Scientists, industrialists, and doctors often need purified versions of common gases like oxygen and nitrogen. The atmosphere is awash with them, but how do we “harvest” them out of the air?

It’s easy to get oxygen, as long as it doesn’t matter to you how pure that oxygen is. Just breathe in. It is only when you want pure oxygen, for use in a lab or in a doctor’s office, that you hit problems. The same goes for nitrogen, neon, and any of the other gases that we are literally immersed in. In order to get them out of the atmospheric mix, and into their distilled form, we’ve built air separation plants.

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There are a number of ways that gases can be harvested, but the most common is known as cryogenic distillation. Gases are only gases because they’re too hot. Air separation plants cool the atmosphere until it turns into liquid air.

When it is first cooled off, the air “ingredients” are still all mixed together. The purpose of cooling it down is heating it back up. All the gases boil at different temperatures. By slowly heating the liquid air, air separation plants can boil off nitrogen at one temperature, oxygen at another, neon at another. (This is a process also used by people who make liquor, to separate the poisonous methanol from the consumable ethanol. Methanol boils off first, leaving us with relatively harmless alcohol to drink.)

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Air separation plants’ most abundant products are nitrogen and oxygen, for obvious reasons. They can also produce xenon, neon, krypton, helium, hydrogen, and carbon dioxide.

Image: Thomas Bresson

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