(400i) CPFD Simulation of Solids Residence Time and Mixing Behaviors in a Downer Reactor
A downer reactor is a relatively new type of circulating fluidized bed (CFB), in which the conversion and selectivity of reactions are closely related to the particle's residence time, thus the in-depth understanding of the particle's residence time and its mixing behavior is critical for the design, operation and scale-up of downer reactors. Computational particle fluid dynamics (CPFD) model, in which the fluid phase is calculated with a Eulerian computational grid and the solids are predicted using Lagrangian computational particles, provides an efficient approach to understand the comprehensive hydrodynamics in downer reactors. The objective of the present study is to systematically analyze the hydrodynamics and solids mixing behaviors in the downer using a modified gas-solid drag model based on the CPFD approach. The effects of operating conditions (i.e., superficial gas velocity, solids circulation rate) on the mixing behavior were addressed. The flowing conclusions were obtained. The comparison between the simulation results and the experimental data indicates that the CPFD method was capable to predict the hydrodynamics and solids mean residence time. The solids RTD featured a narrow peak with large axial Peclet numbers (Pe), indicating that solids flow approximate an ideal plug flow behavior. However, the solids mixing behavior reached higher levels of attainment to ideal plug flow patterns than that of the solids phase. Operating conditions exhibited a certain influence on the characteristics of gas-solid flowing and mixing behaviors in the downer. With the increase of solids circulation rate and the decrease of superficial gas velocity, particles reached the steady state faster and the radial distribution of solids holdup became more uneven, the mean residence time increased with the wider peak and lower peak value of particles RTD, and the Peclet numbers decreased, which mean that the mixing degree increased.