(566e) Membranes for Sweetening Natural Gas with High Carbon Dioxide and Hydrogen Sulfide Concentrations

Natural gas processing is one of the largest industrial markets for gas separation membranes. Virtually all natural gas must be treated to remove carbon dioxide (CO₂) and hydrogen sulfide (H₂S) before entering a pipeline, as these “acid gases” can corrode the pipeline. Cellulose triacetate (CTA) asymmetric hollow fiber membranes are effective for removal of acid gases from natural gas. Under some conditions, however, these gases can plasticize the membrane, which affects membrane permeability and selectivity. In this presentation, CO₂ and H₂S separation from realistic natural gas hydrocarbon mixtures with commercial CTA membranes will be discussed. While there has been significant effort to understand separation of binary mixtures of CO₂ and methane (CH₄), separation of CO₂ and H₂S from complex hydrocarbon mixtures is not frequently reported due to the complexity of multi-gas measurements and the safety measures required for work with H₂S. Gas mixtures containing up to 70% CO₂ and 21% H₂S were studied at feed pressures up to 800 psig and temperatures up to 50 °C. This presentation will explore the relationships among feed gas pressure, temperature, and composition to understand how highly condensable plasticizing gases impact CTA hollow fiber performance in natural gas sweetening. In some feed gas pressure regimes, including the lowest feed pressures tested, the selectivity of the membrane for acid gases over hydrocarbons was essentially unaffected by plasticization. However, the selectivity of the membrane for acid gases over the hydrocarbons in the feed gas mixture was compromised as feed pressure was increased due CTA plasticization. This presentation will describe how feed pressure and temperature can be leveraged to optimize acid gas separation from hydrocarbons in natural gas sweetening.