Nucleation secondary

Secondary nucieation results from the presence of crystals in the supersaturated solution. These parent crystals have a catalyzing effect on the nucieation phenomena, and thus, nucieation occurs at a lower supersaturation than needed for spontaneous nucieation. Although several investigations of secondary nucieation exist, the mechanisms and kinetics are poorly understood. 

Several theories have been proposed to explain secondary nucieation. These theories fall into two categories—one traces the origin of the secondary nuclei to the parent crystal—that include (1) initial or dust breeding (2) needle breeding and (3) collision breeding. Secondary nuclei can also originate from the solute in the liquid phase and the theories that take this into account include (1) impurity concentration gradient nucieation and (2) nucieation due to fluid shear. 

TABLE 2.7 Surface Tension Factor for Heterogeneous Nucleation 

The impurity concentration gradient theory assumes that the solution is more structured in the presence of a crystal. This increases the local supersaturation of the fluid near the crystal, which is the source of crystal nuclie. Changes in the structure of the solution near the crystal surface have been observed experimentally. Dissolved impurities in the solution are known to inhibit nucleation rates. Some of the impurities are incorporated into the crystal surface. Thus, a concentration gradient is formed that enhances the probability of nucleation. Experimental evidence of the theory was presented for the nucleation of potassium chloride in the presence of lead impurities. As expected, stirring the solution causes the impurity concentration gradient to disappear and hence, lower the nucleation rates (Denk 1970). 

Origin of Secondary Nuclei. The various theories proposed in the previous section for secondary nucleation show that secondary nuclei originate either from the seed crystal or from the boundary layer near the growing crystal. 

Secondary nucleation results from the presence of solute particles in solution. Recent reviews [16,17] have classified secondary nucleation into three categories apparent, true, euid contact. Apparent secondary nucleation refers to the small fragments washed from the surface of seeds when they are introduced into the crystallizer. True secondary nucleation occurs due simply to the presence of solute particles in solution. Contact secondary nucleation occurs when a growing particle contacts the walls of the container, the stirrer, the pump impeller, or other particles, producing new nuclei. A review of contact nucleation, frequently the most significant nucleation mechanism, is presented by Garside and Davey [18], who give empirical evidence that the rate of contact nucleation depends on stirrer rotation rate (RPM), particle mass density, Mj , and saturation ratio. 

Typical values of 6 lie between 0.5 and 2.5. These values are much lower than the t5q)ical ones for primary nucleation by either homogeneous or heterogeneous mechanisms, where b values between 6 and 12 are more common. The importance of is first order (i.e.,y = 1), suggesting that contact of the crystals with the walls and impeller is the important phenomenon. However, some systems (i.e., K2SO4 [19] and KCl [20]) have much lower values of j 0.4. Typical values of h range from 0 to 8 but most fall between 2 and 4, which are expected from semi-theoretical models [21]. 

After a particle is nucleated, it can grow by various mechanisms. The kinetics of these growth mechanisms are important in determining the resultant particle structure and size distribution. In the next section, we will discuss the more common growth mechanisms. 

One of the mechanisms of secondary nucleation is the mechanism of Interphase layer. At the solid surface there are a more or less oriented clusters that may be removed by fluid shear back into the bulk of solution 143,188,2161. These clusters. If they are of the critical size, can survive and form new nuclei. 

Some admixtures call forth formation of rough surfaces or even dendrites [100]. Due to fluid dynamic forces or due to partial dissolution these dendrites can be removed back to the bulk of solution, where they serve as new nuclei [61,140]. 

In systems where the admixture can easily be incorporated into the growing crystals lattice, the so-called impurity concentration gradient can be effective [22,611. Nucleatlon in the bulk of solution is hindered due to presence of the admixture at high concentration. Incorporation of the admixture into the crystal lattice leads to a decrease of Its concentration close to the surface so that spontaneous nucleatlon in the Intermediate layer becomes possible again. Presence of growth-restralners also exhibits an effect on nucleatlon [126 (they enlarge the metastable zone width [210]). 

A supersaturated solution nucleates much more readily, i.e. at a lower supersaturation, when crystals of the solute are already present or deliberately added. The term secondary nucleation will be used here for this particular pattern of behaviour to distinguish it from so-called primary nucleation (no crystals initially present) discussed in section 5.1. There have been several comprehensive reviews of the literature on secondary nucleation (Strickland-Constable, 1968 Botsaris, 1976 de Jong, 1979 Garside and Davey, 1980 Garside, 1985 Nyvlt et al., 1985). 

Among the early papers on this subject may be mentioned the work of Ting and McCabe (1934) who demonstrated that solutions of magnesium sulphate nucleated in a more reproducible manner at moderate supersaturations in the presence of seed crystals. Similar observations were made in studies with copper sulphate (McCabe and Stevens, 1951). 

Strickland-Constable (1968) described several possible mechanisms of secondary nucleation, such as initial breeding (crystalline dust swept off a newly introduced seed crystal), needle breeding (the detachment of weak outgrowths), polycrystalline breeding (the fragmentation of a weak polycrystalline 

Clontz and McCabe (1971) showed that at moderate levels of supersaturation, crystal contacts readily caused secondary nucleation of MgS04 7H2O, but crystal-crystal contacts gave up to five times as many nuclei as did crystal-metal rod contacts. Furthermore, the faster growing faces produced fewer nuclei than did the slower growing faces (Johnson, Rousseau and McCabe, 1972) indicating a connection between secondary nucleation and the crystal growth process. 

Several hydrodynamic models of secondary nucleation in agitated crystallizers were applied to experimental data obtained from a 6-L agitated batch crystallizer using potassium sulphate by Shamlou, Jones and Djamarani (1990). They concluded that the secondary nuclei were produced by an attrition process with a turbulent fluid-induced mechanism with critical eddies in the 

Once primary nuclei are formed the ensuing spherulites grow radially at a constant rate. Primary crystallization, which occurs initially on the surface of the primary nucleus and then on the surface of the growing lamellar, also involves a nucleation step, secondary nucleation. It is this step that largely governs the ultimate crystal thickness and which forms the focus of most kinetic theories of polymer crystallization. 

FIGURE 10-31 Schematic diagram showing the parameters used in describing secondary nucleation. 

Polymer crystal growth is predominantly in the lateral direction, because folds and surface entanglements inhibit crystalliza- 4 don in the thickness direction. Neverthe-1 less, there is a considerable increase in the fold period behind the lamellar front during crystallization from the melt and, as we have j seen, polymers annealed above their crys-tallization temperature but below Tm also irreversibly thicken. Nevertheless, in most theories of secondary nucleation, the most i widely used being the theory of Lauritzen and Hoffman,28 it is assumed that once a part of a chain is added to the growing crystal, its. fold period remains unchanged. 

Experimental data on melt crystallization are summarized in Fig. 3.67 in form of the linear crystal growth rate, v. All linear crystal growth rates go exponentially to zero when the temperature approaches the melting temperature, pointing to a nucleation which is similar as developed in the Figs. 3.51-61. These experiments, thus, provide strong evidence that some kind of secondary nucleation must be found to account for the crystal growth of macromolecules [27]. 

6ummary of Linear Crystal Growth Rates from the Melt as Function of Supercooling 

2 on one of the smooth surfaces before the next layer can be grown. The linear crystal growth rate is given by the number of new layers nucleated per second. 

Although this model does not fit well for macromolecules since it neglects the interconnection between the crystallizable units along the molecule, it has been used to develop nucleation models as shown in Fig. 3.69. As soon as a ledge is created on the surface, growth is assumed to occur by adding new molecular segments (stems) 

Two final points need to be made about secondary nucleation. First, that screw-dislocation defects, described in more detail in Sect. 5.3, prodnce indesttnctible secondary nuclei for growth on top of the fold surfaces of polymer lamellae. This surface would otherwise be inactive for further growth and restrict polymer crystals to single lamellae (see Chap. 5). An example of a series of screw dislocations is shown in Fig. 3.72 on the example of poly(oxyethylene) of 6,000 molar mass grown 

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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. 

Supersaturation is modest and secondary nucleation occurs by contact mechanisms througout run... 

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

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

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. 

Several modes of secondary nucleation have been identified (see Garside and Davey, 1980 for a review) ... 

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. 

So, following Botsaris (1976), the rate of secondary nucleation is given by... 

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. 

Garside, J. and Jancic, S.J., 1979. Measurement and scale-up of secondary nucleation kinetics for the potash alum-water system. American Institute of Chemical Engineers Journal, 25, 948. 

PloB, R. and Mersmann, A., 1989. A New Model of the Effect of Stirring Intensity on the Rate of Secondary Nucleation. Chemical Engineering Technology, 12, 137-146. 

We have studied the effect of monomer concentration in the dispersion polymerization of styrene carried out in alcohol-water mixtures as the dispersion media. We used AIBN and poly(acrylic acid) as the initiator and the stabilizer, respectively, and we tried isopropanol, 1-butanol, and 2-butanol as the alcohols [89]. The largest average particle size values were obtained with the highest monomer-dispersion medium volumetric ratios in 1-butanol-water medium having the alcohol-water volumetric ratio of 90 10. The SEM micrographs of these particles are given in Fig. 15. As seen here, a certain size distribution by the formation of small particles, possibly with a secondary nucleation, was observed in the poly-... 

The size distribution of the final product may become wide since the monodispersity cannot be controlled due to the formation of the copolymer particles with a secondary nucleation. 

In the secondary nucleation stage, the remaining amorphous portions of the molecule begin to grow in the chain direction. This is schematically shown in Fig. 16. At first, nucleation with the nucleus thickness /i takes place in the chain direction and after completion of the lateral deposition, the next nucleation with the thickness k takes place, and this process is repeated over and over. The same surface nucleation rate equation as the primary stage can be used to describe these nucleation processes. 

Particle formation in the early stages of a batch reaction is normally quite rapid. Hence the particle surface area produced is able to adsorb the free emulsifier quite early in the reaction (2 to 10% conversion) and particle formation ceases, or at best slows to a very low rate. Particles formed in the beginning of the reaction would have approximately identical ages at the end of the batch reaction. These particles would be expected to be nearly the same size unless flocculation mechanisms, stochostic differences, or secondary nucleation factors are significant. 

The seeding-growth procedure is a popular technique that has been used for a century to synthesize metal particles in solution. Recent studies have successfully led to control the dimensionality of the particles where the sizes can be manipulated by varying the ratio of seed to metal salt [23-25]. The step-by-step particle enlargement is more effective than a one-step seeding method to avoid secondary nucleation [26,27]. This mechanism involves a two-step process, i.e. nucleation and then successive growth of the particles as illustrated in Scheme 1. 

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... 

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