CFD simulations of particle mixing in a binary fluidized bed

It has long been known that fluidization of dissimilar materials can result in either a well-mixed bed or a segregated bed. In a fluidized bed, particle mixing and segregation phenomena are dominated by bubble activity. Depending on operating conditions, lighter or smaller particles ("flotsam") tend to rise to the bed, and larger, heavier particles ("jetsam") tend to sink to the bottom of the bed. A series of unsteady, three-fluid CFD simulations were performed using FLUENT (TM) 6.0. The solids consist of two dissimilar materials, coke and rutile, with different diameters and densities. Simulation parameters (solution technique, grid, maximum packing fraction, drag law) and operating conditions (gas velocity, bed makeup, nozzle location) were each investigated for the relative effects on bubbling and hence on particle mixing and segregation. From the simulations, it is possible to identify the solids wake trailing a rising gas bubble by looking at the vertical solids flux. The mixing mechanisms (particle exchange and circulation) are readily identified, also from the solids flux. A segregation mechanism is observed by watching the difference in the particle velocities (particle slip velocity) caused by a passing bubble. The simulations predict for all conditions investigated a well-mixed fluidized bed with modest localized segregation. However, a collapsed bed simulated by turning off the gas did predict a discernible segregation profile, similar to that measured in laboratory experiments. (c) 2004 Elsevier B.V. All rights reserved.