Crystallization is an attractive alternative for separating close boilers, high melting compounds, and heat sensitive materials that are difficult or impossible to separate by distillation. A large percentage of chemical systems exhibit simple eutectic or compound formation solid-liquid equilibrium (SLE) behavior, which allows crystallization of a pure product. However, some chemicals – especially those with nearly the same molecular shape and size – may form solid solutions. While melt crystallization has been commonly used for isolating high purity products from mixtures with solid solution behavior, it is often not applicable for high melting compounds such as diastereomeric salts or heat sensitive materials such as amino acids. In addition, the presence of eutectic points prevents complete separation. One way to circumvent these problems is to use an appropriate crystallization solvent, which helps to reduce the viscosity, operating temperature, and heat and mass transfer resistance, as well as to overcome any eutectic limitation.

This presentation focuses on the synthesis and design of crystallization-based separation processes for systems involving solid solution and solvent, based on the SLE phase behavior of the system. A generic process is proposed, which consists of two major parts: fractionation to obtain streams that are rich in a desired product and purification to get the final product with the desired purity. SLE phase diagrams are used to guide the selection of suitable unit operations and detemination of operating conditions. Examples are presented to illustrate various scenarios, including different separation objectives (isolating one product vs. separating two components from their mixture) and different phase behavior (such as formation of hydrates).