Article pubs.acs.org/Macromolecules
Cross-Linkable Polyimide Membranes for Improved Plasticization Resistance and Permselectivity in Sour Gas Separations Brian Kraftschik and William J. Koros* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Dr. NW, Atlanta, Georgia 30332-0100, United States S Supporting Information *
ABSTRACT: A series of cross-linkable membrane materials based on the 6FDA-DAM:DABA (3:2) polyimide backbone were synthesized for improved sour gas separation performance, in terms of both membrane stability and permselectivity. Short-chain poly(ethylene glycol) (PEG) molecules were used as cross-linking agents in an esterification-based cross-linking reaction. Pure and mixed gas permeation and pure gas sorption experiments were performed on dense films of these materials. Compared to unmodified 6FDA-DAM:DABA (3:2), higher sour gas permselectivity and membrane stability were achieved under aggressive feed conditions. H2S-induced plasticization was not evident until pure H2S feed pressures greater than approximately 6−8 bar. Pure CO2-induced plasticization only occurred at feed pressures greater than about 25 bar. Under mixed gas feed conditions with 20% H2S, 20% CO2, and 60% CH4 at 35 °C, attractive selectivities above 22 and 27 for H2S/CH4 and CO2/CH4, respectively, were observed for a feed pressure of 62 bar with both triethylene glycol and tetraethylene glycol cross-linking agents.
1. INTRODUCTION According to the Energy Information Administration’s International Energy Outlook 2011, global consumption of natural gas is expected to rise to 152% of its 2008 level by the year 2035.1 In order to meet this anticipated surge in demand, previously unproduced sources of natural gas will have to be exploited. Unconventional natural gas sources, like shale gas, represent one of the major opportunities for expanding the global natural gas supply over the next few decades. Sour gas, which may constitute up to 40% of remaining worldwide natural gas reserves, represents another major opportunity.2−5 Reserves of sour gasnatural gas containing elevated levels of hydrogen sulfide (H2S)have typically been passed over in favor of more economical and technologically feasible opportunities. However, the combination of rising natural gas demand and depletion of higher quality reserves makes it likely that sour gas will play a much more significant role in satisfying natural gas demand in the coming years. Natural gas reserves need to be upgraded through the removal of contaminant species before pipeline transport can occur. A wide range of contaminants exist, including water, carbon dioxide (CO2), higher hydrocarbons, nitrogen, and other inert gases. H2S is another frequently encountered natural gas contaminant, which presents unique challenges due to its high toxicity, flammability, and contribution to SOx emissions when combusted. Both H2S and CO2, referred to as acid gases, can cause corrosion of pipelines and other natural gas processing equipment, reduce the heating value of natural gas, and increase compression and transportation costs. As © 2013 American Chemical Society
such, the concentration of H2S, CO2, and other contaminants in produced natural gas must be reduced to below fixed levels before injection into transportation pipelines. Typical pipeline specifications for the US are shown in Table 1. Table 1. Pipeline Specifications for Natural Gas in the US component
specification
CO2 H2O H2S C3+ content total inerts (N2, He, etc.)
