When a liquid mixture boils at a given temperature

and pressure

, the composition of vapor

is typically different from the liquid composition

., but this is not always the case. With some mixtures it is possible to have a boiling condition

such that the composition of the vapor leaving is the

*same* as the composition of the boiling liquid:

. This boiling mixture is called an azeotrope. Azeotropes (also called constant-boiling mixtures) are classified as either minimum boiling (positive) azeotropes or maximum boiling (negative) azeotropes. In this Demonstration, the azeotropes of binary mixtures containing ethanol are computed. You can select the second species for the binary mixture from the pull-down menu and use a slider to specify the system pressure

. All the azeotropes computed for the ethanol mixtures are minimum boiling azeotropes.

We use a Wilson equation to calculate the liquid activity coefficients

and the Antoine equation to determine the saturation vapor pressures of the pure components

. The gas mixture is assumed to be an ideal gas mixture. The azeotrope condition

at a specified

satifies the equations:

In the plot, the red curve is the first equation and the blue curve is the second equation. The green dot denotes the temperature and ethanol composition at the azeotrope. Since the red curve is always concave up we obtain minimum boiling azeotrope. Clicking the "azeotrope parameters" button in the pane gives a summary of the azeotrope conditions for the selected binary mixture. Parameters for the Antoine equation and the Wilson model were taken from

Aspen HYSYS thermodynamics package.