Phase holdup, liquid dispersion, and gas-to-liquid mass transfer measurements in a three-phase magnetofluidized bed

The gas holdup, epsilon(g), liquid-phase axial dispersion coefficient, D-ax, and volumetric gas-to-liquid mass transfer coefficient, k(l)a, have been measured for a three-phase magneto-fluidized bed (MFB) as a function of gas superficial velocity, liquid superficial velocity, and magnetic field strength. The solid phase consisted of 4 mm diameter calcium alginate spheres within which magnetite powder was entrapped. The liquid and gas phases were water and air, respectively. An axial, direct-current magnetic field having a magnitude between 0 and 300 G was applied by an external solenoid. Six different bed operating regimes were distinguished by visual inspection. Local values of epsilon(g) were measured using a fiber-optic probe, and the average values of epsilon(g) were measured using the valve technique. Average epsilon(g) values decreased by as much as 20% with increasing field strength due to bed contraction and the formation of preferred channels. Local epsilon(g) measurements correlated well with average measurements at low field strengths but became erratic at high field strengths due to bubble channeling. The liquid-phase axial dispersion coefficient was measured using a salt tracer. A fourfold decrease in D-ax was observed in the channel regime. The volumetric oxygen mass transfer coefficient (k(l)a) was determined from measurements of the steady-state oxygen profile across the reactor. A 30% increase in k(l)a was observed in the chain-channel regime. The experimental results for the MFB were compared to published correlations for conventional fluidized bed systems. Copyright (C) 1996 Elsevier Science Ltd.