Equipment For Crystallization

Crystallization is often carried out in a stirred tank or vessel. Depending on the type of crystallization different configurations of the tank/vessel are used. The crystallizers are in some processes combined with external equipment like heat exchangers, settlers/cyclones, etc.

There are 3 main different types of commercial evaporative crystallizers:

  • Forced Circulation (FC) Crystallizers in which the heat for evaporation is added to a circulated suspension. In the evaporator the circulated stream is flashed and the mixing is originating from the impulse of the circulated suspension
  • Draft Tube Baffled (DTB) Crystallizers in which the heat for evaporation is added to a crystal lean stream of liquid which is taken from the settling zone in the crystallizer. The slurry is kept in suspension by a propeller type stirrer in combination with the draft tube.
  • OSLO or fluidized bed crystallizers in which the heat for evaporation is added to the almost crystal-free top stream leaving the fluid bed. After flashing the overheated stream in the evaporator the resulting supersaturated liquid is circulated over the fluidized bed, where the supersaturation is deposited on the crystals in the bed.

The FC crystallizer is the simplest and most robust type of the three, whereas the OSLO crystallizers is the most complex and less robust type. The OSLO crystallizer, however, usually delivers a higher quality of crystals with a larger average size and a narrower size distribution.

There are many different configurations of commercially available cooling crystallizers. The most important types are the batch or continuous Stirred Tank Crystallizers, Fluid Bed Crystallizers or Crystallizers with a Scraped Heat Exchanger. The previously mentioned and FC and DTB crystallizers are examples of Stirred Tank Reactors. The OSLO crystallizer is an example of a FB Crystallizer. In cooling crystallization applications the liquid is not overheated, but undercooled in the heat exchanger. The circulation flow is set at a rate that the fluidized bed can be maintained, while the undercooled solution can remain within the metastable zone to prevent spontaneous nucleation. Usually the temperature difference between the coolant and the crystallizing solution is less than 2°C in crystallizers with non-scraped heat exchangers. The fines removal can be accomplished with equipment like a cyclone. The most common type of crystallizer with a scraped heat exchanger is the double tube configuration. Crystallization takes place in the inner tube, while the coolant flows through the outer tube. The surface of the inner tube is kept free from solids by rotating scrapers. Armstrong Chemtec is a leading supplier of tubular crystallizers with a scraped heat exchanger.

For flash cooling the same crystallizers can be used as for evaporative crystallization (FC, DTB, OSLO). There are less problems with encrustation on the wall as the heat is not withdrawn via the wall. However, problems can occur in the boiling zone and/or near the feed point(s) where locally high supersaturations may be formed. Another potential problem is that the gaseous solvent needs to be recovered and/or purified for recycling or disposal.

Direct contact cooling is also applied to prevent problems with encrustation and it is typically used at low temperatures, where flash crystallization cannot be used easily. A special point of attention is that the coolant may not interfere with the crystallization process. The cooling power for the crystallization is supplied by the evaporation energy or by the cooling caused by the coolant. The recovery and/or purification of the coolant is a potential drawback of direct cooling.

Precipitation is usually carried out in Batch or Continuous Stirred Tank Reactors.