Growth Kinetics of S-Mandelic Acid in Aqueous Solutions in the Presence of R-Mandelic Acid

In this work, the effect of R-mandelic acid on the growth kinetics of S-mandelic acid in aqueous solutions is investigated. Isothermal seeded batch experiments are conducted inside the two phase region where crystals of the S-enantiomer are at equilibrium with an enantiomerically enriched liquid phase. Crystal growth of S-mandelic acid is studied at three different temperature levels, i.e., at 15, 20, and 25 degrees C, and at three different levels of concentration of the R-enantiomer, i.e., at 0 g/g of water, 0.0231 g/g of water, and 0.0393 g/g of water. A previous study reports a very narrow metastable zone width (MSZW) for secondary nucleation (Perlberg et al. Ind. Eng. Chem. Res. 2005, 44, 1012-1020). The MSZW for secondary nucleation is however found to be dependent on the suspension density and on particle size (Kubota et al. J. Chem. Eng. Jpn. 2002, 35, 1063-1071). By decreasing the average particle size and by increasing the mass of seed crystal, the MSZW for secondary nucleation could be increased thus enabling the exploration of a wider range of supersaturations. The concentration profile during the batch experiment is monitored by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and the growth parameters are estimated by solving a population balance model. R-Mandelic acid is found to substantially lower the growth rate of S-mandelic acid. Using a semiempirical, size-independent growth rate expression, the measured concentration profiles can be satisfactorily described by three parameters, two that are independent of the concentration of R-mandelic acid in solution and one which implicitly describes the change in the growth rate due to presence of R-mandelic acid. Moreover, the presence of the R-enantiomer is also found to have a negative impact on crystal purity.