Adding a Second Component to a Fixed-Volume Container

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A constant-volume tank initially contains one component in the vapor phase (either -octane or
-hexane), and you can add the other component at constant temperature using a slider. Change the temperature with another slider. Select the component to be added with buttons. The pressure-composition
diagram has a blue line for the bubble pressure and an orange curve for the dew-point pressure. The black dot is located at the overall composition and the pressure. When two phases are present, the
-hexane mole fractions of the liquid phase (
) and the vapor phase (
) are displayed. The bar graph represents the number of moles of each component in the liquid and vapor phase; purple is for
-hexane and green is for
-octane.
Contributed by: Rachael L. Baumann (February 2016)
Additional contributions by: John L. Falconer
(University of Colorado Boulder, Department of Chemical and Biological Engineering)
Open content licensed under CC BY-NC-SA
Snapshots
Details
The saturation pressures are calculated using the Antoine equation:
,
where is the saturation pressure (bar) of component
;
,
and
are Antoine constants; and
is temperature (K).
Raoult's law for a binary mixture is used to calculate the bubble and dew
pressures:
,
,
where and
are the liquid mole fractions of each component
and
and
are the vapor mole fractions
.
The overall mole fraction of component 1 is , where
and
are the total moles of each component.
When only vapor is present,
,
,
where is pressure (bar),
is the gas constant ([L bar]/[mol K]),
is volume (L) and
are moles of vapor.
The following five equations are solved for ,
,
,
, and liquid moles
when both liquid and vapor are present.
From Raoult's law:
,
.
From overall and species mole balances:
,
.
From a volume balance:
,
where is the molar density of the mixture (mol/L).
The screencast video at [1] explains how to use this Demonstration.
Reference
[1] Adding a Second Component to a Fixed-Volume Container [Video]. (Aug 31, 2016) www.colorado.edu/learncheme/thermodynamics/AddingSecondComponentFixedVolume.xhtml.
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