Modeling the Hydrodynamics of a Fluidized Bed

• Main Author: Deza Grados, Mirka
• Other Authors: Mechanical Engineering
• Format: Others
• Published: Virginia Tech 2014
• Subjects: Pressure Fluctuation Analysis; Numerical Simulations; Biomass; Fluidized Bed; Two-Phase Flow; Computational Fluid Dynamics
• Online Access: http://hdl.handle.net/10919/37554; http://scholar.lib.vt.edu/theses/available/etd-04062012-140118/

Biomass is considered a biorenewable alternative energy resource that can potentially reduce the use of natural gas and provide low cost power production or process heating needs. Biomass hydrodynamics in a fluidized bed are extremely important to industries that are using biomass material in gasfication processes to yield high quality producer gas. However, biomass particles are typically difficult to fluidize due to their peculiar shape and a second inert material, such as sand, is typically added to the bed. The large differences in size and density between the biomass and inert particles lead to nonuniform distribution of the biomass within the fluidized bed, and particle interactions and mixing become major issues. The main goal of this research was to use CFD as a tool for modeling and analyzing the hydrodynamic behavior of biomassas a single material or as part of a mixture in a fluidized bed. The first part of this research focused on the characterization of biomass particles in a fluidized bed and validation of a numerical model with experimental results obtained from pressure measurements and CT and X-ray radiograph images. For a 2D fluidized bed of glass beads, the pressure drop, void fraction and mean bed height expansion were in quantitative agreement between the experiments and simulations using Syamlal-O'Brien and Gidaspow drag models. It was encouraging that the Gidaspow model predictions were in close agreement because the model does not require knowing the minimum fluidization as an input. Ground walnut shells were used to represent biomass because the material fluidizes uniformly and is classified as a Geldart type B particle. Two-dimensional simulations of ground walnut shells were analyzed to determine parameters that cannot easily be measured experimentally. The parametric study for ground walnut shell indicated that the material can be characterized with a medium sphericity (~0.6) and a relatively large coefficient of restitution (~0.85). In the second part of this work numerical simulations of a ground walnut shell fluidizing bed with side air injection were compared to CT data for the gas-solid distribution to demonstrate the quantitative agreement for bed fluidization. The findings showed that 2D simulations overpredicted the fluidized bed expansion and the results did not demonstrate a uniformly fluidizing bed. The 3D simulations compared well for all cases. This study demonstrates the importance of using a 3D model for a truly 3D flow in order to capture the hydrodynamics of the fluidized bed for a complicated flow and geometry. Finally, CFD modeling of pressure fluctuations was performed on sand and cotton-sand fluidized beds operating at inlet velocities ranging from 1.0-9.0Umf with the objective of predicting characteristic features of bubbling, slugging, and turbulent fluidization regimes. It was determined that the fluidized bed can be modeled using MUSCL discretization and the Ahmadi turbulence model. Three-dimensional sand fluidized beds were simulated for different fluidization regimes. Fluidized beds for all the regimes behaved as second-order dynamic systems. Bubbling fluidized beds showed one broad peak with a maximum at 2.6 Hz while slugging and turbulent showed two distinct peaks. It was observed that the peak at low frequency increased in magnitude as the flow transitioned from a slugging to a turbulent fluidization regime. CFD simulations of fluidized beds with the purpose of studying pressure fluctuations have demonstrated to be a useful tool to obtain hydrodynamic information that will help determine the fluidization regime. Prediction of slugging and turbulent fluidization regimes using CFD have not been reported to date. The work presented here is the first of its kind and can be an important advantage when designing a reactor and evaluating different operation conditions without the need to test them in a pilot plant or a prototype. === Ph. D.