Crystallizer with fluidized bed
The crystallizer body is a cylinder or a cone with its apex facing downwards, in which the fluid bed made up of crystals and the rising liquid is situated. Crystals descend slowly as they grow, being captured in the lower part. The supply of supersaturated liquor is ensured by a tube that descends to the lower part of the body. Used liquor (that has lost its supersaturation) is decanted by flooding to the surface of the fluidized bed.
This device is suitable for fragile crystals as agitation is very low within the fluidized bed. It is also suited to the production of large crystals by means of a modest production. Indeed, the liquor’s rate of climb is low and the productivity connected with the horizontal part of the body varies between 30 and 150 kg.h-1.m-2 for crystals in the order of millimeters.
Figure 4.9. A crystallizer with a fluidized bed
Secondary nucleation is zero as it is the clean liquor that circulates through the exchanger and the pump. Moreover, supersaturation is maintained just above the metastable limit to provoke a slight nucleation.
Seeds form throughout the bed but particularly in the lower part in which supersaturation AX is maximum. While they are climbing to the upper part of the bed, they grow and, having reached a size limit, they descend into the bed where they are decanted at the base.