Advanced Separation Processes

Cyclohexane (b.p. 80.7C, at 1atm ) and Benzene (b.p. 80.1C, at 1atm ) are two structurally similar solvents that are prohibitively expensive to separate via conventional distillation processes. In the 1940s an azeotropic distillation method utilizing acetone added as an entrainer to form a low boiling binary azeotrope with cyclohexane was proposed as a method to separate the two species. This approach utilizes an intermediate liquid-liquid column extraction between the two distillation columns with water, to almost completely separate cyclohexane from acetone, hence removing almost pure cyclohexane as product. This two entrainer water acetone stream from the liquid extraction is then separated in its entirety via distillation in a second column and recycled. (i) Draw a process flow diagram for a system to conduct this method of separation of benzene and cyclohexane including reboilers, condensers and make up streams. Recently a more efficient process has been proposed that simultaneously utilizes a conventional extractive distillation using a sulfolane entrainer and a pressure swing between two distillation columns. Using the data below: (ii) Calculate the composition of the bottoms product from the first column. (iii) Draw a process flow diagram for the new proposed extractive distillation system, indicating molar flow rates and mole fractions for all streams. Again including reboilers, condensers and make up streams. Data: - This method utilizes a high boiling entraining solvent sulfolane (b.p. 285C, at 1atm ) that does not form an azeotrope, with either benzene or cyclohexane. - An extractive distillation which operates at atmospheric pressure is conducted in the first column that enables almost pure cyclohexane to be recovered as the top product. The bottom product from the first column is fed to a second column, as the sole feed containing only trace amounts of cyclohexane. - The second column is operated at a pressure of 0.08 atm to minimize reboiler duty and to prevent degradation of sulfolane. This second distillation enables benzene to be taken as an almost pure product directly as the top product of the second column. - The process has been optimized to separate a 100kmol/hr equimolar feed mixture of benzene and cyclohexane to provide close to pure benzene and pure cyclohexane as the two top products, while minimizing the amount of make-up sulfolane and energy input requirements. Such conditions require a 110kmol/hr of sulfolane to be fed near the top of the first column (tray 4) with the feed of the equimolar mixture of benzene and cyclohexane entering slightly below the column midpoint (tray 20 )