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IDAES
CO2 Property Discrepancy
Discussed in https://github.com/IDAES/idaes-pse/discussions/660
Originally posted by NCJazzMan65 January 13, 2022 I have a flowsheet which models a supercritical CO2 cycle. I'm using the Span-Wagner CO2 property package. The flowsheet converges to a solution but when I inspect the results they are not what I expected. I spot checked the state of some streams using the REFPROP add-in for Excel. The enthalpy being calculated by IDAES is way off the enthalpy being calculated by REFPROP. As an example, I have one stream which is at 0 C (273.15 K) and 6.4e6 Pa. IDAES calculates an enthalpy of -13613 J/mol but REFPROP calculates 8668 J/mol for the same conditions. I've double checked everything in my IDAES model and even looked at the Span-Wagner property package to confirm it applies to my range of conditions. I cannot figure out why my IDAES model is giving me incorrect results even though it converged successfully (I understand converging and converging to the "right" solution are two different things).
Can anybody help?
Yes, I think you'll find the problem is just that the reference states are different. You can calculate the enthalpy for two states and compare the difference. The reference state should be in the documentation, but the IDAES model uses the reference state from the Span-Wagner paper. The full reference is in the docs.
As @eslickj said, the issue is most likely the reference states used. You should never directly compare results between two different sets of property calculations (even different implementations of the same model) as most thermophysical properties are dependent on the reference state used.
In order to do a comparison, you either need to compare properties with concrete physical values (e.g. temperature, pressure, density), or look at differential values (e.g. delta enthalpy between two known states). For this example, I would start by looking at the density at the inlet and outlet states (seeing as you are fixing T and P) and the change in enthalpy and entropy between the inlet and outlet states.
Seeing as the has been no activity for a while, I am closing this as resolved.