Multi-stage agglomeration phenomena and properties of titania nanoparticle agglomerates in a lab-scale conical fluidized bed: Evaluation of CFD-DEM simulation by laser-based planar imaging and pressure fluctuation analysis

Abstract

Understanding the agglomeration characteristics of nanoparticles (NPs) helps to improve their fluidization quality. This study aims to ascertain the multi-stage agglomeration of titania NPs in a conical fluidized bed. Fluidization experiments were performed to identify the temporal variation of particle size distribution and pressure fluctuations in different bed zones. A model was built by coupling computational fluid dynamics and discrete element method to ascertain the agglomerate characteristics that could not be identified by experiments. The results showed that the type of flow regime and position of particles in the radial direction have a significant impact on the agglomerate size, particle collision, and therefore bed pressure fluctuations. Primary types of simple-agglomerates (∼25–75 μm) and complex-agglomerates (∼100–150 μm) were mainly detected in the spout and annular zones, respectively. In full fluidization, a continuous break-up of primary complex-agglomerates into secondary simple-agglomerates (∼75–100 μm in size), and re-agglomeration of secondary simple-agglomerates into secondary complex-agglomerates (∼150–200 μm in size) occurred mainly in the annular and spout zones, respectively. In the heterogeneous fluidization, primary types of agglomerates were primarily detected in the spout zone and spout-annulus interface. The highest and lowest pressure fluctuations were obtained in the spout and annular zones, respectively, which was attributed to the effect of particle agglomeration. An increase in the particle cohesion force led to an increase in the probability of complex-agglomerates, as well as a notable deterioration in particle mixing. The effect of particle collision and deagglomeration on particle mixing was much more severe in the annular zone than in both the spout zone and the spout-annulus interface. Inelastic collisions between complex-agglomerates in the annular zone induce a loss in the kinetic energy and granular temperature, with this energy being enough to form secondary-agglomerates, but not enough to break them up, thereby postponing particle mixing.