Fundamental studies in melt crystallization

While solid-liquid separations have been in use for many years, new constraints (environmental as well as economic) and the advent of specialty chemicals have led to a greater interest in these separation systems as process alternatives. Useful to the evaluation of these systems as design alternatives is an equation-based design procedure. Developing such a design procedure for melt crystallization systems requires the equilibrium behavior of the system to be investigated and understood before the complications of crystal growth kinetics are included. This hierarchical approach to the development of the design procedure allows for simpler models (i.e., the equilibrium calculations) to be evaluated and used to reduce the number of design choices. A model is developed for calculating solid-liquid phase equilibria of compound-forming systems. The method treats the compound as a new component containing stoichiometric amounts of the constituent species. The overall method is quite easy to implement and successfully reproduces the phase diagrams for a variety of complex mixtures, including organic melt systems and compound semiconductor systems. This technique is used as the basis for calculating crystallization path maps for ternary equilibrium crystallizers in which both compounds and eutectics occur. Crystallization path maps are similar to residue curve maps in VLE systems, and help to determine overall system behavior especially with regard to feasibility and design of melt crystallization processes. This phase equilibrium model is incorporated into a model of the kinetics of crystal growth in a suspension melt crystallizer. The kinetic model tracks the crystal size (for a monodisperse distribution of crystals), linear growth rate, the rate of solid formation, composition in both the liquid and solid phases, and the temperature of the bulk liquid for a given cooling policy for the crystallizer. A sensitivity analysis of the kinetic model is performed. Also, the phase equilibrium and kinetic models are applied to two example systems and the implications for design are discussed