Investigations On The Mode And Control Mechanism Of Particle Agglomeration In The Gas-solid Fluidized Bed

Agglomerate is a typical mesoscale structure in the gas solid fluidized bed. And particle agglomeration mechanism is always regarded as a research hot spot. The wax/graphite composites with the core-shell structure were used as fluidized particles in this thesis. In the cold mode experiment, the fluidized particles were heated by a novel induction heating method that the rate of heat released could be controlled. After being heated, the wax on the surface of the composites would be sticky and agglomerates would form. By this way, the particle agglomeration behavior during the polymerization reaction was simulated and the aggfomerates were captured. Moreover, the mass, morphology, constitute of the agglomerates were analyzed. These datas were then combined with the variation of pressure drop and bubble size during the particle agglomeration process to study the evolution processes and control methods of the agglomerates. In this thesis, the agglomeration processes were classified into the gradual agglomeration and the sudden agglomeration. And the influences of the operation parameters on these agglomeration behaviors were decided by experimental results. Besides, the population balance model describing the gradual agglomeration process was constructed. Specially, the sudden agglomeration process controlled by the operation parameters and liquid content simultaneously was investigated. The relationship map describing the relationship between liquid content, bed temperature and agglomerates mass was proposed. This map could be used to guide the control of the particle agglomeration that would be helpful to the scaling up and normal operation of the gas phase polymerization reactor. This thesis mainly focused on the following aspects:1. A gas so lid reaction simulation device was constructed, and the feasibility of the method to simulate the polymerization heat by the induction heating technology was evaluated. The results showed that, the increasing rate of the temperature of the gas solid reaction simulation device could be adjusted by regulating the material composition, particle size, and induction heating power. While the device was in the fixed state, the temperature was not uniformly distributed in the radial direction. The temperature gradually decreased when the position approached the center of the bed. Comparing to the fixed state, the device in the fluidized state couid have a more uniform temperature distribution. Moreover, by adjusting the experiment parameters, the range of heat release rate of ethylene polymerization can be covered by the range of heat released rate of wax/graphite composites.2. The gradual agglomeration and the sudden agglomeration were simulated respectively by adjusting the induction heating power in the gas solid reaction simulation device, and the influences of the operation parameters on the gradual and sudden agglomeration behaviors were decided. The results showed that, while the increasing rate of the bed temperature increased and the fluidization gas velocity decreased, the gradual agglomeration rate constants increased. If the time span of the induction heating and the increasing rate of the bed temperature increased, the mass and size of the agglomerates formed in the bed as well as the agglomeration rate would increase greatly. However, the fluidization gas velocity had an adverse effect3. The agglomerates were captured in the gas solid reaction simulation device, and the characterization method about the sudden agglomeration process was constructed. The influence of particle size on the sudden agglomeration behavior was decided by the experimental results. The results showed that, the total bed pressure drop and the diameter of the bubble reduced while the agglomerates occurred. The reduction amplitude in the total bed pressure drop could indicate the mass of the formed agglomerates. Small particles were more easily to agglomerate than the coarse particles. The main forms of the particles agglomeration were small-small particles and small-coarse particles agglomeration. While the mass fraction of the coarse particles was below 87.5 wt%, the frequency of collision between the small-small particles, small-coarse particles was higher than the coarse-coarse particles. According to the energy balance model, the possibility of agglomeration after collision for the coarse-coarse particles was also the lowest.4. Three variation trends of the mass of the agglomerates formed with the liquid content were revealed, a criterion of a stable particle agglomeration based on the relative solid force and the relationship map about the liquid content, bed temperature and agglomerates mass that could be used to guide the control of the particle agglomeration was proposed. The results showed that, while the solid bridge force was larger than the vector sum of the other total force, the solid bridges and the agglomerates would be steady. The operation parameters and liquid content would have a great influence on the solid bridge force. And the influence of the liquid content on the liquid bridge force was relatively small Under the fluidization gas velocity of 0.96 m/s, the varying range of relative solid bridge force was 6-20 in the region where the absolute solid bridge force was approximate with the resultant of other forces. The variations of particle agglomerate mass with the increase of the liquid content under different gas temperatures, superficial gas velocities and heating powers showed three forms, namely, increased monotonously, first increased and then decreased, and decreased monotonously. The criterion of a stable particle agglomeration based on the relative solid force and the relationship map about the liquid content, bed temperature and agglomerates mass all could be used to guide the control of the particle agglomeration.