Order of magnitude for parameters
In order for nucleation to be considerable, we must have a for the supersaturation:
On the other hand, in order for growth to be progressive while remaining economically acceptable, we need:
In this situation, we typically have:
In the industry, the size of crystals is: (L = Gt)
The residence time т , the volume fraction фх of crystals in slurry and the agitation power per kilogram of slurry ? all depend on the type of the crystallizer.
Residence time and supersaturation
We can always develop an installation capable of the thermal power corresponding to the crystallization of a given product.
A further choice presents itself. This is:
- - the volume Vc of a continuous crystallizer;
- - crystallization time т for a batch crystallizer.
Both of these parameters set the supersaturation value.
If they are low, the supersaturation in the slurry will be too high for crystallization to follow the thermal transfer. Nucleation is high, as is the proportion of fine crystals. The granulometry is spread and the mean size low; too high a supersaturation leads to crystals of irregular shape and fragile texture. These crystals can feature inclusions of mother liquor and thus be impure.
Inversely, there is a growth limit of Vc or т. This limit is set by the costs of investment.
Ultimately, the choice of Vc or of т is the result of a compromise. The crystallizer volumes are the result of the value of residence time in the slurry, which can vary according to the product from 1 h to 12 h (viscous liquid). On the other hand, the length of a batch operation is between 2 and 10 h.