Conceptual design of crystallization-based separation processes

A systematic method is presented to synthesize crystallization-based separation schemes. It is assumed that all the streams in the processes are at equilibrium so that the composition can be tracked unit-by-unit on a phase diagram. A procedure to calculate phase diagrams for molecular, ionic, and reactive systems is given. The diagrams are also used to identify thermodynamic barriers which block the complete separation of a mixture and to identify how these barriers change with operating conditions. When the use of a complete phase diagram is impractical, cuts and projections are used. Multicomponent systems can be represented graphically in this manner. A coordinate transform is used to represent reactive systems so that only regions are plotted where both phase and reaction equilibrium are simultaneously satisfied. A different transform, one based on ionic concentrations, is used to represent salt solutions. The method is used to synthesize flow sheets which combine crystallization, distillation, evaporation, and extraction. First, the use of fractional crystallization to separate conjugate salt systems is discussed. Three classes of separation are identified based on the characteristics of the phase diagram and the composition of the feed. Second, drowning-out crystallization is used to show when liquid-liquid phase splits can be utilized to increase crystal yield. The use of decanters and multistage extraction is discussed as well as when to place the crystallizer before the extractor in the equipment train. Third, reactive crystallization processes are investigated. Examples are given which use temperature and pressure swings to selectively crystallize the desired products and to continuously recycle the reaction byproducts. Fourth, crystallization and distillation are combined to provide a solventless-based separation technique. Process alternatives are given to bypass both azeotropes and eutectics. The economics of these processes are compared to distillation as well as to both fractional and adductive crystallization. Fifth, the synthesis of crystallization-based processes is extended to multicomponent systems. The use of solvent swings, phase splits, and reactions are investigated