The result of any crystallization process is a crystalline matter with a certain crystal size distribution (CSD), certain crystal habit and purity. These properties very often are quality requirements and well-defined by market demands. Furthermore, the crystallization process itself requires a minimum CSD, as the suspension leaving the crystallizer still needs to be separated. The separation achieved and the product purity obtained improves with compact crystal habit, and coarser crystal size. Therefore, the quality and efficiency of crystallization as unit separation process is strongly dependent on crystal size and crystal habit. These properties also affect agglomeration during storage, dust generation and bulk density and thus influence the handling of the crystalline product downstream from the crystallizer:
Crystallization theory addresses these critical aspects of the formation of crystalline solids. While crystal size and shape can be regulated by factors outside the crystallizer’s influence (impurities, solvent, etc), of the parameters that the crystallizer can control, the most important is the appropriate handling of superŽsaturation (the driving force of crystallization). Supersaturation can be produced by evaporation or cooling (Fig. 1). In case of “flat” solubility (low dependence with respect to temperature) the evaporation is the optimum choice, while solubility with strong temperature dependence is best suited to cooling crystallizers.
Figure 1 Crystallization process
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