Thermal dependence of carbon dioxide transport through a dense polymeric membrane

Greenhouse Gas Control Technologies, Volume n M. Wilson, T. Morris, J. Gale, K. Thambimuthu (Eds.) © 2005 Elsevier Ltd. All rights reserved THERMAL DEPENDENCE OF CARBON DIOXIDE TRANSPORT THROUGH A DENSE POLYMERIC MEMBRANE X. Duthie^ S. Kentish^*, K. Nagai^ C. Powell', and G. Stevens' 'Cooperative Research Centre for Greenhouse Gas Technologies, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, 3001, Victoria, Australia. ^Department of Industrial Chemistry, Meiji University, Kawasaki, 214-8571, Japan ABSTRACT At present, dense polymeric gas separation membranes are being investigated as a possible CO2 separation process for carbon capture. Commercial use of such membranes, for example in the natural gas industry, is however, currently limited to ambient temperature. From an economic viewpoint, it is desirable to be able to operate separation membranes at elevated temperatures, as this allows reduced cooling of flue gases and hence lower operating costs. This could be particularly relevant in a fossil - fuelled power station where flue gas temperatures can reach 320°C. This study investigates the permeability of polyimide membranes formed from Matrimid 5218 in carbon dioxide at temperatures up to 100°C and pressures up to 2500kPa. Membranes were tested in both as cast and thermally annealed formats. Thermal annealing was found to suppress plasticization of the membranes in this temperature and pressure range. It was shown that some degree of crosslinking occurred during annealing, leading to a reduction of carbon dioxide permeabilities by a factor of 2 - 3. INTRODUCTION Polymeric gas separation membranes are currently used commercially for CO2/CH4 separation in a number of operations worldwide. However, a major technical hurdle associated with such membranes is the plasticization of the membrane with increasing concentrations of polar gases such as CO2. Such plasticization usually increases membrane permeability but decreases selectivity and eventually shortens the useful membrane life. Significant research efforts are being conducted into characterizing and avoiding the plasticization phenomena. Polymeric membranes also show potential for the separation of CO2 from power plant flue gases prior to geosequestration. However, this application introduces another complication: the gas will most likely be supplied at elevated temperatures, of up to 350°C. Cooling to lower temperatures increases costs and also increases the risk of gas condensation, a known fouling mechanism. Consequently it is of value to evaluate membrane performance and susceptibility to plastizisation at elevated temperatures. There is a significant body of research available characterizing the operation of such membranes at temperature, [1] - [4], and on the effects of plasticization [5] - [7]. have investigated plasticization in some detail. Little, if any research however, has been conducted into the plasticization phenomenon as a function of temperature. As an initial starting point, this paper reports data on the plasticization tendencies of a readily available commercial membrane, Matrimid 5218 as a function of temperature. Additionally, the effect of thermal annealing on plasticization is examined. We intend extending this research to other polyimides, particularly those showing promise for flue gas capture in the immediate future. EXPERIMENTAL A commercially available polyimide, BTDA - DAPI (Matrimid 5218, sourced from Vantico Inc., USA) was used as received. The polymer was dissolved in chloroform overnight to form a 2.5'^Vwt% solution, before being twice filtered through 0.25|im glass fibre filters. The solution was cast onto glass Petri dishes, which were then covered and the solvent allowed to evaporate for at least 12hrs. Membranes of diameter 47mm were then cut from the film, before being stored in a vacuum desiccator until use. Membrane thicknesses were measured using a micrometer and were typically 40-45|xm. Thermal annealing of the membranes took place in an oven purged with Argon at 290°C for 30 minutes, before being cooled at a rate of approximately 10°C/min. Permeabilities were measured with a constant pressure, variable volume gas permeation apparatus. Permeability is determined by measuring upstream and downstream pressures, permeate flow rate, as well as

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membrane thickness and area. A fresh membrane was used for each experiment, except where indicated to ensure that membrane history did not affect results. The apparatus operates by supplying feed gas, in this case pure CO2, at a controlled pressure to a pre-heating loop before proceeding to a sealed membrane unit. The unit is a dead end Millipore high - pressure filter holder (# XX45 047 00). Gas permeates through the membrane before entering the downstream section of the apparatus that operates at atmospheric pressure. The heating loop and membrane unit are housed in an oven that is temperature controllable to 300°C. The permeate gas passes into a cooling loop that is housed in a water bath, ensuring a constant measurement temperature of 16 ± 2°C It then passes into a bubble flow meter where a measurement of the gas flow rate is made. CO2 pressures up to 2500kPa were used so as to capture sufficient plasticization data at elevated temperature. It has been noted by previous researchers that polyimides, including Matrimid, can crosslink when subject to elevated temperatures [8]. This can even occur below the glass transition temperature (Tg). Accordingly, polymer structures before and after thermal annealing were investigated using 'H N M R (Varian Unity 400MHz). RESULTS The permeability response data of as cast Matrimid to upstream CO2 pressures between 250 and 2500kPa are presented in Figure 1. Three different temperatures were examined. As expected, the permeability increases with increasing temperature. It can also be seen that all three curves show a minimum in the permeability curve. It is widely accepted that this indicates that the membrane has indeed plasticized [9]. The plasticization pressure is indicated by the curve minimum. The data generally suggests that the plasticization pressure increases with temperature. This result is not unexpected as the exposure to high temperatures will initiate membrane conditioning, leading to a reduction in free volume and polymer mobility. Permeability data obtained from thermally annealed Matrimid membranes over a similar pressure and temperatures range is presented in Figure 2. When compared to as-cast Matrimid, it can be seen that the permeability at each temperature has dropped by a factor of between 2 and 3 as a consequence of the thermal annealing process. Again, this is to be expected since this treatment has the effect of reducing the free volume of the polyimide. Additionally, it is possible to covalently crosslink glassy polymer films by exposing them to elevated temperatures. It can also be seen from Figure 2 that no point of inflexion is apparent in the any of the permeability curves. This suggests that plasticization of thermally annealed Matrimid no longer occurs in the pressure and temperature ranges studied here. The lack of plastizisation was confirmed by repeating the permeability measurements a second time with the same membrane. The repeat measurements showed no increase in permeability, as would be expected had the membrane plasticized.

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Figure 1: Carbon dioxide permeability of as cast Matrimid as aftinctionof temperature and pressure

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The NMR spectra for both as-cast and thermally annealed Matrimid showed a high degree of similarity, as expected. However, the thermally annealed sample displayed a small new peak in the range corresponding to methyl group protons. We believe this small peak is indicative of a small amount of thermal crosslinking of the film. The extent of cross-linking is likely to be small, as the annealed films still dissolved in chloroform, albeit more slowly than the as-cast films. Permeability can be correlated with temperature using a standard Arrhenius relationship as follows:

An Arrhenius plot of permeability against temperature is presented in Figure 3. Data of Bos [9] is also included for reference. This reference data was obtained after conditioning in a vacuum oven for 96 hours at 140°C and so is more comparable with our annealed results. From this, the activation energy, Ep. and Pre-exponential factor, Po, of as cast, as well as thermally annealed Matrimid was calculated and is presented in Table 1. It can be seen that the activation energies calculated are lower than those of Bos and the pre-exponential factor generally higher. Furthermore, the permeability values of Bos agree with the values of obtained for thermally annealed Matrimid. This generally reflects the small temperature range over which Bos conducted their experiments. Over such a range, any experimental error will have a large bearing over the final values determined.

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Transmembrane C02 Pressure (kPa)

Figure 2: Carbon dioxide permeability of thermally annealed Matrimid as a function of temperature and pressure TABLE 1: ACTIVATION ENERGY PREDICTIONS FOR MATRIMID 5218 CO2 PERMEATION Pre-exponential factor, Polymer type Activation Energy, Upstream C02 Po Ep Pressure (kPa) (cm3(STP)cm.cm-2.sec(kJ/mol) l.cmHg-1) 297 X 10-8 Matrimid, as cast 18.87 425 140x10-8 Matrimid, as cast 16.53 1250 12.1x10-8 Matrimid, annealed 13.02 425 8.78x10-8 Matrimid, annealed 12.18 1250 380x10-8 Matrimid [9] 21.6 425

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- - • - - As cast — D — Bos et al —ik - Thermally annealed

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Figure 3: Arrhenius plot of permeability as a function of temperature for an upstream pressure of 4.2 Bar CONCLUSION Thermal annealing of Matrimid polyimide membranes at 290°C suppresses plasticization in the temperature range 18-100°C and carbon dioxide pressures up to 2500kPa. A small degree of crosslinking occurs during this conditioning process, however, further work is required to ascertain whether this crosslinking, or rather free volume reduction leads to plasticization suppression. The permeability data obtained is consistent with other literature data. However, we estimate the activation energy for annealed Matrimid 5218 to be lower than previously suggested with a value of around 12.5 kJ/mol. ACKNOWLEDGEMENTS The authors would like to acknowledge Mr. Raj Rattanavich for assistance with the experimental work for this study. REFERENCES 1.

Norton, F.J., 1963. Gas Permeation Through Lexan Polycarbonate Resin. Journal of Applied Polymer Science, 7{\): p. 1649-1659. 2. Koros, W.J. and D.R. Paul, 1978. Carbon dioxide sorption in poly(ethylene terephthalate) above and below the glass transition. Journal of Polymer Science, Polymer Physics Edition, 16(11): p. 1947-63. 3. Costello, L.M. and W.J. Koros, 1995. Thermally stable polyimide isomers for membrane-based gas separations at elevated temperatures. Journal of Polymer Science, Part B: Polymer Physics. 33(1): p. 13546. 4. Lin, W.H. and T.S. Chung, 2(X)1. Gas permeability, diffusivity, solubility, and aging characteristics of 6FDA-durene polyimide membranes. Journal of Membrane Science. 186(2): p. 183-193. 5. Bos, A., et al., 1999. C02-induced plasticization phenomena in glassy polymers. Journal of Membrane Science. 155(1): p. 67-78. 6. Wessling, M., M.L. Lopez, and H. Strathmann, 2001. Accelerated plasticization of thin-film composite membranes used in gas separation. Separation and Purification Technology. 24(1-2): p. 223-233. 7. Wind, J.D., D.R. Paul, and W.J. Koros, 2004. Natural gas permeation in polyimide membranes. Journal of Membrane Science. 228(2): p. 227-236. 8. Barsema, J.N., et al., 2004. Intermediate polymer to carbon gas separation membranes based on Matrimid PI. Journal of Membrane Science. 238(1): p. 93 - 102. 9. Bos, A., 1996. High pressure CO2/CH4 separation with glassy polymer membranes - Aspects ofC02induced plasticization, in Department of Chemical Technology. University of Twente.