(449u) Seeding Effect on the Purification of Enantiomers By a New Technique Combining Distillation and Crystallization

Stripping crystallization (SC) is a new separation technique which combines distillation and crystallization. Basically, SC is operated at a triple-point condition, in which the liquid is simultaneously vaporized and crystallized due to the three-phase equilibrium. In essence, the process is continued until the liquid phase is completely eliminated and only the pure solid crystals remain in the feed. Thus, crystal washing is not required since only pure solid crystals remain in the feed and no impurities are adhered on the crystal surfaces at the end of SC. In the previous research, SC has been successfully applied to separate p-xylene from the mixed xylenes by Shiau et al. (2005, 2008, 2013).

Ibuprofen, also called 2-(isobutylphenyl)-propionic acid, is a non-steroidal anti-inflammatory drug widely used to treat headaches and minor pains. Although it is known that S-ibuprofen is responsible for the anti-inflammatory effects while R-ibuprofen might cause some long-term side effects, ibuprofen is still sold as a racemic mixture in the market due to the complexity involved in the separation of enantiomers. The experiments show that subcooling of the liquid ibuprofen mixture is observed during the temperature-decreasing process. Ultrasound mixing or magnetic stirring only slightly reduces subcooling. However, once crystallization is induced in the subcooled liquid mixture, crystals are formed abruptly, leading to a product with a lower purity. Due to the structure similarity between S-ibuprofen and R-ibuprofen, R-ibuprofen might be incorporated into the S-ibuprofen crystal lattice when S-ibuprofen crystals are formed abruptly. Therefore, the ibuprofen mixture can not be further purified by SC without seeding.

Subcooling can be nearly eliminated by seeding with ultrasound mixing or magnetic stirring during the temperature-decreasing process. Subsequently, crystals are gradually formed preferentially on the seeds, leading to a product with a higher purity. Thus, seeding with ultrasound mixing or magnetic stirring can be applied to further purify the ibuprofen mixture. As magnetic stirring helps to uniformly disperse the seeds, seeding with magnetic stirring leads to a product with a higher purity compared to seeding with ultrasound mixing.

References:

1. Shiau, L. D.*, Wen, C. C. and Lin, B. S. â??Separation and purification of p-xylene from the mixture of m-xylene and p-xylene by distillative freezingâ?, Ind. Eng. Chem. Res, 44(7), p2258-2265, 2005.

2. Shiau, L. D.*, Wen, C. C. and Lin, B. S. â??Separation of p-xylene from the multicomponent xylene system by stripping crystallizationâ?, AIChE Journal, 54(1), p337-342, 2008.

3. Shiau, L. D.*; Liu, K. F. �Investigations into the effects of the cooling rate on stripping crystallization�, Ind. Eng. Chem. Res, 52, p1716-1722, 2013.

Keywords: crystallization; vaporization; purification; thermodynamics process; ibuprofen