[目的]超临界CO2管输是解决碳捕集、利用与封存(Carbon Capture,Utilization and Storage,CCUS)技术中碳源、碳汇不匹配问题最经济的方式.当管道因维检修需进行放空作业时,由于相变与焦汤效应的影响,放空管段温度可能低于-20 ℃,诱发管道脆断.[方法]利用OLGA软件建立超临界CO2管道放空模型,将模拟结果与参考文献中的实验数据对比,发现OLGA软件在预测CO2相变、温降及压降方面具有较高的准确性.在此基础上,提出间歇放空方案,并模拟分析了不同初始压力与温度下放空阀开度对放空过程与关阀回温过程时长的影响.[结果]在整个干线管道放空段中,放空点是最危险点,其温度会率先降至-20 ℃.随着阀门开度减小,放空点到-20 ℃所需时间呈指数增长.放空阀开度越小,一次开阀放空总质量越大,但平均放空速率越小,放空时间成本越高.放空阀开度增大,一次开阀放空总质量减小,且干线管道放空段整体压力水平高,使将管内介质完全放空至大气压力所需时间增加.关闭放空阀后,管内介质回温过程可分为由轴向换热与径向换热主导的两个阶段,且轴向换热主导阶段的温升速率更大.随着阀门开度的增加,关阀回温时长会迅速增加并趋于稳定.[结论]放空阀开度过大或过小均会导致放空时间过长,工程应用中可以将放空点处的温度或压力与放空阀的动作连锁.为保证控制的可靠性且尽可能缩短总放空时长,需合理确定放空阀每次动作的开度.
Venting design for block valve station of supercritical CO2 pipeline
[Objective]Supercritical CO2 pipeline transmission is deemed the most cost-effective solution to address the carbon source-sink mismatch in Carbon Capture,Utilization,and Storage(CCUS)technology.However,during venting operations required for pipeline maintenance,the vented pipeline segment might drop below-20 ℃ due to the phase change and Joule-Thomson effect,potentially leading to brittle fractures in the pipeline.[Methods]OLGA software was used to establish a venting model for supercritical CO2 pipelines.The subsequent comparison between the simulation results and experimental data demonstrated the accuracy of OLGA software in predicting CO2 phase changes,temperature reductions,and pressure drops.On this basis,an intermittent venting design was proposed.In addition,simulations were conducted to analyze the influence of vent valve openings on the durations of the venting process and the temperature escalation process post valve closure under varying initial pressures and temperatures.[Results]Within the vented pipeline segment of the entire trunk pipeline,the vent point was identified as the most dangerous as it is the first to experience temperature drops under-20 ℃.Decreasing valve openings led to an exponential increase in the duration required to reach-20 ℃ at this point.As the vent valve openings decreased,the total venting volume for a single valve opening operation increased,resulting in lower average venting rates and elevated time costs for venting.Conversely,excessively large vent valve openings led to a reduction in the total venting volume for a single valve opening operation.Moreover,high pressure levels throughout the vented segment of the trunk pipeline also prolonged the duration needed to fully vent the medium in the pipeline until it reached atmospheric pressure.The temperature escalation of the medium in the pipeline following valve closure may be divided into two stages respectively dominated by axial heat transfer or radial heat transfer,and the former stage was observed at a higher rate of temperature rise.As the valve openings increased,the time taken for temperature escalation after valve closure tended to stabilize following initial rapid increments.[Conclusion]Both excessively large or small vent valve openings lead to prolonged venting durations.In engineering applications,the temperatures or pressures at the vent point may be linked with the vent valve actions.However,to guarantee control reliability and minimize total venting durations,it is crucial to rationally choose vent valve openings for each action.
supercritical CO2pipeline transmissionblock valve station ventingintermittent venting
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