Study of Supercritical CO₂ Pipeline Flow Leaks: Effect of Equation of State, Impurity, and Outlet Diameter

The global urgency to mitigate climate change has intensified the development of Carbon Capture, Utilization and Storage (CCUS) technologies. A critical step in CCUS is the safe and efficient pipeline transport of supercritical CO2 (sCO2), where flow dynamics are strongly influenced by phase change phenomena under transient heat transfer or depressurization conditions. Indeed, pressure disturbances, such as leaks or rapid decompression events, can induce vaporization and condensation, processes further complicated by the inevitable presence of impurities (e.g., N2,CH4,Ar) originating from different conditions at sources. These impurities not only shift thermodynamic boundaries but also alter the kinetics of phase transitions, directly impacting pipeline safety and design. In this study, we investigate the effect of impurities on leakage mass flow rate, and decompression waves in sCO2 pipeline transport through computational fluid dynamics (CFD) simulations, benchmarked against experimental data. A real-fluid model (RFM) implemented in the CONVERGE CFD solver is employed for these two-phase simulations, where a tabulation-based approach ensures accurate representation of thermodynamic and transport properties across multiphase regimes. Simulations are performed for varying impurity concentrations, enabling systematic evaluation of their influence on flow rate, and decompression wave propagation and associated flow variables, such as temperature. The results demonstrate strong agreement with experimental observations while providing insights into impurity-driven phase change behavior. The study investigates the effect of outlet geometry, dimensions, and role of Equation of State as well. CPA shows a better fit to the experimental results compared to PR and PC-SAFT for the simulations of supercritical CO2. It is found that for nozzle geometry where there is smooth change in cross-section area, the simulations prediction were quite close to experiment. However, for the case of orifice venting where there is sharp change in cross-section area, the simulations under predict the leakage mass flow rate, implying the influence of head loss due to geometry. Finally, the feasibility of simulating a 50 km industrial pipeline transporting sCO2 was investigated. The role of venting towers and gravity prove to be predominant in this specific case.