Study On Throttling And Unloading Characteristics In Supercritical Carbon Dioxide Pipeline

The transportation of carbon dioxide?CO₂?plays an important role in carbon capture,storage and utilization of CCUS technology chain.Compared with ships,highways and railways,pipeline transportation is considered as the most optimal form of transportation.The venting system is an essential link to ensure the safe operation for CO₂ pipeline transportation.However,the CO₂ pipeline design standards only qualitatively focused on the discharge capacity of venting pipe,temperature drop,blocking and noise of dry ice and so on both at home and abroad nowadays,which lack the systematic research on especially the supercritical CO₂ venting process.In this paper,study on venting and throttling process of CO₂ pipeline is carried out based on the phase equilibrium characteristics of CO₂.The experimental results show that the temperature drop has a linear relationship with the pressure difference before and after the valve.The greater the pressure difference,the greater the temperature drop.Moreover,the water cut will have an effect on the throttling effect coefficient,and the temperature drop increases nonlinearly with the increase of gas and liquid ratio,which decreases gradually.Thus,a supercritical CO₂ venting experiment loop system was developed and experiments on supercritical venting under different initial pressures and temperatures were done.Pressure variations in the pipe section upon venting mainly depend on two factors:the initial pressure and the initial conditions of venting pipe.The outlet pressure will increase sharply due to the opening of releasing valve during the venting process,reaching an extreme value in a relatively short time.The temperature inside the pipeline at both the far and near the end of pipe decreased sharply,and the temperature drop at the far distance from the releasing valve is larger than that near the releasing valve.Besides,the temperature outside drops rapidly which is faster than that in the pipe.The pipe diameter and initial pressure of the releasing pipe have a great influence on the pressure extremum after the releasing valve.At the same time,there will be a violent phase change both in the pipe and outside the releasing pipe upon venting.Due to the low pressure in this experiment,the phase state of the pipeline enters into the gas phase region directly,and few of the experiments enter the dense phase and the liquid phase region.Combined with the venting characteristics at the sites of CO₂ pipeline transportation,the diffusion model for CO₂ venting is established and numerical simulation researches on the influence of venting height and wind speed is completed.Effects of discharge rate and temperature are summarized and analyzed during the supercritical CO₂ venting process based on the experiment study and numerical simulation,which can provide some reference for the design of venting for CO₂ pipeline transportation.