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Enantioseparation using crystallisation by entrainment : some engineering aspects


Elsner,  M. P.
Physical and Chemical Foundations of Process Engineering, Max Planck Institute for Dynamics of Complex Technical Systems, Max Planck Society;

Lorenz,  H.
Physical and Chemical Foundations of Process Engineering, Max Planck Institute for Dynamics of Complex Technical Systems, Max Planck Society;

Seidel-Morgenstern,  A.
Physical and Chemical Foundations of Process Engineering, Max Planck Institute for Dynamics of Complex Technical Systems, Max Planck Society;
Otto-von-Guericke-Universität Magdeburg, External Organizations;

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Elsner, M. P., Lorenz, H., & Seidel-Morgenstern, A. (2003). Enantioseparation using crystallisation by entrainment: some engineering aspects. Poster presented at AIChE Annual Meeting 2003, San Francisco, USA.

The separation of chiral compounds is of large interest because most of the (bio-)organic molecules are chiral and usually only one of the enantiomers exhibits the wanted properties with regard to therapeutic activities or metabolism, whereas the other enantiomer may be inactive or may even cause some undesired effects. For this reason enantiomeric separations have become increasingly important and their application ranges from the pharmaceutical and food industry to the agricultural industry. Generally, chromatographic as well as special reactive methods rank among the common separation processes. An attractive alternative to these methods is the enantioselective crystallisation by the "entrainment" effect. 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 steady state 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 [1] and on its application to separate special chiral systems [2]. Existing studies on preferential crystallisation have usually been developed for certain substances not following methodological pathways. The work to be presented is rather focussed on the concept by itself from the engineering point of view with the intention to understand basic fundamentals and to elucidate general aspects of application. Thus, the goal of our work is to study the preferential crystallisation process in a quasi-continuous operation mode. 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 [3] and the metastable zone width) and knowledge of the temporal concentration changes of the enantiomers during the process are required for successful operation and for mathematical description of the process. Preliminary experimental results using a newly developed technique of online polarimetry in combination with refractometry 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. Results obtained in isothermal experiments for different crystallisation conditions (supersaturation, temperature and enantiomeric excess) in a batch as well as in a quasi-continuous set-up will be shown. The potential and the limitations of this process will be discussed in detail. Finally, based on the experimental results some simple semi-empirical models will be presented and evaluated as a first theoretical approach. [1] Beilles, S., Cardinael, P., Ndzié, E., Petit, S., Coquerel, G. (2001), Preferential Crystallisation and comparative crystal growth study between pure enantiomer and racemic mixture of chiral molecule: 5-ethyl-5-methylhydantoin, Chem. Eng. Sci. 56, 2281-2294 [2] Courvoisier, L., Mignot, L., Petit, M.N., Sprendgard, U., Hedtman, U., Coquerel, G. (2002), Preferential crystallization of (+/-)-5(4'-bromophenyl)-5-methylhydatoin. Comparison between SIPC and AS3PC processes at 2 l and 10 l scales, Proceedings of the 9th International Workshop on Industrial Crystallization BIWIC 2002, Halle-Wittenberg, September, 11th - 12th 2002 [3] Lorenz, H., Sapoundjiev, D., Seidel-Morgenstern, A. (2003), Solubility Equilibria in Chiral Systems and Their Importance for Enantioseparation, Eng. Life Sci. 3, 132-136