Oxygen generators and carbon dioxide scrubbers explained

The latest in our series describing how technology works, by Jake Port.

Astronaut John Swigert holds a carbon dioxide scrubber made from parts found aboard the Apollo 13 spacecraft. An explosion in an oxygen tank forced the astronauts to abort their moon landing and find a way to keep their air breathable.
NASA/Corbis via Getty Images

Providing breathable air is a challenge in space and under the sea, as is disposing of the carbon dioxide submariners or astronauts exhale. So how do submarines and spacecraft create oxygen and handle the toxic build-up of carbon dioxide? The answer lies in a combination of chemical reactions and electricity.

Submarines and the International Space Station produce the majority of the oxygen they need by liberating it from water.

A water molecule consists of two hydrogen atoms and one oxygen atom: H2O. Most of the world’s water is found in the oceans – the roaming ground of vessels such as ships and submarines. So it's convenient to use that seawater as an oxygen source.

First, it must be distilled to remove impurities such as salts. That is done by heating seawater to create water vapour, leaving the salts behind, and then cooling it to condense in a collection tank.

Electricity is passed through the purified water, in a technique known as electrolysis, which separates the water molecule into its constituent parts: hydrogen and oxygen.

When an electric current is passed through water, molecules separate and recombine as illustrated here.

Free atoms of oxygen and hydrogen happily bind to other atoms of the same element, producing hydrogen and oxygen gas, H2 and O2, at a rate of twice as many hydrogen molecules as oxygen.

The positively charged hydrogen ions collect at the negative cathode, while the negatively charged oxygen collects at the positive terminal, or anode. Oxygen is vented into the submarine's air circulation system, and such systems can produce thousands of litres of oxygen each hour.

In space, though, water isn’t as plentiful as in the ocean, but it is still the major source of oxygen.

To access this, the space station must continuously refill its water tanks and reclaim water vapour produced by the astronauts when they exhale.

Electrolysis is powered by the space station's giant solar panels and the oxygen produced hisses into the breathable cabin air system.

At the moment, explosive hydrogen is expelled into space, but that may change. Rather than being simply wasted it could be contained for later use, even as a rocket fuel – an idea being considered to fuel future space missions.

And water (as a liquid, solid or gas) on another planet or asteroid could provide fuel for the trip home or beyond.

But providing the oxygen is only part of the problem solved. While we breathe in oxygen, we also exhale carbon dioxide, or CO2. This gas can be deadly if its concentration in the atmosphere rises above 8%, so controlling it is essential to keep submarine and spacecraft crews alive.

The most straightforward way of removing the gas is to vent it. But it must first be captured through a chemical reaction in a carbon dioxide “scrubber”.

Traditionally, scrubbers use soda lime (a mixture of chemicals including calcium hydroxide, sodium hydroxide and potassium hydroxide) or amines (a derivative of ammonia) to lock onto CO2 molecules.

But in space where every gram counts, lithium hydroxide is used in scrubbers because it has a low molecular weight. When the CO2 reacts with lithium hydroxide it creates create lithium carbonate and water.

While this is the primary scrubber type on the space station it's a back-up on a submarine.

Scrubbers of all types involve chemical reactions that have the added benefit of producing water, which could be used for drinking or to fuel the oxygen generator.

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Jake Port contributes to the Cosmos explainer series.
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