Investigation On Fluidization Dynamics Of Particles With Contact Cohesive Force

Fluidized bed reactors have been widely used due to the high efficiency for heat and mass transfer as well as the capacity for dealing with bulk particles. Due to the extensive application of fluidized beds, the property of processed particles varies. For instance, contact cohesive force presents between particles in terms of liquid or solid bridge force. The presence of the contact cohesive force reduces the fluidity of particles, which causes different fluidization behaviors in comparison to the non-cohesive system, thus influencing the chemical conversion efficiency and the running stability of fluidized bed. Therefore, a detailed understanding of the fluidization dynamics with contact cohesive force may benefit the design and the optimization of these reactors. However, the relevant researches are still scattered, lacking of systematic studies.The objective of this thesis is to study the collision and fluidization dynamics of particles with the presence of contact cohesive force through experiments at the scales of particle and particulate system respectively. The multi-scale analysis could deepen the understanding of the dynamical characteristics of such system. The main results obtained are summarized as follows:(1) A particle-plate collision system with liquid was designed and constructed where the cohesive force between particle and plate was controlled by adjusting the liquid viscosity and layer thickness. The system applies a horizontal disturbing gas flow to drive a free-falling particle to move obliquely so that the oblique impact between the particle and the liquid layers could be realized. The system overcomes the disadvantages of the traditional "plate inclining" method that only feasible for the thin and highly viscous liquid layers. Meantime, the geometrical morphology of the liquid layers and the liquid bridges were captured with the aid of the flood light located below the Perspex plate, and the trajectory and velocity of particles were collected based on the image processing technology. Different from quasi-static process, the formation and development of liquid bridge during collision exhibits hysteresis effect. And more apparent hysteresis was observed under larger liquid Reynolds number viz. higher liquid inertia. Liquid bridge force contributes a lot to the energy loss of rebounding particle in the normal direction while has little effect in the tangential direction. However, in comparison to the other resistances acting during the whole collision, the contribution of liquid bridge force is negligibly small. In the normal direction, the energy loss increases with liquid viscosity and layer thickness, while decreases with increasing collision velocity. In the tangential direction, the energy dissipation process is dominated by layer thickness:at larger thickness, the liquid layer acts as resistance; at smaller thickness, it behaves as "lubricant" Considering the critical role of layer thickness in the energy dissipation process, a modified Stokes number is proposed by introducing the layer thickness into the definition. Moreover, the models for predicting the critical Stokes number and restitution coefficients are also developed by combing the elastohydrodynamic theory and the standard fluid mechanics.(2) The approach of "polymer coating" was used to introduce the contact cohesive force, avoiding the non-uniform distribution of cohesive force and poor reproducibility of traditional methods. The inter-particle cohesive force was adjusted by controlling the temperature of fluidization gas. The statical and dynamical bubble properties of cohesive particles in a two dimensional fluidized bed are collected based on image processing. In comparison to the non-cohesive particulate system, the presence of cohesive force inhibits the bubble coalescence in the medium bed and facilitates the bubble splitting at bed surface. The influence of cohesive force on the fluidization dynamics could be divided into two regimes:as the cohesive force increases initially, the emulsion phase has higher capacity to hold the gas. The bubble shape changes from quasi-circle to oblong shape through frequent vertical coalescence, weakening the particle motion along the sidewalls of the bed. This causes the particles to adhere to the sidewalls with further increasing of cohesive force, along with the sharp decline of bed expansion, finally leading to the failure of fluidization in terms of channeling. The defluidization process is accelerated with the presence of immersed tubes. The tubes reduce the influences of cohesive force on the bubble properties. The bubble profiles around the tubes are controlled by both the cohesive force and the tube location.(3) The X-ray tomography system with multi-sources, built at Delft University of Technology was used to reconstruct bubbles in a 3D bubbling column fluidizing cohesive particles for the first time. Meantime, the in-bed pressure fluctuation analysis was also combined to achieve abundant information for fluidization. The presence of cohesive force facilitates the bubble coalescence, leading to larger bubble sizes and lower bubble frequency, thus inducing slugging and resulting in defluidization at large cohesive force. Different from the fluidization of non-cohesive particles, the particle slug could self-grow with cohesive force, which extends the defluidization region toward the air distributor. Due to the rupture of particle slug, the bed turns to an alternative process of normal fluidization and slugging. The gas slug is more sensitive to the cohesive force in comparison to the free bubbles. Since the slugging is triggered by large bubbles, the parameters that facilitate bubble growth would increase the duration of slugging. The pressure waves generated by the coherent phenomena such as bubble eruption and bed oscillation, etc. dominants the in-bed pressure fluctuation. The attenuation of pressure waves along the bed height under large cohesive force is found to largely depend on the fluidization gas velocity.The comparison between the results obtained in 2D and 3D beds shows that as the cohesive force increases, the fluidization qualities both deteriorate with the reduction in the fluidity of bed materials and the fluidization fails within the same range of bed temperature. However, the differences in the geometrical features of the beds cause different trends of bubble behaviors as well as the defluidization format, indicating the critical role of bed geometry in the fluidization of cohesive particles.