The enthalpy of a real fluid differs from that of an ideal gas due to intermolecular interactions and finite molecular volume effects. The enthalpy of a real fluid may be expressed as the sum of an ideal-gas contribution and a residual contribution that accounts for non-ideal molecular interactions. In other words, the residual enthalpy (also referred to as departure enthalpy) is defined as the difference between the enthalpy of the real fluid and the enthalpy of an ideal gas at the same temperature and pressure.
Residual enthalpy corrections become increasingly important at elevated pressures, near phase boundaries, and in systems exhibiting significant non-ideal behavior. Neglecting these effects may introduce errors in energy balances, heat duty calculations, and equipment sizing. For this reason, SuperPro Designer provides the following options for calculating gaseous residual enthalpy:
This option assumes that all fluids behave ideally and therefore neglects residual enthalpy effects altogether (Hres = 0). The total enthalpy is calculated solely from ideal-gas heat capacity correlations and phase-change contributions. This approach is computationally efficient and generally adequate for low-pressure systems where deviations from ideal behavior are negligible.
For simulations employing an equation of state, residual enthalpy can be calculated directly from the selected EOS (i.e., the PR or SRK model). This method accounts for both attractive and repulsive intermolecular forces and is generally recommended for hydrocarbon systems, light gases, refrigerants, and other mixtures where EOS models provide an accurate description of fluid behavior. The accuracy of the calculations depends on the suitability of the selected EOS and the availability of binary interaction parameters.
As with vapor-liquid equilibrium calculations, the selection of a residual enthalpy model should be guided by the operating pressure, temperature range, mixture composition, and availability of thermodynamic parameters. For low-pressure systems, the Ideal Gas Method is often sufficient, whereas high-pressure hydrocarbon and gas-processing applications typically require EOS-based calculations to obtain reliable energy balance predictions.
Residual enthalpy calculations affect only the energy content of material streams and process states; they do not directly influence phase equilibrium predictions. However, because enthalpy values are used extensively throughout the simulation, the selected residual enthalpy method can significantly impact calculated heating and cooling duties, utility consumption, equipment sizing, and overall process economics.
The gaseous residual calculation options can be viewed and edited on the Vapor Enthalpy Calculation Options Dialog.