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usnistgov / REFPROP-issues

A repository solely used for reporting issues with NIST REFPROP
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Unusual Mixture Reference State Changes #278

Closed HCSpin closed 4 years ago

HCSpin commented 4 years ago

Using the Excel Wrapper on a Mac (Excel 2019), the enthalpy calculated for a mixture of Kr/He changes unexpectedly as follows.

Define the mixture to be (by mole fraction) Kr (.70), He (0.3) at T=350K, P=1200kPa, H = 36.91 kJ/kg

If the mixture is changed to (by mole fraction) Kr (.70), He (0.3), Propane (0.0) at T=350K, P=1200kPa, H = 197.26 kJ/kg

Why does the reference state change for the two "different" mixtures?

Calculating ∆H between two different temperatures/pressures is unaffected and is consistent for both representations of the above mixtures.

ianhbell commented 4 years ago

Indeed, that seems like a bug.

Changes in enthalpy for mixtures is tricky, to say the least, and doesn't always operate the way you think it might in a thermodynamic model. It is a topic that I myself don't fully understand.

EricLemmon commented 4 years ago

This is actually not a bug, but comes about by us humans desiring nice, easy-to-work with numbers for enthalpy and entropy. If we were to get rid of all reference states except nature's true reference, which is h and s equal zero at 0 K, then these problems would go away. For more details, see:

https://github.com/usnistgov/REFPROP-issues/issues/222