Separation and Purification of p-Xylene from the Mixture of m-Xylene and p-Xylene by Distillative Freezing

A new separation technique, called distillative freezing (DF), is described in detail. Basically, the DF process is operated at triple point condition, in which the liquid mixture is simultaneously vaporized and solidified due to the three-phase equilibrium. It results in the formation of pure solid, and liquid phase and vapor phase of mixtures. The process is continued until the liquid phase is completely eliminated and only the pure solid crystals remain in the feed. As the DF process is conducted under an adiabatic condition, the latent heat released in forming the solid crystals is mostly removed by vaporizing portions of liquid mixtures. The DF process is applied in this work to separate and produce solid p-xylene (PX) crystals from liquid mixtures of m-xylene (MX) and PX. The experimental results show that, for a liquid mixture of 100 g comprising10% MX and 90% PX, a final solid PX product of about 51.5 g is obtained when the final operation is at T = - 25.4 (degree) C and P = 30.792 Pa by the proposed DF process. The final purity of solid products analyzed by gas chromatography can reach 99.1% PX.