CFD Analyses On Mass Transfer And Micromixing In A Rotating Packed Bed Reactor

As the intensified process performance and the hight micromixing efficiency, rotating packed bed (RPB) reactors is widely used in many industry fields. However, due to the complex structure of packing in RPB and the limit of computational capabilities, there are few basic researches about the detail information on the complicated flow patterns and mixing behaviors in the RPB reactor.In this study, by appropriate simplification of packing inside the RPB, a three-dimensional RPB model was developed and simulated. The geometric model and mesh generation were built in Design Modeler and Meshing-ICEM, respectively. By means of computational fluid dynamics (CFD), the flow patterns, the breakage of liquid and the process of mixing were simulated. The realizable k-? model, the volume of fluid (VOF) multiphase model, and the dynamic mesh were used to calculate the flow velocity and to capture the gas-liquid two phase interface. The distribution and pattern (film flow, droplet flow and pore flow) of liquid droplets were clearly observed from the simulation results. The coalescence also occurred during the breakage process. The average liquid droplet diameter was significantly affected by the rotating speeds, and slightly changed by the inlet velocity. The increasing rotating speed and liquid inlet velocity, and decreasing of liquid-air contact angle would enhance the specific liquid area, leading to better mass transfer efficiency eventually. As the rotating speed and liquid-air contact angle increased, the residence time would decrease. Compared with the two-dimensional simulation results, the predictions of droplet size, liquid velocity and residence time in the three-dimensional model agreed better with the experiments.The accuracy of VOF model strongly depends on the mesh size, i.e.,big errors will occur when the droplet size is smaller than the minimum mesh size. Because of the limitation of computational capabilities and huge number cells of the three-dimension RPB model, the predicted diameter distribution of droplets within RPB was not satisfactory. By combining experiment and simulation methods, the breakage information including breakage frequency, diameter of satellite droplets,effect of contact position and dripping droplet size were studied, and written into the population Balance (PBM) model. The simulation results on distribution of droplet size within RPB tended to be more reasonable. The needed total number of mesh cells was decreased and computational time was reduced, which have great significance in simulation of big size reactors.In addition, based on the stable flow in RPB, the iodide-iodate reaction system was simulated to study the micromixing efficiency within RPB. The distributions of conversion rate of H+, species H3BO3,species I2 and I3-, and XS along the radial direction in the packings were studied. The CFD simulation results indicated that the micromixing and reaction process mainly occurred within the inner 10 mm zone from inlet(end effect zone). The molar distribution of each reactants and product were clearly shown; the reaction process and the detail of micromixing were revealed. It also showed that XS depends strongly on rotating speed,which agrees well with the experiment trend, and XS was very close to the experimental results at the same radial position of packing.In summary, this work developed a simplified model of RPB to analyze the characteristics of fluid flow, the breakage process of liquid droplets and the micromixing within the RPB for optimization, scaling up and operation.