The Eventual Challenge, of Designing a Closed, Life-Support System

In Science Fiction, a concept which occurs often, is ‘A Closed Life-Support System’. But just like eventual ‘travel at half the speed of light’, a real, closed life-support system has largely remained elusive.

Back in WW2, German Submariners knew a trick to extending the life of the air in their submarines during adverse moments, of sprinkling lime powder – i.e., CaO. But the only real effect this had, was to absorb carbon dioxide out of the air mixture in the submarine, and to hold it in some form of CaCO3 . The lime powder itself did not generate any oxygen, O2, in return for scrubbing the CO2 out of the air mixture. This actually served WW2 submariners well, because until that time, the dangers of CO2 toxicity were usually greater, than the real-life dangers of hypoxia. The era only came later really, that the USA launched various high-altitude platforms, and that hypoxia started to become a common problem, to be dealt with.

CaO reacts vigorously with H2O to form Ca(OH)2 , and nothing else.

This simple fact should not be taken to mean, that WW2 submarines had no way of generating oxygen. It only means, that lime powder as such, or other strong bases, do not generate oxygen by themselves. ( :1 )

What the current International Space Station does, is to collect various forms of moisture from the habitat, and to use ample electricity from its solar collectors, to electrolyze that water into O2 and H2. At the same time, the ISS collects CO2 from the interior air, using ethanolamine, which is a liquid, weak base. And after CO2 has been absorbed by a weak base, this gives the advantage, that the salt which results only needs to be heated to modest temperatures, to re-release the CO2. But a weakness which the ISS exhibits is then to release two waste-gasses back into space: H2 and CO2. If I’m not mistaken in this statement, the ISS falls short of achieving a real, closed life-support system. The CO2 may be released into space, but doing so ‘is made up for’ with fresh carbon introduced into the cycle, in the form of food, which is transported to the ISS.

Well when I was young, I read books, according to which certain technical problems inherent in space travel would soon be solved, which were never solved. One of them was, to devise a catalyst, or some other type of reactor, to combine H2 with CO2, in a way that produces more H2O, which could then be available for electrolysis again, and which would reduce the amount of waste to some unspecified carbonaceous solid. This carbonaceous solid, could then be made up for, in food that Astronauts ate.

But the unfortunate reality which remains is, that reactions that reduce CO2 using H2 remain unharnessed today. The closest to that which we have, is the famous water-gas reactions from the 19th century, that involve some mixture of carbon monoxide ( CO ) and H2. Well unfortunately, CO has not been reduced all the way to Cs .

(Updated 10/27/2018, 15h30 … )

(As of 10/24/2018 : )

1: )

One trick which was known in early submarine design, was not to use lime-powder, but actually to use potassium peroxide crystals ( K2O2 ) . When air was passed over them, several reactions would take place:

  • Moisture was removed from the air, because the solid peroxide itself was hygroscopic: 2H2O + K2O2 -> 2KOH + H2O2
  • Carbon Dioxide was scrubbed from the air, because of the strong, potassium base: CO{2} + KOH -> KHCO3
  • Oxygen was actually released, because peroxide can be reduced to yield plain oxide: 2H2O2 -> 2H2O + O2

But the remaining problem was, that the submarine only had some limited amount of K2O2 to start with, and to regenerate it was difficult. It could theoretically be regenerated, if the mixture of waste-products was heated to a high temperature, which would drive the CO2 back out, and if some external source of oxygen was found, for K2O to react with, thus reforming K2O2 . Yet, because even early submarines were eventually able to surface, or to extend a snorkel, the peroxide could conceivably even be regenerated. ( :2 )

However, for space travel the assumption must be made that there is no external source of O2. And this made peroxide-based resolutions of the issue, useless for space travel.


 

The role that lithium hydroxide played:

During the Apollo Missions, including the Moon Landings, one feature which the (open) life-support systems had, was the use of lithium hydroxide ( LiOH ) . Essentially, this played the same role which lime powder would have played, which was to provide a strong base, to scrub CO2 from the air. The advantage to using LiOH over CaO was, that lithium as an element was much lighter than calcium, and even that lithium ions were much smaller than calcium ions. Thus, a space-capsule could carry a sufficient supply of LiOH into space, to continue removing CO2 from the air mixture, for the duration of extended missions.

But the supply of O2 for those Astronauts, actually came in compressed form, in bottles.


 

Metal Oxide Catalyst?

If I may speculate, I’d think that the mentioned problem, of reducing CO2 using H2, might best be solved using a metal-oxide catalyst, such as maybe one based on thorium oxide or cobalt oxide. Metal oxide catalysts have already been the subject of some research, but their main drawback is the fact that they need to be operated at relatively high temperatures.

What a metal oxide catalyst would do, is to forbid the passage of any other ions, than oxide, O-2 ions. This is the reverse of what some other electrolytes and catalysts will do. But nickel-iron-thorium-oxide catalysts were once used successfully, to convert water-gas into a mixture of hydrocarbons. Their use would be messy, even if directed towards some other goal.

But one way they could be used would be, to set up the catalyst as a semipermeable membrane and heat it to 300⁰C, to pass pressurized CO2 on one side, and to pass H2 at a lower pressure, on the other side. When O-2 ions pass through, they should form water on the H2 side, but leave some form of Cs on the other side. In theory, some small amount of H2 could be mixed along with the CO2, just so that the final waste-product will be semifluid.

But then a drawback which remains is the fact, that Cs is a slightly more-energetic fuel, than H2. This means that CO will want to remain an oxide, more strongly than H2 would want to become water. This difference in energy, since it is pointing in the wrong direction, would need to be supplied somehow, such as maybe, through a supplied electrical voltage, (?)

(Update 10/25/2018, 11h55 : )

There are some major practical issues in trying to use anything related to the Fischer-Tropsch reaction, in space, to recycle CO2:

  • As CO2 is being reduced in this indirect way, carbon monoxide, ( CO ) , will also form. And I can see no way to make sure, that zero CO seeps back into the air-mixture – which is a closed environment – that the Astronauts are breathing. Because CO is a poisonous gas which has been known to kill silently – down on Earth – its levels need to be ensured to be zero, for use in space.
  • If, for the sake of argument, CoO2 is being used as a catalyst, then some small amount of the cobalt carbonyl ( Co2(CO)8 ) will also form, thereby eroding the catalyst over time. This carbonyl is also poisonous. And, in space, one would want an apparatus that functions indefinitely, while on Earth, catalysts can eventually be replaced.
  • Iron, nickel and cobalt are all similar elements, in that each of them will form its respective carbonyl. This means that they can all be used equally-well in Fischer-Tropsch reactions. OTOH, I would assume that ruthenium and thorium do not form carbonyl in quantity, since they each come from a different part of the periodic table. Yet, those latter two elements can also be used as Fischer-Tropsch catalysts.
  • If, on the other hand, thorium oxide was used as catalyst ( ThO2 ) , its main issue is the fact that thorium is a naturally-occurring, radioactive element, only with a very long half-life. It could be off-limits for many applications, just for being radioactive.

 

(Update 10/27/2018, 15h30 : )

2: )

Actually, certain ionic compounds exist which, even though one of the ions is a complex ion, will not decompose when heated. KOH is one such compound. Even when heated to its gaseous form, potassium hydroxide will not dehydrate back into potassium oxide.

I suppose the same is true for Ca(OH)2 / CaO .

What does usually work, is to heat certain bicarbonates, such that H2O is released, forming carbonate, and then to heat the carbonate, such that CO2 is released, forming the oxide.

One important observation to add however, is that the stronger the alkalinity of the base is, the higher the temperature becomes, which the carbonate must be heated to, before the latter releases its CO2. In the case of K2CO3, the substance melts at 891⁰C and decomposes at yet-higher temperatures. This puts in doubt, whether the submarines of WW2 could have done this. This would be too hot, effectively, to be achieved just by burning diesel fuel, and though electrical heating units were available, a heating unit that could reach, say, 1000⁰C, would have to have been made out of special substances, both as the ceramic, and for the wire.

Dirk

 

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