Surface cooled crystallizers are a type of forced circulation crystallizer. Surface cooled crystallizers are generally used where the solution boiling point elevation is extremely high—as in the case of caustic solutions—or when the temperature level is so low that evaporation by vacuum is impractical.
A surface cooled crystallizer consists of a:
Within the crystallizer body is a baffle designed to keep excessively fine crystals separated from the growing magma for size and slurry density control purposes.
The circulation rate through the heat exchanger is normally high enough so the temperature drop is in the range of 1 to 2 °F. Surrounding the tubes is the cooling media, either tempered water circulated through segmented baffles or a vaporized refrigerant.
Because the tube wall is the coldest part of the crystallizing system, the temperature differences between the wall and the slurry being pumped through the tube must be as small as practical. Temperature differences depend on the operating cycles and the properties and characteristics of the materials. Temperature differences limitations ranging from 5 to 15 °F are required to achieve reasonable operating cycles.
Typical applications for the Swenson surface cooled crystallizer are in processing sodium sulfate decahydrate from lithium sulfate solutions, sodium carbonate decahydrate from waste solutions, and sodium chlorate from solutions saturated with sodium chloride.