Batch crystallizing: homogeneous vat
Design of batch crystallizers
We must make the distinction between:
- - crystallizers with solvent vaporization;
- - cooled crystallizers.
- 1) Vaporization crystallizers:
In theory, it is possible to immerse the heating surface in the crystallizer even when agitated. In order to avoid boiling on the wall provoking fast crusting of the tubes and a drop in the system’s thermal power, we must provide a liquid height of 50 cm above the tubes (which are vertical).
Another solution consists of providing an external exchanger, in which we can circulate the slurry at a high velocity (2 m.s-1). To avoid boiling on the exchanger’s wall, we can provide a sufficient hydrostatic height rather than a release valve on the return pipe between the exchanger and the crystallizer.
2) Cooled crystallizer:
We can avoid using the external exchanger and circulation pump by performing the thermal transfer using an immersed coil, which is less expensive. Indeed, the classic coil fits the relationship:
when the crystallizer’s volume does not exceed 5 m3.
When we introduce an undersaturated load, any crusting on the coil is dissolved.
We can use a draft tube fitted with a marine impeller and place the coil in the annular space. This arrangement guarantees high circulation. We must ensure that the temperature difference between the slurry and the cooling fluid is sufficiently low to void crusting. High levels of recirculation may be required.
Crystallizing vats were studied in detail by Wey and Estrin [WEY 73], by Jones [JON 74a] and by Jones and Mullin [JON 74b].