Comparisons of particle clustering phenomenon between gas-solids high-density and low-density circulating fluidized bed risers via numerical study

The clustering phenomenon in gas-solids high-density (Gs ≥ 300 kg/m²s) and low-density circulating fluidized bed (CFB) risers for FCC particles are compared through CFD studies using a cluster-based drag model. Large clouds of clusters can be clearly identified from the contours of local solids holdup distribution from CFD results, which correspond well to experimental observations. The size, shape, and distribution of the cluster clouds, as well as their dynamic properties including the varying velocity and appearance between the HDCFB and LDCFB risers were extracted and further compared. A cluster phase with higher average solids holdup has been distinguished from the dilute solids suspension phase based on the solids holdup's probability density distribution (PDD). Correlations between the cluster phase solids holdup with its fraction or with the overall bed density were proposed. The fraction and the average solids holdup of the dense cluster phase increases with the increase in the overall bed density.