Continuous forced circulation crystallizer (CFCC)
In the external loop, the slurry crosses a tube exchanger that provides the heat or cold as required. This exchanger can be a single tube pass (horizontal or vertical) or double pass (horizontal).
The slurry exits the crystallizer body by a funnel bottom and is directed to the exchanger by an axial pump (generating attrition). It is reintroduced into the body by one of the two standardized devices that will be described below.
Recirculation flow must be such that:
- - the temperature variation in the exchanger does not exceed 2°C;
- - the slurry of the body must, according to the requirements, be renewed in a variable between 1 and 4 mn.
The crystallizer is supplied by means of the pump’s aspiration.
In a CFCC, the crystal growth essentially occurs in the crystallizer body in which the residence time is predominant. On the other hand, secondary nucleation occurs by attrition in the pump.
The slurry’s entry into the body can be:
- 1) tangential entry (Figure 4.7). The slurry is introduced under the free surface in order to avoid sudden vaporization spurts that can be detrimental to the installation’s mechanical integrity. This system is only used in case of vaporization;
- 2) axial entry (Figure 4.8). This system is used in the case of cooling and also in the case of mechanical recompression of condensation. We will see its advantages.
Figure 4.7. Tangential entry
Figure 4.8. Axial entry
The circulation flow in a CFCC is high and we can consider that the slurry flow does not vary, but remains equal to QB between the body entry and exit, even if there is evaporation. Similarly, the crystal surface AC does not vary.
In the case of cooling, the liquor exits from the exchanger super-cooled and supersaturated and, if the installation is correctly designed, the loss of supersaturation in the pipes leading to the body is low (the transit time is between 2 and 3 s and does not exceed the metastable limit).
If we proceed by evaporation, the liquor exits the exchanger reheated and under hydrostatic pressure. As it rises to join the crystallizer, the pressure decreases, boiling occurs and the temperature decreases, approaching that of the body if the solubility increases slightly with temperature. On exiting the exchanger, the liquor is reheated, but ebullition occurs, and the liquor cools and becomes concentrated. It becomes supersaturated in the body where the greater part of ebullition occurs.
Between body exit and entry into the exchanger, secondary nucleation can occur by attrition in the pump.