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Polybenzimidazole composite membranes for high temperature synthesis gas separations
Journal of Membrane Science 415–416 (2012) 265–270
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Polybenzimidazole composite membranes for high temperature synthesis gas separations Kathryn A. Berchtold a,n, Rajinder P. Singh a, Jennifer S. Young b, Kevin W. Dudeck a a
Materials Physics and Applications-Materials Chemistry, Carbon Capture and Separations for Energy Applications (CaSEA) Labs, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b Applied Physics Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
a r t i c l e i n f o
abstract
Article history: Received 5 February 2012 Received in revised form 27 April 2012 Accepted 2 May 2012 Available online 10 May 2012
High temperature gas separation techniques are of great interest for reduction in green-house gas emissions from hydrocarbon fuels such as natural gas, coal or biomass used in power and chemical industry. In this work, a robust industrially viable polybenzimidazole (PBI)/stainless steel composite membrane is developed and evaluated for syngas separations at elevated temperatures for H2 production. A single tube laboratory scale PBI membrane module is tested for H2/CO2 perm-selectivity in pure and simulated dry syngas environments at industrially relevant operating conditions. Additionally, the effects of pressure and temperature on membrane performance are evaluated. The PBI composite membrane demonstrated exceptional long term thermo-chemical stability in the syngas environment even in the presence of H2S and excellent separation performance for H2 over the other syngas components. The H2 permeance and H2/CO2 selectivity for the PBI composite membrane in simulated dry syngas was recorded at 7 GPU (approximately, 88 barrer) and 47, respectively. In comparison to the other H2-selective polymeric membranes, the PBI composite membrane’s performance exceeded the 2008 Robeson upper bound for the H2/CO2 permeability versus selectivity. & 2012 Elsevier B.V. All rights reserved.
Keywords: Polybenzimidazole thin films Organic-inorganic composites Pre-combustion carbon capture Synthesis gas Hydrogen separation membrane IGCC
1. Introduction H2 is the cleanest energy carrier with water being the only product of its combustion [1]. It can be combusted in fuel cells and gas turbines with zero or near zero emissions at a higher efficiency than existing industry standard coal power plants and internal combustion engines. Therefore, transportation and stationary power generation through H2 combustion is highly desirable for energy security, reduced carbon emissions, and efficient natural resource utilization. Besides an energy carrier, numerous other uses of H2 in the chemical industry make it an important commodity. It is used in numerous hydrogenation reactions; in annealing environments for metal processing; for catalyst and sorbent regeneration; and in methanol and ammonia production [2,3]. The demand for H2 is expected to grow significantly in the future to meet the growing demands of utility, chemicals, and fuels industries as we transition into a more environmentally conscious economy. Industrially, natural gas and oil are the primary fuel sources for H2 production, accounting for approximately 80% of the H2 produced daily [2]. H2 production from solid fuel sources such
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as coal and biomass via gasification is also a viable and sustainable alternative. The abundance of these sources makes gasification an ideal path forward for large scale H2 production, especially for power generation. While combustion of H2 is relatively environmentally benign, its production from hydrocarbon fuel sources is not inherently environmental friendly. The simultaneous release of carbon as CO2 upon hydrocarbon fuel processing, is a major environmental concern. CO2 is a greenhouse gas and its presence at historically high concentrations in the atmosphere is linked to global warming and expected to play a dominant role in further increasing average global temperatures by approximately 1.1 to 6.4 1C by the year 2100 [4]. Hence, separation, capture, and sequestration of CO2 produced during hydrocarbon fuel conversion to energy is essential. Typically advanced hydrocarbon fuel processing schemes with integrated carbon capture involves synthesis gas (syngas) production supplemented with shift conversion [5]. Syngas is a mixture of H2 and CO with trace quantities of sulfur compounds, ammonia, metals and particulate matter. After undergoing particulate and sulfur removal, syngas is passed over one or more water–gas shift converters where CO reacts with steam to produce H2 and CO2. In pre-combustion carbon capture pathway, CO2 is separated from syngas prior to its combustion in fuel cells or gas turbines. The captured CO2 can then be compressed and transported for use or sequestration [6].
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While the separation of CO2 from process streams is readily accomplished via standard separation techniques such as pressure swing adsorption and amine scrubbing, the effectiveness of these technologies for separating CO2 on large scale is limited [7–10]. These techniques operate at ambient, sub-ambient, or near ambient temperatures and typically produce a low pressure CO2 stream, resulting in a significant energy penalty for CO2 sequestration. In contrast, gas separation membranes can be economically applied for CO2 separation from syngas since they involve no phase change, are less energy intensive, and typically provide low maintenance operations [8–11]. Polymeric membranes have been used successfully in a number of industrial gas separation/purification applications, including the production of high-purity nitrogen, gas dehydration, removal of acid gases and recovery of hydrogen from process streams for recycle [12–15]. Since the economic benefits of using a membrane separation system are strongly tied to efficient process integration, membrane separation systems with the ability to withstand industrially relevant syngas operating conditions in the presence of the chemically challenging syngas components are desirable [16]. Unfortunately, the commercially available polymeric materials currently employed in the gas separation applications are not stable in pre-combustion syngas environments and operating conditions to the degree required. These membrane materials are often subject to chemical degradation by minor components in the process stream, a problem that is exacerbated at elevated temperatures. Additionally, as operating temperatures approach the glass transition temperature of a membrane material, membrane selectivity and flux are significantly altered, reducing membrane separation efficiency. Consequently, there is a compelling need for membrane materials and subsequently capture systems based on those materials that can operate under more extreme environmental conditions for extended periods of time while providing a level of performance that is economically sustainable by the end user. Heterocyclic PBI-based materials are thermally stable up to temperatures approaching 500 1C owing to their rigid rod like molecular structure. Their glass transition temperatures are often above 450 1C ( 460–500 1C) and typically in close proximity to their degradation temperatures. PBI-based materials possess excellent chemical stability, good mechanical properties and an appropriate level of processability for use in extreme chemical and thermal environments. These properties have led many researchers to investigate the PBI-based materials for applications with operating conditions well outside the typical realm of organic materials. These applications include as electrolyte membranes for fuel cells [17] and gas separation membranes [18–24]. These previous studies on gas separation characteristics of PBI-based materials demonstrate their excellent size sieving ability and potential for small molecule separations such as H2 from CO2. In this work, we report development and characterization of a novel PBI-based composite membrane for syngas separation at elevated temperatures. The perm-selectivity characteristics of the PBI composite membrane are evaluated under industrially relevant realistic operating conditions in simulated dry syngas with a focus on membrane long term durability and stability.
2. Experimental section 2.1. Membrane fabrication The PBI material used in this study is poly(5,50 -benzimida zole-2,20 -diyl-1,3-phenylene). Fig. 1 shows the structure of PBI. A 26.2 wt% PBI solution in dimethyacetamide (DMAC) containing 2 wt% lithium chloride as a phase stabilizer (PBI Performance Products Inc., Columbia, SC) was used for membrane formation. The PBI composite membrane was provided by Pall Corporation,
Fig. 1. Molecular structure of PBI: polybenzimidazole, poly(5,50 -benzimidazole2,20 -diyl-1,3-phenylene).
Fig. 2. Picture of the PBI/zirconia/stainless steel composite membrane and membrane module used in this study.
Cortland, NY and consists of a thin PBI layer, deposited by a proprietary process on the outer surface of a porous stainless steel support with a zirconia intermediate layer [25]. 2.2. Transport measurement The PBI membrane was installed in a stainless steel housing using stainless steel compression fittings resulting in a laboratory scale test module. The module was configured to allow for feed gas flow down the outer diameter of the membrane and use of sweep or vacuum on the permeate side of the module housing for permeance measurement. Fig. 2 illustrates a coated tube alongside the module housing. The PBI composite membrane data presented here was obtained on a single-tube module with a membrane surface area of approximately 31 cm2. The pure gas permeation experiments were performed using a constant-volume/variable-pressure method at a differential pressure of 50 psi with H2, CO2, CH4 and N2. The membrane operating temperature was varied from 150 to 250 1C. The upstream and downstream pressures were measured using high accuracy (70.25% FS) pressure transducers (MKS Instruments, Inc.). The permeation rate was calculated from the slope of the linear part of the permeate pressure rise versus time curve using Eq. (1). dp VT 0 Permeance, P ¼ , ð1Þ dt P 0 T DpA where dp/dt is the pressure rise; V is the downstream volume; Dp is the pressure difference between membrane upstream and downstream side; T0 and P0 are the standard temperature and pressure, respectively, and A is the effective membrane surface area. The ideal selectivity for a gas pair is calculated by taking the ratio of their gas permeances. The mixed gas experiments were performed using the same system in a steady state configuration using a sweep gas. The simulated dry syngas feed mixture containing H2: 55%; CO2: 41%; CO: 1%; CH4: 1%; N2: 1% and H2S: 1% (volume basis) was prepared using mass flow controllers with total feed flow-rate set at 20 sccm. Argon was used as a sweep gas with flow rate set at 5 sccm. The mixed gas data were obtained at 250 1C and 50 psid. The permeances of the feed gas components were computed from the total permeate flow rate measured using a mass flow meter
K.A. Berchtold et al. / Journal of Membrane Science 415–416 (2012) 265–270
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and their concentration in the permeate stream measured by a micro-GC (Agilent 3000). The selectivity is calculated from the ratio of permeances. For all the gas permeation experiments, the gases used were at least 99.9% pure.
3. Results and discussions 3.1. Pure gas permeance The pure gas permselectivity of the PBI composite membrane tested at 50 psid and 250 1C is reported in Table 1. H2 permeance of the PBI composite membrane is 4.67 GPU (4.67 10 6 cm3 cm 2 s 1 cm Hg 1) and the H2/CO2 and H2/N2 ideal selectivities are 43 and 233, respectively. Due to the composite structure of the membrane and potential penetration into support pores, direct determination of membrane thickness is complex. The composite membrane effective thickness is estimated using flat sheet membrane nitrogen permeability at ambient conditions. Thus, the thickness and pure gas H2 permeability of the PBI composite membrane tested here are estimated to be 12.5 mm and 58 barrer (58 10 10 cm3 cm cm 2 s 1 cm Hg 1), respectively. Generally, in polymeric membranes the permeance (or permeability) is represented as the combination of diffusivity and solubility (Eq. (2)) [26]. The diffusivity follows an inverse trend to the size of the permeating gases whereas solubility of gases in the membrane material follows a linear trend with the condensability of the permeating gas. This general principle of gas transport in polymer membranes can be used to understand the perm-selective behavior of polymeric membranes for H2–CO2 separation, two major syngas components. CO2 (critical temperature, Tc ¼ 304 K) polymer solubility is typically significantly higher than that of less condensable permanent gases such as H2 (Tc ¼ 33 K) however diffusivity favors H2 (kinetic diameter¼ 2.89 A˚ [27]) over CO2 (kinetic diameter¼3.30 A˚ [27]). Therefore, to be an efficient H2 selective membrane material, gas transport and selectivity in the polymer must be dominated by diffusivity or size sieving ability commonly referred to as molecular sieving. The high perm-selectivity character of the PBI membrane towards H2 is indicative of a dominant molecular sieving gas transport mechanism. The PBI membrane gas permeance behavior correlates well with gas molecules kinetic ˚ ˚ ˚ ˚ diameter (H2 (kinetic diameter: 2.89 A)4CO 2 (3.3 A)4N2 (3.64 A)4CH4 (3.8 A)). The interplay between diffusivity and solubility as a function of operating conditions can be optimized to enhance the performance of a membrane towards particular separation. Ideally, for H2 separation from syngas, high diffusivity and low solubility is desirable as discussed above. As shown by Eqs. (3) and (4), the diffusivity increases with temperature whereas solubility decreases with temperature. At operating temperatures range of 150 to 250 1C for syngas separations, solubility is not expected to play a significant role. However, the effect of operating temperature on diffusivity can be significant and can be applied as a tool to optimize membrane performance. The H2/CO2 selectivity and permeability of the PBI membrane as a function of temperature are shown in Figs. 3 and 4. Both the H2 and CO2 permeance increases with a temperature increase from 150 to 250 1C consistent with diffusivity dominated gas transport. The PBI membrane H2 permeance approximately doubles as operating temperature is increased over the same temperature range. The permeation activation energies (Ep), calculated from this change in permeance with temperature are reported in Table 1. The activation energies for CO2 and H2 are 26.3 and 18.5 kJ mol 1, respectively leading to an increase in the permeance of both gases with increasing temperature accompanied by a decrease in selectivity. The PBI membrane H2/CO2 ideal selectivity decreased from 58 to 40 as the process temperature increased from 150 to 250 1C. Permeability,P ¼ DnS
ð2Þ
Table 1 Pure and mixed gas permeances and separation factors for the PBI composite membrane tested at 250 1C. Ep is the activation energy of the permeance obtained from the slope of permeance versus inverse of temperature. The number in the parenthesis is the standard error in the values of Ep, reflecting the error in slope determination by curve fitting. Gas Pure gas permeance (GPU)
Pure gas selectivity (H2/Gas)
Mixed gas permeance (GPU)
Mixed gas selectivity (H2/Gas)
Ep (kJ mol 1)
H2 CO2 CH4 N2 H2S CO
– 43.0 387 233 – –
7.000 0.147 0.030 – 0.005* 0.070
– 47.60 233.6 – 1289 99.50
18.5 26.3 23.4 18.4 – –
4.670 0.108 0.012 0.020 – –
(0.7) (2.0) (7.1) (10)
n H2S signal was below the detection limit of our analytical instrument. The permeance reported for H2S is the detection limit of our analytical instrument.
Fig. 3. Effect of permeation temperature on H2/CO2 ideal selectivity of the PBI composite membrane. The line is drawn to guide the eye.
Fig. 4. Effect of permeation temperature on pure gas permeance of the PBI composite membrane. The lines are drawn to guide the eye. Diffusivity, D ¼ D0 eEa =RT ,
ð3Þ
Solubility, S ¼ S0 eDH=RT
ð4Þ
where Ea activation energy of diffusion; DH is the heat of sorption; D0 and S0 are the pre-exponential factors for diffusivity and solubility; R is the universal gas constant; and T is the operating temperature. The ability of PBI membrane to retain its high H2/CO2 selectivity at temperatures up to 250 1C demonstrates the extreme thermal resilience of the PBI macromolecular structure. Typically in polymeric membranes, as temperature increases, the free volume created by the close packing of polymer chains increases due to the enhanced thermal motion of the polymer chains. This increase in polymer free volume renders polymer materials unable to distinguish between the molecules with very small size difference such as H2 and CO2. For example, polypyrrolone membranes, a rigid glassy polymer, tested by Costello et al. [28] showed a significant drop in He/CH4 separation factor from 150 to 30 as the permeation temperature was increased from 35 to 200 1C. Although, Tg of polypyrrolone is 380 1C, significantly higher than the test temperature of 200 1C, a change in the specific volume of the polypyrrolone due to the secondary thermal transition at approximately 130 1C is cited as the reason for the reduction in the molecular sieving ability of polypyrrolone at elevated temperatures. Unlike polypyrrolone, PBI showed an extreme resilience to temperature and maintained its industrially attractive molecular sieving character even at elevated temperatures exceeding 150 1C. PBI’s rigid rod like molecular structure together with the h-bonding between the chains is hypothesized to be the main reason for its exceptional size sieving ability at elevated temperatures.
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In contrast to the temperature effects, increasing the feed pressure has no significant effect on the PBI membrane permeance. As shown in Fig. 5, the H2 permeance is approximately constant when transmembrane pressure is increased from 29 to 59 psi. This result indicates the absence of viscous flow and defects in the membrane module selective layer.
3.2. Mixed gas results The pure gas permeation results discussed above indicate that PBI is an excellent H2 selective membrane material for syngas separations. Inspired by the pure gas permeance data, the PBI composite membrane was exposed to a simulated dry syngas stream containing H2: 55%; CO2: 41%; CO: 1%; CH4: 1%; N2: 1% and H2S: 1% (volume basis) at 250 1C and 50 psid. The results of the mixed gas testing are shown in Table 1 and Fig. 6. A H2 permeance and H2/CO2 selectivity of 7 GPU (88 barrer, estimated) and 47, respectively, were observed during mixed gas testing conducted at 250 1C. The gas permeances during mixed gas experiments are slightly higher than the pure gas permeances with nearly identical H2/CO2 selectivity. The reason for the higher mixed gas H2 permeance is not known at this time. It can be an experimental artifact of using sweep gas and vacuum on the permeate side of the membrane in the mixed and pure gas experiments, respectively. The feed pressure in both pure and mixed gas experiments is identical, however in pure gas experiments the PBI composite membrane is subjected to higher total pressure differential due to lower permeate pressures than that compared to mixed gas experiments. This
higher total differential pressure can potentially lead to a greater degree of the PBI membrane compaction in pure gas experiments than mixed gas experiments thereby reducing the gas permeance [29]. Similar H2/CO2 selectivity during mixed and pure gas experiments clearly demonstrate that the H2 separation performance of the PBI membrane is not adversely affected by the presence of the other primary syngas components especially severely plasticizing and corrosive gases such as CO2 and H2S. The PBI membrane is also highly selective for H2 over CO and H2S. Their presence in the feed mixture up to 1 (vol) % has no effect on the H2 permeance. H2/ CO and H2/H2S selectivities of 100 and 1400, respectively were obtained. H2S permeance was below the detection limit of the utilized analytical system. Therefore, the H2S permeance and H2/H2S selectivity reported in this paper are taken from system’s lower detection limit. As a result, the H2S permeance is likely to be lower than reported, between zero and the detection limit. 3.3. Long term durability To demonstrate the long term durability, the PBI composite membrane was exposed to the syngas components in pure and mixed form at 250 1C for extended periods of time. During this testing, the PBI composite membrane was continuously held at 250 1C for over 330 day and tested intermittently for gas permselectivity. This test protocol involved exposure to a variety of environments including inert gas, H2, CO2 and dry syngas with or without H2S. For the pure gas permeation measurements, the feed side of the membrane was depressurized and purged with pure gas prior to permeance measurement. These pure gas experiments repeated periodically during this period demonstrated another critical feature of the PBI membrane that is its resilience to pressurization and depressurization in variety of environments ranging from inert to highly corrosive syngas. The H2 and CO2 permeance are approximately constant over the entire test period whereas N2 and CH4 permeance appears to decrease initially and then became constant as shown in Fig. 6. This apparent decrease in N2 and CH4 permeance can be due to structural rearrangement or loss of residual solvent from the membrane influencing the permeance measurement. The membrane maintained near constant permeance for all syngas components over the entire test period demonstrating exceptional thermal stability and resilience to syngas components. This is a pivotal development as temperature limitations and intolerance to sulfur compounds often leads to membrane failure.
4. Comparison to other polymeric membranes Critical membrane performance is generally subjected to an upper bound or trade-off between permeability and selectivity. Numerous analyses have been published in the literature discussing this trade-off behavior between various gas pairs such as CO2/N2, H2/CO2, O2/N2 etc. Fig. 7 shows the trade-off curve for Fig. 5. Effect of trans-membrane pressure on the H2 permeance of the PBI composite membrane.
Fig. 6. Long term durability demonstration of the PBI composite membrane held at 250 1C in feed environments varying from pure to dry simulated syngas for almost one year. The lines are shown to guide the eyes.
Fig. 7. Robeson plot comparing the PBI composite membrane with other polymeric membranes tested for the H2/CO2 separation. The lines represents the 1991 and 2008 upper bound from Robeson [30], the black circles are the corresponding experimental data from Robeson [30]. The hexagons represent modified PBI membranes [19] and the diamonds and cross represent the thermally rearranged (TR) PBI-based polymer membranes [24,31].
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H2/CO2 published in 2008 by Robeson [30]. Membranes performing above the trade-off line (data points in the upper right hand quadrant) are generally considered attractive for the industrial implementation. The further away it lies on the top-right quadrant of the trade-off plot, better is its potential for the cost effective implementation because high permeability and selectivity reduces required membrane areas and thus, the capital and operating costs. The literature data, taken from Robeson [30] presented in Fig. 7 is reported at or near ambient temperatures (35 1C), a standard temperature used in academia and industry to quantify the membrane properties. Additionally, literature data on modified PBI-based materials reported at 35 1C [19], and thermally rearranged [24] PBI-based materials reported at 35 and 120 1C [31] is also shown. The studies involving polymer membranes at high temperature ( 4150 1C) are scarce as polymers, with few exceptions, do not have sufficient thermal stability for use in this temperature regime. In an attempt to show the commercialization potential of the PBI membrane, the mixed and pure gas H2 permeance data obtained at 250 1C is incorporated in Fig. 7. The PBI membrane clearly exceeds the Robeson upper bound for selectivity versus permeability and falls in much desired upper right hand quadrant illustrating its outstanding potential for the proposed separation. More importantly, PBI illustrates this potential at the process relevant temperature of 250 1C. This PBI membrane ability to conduct syngas separation at high temperature in close vicinity of WGS convertor exist temperature allows energy efficient integration into a coal gasification or natural gas reforming power plant. This is a significant advantage of H2 selective PBI membrane over the low temperature standard H2/CO2 separation technologies as well as other polymeric membranes for this application.
5. Conclusions A polybenzimidazole-stainless steel composite membrane was developed to address the H2 separation from syngas challenge encountered in the H2 production from abundant hydrocarbon fuel sources. In the pure gas experiments, PBI composite membrane was highly selective for H2 over all other gases tested mainly CO2. The H2 permeance and H2/CO2 ideal selectivity was 4.67 GPU and 43, respectively, at 250 1C, most attractive temperature for the pre-combustion CO2 separation and clean H2 production. In accordance with the activated diffusion phenomena, H2 and CO2 permeance increased with temperature. The H2 permeance approximately doubled whereas the H2/CO2 ideal selectivity decreased from 58 to 47 as permeation temperature increased from 150 to 250 1C. Excellent commercialization potential of the PBI composite membrane was demonstrated by the successful testing in dry simulated syngas environments containing H2S for extended period of time exceeding 330 day without any performance degradation. The composite membrane maintained a steady H2 permeance of 7 GPU and H2/CO2 separation factor of 47 during this year long testing. In comparison to other polymeric membrane tested for H2/ CO2 separation, the PBI composite membrane’s performance exceeded the 2008 Robeson plot for H2/CO2 gas pair and fall in the highly desired upper right hand quadrant of the plot.
Acknowledgement This project supports the U.S. DOE National Energy Technology Sequestration Program project portfolio focused on the capture and separation of CO2 from the power sector. The authors gratefully acknowledge the U.S. DOE National Energy Technology
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Sequestration Program for financial support of the presented work. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC for DOE/NNSA under Contract DE-AC5206NA25396. The LANL identifier for this paper is LA-UR-12– 10243. The authors gratefully acknowledge Pall Corporation Research and Development at Cortland, NY for their collaborative efforts especially those related to the polymer composite membrane fabrication and their fabrication of the membrane utilized in the studies reported in this paper. References [1] G. Marban, T. Valdes-Solis, Towards the hydrogen economy? Int. J. Hydrogen Energy 32 (2007) 1625–1637. [2] G.J. Stiegel, M. Ramezan, Hydrogen from coal gasification: an economical pathway to a sustainable energy future, Int. J. Coal Geol. 65 (2006) 173–190. [3] R.B. Gupta, Hydrogen Fuel: Production, Transport, and Storage, 1st edn., CRC Press, Boca Raton, 2009. [4] D.P. Van Vuuren, M. Meinshausen, G.K. Plattner, F. Joos, K.M. Strassmann, S.J. Smith, T.M.L. Wigley, S.C.B. Raper, K. Riahi, F. de la Chesnaye, M.G.J. den Elzen, J. Fujino, K. Jiang, N. Nakicenovic, S. Paltsev, J.M. Reilly, Temperature increase of 21st century mitigation scenarios, PNAS 105 (2008) 15258–15262. [5] J.D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R.D. Srivastava, Advances in CO2 capture technology—the US Department of Energy’s Carbon Sequestration Program, Int. J. Greenhouse Gas Control 2 (2008) 9–20. [6] P.H. Stauffer, G.N. Keating, R.S. Middleton, H.S. Viswanathan, K.A. Berchtold, R.P. Singh, R.J. Pawar, A. Mancino, Greening coal: breakthroughs and challenges in carbon capture and storage, Environ. Sci. Technol. 45 (2011) 8597–8604. [7] A. Brunetti, F. Scura, G. Barbieri, E. Drioli, Membrane technologies for CO2 separation, J. Membr. Sci. 359 (2010) 115–125. [8] K.C. O’Brien, G. Krishnan, K.A. Berchtold, S. Blum, R. Callahan, W. Johnson D.-L. Roberts, D. Steele, D. Byard, J. Figueroa, Towards a pilot-scale membrane system for pre-combustion CO2 separation, Energy Procedia 1 (2009) 287–294. [9] G. Krishnan, D. Steele, K. O’Brien, R. Callahan, K. Berchtold, J. Figueroa, Simulation of a process to capture CO2 from IGCC syngas using a high temperature PBI membrane, Energy Procedia 1 (2009) 4079–4088. [10] S. Shelly, Capturing CO2: membrane systems move forward, Chem. Eng. Prog. 105 (2009) 42–47. [11] P. Bernardo, E. Drioli, G. Golemme, Membrane gas separation: a review/state of the art, Ind. Eng. Chem. Res. 48 (2009) 4638–4663. [12] T.M. Nenoff, R.J. Spontak, C.M. Aberg, Membranes for hydrogen purification: an important step towards a hydrogen based economy, MRS Bull. 31 (2006) 735–744. [13] W. Ho, K. Sirkar, Membrane Handbook, 1st edn., Springer, 1992. [14] K. Ghosal, B.D. Freeman, Gas separation using polymer membranes: an overview, Adv. Polym. Tech. 5 (1994) 673–697. [15] R.W. Baker, Membrane Technology and Applications, 2nd edn., John Wiley and Sons Ltd, Chichester, West Sussex, 2004. [16] K. Gerdes, D. Gray, J.M. Klara, J. Plunkett, S Salerno, G Tomlinson, C.W. White, Current and future technologies for gasification-based power generation, Volume 2, A Pathway Study Focused on Carbon Capture Advanced Power Systems R&D Using Bituminous Coal, Revision 1 (2010). [17] L. Xiao, H. Zhang, E. Scanlon, L.S. Ramanathan, E.-W. Choe, D. Rogers, T. Apple, B.C. Benicewicz, High-temperature polybenzimidazole fuel cell membranes via a sol–gel process, Chem. Mater. 17 (2005) 5328–5333. [18] B. S. Jorgensen, J. S. Young, B. F. Espinoza, Cross-linked polybenzimidazole membrane for gas separation, US Patent 6946015 (2003). [19] S.C. Kumbharkar, P.B. Karadkar, U.K. Kharul, Enhancement of gas permeation properties of polybenzimidazoles by systematic structure architecture, J. Membr. Sci. 286 (2006) 161. [20] D.R. Pesiri, B. Jorgensen, R.C. Dye, Thermal optimization of polybenzimidazole meniscus membranes for the separation of hydrogen, methane, and carbon dioxide, J. Membr. Sci. 218 (2003) 11–18. [21] M. Sadeghi, M.A. Sesarzadeh, H. Moadel, Enhancement of the gas separation properties of polybenzimidazole (PBI) membrane by incorporation of silica nano particles, J. Membr. Sci. 331 (2009) 21–30. [22] J. S. Young, G. S. Long, B. F. Espinoza, Cross-linked polybenzimidazole membrane for gas separation, US Patent (2004). [23] S.C. Kumbharkar, Y. Liu, K. Li, High performance polybenzimidazole based asymmetric hollow fibre membranes for H2/CO2 separation, J. Membr. Sci. 375 (2011) 231–240. [24] H.B. Park, S.H. Han, C.H. Jung, Y.M. Lee, A.J. Hill, Thermally rearranged (TR) polymer membranes for CO2 separation, J. Membr. Sci. 259 (2010) 11–24. [25] R.P. Singh, T. Powers, K. Rekciz, S. Hopkins, C. Love, Supports for high temperature gas separation membranes, Proc. Int. Conf. Inorg. Membr. 9 (2006) 674–677. [26] S. Matteucci, Y. Yampolskii, B.D. Freeman, I. Pinnau, Transport of gases and vapors in glassy and rubbery polymers, in: B. Freeman, Y. Yampolskii,
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Polybenzimidazole composite membranes for high temperature synthesis gas separations Kathryn A. Berchtold a,n, Rajinder P. Singh a, Jennifer S. Young b, Kevin W. Dudeck a a
Materials Physics and Applications-Materials Chemistry, Carbon Capture and Separations for Energy Applications (CaSEA) Labs, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b Applied Physics Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
a r t i c l e i n f o
abstract
Article history: Received 5 February 2012 Received in revised form 27 April 2012 Accepted 2 May 2012 Available online 10 May 2012
High temperature gas separation techniques are of great interest for reduction in green-house gas emissions from hydrocarbon fuels such as natural gas, coal or biomass used in power and chemical industry. In this work, a robust industrially viable polybenzimidazole (PBI)/stainless steel composite membrane is developed and evaluated for syngas separations at elevated temperatures for H2 production. A single tube laboratory scale PBI membrane module is tested for H2/CO2 perm-selectivity in pure and simulated dry syngas environments at industrially relevant operating conditions. Additionally, the effects of pressure and temperature on membrane performance are evaluated. The PBI composite membrane demonstrated exceptional long term thermo-chemical stability in the syngas environment even in the presence of H2S and excellent separation performance for H2 over the other syngas components. The H2 permeance and H2/CO2 selectivity for the PBI composite membrane in simulated dry syngas was recorded at 7 GPU (approximately, 88 barrer) and 47, respectively. In comparison to the other H2-selective polymeric membranes, the PBI composite membrane’s performance exceeded the 2008 Robeson upper bound for the H2/CO2 permeability versus selectivity. & 2012 Elsevier B.V. All rights reserved.
Keywords: Polybenzimidazole thin films Organic-inorganic composites Pre-combustion carbon capture Synthesis gas Hydrogen separation membrane IGCC
1. Introduction H2 is the cleanest energy carrier with water being the only product of its combustion [1]. It can be combusted in fuel cells and gas turbines with zero or near zero emissions at a higher efficiency than existing industry standard coal power plants and internal combustion engines. Therefore, transportation and stationary power generation through H2 combustion is highly desirable for energy security, reduced carbon emissions, and efficient natural resource utilization. Besides an energy carrier, numerous other uses of H2 in the chemical industry make it an important commodity. It is used in numerous hydrogenation reactions; in annealing environments for metal processing; for catalyst and sorbent regeneration; and in methanol and ammonia production [2,3]. The demand for H2 is expected to grow significantly in the future to meet the growing demands of utility, chemicals, and fuels industries as we transition into a more environmentally conscious economy. Industrially, natural gas and oil are the primary fuel sources for H2 production, accounting for approximately 80% of the H2 produced daily [2]. H2 production from solid fuel sources such
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as coal and biomass via gasification is also a viable and sustainable alternative. The abundance of these sources makes gasification an ideal path forward for large scale H2 production, especially for power generation. While combustion of H2 is relatively environmentally benign, its production from hydrocarbon fuel sources is not inherently environmental friendly. The simultaneous release of carbon as CO2 upon hydrocarbon fuel processing, is a major environmental concern. CO2 is a greenhouse gas and its presence at historically high concentrations in the atmosphere is linked to global warming and expected to play a dominant role in further increasing average global temperatures by approximately 1.1 to 6.4 1C by the year 2100 [4]. Hence, separation, capture, and sequestration of CO2 produced during hydrocarbon fuel conversion to energy is essential. Typically advanced hydrocarbon fuel processing schemes with integrated carbon capture involves synthesis gas (syngas) production supplemented with shift conversion [5]. Syngas is a mixture of H2 and CO with trace quantities of sulfur compounds, ammonia, metals and particulate matter. After undergoing particulate and sulfur removal, syngas is passed over one or more water–gas shift converters where CO reacts with steam to produce H2 and CO2. In pre-combustion carbon capture pathway, CO2 is separated from syngas prior to its combustion in fuel cells or gas turbines. The captured CO2 can then be compressed and transported for use or sequestration [6].
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While the separation of CO2 from process streams is readily accomplished via standard separation techniques such as pressure swing adsorption and amine scrubbing, the effectiveness of these technologies for separating CO2 on large scale is limited [7–10]. These techniques operate at ambient, sub-ambient, or near ambient temperatures and typically produce a low pressure CO2 stream, resulting in a significant energy penalty for CO2 sequestration. In contrast, gas separation membranes can be economically applied for CO2 separation from syngas since they involve no phase change, are less energy intensive, and typically provide low maintenance operations [8–11]. Polymeric membranes have been used successfully in a number of industrial gas separation/purification applications, including the production of high-purity nitrogen, gas dehydration, removal of acid gases and recovery of hydrogen from process streams for recycle [12–15]. Since the economic benefits of using a membrane separation system are strongly tied to efficient process integration, membrane separation systems with the ability to withstand industrially relevant syngas operating conditions in the presence of the chemically challenging syngas components are desirable [16]. Unfortunately, the commercially available polymeric materials currently employed in the gas separation applications are not stable in pre-combustion syngas environments and operating conditions to the degree required. These membrane materials are often subject to chemical degradation by minor components in the process stream, a problem that is exacerbated at elevated temperatures. Additionally, as operating temperatures approach the glass transition temperature of a membrane material, membrane selectivity and flux are significantly altered, reducing membrane separation efficiency. Consequently, there is a compelling need for membrane materials and subsequently capture systems based on those materials that can operate under more extreme environmental conditions for extended periods of time while providing a level of performance that is economically sustainable by the end user. Heterocyclic PBI-based materials are thermally stable up to temperatures approaching 500 1C owing to their rigid rod like molecular structure. Their glass transition temperatures are often above 450 1C ( 460–500 1C) and typically in close proximity to their degradation temperatures. PBI-based materials possess excellent chemical stability, good mechanical properties and an appropriate level of processability for use in extreme chemical and thermal environments. These properties have led many researchers to investigate the PBI-based materials for applications with operating conditions well outside the typical realm of organic materials. These applications include as electrolyte membranes for fuel cells [17] and gas separation membranes [18–24]. These previous studies on gas separation characteristics of PBI-based materials demonstrate their excellent size sieving ability and potential for small molecule separations such as H2 from CO2. In this work, we report development and characterization of a novel PBI-based composite membrane for syngas separation at elevated temperatures. The perm-selectivity characteristics of the PBI composite membrane are evaluated under industrially relevant realistic operating conditions in simulated dry syngas with a focus on membrane long term durability and stability.
2. Experimental section 2.1. Membrane fabrication The PBI material used in this study is poly(5,50 -benzimida zole-2,20 -diyl-1,3-phenylene). Fig. 1 shows the structure of PBI. A 26.2 wt% PBI solution in dimethyacetamide (DMAC) containing 2 wt% lithium chloride as a phase stabilizer (PBI Performance Products Inc., Columbia, SC) was used for membrane formation. The PBI composite membrane was provided by Pall Corporation,
Fig. 1. Molecular structure of PBI: polybenzimidazole, poly(5,50 -benzimidazole2,20 -diyl-1,3-phenylene).
Fig. 2. Picture of the PBI/zirconia/stainless steel composite membrane and membrane module used in this study.
Cortland, NY and consists of a thin PBI layer, deposited by a proprietary process on the outer surface of a porous stainless steel support with a zirconia intermediate layer [25]. 2.2. Transport measurement The PBI membrane was installed in a stainless steel housing using stainless steel compression fittings resulting in a laboratory scale test module. The module was configured to allow for feed gas flow down the outer diameter of the membrane and use of sweep or vacuum on the permeate side of the module housing for permeance measurement. Fig. 2 illustrates a coated tube alongside the module housing. The PBI composite membrane data presented here was obtained on a single-tube module with a membrane surface area of approximately 31 cm2. The pure gas permeation experiments were performed using a constant-volume/variable-pressure method at a differential pressure of 50 psi with H2, CO2, CH4 and N2. The membrane operating temperature was varied from 150 to 250 1C. The upstream and downstream pressures were measured using high accuracy (70.25% FS) pressure transducers (MKS Instruments, Inc.). The permeation rate was calculated from the slope of the linear part of the permeate pressure rise versus time curve using Eq. (1). dp VT 0 Permeance, P ¼ , ð1Þ dt P 0 T DpA where dp/dt is the pressure rise; V is the downstream volume; Dp is the pressure difference between membrane upstream and downstream side; T0 and P0 are the standard temperature and pressure, respectively, and A is the effective membrane surface area. The ideal selectivity for a gas pair is calculated by taking the ratio of their gas permeances. The mixed gas experiments were performed using the same system in a steady state configuration using a sweep gas. The simulated dry syngas feed mixture containing H2: 55%; CO2: 41%; CO: 1%; CH4: 1%; N2: 1% and H2S: 1% (volume basis) was prepared using mass flow controllers with total feed flow-rate set at 20 sccm. Argon was used as a sweep gas with flow rate set at 5 sccm. The mixed gas data were obtained at 250 1C and 50 psid. The permeances of the feed gas components were computed from the total permeate flow rate measured using a mass flow meter
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and their concentration in the permeate stream measured by a micro-GC (Agilent 3000). The selectivity is calculated from the ratio of permeances. For all the gas permeation experiments, the gases used were at least 99.9% pure.
3. Results and discussions 3.1. Pure gas permeance The pure gas permselectivity of the PBI composite membrane tested at 50 psid and 250 1C is reported in Table 1. H2 permeance of the PBI composite membrane is 4.67 GPU (4.67 10 6 cm3 cm 2 s 1 cm Hg 1) and the H2/CO2 and H2/N2 ideal selectivities are 43 and 233, respectively. Due to the composite structure of the membrane and potential penetration into support pores, direct determination of membrane thickness is complex. The composite membrane effective thickness is estimated using flat sheet membrane nitrogen permeability at ambient conditions. Thus, the thickness and pure gas H2 permeability of the PBI composite membrane tested here are estimated to be 12.5 mm and 58 barrer (58 10 10 cm3 cm cm 2 s 1 cm Hg 1), respectively. Generally, in polymeric membranes the permeance (or permeability) is represented as the combination of diffusivity and solubility (Eq. (2)) [26]. The diffusivity follows an inverse trend to the size of the permeating gases whereas solubility of gases in the membrane material follows a linear trend with the condensability of the permeating gas. This general principle of gas transport in polymer membranes can be used to understand the perm-selective behavior of polymeric membranes for H2–CO2 separation, two major syngas components. CO2 (critical temperature, Tc ¼ 304 K) polymer solubility is typically significantly higher than that of less condensable permanent gases such as H2 (Tc ¼ 33 K) however diffusivity favors H2 (kinetic diameter¼ 2.89 A˚ [27]) over CO2 (kinetic diameter¼3.30 A˚ [27]). Therefore, to be an efficient H2 selective membrane material, gas transport and selectivity in the polymer must be dominated by diffusivity or size sieving ability commonly referred to as molecular sieving. The high perm-selectivity character of the PBI membrane towards H2 is indicative of a dominant molecular sieving gas transport mechanism. The PBI membrane gas permeance behavior correlates well with gas molecules kinetic ˚ ˚ ˚ ˚ diameter (H2 (kinetic diameter: 2.89 A)4CO 2 (3.3 A)4N2 (3.64 A)4CH4 (3.8 A)). The interplay between diffusivity and solubility as a function of operating conditions can be optimized to enhance the performance of a membrane towards particular separation. Ideally, for H2 separation from syngas, high diffusivity and low solubility is desirable as discussed above. As shown by Eqs. (3) and (4), the diffusivity increases with temperature whereas solubility decreases with temperature. At operating temperatures range of 150 to 250 1C for syngas separations, solubility is not expected to play a significant role. However, the effect of operating temperature on diffusivity can be significant and can be applied as a tool to optimize membrane performance. The H2/CO2 selectivity and permeability of the PBI membrane as a function of temperature are shown in Figs. 3 and 4. Both the H2 and CO2 permeance increases with a temperature increase from 150 to 250 1C consistent with diffusivity dominated gas transport. The PBI membrane H2 permeance approximately doubles as operating temperature is increased over the same temperature range. The permeation activation energies (Ep), calculated from this change in permeance with temperature are reported in Table 1. The activation energies for CO2 and H2 are 26.3 and 18.5 kJ mol 1, respectively leading to an increase in the permeance of both gases with increasing temperature accompanied by a decrease in selectivity. The PBI membrane H2/CO2 ideal selectivity decreased from 58 to 40 as the process temperature increased from 150 to 250 1C. Permeability,P ¼ DnS
ð2Þ
Table 1 Pure and mixed gas permeances and separation factors for the PBI composite membrane tested at 250 1C. Ep is the activation energy of the permeance obtained from the slope of permeance versus inverse of temperature. The number in the parenthesis is the standard error in the values of Ep, reflecting the error in slope determination by curve fitting. Gas Pure gas permeance (GPU)
Pure gas selectivity (H2/Gas)
Mixed gas permeance (GPU)
Mixed gas selectivity (H2/Gas)
Ep (kJ mol 1)
H2 CO2 CH4 N2 H2S CO
– 43.0 387 233 – –
7.000 0.147 0.030 – 0.005* 0.070
– 47.60 233.6 – 1289 99.50
18.5 26.3 23.4 18.4 – –
4.670 0.108 0.012 0.020 – –
(0.7) (2.0) (7.1) (10)
n H2S signal was below the detection limit of our analytical instrument. The permeance reported for H2S is the detection limit of our analytical instrument.
Fig. 3. Effect of permeation temperature on H2/CO2 ideal selectivity of the PBI composite membrane. The line is drawn to guide the eye.
Fig. 4. Effect of permeation temperature on pure gas permeance of the PBI composite membrane. The lines are drawn to guide the eye. Diffusivity, D ¼ D0 eEa =RT ,
ð3Þ
Solubility, S ¼ S0 eDH=RT
ð4Þ
where Ea activation energy of diffusion; DH is the heat of sorption; D0 and S0 are the pre-exponential factors for diffusivity and solubility; R is the universal gas constant; and T is the operating temperature. The ability of PBI membrane to retain its high H2/CO2 selectivity at temperatures up to 250 1C demonstrates the extreme thermal resilience of the PBI macromolecular structure. Typically in polymeric membranes, as temperature increases, the free volume created by the close packing of polymer chains increases due to the enhanced thermal motion of the polymer chains. This increase in polymer free volume renders polymer materials unable to distinguish between the molecules with very small size difference such as H2 and CO2. For example, polypyrrolone membranes, a rigid glassy polymer, tested by Costello et al. [28] showed a significant drop in He/CH4 separation factor from 150 to 30 as the permeation temperature was increased from 35 to 200 1C. Although, Tg of polypyrrolone is 380 1C, significantly higher than the test temperature of 200 1C, a change in the specific volume of the polypyrrolone due to the secondary thermal transition at approximately 130 1C is cited as the reason for the reduction in the molecular sieving ability of polypyrrolone at elevated temperatures. Unlike polypyrrolone, PBI showed an extreme resilience to temperature and maintained its industrially attractive molecular sieving character even at elevated temperatures exceeding 150 1C. PBI’s rigid rod like molecular structure together with the h-bonding between the chains is hypothesized to be the main reason for its exceptional size sieving ability at elevated temperatures.
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In contrast to the temperature effects, increasing the feed pressure has no significant effect on the PBI membrane permeance. As shown in Fig. 5, the H2 permeance is approximately constant when transmembrane pressure is increased from 29 to 59 psi. This result indicates the absence of viscous flow and defects in the membrane module selective layer.
3.2. Mixed gas results The pure gas permeation results discussed above indicate that PBI is an excellent H2 selective membrane material for syngas separations. Inspired by the pure gas permeance data, the PBI composite membrane was exposed to a simulated dry syngas stream containing H2: 55%; CO2: 41%; CO: 1%; CH4: 1%; N2: 1% and H2S: 1% (volume basis) at 250 1C and 50 psid. The results of the mixed gas testing are shown in Table 1 and Fig. 6. A H2 permeance and H2/CO2 selectivity of 7 GPU (88 barrer, estimated) and 47, respectively, were observed during mixed gas testing conducted at 250 1C. The gas permeances during mixed gas experiments are slightly higher than the pure gas permeances with nearly identical H2/CO2 selectivity. The reason for the higher mixed gas H2 permeance is not known at this time. It can be an experimental artifact of using sweep gas and vacuum on the permeate side of the membrane in the mixed and pure gas experiments, respectively. The feed pressure in both pure and mixed gas experiments is identical, however in pure gas experiments the PBI composite membrane is subjected to higher total pressure differential due to lower permeate pressures than that compared to mixed gas experiments. This
higher total differential pressure can potentially lead to a greater degree of the PBI membrane compaction in pure gas experiments than mixed gas experiments thereby reducing the gas permeance [29]. Similar H2/CO2 selectivity during mixed and pure gas experiments clearly demonstrate that the H2 separation performance of the PBI membrane is not adversely affected by the presence of the other primary syngas components especially severely plasticizing and corrosive gases such as CO2 and H2S. The PBI membrane is also highly selective for H2 over CO and H2S. Their presence in the feed mixture up to 1 (vol) % has no effect on the H2 permeance. H2/ CO and H2/H2S selectivities of 100 and 1400, respectively were obtained. H2S permeance was below the detection limit of the utilized analytical system. Therefore, the H2S permeance and H2/H2S selectivity reported in this paper are taken from system’s lower detection limit. As a result, the H2S permeance is likely to be lower than reported, between zero and the detection limit. 3.3. Long term durability To demonstrate the long term durability, the PBI composite membrane was exposed to the syngas components in pure and mixed form at 250 1C for extended periods of time. During this testing, the PBI composite membrane was continuously held at 250 1C for over 330 day and tested intermittently for gas permselectivity. This test protocol involved exposure to a variety of environments including inert gas, H2, CO2 and dry syngas with or without H2S. For the pure gas permeation measurements, the feed side of the membrane was depressurized and purged with pure gas prior to permeance measurement. These pure gas experiments repeated periodically during this period demonstrated another critical feature of the PBI membrane that is its resilience to pressurization and depressurization in variety of environments ranging from inert to highly corrosive syngas. The H2 and CO2 permeance are approximately constant over the entire test period whereas N2 and CH4 permeance appears to decrease initially and then became constant as shown in Fig. 6. This apparent decrease in N2 and CH4 permeance can be due to structural rearrangement or loss of residual solvent from the membrane influencing the permeance measurement. The membrane maintained near constant permeance for all syngas components over the entire test period demonstrating exceptional thermal stability and resilience to syngas components. This is a pivotal development as temperature limitations and intolerance to sulfur compounds often leads to membrane failure.
4. Comparison to other polymeric membranes Critical membrane performance is generally subjected to an upper bound or trade-off between permeability and selectivity. Numerous analyses have been published in the literature discussing this trade-off behavior between various gas pairs such as CO2/N2, H2/CO2, O2/N2 etc. Fig. 7 shows the trade-off curve for Fig. 5. Effect of trans-membrane pressure on the H2 permeance of the PBI composite membrane.
Fig. 6. Long term durability demonstration of the PBI composite membrane held at 250 1C in feed environments varying from pure to dry simulated syngas for almost one year. The lines are shown to guide the eyes.
Fig. 7. Robeson plot comparing the PBI composite membrane with other polymeric membranes tested for the H2/CO2 separation. The lines represents the 1991 and 2008 upper bound from Robeson [30], the black circles are the corresponding experimental data from Robeson [30]. The hexagons represent modified PBI membranes [19] and the diamonds and cross represent the thermally rearranged (TR) PBI-based polymer membranes [24,31].
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H2/CO2 published in 2008 by Robeson [30]. Membranes performing above the trade-off line (data points in the upper right hand quadrant) are generally considered attractive for the industrial implementation. The further away it lies on the top-right quadrant of the trade-off plot, better is its potential for the cost effective implementation because high permeability and selectivity reduces required membrane areas and thus, the capital and operating costs. The literature data, taken from Robeson [30] presented in Fig. 7 is reported at or near ambient temperatures (35 1C), a standard temperature used in academia and industry to quantify the membrane properties. Additionally, literature data on modified PBI-based materials reported at 35 1C [19], and thermally rearranged [24] PBI-based materials reported at 35 and 120 1C [31] is also shown. The studies involving polymer membranes at high temperature ( 4150 1C) are scarce as polymers, with few exceptions, do not have sufficient thermal stability for use in this temperature regime. In an attempt to show the commercialization potential of the PBI membrane, the mixed and pure gas H2 permeance data obtained at 250 1C is incorporated in Fig. 7. The PBI membrane clearly exceeds the Robeson upper bound for selectivity versus permeability and falls in much desired upper right hand quadrant illustrating its outstanding potential for the proposed separation. More importantly, PBI illustrates this potential at the process relevant temperature of 250 1C. This PBI membrane ability to conduct syngas separation at high temperature in close vicinity of WGS convertor exist temperature allows energy efficient integration into a coal gasification or natural gas reforming power plant. This is a significant advantage of H2 selective PBI membrane over the low temperature standard H2/CO2 separation technologies as well as other polymeric membranes for this application.
5. Conclusions A polybenzimidazole-stainless steel composite membrane was developed to address the H2 separation from syngas challenge encountered in the H2 production from abundant hydrocarbon fuel sources. In the pure gas experiments, PBI composite membrane was highly selective for H2 over all other gases tested mainly CO2. The H2 permeance and H2/CO2 ideal selectivity was 4.67 GPU and 43, respectively, at 250 1C, most attractive temperature for the pre-combustion CO2 separation and clean H2 production. In accordance with the activated diffusion phenomena, H2 and CO2 permeance increased with temperature. The H2 permeance approximately doubled whereas the H2/CO2 ideal selectivity decreased from 58 to 47 as permeation temperature increased from 150 to 250 1C. Excellent commercialization potential of the PBI composite membrane was demonstrated by the successful testing in dry simulated syngas environments containing H2S for extended period of time exceeding 330 day without any performance degradation. The composite membrane maintained a steady H2 permeance of 7 GPU and H2/CO2 separation factor of 47 during this year long testing. In comparison to other polymeric membrane tested for H2/ CO2 separation, the PBI composite membrane’s performance exceeded the 2008 Robeson plot for H2/CO2 gas pair and fall in the highly desired upper right hand quadrant of the plot.
Acknowledgement This project supports the U.S. DOE National Energy Technology Sequestration Program project portfolio focused on the capture and separation of CO2 from the power sector. The authors gratefully acknowledge the U.S. DOE National Energy Technology
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Sequestration Program for financial support of the presented work. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC for DOE/NNSA under Contract DE-AC5206NA25396. The LANL identifier for this paper is LA-UR-12– 10243. The authors gratefully acknowledge Pall Corporation Research and Development at Cortland, NY for their collaborative efforts especially those related to the polymer composite membrane fabrication and their fabrication of the membrane utilized in the studies reported in this paper. References [1] G. Marban, T. Valdes-Solis, Towards the hydrogen economy? Int. J. Hydrogen Energy 32 (2007) 1625–1637. [2] G.J. Stiegel, M. Ramezan, Hydrogen from coal gasification: an economical pathway to a sustainable energy future, Int. J. Coal Geol. 65 (2006) 173–190. [3] R.B. Gupta, Hydrogen Fuel: Production, Transport, and Storage, 1st edn., CRC Press, Boca Raton, 2009. [4] D.P. Van Vuuren, M. Meinshausen, G.K. Plattner, F. Joos, K.M. Strassmann, S.J. Smith, T.M.L. Wigley, S.C.B. Raper, K. Riahi, F. de la Chesnaye, M.G.J. den Elzen, J. Fujino, K. Jiang, N. Nakicenovic, S. Paltsev, J.M. Reilly, Temperature increase of 21st century mitigation scenarios, PNAS 105 (2008) 15258–15262. [5] J.D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R.D. Srivastava, Advances in CO2 capture technology—the US Department of Energy’s Carbon Sequestration Program, Int. J. Greenhouse Gas Control 2 (2008) 9–20. [6] P.H. Stauffer, G.N. Keating, R.S. Middleton, H.S. Viswanathan, K.A. Berchtold, R.P. Singh, R.J. Pawar, A. Mancino, Greening coal: breakthroughs and challenges in carbon capture and storage, Environ. Sci. Technol. 45 (2011) 8597–8604. [7] A. Brunetti, F. Scura, G. Barbieri, E. Drioli, Membrane technologies for CO2 separation, J. Membr. Sci. 359 (2010) 115–125. [8] K.C. O’Brien, G. Krishnan, K.A. Berchtold, S. Blum, R. Callahan, W. Johnson D.-L. Roberts, D. Steele, D. Byard, J. Figueroa, Towards a pilot-scale membrane system for pre-combustion CO2 separation, Energy Procedia 1 (2009) 287–294. [9] G. Krishnan, D. Steele, K. O’Brien, R. Callahan, K. Berchtold, J. Figueroa, Simulation of a process to capture CO2 from IGCC syngas using a high temperature PBI membrane, Energy Procedia 1 (2009) 4079–4088. [10] S. Shelly, Capturing CO2: membrane systems move forward, Chem. Eng. Prog. 105 (2009) 42–47. [11] P. Bernardo, E. Drioli, G. Golemme, Membrane gas separation: a review/state of the art, Ind. Eng. Chem. Res. 48 (2009) 4638–4663. [12] T.M. Nenoff, R.J. Spontak, C.M. Aberg, Membranes for hydrogen purification: an important step towards a hydrogen based economy, MRS Bull. 31 (2006) 735–744. [13] W. Ho, K. Sirkar, Membrane Handbook, 1st edn., Springer, 1992. [14] K. Ghosal, B.D. Freeman, Gas separation using polymer membranes: an overview, Adv. Polym. Tech. 5 (1994) 673–697. [15] R.W. Baker, Membrane Technology and Applications, 2nd edn., John Wiley and Sons Ltd, Chichester, West Sussex, 2004. [16] K. Gerdes, D. Gray, J.M. Klara, J. Plunkett, S Salerno, G Tomlinson, C.W. White, Current and future technologies for gasification-based power generation, Volume 2, A Pathway Study Focused on Carbon Capture Advanced Power Systems R&D Using Bituminous Coal, Revision 1 (2010). [17] L. Xiao, H. Zhang, E. Scanlon, L.S. Ramanathan, E.-W. Choe, D. Rogers, T. Apple, B.C. Benicewicz, High-temperature polybenzimidazole fuel cell membranes via a sol–gel process, Chem. Mater. 17 (2005) 5328–5333. [18] B. S. Jorgensen, J. S. Young, B. F. Espinoza, Cross-linked polybenzimidazole membrane for gas separation, US Patent 6946015 (2003). [19] S.C. Kumbharkar, P.B. Karadkar, U.K. Kharul, Enhancement of gas permeation properties of polybenzimidazoles by systematic structure architecture, J. Membr. Sci. 286 (2006) 161. [20] D.R. Pesiri, B. Jorgensen, R.C. Dye, Thermal optimization of polybenzimidazole meniscus membranes for the separation of hydrogen, methane, and carbon dioxide, J. Membr. Sci. 218 (2003) 11–18. [21] M. Sadeghi, M.A. Sesarzadeh, H. Moadel, Enhancement of the gas separation properties of polybenzimidazole (PBI) membrane by incorporation of silica nano particles, J. Membr. Sci. 331 (2009) 21–30. [22] J. S. Young, G. S. Long, B. F. Espinoza, Cross-linked polybenzimidazole membrane for gas separation, US Patent (2004). [23] S.C. Kumbharkar, Y. Liu, K. Li, High performance polybenzimidazole based asymmetric hollow fibre membranes for H2/CO2 separation, J. Membr. Sci. 375 (2011) 231–240. [24] H.B. Park, S.H. Han, C.H. Jung, Y.M. Lee, A.J. Hill, Thermally rearranged (TR) polymer membranes for CO2 separation, J. Membr. Sci. 259 (2010) 11–24. [25] R.P. Singh, T. Powers, K. Rekciz, S. Hopkins, C. Love, Supports for high temperature gas separation membranes, Proc. Int. Conf. Inorg. Membr. 9 (2006) 674–677. [26] S. Matteucci, Y. Yampolskii, B.D. Freeman, I. Pinnau, Transport of gases and vapors in glassy and rubbery polymers, in: B. Freeman, Y. Yampolskii,
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