How Do You Get Nitrogen Buffer Gas On Mars?

Dook · 2017-05-06 12:05:18

If we're going to use nitrogen buffer gas in the habitats then where are you going to get it on Mars?

Tom Kalbfus · 2017-05-06 12:33:52

Here's a place.

louis · 2017-05-06 14:12:53

I guess if you extract all the reactive gases from concentrated Mars atmosphere you end up with nitrogen? No chemist I, though, so not sure if that would work.

RobS · 2017-05-06 15:47:11

The atmosphere is 96% carbon dioxide, 1.93% nitrogen, and 1.89% argon. You take the air, compress it, and let it cool off at night; most of the CO2 will become solid. Then you separate off the CO2 snow and the rest will be 50-50 nitrogen and argon. That combo would be fine for a buffer gas. It might lower people's voices slightly, but argon is actually better than nitrogen in that it won't give you the bends.

The rest of the atmosphere, by the way, is 0.15% oxygen and 0.5% carbon monoxide. Burn them together and you will get energy, carbon dioxide, and left over oxygen. But they are such a small trace of the atmosphere that you would need to concentrate them a lot to get a flammable mixture, and the energy expended compressing and separating probably exceeds the amount produced by the reaction.

louis · 2017-05-06 16:11:58

Thanks for that detailed explanation!

RobS wrote:

The atmosphere is 96% carbon dioxide, 1.93% nitrogen, and 1.89% argon. You take the air, compress it, and let it cool off at night; most of the CO2 will become solid. Then you separate off the CO2 snow and the rest will be 50-50 nitrogen and argon. That combo would be fine for a buffer gas. It might lower people's voices slightly, but argon is actually better than nitrogen in that it won't give you the bends.
The rest of the atmosphere, by the way, is 0.15% oxygen and 0.5% carbon monoxide. Burn them together and you will get energy, carbon dioxide, and left over oxygen. But they are such a small trace of the atmosphere that you would need to concentrate them a lot to get a flammable mixture, and the energy expended compressing and separating probably exceeds the amount produced by the reaction.

RobertDyck · 2017-05-06 20:50:10

I presented this at the 2005 Mars Society convention: ISRU Atmosphere Harvesting

Mars atmosphere as measured by Viking 2 lander:
Carbon Dioxide (CO2) 95.32%
Nitrogen (N2) 2.7%
Argon (Ar) 1.6%
Oxygen (O2) 0.13%
Carbon Monoxide (CO) 0.07%
Water (H2O) 0.03%
Neon (Ne) 0.00025%
Krypton (Kr) 0.00003%
Xenon (Xe) 0.000008%
Ozone (O3) 0.000003%

Method: Draw Mars atmosphere with a compressor. Use a filter to remove dust. Compress to 100 bar. Freezer coils at the bottom of the container will freeze to -100°C. At the same time a small rhodium based catalyst at the top of the container will combine CO with O2 to produce CO2. NASA produced this catalyst after 9/11, the idea is a face mask for people escaping a burning building. It's rhodium on a substrate, and requires +24°C to work. So this small metal catalyst at the top of the same container will be warmed to +24°C.

Compress to 100 bar pressure (1,450 psi). Going from ~7 mbar to 100 bar will require a multi-stage compressor. That much compression will heat the gas. A pressure transducer will detect pressure, once pressure hits 100 bar, stop the compressor. When pressure drops, start the compressor again. Freezer coils will freeze CO2 to dry ice. Removing water is not intended, but at that temperature water will reduce to vapour pressure of 10^-6150 which is so ridiculously low it's practically zero. Keep operating until temperature hits -100°C. Then keep operating anyway. As temperature drops, pressure will drop, causing the compressor to turn on. As CO2 is frozen to dry ice, pressure will drop, again causing the compressor to turn on. The catalyst will combine CO with O2 to produce CO2, which will freeze as dry ice, again causing pressure to drop. The catalyst will also decompose ozone (O3): 2 O3 → 3 O2. There is more O2 than CO to start with, and one molecule of O2 will combine with 2 molecules of CO, and with even more O2 provided by decomposition of ozone, there's plenty of O2 for this purpose. Once the temperature in the bottom stabilizes at -100°C, and pressure stabilizes at 100 bar, and carbon monoxide drops below detection threshold for the sensor, then turn off the heater for the catalyst. At this point do not allow the compressor to turn on again; pressure will drop a little as temperature at the top of the canister drops to the same -100°C as the bottom, but just let it, do no let the compressor draw in fresh atmosphere. Wait for pressure and temperature to stabilize again. Once it has, open a valve to a gas tank that has complete vacuum. This will draw off the gas quickly. It's important to do this quickly, because dropping pressure over dry ice will cause it to sublimate. You want to draw off the gas quickly before dry ice has a change to do that. Once the two tanks have the same pressure, seal the valve. Then open a valve to another tank, and heat the dry ice. This will sublime the dry ice to CO2 gas. So this last tank will collect CO2 gas. It will self-pressurize as dry ice sublimes to gas. After you have collected all the CO2, vent the rest to Mars atmosphere, then start another cycle.

This gas can be mixed with oxygen, then used as breathing air in a habitat. Because it's used to dilute oxygen, I call it "diluent gas". There isn't much carbon monoxide in Mars atmosphere, but after removing CO2 everything else will be concentrated. If carbon monoxide is not dealt with, it will be lethal. That's why the catalyst.

Estimated gas after this procedure:
CO2 0.75%
N2 61.0%
Ar 36.1%
O2 2.1%
Water 0.00%
Ne 0.0056%
Kr 0.00068%
Xe 0.00018%
O3 0.00%

This produces a mix of nitrogen and argon with the same ratio as Mars atmosphere. I have also argued to reduce spacesuit pressure to 3.0 psi pure oxygen. Air pressure on Earth is 14.7 psi, oxygen is 20.9%, resulting in partial pressure O2 on Earth at sea level 3.0 psi. Apollo used 3.0 psi O2 + 2.0 psi N2 for a total 5.0 psi pressure in the Command Module. They were originally going to use 3.3 psi pure oxygen in suits so that suits could withstand a 10% pressure loss and O2 would still be the same as the spacecraft. They increased pressure a bit, but using that same justification, give a Mars habitat 10% less partial pressure O2 as Mars spacesuits. That gives the hab 2.7 psi O2. Boulder Colorado has 2.54 psi O2, so this is less oxygen than KSC but more oxygen than Boulder.

You don't want to waste any time prebreathing oxygen before getting in a spacesuit to go outside. Apollo used low spacecraft pressure, so they didn't have to prebreathe. Shuttle and ISS use the same pressure as Earth at sea level, the same pressure as the launch site. As I said, even Boulder Colorado has lower pressure. Shuttle astronauts had to spend 17 hours prebreathing oxygen to flush nitrogen out of their blood before decompressing to spacesuit pressure. And Shuttle suits used higher pressure than Apollo suits. You don't want that on Mars. SCUBA diving studies found a ratio: maximum partial pressure nitrogen in the higher pressure environment to total pressure in the lower pressure environment is 1.2:1. For spacesuit pressure of 3.0 psi that means maximum partial pressure N2 in the habitat is 3.6 psi. Then keeping the ratio of N2 to Ar the same as Mars atmosphere, just so we don't have to waste energy separating them, that results in 2.133 psi argon. Again using Mars atmosphere as recorded by Viking 2 lander. There is also a maximum partial pressure of argon for zero prebreathe, but this is below that. That gives the habitat 2.7 psi O2 + 3.6 psi N2 + 2.133 psi Ar = 8.433 psi total. You could reduce diluent gas if you wish, but this is maximum for zero prebreathe time.

Blended air for the habitat:
CO2 0.52%
N2 42.4%
Ar 25.1%
O2 32.0%
Ne 0.0039%
Kr 0.00047%
Xe 0.00013%

This has CO2 above Earth's atmosphere, but below the 2.0% require to get a headache. It would smell stuffy, but you could breathe it. A freezer cannot reduce CO2 below this level, a sorbent must be used to remove the remaining CO2. But the habitat life support system has such a sorbent. Just operate the life support system to reduce CO2 to final level.

If anyone is going to obsess about trace gasses, I have to point out they're in Earth's atmosphere. Earth has:
Ar 0.9340%
Ne 0.001818%
Kr 0.000114%
Xe 0.000005% (one part in 20 million)
helium 0.000524%
methane 0.000179%
hydrogen 0.000055%

Last edited by RobertDyck (2017-05-06 21:01:07)

SpaceNut · 2020-09-12 09:13:09

Bump large co2 atmospheric processing...

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#1 2017-05-06 12:05:18

How Do You Get Nitrogen Buffer Gas On Mars?

#2 2017-05-06 12:33:52

Re: How Do You Get Nitrogen Buffer Gas On Mars?

#3 2017-05-06 14:12:53

Re: How Do You Get Nitrogen Buffer Gas On Mars?

#4 2017-05-06 15:47:11

Re: How Do You Get Nitrogen Buffer Gas On Mars?

#5 2017-05-06 16:11:58

Re: How Do You Get Nitrogen Buffer Gas On Mars?

#6 2017-05-06 20:50:10

Re: How Do You Get Nitrogen Buffer Gas On Mars?

#7 2020-09-12 09:13:09

Re: How Do You Get Nitrogen Buffer Gas On Mars?

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