Low hydrocarbon solubility polymers: Plasticization-resistant membranes for carbon dioxide removal from natural gas

Hydrocarbon polymers developed for CO2 removal from natural gas often lose their superior separation ability at field conditions. This deterioration in performance is primarily a result of polymer plasticization by natural gas components like higher hydrocarbons, which have high solubilities in these polymers. Polymers that have low solubilities for higher hydrocarbons may be less susceptible to plasticization by these penetrants and therefore exhibit more stable separation properties in actual field conditions. This study was undertaken to investigate the above premise through identification of low-hydrocarbon-solubility polymers and performing a fundamental study to assess the potential of such materials to be stable membranes for CO 2 removal from natural gas.; Hydrocarbon and fluorocarbon gas solubility measurements in hydrocarbon polymers and fluoropolymers reveal that interactions between hydrocarbon and fluorocarbon species result in lower solubilities of hydrocarbons in fluorocarbon polymers, and vice versa, than expected on the basis of empirical correlations. The influence of these interactions on gas permeability is greater in lower free volume materials. Interestingly, hydrocarbon solubility in fluoropolymers increases much less with increasing penetrant condensability than in hydrocarbon polymers, implying that large hydrocarbon compounds will exhibit much lower solubility in fluoropolymers than in hydrocarbon polymers.; A commercial fluoropolymer, Hyflon AD 80, has much higher CO2 permeability than typical hydrocarbon polymers, but its CO2/CH 4 selectivity is lower than these polymers. However, Hyflon AD 80 exhibits more stable gas separation properties than typical hydrocarbon polymers in the presence of CO2 and moderate amounts of large hydrocarbons.; Materials selection guidelines for using fluoropolymers as plasticization-resistant coatings on existing hydrocarbon membranes require the fluoropolymer to have a lower ratio of higher hydrocarbon to CO2 (or CH4) solubility than the hydrocarbon polymer. The guidelines also require the coating to have a similar, or greater, diffusivity selectivity (size-selectivity) for gases than that of the hydrocarbon polymer.; Permeability of highly condensable penetrants is often a function of their sorbed concentration in the polymer. A model is presented to rationally predict concentration and temperature dependent gas permeability in rubbery polymers, based on limited experimental data. The model satisfactorily describes vapor permeation in a commercial membrane, poly(dimethyl siloxane), and in poly(ethylene).