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

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 0.1448   g   (g   H 2 O) −1 and 0.4   g   (g   H 2 O) −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, K PX / K MX , 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 59.71   wt%. After repeating the extraction process 4 times, the concentration of p -xylene in the extraction phase can reach 81.48   wt%.