Abstract
An attractive process for gaining pure enantiomers from racemic mixtures is the so-called preferential crystallization (PC) which has been realized up to now usually in a discontinuous operation mode. For elucidating the principle of a continuous enantioselective process one might consider a suspension crystallizer revealing MSMPR characteristics, i.e. a perfectly mixed tank (concerning both phases), which is continuously fed with a solution possessing a racemic composition of two enantiomers. Solid particles and liquid phase are continuously withdrawn. By a continuous supply of homochiral seed crystals of the preferred target enantiomer the preferential crystallization (PC) of only this enantiomer is initialized, i.e. growth of the seed crystals and possibly secondary nucleation of crystals of the seeded enantiomer, provided the crystallization takes place within the metastable zone where spontaneous, uncontrolled primary nucleation is kinetically inhibited. During a starting-up period, which strongly depends on the properties of the system as well as on the process parameters, the concentration of the target enantiomer is decreasing until a steady state is reached where the composition is determined by the mean residence time. Due to different kinetic mechanisms and their inherent different time constants, a different depletion of the supersaturation for each enantiomer can be realized by an appropriate choice of the process conditions. As long as a critical mean residence time, where primary nucleation may appear, is not exceeded, the concentration of the undesired counter enantiomer remains constant during the whole time. This fact reveals a benefit of this continuous process in comparison to the batch one. An optimal selection of the process conditions allows a constant production of the goal enantiomer at a high purity level.
In this work, a dynamic mathematical model is derived for continuously operated ideally mixed preferential crystallizer equipped with a fines dissolution loop. The fines dissolution is included as recycle streams around the MSMPR crystallizer. Moreover, primary heterogeneous and secondary nucleation mechanisms along with size-dependent growth rates are taken into account. Different seeding and operating strategies are numerically investigated. The high resolution finite volume scheme is employed to solve the model. Several numerical case studies are carried out. To judge the quality of the process some goal functions are used, such as product purity, productivity, yield and mean crystal size of the preferred enantiomer. These goal functions give detailed information about the success and potential of continuous preferential crystallization. The results obtained results could be used to find the optimum operating conditions for improving the product quality and for reducing the operational cost of continuous preferential crystallization. Altogether, the process appears to possess large potential and deserves practical realization which is currently in progress.
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