The generated supersaturation (4) can be adjusted by varying the recirculation through the crystallizer. As more solution is recirculated, points (2) and (4) approach nearer to point (3). The coordinates of point (1), i.e. the feed mass flow and the crystallizer cooling (operating) temperature, define the production capacity of the system. The recirculation in the crystallizer is adapted to this production capacity, and the supersaturation is kept within the metastable range. The circulation flow, therefore, is a major design criterion. The circulation flow can be calculated by equation 1.
This is a limiting relationship for the production capacity of a crystallizer, and if exceeded, spontaneous nucleation can occur. Relatively high circulation flows are usually necessary even for minor production capacities, because the typical metastable field has a range of a only few g/l: as an example, for a production of 1 t/h and a Delta C of 1 g/l, the circulation flow would be 1000 m
3/h.
Fig. 6 illustrates the relationship between desupersaturation rate and total crystal surface area. The generation of supersaturation drives crystal growth on the crystal surface area A1 while the suspension is circulated through the crystallizer. Looking at the recirculation as a series of differential elements, a typical saw-tooth function is evident. Because residual supersaturation is added to the fresh supersaturation generated in the element that follows, all supersaturation should preferably be consumed within a single loop. If there is a large amount of crystal surface area available (A2), the desupersaturation (measured in kg mass/m2 of crystal surface area) of the liquid is faster, resulting in lower residual su-persaturation at the completion of the loop. Suspension densities (mass of crystals/mass of suspension) between 15 and 25 wt.-%, usually satisfy this requirement.
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