Drowning-out crystallizations

Case study 2 Drowning-out Crystallization Solvent Design... 

Table 3 Design results of optimal solvent for drowning out crystallization...

If the solute solubility is relatively temperature-independent, add heat to remove the solvent or add an antisolvent to drown out crystals. For evaporation, 14 to 20 g vapor evaporated/s-m exchanger area. For the exchangers, use 2.5-cm dia. tubes with fluid velocities 1.5 to 3 m/s to minimize plugging. Caution if the vapor pressure rise >3.4 kPa/°C, then potential problems with control. 

Figure 9.23 Batch crystallization by nonsolvent dilution (drown-out crystallization).

Sometimes a drowning-out medium is added to a solution. It reduces the solubility of the solute and hence leads to supersaturation. This is called drowning-out crystallization. The solubility of many aqueous solutions of inorganic salts can be reduced by the addition of organic solvents (e.g., acetone, methanol). In reactive crystallization two or more reactants form a product which is less soluble and therefore crystallizes. For example, reactions between an acid and a base lead to the precipitation of a solid salt. This is called precipitation crystallization. However, it should be mentioned that this term is neither clearly defined nor uniformly used. 

Fleischmann, W. Mersmann, A. Drowning out crystallization of sodium sulphate using methanol, in Proc. 9 Symp. on Industrial Crystallization (S. J. Janie and E. J. de Jong, eds.) Elsevier, Amsterdam (1984)... 

As a further example, referring to crystallization from solution, in Figure 7.8 the depletion behavior of three different impurities for a slow and fast drowning-out crystallization of the drug Abecarnil are compared. Not surprisingly, the purification during the slow crystallization is superior to that of the precipitation. This effect is found for all impurities, although most pronounced for the impurity (B). 

Figure 12.4 (a) Path of a classical and an inverse drowning-out crystallization in the solubility diagram and (b) mixing of solution and antisolvent on the droplet level. The diffusion of... 

The presence of the second solvent during drowning-out crystallization has a marked effect on the crystallization kinetics and consequent particle... 

The concept of programmed operation can also be applied to other types of batch crystallization e.g. precipitation via drowning-out with miscible solvents (Jones and Teodossiev, 1988). 

Figure 9.3 Potassium sulphate crystals formed by. a) cooling, (h) drowning-out with acetone Jones etai, 1987)...

Berry, D.A., Dye, S.R. and Ng, K.M., 1997. Synthesis of drowning-out based crystallization separations. American Institute of Chemical Engineers Journal, 43, 91. 

For drown-out, look for MSA in which the component to be crystallized has low solubility, while other components have high solubility (MSA should be completely miscible with feed mixture). 

Acid Pasting. The crude pigment is dissolved in a massive excess of concentrated sulfuric acid, from which it is then reprecipitated in a controlled manner by drowning out the acid solution into iced water to obtain the red-shade, crystallizing-grade, alpha copper phthalocyanine blue. 

Figure 12.1 Geometries for the addition of components in a drowning-out or pH-shift precipitation. Depending on the local supersaturation, either large crystals (left) or smaller particles (right) are obtained. The local supersaturation is a function of intensity of mixing.

Figure 12.9 Crystals of an active compound crystallized by an antisolvent crystallization using supercritical carbon dioxide. In the case shown, an inverse drowning-out mixing scheme was used, leading to small particles.

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