Optimized polythermal operation of coupled preferential crystallization

Kinetically controlled crystallization can be used as a unit operation to separate enantiomers that belong to the conglomerate forming type [1]. This process is carried out in a region of metastability where selective crystallization of the target enantiomer is induced by seeding. Nucleation of the 50% impurity (i.e. the unseeded enantiomer) poses a challenge, since it cannot be reliably predicted due to its stochastic nature. As a consequence, the product yield is usually low after one crystallization step and further cooling during the process is limited by the metastable boundary. The problem of nucleation can be circumvented by coupling two crystallizers via the liquid phase and seeding both with opposite enantiomers [2]. Thus, the respective impurity crystallizes in a separate vessel and the chance of unspecific nucleation is significantly reduced. It is therefore possible, to increase the yield by a suitable cooling policy, which is state of the art for most other crystallization processes. In this contribution, a moment model, based on a one-dimensional population balance model, was used to calculate optimal temperature trajectories for coupled preferential crystallization. During the optimization, the product mass of each seeded enantiomer was to be maximized, considering purity constraints. Subsequently, the generated cooling profiles where validated experimentally. [1] Lorenz H., Perlberg A., Sapoundjiev D., Elsner M.P., Seidel-Morgenstern A., Crystallization of enantiomers. Chemical Engineering and Processing, 2006, 45, 863-873. [2] Elsner M.P., Ziomek G., Seidel-Morgenstern A., Simultaneous preferential crystallization in a coupled batch operation mode. Part II: Experimental study and model refinement. Chemical Engineering Science, 2011, 66(6), p. 1269-1284