Study on phase characteristics and impurity content of dense phase / supercritical CO2 system under pipeline transportation conditions

• This paper optimizes the equation of state suitable for CO 2 system containing impurities by experiments. • This paper reveals the influence mechanism of different impurities on the phase characteristics and critical point of supercritical/dense phase carbon dioxide. • In this paper, the impurity content control index of supercritical/dense phase carbon dioxide system based on the principle of pipeline transportation stability is proposed. Dense phase / supercritical CO 2 pipeline transportation is a critical component of CCUS (Carbon Capture, Utilization, and Storage), where impurity content significantly affects the phase state and stability of CO 2 pipeline transportation. To address the challenge that the influence mechanisms of various impurities on the phase characteristics of CO 2 under pipeline conditions are complex and difficult to predict, a phase equilibrium theoretical model was developed. Additionally, the equation of state (EOS) suitable for impurity-containing CO 2 systems was optimized through experimental validation. Based on this, the sensitivities of the bubble point, dew point, and critical point during pipeline transportation were studied. The following conclusions were drawn: the preferred Peng-Robinson (PR) equation of state is the most suitable for CO 2 systems containing impurities, with an average deviation of 4.835 %. From the perspective of impurity molecular polarity and intermolecular forces, the phase characteristics of CO 2 systems were analyzed. By defining a deviation degree, the average deviations of CO 2 systems with N 2 , CO, H 2 , Ar, and CH 4 impurities were 0.337, 0.364, 0.671, 0.268, and 0.211, respectively. Therefore, the influence of impurities on the phase envelope region, i.e., the impact per unit impurity, follows the order: H 2 > CO > N 2 > Ar > CH 4 . Based on pipeline transportation conditions, the effects of changes in bubble point, dew point, and critical point shifts on pipeline transport were analyzed. This research provides a foundation for establishing impurity content limits in CO 2 systems under pipeline transportation conditions.