Theoretical and experimental investigations of coupling chromatography with preferential crystallization for enantioseparation

In the recent years, the rise of effort for improving of already established separation process has been motivated by the necessity of separation of the racemic mixtures to the enantiomeric pure components. The most commonly used classical resolution methods are chromatographic, crystallization and membrane technologies. Enantioselective preferential crystallization [1] is an attractive, inexpensive method for separation of mixture that crystallizes as a conglomerate i.e., a physical mixture of crystals where each crystal is enantiomerically pure. In solution such systems tend to reach an equilibrium state 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, it is possible to preferentially produce just one of the enantiomers after seeding with homochiral crystals. Perfectly mixed batch crystallizers can be described mathematically using a dynamic, one dimensional model which includes experimentally determined kinetic parameters. Such a model was applied in order to describe the process for the D, L-threonine-H2O system [2]. Based on the simplified approach, attractive and more effective operation modes using several crystallizers can be studied e.g., two crystallizers coupled via liquid phase where in each vessel one of both enantiomers is crystallizing simultaneously and an exchange of the liquid phase between the crystallizers (see Fig. 2) leads to an increase of the process productivity [3]. Preferential crystallization processes are sometimes carried out with an initial enantiomeric excess of the preferred enantiomer. Fig. 2 shows that by an increase of the initial enantiomeric excess higher process productivity might be obtainable using a coupled batch operation mode. The intention of this work is the analysis and optimization of the process combination of chromatography and simultaneous preferential crystallization [4]. The chromatography will be used to gain certain amount of pure enantiomer required for a given enantiomeric excess. The optimized process parameters for both separate processes as well as for the hybrid process will be given in this presentation. Parallel to the theoretical analysis, an experimental validation of this process is currently performed and results will be also presented. [1] COLLINS, A.N., SHELDRAKE, G.N., CROSBY, J. (1994): Chirality in Industry: The Commercial Manufacture and Applications of Optically Active Compounds, John Wiley & Sons [2] ELSNER, M.P., FERNÁNDEZ MENÉNDEZ, D., ALONSO MUSLERA, E., SEIDEL-MORGENSTERN, A. (2005): Experimental study and simplified mathematical description of preferential crystallization, Chirality 17 (S1), S183-S195 [3] ELSNER M.P., ZIOMEK, G., SEIDEL-MORGENSTERN, A. (2007): Simultaneous preferential crystallization in a coupled, batch operation mode. Part I: Theoretical analysis and optimization. Submitted to Chemical Engineering Science. [4] KASPEREIT, M. (2006): Separation of Enantiomers by a Process Combination of Chromatography and Crystallisation. PhD thesis, Otto von Guericke University, Magdeburg