Crystallization Rate and Purity of Produced Crystals

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In industrial crystallization, high purity substance is needed through purification and separation process with saving energy. Concerning separation from cutectic melt, mother melt happens to be included on the surface of crystals, to make the purity lower, and so sweating operation has been selected to improve the purity. In this study, correlation between basic crystallization phenomena and crystal purity wereinvestigated focusing nucleation and growth phenomena, to aim to propose the new theory on the mother inclusion in the process of crystallization, and optimum condition for high purification performance. In this study for the cutectic system of benzene and cyclohexane, experiments of benzene crystallization were performed using semi batch apparatus, to measure the time course of particle size distributions (PSD) and purity of crystals in the determined size range. From the time course of PSD,crystal growth was recognized to proceed by the mechanism of uniting primary nuclei wach … More other and attaching them, and secondary nuclei were observed to be produced periodically. And also crystal purity after wiping operation, was correlated quantitatively with crystal size, to make clear that there are three stages in which (1) formation of small crystals including much mother melt by the agglomeration mechanism of fineparticles, (2) formation of middle sizeone having low content of mother melt : in which find particles are thought to be incorporated into the large crystal, (3) formation of large crystals, in which mother melt are kept between the fine crystals attached to the surface of large crystal. And also batch experiments were performed in the cutectic system of paraxylene and cyclohexane. By the result, initial primary nucleation phenomena affected the purity, and also the mechanism of impurity discharge was shown by our model, considering crystal growth phenomena in the equipment. Based on the result described above, the mechanism and kinetics of impurity inclusion and discharge were considered to propose the optimum crystallization operatin condition for high purification. Less