Mass Transfer Characteristics in a Three-Phase Fluidized Bed Containing Low-Density and/or Small-Size Particles

Experiments were conducted to investigate the mass transfer characteristics in a three-phase fluidized bed under conditions of gas-liquid upward flow in a column of 6.5cm inside dimeter and 157cm height. The particles employed were five kinds of low-density and small-size particles : nylon-6 particles with diameter of 6.32 and 9.52mm, polystyrene particles of 3.1 mm, activated carbon particles of 650μm and glass beads of 550μm. The three-phase fluidized beds showed two distinct regimes, i.e., the complete three-phase fluidization regime and the partial suspension regime in which some particles exist in the freeboard region, depending on gas and liquid velocities. The liquid-phase volumetric mass transfer coefficient, kLa, was calculated using the axial dispersion model together with the axial oxygen concentration profile. It was found that kLa increases with increasing gas velocities. This trend is almost the same as that in conventional three-phase fluidized beds and bubble columns, whereas the absolute values of kLa in the case of low-density particles are always larger than those for bubble columns. The behavior of kLa values is correlated as a function of gas holdup and the modified particle Reynolds number based on the density difference between particles and fluids.