Characterization of fluidized nanoparticle agglomerates by using adhesive CFD-DEM simulation

Abstract

Nanoparticles form multi-stage agglomerates when they are fluidized: simple agglomerates (a few dozens of micrometres), and complex agglomerates (a few hundreds of micrometres). This paper studies nanoparticle fluidization by using adhesive CFD-DEM (Discrete Element Model) simulation, which includes an adhesive contact model having the ability to model the contact and bounce/stick between elastic-plastic and cohesive particles. The discrete elements represent the simple agglomerates of nanoparticles. The effects of particle density, fluidizing gas velocity, and van der Waals force on gas-solid flow and agglomerate behaviors are investigated. It is shown that the particles, representing simple agglomerates, form large agglomerates during fluidized process. As the agglomerates move around in the bed, they continuously break and recombine. The solid movement is evaluated by a dispersion coefficient and it decreases with increase in cohesion. By monitoring the evolution of contacts, the agglomerate breakage is visualized and compared for the different conditions. Increasing fluidizing gas velocity and reducing particle adhesion both increase agglomerate breakage rate. The structure parameters of agglomerates are further analyzed: averaged coordination number of agglomerates around 3.0, packing fraction around 0.2–0.3, and fractal dimension around 1.9–2.3, which can be used to indicate structures and formation mechanisms of agglomerates.