Author Topic: Water, Methane, and Oxygen ISRU on Mars  (Read 22373 times)

Offline Russel

  • Full Member
  • ****
  • Posts: 966
  • Liked: 84
  • Likes Given: 1
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #60 on: 02/01/2018 12:39 AM »
I believe the compression is needed to get sufficient flow rate through the zeolite beds to achieve the water extraction rate needed.  Cannot just open a canister of desiccant and expect a significant capture rate by diffusion alone -- gotta pump 250,000cubic meters of Martian atmosphere through the beds to get one liter of captured water.
There may be ways to use natural flow to extract water from the atmosphere.

In fact, you could mine gypsum, extract water from the gypsum, and dump the anhydrite back onto the surface where it will slowly reabsorb water from the atmosphere and become gypsum again. In fact, you could have sheets of something like gypsum or other hydrated minerals that you harvest periodically, dehydrate, then place back onto the Martian surface to reabsorb water. Perhaps arranged vertically along with the direction of the wind to maximize flow rates and areal density of plates.

I bet that'd be more energy efficient.

This is working its way back to using specialised reusable adsorption media like zeolites. What I wonder is whether there aren't better materials even if the process of re-extracting the water is a bit more energy intensive than an addorptive media.

Offline Russel

  • Full Member
  • ****
  • Posts: 966
  • Liked: 84
  • Likes Given: 1
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #61 on: 02/04/2018 09:43 AM »
The following may end up going in the advanced concepts section if I signed up.
However I'll post it here for now and see what you think.

I've been trying to devise a method to extract water vapour from Martian air that relies upon a simple heat pump. The energy required to extract the vapour is itself not much. Its just the heat of vaporisation of water or about 0.6KWhr per Kg. Of course no real process will get close to this but as an order of magnitude issue its not a real concern.

The issue is that the water vapour is a tiny fraction of the air. For what follows, I'm going to use the figure of 20 parts per million. That's the fraction of water vapour extracted (not all of the available water vapour).

So, for 1Kg of water we need to process 50,000Kg of air, which for current purposes can be treated as if its all CO2. Of course this is back of the envelope but if you were to take 50,000Kg of CO2 and lower it by 50 degrees (sufficient to freeze most of the water and yeah I know it does vary) then its going to cost you 2GJ of energy. About 555KWhr. With a steady 10KW of power that's 400 grams of water per day. Perfectly fine for life support but we can do better.

You're wasting a lot of energy cooling CO2 and venting it. Why can't you use that cold CO2 to cool the incoming CO2? In fact you can do just this and its the overall coefficient of performance that multiplies the quantity of water you get out.

What does it look like in practice? Just like a refrigerated dehumidifier on Earth, only with some attention to efficiency. A working fluid (nitrogen? argon? refrigerant?) is compressed and run through a heat exchanger. This transfers heat from the working fluid to the outgoing air. Its a reverse flow heat exchanger which reduces the effective temperature differential the heat pump is working with and that's where high COP is possible.

The working fluid passes through a mechanical expander (to recover some energy). It then goes through another reverse flow heat exchanger where the working fluid collects heat from the incoming air. In this way heat is being pumped in a loop out of the incoming air and then back into the outgoing air. There's nothing particularly "advanced" about this. It just requires careful design.

Now, what does require some special thought is the efficiency of the heat exchangers, the surface area onto which ice can form and minimising the flow loses. Sounds like a job for a PhD thesis.

My take on this is a composite graphene/polymer heat exchanger that is 3D printed for maximum surface area to mass ratio. It may even be coated or otherwise surface treated to increase the surface area. The incoming air is filtered (down to a few microns) and then run through the heat exchanger. Water molecules form a frost on the surface of the heat exchanger. Remaining dust particles may actually help the process here.

At some point heat is applied to the heat exchanger and the ice turns to water and runs off. It takes with it the finer fraction of Martian dust which is itself filtered out.

The cold (and dryer) Martian air then passes through the other heat exchanger (which is probably going to be of a similar design).

I'd like to see an overall performance of under 100KWhr per Kg but under 50KWhr/Kg is entirely possible. If that pans out then a 10KW source of electricity will get you 4.8Kg of water per day. That's getting close to being useful as a source of propellant.

Incidentally that 4.8Kg/day of water means an air flow of about 140m3/s. Yes, its big, but its also implementable. This is after all low density air.

Now all of this is truly back of the envelope. It does depend on humidity and there's a lot of things that have to be designed very carefully. But I hope it inspires someone with better thermodynamics knowledge to comment.

Once you have a source of clean, dry Martian air its then possible to tap off a small fraction and from this freeze off the CO2. What you have left is primarily Argon, Nitrogen, Oxygen and Carbon Monoxide - all of which are useful.
« Last Edit: 02/04/2018 09:45 AM by Russel »

Offline Lar

  • Fan boy at large
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 10059
  • Saw Gemini live on TV
  • A large LEGO storage facility ... in Michigan
  • Liked: 6884
  • Likes Given: 4671
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #62 on: 02/04/2018 03:33 PM »
Russel:

Sounds very reasonable. (I got a D in P-Chem which included heat transfer so I am not competent to comment on the math)

I would be interested in further trades on this vs. water mining from the soil vs drilling and heating. But your filter and dewatered atmosphere stream feeds nicely into a trace gasses extraction unit. That, however, may push the heat exchange somewhat downstream as it will benefit from the coldest possible stream input.

See the prior thread on chemical industry on Mars.

Here it is:
Proposed ITS Cargo Modules to Initiate a Chemical Industry on Mars
http://forum.nasaspaceflight.com/index.php?topic=42053.0
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline sghill

  • Full Member
  • ****
  • Posts: 1353
  • United States
  • Liked: 1505
  • Likes Given: 2147
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #63 on: 02/04/2018 03:56 PM »
One thing I'd like to ask here. Fairly simple.

If you only consider sites on Mars that are of relevance for exploration. Which I presume tends to limit things to under 20-25 degrees from the Equator.

What do we actually know about the presence of water, how deep and in what form and quantity?

In other words, given realistic landing sites, what are we dealing with and also how probable is it? In other words do we need to land a probe beforehand to be certain of the resource?

It seems to me that ISRU for water on Mars comes on one of the following forms:

1. Water vapour
2. Hydrates and other forms of water bound to soil.
3. Thin layers of ice of some reasonable purity of unpredictable depth and possibly localised scope near surface (1-2m at most)
4. Thicker ice, but with some certainty and located deeper (up to about 30m).

Lets consider those in turn.


You are over thinking this. To make methane and oxygen rocket fuel from the atmosphere, you need to capture the mix of ambient gases first, then you must separate the gases from each other.

The simplest means to do this is cryogenic air separation. You store martian atmosphere, then cool it off.  As the gas gets colder, each molecule liquifies. First all the water comes out, then the CO2, then the argon, nitrogen, oxygen, hydrogen, etc.

H2O, argon, and nitrogen are found in very small amounts in the martian atmosphere, but they are extremely valuable, and have to be separated out first anyway to get down to the oxygen.

Thus, if you are making rocket fuel, the process gets you all the other stuff too.

Air separation is a critical path technology for Mars, along with electricity generation.  These two technologies will be on the first BFR flights before people get there, IMHO. Otherwise, no return trips.

https://en.wikipedia.org/wiki/Air_separation
From Wikipedia, here's how the process works:

To achieve the low distillation temperatures an air separation unit requires a refrigeration cycle that operates by means of the Joule–Thomson effect, and the cold equipment has to be kept within an insulated enclosure (commonly called a "cold box"). The cooling of the gases requires a large amount of energy to make this refrigeration cycle work and is delivered by an air compressor. Modern ASUs use expansion turbines for cooling; the output of the expander helps drive the air compressor, for improved efficiency. The process consists of the following main steps:

Before compression the air is pre-filtered of dust.

Air is compressed where the final delivery pressure is determined by recoveries and the fluid state (gas or liquid) of the products. Typical pressures range between 5 and 10 bar gauge. The air stream may also be compressed to different pressures to enhance the efficiency of the ASU. During compression water is condensed out in inter-stage coolers.

The process air is generally passed through a molecular sieve bed, which removes any remaining water vapour, as well as carbon dioxide, which would freeze and plug the cryogenic equipment. Molecular sieves are often designed to remove any gaseous hydrocarbons from the air, since these can be a problem in the subsequent air distillation that could lead to explosions.[6] The molecular sieves bed must be regenerated. This is done by installing multiple units operating in alternating mode and using the dry co-produced waste gas to desorb the water.

Process air is passed through an integrated heat exchanger (usually a plate fin heat exchanger) and cooled against product (and waste) cryogenic streams. Part of the air liquefies to form a liquid that is enriched in oxygen. The remaining gas is richer in nitrogen and is distilled to almost pure nitrogen (typically < 1ppm) in a high pressure (HP) distillation column. The condenser of this column requires refrigeration which is obtained from expanding the more oxygen rich stream further across a valve or through an Expander, (a reverse compressor).

Alternatively the condenser may be cooled by interchanging heat with a reboiler in a low pressure (LP) distillation column (operating at 1.2-1.3 bar abs.) when the ASU is producing pure oxygen. To minimize the compression cost the combined condenser/reboiler of the HP/LP columns must operate with a temperature difference of only 1-2 K, requiring plate fin brazed aluminium heat exchangers. Typical oxygen purities range in from 97.5% to 99.5% and influences the maximum recovery of oxygen. The refrigeration required for producing liquid products is obtained using the Joule–Thomson effect in an expander which feeds compressed air directly to the low pressure column. Hence, a certain part of the air is not to be separated and must leave the low pressure column as a waste stream from its upper section.

Because the boiling point of argon (87.3 K at standard conditions) lies between that of oxygen (90.2 K) and nitrogen (77.4 K), argon builds up in the lower section of the low pressure column. When argon is produced, a vapor side draw is taken from the low pressure column where the argon concentration is highest. It is sent to another column rectifying the argon to the desired purity from which liquid is returned to the same location in the LP column. Use of modern structured packings which have very low pressure drops enable argon with less than 1 ppm impurities. Though argon is present in less to 1% of the incoming, the air argon column requires a significant amount of energy due to the high reflux ratio required (about 30) in the argon column. Cooling of the argon column can be supplied from cold expanded rich liquid or by liquid nitrogen.

Finally the products produced in gas form are warmed against the incoming air to ambient temperatures. This requires a carefully crafted heat integration that must allow for robustness against disturbances (due to switch over of the molecular sieve beds[7]). It may also require additional external refrigeration during start-up.
« Last Edit: 02/04/2018 03:59 PM by sghill »
Bring the thunder Elon!

Offline Lar

  • Fan boy at large
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 10059
  • Saw Gemini live on TV
  • A large LEGO storage facility ... in Michigan
  • Liked: 6884
  • Likes Given: 4671
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #64 on: 02/04/2018 06:07 PM »
That's some fascinating info but some of it will need modification, you can't separate out the CO2 in an exchanger bed, it's a major component.  Russel's subseaquent post seems to be congruent with what you posted but could benefit from a critique that was more specific to what was sketched out.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Online A_M_Swallow

  • Elite Veteran
  • Senior Member
  • *****
  • Posts: 8373
  • South coast of England
  • Liked: 305
  • Likes Given: 132
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #65 on: 02/05/2018 01:29 AM »
That's some fascinating info but some of it will need modification, you can't separate out the CO2 in an exchanger bed, it's a major component.  Russel's subseaquent post seems to be congruent with what you posted but could benefit from a critique that was more specific to what was sketched out.

Carbon Dioxide has a significantly higher triple point than the other gasses. Is there some way of turning it into snow that falls into a container whilst filtering off the gasses?

Offline sghill

  • Full Member
  • ****
  • Posts: 1353
  • United States
  • Liked: 1505
  • Likes Given: 2147
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #66 on: 02/05/2018 03:54 PM »
That's some fascinating info but some of it will need modification, you can't separate out the CO2 in an exchanger bed, it's a major component.  Russel's subseaquent post seems to be congruent with what you posted but could benefit from a critique that was more specific to what was sketched out.

You freeze out the CO2 and water vapor, then remove the pure precipitates. You're then left with the remaining gases and can utilize gas separation to purify and capture each element.

You have the advantage that ambient temps at night are already cold enough to precipitate CO2, which lowers your energy need.
Bring the thunder Elon!

Offline Russel

  • Full Member
  • ****
  • Posts: 966
  • Liked: 84
  • Likes Given: 1
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #67 on: 02/06/2018 04:41 AM »
That's some fascinating info but some of it will need modification, you can't separate out the CO2 in an exchanger bed, it's a major component.  Russel's subseaquent post seems to be congruent with what you posted but could benefit from a critique that was more specific to what was sketched out.

Carbon Dioxide has a significantly higher triple point than the other gasses. Is there some way of turning it into snow that falls into a container whilst filtering off the gasses?

I've toyed with this problem for quite some time. It works out to be two different problems. One is extracting water vapour. The other is extracting the minor gasses, Ar, N2, O2 and CO.

To obtain enough water vapour you need to process very large quantities of air. Roughly 50,000Kg of air for 1Kg of water.

The only way I can see doing this that approaches the ideal therodynamic limit is to cool the air, extract the water ice and then warm the air. The key to efficiency is using reverse flow heat exchangers and keeping the difference between Thot and Tcold for the heat pump  as small as possible. Remember that because of the heat exchangers that temperature difference is not the change in temperature of the air being processed. You might cool air from -20C to -100C, but the heat pump may only see a 25C differential.

Getting the minor gasses is a different proposition. For every Kg of O2 you need about 700Kg of air. But you need to extract the CO2 in a thermodynamically reversible fashion.

My initial take on this is to compress the CO2 in a multi stage process. The heat being transferred to several heat exchangers. Eventually the CO2 ends up as a warm gas at about 20-30 atm.

A heat pump lowers the temperature to the point that the CO2 liquifies. This liquid is now separated (along with a residual ice fraction).The gases are Ar, N2, O2 and CO. This is your fedstock.

Now the liquid CO2 is pumped past its critical pressure and heated (energy recovery from compression). You now have supercritical CO2. To this you can add any waste heat source (such as nuclear). The supercritical CO2 now spins a turbine and generates electricity.

Separating N2/Ar from O2 would be easy if not for the fraction of CO. These have close melting/boiling points and its unclear to me if ordinary fractionation can reduce the concentration of CO  sufficiently. The answer may lie in separation/freezing at different pressures. But its a non trivial problem.


Online speedevil

  • Senior Member
  • *****
  • Posts: 2045
  • Fife
  • Liked: 1006
  • Likes Given: 1123
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #68 on: 02/06/2018 01:42 PM »
Separating N2/Ar from O2 would be easy if not for the fraction of CO. These have close melting/boiling points and its unclear to me if ordinary fractionation can reduce the concentration of CO  sufficiently. The answer may lie in separation/freezing at different pressures. But its a non trivial problem.

I have not looked up the temperatures at elevated pressures, or mixed gasses, but at 1 bar, CO is further from O2 than N2 is. If you think you can separate out N2 from O2 easily, why is CO harder?

Ar is annoyingly close to O2, being ~2C away, not >10C.

Offline Russel

  • Full Member
  • ****
  • Posts: 966
  • Liked: 84
  • Likes Given: 1
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #69 on: 02/08/2018 03:50 AM »
Separating N2 from O2 on Earth is relatively easy because you're not interested in absolute purity. CO on the other hand you need to get below a very low percentage not to be poisonous.

Offline guckyfan

  • Senior Member
  • *****
  • Posts: 6650
  • Germany
  • Liked: 1721
  • Likes Given: 1665
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #70 on: 02/08/2018 07:19 AM »
Separating N2 from O2 on Earth is relatively easy because you're not interested in absolute purity. CO on the other hand you need to get below a very low percentage not to be poisonous.

It should be possible to remove any impurities catalytic. Oxidize the CO to CO2. As a last step so not much of the O2 is lost.

Online speedevil

  • Senior Member
  • *****
  • Posts: 2045
  • Fife
  • Liked: 1006
  • Likes Given: 1123
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #71 on: 02/08/2018 11:36 AM »
It should be possible to remove any impurities catalytic. Oxidize the CO to CO2. As a last step so not much of the O2 is lost.

Also, for some uses, mixed O2/CO might be fine.
(low-ISP oxidiser)

Offline Russel

  • Full Member
  • ****
  • Posts: 966
  • Liked: 84
  • Likes Given: 1
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #72 on: 02/11/2018 05:45 AM »
Separating N2 from O2 on Earth is relatively easy because you're not interested in absolute purity. CO on the other hand you need to get below a very low percentage not to be poisonous.

It should be possible to remove any impurities catalytic. Oxidize the CO to CO2. As a last step so not much of the O2 is lost.

O2 0.13%
CO 0.08%

If that's by mass then you need 0.57g of O2 to react with every 1g of CO.

So you lose about a third of your oxygen doing that. I think its possible to lose most of the CO through a physical process and then use a chemical process to further purify.

Otoh if electrolysis of CO2 works out to be robust, low maintenance and energy efficient then you wouldn't bother extracting it this way. Instead you'd worry about getting the N2/Ar. Electrlysis is still an unproven in my book. I don't trust the high temps involved. We'll see.

Offline sevenperforce

  • Full Member
  • ****
  • Posts: 873
  • Liked: 250
  • Likes Given: 267
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #73 on: 02/12/2018 02:30 PM »
What about LOX-only ISRU on Mars?

Offline Russel

  • Full Member
  • ****
  • Posts: 966
  • Liked: 84
  • Likes Given: 1
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #74 on: 02/13/2018 01:17 AM »
What about LOX-only ISRU on Mars?

Given that oxygen is 80% of the mass of propellant, that idea sits comfortably with me. The interesting question for me is whether the proces of electrolysis (cracking CO2) is going to be to be robust and efficient or whether it is better to extract oxygen direct from the atmosphere.

Offline Michael Bloxham

  • Regular
  • Full Member
  • ****
  • Posts: 632
  • Auckland, New Zealand
  • Liked: 2
  • Likes Given: 6
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #75 on: 02/13/2018 07:00 AM »
What about the idea of bringing seed hydrogen for fuel production? That's 180 tonnes of methane + oxygen (enough to refuel the BFS) for every 10 tonnes of Hydrogen, no?

Online speedevil

  • Senior Member
  • *****
  • Posts: 2045
  • Fife
  • Liked: 1006
  • Likes Given: 1123
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #76 on: 02/13/2018 11:01 AM »
What about the idea of bringing seed hydrogen for fuel production? That's 180 tonnes of methane + oxygen (enough to refuel the BFS) for every 10 tonnes of Hydrogen, no?

One major annoyance is that lifting seed hydrogen to Mars is hard in its raw form.
Hydrogen is around a tenth as dense as methane,which means that it's almost always going to be easiest just to bring methane.

Offline Russel

  • Full Member
  • ****
  • Posts: 966
  • Liked: 84
  • Likes Given: 1
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #77 on: 02/19/2018 12:53 PM »
I'm puzzled by the 180 tonne figure just used for the BFS. I'd have thought it required a lot, lot more.

Back to the reality of initial scientific/exploratory missions. Its easiest to import hydrogen in the form of liquid methane and simply ISRU the oxygen.

Another portable (and non cryogenic) form of hydrogen is common ammonia which is 17% hydrogen by mass and has a room temperature density of 0.73 tonne per m3.

In terms of density, liquid methane is 105Kg of hydrogen per m3 and ammonia is 129Kg of hydrogen per m3. Plus it is a source of nitrogen.

Online speedevil

  • Senior Member
  • *****
  • Posts: 2045
  • Fife
  • Liked: 1006
  • Likes Given: 1123
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #78 on: 02/19/2018 01:37 PM »
I'm puzzled by the 180 tonne figure just used for the BFS.

180 tons of methane is about right for the methane capacity of RP1, and about 800 tons of oxygen.

Offline sghill

  • Full Member
  • ****
  • Posts: 1353
  • United States
  • Liked: 1505
  • Likes Given: 2147
Re: Water, Methane, and Oxygen ISRU on Mars
« Reply #79 on: 02/19/2018 01:49 PM »
I'm puzzled by the 180 tonne figure just used for the BFS. I'd have thought it required a lot, lot more.

Back to the reality of initial scientific/exploratory missions. Its easiest to import hydrogen in the form of liquid methane and simply ISRU the oxygen.

Another portable (and non cryogenic) form of hydrogen is common ammonia which is 17% hydrogen by mass and has a room temperature density of 0.73 tonne per m3.

In terms of density, liquid methane is 105Kg of hydrogen per m3 and ammonia is 129Kg of hydrogen per m3. Plus it is a source of nitrogen.

I'm sorry, I just don't see or support the argument for importing ammonia or liquid methane to the martian surface. This discussion is a distraction, IMHO.

Yes, you can transport various gases there in stable forms. And if you do, you are taking many trips to the surface all the way from Earth to collect the necessary ingredients to return once.  Until you send up ISRU equipment, you are stuck in this unsustainable and uneconomical paradigm.

The necessary gasses are already on Mars in various forms. Identify the path for extracting each (the purpose of this thread), and send the required ISRU equipment instead on early missions.
Bring the thunder Elon!

Tags: