Experimental study on the influence of venting on temperature and pressure changes in impurity-containing supercritical CO2 pipeline

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.