In designing a crystallizer, all the parameters discussed in Part 3 above
need to be com-bined, so as to provide the optimal environment for crystal
growth. Crystallizers with longer retention times operate at lower specific
energy input, resulting in lower nucleation rates. Crystal-to-impeller impacts
are the most effective source for nuclei production, as they usually generate
at least a hundred-fold more nuclei than crystal-to-wall and crystal-to-crystal
collisions. Types of crystallizers, therefore, differ from one another mainly
in the design and location (in clear liquor or in suspension) of the
circulation pump (compare fig.
4).
In summary, crystals grow larger by:
- Maintaining proper supersaturation levels,
- Decreasing the energy input,
- Decreasing the mechanical stresses,
- Decreasing the attrition, and, at the same time,
increasing the
retention time
Also, conditions for crystallization improve at higher temperatures.
The nucleation rate in a crystallizer is influenced not only by the
specific energy input, but also by the mixing intensity. The specific energy
input by impeller pumps can be defined as:
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