Injecting a Liquid into an Evacuated Tank

In this Demonstration, liquid propane or liquid toluene is injected into an evacuated 2 liter spherical tank that is held at constant temperature. When propane is injected, it either vaporizes completely or forms two phases in vapor-liquid equilibrium (VLE), depending on the temperature and the number of moles injected (control both with sliders). Because toluene has a much lower saturation pressure, only a small amount of injected toluene liquid vaporizes. When the component is in VLE, the tank pressure equals the saturation pressure; otherwise, the pressure is calculated using the ideal gas law. The pressure is displayed at the top of the tank. Click the play button next to "inject liquid" to inject liquid into the tank. The intensity of the blue color of the vapor is proportional to the vapor density. The liquid volume in the tank is exaggerated relative to the vapor volume for better visualization. The bar graph on the right shows the number of moles of liquid and vapor in the tank.


  • [Snapshot]
  • [Snapshot]


When propane or toluene is in VLE, the pressure in the tank is the saturation pressure, which is calculated using the Antoine equation:
where is the saturation pressure (bar), , and are Antoine constants, and is temperature (K).
When only vapor is present, the ideal gas law is used to calculate the pressure :
where is the total moles (moles of vapor in this case), is the ideal gas constant ([L bar]/[mol K]) and is the total volume (L).
When the component is in VLE, the sum of the liquid and vapor volumes equal the volume of the tank :
and the total moles (moles injected) equals the sum of the liquid and vapor moles:
where is the liquid molar density (mol/L).
Solve for :
The screencast video at [1] explains how to use this Demonstration.
[1] Injecting a Liquid into an Evacuated Tank [Video]. (Oct 12, 2016) www.colorado.edu/learncheme/thermodynamics/InjectingLiquidEvacuatedTank.xhtml.
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