(472a) Hollow Fiber Membrane Contactor for Characterization of Gas/solvents Thermodynamics and Mass Transfer Properties

The control of greenhouse gases (i.e. CO2, CH4, N2O,...) emissions before their release to the atmosphere has became a worldwide concern over the last few years[1]. The principal source of CO2 emissions comes from fossil fuel combustion from refineries and the energy industry. Several technologies are available to reduce CO2 emissions from industrial gas streams but the most popular one involves a liquid solvent which will selectively capture the CO2 through a molecular affinity (physical solvent) or a chemical reaction (chemical solvent). For instance, for low to moderate CO2 pressures, chemical absorption with alkanolamines solutions usually permits the removal of 75 to 90% of the CO2 contained in the gas streams[2]. In order to find the most efficient solvents for CO2 capture, there is therefore an increasing need to understand the mass transfer mechanisms occurring in these systems. These properties are often characterized from lab-scale experiments through the determination of Henry constant (H) and diffusion coefficient (D) of a molecular species within a fluid phase.

However the different experimental techniques used to determine these properties can be time consuming as a set of multiple experiments is required to thoroughly determine the physical properties of the gas/liquid system under study. Moreover these different techniques are often limited by the difficulty to control the interfacial contact area between the gas and the liquid phase due to boundary effects[3].

Different contactors can be used to study gas/liquid systems but hollow fiber membrane contactor (HFMC) geometry has recently arisen as an interesting solution for separation technology[4]. For instance, on a process scale, the use of HFMC has been considered to replace packed columns, to avoid technical problems such as flooding, entrainment and foaming[5-7]. In this geometry, the liquid is usually flowing in the lumen side of solvent repellent hollow fibers while the gas flows in the core shell and diffuses through the pores of a microporous membrane before contacting the immobilized solvent. The contactor geometry ensures a large interfacial contact area between the two phases and prevent any problems induced by the dispersion of one phase into another. Unlike classical membrane separation processes, the membrane does not impact on the separation efficiency but provides an interfacial area for the contact between the two non miscible phases. The thermodynamics of the separation is hence solely controlled by the choice of the solvent flowing in the lumen of the fibers. Besides, hollow fiber membrane contactor displays a fixed and known interfacial contact area which makes it very suitable for gas/liquid characterization.

In this work, we perform transient hollow fiber membrane contactor experiments where a physical solvent is progressively saturated with a gas (CO2 or N2O). We also develop a theoretical model representing the gas/liquid mass transfer occurring during such experiments. The developed methodology allows the determination of gas/liquid thermodynamics and mass transfer properties like the Henry constant and the liquid phase diffusion coefficient. Both parameters are obtained within a single experiment at a constant temperature and the comparison with temperature-dependant correlations yields excellent agreement over the whole range of temperature studied in this work. The plasticizer role of CO2 inside the membrane contactor fibers is also highlighted as simulations show a partial wetting of the membrane pore by the liquid meniscus during a contact between CO2 and H2O. We also discuss the possible extension of this methodology to chemical solvents.

References

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[2] Bonenfant, D.; Mimeault, M.; Hausler, R., 2007. Estimation of the CO2 Absorption Capacities in Aqueous 2-(2-Aminoethylamino)ethanol and Its Blends with MDEA and TEA in the Presence of SO2. Ind. Eng. Chem. Res., 46, 8968-8971.

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[4] Li, J.-L.; Chen, B.-H., 2006. Review of CO2 absorption using chemical solvents in hollow fiber membrane contactors. Sep. Purif. Technol. 279, 301-310.

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[6] De Montigny, D.; Tontiwachwuthikul, P.; Chakma, A., 2005. Comparing the absorption performance of packed columns and membrane contactors. Ind. Eng. Chem. Res. 44, 5726-5732.

[7] Karoor, S.; Sirkar, K.K., 1993. Gas adsorption studies in microporous hollow fiber membrane modules. Ind. Eng. Chem. Res. 32, 674-684.