Batch preferential crystallization, purity optical resolution

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... 

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

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. 

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. 

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