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pressure based air separation

separate air using ocean pressure.
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(+3, -2)
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A system that uses pressure rather than very cold temperatures to distill gases into their component parts could be much more efficient than cryogenic distillation(currently the most used way of splitting air into it's component parts), where refrigeration losses are an obstacle to efficiency and which requires highly expensive equipment.

the device I propose consists of a large flexible bag capable of holding a large amount of air with a huge weight attached to it's base. a computer controlled valve at the top of said bag allows gas to go from the main chamber into an expandable temporary storage bag on top of this the top of thefirst bag, another valve allows fluids to drain from this bag to one of several compressed gas cylinders above it. once the main chamber is filled with air you lower it on a neutrally buoyant cable to a depth at which all but one of the gases condenses ,that gas is drained into the upper bladder at which point the device descends further liquefying this gas and then stores it in one of the cylinders. as it is raised back to the surface it does the same for the other gases storing each in the proper cylinder.

such a system would have a low capital cost(no compressors or expansion valves, high pressure equipment, heat exchangers... ect) low operating costs(cryogenic distilation requires lot's of energy) and losses would be mostly due to water drag.

high purity gases are usually produced in situ for industrial processes and so to reduce transportation costs it would probably be best to locate such a facility as close to shore and the user of the gas as possible.

RichardT, May 22 2010

worth a read http://www.polyquip...hnology&category=10
[Ling, May 22 2010]


       You won't have much success with air, as the constituents won't condense at ocean pressures. Also, this isn't energy free, as you get that from the "huge weight," which you have to drop at the bottom if you want to pull the bag up again without undue expense.
ldischler, May 22 2010

       the energy you lose pulling up the weight went towards compressing the gases, which isn't really necessary if they don't need to be liquefied. if you wanted just to separate the gases you could replace the high pressure gas cylinders with a set of bladders. that would also eliminate the temporary storage bag. you would have to compress it eventually though if you are going to pipe it somewhere.   

       are ocean pressures really not enough? I mean if you drop one of these things into the Mariana trench the gas inside has got to condense, right.   

       CO2 will condense at ocean pressures, I know that I don't however know the math to determine gas condensation temperature pressure relations for the other ones.   

       edit: just found out, it's fairly simple. nitrogen the gas with the highest vapor pressure has a vapor pressure of 632 atm at 20 degrees celcius, that means put 632 atm of pressure on it and it liquifies, a large portion of the ocean is below that depth but as I said before nitrogen is the most difficult to condense you could remove it w/o condensing it all you need to do is condense the oxygen which requires only 542 atm remember you get a little more than 1 atm / 10 meters descent, so 5500 meters conservative estimate.   

       Depending on which gases you want to separate you could depth changes also these figures should go further down as temperature does this is usually anywhere from 0-10 degrees celcius. if you went to the arctic you'd get the same pressures but lower temperature which might help but not so much.   

       Anyways putting this near any landmass is out of the question. a pipeline would be super expensive so not much practical use. you could move the refineries and steel plants out there though. very convenient for shipping.
RichardT, May 22 2010

       If nothing else it would require less parasitic energy as mentioned... of course now you have to worry about streamlining... perhaps a continuous string of sausages.
FlyingToaster, May 22 2010

       I'm still leaning towards the monolithic design. low friction coatings plus a streamlined shape would help. also if liquids were drained from the bottom they could be sent up to the surface as soon as they condensed, along with their attached weights, that way separation of oxygen and nitrogen would be a little bit easier.   

       That would be a very long string of sausages it would have to go down 5000 meters or more.
RichardT, May 22 2010

       Okay, Richard, let's say you take a standard shipping container and sink it 4 miles. To get it to sink, you'll need some 85 tons of ballast (sand or gravel would be fine). The ballast has to be disposable, as you don't want to haul all that dead weight from that far down (requiring more than a megawatt hour, not even considering drag). While sinking, the container fills with water, eventually condensing the air into roughly 4 cubic feet of liquid--a mix of mostly nitrogen and oxygen that will collect at the top, and which you'll have to distill if you want to separate them. This will produce about 40 pounds of LOX, worth about $5 (if it were at cryogenic temperatures).   

       So it does seem workable, in principle.
ldischler, May 22 2010

       I don't think ballast has to be a problem or needs to be disposable. As gas is compressed it would require less weight to push it down. so if the ballast was removed as the device descends to compensate for contraction of the gases moving up or down ideally, not considering friction could require a fairly small amount of energy.   

       by splitting the ballast into sections and stringing these sections on a long cable the end of which is held at the surface. as the device descends weight is removed and held by the cable rather than pushing down on the device. such a system could compensate for changes in buoyancy.   

       As for need of distilation afterwards. This was never meant as a means of compressing the gases. That would be very impractical. the intention was to separate them without compression, or at least using easily reversible compression.   

       if the device descends to a point where only the nitrogen gas remains gaseous, it can be drained off. as the device ascends the oxygen boils and can be drained offinto a separate expandeable bladder, the same can be done for all the other gases. same as with fractional distilation except instead of using temperature as a means of boiling and condensing gases in this case pressure is used.
RichardT, May 22 2010

       //So it does seem workable, in principle//
But all that "disposable" ballast will accumulate at the bottom of the ocean, reducing the depth, and so reducing the efficiency of your gas mining operation, and at the same time, causing ocean levels to rise, drowning potential markets (and possibly meerkats).
AbsintheWithoutLeave, May 22 2010

       I'm sure someone said, in an anno on another idea, that you couldn't liquefy nitrogen or oxygen by pressure at ambient temperatures. I thought they were wrong.   

       Anyway. One small point. Just because a given gas liquifies under a certain combination of temperature and pressure, it doesn't follow (iirc) that it will liquify out of a gas *mix* under those conditions. The more volatile gases will, I suspect, act as carriers for the less volatile. But I may be not right.
MaxwellBuchanan, May 22 2010

       //I don't think ballast has to be a problem or needs to be disposable...if the device descends to a point where only the nitrogen gas remains gaseous, it can be drained off //   

       Even if you started removing ballast as it goes down, you still have to haul it up, just not quite as far, on average. And now your rope has to be much thicker. As for producing pure nitrogen or oxygen, I doubt that the purity will be any better than you get with non-cryogenic methods.
ldischler, May 22 2010

       //Anyway. One small point. Just because a given gas liquifies under a certain combination of temperature and pressure, it doesn't follow (iirc) that it will liquify out of a gas *mix* under those conditions. The more volatile gases will, I suspect, act as carriers for the less volatile. But I may be not right.//   

       You are right. water will disolve in air even though it is below it's boiling point (everyday example). however if this is a problem just increase the pressure further(in a cryo plant they use lower temperatures than necesary for the same reasons), another option is to do the process multiple times.   

       anyways the real idea here isn't a way of distilling air, it's a way of making pressure easier to obtain. many industrial processes can use pressure to change chemical equilibria, or to substitute for temperature. the Haber process for example,(nitrogen + hydrogen --> ammonia (NH3), would benefits from higher pressure as that pushes the reaction towards ammonia production If you read the wiki article thereon it states that "pressure is an expensive commodity" so this is, at heart, a way of reducing the "expensiveness" of pressure.
RichardT, May 23 2010

       A "two" way elevator can be used for this. The ballast going down can pull a second cord pulling the liquids up. You could have a single, one-stage, system for each type of gas.
pashute, Oct 30 2012


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