Abstract
Using a circulating fluidized bed (CFB) as a typical column-type solid/gas system, this study aimedto determine the minimum gas velocities restraining the downward-flowing particles through thecontraction midway of the CFB riser, and to formulate an algebraic expression of these criticalvelocities. Using glass-beads as circulating particles, change in the amount of downward-flowingparticles at the contraction with change in gas velocity was examined experimentally for different solidfeeding rates. Algebraic expression of critical velocities was performed with a growing chain model(GCM) and a revised GCM (RGCM). The GCM proposed by us and others is an ideal cluster model thatassumes a vertical chain of particles of a constant size. The RGCM further takes account of the sizedistribution of circulating particles. The experimental results with circulating particles of different sizeranges of 125–280, 90–225 and 28–90 μm revealed that the critical gas velocities are proportional to thesolid feeding rates, and the slopes of those curves are almost the same, irrespective of the size distribu-tion of circulating particles. The critical velocities obtained by extrapolating the curves of experimentalcritical velocity towards zero of solid feeding rate showed a linear relations with the theoretical terminalgas velocities of an infinitely long chain of particles given by the GCM and RGCM. From this, the criticalvelocity can be presented as a linear expression of the solid feeding rate and the theoretical terminal gasvelocity of an infinitely long chain of particles. The theoretical expression of the critical velocity usingthe theoretical terminal gas velocity of an infinitely long chain of particles by the RGCM is confirmedto be adequate, since this expression can also predict experimental critical velocities for 28–280 umcirculating particles with an error of only ±10%.
