Axial flow structure in the entrance region of a downer fluidized bed: Effects of the distributor design

Compared to riser reactors, downer reactors have reduced gas and solids :se gregation and more uniform flow structure which enhance the gas-solids cont acting efficiency and eventually result in more uniform product distributio ns within shorter residence times. However, little research has been report ed on the entrance region and/or on the effect of distributor design, altho ugh this is a very important issue. In this study, a cold model downer unit (9.3 m tall and 0.10 m i.d.) with three distributor designs was used to te st the gas-solids flow conditions in the entrance region. Pressure gauges w ere employed to measure the pressure gradient profiles, while fibre-optic p robes were used to measure particle velocity and solids holdup. The develop ment of the axial gas-solids flow is found to be highly dependent on distri butor type. Pre-acceleration of the particles inside the solids distributor tubes and high-velocity gas nozzles that provide significant momentum can effectively shorten the particle acceleration length. Increased wall fricti on can increase the acceleration length. For higher superficial gas velocit y, the rate of acceleration increases given the increased gas drag, but the acceleration length is also extended because the equilibrium particle velo city is higher. (C) 1999 Elsevier Science Ltd. All rights reserved.