I think with membrane separation you can get staged concentration, like they do with say, isotopic separation of heavy water (H2O vs D2O). With a 10-fold increa

I think with membrane separation you can get staged concentration, like they do with say, isotopic separation of heavy water (H2O vs D2O). With a 10-fold increase at each stage (400ppm -> 4000ppm -> 40,000ppm -> -> pure CO2) you end up with a pure CO2 stream (which is typically what you want for efficient high-pressure, high-temperature industrial chemistry). The real question is what kind of scale is needed to generate fuel at the levels desired - powering the concentrators, the water electrolyzers, the actual process (high pressure high temperature F-T synthesis). Ten square kilometers of solar panels and wind turbines for one oil tanker's worth per month? Of course, if the world had exhausted its fossil fuel reserves by say 1970 we'd already be doing this at scale.