Crystallization Methods and Supersaturation

Crystallization only occurs when supersaturation is created that acts as the driving force for crystallization. The crystallization method is mainly chosen on the basis of the thermodynamic and physical properties of the compound and the solvent, as well as on the required purity of the product. There are several ways to represent the phase diagram, depending on the mode of crystallization. For evaporative and cooling crystallization a solubility diagram is mostly used, in which the solubility of the compound is expressed as a function of temperature. In precipitation the solubility is always very low, and the solubility product at the operating temperature is needed. In antisolvent precipitation, the solubility diagram mostly has to be determined for the particular three-phase system. In melt cooling crystallization T-x diagrams are used, while for melt pressure crystallization pT-x diagrams are needed.

A decision scheme for the selection of the appropriate crystallization method is presented in Figure 1. For a high purity product or when the use of a solvent poses environmental or safety problems, melt crystallization is chosen. A melt temperature lying between 0 and 100°C is preferred, since at higher temperatures many organic compounds become unstable, while at temperatures below 0°C operating the process becomes more difficult, although not impossible. A high viscosity of the melt hampers the operation.

At solubilities below about 1 mass %, the process is designed as a precipitation process to obtain a reasonable production rate. The supersaturation that is generated by the mixing of reactants often reaches high values. Also in antisolvent crystallization, low solubility of the solute and high supersaturation are reached by mixing two solvents.

Finally, for the more easily soluble substances the choice between cooling and evaporative crystallization

Design information

Feed composition Thermodynamics Physical properties

Figure 1 Decision diagram for choosing the method of crystallization.

Specifications

Purity Yield

Average size is made on the basis of the solubility curve, and the prevailing supersaturations are generally low. Especially where water is the solvent, cooling crystallization is more favourable than evaporative crystallization, except for highly soluble substances. Therefore, multi-effect flash evaporation where the vapour is used for heating next crystallizer or the feed stream is frequently applied to reduce the energy costs, if evaporative crystallization is applied.

In cooling crystallization direct cooling, flash cooling or indirect cooling can be applied. Where indirect cooling is used encrustation on the cooling surfaces must be minimized. In flash cooling this problem is largely avoided at the expense of a more complicated installation. Also direct cooling uses an inert cooling medium or refrigerant that is bubbled directly into the solution and evaporates does not suffer from abundant incrustation, but needs recompression of the cooling medium.

So, in these various methods of crystallization, supersaturation is created by cooling, by evaporation of solvent or by a combination of the two in the case of flash evaporation or flash cooling, or by mixing two reactants or solvents. In all these cases the actual concentration is higher than the equilibrium concentration, and a driving force for crystallization is achieved.

From a thermodynamic point of view this driving force is reflected by the difference in chemical potential of the solute in the liquid and in the solid phase at temperature T:

For cooling crystallization from the melt or the solution, eqn [1] can be transformed into:

For melt pressure crystallization:

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