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Ind. Eng. Chem. Res. 1995,34,158-163
MATERIALS AND INTERFACES Characterization of Ultramicroporous Carbon Membranes with Humidified Feeds Cheryl W. Jones* and William J. Koros Separations Research Program, Center for Energy Studies, The University of Texas at Austin, Austin, Texas 78712
Carbon molecular sieving membranes offer many advantages for difficult gas separations. The selectivities achieved with these membranes are much higher than those typically found with polymeric materials, without sacrificing productivity. While they perform very well with highpurity, dry feeds, the work in this paper will show that they are vulnerable to adverse effects from exposure to water vapor. Tests made with humidified air feeds ranging from 23 to 85% relative humidity show that some performance losses occur at all levels, with the losses increasing as the humidity level increases. In addition, most of the performance loss occurs in the initial period of exposure to water vapor. Despite this vulnerability, it was observed that significant membrane function was maintained at the lower levels of exposure. The effects of other factors, including feed pressure, membrane orientation, and the nature of the carbon itself, are also considered.
Introduction In response to the demand for new materials in the field of membrane technology, ultramicroporous carbon membranes have emerged as a very promising candidate for gas separation applications (Soffer et al., 1987). These membranes contain constrictions in the carbon matrix which approach the molecular dimensions of the adsorbing species. These regions of ultramicroporosity (17 A) allow the passage of the smaller species and restrict the larger ones through a molecular sieving mechanism. In this manner, gas pairs with very similar dimensions can be effectively separated (Kapoor and Yang, 1989; Koresh and Soffer, 1980, 1983, 1987). A useful model for describing the pore structure of carbon molecular sieves has been proposed by Koresh and Soffer (1980). This model describes a nonhomogeneous pore structure that is comprised of relatively wide openings with a few constrictions responsible for the sieving mechanism. The wide openings are the major part of the pore volume and adsorption capacity of the material. Thus, the selectivities obtained with these membranes are much higher than those typically found in polymeric membranes and are achieved without sacrificing productivity. While these membranes perform very well with highpurity gas feeds, their viability as a commercial product depends on their performance in actual industrial applications. One compound found in various concentrations in a large number of process streams is water vapor. While a significant body of research exists regarding H20 sorption in various microporous carbon materials (Barton and Koresh, 1983a-c; Bradley and Rand, 1993; Carrott, 1992; Carrott et al., 1991; Dubinin, 1966,1980; Pierce et al., 1949;Walker and Janov, 1968; Youssef et al., 1982), the effect of H2O vapor on the performance of ultramicroporous carbon membranes has not been fully explored. Thus, the following study begins to evaluate carbon membrane performance in the presence of water vapor.
Carbons generally have a hydrophobic nature. They readily chemisorb oxygen on exposure to air however, and the resulting oxygen-containing surface complexes act as primary sites for water sorption. Sorbed H20 molecules then attract additional water molecules through hydrogen bonding, leading to the formation of clusters. The clusters grow and coalesce, leading to bulk pore filling. Studies have shown that, as the quantity of sorbed water in microporous carbon adsorbents increases, the capacity for other species is diminished (Suzuki and Doi, 1982). On the basis of observed H20 sorption behavior in microporous carbon adsorbents, adverse effects from humidity exposure on ultramicroporous membrane performance would be expected. The study described below looks at carbon membrane performance over a range of humidity levels. In addition, factors such as feed pressure, membrane orientation, and the nature of the carbon itself are considered.
Experimental Section Making Carbon Membranes. Carbon molecular sieving membranes are easily produced by pyrolyzing polymeric precursor materials. A number of variables affect the pyrolysis process, and protocols must be optimized for specific polymer precursors and for specific applications. For a given precursor material, small changes in the pyrolysis conditions can dramatically influence the final carbon membrane properties. The membranes described in this study were produced by pyrolyzing an asymmetric hollow fiber polyimide precursor derived from a reaction of 2,4,6-trimethyl-1,3phenylenediamine,5,5’-[2,2,2-trifluoro-l4~uoromethyl)ethylidenel-l,3-isobenzofurandione,and 3,3’,4,4’-biphenyl tetracarboxylic acid dianhydride. The polyimide was pyrolyzed under vacuum, at temperatures of 500 and 550 “C. Properties of Resulting Membranes. The degree of carbonization of the polyimide precursor increases with pyrolysis temperature, and a temperature of 500
0888-588519512634-0158$09.0010 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 159 wall thickness = 35p
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Figure 2. Construction of carbon membrane modules for bore side feed.
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between 11.0 to 14.0 and 0 2 fluxes ranging from 15 to 40 GPU:
GPU =
T c .
pyrolyzed at 550°C Figure 1. Eff& of pyrolysis temperature on membrane porosity.
"C was found to be sufficient to produce a highly carbonized membrane. Electron spectroscopy for chemical analysis (ESCA) of a 500 "C membrane revealed the presence 95.0 atomic % carbon with 5.0 atomic % surface oxygen. After Argon ablation of tens of angstroms of fiber surface, repeat analysis showed 98.7 atomic % carbon, 1.3 atomic % oxygen, and a barely detectable N2 peak. These results confirm that the membranes are primarily carbon. Results of scanning electron microscopy (SEM) performed on membranes from both 500 "C and 550 "C pyrolysis protocols are shown in Figure 1and indicate that the 550 "C membranes have a denser macropore structure than the 500 "C membranes. In both cases, the asymmetry of the precursor is carried through the pyrolysis process, and the resulting carbon membranes exhibit a distinct porosity gradient through the film thickness. The carbon membranes have outer diameters of 170-180 pm with wall thicknesses of 30 to 35
The lower temperature pyrolysis produces a membrane with O n 2 selectivities ranging between 8.5 and 11.5 and 0 2 fluxes ranging between 20 and 50 GPU. The higher temperature pyrolysis produces a more selective membrane with O n 2 selectivities ranging
cm3(STP) x s.cmHgcm2
Single Fiber Module Construction. Characterization work has been done with single fiber test modules constructed from 114411. stainless steel tubing and Swagelok 1/4-in. tees. A small length of tubing is attached to each arm of the tee to form a housing as shown in Figure 2. The hollow fiber carbon membrane is threaded through the housing so that a length of carbon fiber extends on each end. Five-minute epoxy is used to plug the ends of the tubing, and the ends of the carbon membrane are snapped off after the epoxy hardens. Membrane Test System. A diagram of the membrane test system is shown in Figure 3. The membrane module is attached in a bore feed method of operation, and the feed air is supplied from compressed gas cylinders. The feed gas can either be used dry or passed through a humidity chamber prior to the membrane module. The humidity chamber consists of a stainless steel canister in which different saturated salt solutions are used to control the water activity level. The relative humidity is also independently verified with a relative humidity meter at an exit port. The system is at ambient temperature. Vacuum is maintained on the shell side of the hollow fiber membrane, and the permeate is pulled through calibrated sample volume connected to an MKS Instruments, Inc. Baratron pressure transducer (0-10 Torr). By closing the sample volume valve to vacuum, the permeate pressure increase over a small period of time can be measured, and the flux can be calculated from the ideal gas law. Composition of the permeate is determined by gas chromatography, and the flux of individual species is calculated. The thermal condudivity detector (TCD) on the GC does not "see" water, so this method is used only for dry samples.
160 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 BARATRON
PERMEATE PULLED BY VACUUM TO O.C. SAMPLE LOOP
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A different testing protocol is used for runs with humidified feeds and evaluates changes in membrane performance in terms of selectivity and 0 2 flux. Since H2O in the permeate is not detected by the GC, it is not possible to use the total flux measured by pressure change as was used for dry samples. As a result, the H2O flux is not directly measured in these tests. Instead, we measure the changes in the 0 2 and N2 fluxes resulting from the presence of the H20 vapor. Advantage is taken of the fact that the GC response to 0 2 and NZis linear in the test pressure and composition range, and the GC response is not affected by the presence of H2O vapor. Prior to introducing humidified feed to the membrane, the number of area counts for the oxygen peak is measured for the dry permeate under the desired pressure conditions. Without changing the pressure of the feed gas coming from the compressed gas cylinder, the feed is passed through the humidity chamber. Once the feed gas is humidified, productivity losses are determined by changes in the number of area counts that the GC measures for the oxygen peak. Productivity losses are determined by comparing the 0 2 area counts measured with humidified feed to those measured with dry feed, and the results are reported as a percentage. Since the GC calibration is not affected by the presence of H2O vapor, composition measurements for 0 2 and Nz are used to calculate selectivities in the same manner as described previously for dry feeds. Results and Discussion Effect of Relative Humidity. To evaluate the performance of carbon membranes at various water activity levels, runs were made at relative humidity levels ranging from 23% to 85%. All runs were made at low feed pressures (1.5-3.3 psig), and results of these runs are shown in Figures 4-6 and Table 1. It can be seen from the runs at 23% and 44% RH (Figures 4a,b and 5a,b) that significant membrane
20
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Run time ( h n ) Figure 4. (a)Selectivity stability at 23% RH.(b) 02 flux stability a t 23% RH.
function is maintained a t the lower relative humidity levels. In these cases, membrane selectivity remained unchanged after exposure to humidity for a period of days, and membrane performance losses were limited to decreases in productivity. Not surprisingly, however, greater losses in membrane performance were seen at the higher humidity levels. As illustrated in Figure 6a,b, decreases in both selectivity and productivity were observed at 85% RH. The relationship between relative humidity level and degree of membrane function loss is perhaps best illustrated in Table 1. All runs shown in Table 1were made with the same carbon membrane. After each exposure to humidity, the membrane was "regenerated by drying in an 80 "Cvacuum oven for a period of hours. As clearly seen, membrane productivity losses increase as the relative humidity level increases. It can also be noted from these runs that most of the productivity losses occur within the first few hours of exposure. After this initial exposure phase, the rate of productivity loss becomes much slower. This changing rate of productivity loss undoubtedly relates to the HzO sorption behavior in the ultramicropores. While we do not have H2O sorption data on these membranes at this point, the pattern observed is consistent with a scenario in which active sites, some a t critical constrictions, are rapidly blocked. This initial phase of sorption is then followed by a more gradual pore filling (Bradley and Rand, 1993; Dubinin, 1980). A number of other runs, not shown in this paper, consistently showed the same patterns illustrated in the results above; namely, some degree of membrane productivity is lost at all levels of humidity exposure. As the relative humidity level increases, so does the degree of productivity loss. However, a significant degree of
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Figure 5. (a) Selectivity stability a t 44%RH. (b) 0 2 flux stability at 44% RH.
Figure 6. (a) Selectivity stability at 85%RH.(b) 0 2 flux stability at 85% RH.
membrane function is maintained at lower levels of humidity exposure with selectivities generally unaffected. At higher levels of exposure, in addition to higher productivity losses, the effect on selectivity becomes unpredictable. While the membrane selectivity remains the same in a few cases, in the majority of cases it drops. The above runs illustrate the role that the partial pressure of water plays in membrane performance losses with humidified feeds. The next runs were made t o evaluate the influence of other factors. Effect of Feed Pressure. To evaluate the effect of total feed pressure on membrane performance, runs were made at two different pressure settings, at both low (33%)and high (85%)relative humidity levels. For the 33%RH run, the same membrane was used for both pressure settings and regenerated &r the low pressure humidity exposure. Two different membranes with very similar properties were used to evaluate pressure effects at 85% RH. The results of these runs are shown in Table 2 and show that membrane performance is indeed influenced by the total feed pressure. At both humidity levels, the performance losses were lowest a t the lower feed pressure. As the feed pressure increased, performance losses were significantly higher, both in terms of selectivity and productivity. According to a Poynting factor calculation, there is no meaningful change in the vapor pressure of water over the test pressure range (Reid et al., 1987). Thus, these results indicate that the water flux through the membranes is influenced by more than the partial pressure of water. Runs were next made to evaluate the effect that other permeating species might have on the water flux. Recall that the net flow of a species in a mixture is due to a diffusion contribution and a bulk flow component
Table 1. Effect of Relative Humidity Level on Productivity Losses preexposure flux (GPU) 23.1 25.2 25.5 22.0
0 2
RH level (%) 33 44 67 85
% of dry feed 0 2 area counts by GC -52 after 19 h -51 after 19 h -44 after 17.5 h -30 after 2.5 h
Table 2. Feed Pressure Effects with Humidified Air Feeds feed P (psig)
AOfl2 % of dry feed 0 2 preexposure selectivity area counts by GC flux(GPU) Relative Humidity = 33% 1.15 24.5 11.4 11.6 -63 after 24 h 24.87 20.7a 11.8 11.3 -49 after 24 h Relative Humidity = 85% 1.05 21.0 11.9 10.4 -41 after 24 h 99.0 16.gb 12.6 8.4 -13 after 24 h "At 1.4 psig, the dry gas 0 2 flux was 26.9 GPU, and the selectivity was 11.2 after regeneration. At 4.9 psig, the dry gas 02 flux was 22.7 GPU and selectivity was 12.6. 0 2
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(Hines and Maddox, 1985). Thus, the increase in water flux at higher pressures may result from an increase in the bulk flow of oxygen andor nitrogen. If this is the case, even greater effects with a faster diffusing gas such as helium would be expected. Thus, runs were made to evaluate feed pressure effects with humidified helium at both 33% and 85% RH levels. At each relative humidity level, the same membrane was used for both pressure settings and regenerated afier the low pressure humidity exposure. The pressure settings were chosen to correlate with the humidified air runs described above. The response of the GC to helium is not linear over the test pressure range, so the
162 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 Table 3. Feed Pressure Effects with Humidified Helium Feeds relative humidity (%I 33 33 85 85
feed P (psig) 2.0 25.0 1.3 100.0
preexposure He flux (GPU) 181.1 138.2a 136.3 108.1b
9c of dry feed total flux (mUs) 71 after 24 h 36 after 24 h 59 after 24 h 7 after 24 h
a He flux = 171.0 GPU a t 2.5 psig after regeneration. He flux = 146.3 GPU at 1.6 psig after regeneration.
Table 4. Effect of *olysis Humidified Feeds pyrolysis temp ("C) 500 550 500 550
RH level (%) 31-33 31-33 83-85 83-85
Temperature with
preexposure 0 2 flux (GPU) 23.1 20.2 30.7 31.1
AOZ/N2 selectivity
11.2 13.8 10.5 13.4
-- 13.4 10.7 - 10.7
-
470 of dry feed area counts by GC
0 2
-53 -69 -42 12.8 -50
after 19 h after 24 h after 24 h after 24 h
change in area counts was not a reliable method for measuring productivity loss. Thus, membrane changes were monitored by measuring the drop in total flux as calculated from the pressure change method described previously. Since this value reflects both He and H2O flux, the actual decreases in helium flux are even greater than reflected in Table 3. The differences in membrane performance between the low and high pressure helium feeds at each relative humidity level are even greater that those observed with humidified air feeds. These results appear to confirm that the bulk flow of other permeating species can have a significant effect on the water flux through these membranes. This finding points t o the importance of the total feed pressure as an operating parameter. Effects of Membrane Porosity and Surface Polarity. The polarity of the carbon surface and the size and shape of the micropores are both critical factors in defining the HzO sorption characteristics for any given microporous carbon material. Assuming that the sorption mechanism observed in microporous carbons also describes water sorption in ultramicroporous carbon membranes, differences in porosity and surface chemistry should significantly affect membrane performance with humidified feeds. To verify this assumption, runs were made to compare performance of 500 and 550 "C membranes at both high and low relative humidity levels. Recall that SEMs of these two membrane types showed that the 550 "C membranes have a denser macropore structure than the 500 "Cmembranes. It is also likely that the difference in pyrolysis temperature results in surface chemistry differences as well. Comparison runs were made at 31-33% RH and at 83-85% RH, a t low feed pressures (1-2 psig). At each relative humidity level, membranes were chosen for comparison that had very similar 0 2 flux values. Not surprisingly, there were differences in membrane performance at both water activity levels as shown in Table 4. The performance losses were lowest in the 550 "C membranes at both humidity levels, and the difference was most pronounced a t the lower relative humidity level. One explanation for the observed results might be that the higher temperature pyrolysis produces a membrane with more regions of ultramicroporosity. These regions are comprised of more ordered carbon layers with hydrophobic basal planes. The reduction of carbon layer disruptions, and hence sites for oxygen
Table 5. Effect of Membrane Orientation with Humidified Feed feed method shell bore bore
RH level (%) 42-49 44 67-70
AOD2
selectivity 10.8 7.3 9.6 9.5 9.3 8.9
--
% of dry feed 02 area counts by GC -9 &er 19 h -51 aRer 19 h -44 after 18 h
chemisorption, results in fewer active sites for water sorption (Bansal et al., 1988; Bradley and Rand, 1993; Mattson and Mark, 1971). According to Carrott et al. (19911, the adsorption of water vapor a t low relative humidity levels is determined by specific adsorbentadsorbate interactions, while the micropore size and shape control sorption at higher humidity levels. Therefore, it is entirely reasonable to see differences in the two carbons at the low relative humidity level. The situation is more complicated at the higher relative humidity level because H20 sorption is strongly influenced by pore size and shape effects as well as surface chemistry. The difference in performance between the two carbons is reasonable in light of the visual differences in macroporosity observed by SEM. Without sorption isotherms on these membranes, however, data are insufficient at this point to fully explain the observed results. There does appear to be a consistent pattern, however, with the membranes produced by higher temperature pyrolysis being more hydrophobic. Feed Flow Configuration. The issues of porosity and surface polarity also come into play when considering the orientation of the membrane in relation t o the feed stream. The polyimide precursor is an asymmetric, hollow fiber membrane with the selective layer on the outer surface, and the resulting carbon membranes have a porosity gradient across the wall. Significant differences in membrane performance between shell side and bore side feed flow configurations have been observed. As shown in Table 5, performance losses are much lower with bore side feeds where the water first contacts the more open pore regions of the membrane. With a bore feed orientation, the more open pore regions appear to serve as a hydrophobic guard layer for the more selective layer. It is likely that the weakly hydrophobic nature of the porous carbon establishes a small resistance to water vapor permeation, resulting in a drop in the water activity level between the bore concentration and the selective layer. Thus, adjusting the feed flow configuration to minimize the water activity level at the selective layer can significantly improve membrane performance.
Conclusions As predicted from studies of H20 sorption in microporous carbons, ultramicroporous carbon membranes are adversely affected by exposure to water vapor. As the water activity level increases, membrane performance losses in terms of selectivity and productivity also increase. The water flux through the membrane is determined not only by the partial pressure of water, but by the bulk flow of other permeating species as well. Thus, the feed pressure becomes an important operating parameter. In addition, factors such as membrane porosity, surface chemistry, and feed flow configuration significantly influence performance with humidified feeds. Membrane performance is significantlyimproved by choosing the feed flow configuration that minimizes the water activity level at the selective layer.
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Acknowledgment The authors would like to gratehlly acknowledge the financial support from Medal.
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Received for review March 7 , 1994 Revised manuscript received September 1, 1994 Accepted September 21, 1994"
IE940127V @
Abstract published in Advance ACS Abstracts, November
15, 1994.
