Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane

Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane

Journal of Membrane Science 280 (2006) 202–209 Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane夽 S. Sridhar...

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Journal of Membrane Science 280 (2006) 202–209

Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane夽 S. Sridhar a , B. Smitha b , M. Ramakrishna b , Tejraj M. Aminabhavi a,∗ a

b

Membrane Separations Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad 580003, India Membrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology, Hyderabad 500007, India Received 22 September 2005; received in revised form 9 January 2006; accepted 9 January 2006 Available online 2 March 2006

Abstract Two types of poly(phenylene oxide) (PPO) membranes were prepared: one by chemical modification through sulfonation using chlorosulfonic acid and another by physical incorporation with a heteropolyacid (HPA), viz., phosphotungstic acid. These membranes were tested for the separation of CO2 /CH4 mixtures. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction techniques were used to confirm the modified structure of PPO as well as to understand its interactions with gaseous molecules. Scanning electron microscopy (SEM) was used to investigate the membrane morphology. Thermal stability of the modified polymers was assessed by differential scanning calorimetry (DSC), while the tensile strength was measured to evaluate their mechanical stability. Both chemical and physical modifications did not adversely affect the thermally and mechanical stabilities. Experiments with pure CO2 and CH4 gases showed that CO2 selectivity (27.2) for SPPO increased by a factor of 2.2, while the PPO–HPA membrane exhibited 1.7 times increase in selectivity with a reasonable permeability of 28.2 Barrer. An increase in flux was observed for the binary CO2 /CH4 mixture permeation with an increasing feed concentration (5–40 mol%) of CO2 . An enhancement in feed pressure from 5 to 40 kg/cm2 resulted in reduced CO2 permeability and selectivity due to the competitive sorption of methane. Both the modified PPO membranes were found to be promising for enrichment of methane despite exhibiting lower permeability values than the pristine PPO membrane. © 2006 Elsevier B.V. All rights reserved. Keywords: Gas separation; Heteropolyacids; Sulfonated poly(phenylene oxide); Mixed matrix membranes; CO2 /CH4 separation

1. Introduction Separation and purification of gases by selective permeation of one or more components of a gaseous mixture through a polymeric membrane has attracted considerable interest over the past decades [1–4]. One of the challenging gas separation problems in process engineering is that of CO2 /CH4 system due to its significance in natural gas purification. However, identification of new polymeric membrane materials for this separation has been an important objective of membrane researchers [5–10]. Apart from mechanical strength, chemical resistance and durability, the two most important criteria governing the membrane selection for CO2 /CH4 separation or any other gaseous mixture, are the productivity and separation efficiency [2]. Generally, a trade-off exists between gas permeability and separation factor

夽 ∗

This paper is CEPS Communication #99. Corresponding author. Tel.: +91 836 2215372; fax: +91 836 2771275. E-mail address: [email protected] (T.M. Aminabhavi).

0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.01.019

for the common engineering polymers [11]. Modifications in morphology or chemical structure of the polymer bring about improvements in either flux or selectivity [12–14]. Development of mixed matrix materials such as polymer membranes filled with nano-sized particles has been under focus in recent times [15–19]. Duval et al. [15] observed that zeolites such as silicalite-1, 13X and KY tend to improve CO2 /CH4 separation properties of the poorly selective rubbery polymers. Martin et al. [17] used the interfacial polymerization technique to deposit thin films of polypyrrole, poly(N-methylpyrrole) and polyaniline onto the surfaces of microporous support membranes. The CO2 /CH4 selectivities of 16.2 and 31.9 were obtained for undoped and doped poly(Nmethylpyrrole) membranes, respectively, but their poor mechanical strength restricted any further technological advancement. This has prompted researchers to develop mixed matrix membranes using the conductive fillers that are capable of enhancing gas separation properties without compromising on mechanical strength. Examples of such fillers are zirconium phosphate (Zr(HPO4 )2 ·nH2 O), phosphotungstic acid

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(HPA) (H3 PW12 O40 ·nH2 O) and silicotungstic acid (SiWA) (H4 SiW12 O40 ·nH2 O). Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is a well known polymer with a high glass transition temperature (Tg > 200 ◦ C) and good mechanical strength. PPO is known to exhibit highest permeability to gases among many other aromatic polymers, but its low selectivity is due to the absence of polar groups on the polymer backbone [20]. Hence, PPO has been modified by different methods to improve its permselective property [21–23]. Previous researchers have demonstrated that selectivity of PPO could be enhanced by introducing polar groups through sulfonation, which would induce stronger interactions within the polymer matrix as well as between the membrane and polar feed gases such as CO2 [24–27]. The recent publication by Hamad and Matsuura [27] reported a two-fold increase in selectivity from 16.7 to 33.2 with a reasonably high permeability of 58.8 for the sulfonated brominated PPO membrane of 19.7% degree of sulfonation (DS). However, the majority of literature on modified PPO membrane is based on permeability studies with pure gases, but a comprehensive study with binary mixtures is lacking. The present study attempts to fulfill this requirement. In this research, heteropolyacid (HPA), a known proton conducting agent, was evaluated as an additive to enhance the gas permeability characteristics of high-performance PPO polymer for the separation of industrially important CO2 /CH4 gaseous mixtures. Results obtained for PPO–HPA were compared with those obtained from the chemically modified PPO of 20% DS. The PPO-based membranes were characterized by FTIR, XRD, DSC and SEM as well as free volume fraction (FVF) determination. Furthermore, the effect of feed composition and pressure gradient on the performance of PPO membrane and its modified forms were investigated in detail. 2. Experimental 2.1. Materials and methods Chloroform, methanol, sulfuric acid and phosphotungstic acid, H3 PW12 O40 ·6H2 O (HPA), were purchased from Loba ¯ ¯ n = 32000 Chemie, Mumbai, India. PPO polymer of M and ¯ w = 244, 000 with a density of 1.06 g/cm3 at 25 ◦ C was purM chased from Aldrich Chemical Co. (Milwaukee, WI, USA). High molecular weight grade was chosen keeping in view the mechanical stability required during high-pressure experiments. Its Tg was 211 ◦ C and melting temperature, Tm , was 268 ◦ C. Experimentally determined values of density and Tg were 1.014 g/cm3 and 215 ◦ C, respectively. 2.1.1. Preparation of PPO–HPA blend It was found that HPA was highly soluble in methanol, while PPO was soluble in chloroform. HPA was blended with the polymer using a mixed solvent system of methanol and chloroform, since both the solvents are highly miscible. A homogeneous solution of HPA (1.22 wt.%), PPO (6.9 wt.%), methanol (4.41 wt.%) and chloroform (87.47 wt.%) was prepared at the ambient temperature. This solution was spread over a clean

203

Fig. 1. Chemical structures of (a) PPO and (b) SPPO membranes.

dust-free glass plate with a uniform thickness, and subsequently dried in ambient condition (56% relative humidity) to obtain the HPA–PPO blend membrane. 2.1.2. Sulfonated polymer synthesis and membrane preparation Sulfonation was carried out in chloroform at ambient conditions using chlorosulfonic acid as a sulfonating agent [28]. About 10 g of the neutralized PPO was then added to 100 mL of neutralized chloroform taken in a three-necked round bottom reaction flask, and the mixture was stirred for about 30 min at ambient temperature to form a 10 wt.% solution. A 5% (v/v) solution of chlorosulfonic acid prepared in 100 mL of chloroform was transferred to a cone-shaped dropping funnel and part of it was gradually added to the polymer solution over a period of 20 min, and the solution was stirred vigorously at ambient temperature. The precipitated polymer SPPO was washed with distilled water repeatedly and dried in air for 24 h at ambient temperature followed by vacuum drying for about 48 h. Chemical structures of PPO and SPPO polymers are given in Fig. 1. 2.2. Membrane characterization 2.2.1. FTIR studies FTIR spectra of the membranes were scanned between 4000 and 400 cm−1 using a Perkin-Elmer-283B FTIR Spectrophotometer. 2.2.2. XRD studies A Siemens D 5000 powder X-ray diffractometer was used to study the solid state morphology of PPO and its modified ver˚ wavelength were sions in powdered form. X-rays of 1.5406 A generated by a Cu K␣ source. The angle of diffraction was varied from 0◦ to 65◦ to identify changes in crystal structure and intermolecular distances between intersegmental chains of the polymer matrix.

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2.2.3. Thermal analysis Thermal stability of the polymer films was examined using a Seiko 220TG/DTA analyzer from 25 to 700 ◦ C at a heating rate of 10 ◦ C/min with inert nitrogen gas flushed at 200 mL/min.

measured with an accuracy of ±0.001 by floatation method at ambient temperature using the mixtures of ethylene glycol and water [21]. 2.3. Gas separation experiments

2.2.4. Mechanical strength analysis Tensile strength and %elongation at break of the polymer membranes were measured by using Universal Testing Machine (AGS-10kNG, Shimadzu, Japan). The films of thickness ∼0.2 mm, gauge length 50 mm and width of 10 mm were stretched at a crosshead speed of 20 mm/min. 2.2.5. Determination of ion exchange capacity and degree of substitution Ion exchange capacity (IEC) indicates the number of milliequivalents of ions in 1 g of the dry polymer. The degree of substitution (DS) indicates the average number of sulfonic groups present in the sulfonated polymer. To determine the degree of substitution by acid groups, sulfonated membranes and unmodified specimens of similar weight were soaked in 50 mL of 0.01N sodium hydroxide solution for 12 h at ambient temperature. Then, 10 mL of the solution was titrated with 0.01N sulfuric acid [29]. The sample was regenerated with 1 M hydrochloric acid, washed free of acid with water and dried to a constant weight. The IEC was calculated as: IEC =

B − P × 0.01 × 5 m

(1)

where IEC is the ion exchange capacity (mmol/g (meq./g)), B the sulfuric acid used to neutralize the blend sample soaked in NaOH (mL), P the sulfuric acid used to neutralize the sulfonated membranes soaked in NaOH (mL), 0.01 the normality of sulfuric acid, 5 a numerical factor corresponding to the ratio of amount of NaOH taken to dissolve the polymer to the amount used for titration and m is the sample weight (g). The relationship between DS and IEC is: DS =

120IEC . 1000 + 120IEC − 200IEC

(2)

2.2.6. Determination of free volume fraction FVF was calculated according to the following equation [27]: FVF = 1 −

Vw ρ M

(3)

where ρ is the membrane density, Vw the van der Waals molar volume of the polymer and M is its molecular weight. These quantities are calculated using the equations:      244000 DS DS Vw = 71.1 1 − + 96 (4) 120 100 100      DS DS 244000 120 1 − + 198.9 . (5) M= 120 100 100 Constants in the above equations were determined by group contribution method [27]. The density of the membranes was

A permeability cell of stainless steel-316 was designed and fabricated indigenously. The effective membrane area in the cell was about 10 cm2 . Feed and permeate lines in the manifold were made of 0.635 × 10−2 m (1/4 in.) SS piping connected together by means of compression fittings. The vacuum line consisted of a network of high vacuum rubber-glass valve connections capable of giving a pressure of as low as 0.05 Torr. Detailed description and process flow diagram of the set-up was reported earlier [30]. The continuous flow method was used to carry out the permeability studies. 2.3.1. Detailed experimental procedure Pure gas permeabilities of CH4 and CO2 were determined through the membranes by maintaining a constant pressure differential of 30 kg/cm2 across the membrane. For the binary mixtures of CO2 and CH4 , both composition and pressure were individually varied keeping the other parameters constant. All experiments were performed at 30 ◦ C. Feed and permeate lines were evacuated by means of a vacuum pump (Hind High Vacuum Co., Model ED-18, Mumbai, India). Feed gas was introduced slowly into the upper chamber by means of a mass flow controller (MFC), keeping the outlet valve closed until the dial gauge indicated the desired pressure. High purity (99.9%) grade nitrogen was used as a carrier to sweep the permeate to SS-316 gas sample containers (capacity 100 mL) for subsequent analysis. Carrier gas was introduced after 3 h of equilibration of the membrane with the feed gas to ensure steady state. Permeate sample was collected for about 4–6 h at a low pressure of 0.5 kg/cm2 . Even though back diffusion of N2 carrier gas was expected to occur, it was assumed to be negligible since the pressure of sample collection in permeate side was very low compared to the feed side pressures, which were as high as 30 kg/cm2 . Feed gas was analyzed before and after the experiment and its composition remained the same, which means that N2 back permeation into the feed chamber was minimum. Composition of the feed and permeate streams were determined by a gas chromatograph. The permeability coefficient, K was calculated using: K=

Q l tA(P1 − P2 )

(6)

where Q is the permeation volume of the gas [cm3 (STP)], t the permeation time (s), A the effective membrane area (cm2 ) for gas permeation, l the membrane thickness (cm) and P1 and P2 are the feed side and permeate side partial pressures (1.333224 × 103 Pa (cmHg)), respectively. Selectivity was determined as the ratio of permeability coefficients of the two gases: α=

KCO2 . KCH4

(7)

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2.3.2. Analytical procedure Feed and permeate compositions were determined with a Nucon gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) using a Haysep ‘Q’ packed column of 2 m length and 0.3175 × 10−2 m (1/8 in.) i.d. The oven temperature was maintained at 50 ◦ C, while injector and detector temperatures were set at 150 ◦ C each. Hydrogen at 0.9 kg/cm2 pressure was used as the carrier gas for analysis. The sample size was 1 mL throughout. GC was calibrated for response factors of the detector for CO2 and CH4 gases, which had different thermal conductivity coefficients. 3. Results and discussion 3.1. Effect of blending ratio HPA was incorporated in varying amounts (0–30 wt.%) into PPO matrix. It was noticed that an increase in HPA content (>15 wt.%) in the blend renders the membrane brittle as evidenced by the membrane stability test. Stability of the membrane was assessed by bending test. The membrane is considered to be stable if its mechanical strength is restored after bending, i.e., it does not break upon bending. Hence, the polymer matrix containing 15 wt.% HPA was considered in the present studies. 3.2. Effect of sulfonation on membrane performance PPO was sulfonated from 0 to 60%. Sulfonated membranes were tested for their stability under a pressure of 30 kg/cm2 . The sulfonated membranes with DS > 20% failed to withstand pressures due to a loss in mechanical strength. Hence, SPPO with DS = 20% was considered in all our experiments. 3.3. Membrane characterization Membranes were characterized by FTIR, XRD, DSC, SEM and tensile testing techniques. 3.3.1. FTIR studies FTIR spectra of PPO, PPO–HPA and SPPO are shown in Fig. 2A. Characteristic bands of HPA are observed at 792, 887, 981 and 1080 cm−1 , respectively, in Fig. 2B. Test conditions employed, i.e., 140 ◦ C after vacuum drying, should generate an infrared spectrum of pure HPA that represents its stable secondary structure, which contains six protonated water molecules per polyanion. Specific interactions in the PPO–HPA polymer blend represent one of the most important factors influencing aggregation, physico-chemical and mechanical properties of the blend membrane. Fig. 2A(b) pertaining to PPO–HPA shows the characteristic band of HPA at 1080 cm−1 , implying that HPA retains its Keggin-type structure even after blending with the polymer. Other three bands coincide with the bands of PPO itself, indicating no chemical interaction between HPA and PPO; this further suggests that interactions are mostly physical in nature. From the FTIR spectra of SPPO shown in Fig. 2A(c), a sharp peak observed at 780 cm−1 corresponding to mono-substitution seen in Fig. 2A(a) for the unmodified PPO has disappeared,

Fig. 2. (A) FTIR spectra of (a) PPO, (b) PPO–HPA and (c) SPPO membranes. (B) FTIR spectra of pure HPA.

but a new peak at 520 cm−1 representing para-substitution has occurred. This is indicative of the attachment of sulfonic acid groups at the para position of the benzene ring. The spectra of SPPO shows the SO3 symmetric stretching vibrations in the region 1000–1060 cm−1 . The peak observed at 1360 cm−1 is due to asymmetric stretching of S O bond. Symmetric vibration of this bond produced a characteristic split band around 1150–1185 cm−1 . 3.3.2. XRD studies Fig. 3(a–c) shows the wide-angle X-ray diffractograms of PPO, PPO–HPA and SPPO polymers. XRD pattern of PPO (a) shows sharp peaks at 2θ = 10◦ and 20◦ , indicating the crystalline nature of the polymer. The XRD patterns of PPO–HPA (b) exhibits few sharp peaks at 2θ = 10◦ and broad peaks at 2θ = 20◦ , indicating that HPA added to PPO did not exist in a crystal form, but it could be in an amorphous state. Thus, HPA may be present as agglomerates in PPO in some regions as well as finely dis-

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Fig. 4. DSC tracings of (a) PPO and (b) SPPO membranes. Fig. 3. XRD diffractograms of (a) PPO, (b) PPO–HPA and (c) SPPO.

persed particles in other regions of the polymer matrix. This is attributed to the blending pattern of PPO with HPA, which requires the usage of two miscible solvents. The diffractogram of SPPO (c) shows two broad peaks around 10◦ and 20◦ of 2θ, indicating the amorphous nature of the sulfonated polymer. 3.3.3. DSC results DSC tracings of PPO and SPPO are shown in Fig. 4. The Tg value decreased from 215 ◦ C for PPO (a) to 140 ◦ C for SPPO (b). Sulfonic groups attached to aromatic ring of the PPO backbone are not thermally stable and they could decompose at 175 ◦ C or even at lower temperatures [26]. Higher the degree of substitution, greater would be the free volume of the sulfonated product, thus enabling a change in the state of the polymer from a more crystalline state to a more amorphous state, thereby resulting in a reduction of Tg . The reduction in Tg may be attributed to the plasticization by sorbed water at ambient humidity occurring due to strong interaction between polar groups of the chains and water vapor present in the air. However, the Tg value is still high

enough for employing SPPO as a membrane for the removal of CO2 from natural gas mixtures, which are generally available at 45–50 ◦ C at off-shore wells. 3.3.4. SEM studies Fig. 5 shows SEM micrographs of PPO (a) and PPO–HPA (b) containing 15 wt.% of HPA. A distinctive difference can be seen between the surface morphologies. The HPA-free polymer appears dense and homogeneous. The surface of PPO–HPA blend shows clusters in few regions, indicating that HPA is not only partially dispersed as agglomerates, but also is concentrated more on the surface of the polymer matrix. Thus, HPA exists as agglomerates in some parts and is distributed uniformly in other parts of the polymer surface [31]. When HPA is blended along with PPO, the highly acidic groups of HPA increase intermolecular interactions through the polar ionic sites, which is called an ionomer effect. However, due to high volatility of the solvent, ion sites in the ionomers aggregate into clusters and will remain as agglomerates in some parts of the polymer matrix. Undoubtedly, there might be some different and complex inter-

Fig. 5. SEM pictures of (a) PPO and (b) PPO–HPA membranes.

S. Sridhar et al. / Journal of Membrane Science 280 (2006) 202–209 Table 1 Single gas permeability and selectivity of PPO membrane and its modified forms (feed pressure = 30 kg/cm2 ) Membrane

PPO PPO–HPA SPPO a

Permeability (Barrera ) CO2

CH4

43.7 28.2 18.4

3.6 1.36 0.67

Selectivity P (CO2 /CH4 )

12.1 20.6 27.2

1 Barrer = 10−10 cm3 (STP) cm/cm2 s cmHg.

actions between blending components depending upon the kind of polymer material used. However, the fundamental reasons for this non-uniform dispersion have not been fully elucidated yet [32]. The SEM micrograph of PPO–HPA blends corroborates well with the XRD results. 3.3.5. Mechanical strength Tensile strength studies were conducted to assess the mechanical stability of all the polymers before and after modification. From UTM studies, it was noticed that the unmodified PPO membrane exhibited a tensile strength of 176.8 N/mm2 with an elongation at break of 13.45%. The physically modified PPO, i.e., HPA–PPO exhibited a tensile strength of 173.3 N/mm2 and %elongation of 12.78, whereas the chemically modified form, i.e., sulfonated PPO caused a reduction in tensile strength to 146.6 N/mm2 and %elongation at break to 5.67. Therefore, both the modified forms of PPO showed relatively lower mechanical stability than the unmodified polymer membrane. 3.4. Gas permeation experiments 3.4.1. Single-gas permeation behavior of CO2 and CH4 From the values of pure gas permeability and ideal selectivity determined at 30 kg/cm2 (Table 1), it is evident that of the three membranes employed, the unmodified PPO membrane showed the highest permeability of 43.7 Barrer with the lowest selectivity of 12.1. On the other hand, SPPO gave the highest selectivity of 27.2, which is 2.2 times higher than that of the unmodified PPO. The PPO–HPA membrane showed a lower permeability (28.2 Barrer) as compared to the unmodified PPO, but a higher selectivity of 20.6. Improvement in the performance of both the modified membranes in terms of selectivity may be attributed to the availability of large number of polar sites in the membranes for the sorption of CO2 gas [26]. The CO2 gas interacts with the polar HPA filler particles through hydrogen bonding. The other component CH4 has a low affinity for HPA particles and prefers to move into the bulk polymer rather than adsorb on the filler surface. However, the filler acts as an obstacle for both CO2 and CH4 gases by determining a more tortuous path during the permeation process [19]. Even though HPA loading was only 15 wt.% of the polymer matrix, SEM studies have shown that most of the particles remain on the surface of the membrane itself, which reduces the permeability. On the other hand, in case of SPPO, a reduction in free volume fraction from 0.39 to 0.35 is expected to occur due to an increased stiffening of the polymer backbone upon sulfonation (see Table 2). Once

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Table 2 Free volume fractions of PPO and sulfonated PPO membranes Polymer

ρ (g/cm3 )

Vw (cm3 /mol)

M (g/mol)

FVF

PPO SPPO

1.014 1.152

144570 155672

244000 276086

0.399 0.350

again, hydrogen bonding interactions between –SO3 H groups of the adjacent chains are responsible for increased compactness, which explains the reduction in gas permeability. Therefore, in this study we have restricted the degree of sulfonation to an optimum level of 20% [24]. 3.4.2. Effect of feed concentration PPO and its modified membranes were tested for the separation of CO2 /CH4 binary mixtures at 30 ◦ C by varying the feed CO2 concentration between 5 and 40 mol% at the constant pressure of 30 kg/cm2 . Flux and selectivity data observed for the binary gas mixtures were expectedly lower than those obtained with the pure gases owing to the reduced partial pressure of CO2 gas. From Fig. 6, it is observed that an increase in CO2 concentration from 5 to 40 mol% caused a corresponding increase in flux of the unmodified PPO membrane from 10.36 × 10−5 to 84.6 × 10−5 cm3 /cm2 s. Sulfonated and HPA incorporated membranes have shown similar trends with the SPPO membranes by exhibiting an increase in flux from 4.3 × 10−5 to 39.5 × 10−5 cm3 /cm2 s, while the PPO–HPA membrane exhibited an enhancement from 6.5 × 10−5 to 60.0 × 10−5 cm3 /cm2 s. The rise in flux may be attributed to the increasing partial pressure gradient of the preferentially permeating CO2 gas, which is the driving force for gas transport across the membrane [1]. The corresponding enhancement in selectivities is displayed in Fig. 7. The electronegative oxygen atoms in CO2 are expected to hydrogen-bond with the hydrogen atoms present in phosphotungstic acid of HPA–PPO blend and the –SO3 H group of SPPO polymer. However, increasing feed concentration brings about greater sorption of the gas in the polymer membrane due to the availability of more number of CO2 molecules for interaction with the membrane. Furthermore, the permeation of CH4

Fig. 6. Variation of CO2 flux with feed concentration (feed pressure: 30 kg/cm2 ).

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Fig. 7. Variation of membrane selectivity with feed concentration (feed pressure: 30 kg/cm2 ).

Fig. 9. Effect of varying feed pressure on membrane selectivity (feed composition: 5% CO2 + 95% CH4 ).

molecules would be impeded due to increasing polarization of CO2 molecules near the membrane surface. SPPO gave a reasonable selectivity of 5.2 at 5 mol% of CO2 which increased to 13.5 at 40% feed concentration. The selectivity values for PPO–HPA and unmodified PPO were in the range 3.7–9.5 and 2.1–5.8, respectively.

However, with rising pressure, sorption of the secondary components, such as CH4 , becomes more and more competitive in these voids, which results in the exclusion of CO2 molecules [3]. Thus, membrane selectivity dropped gradually from 3.3 to 2.0 for PPO, 5.5 to 3.6 for PPO–HPA and 7.3 to 5.1 for SPPO membrane as revealed in Fig. 9.

3.4.3. Effect of feed pressure Fig. 8 displays the relationship between CO2 permeability and feed pressure for the three membranes tested at a constant temperature of 30 ◦ C and feed composition of 5% CO2 and 95% CH4 . It was noticed that unmodified PPO membrane exhibited a reduction in CO2 permeability from 4.7 to 2.5 Barrer with increasing feed pressure from 5 to 40 kg/cm2 . PPO–HPA and SPPO membranes have shown similar trends of reduction in permeability from 3.2 to 1.6 and 2.1 to 1.2 Barrer, respectively. In case of semi-crystalline polymer membranes sorption is known to occur in the amorphous phase, which consists of microvoids.

4. Conclusions In this research, PPO was successfully modified physically by incorporating HPA filler and chemically by sulfonation. Keeping the mechanical strength in view, the concentration of HPA and degree of sulfonation were restricted to 15 and 20 wt.%, respectively. Incorporation of an inorganic filler into the PPO matrix as well as modification by sulfonation rendered the polymer amorphous. These effects were confirmed by XRD. All the membranes exhibited good thermal and mechanical stability during the experimental conditions. Studies with single gas as well as binary mixtures showed that SPPO membranes gave the highest selectivity, while unmodified PPO membrane was the most permeable. It is further demonstrated that incorporation of inorganic material such as HPA could dramatically increase the CO2 selectivity of the membrane due to interactions between polar sites of the membrane and the gas molecules. Increasing the CO2 feed concentration had a positive effect on flux and selectivity due to increasing partial pressures, whereas a rise in feed pressure showed a negative impact on membrane performance due to the competitive sorption of CH4 molecules. Modified PPO membranes of this study have shown a good potential to separate the CO2 /CH4 mixtures from natural gas/landfill gas purification sites. The preparation of highly selective SPPO as a thin film blend membrane would be ideal for commercial application since the drawback of low permeability would be overcome. Acknowledgements

Fig. 8. Effect of varying feed pressure on CO2 permeability (feed composition: 5% CO2 + 95% CH4 ).

Professor T.M. Aminabhavi is thankful to University Grants Commission (UGC), New Delhi, for a major support to establish

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