Direct Contact Refrigeration Crystallization
When crystallization occurs at such a low temperature that it is impractical to use surface cooling or when the rapid crystallization of solids on the tube walls would foul a conventional surface cooled crystallizer, a draft tube baffle crystallizer (or a forced circulation unit) utilizing the direct contact refrigeration technique can be employed.
In this operation, a refrigerant is mixed with the circulating magma within the crystallizer body where it absorbs heat and is vaporized. Refrigerant vapor leaves the surface of the crystallizer similar to water vapor in a conventional evaporative crystallizer. It then must be compressed, condensed and circulated to the crystallizer to maintain continuous operating conditions. Refrigerants must be relatively insoluble in the solutions processed and have the thermodynamic characteristics to minimize compressor horsepower.
Examples are the crystallization of caustic dihydrate with Freon or Propane, and that of Paraxylene with liquid Propane refrigerant.