Adsorption onto zeolites is also plausible, but the issue is that it requires alternately exposing the zeolites to high air pressure and low air pressure, and high airflow is required. We can idealize a perfect CO2 stripper as a magic box that inhales air and spits it out at 0 ppm. If you want a steady-state concentration of 500 ppm for 2 people, then we can see how much air-flow is required to lock up 2 kg of CO2 in 24 hours. This comes out to about 100 cubic feet per minute. This is the bare minimum air flow for any CO2 stripper, but in this particular case, it corresponds to a 25 horsepower air compressor, which is 18 kilowatts.
I’m not sure this is as much of a barrier as it sounds, at least if you have access to a window you can vent to. Imagine if I had a membrane that let only CO2 through, and a 50-micron vacuum pump (typically 1⁄2 HP). I could pump CO2 out of the room about as fast as the CO2 would diffuse across the membrane. In this setup, the amount of gas pulled out would be pretty tiny, because you’d be pumping only the CO2, but the effective amount of air being processed would be quite large.
So if you have a vacuum swing adsorption machine, with some zeolites in an array of mixing chambers, that can be alternately mixed with the air in the room and then evacuated outside, what matters is the equilibrium adsorption of each component of air, and the speed of adsorption / desorption. The air mixing can be done with a simple fan (providing the effective high volumes of air processing).
That said, there are other major problems with zeolites. The main one I’ve been struggling with is that zeolites really love to adsorb water vapor, nearly as much as CO2, and there’s a lot more water vapor than CO2 in the air. Competition for adsorption sites isn’t well-understood, but one study shows that water vapor in the air seriously decreases the amount of CO2 adsorbed. This also means that the system described above would function as a dehumidifier as much as a CO2 pump.
[epistemic status: very uncertain; writing as though I were more certain because I think it’s more fun / engaging]
I’m not sure this is as much of a barrier as it sounds, at least if you have access to a window you can vent to. Imagine if I had a membrane that let only CO2 through, and a 50-micron vacuum pump (typically 1⁄2 HP). I could pump CO2 out of the room about as fast as the CO2 would diffuse across the membrane. In this setup, the amount of gas pulled out would be pretty tiny, because you’d be pumping only the CO2, but the effective amount of air being processed would be quite large.
So if you have a vacuum swing adsorption machine, with some zeolites in an array of mixing chambers, that can be alternately mixed with the air in the room and then evacuated outside, what matters is the equilibrium adsorption of each component of air, and the speed of adsorption / desorption. The air mixing can be done with a simple fan (providing the effective high volumes of air processing).
That said, there are other major problems with zeolites. The main one I’ve been struggling with is that zeolites really love to adsorb water vapor, nearly as much as CO2, and there’s a lot more water vapor than CO2 in the air. Competition for adsorption sites isn’t well-understood, but one study shows that water vapor in the air seriously decreases the amount of CO2 adsorbed. This also means that the system described above would function as a dehumidifier as much as a CO2 pump.
[epistemic status: very uncertain; writing as though I were more certain because I think it’s more fun / engaging]