Published: 17 February, 2005

Compressed oxygen gets pumped into tanks

How do they compress oxygen into pure oxygen tanks?

A gas compressor is basically a larger, motorized version of a bicycle pump. When you lift up on the handle of the bicycle pump, it pulls up a small piston in a cylinder, and sucks in air through a valve to fill up the cylinder at atmospheric pressure. Then when you press down on the pump handle, this inlet valve closes (by pressure from a spring), and all the gas in the cylinder is squeezed into a progressively smaller volume. When the pressure in the cylinder gets higher than the pressure in the tire, an outlet valve opens up, and all the gas in the pump cylinder gets squeezed out into the tire. The more gas there is in the tire, the higher the pressure, so each stroke you give to the pump builds up the tire pressure a bit more. You may notice that as the bicycle tire pressure gets higher, you have to push harder on the pump handle. You can imagine that the motor in a compressor has to work quite hard to push gas into a tank with pressures 30 to 50 times higher than those in typical bike tires!

The biggest difference between compressing air and a pure gas is that the inlet needs to come from a source of pure gas, rather than just the room. For oxygen, the next big difference is that the seals and lubricants (rubber and oils) that are used to keep the high pressure gas from leaking past the piston and escaping into the atmosphere could easily burn because oxygen is so reactive. So, special fluoridated materials (related to Teflon®) are used instead.

In principle there are many things that could be used as a source of pure oxygen. For example, it would be possible to pass an electric current through water, and collect hydrogen at one electrode and oxygen at the other. But in practice, almost all pure oxygen produced commercially is obtained by liquefying air, then distilling it to separate out liquid nitrogen, oxygen, argon, and neon (and usually throwing away water, carbon dioxide and other contaminants). Small consumers of oxygen then use compressed gas from this liquid after it has been warmed up as a convenient means of storage without having to provide refrigeration. More major users like the steel industry or big chemical plants are likely not to bother with warming it and compressing it. They will buy it in the form of liquid transported by tank trucks that look from the outside much like an oil tanker, but which are really giant 10,000 gallon thermos bottles. Then heat is just supplied at a rate to supply moderate pressure oxygen gas as needed.