Continuous secondary nucleation

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

Evidence for secondary nucleation has came from the early continuous MSMPR studies. MSMPR crystallization kinetics are usually correlated with supersaturation using empirical expressions of the form... 

The polymerization of more hydrophilic monomers is also possible, as shown for MMA and vinyl acetate [36, 56, 73]. In the case of monomers with a pronounced water solubility, the nucleation in water should be efficiently suppressed in order to avoid secondary nucleation in the water phase. This can be achieved, e.g., by using an oil-soluble initiator and the polymerization of acrylonitrile or by adding a termination agent to the continuous phase. A typical calorimetric curve of MMA polymerization using a hydrophobic initiator shows a fast conversion. 

The addition of an antisoivent can be earned out in different ways, as indicated in Fig. 9-1, where the concentration of product is shown on the ordinate and the amount of antisoivent added is shown on the abscissa. A typical equilibrium solubility curve is indicated as A-B-C. (This curve could be concave or linear but is shown as convex for clarity.) The metastable region is indicated as the area between B-C and E-D. From point A to point B, addition of antisoivent will proceed without crystallization because the solution concentration is below the equilibrium solubility. At point B, equilibrium solubility is reached. As the addition of antisoivent continues, supersaturation will develop. The amount of supersaturation that can be developed without nucleation is system specific and will depend on the addition rate, mixing, primaiy and/or secondary nucleation rate, and growth rate, as well as the amount and type of impurities present in solution. 

This polymerization is governed by multiple simultaneous mechanisms such as particle coalescence and break-up, secondary nucleation and the diffusion of monomer to the interface, as Sdnchez-Silva et al. exposed in their study. The collective effect of these mechanisms confers the size, the structure, and the surface properties to the microcapsules. It was developed a method based on a free radical polymerization suspension process to fabricate nonpolar MPCM by Sanchez et al. Also, these authors studied the influence of the temperature in a reaction, stirring rate and the mass ratio of paraffin to styrene on the thermal properties of MPCM. More studies using this process were done by Borreguero et al., who considered two main steps a continuous one with deionized water and the stabilizer (polyvinyl-pyrrolidone, PVP), and a discontinuous one containing the styrene monomer, the paraffin wax, and the benzoyl peroxide. 

Even though significant improvements in product CSD can be made by employing controlled operation, the benefits are often diminished because fine crystals are created by secondary nucleation (Mullin and Nyvlt, 1971 Jones and Mullin, 1974). In an attempt to combat this problem, the effects of fines destruction (FD), commonly employed to improve product CSD in continuous crystallizers (section 9.1.2) was investigated by Jones, Chianese and Mullin (1984). 

Crystallization is the competition between two processes nucleation and crystal growth. Nucleation is the formation of small sites (nuclei) from which crystallites can grow. Primary nucleation creates the initial nuclei. Crystallites develop aroimd these nuclei. Then in secondary nucleation, the surfaces of the crystallites are nucleated. More polymer chains diffuse to the crystallite surfaces and growth continues. 

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