Recompression Evaporation-Crystallization

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

Crystallization that occurs during evaporation can potentially be intensified by use of vapor recompression and spinning discs. In this scenario, the evaporated vapor is compressed and then condensed on the bottom of the discs to heat the crystallizing fluid (58). This approach may permit operation at higher temperatures, lower surface area, and less time. 

As the cost of energy has increased in recent years, attention is again being directed to mechanical vapor recompression, which by its nature, permits substitution of electrical energy for evaporation and crystallization rather than requiring heat energy (steam). A typical recompression crystallizer flow sheet is shown in Figure... 

Some typical applications of mechanical vapor recompression units are evaporation of sea water to give distilled water, evaporation of kraft black liquor in the paper industry (L2), evaporation of heat-sensitive materials such as fruit juices, and crystallizing of salts having inverse solubility curves where the solubility decreases with increasing temperature (K2, M3). 

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