Venus ISRU: Condenseables [Updated]

[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:

  1. As the Venusian atmospheric constituents with the highest boiling and melting points, they are probably the easiest to extract from the atmosphere.
  2. The four easily condenseable constituents with hydrogen are the only local sources of hydrogen for a Venusian colony, and thus extremely valuable
  3. 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.

Water Extraction
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.

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18 Responses to Venus ISRU: Condenseables [Updated]

  1. gbaikie says:

    “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.”

    Or with a pan or tarp.

    “As a point of interest, it should be noted that the frequent sulphuric acid rain showers on Venus never reach the surface of the planet. Falling from the cloud layer between 48 and 58 km, these acid droplets will encounter such high temperatures at 30 km that they evaporate. Sulphuric acid evaporates at about 300°C ; decomposing into water and sulphur dioxide. These gases then rise to feed the clouds. Contrary to what one might think, acid rain on Venus is therefore not a major cause of surface erosion. ”
    http://venus.aeronomie.be/en/venus/rain-thunder.htm

    But I can’t find any specific on how much it rains. But below atmosphere at over 300 C you don’t have Sulfuric acid or more than 1/2 mass of atmosphere is 300 C or higher.
    So pure Sulfuric acid has water in it, and pure Sulfuric acid is attractive to water. And when Sulfuric acid is heated on venus to 300 C it decomposes into sulfur trioxide. And sulfur trioxide and water are explosive [it really likes water].
    So at warmer than 300 C you can have water and sulfur trioxide
    together. Since most atmosphere is hotter than 300 C one will have more sulfur trioxide than Sulfuric acid on Venus. And Sulfuric acid is make in cooler temperatures if water available. But I seen some mention of Sulfuric acid being diluted water, or say, Sulfuric acid being at 50% concentration.

    So it seems to me one will have variation, and one variation [due to variety in weather conditions] *could* have conditions of near constant rain. And/or rain may be only and uniformly like mist or dew, or maybe it’s varying so there is large droplets or like Earth down pours of raining type events.
    What is constantly stated is Venus has a lot clouds. In pictures of Venus it looks there are denser regions. But also it does seem to be like Earth clouds [big thunder clouds with billions of lbs of water in them:
    “Q: How many gallons of water are stored in an average thunderstorm cloud?
    A: According to calculations from Peggy LeMone, senior scientist at the National Center for Atmospheric Research, a thunderstorm cloud contains approximately 275 million gallons of water. With 750,000 gallons of water going over Niagara Falls each second, it would take six minutes for an equal amount of water to go over the Falls.”

    So huge clouds of Venus may be mostly light fog but great depth
    Or they could as uneven as clouds are on Earth.

  2. Paul D. says:

    You have pressed one of my pet peeve buttons!

    The element is flUOrine, not flOUrine. It’s a halogen, not a baking ingredient.

  3. Eldritch says:

    I’ve done the calculations on this, and the dew point of water at that concentration is significantly below freezing.

    According to this calculator
    (http://www.dew-point.com/equivalents.asp)
    the dew-point of water at Venerian concentrations in 1 atm of CO2 is around -55 to -52 degrees C.

  4. Jonathan Goff says:

    Paul,
    Good catch. I was typing most of this up late at night while dealing with one of my boys who had the croup. So I didn’t have a chance to spell check things very thoroughly.

    ~Jon

  5. Jonathan Goff says:

    Eldritch,

    Thanks for running the numbers. So that means that you’ve got a few options–go with a wet desiccant approach (either with sulfuric acid or with brine), or possibly just let it freeze onto the collector surfaces and have a method for removing the frozen water from the collector surface.

    ~Jon

  6. Karl Hallowell says:

    Another condensation to consider here is freezing out CO2 (and condensing SO2) to separate out nitrogen and other low boiling point gases (argon, oxygen, carbon monoxide, helium, and neon).

  7. Karl Hallowell says:

    Another condensation to consider here is freezing out CO2 (and condensing SO2) to separate out nitrogen and other low boiling point gases (argon, oxygen, carbon monoxide, helium, and neon).

  8. George Turner says:

    One use for the sulfur would be in large molten sodium-sulfur batteries whose operating temperatures of 300 to 350C would be ideally suited to powering airships near the planet’s surface, where temperatures and the dim light rule out solar cells.

    Perhaps such ships could either directly dig at the surface for resource extraction or anchor themselves near the surface and run electrostatic dust filters to pick up particulate matter carried on the wind, then periodically return to high altitudes to drop off the dust and recharge their batteries.

  9. Jonathan Goff says:

    Karl,
    That’s where I was going to go next. If you’ve chilled the air enough to get the water out of it, you’re almost cold enough to get the CO2 out.

    Also, I forgot to look up SO2. With a concentration of 150ppm, and a boiling point of -10C, SO2 is just as much a “condenseable” as water. I’ll update the blog to reflect that one.

    ~Jon

  10. George Turner says:

    It strikes me that you can get CO2, nitrogen, argon, and helium from a welding shop, and Walmart sells concentrated sulfuric acid in the plumbing section. It would be pretty trivial to make up a mix that matches the atmosphere of Venus, add a touch of water, and completely debug the Venus ISRU processes in a garage on Earth.

  11. Jonathan Goff says:

    George,
    Trivial? Probably not. But definitely feasible to do a proof of concept. The challenge is that I don’t think we actually know the Venusian atmosphere that well. That said, I think you could simulate the relevant environment sufficiently well to learn a lot before having to spend the big bucks on an ISRU demonstrator. Not a bad idea. My guess is that you could probably build a Venus simulator chamber and work out the process of creating and validating the constituent concentrations within the scope of an SBIR-sized contract. Interesting idea.

    ~Jon

  12. Jonathan Goff says:

    Following up on my previous comment, I checked the currently open NASA SBIR/STTR solicitation, and while they have a Mars atmospheric ISRU topic open, they don’t have anything relevant to Venus. It might be possible next year though for someone with some sway in the Venus community (Geoffrey Landis?) to submit a Venus ISRU topic that could pay for research in this area. I think it would be a good idea for VEXAG to prioritize getting a topic in, since it doesn’t necessarily cost them any money, and it can encourage active research in the area. They could put it either as an SBIR topic or an STTR topic (if they wanted to encourage work together with a University).
    ~Jon

  13. Andrew W says:

    I don’t know if this could be useful:
    How about a system anchored to the surface that’s dragged through the smog layer that uses a system to generate condensation with a low pressure zone creating air foils, as is sometimes seen with maneuvering aircraft?

  14. George Turner says:

    Well, I guess by trivial I mean that since the goal is to run such equipment at roughly Earth’s atmospheric pressure and temperature, experiments won’t require a large vacuum chamber (as a Mars atmosphere simulator would, and that is very expensive).

    Aside from the slight sulfuric acid issue, the environment is going to be very benign for things like pumps and bearings, and the target environment is close enough to 1G that anything that works in your “garage” (sealed off with drop cloths and duct tape if your altitude matches the target altitude, whether sea-level, Denver, or the high plains of Chile, to ensure the atmosphere stays at a constant pressure even as you extract components of it) will probably work just the same as it would floating in the upper atmosphere of Venus. Of course a real test would use a much fancier room with lots of chemical monitoring of the gas components, but there’s nothing particularly high-dollar required to recreate the target environment. The major gas components you need are at your local welding supply house or Walmart, aside from a few oddballs like OCS (carbonyl sulfide?) whose existence is still a mystery.

    A university engineering department would surely get a kick out of the project. I could see some student design competitions, pitting the heavy but efficient conventional refrigeration equipment against lightweight but less efficient Peltier coolers, etc, and perhaps adding some rocking motions to simulate an airship so they can’t absolutely depend on water flowing like they want due to slight changes in gradient.

    And then the follow on contests would involve automated ways to make useful materials from the materials the equipment extracts (plastic precursors, plant food, and fuels), and then the big challenge of making all or almost all of the necessary equipment to do that using only components made in situ, so that there’s a natural growth potential aside from perhaps required silicon or aluminum components like solar cells, which will have to take a different route.

  15. George Turner says:

    Andrew, that’s interesting, but generally the condensation from an airfoil just gets dumped back into the airstream. However, the slight change in pressure that creates the condensation comes from a slight pressure drop, and given the lift potential at depth on Venus, where a strong rigid structure is flyable, you could perhaps just ingest some atmosphere into a tank, pump out a bit of gas to drop the internal pressure, and your condensibles will fall out inside the pressure vessel for collection.

    But if you instead inflated a balloon to raise the pressure vessel higher, thus cooling it naturally, the condensibles (sulfuric acid primarily) will fall out like rain normally would at higher altitudes, yet still be trapped inside the vessel.

    However, your idea might be ideal if it was scaled up so that there was condensation imparted in a relatively closed structure where it could be harvested. From an energy standpoint, I’m not sure how efficient the method is. I’ve heard of all sorts of desert dew traps to collect moisture, but never an airfoil trap. Generally, when the air is extremely dry, you’re just not going to see any condensation off an airfoil.

  16. Egad says:

    Mention of sodium up-thread causes me to wonder: Do we know if the Venusian atmosphere has a meteorically-deposited sodium layer like the Earth’s?

  17. Karl Hallowell says:

    Glancing at George’s comment, it strikes me that in the future, a terrestrial mix of gases similar to what is though to be in Venus’s atmosphere, which I guess would be an “atmosphere simulant” analogous the Lunar and Martian “soil simulants” that can be purchased today, might have a niche market.

  18. Jardinero1 says:

    One could grow algae in bags to extract the CO2. As the algae takes up the CO2 you get edible food and the algae emits oxygen and hydrocarbons which can be refined into plastics or used for fuel.

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