I took two LP gas bottles to be refilled today in Tonga and was told, after they tried, that they could not fill them. The tanks were previously filled in Mexico. Some research on the internet taught me that Mexico gets almost all of its LP gas from the US, and the US refines its LP gas so that it is almost pure propane. The LP gas in Tonga is a mixture of propane and butane. Because the former is more volatile than the latter, Tongan LP gas cannot be condensed into a tank that has US LP gas in it. It is also noteworthy that propane burns hotter than butane on a per molecule basis. So the question is, can you expect a tank of liquid propane to last longer than a tank of butane (or propane/butane mixture)? The heat of combustion is related to the H/C ratio for hydrocarbons and propane has a higher H/C ratio than butane. The formulas for combustion are:
C3H8 + 8 O2 → 4 H2O + 3 CO2
2 C4H10 + 13 O2 → 10 H2O + 8 CO2
The additional heat for propane vs. butane per molecule is given roughly by:
(8/3)/(10/4) = 1.0667
The heat of combustion (at standard temperature and pressure) of propane is 2220.1 kJ/mol and for butane is 2877.5 kJ/mol. The density of liquid propane at room temperature (25°C) is approximately 0.493 grams per cubic centimeter. For butane it is 0.599 g/cm3. So if we assume are tank is 1 cm3 (does not matter what size it is for the calculation), propane produces:
0.493 g/cm3 X (1/44.10 g/mol) X 2229.1 kJ/mol = 24.92 kJ/cm3
Butane produces:
0.599 g/cm3 X 58.12 g/mol X 2877.5 kJ/mol = 29.66 kJ/cm3
So, for a given volume of liquid gas at room temperature, butane produces more energy than propane (29.66/24.92=1.19, about 20%). While propane has a higher calorific value per unit of mass, butane’s higher specific gravity (density) means it packs more mass into the same volume, resulting in a higher overall energy output per volume.
There is a potential issue regarding mixtures of gases (if we are not dealing with pure propane or butane), namely if the molecules interact in some way, but hydrocarbons are generally considered to be “ideal gases” (and ideal liquids).
Bottom line … same-sized LP tanks from Mexico should not last as long as LP tanks from Tonga.
You might ask why there are different formulas for LP gas. Propane is more volatile than butane, so it is more suitable for the cooler climates. Tonga does not have to worry about vaporizing butane.