GAS SOLID CONTACT IN A CIRCULATING FLUIDIZED-BED - THE EFFECT OF PARTICLE-SIZE

A recently developed technique for measuring the efficiency of contact between gas and solid phases has been used to investigate the fundamental relationship between particle size and contact efficiency in a circulating fluidized bed. Contact efficiencies have been measured for two grades of sand (140 and 275-mu-m) and 71-mu-m FCC catalyst. The measurement technique is based on hot gas as a reacting tracer, with detection by a shrouded rapid-response thermocouple. The rate at which hot gas is thermally equilibrated with the bed is measured and this is interpreted in terms of the degree to which gas and solid come into intimate contact. The results show that, as particles become finer, contact efficiencies improve. However, this improvement is significantly less than the corresponding increase in total gas-solid surface area in the system. Finer particles are likely to form clusters or streamers more readily, and it appears that coarse particles are more surface-efficient in that they expose a greater proportion of external surface to the main gas stream. In other words, the difference in contact efficiency performance between a fine and a coarse powder is reduced by the tendency of the finer powder to aggregate more strongly.