Discrete particle simulation of gas fluidization of particle mixtures

Abstract

AbstractThis report presents a numerical study of segregation and mixing of binary mixtures of particles in a gas‐fluidized bed by means of discrete particle simulation, where the motion of individual particles is 3‐D and the flow of continuous gas is 2‐D. Periodic boundary conditions are applied to the front and rear walls to represent a bed of large thickness with a relatively small number of particles. Two initial packing conditions are used in this simulation: completely separated, with the flotsam (1 × 10−3 m in diameter) on the top of the jetsam (2 × 10−3 m in diameter), and well mixed. The flotsam and jetsam are of the same density, with each counting 50% in weight. Gas is injected uniformly at the bottom. Two superficial gas velocities, 1.0 and 1.4 m/s, are used in the simulation, producing significant segregation and good mixing, respectively. The results show that the degree and rate of segregation or mixing are significantly affected by gas velocity and the final equilibrium states are not affected by the initial packing states for a given gas velocity. Significant segregation occurs at a gas velocity of 1.0 m/s, with the top fluidized layer rich in flotsam and the bottom defluidized layer rich in jetsam, whereas there was less segregation at 1.4 m/s with most of the bed fluidized. The simulated results are qualitatively comparable with those observed in the physical experiments conducted under similar conditions. On this basis, the mixing kinetics obtained from the numerical simulation is quantified with a weighted Lacey mixing index and explained in terms of microdynamic results in relation to particle–particle and particle–fluid interactions. It is proposed that an appropriate sampling size should be able to describe properly the two extremes: well mixed and fully segregated. The results also demonstrate that size segregation occurs as a result of the strong fluid drag force lifting the flotsam before a dynamical equilibrium is reached, and the particle–particle interaction, like the particle–fluid interaction, plays an important role in achieving uniform fluidization. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1713–1728, 2004