Calculating enthalpies of gas mixtures

[ahog]

Hi all,
I'm calculating the temperature drop of a gas during a throttling process. When using ideal gas theory, of course throttling is an isothermal process. Now i want to consider a real gas.

My problem: I have a gas, for example containing N2, O2 and H2O in the gaseous phase at a certain temperature and pressure. When considering pure components at system temperature and pressure, N2 and O2 are gaseous and H2O is liquid. In reality, the H2O partial pressure is below the saturation pressure, so H2O is also gaseous. Now I'm using partial pressures for enthalpy calculation of the mixture.

My question: Is it the right approach? Do I always have to use partial pressures, even when considering nearly ideal gases like N2 and O2, because they do show very small deviations from ideal gas law?

Thanks in advance.

 

[faizanrauf612]

Hi @ahog
Yes, using partial pressures is the right approach—even for nearly ideal gases like N₂ and O₂—because enthalpy in real gas mixtures depends on each component’s partial pressure, not just the overall conditions.

For a deeper explanation and supporting data, this paper may help:
https://pubs.acs.org/doi/10.1021/je00103a025

 

[Chestermiller]

Do you know how to determine the temperature change for a throttling operation on a pure real gas? If you can't do that, then you certainly won't be able to do it for a gas mixture. Is the pressure low enough to consider the gas mixture an ideal gas? Are you just trying to determine the JT coefficient, or is the pressure change substantial?

 

[ahog]

Hi @ahog
Yes, using partial pressures is the right approach—even for nearly ideal gases like N₂ and O₂—because enthalpy in real gas mixtures depends on each component’s partial pressure, not just the overall conditions.

For a deeper explanation and supporting data, this paper may help:
https://pubs.acs.org/doi/10.1021/je00103a025

Thanks for your reply, the paper is very infomative.