Cocurrent gas-solid downflow fluidized bed (downer) reactors: II hydrodynamics

Published results on hydrodynamic studies of downer reactors are summarized in this paper. The flow structure of gas-solid two-phase flow in downers is complex. In the axial direction, the flow is characterized by two particle acceleration sections(the first and the second acceleration sections) and a constant velocity section. In the radial direction, the distributions of local gas velocity, particle velocity and particle concentration in downers are more uniform than those in risers, with rather constant flow in the core region but somewhat increased gas velocity, particle velocity and particle concentration in the near wall region. The rather uniform gas-solid two-phase flow structure is due to the fact that gas and solid are flowing in the same direction as gravity. Compared with riser reactors, downer reactors have improved gas-solid contact efficiency and can accommodate ultrashort contact time reactions, so that the downer reactor is a new high efficiecy gas-solid short contact reactor.