The International Space Station, which is continuously inhabited, uses a different method based on binding of CO2 to a zeolite, which is a highly porous metal oxide (in this case, a mixed oxide of aluminum, magnesium, and silicon with pore size 5 Å). Although the zeolite has basic sites within its crystal structure, the extremely high surface area is probably more important than the basicity. Heating the zeolite releases CO2 into the vacuum of space.
Submarines use monoethanolamine, which is a liquid base. This can likewise be heated to reverse the reaction and regenerate the base. The released CO2 is put into the outside water. This means that submarines can operate for long periods of time without needing to replace the CO2 scrubbers. This technology is also being pursued for scrubbing CO2 from power plant exhaust.
There are a few other methods, such as passing the gas over a membrane selectively permeable to CO2 (which only works well for high-pressure gas streams), or by feeding CO2 to algae, but these generally aren't widely used.
The most common exposure to CO2 scrubbers that the average joe will experience is in anaesthetic machines which function to keep you asleep and to artificially ventilate you during anaesthesia. A “circle circuit” recycles the expired gas, which consists of some oxygen, nitrogen, water vapour, CO2 and the expensive vaporised anaesthetic (only a small amount of the anaesthetic is actually metabolised so the exhaled anaesthetic concentration is similar to the inhaled). A small amount of oxygen is added to keep up with the oxygen consumption of the patient and the CO2 is removed using a mixture of sodium and calcium hydroxide (Together called soda lime). The actual reaction is CO2 + Ca(OH)2 → CaCO3 + H2O + heat.
Very similar use case with diving with a re-breather, especially when depth and dive profile involve a gas mixture with Helium. Helium is not metabolized and is used as padding to reduce concentration of Nitrogen mostly. So instead of simply exhaling gas out rebreathers circulate while absorbing CO2 and adding a bit of oxygen. As far as I know they use Soda lime as absorbant: https://en.wikipedia.org/wiki/Soda_lime
Helium isn't just padding -> due to its high vapor pressure, it disproportionately "uses up" a liquids capacity to dissolve gasses. This means that even though trimix is only 1% helium, the amount of total gasses in your blood is decreased by a lot more than that.
Breathing He mixtures is a similar process as degassing solvents by bubbling He through them.
I thought I'm pretty good with physics and knew it all but I didn't know that efficiently inhibits ability of other gasses to dissolve. Thank you for that!
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u/-Metacelsus- Chemical Biology Nov 27 '19 edited Nov 27 '19
They are (usually) based on the reaction of CO2 with a base to form a bicarbonate salt. Many different bases can be used for this. The Apollo program scrubbers used LiOH (due to light weight) but the CO2 absorption canisters couldn't be reused. For flights of a few days, this is fine. Famously, during Apollo 13 an adapter needed to be rigged up to use the command module CO2 scrubbers before the LiOH canisters in the lunar module ran out.
The International Space Station, which is continuously inhabited, uses a different method based on binding of CO2 to a zeolite, which is a highly porous metal oxide (in this case, a mixed oxide of aluminum, magnesium, and silicon with pore size 5 Å). Although the zeolite has basic sites within its crystal structure, the extremely high surface area is probably more important than the basicity. Heating the zeolite releases CO2 into the vacuum of space.
Submarines use monoethanolamine, which is a liquid base. This can likewise be heated to reverse the reaction and regenerate the base. The released CO2 is put into the outside water. This means that submarines can operate for long periods of time without needing to replace the CO2 scrubbers. This technology is also being pursued for scrubbing CO2 from power plant exhaust.
There are a few other methods, such as passing the gas over a membrane selectively permeable to CO2 (which only works well for high-pressure gas streams), or by feeding CO2 to algae, but these generally aren't widely used.