Performance of natural gas membranes in the presence of heavy hydrocarbons

Intrinsically defect-free asymmetric hollow fiber polyimide membrane modules were studied in the presence and absence of saturated and aromatic components. Under some conditions for the ternary system, the permselectivity of the membrane is scarcely affected, while under other conditions, permselectivity is negatively affected by as much as 20%. In most cases, for the ternary feeds, significant depression in fluxes was observed due to competition between the CO2, CH4 and heavier hydrocarbons, but the effect was even more pronounced for the toluene. In addition to steady state tests in the presence and absence of n-heptane and toluene, modules were conditioned for 5 days with ternary mixture of CO2, CH4 and one or the other of these heavy hydrocarbons. Following this conditioning process, the modules were studied with a simple binary 10% CO2/90% CH4 mixture. These conditioning studies provide insight into the fundamental effects induced in the membrane due to the long-term exposure to the complex mixtures. Following exposure to the ternaries containing n-heptane, negligible CO2 permeance increase was seen, while significantly increased permeances were seen under some conditions following toluene exposure even at low pressures of the ternary toluene/CO2/CH4 conditioning gas mixture. Although a more protracted process occurs in the case of n-heptane/CO2/CH4 at 35 °C and 500 ppm, a significant loss in selectivity does still occur in the actual ternary tests after exposure for 5 days. The problem caused by 300 ppm toluene at 35 °C is more immediately apparent than for n-heptane exposure, but the ultimate selectivity loss is similar for both hydrocarbons. In addition to the selectivity, in the presence of toluene the permeability is also depressed significantly, presumably due to a greater capability of toluene to compete for added free volume elements introduced in the conditioning process. The conditioning treatment has negligible effect at 55 °C, suggesting that that the sorption affinity of toluene decreases with increasing temperature.Results also suggest that an essentially defect-free, non-nodular morphology offers advantages in stability under demanding operating conditions. Earlier work [G. Djoekita, Characterization and Analysis of Asymmetric Hollow Fiber Membranes for Natural Gas Purification in the Presence of Hydrocarbons, The University of Texas at Austin, 2000] showed serious losses in performance of membranes comprised of similar materials, when the selective layer had a pronounced fused nodular nature as opposed to the intrinsically defect-free skin layers reported on here.