Carbon Molecular Sieve Hollow Fiber Membranes for CO2 Removals
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New technologies that are more energy efficient could, if applied in the large-scale CO2 removal process, reduce carbon dioxide emissions, and achieve a low-carbon future. Compared to traditional amine-based absorption, membrane-based separation processes have been considered as promising technologies for gas separations, such as CO2 removal in natural gas sweetening and hydrogen purification, due to their simplicity of operation and lower investment cost, intrinsic higher energy efficiency, and environmental friendliness. Carbon molecular sieve (CMS) membranes are promising candidates as temperature- and pressure-resistant materials for these challenging separation scenarios where polymer membranes are difficult to be used, such as high pressure of feed stream and high operating temperatures. This thesis is funded by Norway Research Council through the Petromaks2 program in the CO2Hing project, aiming to develop carbon hollow fiber membranes (CHFMs) with superior separation performances, facile fabrication processes, and easier upscaling capabilities. This thesis demonstrates a new method to fabricate CHFMs from renewable material of cellulose and “green solvent” of an ionic liquid. The preparation of CHFMs is optimized by a fractional factorial experiment design method. The symmetric CHFMs show ultra-selectivities of CO2 over CH4 at a feed pressure up to 50 bar, illustrating that these carbon membranes are potential candidates for CO2 removal from high-pressure natural gas streams. Furthermore, asymmetric CHFMs with ultramicropores of 3-4 Å, consisting of a porous inner layer and a dense outer layer, have been developed. The CHFMs carbonized at different temperatures have been tested and superior H2 separation performances under humidified conditions were documented. The tunable ultramicropores of CHFMs were found to be a result of the controlled conversion of sp3 to sp2 hybridized carbon by tuning carbonization temperature. Afterward, a two-stage membrane system was designed and investigated by HYSYS simulation to evaluate the techno-economic feasibility of the developed asymmetric CHFMs for H2 purification. Operating carbon membrane system at higher temperatures of above 150 °C has been evaluated to be technologically and economically feasible to achieve the H2 purity of 99.95 vol.%. The separation performances of the developed membranes show remarkable selectivity and stability under either high pressures and/or high temperatures, and the methods to fabricate CMS membranes from biomaterial of cellulose and “green solvent” of ionic liquid demonstrates a green material development process for energy-efficient gas separations.