Crystallization from solution classified-suspension crystallizer

While in batch operation, the fresh solution is fed into the crystallizer all at once the crystallization process carries on for a certain period, then the suspension is discharged from the crystallizer and is conveyed to the device for solid-liquid separation. For such operation the classifying leg (9), the overflow cofferdam (7), and the pump A are not needed. 

The solution upflow velocity, u, is not an easy quantity to specify. The solution upflow should balance the settling velocity of the crystals, but this is difficult to measure and virtually impossible to predict with accuracy. The trouble stems from the fact that the crystal suspension contains crystals of all sizes between product size and freshly generated nuclei. To choose the upflow velocity as the free settling velocity of a classified product crystal would result in the smaller crystals being swept out of the crystallizer. An upflow velocity taken as the free... 

Crystal slurry is classified (crystal size classification) by the circulation amount of the outer cycle which is controlled by the suction nozzles. Small crystals from the solution are entrained from the crystal bed and recycled to the inner circulation driven by the propeller pump 3, where supersaturation is reduced due to degradation of crystal fines. Different crystal concentrations and sizes in inner and outer suspension cycles are caused by different circulation amounts in each cycle. With known crystallization behavior the cycle may be controlled in a way, that in the inner cycle at a low suspension concentration and a relatively high supersaturation nucleation occurs. Otherwise, in the outer cycle at a longer residence time and smaller supersaturation crystal growth is favored. Crystal sizes 0.8-4 mm No crystal fines below 0.3 mm... 

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