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I found this diagram of the ISS ECLSS material flow and noticed the $CO_2$ being vented off the station.

ISS ECLSS SpaceStationCycle

source

Now, I understand this is outdated since they added a Sabatier reaction system to combine the $CO_2$ from the atmosphere and $H_2$ from hydrolysis to form $H_{2}O$ and $CH_4$. But the $CH_4$ is still vented off the station?

Sure, in the end the carbon is from food the astronauts eat, and on the ISS that'll always have to be shipped up from Earth. So the carbon cycle won't ever be closed. I guess food also supplies hydrogen, so venting it with the carbon would leave the same loop unclosed.

But it sounds like oxygen is now being almost totally recycled? Is that true? So is the only major biomaterial they have to ship up at this point food?

Basically, I guess I'm wondering what the current ECLSS diagram looks like.

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    $\begingroup$ AFAIK the ISS is not totally airtight. $\endgroup$ – Martin Schröder Apr 13 '18 at 8:52
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It's not as pretty as your diagram but here's one that includes the Sabatier

enter image description here

The paper this came from states that there is still a "long-term deficit of water" resulting in the need to supply makeup water from the ground.

Acronymology:

  • CDRA - Carbon Dioxide Removal Assembly
  • CWC - Contingency Water Container
  • CWC-I - Contingency Water Container - Iodine (despite the name, which is a Shuttle heritage term, use of these containers is nominal)
  • ECLSS - Environmental Control and Life Support System

  • OGA - Oxygen Generator Assembly

  • UPA - Urine Processor Assembly
  • WPA - Water Processor Assembly
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  • $\begingroup$ And so to the question of "How close to closed...?" can something be said? For example, would oxygen atoms cycle through the astronauts say five or ten times before becoming irreversibly lost or vented? $\endgroup$ – uhoh Apr 12 '18 at 12:27
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    $\begingroup$ I attempted to answer the question in the body "Basically, I guess I'm wondering what the current ECLSS diagram looks like." not the question in the title. $\endgroup$ – Organic Marble Apr 12 '18 at 12:34
  • $\begingroup$ Ah, I see; the question substantially "evolved" from top to bottom. $\endgroup$ – uhoh Apr 12 '18 at 12:36
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According to the Space.com article Astronaut Says It 'Smells Great' Inside the International Space Station (Video):

for CO${}_2$ $\rightarrow$ O${}_2$: almost 50%

NASA scientists are also trying to improve the percentage of carbon dioxide that gets recycled back into oxygen. Right now, the life support system converts a little less than 50 percent, but they hope that future technology will be able to recycle at least 75 percent, if not all of the carbon dioxide on board.

The narrator of the video shown there says:

On the station, if all the systems are working, we can recycle a little less than 50% of the carbon dioxide back into oxygen.

enter image description here


for H${}_2$O: 93%

"We want to increase the level of recycling wastes beyond what we do on the station now. Our ISS water system can recycle about 93 percent of the wastewater back to clean water," Molly Anderson, a principal technologist at NASA, says in the video. NASA scientists plan to fly a demonstration technology to the station soon that should be able to recover most of the other 7 percent, which is referred to as "brine."

enter image description here

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So the diagram Organic Marble posted is just the sort of thing I was looking for. But I thought I'd post some information I discovered on the chemistry of what remains unclosed in the system.

This is actually from a Wikipedia link that uhoh added to my question, and it answers everything I wanted to know, chemistry-wise.

The current system

The article discusses how the Sabatier system basically allows them to recover the oxygen from exhaled $CO_2$ by combining it with hydrogen from water:

$$2H_{2}O \overset{hydrolysis}{\rightarrow} 2H_2 + O_2 + \overset{food}{C} \overset{respiration}{\rightarrow} CO_2 + 2H_2 + \overset{added}{2H_2} \overset{sabatier}{\rightarrow} 2H_{2}O + \overset{discarded}{CH_4}$$ source

You end up with the same amount of water out that you put in, so removing those two oxygens and four hydrogens, leaving only what's consumed and produced, you get:

$$\overset{food}{C} + \overset{added}{2H_2} \rightarrow \overset{discarded}{CH_4}$$

Where does the extra hydrogen come from? The Wikipedia article points out that this is supplied by hydrolysis of water, which is where the "long-term deficit of water" that Organic Marble mentioned comes from.

Closing the loop

The Wikipedia article goes on to mention possible ways to recover the hydrogen as well.

Pyrolysis is a simple reaction which uses heat to separate carbon from hydrogen:

$$CH_4 \overset{heat}{\rightarrow} C + 2H_2$$

The article mentions the carbon would end up as an "easily removed" deposit of graphite. As the "[citation needed]" indicates, the practicality of this very much remains to be seen.

To avoid astronauts having to deal with chiseling out a bunch of graphite, you could perform incomplete pyrolysis, wasting some hydrogen but leaving the carbon in gaseous form as acetylene:

$$2CH_4 \overset{heat}{\rightarrow} C_{2}H_{2} + 3H_2$$

An alternative they're apparently investigating is replacing the Sabatier with the Bosch reaction, which wastes no hydrogen, but once again leaves astronauts to deal with the carbon as graphite:

$$CO_2 + 2H_2 \overset{heat}{\rightarrow} C + 2H_{2}O$$

Postscript

If you're thinking "Wait, we don't just eat pure carbon as food!" then yes, you're right, even the first equation isn't complete.

Food is complicated, but if we simplify it to sugars like glucose ($C_{6}H_{12}O_{6}$), which are basically groups of $CH_{2}O$, then it looks like:

$$2H_{2}O \overset{hydrolysis}{\rightarrow} O_2 + 2H_2 + \overset{food}{CH_{2}O} \overset{respiration}{\rightarrow} H_{2}O + CO_2 + 2H_2 + \overset{added}{2H_2} \\ \overset{sabatier}{\rightarrow} 3H_{2}O + \overset{discarded}{CH_4}$$

Basically, the hydrogen and oxygen from food ends up yielding more water, which ends up in the station system. Food ends up being another way of re-stocking the station's water supplies.

Indeed, respiration of food hydrates you completely independently of its actual $H_{2}O$ content! There are desert rodents which actually drink no water, surviving only on the water liberated from their food.

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