SELF-TUNING CONTROL OF CRYSTAL SIZE DISTRIBUTION IN A COOLING BATCH CRYSTALLIZER

In this paper the theoretical basis of a control scheme, previously implemented on a laboratory scale potash alum batch cooling cystallizer (Rohani, Can. J. chem. Engng, in press, 1990), is presented. A comparison is made between the experimental and simulation results. A mathematical model of a seeded batch cooling crystallizer equipped with a fines dissolving loop and a fines suspension measuring device is developed. The mean crystal size and the coefficient of variation of an ideal batch crystallizer are derived analytically and it is shown that a batch crystallizer utilizing the proposed control scheme approaches the ideal case. Using the traditional control schemes based on operation along an optimal cooling trajectory results in a terminal CSD far from the ideal case. It is also shown that due to the time-varying characteristics of a batch crystallizer, a minimum variance self-tuning controller renders tighter control and improves the product CSD to a larger extent (a smaller coefficient of variation and a larger mean crystal size of the weight distribution) when compared with a conventional PI-controller. © 1990.