高压聚乙烯装置分解反应原因分析及控制对策

Cause Analysis and Control Measures for Decomposition Reactions in LDPE Plant

聚乙烯、乙烯分解反应是剧烈的放热反应,会对聚乙烯装置安全运行造成较大影响。通过对某企业釜式法高压聚乙烯装置在开车建立反应初期发生的分解反应进行原因分析,表明开车前系统中氧、水等活性杂质引入,检修过程中釜内乙烯聚合物热氧化,建立反应过程中引发温度低,引发注入管线止逆阀故障等均会导致发生分解反应。为有效控制聚乙烯分解反应,开车投引发剂前,必须使用乙烯气对装置进行升温置换,确保氧含量控制在10mL/m^3以下,水含量控制在20mL/m^3以下。装置检修前应倒空釜内聚乙烯并将反应釜冷却至120℃以下,避免残存聚合物热交联和氧化导致的聚合物熔点升高和水、氧等微量杂质的引入。另外,需确认引发剂注入管线置换彻底,导通状态良好。引发剂注入时,应适当提高引发温度,便于快速建立反应。通过采取以上措施,该装置多次开工未再发生分解反应。

The decomposition reaction of polyethylene and ethylene is a violent exothermic reaction which has a great impact on the safe operation of low density polyethylene(LDPE)plant.This paper investigates the causes of decomposition reaction at the initial stage of reaction establishment during the startup of a LDPE plant using continuous kettle-reactor.The results show that the factors such as the introduction of oxygen,water and other active impurities into the process system before plant startup,the thermal oxidation of ethylene polymer in the kettle during maintenance,the low initiation temperature during the establishment of reactions and the failure of check valve of initiator injecting pipeline will probably cause decomposition reactions.In order to effectively control the decomposition reaction of polyethylene,the plant shall be heated up and replaced with ethylene gas before the initiators are injected,so as to control the oxygen content and water content at below 10 mL/m^3 and 20 mL/m^3 respectively.Prior to the maintenance,the polyethylene in kettle-reactor should be emptied and the reactor should be cooled to below 120℃to avoid the increase in the melting point of polymers caused by the thermal cross linking and oxidation of residual polymers and the introduction of trace impurities such as water and oxygen.In addition,it is necessary to confirm that the initiator injecting pipeline is completely cleaned and in good conduction state.When the initiators are injected,the initiation temperature should be appropriately increased to rapidly establish the reactions.By taking these measures,no decomposition reaction has occurred during the repeated startup of the LDPE plant.