Hydrodynamics and flow development in two circulating fluidized bed risers using fcc and sand particles

The hydrodynamics and flow development is studied in two long riser circulating fluidized bed reactors (10.2 m and 15.1 m tall). Optic fiber probes were used for the measurement of local solids distribution. Pressure drops were also measured with pressure transducers along the riser. Results show that the nozzle gas distributor design has different effects on the axial and radial flow structure for the FCC and sand particles. At lower superficial gas velocity of less than 8.0 m/s, the upward gas-solid flow of the sand particle decelerates to various degrees with the disappearing of nozzle gas distributor effect. The upward gas-solid flow of the FCC particles, however, does not have noticeable deceleration within the range of this study. In the acceleration section, the radial distributions of local solids holdup of the FCC particles are more uniform than those of sand particles under the same operating conditions; while in the fully developed zone, the radial distributions of local solids holdup of the sand particles are more uniform than those of the FCC particles under the same operating conditions. The radial solids holdup distribution and flow development for FCC and sand particles were also discussed. Results show that flow developments in the core and the wall region are not simultaneous and the operating conditions differently affect the flow development process in different radial regions. The flow development in the wall region is mostly represented by the reduction of the solids holdup towards the riser top at r/R from about 0.90 to 1.00, while the flow development in the riser centre is nearly instant with solids holdup remaining constant and low. Increasing Gs significantly slows down the flow development process while increasing Ug accelerates it, especially for the flow development in the annulus region.