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Asymmetric carbon molecular sieve (CMS) membranes prepared from cellulose hollow fiber precursors were investigated for H2/CO2 separation in this work. The prepared carbon membrane shows excellent separation performance with H2 permeance of 111 GPU and an H2/CO2 selectivity of 36.9 at 10 bar and 110 °C dry mixed gas. This membrane demonstrates high stability under a humidified gas condition at 90 °C and the pressure of up to 14 bar. A two-stage carbon membrane system was evaluated to be techno-economically feasible to produce high-purity H2 (>99.5 vol%) by HYSYS simulation, and the minimum specific H2 purification cost of 0.012 $/Nm3 H2 produced was achieved under the optimal operating condition. Sensitivity analysis on the H2 loss and H2 purity indicates that such membrane is still less cost-effective to achieve ultrapure hydrogen (e.g., >99.8 vol%) unless the higher operating temperatures for carbon membrane systems are applied.

Carbon molecular sieve (CMS) membranes with rigid and uniform pore structures are ideal candidates for high temperature- and pressure-demanded separations, such as hydrogen purification from the steam methane reforming process. Here, we report a facile and scalable method for the fabrication of cellulose-based asymmetric carbon hollow fiber membranes (CHFMs) with ultramicropores of 3–4 Å for superior H2 separation. The membrane fabrication process does not require complex pretreatments to avoid pore collapse before the carbonization of cellulose precursors. A H2/CO2 selectivity of 83.9 at 130 °C (H2/N2 selectivity of >800, H2/CH4 selectivity of >5700) demonstrates that the membrane provides a precise cutoff to discriminate between small gas molecules (H2) and larger gas molecules. In addition, the membrane exhibits superior mixed gas separation performances combined with water vapor- and high pressure-resistant stability. The present approach for the fabrication of high-performance CMS membranes derived from cellulose precursors opens a new avenue for H2-related separations., Energy-efficient hydrogen purification technologies are needed for the hydrogen economy. Here the authors report facile and scalable fabrication of asymmetric carbon molecular sieve membranes for the separation of hydrogen and carbon dioxide.

This chapter describes the gas separation performances of different types of carbon membranes such as self-supported hollow fibers, supported flat sheets, and those supported on ceramic tubes. Unsupported flat-sheet carbon membranes have been widely studied to investigate material properties. It is, however, difficult to upscale their production because of the fragility of the carbon membranes. Thus, the development of high-performance supported tubular carbon membranes and self-standing carbon hollow fiber membranes is crucial to meet industrial needs. Self-supported hollow fibers potentially have a lower cost and are applicable for larger gas volumes, whereas the supported carbon membranes, which have a lower packing density, will probably be a good choice for medium to small gas volumes. Asymmetric carbon hollow fiber membranes produced without complex pretreatment have great potential if the problem of continuous production can be solved.

Biohydrogen production from biomass through dark fermentation provides a great potential to achieve a green hydrogen economy for sustainable energy development. However, the purification of fermentative biohydrogen usually contains 30–40 vol% CO2 at small-scale plants requires advanced separation technologies. Carbon molecular sieving membranes are considered as an alternative solution in this application. This work focuses on the techno-economic feasibility analysis of H2-selective carbon membrane systems for biohydrogen enrichment by investigation of process design, optimization of operating parameters, and the selection of membrane materials. High vacuum operation on the permeate is favorable to reduce the specific cost as the membrane-related capital cost is dominating the total cost. While the operation with feed gas compression provides better separation performance, and the minimum specific cost of $ 0.026/Nm3 at 6 bar was identified to achieve the hydrogen recovery of 90%. A two-stage carbon membrane system is technically feasible to reach the biohydrogen purity of >99.5 vol% with the specific cost of $ 0.06/Nm3, which is lower than pressure swing adsorption. The sensitivity analysis indicates that the carbon membrane system is scalable and flexible in responding to the variation of plant capacity with the feed flow ranges from 500 to 2500 Nm3/h without significant changes in production cost. Compared to the enhancement of membrane selectivity, improving hydrogen permeance by developing submicrometer asymmetric carbon membranes is urgently needed to increase its competitiveness for biohydrogen purification.

CO2 removal from gas streams using energy-efficient and environmentally friendly separation technologies can contribute to achieving a low-carbon energy future. Carbon membrane systems for hydrogen purification, post-combustion CO2 capture, and natural gas (NG) sweetening are considered as green processes because of their low energy consumption and negligible environmental impact. Much effort has been devoted to enhancing gas permeance and/or selectivity of carbon membranes by tailoring micropore structures to accomplish different CO2 removal processes. In this review, the status of tuning microstructure and fabrication of the ultrathin selective layer of carbon membranes, as well as membrane module upscaling was analyzed. The precursors made from a clean process using the solvent of ionic liquids have a particular interest, and high-performance asymmetric carbon hollow fiber membranes (CHFMs) without complex pre-treatment were highlighted towards technology advances of carbon membrane development. Energy-efficient processes of carbon membranes for CO2 removal in oil/gas/chemical industries and power plants were discussed for decreasing production costs, environmental impact, energy consumption, and improving process flexibility. Future perspectives on advanced carbon membrane material development based on renewable precursors and simple carbonization processes, as well as module design and process optimization, were proposed. This is an open access article distributed under the terms of the Creative Commons CC-BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Carbon hollow fiber membranes (CHFMs) were fabricated based on cellulose hollow fiber precursors spun from a cellulose/ionic liquid system. By a thermal treatment on the precursors using a preheating process before carbonization, the micropores of the prepared CHFMs were tightened and thus resulting in highly selective carbon molecular sieve (CMS) membranes. By increasing the drying temperature from RT to 140 °C, the cellulose hollow fiber precursors show a substantial shrinkage, which results in a reduction of average pore size of the derived CHFMs from 6 to 4.9 Å. Although the narrowed micropore size causes the decrease of gas diffusion coefficient, stronger resistance to the larger gas molecules, such as CH4, eventually results in an ultra-high CO2/CH4 ideal selectivity of 917 tested at 2 bar for CHFM-140C due to the simultaneously enhanced diffusion and sorption selectivity. The CHFM-140C was further tested with a 10 mol%CO2/90 mol%CH4 mixed gas at 60 °C and feed pressure ranging from 10 to 50 bar. The obtained remarkable CO2/CH4 separation factor of 131 at 50 bar and good stability make these carbon membranes great potential candidates for CO2 removal from high-pressure natural gas. https://doi.org/10.1016/j.memsci.2020.118529. 0376-7388/© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license http://creativecommons.org/licenses/by/4.0/

Novel carbon hollow fiber membranes (CHFMs) have been, for the first time, prepared from cellulose precursors directly spun with a cellulose/(1-ethyl-3-methylimidazolium acetate (EmimAc) + dimethyl sulfoxide (DMSO)) system. The spinning parameters such as air gap, dope and bore flow, bore fluid composition, and take-up speed are investigated by a factorial design method to screen hollow fiber precursors. All the precursors were carbonized using the same controlled protocol, and the prepared CHFMs show good performance that are above the Robeson upper bounds of CO2/CH4 and O2/N2. The best obtained CHFMs shows a CO2 permeability of 239 barrer and a CO2/CH4 selectivity of 186 from single gas permeation measurement. The CHFM shows attractive CO2/CH4 selectivities of 75 and 50 from 10% CO2/90% CH4 permeation tests at 25 °C with a feed pressure of 28 bar, and at 60 °C with 8 bar, respectively. Thus, the developed cellulose-based CHFMs show potential for gas separation.

Compared with commercial polymeric membranes for natural gas (NG) sweetening, the facilitated transport (FT) membranes present a great potential to reduce methane loss with the improved CO2/CH4 selectivity at high pressure operation. Different types of FT membranes together with their advantages and challenges have been reviewed. The carbon nanotube–reinforced polyvinylamine-based fixed-site-carrier membranes were identified as one of the most promising materials in this application. This chapter also provided a clear overview on the process parameter influences on the membrane separation performance for CO2 removal from NG. The technology feasibility was also discussed from the process simulation point of view. Finally, the future perspective on the membrane material and process development for NG sweetening was described.