Gas absorption into emulsions

Abstract

In the present work, absorption of pure oxygen (O2) and pure carbon dioxide (CO2) into aqueous emulsions of three different n-alkanes (n-heptane, n-dodecane and n-hexadecane) has been studied in a stirred tank at the stirring speed of 1000 min-1. The interfacial tensions have been measured with a tensiometer using the ring method. Positive spreading coefficients are calculated for all oils but the value for n-heptane is noticeably higher than the values for n-dodecane and n-hexadecane. The full range of oil volume fraction (0-100%) has been covered. Phase inversion (O/W to W/O) occurs at 60-65 % n-dodecane and n-hexadecane as identified with an electrical conductivity sensor, whereas observation of de-emulsification suggests the inversion region to be 50%-60% n-heptane. The volumetric mass transfer coefficient, kLa, has been evaluated from the pressure decrease under isochoric and isothermal (298.2 K) conditions with a pseudo-homogeneous mass transfer model. As expected, the solubility of both gases increases linearly with oil volume fraction; only for CO2-water-n-heptane system the values are low as compared to linear interpolation of the measured pure phase solubilities. In addition, a fluorescence technique with an oxygen-sensitive fluorophore, soluble only in the water phase, has been applied to measure the oxygen-water volumetric mass transfer coefficient, (kLa)GW, in emulsions of n-dodecane and n-hexadecane in order to gain more insight into the mass transfer mechanism. For O/W emulsions, both O2 and CO2 have similar trends in the variation of kLa with increasing dispersed oil volume fraction. A kLa maximum at low oil volume fractions (1-2%) is found for both n-dodecane and n-hexadecane; the “shuttle” mechanism might be responsible for this maximum. Differently, O/W emulsions of n-heptane always show higher kLa than pure water; this can be explained by a bubble covering mechanism enabled by its high positive spreading coefficient. The oxygen mass transfer rate is strongly enhanced by up to the factor of 38 at 50% n-heptane. Such strong effects have never reported before. For O2 absorption into O/W emulsions of n-dodecane and n-hexadecane, both (kLa)GW and kLa have the same trend with increasing oil volume fraction. The low values of spreading coefficient for n-dodecane and n-hexadecane and the strong decrease of both (kLa)GW and kLa towards the phase inversion region suggest the serial transport mechanism: gas → water → oil. For W/O emulsions, both O2 and CO2 have similar trends in the variation of kLa with increasing dispersed water volume fraction. In W/O emulsions of n-hexadecane, kLa decreases monotonously as expected due to increasing viscosity. For W/O emulsions of both n-dodecane and n-heptane, the kLa values do not decrease but increase substantially. The reason for this surprising trend is not clear.