A downer-riser circulating high velocity fluidization apparatus was developed to study the fundamentals of downflowing gas-solid particle mixtures. The acceleration and deceleration of solids due to the influences of the entrance and exit sections result in a relatively, uniform axial solids distribution. Radial solid density profiles detected with an X-ray imaging system in the downer show the existence of a core-annulus flow with a dilute cole surrounded by a denser wall region. Local solids flux profiles were obtained with an aspirating probe device and the solid velocity profile obtained from the two measured ed quantities. These confirm that the majority of solids segregates in a wall region that flows faster than the dilute core region. Thus, the shorter residence time in the high-speed downer wall region is coupled with faster reaction rates due to the accompanying high concentration of catalyst, while the dilute core has slower reaction rates with longer residence time due to the lower catalyst concentration and flow, velocity. This results in much more uniform reaction extent over the cross-sectional area of the downer and, therefore, should improve the product selectivity.