Using a Cubic EOS to Determine Vapor-Liquid Equilibrium: Interactive Simulation

This simulation was prepared using Mathematica. Download the free Wolfram player, and then download the simulation CDF file (link given below or click on figure to download). Try to predict the behavior when a parameter changes before using a slider to change that parameter. A screencast below explains how to use this simulation.

This Demonstration uses the Peng-Robinson equation of state for mixtures to plot isotherms for n-butane(1)/n-octane(2) mixtures on a log pressure versus log volume graph. Use the sliders to select the temperature. Selecting the mole fraction x1 of the liquid yields the isotherm for the liquid in blue. The green isotherm is for the vapor phase (mole fraction y1) that is in equilibrium with the liquid; the values of y1 and the pressure are determined from Raoult’s law and are displayed in the upper-right corner. The dashed black line connects the blue isotherm to the green isotherm at the VLE pressure.

Try to answer these questions before determining the answer with the simulation. We suggest that you write down the reasons for your answers.
  1. For a liquid mixture of n-butane and n-octane, as the mole fraction of n-butane increases at constant temperature, how does the bubble pressure change?
  2. For a liquid mixture of n-butane and n-octane, as the temperature increases, does the pressure for vapor-liquid equilibrium increase or decrease?