Why is propane stored in household tanks but natural gas is not?

Category: Chemistry      Published: May 2, 2013

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University

Public Domain Image, source: Christopher S. Baird.

In order to get a useful amount of gaseous fuel into a reasonably-sized tank, you have to liquify it. Some fuels are easier to liquify than others. According to the textbook Organic Chemistry by Joseph M. Hornback, propane has a boiling point of -44° F (-42° C) at atmospheric pressure, but methane (natural gas), has a boiling point of -260° F (-162° C) at atmospheric pressure. This means that methane has to be cooled to a much lower temperature than propane in order to be turned to a liquid that can be stored in a tank. Propane molecules consist of three carbon atoms bonded in a chain with eight hydrogen atoms bonded to these carbon atoms. In contrast, a methane molecule is just one carbon atom bonded to four hydrogen atoms. Methane molecules have a high degree of symmetry. As a result, they do not have a permanent electric dipole. Bonding between permanent dipoles is the dominant bonding mechanism between molecules as they liquify for many substances such as water. Methane's symmetry, and therefore lack of a permanent electric dipole, means that its molecules can only bond through a much weaker effect known as the London dispersion force or the van der Waals force. In this effect, molecules induce temporary dipoles in each other, and these dipoles then bond. Because this bonding mechanism is so weak, the methane molecules have to be cooled to a low temperature until they are still enough to bond and form a liquid. In contrast, propane does not require as low a temperature to liquify.

But household propane is not usually kept in a liquid state by a low temperature. Instead, high pressure is used. In order to keep propane a liquid at room temperature (70° F or 21° C), it has to be held in a tank at a pressure of about 850 kPa. This can be accomplished with a strong metal tank. In contrast, to keep methane a liquid at room temperature requires a tank that can maintain a pressure of about 32,000 kPa. Household metal tanks cannot withstand this pressure. In short, methane is not stored in household tanks because the symmetry of its molecule makes it hard to liquify. You could in principle store methane in a tank in the gas state, but methane has such low density in the gas state that you could not store a usable amount. Instead, natural gas is processed and stored at refinery plants and then pumped to households in the gas state through pipes. The properties of different basic fuels are summarized below, showing nicely the trend in room temperature liquid pressures. Note that the pressures are approximate.

Topics: fuel, liquid, methane, natural gas, phase, phase diagram, pressure, propane, propane storage, propane tanks, vapor pressure