Crystallization metastable region

Some guidelines have been provided for defining the metastable region. If the seed crystals dissolve when added to the metastable solution, this implies that saturation conditions have not been reached. If the addition of the seed leads to the formation of an oil dispersion, it may be concluded that supersaturation has been realized (Anderson, 2000). 

In this section, a brief description of the necessary experiments to identify the kinetic parameters of a seeded naphthalene-toluene batch crystallization system is presented. Details about the experimental apparatus and procedure are given by Witkowski (12). Operating conditions are selected so that the supersaturation level is kept within the metastable region to prevent homogeneous nucleation. To enhance the probability of secondary nucleation, sieved naphthalene seed particles are introduced into the system at time zero. 

Limiting Supersaturation for Nucleation. Like sucrose, D-fructose solutions can tolerate a high degree of supersaturation without nucleating, even in the presence of seed crystals. This is the metastable region on the Miers supersolubility diagram (9). 

Occasionally, long-range disorder and/or different phases may coexist within a crystalline material. Arrangement of molecules in the different regions will necessarily be different in at least some respects. One of the earliest reports of invocation of this phenomenon involves the photodimerization of anthracene in the crystalline state [219]. In the crystal structure of anthracene, the faces of no molecules are separated by <4 A. Yet upon irradiation, a dimer is readily formed. Thomas, Jones, and co-workers used electron microscopy to reveal the coexistence inside normal anthracene crystals of regions of a metastable phase. In the minor phase (space group PI), the C9- -C9. distance is 4.2 A, whereas in the stable crystal it is 4.5 A. The dimerization is proposed to originate in the minor phase of the crystal. 

Crystallization can occur only from supersaturated solutions. Growth occurs first by formation of nuclei and then by their gradual growth. At concentrations above supersaturation, as at point d on Figure 16.1(d), nucleation is conceived to be spontaneous and rapid. In the metastable region, nucleation is caused by mechanical shock or friction and secondary nucleation can result from the breakup of already formed crystals. It has been observed that the rate depends on the extent of supersaturation thus... 

Figure 3.4a shows a normal crystallization curve with the spinodal (supersaturation limit) curve (CD) and equilibrium curve (AB). At point P neither nuclei nor crystal growth will occur since the solution is superheated by the amount RP. Once the saturation line (AB) is crossed, either through cooling or increase in concentration, nuclei and crystals may or may not form in the metastable region. Metastable point Q is shown between point R and the crosshatched line CD. 

FIGURE 3.20 Successive cooling curves for hydrate formation with successive runs listed as Sj < S2 < S3. Gas and liquid water were isochorically cooled into the metastable region until hydrates formed in the portion of the curve labeled Sj. The container was then heated and hydrates dissociated along the vapor-liquid water-hydrate (V-Lyy-H) line until point H was reached, where dissociation of the last hydrate crystal was visually observed. (Reproduced from Schroeter, J.R, Kobayashi, R., Hildebrand, M.A., Ind. Eng. Chem. Fundam. 22, 361 (1983). With permission from the American Chemical Society.)... 

When the goal is the production of fine particles it is important to save the primary size of the crystals as they appear first in the solution, i.e. the nucleation has to be promoted over the growth and aggregation steps. In the case of undersaturated, weak initial solutions the precipitation takes place near the metastable region where the kinetic processes are rather slow. For example, the induction time which was necessary for crystallization in the weakest NaCl solutions approaches to 60 min. Repeating the precipitation (where the ethanol content was the same 99.6%) with saturated aqueous solution there was no measurable induction time and the particle size changed considerably within the applied 60 min operational time the d was 4.41 p,m after 10 min, 8.86 pm at 20 min and finally 16.27 pm at 60 min. It is obvious that in the latter case not the smallest available size was measured after 60 min, but for the sake of comparison the same operational time had to be applied. 

To ensure that all the overall crystal-growth rate coefficients are measured under the conditions without nucleation, the metastable region of the solution has to be determined first and therefore the solubility and super solubility need to be measured. 

The method used for the measurement of the metastable region is also the traditional one, i.e., using the light scattered by the newly formed fine crystals to detect the nucleation status. The experimental equipment is shown in Fig. 12.1. The water bath tank is surrounded with a light-proof black screen, with two windows vertical to each other with dimension of 2x3 cm for light beam casting and visual observation, respectively. To parallel the industrial condition of induced nucleation, all the measurements are carried out under conditions with the existence crystal seeds. 

Wu, Yuan (1985). Metastable region and crystal-growth rate of commercial trisodium phosphate. Chemical Engineering. 13(4) 41-47, 26, 1985 (in Chinese). 

In the crystallization of melts, where relatively large degrees of supersaturation are attainable, nucleation and growth phenomena are more easily separated and studied experimentally than in crystallization from solution, which is characterized by rather narrow metastable regions. However, the basic concepts of nucleation are the same in both types of processes. In fact, much of the experimental verification of nucleation theory has come from studies of condensation and precipitation from the vapor phase. The highly publicized rainmaking experiments of several years ago made significant contributions (S6). 

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