Interfacial tension of tri-ethylene glycol-water mixtures in carbon dioxide at elevated pressures

Abstract

The study of interfacial tension (IFT) of fluid/liquid systems is vital to optimize the design and operation of liquid-fluid contactors, and in this case, the design and performance of carbon dioxide (CO2) dehydration by absorption using tri-ethylene glycol (TEG). Several studies on improving CO2 dehydration with an absorption column containing TEG focused on system simulation and modeling but so far neglect the impact of interfacial tension of TEG–CO2 on the overall system performance. This study analyzes the interfacial tension, fluid mixture densities and drop volumes of TEG and TEG + water in CO2 medium. The pendant drop and the oscillating u-tube methods with a high-pressure densitometer were used to measure the IFT and the mixture densities of TEG–CO2 and TEG + water–CO2, respectively, at temperatures ranging from 10 °C to 50 °C and pressures up to 250 bars. The saturated TEG–CO2 and TEG + water–CO2 densities were used to evaluate the IFT from recorded drop profiles. The static IFT of TEG–CO2 shows a strongly decreasing trend from above 40 mN/m to less than 5 mN/m as the pressure increases up to 250 bars at a constant temperature (30 °C) but strongly increases by a factor of nearly 6 as the temperature is raised from 10 °C to 50 °C at a fixed pressure of 70 bars. The same trend is observed for IFT of TEG–water–CO2, although a slight increase in IFT (sim 2 mN/m) is observed due a water content of 5 wt% which would have a negative effect on the mass transfer efficiency. Conversely, a lower interfacial tension at higher pressures and lower temperatures favors the formation of a larger interface which is advantageous for mass transfer. Dynamic IFT of TEG–CO2 attains a constant after around 5 min at 70 bars and 50 °C while the drop volume increases leveling off after 10 min corresponding to a volumetric expansion of nearly 14% which corresponds to a CO2 solubility in TEG of 14%. The drop volume of a pendant drop at higher pressures hardly depends on temperature due to the superposed counteracting effects of a slightly increasing buoyancy and slightly dropping IFT. For a process design, static values of IFT can be applied with a reasonable precision, e.g., for using We-correlations to estimate drop sizes.