EQUIPMENT DESIGN
The feed slurry is first heated in a heat exchanger, then pumped to the main body of the crystallizer. Vaporization occurs at the top surface of the slurry, while nucleation occurs near the bottom of the crystallizer body. The crystals are removed and vaporized solvent is condensed and returned to the crystallizer body.
There are several adaptations that can be added to a forced-circulation crystallizer to help narrow the crystal size distribution. Options include baffling, a conical entrance, and an elutriation step. A conical entrance promotes more thorough mixing, which creates a more uniform slurry mixture. Baffling is used to remove fines from the mixture, so that they can be recirculated for further growth. Elutriation, like baffling, removes smaller particles from the slurry, except elutriation separates particles based on weight rather than size. Elutriation is the process of flowing air over particles. Lighter particles are picked up by the air stream, while heavier ones remain in place.
USAGE EXAMPLES
Forced-circulation crystallizers are commonly used to produce salts and chemicals in industry. The forced-circulation crystallizer shown on the left is used to produce sodium sulfate, while that on the right is used in the agricultural industry.