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Abstract:
Compared to other enantioseparation techniques (e. g. chromatography) crystallisation by the "entrainment" effect can be a cheap alternative to obtain high enantiopurities. As is generally known such systems also tend to reach equilibrium in which the liquid phase will have racemic composition and the solid phase will consist of a mixture of crystals of both enantiomers. However, before approaching this state of equilibrium it is possible to preferentially produce just one of the enantiomers after seeding with homochiral crystals under particular conditions. The process is based on the different initial surface areas of each enantiomer and the specific driving forces due to the different supersaturations. The potential of this so-called "preferential crystallisation" as an effective and alternative technology for the production of pure enantiomers has been the subject of some considerable academic attention in the recent years with an emphasis on its chemistry and on its application to separate special chiral systems.
The focus of our investigation is rather concentrated on the concept by itself from the engineering point of view with the intention to understand basic fundamentals and to elucidate some general aspects of application. Thus, the aim of our work is to study the preferential crystallisation process in a quasi-continuous operation mode which seems to be much more interesting for any kind of industrial application. In first experiments the amino acid threonine was used as a model system. Reliable thermodynamic data (the ternary solubility phase diagrams of threonine in the used solvents), the metastable zone width as well as the knowledge of the temporal concentration changes of the enantiomers during the process are required for successful and effective operation and for a mathematical description of the process. Experimental results using a newly developed technique of online polarimetry in combination with offline/online refractometry and an inline FBRM probe proved the general applicability of this process. The technique suggested enables the direct monitoring of the resolution progress and the recognition of the region of "safe" resolution. Preliminary results obtained in isothermal experiments under different crystallisation conditions (supersaturation, temperature and enantiomeric excess) in a batch as well as in a quasi-continuous configuration will be presented. The potential and the limitations of this process will be discussed in detail.
