Effect of surfactants on the hydrodynamics of bubble columns and three-phase fluidized beds

Abstract

Most hydrodynamic correlations and models used for gas-liquid-solid fluidized beds implicitly assume that the major physical properties of the liquid (density, viscosity and surface tension) are sufficient to characterize bubble dynamics. While this is true for pure liquids, multi-component liquids require additional properties due to surface-active agents preferentially accumulating at the gas-liquid interface. Experiments were first conducted to evaluate the effects of surface-active agents (alcohols as well as commercial anionic, cationic and non-ionic surfactants) on the phase holdups and minimum liquid fluidization velocities in a fluidize bed composed of air, water and glass beads of 1.2 and 5 mm in diameter. The gas-liquid interfacial behaviour of the liquid solutions was then characterized by measuring the foamability as well the dynamic surface tension and dilatational surface elasticity using the maximum pressure bubble method. The presence of surface-active agents decreases the minimum liquid fluidization velocity and the effect is more important for the larger particles. The effect of surfactants on the bed phase holdups, and in particular the gas holdup, is significant and more pronounced in the coalesced bubble flow regime where foaming occurred. Further, the bed gas holdups are lower than those observed in the freeboard. The widely used gas holdup correlations proposed by Han et al. (1990) and Larachi et al. (2001) failed. Furthermore, the approach of Gorowara and Fan (1990) which uses the dynamic surface tension also failed as all surfactant solutions present similar phase holdups despite very different surface tension behaviour. More fundamental work is required to elucidate the complex relationship between the physical properties of a multi-component liquid, the degree of bubble coalescence and the hydrodynamic features of multiphase reactors.