Numerical investigation of turbulent mass transfer processes in turbulent fluidized bed by computational mass transfer

Abstract

Turbulent fluidized bed possesses a distinct advantage over bubbling fluidized bed in high solids contact efficiency and thus exerts great potential in applications to many industrial processes. Simulation for fluidization of fluid catalytic cracking (FCC) particles and the catalytic reaction of ozone decomposition in turbulent fluidized bed is conducted using the Eulerian–Eulerian approach, where the recently developed two-equation turbulent (TET) model is introduced to describe the turbulent mass diffusion. The energy minimization multi-scale (EMMS) drag model and the kinetic theory of granular flow (KTGF) are adopted to describe gas–particles interaction and particle–particle interaction respectively. The TET model features the rigorous closure for the turbulent mass transfer equations and thus enables more reliable simulation. With this model, distributions of ozone concentration and gas–particles two-phase velocity as well as volume fraction are obtained and compared against experimental data. The average absolute relative deviation for the simulated ozone concentration is 9.67% which confirms the validity of the proposed model. Moreover, it is found that the transition velocity from bubbling fluidization to turbulent fluidization for FCC particles is about 0.5 m·s−1 which is consistent with experimental observation.

1. Introduction

Compared with bubbling fluidization and fast fluidization, turbulent fluidization has a lot of advantages such as higher gas–solid contact efficiency, higher solid volume fraction and lower axial dispersion of gas. By virtue of these advantages make the turbulent fluidized bed become a promising multiphase reactor for the catalyst cracking, gasification, methanol-to-olefins (MTO) and combustion that involves strong exothermic reaction [1]. However, vigorous gas–solid interaction results in complex flow hydrodynamics including dramatic movement of particles and turbulence of both phases, which leads to great difficulty in experimental and theoretical analysis on turbulent fluidization, thereby hindering the design and optimization of turbulent fluidized bed [2].

Nevertheless, thanks to the substantial development of computer technology, numerical simulation (i.e., computational fluid dynamics) can make complements to experimental and theoretical analysis while overcoming their limitations and is adopted as an effective way to investigate the gas–solid two-phase flow in turbulent fluidization process [3]. Generally, the gas phase is deemed as continuous phase and is treated under the Eulerian framework, whereas the solid phase is either regarded as continuous phase by Eulerian approach [4] or regarded as dispersed phase by Lagrangian approach [5]. Since trajectory of each particle should be tracked under Lagrangian framework, it requires large amount of computer resources. Therefore, the treatment of solid phase as continuous by Eulerian approach is often used for fast simulation of numerous particles fluidization [3,6–8].