A basic crystallizer design feature is the way supersaturation is controlled and kept within the metastable range. The method is independent of the crystallization process. Fig. 5 illustrates this method for a vacuum-cooled crystallizer. This principle is also valid for va-cuum evaporation, surface cooling and reaction crystallization.

Figure 5 Supersaturation Control in a Vacuum Cooled Crystallizer

The unit illustrated is a Forced-Circulation (FC) crystallizer which is equipped with an ex-ternal circulation loop. The simplified solubility graph shown includes the metastable zone (dashed line). The events occurring in one circulation loop are indicated by numbers in both the solubility diagram and the FC crystallizer sketch. The suspension (3) in the crys-tallizer is mixed with feed liquor (1). In cooling crystallization the feed is hotter and more concentrated than the circulated suspension. As a consequence, the resulting mixture (2) is more concentrated and hotter than the original suspension. The mixture is pumped to the Vapor/Liquid separator vessel (4) which is maintained at the boiling point of the suspension. There, the supersaturated solution temperature drops as it flashes to its the boiling point adiabatically. The boiling creates supersaturation C, which is represented by the line 3 - 4. This C is consumed by crystal growth, and the solution returns to its steady state.
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