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Facilitated transport behavior of humidified gases through thin-film composite polyamide membranes for carbon dioxide capture
Journal of Membrane Science 429 (2013) 349–354
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Facilitated transport behavior of humidified gases through thin-film composite polyamide membranes for carbon dioxide capture S. Andrew Lee, Geoff W. Stevens, Sandra E. Kentish n The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, VIC 3010, Australia
a r t i c l e i n f o
abstract
Article history: Received 16 August 2012 Received in revised form 11 November 2012 Accepted 21 November 2012 Available online 29 November 2012
In this work, commercial nanofiltration membranes from the water treatment industry have been shown to give good performance in a carbon dioxide capture application. Permeation of humidified gases through the membrane showed a carbon dioxide permeance of up to 340 GPU and a selectivity of 50, although the performance of a dry membrane with dry gas was significantly lower (6–8 GPU). The permeance of carbon dioxide was dependent on the temperature, pressure and pH of the water used to condition the membranes. The results indicate that a facilitated transport mechanism is dominant when humidified gases are present, probably due to the presence of piperazine functional groups and free amine within the membrane structure. The stability of membrane performance largely depends on the presence of water. This approach has strong potential for commercialisation, as the reliance on a commercial membrane material means that the development pathway can be fast tracked and membrane fabrication costs reduced. & 2012 Elsevier B.V. All rights reserved.
Keywords: Polypiperazine amide Carbon dioxide capture Facilitated transport Nanofiltration
1. Introduction Membrane processes for gas separation have been studied for decades and were first commercialized for this purpose in the 1980s. Membranes that are selective for carbon dioxide are of importance to natural gas sweetening, and more recently have gained a high profile due to their potential for use in the capture of carbon dioxide from combustion flue gases. To be highly selective towards condensable carbon dioxide over other gases such as nitrogen and hydrogen, it is beneficial that the membrane material has a high solubility selectivity rather than diffusivity selectivity. Thus, rubbery polymers are good candidates, while the presence of polar groups in the membrane structure that can interact with carbon dioxide, is known to be advantageous. The best commercial membranes, designed for natural gas sweetening, have a CO2 permeance of around 100 GPU and a CO2/N2 selectivity of 30 [1]. Conversely, rubbery membranes under development for CO2 capture have permeance values of around 1000 GPU and a CO2/N2 selectivity of 40 [1]. Facilitated transport systems have also shown potential, with selectivities well above those of conventional membranes that follow the solution-diffusion mechanism [2–4]. CO2/N2 selectivities in the range of 50 to 1000 have been reported [5] while CO2
*
Corresponding author. Tel:þ 61 3 8344 6682. E-mail address: [email protected] (S.E. Kentish).
0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.11.047
permeance values can range from 10 to 1200 GPU [6–8]. In this case, the carbon dioxide reacts with either a mobile or fixed carrier species and is carried across the membrane in a reacted state. However, in most cases, the facilitated transport mechanism relies on the membrane being water saturated, as the chemical reaction occurring is generally a Lewis acid–base interaction. It is expected that hydrogen bonding between the CO2 and amine groups in the membrane also play an important role in increasing the selectivity [9–11]. In addition, it has been shown that the diffusivity of carbon dioxide through membranes increases when the membrane is swollen by water [4,12]. While showing potential, facilitated transport membranes have limited durability due to the membrane drying out. Different methods have been tested to maintain performance such as additional water injection [9], continuous water spraying [13,14] or the use of a wet sweep gas [8,15]. While thousands of membrane materials have been identified as prospective for CO2 capture, only a limited number of materials have been commercialized [16]. One of the reasons behind this lack of success has been the need to form the membrane material into an ultrathin layer, of the order of 100 to 200 nm in thickness. Asymmetric structured membranes and thin film composite (TFC) membranes are the two methods commonly employed to achieve this objective. In particular, the selective upper layer and porous support substrate of TFCs can be separately optimised, therefore giving high permeability and selectivity at a cheaper price than most asymmetric single material membranes.
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Fig. 1. Schematic of the Dow FilmtecTM NF3838/30FF membrane chemistry, based on the description provided by the manufacturer [32].
Polyamide TFC membranes made by interfacial polymerisation (IP) have been widely used for nanofiltration, reverse osmosis and pervaporation processes [17]. The defect-free nature and ease in scale up of IP makes this method viable for manufacturing commercial membranes [10]. In situ polycondensation of polyfunctional amine monomers such as 1,3-benzenediamine and piperazine in the aqueous phase with polyfunctional acid chlorides such as trimesoyl chloride in the organic phase is the most well-known method to make commercial polyamide membranes [18–20]. Many different combinations of monomers and their polymerization methods have been trialled [21–23]. Porous polysulfone or polyethersulfone membranes have been widely used as a substrate due to their superior chemical and mechanical stability and relatively low price. While polyamide TFC membranes have been extensively deployed for NF and RO, they have also been trialed for gas separation applications. Recent results have shown that polyamide RO membranes and their derivatives are possible candidates for CO2/N2 or CO2/CH4 separation purposes [4,10,24–27]. In this project, we initially examined a range of commercial polyamide membranes for their potential for carbon dioxide capture. The advantage of using a TFC membrane that is already a commercial product has obvious advantages—the ability to manufacture at scale has clearly been demonstrated. All of the membranes we selected are believed to be based on a piperazine monomer (Fig. 1), either by declaration on the manufacturer’s website, or from the literature [14]. Piperazine is commonly used as a rate promoter in solvent based carbon dioxide capture and is known to have a very high rate of reaction with carbon dioxide [28,29], more than double that of the more common monoethanolamine at 40 1C [30]. The reaction with carbon dioxide at low carbon dioxide concentrations produces both piperazine carbamate and protonated piperazine [31]:
The reaction is known to increase in rate with temperature, but the loading of carbon dioxide that can be achieved decreases with temperature due to unfavourable reaction equilibria [30]. The permeance of pure and mixed gases in both a dry and wet membrane, the effect of conditioning and the long-term stability of the membranes were tested and analysed.
2. Materials and methods 2.1. Materials Polyamide/polysulfone composite membranes were used in this research. These membranes have a polyester support,
Fig. 2. Schematic diagram of the experimental setup.
microporous polysulfone substrate and an ultra thin (approx. 200 nm) polyamide selective barrier on the surface. Dow Filmtecs membrane NF3838-30FF was kindly donated by DOW Water and Process Solutions. According to the manufacturer [32], the surface layer is an aromatic/aliphatic polyamide with the presence of free amine and carboxylate end groups (Fig. 1). This particular membrane is intended for use in food and dairy environments. Other commercial polypiperazine amide membranes, GE Osmonicss HL, Triseps TS40 and Triseps XN45, were purchased and tested under the same conditions without modification or pre-treatment. Some membranes were conditioned before use. In this case, deionised water was deposited on the membrane surface homogeneously then excessive water was removed carefully with a paper tissue before installing the membrane in the cell. The bottom of the membrane was kept dry to prevent water penetration into the substrate pores that could retard gas permeation. Pure nitrogen, argon and carbon dioxide gases (high purity, BOC) and a gas mixture (10.2% CO2 in nitrogen balance, BOC) were used for the permeation tests.
2.2. Permeation tests Membrane samples (13.85 cm2) were installed in a stainless cell for gas permeation tests. Fig. 2 shows the schematic diagram of the permeation rig. Feed gases were introduced to the membrane cell either dry or humidified by passage through an upstream gas bubbler filled with deionised water. In separate experiments, the relative humidity of the gas stream produced by the gas bubbler was monitored by a humidity meter (Vaisala MI70) and a value above 95% RH was recorded for up to five days. This corresponds to a water partial pressure of 5.6 kPa at 35 1C. Permeation tests were performed under a range of pressures and temperatures. A cold trap was used to remove water from the permeate gas before analysis. Pure gas permeation tests relied on a simple measurement of the permeate flow rate to determine permeance. For mixed gas experiments, an argon sweep gas (10 ml/min) was introduced to the downstream side of the membranes and the concentration of permeate CO2 was analysed by gas chromatography (Varian CP3800) using a thermal conductivity detector. In some cases, the sweep gas was also humidified by passage through a gas bubbler to minimise water loss from the membrane surface. In these instances, the downstream sweep gas humidity was again checked with a humidity probe (Vaisala MI70) and recorded as above 95% RH.
S. Andrew Lee et al. / Journal of Membrane Science 429 (2013) 349–354
3. Results and discussion 3.1. Effect of moisture in pure gas permeation Results of permeation test of pure gases are described in Table 1. None of the membrane showed high permeance and selectivity when the membrane and gases were dry. The GE Osmonicss HL membrane clearly showed a behaviour typical of Knudsen diffusion when the gases were dry. The Triseps XN45 and Dow Filmtecs NF3838/30FF provided better results with CO2/ N2 selectivities around 3. Somewhat surprisingly, there was little correlation between the dry gas permeation results and the published results for the flux, molecular weight cut-off (MWCO) or rejection of salt solutions through each membrane. When wet gases were applied to the system, the permeance of nitrogen decreased through all the membranes. The permeance of carbon dioxide, however, showed interesting behaviour depending upon the membrane. The Triseps XN45 and Dow Filmtecs NF3838/30FF membranes showed a higher permeance and significantly greater CO2/N2 selectivity when humidified carbon dioxide was introduced. This is presumably due to facilitated transport of carbon dioxide through Lewis acid-base interactions. While both of these membrane materials showed promise, the Dow Filmtecs NF3838/30FF membrane was selected for further testing, as it showed the biggest change in carbon dioxide permeance when water was added and a slightly higher selectivity. Thus, the following experiments were performed using the Dow Filmtecs NF3838/30FF membranes only. More detailed results of the permeance of dry gases through a dry Dow Filmtecs NF3838 membranes showed low permeance values as shown in Fig. 3(a) which did not change significantly with temperature. These results show that the dry NF membrane is not appropriate for gas separation service. Although higher permeance can be expected at higher temperatures due to greater diffusivity, this figure shows similar results in this temperature range presumably due to the lower solubility of carbon dioxide at the higher temperatures. The use of humidified feed gases dramatically improved the carbon dioxide permeance and selectivity as shown in Fig. 3(b). Wet membranes can be swollen and plasticised by water molecules. However, if this were the cause of the increased permeance, it would be expected to affect both carbon dioxide and nitrogen, which is not observed. It is more likely, given the presence of amide functional groups, as well as some free amine, that the membrane is acting in a facilitated transport mode for carbon dioxide. Water enhances the solubility of carbon dioxide due to the acid–base interaction. Carbon dioxide, considered as a Lewis acid, reacts with the free amide functional groups and free amine in the membrane. The presence of amine moieties as well as membrane charge plays an important role in enhancing the carbon dioxide permeation when the membrane matrix is moist.
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When temperature increases, the solubility of gases in the membrane should decrease, while diffusivity increases by thermal energy. When facilitated transport is involved, higher temperatures can accelerate the reaction rate between the amine and carbon dioxide, but decrease the capacity of the membrane to take up the CO2 due to unfavourable equilibria. Higher temperatures will also mean that there is a larger driving force for evaporation of water from the membrane. Such accelerated evaporation of water from the membrane matrix will make the membrane drier and denser. The permeance and selectivity recorded will be determined as a result of these competitive factors.
Fig. 3. The effect of temperature on the pure gas permeance and ideal selectivity of (a) dry gases and (b) humidified gases through a Dow Filmtecs NF3838/30FF membrane at 2 bar feed pressure. Permeability of CO2 (’), Permeability of N2 (K) and ideal selectivity (m).
Table 1 Permeation results of dry and humidified pure gases at 35 1C and 400 kPa pressure through a variety of polypiperazine based nanofiltration membranes. Flux/DP (GFD@psi) In 2 g/l salt solutiona
GE HL Trisep TS40 Trisep XN45 Dow NF3838/30FF a
29/110 12/110 25/110
Data provided by manufacturer.
Rejection of NaCla(%)
10–30
Rejection of MgSO4a(%)
98 99 95
MWCOa
150–300 200–300 500 200
Dry gases
Wet gases
P(N2) (GPU)
P (CO2) (GPU)
Selectivity
P(N2) (GPU)
P (CO2) (GPU)
Selectivity
207 29 9.8 3.7
170 31 29 10
0.82 1.0 3.0 2.7
96 14 2.4 1.8
116 33 34 34
1.2 2.3 14 19
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3.2. Effect of pressure Fig. 4 illustrates the effect of pressure on humidified gas permeation. Permeation tests were performed at 35 1C with humidified pure feed gases (N2 and CO2) at different pressures. At higher pressures, the flux of nitrogen showed an insignificant change. The flux of carbon dioxide increased as expected with increasing pressure but the permeance declined. These results support the hypothesis that carbon dioxide permeation is mainly driven by facilitated transport. As the pressure increases, permeance is attenuated because the carriers on the feed side of the membrane become saturated and so the complexation reaction rate stabilises [33]. It is also obvious that the selectivity declines at higher pressures. 3.3. Effect of conditioning When membranes were pre-conditioned with Milli Q water (pH 7) before installation in a permeation cell, a complex behavior was observed. The flux was initially low, presumably due to the barrier effect of excessive liquid water. This flux increased with time, presumably due to the evaporation and/or permeation of this excess water until reaching a maximum. The permeance then gradually decreased, presumably due to the membrane drying out, before stabilising. In Fig. 5, the permeance calculated at the point of maximum flux is presented, corresponding to a fully wet membrane. The general trend in this figure is similar to the data presented in Fig. 3(b) but the permeance of CO2 as well as the selectivity shows a much higher absolute value. The CO2 permeance increases with temperature slightly, which can be related to both increasing diffusivity and reaction rate. However, the increase is limited by the declining solubility of the species and unfavourable reaction equilibria. These effects will influence carbon dioxide more significantly than nitrogen. Thus, lower selectivity values were obtained at higher temperature. The difference in CO2 permeance between the pre-conditioned membranes (Fig. 5) and the unconditioned membranes (Fig. 3(b)) shows that humidification using a simple bubbler is insufficient to compensate for water evaporation from the membrane surface. The gradual decrease in permeance of CO2 through preconditioned membranes also supports this conclusion. Better humidification methods, such as the use of a wet sweep gas, or deliberate injection of water is required to keep the membrane sufficiently wet and to maximise the performance.
Fig. 5. The effect of temperature on the pure gas permeance and ideal selectivity through a Dow Filmtecs NF3838/30FF membrane, pre-conditioned by wetting with water, in a humidified gas stream at 2 bar feed pressure. Permeability of CO2 (’), Permeability of N2 (K) and ideal selectivity (m).
Fig. 6. The effect of the pH of the conditioning water used to wet the membrane, on the pure gas permeance and ideal selectivity through a Dow Filmtecs NF3838/ 30FF membrane. Measurements conducted at 35 1C and 2 bar feed pressure. Permeability of CO2 (’ ), Permeability of N2 (K) and ideal selectivity (W).
3.4. Effect of pH of conditioning water Membranes were also conditioned with pH controlled Milli Q water. The permeance and selectivity of humidified pure gases through these membranes was tested at 2 bar feed pressure and 35 1C. The results are illustrated in Fig. 6. The CO2 permeance increases with pH, reaching a maximum of 340 GPU at pH 12, while that of nitrogen was unchanged. As the pH increases, the conversion of CO2 into bicarbonate ions is favoured, which in turn increases the solubility of the species and the formation of carbamate ions as per Equation 1. The CO2/N2 selectivity reaches a maximum value of 50. 3.5. Effect of moisture in mixed gas permeation
Fig. 4. The effect of pressure on the pure gas permeance through a Dow Filmtecs NF3838/30FF membrane in a humidified gas stream at 35 1C. Permeability of CO2 (’), Permeability of N2 (K).
Permeation of a 10% CO2 in nitrogen mixture at a total feed pressure of 2 bar (partial pressure of 0.2 bar) was then trialled. The dry gas results (Fig. 7(a)) were comparable to those of the pure gases (Fig. 3(a)). Both pure gas pairs and gas mixtures showed no significant separation performance. The CO2 permeance and selectivity of the humidified mixed gas are illustrated in Fig. 7(b). These experiments used both a humidified feed and sweep gas. The CO2 permeance in the mixed gas remained significantly higher than for the dry mixed gas, but
S. Andrew Lee et al. / Journal of Membrane Science 429 (2013) 349–354
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was lower than expected based on a pure wet gas at a comparable CO2 pressure of 0.2 bar (Fig. 4). As shown in Fig. 4, higher CO2 permeance can be expected at lower partial pressure, if facilitated transport is the dominant mechanism. In the mixed gas case, the CO2 permeance also fell with increasing temperature, contrary to that for the pure gas. Again, such changes with temperature relate to an interplay between water evaporation rates, reaction rates and diffusivity that may change between these two cases. The experiment in Fig. 7(b) at 35 1C was continued for over a week using a humidified sweep gas and the longer-term performance was monitored. Results in Fig. 8 showed stable CO2 permeance for this period. There was no evidence of membrane plasticisation or degradation, presumably due to the heavily crosslinked structure of this polymer material. The use of a wet sweep gas is thus a practical method to minimise the partial pressure difference of moisture on both side of the membrane in a laboratory situation. However, this approach will be less practical in an industrial application.
4. Conclusions Commercial thin-film composite polyamide/polysulfone membranes showed good CO2 separation performance in the presence of water. The high permeance is related to a facilitated transport mechanism where the carbon dioxide converts to a carbamate ion and transfers across the membrane in a reacted state. Results
Fig. 8. Long-term stability test of a Dow Filmtecs NF3838/30FF membrane in a mixed gas stream (10% CO2 in N2) at a total feed pressure of 2 bar and 35 1C. The data shown is the percentage concentration of CO2 in the permeate stream.
showed that the highest CO2 permeance and selectivity could be achieved under low pressure and low temperature conditions as expected for a facilitated transport mechanism. Stable membrane humidification is required in order to maintain the maximum efficiency and productivity of the membrane. More work is required to provide viable methods for retaining a wet membrane in a commercial scale application. The results suggest that the use of commercial NF membranes show strong potential for carbon capture applications if the issues with maintenance of humidity can be addressed. The permeance and selectivity is comparable to that of commercial natural gas membranes but still below that of rubbery membranes under development. However, the use of a membrane that is already a commercial product dramatically reduces the time and development costs required to bring the application to an industrial scale. Further, the relatively cheap price of NF membranes, relative to those used currently in gas separation applications means that carbon capture costs can be substantially reduced.
Acknowledgements The authors would like to acknowledge funding for this project provided by the Australian Government through its Cooperative Research Centre program.
References
Fig. 7. Permeation performance of a Dow Filmtecs NF3838/30FF membrane in a mixed gas stream (10% CO2 in N2) at a total feed pressure of 2 bar and 35 1C. Results are for a (a) dry and (b) humidified gas mixture. Permeability of CO2 (’), Permeability of N2 (K) and mixed gas selectivity (m).
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Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Facilitated transport behavior of humidified gases through thin-film composite polyamide membranes for carbon dioxide capture S. Andrew Lee, Geoff W. Stevens, Sandra E. Kentish n The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, VIC 3010, Australia
a r t i c l e i n f o
abstract
Article history: Received 16 August 2012 Received in revised form 11 November 2012 Accepted 21 November 2012 Available online 29 November 2012
In this work, commercial nanofiltration membranes from the water treatment industry have been shown to give good performance in a carbon dioxide capture application. Permeation of humidified gases through the membrane showed a carbon dioxide permeance of up to 340 GPU and a selectivity of 50, although the performance of a dry membrane with dry gas was significantly lower (6–8 GPU). The permeance of carbon dioxide was dependent on the temperature, pressure and pH of the water used to condition the membranes. The results indicate that a facilitated transport mechanism is dominant when humidified gases are present, probably due to the presence of piperazine functional groups and free amine within the membrane structure. The stability of membrane performance largely depends on the presence of water. This approach has strong potential for commercialisation, as the reliance on a commercial membrane material means that the development pathway can be fast tracked and membrane fabrication costs reduced. & 2012 Elsevier B.V. All rights reserved.
Keywords: Polypiperazine amide Carbon dioxide capture Facilitated transport Nanofiltration
1. Introduction Membrane processes for gas separation have been studied for decades and were first commercialized for this purpose in the 1980s. Membranes that are selective for carbon dioxide are of importance to natural gas sweetening, and more recently have gained a high profile due to their potential for use in the capture of carbon dioxide from combustion flue gases. To be highly selective towards condensable carbon dioxide over other gases such as nitrogen and hydrogen, it is beneficial that the membrane material has a high solubility selectivity rather than diffusivity selectivity. Thus, rubbery polymers are good candidates, while the presence of polar groups in the membrane structure that can interact with carbon dioxide, is known to be advantageous. The best commercial membranes, designed for natural gas sweetening, have a CO2 permeance of around 100 GPU and a CO2/N2 selectivity of 30 [1]. Conversely, rubbery membranes under development for CO2 capture have permeance values of around 1000 GPU and a CO2/N2 selectivity of 40 [1]. Facilitated transport systems have also shown potential, with selectivities well above those of conventional membranes that follow the solution-diffusion mechanism [2–4]. CO2/N2 selectivities in the range of 50 to 1000 have been reported [5] while CO2
*
Corresponding author. Tel:þ 61 3 8344 6682. E-mail address: [email protected] (S.E. Kentish).
0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.11.047
permeance values can range from 10 to 1200 GPU [6–8]. In this case, the carbon dioxide reacts with either a mobile or fixed carrier species and is carried across the membrane in a reacted state. However, in most cases, the facilitated transport mechanism relies on the membrane being water saturated, as the chemical reaction occurring is generally a Lewis acid–base interaction. It is expected that hydrogen bonding between the CO2 and amine groups in the membrane also play an important role in increasing the selectivity [9–11]. In addition, it has been shown that the diffusivity of carbon dioxide through membranes increases when the membrane is swollen by water [4,12]. While showing potential, facilitated transport membranes have limited durability due to the membrane drying out. Different methods have been tested to maintain performance such as additional water injection [9], continuous water spraying [13,14] or the use of a wet sweep gas [8,15]. While thousands of membrane materials have been identified as prospective for CO2 capture, only a limited number of materials have been commercialized [16]. One of the reasons behind this lack of success has been the need to form the membrane material into an ultrathin layer, of the order of 100 to 200 nm in thickness. Asymmetric structured membranes and thin film composite (TFC) membranes are the two methods commonly employed to achieve this objective. In particular, the selective upper layer and porous support substrate of TFCs can be separately optimised, therefore giving high permeability and selectivity at a cheaper price than most asymmetric single material membranes.
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Fig. 1. Schematic of the Dow FilmtecTM NF3838/30FF membrane chemistry, based on the description provided by the manufacturer [32].
Polyamide TFC membranes made by interfacial polymerisation (IP) have been widely used for nanofiltration, reverse osmosis and pervaporation processes [17]. The defect-free nature and ease in scale up of IP makes this method viable for manufacturing commercial membranes [10]. In situ polycondensation of polyfunctional amine monomers such as 1,3-benzenediamine and piperazine in the aqueous phase with polyfunctional acid chlorides such as trimesoyl chloride in the organic phase is the most well-known method to make commercial polyamide membranes [18–20]. Many different combinations of monomers and their polymerization methods have been trialled [21–23]. Porous polysulfone or polyethersulfone membranes have been widely used as a substrate due to their superior chemical and mechanical stability and relatively low price. While polyamide TFC membranes have been extensively deployed for NF and RO, they have also been trialed for gas separation applications. Recent results have shown that polyamide RO membranes and their derivatives are possible candidates for CO2/N2 or CO2/CH4 separation purposes [4,10,24–27]. In this project, we initially examined a range of commercial polyamide membranes for their potential for carbon dioxide capture. The advantage of using a TFC membrane that is already a commercial product has obvious advantages—the ability to manufacture at scale has clearly been demonstrated. All of the membranes we selected are believed to be based on a piperazine monomer (Fig. 1), either by declaration on the manufacturer’s website, or from the literature [14]. Piperazine is commonly used as a rate promoter in solvent based carbon dioxide capture and is known to have a very high rate of reaction with carbon dioxide [28,29], more than double that of the more common monoethanolamine at 40 1C [30]. The reaction with carbon dioxide at low carbon dioxide concentrations produces both piperazine carbamate and protonated piperazine [31]:
The reaction is known to increase in rate with temperature, but the loading of carbon dioxide that can be achieved decreases with temperature due to unfavourable reaction equilibria [30]. The permeance of pure and mixed gases in both a dry and wet membrane, the effect of conditioning and the long-term stability of the membranes were tested and analysed.
2. Materials and methods 2.1. Materials Polyamide/polysulfone composite membranes were used in this research. These membranes have a polyester support,
Fig. 2. Schematic diagram of the experimental setup.
microporous polysulfone substrate and an ultra thin (approx. 200 nm) polyamide selective barrier on the surface. Dow Filmtecs membrane NF3838-30FF was kindly donated by DOW Water and Process Solutions. According to the manufacturer [32], the surface layer is an aromatic/aliphatic polyamide with the presence of free amine and carboxylate end groups (Fig. 1). This particular membrane is intended for use in food and dairy environments. Other commercial polypiperazine amide membranes, GE Osmonicss HL, Triseps TS40 and Triseps XN45, were purchased and tested under the same conditions without modification or pre-treatment. Some membranes were conditioned before use. In this case, deionised water was deposited on the membrane surface homogeneously then excessive water was removed carefully with a paper tissue before installing the membrane in the cell. The bottom of the membrane was kept dry to prevent water penetration into the substrate pores that could retard gas permeation. Pure nitrogen, argon and carbon dioxide gases (high purity, BOC) and a gas mixture (10.2% CO2 in nitrogen balance, BOC) were used for the permeation tests.
2.2. Permeation tests Membrane samples (13.85 cm2) were installed in a stainless cell for gas permeation tests. Fig. 2 shows the schematic diagram of the permeation rig. Feed gases were introduced to the membrane cell either dry or humidified by passage through an upstream gas bubbler filled with deionised water. In separate experiments, the relative humidity of the gas stream produced by the gas bubbler was monitored by a humidity meter (Vaisala MI70) and a value above 95% RH was recorded for up to five days. This corresponds to a water partial pressure of 5.6 kPa at 35 1C. Permeation tests were performed under a range of pressures and temperatures. A cold trap was used to remove water from the permeate gas before analysis. Pure gas permeation tests relied on a simple measurement of the permeate flow rate to determine permeance. For mixed gas experiments, an argon sweep gas (10 ml/min) was introduced to the downstream side of the membranes and the concentration of permeate CO2 was analysed by gas chromatography (Varian CP3800) using a thermal conductivity detector. In some cases, the sweep gas was also humidified by passage through a gas bubbler to minimise water loss from the membrane surface. In these instances, the downstream sweep gas humidity was again checked with a humidity probe (Vaisala MI70) and recorded as above 95% RH.
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3. Results and discussion 3.1. Effect of moisture in pure gas permeation Results of permeation test of pure gases are described in Table 1. None of the membrane showed high permeance and selectivity when the membrane and gases were dry. The GE Osmonicss HL membrane clearly showed a behaviour typical of Knudsen diffusion when the gases were dry. The Triseps XN45 and Dow Filmtecs NF3838/30FF provided better results with CO2/ N2 selectivities around 3. Somewhat surprisingly, there was little correlation between the dry gas permeation results and the published results for the flux, molecular weight cut-off (MWCO) or rejection of salt solutions through each membrane. When wet gases were applied to the system, the permeance of nitrogen decreased through all the membranes. The permeance of carbon dioxide, however, showed interesting behaviour depending upon the membrane. The Triseps XN45 and Dow Filmtecs NF3838/30FF membranes showed a higher permeance and significantly greater CO2/N2 selectivity when humidified carbon dioxide was introduced. This is presumably due to facilitated transport of carbon dioxide through Lewis acid-base interactions. While both of these membrane materials showed promise, the Dow Filmtecs NF3838/30FF membrane was selected for further testing, as it showed the biggest change in carbon dioxide permeance when water was added and a slightly higher selectivity. Thus, the following experiments were performed using the Dow Filmtecs NF3838/30FF membranes only. More detailed results of the permeance of dry gases through a dry Dow Filmtecs NF3838 membranes showed low permeance values as shown in Fig. 3(a) which did not change significantly with temperature. These results show that the dry NF membrane is not appropriate for gas separation service. Although higher permeance can be expected at higher temperatures due to greater diffusivity, this figure shows similar results in this temperature range presumably due to the lower solubility of carbon dioxide at the higher temperatures. The use of humidified feed gases dramatically improved the carbon dioxide permeance and selectivity as shown in Fig. 3(b). Wet membranes can be swollen and plasticised by water molecules. However, if this were the cause of the increased permeance, it would be expected to affect both carbon dioxide and nitrogen, which is not observed. It is more likely, given the presence of amide functional groups, as well as some free amine, that the membrane is acting in a facilitated transport mode for carbon dioxide. Water enhances the solubility of carbon dioxide due to the acid–base interaction. Carbon dioxide, considered as a Lewis acid, reacts with the free amide functional groups and free amine in the membrane. The presence of amine moieties as well as membrane charge plays an important role in enhancing the carbon dioxide permeation when the membrane matrix is moist.
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When temperature increases, the solubility of gases in the membrane should decrease, while diffusivity increases by thermal energy. When facilitated transport is involved, higher temperatures can accelerate the reaction rate between the amine and carbon dioxide, but decrease the capacity of the membrane to take up the CO2 due to unfavourable equilibria. Higher temperatures will also mean that there is a larger driving force for evaporation of water from the membrane. Such accelerated evaporation of water from the membrane matrix will make the membrane drier and denser. The permeance and selectivity recorded will be determined as a result of these competitive factors.
Fig. 3. The effect of temperature on the pure gas permeance and ideal selectivity of (a) dry gases and (b) humidified gases through a Dow Filmtecs NF3838/30FF membrane at 2 bar feed pressure. Permeability of CO2 (’), Permeability of N2 (K) and ideal selectivity (m).
Table 1 Permeation results of dry and humidified pure gases at 35 1C and 400 kPa pressure through a variety of polypiperazine based nanofiltration membranes. Flux/DP (GFD@psi) In 2 g/l salt solutiona
GE HL Trisep TS40 Trisep XN45 Dow NF3838/30FF a
29/110 12/110 25/110
Data provided by manufacturer.
Rejection of NaCla(%)
10–30
Rejection of MgSO4a(%)
98 99 95
MWCOa
150–300 200–300 500 200
Dry gases
Wet gases
P(N2) (GPU)
P (CO2) (GPU)
Selectivity
P(N2) (GPU)
P (CO2) (GPU)
Selectivity
207 29 9.8 3.7
170 31 29 10
0.82 1.0 3.0 2.7
96 14 2.4 1.8
116 33 34 34
1.2 2.3 14 19
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3.2. Effect of pressure Fig. 4 illustrates the effect of pressure on humidified gas permeation. Permeation tests were performed at 35 1C with humidified pure feed gases (N2 and CO2) at different pressures. At higher pressures, the flux of nitrogen showed an insignificant change. The flux of carbon dioxide increased as expected with increasing pressure but the permeance declined. These results support the hypothesis that carbon dioxide permeation is mainly driven by facilitated transport. As the pressure increases, permeance is attenuated because the carriers on the feed side of the membrane become saturated and so the complexation reaction rate stabilises [33]. It is also obvious that the selectivity declines at higher pressures. 3.3. Effect of conditioning When membranes were pre-conditioned with Milli Q water (pH 7) before installation in a permeation cell, a complex behavior was observed. The flux was initially low, presumably due to the barrier effect of excessive liquid water. This flux increased with time, presumably due to the evaporation and/or permeation of this excess water until reaching a maximum. The permeance then gradually decreased, presumably due to the membrane drying out, before stabilising. In Fig. 5, the permeance calculated at the point of maximum flux is presented, corresponding to a fully wet membrane. The general trend in this figure is similar to the data presented in Fig. 3(b) but the permeance of CO2 as well as the selectivity shows a much higher absolute value. The CO2 permeance increases with temperature slightly, which can be related to both increasing diffusivity and reaction rate. However, the increase is limited by the declining solubility of the species and unfavourable reaction equilibria. These effects will influence carbon dioxide more significantly than nitrogen. Thus, lower selectivity values were obtained at higher temperature. The difference in CO2 permeance between the pre-conditioned membranes (Fig. 5) and the unconditioned membranes (Fig. 3(b)) shows that humidification using a simple bubbler is insufficient to compensate for water evaporation from the membrane surface. The gradual decrease in permeance of CO2 through preconditioned membranes also supports this conclusion. Better humidification methods, such as the use of a wet sweep gas, or deliberate injection of water is required to keep the membrane sufficiently wet and to maximise the performance.
Fig. 5. The effect of temperature on the pure gas permeance and ideal selectivity through a Dow Filmtecs NF3838/30FF membrane, pre-conditioned by wetting with water, in a humidified gas stream at 2 bar feed pressure. Permeability of CO2 (’), Permeability of N2 (K) and ideal selectivity (m).
Fig. 6. The effect of the pH of the conditioning water used to wet the membrane, on the pure gas permeance and ideal selectivity through a Dow Filmtecs NF3838/ 30FF membrane. Measurements conducted at 35 1C and 2 bar feed pressure. Permeability of CO2 (’ ), Permeability of N2 (K) and ideal selectivity (W).
3.4. Effect of pH of conditioning water Membranes were also conditioned with pH controlled Milli Q water. The permeance and selectivity of humidified pure gases through these membranes was tested at 2 bar feed pressure and 35 1C. The results are illustrated in Fig. 6. The CO2 permeance increases with pH, reaching a maximum of 340 GPU at pH 12, while that of nitrogen was unchanged. As the pH increases, the conversion of CO2 into bicarbonate ions is favoured, which in turn increases the solubility of the species and the formation of carbamate ions as per Equation 1. The CO2/N2 selectivity reaches a maximum value of 50. 3.5. Effect of moisture in mixed gas permeation
Fig. 4. The effect of pressure on the pure gas permeance through a Dow Filmtecs NF3838/30FF membrane in a humidified gas stream at 35 1C. Permeability of CO2 (’), Permeability of N2 (K).
Permeation of a 10% CO2 in nitrogen mixture at a total feed pressure of 2 bar (partial pressure of 0.2 bar) was then trialled. The dry gas results (Fig. 7(a)) were comparable to those of the pure gases (Fig. 3(a)). Both pure gas pairs and gas mixtures showed no significant separation performance. The CO2 permeance and selectivity of the humidified mixed gas are illustrated in Fig. 7(b). These experiments used both a humidified feed and sweep gas. The CO2 permeance in the mixed gas remained significantly higher than for the dry mixed gas, but
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was lower than expected based on a pure wet gas at a comparable CO2 pressure of 0.2 bar (Fig. 4). As shown in Fig. 4, higher CO2 permeance can be expected at lower partial pressure, if facilitated transport is the dominant mechanism. In the mixed gas case, the CO2 permeance also fell with increasing temperature, contrary to that for the pure gas. Again, such changes with temperature relate to an interplay between water evaporation rates, reaction rates and diffusivity that may change between these two cases. The experiment in Fig. 7(b) at 35 1C was continued for over a week using a humidified sweep gas and the longer-term performance was monitored. Results in Fig. 8 showed stable CO2 permeance for this period. There was no evidence of membrane plasticisation or degradation, presumably due to the heavily crosslinked structure of this polymer material. The use of a wet sweep gas is thus a practical method to minimise the partial pressure difference of moisture on both side of the membrane in a laboratory situation. However, this approach will be less practical in an industrial application.
4. Conclusions Commercial thin-film composite polyamide/polysulfone membranes showed good CO2 separation performance in the presence of water. The high permeance is related to a facilitated transport mechanism where the carbon dioxide converts to a carbamate ion and transfers across the membrane in a reacted state. Results
Fig. 8. Long-term stability test of a Dow Filmtecs NF3838/30FF membrane in a mixed gas stream (10% CO2 in N2) at a total feed pressure of 2 bar and 35 1C. The data shown is the percentage concentration of CO2 in the permeate stream.
showed that the highest CO2 permeance and selectivity could be achieved under low pressure and low temperature conditions as expected for a facilitated transport mechanism. Stable membrane humidification is required in order to maintain the maximum efficiency and productivity of the membrane. More work is required to provide viable methods for retaining a wet membrane in a commercial scale application. The results suggest that the use of commercial NF membranes show strong potential for carbon capture applications if the issues with maintenance of humidity can be addressed. The permeance and selectivity is comparable to that of commercial natural gas membranes but still below that of rubbery membranes under development. However, the use of a membrane that is already a commercial product dramatically reduces the time and development costs required to bring the application to an industrial scale. Further, the relatively cheap price of NF membranes, relative to those used currently in gas separation applications means that carbon capture costs can be substantially reduced.
Acknowledgements The authors would like to acknowledge funding for this project provided by the Australian Government through its Cooperative Research Centre program.
References
Fig. 7. Permeation performance of a Dow Filmtecs NF3838/30FF membrane in a mixed gas stream (10% CO2 in N2) at a total feed pressure of 2 bar and 35 1C. Results are for a (a) dry and (b) humidified gas mixture. Permeability of CO2 (’), Permeability of N2 (K) and mixed gas selectivity (m).
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