Highly productive ester crosslinkable composite hollow fiber membranes for aggressive natural gas separations

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Ma, Canghai
Koros, William J.
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Despite intrinsically high separation performance, conventional polymeric membranes suffer from CO₂ induced plasticization, which reduces CO₂/CH₄ separation efficiency significantly. Covalent ester-crosslinking can improve the plasticization resistance by controlling the segmental chain mobility in the polymer; however, only relatively thick selective skin layers and lower separation productivity have been reported to date. On the other hand, the high cost of crosslinkable polymers makes the approach challenging, especially for large-scale gas separations which require large membrane areas with high feed pressures. Dual-layer hollow fiber spinning can be used to reduce the cost of membrane production by integrating a low-cost supporting core polymer with the expensive crosslinkable sheath polymer. However, the complexity of interfacial interaction between the sheath/core layers and subsequent crosslinking required can delaminate the sheath/core layers and collapse the core layer polymer. This can reduce mechanical strength and the separation productivity significantly. This work aimed to develop thin-skinned high-performing ester-crosslinked hollow fiber membranes with improved CO₂ plasticization resistance. The skin layer thickness of hollow fibers was first optimized by simultaneous optimization of the polymer dope and spinning process variables. Moreover, this study also addresses the solutions of challenging in transitioning the monolithic hollow fiber to composite hollow fiber format. The ester-crosslinked hollow fibers were subjected to high feed pressures and high-level contaminants to probe their CO₂ plasticization and hydrocarbon antiplasticization resistance, respectively. The resultant ester-crosslinked monolithic hollow fibers show significantly reduced skin layer thickness and improved separation productivity under extremely challenging operation conditions. They also demonstrate strong stability under high feed pressures and reversibility after contaminant exposure. Moreover, this study presents a newly discovered core layer material, Torlon®, which demonstrates excellent compatibility with the crosslinkable polymer and superior thermal stability during crosslinking without sheath/core layer delamination or collapse. The characterization under aggressive feed conditions clearly suggests that ester-crosslinked composite hollow fibers can achieve high separation performance and reduce membrane cost simultaneously. This provides a significant advance in state of the art for natural gas separations under realistic operation environments
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