Benzene-cyclohexane azeotrope

Terada T, Hohjoh T, Yoshimasu S, Ikemi IM, and Shinohara T. Separation of benzene/cyclohexane azeotropic mixture through polymeric membranes with microphase separated structures. Polym J 1982 14(5) 347-353. 

Sakata et al. made porons carbon membrane plates (PCMP) to separate benzene-cyclohexane azeotrope by pervaporation [9]. PCMPs were prepared from phenol resin powder (BELLPEARL S-870, Kanebo) by pressurizing the powder in a hydraulic press. The disk was heated at 300°C for 1 h in air stream and then carbonized at 800°C in nitrogen stream (A) or CO2 stream at 800°C (B) or at 850°C (C). A had only micro-pores, B had much wider pore size distribution than A and C had both micro- and meso-pores. The separation factor (benzene over cyclohexane) of 2.8 was obtained by pervaporation at 60°C. Flux of pure benzene and pure cyclohexane was 0.34 and 0.57 kg/m h, respectively. 

Benzene-cyclohexane Minimum-hoiling azeotrope Anihne ... 

Azeotrope formers, generally polar compounds, have the ability to form, with hydrocarbons, nonideal mixtures having vapor pressures higher than either component in the mixture and therefore lower boiling points. Fortunately, different types of hydrocarbons show different degrees of nonideality with a given azeotrope former. For example, benzene and cyclohexane boil at about 176° F., while the methanol-cyclohexane azeotrope boils at 130° F., and the methanol-benzene azeotrope boils at 137° F., a difference of 7° F. Hence, fractionation of a mixture of benzene and cyclohexane in the presence of methanol effectively separates the two hydrocarbons. 

Figure 9.20(b) illustrates the use of pervaporation with two distillation columns to break a binary azeotrope such as benzene/cyclohexane. The feed is supplied at the azeotropic composition and is split into two streams by the pervaporation unit. The residue stream, rich in cyclohexane, is fed to a distillation column that produces a pure bottom product and an azeotropic top stream, which is recycled to the pervaporation unit. Similarly, the other distillation column treats the benzene-rich stream to produce a pure benzene product and an azeotropic mixture that is returned to the pervaporation unit. 

Benzene-cyclohexane Minimum-boiling azeotrope Aniline ... 

FIGURE 2.9 Single-stage azeotropic separation, benzene-cyclohexane-ethanol at 103.4 kPa. 

A stream of 100 kmol/h contains 70% mole benzene (1) and 30% mole cyclohexane (2). These components form a minimum-boiling azeotrope, which prevents their separation by conventional distillation. They can be separated by azeotropic distillation using acetone (3) as the entrainer. Acetone forms a minimum-boiling azeotrope with cyclohexane at the column pressure of 100 kPa, where the azeotropic composition is 73.9% mole acetone and 26.1% mole cyclohexane. The entrainer, at the rate of 75 kmol/h, is mixed with the feed and sent to the column. The distillate contains 99% mole acetone-cyclohexane azeotrope and 1% mole benzene, and the bottoms product is mostly benzene with a small percentage of cyclohexane. Use material balances to calculate the flow rates and compositions of the products. 

In general, the selectivity of pervaporation is high, as demonstrated in Figure 6.41 for the system "benzene-cyclohexane, polyethylene membranes". The separation characteristic is shifted to much more favorable figures compared to the thermodynamic equilibrium curve the azeotropic point can be suppressed.28-30 Parameter in the diagram is the ratio of total pressure p at the permeate side to saturation pressure Pl = TjXjP + TjXjpj. 

Liquid-liquid extraction can be used to recover one of the components to be separated, as well as the entrainer to be recycled. A typical process is the separation of cyclohexane/benzene, with nbp s at 80.8 °C and 80.1 °C and minimum-boiling azeotrope at 77.6 °C (Seader and Henley, 1998). If acetone is used as entrainer (nbp 56.2 °C), then an azeotrope appears between acetone and cyclohexane (nbp 53.1 °C). RCM shows a distillation boundary between the components to be separated (Fig. 9.32). To simplify the process, consider an initial azeotropic mixture. After mixing with entrainer the feed f, is separated in two products, benzene in bottoms and acetone-cyclohexane azeotrope in top. From the last mixture cyclohexane can be extracted with water. Finally acetone and water are separated by simple distillation. 

From the azeotropic composition for the benzene-cyclohexane system in Problem... 

Methyl Cellosolve does not form a ternary azeotrope with water and the hydrocarbons benzene, cyclohexane or toluene, although it does with xylene and ethylbenzene. As for ethyl Cellosolve, methyl Cellosolve can be dehydrated with toluene, provided a continuing use for the resulting methyl Cellosolve/toluene mixture can be found. If not, a Cj normal/isoalkane mixture can be used with a phase separation to remove the hydrocarbon when the feed has been dried. This will best be carried out in a hybrid or batch still and, if the same unit is used as for drying, facilities for rejecting both the denser (water) phase and the less dense (heptane) phase will be needed. 

One method for ethanol dehydration is heterogeneous azeotropic distillation, which has been used for many decades. A suitable light entrainer component (benzene, cyclohexane, isooctane, ethylene glycol, and so on) is added to modify the relative volatilities. The water is driven overhead with the entrainer and a high-piu ity ethanol bottoms stream is produced in the azeotropic column. The overhead vapor is condensed and fed to a decanter. The organic phase is refluxed back to the column. The aqueous phase is fed to another column that produces a bottoms product of high-purity water and a distillate that is recycled back to the azeotropic column. A third column in the front end of the process is used to preconcentrate the low-concentration stream from the fermenter up to a concentration closer to the azeotrope before feeding this into the azeotropic column. 

Examine with the help of the regular solution theory, UNIFAC and modified UNIFAC if the binary systems benzene-cyclohexane and benzene-n-hexane show an azeotropic point at 80 C. In case of the regular solution theory, calculate the solubility parameter from the saturated liquid density and the heat of vaporization using Eq. (5.70). All required data are given in Appendices A, H, and 1. 

Figure 11.16 Separation of azeotropic systems by azeotropic (feed F pyridine-water, solvent toluene) and extractive distillation (feed benzene-cyclohexane).

Aromatics/aliphatics separation is accomplished by solvent extraction. A number of solvents have been used (Bailes, 1983). Although these separation processes are efficient, they are energy intensive, and more importantly, the solvents (such as sulfolane) increasingly pose as environmental hazards. Another possible separation technique is fractional distillation. It is, however, difficult because of the close relative volatilities. For benzene/cyclohexane, the mixture has a minimum azeotrope at about 53%. Therefore, acetone is added as an entrainer and a complex hybrid system (distillation combined with extraction in this case) can be used for separation (Stichlmair and Fair, 1998). 

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