Purification of Aggressive Supercritical Natural Gas Using Carbon

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Purification of aggressive supercritical natural gas using carbon molecular sieve hollow fiber membranes Chen Zhang, Graham B. Wenz, P. Jason Williams, Joseph M. Mayne, Gongping Liu, and William J. Koros Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03018 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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Industrial & Engineering Chemistry Research

Purification of aggressive supercritical natural gas using carbon molecular sieve hollow fiber membranes Chen Zhang a, Graham B. Wenz a, P. Jason Williams b, Joseph M. Mayne b, Gongping Liu ac, and William. J. Koros*a a

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta GA USA30332

b

Shell International Exploration and Production Inc., 3333 Highway 6 South, Houston TX USA 77082

c

State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing, China 210009

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ABSTRACT In this paper, we describe polyimide-derived carbon molecular sieve (CMS) hollow fiber membranes with CO2/CH4 separation factors ~60 under a supercritical (1800 psia) natural gas feed comprising 50% CO2 and 500 ppm highly condensable C7 hydrocarbons. Longterm tests extending for 200 hours proved membrane stability. Temperatures ranging from -50 to 100 oC were also tested and the membrane showed attractive performance under the diverse conditions studied. With attractive and stable separation performance, the CMS hollow fiber membranes studied in this work can potentially enable next-generation CO2 removal processes for challenging natural gas feeds.

KEYWORDS Natural gas purification, carbon molecular sieve membrane, hollow fiber membrane, supercritical fluids, membrane stability


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1. Introduction World natural gas production is expected to rise by over 50% by 2040, with U.S. shale gas production comprising a large portion of the growth.1-2 As the cleanest fossil fuel, natural gas emits 50% less carbon dioxide (CO2) than coal and is seen as “fuel for the 21st century”.3 The processing of raw natural gas into pipeline-quality product gas can comprise a significant portion of overall cost. Conventional natural gas processing plants use energy-intensive thermally-driven processes (e.g. amine absorption and fractional distillation) to remove impurities (e.g. CO2, hydrogen sulfide [H2S], nitrogen [N2], etc.) and recover heavy hydrocarbons (e.g. ethane, propane, butanes, etc.).4 Breakthroughs in natural gas processing technologies will clearly reduce the energy intensity and consequently environmental impacts of the natural gas industry.

Removing CO2 from natural gas using membranes is well-established.5-8 Membrane processing is particularly attractive for remote and offshore natural gas installations where low maintenance, flexibility, and reduced equipment footprints are highly desirable.9-10 Commercially-available membrane gas separators are predominantly derived from un-crosslinked polymers, with cellulose acetate being the most common. In addition to the permeability-selectivity trade-off11, another challenge associated with un-crosslinked polymers is plasticization-induced selectivity loss under aggressive feeds.12 Raw natural gas can comprise high level of CO2 and H2S, in addition to heavy hydrocarbons.10,

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In general, sorption of condensables causes increased

mobility of polymer chain segments, thereby increasing diffusivities of all penetrants (especially CH4) and reducing membrane permselectivity.14-15 For example, cellulose acetate loses 70% of its CO2/CH4 permselectivity when exposed to high pressure natural gas feeds comprising


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condensable hydrocarbons.16 The permselectivity loss reduces membrane CH4 recovery, and thus membrane separation becomes less attractive under these conditions.

Due to these limitations, scalable and more robust membranes are needed with attractive CO2/CH4 selectivity under aggressive natural gas feeds. Extensive efforts were made in the past two decades to improve plasticization resistance of polymer membranes. A notable class of advanced polymers with enhanced plasticization-resistance comprise crosslinked polyimides in which polymer swelling is suppressed via covalently formed linkages between polymer chains.1719

More recently, thermally-rearranged polymers20-21, polymers with intrinsic microporosity

(PIMs)22-23, hydroxyl-functionalized polyimides24, and mixed-matrix materials25-26 have also been reported to possess improved plasticization-resistance over commercially-available polymer membranes. Inorganic materials (e.g. zeolites, ceramics, carbon molecular sieves) and inorganicorganic hybrids (e.g. metal-organic frameworks) with more rigid framework generally provide good plasticization-resistance when exposed to aggressive feeds.27-28 Among these candidate materials, carbon molecular sieves (CMS) appear to offer the best balance between performance and scalability.29-32 CMS membranes are formed by controlled pyrolysis of polymer precursors. The unique pore structure of CMS offers excellent selectivity and permeability that well exceed the polymer upper bounds.33-38 The micropore “chambers” provide surface area for penetrant sorption as well as controllable penetrant jump lengths, and are formed by packing imperfections of disordered sp2-hybridized defective graphene-like plates. CMS ultramicropore “windows” provide entropically-enabled diffusion selectivities, and are primarily defects within the graphene-like plates. In a recent work37, it was shown that sorption selectivities in CMS can be tuned by forming ultramicropores and micropores that selectively exclude target penetrant


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molecules. CMS materials are derived from polymer precursors, thereby allowing translation into asymmetric hollow fibers for efficiently packed high-density hollow fiber modules.39-40

Existing work on high-pressure membrane-based natural gas purifications are usually evaluated at CO2 partial pressure

Purification of Aggressive Supercritical Natural Gas Using Carbon

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