Flow maldistribution was investigated in an 8.5 in high circulating fluidized bed with a rectangular cross section of 1 x 0.3 in 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, V-max/V-av). 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, Delta P-a/Delta P-r). (c) 2005 American Institute of Chemical Engineers.