Mass transfer and selectivity analysis of a dense membrane contactor for upgrading biogas

• Transmembrane pressure difference increases performance of dense membrane contactor.
• Mass transfer resistance of composite membrane is not limiting contactor performance.
• Dense membrane contactors play out their strengths in combination with physical solvents.

The mass transfer of a dense membrane contactor for the removal of CO2 from a biomethane-CO2 mixture was investigated in detail. Pure gas CO2 experiments as well as mixed gas experiments were conducted in order to determine the transmembrane flux and the selectivity of the contactor as a function of the pressure level and the transmembrane pressure difference. Two membranes, a PDMS coated and a Teflon-AF coated composite flat-sheet gas permeation membrane (PolyActive™) were tested in the self-constructed membrane module. The mass transfer was analyzed using a rigorous mass transfer model. With this model, it was possible to determine the mass transfer resistance of the membrane and the liquid boundary layer. It was found that both resistances do significantly contribute to the overall mass transfer resistance that is governing the membrane flux and the selectivity of the dense membrane contactor. Nevertheless, an increased driving force due to an increased pressure difference across the membrane could easily compensate for the additional mass transfer resistance of the membrane. A transmembrane pressure difference of up to two bars and absorbent liquid phase pressures at the permeate side of up to five bars were applied. It could be shown that a higher permeate pressure and a large transmembrane pressure difference is favorable for both, the permeance and the selectivity of the dense membrane contactor.