Large and Mono-Sized Product Crystals from Natural Cooling Mode Batch Crystallizer.

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Abstract
The performance of a 12.2L batch crystallizer operated under natural cooling mode was remarkably improved with seeding. The product crystal size distribution (CSD) was bi-modal at low seed concentrations, reflecting the contribution of crystals newly generated by a secondary nucleation mechanism. With increasing seed concentration, the fraction of the newly generated particles decreased. At high seed concentrations, a large and mono-dispersed product was obtained. A comprehensive seed chart for potassium alum crystallization incorporating the effects of seed concentration, seed size and seed quality on the product weight mean size was developed. With the aid of this chart, an appropriate seed size and seed concentration can be chosen either to maximize growth or to minimize nucleation in a given batch crystallizer configuration. In addition to the natural cooling mode, the other two cooling policies, either of which requires a respective temperature control, were examined. The bi-modal crystal size distribution characteristics remained the same at low seed concentrations. At high seed concentrations, no effect of cooling mode on the CSD was observed. Hence, natural cooling, a simple and easily operable mode with no temperature control, can be beneficially applied to obtain large and mono-dispersed product crystals with adequate seed concentration.