Values obtained for T above maximum for model

[Matt Yates]

What does CoolProp do when you ask it to calculate state properties outside the T range of the model?  For example the propane model:

https://coolprop.org/fluid_properties/fluids/n-Propane.html

says the maximum temperature is 650 K.  However, nothing in CoolProp stops me from calculating values well above that temperature.  Should I just assume calculated values above 650 K are meaningless?

I have been working on calculating enthalpy and Gibbs energy of chemical reactions.  I posted code on github to answer this issue:

https://github.com/CoolProp/CoolProp/issues/1360

It would be very useful for chemical reactions if higher T values outside the model limits could be used.  If CoolProp cannot be used above the T limit of each model, it would require switching to a different model (like ideal gas with a polynomial fit of Cp or an empirical correlation) when the T limit of the CoolProp model is reached.

-Matt Yates

[Ian Bell]

There are flags to skip the checks, but for consistency with REFPROP, you are permitted to evaluate up to 1.5*Tmax. The model is extrapolated, which may or may not be reasonable depending on the model, and caveat emptor

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[Matt Yates]

Ian,

Thank you for the extra information.  I just tried an experiment with the ammonia formation reaction.  CoolProp ammonia model upper limit in T is listed as 725 K, but it allows me to calculate data using T much higher than 1.5*725.

JANAF has ammonia formation data here up to 6000 K: https://janaf.nist.gov/tables/H-083.html

I used the CoolProp model extrapolated to 6000K at the same 0.1MPa pressure for the JANAF table to generate estimates of ΔG and ΔH of this reaction from 300 K to 6000 K.  It does a very good job approximating values shown by NIST in the JANAF tables.  Maybe it is a fluke for ammonia and a bad idea to extrapolate willy nilly.  I attached plots of the difference in value estimated by coolprop to that given by JANAF in units of kJ/mol.  As a percentage difference, the maximum difference for ΔG is less than -0.5%.  It does a good job with estimating ΔH up to ~3000 K, but falls off after that.  As a percentage, the maximum error in ΔH from the CoolProp model is above 20% at 6000 K.  The enthalpy estimate at extremely high temperatures isn't great, but still doesn't seem terrible.

-Matt

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