Importance of macromixing in batch cooling crystallization

The importance of spatial variations of the conditions in industrial-scale, agitated batch cooling crystallizers is investigated by computer simulations. A three-compartment model is developed considering primary and magma density-dependent secondary nucleation. An increasing crystallizer size is described by an increasing suspension turnover time. It is shown that accumulation of larger crystals in the bottom region, localized supersaturation generation and variations in the secondary nucleation rate due to varying local mixing intensity will, under normal conditions, exert a low and often negligible influence on the product-size distribution of an industrial unit. In a batch process, the product-size distribution is governed mainly by the conditions early in the process. During this period the supersaturation half-life is much longer than the suspension turnover time, and the influence of local variations becomes weak.