Online microscopy has received much attention in the field of crystal shape control over recent years, since commonly used measurement techniques cannot provide enough information for this purpose. In this work, we present an estimation scheme that serves to reconstruct the 3D crystal shape from the measured 2D crystal projection. The boundary curves of the crystal projections are parametrized by Fourier descriptors, which are subsequently compared with a precomputed database. The procedure is evaluated in comparison with various effects that might impair the estimation. A good agreement between the true and estimated values is found in all cases. The presented methods are applied to batch cooling crystallizations of potassium dihydrogen phosphate (KDP) dissolved in water. As a result, the face specific growth rates are determined as a function of supersaturation. To validate the performance of this scheme, the calculated face specific growth rates are used in a model-based prediction of the supersaturation profiles, which agrees well with the experimental data.
An integrated process for the chiral separation of the industrially relevant substance 2′,6′-pipecoloxylidide (PPX), an intermediate in the manufacture of a number of anesthetics, was developed. By combining three different techniques, chromatography, crystallization, and racemization, high productivity was achieved. All unit operations were executed using a common solvent system, full recycling, and a minimum of solvent exchanges or removals. The target molecule was obtained with an enantiopurity of >99.5 wt %.
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.
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