(70c) Eulerian Simulation of Dense Particle-Gas Flows: Effect of Reactor Scale on Bubbling Fluidization

Fluidized beds constitute the most widely employed paradigm of gas-solid flow with applications ranging from energy and chemicals production to pharmaceutical and agricultural processing, because of their excellent mass and heat transfer characteristics,. The importance of these devices across many industrial fields renders the optimization of their operation a worthy modeling challenge. While recent efforts in CFD and experimental techniques have improved our fundamental understanding and predictive capability, there is still considerable uncertainty because of the limited experimental data for large-scale fluidized beds along with the high computational cost of accurate 3D simulations.

 While lab-scale setups provide useful insights into the hydrodynamics including flow visualization, characterization and quantification, the small-scale geometry of these setups confines the flow and as such, conclusions from these studies may not be applicable for larger scales. This is because even though the nature of the particle-particle and particle-gas interactions remains unaltered (micro and meso-scale interactions), the overall hydrodynamics are strongly impacted by the geometry. It is well established that the presence of walls aids bubble coalescence and growth forming larger, longer bubbles (slugs) which are characteristic to thin and long lab-scale fluidized beds. Further, since bubble dynamics and solids circulation are inherently coupled, it follows that gas distribution, mixing time scales and other quantities of interest not only depend on the operating parameters (for instance superficial velocity, particle properties) but are also influenced by the bed geometry. While there are many studies in literature which separately characterize the bubble dynamics and the solids circulation, there does not exist a thorough investigation analyzing both together and their dependence on the bed geometry (diameter, initial height) and operational parameters (inlet gas velocities, particle properties) at large-scales.

 This study is aimed at investigating the impact of scale on the fluidization hydrodynamics for distinctly sized particles and fluidization regimes using accurate 3D CFD simulations. The Two-Fluid Model (TFM) is employed describing the solid and gas phases as interpenetrating continua. Sophisticated tools are first developed to quantify the hydrodynamics in the bed. These include circulation fluxes and times for the solids phase motion and bubble dynamics for gas motion. Recognizing that bubble dynamics in large beds may not be accurately captured using the commonly employed 2D techniques, a full 3D and scalable bubble statistics technique has been developed to accurately describe the bubble size, shape, location and velocity in the bed. Using these metrics, the simulation tools are validated against experimental measurements from several lab and pilot-scale beds. Finally, the validated tools are used to draw useful insights from simulations of 15, 30 and 50 cm sized beds for three superficial velocities (2-4 Umf) and distinct Geldart B particles (0.5 mm glass and 1.1 mm LLDPE). Based on this computational study, beds of diameter 50 cm may be suitable for scale-up to industrial-scale bubbling fluidized beds.

The numerical tool used to solve the governing equations is the Multiphase Flow with Interphase eXchanges (MFIX) code developed by the U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL).