The control of super saturation, crystal suspension density and
crystal breakage requires forced circulation evaporation systems. Depending on
the vessel size and the materials of construction, integrated heating chambers
or external heating circuits can be used. Where special requirements are
specified with respect to the quality of the crystals, crystallizers with
additional circuit combinations are used.
All this is considered in modern types of continuous crystallizers.
Crystallizers with longer retention times are operated with less specific
energy input, resulting in lower nucleation rates. The impacts between crystals
and the impeller pump blades are the most effective source for the nuclei
production. These impacts are at least 100fold more effective than crystal/wall
and crystal/crystal impacts. Therefore, types of crystallizers differ mainly in
design and the position of the impeller pump.
Typical examples of such processes are:
Crystallization by evaporation or vacuum cooling for the production
of various inorganic and organic compounds such as but not limited to sodium
chloride (salt from brines), potassium chloride, ammonium sulphate, sodium
sulphate and adipic acid.
Crystallizers are of continuously operated forced circulation type.
They may be designed with internal or external circulation, with or without
baffle zones and with no, partial or total classification.