Peng–Robinson\nor Redlich–Kwong? Twu\nor Soave α‑Function? Which Combination of Cubic Equation\nof State (CEoS) and α‑Function Produces More Accurate\nand Consistent Results for Pure Components

This study tested the accuracy and thermodynamic consistency\nof\nfour CEoS/α-function models. The objective was to find the most\nsuitable CEoS/α-function combo for producing accurate and consistent\nphysical and derivative properties for nonpolar, polar and hydrogen\nbonding components at subcritical conditions. The models tested were\nPR-Twu, PR-Soave, RK-Twu, and RK-Soave. The first term in the model’s\nname refers to the CEoS used: Peng–Robinson (PR) or Redlich–Kwong\n(RK). The second term indicates the α-function used, i.e., Twu’s\nor Soave’s. The models were tested on a data set containing\nsaturation pressure, enthalpy of vaporization and saturated liquid\nheat capacity of 147 pure components classified as polar, nonpolar,\nand hydrogen bonding. The three Twu α-function parameters were\nfitted to data and constrained to produce thermodynamic consistent\nvalues across the phase diagram; and, the Soave α-function parameter\nwas predicted from a well-known correlation. The thermodynamic consistency\nof the models was assessed by calculating the Waring number and the\nsaturated liquid speed of sound of 147 and 79 pure components, respectively.\nThe results showed that PR-Twu and RK-Twu produced more accurate pure\ncomponent properties compared to those from PR-Soave and RK-Soave.\nHowever, there was not a significant difference between the performance\nof PR-Twu and RK-Twu for calculating pure component properties. The\nsame result was obtained when comparing PR-Soave and RK-Soave. Interestingly,\nthe consistency analysis showed that only PR-Twu and PR-Soave produced\nconsistent Waring number trends for components with acentric factors\nbelow 0.7. It was also observed that the saturated liquid speed of\nsound calculated from all four models tested was not accurate as the\nmodels cannot produce precise −(∂<i>P</i>∂<i>v)</i><sub><i>T</i></sub> and liquid volumes. Besides,\nusing volume translation is detrimental to the accuracy of the calculated\nsaturated liquid speed of sound. The most accurate and consistent\nmodel was PR-Twu; however, caution should be exercised when modeling\nthe saturated liquid heat capacity of hydrogen bonding components.