Stabilizing control of crystal size distribution in continuous crystallization processes

Crystallization processes often suffer from unstable operation arising from sustained oscillation of the crystal size distribution (CSD). This calls for stabilizing control of CSD, Because of its distributed nature, however, CSD cannot be used as a controlled variable. A detailed model of crystallizer is analyzed in order to determine controlled and manipulated variables which are effective to suppress CSD oscillation. This analysis focuses on phenomena at start-up and sustained oscillation in particular. It is shown that a multi-loop configuration, consisting of two SISO control loops, is effective in suppressing the CSD instability: one SISO loop controls fines population density using fines flow rate, and the other loop regulates the third moment of large crystals by manipulating the product flow rate. Further, it is found that the suggested multi-loop scheme works effectively, even though the input variables are constrained.