Cosolvent Selection for Benzene-Cyclohexane Separation in Extractive Distillation

The Krupp-Koppers (K-K) extractive distillation method with N-formylmorpholine (NFM) as the solvent is one of the most important processes for catalytic hydrogen refining of rude benzene. To increase the capacity and selectivity of the solvent and decrease the ratio of solvent to feed in this process, ethylene glycol (EG), N,N-dimethylformamide (DMF), and N-methylpyrrolidone (NMP) were introduced as the cosolvent with NFM. The relative volatility of cyclohexane to benzene (alpha) at a certain R-stf (liquid-phase ratio of solvent to feed) was considered as a criterion of the performance of cosolvent. The vapor-liquid equilibrium (VLE) data for benzene + NFM, EG + NFM, benzene + cyclohexane + NFM, benzene + cyclohexane + NFM + EG, benzene + cyclohexane + NFM + DMF, and benzene + cyclohexane + NFM + NMP were measured at atmospheric pressure. VLE data of benzene + NFM, EG + NFM, benzene + cyclohexane + NFM, and benzene + cyclohexane + NFM + EG were calculated by the nonrandom two-liquid (NRTL) model. The average temperature deviations of benzene + cyclohexane + NFM and benzene + cyclohexane + NFM + EG systems are (1.40 and 3.19) K, respectively. The average deviations of the vapor-phase mole fraction of benzene are 0.030 and 0.066, respectively.