Separation of the Benzene/cyclohexane Mixture by a Distillative Crystallization Technology

Lie-Ding Shiau, Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Taoyuan, 333, Taiwan and Chia-Chen Yu, Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Taoyuan, 333, Taiwan.

Cyclohexane (Cx) is mainly produced by catalytic hydrogenation of benzene (Bz) in the petrochemical industry. Due to close boiling points (Bz:80.1C, Cx:80.7C) and azeotrope formation, it is very difficult to separate Cx and Bz by conventional distillation. Although azeotropic distillation and extractive distillation can be used for this separation, these two processes suffer from high capital and operation cost. Therefore, searching for new alternatives to separation of the Bz/Cx mixture has been an important research issue.

A distillative crystallization technology, called distillative freezing (DF) or stripping crystallization (SC), has been applied to separate xylene mixtures with close boiling temperatures. In principle, DF 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, DF results in the formation of pure crystals, and liquid phase and vapor phase of mixtures.

Distillative freezing (DF) will be applied in this work to separate the mixture of benzene (Bz) and cyclohexane (Cx). The experiments show that three DF operations are required to purify Cx (B) from to in the Cx-rich mixture. On the other hand, only one DF operation is required to purify Bz (A) mixture from to in the Bz-rich mixture. Thus, it is easier to separate Bz from the Bz-rich mixture than to separate Cx from the Cx-rich mixture. These experimental results are consistent with the DF simulations predicted by the proposed model.

In essence, the DF operation is a combination of distillation and crystallization. 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] L.D. Shiau, C.C. Wen, B.S. Lin, Application of distillative freezing in the separation of o-xylene and p-xylene, AIChE J. 52 (2006) 1962-1967.