Permeability enhancement of chemically modified and grafted polyamide layer of thin-film composite membranes for biogas upgrading

Abstract

Membrane separations enable biogas upgrading, but their energy efficiency must still be improved for industrial upscaling. Nevertheless, UV treatment affects the permeation properties of the polyamide functional layer of reverse osmosis (RO) and nanofiltration thin film composite (TFC) membranes. In this work, after membrane activation via Piranha solution, cysteamine grafting and UV irradiation, we determined the gas permeability of dry and swelled samples. The samples exhibited higher permeability to gases (CO2, CH4 and N2) than pristine membranes, reaching a 100% increase in RO membranes grafted with cysteamine after UV activation. Permeability increased more than twofold compared to TFC RO membranes activated by diode discharge plasma, as recently reported. Separation favored smaller gas molecules, and the increase in permeability resulting from all modifications did not adversely affect selectivity. CO2/CH4 selectivity remained almost constant over the range of trans-membrane pressure difference to 400 kPa. The grafting with cysteamine to the activated functional layer at the RO membrane positively affected permeability despite the detrimental effect of activation with a Piranha solution. The same activation or cysteamine grafting method at the nanofiltration membrane led only to a very short operation time, although the pristine nanofiltration membrane was stable. The pristine nanofiltration membrane were less permeable to all gasses than all RO membranes. Mixed gas separation of model binary biogas mixtures enhanced CH4 and CO2 permeability only in membranes activated with UV radiation. Decrease of mixed gas selectivity with the growing feed pressure showed that the gas mixture is more effectively separated at lower trans-membrane pressures. Therefore, our model for describing gas mixture separations in cylindrical permeation cells can be utilized to better evaluate the mass transfer coefficient and assess the strength of the coupling effect.