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Adv. Mater. 2006, 18, 39–44

DOI: 10.1002/adma.200501409

High-Performance Polymer Membranes for Natural-Gas Sweetening** By Haiqing Lin, Elizabeth Van Wagner, Roy Raharjo, Benny D. Freeman,* and Ian Roman

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[3] R. Z. Valiev, E. V. Kozlov, Yu. F. Ivanov, J. Lian, A. A. Nazarov, B. Baudelet, Acta Metall. Mater. 1994, 42, 2467. [4] R. Z. Valiev, A. V. Sergueeva, A. K. Mukherjee, Scr. Mater. 2003, 49, 669. [5] N. Q. Chinh, G. Vörös, P. Szommer, Z. Horita, T. G. Langdon, Mater. Sci. Forum 2006, 503–504, 1001. [6] N. Q. Chinh, J. Gubicza, Zs. Kovács, J. Lendvai, J. Mater. Res. 2004, 19, 31. [7] J. Man, K. Obrtlík, J. Polák, Mater. Sci. Eng. 2003, A351, 123. [8] J. Man, M. Petrenec, K. Obrtlík, J. Polák, Acta Mater. 2004, 52, 5551. [9] Y. Choi, H.-S. Lee, D. Kwon, J. Mater. Res. 2004, 19, 3307. [10] N. Q. Chinh, G. Horváth, Z. Horita, T. G. Langdon, Acta Mater. 2004, 52, 3555. [11] R. C. Gifkins, T. G. Langdon, J. Inst. Met. 1964, 93, 347. [12] A. Vinogradov, S. Hashimoto, V. Patlan, K. Kitagawa, Mater. Sci. Eng. 2001, A319–321, 862. [13] Y. Huang, T. G. Langdon, Mater. Sci. Eng. 2003, A358, 114. [14] H. Hahn, K. Padmanabhan, Philos. Mag. B 1997, 76, 559. [15] H. Van Swygenhoven, A. Caro, Appl. Phys. Lett. 1997, 71, 1652. [16] J. Schiøtz, F. D. Di Tolla, K. W. Jacobsen, Nature 1998, 391, 561. [17] H. Van Swygenhoven, M. Spaczer, A. Caro, D. Farkas, Phys. Rev. B 1999, 60, 22. [18] H. Van Swygenhoven, P. M. Derlet, Phys. Rev. B 2001, 64, 224 105. [19] H. Van Swygenhoven, A. Caro, D. Farkas, Mater. Sci. Eng. 2001, A309–310, 440. [20] H. Conrad, J. Narayan, Scr. Mater. 2000, 42, 1025. [21] V. Yamakov, D. Wolf, S. R. Phillpot, A. K. Mukherjee, H. Gleiter, Nat. Mater. 2002, 1, 45. [22] A. Hasnaoui, H. Van Swygenhoven, P. M. Derlet, Phys. Rev. B 2002, 66, 184 112. [23] J. Markmann, P. Bunzel, H. Rösner, K. W. Liu, K. A. Padmanabhan, R. Birringer, H. Gleiter, J. Weissmüller, Scr. Mater. 2003, 49, 637. [24] R. J. Asaro, P. Krysl, B. Kad, Philos. Mag. Lett. 2003, 83, 733. [25] Z. Shan, E. A. Stach, J. M. K. Wiezorek, J. A. Knapp, D. M. Follstaedt, S. X. Mao, Science 2004, 305, 654. [26] Y. T. Zhu, X. Z. Liao, R. Z. Valiev, Appl. Phys. Lett. 2005, 86, 103 112. [27] R. J. Asaro, S. Suresh, Acta Mater. 2005, 53, 3369. [28] R. Z. Valiev, Adv. Eng. Mater. 2003, 5, 296. [29] C. Y. Yu, P. L. Sun, P. W. Kao, C. P. Chang, Scr. Mater. 2005, 52, 359. [30] H. Conrad, K. Jung, Scr. Mater. 2005, 53, 581. [31] J. Weissmüller, J. Markmann, Adv. Eng. Mater. 2005, 7, 202. [32] Y. M. Wang, E. Ma, M. W. Chen, Appl. Phys. Lett. 2002, 80, 2395. [33] R. Z. Valiev, Nat. Mater. 2004, 3, 511. [34] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T. G. Langdon, Scr. Mater. 1996, 35, 143. [35] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T. G. Langdon, Mater. Sci. Eng. A 1998, 257, 328. [36] K. Oh-ishi, Z. Horita, M. Furukawa, M. Nemoto, T. G. Langdon, Metall. Mater. Trans. A 1998, 29, 2011. [37] Y. Iwahashi, Z. Horita, M. Nemoto, T. G. Langdon, Acta Mater. 1998, 46, 3317.

Polymers containing ethylene oxide units have attracted significant scientific interest since ether oxygen linkages lead to flexible polymer chains and specific interactions with metal ions,[1] polar molecules such as H2O and H2S,[2] and quadrupolar molecules such as CO2.[3] Amorphous polymers containing poly(ethylene oxide) (PEO) have been studied extensively in applications such as high-energy density batteries[4] and drug delivery.[5] Herein, we demonstrate a strategy to design rubbery membrane materials for the removal of acid gases such as CO2 and H2S from natural gas (mainly CH4) using a highly branched, crosslinked PEO hydrogel. Unlike conventional size-sieving membrane materials, which achieve high permeability selectivity mainly via high diffusivity selectivity,[6] these polar rubbery membrane materials exhibit high CO2 permeability, high CO2/CH4 mixed-gas selectivity, and excellent stability to contaminants in natural gas due to high gas diffusivity and high CO2/CH4 solubility selectivity. Natural gas is a vital energy source, and approximately 20 % contains excess CO2 which must be removed to meet pipeline specifications.[6] Due to the large volume of natural gas produced annually (6.8 × 1011 m3 (at standard temperature and pressure, STP) in 2003 in the US alone[7]), even small improvements in CO2-removal efficiency could lead to considerable cost reductions in purifying natural gas. Membrane technology has attracted interest for this application, especially in remote areas such as off-shore platforms,[6] because membrane units are compact (they have a small footprint), energy

– [*] Dr. B. D. Freeman, Dr. H. Lin,[+] E. Van Wagner, R. Raharjo Center for Energy and Environmental Resources Department of Chemical Engineering The University of Texas at Austin 10100 Burnet Road, Building 133, Austin, TX 78758 (USA) E-mail: [email protected] I. Roman MEDAL L.P., Willow Bank Plant 305 Water Street, Newport, DE 19804 (USA) [+] Present address: Membrane Technology and Research Inc., 1360 Willow Road, Suite 103, Menlo Park, CA 94025, USA. [**] We gratefully acknowledge partial support of this work by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (Grant No. DE-FG03-02ER15362). This research was also partially supported by the United States Department of Energy’s National Energy Technology Laboratory under a subcontract from Research Triangle Institute through their Prime Contract No. DE-AC2699FT40675. Partial support from the National Science Foundation under grant number CTS-0515425 is also acknowledged.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

39

(1)

where NA is the steady state flux, p2 and p1 are the upstream and downstream pressures, respectively, and x2A and x1A are the mole fractions of gas A at the upstream and downstream faces of the membrane, respectively. f1A and f2A are the fugacity coefficients of gas A at the upstream and downstream faces of the membrane, respectively, which account for the influence of gas phase non-idealities on the driving force for gas permeation. If Fickian diffusion is the rate-controlling step for gas permeation, and p2 x2Af2A is much greater than p1 x1Af1A, PA is given by[6] PA ˆ SA × DA

aA=B

www.advmat.de

40

30 XLPEO 20

pure gas

10

mixed gas

0 0

5

10

15

20

CO Partial Pressure [atm] 2

(3)

where SA/SB is the solubility selectivity and DA/DB is the diffusivity (or mobility) selectivity. Solubility selectivity is often controlled by the relative condensability of gases A and B and the relative affinity of gases A and B for the polymer; diffusivity selectivity is governed mainly by relative penetrant size and the size-sieving ability of the polymer.[16] In CO2/CH4 separation, CO2 is more condensable (as characterized by a higher critical temperature) and more mobile (as characterized by a smaller kinetic diameter) than CH4.[16] Therefore, membranes favor CO2 permeation over CH4 since both CO2 solubility and diffusivity are higher than those of CH4. For this reason, purified CH4 is produced at or near feed pressure, so expensive recompression of CH4 to pipeline pressure is minimized or avoided. Very permeable and highly selective membrane materials are desired for natural-gas purification. During the last two decades, research has focused primarily on polymer materials that achieve high overall selectivity via high CO2/CH4 diffusivity selectivity.[6,14] Such materials are rigid, glassy polymers

40

35ºC 6FDA-mPD (mixed gas)

(2)

where SA is the gas solubility and DA is the effective, average gas-diffusion coefficient in the polymer. The ideal selectivity of gas A over gas B in a polymer, aA/B, is given by[6,15] P S D ˆ Aˆ A × A PB SB DB

50

4

NA l p2 x2A f2A p1 x1A f1A

2

PA ˆ

with strong size-sieving ability, including cellulose acetate and various polyimides.[6] However, an inherent limitation of this approach is a trade-off between permeability and selectivity (higher selectivity often results in lower permeability) which was demonstrated empirically by Robeson[17] and modeled by Freeman.[18] More importantly, raw natural gas often contains CO2, H2S, H2O, and other hydrocarbons (i.e., C2H6, C3H8, n-C4H10, and higher aromatic and aliphatic hydrocarbons),[2,6] which are condensable and, potentially, strongly sorbed by polymers. The dissolved penetrants sorb into the polymer, increasing overall system free volume and polymer-chain mobility; that is, they plasticize the polymer. This effect can be observed, in some cases, by a significant depression of the glass–rubber transition temperature, Tg,[19,20] and a decrease in the size-sieving ability of the polymer matrix, which reduces permeability selectivity.[11,14,21] Figure 1 presents the effect of CO2 partial pressure on mixed-gas CO2/CH4 selectivity in a glassy polyimide (6FDA-mPD, 4,4′-(hexafluoroisopro-

CO /CH

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efficient (no phase change is required to achieve separation), and reliable (no moving parts).[8–10] However, membranes are typically used for the separation only after the natural gas has been subjected to rigorous and energy-intensive pretreatment steps to remove components (e.g., higher hydrocarbons)[6] that could plasticize the membranes and compromise their separation performance.[11] High partial pressures of CO2 in natural gas can also induce plasticization and reduce selectivity.[12–14] This work explores new families of membrane materials that would exhibit good separation properties and be more robust than conventional materials, potentially reducing the need for rigorous feed pretreatment prior to acid-gas removal. The steady-state permeability of gas A, PA, through a nonporous polymeric film of thickness l is defined as[6,15]

Figure 1. Effect of CO2 partial pressure on pure and mixed-gas CO2/CH4 selectivity at 35 °C in XLPEO (a copolymer of 30 wt.-% poly(ethylene glycol) diacrylate and 70 wt.-% poly(ethylene glycol) methyl ether acrylate) and in a polyimide, 6FDA-mPD [14]. The CO2/CH4 feed gas mixture was 50:50 for the polyimide. The feed gas mixtures for XLPEO were 10:90 (䊉), 50:50 (~), and 80:20 (䉲) (mol/mol) CO2/CH4. The lines serve as guides to the eye. While these results are reported as a function of CO2 partial pressure, the permeability and selectivity are based on calculations employing Equation 1, which properly accounts for the influence of gas phase non-idealities on the results.

pylidene)diphthalic anhydride m-phenylene diamine) at 35 °C.[14] At low CO2 partial pressure, the polyimide exhibits high CO2/CH4 selectivity due to high diffusivity selectivity. However, the selectivity decreases by more than an order of magnitude at a CO2 partial pressure of 10 atm[14] (1 atm ∼ 101 kPa), which is in the range of typical CO2 partial pressures in natural gas. While this particular polyimide is particularly sensitive to losses in selectivity with increasing CO2 partial pressure, and other polyimides are reported to be somewhat more resistant to this phenomenon,[13] this problem

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adv. Mater. 2006, 18, 39–44

Adv. Mater. 2006, 18, 39–44

Table 1. Pure and mixed-gas CO2/CH4 separation properties in a strongly size-selective aromatic polyimide (6FDA-mPD), cellulose acetate (CA), and XLPEO containing 30 wt.-% PEGDA and the balance PEGMEA. Polymer

6FDA-mPD [14] CA [21,25] XLPEO

Temp. [°C]

PCO2 [a] [Barrer]

35 35 35 –20

12 7.2 420 130

SCO2 =SCH4 [c]

aCO2 =CH4 [b] Pure Gas

Mixed Gas

40 36 18 350