CONTROL OF CRYSTAL SIZE DISTRIBUTION IN A BATCH COOLING CRYSTALLIZER

A control scheme for crystal size distribution (CSD) in a batch crystallizer, based on indirect measurement of fines suspension density and manipulation of fines dissolving rate, is proposed and implemented on a 27 L laboratory batch cooling crystallizer using the potash alum‐water system. The measured variable was a temperature difference related to the fines suspension density detected by a new fines sampling/suspension density measuring device proposed by Rohani and Paine (1987). Servo‐control of the fines suspension density was achieved using a conventional PI control mode. Two different cooling policies, namely, linear cooling and isothermal operation were examined and improvement in the final CSD was observed in both cases. The weight‐mean crystal size and the coefficient of variation of the end product showed a maximum improvement (larger mean size and smaller coefficient of variation) of 80% and 31% over the uncontrolled experiments, respectively. The weight fraction of fines (smaller than 150 μm) in the end product was decreased by a maximum of 99% over the uncontrolled run. Higher overall rates of fines dissolution led to a more uniform product with a larger weight‐mean crystal size at the expense of a small reduction in the rate of solids make. Copyright © 1990 Canadian Society for Chemical Engineering