Carbon Hollow Fiber Membranes
Membranes with the hollow fiber configuration have the advantage of packing density compared with spiral wound, plate-and-frame, and tubular modules, with 30,000 m2 nr3 configurations possible , which is the industrially preferred configuration. Carbon membranes in the form of hollow fibers have the ability to withstand high transmembrane pressures . Thus, CHFMs show promising applications in different separation scenarios, such as CO, removal from natural gas [31-34], organic solvent reverse osmosis , and H, purification [36, 37]. Hollow fiber precursor spinning and the following carbonization process are essential to fabricate high-performance CHFMs in symmetric or asymmetric structures. Normally, to obtain symmetric CHFMs, anti-collapse treatment during the carbonization process is needed.
Symmetric CHFMs with a thicker selective layer normally provide remarkable gas-pair selectivity compared with asymmetric structures with a skin selective layer. Zhang et al.  reported ultrahigh permselectivities in CHFMs derived from Matrimid polyimide precursors. The reported CHFMs display a symmetric structure with a well-defined separation layer and properties, which are convenient for understanding the material's intrinsic permeation properties. The membrane carbonized at 900 °C presents the ideal selectivity for different gas pairs: a [CO,/CHj = 3650, a [N2/CH4] = 63, a [0,/N,] = 21, a [He/ CHJ = 16700, a [H,/CH4] = 40350, and a [H,/N,] = 640, which are the highest selectivities reported to date. Using the time-lag method, the authors deconvo- luted gas permeability into diffusivity and sorption coefficients to explain such unprecedentedly high selectivities. Based on the calculated diffusivities and sorption coefficients, the selectivities of different gas pairs were also divided into diffusion selectivity and sorption selectivity. The authors found that that increased CO,/CH4 selectivity following increased carbonization temperature is due to simultaneously improved diffusion selectivity and sorption selectivity. On the one hand, elevating carbonization temperature tends to form a more ordered graphitic carbon structure (sp2 hybridized carbon) that results in denser packing and smaller micropore and ultramicropore sizes. The refined ultramicropores improve the gas-pair diffusion selectivity. On the other hand, the smaller pore structure restricts sorption of larger gases like CH4. As a result, both diffusion selectivity and sorption selectivity are enhanced.