Objective Impurities are inevitably present in CO₂ transmitted through pipelines, and these substances have significant impacts on the venting process of supercritical CO₂ pipelines, including temperature reductions, pressure drops, and phase changes of the medium inside the pipeline. However, experimental research on the venting of large-diameter industrial-scale supercritical CO₂ pipelines remains deficient.

Methods Utilizing a specially designed experimental setup for throttling and venting impurity-containing supercritical CO₂ pipelines, a study was conducted to delve into the influences of different contents of CH₄ or N₂ on temperature reductions, pressure drops, and phase changes in both the main pipeline and vent piping of the supercritical CO₂ pipeline during the venting process. Furthermore, the phase changes of fluid in the main pipeline of the impurity-containing supercritical CO₂ pipeline during venting were summarized into trend charts.

Results During the venting process of the impurity-containing supercritical CO₂ pipeline, the mixed impurities significantly affected both the CO₂ filling quality and the duration of venting. Moreover, the mixed N₂ caused a temperature increase of CO₂ after throttling in the venting piping. The mixed CH4 or N₂ led to an apparent increase in the minimum temperature of the fluid in the main pipeline during venting, while reducing the radial temperature difference of the fluid in the pipeline. With higher contents of impurities mixed, the minimum temperature of the fluid in the pipeline rose while the radial temperature difference decreased. The positions of the minimum temperature in the main pipeline during venting showed contrasting patterns among the scenarios involving pure CO₂, impurities at an amount fraction of 1%, and impurities at an amount fraction of 3%. Specifically, during the CO₂ pipeline venting, the minimum temperature occurred at the position farthest from the vent piping in the scenarios of pure CO₂ and impurities at an amount fraction of 1%. Conversely, in the scenario with impurities at an amount fraction of 3%, the minimum temperature was observed closest to the vent piping.

Conclusion The study findings can serve as valuable references for designing venting systems, preventing dry ice formation, and protecting pipes during the venting process of impurity-containing supercritical CO₂ pipelines.