Installation possibilities of a CHC (population density)
1) Separate decanting of clean liquor.
Decanting the product at flow QP is performed on the lower part of the continuous homogenous crystallizer. Decanting of clean liquor is performed separately at flow QCL at the upper part of the classification zone.
The cutoff dimension is LF between the fines (decanted at flow QP + Qcl) and the product decanted at QP. The corresponding residence times are:
Figure 4.4. Separate decanting of clean liquor
As the dominant crystal size of the product is equal to LD = 3Gt, we can expect that this size will be increased as:
However, as growth G decreases little with residence time, the crystal size will increase less than proportionally with t.
2) Destruction of fines:
VCLA : overflow (fines + liquor)
QP: underflow (crystal slurry)
In the laboratory, with R = 2, we observe an increase of 15% in LDominant. The higher the R, the longer the residence time TD of the product. The residence time for fines decantation (L < LF) is:
For the product (L > Lf):
We accept that the no of seeds is identical with and without fines destruction. We also accept that tp is the same with and without fines destruction. However, growth GF with fines destruction is higher than that of GO without destruction as the crystals are larger.
Depending on whether L is greater than or less than LF, the variation law of n according to L is different:
For L = LF , there is no discontinuity of n(L). Accordingly:
Gf is greater than Go which means that with fines destruction, supersaturation is increased.
Figure 4.5 represents what has been explained above.
Figure 4.5. Fines destruction
Typically, the previous results are only verified for a continuous homogenous crystallizer without attrition, that is, in the laboratory.
3) Product classification:
After crystallizer decantation, slurry containing the product is cleared of fines in a classifying device that may be:
- - an elutriation column;
- - a hydrocyclone.
Compared with the crystallizer body, the product exits at the global flow zQp.
The fines leave in flow zVP but return in flow (z - 1) Qp, so, in total, decantation has the flow:
Figure 4.6. Product classification
The residence times are:
LC is the “cutoff dimension” of crystals in the classifier.
Hence, the expression of n(L):
Thus, the slope of the straight line corresponding to the product (Ln(n) = f(L)) is increased. The residence time of the product is reduced and consequently, so is its mean dimension. Size distribution is also tightened due to the elimination of fines in the product by classification.
On the other hand, the quantity of fines increases in the crystallizer, which tends to decrease supersaturation and also the mean size of the product.
This is why we often associate destruction of fines with product classification.
4) Destruction of fines and product classification:
The corresponding expressions for n = f (L) are: