Extractive distillation methylcyclohexane/toluene /phenol

The results obtained in the solution of a sample problem are summarized here to illustrate the application of the method. An extractive distillation problem from Oliver (6) was used in which methylcyclo-hexane is separated from toluene by adding phenol. The column contains 11 stages (including the reboiler and condenser) and has a feed of 0.4 moles/unit time of methylcyclohexane and 0.6 moles/unit time of toluene to the fourth stage from the reboiler and 4.848 moles/unit time of phenol to the fourth stage from the condenser. We used the same physical property correlations as Oliver. The activity coefficients were obtained from a multicomponent form of the Van Laar Equation (7). 

Figure 13.24. Composition profiles and flowsketches of two extractive distillation processes, (a) Separation of methylcyclohexane and toluene with phenol as solvent (data calculated by Smith, 1963). (b) Separation of aqueous ethanol and isopropanol, recovering 98% of the ethanol containing 0.2 mol % isopropanol, employing water as the solvent. Flow rates are in mols/hr (data calculated by Robinson and Gilliland, 1950).

Typical mixtures that can be separated by extractive distillation in processes similar to the one described above include cyclohexane and benzene, and toluene and methylcyclohexane, both using phenol as the solvent. In another process, isobutane and 1-butene are separated using furfural as the solvent. 

An example of an extractive distillation process is the separation of methylcyclohexane (MCH) from toluene using a phenol solvent, as shown in Figure 12.17. Since MCH boils at 101.0°C and toluene boils at 110.7°C (1 atm), their separation by ordinary distillation is very difficult even though they do not form an azeotrope. Phenol is an effective solvent, since it has a structure more similar to the aromatic than to MCH (a naphthene), and it is relatively nonvolatile. The rectification... 

Determination of stage requirements for extractive distillation follows the approaches discussed in Section 5.3. It is necessaiy to have vapor-liquid equilibrium data, and these are measured conventionally. A representative relation between solvent/nonsolvent ratio and relative volatility is shown in Rg. 5.5-4, taken from the papers of Oerster and Drickamer et al. From the figure it is clear that vatious combinations of solvent content and stage requirements are possible, and the optimum solvent ratio must be determined. Figure 5.5-4 deals with the separation of toluene fium methylcyclohexane using phenol as the solvem. The natural volatility of the binaiy mixture is shown as the bottom line (no phenol presem) and the enhancement of this volatility by phenol addition is significant. A ffow diagram of this separation process te shown in Fig. 5.5-5. 

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