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Process Conditions and Granulometric Properties of Crystals

Granulometric properties of crystals (mean crystal size, crystal size distribution and crystal shape) are very sensible to process conditions (mixing rate, cooling rate, cooling profile, seeding, type of impeller, batch time, etc). Selection of the appropriate process condition, in order to obtain the required product quality, is of a great importance. To achieve that, the influence of various process conditions on the granulometric properties of crystals, have been investigated on a laboratory scale batch cooling crystallizer. Three different salts were used for crystallization from water solution: two inorganic (potassium chloride, potassium sulphate) and one organic (penthaerythritol). The intensity of cooling and mixing strongly influence the shape, the mean crystal size and the CSD of the KCl crystals. To produce crystals of better quality (regular shape, bigger size and narrow CSD) the process must be conducted at slow cooling and high mixing rate. Cooling rate does not influence the shape of the K2SO4 crystals, but it affects the crystal size and CSD in the same manner as on the KCl. The usage of the impeller that produces the axial flow is recommended, because the improvement in the final CSD (it shifts to the larger size ranges) was observed, when compared with the impeller with radial flow. The penthaerythritol crystal shape does not depend on the applied operating conditions. Well mixed conditions (higher mixing rate) result with production of bigger crystals. The final CSDs are bimodal for all cooling modes (convex, natural and linear), higher batch time and low mixing rate. Crystals obtained from seeded crystallization experiments results with unimodal CSD. The mean crystal size remains the same for all seed sizes and loadings.

batch crystallization; cooling rate; granulometric properties; mixing rate; seeding

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