Batch cooling crystallization at constant supersaturation: Technique and experimental results

A new laboratory plant for batch cooling crystallization is described, which allows to maintain the supersaturation at a constant level during an experimental run. A method for continuous supersaturation measurements has been developed, using a density meter and a hydrocyclone. Experimental results obtained from a seeded solution in constant supersaturation cooling mode and at constant cooling rate are compared. In the case of a constant cooling rate, the supersaturation passes through a sharp maximum, whereby effective nucleation is enhanced. As a consequence, a smaller median crystal size is obtained than for cooling at constant supersaturation. Equation for controlled cooling at a constant supersaturation require an accurate description of the effective rate of nucleation. Therefore, the mechanical stress, to which the crystals are subjected, must be included as the main source of new nuclei. This is shown by the influence of stirrer on batch time.