This question is pretty much what it says on the tin: what would be an efficient (low pressure drop, reasonable energy consumption), low overhead (simple, lightweight) means to scrub acid gases from a feedstock stream without water loss (and ideally, with water capture)?

Say, the stream is primarily $\ce{CO2}$, a few percent nitrogen, 80kPa / 25°C; the $\ce{H2O}$ partial pressure is ~5 Pa (~20ppm); and there's a variety of acid gases in the dozens to hundreds of ppm range, and you'd like to scrub at least 90% of the $\ce{H2O}$ and acid gases, preferably more. Obviously if you were to just, say, run it through a packed bed with water you're just going to humidify the gas stream in the process of scrubbing, because the vapour pressure of water is three orders of magnitude higher than the $\ce{H2O}$ partial pressure already in the stream. Obviously if the gas stream were compressed to several hundred atm or more then you could do that, but such levels of compression would neither be efficient nor low overhead. You could likewise freeze out $\ce{CO2}$ to increase the partial pressures of everything else, and even distill the remaining gases - but these are anything but efficient, low-overhead solutions. I thought about the use of dilute sulfuric acid as the scrubbing medium to drop the vapour pressure, but the vapour pressure drop isn't nearly enough until high molarity, which would be relatively useless for scrubbing.

I'm sure there's a "right" solution for such a case, but I'm not sure what it is. Adsorption into a solid chemical bed, with subsequent regeneration (and if so, what sort of chemical)? Multistage, with say acid gas scrubbing first, then water scrubbing (and if so, what method for the latter to get down to under a few pascals partial pressure)? Selective membranes - say, to exclude $\ce{CO2}$, with a partial vacuum drawn on the other side? Some sort of ion pump?

  • $\begingroup$ Are you trying to scrub the carbon dioxide along with the other acid gases? $\ce{CO2}$ is itself an acid gas, so just about anything that'll pull down $\ce{SO_x}$/$\ce{NO_x}$ will also take up $\ce{CO2}$. You'll consume absorption media like crazy. $\endgroup$ – hBy2Py Feb 7 '17 at 15:10
  • $\begingroup$ Not trying to scrub the CO2, just everything else (HCl, SO2, SO3, some H2SO4 mist, small amounts of other acids, etc). I don't know about other media, but simple Henry's Law calculations show that water can easily scrub everything else without taking a significant amount of CO2, given adequate contact time/area. The problem is that it would also humidify the CO2 stream. $\endgroup$ – KarenRei Feb 7 '17 at 15:29
  • $\begingroup$ Taking away aqueous scrubber solutions is limiting. Do you need to roughly maintain that 5Pa value, or what is the limit for water vapor? $\endgroup$ – airhuff Feb 7 '17 at 16:04
  • $\begingroup$ Ideally even lower than the 5Pa value. Minimizing water remaining is critical. I'm thinking, perhaps a 3-stage process is best - an aqueous scrubber, than an easily reversible desiccation bed (say, silica gel), then a very aggressive desiccator like phosphorus pentoxide to scrub what remains. Hmm... come to think of it, I don't know how to calculate how rapidly water would move into or out of the absorber to determine if a given arrangement is plausible... $\endgroup$ – KarenRei Feb 7 '17 at 16:18
  • $\begingroup$ How about water absorption of acid gases followed by a CO2 membrane separator? IIUC there are some really good CO2-selective membranes starting to reach commercial viability, which I think should be able to pull CO2 and leave behind water and N2. $\endgroup$ – hBy2Py Feb 7 '17 at 16:22