MODELING AND COMPUTATIONAL FLUID DYNAMICS SIMULATION OF A BUBBLING FLUIDIZED BED PROCESS AT DIFFERENT SCALES
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In recent years there has been increased research activity in the experimental and numerical study of gas-solid flow system in the bubbling fluidized bed process. The bubbling fluidized bed process have numerous applications in the energy, pharmaceuticals, and chemicals process industries since it has provides a number of advantages such as large heat capacity inside a bed, and rapid heat and mass transfer rate. A reliable design and scale-up approach for a bubbling fluidized bed process requires a very detailed model based on the fundamentals of multiphase transport phenomena. The present works address the simulation and scale-up of rather complex gas-solid flow behavior in bubbling beds using Computational Fluid Dynamics (CFD) approach. The CFD model developed in this study which is based on two fluid model was used to optimize the performance and utilized as a scale-up tool for an isothermal and a non-isothermal bubbling fluidized bed process. For isothermal case, 2-Dimensional and 3-Dimensional simulations of bubbling beds for both PSRI laboratory and large scales fluidized beds using a kinetic theory approach were performed. The FLUENT code was used to conduct the simulations. Our simulation results were validated and refined by comparing them with the laboratory-scale experimental data of PSRI. Then, our modified 2-D and 3-D CFD models were used to predict the large-scale PSRI bubbling fluidized bed performance at different operating conditions. In our 3-D simulations, we used exactly the same bed dimensions and inlet configurations (such as air distributor) as the experimental one to predict the characteristics of gas-solid flow patterns in the PSRI large-scale bubbling fluidized bed. The numerical simulation results compared well with both PSRI large scale experimental xx data on pressure drop and time-averaged void fraction near the wall, which could be a very good proof for demonstrating the capability of CFD as a tool to be used in the design and scale-up of bubbling fluidized bed systems. For non-isothermal case, the set of equations necessary to describe the flow patterns and heat/mass transfer phenomena of bubbling beds at three different scales were developed. CFD simulations were performed to investigate the characteristics of pharmaceutical particle drying process in bubbling fluidized beds at three different scales (e.g., lab, kilo, and 10-kilo scales). The results of CFD simulation were compared with the experimental data obtained at laboratory-scale (Duquesne University experiments), to validate and refine our CFD model. The modified model was used to simulate the drying of the same material in Abbott laboratory kilo and 10-kilo scale units. Our simulation results for solid particles drying as a function of dimensionless time showed that our CFD model along with similar dimensionless group similarity approach can be used as a tool to scale-up the drying process from experimental scale to both kilo-scale and 10-kilo scale fluidized bed dryer. Moreover, to determine the optimum particle mixing, numerical simulations were performed at different particle diameters, bed heights, inlet velocities and inlet velocity distributions, respectively. The numerical simulation results compared well with the experimental data (performed by Duquesne University and Abbott laboratory) on moisture removal rate and outlet gas temperature. This also could be a very good proof for demonstrating the capability of CFD as a tool to be used in the design and scale-up of non-isothermal bubbling fluidized bed processes.