FLOW REGIMES OF THE 3-PHASE CIRCULATING FLUIDIZED-BED

Gas-solids circulating fluidized beds have been successfully used in c atalytic cracking of heavy oil, coal combustion, and some metallurgica l and physical processes (Grace, 1990). Gas-liquid-solids fluidized be ds are operated mainly in conventional fluidization regimes without so lids circulation or in the transport regime with low solids holdups (l ess than 5%) (Fan, 1989). A circulating/fast fluidization regime, howe ver, has not been studied. A three-phase circulating fluidized bed has several potential applications in biochemical and chemical processes. Three-phase fluidized-bed bioreactors generally use light and small p articles (Berk et al., 1984). Circulating operation can promote solids mixing and increase product throughput per unit bed cross section, wh ile high shear stress can promote biofilm renewal (Pirozzi et al., 199 0). In three-phase hydrotreating reactors, solids catalysts lose their activity due to the deposit of metal and coke on the surface. Circula ting operation not only regenerates deactivated catalyst continuously using accompanying downcomers but also transfers heat to and from the reactor. This article discusses the flow regimes of the three-phase ci rculating fluidized bed.