glycerol triacetate gel membranes

Journal of Membrane Science 469 (2014) 43–58

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Improvement in CO2/H2 separation by fabrication of poly(ether-b-amide6)/glycerol triacetate gel membranes Hesamoddin Rabiee a, Mohammad Soltanieh a,n, Seyyed Abbas Mousavi a, Ali Ghadimi b a b

Department of Chemical and Petroleum Engineering, Sharif University of Technology, Azadi Avenue, P.O. Box 11155 9465, Tehran, Iran National Petrochemical Company, Petrochemical Research and Technology Company, P.O. Box 14358 84711, Tehran, Iran

art ic l e i nf o

a b s t r a c t

Article history: Received 28 February 2014 Received in revised form 12 June 2014 Accepted 13 June 2014 Available online 21 June 2014

The purpose of this study is to investigate separation performance of poly(ether-b-amide6) (Pebax1657)/ glycerol triacetate (GTA) gel membranes for CO2 removal from H2, N2 and CH4. GTA as a low molecular weight and highly CO2-phill compound was added to membrane structure at various weight fractions, 20%, 40%, 60% and 80% of Pebax, to fabricate a new high solubility selective membrane with improved performance. Permeation of pure gases was studied at different temperatures from 25 to 65 1C and pressures from 4 to 24 bar and ideal selectivities were calculated. Results indicated enhancement in permeation for all tested gases. For example, at a pressure of 4 bar and a temperature of 25 1C, membrane permeability with 80 wt% GTA for CO2, H2, N2 and CH4 increased by 8, 4, 13 and 18 times, respectively. Although CO2/H2 selectivity almost doubled, opposite results were observed for CO2/N2 and CO2/CH4 separations. However, the overall performance of membranes for CO2/N2 separation improved to the upper bound of Robeson graph, whereas CO2/CH4 separation did not improve. Morphology of membranes was characterized by SEM that showed remarkable changes. Also, DSC, FTIR spectroscopy and tensile analyses were applied to study thermal properties, peaks of functional groups and mechanical strength of fabricated membranes, respectively. & 2014 Elsevier B.V. All rights reserved.

Keywords: Membrane gas separation CO2 capture Glycerol triacetate (GTA) Pebax1657/GTA blended membranes Solubility selectivity

1. Introduction CO2 capture and sequestration is an ongoing technology in order to reduce greenhouse gas emissions. From environmental point of view, CO2 contributes to 64% of the greenhouse effect and is the main reason for climate change [1]. On the other hand, as more than 80% of the total energy in the world is supplied by fossil fuels and almost 40% of CO2 emission is due to this source of energy, utilization of new and efficient approaches for CO2 removal from fossil fuel combustion, is highly demanded [2]. CO2 capture from N2 and H2 in post-combustion and precombustion processes, respectively, is the leading field for CO2 removal. Moreover, CO2/CH4 separation is of great importance in natural gas sweetening. CO2 removal is currently being performed in various ways such as: amine absorption process as the most extensive method, adsorption with CO2 favorable adsorbents like activated carbon, lithium compounds or molecular sieve adsorbents and membrane separation. Pre-combustion process is

n

Corresponding author. Tel.: þ 98 21 6616 5417; fax: þ 98 21 6602 2853. E-mail address: [email protected] (M. Soltanieh).

http://dx.doi.org/10.1016/j.memsci.2014.06.026 0376-7388/& 2014 Elsevier B.V. All rights reserved.

associated with CO2 separation from H2 and produce purified hydrogen, which is an important chemical for production of ammonia, methanol, power generation, and as a feed for fuel cells that provide a clean source of energy. Production of high-purity hydrogen is currently being done by pressure swing adsorption and amine scrubbing and they both suffer from high energy consumption of sorbent regeneration [3]. For the last 2 decades, membrane technology, due to its interesting separation efficiency, being less energy intensive and low operating costs compared to other methods, has obtained a great deal of attention [4,5] and the number of patents for CO2 capture with membrane has increased more than 3 times [6]. Both glassy and rubbery membranes have limitations and it has been found that membranes with low selectivity are more permeable and vice versa. Thus, there is a trade-off between permeation and selectivity, called “Robeson upper bound”, to develop membrane technology for industrial applications [7–9]. There are several strategies to overcome this upper bound such as facilitated transport membranes, block copolymers and mixed matrix membranes [10]. For the case of CO2 separation from light gases such as CH4, N2, and H2, it is highly desirable to remove CO2 from these gases in order to produce CO2-free gases near their feed pressure

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to eliminate a recompression unit and lower the investment cost [11]. Thereby, finding new methods and materials to produce CO2selective membranes is of great importance. It has been confirmed that membranes having ether, acetate and carbonyl functional groups are appropriate options for acid gas removal such as CO2 and H2S from light gases [12,13], especially for CO2/H2 separation [14]. In recent years many studies have focused on incorporating Ethylene Oxide (EO) groups into the polymeric membrane, in particular co-polymers, in order to enhance their separation properties [15–21]. Poly(ethylene glycol) (PEG) is the most commonly studied additive, containing EO groups, to improve the performance of polymeric membranes [17,22,23]. One of the most promising polymers for CO2/light gases separation is poly(ether block amide), commercially available under the name of Pebax. Pebax is a co-polymer consisting of polyamide (PA) as the hard segment which provides mechanical strength and does not show noticeable permeability, and poly(ethylene oxide) (PEO) that is responsible for gas separation properties of membrane because of its high chain mobility in the soft segment [24,25]. Interaction between ether oxygen and CO2 with polar molecules in EO group of PEO leads to high solubility of CO2 in PEO chains [26]. Moreover, permeation trends for Pebax membrane, as an elastomer thermoplastic, is similar to rubbery membranes at different operating conditions which confirms the rule of soft segment in separation and solubility-controlled permeation through this membrane [27]. Several studies have concentrated on the modification of Pebax to improve its separation properties. Car et al. studied Pebax1657– PEG blended membrane for CO2 removal and found great increment in permeation of the tested gases and enhancement in CO2/ H2 ideal permselectivity [16]. Reijerkerk et al. obtained interesting improvement in CO2 permeation and CO2/H2 permselectivity along with a slight decrease in CO2 selectivity over CH4 and N2, by addition of PEG–PDMS blend to Pebax1657 [28]. Yave et al. also reported improvement in both solubility and diffusion coefficients of Pebax membrane by addition of PEG due to the incorporation of EO groups and increment in free volume in membrane [19]. Also [BMIM][CF3SO3], a type of ionic liquid, was added to Pebax1657 matrix and increment in permeability of O2, CH4, He, H2 and CO2, along with slight selectivity reduction was observed [29]. Shishatskiy et al. added quaternary ammonium compounds to Pebax1657 and increased solubility selectivity of membranes up to almost 1300–1500 for some modified membranes but diffusion selectivity decreased considerably so that the overall, permeation selectivity did not improve and even decreased in some cases [30]. Incorporation of acetate groups, as another CO2-phill group, has been recently investigated by addition of glycerol triacetate (GTA) in Pebax1074 membrane and attractive observations were reported for permeation and selectivity through this membrane because of plasticization effect and CO2-phillic behavior of this additive which positively influences CO2 diffusion and solubility coefficients [31]. Besides, GTA showed excellent results as a solvent for CO2 capture for integrated gasification combined cycle (IGCC) process along with other CO2-philic oligomers which contain EO groups [32]. Also its ability as a liquid membrane for CO2/H2 separation was quite attractive and comparable with other EO-contained materials [33]. Ghadimi et al. recently studied the effect of poly(ethylene glycol diacrylate) in Pebax1657 membrane for CO2/H2 separation. Their results show enhancement in CO2 permeability and selectivity over CH4, N2 and H2 due to decreasing the effect of diffusion selectivity and strengthening the influence of solubility by increasing the Tg of membranes [34]. Apart from blending modification, inorganic additives have also been taken into account in order to prepare mixed matrix and nanocomposite Pebax-based membranes with enhanced separation properties [35–38].

Various grades of Pebax have different soft segment, hard segment or soft segment/hard segment ratios which leads to different permeation and solubility properties for the tested gases. Also, different materials in hard and soft segments show unique structural properties such as crystallinity or thermal properties which have many effects on transport properties and the field they could be used [27]. Pebax1657 is a great grade of poly (ether block amide), which has been widely studied and modified for better CO2/H2 separation. Based on solubility isotherms of N2, H2, CH4 and CO2 in different grades of Pebax, Pebax1657 with the most polar components in both soft and hard segments shows the lowest solubility for all gases in comparison with other grades (1074, 2533 and 4033), except for quadrupolar CO2. Thereby, for the case of CO2/CH4, CO2/H2 and CO2/N2 separation, Pebax1657 shows the best solubility selectivity among other grades [24], which makes it a great option in order to prepare solubility-based selective membrane. In order to strengthen the effect of solubility selectivity through a membrane, the difference between diffusion coefficients of penetrant gases should be decreased; hence solubility becomes the main criterion of separation of different gases. For reduction of the differences among diffusion coefficients of different gases the membrane matrix should become more flexible and less sizeselective. Therefore, CO2-phill compounds, which make the membrane more flexible and lead to even more solubility selective behavior the in membrane, are preferred. Having considered the criteria for material selection mentioned above, in this study Pebax1657 and GTA were chosen for membrane preparation based on their excellent transport properties. Pebax1657, as the most solubility selective grade of Pebax, is a perfect option to fabricate highly permeable blended membranes, which separate CO2 from light gases based on solubility. Also, GTA which is a CO2-phill compound was considered to increase permeation through the membrane with increasing diffusion coefficient, and enhance CO2 selectivity over light gases due to its affinity to CO2. For industrial applications, CO2 capture and especially H2 purification are demanded at different operating conditions, in particular high pressures. Therefore, this study tries to prepare a new high-solubility selective membrane for CO2 capture from N2, H2 and CH4 and also for the first time investigates in depth the possibility of using Pebax1657/GTA blended membranes at a wide range of operating conditions near industrial applications, which has not been taken into account in the literature. Membranes were characterized by DSC, FT-IR, SEM and tensile tests and their permeation performances were measured with a constant volume membrane performance test setup. Also, the activation energy of permeation for different gases through the neat and blended membranes was calculated. The obtained results showed great increment in permeability of all tested gases and the best CO2/H2 separation factor in comparison with other similar Pebax-based membranes was observed. Also, for the case of CO2/N2, in spite of reduction in permeation selectivity, GTA addition led to pass the upper bound limit of the Robeson graph, which shows that the prepared membranes are suitable for this separation application as well.

2. Experimental 2.1. Material Pebax1657 (comprising 60 wt% PEO and 40 wt% PA-6) was supplied from Arkema Inc., France, in the form of elliptic pellets. Glycerol triacetate (CAS number ¼102-76-1) was purchased from Sigma-Aldrich, Germany. 1-Butanol, used as solvent, was purchased from Merck, Germany. N2, CO2, CH4, and H2 gases of

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research grade were bought by Technical Gas Services Inc., Tehran, Iran. All the purchased chemical materials were used as received. Some Teflon petri dishes, oven (Memmert, Germany), a stirrer (Heidolph, Germany) and a vacuum oven (Wisd, WiseVen, Germany) were used in the membrane preparation process. Mitutoyo digital micrometer, The US (7 1 mm accuracy), was used to measure the membranes thickness. 2.2. Membrane preparation Polymeric solution with 3 wt% concentration was prepared by dissolving Pebax1657 in 1-butanol at 80 1C under reflux in order to prevent solvent loss due to evaporation during polymeric solution preparation. After 48 h, a homogenous solution was obtained, subsequently various amounts of GTA were added to have a certain solution concentration from 20 wt% to 80 wt% based on Pebax weight. The solution was allowed to stir for another 3 h to make sure that the solution is quite homogenous. Afterwards, the solution was cast on Teflon petri dish and placed in an oven at 60 1C for solvent evaporation for 24 h. After solvent evaporation, membranes were easily peeled off from the petri dish and were further dried for another 24 h in a vacuum oven at 30 1C to remove the residual solvent. Membranes were kept in a desiccator for performance and characterization tests. It is interesting to mention that the more usual solvent for Pebax1657 is a mixture of water/ ethanol (3:7), which has been widely used for nano-composite and blended membranes [16,18,34,37]. In this study, despite the fact that GTA is soluble in mixture of water/ethanol due to the presence of ethanol, as GTA is slightly miscible with water, membranes fabricated with a mixture of water/ethanol had some amount of GTA left on the surface of the membrane after solvent removal. This shows that during solvent evaporation ethanol evaporates quickly and the casted solution loses its homogeneity and all the added GTA could not be mixed with polymer matrix. Thus, some of the added GTA may not be trapped in the membrane body and to prevent this occurrence, 1-butanol, which is completely miscible with GTA, was used as the solvent. 2.3. Membrane characterization 2.3.1. Fourier transform infrared, FT-IR Structural characterization of fabricated membranes was studied by FTIR analysis in ATR (Attenuated Total Reflectance) mode. The spectra were recorded by a BRUKER (VERTEX 80, Germany) spectrometer and the scanning range was from 4000 to 600 cm  1 under ambient conditions, with a spectral resolution of 2 cm  1. 2.3.2. Differential scanning calorimetry, DSC DSC analyses for fabricated membranes in this study were obtained by METTLER TOLEDO DSC 822 (Switzerland) instrument. Temperature range of 100 to 220 1C with a heating rate of 10 1C/min in nitrogen atmosphere was applied to record DSC thermograms. 2.3.3. Mechanical properties Tensile analysis was applied to investigate mechanical strength of the prepared membranes, using INSTRON (5566, England) instrument at room temperature. As fabricated membranes, specially blended ones with high GTA content, were very gel like, an adhesive rubber was used to prevent slipping or damage of membranes. Testing speed, operating head load and grip length were fixed at 12.5 mm/min, 5 kN and 5 cm, respectively, and all the tests were taken at least 5 times and the average results are reported.

Fig. 1. The schematic of gas permeation apparatus.

2.3.4. Scanning electron microscope, SEM Morphological structures of neat and blended membranes were examined using scanning electron microscopy (VEGA\\TESCAN SEM, Czech Republic). In order to investigate the crosssectional morphology, the prepared membranes were fractured in liquid nitrogen and coated with gold before SEM analysis.

2.4. Gas permeation and sorption apparatus 2.4.1. Set up and module Gas permeation and sorption measurements of the neat and blended membranes were investigated by the module and special apparatus shown in Fig. 1. This setup is submerged in water with a water circulator and an immersion thermostat that controls the temperature of system. After changing the temperature, the tests started at least after 30 min to ensure the system is iso-thermal. Also pressure regulators were used to control pressure, precisely, as shown in Fig. 1. The setup is designed to measure permeability of the membranes using the constant volume method. The permeability of the tested gases through the membranes was calculated as follows: P¼

V 273:15 l dp A T ps Δp dt

ð1Þ

where V, A, ps and l are the volume of permeate (cm3), membrane area (cm2), standard pressure (1 bar) and thickness of membrane (cm), respectively. T is the operating temperature (K), Δp is the pressure difference across the membrane (mmHg), and dp=dt represents pressure increment with time (bar/s). P is the permeability coefficient (barrer, 1 barrer¼ 10  10 cm3 (STP)cm cm  2 s  1 cmHg  1). The module is a stainless steel (grade 316) cross-flow membrane cell, shown in Fig. 2. This module has an effective area of approximately 20 cm2 and consists of two detachable parts which has two concentric rubber O-rings. The inner O-ring is for pressure-tight seal between the membrane and the cell and the outer one for preventing water from wetting the membrane. The concentration of dissolved gases at a particular temperature and pressure was measured using the gas sorption part of the

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setup and the following equation: C¼

22; 414 Vm ðP 1  P 2 Þ RT Vp

ð2Þ

where V m and V p are the volumes of the module and membrane sample (cm3), respectively. P 1 and P 2 refer to initial and final pressures in the module, respectively. 22,414, R and T are Avogadro's (number of gas molecules per mole at standard condition), gas constant and the absolute temperature, respectively. Solubility coefficients (S) are calculated by dividing concentration (C) by operating pressure (P), as follows: S¼

C p

ð3Þ

Considering the solution–diffusion transport mechanism, diffusion coefficients (D) were directly obtained using the following equation: D¼

P S

ð4Þ

where P is the permeability of permeants.

to 1640 cm  1 for Pebax/80%GTA blended membrane). Also for –N– H peaks, wave numbers shift from 3293 cm  1 to 3297 cm  1. This indicates that GTA addition as a plasticizer causes hindrance in Hbond formation. It has been proposed that these bonds are like intramolecular bonds in polymer structure and their reduction could lead to a lower degree of crystallinity in membranes [34,40]. Then, GTA content leads to lower crystallinity, which will be mentioned in DSC discussion. 2. For –C–O–C, the position shifts to higher wave numbers and its intensity becomes lower when GTA content increases. This is due to reduction of intramolecular H-bond between PA and PEO in Pebax1657 [31]. 3. For the case of asymmetric and symmetric –C–O, the peaks shift to lower wave numbers in comparison with pure Pebax1657 and pure GTA, which could be probably due to molecular interaction and perfect mixing between GTA and Pebax1657. 4. For the case of O–CQO characterization peaks of blended membranes are slightly lower than pure GTA and Pebax1657 that is due to the weak tendency of O–CQO in GTA to form Hbond intermolecular with Pebax1657 [41].

3. Results and discussion 3.1. Characterization tests 3.1.1. FTIR Structural characterization of neat Pebax1657 and Pebax/GTA blended membrane was investigated by FTIR-ATR analysis as shown in Fig. 3. Polyamide segment of Pebax shows two characteristic peaks around 1640 cm  1 and 3293 cm  1 that are attributed to H–N–CQO and –N–H functional group, respectively. Another carbonyl group in Pebax1657 (O–CQO) is saturated esters and occurs at stronger frequency (1731 cm  1) compared to that in polyamide, which is influenced by resonance effect. Strong bond around 1099 cm  1 is related to –C–O–C of ether groups [27,31,39]. For the case of pure GTA, main characteristic peaks are located around 1733 cm  1 and 1211 cm  1, attributed to carbonyl and asymmetric –C–O stretching in acetate groups, respectively. Another peak around 1047 cm  1 indicates symmetric –C–O cm  1 stretching vibration of GTA. All the main characterization peaks are shown in Fig. 3. Addition of GTA to Pebax1657 leads to change of peak positions to some extent due to interaction between two materials, which have been also observed in other researches, as follows: 1. The position of H–N–CQO shifts to higher wave numbers on increasing the content of GTA (from 1636 cm  1 for neat Pebax1657

3.1.2. DSC Thermal properties of the neat and blended membranes were investigated by DSC measurements and changes in melting temperature (Tm), glass transition temperature (Tg) and crystallinity were observed with increasing GTA addition. DSC thermograms were prepared using three run tests (heating, cooling and again heating). In the first run samples were heated to 100 1C and maintained steady to eliminate humidity or any residual solvent, then they were cooled to  100 1C and finally heated again to 220 1C to investigate thermal properties. The degree of crystallinity in hard and soft segments of the polymer was calculated as follows: X crystallinity ¼

ΔH m  100 ΔH0m

ð5Þ

where ΔH 0m ðJ=gÞ is a reference value and represents the heat of melting if the polymer were 100% crystalline and ΔH m ðJ=gÞ is the heat of melting of crystals determined by integrating the areas under the peaks. ΔH 0m for soft and hard segments are 166.4 J/g and 230.0 J/g, respectively [16,42]. ΔH m is calculated by dividing heat of melting obtained from integral peak of melting, by the weight of each phase in sample (60% PEO and 40% PA). It should be mentioned that for crystallinity calculation, the weight of polymer in the sample must be considered [34,43]. Total crystallinity

Fig. 2. Membrane module.

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47

Fig. 3. FTIR-ATR spectra of GTA, the neat and blended membranes.

Fig. 4. DSC thermograms of the neat and blended membranes (curves are shifted vertically for comparison).

obtained for neat Pebax1657 was 28.7%, comprising 35.8% in PA and 18% in PEO that is fairly consistent with other studies [16,18,44]. GTA, as a plasticizer, could locate between polymer chains and leads to more chain mobility in the polymer matrix and as a result, Tg occurs at lower temperature [16,19]. As shown in Fig. 4, Tg of membranes reduces from  50.1 1C, for pure Pebax1657, to almost 67 1C on increasing the content of GTA up to 80 wt% of polymer. This Tg is for PEO and that of PA cannot be detected. The Tg of GTA is around  75 1C and DSC thermograph shows just one Tg for each sample which verifies the exact miscibility of GTA in the polymer matrix. Also sharp peaks around 14 1C and 205 1C which are attributed to melting temperature of PEO and PA segments in Pebax1657, respectively, shift to the left side, specifically for PEO. Ultimately, at very high GTA loading the peak related to melting point of PEO becomes too weak. Existence of two melting points in DSC diagrams represents microphase-separated structure in block copolymers. Similar results are reported about glass transition and melting temperatures for Pebax 1657 in the literature [16,24,34,44]. Weakening of PEO melting peak could be due to behavior of GTA in the polymer matrix that acts as a solvent for PEO and

results in having weaker and smaller crystals [16,19,31]. Table 1 shows the main thermal properties of the neat and blended membranes. It can be seen that the crystallinity of fabricated membrane reduces almost three times by addition of 80 wt% GTA, which is due to plasticization behavior of GTA in the polymer matrix [29]. Decreasing crystallinity in both PA and PEO segments and having more amorphous structure lead to more fractional free volume and more volume for solubility in membrane which are also supported by decline in Tg that influences diffusion and solubility of penetrants, remarkably [19]. This point will be further discussed later in permeation results. Tg of the blended membranes could be estimated using the well-known Fox equation [45,46], as follows: 1 W1 W2 ¼ þ T g T g 1 T g2

ð6Þ

where T g , T g1 and T g2 are glass transition temperature of the blended membrane, Pebax1657 and GTA, respectively. W 1 and W 2 are mass contents of Pebax1657 and GTA, respectively. This equation estimates that Tg of the blended membranes decreases monotonically with addition of more GTA. This is also observed from experimental results (Fig. 5).

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Table 1 Thermal properties of the neat Pebax and blended membranes. Membrane

weight of polymer in sample [mg]

Integral of PEO melting peak [mJ]

Integral of PA melting peak [mJ]

Tg [1C]

Xc (PEO) [%]

Xc (PA) [%]

Tm (PEO) [1c]

Xc (Total) [%]

Pebax1657 Pebax/20%GTA Pebax/40%GTA Pebax/60%GTA Pebax/80%GTA

5.8 5.5 5.6 5.8 5.9

104.28 91.09 69.33 51.72 34.37

191.38 148.35 125.91 102.21 76.42

 50.12  52.33  56.88  60.28  66.59

18.0 16.5 12.4 8.9 5.8

35.8 29.31 24.4 19.15 14.1

14.24 12.55 9.58 3.76  1.94

28.7 24.2 19.6 15.1 10.7

One of the successful empirical equations that has been suggested to describe the lowering of the Tg of polymers by plasticizer [47] is Jenckel and Heusch's derivation [48]: T g ¼ w1 T g1 þ w2 T g2 þ w1 w2 bðT g2  T g1 Þ

ð7Þ

where b is a constant of fitting [49]. The quadratic term in this equation has been introduced to account for specific interactions existing in the mixture. Curves that fit the experimental data with reasonable accuracy can be produced with values of b¼ 0.25–0.3 and the sum of the squared errors minimizes at b ¼0.27. As it can be seen in Fig. 5, this derivation gives closer results to the experimental data, compared to that of the Fox equation. Another equation used in this study to predict Tg of the blended membranes, is the Kwei equation [50,51], as follows: Tg ¼

w1 T g1 þ K kwei w2 T g2 þ qw1 w2 w1 þ K kwei w2

ð8Þ

where K kwei and q are adjustable parameters that can be fitted using experimental data. The results show that with q¼  5.5 and k ¼1.1, the closest estimation to experimental data could be obtained, as shown in Fig. 5. The results from this equation are close to those from Jenckel and Heusch's equation. However, higher reduction in Tg of the blended membranes and larger difference between experimental and predicted data, at higher GTA content, is probably due to significant decline in crystallinity that leads to stronger decrease in Tg than at lower GTA content. The larger difference between the results from experiment and Eqs. (6)–(8), at higher GTA content could be also due to more interactions between GTA and Pebax1657. However, as these equations are mainly proposed for polymer blending system (here GTA is an oligomer) and also the weight percentage of GTA in soft and hard segment of Pebax is not clear, one should be cautious when using these equations to predict Tg of the blended membranes [34].

3.1.3. SEM SEM analysis was applied to observe the cross-sectional morphology of membranes as shown in Fig. 6. The morphology of the neat Pebax1657 membrane is more uniform and regular in comparison with Pebax1657/80 wt% GTA blended membrane. This observation is consistent with the DSC results, which indicated that crystallinity of membranes decreases with GTA addition. Crystallinity reduction leads to more amorphous and unsymmetrical structure in membrane body. These crystals are like scaffold and physical crosslinks in the membrane body to keep it homogenous. Therefore, as can be seen in Fig. 6 for a membrane with 80 wt% GTA content, amorphous structure in the membrane body is quite clear which is due to more than 50% loss in their total crystallinity as mentioned in Table 1. These observations are fairly consistent with other studies about the effect of plasticizer additive on Pebax membrane morphology [16,31,34]. Increment in amorphous structure of membrane body has a considerable

Fig. 5. Comparison between Tg values of Pebax1657/GTA blended membranes obtained from experiment and different predictive equations of Fox [45] and Jenckel–Heusch [48] and Kwei [51].

effect on diffusion of penetrants through membrane, which will be mentioned in the permeation results.

3.1.4. Tensile tests Mechanical strength measurements of the neat and blended membranes were investigated by tensile tests at room temperature. The membranes with 20 wt%, 40 wt% and 60 wt% GTA content had enough mechanical stability to be handled and used in gas permeation test up to 24 bar, without any problems during tests. However for Pebax/80%GTA membrane, it could not be tested above 12 bar due to the great deal of loss in its mechanical strength and crystallinity. To evaluate mechanical properties of samples, each membrane was tested at least 5 times to have reasonable repeatability. The results show that membranes become softer as GTA content increases. The reason is due to decreasing membrane crystallinity and Tg, as observed from DSC analysis, thereby membranes lose their mechanical strength. Moreover, GTA is a plasticizer, which leads to more chain mobility and movement of polymer matrix and its presence in membrane causes less elongation compared with neat Pebax1657 membrane. As shown in Table 2 tensile modulus of the prepared membranes reduces from 154 MPa for neat membrane to 21 MPa for membrane with 80 wt% GTA that represents lower rigidity for blended membranes. Lower elasticity for blended membranes is confirmed by tensile strain results that decrease with GTA addition. The current observations are in good accordance with mechanical properties of another gel-like Pebax1657 membrane [29].

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Fig. 6. SEM of cross-sectional morphology of the neat and Pebax/80 wt% GTA membranes.

Table 2 Mechanical properties of the prepared membranes. Membrane

Tensile modulus [MPa]

Tensile strain [%]

Neat Pebax1657 Pebax/20%GTA Pebax/40%GTA Pebax/60%GTA Pebax/80%GTA

154.25 111.34 84.24 41.95 21.65

254.23 173.43 121.74 64.95 44.42

It should be mentioned that several parameters like membrane preparation method, drying conditions, the solvent used and aging can influence the mechanical and transport properties of the membranes and lead to different results [34,52]. 3.2. Gas transport properties 3.2.1. Effect of GTA loading Effect of GTA content on the permeation of pure CO2, CH4, N2 and H2, was investigated at pressures from 4 to 24 bar and temperatures from 25 1C to 65 1C. Also, solubility and diffusion coefficients of the tested gases for the neat and blended membranes were measured at 4 bar and 25 1C to investigate their effects on permeability. As Fig. 7(A) shows, permeability of the tested gases increases on increasing the content of GTA. This is due to enhancement in both the solubility and diffusion coefficients of the membranes for the following two reasons: 1. GTA as a low molecular weight additive increases the chain mobility of polymer in the soft segment that is responsible for permeation. Increase in chain mobility leads to higher free volume in the membrane matrix and consequently gas diffusion increases. Besides, GTA, as a plasticizer, could be located between polymer chains and makes interconnected paths between polymer chains that accordingly creates higher intermolecular space in the membrane body, resulting in increased chain movement and diffusion rate of permeant gases [19]. As shown in Fig. 9(A), diffusion coefficients of all the tested gases increase with increment of GTA content in the membrane matrix. Similar results are reported in other researches about increasing diffusion coefficient after modification with low molecular weight additives [16,18,29]. Moreover, based on the

Fig. 7. Permeability and selectivity of tested gases at T ¼25 1C and Δp¼ 8 bar.

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Fig. 8. Permeability and pair selectivity of gases through the neat and blended membranes at T¼ 25 1C and various pressures.

DSC results, GTA leads to lower crystallinity and Tg, which confirm higher FFV and diffusion for all penetrant gases. The same reasoning is explicitly presented by other similar researchers and they clearly concluded that reduction in crystallinity and Tg, leads to higher FFV and chain mobility, thereby diffusion increases [16,18,34]. Increase in diffusion coefficient leads to weak size-sieving permeation through membrane, thereby the effect of solubility on permeation becomes superior, which helps to have a membrane with greater solubility-selective effect [14]. This assumption is based on the permeation mechanism through dense membranes, which is influenced by diffusion and solubility. This mechanism is widely explained by different researches [53,54] and is formulated as follows: P ¼ SD p A;B ¼

ð9Þ P A SA DA ¼ P B S B DB

ð10Þ

where p A;B is selectivity of permeants through membrane. 2. Addition of GTA, as a CO2-phil material, increases CO2 solubility through the membrane matrix, significantly. It is due to interaction between oxygen in the acetate group in GTA and CO2 [31,55]. Generally, high-oxygenated materials show interesting affinity with acidic gases like CO2, H2S and SO2 as a result of this interaction [26]. As shown in Fig. 9(B), the solubility coefficients of the tested gases increase slightly by addition

of GTA. This increment is not as much as the increment in diffusion coefficient and has been observed in other similar studies, either [18,29]. Also, GTA addition results in the reduction of polymer crystallinity, which is in fact the impermeable part of the polymer matrix that acts as a non-sorbing segment; therefore the solubility of the tested gases improves. The first effect causes growth in permeation of all gases due to increment in diffusion and relative to their molecule sizes. This means that increment in diffusion for CH4, N2 and CO2 with larger kinetic diameter is more than that of H2. The results show that the diffusion coefficient for H2, CO2, N2, and CH4 increases in the ascending order of 3.6, 6.5, 9.5, and 11.5 times, respectively, for Pebax/80%GTA membrane (Fig. 9(A)). This is because H2, with its low molecular size, had already the highest diffusion coefficient and it remained the highest with GTA addition, but its diffusion increases less than that of larger molecules, comparatively. This comparison could be clearly observed in Fig. 9(A, red graph) which is plotted for better observation of changes in diffusion and shows that increment in diffusion of gases ðDBlended =DNeat Þ is higher for larger molecules at all the blending ratios. Thus, according to Fig. 9 (C), by GTA addition, DCO2 =DH2 , which was about 0.16 for the neat membrane, because of the high coefficient of diffusion for H2, increases 1.8 times and tends to approach 1 (it became 0.28 after 80 wt% GTA addition). This is the main reason for the remarkable increment in CO2/H2 selectivity with GTA addition. Therefore, unfavorable diffusion selectivity, resulting from more H2 diffusion,

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Fig. 9. Solubility and diffusion coefficients of gases through the neat and blended membranes at T ¼ 25 1C and P ¼4 bar. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

becomes less effective. That is the main concept to design CO2selective membrane for CO2/H2 separation in which the effect of solubility selectivity (ðSCO2 =SH2 Þ) becomes superior along with minimizing the adverse effect of diffusion selectivity (DCO2 =DH2 ) [14,56]. However, the opposite trend is observed for diffusion selectivity of CO2 over CH4 and N2. Fig. 9(C) shows that DCO2 =DCH4 and DCO2 =DN2 , which were above and near 1, respectively, for the neat Pebax1657 membrane, decrease continuously almost 1.7 and 1.5 times, respectively. Thus, in spite of the slight increase in solubility, permeation selectivity reduces which is dominantly due to reduction in diffusion selectivity [18]. The second effect of GTA addition is on the solubility of gases in the polymer, which should be considered simultaneously to explain the changes in permeation trends with GTA addition. Solubility, as shown in Fig. 9(B), enhances slightly for all the tested gases. However, solubility selectivity does not change very much, compared to diffusion selectivity. As shown in Fig. 9(D), CO2/H2 solubility selectivity enhances slightly and its effect can add up to improvement in diffusion selectivity; consequently, permeation selectivity enhances considerably. Other researchers also have reported a similar trend for small changes in solubility selectivity that confirms the dominant effect of changes in diffusion

selectivity on permeation selectivity [16,18,29]. For instance in a similar work, Bernardo et al. observed a slight reduction in CO2/N2 and CO2/CH4 solubility selectivity, which is consistent with the obtained results in the current study [29]. The order of permeation through Pebax is attributed to penetrant size and solubility (condensability) of gases but since permeation mainly occurs in soft segment, it is dominantly controlled by solution mechanism and that is why CO2 permeation is much more than that of other gases [27]. Table 3 represents TC, VC and kinetic diameter of the tested gases in this study. In fact the order of gas permeation is pretty similar to the order of TC, that is the main parameter to evaluate condensability of gases and the more TC, the more condensable gas is, except for H2 which has the least TC but its permeation through the neat Pebax membrane is more than that of N2 and CH4. This phenomenon is because of low molecule size of H2 which can be described by VC or kinetic diameter; hence, H2 can diffuse much more easily than the other tested gases through Pebax. Briefly, it can be said that permeation of H2 through Pebax takes place via diffusion because of its small molecular size, even though for CO2 that is highly condensable, N2 and CH4 (gases with larger molecular size) solution is the more effective mechanism.

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As shown in Fig. 7(A) permeation of all tested gases is on the rise when GTA content increases because of enhancement in solubility or diffusion coefficients of all the gases, as discussed earlier. The permeability of pure GTA for CO2 and H2 has been reported to be respectively about 1200 and 85 barrer at 25 1C and 0.2 atm [33]. These permeabilities for the blended membranes are always between that of the neat Pebax1657 and pure GTA, which means that the permeability of the blended membranes can be correlated with the permeability of pure components. Similar work has used some predictive models in order to estimate the permeation of the blended membranes, based on the permeation of pure components [23]. Their results show that these models also estimate the permeability of the blended membranes between the permeability of pure components and a similar trend is observed in experimental data of this study.

Table 3 TC, VC and kinetic diameter of tested gases in this study. Gas

TC (K)

VC (cm3/mol)

Kinetic diameter (Å)

CO2 CH4 H2 N2

304.2 190.6 33.19 126.2

94 98.6 64.1 86.2

3.3 3.8 2.89 3.64

However, the comparison between changes in permeation selectivities shows increment for CO2/H2 and reduction in CO2/ N2 and CO2/CH4 (Fig. 7(B)), even though for CO2/N2 separation, the blended membranes provide still sufficient permselectivity. The reason is because of solubility and diffusion enhancement, which is more remarkable for CO2, compared to H2 and also high affinity between CO2 and GTA. In brief, it could be said that permeation growth for H2, because of its very weak tendency to condense and low increment in diffusion due to its small molecular size, is the least among other tested gases. But for the case of N2 and CH4, their relative growth in diffusion (based on molecular size in Table 3) is more than that of CO2 and also they show more solubility-dependent behavior compared to H2 (based on the TC in Table 3). Thereby, unlike CO2/H2, CO2 permeation selectivity over N2 and CH4 decreases. This means relative increment in CO2 permeation is more than that of H2 and less than those of N2 and CH4. These results are fairly consistent with another Pebax-based membrane, modified by CO2-phill agent [18]. For example at an operating pressure of 4 bar, permeability of CO2, CH4, N2 and H2 increases by 8, 18, 13 and 4 times, respectively, for Pebax1657/80% GTA blended membrane and about 80% of this increment is due to diffusion growth. Then, it could be said that the order of increasing permeation is CH4 4N2 4CO2 4H2, which is quite similar to the order of kinetic diameter of gases and shows the role of diffusion in permeability after GTA addition. Permeability and selectivity of the tested gases at different GTA content and pressures are shown in Fig. 8. The left column is for

Fig. 10. Effect of operating pressure on permeability and pair selectivity of the neat and blended membranes at 25 1C.

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permeability results and the right column is for the corresponding selectivities. The trends are followed as discussed above; however, some unusual trends will be explained in the following section. In spite of reduction in CO2/N2 permeation selectivity with GTA addition, it is still over 50 and suitable for industrial application in which permeation selectivities between 30 and 50 are practical and acceptable [57,58]. It should be pointed out that CO2/H2 selectivity of pure GTA as a Supported Liquid Membrane (SLM) at operating pressure of 0.2 bar is about 13 and 9 for 25 1C and 37 1C, respectively [32,33]. However, in this study GTA has been used to modify the polymeric membrane and description of FFV, chain mobility and diffusion in polymeric-based membrane is not comparable with that in liquid membrane. Also, because of differences in the nature of materials and mechanism of transport properties through liquid and polymeric membranes and different operating pressures, the results of the current study cannot be exactly compared with the SLM membrane. This high increment in CO2/H2 selectivity is also reported in another similar work [31]. It is remarkable to mention that GTA addition leads to changing in permeation order from CO2 4H2 4CH4 4 N2 for the neat membrane to CO2 4 CH4 4 H2 4N2 after addition of 40 wt% GTA (Fig. 7 (A)). This explains the influence of GTA on transport properties of membrane based on absorption capacity of GTA that absorbs CH4 much greater than H2 [31,59]. This observation also confirms that at higher GTA additions, permeation is highly influenced by GTA.

3.2.2. Effect of operating pressure Fig. 10 shows the effect of operating pressure on gas permeation properties of the neat and blended membranes from 4 to 24 bar at 25 1C. Increasing pressure, on the one hand, leads to more gas diffusion of gases through free volume in membrane because of the higher driving force and on the other hand, enhances concentration of condensable gases in polymer matrix. Besides, plasticization effect of CO2 on PEO segment that is more considerable with addition of GTA leads to reduction in polymer cohesive energy density and, as a result, lower energy is demanded to create a molecular-scale hole in polymer matrix and permeation increases [24,60,61]. Moreover, increasing pressure causes compactness and contraction in polymer chains and influences on Fractional Free Volume (FFV). Based on transport mechanism in dense membranes, in order to investigate the effect of operating pressure on permeability, changes in solubility and diffusion with pressure should be taken into account, simultaneously. The most usual trend for permeation with increasing pressure, which has been widely observed in experimental studies, is increment for condensable gases such as CO2 or vapors (like C3H8), due to high condensability and concentration of these gases. In addition, higher pressures act as a driving force for more diffusion through the membrane [62]. However, for non-condensable gases such as N2, O2, CH4, and H2, constant or decreasing trend was reported due to low increment in gas concentration in membranes based on Eq. (3), leading to lower the solubility of penetrants when the pressure increases. Also, decrease in diffusion of gases due to the compactness of polymer chains and lower FFV in membrane body, adds up to the solubility effect. Thus, for the discussion on the effect of pressure, change in diffusion due to compactness and solubility (which is correlative with concentration based on Eq. (3)) due to condensation of gases should be considered. Changes in solubility and concentration depend on condensability of the gases. However, recently, Ghadimi et al. observed a slight increasing pattern for permeation of light gases through polymeric dense Pebax membrane [34]. They explained their observations based on

53

two reasons: (1) the more the feed pressure, the more the gas concentration (not solubility). (2) They also expressed that semicrystalline nature of Pebax with a high percentage of crystal (about 28% in this study, obtained from DSC results) in its body, prevents compactness of the membrane which results in enhanced diffusion. This increment in diffusion can overcome reduction in solubility, due to increasing pressure (based on Eq. (3)), resulting in improved permeation. Because the same polymer matrix that was used in this study as that of Ghadimi et al. [34], a similar trend was observed in the current study. As shown in Fig. 10, increasing pressure leads to growth in permeability of all tested gases. However, when GTA content rises, as observed in DSC results, crystallinity of membranes decreases remarkably, and for the case of membranes with 60 wt% and 80 wt% GTA content, total crystallinity became almost less than half that of the neat membrane. This phenomenon is the reason why membranes with 80 wt% GTA did not have enough mechanical strength to be tested with pressures more than 12 bar. Membranes with 80 wt% GTA content defected and could not be tested after a pressure of 12 bar. Also the growing trend of permeation with pressure as mentioned above surpassed for CH4 and N2 after 8 bar and their permeations increased more slightly. The reason is probably due to compactness of membrane body that is expected because of much lower crystallinity, compared to the neat membranes. For the Pebax/60% GTA membrane, for pressures of more than 12 bar, reduction in N2 and CH4 permeation growth was also observed and permeation remained constant or increased at a lower rate than that for lower pressures. It should be noted that permeation tests for each membrane were done at least 3 times in order to ensure the repeatability of results. However, this reduction in permeability-growth trend for Pebax/60%GTA and Pebax/80%GTA was not detected for CO2 and H2, as could be seen in Fig. 10. The reason is probably that CO2 because of its high condensability at higher pressures and H2 due to its low kinetic diameter continued the rising trend. As a result, compactness and, consequently, reduction in diffusion did not influence the trend of CO2 and H2 permeation. Thus, the unusual observation in CO2/N2 and CO2/CH4 pair selectivity could be explained by observing the permeation changes. As shown in Fig. 10, increment in pressure leads to increased ideal permeation selectivity for all the membranes. Higher CO2/N2 and CO2/CH4 selectivities of Pebax/80%GTA membrane, compared to that of Pebax/60%GTA and also higher CO2/N2 selectivity of Pebax/60%GTA compared with Pebax/40%GTA, are attributed to the reduction of permeation in these membranes, as discussed earlier. 3.2.3. Effect of operating temperature Effect of operating temperature from 25 1C to 65 1C on permeability and ideal permeation selectivity of the tested gases was investigated at an operating pressure of 4 bar for the neat Pebax1657 and Pebax/GTA blended membranes. As shown in Fig. 11, permeability of all gases follows an increasing trend when the temperature increases. Increasing temperature leads to more chain mobility in the polymer matrix and as a result, permeability enhances. Effect of temperature on transport properties of a gas through a dense polymeric membrane could be predicted by the Arrhenius type of equation as follows:   Ep P ¼ P 0 exp  ð11Þ RT   ΔH s S ¼ S0 exp  RT

ð12Þ

  E D ¼ D0 exp  d RT

ð13Þ

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Fig. 11. Effect of operating temperature on permeation and pair selectivity of the neat and blended membranes at operating pressure of 4 bar.

E p ¼ ΔH s þ E d

ð14Þ

where Ep ; Ed and ΔH s are the activation energies for permeation and diffusion and the heat of solution, respectively. Activation energy of permeation is interpreted as the barrier for permeation through the membrane. For condensable gases like CO2, with higher permeability, the activation energy is lower than that of light gases such as H2, N2 and CH4. This is due to the negative amount of ΔH s for CO2 because activation energy of diffusion for all tested gases is positive. Thereby, based on Eq. (14) Ep for these gases is lower [27]. Lower Ep for CO2 leads to higher permeation value based on Eq. (11), however, when temperature increases, increment in permeation is directly related to the amount of Ep and that is why for light gases, permeation growth is higher than that of CO2. Solubility through dense polymeric membrane is a thermodynamic process and two contributions play a role in this process. Thus ΔH s is described by

ΔHs ¼ ΔH Condensation þ ΔH Mix

ð15Þ

where ΔH Condensation is the molar heat of condensation of the permeant, and ΔH Mix is the partial molar heat of mixing of gas with the polymer matrix. In this process, the first contribution is the condensation of penetrants and transition from gas phase to condensed phase. Then, as the second contribution, the integration of the molecules into the polymer matrix and mixing of the two condensed phases take place. Condensation is an exothermic process and thereby ΔH Condensation is always negative and for condensable

gases it is much more dominant than ΔH Mix , which is generally positive [61]. However, for non-interacting gases such as H2, N2 and CH4, which are all above their critical temperature at room temperature, unlike soluble gases, ΔH Condensation is negligible and ΔH Mix , as a result of weak interaction between gas and polymer, plays a key role. Kim et al. reported Ep ; Ed and ΔH s for CO2, N2, He, and O2 in Pebax [27]. Their results show positive value of ΔH s for N2, He and O2, while it is negative for CO2. Briefly, for gases like CO2 with negative ΔH s , an increase in temperature, results in solubility reduction whereas for noncondensable light gases the opposite trend is expected and solubility grows. Hence solubility selectivity for CO2 over light gases declines with increasing temperature [61,63]. As mentioned earlier, Ed is always positive and usually much more than ΔH s . Then, because of the higher temperature sensitivity of diffusion, compared to permeation and solubility, Ep is positive and consequently permeation increases with temperature. No opposite observation for the effect of temperature on gas permeation in rubbery dense polymeric membrane has been reported. Moreover, Ed , which describes the energy needed for permeant to diffuse in the polymer matrix, is naturally higher for large molecules. Therefore, with increasing temperature, diffusion enhances more for larger gases than for small ones. Thus, it could be said that the order of diffusion increment with temperature for the tested gases is CH4 4 N2 4CO2 4H2, which is similar to their kinetic diameter order. For N2, He, O2 and CO2 also similar trend

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between kinetic diameter and coefficient of diffusion is observed [27]. Because of increment in free volume and chain mobility of membrane with temperature, the ability of the membrane to distinguish between molecule sizes, reduces, which leads to reduction in diffusion selectivity [14]. Also solubility increment for CO2 (with negative ΔH s ), based on Eq. (12), is the least among other gases (the ones with positive ΔH s ); thus CO2 selectivity over these light gases reduces with temperature. CO2 diffusion could be also affected by swelling in the polymer matrix, caused by the high solubility of CO2. For Pebax/GTA blended membranes a similar trend was observed. It is notable that for higher GTA loading, increment in permeation of CH4 and N2 (with larger molecular size) is more considerable with increasing temperature. It is probably due to the order of diffusion increment with temperature. Also at higher GTA content, both GTA content and temperature lead to high increment in diffusion and permeation, as discussed earlier. Changes in CO2 selectivity over light gases for neat Pebax and blended membranes are shown in Fig. 11. As shown in this figure, all the pair selectivities decline with rising temperature because of more enhanced permeation of light gases in compared with CO2. In summary, based on the discussions above, for the gas pairs CO2/N2 and CO2/CH4 both solubility and diffusion selectivities decrease that is probably due to the higher increment in solubility, diffusion and, ultimately, the permeability for CH4 and N2 with temperature, compared to CO2. For the case of CO2 and N2, their molecular sizes are very close to each other and in some works their diffusion selectivity is measured to be unity for the neat Pebax1657 membrane [30]. That means reduction in solubility selectivity with temperature is probably more dominant. But for CO2/H2 only solubility selectivity decreases. However, as stated earlier, CO2 permeation through Pebax is mainly governed by its solubility in soft segment and reduction in solubility selectivity is probably higher than the rise in diffusion selectivity that results in decrease in CO2/H2 selectivity. According to Eq. (11), at higher Ep , permeation increases more rapidly with increasing temperature and this is probably the reason for the higher reduction in CO2/N2 compared with other permeation selectivities, as illustrated in Fig. 12. For example for Pebax/60%GTA membrane at 4 bar, CO2/N2 permeation selectivity decreased from 50 to around 17 (64% reduction) with increment in temperature from 25 1C to 65 1C. However for CO2/CH4 and CO2/H2, this decline was about 35% and 22%, respectively.

3.2.4. Effect of GTA addition on Ep of penetrants Fig. 13 shows the activation energy of permeation for all neat and blended fabricated membranes, calculated from changes in permeation at 4 bar and various temperatures from 25 1C to 65 1C. It is clear from this figure that the activation energy of permeation for all gases decreases with the addition of GTA. It was also observed that GTA addition leads to an increase in permeation of all gases. Since, activation energy of permeation is responsible for the total energy needed for penetrants to permeate through the membrane matrix, reduction in Ep with GTA addition is expected. These calculated activation energies of permeation are in excellent agreement with the obtained results of permeability. Separation performance of neat and blended membranes were investigated by plotting the obtained data on Robeson upper bound graphs [8], as shown in Fig. 14. For CO2/CH4, the results show that membrane performance moves towards lower right of the Robeson graph with GTA addition. It is due to increment in permeability of the membrane along with reduction in selectivity. For CO2/N2, the performance of membrane is close to the upper bound and ultimately passes it at higher GTA content which shows that despite the decline in selectivity, permeation has been raised

55

Fig. 12. Effect of temperature on pair selectivity of tested gases for the blended membrane with 60 wt% GTA at 4 bar.

Fig. 13. Activation energy of permeation as a function of GTA content for the neat and blended membranes.

enough to be suitable for CO2/N2 separation. For the case of CO2/H2 separation, the obtained results seem more promising compared with other modified Pebax-based membranes. Fig. 15 shows the comparison of permeability–selectivity trade-off in CO2/H2 separation, between the obtained results in this work and those reported in the literature for the neat Pebax and their blended membranes. The comparison shows excellent improvement in CO2/H2 separation that has provided both high permeation and the desired selectivity. The results obtained at 25 1C and 4 bar are used in this plot. CO2-selective membranes for CO2/H2 separation are supposed to perform at relatively high pressures and in spite of the unsuccessful performance of Pebax/80%GTA membrane for pressures more than 12 bar, Pebax/40%GTA and Pebax/60%GTA blended membranes showed hopeful enhancement in transport properties of neat Pebax1657 membrane. CO2/H2 selectivity for Pebax/60%GTA and Pebax/40%GTA membranes was improved by more than 75% and 45%, respectively, at different operating pressures up to 24 bar. Our results also show better separation performance compared with another GTA-modified membrane at almost the same operating conditions (4 bar and 25 1C in this study, 5 bar and 25 1C in [31]). This confirms the desirable

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Fig. 16. Performance of Pebax1657/GTA membrane in temperature-dependent upper bound for CO2/H2 separation [64].

performance of Pebax1657-based solubility selective membranes for CO2/H2 separation and H2 purification at different operating conditions near industrial application ranges. Separation performance of fabricated membranes for CO2/H2 is also shown in temperature-dependent upper bound of the Robeson chart as shown in Fig. 16. As it can be observed CO2/H2 selectivity for all the prepared membranes, is far above the upper bound, presented by Rowe et al. [64]. As temperature increases, the upper bound shifts to lower selectivity, which indicates reduction in membrane selectivity with temperature increment, as observed in the current research.

4. Conclusion

Fig. 14. Selectivity of CO2 over CH4 and N2 versus CO2 permeability for the neat and blended membranes at 25 1C and 4 bar in the presence of Robeson upper bound.

Fig. 15. Performance of Pebax-based membranes in this work for CO2/H2 separation at 25 1C and 4 bar and comparison with literature results: Ghadimi et al. [34], Feng et al. [31], Car et al. [16], Yave et al. [18,19], Bernardo et al. [29], and Reijerkerk et al. [28]

The effect of GTA addition to Pebax1657 membrane on gas separation and permeation properties of this membrane was studied at different temperature and pressure operating conditions. Activation energies of permeation and pair permeation selectivities of CO2 over CH4, N2 and H2 were calculated for the neat and blended membranes. The presence of GTA as a plasticizer and low molecular weight additive leads to more chain mobility in the polymer matrix. Consequently, FFV increases that causes an increment in the diffusion of gaseous penetrant. Besides, GTA is highly oxygenated that possesses ether oxygen in its acetate groups, which could act as a CO2-phill agent in the membrane, resulting in enhanced solubility of CO2. Lower Tg of blended membranes compared with the neat membranes increased permeability of light gases, along with CO2. As a result, CO2/N2 and CO2/CH4 pair selectivities decreased with GTA addition. In contrast, lower increase in H2 permeation increased CO2/H2 selectivity by almost 50% with 40 wt% GTA addition membrane and by almost twice (100%) by the 60 wt% and 80 wt% GTA addition membranes. These results clearly indicate excellent performance of the prepared membranes for high purity H2 production at high pressures, which is extremely demanded, and seem promising compared to similar studies. Finally, it can be concluded that Pebax1657/GTA blended membranes are valuable to be utilized in CO2/H2 separation (pre-combustion capture and syngas process), and also for CO2/N2 separation (post-combustion). The latter is because in spite of reduction in selectivity, CO2 permeation increased significantly

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while the selectivities are still more than 50, which is useful for industrial applications. However, the membranes fabricated in this work do not show acceptable performance for CO2/CH4 (natural gas) separations.

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