气相法聚乙烯工艺冷凝态操作模式的稳定性和动态行为

Stability and dynamic behaviors of gas-phase ethylene polymerization process under condensed mode operation

气相法聚乙烯工艺冷凝态操作模式由于显著提高了循环气移热能力和反应器时空产率,已成为流化床乙烯聚合工艺的主流操作模式。建立了气相法聚乙烯工艺冷凝态操作模式的数学模型,包括流化床反应器模型,多级换热器模型和反应温度、压力以及循环气组成的控制模型。基于此,采用流程模拟方法,计算了系统在反应器温度采用闭环控制时的稳态解;根据系统对小扰动的动态响应特点,定性判断了反应器温度采用开环控制和闭环控制时聚合反应系统的稳定性;考察了系统对1-己烯分压和催化剂进料速率的阶跃响应特性。结果表明,反应器温度采用闭环控制时,聚合反应系统在所考察操作条件下均是稳定的,而采用开环控制时,解曲线被分叉点分割为稳定区域和不稳定区域。反应器温度对1-己烯分压阶跃变化的动态响应表明聚合反应系统存在长、短周期两类振荡,表明冷凝态操作模式下乙烯聚合反应过程是一个多控制回路耦合的复杂过程。

Due to significant enhancement in heat removal capacity of recirculating gas and space-time yield of fluidized bed reactors, condensed mode operation has become most popular in gas-phase ethylene polymerization.A mathematical model for condensing mode operation of gas phase ethylene polymerization process was established, which included fluidized-bed reactor model, stage heat exchanger model, and control model of reactor temperature, pressure, and component partial pressure of recirculating gas. Process simulation was applied to obtain steady state solution when reactor temperature was controlled by close-loop, and dynamic behavior of the system under small perturbation was used to qualitatively analyze stability of ethylene polymerization when reactor temperature was controlled by open-loop and close-loop. Finally, stepwise response characteristics of ethylene polymerization close-loop system was studied under consideration of 1-hexene partial pressure and catalyst feed rate. Simulation results show that the steady state solutions are stable within the scope of studied conditions when reactor temperature is close-loop controlled, whereas the steady state solutions are divided into stable and unstable zone by bifurcation points when reactor temperature is open-loop controlled. The long and short period oscillations observed in stepwise response of reactor temperature to 1-hexene partial pressure reveal that ethylene polymerization system under condensed mode operation is a complex process with multiple coupling control loops.