(611h) Hydrodynamics of High Velocity Circulating Fluidized Bed Risers

Circulating fluidized bed (CFB) risers are extensively used in fluid catalytic cracking (FCC) of heavy oil.  Typically, the FCC risers operate at solids circulation fluxes of 400 – 1400 kg/s.m² and superficial gas velocities as high as 15 – 20 m/s.  Although extensive studies have been conducted of CFB systems almost all reported CFB data are for systems operating at relatively low gas velocities (< 10 m/s) and modest solids circulation rates (< 200 kg/m2s).  Academic research laboratory units usually have limited capacities of achieving higher gas velocity and solids flux conditions.  CFB combustor risers operate at much lower gas velocities and solids circulation fluxes and 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.  The height of the dense region increases or decreases depending on the gas superficial velocity and solids circulation rate.  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 suggest also an existence of a core-annular flow structure, but with little or no net downflow of solids in the annulus.  This paper discusses tests conducted in three CFB risers 15, 22, and 24 m in height and all having a diameter of 0.3 m using fluid catalytic cracking catalyst particles.  The three units were operated for superficial gas velocities of between 12 and 16 m/s and solids fluxes of about 70 to 700 kg/s.m².  Total riser pressure drop and axial apparent density profiles were measured using differential pressure transmitters.  Local solids flux profiles were measured at one elevation, approximately halfway, in each of the risers using the solids extraction tube technique.  Results showed that at low solids circulation fluxes the apparent density decreased exponentially with increasing height but a dense lower region started to form as the solids flux was increased.  The height of the dense region increased to occupying nearly half of one of the risers’ height at the highest solids flux.  The solids flux measurements showed the risers having different shapes of radial profiles.  Nearly flat profiles, parabolic profiles with maxima in the core as well as bell shaped profiles with highest fluxes near the riser walls were observed.  The net solids flow direction at all radial locations was found to be upward for the conditions used in this study.