Absorption of CO2 from flue gas under oscillating gas flow conditions in gas-solvent hollow fibre membrane contactors

Abstract

Membrane gas-solvent contactors are a viable approach for the absorption of carbon dioxide (CO2) into solvents, such as potassium glycinate, compared to traditional solvent absorption. The hollow fibre membrane design does have limitations, as the presence of the membrane adds extra resistance to mass transfer compared to traditional solvent absorption. To reduce the impact of this extra resistance it is necessary to increase mass transfer in the solvent and gas phase boundary layers. This investigation aimed to increase mass transfer in the gas phase boundary layer of a membrane contactor process by applying oscillating gas flow conditions. For an asymmetric polydimethylsiloxane membrane contactor undergoing oscillating gas flow conditions, an average increase of 19% in overall mass transfer coefficient (KG) compared to non-oscillating gas flow was observed, for 15 wt% potassium glycinate solvent. The oscillating frequency of the gas phase had only a minor impact on performance, above 2 Hz, implying that oscillation of the gas phase was important for mixing, not the rate of the oscillation. Alternatively, KG approximately doubled in value as a function of increased oscillating amplitude. This was due to the resulting pressure wave producing localized increases in the partial pressure driving force for mass transfer across the membrane. For the membrane contactors studied, mass transfer correlations, in terms of Sherwood number, were proposed for the oscillating and non-oscillating flow conditions and fitted to the experimental data. This outcome highlighted the importance of inducing mixing within the gas phase to maximise mass transfer in membrane contactors.