Gas-solids contact efficiency in the entrance region of a co-current downflow fluidized bed (downer)

The contact efficiency between gas and solid phases has been investigated in the entrance region of a 9.3 m tall, 100 mm diameter gas-solids cocurrent downflow fluidized bed (downer) using FCC particles. The contact efficiency between gas and solids was estimated based on a thermal method by measuring the temperature changes of hot fluidized air in the bed. Three different distributor designs were tested under gas velocities in the range 5.2-9.3 m s^-1 and solids circulation rates up to 180 kg m^-2 s^-1. The results show that the initial gas-solids contact efficiency is very high immediately below the distributor and that the gas and solids entrance structure (distributor design) largely influences the gas-solids contact efficiency. The results also reveal that a close relationship exists between the gas-solids mixing behaviour and the hydrodynamics in the downer. Rapid changes of the flow structure are responsible for increased gas-solids contact efficiency. The contact efficiencies also change with the operating conditions, but the effects of solids circulation rate and gas velocity seem to be different under different distributor designs.