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 segregation and more uniform flow structure which enhance the gas-solids contacting efficiency and eventually result in more uniform product distributions within shorter residence times. However, little research has been reported on the entrance region and/or on the effect of distributor design, although 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 test the gas-solids flow conditions in the entrance region. Pressure gauges were employed to measure the pressure gradient profiles, while fibre-optic probes were used to measure particle velocity and solids holdup. The development of the axial gas-solids flow is found to be highly dependent on distributor 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 friction can increase the acceleration length. For higher superficial gas velocity, the rate of acceleration increases given the increased gas drag, but the acceleration length is also extended because the equilibrium particle velocity is higher.