制冷系统乙烯回收工艺的优化设计与安全分析

发布时间：2019-03-16 21:48

【摘要】：安全问题一直是化工行业的首要问题,实现安全生产与经济效益“共赢”是企业的目标。乙烯压缩制冷系统由于在负压下操作会引进空气产生不凝性气体(主要是N2和02),系统中乙烯与02共存易形成爆炸性混合物,为了保证生产安全性,系统必须定期排放不凝气,富含乙烯的不凝气直接排放造成严重浪费。此外,乙烯压缩制冷系统开车过程需要用高纯乙烯置换氮气,目前工厂将乙烯置换气直接排放至火炬燃烧,没有采取回收措施,也造成乙烯资源的浪费。对排放乙烯进行合理回收,能够提高企业的经济效益。负压操作下的压缩制冷系统正常工况和开车工况年总排放420吨乙烯。现有工艺采用冷凝法和膜法回收正常工况排放的乙烯,冷凝工艺由于气液分离罐存在严重的雾沫夹带现象,造成气液分离不彻底使尾气带液,直接冷凝达到临界液化温度会导致O2被液化乙烯吸收排不出去,而在系统中积累,存在氧含量超标的安全隐患。通过减少尾气的带液量,改变冷凝起点,可提高冷凝温度的操作弹性,降低系统中氧含量,提高安全系数。采用的膜分离回收工艺,由于膜渗透侧为负压,会增加渗入空气的概率,同样导致02在系统中积累。通过使用限流孔板产生压降,将膜渗透侧压力提高到正压,可有效避免空气渗入。针对现有回收工艺存在的氧积累的安全隐患问题,提出使用精密过滤器减少尾气带液量,膜渗透侧直接设置为正压的方法,并设计了解决安全问题的改进方案。通过UniSimDesign软件对操作参数进行模拟优化,考察了冷凝温度对系统中氧含量及经济效益的影响。结果表明：改进方案冷凝温度范围均从-44～-46℃提高到-50-55℃,提高了冷凝温度的操作弹性,降低了系统中的氧含量,提高了安全性,同时还可保持较高的经济效益。其中膜回收法可使系统中氧含量降低到150 ppm,经济效益达到375万元·a-1。针对开车工况尾气间歇排放,瞬间排放量大(8500 Nm3.h-1),乙烯含量变化幅度大(C2H4 wt%：O～100%),且尾气带压的特性,提出了采用透平膨胀制冷开车尾气回收方案,并采用UniSim Design软件进行参数优化。结果表明,冷凝温度为-120～-130℃时,经济效益最大为14.5万元·a-1。设计的冷箱的换热面积为450 m2,透平膨胀机输出功为90 kW。
[Abstract]:Safety problem has always been the primary problem in chemical industry. It is the goal of enterprises to realize "win-win" between production safety and economic benefit. Ethylene compression refrigeration system can produce non-condensable gases (mainly N2 and 02) due to the introduction of air under negative pressure. The co-existence of ethylene and 02 is easy to form explosive mixture. In order to ensure the safety of production, The system must discharge non-condensable gas regularly, and direct emission of ethylene-rich non-condensing gas results in serious waste. In addition, it is necessary to replace nitrogen with high purity ethylene during the start-up process of ethylene compression refrigeration system. At present, ethylene replacement gas is directly discharged into torch combustion in the factory, and no recovery measures have been taken, which also results in the waste of ethylene resources. Reasonable recovery of ethylene emissions can improve the economic benefits of enterprises. Compression refrigeration system under negative pressure operation under normal and start-up operation of the total annual emissions of 420 tons of ethylene. The existing process uses condensation method and membrane method to recover ethylene discharged under normal working conditions. Because of the serious entrainment phenomenon in the gas-liquid separation tank, the gas-liquid separation does not completely bring the tail gas to the liquid. Direct condensation to the critical liquefaction temperature will lead to O _ 2 being absorbed and discharged by ethylene liquefaction, but accumulation in the system, there is a hidden danger that the content of oxygen exceeds the standard. By reducing the amount of liquid in tail gas and changing the starting point of condensation, the operating elasticity of condensing temperature can be increased, the oxygen content in the system can be reduced and the safety factor can be improved. The membrane separation and recovery process, because the membrane penetration side is negative pressure, will increase the probability of infiltration into the air, also lead to the accumulation of 02 in the system. Air infiltration can be effectively avoided by increasing the pressure of membrane infiltration side to positive pressure by using flow-limiting orifice plate to produce pressure drop. In view of the hidden danger of oxygen accumulation existing in the existing recovery process, the method of using precision filter to reduce the amount of liquid in tail gas and to set the membrane infiltration side directly to positive pressure is put forward, and the improvement scheme to solve the safety problem is designed. The influence of condensation temperature on the oxygen content and economic benefit of the system was investigated by simulating and optimizing the operating parameters by UniSimDesign software. The results show that the range of condensation temperature is increased from-44 鈩，

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