纯CO_2的节流特性

Throttle characteristics of pure CO_2

CO_2管道输送是碳捕集与封存技术的一个重要环节。当CO_2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CO_2节流特性对于CO_2管道安全泄放控制具有重要意义。为此,建立纯CO_2等焓节流模型,给出了考虑相变的CO_2节流后温度的计算方法,并与天然气主要成分CH_4对比,分析了CO_2的焦耳一汤姆逊效应。研究结果表明,处于正常输送工况(20 MPa 1,以下)的CO_2节流时将产生温降效应;由于CO_2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CO_2不易进入两相区,但使密相CO_2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CO_2管道的节流控制和干冰堵塞预防具有指导意义。

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 CO2 will present a two-phase zone instead of super- critical CO2 if the inlet temperature increases. If the throttle is at the two-phase zone, the outlet temperature will not be affected by the in- let 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.