Editing Crystallization (section)

====Cooling crystallizers====
[[File:1-cooling-crystallizer-schladen.JPG|thumb|upright|Vertical cooling crystallizer in a beet sugar factory]]
The simplest cooling crystallizers are tanks provided with a [[Industrial mixer|mixer]] for internal circulation, where temperature decrease is obtained by heat exchange with an intermediate fluid circulating in a jacket. These simple machines are used in batch processes, as in processing of [[pharmaceuticals]] and are prone to scaling. Batch processes normally provide a relatively variable quality of the product along with the batch.

The ''Swenson-Walker'' crystallizer is a model, specifically conceived by Swenson Co. around 1920, having a semicylindric horizontal hollow trough in which a hollow [[screw]] conveyor or some hollow discs, in which a refrigerating fluid is circulated, plunge during rotation on a longitudinal axis. The refrigerating fluid is sometimes also circulated in a jacket around the trough. Crystals precipitate on the cold surfaces of the screw/discs, from which they are removed by scrapers and settle on the bottom of the trough. The screw, if provided, pushes the slurry towards a discharge port.

A common practice is to cool the solutions by flash evaporation: when a liquid at a given T<sub>0</sub> temperature is transferred in a chamber at a pressure P<sub>1</sub> such that the liquid saturation temperature T<sub>1</sub> at P<sub>1</sub> is lower than T<sub>0</sub>, the liquid will release [[heat]] according to the temperature difference and a quantity of solvent, whose total [[latent heat]] of vaporization equals the difference in [[enthalpy]]. In simple words, the liquid is cooled by evaporating a part of it.

In the sugar industry, vertical cooling crystallizers are used to exhaust the [[molasses]] in the last crystallization stage downstream of vacuum pans, prior to centrifugation. The massecuite enters the crystallizers at the top, and cooling water is pumped through pipes in counterflow.