Question:

Compressing Natural Gas?

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I want to compress natural gas into a holding tank and have the follwing questions:

1)How high of a psi can it be compressed to?

2)At the highest psi how big of a tank would it take to hold 200,000 CCF's which is about what I use per year?

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  1. CNG can be compressed to about 220 atm

    Therefore 200,000 scf will compress to 200,000scf /220 atm = 215 cu-ft.

    CNG is typically stored in spheres so a sphere containing 215 cu-ft would have a diameter of (6*215/pi)^(1/3) = 7.43 ft diameter sphere

    Sorry my arithmetic is off. 200,000scf/220atm=909cu-ft. That puts the Sphere Diameter to 12 feet.


  2. The other answers look fairly reasonable except that I don't understand the assertion of  BRUZER's that "Therefore 200,000 scf will compress to 200,000scf /220 atm = 215 cu-ft." I get 909cf using Boyle's law, but that is complicated by the fact that don't think natural gas is an ideal gas at those pressures. Possibly BRUZER knows something I  don't, but the Wikipedia article on CNG says it can be compressed "to less than 1% of its volume at standard atmospheric pressure", while BRUZER has it at about 0.1% (which is, true enough, less than 1%)

    And also except for the fact that CCF is hundreds of cubic feet, so you need to store 20 million cubic feet, compressed to 1% that is still, 200,000cf , if you were to store it in a single spherical tank (not that you would put in into a single tank at over 200atm) the tank would be about 70 feet in diameter, probably not really too practical.

    But then that is a LOT of gas, so you may have meant cubic feet in the first place. Compressed to 1% (2000cf) ,  200,000cf would fit in a sphere about 16ft diameter

    [EDIT] I don't know much about the cost of large compressed gas storage, but I came across this in a PDF about hydrogen transport;

    "...cost is accounted for by the cost of the trailers themselves ($165,000 for a 9 tube trailer with a total internal volume of 23.4 m3)"

    23.4m³ is roughly 830cf, it is plausible that at 3500psi, 200,000cf (at 1 atm)  will fit into one trailer. You might save a little since you might not need the trailer part, but that might give you a ballpark idea of the cost, perhaps 50 or so times the cost of the gas. So if you want to store gas as a hedge against price increases, it probably won't pay off. If you want to to store it to guard against supply interruptions, only you know what that is worth to you,  but you'd probably be better off with propane that can be stored as a liquid at reasonable pressure, or even diesel which can be stored at atmospheric pressure. However diesel  would require large changes in the equipment that burns it, or complete independent backup equipment, but for $165,000 it might be an option.

  3. Natural Gas can be compressed to whatever pressure the equipment for its storage can safely take.

    (Its design pressure should be 1.5 times the expected operating pressure.

    The Compression to very high pressure must be carried out in stages with inter-coolers between each stage and a final after-cooler. This is necessary as the pressure increase will also cause a high temperature increase.

    Air-fin coolers or Water cooled exchangers are generally used for this process.

    The Gas will NOT condense as it has to be at its 'Critical' temperature and pressure in order to liquefy.

    These parameters are:

    Critical Temp: -82°C (-116°F);

    Critical Press: 46atm (673.1 psia).

    At ambient temperature about 20°C.

    200,000ft³ at 1 atm = 4,350ft³ at 46atm. (680psi) which is quite a high pressure.

    By doubling your storage capacity to about 9,000ft³, you will halve the pressure to 340psi.

    Storage vessel desigh pressure should therefore be 1,020psi or 510psi. Safety relief valvesshould also be installed.

    Using your figure of 3,500psi (238atm), the Volume will be 200,000ft³ / 238atm = 840ft³

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