(376bn) Purification of L-Menthol Enantiomers from the Racemic Mixture By Stripping 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).

Menthol is widely used in perfumery, cigarette, cough drops, and nasal inhalers for its cooling effect and refreshing flavor. Only L-menthol has been reported to have analgesic effect whereas D-menthol is lack of analgesic and flavor properties. Although natural L-menthol is obtained by distillation from the leaves of various subspecies of mint, menthol can also be synthesized via a number of routes, by which a racemic mixture is obtained. Thus, separation and purification of L-menthol from the racemic mixture has been an important research issue. A thermodynamic model is developed in this work to simulate the three-phase equilibrium during the SC operation and to direct the batch SC experiments. The experiments show that no crystallization occurs during SC without stirring due to subcooling of the liquid enantiomers. Stirring can eliminate subcooling during SC, leading to the L-menthol crystals gradually formed. The experimental results, including the final enantiomeric purity and recovery ratio of L-menthol, are consistent with the simulation results predicted by the model.

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

References:

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

2. Shiau, L. D.*, Wen, C. C. and Lin, B. S. “Separation of p-xylene from the multicomponent xylene system by stripping crystallization”, 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.