Absorption of CO₂ into Alkane/Water Emulsions in a Stirred Tank

Abstract

Absorption of pure carbon dioxide into aqueous emulsions of three different n-alkanes (n-heptane, n-dodecane and n-hexadecane) has been studied at 0 to 100% oil volume fraction in a stirred tank at a high stirring speed (1000 min⁻¹). Phase inversion (o/w to w/o) occurs for n-dodecane and n-hexadecane at 60–65% oil volume fraction; for n-heptane, there is no clear inversion point. The volumetric mass transfer coefficient k_La was evaluated from the pressure decrease during saturation under isochoric and isothermal (298.2 K) conditions. The o/w emulsions of both n-dodecane and n-hexadecane show the same trends: k_La first increases to a maximum at 2% oil fraction, then decreases towards the phase inversion region. The maximum might indicate an additional transport mechanism; the decrease at higher oil fractions is expected based on the increase in emulsion viscosity. Differently, the o/w emulsions of n-heptane always show higher k_La values compared to pure water. The increase in k_La (by +280% at 25% n-heptane) might be explained by a bubble covering mechanism enabled by the high spreading coefficient. In w/o emulsions of n-hexadecane, from 100% oil to the phase inversion region, k_La monotonously decreases with increasing fraction of the dispersed water phase as expected due to the viscosity effect. Surprisingly, for both n-heptane and n-dodecane, k_La does not decrease, but rather increases substantially. At the phase inversion region, the relative k_La increase compared to pure oil is by +107% for n-dodecane, and even +143% for n-heptane.