LIQUID-MIXING AND PHASE HOLD-UPS IN GAS PRODUCING FLUIDIZED-BED BIOREACTORS

A large number of biological processes are developed with fluidized be ds as reactor. This technology allows a higher biomass retention and t hus a better efficiency. Some anaerobic processes such as ethanol ferm entation, anaerobic digestion or denitrification are characterized by an important gas production. Nevertheless, gas production is seldom ta ken into account in the description of liquid mixing. In this paper, w e present an experimental study of mixing and phase hold-ups in gas pr oducing fluidized beds. The properties of gas producing and classical gas injected three-phase fluidized beds are compared. For a same gas f low rate, gas hold-ups in gas generated systems appears to be much hig her than in gas injected reactors. This is attributed to the gas gener ation mechanism, which provides smaller bubbles at lower velocities. T he degree of axial mixing is also different when gas in generated in s itu than when it is injected. The dispersed plug-flow model can be rea sonably used to describe the phase mixing in the reactor. The nature o f bubble population appeared to be the main origin of differences betw een the two modes of gas introduction. (C) 1998 Elsevier Science Ltd. All rights reserved.