Study On Gas-Solids Fluidization Behaviors In Centrifugal Fluidized Beds

Centrifugal fluidization is a new type of gas-solid contacting technology. Compared with conventional fluidized bed, it has much higher heat and mass transfer rate because of the gas-solid interactions intensified by super-gravity. To investigate the gas-solids fluidization behaviors under centrifugal field and develop corresponding hydrodynamic models are clearly of essential importance not only for the further studying of transfer processes, but also for the designing of centrifugal fluidized beds (CFB). By considering the operation of CFB and the factors affecting gas bubble size, the principles that CFB controls bubble formation and growth and achieves particulate fluidization are illustrated. Based on Geldart's particle classification and Ergun equation, and by taking into account the relative magnitude of viscous force and inertial force acted on the particles, a new transition equation between particle groups B and D is obtained. Moreover, by using super-gravity factor, the transition equations of particle groups C/A, A/B and B/D in centrifugal fields are developed. These equations indicate that with the increasing of the super-gravity factor, the transition boundaries of particle classification could be shifting, so that, for example, group C particles could behave like group A particles under some higher gravity. This result provides the theoretical foundation for achieving a good fluidization of group C particles in CFB and, therefore, has practical importance for fluidization operation. By taking super-gravity fields'influence into account, a dimensionless number for powder agglomeration is defined. It shows that the increased effective particle gravity helps overcome the shearing force between particles and take the agglomerated particles apart, so that fluidization performance (especially for the Geldart C particles) can be improved. A model for predicting bed pressure drop in a centrifugal gas-solid fluidization system is developed according to the momentum balance by analyzing the overall force. Compared with the experimental data, the model predictions agree well with the first type of pressure drop curve. Then, for the prediction of the second type of pressure drop curve, the model is modified by taking the effect of particle sliding into account. The improved model agrees better with the experimental results. Finally, the heat transfer between gas and solids and the influencing factors on it are also analyzed in CFB.