polyethylene glycol membranes

polyethylene glycol membranes

Journal of Petroleum Science and Engineering 173 (2019) 13–19 Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering...

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Journal of Petroleum Science and Engineering 173 (2019) 13–19

Contents lists available at ScienceDirect

Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol

Assessment of gas separation properties and CO2 plasticization of polysulfone/polyethylene glycol membranes


Solmaz Karimi, Ehsan Firouzfar∗, Mohammad Reza Khoshchehreh Department of Chemical Engineering, Quchan Branch, Islamic Azad University, Quchan, Iran



Keywords: Polysulfone Polyethylene glycol Gas separation Plasticization Selectivity

In the current paper, the effect of various concentrations of polyethylene glycol (PEG) on properties of PEGblended polysulfone (PSU) gas separation membranes was investigated. Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and x-ray diffraction analysis were applied to assess membrane morphology as well as chemical and phase structure, respectively. The obtained results confirmed the dense asymmetric structure in membrane cross sectional morphology. Moreover, the PSU/PEG membranes were found to be consisted of amorphous regions without any crystalline structure. In addition, gas permeability and selectivity of obtained membranes were investigated with respect to pure gases of N2, O2, CH4, and CO2 and membrane CO2 plasticization was studied at varying pressures. According to the results, CO2 accounted for the highest gas permeability at all PEG concentrations while CH4 and N2 represented the lowest gas permeability through the pure PSU and PEG-blended PSU membranes, respectively. CO2 permeability was found to decrease and then increase with increasing feed pressure at various PEG concentrations with the highest variation of permeability at 20 wt % PEG.

1. Introduction In recent years, membrane separation processes are known as one of the emerging technologies which have gained a substantial improvement because of a wide variety of its practical applications. Nowadays, membranes are applied in production of drinking water, treatment of industrial effluents, recovery of valuable constituents, purification or fractionation of macromolecular mixtures in food industry as well as medicine industry, and separation of mixtures of gases and vapors. Moreover, membranes are potentially utilized in energy conversion systems, artificial organs, and drug delivery devices as well (Bharali et al., 2017). The annual market of polymeric gas separation membranes has recently been estimated in the range of 150–230 million dollars, with a growth rate of 15% per year (Baker, 2002). Several gas separation modules with polymeric membranes are operating throughout the world which can be categorized into four major application sectors including nitrogen (N2) separation from air (50%), carbon dioxide (CO2) separation from natural gas (20%), hydrogen (H2) recovery (17%), and vapor recovery (13%) (Favre, 2017). Among its various applications, membrane separation processes have been considered as a promising technique for CO2 capture thanks to its advantages such as high energy efficiency, environmental sustainability and operational ∗

simplicity (Quan et al., 2017). Unfortunately, the increase in the concentration of various hazardous gases in the atmosphere has become an issue of great concern regarding numerous environmental problems, namely global warming, greenhouse effect, etc (Zhang et al., 2002a). Therefore, a large number of research studies have been conducted on CO2 permeation through membranes (Zhang et al., 2002a, 2002b; Eltahir Mustafa et al., 2017; Ur Rehman et al., 2017). Improvement of separation properties (i.e. permeability as well as selectivity) of polymeric membranes is a matter of great concern expressed by the researchers involved in the development of enhanced membranes, especially for CO2 separation (Yu et al., 2017; Wang et al., 2016, 2017). However, the commercial membrane materials such as polyimide (PI) and cellulose acetate (CA) have not provided the requirements in terms of high permeability and selectivity (Sadeghi et al., 2008a). One of the most widely studied glassy polymer membrane materials for CO2/CH4 separation is polysulfone (PSU) (Julian and Wenten, 2012). Polysulfones comprise a class of amorphous thermoplastic polymers characterized by supreme mechanical strength and high thermal resistance (Kiani et al., 2016, 2017). PSU pure and mixed gas permeation properties have been extensively investigated for its application in gas separation due to its low price, mechanical strength, and chemical stability (Hachisuka and Ikeda, 1999). Compared to CA,

Corresponding author. E-mail address: Dfi[email protected] (E. Firouzfar).

https://doi.org/10.1016/j.petrol.2018.10.012 Received 19 June 2018; Received in revised form 9 September 2018; Accepted 5 October 2018 Available online 06 October 2018 0920-4105/ © 2018 Elsevier B.V. All rights reserved.

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2. Materials and methods

PSU exhibits lower CO2 permeability and CO2/CH4 selectivity (Mulder, 1996), though higher plasticization pressure (Bos et al., 1999). The practical importance of plasticization is attributed to its influence on membrane selectivity. Membranes with higher plasticization pressure would maintain their selectivity better in comparison with membranes with lower plasticization pressure (Julian and Wenten, 2012). Various types of membrane based on PSU have been developed for CO2 separation but two limitations have restricted their application. The first challenge is obtaining higher selectivity with at least similar productivity and the second one is sustaining membrane performance at harsh conditions (Koros and Mahajan, 2000). Many researches have been conducted to address the mentioned challenges by variation of the membrane formulation and manufacturing process. Fabrication of novel membranes based on blend polymers is one of the main areas of research. In this case, separation properties is not only influenced by individual polymers, but can also be affected by the characteristics of the polymer blend. Indeed, during membrane fabrication via this method, a polymer with hard segments (e.g. PSU) that provides high mechanical and chemical stability is blended with another polymer such as polyethylene glycol (PEG) to obtain better separation properties. In this regard, the soft segments of PEG provide a high permeation rate on account of its chain flexibility (Sadeghi et al., 2008a). PEG has been blended with a number of polymers to prepare gas separation membranes. For instance, Kawakami et al. (1982) prepared cellulose nitrate/PEG blend membrane and found that CO2/N2 selectivity increased with increasing PEG concentration because of the affinity of PEG to CO2. Okamoto et al. (1995) and Yoshino et al. (2000) added PEG to polyimide membranes and reported that the blend membranes exhibited a high CO2/N2 selectivity due to the strong interactions between PEG and CO2 and thus the resultant high solubility-selectivity. Car et al. (2008) and Yave et al. (2010) prepared PEBAX/PEG gas separation membranes and found that increasing PEG concentration in the blend membranes resulted in an increase in the CO2 permeance and CO2/N2 selectivity of both dense films and composite membranes. Having said that, the addition of PEG to PSU gas separation membranes has not been studied thoroughly. Woo et al. (2014) synthesized PSU-PEG graft copolymer and reported that CO2 permeability and CO2/ N2 selectivity was enhanced with increasing PEG content in the copolymer because of the increased CO2 solubility. Even so, the difficulty involved in the synthesis of graft copolymers is undeniable. In another research, Mansourizadeh and Ismail (2010) prepared PSU hollow fiber membranes using PEG 200 as the low molecular weight additive via a phase inversion method. The prepared membranes were applied in a gas-liquid membrane contactor for CO2 separation and it was found that the addition of PEG affected the membrane porosity and consequently the gas separation performance. However, the effect of PEG on the solubility-selectivity of the resultant membranes was not reported in the mentioned paper which might be due to the fact that the low molecular weight PEG does not remain in the membrane during the phase inversion method. With respect to what has been mentioned before, blending PSU with PEG to prepare gas separation membranes via dry-casting has not been reported, yet. Dry-cast PSU/PEG membranes take advantage of both enhanced CO2 selectivity and versatile preparation method. Consequently, the objective of the present research is the preparation and characterization of blended PSU/PEG gas separation membranes. Morphology of the obtained membranes was assessed through scanning electron microscopy (SEM) imaging while their chemical structure as well as phase structure was investigated via Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analysis, respectively. Performance of the PSU/PEG membranes in gas separation was studied using pure gases of N2, O2, CH4, and CO2 and the permeability and selectivity of the membranes was determined. Moreover, CO2 plasticization of membranes was examined at various gas pressures.

2.1. Materials Polysulfone (PSU) was obtained from BASF. Dimethylformamide (DMF) and Polyethylene glycol (PEG) with average molecular weight of 10,000 g/gmol were supplied from Merck and applied as the solvent and membrane additive, respectively. N2, O2, and CO2 with 99.99% purity were provided by Ardestan Gas (Iran) while CH4 with 99.5% purity was obtained from Technical Gas Service (USA). 2.2. Membrane fabrication PSU/PEG casting solutions were obtained by addition of a specified amount of PSU and various amounts of PEG to DMF with subsequent stirring at 60 °C for 4 h. A series of such polymeric solutions were prepared by varying the concentration of PEG at 0, 5, 10, 15, and 20 wt % while PSU concentration was kept constant at 10 wt %. The resultant solutions were applied in preparation of PSU/PEG gas separation membranes through the so-called dry process. During the dry process, dense membranes are fabricated without immersion in precipitation bath (Julian and Wenten, 2012). The obtained homogenous solutions were poured on a clean glass plate followed by heating in a Nuve EN-120 oven (Turkey) at 65 °C for 24 h. The obtained membranes were then placed in a Lab Tech LVO-240 vacuum oven (Korea) at 75 °C for additional 4 h to remove the residual solvent. 2.3. Characterization 2.3.1. SEM characterization Cross sectional morphology of the membranes was investigated using KYKY EM-3200 scanning electron microscope (China). Prior to characterization, the membrane samples were ruptured in liquid nitrogen and further sputtered by SC7620 Au sputter coater (England). 2.3.2. FTIR characterization FTIR analysis was applied as a means to investigate the chemical structure of the membranes. FTIR spectra of the membrane samples were collected using a Perkin Elmer Spectrum-65 spectrometer (USA) in the wave number range of 400–4000 cm−1 in transmission mode. 2.3.3. XRD analysis The membrane samples were studied in terms of phase structure by applying XRD analysis (D8-Advance Bruker, Germany). The analysis was performed using Cu-Kα radiation (λ = 0.15406 nm) in 2θ range of 10–100°. 2.3.4. Gas permeation studies In the present research, delay time method was applied to investigate the permeability of pure gases. Delay time method, as the most common method in gas permeability studies in polymeric membranes (Semsarzadeh and Ghalei, 2012), includes measurement of gas permeation through membrane from zero-point time to the point polymer permeation velocity reaches constant value. Constant pressure technique, in which the variation of the volume of gas permeating through the membrane is recorded, was utilized to determine gas permeability. A schematic diagram of the experimental setup utilized in gas permeation test is shown in Fig. 1. The setup mainly included a compressed gas cylinder, a membrane module, and a U-shaped tube filled with water. Accordingly, pure gas entered the membrane module at the membrane upstream, passed through the membrane, and its discharge rate was measured at the membrane downstream via the U-shaped tube. In this method, gas permeation at transition as well stable stage is studied. Measurement of gas discharge rate in terms of time can be employed as a means to determine gas permeability. 14

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equation below (Macchione et al., 2006):


Q. l Δp . A


where P is the gas permeability coefficient through the membrane (cm3(STP).cm/cm2.s.cmHg) and Q is the permeated gas flow rate trough the membrane (cm3(STP)/s) which was obtained by determination of the slope of the line at stable stage. Moreover, Δp and A are the transmembrane pressure (cmHg) and membrane effective surface area (cm2), respectively. It should be noted that the gas permeability coefficient is generally expressed in terms of barrer which is 10−10 cm3(STP).cm/cm2.s.cmHg, and so is in the current paper. In addition, ideal membrane selectivity (α ) with respect to the gases i and j was calculated as the ratio of their permeability coefficients: Fig. 1. Schematic diagram of the experimental setup.

α= Experiments were carried out at constant pressure and temperature. In the first step, the membrane module inlet gas tube was connected to the gas cylinder regulator. Ensuring that the regulator was close, the main cylinder valve was opened. The U-shaped tube was filled with water to a specified height and fastened to the membrane module downstream. Air blower and thermostat-controlled heater were turned on. Once the desired temperature was set, the blower started working. It took almost 30 min for the inlet coil and membrane module to reach a constant temperature. Eventually, the membrane specimen was placed into the cell, leaving both the retentate and the permeate outlet valves open. On fastening the cell, the regulator was opened for a few seconds in order to let the test gas replace the air within the cell and adjacent tubes. Gas replacement step was repeated for each test gas. Then, the retentate outlet valve was closed. 15 min duration was required to ensure equality of membrane and cell temperatures after which the regulator was opened, transmembrane pressure was set to the desired value, permeate outlet was closed, and water height in the U-shaped tube was recorded at certain time intervals. Gas permeation tests were conducted using a variety of pure gases. The sequence of permeation test followed the condensability trend of gasses, namely the test initiated with the less condensable gas, i.e. N2, followed by O2, CH4, and CO2, respectively. Once the transmembrane pressure was set for the most condensable gas (CO2), the permeate outlet valve was left open for 30 min to ensure that its plasticizing effect had completely accomplished. Then, the experiment was initiated. Gas permeation experiment was repeated three times for each gas and average of the obtained results was reported. Variation of the water height in the U-shaped tube, as a criterion for the permeated gas volume, was depicted in terms of time. Fig. 2 is a typical of the obtained figure. Afterwards, gas permeability coefficient was obtained using the

Pi Pj


3. Results and discussion 3.1. Morphological studies Morphology of the obtained membranes was investigated by means of SEM characterization. Cross sectional SEM images of the membranes at selected PEG 10,000 concentrations is shown in Fig. 3. As shown in Fig. 3, both the pure PSU membrane and PSU/PEG membranes reveal a symmetric dense structure throughout the membrane thickness. The obtained structure gives rise to higher membrane selectivity. 3.2. Investigation of membrane chemical structure FTIR analysis was employed to study the membrane chemical structure and the obtained spectra for the pure PSU membrane, pure PEG 10,000, and PEG-blended PSU membranes is depicted in Fig. 4. In line with Fig. 4, both pure and PEG-blended PSU membranes showed typical spectra of PSU membrane. For instance, the peaks at 1150 and 1293 cm−1 are attributed to the symmetric and asymmetric stretching band of (S]O)2, respectively, while the bands at 1100 and 1240 cm-1 are assigned to the symmetric and asymmetric stretching of CeO (Kibechu et al., 2017). Other characteristic peaks of PSU are listed in Table 1. With respect to Fig. 4, no additional peak was inspected in the FTIR spectra by addition of PEG in comparison with the pure PSU membrane since the strongest PEG bands overlapped with PSU bands. Having said that, an increase in transmittance was found at 1110 cm−1 which was due to additional intensity of CeO bond stretch and confirms the presence of PEG in the PSU/PEG membrane. The obtained results are in agreement with previous studies on polyphenylsulfone (PPSU)/PEG 400 membranes (Kiani et al., 2015) as well as Polyethersulfone (PES)/ PEG membranes (Susanto and Ulbricht, 2009). Furthermore, the peaks at 2872 cm−1 and 2853 cm−1 which are assigned to the symmetric stretching vibrations of methylene group (eCH2e) in PEG structure and methyl group (eCH3) in PSU structure, respectively (Pavia et al., 2014), are obviously overlapped. However, its increased intensity in the PSU/ PEG membrane was another evidence for the presence of PEG in the blend membrane. 3.3. Crystallography studies Fig. 5 illustrates the XRD patterns of pure PSU as well as PSU/PEG membranes. It is widely known that a large crystalline region in a polymeric material brings about the appearance of a sharp peak with strong intensity in the XRD patterns of the polymer whereas that of amorphous polymer is quite broad (Sadeghi et al., 2008b). Regarding Fig. 5, neither membrane exhibits any sharp peak and

Fig. 2. Typical curve obtained for time variation of water height in the Ushaped tube used for evaluation of membrane performance in gas permeability experiment. 15

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Fig. 4. FTIR spectra obtained for (a) pure PSU membrane, (b) pure PEG 10,000, (c) PSU/PEG membrane at 10 wt % PEG, and (d) PSU/PEG membrane at 20 wt % PEG.

Table 1 FTIR characteristic peaks assigned to PSU structure (Kim et al., 2013). Wave number (cm−1)

Bond attributions

3092, 3061 1580, 1478, 1405 1310, 1293, 1150, 1070, 1000 867, 826 717, 696

CeH C=C S(=O)2 CeH C=C

3.4. Investigation of gas permeability and separation performance of membranes Permeation of pure CO2, CH4, O2, and N2 through obtained PSU membranes at 30 °C and 10 bar is illustrated in Fig. 6. As expected, CO2 permeability accounted for the highest value of gas permeability, at 5.613 barrer, which was significantly higher than the other gases. With respect to the solution-diffusion mechanism, the permeability coefficient (P) of gases through polymeric membranes is expressed as a combination of diffusion (D) and solution (S) coefficients (Shoghl et al., 2017):

P = S∗D


The solubility coefficient, as a thermodynamic term, defines the affinity between a gas and polymer phase (Yampolskii, 2017). A variety of parameters influence the solubility coefficient including the inherent condensability of the penetrant. Typically, higher gas critical temperature brings about its higher solubility in membranes (Quan et al., 2017). Moreover, the diffusion coefficient, as a kinetic property, is a criterion for the penetrant mobility within the polymer matrix, and consequently, the size of the penetrant molecule controls it (Sadeghi et al., 2008b). Table 2 shows the inherent gas properties including the critical temperature as well as kinetic diameter. Accordingly, CO2 accounts for the highest critical temperature (i.e. higher condensability) and lowest kinetic diameter in comparison with CH4, O2, and N2 which results in its highest permeability among the other gases. In addition to what has been mentioned above, the fractional free volume is the most important property of polymers that influence their diffusion, sorption, and permeation characteristics (Yampolskii, 2017). It should be noted that at high gas solubility, the properties of polymer matrix is affected since the gas molecules plasticize it (Yampolskii, 2017). Therefore, CO2 with high solubility coefficient is considered as a plasticizer. The presence of plasctcizers within the membrane matrix increases the chain mobility of polymer as well as its fractional free volume which in turn contributes to high CO2 permeability through the membrane (Zhao et al., 2017). In comparison with other test gases, CH4 represented the lowest gas

Fig. 3. Cross sectional SEM images of the PSU membranes with respect to PEG 10,000 concentration: (a) 0 wt %, (b) 10 wt %, and (c) 20 wt %.

only a broad peak is observed in the XRD patterns. The obtained result confirms that all of the membranes were consisted of amorphous regions without any crystalline structure. Moreover, the broad peaks of the membranes were observed due to compatibility and complete homogeneity among the membrane components (Bharali et al., 2017). The occurrence of this band has been previously reported as a result of the simultaneous existence of ordered entanglement regions of PSU chains (Ionita et al., 2015).


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Fig. 5. XRD patterns of PSU/PEG membranes with respect to PEG concentration: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, (e) 20 wt %.

permeability through the pure PSU membrane at 0.207 barrer. With respect to Fig. 6, gas permeability of the pure PSU membrane varied in the following order: CO2 > O2 > N2 > CH4 Indeed, the observed trend is in agreement with the inherent characteristics of PSU. Gas permeation through PSU as a glassy polymer, is principally governed by the size of permeant (Favre, 2017). Therefore, in spite of the different trend in critical temperature and thus compressibility (Table 2), gas permeability through pure PSU membrane followed the sequence of kinetic diameter. The obtained results are consistent with the results of previous literature (Favre, 2017; Sadeghi et al., 2008b). Having said that, as shown in Fig. 6, a different trend was observed in gas permeability of PEG-blended PSU membranes: CO2 > O2 > CH4 > N2 in which CH4 permeability was higher than that of N2, indicating the dominant effect of gas condensability. Actually, in the case of PSU/PEG membranes, the presence of PEG molecules as the plasticizer interrupted the intermolecular interaction and created higher intermolecular space, increasing PSU chain mobility (Zhang et al., 2017). In this case, the blended PSU/PEG membrane acted as a rubbery polymer in which the permeant condensability became the major governing factor (Favre, 2017). A similar trend has been previously observed in polyvinylchloride (PVC)/PEG membranes (Sadeghi et al., 2008b). Regarding Fig. 6, the highest permeability at various PEG concentrations was represented by CO2 as compared to other test gases. The observation is attributed to CO2 higher condensability, smaller kinetic diameter and its plasticization effect as well, which has been discussed earlier. However, an improvement in CO2 permeability was obvious with increasing PEG concentration which is explained by the high condensability of CO2 and the high affinity of polar CO2 molecule to polar PEG segments (Sadeghi et al., 2008b). Regarding membrane selectivity (Fig. 6), it was found that the pure PSU membrane accounted for the highest CO2/CH4 as well as CO2/N2 selectivity with slightly lower value for the latter. Indeed, the dominant effect of gas kinetic diameter on glassy polymers (pure PSU) resulted in much higher CO2 permeability in comparison with other test gases, however, incorporation of PEG molecules into the PSU matrix turned it into a rubbery polymer with increased CH4 and N2 permeability. Undoubtedly, increasing the value of denominator during selectivity calculation reduces the final result. In other words, CO2/CH4 and CO2/N2

Fig. 6. Membrane permeability and selectivity towards various gases at 30 °C and 10 bar with respect to PEG concentration. Table 2 Psychochemical properties of studied gases (Favre, 2017). Gas

Kinetic diameter (Å)

Critical temperature (K)

CO2 O2 N2 CH4

3.3 3.46 3.64 3.87

304 155 126 191


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pressure (Shoghl et al., 2017). In addition to the discussed results, Fig. 7 reveals that CO2 permeability of PSU/PEG membranes was higher than that of pure PSU membranes at all feed pressures as well as all PEG concentrations. The presence of PEG molecules and their plasticizing effect contributed to the higher gas permeability. Supposedly, the polymer matrix with a loosened patch structure provides higher gas permeability (Shoghl et al., 2017). Moreover, just like the pure PSU membrane, PSU/PEG membranes permeability underwent a decreasing trend up to 4 bar feed pressure while an increasing trend was observed afterwards. Similar phenomena as the pure PSU membranes were influential on the obtained results. Indeed, CO2 plasticization at 4 bar intensified gas permeability.

Fig. 7. CO2 permeability through PSU/PEG membranes with respect to the feed pressure and PEG concentration.

4. Conclusions

selectivity decreased with PEG addition. Besides, higher N2 permeability in comparison with that of CH4 in glassy pure PSU membrane contributed to higher CO2/CH4 selectivity compared to that of CO2/N2. Having said that, the rubbery PSU/PEG membrane exhibited higher selectivity CO2/N2 compared to CO2/CH4 as a result of improved CH4 permeation thorough the blended membrane. The obtained results indicated a significantly low N2/O2 selectivity because of their roughly similar condensability and kinetic diameter.

The aim of the present research is the investigation of blend PSU/ PEG gas separation membranes properties through addition of PEG 10,000 to PSU at various concentrations. Morphology of the obtained membranes was assessed through scanning electron microscopy (SEM) imaging and a dense symmetric structure in cross sectional SEM images was confirmed. Moreover, membrane chemical structure as well as phase structure was investigated via Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analysis, respectively. Performance of the PSU/PEG membranes in gas separation was studied using pure gases of N2, O2, CH4, and CO2 and the permeability and selectivity of the membranes was determined. It was found that the pure PSU membrane represents the highest CO2/CH4 as well as CO2/N2 selectivity while the mentioned selectivity values decreased with PEG addition. Moreover, CO2 plasticization of membranes was examined at various gas pressures and a decreasing and then increasing trend with pressure increase was observed in CO2 permeability at all PEG concentrations.

3.5. Evaluation of membrane CO2 plasticization CO2 plasticization of PSU/PEG membranes was assessed at a variety of feed pressures ranging from 2 to 10 bar and the determined gas permeability is depicted in Fig. 7. As shown in Fig. 7, CO2 permeability through the pure PSU membrane decreased with increasing feed pressure from 2 to 6 bar, however, further increase in feed pressure increased CO2 permeation. The observed decreasing and increasing trend has been previously reported for the glassy polymers (Shoghl et al., 2017; Yampolskii, 2017). Gas solubility in the glassy polymers is generally explained according to the dual mode sorption model. Based on the dual mode sorption model, the glassy membranes exhibit non-equilibrium behavior which is be explained by the Henry's law (linear term) and Langmuir's law (non-linear term) (Shoghl et al., 2017):

P = kD DD +

C ′H DH b (1 + bp)

Declarations of interest None. References Baker, R.W., 2002. Future directions of membrane gas separation technology. Ind. Eng. Chem. Res. 41, 1393–1411. Bharali, P., Borthakur, S., Hazarika, S., 2017. Selective permeation of CO2 through amine bearing facilitated transport membranes. J. Membr. Sci. Technol. 6, 167. Bos, A., Pünt, I.G.M., Wessling, M., Strathmann, H., 1999. CO2-induced plasticization phenomena in glassy polymers. J. Membr. Sci. 155, 67–78. Car, A., Ctropnik, C., Yave, W., Peinemann, K.V., 2008. Pebax/polyethylene glycol blend thin film composite membranes for CO2 separation: performance with withed gases. Separ. Purif. Technol. 62, 110–117. Eltahir Mustafa, S.G.E., Mannan, H.A., Nasir, R., Mohshim, D.F., Mukhtar, H., 2017. Synthesis, characterization, and performance evaluation of PES/EDA-functionalized TiO2 mixed matrix membranes for CO2/CH4 separation. J. Appl. Polym. Sci. 134 45346-n/a. Favre, E., 2017. Polymeric membranes for gas separation. In: Comprehensive Membrane Science and Engineering, second ed. Elsevier, Oxford, pp. 124–175. Hachisuka, H., Ikeda, K., 1999. Inventors. Polysulfone Semipermeable Membrane and Method of Manufacturing the Same US Patent 5888605. Ionita, M., Vasile, E., Crica, L.E., Voicu, S.I., Pandele, A.M., Dinescu, S., Predoiu, L., Galateanu, B., Hermenean, A., Costache, M., 2015. Synthesis, characterization and in vitro studies of polysulfone/graphene oxide composite membranes. Compos. B Eng. 72, 108–115. Julian, H., Wenten, I.G., 2012. Polysulfone membranes for CO2/CH4 separation: state of the art. IOSR J. Eng. 2, 484–495. Kawakami, M., Iwanaga, H., Hara, Y., Iwamoto, M., Kagawa, S., 1982. Gas permeabilities of cellulose nitrate/poly(ethylene glycol) blend membranes. J. Appl. Polym. Sci. 27, 2387–2393. Kiani, S., Mousavi, S.M., Shahtahmassebi, N., Saljoughi, E., 2015. Hydrophilicity improvement in polyphenylsulfone nanofibrous filtration membranes through addition of polyethylene glycol. Appl. Surf. Sci. 359, 252–258. Kiani, S., Mousavi, S.M., Shahtahmassebi, N., Saljoughi, E., 2016. Preparation and characterization of polyphenylsulfone nanofibrous membranes for the potential use in liquid filtration. Desalination Water Treat. 57, 16250–16259. Kiani, S., Mousavi, S.M., Saljoughi, E., Shahtahmassebi, N., 2017. Novel high flux


where kD is the Henry's law solubility coefficient for (characterizing sorption in the densified equilibrium matrix of the glassy polymer (Yampolskii, 2017)), C'H is the Langmuir capacity constant (which characterizes sorption in the nonequilibrium excess volume of the glassy polymer (Yampolskii, 2017)), b is the affinity constant and DD and DH are the Henry's law and the Langmuir mode diffusion coefficients, respectively. Eq. (4) describes the sorption data in a wide range of pressures (Yampolskii, 2017). Regarding the dual-mode sorption model, CO2 content within the glassy membrane increases with increasing feed pressure. Meanwhile, the gas molecules are sorbed more into the Langmuir mode sites than into Henry's mode sites. Thus the Langmuir capacity is saturated at higher pressure and Henry's mode becomes the dominant solubility reason for further sorption. Consequently, gas sorption in glassy PSU membrane is more like the parabolic concave shape to the pressure axis, especially for the penetrants with high gas solubility like CO2. As a result, gas permeability through the membrane decreases. However, higher CO2 sorption at high feed pressure brings about higher free volume within the membrane matrix due to CO2 plasticization effect and higher mobility of polymer chains, and therefore, gas diffusion in the membrane is enhanced. In a word, the PSU plasticization at 6 bar and the resultant improvement in gas sorption as well as its diffusion lead to an increase in the permeability of CO2 with the operating 18

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