Suspension densities in a high-density circulating fluidized bed riser

Flow behaviour of fluid catalytic cracking,particles were investigated in a riser with volumetric solids concentrations of 20% and more. The air veloc ity and solids circulation flux had only a small influence on the solids ho ld-up once high-density conditions were attained. At these high-density con ditions, refluxing of solids near the riser wall, commonly observed in low- density CFB risers, disappeared and was replaced by a more homogeneous flow structure. The slip velocity increased with solids hold-up and, for consta nt superficial gas velocity, there appears to be a unique relationship betw een the two. Slip factors as high as 10 were obtained in the developed regi on of the riser, compared to values of about 2 to 5 reported in the literat ure for more dilute fully developed flows in the fast fluidization flow reg ime. Differential pressure fluctuations increased with increasing suspensio n density and were not significantly affected by superficial gas velocity a nd height. A transition point is defined to distinguish dilute from high de nsity conditions in the CFB riser. (C) 1999 Elsevier Science Ltd. All right s reserved.