(229e) Separation of O-Xylene from the Mixed Xylenes by Distillative Freezing

Xylenes have very broad applications in chemical industries. The mixed xylenes produced mainly consist of p-xylene (PX), m-xylene (MX), o-xylene (OX) and ethylbenzene (EB). Due to their similar molecular structures, these isomers have close boiling points. Therefore, it is very difficult to separate them from each other by distillation. The cost for separation and purification of these isomers is generally very high.

A new separation technique, called distillative freezing (DF), has been successfully applied to separate PX from the binary xylene mixture (MX+PX and OX+PX, respectively) (Shiau et al., 2005; 2006). In principle, the DF process is operated at a triple point condition, in which the liquid mixture is simultaneously vaporized and crystallized due to the three-phase equilibrium. Thus, DF is a distillative crystallization technology, which combines distillation and crystallization to produce pure crystals. By lowering temperature and reducing pressure during the operation, the DF process results in the formation of pure crystals, and liquid phase and vapor phase of mixtures. The process is continued until the liquid phase is completely eliminated and only the pure crystals remain in the feed. Distillative freezing (DF) will be applied in this work to separate and produce OX crystals from the mixed xylene mixtures containing MX, PX, OX and EB. The experimental results indicate that, when the DF experiments are performed based on the simulated adiabatic DF operation, OX crystals can be produced and the purity of OX crystals can reach 99%~99.5% for the mixed xylene mixtures of MX, PX, OX and EB.

There are several unique features of the DF operation: (1) The DF process is conducted under an adiabatic condition at a three-phase equilibrium, where the latent heat released in crystallization is removed by vaporization. Thus, it is an energy-conserving separation method compared to distillation. (2) The addition of solvent is not needed in DF compared to azeotropic or extractive distillation in the separation of the close-boiling mixture. Thus, it is a clean separation technique. (3) The DF process is continued until the liquid phase is completely eliminated and only the pure crystals remain in the feed. Compared to crystallization, subsequent filtration or centrifugation is not needed since no mother liquor is present with the pure crystals. (4) Crystal washing is not required since only the pure crystals remain in the feed and no impurities are adhered on the crystal surfaces at the end of the DF operation.

[1]Shiau LD, Wen CC, Lin BS. Separation and purification of p-xylene from the mixture of m-xylene and p-xylene by distillative freezing. Ind. Eng. Chem. Res. 2005;44: 2258-2265.

[2]Shiau LD, Wen CC, Lin BS. Application of distillative freezing in the separation of o-xylene and p-xylene. AIChE Journal. 2006;52:1962-1967.