Adding One Component to a Binary Vapor-Liquid Equilibrium (VLE) Mixture

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Initially an -hexane/-octane mixture (20 mol of liquid, 3 mol of vapor) is in vapor-liquid equilibrium in a cylinder with a movable piston. Additional -hexane or -octane is added when the inject isothermally play button is pressed. Change the number of moles injected at constant temperature and pressure with the slider. The system is modeled by Raoult’s law. The -- diagram shows what happens when additional alkane is injected. The moles in the liquid phase and the vapor phase are displayed, as are the corresponding mole fractions of -hexane in liquid, in vapor). Use the reset button to start the process again.

Contributed by: Rachael L. Baumann (March 2015)
Additional contributions by: John L. Falconer and Nick Bongiardina
(University of Colorado Boulder, Department of Chemical and Biological Engineering)
Open content licensed under CC BY-NC-SA



This system is assumed to obey Raoult's law. The saturation pressures (bar) of -hexane () and -octane () are calculated from the Antoine equation, where the Antoine coefficients , , and are constants and (°C) is temperature:


In order for the components to be in vapor-liquid equilibrium (VLE), the total pressure must equal the sum of the partial pressures:


Here, and are the liquid mole fractions of -hexane and -octane. The mole fractions of -hexane and -octane in the vapor phase are and . Initially the system contains -hexane and -octane in VLE. When -hexane (the component with the higher vapor pressure) is added at constant temperature, vaporization returns the mixture to VLE. By contrast, if -octane (lower vapor pressure) is added at constant temperature, condensation returns the mixture to VLE.

The screencast video at [1] explains how to use this Demonstration.


[1] Adding One Component to a Binary VLE Mixture [Video]. (Dec 16, 2020)

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