9814

Separating Binary Azeotropes Using Pressure Swing Distillation

Pressure swing distillation (PSD) is a complex distillation technique in which two distillation columns, one operating at low pressure and the other at high pressure, are used to separate homogeneous azeotropic mixtures. For a successful separation, it is necessary that the azeotrope compositions at the high and low pressures be sufficiently different. This Demonstration considers a binary mixture of acetonitrile and water. This mixture is a good candidate for PSD because it exhibits a pressure-sensitive minimum-boiling azeotrope [1, 2].
The composition of the azeotrope at low (e.g., 101.325 kPa) and high (e.g., 278 kPa) pressures are equal to 70.4 mole% and 64.1 mole%, respectively. The mixture is assumed to obey the modified Raoult's law and activity coefficients are predicted using the Wilson model.
The first part of the Demonstration computes the necessary VLE data in the -, plane and also shows how the azeotrope composition varies with pressure. The azeotrope location is indicated in the graphic by the blue point. For a binary mixture, the azeotrope position is always the intersection of the equilibrium curve and the line .
The flow chart for the two distillation units is shown in the PSD setup tab. The PSD system is drawn with reboilers indicated in red, the distillation accumulator (DA) in gray, and the condenser in blue [2]. In a typical operation the pressure levels for the two columns are specified, which in turn specifies the compositions at the two distillate streams leaving the columns. For a large number of separation plates, the distillate compositions approximate the respective azeotrope compositions at the two pressures. With the PSD setup tab, you can select the pressure levels for the two distillation columns. Then the compositions of distillate streams leaving the column are computed and displayed on the flow chart. Note that pure water and pure acetonitrile leave the respective columns, thereby illustrating how PSD is capable of separating an azeotropic mixture.

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References
[1] J.-U. Repke and A. Klein, "Homogeneous Azeotropic Pressure Swing Distillation: Continuous and Batch Process," Computer Aided Chemical Engineering, 20, 2005 pp. 721–726. doi:10.1016/S1570-7946(05)80242-1.
[2] J. P. Knapp and M. F. Doherty, "A New Pressure-Swing-Distillation Process for Separating Homogeneous Azeotropic Mixtures," Industrial & Engineering Chemistry Research, 31(1), 1992 pp. 346–357. doi:10.1021/ie00001a047.
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