Characterization of downflowing high velocity fluidized beds

A downer-riser circulating high velocity fluidization apparatus was develop ed to study the fundamentals of downflowing gas-solid particle mixtures. Th e acceleration and deceleration of solids due to the influences of the entr ance and exit sections result in a relatively, uniform axial solids distrib ution. 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 obtaine d with an aspirating probe device and the solid velocity profile obtained f rom the two measured ed quantities. These confirm that the majority of soli ds segregates in a wall region that flows faster than the dilute core regio n. Thus, the shorter residence time in the high-speed downer wall region is coupled with faster reaction rates due to the accompanying high concentrat ion of catalyst, while the dilute core has slower reaction rates with longe r residence time due to the lower catalyst concentration and flow, velocity . This results in much more uniform reaction extent over the cross-sectiona l area of the downer and, therefore, should improve the product selectivity .