Distillation is energy-intensive for solutions that form isomers and azeotropes. In these cases, crystallization can provide better separation and a more energy efficient process. Melt crystallization is the process of separating the components of a liquid mixture by cooling until a quantity of crystallized solid is deposited from the liquid phase. Therefore, melt crystallization is often considered to be commercially attractive, compared with distillation, for the separation of close-boiling organic substances. The relatively low energy demand of the freezing process and high selectivity of crystallization are regarded as the main advantages of melt crystallization . The operation at much lower temperatures than distillation is a further benefit, which makes it a very valuable operation for processing thermally unstable substances . The heats of fusion for the majority of compounds with industrial importance are by the factor 0.2 to 0.5 lower than the heats of vaporization. The example of water can demonstrate the low energy demand: the heat of vaporization is 2260kJkg-1, whereas the heat offusion is only 334kJkg-1.
However, in batch crystallization the energy requirements of heating and cooling of the whole crystallizer equipment has to be considered, which can reduce the energy benefits of melt crystallization compared with distillation .
A hybrid purification process consisting of a combination of crystallization with another separation process can provide an effective measure for the production of high purity products with much greater efficiency than the single process alone. An example is a distillation process as an initial separation step followed by the crystallization-based process as a second step to achieve the pure product. A typical hybrid purification process with a distillation column and crystallization
for the separation of diphenylmethane diisocyanate (MDI) is shown in Figure 7.23 .
Evaporative Crystallization from Solutions
In evaporative crystallization, the solution is heated and evaporated to remove the solvent from the solution. One of the disadvantages of evaporative crystallization is therefore its large energy requirement. Comparable to evaporation (see Section 7.5.2) there are three basic possibilities to save energy:
• feed preheating;
• multistage evaporation;
• vapor recompression.
The energy efficiency of the evaporative crystallization can be improved by feed preheating and reusing the hot vapor discharged from a crystallizer. In a typical process for crystallization of salt for example, usually three or four evaporative crystallizers are configured in a multistage concept, comparable to multistage evaporation. The energy efficiency of the multistage process can be improved further by vapor recompression and recovering the enthalpy from the condensed liquid (see Section 7.5.2).
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