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Topic: Phases of compounds in a compressed gas cylinder  (Read 2926 times)

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Offline temaire

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Phases of compounds in a compressed gas cylinder
« on: October 15, 2017, 08:22:37 PM »
I'm currently in the process of selecting gas mixtures to calibrate a gas chromatograph. One of the gas mixtures I'm interested in has the following composition by percent volume:

methane (CH4): 95%
carbon dioxide (CO2): 1%
butane (C4H10): 1%
acetylene (C2H2): 1%
nitrogen (N2): 2%

The gas cylinder pressure is 1345 psig. I plan on calibrating the gas chromatograph at room temperature (24oC) and need to ensure that all compounds in the cylinder are in gas phase. I know that the CH4 and N2 will be in gas phase since their critical temperatures are well below room temperature. However, I'm unsure about whether the CO2, butane, and acetylene are in liquid or gas phase. For example, CO2 has a critical point of ~ 31oC and 1070 psi. Since the gas cylinder will be pressurized above this at 24oC, will it be in liquid phase? Am I approaching this problem the right way?

Offline mjc123

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Re: Phases of compounds in a compressed gas cylinder
« Reply #1 on: October 16, 2017, 04:43:45 AM »
You should be able to find tables or graphs of vapour pressure vs. temperature for these compounds. The important quantity is not the cylinder pressure of 1345 psig, but the partial pressure of each gas. If this is above the equilibrium vapour pressure of the compound at the given temperature, some of it will condense. For example, the partial pressure of CO2 is 1% of 1345 = 13.45 psi. The vapour pressure at 31°C (you tell me) is 1070 psi. Do you think the CO2 will liquiefy at 24°C?

Offline temaire

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Re: Phases of compounds in a compressed gas cylinder
« Reply #2 on: October 16, 2017, 04:01:11 PM »
Thanks for clarifying this problem for me. My main point of confusion was about whether you're supposed to compare the compound's vapor pressure to its partial pressure or total system pressure when determining its phase. I initially thought that you compare vapor pressure to total pressure because to me it seemed that this is what the compound "feels." But I guess this doesn't make sense since according to the ideal gas law, each gas in a mixture behaves independently of other gases and has its own pressure.

With this being said, the partial pressures of CO2, butane, and acetylene are each 13.45 psig (28.14 psia). Based on NIST chemistry webBook, the vapor pressures for CO2, butane, and acetylene at 24oC are approximately 912 psia, 34.2 psia, and 700 psia. So it looks like all compounds in the cylinder should be in gas phase.

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