Effect of inter-particle forces on solids mixing in fluidized beds

Abstract

Mixing of particles is important in fluidized beds. For a more accurate and realistic investigation of this process, it is necessary to consider the inter-particle forces. The effect of four inter-particle forces (van der Waals, liquid bridge, electrostatic, and hydrogen bond) on the particles mixing in a fluidized bed is investigated in this work. A pseudo-2D fluidized bed was simulated in two cases; particles of the same size with different densities, and particles of the same density with different diameters. The effect of each force alone on the axial mixing of particles was investigated, and results were compared with a bed in the absence of inter-particle force. The combined technique of computational fluid dynamics and discrete element method was adopted for simulations. For better comparison, each force was normalized with respect to the weight of particles. It was observed that the hydrogen bond force has the most significant impact on the size of bubbles, thus the mixing of particles. The porosity distribution in the bed was examined, and it was observed that the bubble phase fraction increases in the presence of inter-particle force. It was found that diffusivity increases with increasing the gas velocity in the presence of inter-particle forces due to the formation of larger bubbles. Finally, the Ashton mixing index was evaluated for the two simulation cases in the presence of each inter-particle force, and the results showed that although diffusivity is greater in the presence of inter-particle force due to the formation of larger bubbles, the mixing time is higher due to slower mixing in the particle scale. • The effect of four principal inter-particle forces in a fluidized bed was studied. • The pseudo-2D fluidized bed was simulated by the CFD-DEM method. • The bubbles' size, porosity distribution, and diffusivity were studied. • The Ashton mixing index was evaluated in the presence of each inter-particle force. • It was observed that the hydrogen bond force had the most impact on the mixing.