On the use of departure function correlations for hydrocarbon isobaric liquid phase heat capacity calculation

Constant pressure heat capacities for pure liquids and mixtures are by default evaluated indirectly, in process simulators and in general purpose calculations, using ideal gas isobaric heat capacity values to which equation of state based departure functions are added. As ideal gas heat capacities are known or can be calculated from theory with small uncertainties and typically comprise 75% of liquid heat capacity values, the large relative deviations present in departure function calculations appear to be tolerated or ignored because deviations between indirectly calculated and measured constant pressure liquid heat capacities are typically less than 15% and large deviations are uncommon. For hydrocarbon and petroleum applications, non-cubic equations of state have a limited range of application and a cubic equation of state departure function calculations possesses a systematic skew with respect to absolute and relative temperature. This provides application windows for stand alone departure function correlations: by Tyagi that requires the input properties (Tc, Pc, ω, M) at high reduced and absolute temperatures; and a difference calculation based on correlations by Dadgostar and Shaw (for hydrocarbon liquids) and Laštovka and Shaw (for ideal gases) which extend the range of fluids for which liquid state heat capacity departure functions can be evaluated to include poorly defined (Tc and M are available) individual hydrocarbons or mixtures or ill-defined (only elemental analysis is available) hydrocarbon mixtures. However, none of these approaches provides consistently low deviations from measurements and consequently, direct calculation of liquid phase heat capacities is recommended for detailed engineering calculations.