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I recently acquired a 5-liter mixed gas cylinder that has 70% nitrogen and 30% carbon dioxide, used for dispensing beer. The cylinder pressure is 200 bar (2900 PSI) and my room temperature is around 20C to 22C (68F to 72F).

I've started to wonder whether the gas ratio can be disturbed by the CO2 changing its thermodynamic state due to room temperature fluctuations. Given the parameters, should I expect the CO2 to be in gaseous or liquid form?

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  • $\begingroup$ To a first approximation, it seems the carbon dioxide is near its liquid/gas equilibrium curve, though it's hard to be precise because the ideal gas approximation probably isn't very good in these conditions. If any of it is liquid, then yeah the ratio is going to be affected. If you gently swirl/rock the cylinder, can you detect any sloshing? $\endgroup$ Oct 27, 2023 at 10:54
  • $\begingroup$ Without particular gas data it is hard to say the gas condition. For illustration: Nitrous gas N2O has comparable critical temperature as CO2. While it is liquid at room T at 100 bar, the mixture 1:1 (V/V) with oxygen is at 200 bar gaseous. $\endgroup$
    – Poutnik
    Oct 27, 2023 at 11:29
  • $\begingroup$ I can only assume that the store that sold you the tank knows what they are doing and that it will work for the intended purpose. So use it and don't worry. $\endgroup$
    – MaxW
    Oct 27, 2023 at 12:26

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Calculated CO2-N2 Pressure-temperature phase diagrams, albeit at lower N2 concentrations, can be found in an Energia paper. With the addition of N2, the line that was the gas-liquid boundary (terminating at the critical point) now splits into a vapor saturation line and a liquid saturation line which meet at a critical point. You are concerned that your gas mix may fall into the two-phase (liquid+gas) area between those two curves.

In Figure 4 of the paper (below, annotated by Karsten) they authors show the calculated phase diagram for a 80% CO2 - 20% N2 mixture. For pure CO2 the critical point is at (30.978C, 73.773 bar). With only 20% N2 in the mix the critical point has already moved to (14.55C, 105.47 bar).

With 70% N2 in the mix, you will not be in the two phase region for any conditions you are likely to encounter.

20%N2 in CO2 P-T diagram

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    $\begingroup$ @Karsten - thank you! I put that in. I guess I'm just too used to phase diagrams... That is much clearer. $\endgroup$
    – Jon Custer
    Oct 27, 2023 at 13:12
  • $\begingroup$ Thank you a ton, both of you! I also assume that the CO2 does not settle to the bottom of the cylinder even though it's denser than nitrogen? I'm just wondering because the cylinder has a dip tube in it, drawing the gas from the bottom. $\endgroup$ Oct 27, 2023 at 13:20
  • $\begingroup$ There is enough mixing to keep it uniform throughout the cylinder. I used to run a Van de Graaff accelerator with a giant pressure vessel holding an N2-CO2-SF6 mix to hold off voltage. There was no indication that the SF6 separated out, much less CO2 vs N2. $\endgroup$
    – Jon Custer
    Oct 27, 2023 at 13:32
  • $\begingroup$ Got it. One concern though. In the diagram, the percentage of gas is reported as mol%. I was wondering if it would make a difference if the gases were measured by volume or mass? I see some information floating around that the CO2 partial pressure would be 60 bar, which would mean that my margins for being either in liquid or gas state are very fine, given my room temperature is 20C. $\endgroup$ Oct 27, 2023 at 14:21
  • $\begingroup$ The margins would be fine if you had 80% CO2 in the mix. This just shows how quickly the critical point (and the 2-phase region) move with adding a bit of N2. At your mix, the phase diagram will look more like N2, so you are far away from any issues. $\endgroup$
    – Jon Custer
    Oct 27, 2023 at 16:27

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