Fugacity Dependence on Pressure in an Ideal Binary Mixture

This Demonstration shows how the fugacities of benzene () and toluene () in a binary mixture change with pressure and composition at constant temperature. The liquid mixture is modeled as an ideal solution and the gas phase is assumed ideal, thus Raoult's law models vapor-liquid equilibrium. Use sliders to vary the temperature and mole fraction of benzene. Use buttons to view the pressure-composition diagram (--), the fugacity-pressure plot or both plots at once. Black dots on the diagrams represent the pressure (on the -- diagram) or fugacity (on the fugacity plot) for the given temperature and pressure.
  • Contributed by: Neil Hendren
  • Additional contributions by: John L. Falconer
  • (University of Colorado Boulder, Department of Chemical and Biological Engineering)


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The saturation pressures were calculated using the Antoine equation:
where represents either benzene or toluene ( or ), is temperature (°C), and , and are Antoine constants.
The fugacity of a component depends on temperature and molar composition.
In the liquid region:
where is the liquid mole fraction and both and have units of bar.
In the vapor region:
where is the vapor molar composition and is the total pressure (bar).
When vapor and liquid are in equilibrium (VLE):
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