CFD MODELING AND SIMULATION OF HYDRODYNAMICS IN GAS-SOLID BUBBLING FLUIDIZED BED REACTOR

Abstract

Despite their widespread application of Fluidised Bed Reactors, much of the development and design of these reactors has been empirical as the complex flow behavior of gas—solid flow in these systems makes flow modeling a challenging task. The fundamental problem encountered in modeling hydrodynamics of a gas—solid fluidized bed is the motion of two phases where the interface is unknown and transient, and the interaction is understood only for a limited range of conditions. With the advent of increased computational capabilities computational fluid dynamics, CFD is emerging as a very promising new tool in modeling hydrodynamics especially multiphase flows like those occurring in fluidized bed reactor. Firstly, sensitivity analysis was carried out to investigate the behavior of grid size on bubble behavior. Numerical simulation was carried out both, in two and three dimensions. The commercial CFD package FLUENT 6.3.26 has two built in drag models, Gidaspow and Syamlal-O'brien. The effect on hydrodynamics of the fluidised bed due to these drag models have been investigated. Also, the influence of model parameters such as wall boundary conditions, coefficient of restitution, operating velocity and particle size distribution was investigated in Eulerian—Eulerian numerical simulations of a bubbling fluidized bed. A comparative study involving different shaped distributor was also carried out. The results from the simulations with these two drag models do not differ significantly from each other. Both the models give high bed expansion, and a rather low bubble frequency. The Syamlal & 0 Brian drag model gives slightly higher bubble frequency than the Gidaspow drag model. Simulations conducted under different conditions and comparisons with available experimental solid velocity and bubble property data indicate that the solid-phase wall boundary condition needs to be specified with great care as it has substantial impact on the hydrodynamics. Many important parameters like bed expansion, voidage distribution, bubble frequency depends on wall effects and consequently, effects hydrodynamics of fluidised bed