Liquid-liquid Equilibrium For Extraction Separation Benzene And Cyclohexane Using N,N-Dimethylformamide And Potassium Thiocyanate

As a very important bulk chemical, cyclohexane is produced via catalytichydrogenation of benzene. The unreacted benzene remain in the reactor’s effluent streamand the product which must be removed for pure cyclohexane recovery. However,separation of benzene and cyclohexane mixtures has been proved one of the mostdifficult tasks in the petrochemical industry because of their close boiling points（benzene:353.25K; cyclohexane:353.85K）, approximately equal molecular volumes, lowrelative volatility in the whole composition range, and easy formation of azeotropicmixture. Conventional distillation can not achieve the task in practice. Specialdistillation such as azeotropic distillation and extractive distillation are usuallyemployed for benzene/cyclohexane separation. These two processes, however, sufferfrom complexity and high energy consumption. For all these reasons, the industry hasalways been eager to look for a viable alternative to the conventionalbenzene/cyclohexane separation processes.Liquid-liquid extraction is widely used industrial separation process for ahomogeneous liquid mixture which is a viable alternative to the conventional techniquesboth from economical and technical points of view. In this work, the complex solventsystem, ammoniutn thiocyanate （NH₄SCN） dissolved in N,N-dimethylformamide（DMF）, was found to be a good extractant for the separation of benzene andcyclohexane because the solvent is highly selective and very cheap. Phase equilibriumdata are required for the evaluation of solvent combinations and design of extractionequipment, so the experimental liquid-liquid equilibrium data were measured forbenzene+cyclohexane+DMF+NH₄SCN at298.15K and303.15K under atmosphericpressure. The selectivity coefficients of DMF+NH₄SCN for benzene are1.88to18.95.Distribution coefficients of DMF+NH₄SCN for benzene are0.14to1.08. After fivestages cross current extraction, the highest mole fraction of cyclohexane in the raffinatereaches0.951. With the increase of the mass fraction of NH₄SCN in the extractant, theyield cyclohexane reaches0.949. The results revealed that the selectivity coefficientscomplex solvent increases with the increasing of the NH₄SCN concentration in the solvent, the selectivities decrease with the increasing of the mass fraction of benzene inthe raffinate phase. Considering the high selectivity of benzene, DMF+NH₄SCN may beused as a potential extracting solvent for the separation of benzene and cyclohexane.The experimental data was correlated using the Othmer-Tobias correlation.Triangular phase diagrams and tetrahedron phase diagrams were drew according to theliquid-liquid equilibria data（LLE）. The LLE data were analyzed by NRTL model withtemperature-dependent binary parameters determined from the experimental LLE data,both as programmed by the ChemCAD simulation software. Based on the analysis ofthese data, the NRTL model represented the experimental data with sufficient accuracyas revealed from the very small values of the root mean square error （RMSE） and theaverage absolute deviation （AAD） in composition. The feasibility of extraction benzenefrom cyclohexane with DMF+NH₄SCN was investigation by ChemCAD simulator.Some effects were investigated, such as the flow ratio, the composing of the feed, themass fraction of NH₄SCN in the extractant. The emulation results indicated thatDMF+NH₄SCN were able to use extraction benzene from cyclohexane, when the theflow ratio is3, the mass ratio of benzene in the feed is5:95, the mass fraction ofNH₄SCN in the extractant is5:95, after five-stage countercurrent extraction, the massfraction of cyclohexane in the raffinate is97.81%, while the mass fraction of benzene isonly0.95%.