by jasper » 29 December 2012, 14:37
If EnthalpyF is available, this is always a good choice. Heat of formation however is not always known for all compounds. In this case a property package may not be able to supply EnthalpyF. In this case, heat of reaction may be known and is required to close the heat balance. But now it gets a bit more complex to get things to be consistent.
In case heat of reaction is known (at reference conditions that should match those of the thermo!), you can use an enthalpy that does not contain heat of formation. EnthalpyNF is sure to not contain formation terms. For Enthalpy (without F or NF) it depends on the property package implementation (and perhaps configuration) whether heat of formation is or is not included. For TEA, the property Enthalpy does not include heat of formation.
TEA always has its reference point at 298.15 K, 1 atm, in the ideal vapor phase. So if you configure a heat of reaction, it should be either at the same reference conditions, or you should adjust the material object to match the reference conditions of the reaction package's heat of reaction. COFE can do this; you can change the reference conditions (pressure, temperature and phase) in the flowsheet setup. Effectively this results in COFE calculating enthalpy at the chosen reference conditions for the pure compounds (once) and correcting all calculated enthalpies by the sum of composition times compound reference enthalpties (and the same holds for entropies): . Note that with such reference state correction, you can be sure that any formation terms that might or might not be present in Enthalpy are cancelled out. So Enthalpy as seen by the reactor unit operation, in case COFE's reference state correction option is selected, is always at known reference state conditions and without formation terms, no matter which thermo is used.
So in short:
- if formation heats are known, use EnthalpyF, don't use heat of reaction
- if heat of reaction is known, set COFE's reference state correction to the conditions at which heat of reaction is known
It is all a bit complex, and it would help if the reaction terms and the thermodynamics would come from the same source, as then more likely they will be consistent. This will be possible with the next version of the reactions standard, which is in the making (part of the work of the Thermo Special Interest Group (Thermo SIG) of CO-LaN).