Numerical study of gas–solid flow behavior in a two-stage high-density riser using EMMS-based drag model

• EMMS/matrix drag model was used in the simulation of two-stage riser.
• Choking phenomena in the two-stage riser have been predicted.
• Comparison of flow behaviors in novel and typical risers has been performed.
• Parametric studies of diameter and height of the first zone have been conducted.

The two-fluid model coupled with the EMMS/Matrix drag model was employed to investigate the gas–solid flow behavior in the novel two-stage high-density riser with an enlarged bottom section. The choking phenomenon in the bottom zone due to the low superficial gas velocity has been predicted. Through the comparison of flow behaviors in the constant-diameter and the novel risers under high solids circulating flux (660 kg/m2s), the characteristics of turbulent fluidization was found in the bottom zone, and the dense suspension upflow regime was achieved in the upper zone of the novel riser, but it held much more solids inventory than the typical riser. The larger diameter of the bottom zone made the flow regime transition from fast fluidization to turbulent fluidization, as revealed from probability density distributions. The larger height of the bottom zone extended the horizontal segment of the choking curve. With the increase of the height or the diameter of the bottom zone, it needed more solids inventory to achieve dense suspension upflow in the upper zone, but the solids residence time in the novel riser increased significantly.