Importance of Efficient Force Distribution on Agitators in Continuous Stirred Tank Slurry Reactors

摘要

In Reliance, Multimodal HDPE is produced through two Continuous Stirred Tank Slurry Reactors in Series. The multimodal molecular weight distribution is built in the second reactor. The heat of reaction is removed through two external cooling legs and the reactor is agitated using 4 Intermig agitators separated along the axial direction. In the second reactor, the injection point of two recirculation legs was separated radially by 30° angle. To improve the cooling (production capacity) as well as fouling behaviour in the second reactor, the recirculation rates of the slurry were increased by 1.5 times. After this change, the top impeller blade started breaking frequently. Detailed CFD study was carried out to understand root cause for the behaviour through analysing velocity, force and torque distribution profiles inside the reactor due to this change. The study revealed that materials from two recirculation legs communicated with each other leading to very strong jet hitting on one side of top impeller blade creating huge force imbalance on the agitator which led to the frequent breakage. A simple modification was proposed, as result of CFD studies, one of the recirculation streams was moved away from another by 150° thereby avoiding the cross-communication between two recirculation streams. After it was implemented, no agitator blade breakage was seen for very long time and plant has been running very smoothly. This case study implied that force balance needs to be given importance while debottlenecking agitated vessels with recirculation loops. Computational tools like CFD can be effectively leveraged to precisely indicate the potential areas of issues while carrying out debottlenecking or capacity expansion.