(400y) Investigation of the Agglomeration Behaviors in Gas-Solid Fluidized Beds with Side-Wall Liquid Injecting

Qiang Shi, Shaoshuo Li, Sihang Tian, Zhengliang Huang, Jingdai Wang, Yongrong Yang

State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China

Abstract: Agglomeration exists in the wide range applications of fluidized beds, such as fluidized catalytic cracking, drying, solid fuel conversion, direct reduction of iron ore, biomass conversion, coal combustion and gas phase polymer production. The understanding of agglomeration mechanisms is critical to effectively control its evolution process. In gas phase olefin polymerization operated under condensed mode, liquid evaporation enhanced the heat removal efficiency, thereby the productivity. However, after being introduced to the bed, liquid adheres to the particles and forms liquid bridges among them which results in liquid bridge force. Meanwhile, as the polymerization is a highly exothermic process, particles soften if the surface temperature is high. Chain entanglements among particles form solid bridge, thereby the solid bridge force. Thus, under the actions of the two different strong interparticle forces, how does the bed hydrodynamics change ? And what’s the mechanism of particle agglomeration ? We have to get into insight into it so as to guide the operation of industrial fluidized bed reactors.

In order to mimic the exothermic process in an olefin polymerization fluidized bed reactor, we firstly designed a novel cold-mode fluidized bed based on induction heating method. Graphite particles coating with polyethylene wax on their surface were used as fluidized particles. Eddy currents formed on the surface of graphite particles generated heat and then heated up the particles. Polyethylene wax became sticky and formed solid bridge among particles after being heated. Liquid was injected into the bed through the side wall nozzle, and then adhered to the particles and formed liquid bridge among them. In this way, liquid bridge force and solid bridge force simultaneously existed in the fluidized bed. On this basis, we adjusted the liquid injecting rates so as to change the bed temperature and liquid contents among particles to investigate the dominant mechanisms of agglomeration under different interaction intensities of solid bridge force and liquid bridge force. Solid agglomerates (caused by solid-bridge force) sampled after experiments and liquid agglomerates (caused by liquid bridge force) deduced from bed pressure drop were used as characteristic variables to validate the mechanistic assumptions. Results showed that the interaction intensity of liquid bridge force enhanced with increasing liquid injecting rate, and more liquid agglomerates were formed till bed defluidized. Under low liquid injecting rate, liquid evaporation process dominated, leading to the decrease of bed temperature. The solid bridge force was weakened and the amount of solid agglomerates decreased. In this condition, gas came from liquid evaporation resulted in the increase of bubble diameter which was consistent with the variations of bed pressure fluctuations and standard deviations. However, further increasing liquid injecting rate, the enhancement of liquid bridge force and the formation of liquid agglomerates lead to the decrease of bubble diameter and the deterioration of bed fluidization quality, as agglomerates segregated in the distributor and formed dead zones. Heat accumulated in dead zones caused the overheating of particles surface and solid bridges were formed among them, so the amount of solid agglomerates increased. That is to say, dead zones caused by the segregation of liquid agglomerates created the acting environment for solid bridge force. Therefore, with increasing liquid injecting rate, the dominant mechanisms of agglomeration can be divided into three stages: solid bridge force dominated, dynamic equilibrium of liquid evaporation and liquid bridging, cooperated control of liquid bridge force and solid bridge force. Besides, we also investigated the effects of process variables and found that increasing heat releasing rate and the distance of the nozzle from distributor, the range of safe injecting rate of liquid widened.

Keywords: Induction heating; Liquid bridge force; Solid bridge force; Liquid evaporation; Cooperative control;