Axial holdup distributions of FCC catalysts in a small-diameter riser with abrupt exit in circulating fluidized bed

Abstract

Effect of exit geometry on solid holdup distributions in a small-diameter riser (0.01 m i.d. × 3.07 m high) has been determined with three fluid catalytic cracking (FCC) catalysts of different physical properties. The axial holdup distributions of the catalysts showed the C-shape with relatively dense bottom and upper regions of the riser. The solids holdups (εs) in dilute phase increased with decreasing gas velocity and increasing solids circulation rate. Higher εs in the upper region of the riser were observed with larger catalyst diameter and density. As the exit diameter decreased, the εs in the upper region were increased. The T-shaped exit with a projected roof increased the solids holdups near the riser top region compared to the L-shaped exit. No significant change in solid holdups was observed with increasing the projected roof height above a certain height. Interestingly, decay constants affected by the riser exit (ae) increased with increasing solid circulation rate in the small-diameter riser due to the increased ratio of perimeter to unit area of the riser. Empirical correlations on ae and reflux constant (Ce) for the small-diameter riser were proposed. Finally, a model was proposed for prediction of the axial εs distribution in the small-diameter riser with abrupt exit.