(254h) Solids flux profiles in high velocity CFB risers of FCC catalyst particles

Circulating fluidized bed (CFB) risers are widely used in the fluid catalytic cracking (FCC) process. FCC risers typically operate with solids circulation fluxes of 400 – 1400 kg/s.m² and superficial gas velocities as high as 15 – 20 m/s. Although extensive work has been conducted on CFB risers over the years, almost all the reported data are for low gas velocities (< 10 m/s) and modest solids circulation rates (< 200 kg/m²s). Limited capacities have often prevented academic studies from achieving higher gas velocity and solids flux conditions. CFB combustor risers operate at much lower gas velocities and solids circulation fluxes, and they utilize Group B solids. The hydrodynamics literature shows that CFB risers consist of a dilute region towards the top and a relatively dense region near the bottom, with the height of the dense region increasing or decreasing depending on the gas superficial velocity and solids circulation rates. The top region has a dilute core in which solids flow rapidly upward, surrounded by a descending dense annulus. The bottom zone has not been widely studied, but the few available studies also suggest an existence of a core-annular flow structure with little or no net downflow of solids in the annulus. This presentation discusses tests conducted in four CFB risers 15 to 25 m in height, all having a diameter of 0.3 m using FCC catalyst particles. Operating superficial gas velocities were between 9 and 17 m/s, and the riser solids fluxes ranged from about 70 to 700 kg/s.m². Axial apparent density profiles were determined from pressure gradients measured by differential pressure transmitters. Local solids flux profiles were obtained by solids extraction tubes. High speed video images were acquired in one of the risers. The results revealed local solids flux profiles of various shapes, which depended to a great extent on the superficial gas velocity. They included parabolic profiles with solids downflow near the wall, all upward flow parabolic profiles with maxima in the core, nearly flat profiles, as well as bell-shaped profiles with the highest fluxes near the riser walls. For typical FCC riser operating conditions, the net solids flow direction at all radial locations was found to be upward. This was also confirmed from the high-speed video images.