Batch preferential crystallization, purity

Batch crystallizers, applications, 102 Batch preferential crystallization, purity decrease of L-threonine crystals in optical resolution, 251-259 Bayer proce description, 329 Benzene, hi -pressure crystallization from benzene-cydohexane mixture, 281-289... 

In the optical resolution of DL-threonine mixtures by batch preferential crystallization, changes of solution concentration and crystal purity were measured. The mechanism of nucleation of the un-seeded enantiomer was discussed to explain the purity decrease of the resolved crystals. From the observation of crystallization behavior of the seed crystals of L-threonine, it was concluded that the existence of the D-enantiomer on the surface of the seed caused the sudden nucleation when they grew to attain sufficient amounts. 

Purity Decrease of L-Threonine Crystals in Optical Resolution by Batch Preferential Ciystalli tion... 

Another form of a direct crystallization is a preferential crystallization, which is also called resolution by entrainment of one enantiomer. The process involves solution formation, e.g., under elevated temperature that will then be subcooled and seeded with crystals of the preferred species. Preferential crystallization is a potent method for the improvement of the product quality and at the same time achievement of a higher yield and productivity. For example, by preferential crystallization of DL-threonine from the aqueous solution, it is possible already during a simple batch to gain highly pure enantiomers up to a certain point after which the opposite enantiomer nucleates from the solution and leads to a purity drop. If (—)-hydrobenzoin seed crystals are added to an ethanol solution of ( )-hydrobenzoin, the ( )-enantiomer will crystallize out and after 15 cycles, 97% optical purity can be obtained. 

Occurrence of Spontaneous Crystallization of an Unseeded Enantiomer. When we consider the purity drop on the phase diagram, it is the prerequisite that the seeds added to cause the preferential resolution are completely pure. However if the seeds are contaminated by the undesired enantiomer, the phenomena could be completely different. Since in general seeds are taken from a previous batch, they are likely to have been contaminated by the adhered mother liquor due to incomplete phase separation or by some non-equilibrium crystallization phenomena such as inclusion of mother liquor or agglomeration of crystalline particles. For the case of the former, drying of the crystals generates small particles of the undesired enantiomer which will then be introduced to a racemic solution as seeds. 

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