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
Effect of superficial gas velocity on the particle temperature distribution in a fluidized bed with heat production
The heat transfer behavior of particles and gas in an olefin polymerization fluidized bed was numerically analyzed using an in-house developed 3-D computational fluid dynamics discrete element model (CFD-DEM). First the implementation of the model was verified by comparing simulation results with analytical results. A constant volumetric heat production rate was implemented in the particle energy equation to mimic the heat production due to the polymerization reaction. It was found that the probability density function (PDF) of the particle temperature becomes more homogeneous with increasing superficial gas velocity. Furthermore, instantaneous snapshots of the thermal driving force (the difference between the single particle temperature and bed-average gas temperature, Tp− ) for different heat production rates provide detailed insight in the particle temperature distribution inside the fluidized bed. The time- and bed-averaged particle convective heat transfer coefficient, which was calculated by Gunn׳s correlation, was found to be independent of the superficial gas velocity. This is explained by the fact that the relative velocity of gas and particles in the emulsion phase, where most of the particles and gas interact, is hardly influenced when increasing the gas superficial velocity. From the spatial distribution of Nusselt number, it becomes apparent that the high heat transfer regions are found in the wake of rising bubbles, whereas low heat transfer rates are found in the clouds of the bubbles.
Gas phase solid-catalysed chemical reaction in spouted beds
Abstract Previously reported experimental work on the decomposition of ozone in a 0.15 m diameter spouted bed of angular iron oxide catalyst particles has been extended to spherical catalyst particles in both 0.15 m and 0.22 m dia. columns. Other variables, aside from column diameter, included catalyst particle size, bed depth, spouting velocity, and reaction rate constant (varied by adjusting the bed temperature). The use of a photometric analyser allowed analysis of concentrations at the inlet and exit from the bed, at different radial positions at the bed surface, and at several positions within the bed itself. Very little effect of particle size was found over the range studied (2.58 to 4.38 mm). Contrary to previous expectations, the conversion was found to decrease as the bed diameter was increased. The concentration gradients observed in the annulus indicate that the streamtube model is superior to the one-dimensional model, despite the fact that overall conversions predicted by these previously derived models are again similar. The observed gradients were not, however, as pronounced as those predicted by the streamtube model, and this suggests that radial dispersion in the annulus cannot be entirely neglected.