Crystallization secondary nucleation

Secondary nucleation is an important particle formation process in industrial crystallizers. Secondary nucleation occurs because of the presence of existing crystals. In industrial crystallizers, existing crystals in suspension induce the formation of attrition-like smaller particles and effectively enhance the nucleation rate. This process has some similarity with attrition but differs in one important respect it occurs in the presence of a supersaturated solution. 

After the bath attained its equilibrium temperature, the crystallizer was charged with about 400 ml of liquid and was inserted into the bath. After about 30 minutes the system attained a constant temperature and a subcooling (difference of equilibrium temperature and constant temperature before initiation of crystaUization) was established. Introduction of the seed crystals ( ter being allowed to warm for a period of a few seconds) on the specially prepared stirrer initiated crystallization (secondary nucleation) and resulted in a change in the temperature of the crystallizer (figure 2). The temperature of the crystallizer attained an uilibrium value of a few minutes after nucleation occurred. The concentration of the sucrose solutions was measured using a refractometer (.1% accuracy). 

Keywords entanglement, disentanglement, cross-hatching, lamellae, crystallization, nucleation, reptation, nucleation (crystallization) regimes, nucleation agents, nucleation rate, spherulitic growth rate, Avrami-equation, Ozawa-equation, isothermal crystallization, nonisothermal crystallization, secondary nucleation, supercooling. 

Polymer crystallization theory is a mature area, and there are several review articles available that present and discuss the different theories in great detail (eg, 103,104). Having said that, over the last 5 years or so there has been a flurry of new interest becanse of the increase in computational power, which has the potential to decisively enter the debate in some areas. In the following the underlying themes of the two principle theories of polymer crystallization, secondary nucleation theory and rough-surface or entropic barrier theory, are outlined. The results of more recent simulations are then briefly discussed, in which the constraints of the above theories, introduced to provide analytical solutions, have been relaxed. Finally, some of the more fundamentally different ideas that have recently appeared are discussed. 

When a folded layer on the surface of a crystal has finished growing, a new nucleus needs to form on the surface for continued growth of the crystal. Secondary nucleation requires a high undercooling because it has a low temperature dependence. The secondary crystallization layers are completed by an attachment-detachment mechanism [1]. 

The process of crystallization of a polymer from the melt can be divided in three stages (1) primary nucleation (2) growth of the crystals (secondary nucleation) (3) secondary crystallization. 

Deviations from the Avrami equation are frequently encountered in the long time limit of the data. This is generally attributed to secondary nucleation occurring at irregularities on the surface of crystals formed earlier. 

Secondary nucleation is crystal formation through a mechanism involving the solute crystals crystals of the solute must be present for secondary nucleation to occur. Thorough reviews have been given (8,9). 

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. 

Seed crystals grow and participate in secondary nucleation by contact mechanisms throughout run... 

Secondary nucleation (induced by presence of existing crystals)... 

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... 

A number of authors have developed mechanistic descriptions of the processes causing secondary nucleation in agitated crystallizers (Ottens etal., 1972 Ottens and de Jong, 1973 Bennett etal., 1973 Evans etal., 1974 Garside and Jancic, 1979 Synowiec etal., 1993). The energy and frequency of crystal collisions are determined by the fluid mechanics of the crystallizer and crystal suspension. The numbers of nuclei formed by a given contact and those that proceed to survive can be represented by different functions. 

Although programmed cooling crystallization clearly results in a larger mean crystal size than that from natural cooling it is also evident that some fines i.e. small crystals are also present in the product. Since the solution was seeded these fine crystals must clearly have arisen from crystal attrition or secondary nucleation (see Chapter 5). 

Botsaris, G.D., 1976. Secondary nucleation A review. In Industrial Crystallization, Ed. J.W. Mullin. Plenum Press New York, pp. 3-22. 

Chianese, A., Mazzarotta, B., Huber, S. and Jones, A.G., 1993. On the Effect of Secondary Nucleation on the Size Distribution of Potassium Sulphate Fine Crystals from Seeded Batch Ci-ystallization. Chemical Engineering Science, 48, 551-560. 

Secondary nucleation requires the presence of crystalline product. Nuclei can be formed through attrition either between crystals or between crystals and solid walls. Such attrition can be created either by agitation or by pumping. The greater the intensity of agitation, the greater... 

As pointed out in Chapter 10, another way to create secondary nucleation is to add seed crystals to start the crystals growing in the supersaturated solution. These seeds should be pure product. 

As mentioned previously, scale-up of crystallization processes from the laboratory is far from straightforward. Various parameters need to be maintained to be as close to those used in the laboratory as possible in order to reproduce the results from the laboratory. For scale-up, supersolubility, agitation (and its effect on secondary nucleation throughout the vessel), fraction of solids in the slurry, seed number and sizes, contact time between growing crystals and liquid all need to be maintained. 

Before discussing the effect of short-chain branching on the kinetics of crystallization process, it is necessary to revisit the theory of secondary nucleation and the concept of regimes as given by Lauritzen and Hoffmann... 

Figure 15 Schematic representing the secondary nucleation process and growth at the crystal face by spreading. The rates of these two processes are / and g, respectively. (See Color Plate Section at the end of this book.)...

Secondary nucleation is essentially a crystal growth process. Secondary nucleation occurs by the deposition of a stem of the polymer molecule on a preexisting crystal-face as shown in Figure 15. The overall rate of this process is given by the following expression [58],... 

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