[mbbutler]

The tech for Direct Air Capture (DAC) is already quite efficient at removing CO2 from the atmosphere (80%+ depending on the particular process). At best, new innovation on the chemistry of the removal process can only increase the efficiency by 25%.

The real issue with DAC is that it is incredibly difficult to innovate around the fact that CO2 in air is just immensely dilute. You need to process enormous amounts of air to remove an appreciable amount of CO2 and, even worse, as the plant operates and recirculates processed air, the local air around the plant becomes more and more devoid of CO2 leading to a decreased amount of CO2 captured per unit volume of air.

The only real improvement that I could see occurring in this space is coming up with a process that creates less back-pressure against the fans pumping in unprocessed air which could bring down the energy cost per ton of CO2 removed. But even in that case the back pressure created by a DAC process is normally caused by flowing air through a porous catalyst, which is essential for high efficiency. So there's a trade-off there as well.

Ultimately I am not very sanguine about DAC and I have been disappointed to see news agencies reporting as if DAC is even in the top 10 technologies most important to reduce carbon emissions.

[photochemsyn]

Staggered extraction cycles, where each round leads to a 10X increase in concentration, is one option. If you look up how D2O (heavy water, where the hydrogens are replaced by deuterium) is produced, the first step is often done that way. The initial stage is the concentration of HDO from H2O, which is naturally found at about 1 part in 3200. 400 ppm CO2 is 1 part in 2500, so it's comparable.

Proof-of-process for Starship fueling would be generating a stream of pure CO2 from air at a rate sufficient to feed into a methane production facility (which would require a similar stream of hydrogen from water processing) to produce around 500 tons of liquid methane in a reasonable amount of time. Possible?