What to Learn from Phase Diagrams

Phase diagrams provide fundamental information about the physico-chemical interaction between the substances in a mixture to 

Eutectic forming mixtures provide a theoretical opportunity to achieve one component by crystallization that has 100% purity. This is of course only possible if, on the one hand, the crystal growth rates are so extremely slow that perfect crystals are created and, on the other hand, the necessary solid-liquid separation after the growth process is also perfect. 

Solid solutions, which represent the other extreme in the phase diagrams, show that only an enrichment of one component is possible. Therefore, a number of steps would be necessary to achieve the desired purity. 

In chemical engineering practice, however, no separation process yields a perfect separation. Therefore, the effectiveness of the process has to be considered. This means more steps are necessary than indicated by the equilibrium phase diagram. 

If the melting points of the components are too close, the separation will be difficult for solid solutions. If the feed composition of an eutectic forming mixture is at or near the eutectic point, then no or little product can be recovered. This is true for solution crystallization as well unless the solvent-solute interaction is far from ideal, resulting in different activity coefficients for the components. In such cases other separation technologies should be used. 

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