The influence of gradual classification on continuous crystallization

Abstract

Classification occurs when the mean residence time of a crystal of size L, τ( L), differs from that of the liquid in the crystallizer. The classification function p = τ/τ( L) characterises this difference and, when coupled with the population balance, predicts the crystal size distribution produced in a classifying crystallizer. This is exemplified here for size-dependent growth kinetics as well as when growth dispersion occurs. A contoured-base tank, characterised by a constant cross-section for magma circulation and unusually mild agitation, was operated continuously. In 13 runs potash alum was crystallized under MSMPR conditions, while in 17 runs lower stirrer speeds induced classification of the type τ( L) ≥ No discontinuities in growth and nucleation kinetics were observed. The mean crystal size increased substantially, while the coefficient of variation r only slightly in the classified regime. Classification had to be taken into consideration when predicting the crystal size distribution. The classifica function of potash alum crystals could not be predicted by measuring p for glass beads having the same settling rates. The limiting step in complet mixing the suspensions was lifting particles out of the boundary layer on the tank base.