Adding a Second Component to a Single-Component Vapor-Liquid Equilibrium (VLE) Mixture

piston-cylinder

P-x-y diagram

add n-octane

add n-hexane

moles to inject

2

inject isothermally

Initially, 20.1 moles of either

n

-hexane or

n

-octane are in vapor-liquid equilibrium (VLE) in a cylinder with a movable piston. You can add

n

-octane to

n

-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

P

-

x

-

y

diagram. Because the saturation pressures are different for

n

-hexane and

n

-octane but the total pressure is constant, the final equilibrium state is all liquid or all vapor, depending on which alkane is injected.