Characterization of high-density gas-solids downward fluidized flow

Experiments were carried out in a specially designed 5 m tall, 0.025 m i.d. high-density gas-solids downflow fluidized bed to measure the axial pressu re gradient profiles along the downer and the actual solids holdup in the f ully developed region. Fluidized catalytic cracking (FCC) particles ( rho ( p) = 1300 kg/m(3), d(p) = 70 mum) and two types of glass beads ( rho (p) = 2500 kg/m(3), d(p) = 123 and 332 mum) were used. A particle acceleration re gion and a fully developed region were identified along the column from the pressure gradient profiles. In the fully developed region, solids holdup d ecreases with the gas velocity but increases linearly with the solids circu lation rate. The latter results in nearly constant particle velocities over a large range of solids flux at given gas velocities. Particle velocity al so increases linearly with the gas velocity. Particle properties seem not t o affect the mean particle velocity much, but smaller and/or lighter partic les give larger solid holdups. A solids holdup as high as 10-20% has been a chieved. Comparison of the results obtained here with those from an upflow riser shows inherent similarities between the two gas-solids co-current flo w systems. In the fully developed region, the apparent solids holdup calcul ated from the pressure gradient agreed well with the actual solids holdup m easured by a pair of pinch valves under not very high gas velocities, but w as underestimated at higher gas velocities due to the increased wall fricti on loss. (C) 2001 Elsevier Science B.V. All rights reserved.