Carbon Dioxide Capture Using a CO2-Selective Facilitated Transport

Jan 16, 2008 - Saurav Datta , Michael P. Henry , YuPo. J. Lin , Anthony T. Fracaro , Cynthia S. Millard , and Seth W. Snyder , Rebecca L. Stiles , Jit...
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Ind. Eng. Chem. Res. 2008, 47, 1261-1267

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Carbon Dioxide Capture Using a CO2-Selective Facilitated Transport Membrane Jin Huang, Jian Zou, and W. S. Winston Ho* Department of Chemical and Biomolecular Engineering, Department of Materials Science and Engineering, The Ohio State UniVersity, 140 W. 19th AVenue, Columbus, Ohio 43210-1180

A novel CO2-selective membrane with the facilitated transport mechanism has been synthesized to capture CO2 from the industrial gas mixtures, including flue gas. Both mobile and fixed amine carriers were incorporated into the cross-linked poly(vinyl alcohol) (PVA) during the membrane synthesis. The membrane showed desirable CO2 permeability (with a suitable effective thickness) and CO2/N2 selectivity up to 170 °C. In the CO2 capture experiments from a gas mixture with N2 and H2, a permeate CO2 dry concentration of >98% was obtained, using steam as the sweep gas. The effects of the feed flow rate and the sweep:feed molar ratio on the membrane separation performance were investigated. A one-dimensional isothermal model was established to examine the performance of a hollow-fiber membrane module composed of the described CO2selective membrane. The modeling results show that a CO2 recovery of >95% and a permeate CO2 dry concentration of >98% are achievable from a 1000 standard cubic feet per minute (SCFM) (or 21.06 mol/s) flue gas stream with a 2 ft (0.61 m) hollow-fiber module that contained 980 000 fibers. 1. Introduction The increasing public concern over global warming has been concentrated on the anthropogenic (man-made) emissions of greenhouse gases, especially carbon dioxide (CO2). CO2 is primarily emitted as the result of the combustion of fossil fuels, and also from nature gas sweetening, the production of synthesis gas, and certain chemical plants.1 Currently, the principal CO2 capture technologies include chemical and/or physical absorption, physical adsorption, membrane separation, and cryogenic distillation.2 Compared to other technologies, membrane-based CO2 capture has the advantages of low-energy consumption, simplicity of operation, and an absence of moving parts. It has shown great potential in natural gas purification, CO2 capture from emissions of coal-fired power plants, and metabolic CO2 removal from enclosed breathing atmospheres (for example, in a space shuttle or space station).3-7 Moreover, it may also be used in the CO2-selective water-gas-shift membrane reactor for the purification of H2 in fuel-cell fuel processing, which is a key challenge for the proton-exchange membrane fuel cells (PEMFCs). We have proposed and verified this idea in earlier publications.8,9 However, today’s commercial polymeric membranes, such as cellulose acetate and polyimide, suffer from low CO2 permeability and selectivity, because they are based on a solution-diffusion mechanism and rely primarily on the subtle size differences of the penetrants to achieve separation.3,10,11 Also, these membranes are not suitable for high-temperature and high-humidity operation, which is required in the direct CO2 capture from the flue gas in power plants.11,12 For the specific application of CO2 capture from the flue gas, it has been reported that a CO2/N2 selectivity of >70 and a minimum CO2 permeability of 100 Barrers for a membrane thickness of 0.1 µm (a permeance of 1000 GPU (gas permeation unit), where 1 GPU ) 10-6 cm3 (STP)/(cm2 s cm Hg)) are required for the economic operation.13,14 As an alternative to conventional polymeric membranes, facilitated transport membranes have shown better promise to satisfy these goals.4,15,16 * To whom correspondence should be addressed: E-mail address: [email protected].

In this type of membrane, the incorporated carrier agents can react reversibly with the target gas component. Therefore, the reaction in the membrane creates another transport mechanism, in addition to the simple solution-diffusion mechanism. The target gas component dissolves in the membrane first, and it can diffuse down its own concentration gradient or diffuse down a concentration gradient of a carrier-gas complex. Meanwhile, the gas components that do not react with the carrier agents will permeate exclusively via the solution-diffusion mechanism.16 The facilitated transport mechanism for CO2 removal is shown schematically in Figure 1. The reaction between CO2 and the amine carrier can be described by the zwitterions mechanism, which was originally proposed by Caplow17 and reintroduced by Danckwerts.18 First, CO2 reacts with primary or secondary amines (RR′NH, where R is a functional group and R′ is a functional group or hydrogen) to form zwitterions as an intermediate.

CO2 + RR′NH h RR′NH+COO-

(1)

The zwitterion then is deprotonated by bases such as the amine itself and H2O to form the carbamate ion.

RR′NH+ COO- + RR′NH h RR′NCOO- + RR′NH2+ (2) RR′NH+ COO- + H2O h RR′NCOO- + H3O+

(3)

If the carbamate ion of the amine carrier is not stable, it will react with H2O to form bicarbonate, HCO3-.

RR′NCOO- + H2O h RR′NH + HCO3-

(4)

In addition, a water-swollen condition may provide a better facilitation effect than a dry condition. This is due to the fact that the CO2 hydration reaction in a water-swollen membrane would be enhanced in the presence of amino groups, which work as weak base catalysts. Therefore, CO2 transport is facilitated in the forms of carbamate and bicarbonate.19 Under the partial pressure difference in the gas separation process, these carriergas reaction products (i.e., carbamate and bicarbonate) will diffuse down their concentration gradient or pass off to the next carrier agent.

10.1021/ie070794r CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008

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Figure 1. Schematic of the facilitated transport mechanism.

Extensive studies have been done on the CO2 facilitated transport membranes since the 1960s.8,9,15,16,19-36 Based on the carrier mobility, facilitated transport membranes can be divided into two categories: mobile-carrier membranes and fixed-carrier membranes. In mobile-carrier membranes, the carrier can diffuse in the membrane. Generally, they were prepared by immersing the microporous supports in the carrier solutions or a reactive solvent, which was known as the immobilized liquid membrane (ILM).22-24 However, this configuration is relatively unstable (e.g., the membrane can be dried out at high temperatures and the carrier agents can be lost). To improve the membrane stability, the ion-exchange membrane was first proposed as the support of ILM by LeBlanc et al.25 When the ionic mobile carriers such as monoprotonated ethylenediamine were immobilized in an ion exchange membrane by the electrostatic force, the carrier washout would be reduced, to some extent.25-28 In fixed-carrier membranes,19,29-36 in which the reactive carrier agents are covalently bonded to the polymer backbone, CO2 reacts at one carrier site and then hops to the next carrier site along the direction of the concentration driving force via the “hopping” mechanism.20 Even though mobile-carrier membranes have shown higher CO2 transport rates than fixed-carrier membranes, because of the higher reactive diffusivity, fixedcarrier membranes were considered to be the most stable configuration.16 In the present study, a novel CO2-selective polymeric membrane that contained both mobile and fixed amine carriers was prepared, and the permeation properties were measured at g100 °C. The behaviors of CO2 capture from the gas mixtures were studied with a simulated feed gas while using steam as the sweep gas. Based on the measured CO2 permeability and CO2/N2 selectivity, a mathematical model was developed to evaluate the separation performance of a hollow-fiber module that was composed of the described membrane. 2. Experimental Section 2.1. Chemicals and Materials. Poly(vinyl alcohol) (PVA) with 99+% hydrolysis [from poly(vinyl acetate)] and a molecular weight of 89 000-98 000, potassium hydroxide (KOH) (85+ wt %), formaldehyde (37 wt % solution in water), and 2-aminoisobutyric acid (AIBA) were purchased from Aldrich Chemicals (St. Louis, MO). Poly(allylamine hydrochloride), with a molecular weight of ∼60 000 was obtained from Polysciences, Inc. (Warrington, PA). Deionized (DI) water was used as the solvent during the membrane preparation. All these chemicals were used as received, without further purification. Two certificated gas mixtures were used as the feed gases. The gas compositions were (1) 20% CO2, 40% H2, and 40% N2, and (2) 1% CO, 17% CO2, 45% H2, and 37% N2. The second gas mixture simulated the composition of synthesis gas from the autothermal reforming of gasoline with air. Prepurified

argon (99.998%) was used as the sweep gas in the gas permeation measurements, for ease of gas chromatography (GC) analysis. All of these gases were obtained from Praxair, Inc. (Danbury, CT). The microporous poly(tetrafluoroethylene) (PTFE) support (thickness of 60 µm, average pore size of 0.2 µm, and 80% porosity) was donated by BHA Technologies (Kansas City, MO). 2.2. Synthesis of CO2-Selective Membrane. The membrane of PVA that contained amino functional groups in the thinfilm composite structure was prepared using the solution casting method. PVA was first dissolved in DI water at 80 °C under stirring. A stoichiometric amount of formaldehyde and a certain amount of KOH were added into the PVA aqueous solution to achieve a 60 mol % degree of crosslinking, based on the hydroxyl groups of PVA. The PVA/formaldehyde/KOH solution was heated at ∼80 °C for 20 h under stirring. The crosslinking of PVA with formaldehyde is a condensation reaction, which is shown schematically in Figure 2. Free polyallyamine was prepared by mixing poly(allylamine hydrochloride) with a stoichiometric amount of KOH in methanol overnight. 2-Aminoisobutyric acid (AIBA) potassium salt (AIBA-K) was synthesized by mixing AIBA with a stoichiometric amount of KOH in DI water for 30 min. The chemical structures of these compounds are illustrated in Figure 3. The designated amounts of these two chemicals were then added into the cross-linked PVA solution as the fixed and mobile carriers, respectively. The resulting solution was centrifuged to remove air bubbles and cast to a uniform thickness with a GARDCO adjustable micrometer film applicator (Paul N. Gardner Company, Pompano Beach, FL) on the microporous PTFE support. The solution was viscous enough to avoid penetrating the pores of the support, which was verified by SEM cross-sectional pictures. The gap setting of the film applicator was adjusted to control the membrane thickness. The membrane was dried first in a fume hood under ambient conditions overnight, to remove most of the water, and then was further cured in a 120 °C furnace for 6 h to complete the cross-linking reaction. The final composition of the dense, active layer was 50 wt % PVA (60 mol % crosslinking), 10 wt % AIBA-K, 20 wt % KOH, and 20 wt % polyallylamine. The thickness of the active layer of the membranes ranged from ∼5 µm to ∼80 µm after heat treatment; this was referred to as the thickness of the membrane. Details about the preparation procedure and characterization of this membrane can be found elsewhere.35 2.3. Gas Permeation Measurements. The transport properties of the membrane were measured using a gas-permeation unit, such as that shown in Figure 4. In the experiments, a membrane with a thickness of 30 µm was cut into a circular piece and mounted into a circular permeation cell with an effective area of 45.60 cm2. The circular permeation cell that was used had flow channels on the feed and permeate sides that were designed in a way that both the retentate and the permeate were well-mixed, respectively, inside the cell, so that we could neglect gas-phase boundary layer resistances. A ternary gas mixture that contained 20% CO2, 40% H2, and 40% N2 at a flow rate of 60 mL/min (under ambient conditions) was used as the feed gas. We included H2 in the gas mixture to study the potential of CO2-selective membranes in H2 purification for fuel cells. The total pressure on the feed side was measured using a pressure gauge and set at 2 atm via a back-pressure regulator (Model 44-2362-24, Tescom Co., Elk River, MN). The total pressure of argon sweep gas was set at 1.1 atm, and the flow rate was 30 mL/min (under ambient conditions).

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Figure 2. Synthesis of the cross-linked PVA with formaldehyde.

3. Modeling Work

Figure 3. Chemical structures of (a) free polyallylamine and (b) AIBA-K.

On each of the feed and sweep sides, DI water at a flow rate of 0.03 mL/min was pumped into the gas permeation unit by a solvent delivery pump (Prostar 210, Varian, Inc., Palo Alto, CA). Water vapor and each of the dry gases were mixed well in a humidifier. The testing temperature was adjusted with an oven temperature controller (Model FTU4.6+50×350C, Bemco Inc., Simi Valley, CA). After leaving the gas permeation cell, the water vapors in both the retentate and the permeate were trapped in respective water knockout vessels. The dry gas compositions of both gas streams were analyzed using a gas chromatograph that was equipped with a thermal conductivity detector (Model 6890N, Agilent Technologies, Palo Alto, CA) and a stainless steel micropacked column (80/100 mesh Carboxen 1004, SigmaAldrich, St. Louis, MO). A counter-current flow configuration was applied to offer the maximum driving force across the membrane. 2.4. CO2 Removal Capacity. To illustrate the CO2 removal capacity of the flat-sheet membrane, we constructed a rectangular gas permeation cell, which had a larger effective membrane area of 342.7 cm2 (19.10 cm × 17.95 cm) and welldefined gas-flow channels on both the feed and sweep sides. The feed gas consisted of 1% CO, 17% CO2, 45% H2, and 37% N2 (on a dry basis). The water contents on the feed and sweep sides were ∼35% and ∼95%, respectively. Membranes with the same composition as the previous one but with thicknesses of 20-80 µm were used. The retentate CO2 dry concentrations, from various feed flow rates (ranging from 10 mL/min to 130 mL/min, under ambient conditions), were measured. 2.5. CO2 Capture Experiments. The experiments on CO2 capture from a gas mixture were performed in the circular permeation unit previously described. Compared to other gases, steam is the more desirable sweep gas in the CO2 capture process, because water will condense under ambient conditions and a high-purity CO2 product can be readily obtained. The ternary gas mixture that contained 20% CO2 was used as the feed gas. Various gas flow rates were applied to study the effects of process parameters, such as the feed flow rate and the sweep: feed molar ratio, on the CO2 recovery and the permeate CO2 purity. The experimental conditions are listed in Table 1. All of the CO2 capture experiments were performed at 110 °C for the optimal membrane transport properties. Feed-side and sweep-side pressures were set at 2 and 1.1 atm, respectively. The membrane thickness was 28 µm. The CO2 recovery was defined as the ratio of the permeate CO2 molar flow rate to the feed inlet CO2 molar flow rate.

It is interesting to evaluate the separation performance of the CO2-selective membrane module through the modeling based on the transport data obtained experimentally. In the modeling work, a one-dimensional isothermal model was configured to simulate the CO2 capture from the flue gas with a hollow-fiber membrane module that was composed of the CO2-selective membrane previously described. The following assumptions were made in the model:8 (1) The module is isothermal and operating at a steady state; (2) The module is operated with a counter-current configuration; (3) There is no axial mixing; (4) The pressure drops on both the lumen and shell sides are negligible. Based on the differential volume element on a single hollow fiber along the length direction, as shown in the lower righthand side of Figure 5, the molar balances for gas species i on the feed (lumen) side and the sweep (shell) side can be expressed as eqs 5 and 6, respectively:

dnfi dz dnsi dz

) -πdinJi

(5)

) -πdinJi

(6)

where Ji is expressed as

( )

J i ) Pi

∆pi l

(7)

The feed gas was assumed to be a simulated flue gas mixture that contained 9% CO2, 11% H2O, and 80% N2 at a flow rate of 1000 standard cubic feet per minute (SCFM) (or 21.06 mol/ s). Pure steam was used as the sweep gas. The operating temperature was assumed to be 110 °C, and both the feed-side and sweep-side pressures were set at 1 atm, considering the real case of the flue gas from power plants. The inlet sweep:feed molar flow rate ratio was set at 3. The bVp4c solver in Matlab was used to solve the aforementioned differential equations of the boundary value problem. During the calculations, the hollow fiber number was adjusted to satisfy a constraint of CO2 recovery (i.e., > 95%). In addition, a similar flat-sheet model was applied to simulate CO2 removal experiments with the rectangular cell described earlier. The same geometric dimensions and process parameters as those in the experiment were used in the calculations. 4. Results and Discussion 4.1. Transport Properties of the CO2-Selective Membrane. The transport properties of the gas separation membrane are

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Figure 4. Schematic of the gas permeation unit. Table 1. Operating Parameters for the CO2 Capture Experiments Feed Side sweep-side dry gas water total water flow rate flow rate flow rate flow rate sweep:feed experiment (mL/min)a (mL/min) (10-5 mol/s) (mL/min) molar ratio 1 2 3 4 5 6 7 8 a

40 60 80 100 80 80 80 80

0.02 0.03 0.04 0.05 0.04 0.04 0.04 0.04

4.58 6.86 9.15 11.44 9.15 9.15 9.15 9.15

0.050 0.075 0.100 0.125 0.050 0.100 0.150 0.200

1.0 1.0 1.0 1.0 0.5 1.0 1.5 2.0

Under ambient conditions.

characterized primarily by permeability and selectivity. The permeability of a gas species i is defined in terms of the steadystate flux and the thickness-normalized pressure driving force. It can be calculated by rearranging eq 7:

Pi )

Ji ∆pi/l

(8)

The common unit of permeability is Barrer (1 Barrer ) 10-10 cm3 (STP) cm/(cm2 s cmHg)). The selectivity between two components i and j in a gas mixture may be expressed as

Rij )

yi/yj xi/xj

(9)

A schematic of the CO2 separation in the permeation cell is shown in Figure 6. Based on the measured gas compositions of both the retentate and the permeate, the mass balance was performed with the assumption of a constant argon molar flow rate on the sweep side. The CO2 permeability and CO2/N2 selectivity were then calculated from eqs 8 and 9. Figure 7 shows the membrane CO2 permeability and CO2/ N2 selectivity, as a function of temperature. As the temperature increased from 100 °C to 110 °C, both the permeability and selectivity increased, which could be explained by the significantly higher CO2/amine reaction rate but relatively stable N2 permeability in the membrane at the elevated temperature. A CO2 permeability of 6196 Barrers and a CO2/N2 selectivity of 493 were obtained at 110 °C. As the temperature increased to

Figure 5. Schematic of the hollow-fiber membrane module.

g120 °C, both permeability and selectivity gradually decreased, because the water content in the membrane decreased with increasing temperature. However, a CO2 permeability of >2000 Barrers and a CO2/N2 selectivity of >40 were still observed at 170 °C, which demonstrated that PVA, as a hydrophilic polymer, was a desirable polymeric matrix for the CO2-selective facilitated transport membrane. Crosslinking was necessary for PVA to hold the membrane structure while maintaining enough water content at the high temperatures. Compared to the traditional facilitated transport membranes immobilized in microporous supports, such as ILMs, embedding carriers in cross-linked PVA matrix may significantly improve the long-term membrane stability, because the matrix has strong interactions with the nonvolatile mobile carrier, including hydrogen bonding. AIBAK, poly(allylamine), and KHCO3-K2CO3 (converted from KOH) were the carriers to facilitate CO2 transport, as discussed previously. In principle, the mobile carrier and the fixed carrier may have advantages on carrier mobility/membrane permeability33 and membrane stability, respectively. The optimal combination of both types of carriers is of great importance to the overall membrane performance, particularly for membrane permeability.33 Almost all of the CO2 facilitated transport studies reported in the literature were conducted at approximately ambient temperature. The excellent transport properties of this CO2selective membrane at the high temperatures would be more

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Figure 6. Schematic of gas separation with the CO2-selective membrane.

Figure 7. CO2 permeability and CO2/N2 selectivity, as a function of temperature. Feed gas: 20% CO2, 40% H2, and 40% N2 (on a dry basis); water content in feed ) 41%; pf ) 2 atm, ps ) 1.1 atm; membrane thickness ) 30 µm.

Figure 8. Exit dry CO2 concentration in the retentate versus the feed flow rate. Feed gas: 1% CO, 17% CO2, 45% H2, and 37% N2 (on a dry basis); water content in feed ) 35%; pf ) 2 atm, ps ) 1.1 atm; membrane thickness ) 20-80 µm.

desirable to CO2 capture from the flue gas, because steam could be used as the sweep gas and operating at higher temperatures would reduce the energy loss from cooling the flue gas. 4.2. CO2 Removal Capacity. A rectangular gas permeation cell was used to study the CO2 removal capacity. The welldefined gas-flow channels of this cell allowed us to compare the experimental data with the results from the one-dimensional model, which assumed no axial mixing. CO2 removal experiments were conducted using the rectangular cell at a temperature of 120 °C with different feed flow rates. As shown in Figure 8, the membrane removed CO2 in the retentate, from 17% to ∼100 ppm, at a feed gas flow rate of 60 mL/min (under ambient conditions) and to 1000 ppm at a flow rate of 120 mL/min (under ambient conditions). Such effectiveness in CO2 removal could be attributed to the high CO2 permeability demonstrated in Figure 7. The variation of data in Figure 8 was presumably due to the membranes with different thicknesses that were used in the experiments, because the thicker membrane, with the higher mass-transfer resistance, resulted in a higher exit dry CO2 concentration on the feed side. The modeling results, based on the CO2 transport properties

Figure 9. Permeate CO2 dry concentration and CO2 recovery versus the feed flow rate. Feed gas: 20% CO2, 40% H2, and 40% N2 (on a dry basis); water content in feed ) 41%; pf ) 2 atm, ps ) 1.1 atm; membrane thickness ) 28 µm.

obtained experimentally, are also shown in Figure 8. The experimental data have agreed well with the modeling results. This suggests that the one-dimensional model can accurately describe the gas permeation process in the rectangular cell. 4.3. CO2 Capture Performance. Although flue gases from different sources might have various compositions, N2, CO2, and water vapor are generally the major components, accounting for >90% of the gas stream. After the dry feed gas was mixed with the water vapor in the vessel in the experimental setup, the total feed gas composition used was ∼12% CO2, ∼24% H2, ∼24% N2, and ∼40% H2O, which would provide a close estimate of the CO2 capture from the real flue gas. Using the circular permeation cell, we have demonstrated the CO2 capture performance with different feed inlet flow rates. The enhancing effect of the sweep gas on the separation performance was also investigated experimentally. 4.3.1. Effect of Feed Inlet Flow Rate. To study the effect of the feed molar flow rate on the membrane separation performance, the CO2 capture experiments were performed with four different flow rates while the feed gas composition and the sweep:feed ratio were kept constant (refer to experiments 1-4 in Table 1). As shown in Figure 9, a permeate CO2 dry concentration of >98% was achieved for feed flow rates as high as 11.44 × 10-5 mol/s (a 100 mL/min dry feed gas flow rate, as shown in Table 1). Furthermore, if we deducted the H2 portion from the permeate, the CO2 dry concentration would be >99%. Obviously, the high CO2/N2 selectivity accounted for this high permeate CO2 purity. A slight increase in the permeate CO2 dry concentration was observed with the increases in the feed flow rate. This was due to the fact that the higher water flow rate that was associated with the higher feed rate (to keep the same water content in the feed gas) was more favorable for keeping a higher water content in the membrane, which was also more favorable for the facilitated transport mechanism. The CO2 recovery decreased as the feed inlet flow rate increased,

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Figure 11. CO2 concentration profiles along the length of the membrane module. Figure 10. Permeate CO2 dry concentration and CO2 recovery versus the sweep-to-feed molar ratio. Feed gas: 20% CO2, 40% H2, and 40% N2 (on a dry basis); water content in feed ) 41%; pf ) 2 atm, ps ) 1.1 atm; membrane thickness ) 28 µm.

which was understandable, considering that the residence time of the gas mixture in the permeation cell was reduced as the feed inlet flow rate increased. 4.3.2. Effect of the Sweep:Feed Ratio. The impact of the sweep gas on the CO2 capture was investigated using four sweep water rates while the gas and water flow rates on the feed side were kept constant. As shown in Table 1 (from experiments 5-8), the inlet sweep:feed molar flow rate ratios were 0.5, 1, 1.5, and 2, respectively. Figure 10 illustrates the effect of the sweep:feed ratio on the permeate CO2 dry concentration and the CO2 recovery. The permeate CO2 dry concentrations were all >95% and did not change significantly as the sweep:feed ratio increased. The slightly declining trend was presumably due to the fact that a slightly larger amount of CO2 was absorbed in the water knockout when a higher water flow rate on the sweep side was used. The CO2 recovery increased significantly as the sweep:feed ratio increased from 0.5 to 1.5. A higher sweep:feed ratio resulted in a lower CO2 concentration on the sweep side and then a higher CO2 permeation driving force. Beyond a sweep:feed ratio of 1.5, higher sweep:feed ratios did not show a higher CO2 recovery, which suggested that the excessively higher steam rate on the sweep side would not significantly increase the permeation driving force, because the sweep-side CO2 concentration was already sufficiently low. 4.4. Modeling Study. As a common type of commercialized membrane modules, the hollow-fiber membrane module has shown excellent mass-transfer performance, because of its large surface area per unit volume.4 In the modeling calculations, hollow fibers were assumed to have a length of 61 cm (∼2 ft), an inner diameter of 0.1 cm, and a porous support with a porosity of 50% and a thickness of 30 µm. The membrane was assumed to have an effective thickness of 5 µm, a CO2/N2 selectivity of 500, and a CO2 permeability of 6000 Barrers. These values of the CO2/N2 selectivity and the CO2 permeability were based on the experimental data shown previously. As reported in an earlier paper from our group,35 a constant CO2 permeability of ∼6000 Barrers for a CO2 partial pressure of 95% of the CO2 and obtain a permeate CO2 concentration of >98% (on a dry basis) from a 1000 SCFM (or 21.06 mol/s) flue gas stream that contained 9% CO2.

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Nomenclature d ) hollow fiber diameter (cm) J ) permeation flux (mol/(cm2 s)) l ) membrane thickness (cm) n ) molar flow rate (mol/s) p ) pressure (atm) P ) permeability (Barrer) x ) retentate molar fraction y ) permeate molar fraction z ) axial position along the length of reactor (cm) Greek Letters R ) selectivity Subscripts f ) feed side i, j ) species in ) inside of the hollow fiber s ) sweep side Acknowledgment We would like to thank Chris Plotz and BHA Technologies, Inc., for giving us the BHA microporous Teflon support used in this work. We would also like to thank the National Science Foundation (NSF), the Ohio Department of Development (through Wright Center of Innovation Grant No. 342-0561), and The Ohio State University for the financial support. Part of this material is based on work supported by the NSF, under Grant No. 0625758. Literature Cited (1) U.S. Department of Energy. Emissions of greenhouse gases in the United States 2004. Available via the Internet at http://www.eia.doe.gov/ oiaf/1605/ggrpt/pdf/057304.pdf, 2006. (2) Aaron, D.; Tsouris, C. Separation of CO2 from flue gas: a review. Sep. Sci. Technol. 2005, 40, 321-348. (3) Zolandz, R. R.; Fleming, G. K. Gas permeation. In Membrane Handbook; Ho, W. S. W., Sirkar, K. K., Eds.; Chapman and Hall: New York, 1992. (4) Ho, W. S. W.; Sirkar, K. K., Eds. Membrane Handbook; Chapman and Hall: New York, 1992. (5) Kesting, R. E.; Fritzche, A. K. Polymeric Gas Separation Membranes; Wiley: New York, 1993. (6) Koros, W. J.; Fleming, G. K. Membrane based gas separation. J. Membr. Sci. 1993, 83, 1-80. (7) Xomeritakis, G.; Tsai, C.-Y.; Brinker, C. J. Microporous sol-gel derived aminosilicate membrane for enhanced carbon dioxide separation. Sep. Purif. Technol. 2005, 42, 249-257. (8) Huang, J.; El-Azzami, L.; Ho, W. S. W. Modeling of CO2-selective water gas shift membrane reactor for fuel cell. J. Membr. Sci. 2005, 261, 67-75. (9) Zou, J.; Huang, J.; Ho, W. S. W. CO2-selective water gas shift membrane reactor for fuel cell hydrogen processing. Ind. Eng. Chem. Res. 2007, 46, 2272-2279. (10) Meisen, A.; Shuai, X. Research and development issues in CO2 capture. Energy ConVers. Manage. 1997, 38 (Suppl.), S37-S42. (11) Koros, W. J.; Mahajan, R. Pushing the limits on possibilities for large scale gas separation: which strategies. J. Membr. Sci. 2000, 175, 181196. (12) Bredesen, R.; Jordal, K.; Bolland, O. High-temperature membranes in power generation with CO2 capture. Chem. Eng. Process. 2004, 43, 1129-1158. (13) Hirayama, Y.; Kase, Y.; Tanihara, N.; Sumiyama, Y.; Kusuki, Y.; Haraya, K. Permeation properties to CO2 and N2 of poly(ethylene oxide)-

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ReceiVed for reView June 8, 2007 ReVised manuscript receiVed November 12, 2007 Accepted November 15, 2007 IE070794R