For operation at temperatures below which it is not economically feasible to use vacuum equipment, or for solutions with very high boiling point elevations, the surface cooled crystallizer is generally specified.
The surface cooled unit is a type of forced circulation crystallizer consisting of a shell and tube heat exchanger through which is pumped the slurry of growing crystals, a crystallizer body to provide retention time, and a recirculation pump and piping. 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. Practical values depend on the operating cycles and the properties and characteristics of the materials. Temperature differences ranging from 5 to 15 °F are required to achieve reasonable operating cycles.
Surface cooled crystallizers are 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.
Typical applications for the Swenson surface cooled crystallizer are in processing sodium chloride from caustic solutions, sodium carbonate decahydrate from waste solutions, and sodium chlorate from solutions saturated with sodium chloride.