Impurities crystal growth rates

Because impurities most often result in reduced crystal growth rate, feedstocks to laboratory and bench-scale units should be as similar as possible to that expected in the full-scale unit. The generation of impurities in upstream process units can depend on the way those units are operated, and protocols of such units should follow a consistent practice. It is equally important to monitor the composition of recycle streams so as to detect any accumulation of impurities that might lead to a reduction in growth rates. 

Crystallization from an overall viewpoint represents transfer of a material from solution (or even a gas) to a solid phase by cooling, evaporation, or a combination of both. But there is more to it. Of considerable importance are economics, crystal size distribution, purity, and the shape of the crystals. Impurities or mother solution are carried along only in the surface or occlusions in the crystals. The partical size distribution depends on the distribution of seed crystals, which are injected into the crystallizer prior to initiation of crystallization (batch) or continuously from recycled undersized particles, the mixing in the system, the crystal growth rate, and the degree of supersaturation of the mother liquor. As in shown in the figures, both batch and continuous crystallization are used in industry. 

For solvent systems with a window of operating temperature, proper selection of the method of supersaturation generation (e.g., cooling and antisolvent addition) and mode of crystallization (e.g., batch vs. semicontinuous) can also affect the overall crystal growth rate. In many instances in which solvent or impurity rejection becomes critical, adequate mixing to avoid local high supersaturation can be critical. Examples 9-2 and 10-4 illustrate two cases of rejection of impurities and residual solvent. These examples show how various means are applied to overcome these complications. 

The ultimate particle size distribution (PSD) from crystallization is dictated by the balance between nucleation and crystal growth rates. The processes indicated above, because of very high supersaturation, often result in rapid nucleation of too many particles and smaller than desired final product. The rapid nucleation and growth occurring may also result in impurity and/or solvent occlusion. 

As we have seen in the previous section, crystalline materials can be characterized in terms of their crystal structures. A given chemical species, however, can have more than one possible crystal structure. The phenomena of a chemical species having more than one possible crystal form is known as polymorphism. The term allotropism is used to describe elements that can form more than one crystal form. Materials crystallize into different crystal forms as a function of the conditions of growth (temperature, pressure, impurity content, growth rate, etc.). 

Typically, crystal habits predicted based on crystal chemistry alone are best compared with crystals grown from sublimation processes, or to solution-based systems where the solvent/impurity interactions are negligible. In fact, significant deviations from structure-based predictions are often best explained by such solvent and impurity interactions (Davey et al. 1992 Winn et al. 2000). There are significant efforts underway to understand such phenomena at the molecular level and to consequently predict the influences of solvent and impurities on crystal growth rates. This will be further highlighted in subsequent sections. 

Many industrial crystallization processes, by necessity, push crystal growth rates into a regime where defect formation becomes unavoidable and the routes for impurity incorporation are numerous. Since dislocations, inclusions, and other crystal lattice imperfections enhance the uptake of impurities during crystallization, achieving high purity crystals requires elimination of impurity incorporation and carry-over by both thermodynamic and non-thermodynamic mechanisms. Very generally, the impurity content in crystals can be considered as the sum of all of these contributions... 

Black and Davey used Eq. (3.22) to study the effect of the tailor-made additive L-glutamic acid on L-asparagine monohydrate crystals. With the use of a linear adsorption isotherm, Eq. (3.22) fit the crystal growth rate data. Consistent with a structural model in which impurities are embedded in the growing crystal surface, the growth rate of the crystals tended to zero at a high L-glutamic level. 

Another example of the crystallization of chemically related compounds is found in the commercial separation of fructose from the impurity difructose dianhydride (Chu et al. 1989). Fructose undergoes irreversible dehydration during the crystallization process to yield several forms of difructose dianhydride impurities. Since the difructose dianhydride molecule consists of two fructose moieties, it exhibits some of the chemical and structural features of the host fructose molecule. In an analogous fashion to a tailor-made additive, the difructose dianhydride impurities appear to incorporate into the crystal (at < 1 wt% level), thus inhibiting the subsequent adsorption and growth of fructose molecules. The resulting fructose crystal growth rates are so low that the crystallization time in fructose manufacture is often on the order of days. 

Small amount of impurities sometimes retard dramatically the crystal growth rate. Chromium(III), for example, suppresses the crystal growth of potassium sulfate(i), ammonium dihydrogen phosphate(2) and ammonium sulfate (5), etc. in aqueous solutions. Other metallic ions, iron(III), aluminium(III) are also effective impurities W... 

In some cases, the crystal growth rate in the presence of impurity decreases gradually over several tens of minutes after an impurity is introduced into the system, and finally reaches a steady state value or zero value. Experimental data of this unsteady state impurity action have been reported fragmentarily in the literature (2, J, 5). But no theoretical explanation has been given so far. 

Thirdly, the other compounds may lead to an impure product due to inclusions that contain the mother liquor or to adhesion onto the crystal surface. Furthermore, other compounds may influence the mass transfer phenomena of the crystallizing substance, which leads to the need for a better understanding of multicomponent diffusion [13], Other components may adsorb onto a certain facet of the crystal surface, thereby affecting the crystal growth rate and the final crystal shape. 

Impurities ean influence crystal growth rates in a variety of ways. They can change the properties of the solution (structural or otherwise) or the equilibrium saturation eoneentration and hence the supersaturation. They ean alter the eharaeteristies of the adsorption layer at the erystal solution interface and... 

It describes the thermodyrranric equiUbrium between the concentrations of impurities X in the solid and y, in the solution. The thermodyrranric eqirilibriirm is orrly achieved at low crystal growth rates v 0. In case of systems without formation of mixed crystals this distribution coefficient should ideally be close to zero. In reahty, however, the crystallized solid will not possess a distribution coefficient of... 

Crystal growth rates are approximately the same in all directions, but crystals are never spheres. Crystal growth rates and sizes are controlled by limiting the extent of supersaturation, S = C/C ra, where C is concentration, usually in the range 1.02 < S < 1.05. Growth rates are infiuenced greatly by the presence of impurities and of certain specific additives that vary from case to case. 

The crystal growth rate increases virtually linearly with the degree of supersaturation of the solution (Fig. 7-24). The rate increases with temperature and relative velocity, Wr and decreases with increasing viscosity of the solution. Furthermore, the rate is influenced by the pH value and the impurities in the solution, and is different for each interfacial area with the same substances (see crystal form, crystal habit in [7.1, 7.3]). The process of reducing supersaturation is described in detail in [7.38, 7.39, 7.44-7.47]. 

To account for deviation from thermodynamics, under real conditions an effective distribution coefficient ke F is defined (Equation 7.3). The equation is similar to Equation 7.2, but the effective distribution coefficient results from parameters measured under real crystallization conditions. That is, the parameters Xir,ip s as the impurity content in the solid phase and ximp.i, as the impurity content in the liquid phase are values obtained from the separation process performed. In contrast, the parameters in Equation 7.2 are directly related to the phase diagram. Thus, the effective distribution coefficient also comprises the influence of the crystallization kinetics, in particular the crystal growth rate and mass transfer limitations. 

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