Residence time of crystals in the installation
If we use one loop, the various residence times are as follows:
- - crystallizer body: 1-4 mn;
- - loop auxiliaries (velocity 2 m.s-1) and particularly:
- - pipes between the exchanger and the body: <3 s,
- - exchanger: 2-6 s.
We observe that, as expected, the residence time in the auxiliaries is short relative to the residence time in the body. The characteristics of the latter are determinant here, and the chief among these is volume.
For tangential entry, experience shows that the residence time of crystals is identical to that of the mother liquor if their size does not exceed 40 pm. For larger sizes, crystals do not follow the fluid streams and their residence time in the installation depends on their size:
Segregation occurs inside the crystallizer. There is no segregation in the auxiliaries since the flow here is of the piston type at a high velocity.
In general, we can consider that the k coefficient is proportional to AP = Pc-Pl
With regard to the axial entry, the larger crystals are projected upwards (within the liquor), taking time to descend so that the k coefficient is lower or even zero.
Entry level in a vaporization body
When the level of slurry within the body is greater than 50 cm or more at the level of the entry tube, a fraction of the arriving slurry does not reach the zone above the tube. This fraction of slurry immediately descends into the exit cone without being freed of all the heat corresponding to its superheating. This is followed by an increase in the circulating slurry temperature and a decrease in the exchanger’s LMTD (logarithmic mean temperature difference). The thermal transfer power of the exchanger is impaired as a result.
However, we cannot have the tube arrive too close to the slurry surface if we wish to avoid vapor spurts, deposits and encrustation on the walls of the gas space.
Thermal short-circuiting is minimized if we choose axial entry.