Upgrading low-quality natural gas with H2S- and CO2-selective polymer membranes -: Part II.: Process design, economics, and sensitivity study of membrane stages with recycle streams

The cost of membrane separation processes for removing CO2 and H2S from low-quality natural gas can be reduced for some concentration ranges of CO2 and H2S by utilizing concurrently two different types of polymer membranes, one with a high CO2/CH4 selectivity and the other with a high H2S/CH4 selectivity. The polymers considered in this exploratory study were 6FDA-HAB polyimide for the removal Of CO2 and [poly(ether urethane urea)] (PEUU) for the removal of H2S. It was required that the concentrations Of CO2 and H2S in low-quality natural gas be reduced to US pipeline specifications ( <= 2 mol% CO2 and <= 4 ppm H2S). Low-quality natural gas was simulated in this study by CH4/CO2/H2S mixtures containing up to 40 mol% CO2 and 10 mol% H2S. Twenty-seven membrane process configurations (PCs) were examined by computer simulations and optimized in order to determine the most economical configurations. Part I of this study considered only PCs without recycle streams [J. Hao, P.A. Rice, S.A. Stern, Upgrading low-quality natural gas with H2S- and CO2-selective polymer membranes. Part I. Process design and economics of membrane stages without recycle streams, J. Membr. Sci. 209 (2002) 177-206]. In Part II, reported below, the study was extended to two- and three-stage PCs with various recycle options. A sensitivity analysis was also made to determine the effects of variations in feed flow rate, feed pressure, membrane module cost, and wellhead price of natural gas on process economics. The economically optimal PCs were found to be either two membrane stages connected in series with or without recycle streams or single stages without recycle, depending on feed composition and selected operating conditions. The optimal two-stage PCs with recycle streams would utilize the H2S/CH4 -selective membranes in the first stage and either the CO2/CH4 or the H2S/CH4 -selective membranes, or both, in the second stage. Three-stage membrane PCs were not found to be economically competitive under the conditions assumed in this study. (C) 2008 Elsevier B.V. All rights reserved.