KSR's Red Mars - Molecular sieve breathing mask

[DaveC426913]

This is about the physics of a molecular sieve, but since it is a fictional technology, I'm obliged to put it in Sci-fi.

It has been a long time since I read Kim Stanley Robinson's Mars series. They had masks that allowed them to breathe in the modified - yet still rarefied - Martian atmosphere. The idea was that the masks preferentially let oxygen through but blocked CO₂. (They were Areforming Mars - releasing oxygen into the atmo).

I am assuming the masks are using some passive technology , not an active pump that forces oxygen into the mask.

It seems to me that such a mask would have two major side-effects:

• it would be crammed on the user's face, like a dive mask, due to the partial pressure of the CO₂. The mask would experience what is efetiely a vacuum inside equivalent to the partial pressue of the CO₂.
• the user would feel like he is suffocating all the time, they would have to suck on the mask with force to get oxygen into his lungs.

I dont know the numbers KSR cited, but let's say the atmo was - I dunno - 5PSI of which 1PSI was oxygen.

So the masks would press on their face with 4PSI. For a mask that covers, say, 20 square inches of their face, that would be 80 pounds of pressure, no?

I'm not sure to work out how hard it would be to actually breathe. They're basically trying to breathe a 1% oxygen atmo.

 

[BillTre]

This how I would think about this:

• Molecular sieves will only remove some chemicals going by until they become saturated (binding sites used up). Bring in the oxygen from a low pressure atmosphere while removing the more common carbon dioxide is not going to work.
• For a human to be able to live off the gas being produced they would need some level of pressure that gets the partial oxygen pressure high enough for survival. Pure oxygen at a low pressure won't do it.
• Some kind of an exchanger with the atmosphere would have to pressurize what gets breathed prior to human consumption. It would use something like a pump to make a pressure increase.
• Changing the hemoglobin oxygen binding properties (genetic engineering or evolution over longer periods) might require less pressures. However, there would be a minimum amount (grams) of oxygen required to sustain a human. If the air is thinner (lower pressure), more air will be needed to fulfill the required amounts.
• Carbon dioxide will also have to be removed, possibly upgradient (depending on the atmospheric carbon dioxide partial pressure) going to the atmosphere. Might also require some pumping.

 

[DaveC426913]

I was under rhe impression in KSR's technology, the CO2 doesn't build up or saturate the mask filter; it merely blocks its passage. That's what supplies the pressure. The partial pressure CO2 treats the mask as having a vacuum on the other side.

 

[BillTre]

Something like a reverse osmosis membrane. These RO machines work by using pressure to push water molecules through a membrane that does not let other chemicals in the water go through. The pressure is to overcome the osmotic pressure that would develop when the the molecules accumulate to higher levels than on the side of the membrane that they came from.
This seems like a similar situation to me.
@Borek might have some ideas on this.

 

[Borek]

it would be crammed on the user's face, like a dive mask, due to the partial pressure of the CO₂. The mask would experience what is efetiely a vacuum inside equivalent to the partial pressue of the CO₂.

No, you can have a third, inert gas inside. Oxygen in, carbon dioxide out, inert gas stays inside and its amount is constant, keeping the pressure identical to the outside.

I don't see how to get a mask working in conditions given without some additional pump. RO doesn't work against a pressure gradient, molecular sieves in portable oxygen concentrators work in cycles where pressure changes through concentrating and cleaning phases.