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

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Abstract

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.

Highlights

► The permeability of NO through polysulfone, Matrimid and 6FDA-TMPDA was measured ► The effect of CO on the permeability of CO2 and N2 in these membranes was determined ► Competitive sorption of CO reduces permeability even when present at only 1000 ppm

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 coal-based 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 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).

Section snippets

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=PAΔpAlwhere the permeability is generally quoted in barrer (10−10 cm3 (STP)

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 μm pore size). A Laurell

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 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

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

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.

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