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Ind. Eng. Chem. Res. 1995,34, 164-167
Carbon Composite Membranes: A Solution to Adverse Humidity Effects Cheryl W. Jones* and William J. Koros Separations Research Program, Center for Energy Studies, The University of Texas at Austin, Austin, Texas 78712
While the separation properties of carbon molecular sieving membranes are superior to those of other materials, a significant drawback has been their vulnerability to adverse effects from exposure to water vapor. Since significant membrane function is maintained at low levels of exposure, efforts were focused on devising a means to lower the water activity at the carbon surface. This was successfully accomplished with the development of carbon composite membranes. These membranes consist of a hollow fiber carbon membrane coated with a thin layer of certain unique polymeric materials. The polymers are highly hydrophobic but do not prohibitively reduce the flux of other permeating species, and the resulting composite membranes are much more resistant to water vapor effects. The performance of the composite membranes is analyzed in terms of the series resistance model. While small losses in selectivity and productivity occur as a result of the resistance added by the polymer layer, the composite membranes are still very attractive as compared to conventional polymer membranes. The polymer barrier type and thickness and the resulting degree of protection are variables that can be tailored in a controlled manner for specific applications.
Introduction Carbon molecular sieving (CMS) materials have proven to be very important in the field of gas separations, and CMS membranes have demonstrated exceptional gas separation properties (Kapoor and Yang, 1989; Koresh and Soffer, 1980,1983; Soffer et al., 1987). The selectivities obtained with these membranes are much higher than those typically found in polymeric materials, and these selectivities are achieved without sacrificing productivity. While the separation properties of ultramicroporous carbon membranes are superior to those of other materials, a significant drawback has been their vulnerability to adverse effects from water vapor exposure. A significant body of research exists regarding the nature of H20 sorption and pore filling in microporous carbon materials (Bansal et al., 1988; Barton and Koresh, 1983a-c; Bradley and Rand, 1993; Carrott, 1992; Carrott et al., 1991; Dubinin, 1966, 1980; Mattson and Mark, 1971; Pierce et al., 1949; Stoeckli and Kraehenbuehl, 1981; Suzuki and Doi, 1982; Walker, and Janov, 1968; Youssef et al., 1982.). It is widely accepted that the key factors in H20 sorption are the porosity of the carbon and the nature and number of oxygen-containing surface groups. While carbon surfaces are generally hydrophobic, oxygen-containing surface groups act as primary sites for the sorption of water molecules. The molecules sorbed on the primary sites attract additional water molecules through hydrogen bonding, leading to the eventual formation of water clusters. Since this process ultimately reduces the pore capacity available to other permeating species, significant membrane function is lost. Previous studies (Jones and Koros, 1995)have shown that some degree of membrane function is lost upon exposure to water vapor at all activity levels. Test results showed that some productivity losses occurred at the lower humidity levels but that selectivity losses were generally minimal. At the higher relative humidity levels, however, productivity losses were much greater and changes in selectivity became unpredictable. In some cases, the selectivity remained constant or 0888-5885/95/2634-0164$09.0QIQ
actually increased a small amount. In the majority of cases, however, selectivity losses were observed. It is likely that water clustering occurs a t critical pore constrictions at the higher humidity levels, greatly diminishing the diffusion rate of other permeating species. The water activity level at which this clustering occurs will vary with different carbons and is dependent on the polarity and porosity of a given material. Tests with the carbon membranes described above have shown that adverse effects from humidity become more pronounced at relative humidity levels above 45%,while a significant amount of membrane function is preserved at lower levels. Thus, the successful use of these membranes depends on a low water activity level at the carbon membrane surface. Obviously the feed stream can be pretreated or dried, but this extra step adds to the complexity and cost of the process. A simpler and more cost-effective approach would be to make the membranes themselves more resistant to water vapor effects. One approach to the problem has been to treat the carbon surface with various agents, such as Ha and Clz, to make it more hydrophobic (Stoeckli and Kraehenbuehl, 1981; Verma and Walker, 1992). While this approach shows promise in reducing water sorption at lower humidity levels, effective use at high water activity levels requires complete surface passivation. In addition, surface modification would most likely change the molecular sieving properties of the membrane. Our approach to the water vapor problem focused on devising a means to lower the water activity level at the carbon surface, and this was successfully accomplished with the development of carbon composite membranes. Composite membranes consist of a hollow fiber ultramicroporous carbon membrane coated with a thin layer of certain unique polymeric materials. These polymers are highly hydrophobic but do not prohibitively reduce the flux of other permeating species. The resulting composite membranes demonstrate a greater resistance to the adverse effects from water vapor while retaining very good separation properties. 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 166 Table 1. Comparison of PMP-Coated and Uncoated Carbon Membranes
humidity exposure (%) 62-65 for 18 h 44 for 19 h 67-70 for 17 h
membrane composition carbon with 0.45 pm PMP uncoated carbon uncoated carbon
preexposure 0 flux (GPU) 23.9 25.2 25.5
Table 2. Permeabilities of Coating Materials (Nemser and Roman. 1991; Roman, 1989)
permeability (Barrers) coating material PMP Teflon AF1600 Teflon AF2400
PO,
PN*
pH20
25 250 990
6.25 96 490
421 1165 4101
Experimental Section Making Carbon Membranes. Carbon membranes are produced by pyrolyzing hollow fiber polymeric materials in a tube furnace. The membranes described in this study were produced by pyrolyzing the polyimide precursor described in Jones and Koros (19951,under vacuum, at a temperature of 500 "C. Membrane Characterization. Characterization work was carried out with the single fiber test modules and the permeation test system described in detail in Jones and Koros (1995). Coating Carbon Membranes. The coating process takes place after the carbon membrane has been mounted in a module. Suitable barrier materials must be hydrophobic without greatly inhibiting the flux of other permeating species, and very few materials meet these criteria. Some initial success was achieved with poly(4-methyl-l-pentene),PMP, as a coating material. The structure of PMP, which is available from Scientific Polymer Products in pellet form, is (Mohr, 1990)
- (CHp -CH)I I
CHZ CH
/ \
CH3 CH3
Far better composite membranes were produced by coating the carbon membranes with DuPont's Teflon AF1600 and AF2400. These amorphous fluoropolymers are copolymers of (a) perfluoro-2,2-dimethyl-1,3-dioxole and (b) tetrafluoroethylene (Squire, 1988,1990).
Coating solutions are made by dissolving the polymeric barrier material in the appropriate solvent so that the polymer concentration is between 0.5 and 2.0% by weight. Since a bore side feed method of operation is used in the test system, the barrier coating is applied to the bore side of the hollow fiber membrane. A helium head pressure is used to force the flow of the coating solution through the bore of the membrane while vacuum is pulled on the shell side of the membrane. While the amount of coating solution fed through the bore varies, it is generally in the range of 0.5-1.0 mL. This procedure leaves enough coating solution on the membrane wall so that a barrier layer in the 0.5-5.0 pm thickness range is formed when the solvent evapo-
2
membrane changes during humidity exposure a02/Nz: 7.3 7.4,02flux loss -43% aOfl2: 9.6 9.6,Oz flux loss -49% aOz/Nz: 9.3 8.9,02flux loss -56%
--
rates. The bore is purged with dry air until solvent removal is complete. Testing Protocol. The O n 2 selectivity and the 02 pressure normalized flux of the carbon membrane are measured prior to coating and again after the coating process is completed. The permeability of the barrier material is determined from flat film measurements. The resistance in series model (eq 1) is then used to calculate the thickness of the composite barrier layer from changes in membrane productivity:
carbon
P=
cm3(STP)cm s*cmHgcm'
polymer
(1)
x 10-lO=Barrersand
As a check, the selectivity of the composite membrane is predicted from the resistance in series model (eq 2) and compared to the measured value:
When these two values closely agree, the membranes are considered to be successfully coated. At a given set of run conditions, both coated and uncoated membranes are tested for comparison. Changes in membrane performance in most of these studies are characterized by changes in O D 2 selectivity and in the 0 2 flux. In one study, changes in HdCH4 selectivity and total flux are measured.
Results and Discussion Composite membranes were first produced by coating the hollow fiber carbon membrane with a 0.45-pmlayer of poly(4-methyl-l-pentene) or PMP. This barrier layer dropped the carbon membrane selectivity from 9.7 to 7.3,but also reduced the membrane performance losses when exposed to air feed a t a 62-65% RH level. For comparison, an uncoated membrane with a selectivity of 9.3 was tested at 44% and 67-70% RH,and the results of these runs are shown in Table 1. Much better composite membranes were produced by coating the carbon with Teflon AF1600 and AF2400 (Koros and Jones, 1994). The Teflon AF materials are also hydrophobic, like the PMP, but are far less restrictive to the flux of 0 2 and Nz. Thus, it is possible to apply much thicker coatings and still have smaller drops in selectivity and productivity as compared to the PMP. A comparison of the permeabilities of the Teflon AF materials and PMP is given in Table 2. As shown in Table 3, the thicker coatings of the Teflon AF materials offer greater protection from water vapor effects.
166 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 Table 3. Comparison of Composite Membranes at 6 2 4 5 % RH
membrane composition carbon with 0.45 pm PMP carbon with 0.9 pm Teflon -1600 carbon with 4.1 pm Teflon AF2400
selectivity A from coating 9.7 7.3 10.7 9.6 9.3 8.3
0 2 flux A (GPU) from coating 45.0 23.9 38.3 34.5 38.7 33.5
--
--
-
membrane changes during humidity 7.4,02 flux loss -43% aOz/Nz: 9.6 9.8, 0 2 flux loss -38% aOz/Nz: 8.3 8 . 8 , O flux ~ loss -20% aOz/N2: 7.3
--
exposure after 18 h after 24 h after 52 h
Table 4. Performance of Composite Membranes at 8 3 4 6 % RH
membrane composition carbon with no coating carbon with 4.1 pm Teflon AF2400 carbon with 3.5 pm Teflon AF1600 Table 5. Membrane Performance at 83-85%
membrane composition carbon with no coating carbon with 2.8 pm Teflon AF1600
preexposure 0 flux (GPU) 30.7 32.3 29.9
---
RH for Hs/CI& Separations preexposure Hz flux (GPU) 97.8 113.8
Both of the Teflon AF materials provide some degree of protection against water, and the material of choice will likely be determined by each application. Composite membranes with comparable thicknesses of Teflon AF1600 and AF2400 were tested at 83435% RH, and the results of these runs are shown in Table 4. The Teflon AF2400 is less restrictive to the flux of all permeating species, including the water. Carbon membranes coated with this material have smaller losses in selectivity and productivity from the coating process but are also less protected from adverse water vapor effects. The Teflon AF1600 material is more hydrophobic, but is also more restrictive to the flux of other permeating species. After an initial trade-off in terms of selectivity loss, however, membranes coated with the Teflon AF1600 are far more resistant to water vapor effects. To further define potential application areas, a Teflon AF1600 coated carbon membrane and an uncoated carbon membrane were tested with a humidified Hz/ CHI feed. (The composite membrane was originally used in O D 2 studies, and so precoating measurements were not made for HdCH4.I The feed gas for this study was a 50%Hz/50% CHI mixture humidified to 8 3 4 5 % RH, and a higher feed pressure of 105 psig was used. The GC response is not linear over the test pressure range for this gas pair, and so the change in area counts could not be used to accurately determine productivity losses. Thus, the change in total flux (mUs of H2 CHI HzO) was measured for this study. Results of this run are shown in Table 5 and again demonstrate how effectively the barrier coating protects against water. It has been consistently observed that most of the membrane performance loss occurs in the first few hours of exposure t o water vapor and that additional changes are gradual. Further, it has been observed that in most cases these composite membranes can be restored to their prewater exposure condition by drying them at 80 "C in a vacuum oven. The carbon membranes used in the above studies were produced with the 500 "C pyrolysis protocol and had initial O D 2 selectivities in the 9-11 range. Since small selectivity and productivity losses occur with the addition of the polymer layer, efforts are underway to extend the coating process to even more selective carbon membranes. Carbon membranes with selectivities ranging from 11.0 t o 14.0 have been produced by increasing the pyrolysis temperature to 550 "C, but it has proved to be more difficult to deposit polymeric coatings on these membranes. This is likely due t o differences in
+
membrane changes after 24 h of humidity exposure a 0 f l z : 10.5 10.7,Oz flux loss -58% aOz/Nz: 8.4 9.6, 0 2 flux loss -41% aOz/Nz: 7.8 7.7,Oz flux loss -11%
2
+
membrane changes during humidity exposure aH2/C&: 520 106, total flux loss -87% after 22 h a & & : 206 196, total flux loss -14% after 24 h
--
the pore structure and surface chemistry of the more selective membranes. Indeed, SEMs confirm that the 550 "C membranes have a denser macropore structure than the 500 "C membranes. With adjustments to the coating protocol, however, it should be possible t o successfully coat the more selective membranes.
Conclusions Carbon composite membranes offer a very attractive solution to the problem of membrane performance losses from exposure to water vapor. The addition of thin layers of certain unique barrier materials to the feed side surface of the carbon membrane significantly reduces adverse effects from humidity exposure. By lowering the water activity level at the carbon surface without prohibitively reducing the flux of other permeating species, the polymeric barrier layer significantly improves membrane performance in humid environments. In process applications where small performance losses can be tolerated, these membranes offer a way t o eliminate a feed stream drying step. The polymer barrier type and thickness and the resulting degree of protection are variables that can be tailored to specific applications. Although small losses in membrane selectivity and productivity result from the resistance added by the polymer layer, the resulting composite membranes still have very attractive separation properties as compared to conventional polymeric membranes.
Acknowledgment The authors would like to gratefully acknowledge the financial support from Medal.
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IE940128N
* Abstract published in Advance ACS Abstracts, November 15, 1994.
