I have to disagree with this a bit. You can definitely preferentially condense LPG mixtures.
If at a given pressure the condensing point of butane is higher than the condensing point of propane, you will “preferentially” condense butane.
In theory, if you can maintain the entire environment (thermodynamic “system”) at or near the condensing point of butane while staying above the condensing point of propane—more butane will condense.
You can see this in the following T-X-Y diagram for a propane butane mixture:
I’ll have to draw something out to expand on this diagram…
If I start with a 50/50 mixture at temperature X and slowly increase to temperature Y, the composition of the liquid phase becomes point C and the vapor phase composition becomes point D.
This is the concept behind distillation:
Condense to C, the reboil that liquid at temperature Z1 up to temp Z2, condense to C1, reboil at Z2 up to temp Z3, condense to C2
Etc, etc.
This is why crude oil (like the kind from the ground) distillation columns fucking TALL (the tallest is almost 370ft tall) 