High-Pressure CO 2 /CH 4 Separation Using SAPO-34 Membranes Shiguang Li, Janna G. Martinek, John L. Falconer,* and Richard D. Noble Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0424 Tracy Q. Gardner Department of Chemical and Petroleum Engineering, Colorado School of Mines, Golden, Colorado 80401 SAPO-34 membranes on stainless steel, tubular supports separated CO 2 from CH 4 at feed pressures up to 3.1 MPa. The highest CO 2 permeance was 2.4 × 10 -7 mol/(m 2 s Pa) for a 50/50 feed mixture at a pressure drop of 0.14 MPa. For a pressure drop of 3 MPa, the CO 2 /CH 4 separation selectivities at 253 K were 140-150; at lower pressure drops, the highest selectivity was 270. The highest CO 2 flux was 21 kg/(m 2 h) at 295 K and a pressure drop of 3 MPa. Separation selectivity decreased as temperature increased because separation was partly based on competitive adsorption. As transmembrane pressure drop increased, both CO 2 flux and CO 2 permeate concentration increased for a 50/50 mixture. The flux pressure dependence was modeled by Maxwell-Stefan diffusion for mixtures. Methane decreased the CO 2 diffusion rate and, thus, decreased the CO 2 flux. The CH 4 flux was also lower in a mixture because CO 2 inhibits CH 4 adsorption. 1. Introduction Carbon dioxide separation from CH 4 is important in natural gas processing because CO 2 reduces the energy content. Also, CO 2 is acidic and corrosive in the presence of water within the transportation and/or storage sys- tem. About 17% of natural gas in the United States is treated to remove CO 2 before it is passed to the pipeline. 1 Pipeline specifications for natural gas require a CO 2 concentration below 2-3%. 2 The technology most widely used for CO 2 removal is amine adsorption, but amine plants are complex and costly. 1 Membrane plants using CO 2 -selective cellulose acetate membranes were installed in the 1980s, and currently the largest mem- brane facility for CO 2 removal operates at 700 million scfd. The inlet CO 2 mole percentage is 36%, and the product specification is <16% CO 2 . 2 Polymeric membranes may be less stable for CO 2 /CH 4 separation at high CO 2 pressures, which can plasticize polymer membranes and decrease their separation ability. 3 Thus, microporous inorganic membranes with pore sizes between 0.2 and 0.8 nm have been studied for gas separation. They have superior thermal, me- chanical, and chemical stability, good erosion resistance, and high pressure stability. Microporous silica, 4 carbon molecular sieve, 5 and zeolite membranes 6,7 have been shown to separate CO 2 from CH 4 . Zeolites are inorganic crystalline structures with uniform-sized pores of molecular dimensions. Small-, medium-, and large-pore zeolites have been used to prepare membranes that separated CO 2 from CH 4. 6-13 Because both CO 2 (0.33 nm kinetic diameter) and CH 4 (0.38 nm) molecules are much smaller than the pores of large-pore and medium-pore zeolites, separation with these membranes was mainly based on competitive adsorption. For ZSM-5 membranes, the CO 2 /CH 4 sepa- ration selectivity at room temperature was 2.4-5.5. 8,14 For Y-type membranes, CO 2 /CH 4 separation selectivities were ∼10; 9 for X-type membranes, CO 2 /CH 4 separation selectivities as high as 28 were obtained. 6 In contrast, the small-pore molecular sieves such as zeolite T (0.41 nm pore diameter), DDR (0.36 × 0.44 nm), and SAPO-34 (0.38 nm) have pores that are similar in size to CH 4 but larger than CO 2 . High CO 2 /CH 4 selectivities were observed for these membranes due to a combination of differences in diffusivity and competi- tive adsorption. Cui et al., 15 using a T-type zeolite membrane, obtained a CO 2 /CH 4 composition selectivity of 400 and a CO 2 permeance of 4.6 × 10 -8 mol/(m 2 s Pa) at 308 K for a transmembrane pressure drop of 0.1 MPa and a vacuum on the permeate side. Tomita et al., 16 using DDR zeolite membranes on porous alumina tubes, obtained a CO 2 /CH 4 selectivity of 220 and a CO 2 permeance of 7 × 10 -8 mol/(m 2 s Pa) at 301 K for a pressure drop of 0.5 MPa. We have reported that SAPO-34 membranes selec- tively separate CO 2 from CH 4 . 7,11,12,17 The SAPO-34 is a silicoaluminophosphate having the composition Si x - Al y P z O 2 where x ) 0.01-0.98, y ) 0.01-0.60, z ) 0.01- 0.52, and x + z ) y. 18 Previous SAPO-34 membranes had a CO 2 /CH 4 selectivity of 67 and a CO 2 permeance of 1.6 × 10 -7 mol/(m 2 s Pa) at 297 K. 11 Adsorption isotherms showed that CO 2 adsorbs more strongly than CH 4 on SAPO-34 crystals, and thus, preferential ad- sorption of CO 2 is partially responsible for the CO 2 /CH 4 selectivity. These membranes also were selective in the presence of H 2 O, N 2 ,C 2 H 4 ,C 3 H 8 , and n-C 4 H 10 impuri- ties. With these five impurities in the feed, the CO 2 / CH 4 selectivity was 48 and the CO 2 permeance was 0.88 × 10 -7 mol/(m 2 s Pa) at 297 K. 12 Almost all studies of gas separations in zeolite mem- branes have used relatively low feed pressures, but feed pressures may be as high as 7 MPa in industrial applications. 1 Methane permeance for T-type mem- branes increased with increasing pressure drop, indicat- ing that viscous flow contributed to CH 4 flux. 16 As the feed pressure increased from 0.1 to 0.5 MPa at 308 K, the CO 2 /CH 4 selectivity in T-type membranes decreased * Corresponding author. E-mail: john.falconer@colorado.edu. Tel.: (303) 492-8005. Fax: (303) 492-4341. 3220 Ind. Eng. Chem. Res. 2005, 44, 3220-3228 10.1021/ie0490177 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/15/2005