纯CO2的节流特性

CO₂管道输送是碳捕集与封存技术的一个重要环节。当CO₂管道超压时可采用节流方式泄压，但节流易导致管内温度场骤变，有可能会造成干冰堵塞或管道低温损伤，因而研究CO₂节流特性对于CO₂管道安全泄放控制具有重要意义。为此，建立纯CO₂等焓节流模型，给出了考虑相变的CO₂节流后温度的计算方法，并与天然气主要成分CH₄对比，分析了CO₂的焦耳-汤姆逊效应。研究结果表明，处于正常输送工况（20 MPa以下）的CO₂节流时将产生温降效应；由于CO₂临界点温度和压力较高，其节流后容易进入两相区，使温度和密度发生突变。因此，进一步讨论了改变节流控制参数（入口温度、入口压力和出口压力）对节流出口温度的影响。结论认为：①在节流压力一定时，提高入口温度使超临界CO₂不易进入两相区，但使密相CO₂更易进入两相区；②若节流进入两相区，在一定范围内改变入口压力对出口温度没有影响；③当节流入口条件一定时，出口压力为临界压力时是形成两相流的分界压力，出口压力为三相点压力时是形成干冰的分界压力。该研究成果对CO₂管道的节流控制和干冰堵塞预防具有指导意义。

Throttle characteristics of pure CO2

CO₂ pipeline transportation is an important link of carbon capture and sequestration. When overpressure occurs in CO₂ pipeline, throttle can be adopted for blowdown, but throttle can induce temperature field jump inside the pipes, which may result in dry ice plugging or pipeline freezing. In order to realize the safety relief and control of CO₂ pipeline, it is significant to study CO₂ throttle characteristics. In view of this, a pure CO₂ isoenthalpy throttle model was built, and calculation methods taking into account of phase change were provided for the temperature after CO₂ throttle. Comparison was conducted with CH₄, the main component of natural gas. And analysis was carried out on Joule-Thomson effect of CO₂. It is shown that temperature drop effect will be generated by CO₂ throttle in normal transportation conditions (lower than 20 MPa). CO₂ critical temperature and pressure are higher, so two-phase zone tends to appear after throttle, and temperature and density change abruptly. Therefore, analysis was further performed on the effect of throttle control parameters (inlet temperature, inlet pressure and outlet pressure) on throttle outlet temperature. It is indicated that when the throttle pressure is kept at a certain value, dense-phase CO2 will present a two-phase zone instead of supercritical CO₂ if the inlet temperature increases. If the throttle is at the two-phase zone, the outlet temperature will not be affected by the inlet pressure if it is changed in a certain range. When the throttle inlet is kept in a certain condition, the outlet pressure will be the critical pressure for the formation of two-phase flow when it is the critical pressure, and it will be the critical pressure for the formation of dry ice when it is the triple-point pressure. The findings can play a guiding role in throttle control and dry ice prevention of CO₂ pipeline.