Permeability enhancement of composite membranes for biogas upgrading using chemical modification of functional layers

A biogas upgrading using membranes, i.e. the removal of unburnable CO₂ from the CO₂ + CH₄ mixture, is a modern technology for energy recovery from renewable sources and waste in the form of methane. However, in order to improve the energy efficiency of the process in industrial applications, it is necessary to increase the gas permeability (permeance) through the membranes while maintaining efficient separation capacity. This is one of the key research directions of the Department of Membrane Separation Processes. A recent paper published under the leadership of Dr. Pavel Izák in the renowned scientific magazine Journal of Membrane Science in collaboration with the Institute of Solid State Engineering at the University of Technology Prague and the Materials Centre at the UJEP dealt with the modification of commercial composite membranes, that are used for reverse osmosis and nanofiltration, for efficient gas separation.

The functional polyamide layers of the membranes were modified by UV irradiation or by a liquid activator (oxidizing agent “Piranha”). Cysteamine molecules, which have previously been shown to affect surface chemistry and wettability, were also attached to the activated polymer surface. In order to improve the separation technology utilizing a condensing water membrane, the gas permeability of the water-swollen membranes was investigated for various gasses in both, the pure form and their mixture. Almost all the modified reverse osmosis membranes showed higher permeability for CO₂, CH₄ and N₂ than the original membranes, and a 100% increase in permeability was achieved for the reverse osmosis membranes grafted with cysteamine after UV activation. The separation favored smaller gas molecules, and the increase in gas permeability by all modifications did not affect the selectivity of the separation.

The same functional layer activation or cysteamine grafting methods applied to the nanofiltration membrane resulted only in very short operating times, although the original nanofiltration membrane was stable for separations under humid conditions. The nanofiltration membrane was also less permeable to all gases than either original or modified reverse osmosis membranes.

The decrease in mixed gas selectivity with increasing inlet pressure showed that the gas mixture was most efficiently separated at lower membrane overpressures, resulting in significant energy savings in the separation process. Also, the computational model from this work can be used for better evaluation of the mass transfer coefficient across the membrane and to assess the intensity of the so-called “coupling effect” for gas mixture separations in testing permeation cells.

• P. Stanovský, M. Benkocká, Z. Kolská, M. Šimčík, P. Slepička, V. Švorčík, K. Friess, Marek C. Ruzicka, P. Izak: Permeability enhancement of chemically modified and grafted polyamide layer of thin-film composite membranes for biogas upgrading. J. Membr. Sci. 2022, 641, 119890. DOI: https://doi.org/10.1016/j.memsci.2021.119890