Kemija u Industriji (Jul 2009)
Batch Crystallizer Scale-up
Abstract
In order to define an appropriate scale-up criterion for a batch crystallizer batch cooling crystallization of potassium chloride from the aqueous solution has been performed. The measurements were undertaken on the laboratory scale in three geometrically similar pieces of equipment (Table 1) under mixing intervals from 300 to 600 min –1 . A stirrer was supplied with pitched (at 45°) 4- blade propeller. Granulometric properties (shape and size distribution) of the obtained crystals were used as a comparison criterion. Hydrodynamic conditions in the crystallizer and size of the equipment had major influence on the properties of the crystals. Under higher mixing intensity and with larger equipment the crystals assumed more regular shape (Fig. 3.), and their size distribution showed finer fractions (Fig. 2, Fig. 4.). Actually, these conditions reduced the amount of agglomerates in the finished product. In addition, better hydrodynamic conditions for heat andmass transfer were achieved, whereas the secondary nucleation (contact) was minor. The comparison between the obtained crystal size distributions in all three pieces of laboratory equipment showed that the mixing rate of n = 500 min –1 yielded satisfactory results (Fig. 4 b.). They were further used to define the scale-up criterion for the batch crystallizer. Applicability of different scale-up criteria to the suspension stirrers was tested. Compliance with the geometric similarity and hydrodynamic conditions did not allow for compliance with the conventional scale-up criteria (Fr = const, Re = const). Based on the scale-up methodology, when the measurements can be undertaken in three or more pieces of equipment, the scale-up criterion, for the batch crystallizer for cooling crystallization of potassium chloride from the aqueous solution, is defined (Fig. 5.). The methodology proposed for the studied crystallizer gives a relatively low mixing intensity (237 min –1), which corresponds to the solution homogeneity 1 (Table 2.). Given that crystallization required suspension homogeneity between 6 and 9 (Chemineer ChemScale),8 implying that the suspension homogeneity was 95–98 % of the liquid height, the mixing intensity for these conditions were determined as well. The obtained values were within the range of the used mixing intensity.
