In addition to the criteria mentioned earlier, the design of specific crystallization processes is influenced by several other factors. Some of the most important are mentioned below.
SURFACE-COOLING CRYSTALLIZATION
The surface-cooling process produces supersaturation directly on the heat exchanger surface. The supersaturation in the heat exchanger is the highest in the entire crystallizer. Incrustations on the heat transfer surface and eventual plugging of the tubes are the nor-mal consequences. This can be an acceptable situation for discontinuous operation, be-cause with each next batch the incrustations may be dissolved again. For continuous processes, however, the surface cooling is only an option if the low operating temperature required in the crystallizer makes vacuum cooling crystallization impractical. If a conti-nuous crystallizer must employ surface-cooling, especially large heat exchanger surface area is supplied, in an effort to increase the operating cycle.
VACUUM-COOLING CRYSTALLIZATION
Vacuum-cooled crystallization is the preferred cooling crystallization method for continuous operation. Because cooling is generated by adiabatic expansion of the solvent, and the condensing of the vaporized solvent is done in a separate heat exchanger, scaling of cooling surfaces is not experienced. Vacuum cooling becomes uneconomical (or imprac-tical) only if operation at very low temperatures is required.
EVAPORATION CRYSTALLIZATION
The evaporative crystallization is generally a vacuum process, much like vacuum-cooled crystallization. The difference is that this process is independ¬ent of the concentration and temperature of the feed solution. External heat can be added to the system and the con-centration of mother liquor can be adjusted by evaporation. Like vacuum-cooled crystalli-zation, there are no special encrustation problems in evaporative crystallization. Operating difficul¬ties may arise in the case of concentration of inversely soluble substances, like some sulfates and carbonates. In such cases the same encrustation model exists as in surface-cooled crystallization. High suspension velocities in the heater tubes and high suspension density (to increase the desupersaturation rate) and can improve the operating cycle. Multiple-effect evaporative crystallization plants are supplied in cases where low energy consumption is especially important.
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