AbstractFlow maldistribution was investigated in an 8.5 m high circulating fluidized bed with a rectangular cross section of 1 × 0.3 m using a 14‐bubble‐cap distributor and a 33‐bubble‐cap distributor. Experimental results indicate that the bubble caps in the center region have a large flow rate and the bubble caps at the wall region have a small flow rate. The extent of the flow maldistribution is quantitatively characterized by the ratio of the maximum flow rate through a bubble cap to the average flow rate through all bubble caps (maldistribution number, Vmax/Vav). A correlation is proposed for predicting flow rate distribution, maldistribution number, in the bubble cap distributor by the ratio of distributor pressure drop to riser pressure drop (pressure‐drop ratio, ΔPd/ΔPr). © 2005 American Institute of Chemical Engineers AIChE J, 2005