Effect of Superficial Gas Velocity on the Solid Temperature Distribution in Gas Fluidized Beds with Heat Production
The hydrodynamics and heat transfer of cylindrical gas–solid fluidized beds for polyolefin production was investigated with the two-fluid model (TFM) based on the kinetic theory of granular flow (KTGF). It was found that the fluidized bed becomes more isothermal with increasing superficial gas velocity. This is mainly due to the increase of solids circulation and improvement in gas solid contact. It was also found that the average Nusselt number weakly depends on the gas velocity. The TFM results were qualitatively compared with simulation results of computational fluid dynamics combined with the discrete element model (CFD-DEM). The TFM results were in very good agreement with the CFD-DEM outcomes, so the TFM can be a reliable source for further investigations of fluidized beds especially large lab-scale reactors
Tracking of particles using TFM in gas-solid fluidized beds
Abstract In this work, a new method is presented to track discrete tracer particles in a two fluid model (TFM). This method is particularly useful for studying features of discrete particles, such as solids mixing. Following the implementation and verification of this method, its accuracy was studied. The results showed that the new method fulfills the continuity equation and it can represent individual solids motion very well. This method may suffer from false diffusion, which can be diminished by selecting a sufficiently small grid size. In addition, it has several advantages over other techniques, like simplicity, ease of implementation, straightforward processing and enabling the calculation of a mixing index based on the initial neighbor distance concept. Moreover, this method can open a new way to combine the TFM with Lagrangian approaches. After analyzing the strengths and drawbacks of our method and finding the proper simulation settings, the effects of superficial gas velocity and restitution coefficient on solids mixing were investigated. The results showed that the solids mixing is enhanced by increasing the gas velocity and/or decreasing the restitution coefficient. The observed trends can be attributed to altered bubble formation and dynamics. These results also confirm our earlier findings on the solids temperature distribution in fluidized beds for polyolefins production (Banaei et al., 2017).
Theoretical and experimental bubble formation at a single orifice in a two-dimensional gas-fluidized bed
An earlier developed, first principles hydrodynamic model of gas-fluidized beds has been employed to study theoretically bubble formation at a single orifice in a two-dimensional bed. For several orifice discharge rates, theoretically predicted bubble sizes, formation times and shapes have been compared with experimental data obtained from triggered photographs. Besides, the present experimental and theoretical results were compared with predictions from two approximate models reported in the literature which are based on an idealized picture of the process of bubble formation. The advanced hydrodynamic model appears to predict the experimentally observed diameters, formation times and shapes of bubbles quite satisfactorily. The observed and calculated bubble diameters fall between the predictions from the two approximate models. Both the experimental and theoretical results clearly indicate that a strong leakage of bubble gas into the surrounding porous emulsion phase occurs, especially during the initial stage of bubble formation