Segregation and dispersion studies in binary solid-liquid fluidised beds: a theoretical and computational study

Solid-liquid fluidised beds (SLFBs) are of high industrial importance due to higher heat and mass transfer rates. In the design of such multiphase fluidised beds, it is important to understand the bed expansion behaviour, as well as the spatial distribution of phase volume fractions, segregation and intermixing of the two solid phases. In this study, these hydrodynamics characteristics were studied analytically utilizing dispersion coefficient correlations. First, a thorough comparison of the relative predictive capabilities of the available correlations was conducted and specifically the effect of specific energy dissipation rate on this parameter was evaluated. It was found that the dispersion coefficient is an increasing function of the specific energy dissipation rate of the system. Dispersion coefficients varied in the range of ~ 5 × 10⁻⁵ to 5 × 10⁻⁴ m² s⁻¹ when energy dissipation rate increased from ~ 0.005–0.01 m² s⁻³. Different dispersion correlations were then utilized to describe the intermixing and segregation behaviour for the binary particle species differing in density in terms of axial particle concentration profile using a one-dimensional convection-diffusion model which agreed well with the experimental data. Additionally, a two-dimensional (2D) Eulerian-Eulerian (E-E) model based on kinetic theory of granular flow (KTGF) was used to simulate axial variation of the binary solids concentration which showed good agreement (~ 10% deviation) with the published experimental data. Axial profile of dispersion coefficient predicted by the various correlations exhibited a sharp variation in the intermixing zone formed in between the lower (higher density) and upper particle bed (low density). In this region, CFD model predicted energy dissipation rate increased significantly with liquid superficial velocity which reflected strong phase interactions in the intermixing zone.