Widom line of supercritical CO2 calculated by equations of state and molecular dynamics simulation

Abstract

Supercritical CO2 has been widely applied in power cycle owing to the exceptional physical property in the supercritical region. The Widom line is very important to reveal the pseudo-boiling mechanism of supercritical CO2 which plays significant influence on design and optimization of power cycle. In this paper, the Widom lines for supercritical CO2 in terms of thermodynamic response functions are constructed using equation of state (EoS) and molecular dynamics (MD) simulation. Different equations of state and CO2 models are proposed and applied to predict the various thermodynamic properties. The results show that the Peng-Robinson (PR) series equations of state play better performance in predicting the thermodynamic properties and the average absolute relative deviation is around 5.7 % compared to the NIST database. The TraPPE model could represent a very good agreement with the experimental values at different pressures with about 1~6 % deviation. Owing to large temperature interval for molecular dynamics, the simulated values could not capture the dramatic change of thermodynamic properties at low temperature region (295 ℃< T < 310 ℃), resulting in that the simulated thermodynamic properties by molecular dynamics are generally lower than that calculated by Peng-Robinson equation of state. When the temperature is larger 335 K, the deviation of Widom lines between the NIST and PR EoS decreases gradually with the temperature. Nonetheless, all the Widom lines could fall into the coexistence zone of liquid-like and gas-like phase which is surrounded by the isobaric temperature boundaries.