RECIRCULATION OF PARTICLES IN FAST FLUIDIZED RISERS

ABSTRACT This work suggests that a fast fluidized riser consists of two parts as follows, (i) The lower part of the riser is occupied by a violently bubbling or slugging fluidized bed (analogous to churn-turbulent gas-liquid flow). (ii) The upper part of the riser contains a core, moving rapidly upwards, in which the particles are entrained by the gas; adjacent to the wall is a thin dense film of particles falling under gravity. Measurements of the flow rate of particles in this fallings film in a 0.15 m diameter fast fluidized bed have been made by means of scoops which collect the particles in the film; the film thickness appears to be about 1 — 2 mm. The flow rate of particles in the film, w, was found to decrease with increasing height, x, above the air distributor: it was found that w = w∞ + δw exp(-Kx) where w∞, δw and K are constants; this implies similar variation of the upward flow rate of solids in the core. These observations are consistent with the hypothesis that there is turbulent mass transfer of particles from the core to the film, and transfer coefficients for transport of particles have been inferred: these transfer coefficients are similar to those obtained for deposition of droplets in annular two-phase flow. The internal recirculation of particles thus generated within the riser may be several times the solids feed rate: this explains the high apparent slip velocities and gas-solids contacting efficiencies reported by other workers.