Optimizing the structure and plasticization resistance of asymmetric polyimide hollow fiber membranes for CO2 removal from natural gas

Abstract

Polymeric membrane has been recognized as an energy efficient and economical approach for many industrial gas separation applications, including natural gas purification. Matrimid 5218® has been used as a benchmark polymer, due to its combination of high selectivity and acceptable permeance, as well as its excellent mechanical properties and commercial availability. Based on series of comprehensive experimental studies, we have evaluated many of the critical fabrication parameters and optimized those parameters as regard to the membrane physical and separation properties. Asymmetric hollow fibers with CO2/CH4 separation factors up to 67 were successfully fabricated under the optimum fabrication conditions. Our further thermodynamic analysis based on Hansen’s solubility parameter and kinetic analysis based on Hayduk and Minhas correlations resulted consistent observations with the experimental results including cloud point experiments, SEM images, and the gas separation properties. In this study, a nodule structure model was developed based on the nucleation and growth and dual mode sorption model to gain a comprehensive understanding of the plasticization phenomenon and to link the bridge between membrane formation mechanisms with membrane morphology and plasticization phenomenon. Gas sorption tests were performed on the in-house fabricated Matrimid hollow fibers with various ranges of separation properties, and the Henry’s and Langmuir sorption isotherms were decoupled and compared against the pressure. The sorption observations accompanied with the CO2 conditioning experimental results confirmed that Matrimid hollow fiber adsorbed more CO2 gas penetrants in Henry’s sites at low-pressure range were prone to the plasticization. In this study, thermal annealing as a post-treatment to enhance membrane’s plasticization resistance was evaluated. CO2 sorption tests provided solid evidence to prove the suppression of plasticization achieved by thermal annealing was partially because the thermal annealing alters the ratio of Henry’s and Langmuir sorption sites and consequently shift the plasticization pressure to a higher level. Other techniques were also adopted in this study to characterize the treated hollow fiber including SEM, DSC measurements as well as dissolution tests.