I’d assume that would take water as a reagent to convert the CO2 back to O2. However submarines don’t use H2O, I think they use KO2 or Na2O2. I’m sure there are tons of ways of doing it.
Either way, if you could create a system where you convert CO2 back into breathable O2 and all the reagents can be renewed back into their active form solely with energy like heat, then you could have something. But I think a lot of our current systems end up using the reagents in the process (H2O, KO2, Na2O2, etc) so you’d have to carry many pounds of that around with you too. Creating a few pounds of O2 would require carrying several pounds of water which gets used up in the reaction.
With battery technology constantly getting better maybe someone will come up with a method.
In my opinion, pure Oxygen (96%+ concentration) is even more dangerous than hydrogen or other inflammable. I’ve worked a lot with it and have seen carbon steel pipes being burnt like paper just because the velocity in the pipe exceeded a limit. You can get an asphalt road to explode if you drop liquid oxygen on it.
There is a 1970’s video on youtube (Warning - Graphic Content) on liquid oxygen and its handling Man from LOx.
Safety is probably the main reason why you wouldn’t want to keep highly pure - highly compressed/liquid oxygen near the human body - even for breathing purposes - (remember we are carbon based life forms and carbon loves oxygen).
As to removing CO2 from our exhalation - commonly KOH / NaOH are used to absorb it. In industrial settings - Amine solutions, glycol complexes, methanol etc are used in loop since they can be regenerated.
CO2 - in an exteremly hypothetical case can be regenerated to O2 if power was not an issue.
Here’s how you will do it :
CO2 + 2H2 ----> C +2H2O (will need new catalysts and may form side products)
2H2O —> 2H2 + O2 (Simple electrolysis) - H2 can be recycled o reaction 1
How much energy would it take to extract the same amount of oxygen from water, rather than CO2?
My thinking:
Water is liquid (and the densest) at room temp. Unlike dry ice, water expands when frozen as a solid. So, using electrolosys, you can yeild 33% of any potable or distilled water into O2.
CO2 would have to be under pressure/very cold to carry the same amount of O2 along. Otherwise, you’ll be back at big tanks.
Then, you could recoup a lot of oxygen by scrubbing the CO2 from your spent exhaled breathing as a secondary measure.
Gotta bring water wherever you go anyway, and electrolysis would yield 66% hydrogen too, which plays nice with O2 for energy/fuel, among a ton of other useful stuff as a raw resource.
Not quite true. Ice (water) expands when you cool it from 4 deg C to 0 deg C only. Cool it any further and it contracts (density increases). See wiki
That’s volume basis. On a mass basis, which is more relevant since you are considering payloads:
100 lb of water will yield 88.9 lbs of Oxygen and 11.1 lbs of Hydrogen.
100 lb of CO2 will yield 72.7 lbs of Oxygen and 27.3 lbs of Carbon
Look at the standard enthalpy of formation here FOr CO2 it is −393.509 kJ/mol and for water it is -285.830 kJ/mol. To make the same amount of Oxygen from water, you will have to spend (2x286/394 = 1.5) times more energy. So you’ll on the best case scenario spend 1.5 times the energy to make Oxygen from water than from CO2 - but this is only a thermodynamic comparison - the real world chemistry is much more complicated.
The problem with this is that there are very few forms of stable elemental carbon, and none of them are easy to work with. Individual atoms of carbon aren’t stable, they have too many free bonds and really, really want to form a molecule with something. When you react carbon dioxide and hydrogen, you don’t get water and solid carbon, you get water and some form of hydrocarbon, since the carbon atoms also need those hydrogens to form molecules. Stable elemental carbon can only exist as diamonds, graphite, or buckeyballs. Technology which can take carbon dioxide, strip off the oxygen, and then assemble the carbon into buckeyballs or something will require atomic-level matter manipulation that is essentially magic, especially considering the required throughput and miniaturization.
A basic piece of info that would be useful here is the oxygen requirements of a person per day. Assuming something that scrubs the CO2 from recycled air we could determine just how much oxygen would be needed to supply it without breaking down CO2.
If you read my post - I said that it needed new catalysts. And graphite is a very stable form of elemental carbon. It is feasible at the laboratory scale to do this - there are numerous references - here is one reference click
For another thread on metabolism, I calculated that the average person is exhaling something between 500-1000 pounds of CO2. The oxygen content of that is something like 70% (32/46). The range in weight obviously depends on the person’s weight and activity level and could easily be much higher for an astronaut in a demanding environment.
You are reading this wrong. It takes less energy to get oxygen from water. I don’t think it matters though. I think the only workable system will be one that recycles waste into Oxygen. I don’t think any material has enough molar density of oxygen to last a full day and still be comfortably worn.
How much Oxygen does an average person breath every day?
As for chemistry of recycling CO2, a variation on the water-gas shift reaction was posted upthread. Ultimately it does give you methane. Graphite is stable, but being a solid it will tend to gum up any catalyst that produces it. A more likely scenario is something more similar to photosynthesis. If solar energy isn’t available, it might be adjusted to nuclear somehow.
From the first google link (so chunk of salt with it, of course), a person breathes about 11000L of regular air, or about 550 liters of pure gaseous O2, a day.
If converted to LOX, that’s about a half liter. Of course, oxygen consumption for astronauts may be somewhat higher than average. For a supply of several days, that’s a couple soda bottles’ worth. With the apparatus to store and convert to gas, that would be a noticeable bulk, if not also heavy, with existing technology.
Actually that’s not that bad. Assuming it’s not a skin tight space suit, there should be room to distribute a couple of liters of LOX. It will still take space for something to remove CO2, and you can’t breathe liquid oxygen, it has to decompress to gaseous form and warm up. But it’s not as bad as it sounded initally.
Yes i asked a research chemist, a doctor of chemistry no less! He thought a Zeolite mesh, that is a structure that one can organize to have exactly the right holes in it to capture specific atoms. Acting rather like palladium does with Hydrogen.
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