Growth kinetics of L-asparagine monohydrate in racemic aqueous solutions as well as nucleation, growth and dissolution kinetics of the same enantiomer crystallized from pure Lasparagine solutions are measured and the kinetic parameters are estimated applying a recently developed shortcut-method. The corresponding experimental procedure is based on a small number of preferential (seeded) cooling crystallizations where the crystal size distribution is monitored with an online microscope. Afterwards, image analysis yields the transient particle size evolution of initially provided crystals, from which the response of the solid phase to the liquid phase driving force can be extracted. Subsequently, parameter estimation is carried out applying this data together with the information of concentration and composition of the liquid phase, to discriminate between different model approaches. The kinetics are validated finally with independent experiments to evaluate their quality. It is proven that growth kinetics of L-and D-asparagine monohydrate from water are identical. In contrast, it can be shown and quantified, that growth kinetics from racemic and enantiopure solutions of asparagine differ significantly from each other. The corresponding calculation of the driving force of enantiomeric systems is discussed in detail by means of ternary phase diagrams.
A recently developed continuous enantioseparation process utilizing two coupled fluidized bed crystallizers is systematically investigated to identify essential correlations between different operation parameters and the corresponding process performance on the example of asparagine monohydrate. Based on liquid phase composition and product crystal size distribution data, it is proven that steady state operation is achieved reproducibly in a relatively short time. The process outputs at steady state are compared for different feed flow rates, supersaturations, and crystallization temperatures. It is shown that purities >97% are achieved with productivities up to 40 g/L/h. The size distribution, which depends almost exclusively on the liquid flow rate, can be easily adjusted between 260 and 330 µm (mean size) with an almost constant standard deviation of ±55 µm.
Continuous preferential crystallization is an innovative approach to the separation of chiral substances. The process considered in this work takes place in a gently agitated fluidized bed located in a tubular crystallizer. The feasibility of the process has been shown in previous work, but it also turned out that choosing suitable operation conditions is quite delicate. Hence, a model based process design is desirable. Existing models of the process are rather complicated and require long computational times. In this work, a simple linear dynamic model is suggested, which captures the main properties of the process. The model is distributed in space and in a property coordinate. Using the method of characteristics, a semi-analytical solution of the linear model is derived. As a challenge to the solution, there is a recycle loop in the process that causes a feedback and couples the boundary conditions at different boundaries of the computational domain. In order to deal with this, a numerical scheme is suggested. The semi-analytical solution provides a deeper insight into the process dynamics. A comparison with a more detailed mathematical model of the process and with experiments shows strengths and limitations of the linear model.
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