Suppose you have a spacecraft on a long journey and you would like to recycle the C02 to 02 + waste graphite. The spacecraft has nuclear-electric power so energy is not a problem, but you don’t want to use plants because you feel that equipment and chemical reactors are more reliable. (not sure if this is likely to be true, but anyways)
How could you do it? Here’s half the reaction :
CO2 + 2H2 -> HCO2H + H2O
HCO2H -> CO + H2O
H2O -> 1/2H2 + O2
To summarize : react the C02 gas you isolated from the air (you can remove the C02 by refrigeration or using a regenerative absorber material) with hydrogen gas in the presence of a catalyst. Heat the water/methanoic acid mixture to high temperature. CO will bubble off. Electrolyze the water to hydrogen and oxygen.
But now what? You have carbon monoxide which is holding on to the remaining oxygen even more tightly than in the dioxide form! You don’t want to vent that CO to space, you want it to cough up the other atom. Google searches all get short circuited by references to the dangers of carbon monoxide.
Oh boy !! Having spent a large part of my career designing industrial systems involving C, CO, CO2, H2, HCOOH, CH4, etc. etc. here’s my critic and some suggestions :
This is sort of water gas shift reaction.. The predominant reaction is CO2+H2 -> CO + H2O. This reaction is equilibrium limited and the equilibrium shifts towards the left (more hydrogen generation) at higher temperatures - at lower temperatures the reaction is very slow. There is always some formic acid (HCOOH) formed - but it is undesirable. The estimates of how much formic acid is formed, and the means to avoid it, is proprietary knowledge of several Chemical companies. Formic acid at these temperatures is extremely corrosive.
You make this sound extremely simple - it is not! There are chemical means of separating CO2 from air - like KOH or NaOH but then the CO2 is turned into a carbonate salt. Some amine solutions will reactively absorb the CO2 - but the surface area of contact has to be huge and there needs to be a regeneration system. Amines degrade in presence of Oxygen - so you have another problem there. If you use Molecular sieves as adsorbent, they will adsorb the CO2 - but you may need a polishing media to bring the CO2 down to acceptable limits. Also - to regenerate the molecular sieve you will need an inert gas like nitrogen to sweep through it and/or high temperature.
So now you say you will use cryogenic methods - well you’ll need a LARGE heat ex-changer to cool all the air down to get the CO2 to freeze out. And even then some CO2 will remain in air because it is soluble.
What about the Hydrogen ?
Even if you successfully designed a system with such complexity - the weight of the system will far far surpass the weight of oxygen taken on board or better water taken onboard which is electrolyzed.
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You can always use the Sabatier reaction:
CO2 + 4 H2 → CH4 + 2 H2O + energy
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This reaction looks very good on paper. Run some Gibbs free energy and you will see that the equilibrium concentration of CH4 is very low (this is the reverse Steam Reforming reaction). To use this reaction - you will need extremely large reactors and compressors and heaters that keep recycling. Oh and separating the small amount of CH4 produced per pass of the reactor will be a nightmare.
[QUOTE=Dr. Strangelove]
And then use pyrolysis to get the hydrogen back:
CH4 + heat → C + 2 H2
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You will need a Plasma reactor to do this. And Plasma reactors are notoriously unreliable, need a lot of baby sitting - and moreover the reaction is not complete. So you will have to separate the little bit of H2 produced from the CH4 , and keep recycling the CH4. On the plus side - the crew will have plenty supply of mascara.
Maybe so, but a Sabatier reactor is already installed on the ISS and basically works. It has had a few issues but nothing that can’t be ironed out over time as best I can tell. It doesn’t seem to need a very large reactor; humans only need a few hundred grams of O2 per day after all, so a production rate of a couple grams per minute would be sufficient for a 6-man crew. Not much.
They don’t yet have a closed cycle, though. Bit more of a challenge there.
Seems to me that if you can afford to discard the carbon, why not simply discard the CO2 (or CO) wholesale and eat the weight for the extra O2 for the sake of simplicity. Or alternatively, close the carbon loop, too, and grow plants for both oxygen and food.
Recycling the oxygen but discarding the carbon means you’re still generating a lot of waste even though you’re doing a lot of processing. The only advantage it buys you is that you don’t have to depend on plants, but it seems to me that that can be made as reliable as chemical machinery with enough care. (I.e., build in the ability to start from scratch if a plant disease breaks out or something.)
The premise of the OP was a long space flight (mars?) and not the ISS. As I read more, the Sabatier reactor on the ISS is continuously replenished by H2 from space mission and it discharges CH4 into the atmosphere. I did not see data - but I am guessing that the catalyst also needs replacement every so many hundred hours of operation. In this NASA published paper- it notes that the catalyst loses activity very quickly.
Having worked with gas processes and pyrolysis processes, I can confidently say that these systems require some baby sitting, are prone to failure and require replenishment or refurbishment on a regular basis - hardly something you want on a long flight with no available resources coming to you.