Pressure Response,Phase Transition And Dispersion Characteristics During CO₂ Pipeline Releases

As a part of the carbon capture and storage pipeline transportation of CO2 is considered an important transportation option in large-scale.Rupture and accidental leakage will inevitably occur in long-distance CO2 pipelines due to various complicated factors.Currently,there isn't a clear understanding of the pressure response,phase transition and dispersion characteristics of pressurized CO2 pipelines.Industrial scale release and venting experiments were performed to study the pressure,temperature,phase and decompression velocity evolutions inside the pipeline,the temperature,CO2 concentration and visible cloud evolutions in the discharge area.The concrete research contents are as follows:(1)An industrial scale pipeline experimental apparatus with a total length of 258 m and the inner diameter of 233 mm was constructed to measure synchronously the pressure and temperature of CO2 inside the pipeline and the wall temperature,the CO2 concentration,the temperature and the visible cloud in the discharge area of gaseous,dense and supercritical CO2 with different orifice(0-233 m)and different release direction(horizontal and vertical).A dual-disc blasting pipe and a reinforced anchor device were was designed to safely and controllably open the pressurized pipeline and initiate full bore rupture experiments.(2)The pressure responses of gaseous,dense and supercritical CO2 were studied during horizontal releases with different orifice diameters.The results showed as follows:In the gaseous CO2 releases,the inventory pressures had a linear sudden drop successively when the decompression wave arrived,and subsequently recovered to the quasi-static level due to the droplet formation and gasification.In the dense CO2 releases,during the pressure response process when the decompression wave arrived there was an obvious slowdown between sharp decline and rapid rise in pressures as a result of bubble nucleation.In the supercritical CO2 releases,the pressure undershoot and rebound occurred near the critical region,and the rate of pressure change stagnated when the inventory pressure passed through the critical pressure.For three phase CO2 releases,the number of the pressure undershoot-rebound increased and the quasi static pressures became greater with decreasing diameter of the orifice.For the full bore releases,after undershoot the pressure only had a slowdown and there was no rebound.(3)The phase transitions of gaseous,dense and supercritical CO2 were studied during horizontal releases with different orifice diameters.The results showed as follows:In the gaseous CO2 releases,during the depressurization process,the CO2 phase was generally gaseous near the orifice.When the release diameter was increased,the gas-liquid or gas-solid-liquid CO2 would appear near the pipe end.In the dense CO2 releases,the dense phase transformed into the gas-liquid phase firstly,and then the CO2 phase was mainly gas-solid or gas-solid-liquid with dry ice forming at the bottom of the pipeline.In the supercritical CO2 releases,the supercritical CO2 successively transformed into gas-liquid phase and gas phase when the inventory pressure fell below the critical pressure.The supercritical CO2 turned into a superheated gas firstly with very great release rate,then changed into the gas-liquid phase with decreasing diameter of the orifice.The dry ice particles were produced when the inventory temperature fell below the triple point.(4)The near-field characteristics and dispersion behavior of gaseous,dense and supercritical CO2 were studied during horizontal releases with different orifice diameters.The results showed as follows:A highly under-expanded flow was developed near the orifice during the release of high pressure CO2.A large quantity of dry ice particles formed in the near-field due to Joule-Thomson cooling,these were carried into the far-field.The dispersing cloud was made visible by both these dry ice particles and condensing water vapor.The dry ice sublimed rapidly and did not settle.For given initial conditions,as the CO2 inventory mass or orifice diameter increased the temperature drop amplitude and the dispersion distance of CO2 in the discharge area increased.The temperature and CO2 concentration distribution was significantly affected by the environmental conditions in the far dispersion area.(5)The pressure response,phase transition and dispersion behaviour of gaseous,dense and supercritical phase CO2 were studied during vertical leakage with small orifice diameter.Some conclusions were demonstrated as follows:Comparing to horizontal releases,a "two cold,intermediate hot" phenomenon was observed during the vertical leakage in the dense and supercritical release due to dry ice particle accumulation near the orifice.For three phase CO2 leakage,the gas-solid two-phase jet entrained a mass of dry ice particles,gaseous CO2,air and condensed water in the near-field,this mixture continued to spread in the far-field.Compared to the gaseous and supercritical CO2 releases,in the dense CO2 release the escaping CO2 more easily formed a relatively high concentration cloud at ground level,and a clearly visible cloud settlement process could be observed.(6)The pressure response,phase change and diffusion behavior during the venting process were studied.The results showed as follows:In the vertical venting process of CO2 pipelines,there weren't pressure undershoots and rebounds inside the main pipeline.The pressure inside the venting pipe quickly rose after electric valve opened,and the rise amplitude became smaller with the distance to the orifice decreased.After electric valve opened,the supercritical CO2 transformed into the gas-liquid two-phase inside the main pipeline,while the gaseous CO2 was filled in the venting pipe at first and then transformed quickly into the gas-liquid two-phase.The length of the venting pipe only changed the height of the venting orifice,and there were little impacts on the release form itself.