Optimization and seeding strategy in batch crystallization processes

This thesis is concerned with the development of recipes for seeded batch crystallization processes. Two major components of such recipes are considered: the growth rate profile (which is related to the superaturation profile) during the batch and the seeding policy. The effect of seeding policy is investigated first. For some crystallization systems, it is possible to almost completely suppress nucleation by using a sufficient quantity of seeds, called the critical seed loading ratio. An analytical short-cut equation is proposed to determine critical seed loading ratio in terms of the nucleation kinetics. Results from the short-cut equation show high consistency with results from simulation using detailed process models. The shortcut equation is further applied to determine the critical seed loading ratio for a set of about thirty crystallization systems. A similar analysis is also applied to determine the feasible range of seed crystal sizes and corresponding seed loading given a desired product crystal size, and the method is also applied to the case of continuous seeding during the batch. In the second part of this thesis, the optimization of the growth rate is investigated. Pontryagin’s Minimum Principle (PMP) is applied to solve the optimization problem. Nine different objective functions based on the moments of the product crystal size distribution are considered. Multi-objective optimization problems are also solved and discussed in this work. Results are compared with standard shortcut trajectories such as the linear and cubic temperature profiles. It is shown that the optimization problem can also be solved knowing only the nucleation kinetics and the optimal growth rate trajectory is determined for the same set of crystallization systems considered in the first part of the thesis.