ISRU Atmosphere Harvesting
Robert Dyck
Mars Atmosphere
CO295.32%
N22.7%
Ar1.6%
O20.13%
CO0.07%
H2O0.03%
Ne0.00025%
Kr0.00003%
Xe0.000008%
O30.000003%
First, this is the raw material we have to work with. Although carbon dioxide is the largest component, it isn’t the only component. No nitrogen compounds have been identified yet in Mars regolith, so atmospheric nitrogen is the only source. Nitrogen is a component in protein so requires for food. Argon can be used to insulate sealed windows; important for the cold climate. There’s also carbon monoxide which has to be controlled to prevent contaminating breathing air. Oxygen is present, too little to be useful directly but important to control carbon monoxide.
CO2 freezer
/ filter dust/finescompress air to 10 bar
freeze to 170°K (-103.15°C)
Vapour pressure:
CO2 = 0.1 bar
H2O = 3.4x10-277 mbar
Pioneer Astronautics developed the MACDOF: Mars Atmosphere Carbon Dioxide Freezer. This unit froze CO2 into dry ice, blowing Mars air across it’s coils without pressurization. It avoided the power for a compressor, but everything other than CO2 was vented. To capture the other gasses we have to use a compressor to pressurize it. Even after freezing to 170°K and pressurizing to 10 bar, residual partial pressure of CO2 will be 0.1 bar.
Carbon Monoxide
After freeze: CO = 0.045%, O2 = 0.096%
CO: 0.001% symptoms of poisoning, 0.5% fatal
O2 + 2 CO → 2 CO2 with catalyst
2 O3 → 3 O2
After compressor stops, continue to heat catalyst until CO below detection threshold of sensor.
After catalyst heater is turned off, continue to run freezer until pressure stops dropping.
Release mix gas to storage tank and rapidly pump out.
Release residual pressure to Mars, then reseal.
Heat dry ice to sublimate, use pressure to fill CO2 tank.
If this residual gas is to be mixed with oxygen as breathing air in the habitat, it can’t contain carbon monoxide. The catalytic convert of a car uses heat to combine carbon monoxide with oxygen to form carbon dioxide, as well as breaking down unburned hydrocarbons. Remaining oxygen is greater than carbon monoxide, so we can convert it to CO2. By placing the catalyst in the freezer canister, synthesized CO2 will be removed with the rest. After 9/11 NASA developed a breathing mask that operates at room temperature to combine carbon monoxide with oxygen to form carbon dioxide. Since our catalyst will be in the same canister as freezer coils, the catalyst heater will fight against the freezer so lower temperature reduces power consumption. Either catalyst will also decompose ozone into molecular oxygen.
Mix gas
Resulting gas, by volume: / Breathing gas:CO2 / 0.74935% / CO2 / 0.52%
N2 / 61.0% / N2 / 42.4%
Ar / 36.1% / Ar / 25.1%
O2 / 2.1% / O2 / 32.0%
Ne / 0.0056% / Ne / 0.0039%
Kr / 0.00068% / Kr / 0.00047%
Xe / 0.00018% / Xe / 0.00013%
@ 8.43333 psi
2.7 psi PP O2
CO2 not noticeable below 2% for several hours.
Resulting mix gas will still have a little CO2, but mostly N2 and Ar. This can be blended with oxygen to form breathing air. The result will still have significant CO2, but it’s impractical to remove CO2 below this level with a freezer. A freezer is good to harvest large volumes of CO2, but scrubbing to extremely low levels requires a sorbent. Standard life support uses a sorbent to scrub CO2; rather than duplicating this system it would be more efficient to use the cabin recycling system to scrub that last bit of CO2. On Earth in 1 atmosphere pressure, CO2 below 2% is not noticeable at all and quite safe for short periods. After the cabin’s recycler scrubs the last CO2, the result is a tri-gas mix; oxygen, nitrogen, and argon. Air on Earth has 0.93% argon, and contains traces of the other noble gasses. This mixture has more argon and less nitrogen, but the NAVA already uses tri-gas for deep diving; it’s safe and well documented.
Spacesuit & habitat pressure
Spacesuit is the primary determinant of habitat pressure.
Shuttle oxygen pre-breathe time prior to EVA: 17 hours.
Mars 1 EVA per day, demands zero pre-breathe time.
Partial pressure N2 vs. suit total pressure: 1.2:1
Partial pressure O2 on Earth at sea level: 3.0 psi
Higher altitude cities have lower pressure but 21% O2, lower partial pressure is fine with acclimation.
2.7 psi habitat pressure is 90% of Earth sea level, reduces risk of fire, easily acclimated.
3.0 psi pure O2 suit reduces movement counterforce but leak of 10% pressure would reduce O2 to habitat level.
According to a pear reviewed paper from NASA’s Advanced Life Support project, the ratio of partial pressure of nitrogen to total spacesuit pressure is 1.2
Equipment size
Compressor: 113 litres/minute (Mars air)
Assumed Mars atmosphere average: 7mbar, -38.15°C
Compression time: ¾ sol (18 hours, 29 min., 41.4 sec.)
Gas collected:
CO2 1,882.9 g
mix gas 66.0675 g
Vessel volume: 5.880 l
Water ice accumulated per cycle: 0.24 g
A vessel 5.88 litres can collect 66 grams of mix gas per day. This volume doesn’t include catalyst or refrigeration coils. Water ice will accumulate only 0.24 grams per cycle, roughly ¼ ml per cycle. Collection volume for a settlement would require a larger vessel and perhaps higher pressure to acquire significant quantities.
Ammonia & Argon
N2 + 3 H2 → 2 NH3
O2 + 2 H2 → 2 H2O
First remove all CO2 with sorbent, any trace would combine with H2 to form H2O and CO.
NH3 + H2O → NH4OH
separate ammonia @ -77.7°C (freeze to solid)
Remaining: all noble gas
Ar 99.98%
Ne 0.0156%
Kr 0.00187%
Xe 0.00050%
Mix gas can be further separated. Burning mix gas with hydrogen will convert nitrogen to ammonia and oxygen to water. It’s important to remove all traces of CO2 prior to adding hydrogen to avoid forming carbon monoxide. Water will bind with ammonia to form hydrous ammonia, but there won’t be much water. Hydration level will be so low the ammonia will be called anhydrous. The result is 99.9% pure argon, with traces of other noble gasses, ammonia vapour and un-combusted nitrogen. Argon is used to fill sealed windows because it conducts less heat than air. Separation of ammonia requires cold, it boils at -33.43°C but has a high vapour pressure; complete separation requires chilling to the freeze/melt temperature. Luckily anhydrous ammonia freezes at a temperature regularly encountered on Mars at night. This also ensures no ammonia condensation within windows.
Uses of ammonia
•Nitrogen fertilizer for greenhouse
•Mars regolith has no nitrogen compounds
•Refrigerant, especially absorption cycle
Membranes
More energy efficient
Hollow fibres thinner than human hair, maintainable on Mars colony?
Membranes can be used with Sabatier, to separate
non-combusted H2 from CH4.
Membrane gas separation systems use hollow fibres thinner than a human hair, composed of polyimide. Membrane systems use filtration to ensure particulates don’t get into the membrane, often using a fine filter at the compressor followed by a carbon bed or molecular sieve to keep oil and water droplets out.
UBE's polyimide membrane is produced by the condensation polymerization of biphenyltetracarboxylic dianhydride and aromatic diamines.