Heterogeneous structure in gas–solid riser flows

Recent measurements by electric capacitance tomography reveal that, under certain operating conditions, the solids concentration distribution in the dense and acceleration regimes of gas–solid riser flows exhibits a strong heterogeneous structure, with the solids concentration near the centerline much higher than that in the annular surrounding flows (except in the wall region) within the same cross-section. This core–annulus–wall heterogeneous structure, significantly different from the commonly known “core–annulus (wall)” two-zone structure in riser flows, becomes unstable with the increase in solids loading, which eventually leads to the occurrence of choking. In this study, we propose a mechanistic model to investigate the formation and distribution of these interesting heterogeneous structures in riser flows. The intrinsic mechanisms include (1) counterflow between back mixing solids and upward moving solids from entrance, (2) the radial migration of solid flow from the wall toward the centerline of riser, and (3) the nonuniform acceleration of solids across the cross-section in the dense and acceleration regimes. The model, assuming one-dimensional two-phase flow in each zone along the riser, consists of a set of coupled ordinary-differential equations developed from the conservation laws of mass, momentum, and energy of both gas and solids phases. The solving algorithm is based on the Runge-Kutta method. Our model calculations show that the commonly known “core–annulus (wall)” flow structure occurs at low solids loadings and/or high gas velocities, whereas the core–annulus–wall flow structure is formed at moderate solids loading and/or at low gas velocities. With a high solids loading at low gas velocity, severe backflow results in a solid concentration peak formed in the core, which not only causes the flow instability but also possibly triggers the choking. Model simulations are validated by direct comparisons against measurements in solids concentrations as well as in the pressure drop along the riser, which shows a fairly good agreement. © 2008 American Institute of Chemical Engineers AIChE J, 2008