CO2/CH4 and Propylene/Propane Separation Using Cross-Linkable Polymeric Membranes

Abstract

Membrane is an emerging technology that holds great promises and displays attractive advantages over conventional methods. Polymeric membranes, especially polyimide membranes, have been widely applied for gas separations due to their attractive permeability, selectivity, and processing characteristics. However, traditional membrane materials cannot always achieve high degrees of separation performance and suffer from an upper-bound relationship for its permeability and selectivity. Their use for natural gas and hydrocarbon separations is also limited by plasticization-induced selectivity losses in feeds with significant partial pressures of CO2 and C3+ hydrocarbons. This greatly constrains the application of polymeric materials for industrial use. In this PhD work, the main focus is to explicitly tailor the properties of cross-linkable glassy polymeric membranes for gas separation application. Four aspects have been thoroughly investigated. Firstly, the new flexible and high performance gas separation membranes were fabricated by grafting various sizes of cyclodextrin to the cross-linkable co-polyimide (6FDA-Durene/DABA (9/1)) matrix and then decomposing them at elevated temperatures. The gas permeability and selectivity of thermally treated pristine polyimide and CD grafted co-polyimide has been determined. Secondly, with the purpose of better fundamental understanding of this class of polymers and aid evaluation of their potential for use in industrial application, the intrinsic gas transport properties of thermally treated cross-linkable 6FDA-based co-polyimide membranes have been studied. Grafting various sizes of cyclodextrin (CD) to the co-polyimide matrix and then thermally decomposing CD at elevated temperatures are an effective method to micro-manipulate microvoids and free volume as well as gas sorption and permeation. Solubility and permeability coefficients of CH4, CO2, C3H6 and C3H8 were conducted at 35 ?C for different upstream pressures. Thirdly, considering the importance of hollow fiber for industry use, thermally cross-linkable co-polyimide dual-layer hollow fiber membranes grafted with ?-Cyclodextrin were fabricated. The fiber membranes were thermally cross-linked at different temperatures and the gas performance of the fibers before and after silicon rubber coating was studied. Fourthly, to continue from the previous works, the mixed matrix membrane (MMM) were fabricated by using three 6FDA-based polyimides (6FDA-Durene, 6FDA-Durene/DABA (9/1), 6FDA-Durene/DABA (7/3)) and nano-size zeolitic imidazolate framework-8 (ZIF-8) with uniform morphology comprising ZIF-8 as high as 40 wt% loading. The gas permeability and selectivity of thermally treated MMMs has been determined.