Study On Effect Mechanism Of Agitation And External Particle Circulation In Gas Phase Fluidized Bed Polymerization Reactor

Gas phase fluidized bed polymerization reactor is widely used in polyolefin production process. Wall sheeting or agglomeration caused by sticky polymer particles or electrostatic effects in fluidized bed reactor, would make the fluidization deterioration. This problem seriously restricts the long term and stable operation of fluidized bed reactor. The traditional solution is using mechanical agitation, and the new solution is external particle circulation. Mastering the reactor characteristics of agitated fluidized bed and external particle circulation fluidized bed is the basis of process control and development for new products. To achieve this goal, four aspects of research works were carried out. Firstly, the key process parameters of agitated fluidized bed were measured by using acoustic emission technology. Secondly, hydrodynamic behavior of agitated fluidized bed was studied. Thirdly, hydrodynamic behavior of external particle circulation fluidized bed was also studied. Finally, effects of mechanical agitation on hydrodynamics were compared with those of external particle circulation.The main research results are as follows:1. Based on the particle acoustic characteristics in the agitated fluidized bed, the initial fluidization velocity, bed level and elutriation rate were measured by acoustic emission technology. By defining agitation effect factor (AEF), a criterion for initial fluidization velocity was proposed from the AEF-gas velocity figure. Subsequently, self-correction bed level measurement method was proposed. This method could improve the measurement accuracy. Finally, the measurement of elutriation rate was realized on the basis of corresponding relationship between AE energy and elutriation rate.2. Effects of agitation on hydrodynamic behavior of fluidized bed were obtained. Based on the experimental results, effect mechanism of agitation in the fluidized bed was clarified. It was found that the agitated fluidized bed showed similar characteristics of turbulent fluidized bed, and there were much smaller size bubbles and lower pressure fluctuation amplitude in the agitated fluidized bed compared to general fluidized bed, due to the bubble repression and breakage caused by mechanical agitation. This could enhance gas-solid contact and intensify particle mixing in the bed. Meanwhile, reduction of entrainment and bed electrostatic potential, improvement of fluidization quality, increase of bed operating stability could also be realized. Finally, selection principles and recommendations for stirring speed were proposed.3. Effects of external particle circulation on hydrodynamic behavior of fluidized bed were obtained. Based on two dimensional CFD simulations, the core-annulus structure flow pattern composed of plug flow in core zone and mixed flow in annulus was proposed and validated by experiments. Then effect mechanism of external particle circulation in the fluidized bed was clarified. External particle circulation broke the bubbles in the core zone and caused them move towards the wall, then concentrated in the annular zone. This could enhance gas-solid contact and intensify particle mixing in the bed, especially near the wall. Meanwhile, reduction of entrainment and bed electrostatic potential, improvement of fluidization quality, increase of bed operating stability could also be realized. However, excessive external particle circulation may adversely affect the particle mixing due to strong PFR effects in the core zone. Ultimately, selection principles and recommendations for external particle circulation flow rate were provided.4. Effects of mechanical agitation and external particle circulation on hydrodynamics behavior in fluidized bed were compared. To establish the evaluation method for mechanical agitation and external particle circulation, five key evaluation indexs and two reference evaluation indexs were put forward. Key evaluation indexes included entrainment reduction, bubble breakage, particle mixing, gas channeling and slugging elimination, fluidization quality improvement. Reference evaluation indexes included electrostatic charge reduction and pressure drop reduction. The analysis results showed that external particle circulation could replace the traditional mechanical agitation.