CFD-DEM Simulation Of Liquid-Solid And Gas-Liquid-Solid Circulating Fluidized Beds

Multiphase flow is a basic phenomenon in chemical equipments and chemicalunit operation. In multiphase systems, the flow behavior of the fluid and the particlesare very complex, not only because of the turbulence but the interaction betweenparticles/fluid and particles/particles. The onset velocity for circulating fluidizationregime and the particle residence time distribution in liquid-solid circulating fluidizedbed risers are investigated by using the CFD-DEM method. For gas-liquid-solidthree-phase flows, this paper establishes a three-phase CFD-DEM model, andinvestigates the particle motion, phase holdup distribution in traditional three-phasefluidized beds.The onset velocity for circulating fluidization regime is the minimum fluidvelocity at which the liquid-solid flow approaches the circulating fluidization regimewith particles circulation in the system. This study proposes a new approach tocalculate the onset velocity from the particle residence time distribution (RTD) usingCFD-DEM simulation. By identifying the intersection of the fitting lines of theparticle mean residence time as a function of superficial fluid velocity, the onsetvelocity is determined and in reasonable agreement with experiments. The simulationindicates that the onset velocity is influenced by the density and size of particles andweakly affected by the column dimensions (bed diameter and height). A power lawfunction is then proposed to correlate the particle mean residence time and superficialliquid velocity. The collisional parameters have a minor effect on the mean residencetime and the onset velocity, but influence the RTD by showing some humps andtrailing. The particle trajectories depicted on the RTD curves suggest the complexflow structure and underlying mechanisms of the RTD.This paper also proposes a gas-liquid-solid E/E/L model by using a two fluidmodel to describe the gas-liquid two-phase, at the same time tracking particles in theLagrange coordinate system. The E/E/L model takes liquid-solid interaction intoaccount and ignores the gas-solid interaction. The gas-liquid-solid E/E/L model isimplemented through open source software OpenFOAM. The phase holdupdistribution, the particles motion in traditional three-phase fluidized beds are simulated. The simulation results show that, in three-phase systems, when thesuperficial gas velocity is low, the gas holdup simulation values is slightly lower thanthe experimental value. There is a great difference in gas holdup between thesimulated value and experimental value at high superficial gas velocity. The gas-solidinteraction can be ignored under low superficial gas velocity, while can not be ignoredunder high superficial gas velocity. The particle residence time distribution curve ofthree-phase systems is more close to the RTD curve of continuous stirring tankreactor, indicating that the backmixing of particles in three-phase system is veryserious. In addition, there are some larger "peak" on the particle residence timedistribution curve in the three-phase system, implying the existence of particleagglomeration. The gas holdup distribution shape is affected by the presence of theparticles, showing a flat gas holdup distribution.