Fluidization Characteristics Of Cohesive Particles In Agitated Fluidized Beds

Gas-solid fluidized bed reactors are widely used in polyolefin industry. Coarse cohesive particles are difficult to be fluidized because of their high cohesiveness. Cohesive particles easily stick to internal bed wall, adhere to other particles and form agglomerates. More seriously, the agglomeration causes defluidization and shut-down in fluidized bed reactor.To deal with the problem of stable fluidization of cohesive particles, an agitator was introduced to general fluidized bed. Three aspects of research were summarized as following:(1) fluidization characteristics of Geldart D particles in agitated fluidized bed; (2) experimental investigation of cohesive particles in agitated fluidized bed; (3) numerical simulation of flow behaviour of cohesive particles. The following innovative results were achieved:(1) Effects of agitation on hydrodynamic behavior of Geldart D particles in fluidized bed were obtained. Due to the effects of suppression and breakage on bubbles caused by agitation, lower pressure fluctuation amplitude and smaller bubbles are found in the agitated fluidized bed compared to general fluidized bed. The synergy between gas flow and agitation occurred in agitated fluidized bed. For a gas-solid fluidized bed with an anchor impeller or a frame impeller, sufficiently high agitation speed is needed to improve fludization performance. However, for self-cleaning agitator, higher agitation speed engenders adverse phenomena of gas short circuit and particles accumulation near the blades, therefore, the feasible agitation speed is recommended for extending the application of the new self-cleaning agitator in industry.(2) A new self-cleaning agitator was developed to reduce even eliminate agglomeration of cohesive particles and maintain the stable fluidization of particles. Fludization and agglomeration behaviours of cohesive particles were studied in the normal fluidized bed and agitated fluidized bed with anchor impeller and self-cleaning agitator. The clumps of agglomeration could be broken by the meshing effect of agitator and baffle, as a result, the bed fluidized in the form of agglomerates. S-arrractor could monitor the agglomeration much earlier, and accurately reflect development tendency of agglomeration.(3) Effects of particle-particle interaction on hydrodynamic behavior of gas-solid flow were studied. Based on computational fluid dynamics (CFD) method, gas-solid flow structure depended strongly on the coefficient of restitution. As the particle collisions became ideal, it reached the homogenous fluidization regime with non-bubbling, low pressure fluctuations. Meanwhile, the agitation of the frame impeller improved fluidization performance with reduced bubble diameters and a high agitation speed was needed to strengthen fluidization for inelastic particles. Based on the experimental results and discrete element method (DEM) simulation in 2d fluidized bed, the agglomeration nuclear formed firstly with cohesive particles. When the cohesiveness was high, more particles will adhered to the agglomeration nuclear and then developed into unilateral or bilateral bridge. As for the weak cohesiveness, the influences of gas flow and interparticle bonding on fluidization were equally important and cohesive particles fluidized in the form of agglomerates.