Pure- and Mixed-Gas Transport Study of Nafion® and Its Fe3+-Substituted Derivative for Membrane-Based Natural Gas Applications
AuthorsMukaddam, Mohsin Ahmed
KAUST DepartmentPhysical Sciences and Engineering (PSE) Division
Chemical and Biological Engineering Program
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AbstractThe focus of this research project was to develop a fundamental understanding of the structure-gas transport property relationship in Nafion® to investigate its potential use as a gas separation membrane material for natural gas (NG) applications including carbon dioxide removal from NG, helium recovery, higher-hydrocarbon removal, and nitrogen separation from methane. Separation processes account for ~45% of all energy used in chemical plants and petroleum refineries. As the drive for energy savings and sustainability intensifies, more efficient separation technology becomes increasingly important. Saudi Arabia ranks among the world’s top 5 NG producers. Commercial hydrocarbon-based glassy polymers often lose their gas separation properties in the presence of condensable, highly sorbing NG components such as CO2, ethane, propane, n-butane, and C5+ hydrocarbons. This deterioration in gas separation performance results from penetrant-induced dilation and plasticization of the polymer matrix, leading to significant methane and higher hydrocarbon losses. Polymers that have intrinsically low affinity to high-solubility NG components may be less susceptible to plasticization and therefore offer better performance under actual field conditions. By virtue of their strong carbon-fluorine bonds and chemical inertness, perfluoropolymers exhibit very low affinity for hydrocarbon gases. Nafion®, the prototypical perfluoro-sulfonated ionomer, comprising hydrophilic sulfonate groups phase-separated from a hydrophobic perfluorocarbon matrix, has demonstrated interesting permeability and selectivity relationships for gas pairs relevant to NG applications. Gas transport properties of Nafion® indicated gas solubility behavior similar to rubbery polymers but with sieving properties more commonly observed in low free volume glassy polymers. Nafion® demonstrated very low solubility for CO2 and hydrocarbon gases; the trend-line slope of solubility versus penetrant condensability in Nafion® was almost 2.5 times lower than that of typical hydrocarbon polymers, highlighting Nafion’s® effectiveness in resisting high-solubility induced plasticization. Additionally, Nafion® showed extraordinarily high permselectivities between small gases (He, H2, CO2) and large hydrocarbon gases (C1+): He/CH4 = 445, He/C3H8 = 7400, CO2/CH4 = 28, CO2/C3H8 = 460, H2/CH4 = 84 and H2/C3H8 = 1400 owing to its tightly packed chain domains. These high selectivities could potentially be harnessed for helium recovery and CO2 removal in natural gas applications, and hydrogen recovery from refinery gas streams. Pressure-dependent pure- and mixed-gas permeabilities in Nafion® were determined at 35 °C. Nafion® demonstrated two divergent pressure-dependent permeability phenomena: gas compression and plasticization. In pure-gas experiments, the permeability of the permanent gases H2, O2, N2 and CH4 decreased with increasing pressure due to polymer compression, whereas the permeability of the more condensable gases CO2, C2H6 and C3H8 increased dramatically due to solubility-induced plasticization. Binary CO2/CH4 (50:50) mixed-gas experiments showed reduced performance with up to 2-fold increases in CH4 permeability from 0.075 to 0.127 Barrer, and a 45% drop in selectivity (from 26 to 14), between 2 and 36 atm total pressure as a result of CO2-induced plasticization. At a typical NG CO2 partial pressure of 10 atm, Nafion® exhibited 24% lower CO2/CH4 selectivity of 19, with a 4-fold lower CO2 permeability of 1.8 Barrer relative to a commercial cellulose acetate (CA) membrane. Ternary CO2/CH4/C3H8 (30:50:20) experiments quantified the effect of CO2 and C3H8 plasticization. The presence of C3H8 reduced CO2 permeability further due to a competitive sorption effect causing a 31% reduction in CO2/CH4 selectivity, relative to its pure-gas value of 29, at 16 atm total feed pressure. The strong cation-exchanging sulfonate groups in Nafion® provided an opportunity to tailor the material properties by incorporating metal ions through a simple ion-exchange process. Nafion® neutralized with Fe3+ was investigated as a potential approach to mitigate CO2-plasticization. XRD results demonstrated an increase in crystallinity from 9% in Nafion H+ to 23% in Nafion Fe3+; however, no significant changes in the average inter chain spacing was observed. Raman and FT-IR technique qualitatively measured the strength of the ionic bond between Fe3+ cation and sulfonate anion. The strong crosslinking effect in Fe3+-cation-exchanged membrane demonstrated substantial increase in permselectivity: N2/CH4 selectivity increased by 39% (from 2.9 to 4.0) and CO2/CH4 selectivity increased by 25% (from 28 to 35). Binary CO2/CH4 (50:50) mixed-gas experiments at total feed pressures up to 30 atm quantified the effect of CO2 plasticization on the CO2/CH4 separation performance. Nafion® Fe3+ demonstrated better resistivity to plasticization enduring approximately 30% CH4 permeability increases from 0.033 Barrer at 2 atm to 0.043 Barrer at 15 atm CO2 partial pressure. At 10 atm CO2 partial pressure, CO2/CH4 selectivity in Nafion® Fe3+ decreased by 28% to 28 from its pure-gas value of 39, which was a significant improvement compared to Nafion® H+ membrane that decreased by 42% to 19 from its pure-gas value of 32.