Allan Issangya

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 velocity and solids circulation flux had only a small influence on the solids hold-up once high-density conditions were attained. At these high-density conditions, 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 constant superficial gas velocity, there appears to be a unique relationship between the two. Slip factors as high as 10 were obtained in the developed region of the riser, compared to values of about 2 to 5 reported in the literature for more dilute fully developed flows in the fast fluidization flow regime. Differential pressure fluctuations increased with increasing suspension density and were not significantly affected by superficial gas velocity and height. A transition point is defined to distinguish dilute from high density conditions in the CFB riser.

Further measurements of flow dynamics in a high-density circulating fluidized bed riser

Abstract Local voidages were determined for conditions corresponding to dense suspension upflow and fast fluidization in a riser of diameter 76.2 mm and height 6.1 m using a reflective-type optical fiber probe at superficial air velocities between 4 and 8 m/s and solids circulation fluxes up to 425 kg/m2 s. High-density flow of cross-sectional mean volumetric solids concentration of about 0.2±0.05 was achieved in the riser with fluid catalytic cracking (FCC) particles of mean diameter 70 μm and density 1600 kg/m3. Local time-mean voidages were nearly as low as emf at the wall and as high as 0.9 at the axis. As in our other recent work, solids refluxing near the wall of dilute CFB risers no longer existed for high-density conditions. Statistical analysis of local voidage fluctuations was used to characterize the local flow dynamics. The core behaves as a relatively uniform dilute flow, interspersed with infrequent particle structures. The number and density of these structures increases with radius. Maximum heterogeneity for high-density conditions occurs at some distance from the wall, unlike dilute risers where the flow is least uniform at the wall. Intermittency index vs. local voidage plots are “bell-shaped”, with a maximum where the local time-mean voidage is approximately 0.75.

A novel method for determination of choking velocities

A novel dynamic method is presented which allows both the type A and the type C choking velocities to be determined sequentially in a single experiment, with ease of operation and considerable experimental economy. Initial results have shown that the type A choking velocity, UcA, and type C choking velocity, UcC, are consistent with earlier measurements using conventional techniques. The average solids holdups corresponding to choking conditions for the FCC particles are about 0.04 for type A choking and 0.25 for type C choking, again in good agreement with available results from the literature.