Nitric oxide and carbon monoxide permeation through glassy polymeric membranes for carbon dioxide separation

chemical engineering research and design 8 9 ( 2 0 1 1 ) 1730–1736

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Nitric oxide and carbon monoxide permeation through glassy polymeric membranes for carbon dioxide separation Colin A. Scholes, George Q. Chen, Geoff W. Stevens, Sandra E. Kentish ∗ Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, VIC 3010, Australia

a b s t r a c t Minor components present in polymeric membrane gas separation can have a significant influence on the separation performance. Carbon monoxide and nitric oxide exist in post-combustion gas streams and can therefore influence CO2 transport through membranes designed for that application. Here, the permeability of nitric oxide (NO) through three glassy polymeric membranes (polysulfone, Matrimid 5218 and 6FDA-TMPDA) was determined and found to be less than the CO2 but greater than the N2 permeability in each membrane. This study also investigated the influence of 1000 ppm CO on the mixed gas permeability of CO2 and N2 for two glassy polymeric membranes; polysulfone and 6FDA-TMPDA. For both membranes, CO competitive sorption resulted in a reduction in the measured permeability of CO2 and N2 even though present at only low concentration. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Gas separation; Polymeric membranes; Carbon dioxide; Carbon monoxide; Nitric oxide; Competitive sorption

1.

Introduction

Carbon dioxide separation from the flue gas of coal-based power stations is known as post-combustion capture (Blok et al., 1992; Ishitani and Johansson, 1995). This carbon capture process is believed to be an important future strategy for reducing carbon emissions, given the large number of coalbased power stations currently in existence. Gas separation glassy polymeric membranes are a technology that has potential for carbon dioxide capture, with a wide range of membrane materials successfully separating CO2 from N2 (Powell and Qiao, 2006). Given the commercial success of membrane technology in the natural gas industry (Sridhar et al., 2007; Stern, 1994), there is significant potential for the successful application of such materials in a post-combustion scenario. Membranes have a number of advantages over other carbon capture technologies, such as reversible solvent absorption (Favre, 2007). In particular, the simplicity of the process and modular nature of membranes make them good candidates for retrofitting to existing coal-based power plants as part of a post-combustion capture process (Kohl and Nielsen, 1997).

∗

A possible disadvantage for the application of membranes is the low partial pressure of carbon dioxide in the flue gas. Another disadvantage against the rapid uptake of membrane technology is the still unknown influence of minor components in the flue gas, such as sulfur oxides (SOx), nitric oxides (NOx) and carbon monoxide, upon membrane separation performance. Flue gases from coal-based power station have a range of minor components present (Scholes et al., 2009), importantly SOx (1000–5000 ppm), NOx (10–500 ppm), CO (<20 ppm) and water (saturated). Hence, their presence within the polymeric membrane can lead to competition with CO2 for separation, as well as chemical degradation and plasticization of the polymeric structure. All of these components can reduce the separation efficiency of the process and potentially lead to membrane failure. The influence of water on polymeric membranes has been considered by a number of researchers (Chern et al., 1983; Park, 1983; Paulson et al., 1983). Similarly, the performance of SOx, specifically SO2 , in membranes has been well reported because of research into the application of membranes for desulfurization (Davis and Rooney, 1971; Kuehne

Corresponding author. Tel.: +61 8344 6682. E-mail address: [email protected] (S.E. Kentish). Received 14 May 2010; Received in revised form 22 November 2010; Accepted 5 April 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.04.001

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and Friedlander, 1980). In contrast, the permeability and separation performance of NOx, in particular the major component NO, has to the authors’ knowledge not been reported for glassy polymeric membranes. And while the permeability of CO has been reported for a range of polymeric membranes (Scholes et al., 2009), the influence of CO on the ability of a membrane to separate CO2 from N2 has not been reported. Such information is vital to quantitatively evaluate the performance of polymeric membranes for post-combustion carbon capture; to determine where in the separation process the minor components are conveyed and how this influences CO2 separation. Here, the permeability of NO through three glassy polymeric membranes, polysulfone, Matrimid 5218 and 6FDA-TMPDA, is reported, along with the influence of CO on CO2 /N2 separation for polysulfone and 6FDA-TMPDA. Polysulfone and Matrimid 5218 were chosen because they are commercially available proven CO2 -selective membranes, while 6FDA-TMPDA is one of a class of new polyimides that has improved performance over existing commercial CO2 -selective membranes (Powell and Qiao, 2006).

where KDA is the Henry’s Law coefficient for species A, CHA is the maximum gas concentration in the Langmuir voids and bA is the Langmuir affinity constant. Thus for these systems, solubility is described by:

2.

This can be related to the gas fluxes, and therefore through Eq. (1) to the permeabilities:

Theory

The selective layer of a polymeric membrane is generally a non-porous film that transports gases across by the solution–diffusion mechanism, where the driving force is the partial pressure difference across the membrane. The average permeability of gas across the membrane (PA ) is dependent on the flux of gas A (NA ), and the fugacity or partial pressure (pA ) across a membrane of thickness l (Matteucci et al., 2006): NA = PA

 p  A

l

(1)

where the permeability is generally quoted in barrer (10−10 cm3 (STP) cm/cm2 s cm Hg). The permeability is experimentally determined from the gas permeation rate (QA ) through the membrane, of known surface area (A) and thickness (l), with the partial pressure difference the driving force of separation: PA =

QA · l A · pA

(2)

According to the solution–diffusion model, this permeability can be related to the diffusivity of species A (DA ) and its concentration within the membrane phase (CA ): PA = DA .

∂CA ∂pA

(3)

The solubility of a species within the membrane phase is defined by: SA =

CA pA

(4)

For the glassy polymers considered here, gas sorption is generally described by a dual mode absorption model which includes a Henry’s Law term and Langmuir sorption term: CA = KDA pA +

C HA bA pA (1 + bA pA )

(5)

SA = KDA +

C HA bA (1 + bA pA )

(6)

For pure gas systems, the ideal selectivity (˛) of one gas, A, over another gas, B, is defined by the permeability ratio (Ghosal and Freeman, 1994): ˛AB =

PA PB

(7)

For binary mixtures, the separation factor (˛∗AB ) is defined in terms of mole fractions in the permeate stream (yi ) and feed stream (xi ) (Coker et al., 1998): ˛∗AB =

˛∗AB =

(yA /yB ) (xA /xB )

(NA /NB ) = (xA /xB )feed

(8)

P p x − p y  A 1 A 2 A PB

xA

xB p2 xB − p1 yB

 (9)

which relates the separation factor to the selectivity by:

 ˛∗AB = ˛AB

p2 − p1 (yA /xA ) p2 − p1 (yB /xA )

 (10)

While Eq. (10) provides the correct definition of the selectivity in a mixed gas system, it is common to present results in terms of the permeability ratio (Eq. (7)) to allow comparison to pure gas literature data, and this will be the approach taken in the present paper.

3.

Experimental

Membranes studied were polysulfone (Aldrich), Matrimid 5218 (Huntsman Chemical Co.) and 6FDA-TMPDA (2-2 -bis(3,4 -dicarboxyphenyl) hexafluoropropane – dianhydrid-2,3,5,6-tetramethyl-1,4-phenylenediamine synthesized in house (Powell et al., 2007)). For NO measurements, asymmetric glassy membranes were produced from these three materials by spin coating 0.5 mL of a 10 wt% solution in chloroform (AR) onto a flat poly tetrafluoroethylene (PTFE) support (Sartorius, 0.2 ␮m pore size). A Laurell WS400A-NPP/Lite spin coater was used at 1000 rpm for 30 s. The thickness of the active layer was increased by spin coating an additional 4 × 0.5 mL amounts of solution. This built up the active layer in controlled steps to 2 ␮m (Scholes et al., 2010). Asymmetric membranes were required to achieve a measureable gas flux (0.1 cm3 /s overall, NO detection threshold 1 ppm) because of the low partial pressure of NO in the feed gas (2500 ppm). For CO measurements, flat dense sheet membranes (thickness 20–60 ␮m) were cast from chloroform for polysulfone and from dichloromethane for 6FDA-TMPDA, following established procedures (Powell et al., 2007). In all cases, membranes were dried at 100 ◦ C for 24 h under vacuum; they were then annealed at 150 ◦ C for 48 h under vacuum. On completion of the annealing process, all membranes were cooled slowly to

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Table 1 – Permeability (barrer) of NO, CO, N2 and CO2 in polysulfone, Matrimid 5218 and 6FDA-TMPDA at 35 ◦ C.

N2 CO NO CO2

Notes

Polysulfone

Matrimid 5218

6FDA-TMPDA

Pure gas 30 ◦ C (McCandless, 1972) 2500 ppm in N2 Pure gas

0.26 0.37 3.1 ± 0.6 5.4

0.29

47

3.7 ± 0.6 7.7

168 ± 16 551

room temperature under vacuum conditions, ∼6 h, and stored in a moisture free environment for at most a week, to minimize aging effects. The pure gas permeability of CO2 and N2 through the membranes was measured on a constant volume, variable pressure gas permeation apparatus discussed elsewhere (Duthie et al., 2007), with the feed pressure at 600 kPa. Mixed gas conditions were tested on a different instrument, based on constant pressure (Anderson et al., 2008), Fig. 1, with permeability calculated using Eq. (2). The membrane films were housed in a custom built test cell on a porous stainless steel support, which provided an exposed area of diameter 47 mm. The test cell was held at constant temperature (±0.2 ◦ C, Oven – S.E.M. Pty. Ltd.). The pressure on the feed side of the membrane was controlled by a gas bottle regulator, with the gas flow rate in both the feed and retentate lines measured by digital mass flow indicators (Aalborg). The retentate flowrate was kept constant at 100 ml/min using a needle valve. The permeate line was kept at or near atmosphere to maximize pressure drop across the membrane, with the permeate gas directed to gas analysis instrumentation, and the flowrate measured by digital mass flow indicators (Aalborg). For CO measurements, a feed stream of 90% N2 –10% CO2 gas mixture (BOC Ltd., Australia) was passed across the retentate side of the membrane. A helium sweep gas was passed across the permeate side at 20 ml/min to minimize concentration polarization. Each membrane was exposed to this feed gas for 3 h until steady-state CO2 /N2 separation had been obtained. The feed gas was then changed to 90% N2 –10% CO2 with 1000 ppm CO gas mixture (BOC Ltd., Australia), with pressure and temperature conditions the same as before. The permeability of CO2 and N2 was measured every 15 min until steady-state had been achieved, and the influence of CO observed. Permeability was determined by Eq. (2), from the measured flux (Q), where the concentrations of CO2 and N2 in

Table 2 – Mixed gas permeabilities (barrer) of N2 and CO2 in polysulfone and 6FDA-TMPDA at 35 ◦ C.

N2 CO2

Notes

Polysulfone

6FDA-TMPDA

90% v/v in CO2 10% v/v in N2

0.13 5.2

23.5 400

the permeate gas determined by gas chromatography (Varian CP-3800, column PORAPAK Q). We were unable to detect CO in the permeate stream due to its low concentration being below the sensitivity of the detectors (2 ppm), with or without sweep gas present. Therefore, to minimize the effect of concentration polarization a sweep gas on the permeate side was used. The change of CO concentration in the retentate was also too small to be detectable because of the small stage-cut. For NO measurements a feed stream of N2 with 2500 ppm NO gas mixture (BOC Ltd., Australia) was passed across the asymmetric membrane at 200 kPa. No sweep gas was employed for NO measurements, due to concerns about diluting the NO concentration of the permeate gas below the detector threshold (sensitivity 0.5 ppm). Initially, pure N2 was purged through the membrane system at a set pressure for 15 min before the NO gas mixture was introduced. The gas composition of the permeate stream, in particular the NO and N2 concentrations, were measured on a Horiba (PG-250) flue gas analyzer, with make-up air supplied to meet the Horiba sampling rate requirements.

4.

Results and discussion

The measured steady-state permeability (barrer or 1010 cm3 cm/cm2 s cm Hg) of NO within the NO/N2 mixture in polysulfone, Matrimid 5218 and 6FDA-TMPDA membranes is provided in Table 1 at 35 ◦ C. Also included are the measured

Fig. 1 – Constant pressure, variable volume (CPVV) apparatus for measuring mixed gas separation performance.

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Fig. 2 – CO2 permeability ratio through polysulfone, relative to no CO present, at pressures between 600 and 1400 kPa (increasing grayscale); (a) 15 ◦ C, (b) 35 ◦ C, (c) 55 ◦ C and (d) 75 ◦ C.

pure gas permeabilities of CO2 and N2 in these glassy membranes at 35 ◦ C, along with the literature reported permeability of CO for polysulfone at 30 ◦ C (McCandless, 1972). The CO2 and N2 values have good agreement with literature, 6FDA-TMPDA has a reported CO2 permeability of 577 barrer (Duthie et al., 2007), while Matrimid CO2 permeability has been reported as 6.5 (Tin et al., 2003) and polysulfone CO2 permeability reported as 5.6 barrer (McHattie et al., 1991). It is clearly evident that CO2 is the most permeable gas, followed by NO and finally N2 , the least permeable. Hence, all three membranes demonstrate a preference towards CO2 , where the CO2 /NO permeability ratio is around 1.8 for polysulfone, 1.8 for Matrimid and 3.4 for 6FDA-TMPDA. For polysulfone, CO permeability appears to be lower than both CO2 and NO, while being greater than N2 . Similarly, Favvas et al. (2007) has reported a CO permeance of 26 GPU, compared to 84.9 GPU for CO2 and 23.1 GPU for N2 for a Matrimid hollow fibre. Therefore, for these glassy polymeric membranes the order of increasing permeability is N2 < CO < NO < CO2 . For glassy membranes, gas permeability is

strongly influenced by gas solubility, especially the amount of gas that can adsorb onto the Langmuir voids of the polymeric matrix (Petropoulos, 1970). The magnitude of this solubility has been shown to be strongly correlated with the condensability of each gas species (Bondar et al., 1999; De Angelis et al., 2007; Ghosal et al., 1995; Stern et al., 1969), and a good indication of this is the critical temperature of the gas. In the present case, the critical temperatures do indeed correspond with the reported gas permeability order N2 (126.2 K) < CO (132.9 K) < NO (180 K) < CO2 (304.2 K). However, direct comparison of the permeabilities between the NO and CO systems should be treated with caution because of the difference in membrane morphology (NO measurements were made using an asymmetric membrane, while that used for CO measurements was dense), which may potentially influence both the diffusivity and solubility of gases. In a 90 v/v% N2 /10% CO2 mixture, the permeability of both CO2 and N2 decrease from the pure gas value, as can be seen in Table 2. This loss in permeability is due to competition sorption inside the membrane. The percentage loss of CO2

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Fig. 3 – CO2 permeability ratio through 6FDA-TMPDA, relative to no CO present, at pressures between 600 and 1200 kPa (increasing grayscale); (a) 15 ◦ C, (b) 35 ◦ C, (c) 55 ◦ C and (d) 75 ◦ C.

permeability under these conditions is 3.7% for polysulfone and 27% for 6FDA-TMPDA. This reduction in permeability is indicative of mixed gas systems (Scholes et al., 2009) and is why the use of pure gas permeability to model real multi-gas systems can lead to significant error. The addition of 1000 ppm CO to the CO2 /N2 mixture further affects the membrane performance, as shown in Fig. 2, for polysulfone, and Fig. 3 for 6FDA-TMPDA. The CO2 permeability is shown relative to the initial permeability for the 90% N2 /10% CO2 feed mixture with no CO present. Upon exposure to CO a reduction in the permeability of CO2 occurs for both glassy membranes, at every temperature and partial pressure. This is due to competitive solubility, where the sorption of CO influences CO2 solubility within the membrane. Importantly, no plasticization of either of these membranes is observed upon exposure to CO, over the time studied, which would be indicated by an increase in CO2 permeability at longer timeframes (Kesting and Fritzche, 1993). This is either as a result of CO inability to plasticize the polymeric membranes or the low concentration present.

The magnitude of the loss in CO2 permeability in these systems, 6–14% for polysulfone and 6–10% for 6FDA-TMPDA upon exposure to CO is significant relative to the low 1000 ppm concentration and signifies that minor components at low concentration can have measurable effects. This is especially true for polysulfone, where CO exposure produces a similar decrease in CO2 permeability, as that observed for the transition from the pure gas case to a 90% N2 mixture. Within experimental error, the permeability loss for polysulfone is the same at all the temperatures tested. However, for 6FDA-TMPDA, the permeability loss is smaller at higher temperatures (Fig. 4), reflecting the stronger contribution of Langmuir sorption at lower temperatures (Duthie et al., 2007). The average activation energy for permeation is −2.5 kJ/mol, which is consistent with previous literature reports for CO2 permeation in this polymer (Duthie et al., 2007; Tanaka et al., 1992). For both polysulfone and 6FDA-TMPDA, N2 permeability also decreases with CO present (see for example Fig. 5). Again, this is because CO undergoes competitive sorption with N2

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tion in both permeabilities is observed. The decrease in CO2 /N2 selectivity implies that CO has a stronger competitive sorption effect on CO2 within these two membranes than N2 .

Acknowledgements The authors would like to thank the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council for access to equipment. Funding for this project is provided by the CRC for Greenhouse Gas Technologies (CO2CRC) through the Australian Government Cooperative Research Centre program.

References

Fig. 4 – A plot of the CO2 permeability in 6FDA-TMPDA as a function of 1/T at a total gas pressure of 1000 kPa. The solid lines represent the exponential relationship between permeability and 1/T expected from an Arrhenius relationship.

Fig. 5 – N2 Permeability (䊉) through 6FDA-TMPDA, 35 ◦ C 600 kPa, along with the permeability ratio of CO2 to N2 (

).

inside the membrane. However, the reduction in N2 permeability is not as pronounced as that observed for CO2 , highlighted by the reduction in the ratio of CO2 and N2 permeabilities upon exposure to CO (Fig. 5). This shows that CO2 permeability is more affected by the presence of CO than N2 .

5.

Conclusion

The permeability of NO has been reported for polysulfone, Matrimid 5218 and 6FDA-TMPDA. There is a clear trend of increasing gas permeability coinciding with increasing gas critical temperature in the order N2 < CO < NO < CO2 . This is a product of the increased solubility affinity of each gas for the polymeric matrices. Under mixed gas conditions, the permeabilities of CO2 and N2 in polysulfone and 6FDA-TMPDA are reduced from the pure gas case, indicative of competitive sorption, and upon exposure to 1000 ppm CO, a further reduc-

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