Three-Phase Fluidized Bed Reactors

Abstract

The particular characteristics of three-phase fluidized bed reactors have been covered in several recent reviews by Ostergaard [1], Wild [2], Epstein [3], Baker [4] and Muroyama and Fan [5]. Epstein [3] distinguished in particular the difference between three-phase fluidized beds and slurry reactors. In slurry reactors the size of the solid particles is normally smaller than 0.1 mm while in three-phase fluidization the particle diameter is bigger than 0.2 mm. The volumetric solid fraction is another significant difference, being 10% or below for slurry reactors and between 20–40% in three-phase fluidized bed units. In three-phase fluidized beds the particles are supported by the liquid and/or the gas while in slurry reactors the solid particles are suspended by the momentum transferred from the gas bubbles to the liquid and from the liquid to the solids. In slurry reactors the solid particles are normally carried into and out of the unit by the liquid stream. In three-phase fluidized beds the solids are not transported out of the unit by the liquid stream, they are fed and withdrawn independently of the liquid stream [3]. Epstein [3] introduced an interesting classification for three-phase reactors and particularly proposed four modes of operation for three-phase fluidized beds: Mode I: Cocurrent upflow circulation of gas and liquid with the liquid as a continuous phase, Mode II: cocurrent upflow circulation of gas and liquid with the gas as a continuous phase, Mode III: Countercurrent circulation of gas (upflow) and liquid (downflow) with the liquid as the continuous phase, Mode IV: Countercurrent circulation of gas (upflow) and liquid (downflow) with the gas as the continuous phase.