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Abstract

The manufacture of chemical products applied either for the promotion of human health or to combat pests which otherwise adversely impact on the human food supply is increasingly cencerned with enantiomeric purity. Despite of significant achievements in the area of asymmetric synthesis there is a demand in developing efficient resolution methods. Although powerful and flexible, resolution methods employing selective crystallization are still far from predictable. The intention of the lecture is to discuss several aspects which are relevant in order to design and optimize crystallization processes which should deliver pure enantiomers. At first a brief overview will be given regarding solid liquid equilibria of eanatiomeric systems and possible types of phase diagrams. Based on this, possible concepts for perfoming a crystallization process will be introduced. The focus of this presentation will be set on crystallization in solution. Options to work in different regions of the phase diagram will be explained. Results of two case studies will be presented. These studies were devoted to separate enantiomers of threonine and mandelic acid. The enantiomers of threonine form a conglomerate. In contrast, the enantiomers of mandelic acid can form a racemic compound. In both cases batch crystallization was perfomed under different operating conditions. To have well defined starting conditions seed crystals were used. In order to describe the observed transients quantitatively it was attempted to estimate growth rates. The results of systematic investigations revealed strong kinetic interactions. A simplified model of a batch crystallizer was applied in order to simulate possible process configurations. Main focus was set on studying variants of performing crystallization by entrainment which is a technique capable to resolve directly conglomerates. Finally, possible process schemes will be explained which combine an enrichment step (based on another separation method, e. g. chromatography) with a subsequent crystallization step. Such hybrid processes provide attractive options to resolve the frequently encountered systems forming racemic compounds.