Numerical analysis of boundary line between liquid-like zone and gas-like zone of supercritical CO₂

Abstract

Supercritical carbon dioxide has gradually been becoming an important research subject in the academic field due to the fact that it has a promising application prospect in the field of extraction, precipitation, thermodynamic cycle and chemical reaction. In recent years, the interest in studying the region near the critical point was aroused and a large variation of the physical properties could be detected due to the change of temperature and pressure. The rapid development of molecular simulation technology benefits the traditional experimental methods to study the variations of relevant physical properties in the near-critical region. In order to find out the Widom line range of supercritical carbon dioxide in the near-critical region and the molecular structure characteristics of the liquid-like gas region, both the molecular dynamics simulation technology and the cluster analysis are used to investigate the relation between variation coefficient and skewness of CO₂ density time series with Widom line and liquid-gas-like interval, under the condition of the temperature and pressure range of 300–350 K and 5.5–18.5 MPa, respectively. The results show that the Widom line of supercritical carbon dioxide in the near-critical region can be determined by connecting the maximum coefficient of variation of the density time series curve. The Widom line begins to extend along with the critical point until it stops at 350 K. The molecular distribution structure of supercritical carbon dioxide liquid-like region and gas-like region can be differentiated by the skewness of the number density distributions. The skewness is positive in the gas-like region, but negative in the liquid-like region, and reaches the maximum at the Widom line.