If the liquor is exhausted faster in the product than in impurities, then batch crystallization is required.
Cascade of serial crystallizers
The value of a cascade is twofold compared with a single large device:
- - to limit supersaturation in each device;
- - to avoid supersaturation instability, which is always possible in a large device.
The concurrent circulation of liquor and slurry has the consequence that, with more devices, we approach more operation of the double envelope tube (in the case of cooling); in other words, we approach batch crystallization. This explains why the median size increases with the number of serial devices.
However, this system includes many pumps, and attrition can be severe.
Particles in the slurry
In the liquid of an industrial crystallizer, we can encounter a certain particle diversity:
- real seeds consecutive to primary nucleation;
- - apparent seeds resulting from the agglomeration of real seeds;
- - fragments of size between 1 and 10 pm resulting from the attrition (wear) of significantly larger crystals;
- - fragments of size from 30 to 100 pm resulting from the shattering of crystals of size 100-1,000 pm, following an impact with the pump impeller
or agitator impeller.
Possible crystal dimensions
Of course, the size obtained for the crystals depends not only on the system in question, but also on the type of crystallizer used:
Crystallizer type Median size (mm)
Forced circulation 0.2-0.5
Draft tube 0.5-2
Fluidized bed (and batch crystallization) 1-10