Extractive Distillation of an Azeotropic Mixture of Isopropyl Alcohol and Water: Effect of Entrainer Feed Temperature

Extractive distillation is one of several methods that can be used to separate an azeotropic mixture. In extractive distillation a third component, called an entrainer, is added to the mixture to break the azeotrope. This Demonstration examines the effect of entrainer temperature on the efficiency of separation [1,3].
The feed to the extractive distillation column is an equimolar mixture of isopropyl alcohol (IPA) and water using dimethyl sulfoxide (DMSO) as an entrainer. The column has 41 equilibrium stages, a partial reboiler, and a total condenser. The feed locations for the entrainer and -water mixture are stages 7 and 35, respectively. (Stages are counted from the top of the column.) The DMSO feed flow is 1708.33 kmol/hr, and the azeotropic mixture (IPA and water) feed flow rate is 1666.67 kmol/hr. For a well-posed problem, we set the distillate flow rate equal to 832.50 kmol/hr and set the reflux stream flow rate to 527.81 kmol/hr. You can vary the temperature of the entrainer.
The Demonstration computes the composition and temperature profiles inside the extractive distillation column. In the composition profile plots, the orange, brown, and green curves correspond to the compositions of IPA, DMSO, and water, respectively. You can easily see that the highest concentration of IPA (almost 100 mole %) is obtained for low entrainer feed temperatures. As the entrainer temperature is increased, the separation becomes less efficient, and the purity of IPA leaving the top of the column decreases.
The various functions of the extractive column can be described as follows: in the rectifying section of the column (above the entrainer feed location) the IPA mixture is purified (little if any water or DMSO is present), while in the stripping section of the column (below the azeotropic mixture feed location) IPA is stripped from the mixture. In the section of the column between the two feeds (also called the extractive section), a high DMSO composition is present. These features are evident on the composition plots.
The bottom product of the extractive distillation column is DMSO and water. DMSO is separated from water in a solvent recovery column (not shown), cooled, and then recycled back to the extractive distillation column.


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Expressions for pure component vapor and liquid enthalpies were adapted from Aspen HYSYS.
The mixture is assumed to obey the modified Raoult's law and activity coefficients are predicted using the Wilson model [4].
[1] M. F. Doherty and M. F. Malone, Conceptual Design of Distillation Systems, Boston: McGraw-Hill, 2001.
[2] E. J. Henley and J. D. Seader, Equilibrium-Stage Separation Operations in Chemical Engineering, New York: Wiley, 1981.
[3] W. L. Luyben and I.-L. Chien, Design and Control of Distillation Systems for Separating Azeotropes, Hoboken, NJ: Wiley, 2010.
[4] G. M. Wilson, "Vapor-Liquid Equilibrium XI: A New Expression for the Excess Free Energy of Mixing," Journal of the American Chemical Society, 86(2), 1964 pp. 127–130.
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