A dense membrane contactor for intensified CO2 gas/liquid absorption in post-combustion capture
Résumé
Membrane contactors based on microporous hydrophobic materials already offer remarkable performances for different separation applications at industrial scale, especially for gas-liquid mass transfer operations. Impressive process intensification effects can indeed be achieved in certain cases, due to the large interfacial area provided by hollow fiber membranes. Nevertheless, depending on the membrane and fluid properties, great differences in mass transfer can be obtained; undesirable effects due to liquid contact such as gradual changes in membrane structure and/or partial wetting of the pores can dramatically affect mass transfer performances. Numerous studies have addressed these difficulties for one of the most attracting and challenging application of membrane contactors: the absorption of CO2 in a chemical solvent in order to achieve post-combustion CO2 capture from flue gases. For this application, given the fast chemical reaction which takes place in the liquid phase, a highly permeable membrane material is absolutely necessary. Additionally, the membrane material has to withstand long term contact with a chemically reactive solvent (typically an amine such as monoethanolamine: MEA) and has to remain non wetted. A possible solution which prevents wetting problems together with a minimal impact on the membrane mass transfer coefficient is reported in this study; the key idea is to make use of a composite membrane based on a thin dense skin, based on a highly permeable glassy polymer, coated on a microporous support. In a first step, screening tests have been performed in order to identify potential polymer candidates for the thin skin, combining a high CO2 permeability and solvent (MEA) compatibility on long time scales. In a second step, composite hollow fibers with a thin skin (Teflon AF®, PTMSP) coated on a porous support (PP) have been prepared and tested. The concept has been finally tested and validated at lab scale for CO2 capture from a gas mixture into aqueous solutions of MEA with hollow fiber modules. Remarkably, the overall mass transfer performances of the composite fibers compete with the most permeable microporous membranes classically proposed for membrane contactor applications. The possible use of these novel composite fibers for other applications and the extension of the concept to different industrial situations are discussed.