Crosslinked Thermally Rearranged (XTR) Hollow Fiber Membranes for Gas Separation

Abstract

This thesis addresses hollow fiber membranes using recently developed crosslinked TR (XTR) material for gas separation applications, especially for CO2 capture in post-combustion process. Hollow fiber spinning parameters have been investigated for XTR polymers resulting in ultrathin-skinned XTR hollow fiber membranes. Thermal densification in XTR hollow fibers is a challenging issue, which causes reduction in gas permeation by thickening the skin layer. Thermal densification, however, was employed to minimize the gas permeation drop in this study.. Chapter 1 describes details of introductive contents on membrane technology and background on gas separation membranes. Representative ‘high free volume’ polymer membranes will be reviewed including TR membranes. Chapter 2 is related with fundamentals of the phase inversion for preparation of asymmetric membrane using phase inversion thermodynamics and kinetics. In addition, the experimental details on preparation of dope solution, hollow fiber spinning and characterization of asymmetric XTR-PBOI hollow fiber membranes were included. Chapter 3 discusses inner-skinned asymmetric XTR-PBOI hollow fibers prepared from crosslinkable co-HPI precursors through a systematic optimization of skin layer as well as the support. As thermal treatment temperature increased, CO2 permeance linearly increased up to 830 GPU until the temperature reached 400 oC due to cavity evolution during the TR conversion process. However, a permeance drop to 690 GPU can be observed at 425 oC which is obviously indicative of thermal densification effect. As a result, the thermal densification in XTR hollow fiber membranes is very disadvantageous for the enhancement in gas permeation performance through the TR conversion process resulting in gas permeance loss. Thus, it should be overcome or minimized to produce high-flux XTR hollow fiber membranes. Chapter 4 introduced a novel ultra-thin skin formation method using thermal densification from defective precursor hollow fiber membranes without the permeation flux drop. The skin layer in defective precursor fibers results in a superior CO2 permeance of 2300 GPU with an excellent CO2/N2 selectivity of 19 at a targeted TR temperature. In Chapter 5, the influence of water vapor on the CO2 capture efficiency was evaluated using XTR poly(benzoxazole-co-imide) (XTR-PBOI) hollow fiber modules, and these were compared to co-HPI module in simulated flue gas and single gases. The results revealed that the permeate CO2 flow rate in the hydrophobic XTR-PBOI module showed enhanced separation performance, whereas the flow rate severely decreased in the relatively hydrophilic co-HPI module, reflecting a disturbance in gas transport from competitive sorption between H2O and CO2 in the co-HPI membranes. Chapter 6 summarizes the conclusions of this work and suggests recommendations for further study and investigation.