Reference states

Each property package that is loaded can have their own reference state for enthalpy and entropy.

To correct for different reference states in different property packages, the Solve options page allows for turning on a reference state correction.

If reference state correction is turned on, the reference state must be selected. The reference state can be in the ideal gas phase, the vapor phase or the liquid phase. It will be at specified temperature and pressure (pressure is irrelevant if the reference state is in the ideal vapor phase).

The reference state correction will apply to properties enthalpy and entropy, as well as to the properties enthalpyNF (enthalpy which does not include heat of formation) and entropyNF (entropy which does not include entropy of formation), but does not apply to enthalpyF (which does include enthalpy of formation) or entropyF (which does include entropy of formation). The reference state correction is also applied to properties idealGasEnthalpy and idealGasEntropy.

The following table shows which property is used for reference state correction, depending on the reference phase:

Reference state correction is applied by calculation the reference state property for each pure compound, and subtracting the sum of products of mole fraction and reference state values from the calculated property before correction.

If reference state is applied, the property packages must provide idealGasEnthalpy for correction of enthalpy, enthalpy.DmolFraction, enthalpy.Dmoles and idealGasEnthalpy. Note that reference states do not apply to temperature and pressure derivatives. The property packages must supply idealGasEntropy for correction of entropy, entropy.DmolFraction, entropy.Dmoles and idealGasEntropy. If reference state correction is turned on, and idealGasEnthalpy is not available from the property package, all the enthalpy terms above will fail to evaluate. Similarly, all above entropy terms will fail to evaluate in case of missing idealGasEntropy.

By default, reference state correction is turned off. There are three scenarios in which you may want to turn on reference state correction:

• When using multiple property packages: in this case different property packages may have different reference states. Correcting for reference states makes sure that enthalpies and entropties from different packages are comparable. When adding a property package after the first one, you will automatically be asked to turn on reference state correction,
• When using reactions: it is not always known whether heat of formation and entropy of formation are included in the property package's enthalpy and entropy calculations. When turning on reference state correction, it is sure that these are not included, and that heat of reaction for example still must be applied,
• When you need the reference state to be known and you are not sure what the property package is doing.