Venus ISRU: Gas Phase Processes

In the last post, we talked about methods to condense out the five most readily condenseable constituents of the Venusian atmosphere (sulfuric acid, water, sulfur dioxide, hydrogen chloride, and hydrogen fluoride). In some ways the next steps of the gas phase processing could be thought of as continuations of that process.

At this point in the process, the remaining constituents (assuming you’ve properly removed the previous-listed five condenseables) can be lumped into two categories:

  • Semi-reactive gasses like Carbon Dioxide, Carbonyl Sulfide, and Carbon Monoxide
  • Neutralish gasses like Nitrogen, Argon, Helium, and Neon

There are a couple of ways of proceeding from here. One thought to keep in mind though is that due to the large amount of CO2 in the atmosphere, you’re least likely to care about collecting it efficiently–it’s the other trace constituents that are worth gathering. The quickest way of separating out the other elements from the CO2 would be to do what someone suggested in the previous comments thread–freeze the CO2. With a freezing point of only -78C, the previous processes have likely already chilled the Venusian atmosphere fairly close to this temperature anyway. The phase change will take a lot of energy, but if after separating out the other constituents you allow a good fraction of the CO2 to be sublimed away on your refrigeration processor’s radiator (assuming you’re using active refrigeration instead of the heat-pipe trick), not all of that energy will go to waste.

In the process of freezing out the CO2, assuming you are doing this in some sort of a batch process, the pressure is going to drop by about a factor of 20x. This may raise the concentration of the carbonyl sulfide enough to get it to condense out as a dew that can be removed, since the air stream will now be lower than the boiling point of the COS, and the effective concentration will be increased by about 20x since you just got rid of most of the atmosphere.

If that’s successful, the only semi-reactive species that will be left is Carbon Monoxide. Carbon Monoxide’s boiling and melting points are much lower, so you probably won’t be able to remove it at this step. You have a few approaches you could take to removing the Carbon Monoxide. You could try to distill it out if you’re processing the rest of the more neutral gasses to individually separate them. Or you could mix the neutral gasses with oxygen and run it over a warmed catalytic converter. Or you could try reacting it with warmed sulfuric acid–since carbon monoxide is a fuel and sulfuric acid is an oxidizer. Or you could just live with it as an impurity. Or there might be some sort of way to filter it out electrochemically. Not sure which approach is best, but my guess is one of the first two.

Once you have the carbon monoxide removed, you’re down to neutral gasses like Nitrogen, Argon, Helium, and Neon. It may make sense to separate these further via distillation if you already have them this cold. This can proceed using similar equipment to what is used terrestrially for producing various industrial gases. For instance using either Linde’s or Claude’s processes to chill and liquify the gasses. If on the other hand you don’t yet have need for the separated gasses, you could just leave them mixed together as the buffer gas to mix with oxygen for breathing purposes. It should be noted that going into this step, if you’re down to just the neutral gasses, the Nitrogen is still about 99.8% of what you have. The concentration of Argon at this point is only 0.2%, which is actually lower than the 0.9% you find on earth, but Helium and Neon are both at much higher concentrations than they are in Earth’s atmosphere. So it might be worth processing them out via some sort of fractional distillation process.

Once you’ve removed the neutral gasses and dealt with (or not) the carbon monoxide, you can take whatever portion of the carbon dioxide you want to keep for chemical processing, and allow the rest to be vented overboard, using them to chill the hot-end heat exchanger on your active refrigeration system if you’re using one.

At this point you’ve now broken the Venusian atmosphere out into its constituents, and possibly chemically altered a few of them (reacting some of the Sulfuric acid to release water, or maybe catalytically converting the carbon monoxide). At this point you’re ready to start doing some basic chemical processes to try and create some chemical precursors.

Next Up: Basic Chemical Precursors ISRU Development Phases

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7 Responses to Venus ISRU: Gas Phase Processes

  1. George Turner says:

    So, I built a spread sheet of the gases of Venus based on Wiki and some information on OCS and H2S04 (which wiki strangely doesn’t list), and adjusting the nitrogen content a bit to 3.407% to make the numbers sum to 1 million ppm of all the gases.

    Removing the early condensates (H2S04, water, S02, HF, HCl) hardly affects the gas volume, leaving 999,827 ppm of the original. After you remove the CO2 you should have 99.67% N2, 0.20% Argon, 0.034% Helium, 0.02% Neon, and 0.048% CO. The CO is 488 ppm, which is above OSHA standards of 50 or 100 ppm, so it will have to be dealt with. I’d go with your thought of reacting it with the oxygen that will have to be added to the gas mix, since the concentration isn’t really high enough to be useful.

  2. Pingback: Venerian Gas-Phase Processes | Transterrestrial Musings

  3. Karl Hallowell says:

    I’m reluctant to throw away that carbon monoxide. At least it can be, along with the free oxygen available, used to make a simple energy releasing reaction. I doubt it’d be viable as rocket propellant, but it might have some use along these lines.

    I wonder if you could extract the carbon monoxide out by reacting with chlorine? That produces phosgene, a very toxic and notorious chemical (was used as a chemical weapon in the First World War), but with a boiling point of roughly 8 C at STP. And apparently, heating phosgene to 200 C decomposes it back into carbon monoxide and chlorine.

  4. born01930 says:

    Hey George…whats the chance of you sharing that spreadsheet? Also, is the Venusian atmosphere homogeneous? Not sure if the pressure gradient would effect the gas distribution. I never realized the high proportion of N2 in the Venusian atmosphere.

  5. George Turner says:

    Oops. I didn’t even save it, Born, but it only took maybe 15 minutes or so to build, just going from the Wiki and some random sources on acid (H2S04, HF, HCl) concentrations. I think perhaps the reason the acid concentrations are sketchy is that they aren’t homogeneous, because they precipitate and evaporate so their concentration, and state would be highly dependent on altitude and weather.

    I was thinking that the airships could start cleansing the atmosphere of sulfur by encasing it in graphite (made from the CO2), and perhaps wrapping that in thin stainless steel foil (commonly available down to 0.001″ thickness), and that may work, but there is an enormous mass of sulfur in the atmosphere.

    Given that Venus is thermodynamically stable, by which I mean that the temperatures and pressures have driven things into the current state, taming it will probably require bucking those stabilities by encasing some of the elements so they’re inaccessible to chemical reactions even at temperatures and pressures that would favor such, or giving them a “timeout” in storage tanks somewhere.

  6. born01930 says:

    George,

    You archive like a climate scientist…
    I would rather leave the atmosphere as is, how else to make those fabulous Thanksgiving turkeys? Make a floating city kind of like that idea someone had of a giant ship that floats around staying tax and duty free. We could revive indentured servitude, have the potential citizens pay for their travels by working in the mines far below.

  7. priusmaniac says:

    You are definitively going to use a thermoacoustic system to process the gases of Venus. It is rather compact and able to reach low temperatures in one single step. Thermoacoustic is part of the trip.

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