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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