Equation of state, density relative to air
Equation of state, density relative to air
(OP)
Hi,
Background:
AGA PAR NX-19 is an equation of state applicable to some compositions of natural gas for estimating the compressibility z. The required input is:
p, T, mol% CO2, mol% N2 and d
where d is the density relative to air (I assume this is at 1 atm). d is often not known or at least not provided.
Case:
From a known d (0.65) for a given composition, the calculated sum(comp1_mol% * MW1 + comp2_mol% * MW2 ...)/avg_MW_air = 0.62, which is off by 5%.
Question:
What would be an accurate way to determine d when only the composition is known? Is the above under Case 'good enough'?
Background:
AGA PAR NX-19 is an equation of state applicable to some compositions of natural gas for estimating the compressibility z. The required input is:
p, T, mol% CO2, mol% N2 and d
where d is the density relative to air (I assume this is at 1 atm). d is often not known or at least not provided.
Case:
From a known d (0.65) for a given composition, the calculated sum(comp1_mol% * MW1 + comp2_mol% * MW2 ...)/avg_MW_air = 0.62, which is off by 5%.
Question:
What would be an accurate way to determine d when only the composition is known? Is the above under Case 'good enough'?





RE: Equation of state, density relative to air
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RE: Equation of state, density relative to air
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Equation of state, density relative to air
We have Design2 but it doesn't integrate well into some of the in-house tools. I'm looking for a reasonable alternative to speed tasks such as orifice sizing and generally get a better understanding equations of state. Properties of gases and liquids by Prausnitz et al describes many equations of state but most of them require large databanks of coefficients in order to be useful.
So it would be interesting to know whether or not the mol weight approach suffices or perhaps there are other equations of state that are of interest.
RE: Equation of state, density relative to air
Thanks for your input. I thought 5% to be a bit much as well. Perhaps there's an error in my program, I'll need to go through it again.
RE: Equation of state, density relative to air
The average MW method you proposed gave a gas density ratio within 0.5% for their data. If you are looking at normal process design calcs then I believe you are safe with this method. If you are doing transfer of custody work then the contracts should specify the calculation methods.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com