Norihito Doki

Seeding policy in batch cooling crystallization

Abstract The effect of seeding in batch cooling crystallization is discussed on the basis of original experimental data on potassium alum–water system obtained for both controlled and natural cooling modes. Regardless of the cooling mode, unimodal product crystal size distributions of grown seeds were obtained, provided that enough seed loadings are above a critical concentration. Slow cooling was not a necessary condition to avoid secondary nucleation. The critical seed concentration can be determined easily from several batch trials with the help of a seed chart, in which mean mass size of product crystals normalized with the seed size is plotted as a function of seed concentration, with seed size as a parameter. This type of seeding effect could be observed for cooling crystallization of other systems and for crystallization employing other supersaturation-generating methods.

Effect of Cooling Mode on Product Crystal Size in Seeded Batch Crystallization of Potassium Alum

Potassium alum was crystallized by seeding in a batch crystallizer under controlled and natural cooling modes. Regardless of the cooling mode, the product crystal size distribution (CSD) became bi-modal at low seed concentrations because of enormous secondary nucleation. The mean mass size of the product was smaller for the natural cooling mode compared to that for the controlled cooling mode with more intensive secondary nucleation. On the other hand, at high seed concentrations, the product CSD became uni-modal with the same mean mass size for both cooling modes, where the crystallization was dominated by seed growth. The low supersaturation caused by the growth of enough seeds plays a key role to produce uni-modal size distribution with suppressed nucleation. Adhering of small crystals (secondary nuclei) to growing seed crystals is also considered to be another mechanism for generating uni-modal CSD.