May 23, 2017
Allan S. Issangya, S.B. Reddy Karri, Ray A. Cocco, Ted Knowlton, 12th International Conference on Fluidized Bed Technology, Krakow, Poland
Fluid catalytic cracking (FCC) risers operate at solids circulation fluxes of 400 to 800 kg/s-m2 and superficial gas velocities as high as 15 to 25 m/s. However, although extensive CFB riser studies have been conducted, most of the reported data are for risers operating at relatively low gas velocities (< 10 m/s) and modest solids circulation rates (< 200 kg/s-m2). There is a lack of hydrodynamics data for conditions similar or close to those of commercial FCC risers. This paper discusses total pressure drop, apparent density and local solids flux measurements obtained from three 0.3-m-diameter risers 15, 22 and 24 m in height using FCC catalyst particles. The risers were operated at superficial gas velocities of 12 to 16 m/s and solids fluxes of about 70 to 700 kg/s-m2. At low solids circulation fluxes the apparent density decreased exponentially from the bottom to the top of the riser. A dense lower region started to form as the solids flux was increased at constant gas velocity. The height of the dense region increased to nearly occupying half of one of the risers’ height at the highest solids flux. A variety of radial solids mass flux profiles were found in the risers depending on the superficial gas velocity and net solids mass flux. These included parabolic profiles with highest fluxes in the core region, flatter profiles, inverted parabolic profiles with the highest values near the riser walls as well as profiles with the highest solids flux near one wall and the lowest at the opposite wall. In contrast to parabolic solids flux profiles found in small diameter CFB risers, the solids mass flux profiles in the 30- cm-diameter risers tested here were relatively more flat. And, except for very few cases attributed to entrance effects, the net solids flow direction at all radial locations was found to be upward for the conditions used in this study. This would suggest that the widely reported upflow core and downflow annulus in low gas and solids flow risers are not representative of what takes place in commercial FCC risers.