110th Anniversary: High Performance Carbon Molecular Sieve

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High performance carbon molecular sieve membranes resistance to aggressive feed stream contaminants Rachana Kumar, and William J. Koros Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00899 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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Industrial & Engineering Chemistry Research

110th Anniversary: High performance carbon molecular sieve membranes resistance to aggressive feed stream contaminants Rachana Kumar and William J. Koros* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States * Corresponding Author Email: [email protected], Phone: 404-385-2845, Fax: 404-385-2683

Abstract Carbon molecular sieve (CMS) membranes show high performance for various critical gas pair separations with excellent scalable manufacturability. For practical applications it is important to evaluate such membranes for resistance to contaminants possibly present in feed streams related to on-board inert gas generation system (OBIGGS) for aerospace applications. Here we report the performance of CMS membranes prepared by optimized dual temperature secondary oxygen doping (DTSOD) in the presence of aggressive contaminants, potentially in the bleed air of aircraft. Such feed can be used to eliminate the need for a separate compressor in membrane systems to create nitrogen enriched stream for fuel tank inerting, and we considered this case using simulated engine bypass air. Our CMS membranes were exposed to different humidity levels, combinations of gaseous contaminants (denoted as Class ‘A’) and ozone to study the membrane performance for O2/N2 separation with time at high temperatures. The CMS shows a robust nature under such aggressive feed conditions especially at or above 160 oF (71 oC) under steady state operation with attractive performance under long term exposure. 1. Introduction Membranes are thin selective barriers1 that offer advantages over conventional thermally driven separation processes in terms of lower energy consumption, low capital cost and small foot prints for offshore and aerospace applications. Ideally membranes can enable separation of molecularly similar components in high efficiency, cost competitive units under complex feed streams and conditions.2,3 Air 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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separation is one of the major application area of membranes wherein either oxygen enriched or nitrogen enriched gas streams can be created.4-6 For offshore petroleum well protection and on board inter gas generation system (OBIGGS) of aircrafts, polymeric hollow-fiber membranes are already used to create nitrogen enriched atmosphere due to their light weight and simple operation.7,8 The feed air in OBIGGS is ideally at high temperature to avoid cooling units and it contains several chemical impurities from heated engine oil as well cruise level ozone.9 State-of-the-art polymeric membrane materials are limited by their performance at elevated temperatures (≥140 oF) and under aggressive contaminant feed stream containing high water levels and chemical impurities.10,11 Carbon molecular sieve (CMS) membranes have already proven to outperform polymer membranes selectivities and productivities in some cases thereby offering next generation membranes for gas pair separations.12-15 Although considerable literature is available on development of new materials and strategies to prepare high performing CMS membranes,16-19 only a few reports are available on CMS membrane performance under contaminant feed stream.20,21 Performance loss of CMS membranes exposed to water vapors has been documented,22 but the use of hydrophobic Teflon AF® layers can avoid this problem, even for high relative humidity feeds.23 Fortunately, at low humidity levels (~25% relative humidity, RH) productivity and selectivity losses were minimal and did not require such Teflon AF® protection. Kiyono showed that oxygen doped CMS membranes offer better resistance to humidity than un-doped CMS membranes24 and Jones et al., also studied CMS membrane performance on exposure to feed stream saturated with C6 and higher hydrocarbons.25 They observed large performance loss in such cases, but demonstrated a simple regeneration process with propylene as cleaning agent to restore the performance. Here we consider CMS to provide robust, highly selective and productive membranes, to operate under such conditions. To the best of our knowledge, there is no report on CMS membrane performance study on exposure to complex mixture of condensable, polar organic chemicals & oxidizing gases including exposure to ozone gas. In the current work, we report CMS membranes, with good selectivity and permeance prepared by using our dual temperature secondary oxygen doping method (DTSOD) method,26,27 performance study under the aggressive feed stream of contaminants. DTSOD is an effective tool to prepare CMS 2 ACS Paragon Plus Environment

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membranes with high and tunable selectivity while maintaining high permeance from defect free precursor polymer fibers. These membranes were tested under humidified stream (40% RH & 80% RH), mixture contaminants feed stream at different temperatures (≥140 oF). Membranes were also tested by exposure to 0.02 ppm ozone at 160 oF which may be present at cruising altitudes. Membrane regeneration has also been performed using propylene gas to regenerate membranes performance when necessary as was reported earlier for other contaminants.25 2. Transport in CMS membranes Gas transport though CMS membranes follows a sorption-diffusion model where the permeability coefficient (or permeability, Pi) is the product of diffusion coefficient (Di) and sorption coefficient (Si) of the penetrant molecule i (Eq. 1).

Pi = Di x Si

(1)

Permeability is defined as the steady flux of penetrant i across the membrane (Ni) normalized by the partial pressure difference across the membrane (DpA) and membrane thickness (l) (Eq. 2).

𝑃" =

$% . (

(2)

∆*+

For asymmetric hollow fiber membranes the exact thickness of separation layer is not readily known, therefore the practical productivity of these membranes is represented in terms of permeance (P/l)i, as shown in Eq. (3). ,% (

$

= ∆*%

(3)

+

Gas molecules first sorb on the upstream side of the membrane and transport though the membrane under a chemical potential gradient and desorb on the downstream of the membrane. CMS comprises disordered amorphous carbon plates formed by ordered aromatized strands during the pyrolysis process.28 The voids between the plates act as primary sorption sites and are called micropores (7-20 Å). The slits between the

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strands are the critical ultramicropores (40 % but selectivity was not highly affected, and in fact it increased after 7 days of exposure. The oxygen doped CMS membranes have better stability in terms of separation performance under humidified feed conditions. Kiyono described the performance change

dum

dm

Unexposed membrane

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Figure 3. Pictorial presentation of molecular flux change due to sorbed water molecules in membrane on exposure to 40% and 80 % relative humidity. Blue shows the Nitrogen molecule while green shows the oxygen molecules. dum = ultramicropore and dm = micropore dimension.

based on sorption study on humidified (80% RH) CMS membranes and compared with unexposed membranes.24 The sorption isotherm showed decrease in sorption capacity of gas molecules by ~30%, but sorption selectivity remained unchanged. Thus the increase in selectivity was basically due to increase in diffusion selectivity related to selective closure of less selective ultramicropores. In the present case also (Figure 3), at low relative humidity the water molecules first sorb at ultramicropores increasing the selectivity of membrane. Fortunately at 160 oF the micropores are negligibly affected at 40% RH and low loss in permeance is observed. Nevertheless, at high RH level the ultramicropores are clogged by the water molecules and result in reduced permeance as well as selectivity as depicted in Figure 3. These studies show that CMS membranes are durable and stable against humidity and regeneration with propylene is very effective tool to restore the membrane performance if exposed to process upset conditions that physically block the selective pores. 4.2 Exposure to Gaseous Contaminants


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CMS membranes were next exposed to a mixture of condensable organic chemicals and gaseous contaminants at different temperatures to probe the effect of contaminants on membrane performance. Table 1 shows the list of contaminants denoted as Class ‘A’, which comprise typical components found in aerospace environment. Table 1. List of class ‘A’ contaminants balanced with Nitrogen gas to which membranes were exposed at different temperatures.9

Contaminant

Composition (ppm by volume)

Gases Nitrogen dioxide Carbon monoxide Toluene Hexane Ethanol Methyl ethyl ketone (Butanone) Acetone Methylene chloride Methyl bromide Methyl tert-butyl ether (MTBE) Major component (balance)

3 25 5 30 30 5 1.5 2 1.0 1 Nitrogen

4.2.1 Exposure to class ‘A’ contaminants at 140 oF (60 oC) Upon exposure to class ‘A’ contaminants at 140 oF we have observed a rapid loss in membrane performance. As can be seen in Figure 4 (and Table S6), a regular drop in oxygen flux as well as in O2/N2 selectivity was seen. After 131 hours 76 % drop in oxygen permeance was observed. The highest drop in oxygen permeance (55 %) was in the very first hour of exposure. A total 47 % loss in selectivity is observed after 131 hours. It seems that after certain time, the membrane reaches steady state as only 4% loss in permeance occurred from 7 hr to 131 hr of exposure. The sorbing species appear to obstruct the ultramicropores and therefore both permeance as well as selectivity loss was observed from the very first hour of exposure.25 The membranes have specific loading capacities for contaminant molecules. As the sorption of molecules proceeds in the micropores, less capacity is available for gas molecules.



Fortunately, an increase in test temperature reduces the contaminants sorption in membrane and would result in better performance. (A)

(B)

140 oF 160 oF 203 oF

1.0

1.2

0.8

1.0 Normalized aO2/N2

Normalized P/lO2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.4 0.2 0.0

0.8 0.6 0.4 140 oF 160 oF 203 oF

0.2 0

50

100 150 200 Exposure Time (hr)

250

300

0.0

0

50

100 150 200 Exposure Time (hr)

250

300

Figure 4. Plot of (a) oxygen permeance (normalized), (b) O2/N2 selectivity (normalized) of CMS membrane with time on exposure to Class ‘A’ contaminants at 140, 160 and 203oF.

4.2.2 Exposure to class ‘A’ contaminants at 160 oF (71 oC) CMS membranes were further exposed to class ‘A’ contaminants at 160 oF (71 oC) (Figure 4, Table S7,). Membranes was exposed to gaseous contaminants up to 256 hours and ~ 10 % oxygen permeance reduction occurred in the first hour of exposure (in contrast to 55 % loss at 140 oF). Enhancement in the selectivity was observed with exposure on extending the hours of exposure. The maximum performance drop was in the first 35 hours (31 % permeance loss), and in the next 221 hours of exposure only a 13 % loss was seen in oxygen permeance which can be interpreted as stated above on the basis of membrane adsorption saturation. A consistent increase in O2/N2 selectivity was observed with ~19% increase after 256 hours of exposure due to sorption of organic molecules on ultramicropores. 4.2.3 Exposure to class ‘A’ contaminants at 203 oF (95oC) CMS membrane were further tested at 203 oF (95 oC), with a rational that at higher temperature gas molecules have lower sorption and we should observe very less effect of gaseous contaminants on CMS membrane. As can be seen in Figure 4 (and Table S8) only minor loss in oxygen permeance and O2/N2 10 ACS Paragon Plus Environment

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selectivity was observed after ~ 100 hours of exposure at 203 oF in accordance to our expectation. We hypothesize that organic contaminant molecules may sorb on ultramicropores and make the slits more selective for N2 molecules thus increasing the O2/N2 selectivity with loss of O2 permeance at 160 oF. However, at higher temperature (203 oF) the sorption of class ‘A’ contaminants is reduced drastically and results in minimal loss in oxygen permeance while maintaining the selectivity within 90% of the initial value. 4.3 Exposure to N2O (2 ppm, balanced N2) at 160 oF Some organic contaminants may neutralize the strongly oxidizing N2O, this contaminant was studied separately as a “worst case”. Specifically, our CMS membrane was exposed to ~ 2ppm (by volume, balanced with nitrogen) of N2O gas at 160 oF. As can be seen in Table 2, no significant effect of N2O on CMS membrane performance was observed even after 5 hours of exposure and therefore wasn’t considered for further exposure. CMS membrane is robust in performance for N2O molecules. Table 2. Performance of CMS membrane on exposure to N2O (balanced with N2) mixed gas with time at 160oF and 60 psia feed pressure.

Time of Exposure (hr)

P/lO2 (GPU)

0 1 5

28 27 27

O2 Permeance loss (%) 3.5 3.5

aO2/N2

Selectivity loss (%)

6.8 6.7 6.7

1 1

4.4 Exposure to NO (1 ppm), SO2 (2 ppm, balanced N2) at 160 oF CMS membranes were also separately exposed to NO-SO2 containing feed stream (balanced with nitrogen) at 160 oF. Both molecules are polar in nature and expected to adsorb on the ultramicropores. Table 3 summarizes the results on exposure for 135 hours. Some loss in oxygen permeance with initial decrease and further increase in selectivity was seen. These molecules are presumably adsorbing on the ultramicropores and decreasing the overall flux of the gas along with increasing molecular sieving capability of the membrane. 11 ACS Paragon Plus Environment


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Table 3. Performance of CMS membrane on exposure to NO-SO2-N2 gas mixture contaminants at 160oF and 60 psia feed pressure. .

Time Time of of Exposure Exposure (hr) (hr)

P/l P/lO2 O2 (GPU) (GPU)

0 0 1 1 3 3 10 24 40 135

28 44 26 26 26 25 25 23 22 20

O O22 Permeance Permeance loss loss (%) (%) 7 41 7 43 11 48 21 31

a aO2/N2 O2/N2

Selectivity Selectivity gain gain (%) (%)

6.8 6 6.4 6.3 6.5 6.1 6.5 6 7 7.2

-6 5 -4 2 -4 0 3 6

4.5 Exposure to Ozone: 0.02 ppm ozone at 160 oF CMS was also tested for performance change study on exposure to ozone at 160 oF with time since it is known to be present at cruising altitudes.19 A drawing of the ozone generator and analyzer system is given in supporting information (Figure S6). Ozone is generature by Water System Ozone Generator using pure dry oxygen feed gas and passes though an ozone analyzer (ThermoFisher i49 Ozone Analyzer) for the concentration of ozone that being exposed to membrane. The unused ozone is destroyed by the destroyer in the end. A carbon membrane module was exposed to 0.02 ppm ozone for 1 hr, 3 hr and 24 hours and change in oxygen permeance and O2/N2 selectivity is shown in Table 4.

Table 4. Change in performance of CMS membrane on exposure to 0.02 ppm Ozone at 160oF and 60 psia feed pressure.

Ozone is highly oxidizing in nature and it was expected to adsorb on the edges of ultra-micropores which were left un-doped while oxygen doping during pyrolysis experiment. A drop in oxygen permeance along 12 ACS Paragon Plus Environment

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with increase in O2/N2 selectivity was observed in first few hours of exposure. Ozone is presumably saturating most of the open sites on ultra-micropores making them more selective but less permeable in the very first hour, so further exposure does not cause much change in permeance and selectivity returns to its starting value. In the above cases, a rapid loss in performance is observed in first few hours of exposure. Presumably reflecting a combined effect of physical aging of the fresh CMS membrane and some sorption on ultramicropores and micropores.32 When the membranes were exposed to propylene for regeneration, humidified membranes could be regenerated (91-98 %). No change in oxygen permeance and selectivity was observed for class ‘A’ contaminates and ozone exposed membranes. This observation indicates that humidity exposure on the fresh CMS shows partial physical aging along with regenerable “sorption aging”. Since performance loss on exposure to organic contaminants and ozone was due to combined effect, therefore could not be recovered on regeneration with propylene gas and other regeneration methods can be applied.33

Conclusions We have studied the performance of CMS membranes for O2/N2 separation on long time exposure to variety of contaminants at different temperatures. Low relative humidity shows minimal effect on CMS performance at 160 oF. Relative humidity of ~80% shows significant effect on CMS membranes but in both the cases membrane performance can be restored by propylene regeneration method. Moreover, as shown earlier Teflon AF® can mitigate high %RH effects. Gaseous contaminants show maximum performance loss in the first few hours of exposure and stabilizes after that. Very low performance loss was observed at high temperature (180 and 203 oF) on exposure to gaseous contaminants. Ozone also shows similar effect, with stabilized performance after 1 hour of exposure. CMS membranes have shown their robustness against humidity as well as gaseous contaminants. Overall, CMS membranes show very attractive performance under such aggressive contaminants feed conditions even on long term exposure. Supporting Information contains details of materials used for hollow polymer precursor membranes preparation, spinning conditions, CMS membranes preparation, pyrolysis protocol, Table of different



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tubesheet materials, high temperature test set up, design of humidity testing set up and design of ozone generation and testing set up. Acknowledgements The authors acknowledge the support of the Georgia Institute of Technology and Office of Basic Energy Science of the U.S. Department of Energy (DE-FG02-04ER1550). Authors also acknowledge The Specialty Separations Center (SSC) at Georgia Institute of Technology for equipment support.


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References 1. Koros, W. J.; Zhang, C. Materials for next-generation molecularly selective synthetic membranes. Nature Mater. 2017, 16, 289-297. 2. Koros, W. J.; Lively, R. P. Water and beyond: Expanding the spectrum of large-scale energy efficient separation processes. AIChe J. 2012, 58, 2624-2633. 3. Koros, W. J. Evolving beyond the thermal age of separation processes: Membranes can lead the way. AIChe J. 2004, 50, 2326-2334. 4. Murali, R. S.; Sankarshana, T.; Sridhar, S. Air Separation by Polymer-based Membrane Technology. Sep. Purif. Rev. 2013, 42, 130-186. 5. Himma, N. F.; Wardani, A. K.; Prasetya, N.; Aryanti, P. T. P.; Wenten, I. G. Recent progress and challenges in membrane based O2/N2 separation. Rev. Chem. Eng. 2018, 34, 1-35. 6. Chong, K. C.; Lai, S. O.; Thiam, H. S.; Teoh, H. C.; Heng, S. L. Recent progress of oxygen/nitrogen separation using membrane technology, J. Eng. Sci. tech. 2016, 11, 1016-1030. 7. Jojic, I.; Snow Jr., R.; Grim, A.; Hart, C. W. Aircraft fuel tank flammability reduction methods and systems. 2016, US 9327243 B2. 8. Alqaheem, Y.; Alomair, A.; Vinoba, M.; Perez, A. Polymeric Gas-Separation Membranes for Petroleum Refining. Int. J. Poly. Sci., 2017, 117, 1-19. 9. Day, G. A. Aircraft cabin bleed air contaminants: A review. Federal Aviation Administration, 2015, Report No. DOT/FAA/Am-15/20. 10. Omole, I. C.; Bhandari, D. A.; Miller, S. J.; Koros W. J. Toluene impurity effects on CO2 separation using a hollow fiber membrane for natural gas. J. Membr. Sci. 2011, 369, 490–498. 11. Ma, C.; Koros W. J. Effects of hydrocarbon and water impurities on CO2/CH4 separation performance of ester-crosslinked hollow fiber membranes. J. Membr. Sci. 2014, 451, 1–9. 12. Zhang, C.; Koros, W. J. Ultraselective Carbon Molecular Sieve Membranes with Tailored Synergistic Sorption Selective Properties. Adv. Mater. 2017, 29, 1701631-n/a.



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13. Rungta, M.; Wenz, G. B.; Zhang, C.; Xu, L.; Qiu, W.; Adams, J. S.; Koros, W. J. Carbon molecular sieve structure development and membrane performance relationships. Carbon, 2017, 115, 237-248. 14. Singh, A.; Koros, W. J. Significance of Entropic Selectivity for Advanced Gas Separation Membranes. Ind. Eng. Chem. Res., 1996, 35, 1231–1234. 15. Sanyal, O.; Zhang, C.; Wenz, G. B.; Fu, S.; Bhuwania, N.; Xu, L.; Rungta, M.; Koros, W. J. Next generation membranes —using tailored carbon. Carbon, 2018, 127, 688-698. 16. Qiu, W.; Zhang, K.; Li, F. S.; Zhang, K.; Koros, W. J. Gas Separation Performance of Carbon Molecular Sieve Membranes Based on 6FDA-mPDA/DABA (3:2) Polyimide. ChemSusChem 2014, 7, 1186-1194. 17. Fu, S.; Sanders, E. S.; Kulkarni, S. S.; Koros, W. J. Carbon molecular sieve membrane structure– property relationships for four novel 6FDA based polyimide precursors. J. Membr. Sci. 2015, 487, 60-73. 18. Steel, K. M.; Koros, W. J. An investigation of the effects of pyrolysis parameters on gas separation properties of carbon materials. Carbon 2005, 43, 1843-1856. 19. Kamath, M. G.; Fu, S.; Itta, A. K.; Qiu, W.; Liu, G.; Swaidan, R.; Koros, W. J. 6FDA-DETDA: DABE polyimide-derived carbon molecular sieve hollow fiber membranes: Circumventing unusual aging phenomena. J. Membr. Sci. 2018, 546, 197-205. 20. Haider, S.; Lindbrathen, A.; Lie, J. A.; Hagg, M-B. Regenerated cellulose based carbon membranes for CO2 separation: Durability and aging under miscellaneous environments. J. Ind. Eng. Chem., 2019, 70, 363-371. 21. Ansderson, C. J.; Tao, W.; Scholes, C. A.; Stevens, G. W.; Kentish, S. E. The performance of carbon membranes in the presence of condensable and non-condensable impurities. J. Membr. Sci. 2011, 378, 117-127. 22. Jones, C. W.; Koros, W. J. Characterization of Ultramicroporous Carbon Membranes with Humidified Feeds. Ind. Eng. Chem. Res. 1995, 34, 158-163.


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23. Jones, C. W.; Koros, W. J. Carbon Composite Membranes: A Solution to Adverse Humidity Effects. Ind. Eng. Chem. Res. 1995, 34, 164-167. 24. Kiyono, M. Carbon molecular sieve membranes for natural gas separations, (Ph.D. thesis) Georgia Institute of Technology, Georgia (2010). 25. Jones, C. W.; Koros, W. J. Carbon molecular sieve gas separation membranes-II. Regeneration following organic exposure. Carbon 1994, 32, 1427-1432. 26. Singh, R.; Koros, W. J. Carbon molecular sieve membrane performance tuning by dual temperature secondary oxygen doping (DTSOD). J. Membr. Sci. 2013, 427, 472-478. 27. Singh, R.; Koros, W. J. Carbon molecular sieve membrane (CMSM) performance tuning by dual temperature secondary oxygen doping (DTSOD). 2013, US 2014/0000454 A1. 28. Adams, J. S.; Itta, A. K.; Zhang, C.; Wenz, G. B.; Sanyal, O.; Koros, W. J. New insights into structural evolution in carbon molecular sieve membranes during pyrolysis. Carbon 2019, 141, 238-246. 29. Clausi, D. T.; Koros, W. J. Formation of defect-free polyimide hollow fiber membranes for gas separations, J. Membr. Sci. 2000, 167, 79–89. 30. Vu, D. Q.; Koros, W. J.; Miller, S. J.; High pressure CO2/CH4 separation using carbon molecular sieve hollow fiber membranes, Ind. Eng. Chem. Res. 2002, 41, 367–380. 31. Pye, D. G.; Hoehn, H. H.; Panar, M. Measurement of gas permeability of polymers. II. Apparatus for determination of permeabilities of mixed gases and vapors. J. Appl. Polym. Sci. 1976, 20, 287–301. 32. Xu, L.; Rungta, M.; Hessler, J.; Qiu, W.; Brayden, M.; Martinez, M.; Barbay G.; Koros, W. J. Physical aging in carbon molecular sieve membranes. Carbon 2014, 80, 155–166. 33. Haider, S.; Lindbrathen, A.; Lie, J. A.; Hagg, M-B. Carbon membranes for oxygen enriched airPart I: Synthesis, performance and preventive regeneration. Sep. Purif. Tech. 2018, 204, 290-297.


1.0

um

dm

CMS attractive performance Propylene under regeneration aggressive contaminants feed conditions

Normalized P/lO2

Industrial & Engineering Chemistry Page Research 18 of 18 d 0.8

0.6

0.4

1Humidity 2exposure 3 4 ACS Paragon Plus Environment 5 d 6= ultramicropores = doped oxygen 0.2

0.0 1.2

Normalized αO2/N2

1.0 0.8 0.6 0.4 0.2

um

dm = micropores

= water molecules

Class 'A'@140 oF Class 'A'@203 oF

0.0

0

50

100

Class 'A'@160 oF 0.02 ppm Ozone@160 oF

150

200

Exposure Time (hr)

250

110th Anniversary: High Performance Carbon Molecular Sieve

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