Calculating k (=Cp/Cv) for Gas Mixture?
Calculating k (=Cp/Cv) for Gas Mixture?
(OP)
Hi guys. Anybody got a shortcut, that doesn't require a simulator, for calculating k=Cp/Cv for a gas mixture? Gas is 65% CO2, 35% CH3, water-saturated. My rules of thumb say stuff like Cp=(9/2)R and Cv=(7/2)R for "triatomic gas", but that doesn't help much for this gas mixture. And Cp-Cv=R gives me negative values for this particular gas, so that's out. I am within ideal gas approximation range if that helps. Thanks guys!





RE: Calculating k (=Cp/Cv) for Gas Mixture?
Have you tried to find published thermodynamic literature for these compounds? I think that's the best route for accurate calculation without simulator.
RE: Calculating k (=Cp/Cv) for Gas Mixture?
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Calculating k (=Cp/Cv) for Gas Mixture?
RE: Calculating k (=Cp/Cv) for Gas Mixture?
for these calc's the obvious reference is the Properties of Gases and Liquids which includes many examples about how to estimate thermodynamic properties,
as already suggested, a EOS allows to estimate the water content and enthalpy departure from ideal gas giving more accurate results...
it may be useful to compare values calculated with different methods,
herebelow the values calculated with ideal gas law, extended version of Peng Robinson available in Prode (Prode PRX) and GERG (2008)
mixture (mol. fract.) CO2 0.635 C3 0.35 H2O 0.015 at 288.15 K 1 Bar
cp (Kj/Kg-K)
ideal 1.13 (Prode database)
PRX 1.136
GERG 1.131
cv (Kj/Kg-K)
ideal 0.94 (as above)
PRX 0.94
GERG 0.935
at these conditions, all the values are in reasonable agreement,
however, if you repeat the same calc's at different conditions, for example 350 K 20 Bar, there are quite large differences in cp calculated with ideal and EOS based models
mixture (mol. fract.) CO2 0.635 C3 0.35 H2O 0.015 at 350 K 20 Bar
cp (Kj/Kg-K)
ideal 1.26 (Prode database)
PRX 1.38
GERG 1.39
cv (Kj/Kg-K)
ideal 1.07 (as above)
PRX 1.09
GERG 1.1
in your specific case in addition to the operating conditions, I would consider the application, for example if you need cp/cv for sizing a PSV (with API formulation) it must be cp/cv of ideal gas (API formulation requires that) or, as alternative you may use HEM or similar rigorous methods,
the same consideration for many other common problems...
RE: Calculating k (=Cp/Cv) for Gas Mixture?
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Calculating k (=Cp/Cv) for Gas Mixture?
I see two interpretations of your "CH3", methane and C3's. Which is it?
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Calculating k (=Cp/Cv) for Gas Mixture?
apetri - I have that text and flat forgot that I had it. I will look.
zdas - Thanks for the tip on the NIST software. That right there was worth the price of admission today. Thanks very much. On reading the FAQ at the NIST site, it sounds like it has problems doing saturated mixtures. Have you done those with it, and does it do flash calculations to determine the composition of the phases for liquid-vapor phase equilibrium?
RE: Calculating k (=Cp/Cv) for Gas Mixture?
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist