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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