Nanoparticles are usually fluidized as agglomerates, which are in dynamic states of agglomeration and fragmentation. It is critical to consider the size distribution of agglomerates in modeling of the fluidization of nanoparticle agglomerates. In this article, the fluidization behavior of nanoparticle agglomerates is investigated using a two-fluid model─population balance model. The model includes the agglomeration and breakage kernel functions based on the continuum theory of cohesive particles developed by Kellogg et al. (J. Fluid Mech. 2017;832:345-382). The ratio of the critical breakage velocity to the critical agglomeration velocity is defined to represent the cohesion of nanoparticles. The predictions of bed pressure drop, bed expansion ratio, and bed collapse curves agree well with those of experiments. By changing the critical agglomeration velocity and the ratio between the critical velocities, the transition from almost defluidization to uniform fluidization is predicted. Finally, the model’s ability to simulate the fluidization of fine particles with a few micrometers is also shown. This study provides a practical tool for simulating the fluidization of nanoparticle agglomerates.

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.

Nanoparticles are fluidized as agglomerates with hierarchical fractal structures. In this study, we model nanoparticle fluidization by assuming the simple agglomerates as the discrete element in an adhesive (Computational Fluid Dynamics—Discrete Element Modelling) CFD-DEM model. The simple agglomerates, which are the building blocks of the larger complex agglomerates, are represented by cohesive and plastic particles. It is shown that both the particle contact model and drag force interaction in the conventional CFD-DEM model need modification for properly simulating a fluidized bed of nanoparticle agglomerates. The model is tested for different cases, including the normal impact, angle of repose (AOR), and fluidization of nanoparticle agglomerates, represented by the particles with the equivalent material properties. It shows that increasing the particle adhesion increases the critical stick velocity, angle of repose, and leads from uniform fluidization to defluidization. The particle adhesion, bulk properties, and fluidization can be linked to each other by the current adhesive CFD-DEM model.