[Note: Karl caught an important oversight in the comments. With a concentration of 150ppm and a boiling point of only -10C, Sulfur Dioxide (SO2) should also be considered a condenseable. It's dew point is likely pretty close to water's. So I've updated this blog post to reflect that important oversight on my part.]
In my opinion the first place to start with ISRU processing of the Venusian atmosphere is to try and remove all five of the easily condenseable atmospheric constituents: Sulfuric Acid, Water, Sulfur Dioxide, Hydrogen Chloride (in the form of Hydrochloric Acid), and Hydrogen Fluoride (either directly or in the form of aqueous Hydrofluoric Acid). I think this is the best course of action for a few reasons:
- As the Venusian atmospheric constituents with the highest boiling and melting points, they are probably the easiest to extract from the atmosphere.
- The four easily condenseable constituents with hydrogen are the only local sources of hydrogen for a Venusian colony, and thus extremely valuable
- Sulfuric Acid is corrosive enough that removing as much of it as possible from the gas stream will probably make all downstream processes a lot easier/more-reliable.
Sulfuric Acid Extraction
In the case of the Sulfuric Acid, the freezing point of the acid is 10C, and the boiling point is very high (337C). This high boiling point and freezing point mean that of the four condenseables, the Sulfuric Acid will probably be the easiest one to extract. Especially when you factor in that in the cloud altitudes, the Sulfuric Acid is probably pretty close to its saturation density. Unfortunately the sources I could readily find didn’t give a clear indication of if this was truly the case or not. I’m not even sure if we know. If it really is at saturation density already, condensing it out of the air might take the form of a fog fence, or more likely some form of atmospheric water generator.
The fog fence would basically be a fine mesh net, probably of PTFE fibers, placed in front of the flow of Venusian air. Some sources I’ve read have indicated that the sulfuric acid droplets are likely positively charged electrostatically, so it might be possibly to electrostatically charge the the fog fence net to increase its ability to capture droplets with less pressure drop through the mesh. I’m not sure what the best method of getting the droplets back out of the mesh in order to collect the liquid. One possibility would be occasionally reversing the electrostatic polarity on the mesh net. Or possibly nothing may be needed as the mesh gets enough sulfuric acid trapped in it.
If the sulfuric acid droplets aren’t high enough density to make the fog fence work, you could chill the air until the sulfuric acid reaches its dew point and starts precipitating onto the cooling surfaces. Due to the higher boiling and melting points of sulfuric acid compared to the water, it should condense out first before the water starts condensing. Once most or all of the Sulfuric Acid has been removed, the remaining ISRU steps should become significantly easier.
After the Sulfuric Acid has been entirely or mostly removed from the gas stream, the next step is to remove the water. There is supposedly more water vapor than sulfuric acid at the altitudes in consideration, if the sources I’m reading are correct. To collect the water, the best approach is probably to chill the air until the water reaches its dew point, and then it will collect on the chilling surfaces. I’m not sure how far you have to chill the air to get this to happen at the concentrations of water we’re talking about. I saw many sources describing the dew point of water in a high-pressure carbon dioxide atmosphere (for CO2 scrubbing systems for plants), but not much on the dew point of water vapor in lower pressure carbon dioxide at this low of concentration of water.
If it turns out for instance that you have to chill the air so far that the water freezes onto the cooling surfaces instead of condensing on it, some sort of wet dessication approach could be used instead. In that approach, you use a brine solution to absorb water from the air, then pull a vacuum on the brine and heat it a bit to boil-off the captured water. I’m not sure which makes more sense in this situation. But those are the two main routes. Alternately, Sulfuric Acid is actually a strong desiccant, absorbing water out of the air to make a more dilute sulfuric acid. So it may be that you can get some of the water vapor out of the air with the sulfuric acid, and then distill out the water via boiling or lowering the pressure till the water boils out.
Sulfur Dioxide Extraction
The next major constituent to extract is the Sulfur Dioxide. With a boiling point of -10C and a freezing point of -72C, the Sulfur Dioxide should be condenseable using similar cooling processes to what was used for the Sulfuric Acid and the water.
Hydrogen Chloride and Hydrogen Fluoride Extraction
There are two possible routes for collecting the remaining two easily condensable species. First, both of them absorb into water to form hydrochloric and hydrofluoric acid. It may be that if the water extraction is done right, it will remove a decent amount of the HCl and HF at the same time–if you can get it to absorb into the condensed water fast enough. Alternately, you could extract them by continuing to chill the air until they condense out. In the case of HF, its boiling point is about room temperature, but in the case of HCl, the boiling point is cold enough (-85C) that it may not be worth trying to get it out if you can’t capture some of it in the water condensation step. Fortunately Fluorine is a more useful element than Chlorine, so the fact that it’s likely easier to extract than the HCl is useful. It may still not be enough to be worth the hassle, but if it can be extracted, HF is a key chemical precursor to creating fluorocarbons, which as one of the few materials that can handle concentrated sulfuric acid, will be really useful for exposed surfaces on these colonies.
Heat Pipe Cooling Source?
One other point worth making is that a potential heat sink for chilling the air was suggested in one of the previous comment threads–heat pipes connected to higher in the atmosphere. The Venusian atmosphere at this altitude drops 30-40K per 5km. I don’t know if it is at all practical to use a say helium balloon to support a heat exchanger at a higher altitude with an insulated heat pipe to transfer heat from the lower altitude collector and dump it into the cooler air above. If it is, it may enable much more rapid processing of the atmosphere since it would provide you with both a low-power way of pulling a ton of heat out of the atmosphere for extracting condenseables, but also as a way of keeping a relative inflow of air into the collector (since higher altitudes have faster winds on Venus).
If that proves to be impractical, wind or solar generated power could be used to run a traditional electric refrigeration circuit. Heat pipes just seem like a more elegant way of solving the problem.
What to do with the Sulfuric Acid?
Once you have the sulfuric acid extracted, there are several things to do with it. First off, it might be worth leaving some of it as sulfuric acid, either diluted with some of the water, or in concentrated form. But most likely most of the sulfuric acid is best broken down chemically to release the hydrogen (in the form of water), and eventually release the sulfur for use in sulfurcrete. The two simplest options I can see for making this work are to react the sulfur either with hot graphite or with hot carbon monoxide. Either of those should result in Sulfur Dioxide, Water, and Carbon Dioxide. The carbon monoxide route is likely easier to get to chemically than graphite, so is probably the better method. In this reaction it’s probably not worth trying to capture the CO2 or SO2 per se, since they’re already fairly abundant in the atmosphere, so really you’re just breaking down the sulfuric acid to release the hydrogen in the form of water.
Once you’ve done all of these steps, the air has had its most corrosive elements removed from it, and you’ve got water which is useful both as water itself, and as a source of hydrogen for all sort of other things (such as rocket propellants and plastics). There’s still a lot of details to be sorted out here on the best approaches for removing condenseables, particularly by someone who has a strong background in chemical engineering. But I think this provides a decent introduction to some of the approaches.
Next up: Gas-Phase processes.