Six chemical species are present in significant amounts in a light-ends petroleum fractionation system: methane, ethane, propane, isobutane, n-butane and n-pentane. A mixture of these species exist in vapor/liquid equilibrium in closed vessel. On how many phase-rule variables in addition to T and P do the compositions of the phases depend?
If T and P are to remain the same, is there any way the compositions of the phases of the total content of the vessel can be changed (by adding or removing material) without affecting the composition of the phases?