9887

Adding a Component to a Vapor-Liquid Mixture

Initially, 20.1 moles of either -hexane or -octane are in vapor-liquid equilibrium (VLE) in a cylinder with a movable piston. You can add -octane to -hexane or vice versa. When you click the inject button, the contents of the vessel return to equilibrium at constant pressure and temperature. The reason for the system behavior, which is modeled by Raoult's law, is shown in the diagram. Because the saturation pressures are different for -hexane and -octane but the total pressure is constant, the final equilibrium state is all liquid or all vapor, depending on which alkane is injected.

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The system is modeled assuming Raoult's law. The diagram shows the behavior when one component is added to the mixture.
The saturation pressures (mm Hg) of -hexane and -octane were calculated using the Antoine equation, where , , and are constants and is temperature (°C):
.
Vapor-liquid equilibrium for this ideal system exists when . When the system initially contains -hexane in vapor-liquid equilibrium and -octane (which has a lower saturation pressure) is added at constant total pressure and constant temperature, then , so only liquid is present at equilibrium. When the system initially contains -octane in vapor-liquid equilibrium and -hexane (which has a higher saturation pressure) is added at constant total pressure and constant temperature, then , and only vapor is present at equilibrium.
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