http://eprints.utm.my/id/eprint/5262/1/ZainalZakaria2005_ThePredictionOfVapourLiquidEquilibrium.pdf
This is a good read for everyone wondering about propane/butane azeotropes. Talks about liquid/vapor equilibrium at several temperatures. It’s dense but it’s helpful—a very important factor is the fact that gases aren’t ideal, we have to consider fugacity (for gas’s phase) and activity (for liquid phase). These phenomena are also compounded by pressure, because the gases are compressible and so are the liquids.
In the context of this discussion though, there is a portion of evaporated gas that is a mixture of propane and butane. In fact, most of the vapor created is a mixture—initially more rich in propane and then more rich in butane. Not one followed by the other, more of a gradient.
Raoult’s Law mentioned earlier is probably the best starting point.
Edit: Here’s an excerpt
