Separation of para-xylene and meta-xylene by extraction process using aqueous cyclodextrins solution

Abstract

The separation of para-xylene (p-xylene) from its mixtures with meta-xylene (m-xylene) using aqueous hydroxypropyl-β-cyclodextrin (HP-β-CD) solution by a two-step liquid–liquid extraction process has been experimentally investigated. The results show that HP-β-CD can form inclusion complexes of different strengthes with p-xylene and m-xylene. It is shown that the different strengthes stem from the differences in size, relative alkalinity and dipole moment between the two xylene isomers. The optimum separation condition has been determined as follows: the concentrations of HP-β-CD and initial xylene mixture are 0.1448g(gH2O)−1 and 0.4g(gH2O)−1, respectively. Under this condition, the separation factor α, which is the most important evaluation index in extraction process, is 1.469. The ratio of the formation constants, KPX/KMX, has been determined to be slightly greater than α, consistent with the theoretical prediction. Moreover, a new method for calculation of α has been developed. The experimental results demonstrate the almost perfect reusability of aqueous HP-β-CD solution. For the experiment that begins with a 1:1 mixture of p-xylene and m-xylene, the p-xylene concentration in the extraction phase is 59.71wt%. After repeating the extraction process 4 times, the concentration of p-xylene in the extraction phase can reach 81.48wt%.