The effects of gas velocity, solids circulation rate and secondary air injection on the axial solid holdup distributions have been determined in a circulating fluidized bed (0.1 m-ID x 5.3 m high) of FCC particles ($d_p$ = 65 ㎛, $ρ_p$ = 1720 kg/㎥).
The solid holdup in the riser increases with increasing solids circulation rate but decreases with increasing gas velocity.
The axial solid holdup distribution has been found to be simple exponential function in the riser. The transition height ($Z_t$) defined as the height of the dense/dilute region interface can be determined from the kink point of the slopes of the axial solid holdup distribution since the solid holdup decreases exponentially with the riser height. These transition heights have been correlated with the pertinent dimensionless groups.
The solid holdup in the dense regione increases with increasing solids circulation rate at lower gas velocities, whereas it is independent of solids circulation rate at higher gas velocities.
The solid holdup in e dilute regione increases linearly with increasing solids circulation rate and decreases with increasing gas velocity. The solid holdup in the region below the secondary air injection point increases with increasing secondary air injection ratio (0-50 %). However, the solid holdup in the region above the secondary air injection point slightly decreases with increasing secondary air injection ratio. Therefore, the gradient of solid holdup distribution increases with increasing secondary air ratio.
The total solid holdup in the riser increases with increasing the height of secondary air injection, and the radial or tangential injection type of secondary air has no significant effects on the solid holdup distribution.
