Usually, low nucleation will produce coarser crystals. Fig. 3 illustrates
this principle. In a very simplified example, assuming a product crystal mass
of 10 g, 10 particles of one-gram crystals are generated if there were only 10
nuclei at the start of crystallization. By comparison, if there were 104 nuclei
present at the start of crystallization, they will result in an equal number of
1 mg crystals, totalling the same 10 g product mass. However, the degree of
possible supersaturation is limited, and the result is that significantly more
time is necessary to crystallize ten one-gram crystals than one-mg crystals.
This relationship is indicated by the time arrows in Fig. 3.
These
principles lead to the following basic crystallizer design rules:
- The supersaturation in crystallizers must be within the metastable range;
- High supersaturation is important for efficient crystal growth rates;
- Secondary nucleation should be minimized by limiting the mechanical energy
input.
Figure 3 Influence of nucleation on crystal size
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