Comparison of pure component thermodynamic properties from CHEMCAD with direct calculation using the Soave–Redlich–Kwong equation of state

Abstract

Gas-phase thermodynamic properties are routinely calculated with equations of state in process design software such as CHEMCAD [1], and users should be able to verify the calculations. In a previous study, we reproduced the enthalpy, entropy, compressibility factors, and fugacity coefficients obtained from CHEMCAD using the Lee–Kesler method [2, 3], and later extended this study to include the Peng–Robinson equation of state [4, 5]. An interesting feature of the earlier work is that compressibility and fugacity coefficients in CHEMCAD did not match up precisely with our independent calculations. We therefore extended the study to include the Soave–Redlich–Kwong (SRK) equation of state [6, 7]. Our results show consistency for most of the 48 molecules studied, with percent error in enthalpy decreasing significantly for the SRK method to generally less than 0.002% compared to about 0.3-1% for the Lee–Kesler and Peng–Robinson methods. Unlike the previous two studies, we now see nearly perfect agreement in the compressibility and fugacity coefficient. Furthermore, we show that compressibility and fugacity coefficient remain constant in CHEMCAD even when the equation of state is changed. For enthalpy, while the results are much closer than in the previous studies, we see deviations for methane, air, carbon monoxide, carbon dioxide, nitrogen, and oxygen. Discrepancies in entropy were also observed for bromine, carbon dioxide, chlorine, hydrogen sulfide, and nitrous oxide.