(344b) Seeding Policy and Optimization of Batch Crystallization Processes

Two important components of a recipe for the operation of a seeded batch crystallization process are the seed properties and the temperature (or supersaturation) profile. When seeds are used in batch crystallization, in most cases the objective is to relieve as much of the generated supersaturation as possible by the growth of the seeds, which corresponds to minimizing the nucleated mass.

Doki et al. (2002) showed that at least for some systems, nucleation could be effectively suppressed by using a sufficient seed mass, called the critical seed loading, and that the required seed mass depends on seed size. Later, Tseng and Ward (2014) showed that the critical seed loading could be determined knowing only crystal nucleation kinetics, which permitted the determination of the critical seed loading ratio for a large number of chemical systems for which nucleation kinetics have been measured. In this contribution we show that similar analysis can be used to determine a feasible range of seed sizes and corresponding seed loadings that will generate product crystals of a desired size (or size distribution) with negligible nucleation.

Furthermore, extending previous work by Hofmann et al. (2010), we show that nearly-analytical solutions for the optimal supersaturation trajectory (and Praeto fronts for multi-objective cases) can be determined for a variety of different objective functions. The nearly-analytical nature of the results allows greater understanding of the effect of the objective function on the shape of the optimal trajectory. This method also requires only knowledge of the nucleation kinetics, and therefore can be applied to determine the optimal supersaturation trajectory for a large number of systems.

Combining the two methods together, we develop a systematic procedure for rapidly determining operating recipes for seeded batch crystallization with a small number of experiments.

References

Doki, N., N. Kubota, M. Yokota and A. Chianese (2002). Determination of critical seed loading ratio for the production of crystals of uni-modal size distribution in batch cooling crystallization of potassium alum. Journal of Chemical Engineering of Japan 35(7): 670-676.

Hofmann, S., and J Raisch (2010). Application of optimal control theory to a batch crystallizer using orbital flatness, 16th Nordic Process Control Workshop, Lund, Sweden, 25â??27 August 2010; Nordic Working Group on Process Control.

Tseng, Y. T. and J. D. Ward (2014). Critical seed loading from nucleation kinetics. AIChE Journal 60(5): 1645-1653.