ABSTRACT
A. MERSMANN Technische Universita¨t Mu¨nchen, Garching, Germany
In addition to the median crystal size, important quality parameters include the crystal size distribution (CSD) and the coefficient of variation (CV). Furthermore, the shape of crystals and their purity play an important role in commercial products. The rates of nucleation, growth, agglomeration, and attrition determine the size distribution and median size L50 of crystals produced in batch or continuously operated crystallizers. For the sake of simplicity, let us consider an (MSMPR) mixed suspension, mixed product removal crystallizer for which the median crystal size L50 depends on the growth rate G, the nucleation rate B, and the volumetric holdup ’T [see equation (4.2.34)]:
L50 ¼ 3:67 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi G
6B0 ’T
s ð0:1Þ
The previous chapters have shown that despite the general occurrence of growth-rate dispersions for primary and secondary nuclei as well as for large crystals, some general statements can be made concerning a large number of
crystals growing in a solution free of impurities and additives. This is not the case with respect to the rates of nucleation because the origin of the nuclei can differ greatly. In general, the rate of nucleation is the sum of the rates of homogeneous ðBhomÞ, heterogeneous ðBhetÞ, and secondary ðBsec) nucleation:
B0 ¼ Bhom þ Bhet þ Bsec withBsec ¼ Ba þ Bsurf ð0:2Þ The rate of secondary nucleation may be subdivided according to various mechanisms, such as cluster detachment, shear stress, fracture, attrition, and needle breaking, or according to the different types of collision, such as crystal-crystal collision and crystal-rotor collision. It is presumed that secondary nucleation dominates in systems of high solubility due to the fact that a coarse product is usually obtained.
