Secondary Nucleation. It is often observed that agitation during
crystallization enhances the number of crystals formed, which presumably
implies a higher nucleation rate. This may be due to a number of
mechanisms. When heterogeneous nucleation occurs at the surface of (parts
of) the equipment, agitation will greatly increase the volume of liquid that
comes into contact with such surfaces. Intensive stirring may lead to
breaking of newly formed crystals upon impact, or in ‘‘scraping off’’ small
pieces from a crystal. True secondary nucleationmeans that nuclei are
formed in thevicinityof a crystal of the same phase (not at thesurface). It
can occur in the absence of agitation.
A clear example of such quiescent secondary nucleation is provided by
some natural fats, i.e., mixtures of several triglycerides. Consider Figure
14.6a. At 15 8 C, the number concentration of catalytic impurities observed
was about 3? 1016 m^3 , which means one impurity per 30mm^3. A droplet of
FIGURE 14.7 Crystallization from solution. Approximate example of the
nucleation rate, the average linear crystal growth rate, and the average crystal size
resulting, as a function of supersaturation lnb. Arbitrary linear scales.