Crystallization is based on the formation of solid crystals precipitating from a solution or melt or more rarely deposited directly from a gas. The physical principle of crystallization is a mass transfer of a solute from the liquid solution to a pure solid crystalline phase. Supersaturation must be achieved before crystals can grow.
The supersaturated condition can be achieved by two main alternatives. One is to decrease the solution temperature carried out in cooling crystallization. However, if the dependence of the saturation concentration on the temperature is not large enough, this technique cannot be applied for crystallization. In such a case, evaporative crystallization can be used. In addition, supersaturation can be achieved by solvent shift, by pH shift or by a chemical reaction between two or more substances with precipitation of one of the reaction products .
Crystallizers are generally classified based on the technique by which supersaturation is achieved, for example, cooling, evaporation, vacuum (adiabatic cooling), reaction, salting out.
While many chemical processes can be optimized purely on the basis of energy considerations, in crystallization plants a number of process parameters must be taken into account. Thus the operating temperature, temperature gradients, the volume of the apparatus etc. are closely related to the kinetic data of the crystallization system and in certain cases can be varied only within very narrow limits.
The crystallization process (evaporation or cooling) is usually determined by the solubility properties of the product that is being crystallized. The first selection criterion is: If the saturation concentration has low temperature dependency then evaporation-crystallization should be carried out. If the saturation concentration varies strongly with temperature, then cooling crystallization should be used.
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