Cross-Linkable Semi-Rigid 6FDA-Based Polyimide Hollow Fiber Membranes for Sour Natural Gas Purification

Robust high-performance hollow fiber membranes are needed for sour natural gas separation. In this study, a semi-rigid, cross-linkable, defect-free 6FDA-based polyimide hollow fiber membrane was developed and evaluated under pure H₂S and three realistic sour natural gas feeds. No cross-linking-induced decline of H₂S permeance was observed, indicating that the delicate asymmetric nanostructures of hollow fibers were well preserved during the cross-linking process due to the rigid molecular structure. For sour natural gas feed with low H₂S concentration (0.5% H₂S, 20% CO₂, 79.5% CH₄), the cross-linked hollow fiber membrane exhibited H₂S permeance and H₂S/CH₄ permselectivity as high as 32 GPU and 30, respectively, at 450 psi and 35 °C. The CO₂ permeance and CO₂/CH₄ permselectivity were also well maintained with values of 35 GPU and 32, respectively. Under more aggressive sour natural gas feeds (25% H₂S, 5% CO₂, 70% CH₄ and 20% H₂S, 20% CO₂, 60% CH₄), plasticization was observed for uncross-linked CF₃ hollow fiber, but this phenomenon actually was beneficial for H₂S separation. Specifically, H₂S permeance and H₂S/CH₄ selectivity increased somewhat for the uncross-linked vs the cross-linked sample. In addition, the effects of sour gas composition on the performance of hollow fiber membranes were also considered. The results suggest increased H₂S concentration in sour natural gas exhibits more pronounced effects on both H₂S and CO₂ permeance and selectivity than does increased CO₂ concentration in the feed. Considering the impressive H₂S and CO₂ separation performance as well as the robustness of hollow fibers under highly aggressive sour natural gases, our work shows promising potential to simultaneously remove acid gases from sour natural gas with energy-efficient membrane processes under challenging feed conditions.