Absorption into Stirred Emulsions

Abstract

The volumetric mass transfer coefficient for carbon dioxide was determined by physical absorption in a surface-aerated stirred vessel (1000 rpm) in emulsions of n-alkanes (n-heptane, n-dodecane and n-hexadecane) at 0-100% oil volume fraction. Additionally, the effect of the anionic surfactant sodium dodecyl sulfate (SDS) was studied at 0.22 g/L in the aqueous phase. Some preliminary experiments were performed to determine the specific interfacial area in O/W emulsions of n-hexadecane by cobalt-catalyzed sulfite oxidation. The volumetric mass transfer coefficient showed a maximum at oil volume fractions of 1-2%, for all organic liquids studied. At the same low n-hexadecane loadings, the specific interfacial area determined by sulfite oxidation dropped from the value in oil-free sulfite solutions. This excludes a coalescence hindering effect of the oil often suggested in the literature. An additional transport mechanism (shuttle effect) could still be an explanation as this would be less effective at the smaller effective film thickness in the absorption regime of fast chemical reaction (Ha = 5.2). Upon further increase in oil fraction, for all O/W emulsions (except surfactant-free n-heptane), the volumetric mass transfer coefficient decreased towards the phase inversion region because of the viscosity effect. This trend can be uniformly correlated with emulsion viscosity to the power of -0.72. The decrease in interfacial area is less strong; this indicates an additional effect on Liquid-side mess transfer coefficient. Different from the other oils, the addition of n-heptane increased the volumetric mass transfer coefficient even up to the phase inversion region. This trend had been observed previously and tentatively been attributed to bubble covering by oil spreading. In this study, addition of the ionic surfactant SDS eliminated the high mass transfer rates in n-heptane emulsions and induced the same viscosity effect as in the other W/O emulsions. This indicates that spreading of n-heptane on the bubble surface is indeed the mechanism causing the high absorption rates. In W/O emulsions, the volumetric mass transfer coefficient decreased with increasing dispersed water volume fraction for all emulsions studied. The trends cannot be uniformly correlated to the volume fraction or the viscosity, respectively, but depend on the oil. SDS had no effect on the volumetric mass transfer coefficient in W/O emulsions as it accumulates at the oil/water interface without having contact to the gas bubbles.