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Permeation of Carbon Dioxide and Methane Gases through Novel Silver-Incorporated Thin Film Composite Pebax Membranes S. Sridhar,†,§ T. M. Aminabhavi,*,† S. J. Mayor,‡ and M. Ramakrishna§ Membrane Separations DiVision, Center of Excellence in Polymer Science, Karnatak UniVersity, Dharwad 580 003, India; Permionics Membranes PVt. Ltd., Vadodara 390 016, India; and Chemical Engineering DiVision, Indian Institute of Chemical Technology, Hyderabad 500 007, India
The selectivity of a membrane for CO2/CH4 separation is critical for purification of natural gas, landfill gas, and biogas since CO2 is the major impurity present in gas mixtures while CH4 is the major hydrocarbon constituent. The performance of poly(ether-block-amide) (Pebax 2533) membrane and its silver-incorporated form were investigated with respect to their permeation properties for these two gases. Thin-film composite (TFC) membranes of Pebax were prepared by dissolving 20 g of the polymer in 80 mL of 3:7 volumetric ratio of m-cresol/isopropanol solvent mixture, followed by casting on polyvinylidene fluoride (PVDF) ultrafiltration substrate. Silver incorporation was carried out by loading silver tetrafluoroborate (AgBF4) at 36% of the Pebax polymer weight in the same dope solution. Membranes were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to study intermolecular interactions, crystalline nature, and surface and cross-sectional morphologies, respectively. At a feed pressure of 1 MPa, Pebax gave a CO2 permeance of 32.8 GPU at a CO2/CH4 selectivity of 15.3, whereas Ag-Pebax exhibited an enhanced selectivity of 36.2, even though its permeance was reduced to 12.1 GPU. The Ag-Pebax membrane was further upscaled as a spiral-wound module of 0.20 m2 effective area to test its commercial feasibility. 1. Introduction Membrane-based processes, because of some inherent advantages, are becoming increasingly more competitive than the conventional gas separation processes.1 Receiving particular attention in recent years is the use of membranes in a solubilityselective mode, wherein more soluble gaseous species permeate preferentially through the membranes.2,3 For specific applications, we require the removal of heavier species present in dilute concentrations, such as CO2 and H2S, from the lighter species, but for the separation of the major constituent, CH4 in natural gas or biogas, the membrane-based method is particularly attractive. The preferential permeation of heavier species translates into lower surface area requirements, and the cleaned lighter component (CH4) is available at a pressure close to that of the feed. CO2 and H2S would cause corrosion and lowering of the calorific value, and hence, these are presently separated by absorption in aqueous solutions of monoethanolamine and diethanolamine.4,5 However, the conventional method is not only energy-intensive and polluting but also has other drawbacks such as solvent-regeneration problems, large space requirements, high labor costs for control and maintenance, and corrosive nature of the solvents. Therefore, the requirement of an economical, safe, and ecofriendly alternative separation technology is the need of the day. In this context, separation using membranes is expected to play an important role in tomorrow’s chemical industry because of lower capital costs, high recovery rates, greatly reduced electric power and fuel consumption, and easy and cleaner mode of operation involved compared to the conventional separation techniques.2 Further, the membrane * Corresponding author. E-mail:
[email protected]. Phone: 91-836-2215372. Fax : 91-836-2771275. † Karnatak University. ‡ Permionics Membranes Pvt. Ltd. § Indian Institute of Chemical Technology.
units can be assembled into compact modules, resulting in minimal space requirements and economical installation costs. Particularly, the membrane-based gas separation looks quite attractive for purification of natural gas mixtures, provided suitable polymeric materials are developed that give high CO2 permeability as well as good separation characteristics for the CO2/CH4 mixture.6 The development of inexpensive, but upscalable polymeric membranes capable of giving high separation factors and permeabilities is a challenging task. Numerous attempts have been made to develop membranes giving high permeability and selectivity for CO2/CH4 separation.7-11 Among the many polymers studied, cellulosic polymers as well as aromatic polymers such as polycarbonates, polyimides, and polysulfones have offered optimum results.12 In our earlier work,13 we pursued the development of modified poly(phenylene oxide) membranes for the separation of CO2 from CH4. Poly(ether-block-amide) (Pebax) is a nonplasticized flexible thermoplastic elastomer having a soft polyether block and a rigid polyamide block.14 Pebax is reported to show a microphase separation morphology in the solid state because of the high polarity difference between its hard and soft segments.15-17 Its high mechanical strength, good temperature, and chemical resistance properties appear to be promising for gas-separation studies involving the removal of CO2 and H2S from the natural gas streams.18-20 Membrane Technology and Research, Inc. (Menlo Park, CA) reported21 a selectivity of 13 for CO2/CH4 and 50 for H2S/CH4 gas mixtures with Pebax 4011 for a mixed-gas feed composition of 4% CO2 and 100 ppm H2S (balance CH4) at a feed pressure of 7 MPa. Pebax has been reported to be quite hydrophilic,21,22 which means that H2O vapor present in natural gas can also be removed at a rapid rate along with CO2 and H2S. In order to improve its gas-permeation characteristics, Pebax has been modified by doping with inorganic fillers such as silica23 or titanium oxide.24 Muller et al.25 modified Pebax 2533 and 4011 polymers by silver incorporation using AgClO4 and AgBF4 and
10.1021/ie070114k CCC: $37.00 © 2007 American Chemical Society Published on Web 10/30/2007
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Figure 1. (a) General structure of Pebax polymer and (b) photographs of Pebax 2533 membrane (white) and Ag-Pebax membrane (reddish-brown).
Figure 2. (a) Experimental manifold for gas permeability measurement, (b) design drawing of spiral-wound Ag-Pebax gas-separation element, and (c) photograph of spiral membrane module with stainless steel housing.
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Figure 3. FTIR spectra of (a) Pebax-htc and (b) Ag-Pebax membranes.
reported their selectivities to be as high as 110 for ethane/ ethylene separation for Pebax 2533 loaded with 36% Ag+. In the present study, Pebax 2533 was loaded with 36% AgBF4 to improve its CO2 selectivity. Silver is superior to other metals in electrical property, antimicrobial effect, optical properties, and oxidation catalysis.26 Silver salts such as AgBF4 readily dissolve in commercially available polar polymers such as Pebax, which has the ability to solvate silver salts into active Ag+ for complexation with the polymer matrix.27 The electropositive silver metal cations undergo ion-dipole interactions with electron-rich oxygen atoms such as those present in the ether block of Pebax to form a stable composite membrane.26 The silver-incorporated Pebax membranes of this study were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Further, the Ag-Pebax 2533 membrane was spirally wound into a gas-separation element and tested for its commercial feasibility. 2. Experimental Section 2.1. Materials. Pebax 2533 (polymer grade) was purchased from Atofina Chemicals, France. Dimethyl formamide, m-cresol, and isopropanol solvents were purchased from Loba Chemie, Mumbai, India. Poly(vinylidene fluoride) (PVDF) was supplied by Solvay Advanced Polymers, OH. Its molecular weight (M h n) was 146 000, density (F) ) 1.02 g/cm3, and glass transition temperature (Tg) ) -30 °C. Fabrication of the spiral element was done at Permionics Membranes Pvt. Ltd., Vadodara, India. AgBF4 was supplied by Aldrich Chemical Co., Milwaukee, WI. Cylinders of CH4 and CO2 gases of >99.9% purities were supplied by Inox Air Products Ltd., Mumbai, India. 2.2. Membrane Preparation. Ultraporous PVDF substrate was prepared by phase-inversion technique using a solution containing 25 g of polymer in 71 mL of N,N′-dimethyl formamide solvent with 4 mL of propionic acid as a surfactant additive. This particular additive was expected to cause shrinkage of the pores during membrane formation. The solution was cast on a nonwoven polyester membrane support fabric, which had already been fastened tightly onto a clean glass plate without any air gaps. The glass plate was then immersed in an ice-cold water bath for 5 min for gelling the polymer film. The molecular weight cutoff (MWCO) of the PVDF support was ∼10 000 Da using the aqueous solutions containing 2 000 ppm of polyeth-
ylene glycol (PEG) of different molecular weights. The support was found to reject 80-85% of PEG of 10 000 molecular weight at 0.35 MPa pressure. Thin-film composite (TFC) membranes of Pebax were prepared on a PVDF ultrafiltration substrate by solution casting and solvent-evaporation technique. Pebax (20 g) was dissolved in 80 mL of 3:7 volumetric ratio of m-cresol (24 mL) and isopropanol (56 mL) solvent mixture, and the bubble-free solution was cast onto the PVDF support to the desired thickness using a thin-layer chromatography (TLC) applicator (model AS 30, Desaga), which consisted of an aluminum casting blade. A doctor’s blade was used to adjust the gap between the ultraporous PVDF substrate and the aluminum casting blade to ∼200 µm. Solvent was evaporated in an oven at 150 °C for 3-5 min to obtain Pebax-htc (high-temperature cured) TFC membrane, which was used in separation studies. The chemical structure of Pebax polymer is given in Figure 1a, where x represents the average number of soft polyether repeat units in a segment and y is the average number of rigid polyamide repeat units of the segment. In the case of Pebax 2533, polyether units comprised 80% of the polymer chain and polyamide constituted 20%. Silver incorporation was carried out by adding 7.2 g of AgBF4 to 12.8 g of Pebax (36% loading) followed by dissolution in 80 mL of the same mixed-solvent medium. The procedure followed in this study for making Pebax-htc membrane was repeated to obtain high-temperature cured Ag-Pebax membrane. In view of the instability of silver ions, the Ag-Pebax membranes were cast in a fume cupboard to avoid direct sunlight with continuous purging of inert N2 gas. All the membranes were vacuum-dried in an oven at 40 °C for 24 h to ensure the complete removal of residual solvents. The total membrane thickness was determined using a micrometer (accuracy ( 0.1 µm) and was found to be 157 µm, which also includes the nonwoven polyester fabric (100 µm) and the ultraporous PVDF substrate (45-50 µm) layers. The exact thickness of the effective Pebax skin layer is unknown since the extent of its penetration into the ultraporous PVDF substrate could not be determined with SEM. Moreover, the Pebax layer could not be peeled off exclusively from the PVDF substrate. Therefore, the permeance of the membranes is reported instead of permeability. The casting thickness for Pebax and Ag-Pebax was kept constant at 200 µm for appropriate comparison of their performances. The addition of silver does not affect the thickness, but casting very thin Ag-Pebax membranes induced defects in the membranes formed. A photograph of Pebax and Ag-Pebax membranes is shown in Figure 1b. 2.3. Permeability Studies. A schematic diagram of the highpressure gas separation manifold used in permeability studies is given in Figure 2a. A permeability cell of stainless steel 316 having an effective area of 0.0042 m2 was designed and fabricated in-house. Feed and permeate lines in the manifold were made of stainless steel piping of 1/4 in. outer diameter (o.d.) connected together by means of compression fittings. The carrier gas cylinder was used only to determine the permeation of methane at lower pressures (1-2 MPa) by the continuous flow method, which was described in detail earlier along with the requisite analytical procedure.13 For all other experiments, the permeate flow rate was sufficient enough for direct measurement of its flow through a soap bubble meter or rotameter, which meant that only the valve to the flow indicator was kept open, whereas all other needle valves in the permeate line were closed. While working with the spiral module, the soap bubble meter was replaced with a wet gas meter to account for high permeate rates. Permeate outlet was also kept partially open to avoid the
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Figure 4. X-ray diffractograms of (a) Pebax and (b) Ag-Pebax membranes.
buildup of pressure more than 0.5 MPa. Housing of the spiral module was also provided with threaded-end connections, which suited 1/4 in. nut and ferrule fitting, to enable the easy replacement of the permeability cell with the module in the same manifold. Any variation in feed pressure due to high permeate flux was compensated by a constant throttling of the feed cylinder regulator. Experiments were conducted at ambient temperature (30 °C) and repeated thrice to ensure reproducibility of the data. Instead of permeability coefficient, permeance (K) is reported throughout this work, which was calculated as
K)
Q tA(P1 - P2)
(1)
where Q is the volume of permeated gas (m3) at STP; t is the permeation time (s); A is the effective membrane area (m2) for gas permeation; and P1 and P2 are feed side and permeate side partial pressures (Pa), respectively. The unit of permeance is denoted as GPU [1 GPU ) 7.5 × 10-12 m3(STP)/(m2‚s ‚Pa)]. However, it is worth mentioning that the expression in eq 1 assumes well-mixed residue and permeate streams. Selectivity was determined as the ratio of permeances of CO2 and CH4, respectively, using the equation
R ) KCO2/KCH4
(2)
2.4. Fabrication of Spiral Ag-Pebax Gas-Separation Element. The spiral-wound element is one of the most economical configurations in terms of accommodating maximum surface area of the membrane in a given volume (900 m2 m-3). The spiral Ag-Pebax element fabricated in-house consisted of essentially five components (parts b and c of Figure 2): (i) perforated permeate central tube, which collects the permeate; (ii) permeate spacer, which carries the permeate to the central tube; (iii) membrane, which is the active separation media of 0.2 m2 effective area; (iv) feed spacer that separates the two membrane leaves, facilitating the flow of feed gas across the
membrane; and (v) gas seal, which is similar to a brine seal in a reverse-osmosis spiral module. The seal provides a tight-fitting junction at the feed inlet end to prevent the bypass of feed in the membrane housing by ensuring that the entire gas enters the membrane module without escaping into the annular space between membrane module and housing. Glue was applied over the carrier on three sides at the edges. The glue line was at the borders having a width of ∼1.5 in. The membrane was folded over in half and inserted into the region where the carrier met the central tube. The membrane was then wound along with the carrier cloth after inserting the feed spacer between the folded membranes, thus separating the two membrane surfaces from each other. In the case of a small element, only one membrane envelope is wound as in the present work. Once the diameter is reached, an adhesive tape was wrapped around to keep the element from opening up. The module was then kept aside overnight for the glue to harden/ cure. Next day, the sides of the element were trimmed and then the element was finished and readied for testing after connecting it to the feed gas cylinder. Figure 2b represents the design drawing of the miniature spiral Pebax 2533 module having an effective membrane area of 0.2 m2. The total length of permeate central tube was 15 in. (0.375 m), whereas the wounded membrane part was 11 in. long. The outer diameter of the permeate tube was 0.75 in., whereas the maximum diameter of the element was 1.4 in. at any point. The module was housed in a stainless steel 316 pressure vessel, which consisted of a cylindrical tube of 14 in. length, 1.5 in. internal diameter (i.d.), and thickness 0.6 in. with flange fittings of 5.6 in. diameter welded at both ends. The gas seal was slipped onto the element and then taped to it. The module was then inserted into the housing. Detachable stainless steel 316 end flanges of matching dimensions were fixed at each end by means of six pairs of nuts and bolts. The thickness of each flange was 0.4 in. such that the total length of the housing was 15.6 in. (0.39 m). Two small O-rings, each of i.d. equal to the outer diameter (o.d.) of the permeate central tube, were provided in
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Figure 5. SEM pictures representing the surface morphologies of (a) PVDF, (b) Pebax, and (c) Ag-Pebax membranes.
the internal grooves of the end flanges for a leak-tight arrangement, which prevented the mixing of feed and permeate gases. Figure 2c is the actual photograph of the fabricated spiral AgPebax module with stainless steel housing. 2.5. Membrane Characterization. 2.5.1. FTIR Studies. FTIR spectra of the Pebax membrane dried at room temperature (Pebax-rtd) and membrane prepared after high-temperature drying (Pebax-htc) as well as Ag-Pebax were scanned between 4000 and 400 cm-1 using Perkin-Elmer-283B FTIR spectro-
meter (Boston, MA). Permeability studies were carried out with Pebax-htc membrane only. 2.5.2. XRD Analysis. Siemens D 5000 powder X-ray diffractometer (NJ) was used to measure the solid-state morphology of Pebax 2533 and Ag-Pebax in powder form. X-rays of 1.5406 Å wavelength were generated by Cu K_ source. The angle (2θ) of diffraction was varied from 0° to 65° to identify the crystal structure and the intermolecular distances between intersegmental chains.
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2.5.3. SEM. The morphology of Pebax 2533 and Ag-Pebax membranes were studied by SEM using a Hitachi S2150 microscope (Ibaraki, Japan). 3. Results and Discussion Pebax would resemble polyurethane showing limited compatibility between two types of segments because of the hydrogen bonding that links the N-H group in the hard segment to either the ether group in the soft segment or the carbonyl group of the ester.28 Two different methods have been developed to monitor the evolution of hydrogen bonding. One is to study the area change of the N-H stretch band,29 and the other is to trace the area changes of the stretch bands of the hydrogenbonded carbonyl group as well as the free carbonyl group. We adopted the latter approach and concentrated on investigating both stretch bands of the hydrogen-bonded and free carbonyl group in the ester linkage between the soft segments. The second reason is that the FTIR spectra of the latter will directly reveal the information about the compatibility of hard and soft segments. 3.1. FTIR Studies. FTIR spectra of Pebax-rtd (roomtemperature dried membrane), Pebax-htc (high-temperature cured membrane), and Ag-Pebax were taken in the wave number range of 4000 to 400 cm-1. However, only the spectra pertaining to Pebax-htc (a) and Ag-Pebax (b) are shown in Figure 3 to avoid confusion. The characteristic peaks at 1740 and 1100 cm-1 in Pebax-rtd and Pebax-htc are attributed to -CdO and -CO- stretching vibrations, respectively. Another two peaks observed at 1640 and 3300 cm-1 indicate the presence of H-N-CdO and N-H groups, respectively. The absorption band attributed to the stretching of the ester carbonyl group appears around 1700 to 1800 cm-1. Two absorption bands have overlapped in this range: one centers at 1739 cm-1, which is the absorption band of hydrogen-bonded carbonyl, while the other centers at 1780 cm-1, representing that of free ester carbonyl, and this band appears as a shoulder attached to the first band. From the spectra of Pebax-rtd (not shown in the figure), it was observed that, at elevated temperature, the absorption band of the ester carbonyl broadens and shifts toward higher wave numbers. However, the change in the area fractions of the hydrogen-bonded carbonyl is negligible in the case of Pebax-htc (Figure 3a). The existence of a hydrogen-bonded carbonyl at the elevated temperature indicates that, within the spherical domains, there is some degree of mixing between the soft and hard segments, and thus, a decrease in the area fraction of the hydrogen-bonded carbonyl implies the dissociation of hydrogen bonding between the two segments. The hightemperature cured Pebax-htc was used in gas permeability studies and is referred to as Pebax throughout this manuscript. Figure 3b shows the FTIR spectra of Ag-Pebax. The metalligand interaction bands are seen28 distinctly in the lowfrequency region (600-400 cm-1). Silver exhibits a distinct new band at 650 cm-1 (due to metal-oxygen interaction) and a small shoulder band at 890 cm-1. Thus, from the FTIR spectra, one can observe that metals have shown interaction with the electrondonating sites, i.e., oxygen sites present in the polymer backbone. 3.2. XRD Studies. Figure 4 shows the XRD results of Pebax and silver-incorporated Pebax. Both diffractograms show strong reflections at 14°, 17.6°, 22.6°, and 25.0° of 2θ. The strong crystalline peak at 2θ ) 25° results mainly from the crystalline region of the polyamide block through the interchain hydrogen bonding.25 In the silver-loaded Pebax (Figure 4b), no reflections of AgBF4 appeared under the measurement conditions. Thus,
Table 1. Permeation Properties of Flat-Sheet Pebax Membranes at 1 MPa membrane type Pebax Ag-Pebax a
flux (J) [10-5 m3(STP)/m2‚s]
permeance (K) (GPUa)
selectivity
CO2
CH4
CO2
CH4
KCO2/KCH4
23.6 8.71
1.54 0.24
32.8 12.1
2.14 0.33
15.3 36.2
1 GPU ) 7.5 × 10-12 [m3 (STP)/(m2‚s ‚Pa)].
it can be shown that the crystals of AgBF4 disappeared in moist conditions. Furthermore, the XRD spectra of Pebax showed a higher number of counts as compared to that of the metalincorporated matrix, thereby rendering the former semicrystalline. The Ag-Pebax appears comparatively more amorphous. The deff, which is the direct measure of polymer packing density (i.e., free volume of the polymer), was calculated for the highest intensity peak. The deff was calculated using Bragg’s first-order X-ray diffraction equation (deff ) λ/2 sin θ), where λ )1.5406 Å. From the spectra, it can be evidenced that the deff was reduced from 5.24 to 5.10 Å upon metal incorporation. However, because of the charged nature of the metal, there seemed to be a shrinkage in the molecular structure of the polymer at the sites of interaction, which, in turn, affected the polymer deff value slightly.31 This also indicates the metal-polymer complexation. This reduction in deff improves the ability of the metal-polymer system to discriminate between the permeating species. 3.3. SEM Studies. SEM pictures of PVDF, Pebax, and AgPebax are shown in Figure 5, wherein it can be observed that the surface morphology of PVDF has pores that are distributed uniformly across the membrane surface. Pores in the support PVDF ultrafiltration membrane disappear after coating with Pebax, as evidenced in Figure 5b at the same magnification factor to reveal a nonporous defect-free surface. Figure 5c, on the other hand, shows the homogeneous dispersion of silver particles in a fibrous form onto the surface of Pebax. A uniform scattering of the incorporated silver occurs because of the interaction between electron-donating sites (i.e., oxygen atoms) in the Pebax repeat unit with silver. However, the hightemperature curing at 150 °C during the membrane-formation process causes some of the silver cations in Pebax to reduce to metallic silver, which would form clusters, as indicated by the presence of some crystal-like objects in Figure 5c.26 3.4. Permeability Results. Table 1 compares the CO2 permeance and selectivities of flat sheet Pebax 2533 membrane and its silver-incorporated form (Ag-Pebax) at a constant feed pressure of 1 MPa and a total barrier thickness of 157 µm. Pebax exhibited a high permeance of 32.8 GPU at a selectivity of 15.3, whereas Ag-Pebax exhibited a selectivity of 36.2, which was nearly two and half times greater with a lower permeance of 12.1 GPU. However, CO2 does not exert any interaction with silver ions.32 The addition of silver salt to Pebax would simply increase the cohesive energy density of the membrane, reducing the permeance and increasing the size-selective nature of the matrix. In other words, adding silver ions would increase the diffusive selectivity of the membrane, which favors the transport of smaller CO2. The diffusion coefficient of CO2 is, thus, higher than that of CH4, since its kinetic diameter is 3.3 Å as compared to 3.8 Å for CH4.12 Figures 6 and 7 display the effect of varying pressure (1-4 MPa) of pure CO2 and CH4 gases on the performance of the composite membranes in flat-sheet form. A gradual increase in selectivity and permeance for both membranes could be attributed to increased sorption of the more soluble CO2 gas in
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Figure 6. Selectivity of Pebax and Ag-Pebax flat-sheet membranes for CO2/CH4 system at different pressures.
Figure 8. Performance of spiral-wound Ag-Pebax module for CO2/CH4 system.
at lower thickness, there could be an absence of continuous polymer layer along the cross section, resulting in the creation of microvoids. Figure 8 displays the performance of the spiral-wound AgPebax gas-separation element over the same range of pressure employed in the case of flat-sheet membranes. The permeance obtained through the module was relatively greater when compared to the flat sheet, and it varied from 15.4 to 21.3 GPU. However, a marginal drop in selectivity (27.8-36.6) was observed, which is attributed to the stretching process undergone by the TFC membrane during machine-based spiral winding. This stretching may cause the thickness of the skin layer at the edges to reduce slightly, which would cause a rise in flux. Reproducibility was verified experimentally by testing the silverincorporated spiral module three times a week over a period of 6 months, and the results obtained were consistent. 4. Conclusions
Figure 7. Effect of feed pressure on CO2 permeance of Pebax and AgPebax flat-sheet membranes.
the membranes. Earlier, we reported increased propylene sorption in ethyl cellulose membrane under similar operating conditions.33 The selectivity of plain Pebax varied from 15.3 to 20.4, whereas that of Ag-Pebax ranged from 36.2 to 47.6, which indicates a significant increase after incorporation of silver (Figure 7). However, the permeance in the case of Ag-Pebax (12.1-15.7 GPU) was reduced to almost one-third the values obtained for pristine Pebax (32.8-44.3 GPU). SEM studies have shown that most of the silver particles are uniformly distributed on the surface as well as on the cross section of the membrane. An inert impermeable filler that has good compatibility with the organic polymer matrix usually decreases the gas permeability, because of the reduction in polymer area through which the gas molecules will diffuse, as well as the tortuous path created for their transport.34 The thickness of the unmodified Pebax membrane can be scaled down in a straightforward manner to achieve a higher permeance. However, this is not possible in the case of AgPebax, for which a minimum thickness of 10-12 µm has to be maintained at this particular loading (36%) to avoid defects that arise through nonuniform deposition of the inorganic filler. Thus,
The present study demonstrated the feasibility of preparing TFC membranes of Pebax 2533 as well as Ag-Pebax on PVDF supports for CO2/CH4 separation. FTIR studies revealed the dissociation of H-bonds between the hard and soft segments of Pebax polymer upon curing the membrane at 150 °C. FTIR showed that silver metal particles have exerted interactions with the electron donating sites, viz. oxygen sites of the polymer backbone, but these are not merely present as fillers. XRD showed that deff got reduced from 5.24 to 5.10 Å upon metal incorporation, which made the membrane more selective to CO2. SEM photographs showed the presence of a nonporous Pebax layer on the porous PVDF substrate apart from exhibiting uniform distribution of silver particles onto the Ag-Pebax membrane surface. The CO2 sorption was enhanced with increasing feed pressure, which improved both flux and selectivity. Ag-Pebax membranes exhibited higher selectivities due to increased cohesive energy density of the membrane, but the permeance was reduced because of the impermeable nature of metal particles, which decreased the membrane area available for permeation and created a tortuous path for transport of gas molecules. In the case of Ag-Pebax, a minimum effective membrane layer thickness of 10-12 µm has to be maintained at this particular loading (36%) to avoid any defects arising from the deposition of the inorganic filler. The spiral-wound Ag-Pebax element was found to be promising for the recovery of CO2 from its mixtures with CH4
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by exhibiting results that not only matched those observed for flat-sheet membranes but were also reproducible over a period of time. However, the stability of the silver-loaded membrane in gas mixtures containing hydrogen sulfide has to be assessed, since silver ions could react with hydrogen sulfide to form impermeable silver sulfide crystals. This will reduce the permeance because of increased tortuosity and decreased selectivity compared to the original polymer value because of a reduction in cohesive energy density. In such a case, a hybrid process of H2S removal through a ZnO adsorption column, followed by CO2 separation using the proposed Ag-Pebax membrane, may be worth pursuing. Acknowledgment T.M.A. thanks the University Grants Commission, New Delhi (Grant No. F1-41/2001/CPP-II), for a financial support to establish Center of Excellence in Polymer Science. S.S. thanks the Department of Scientific and Industrial Research (DSIR), New Delhi, India, for a financial grant to IICT, Hyderabad and Engineers India Ltd. (EIL), Gurgaon, India, for their support. Literature Cited (1) Kesting, R. E.; Fritzsche, A. K. Polymeric Gas Separation Membranes; Wiley: New York, 1993. (2) Koros, W. J.; Fleming, G. K. Membrane-based Gas Separation. J. Membr. Sci. 1993, 83, 1. (3) Freeman, B. D.; Pinnau, I. Separation of Gases using Solubility Selective Polymers. Trends Polym. Sci. 1997, 5, 167. (4) Astarita, G; Savage, D. W.; Bisio, A. Gas Treating with Chemical SolVents; John Wiley & Sons: New York, 1983. (5) Kohl, A. L.; Riesenfeld, F. C. Gas Purification, 2nd ed.; Gulf Publishing Co.: Houston, TX, 1974. (6) Hao, J.; Rice, P. A.; Stern, S. A. Upgrading Low-Quality Natural Gas with H2S- and CO2-Selective Polymer Membranes. Part I. Process Design and Economics of Membrane Stages without Recycle Streams. J. Membr. Sci. 2002, 209, 177. (7) Koros, W. J.; Mahajan, R. Pushing the Limits on Possibilities for Large Scale Gas Separation: Which Strategies? J. Membr. Sci. 2000, 175, 181. (8) Kim, K. J.; Park, S. H.; So, W. W.; Ahn, D. J.; Moon, S. J. CO2 Separation Performances of Composite Membranes of 6FDA-based Polyimides with a Polar Group. J. Membr. Sci. 2003, 211, 41. (9) Wind, J. D.; Paul, D. R.; Koros, W. J. Natural Gas Permeation in Polyimide Membranes. J. Membr. Sci. 2004, 228, 227. (10) Tin, P. S.; Chung, T. S.; Liu, Y.; Wang, R. Separation of CO2/ CH4 through Carbon Molecular Sieve Membranes Derived from P84 Polyimide. Carbon 2004, 42, 3123. (11) Bhide, B. D.; Voskericyan, A.; Stern, S. A. Hybrid Process for the Removal of Acid Gases from Natural Gas. J. Membr. Sci. 1998, 140, 27. (12) Ho, W. S. W., Sirkar, K. K., Eds. Membrane Handbook; Van Nostrand Reinhold: New York, 1992. (13) Sridhar, S.; Smitha, B.; Ramakrishna, M.; Aminabhavi, T. M. Modified Poly(phenylene oxide) Membranes for the Separation of Carbon Dioxide from Methane. J. Membr. Sci. 2006, 280, 202. (14) Sheth, J. P.; Xui, J.; Wilkes, G. L. Solid State Structure-Property Behavior of Semicrystalline Poly(ether-block-amide) Pebax Thermoplastic Elastomers. Polymer 2003, 44, 743.
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ReceiVed for reView January 18, 2007 ReVised manuscript receiVed May 27, 2007 Accepted July 23, 2007 IE070114K
