Circulating fluidizing system of binary Geldart C powders and Geldart A particles was formed, and was called a circulating powder-particle fluidized bed (CPPFB). Solid residence in the CPPFB was concerned in two aspects, namely, bed turnover time and average turnover time of fine powders. The former represented the average time needed for all the bed particles to be circulated, while the latter represented the time needed for all the fine powders in the bed to be discharged out of the bed. Both parameters were investigated under different operating conditions as to the superficial gas velocity, size and hold-up of fine powders. FCC particles of 66 mu m were used as coarse particles and 1-5 wt.% Al(OH)(3) powders of different sizes ranging from 0.5 to 15 mu m were used as fine powders. The bed turnover time decreased with increasing the size of fine powders to a certain level and then became almost constant with further increase of the size of fine powders. When the size of fine powders was larger than this critical size, neither the size nor the hold-up of fine powders affected the bed turnover time. The bed turnover time drastically increased with increasing the hold-up of fine powders for the cases of using very fine powders of 1.0 mu m or smaller. On the other hand, the average turnover time of fine powders decreased with increasing the size of fine powders to a minimum at around 3.5 mu m and then increased with increasing the size of fine powders. It also decreased with increasing the gas velocity and decreasing the hold-up of fine powders in the bed. The average turnover time of fine powders was several times larger than the bed turnover time at the same operating conditions. (c) 2005 Elsevier B.V. All rights reserved.