Vacuum cooling crystallization

Principle

By flashing the solution in vacuum, the solution temperature is gradually reduced. This reduces the solubility of the salt and causes crystallization. The resulting vapour steam can be used for preheating.

Process description

Vacuum cooling crystallization is mainly used for salts with a decreasing solubility at decreasing temperature. A warm, almost saturated solution enters the multi-stage vacuum crystallizer K1-4. In this case, the pressure is gradually reduced, whereby solvent (water) evaporates and the solution cools down to the boiling temperature. By lowering the temperature, salt crystallizes. This is held in suspension by air sucked in at the bottom of the crystallizer (air agitation) and is transported to the outlet. From there, the suspension is pumped to the cyclone ZY for thickening by means of the suspension pump SP. The liquid is then separated in the centrifuge Z. The salt is dried in dryer T. The resulting vapour steam can be used to preheat the mother liquor in the bath condenser BK or condensed with cooling water in the mixing condenser MK. The stage pressure and thus the solution temperature can be reduced even further by use of steam jet pump D. The inert gases are extracted with a vacuum pump VP.

Characteristics

• Since no cooling surfaces in the form of heat exchangers are necessary for evaporation, no cooling surface incrustation (fouling) occurs. This results in maximum operating times between cleaning procedures.
• Heat recovery is possible to reheat draining solution after salt separation.
• Flexible partial load operation is possible.
• Suitable for various applications.

Variants

• Increase in the number of stages to improve heat recovery.
• With or without salt separation.
• Horizontal crystallizer with air agitation or vertical crystallizers with draft tube and bottom flange stirrer (Draft Tube Baffled, DTB) or external circulation circuit (Forced Circulation, FC).
• Vacuum generator by vacuum pump and / or steam jet pumps.
• Decrease of the pressures and thus discharge temperature of the solution / suspension by acid condensation or use of refrigerating machines and refrigerants.
• Vapour condensation of the vapour of the final stage in mixing or surface condensers.