Growth rate solvent effect

The use of tailor made additives holds great promise in the area of crystal growth and morphology control. The routine selection and use of these type of additives will require a fundamental understanding of the mechanism which the additives work on a molecular basis. At the same time, the effect of solvent molecules on the crystal growth process is another related and important problem. In both instances, the relationship between internal aystal structure, aystal growth rate, solvent and impurities are needed to predict the habit of a crystal and thus allow seleaion of the proper conditions and components required to obtain a desired habit... 

The solvent from which a material is crystallized influences crystal morphology and growth rate. These effects have been attributed to two sets of factors. One has to do with the effects of solvent on viscosity, density, and dif-fusivity and, therefore, mass transfer. The second factor is concerned with the structure of the interface between crystal and solvent a solute-solvent system that has a high solubility is likely to produce a rough interface and, concomitantly, large crystal growth rates. 

Economic Aspects. The 1992 MEK nameplate capacity for the United States, East Asia, and Western Europe is Hsted in Table 5. During the period 1980—1989 MEK achieved a negative growth rate as demand dropped from 311,000 (48) to 228, 000 t/yr (49). Stricter VOC regulations were largely responsible for the decline, and the trend will continue as solvent recovery and recycling, as well as substitution away from MEK, take effect. 

Effects of Impurities nd Solvent. The presence of impurities usually decreases the growth rates of crystalline materials, and problems associated with the production of crystals smaller than desired are commonly attributed to contamination of feed solutions. Strict protocols should be followed in operating units upstream from a crystallizer to minimize the possibiUty of such occurrences. Equally important is monitoring the composition of recycle streams so as to detect possible accumulation of impurities. Furthermore, crystalliza tion kinetics used in scaleup should be obtained from experiments on solutions as similar as possible to those expected in the full-scale process. 

As discussed in section 2.4.4 the coordinating ability of a solvent will often affect the rate of nucleation and crystal growth differently between two polymorphs. This can be used as an effective means of process control and information on solvent effects can often be obtained from polymorph screening experiments. There are no theoretical methods available at the present time which accurately predict the effect of solvents on nucleation rates in the industrial environment. 

Figure 16.5. Supersaturation behavior, (a) Schematic plot of the Gibbs energy of a solid solute and solvent mixture at a fixed temperature. The true equilibrium compositions are given by points b and e, the limits of metastability by the inflection points c and d. For a salt-water system, point d virtually coincides with the 100% salt point e, with water contents of the order of 10-6 mol fraction with common salts, (b) Effects of supersaturation and temperature on the linear growth rate of sucrose crystals [data of Smythe (1967) analyzed by Ohara and Reid, 1973],...

Impurity(ies) can have a profound effect on the growth rate and, in this case, on the approach to equilibrium solubility. As in many cases due to experimental time constraints, the actual cause of the improvement was not clearly established. A very effective method of determining the impact of impur-ity(ies) from the process in question is to recrystallize the compound from a pure solvent(s) and from its own mother liquor and compare crystal size, growth rate, and morphology. 

Solvent occlusion is shown to be directly related to the ratio of nucleation rate over crystal growth rate. To minimize the occlusion, fine seed, extended addition time, and better mixing were found to be effective. 

Lastly, the mass transport processes at the crystal-liquid interface play a central role in crystallization. The influence of solvent and impurities on the structure and growth rates of faces is considered in this chapter along with its effect on the incorporation of impurities. The solvent solute-impurities interactions in solution will also be shown to interact in subtle, but important, ways with the interface during the crystallization process. With appropriate thermodynamic analysis it is shown how these interactions ultimately affect crystallization as a purification process. 

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