Continuous homogeneous crystallizer (CHC)
The device is perfectly mixed. Throughout the useful volume (occupied by crystal slurry), the liquor composition is the same and the crystal concentration is also the same throughout; the device is equipped with:
- - a supply;
- - a slurry exit;
- - (possibly) a water vapor exit if we proceed by vaporization.
Crystallizer with a draft tube and an internal exchanger
The industrial system that leads to the lowest attrition is the draft tube crystallizer, with an internal exchanger as represented in Figure 4.2. This design is applicable to both vaporization and cooling.
The draft tube extends above the body, as a significant level of slurry above the body is required in case of vaporization in order to avoid boiling in the tubes. The diameter of the draft tube is a third or half that of the crystallizer itself.
For a given velocity in the tubes (1.5 m.s-1), the thermal transfer and surface transfer coefficients are invariable. We can then choose between:
- - many short tubes (length: 1.5 m) and consequently a significant circulating flow;
- - few long tubes (length: 4-7 m) and a lower circulating flow.
Figure 4.2. A crystallizer with a draft tube and an internal exchanger
The first solution is appropriate for the CHC, and is also more expensive (due to the price of tube plates). On the other hand, the discharge pressure required for the impellor is lower, and attrition less significant. The second solution makes the crystallizer closer to an external exchanger and is also less expensive. In reality, the shorter the tubes (due to the drop in pressure), the higher the magma’s viscosity (which is high in the case of sugar)
We should note here that crystallizers with an internal heat exchanger are more of an economical investment than devices with an external exchanger (that is, with forced circulation).