Influencing mechanisms of various impurities on the physical properties of supercritical/dense-phase CO₂

CO₂ transport is a crucial component linking the upstream and downstream segments of the Carbon Capture, Utilization and Storage (CCUS) industry chain. Due to variations in carbon sources and capture methods, pipeline-transported CO₂ often contains various impurities, which can significantly impact its physical properties. In this study, the physical properties and phase behavior of pure CO₂ were analyzed at first, confirming that the supercritical/dense-phase was optimal for large-scale, long-distance transport. Using the equation of state, gas interaction models, critical temperature and pressure models, and the gas-liquid equilibrium equation, the physical properties of CO₂ with impurities were calculated. Results indicated that adding impurities such as CH₄, N₂, and CO at molar fractions below 5% effectively lowered the critical temperature and pressure of CO₂. The optimal transport conditions were approximately 14 MPa and a temperature near the critical point. Multiple regression analysis, combined with the equation of state, revealed that the influence of impurities on CO₂'s critical temperature and pressure was positively correlated with the impurities' own critical parameters. When the molar fraction of impurities was less than 5%, their overall impact on CO₂ properties such as density and viscosity was minimal. For pipeline transport, particular attention should be given to the effects of impurities on the critical temperature and pressure of CO₂. This study provides design basis and theoretical foundation for the safe, efficient, and large-scale transport of supercritical/dense-phase CO₂ by pipeline.