Doctor of Philosophy
Chemical and Biochemical Engineering
Cedric Louis Briens
In this study, the multi-phase Eulerian-Eulerian two-fluid method (TFM) coupled with the kinetic theory of granular flow (KTFG) was used to investigate hydrodynamics of particle flows (Geldart Group B) in a lab-scale fluidized bed Geldart Group B particles, operating in bubbling and turbulent regimes. The effect of gas distributors and baffles on the distribution of the gas bubbles, and the mixing of gas and solids were investigated under various superficial gas velocities.
The numerical model is validated by experimental results from two different measurement methods with various gas distributor configurations and a superficial gas velocity ranging from 0.4 m/s to 1.0 m/s; the E-probe method measured the local gas flux while the radiation transmission method provided the local solid hold-up. Simulation with different gases and particles spanned the range from lab to industrial conditions. The gas distributor configuration and angle were found to have a significant impact on the gas bubble distribution.
Baffles are used to modify fluidized bed hydrodynamics in industrial processes. This work simulated the impact of various baffles on fluidized bed hydrodynamics. A ring baffle can redirect gas bubbles and induce strong liquid recirculation currents. Adding a vertical fluxtube to a baffle can significantly modify its impact on the gas flow patterns. With a fluxtube that does not extend past the baffle lip, the gas is more evenly distributed in the fluidized bed. The fluxtube length has a stronger impact than the fluxtube diameter on the fluidized bed hydrodynamics.
New methods were developed to characterize the gas and solids mixing patterns from the simulation results. Gas and solids mixing in both horizontal and vertical directions are affected by the gas distributor configuration and the presence of a ring baffle. The ring baffle separates the bed into two regions and reduces the back mixing of gas and solids between the upper and lower regions
Summary for Lay Audience
In fluidized beds, gas is injected into a bed of particles to impart a liquid-like behavior to the gas-solids mixture. Fluidized beds are used in many industrial processes ranging from oil refining and biofuels production to pharmaceuticals and food applications. This thesis uses a CFD numerical method to simulate fluidized bed systems with a set of governing equations. Due to the fast development of computer technology, CFD modelling has become an effective and economical tool to investigate fluidized beds. Different numerical methods have been developed in this work. New technology is introduced to track the particle and gas molecule in the fluidized bed using the two-fluid model. The statistical time distributions, dispersion rate, and the mixing rate in the lateral and vertical direction for solid and gas were investigated.
For this thesis, the strategy was to use experimental data obtained in a small laboratory fluidized bed to validate CFD modelling tools. These tools were applied to verify that the lab experimental data were obtained under conditions that would be relevant to industrial processes. They were then used to show that the performance of the lab-scale fluidized bed could be greatly improved by using baffles and/or modifying the initial gas distribution into the bed. In the future, these tools will be applied to the optimization of industrial fluidized beds in which it would be very difficult, unsafe and extremely costly to run experiments. Optimizing industrial beds reduces their cost and minimizes their environmental impact.
Xing, Xuelian, "Numerical study of the effect of gas distributors and baffles on the bubble distribution, gas and solid mixing in a fluidized bed" (2020). Electronic Thesis and Dissertation Repository. 7474.
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