for fines removal. This fines removal system gives rise to a mass accumulation in the large annular zone (4), which affects the process dynamics at least under unsteady conditions. It is shown, that implementation of this fines destruction system into the simulation program has some implications for the algorithm used. The Influence of the mass accumulation in the annular zone on the process dynamics is also discussed. 

It is implicit that increasing the value of Lf will raise the supersaturation and growth rate to levels at which mass homogeneous nucleation can occur, thereby leading to periodic upsets of the system or cycling [Randolph, Beer, and Keener, Am. Inst. Chem. Eng. 19, 1140 (1973)]. That this could actually happen was demonstrated experimentally by Randolph, Beckman, and Kraljevich [Am. Inst. Chem. Eng. 23,500 (1977)], and that it could be controlled dynamically by regulating the fines-destruction system was shown by Beckman and Randolph [ibid., (1977)]. Dynamic control of a crystallizer with a fines-destruction baffle and fine-particle-detection equipment... 

The design of Fig. 18-65 contains a fines-destruction feature comprising the settling zone surrounding the crystallizer body, the circulating pump, and the heating element. The heating element supplies sufficient heat to meet the evaporation requirements and to raise the temperature of the solution removed from the settler so as to destroy... 

This type of equipment can also be used for apphcations in which the only heat removed is that required for adiabatic cooling of the incoming feed solution. When this is done and the fines-destruction feature is to be employed, a stream of liquid must be withdrawn from the settling zone of the crystallizer and the fine crystals must be separated or destroyed by some means other than heat addition—for example, either dilution or thickening and physical separaUon. 

If an elutriation leg or other product-classifying device is added to a crystallizer of the MSMPR type, the plot of the population density versus L is changed in the region of largest sizes. Also the incorporation of an elutriation leg destabilizes the crystal-size distribution and under some conditions can lead to cycling. To reduce cycling, fines destruction is usually coupled with classified product removal. The theoretical treatment of both the crystallizer model and the cycling relations is discussed by Randolph, Beer, and Keener (loc. cit.). 

If prevention of lha format ion of nuclei is unsuccessful in achieving the desired crystal size distribution, then fines destruction techniques may be employed. Approaches include the following ... 

Calculation of the optimum fines cut size in a DDO crystallizer is discussed by White and Randolph. The optimiun cut size for fines destruction with solute recycle, with design charts for DDO and fines-destruction crystallizers, is given by Sutradhar and Randolph. ... 

A possible method for changing a CSD from an MSMPR crystallizer is the use of fines destruction or classified product removal. In the case of fines destruction, it is assumed that small crystals in the size range from 0 to Lf are withdrawn at a flow rate (R - )Qo, while crystals in the range Lf to co are withdrawn at go-R is the ratio of product to fines drawdown times and... 

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