Crystallization has been widely used for product separation and purification in the chemical, pharmaceutical and food industry because it can separate one or more components from a liquid mixture in pure solid form. However, to obtain these components with desired attributes has remained a challenge, owing to the difficulties in understanding the crystallization thermodynamics and kinetics of the system under consideration, as well as in utilizing such information for process development and synthesis. Compounded by the persistent pressure to meet the ever-changing market demand, developing systematic methods that can facilitate the synthesis of crystallization-based processes for producing crystals of required qualities are thus of practical significance. In this research, we achieve t...[ Read more ]

Crystallization has been widely used for product separation and purification in the chemical, pharmaceutical and food industry because it can separate one or more components from a liquid mixture in pure solid form. However, to obtain these components with desired attributes has remained a challenge, owing to the difficulties in understanding the crystallization thermodynamics and kinetics of the system under consideration, as well as in utilizing such information for process development and synthesis. Compounded by the persistent pressure to meet the ever-changing market demand, developing systematic methods that can facilitate the synthesis of crystallization-based processes for producing crystals of required qualities are thus of practical significance. In this research, we achieve this goal by developing methods that rely on solid-liquid equilibrium (SLE) phase diagrams and workflow application to tackle problems in crystallization thermodynamics and kinetics.

Three different projects are used to illustrate the important role of SLE phase diagrams on understanding crystallization processes and how phase diagram-aided process synthesis can effectively lead to the separation of crystals of targeted component(s). The focus of the first project is placed on the determination of thermodynamic data of a simplified multicomponent salt lake system, which comprises Li⁺, Na⁺, K⁺, Mg²⁺//Cl^-, SO₄^2- - H₂O at 25°C and 1 atm. A method integrating three interrelated activities - representation of phase behavior as a phase diagram/thermodynamic model, experimental measurement of necessary SLE data, and visualization of high-dimensional phase diagrams - is presented to show how solid-liquid phase behavior data salient for process synthesis can be obtained in a systematic and objective-oriented manner. Application of the resulting phase diagram to synthesize a recovery process for salts of high lithium content is also briefly discussed. The second project demonstrates the course of developing a crystallization-based process that can simultaneously separate high-purity C₆₀ and C₇₀ solids from a fullerene mixture with the scientific information obtained from the ternary phase diagram of C₆₀, C₇₀ and o-xylene. The practicability of such a process is verified by a batch experiment, in which C₆₀ and C₇₀ fullerene solids of purity greater than 99 wt% can be produced from a commercially-available fullerene mixture that is pretreated with adsorption to remove higher fullerenes. In the third project, we design and build up a bench-scale prototype of a novel apparatus, namely, evaporative column crystallizer, and demonstrate its feasibility in separating C₆₀-rich solid solution into a solid phase of high purity of C₆₀ and a liquid phase enriched with C₇₀ using the solvent o-xylene under semi-batch operations. The effect of solvent feed rate on the results of separation is investigated and the SLE phase diagram of C₆₀, C₇₀ and o-xylene is used to explain the change in the experimental results.

In addition to chemical identity, the product crystal population is also required to meet specifications on other attributes, such as size, size distribution, impurity level and shape, which are mainly governed by crystallization kinetics, i.e. the nucleation and growth rate of crystals. The application of seeding technique is regarded as the most effective way to control such kinetics-related attributes of product crystals. A workflow that systematically guides the decision making for seeding application in batch crystallization is thus presented in the fourth project of this thesis, with the aim of minimizing the time and effort to get to the product crystals of desired attributes. The heart of the workflow lies on two causal tables that summarize the qualitative relationships between kinetic phenomena and attributes of product crystals and those between seed qualities/seeding practice and kinetic phenomena, respectively. Execution of the workflow is illustrated with an example, in which paracetamol crystals of specified mean size and impurity level are to be obtained from cooling crystallization in water in the presence of 4-nitrophenol, an impurity.