Transport and Morphological Characteristics of ... - ACS Publications

Ind. Eng. Chem. Res. 1999, 38, 3367-3380

3367

Transport and Morphological Characteristics of Polyetherimide-Based Carbon Molecular Sieve Membranes Mehran G. Sedigh, Lifang Xu, Theodore T. Tsotsis,* and Muhammad Sahimi Department of Chemical Engineering, University of Southern California, Los Angeles, California 90089-1211

A new class of carbon molecular sieve membranes (CMSMs) has been prepared by carbonization of polyetherimide-coated mesoporous tubular supports. The membranes show higher permeance and better separation factors than other supported CMSMs reported in the literature for the CO2/CH4 and H2/CH4 binary mixtures as well as for the CO2/H2/CH4 ternary mixture. CO2/CH4 separation factors as high as 145 for the equimolar binary and 155 for the ternary mixture were obtained with a CO2 permeance about 0.15 (cm3/cm2‚psi‚min). The corresponding H2/CH4 separation factors for the equimolar binary and ternary mixtures were 68 and 50, respectively, with a H2 permeance of 0.13 (cm3/cm2‚psi‚min). The membrane also shows good stability when tested with CO2 and Ar single gases, as well as with an equimolar mixture of CO2/CH4. To study the mechanism of permeation and separation in CMSMs, tests with single gases as well as with binary and ternary mixtures were performed at different temperatures, transmembrane pressure differences, and feed compositions. Elemental analysis, scanning electron microscopy, and gas adsorption were also employed to study the morphology of the resulting membranes. Elemental analysis shows that although the structure consists mostly of carbon, it also still contains oxygen, nitrogen and hydrogen. Scanning electron microscopy of the cross section of the carbonized membrane shows that the carbonized layer lies essentially within the mesoporous γ-alumina layer, a result also verified by N2 adsorption analysis at 77 K. The experimental data were compared with simulation results with the same mixtures using a nonequilibrium molecular dynamics method. Introduction The ever-increasing amounts of energy consumed in separation processes in chemical and petrochemical plants, with their adverse impacts on the environment and process operating costs, have motivated to date extensive studies for replacing the more energy-demanding, conventional separation processes with new, more energy-efficient technologies. A well-known class of such technologies is membrane-based separations. Such processes find today increased use in chemical and petrochemical plants either as stand-alone units or, most commonly, in combination with conventional processes in order to reduce the energy consumption in various gas separation applications. The majority of commercial gas-phase applications for the purification of a variety of gaseous mixtures currently utilize nonporous polymeric membranes. In recent years, dense nonporous (SiO2 and metal) and porous inorganic membranes have also been studied for a variety of applications (a brief review of the development status of these types of membranes has been presented in a recent paper by our group1). Carbon molecular sieve membranes (CMSMs), prepared by the carbonization of polymeric precursors, have been studied in the past few years as a promising alternative to both inorganic and polymeric membranes. These membranes have been shown to have, for many commercially interesting separations, comparable or even higher permselectivity than polymeric membranes and high permeabilities, similar to those reported for microporous inorganic membranes. These characteristics make them potentially attractive for a number of industrial applications. An example of such an application, currently motivating the development of CMSMs

(and of microporous membranes in general), is the separation of CO2 from gaseous mixtures. Typical applications here include the processing of reformate mixtures, the upgrading of biogas and landfill gas, and the treatment of flue gas. CMSMs are thought to have potentially an important role to play in this technological area. The CMSMs that have been reported so far have been prepared through either (1) the carbonization of preexisting polymeric substrates (e.g., hollow fibers or selfsupporting thin polymeric films) or (2) the carbonization of films deposited on underlying macro- and mesoporous supports by pyrolysis, typically in an inert atmosphere. The pyrolysis is carried out at temperatures of 5001000 °C, as determined by the polymer and the required final membrane structure. Depending on the conditions, the pyrolysis process removes most of the heteroatoms, originally present in the polymeric macromolecules, while leaving behind a cross-linked and stiff carbon matrix. The CMSMs so prepared have an amorphous porous structure created by the evolution of gases generated during the pyrolysis of the polymeric precursors. Though amorphous in nature, one still finds in CMSMs subdomains, where the structure of the polymeric precursors is still recognizable. This subdomain structure determines, in part, the differences that are found in the performance of CMSMs derived from various polymeric precursors. The extent of this subdomain structure also depends on the pyrolysis conditions. The higher the temperature and the longer the pyrolysis period are, the less similarity one finds between the final carbon matrix and the initial structure of the polymeric precursor. For high carbonization temperatures, it has

10.1021/ie9806592 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/13/1999

3368 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999

been even observed in some instances that the final carbon structure is made up of graphitic layers.2-7 Koresh and Sofer,8 in their pioneering studies, prepared CMSMs by the carbonization of a polymeric hollow fiber, whose composition was not disclosed. On the basis of single gas permeation tests, they reported a separation factor of 2.0 for the He/N2 gas pair and a separation factor of 9.0 for the CO2/N2 pair. A number of studies have since followed. Jones and Koros,9 for example, prepared CMSMs by the carbonization of commercially available, asymmetric hollow fiber polyimide membranes. Carbonization was carried out at two different temperatures, 500 and 550 °C. They reported a CO2/CH4 separation factor of 190 and a H2/CH4 separation factor of about 450. Exposure of these membranes to volatile organic compounds (VOCs) at ambient temperatures resulted in losses in both permeance and selectivity.10 For applications with “real” feeds, this still remains a problem with all microporous membranes; the presence of VOCs in such feeds is, of course, also known to be detrimental to polymeric membranes. They also studied the effect of humidified feeds on O2/N2 selectivity and permeability using feeds with relative humidity levels between 23 and 85%.11 Some performance losses occurred at all humidity levels, with the losses increasing with increasing humidity level. This sensitivity to humidity at ambient conditions was reduced by rendering the membrane surface hydrophobic by coating it with a thin layer of Teflon.12 It was also shown, in addition, that membranes carbonized at higher temperatures show smaller losses in performance in the presence of water vapor. In subsequent studies by the same group, the effect of different pyrolysis conditions (in particular, the effect of the type and flow rate of the inert gas used) on the properties of CMSMs were studied.13 Haraya et al.14 also prepared a CMSM by carbonization of a Kapton hollow fiber membrane at 950 °C. These membranes were, in some instances, further treated by coating them with a thin layer of poly(dimethyl siloxane) (PDMS) to prevent gas flow through defects in the structure. The separation factor for the CO2/N2 gas pairs (on the basis of single gas permeation tests) was reported to be ∼5 at 200 °C. Shusen et al.15 also reported the preparation of asymmetric CMSMs from the pyrolysis of thin selfsupporting films of a thermosetting phenol-formaldehyde resin (50-100 µm average thickness), followed by controlled oxidation of only one side of the film. A separation factor of 23.6 for the H2/N2 mixture (on the basis of single gas permeation studies) was reported at ambient temperatures. More recently, Kita et al.16 prepared unsupported CMSMs by carbonizing a polypyrrolone thin flat plate initially cast on a glass plate. The polymer film was then carbonized at 700 °C. A CO2/CH4 separation factor of about 230 was reported for this membrane. Noborio et al.17 also prepared CMSMs from polyimide thin flat plates by carbonizing these thin films at 700 °C. The CO2/CH4 separation factor of the resulting carbon membranes was reported to be about 175. A number of groups have also reported the preparation of carbon membranes by carbonization of polymeric films previously deposited on porous inorganic and metal substrates. Rao et al.,18 for example, prepared CMSMs by carbonization at 1000 °C (in a N2 atmosphere) of a thin, uniform layer of poly(vinylidene

chloride)-acrylate terpolymer latex film deposited on a macroporous graphite support from an aqueous suspension. They measured pure gas permeabilities for He and H2 as well as mixed gas permeabilities of H2/hydrocarbon and H2/CO2/CH4 mixtures. For a ternary mixture of CO2, CH4, and H2, a CO2/CH4 separation factor of 2.4 was measured. Hayashi et al.19 prepared CMSMs by carbonization of thin polyimide films deposited on the outer surface of R-alumina tubular supports (mean pore diameter ∼1400 Å) at various temperatures between 500 and 900 °C. The resulting membranes had a CO2/CH4 separation factor of about 100 (for a carbonization temperature of 800 °C) at 30 °C. They studied the effect of carbonization temperature on permselectivity and permeance of the membrane and showed that for the CO2/N2 and CO2/ CH4 binary mixtures the higher the carbonization temperature, the greater is the selectivity and the lower is the permeance of the resulting membranes. They also studied the permeation and selectivity of the C2H6/C2H4 and C3H6/C3H8 binary mixtures in a membrane that was carbonized at 700 °C.20 The membrane had separation factors of 7, 56, and 40 for the C2H4/C2H6, C3H6/C3H8, and CO2/N2 binary mixtures, respectively. The same group also studied the feasibility of modifying the pore size of carbon membranes by the pyrolysis of propylene within the membrane’s pores.21 In the study, a separation factor of 72 was reported for the CO2/N2 mixture. In a recent study by Fuertes and Centeno,22 CMSMs were prepared by carbonization of thin polyimide films coated on macroporous carbon disks. After preparing a smooth crack-free carbon disk, Fuertes and Centeno 22 deposited on it a polymeric film using a polyamic acid solution. A spin-coating technique at a speed of 1600 rpm was used for coating the substrate. The polymeric film was then gelled by immersion into a coagulant bath (acetone or isopropyl alcohol). The resulting polymeric layer was dried in air at room temperature, followed by drying in air at 150 °C for 1 h. This dried polymeric precursor film was then imidized at 380 °C for 1 h and carbonized in a vacuum for an additional 1 h. They also reported on the preparation of an asymmetric polyetherimide-based CMSM on macroporous carbon disks using a phase-inversion technique.23 They reported separation factors for the CO2/N2 and CO2/CH4 mixtures of 15 and 25, respectively. Foley et al.24 prepared a CMSM by pyrolysis of PFFA [poly(furfuryl alcohol)] films deposited on a macroporous, sintered stainless steel flat plate. They measured the permeation of single gases (H2, He, Ar, O2, N2, and SF6) to determine the permselectivity of the membrane. Separation factors of 2-3 for the O2/N2 and up to 30 for the H2/N2 gas pairs were obtained from single gas permeation experiments at 20 °C. CMSMs, resulting by the carbonization of PFFA previously deposited on mesoporous tubular supports, have also been prepared by our group.1 We have obtained a CO2/CH4 separation factor of 34-37 for the binary CO2/CH4 mixture as well as for the four-gas mixture of CO2/CO/CH4/H2. Although, as noted from the above discussion, significant progress has already been made in improving the techniques utilized for the preparation of CMSMs, room for further significant improvement still remains in terms of both the reported separation factors and the permeances. Furthermore, the fundamental understanding of the phenomena determining the transport characteristics of these membranes still remains incomplete.

Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3369

It is the main goal of our studies to offer further insight into the fundamental processes determining the transport and sorption behavior of these membranes, with the hope that such improved fundamental understanding will eventually lead to membranes with better permeances and separation factors. The emphasis here is on supported membranes. Unsupported membranes have proven of value in providing useful information about the transport and structural characteristics of CMSMs. Their mechanical strength and durability remain, nevertheless, an issue that requires further attention, especially for high temperature or reactive applications. As already noted, we have already reported, in an earlier publication, on the preparation of CMSMs using a poly(furfuryl alcohol) (PFFA) resin. We have investigated the transport characteristics of these membranes under different temperature and pressure conditions for a number of single gases, the binary CO2/CH4 mixture, and the fourgas (CO2/CO/H2/CH4) mixture.1 In this paper, we present the results of our studies on the preparation and transport and morphological characteristics of a new class of CMSMs prepared by the carbonization of polyetherimide-coated mesoporous γ-alumina tubes. This new type of CMSMs has shown significantly higher separation factors, permeance, and stability than the PFFA-based CMSMs prepared by our group, as well those prepared by other investigators. Elemental analysis, scanning electron microscopy, and gas adsorption were also used in order to obtain a better understanding of the morphology of the membranes. To expand our knowledge on the transport characteristics of these CMSMs, extensive permeability tests were performed at different temperatures, transmembrane pressure differences, and feed compositions. The results of these tests are presented and discussed in the following sections. Experimental Techniques Membrane Preparation. The molecular structure of the starting polymeric precursor is among the key factors that affect the performance of a CMSM. To prepare a CMSM with high permeance and separation factor and good chemical and thermal stability, it is important to select the appropriate starting polymer. Subsequently, the coating and carbonization procedures must be optimized. Useful in this area is the knowledge that has been generated in recent years by investigators who have studied the effect of the molecular structure on the separation factor and permeance of polymeric membranes. Polyimides have received particular attention through the years due to their superior separation factors, permeance, and stability, when compared with the other polymers that have also been used to prepare polymeric membranes. Tanaka et al.,25 for example, prepared 18 different polyimides by varying the dianhydride and dianiline moieties that they utilized. Changing these moieties resulted in polyimides with a free space varying in the range of 0.12-0.19. Polyimides, for example, containing -C(CF3)2- groups in the dianhydride or dianiline structure exhibited a free space of 0.16-0.19. Larger free volumes are typically associated with enhanced penetrant diffusivity as well as sorptivity, although in the latter instance to a smaller extent. This is because the bulky groups prevent intersegmental packing. This, in turn, improves the permeance through

the membrane. In most cases, however, the increase in permeance is typically accompanied by a simultaneous decrease in the separation factor. This, however, may be prevented by the introduction of stiff molecular units that restrict the rotational mobility of the structure. Kim et al.,26 for example, reported that in polyimides replacement of the PMDA unit by a corresponding 6FDA unit increases the permeance without an accompanying decrease in the separation factor. Tanaka et al.27 also showed that the introduction of a rigid and bulky dianiline structure inhibited the rotation around the -CH2- or -O- linkages and increased the permeance while causing only a minimal loss in the separation factor. The above studies indicate that, from a structural point of view, it is essential to have a molecular structure with a high free volume together with restricted intra- and intersegmental mobility in order to improve the performance of the membrane, both in terms of permeance and separation factor. In the first part of our studies, we chose PFFA resin as the polymeric precursor for preparing CMSMs. As mentioned in our recent paper,1 the choice of PFFA was based on a number of considerations. PFFA is an amorphous polymer with a nongraphitizable structure, potentially a good precursor to prepare CMSMs. Because of its simple molecular structure (PFFA is a lowmolecular-weight resin, with a single furan ring in its monomer) and formation mechanism, it is an appropriate material for fundamental experimental and atomistic simulation studies. When compared to other CMSMs (see Table 1), PFFA-based CMSMs have shown a good CO2/CH4 separation factor for both the binary and the four-gas, CO2/CO/H2/CH4, mixture together with a reasonable CO2 permeance. We have found it, however, difficult to improve upon the separation factor of the PFFA-based membranes without significantly further impacting on their permeance. It became clear, during our earlier studies with the PFFA-based CMSMs, that in order to prepare CMSMs with better performance, in terms of both the separation factor and permeance, it is necessary to look for a better starting precursor. Polyetherimides (PEIs) are one of the polymeric precursors we have investigated. They are one of the newest generic groups of engineering plastics and have a number of advantages as membrane materials. Studies on gas permeation through PEI dense membranes have shown that they exhibit impressive separation factors as well as good chemical and thermal stability.28,29 Several groups have recently studied the effect of molecular structure and preparation techniques on gas separation performance of PEI dense membranes. Teo et al.,28 for example, prepared asymmetric PEI hollow fiber membranes using Ultem 1000. Hollow fibers with an outer diameter of about 600 µm and an inner diameter of about 300 µm were prepared, and their structure and gas separation properties were examined. Li et al.30 studied the permeability of a number of single gases (H2, CO2, O2, N2, and CH4) at different temperatures and pressures in a series of PEIs that were prepared from 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride (HQDPA) and various aromatic diamines. They found that the permeability and separation factor of the PEI membranes varies with their molecular structures. Generally speaking, for most of the molecular structures investigated, the ideal separation factors decreased as permeability increased. However, the HQDPA-DMoBZD (3,3′-dimethoxy benzidine)

3370 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 Table 1. Summary of Carbon Molecular Sieve Membranes Prepared and Characterized by Different Investigators polymeric precursor undisclosed polymer asymmetric hollow fiber polyimide polyamic acid thermosetting phenol formaldehyde resin BTDA-polypyrrolone 6FDA-DABZ polyimide PVC-acrylate terpolymer latex BPDA-pp′ODA polyimide BPDA-pp′ODA polyimide BPDA-pp′ODA polyimide poly(furfuryl alcohol) resin bectron polyimide polyetherimide

membrane type hollow fiber

test temp (°C)

carbonization atmosphere

800

inert gas

CO2:1.39 × 10-2

permeance (cm3/cm2‚psi‚min)

CO2:2.26 × 10-2 H2: 5.3 × 10-2 CO2:1.55 × 10-3

15

CO2:1.85 × 10-3

CO2/N2 ) 72

30

CO2:4.0 ×

10-2

CO2/CH4 ) 35

1

25

CO2:4.5 ×

10-3

25

CO2:1.5 ×

CO2/CH4 ) 37 CO2/N2 ) 18 CO2/CH4 ) 25 CO2/N2 ) 15

22

10-3

vacuum

20

hollow fiber

950

inert gas

200

20

700 700 1000

N2 N2 N2

35 35 20

15 15 13

H2:∼3.16 × 10-4 N2: ∼1.34 × 10-5 CO2:1.55 × 10-3 CO2:1.24 × 10-3 CO2:3.35 × 10-2

800

N2

30

15

CO2:1.86 × 10-3

700

Ar

700

Ar

inert gas

600 550

and HQDPA-DMoMDA (3,3′-dimethoxy-4,4′-methylenedianiline) PEIs, both of which have bulky methoxy side groups on the aromatic rings of the diamine residue, displayed both high permeability and separation factors. They are, according to Li et al.,30 good candidates for gas separation applications. Eastmond et al. 31,32 synthesized a series of bis(ether anhydride)s with large, motion-hindering substituents. These bis(ether anhydride)s were then incorporated into PEIs by polymerization with diamines, with and without alkyl substituents. Eastmond et al.31,32 showed that the flexibility of the polymer backbone and substituents influences the chain rigidity on packing and, as a result, the glass transition temperature, as well as the gas permeability and selectivity. PEIs can be produced by either a nucleophilic substitution reaction process or the more conventional technique involving the condensation of diamines and dianhydrides.33 The general structural formula of PEIs is as follows:

'

Many different resins can be made by varying R and R′, as in the Ultem 1000 resin:

As can be seen, the presence of a bulky C(CH3)2 group combined with a highly stiff structure of the polymer makes the Ultem 1000 polymer a very promising candidate for polymeric membranes. Since during carbonization at moderate temperatures the backbone of the starting polymer remains, most likely, unchanged, one expects CMSMs utilizing PEIs as polymeric precursors to maintain their superior performance as well.

35

Ar

20

vacuum vacuum

ref

CO2:2.82 × 10-2

550

thin self-supported film

separation factor CO2/N2 ) 9.0 (single gas) CO2/CH4 ) 190 H2/CH4 ) 450 CO2/N2 ) 5 (single gas) H2/N2 ) 23.6 (single gas) CO2/CH4 ≈ 230 CO2/CH4 ≈ 175 CO2/CH4 ) 2.4 CH4/H2 ) 7.5 CO2/CH4 ≈ 100 CO2/N2 ≈ 27 CO2/N2 ≈ 40

asymmetric hollow fiber

unsupported flat plates unsupported flat plate supported on graphite disks supported on ∝-alumina tubes supported on ∝-alumina tubes supported on ∝-alumina tubes supported on γ-alumina tubes supported on macroporous carbon disks supported on macroporous carbon disks

test pressure (psig)

carbonization temp (°C)

8 9 14 15 16 17 18 19 20 21

23

To prepare CMSMs from PEI, mesoporous tubular substrates were dip-coated in PEI solution. The support substrates were cylindrical ceramic tubes (7 mm id, 10 mm od, and 4.5 cm long), manufactured by U. S. Filter, with a thin inside layer of γ-alumina.1 To prepare the PEI solution, Ultem 1000 (supplied by GE) was dissolved in 1,2-dichloroethane (DCE). The dissolution process lasted about 24 h with a slight heat applied in the early stages of dissolution under continuous total reflux conditions. The outer surface of the support substrate was wrapped with Teflon tape. The support substrate was then dip-coated into a 6% PEI/DCE solution for 3 min and was pulled out of the solution at a constant rate of 1.5 cm/min. After coating, the membrane was dried in the air for 24 h and was tested with He and N2 to make sure that the layer was free of any pinholes. An ideal He/N2 separation factor of 2025 was used as the yardstick, indicating that a relatively pinhole- and crack-free selective layer had been prepared prior to carbonization. Membranes which failed this test were dip-coated additional times using a 2% PEI/DCE solution. After coating with the PEI, the membrane was carbonized in the presence of flowing 99.997% pure argon (purchased from the Southern California Gas Company) in a cylindrical furnace, 2 in. in diameter and 2 ft long, externally heated and controlled by a programmable Omega CN3000 controller. An argon flow rate of ∼60 cm3/min was used. The carbonization protocol involved raising the temperature slowly (1 °C/min) to prevent any crack formation during carbonization and holding it constant first at 350 °C for 30 min and then at 600 °C for 4 h. Subsequently, the membrane was cooled to 180 °C at a rate of 2 °C/min and to room temperature at a rate of 5 °C/min. The coating/carbonization procedure was repeated as many times as required to modify the selective layer to achieve as high a separation factor as is possible with the membrane, without damaging its other desirable properties. The permeance of CH4 and CO2 through the carbonized membrane was measured, after each carbonization step, to evaluate the performance of the membrane (see below). Transport Investigations. After each membrane was prepared, its transport and separation characteristics were tested. In the area of membrane separation,


the focus in our group is on applications involving landfill gas, biogas, and reformate mixtures. We have, therefore, investigated the transport of the single gases CH4, H2, and CO2, which are of relevance to such mixtures, and that of Ar, which is the inert gas used during membrane preparation. We have also systematically investigated the transport characteristics of three binary mixtures of CO2/CH4, H2/CH4, and CO2/H2, as well as those of the ternary mixture of CO2/H2/CH4. The experimental apparatus utilized for the transport investigations has been described in detail elsewhere.34 Argon, methane, hydrogen, carbon dioxide, helium, and nitrogen were all supplied from high-pressure gas cylinders equipped with regulators. The flow rates were controlled with Tylan FC-260 mass flow controllers. All the gases used were 99.95% pure or better. They were, in addition, purified by passing them through Drierite, to capture any water vapor impurity that might be present, and heated in a preheater (a stainless steel coil equipped with a pair of semicylindrical heaters controlled by an Omega CN2000 controller), which operated at the testing module temperature. They were then fed into the testing module containing the CMSM, a detailed description of which has also been provided elsewhere.34 The membrane was sealed in the module using compressible graphite tape and compression fittings. The testing module was made of 316 SS and was supplied with inlet and outlet ports for its tube and permeate sides. Heating of the module was accomplished with a pair of semicylindrical heaters controlled by an Omega CN2000 temperature controller. The compositions of the permeate- and tube-side effluents (for the gas mixture studies) were measured on-stream using a Varian 3400 gas chromatograph. Pressure in the system was measured with an accuracy of 0.1 psia by Omega DP2000 pressure transducers. The tube-side pressure was controlled by a needle valve placed in the outlet. The permeate side was maintained at atmospheric pressure. Measurement of the outlet gas flow rates was accomplished using a soap-bubble flowmeter. Membrane permeances were calculated by the following relationship:

Pj )

VPmT0 TmP02πRL(∆Pj)

(1)

In the above equation, Pj is the permeance of species j in cm3(STP)/cm2‚min‚psi, V is the volumetric flow rate of the gas across the membrane in cm3/min, Pm is the pressure in psia at which the volumetric flow of tube side and permeate side was measured, Tm is the corresponding temperature in K, L is the membrane length in cm, R is the membrane radius in cm, P0 ) 14.696 psia, and T0 ) 273.15 K. ∆Pj is the log-mean pressure difference between the tube and permeate sides of species j defined as

∆Pj )

(P tj 1 - P pj 1) - (P tj 2 - P pj 2) log

P tj 1 - P pj 1

(2)

P tj 2 - P pj 2

In the above equation, P jp1 is the pressure at the permeate-side inlet, P jp2 the pressure at the permeateside outlet, P jt1 the pressure at the tube-side inlet, and P jt2 the pressure at tube-side outlet, all for species j in

psia. log-mean pressure difference is usually chosen in the membrane transport literature in order to account for the change of pressure along the direction of flow in both the tube and permeate sides. This is in analogy with the log-mean temperature difference utilized in modeling shell and tube heat exchangers. In our experiments, however, due to the short membrane length, only minor pressure drops existed in both the permeate as well as the tube side. Using a transmembrane pressure difference, simply calculated from the tube- and shellside average pressures, would not have introduced any significant error; thus, direct comparison of our results can also be made with studies where the log-mean pressure difference was not utilized. The separation factor for each pair was then calculated by dividing the individual permeances obtained from eq 1. Results and Discussion To understand the mechanisms of transport and separation of gas mixtures in PEI-based CMSMs and the effect of various operating conditions on their performance, the permeances and separation factors of different gas mixtures were studied as functions of transmembrane pressure difference and temperature. They have also been compared to the ideal separation factors, defined as the ratio of the permeances of the individual gases as they permeate through the membrane separately. In what follows, we discuss first the effect of the temperature, transmembrane pressure difference, and the number of coating/carbonization steps on the permeances and the ideal separation factors for single gases. Subsequently, the effect of the same factors, as well as that of the feed composition on the permeances and separation factors of the three binary mixtures, namely, CO2/CH4, H2/CH4 and CO2/H2, as well as of the pair separation factors in the ternary mixture of CO2/CH4/H2 are presented and discussed. The experimental transport results presented in this paper were obtained by testing two membranes, denoted as membranes 1 and 2. Membrane 1 was prepared following the procedure described above and was used in the early tests. This membrane was inadvertently damaged during the tests, by exposure to air at a temperature around 400 °C; to continue with the tests, the membrane was dip-coated in a 2% PEI/DCE solution and carbonized following the procedure described earlier. The resulting new membrane (membrane 2) was used in the latter part of our transport studies. As the data presented below indicate, not only did the new layer successfully repair the damaged membrane, but it also improved the separation characteristics of the membrane, resulting in higher separation factors for the various gas pairs. Single Gas Tests. The single gas permeation tests indicate that the membrane permeances and separation factors are a strong function of the number of coating/ carbonization steps. Figure 1, for example, shows the permeances (and the corresponding ideal separation factor for the gas pair) for CO2 and CH4. After each coating/carbonization step, the permeances of both gases decrease and the ideal separation factor increases. The increase in the separation factor may be attributed to some pore structure narrowing but, likely, mostly because the additional coating tends to repair the existing pinholes and cracks. This improvement in the membrane separation performance comes at the tradeoff of diminished permeances, something that was also observed with uncarbonized polymeric membranes.


Figure 1. Effect of the number of layers on the permeance and ideal separation factor of the membrane: T ) 20 °C and ∆P ) 30 psi.

Figure 2. Effect of temperature on permeance and ideal separation factor of the membrane: ∆P ) 30 psi.

Figure 2 shows the results of single gas permeation tests with the four gases (CH4, CO2, H2, and Ar) at four different temperatures. The permeances of CH4 and Ar increase with increasing temperature, indicative of an activated diffusion process. The CO2 permeance, on the other hand, decreases, and as a result, the CO2/CH4 ideal separation factor decreases from 91 at 20 °C to 24 at 150 °C. This result is consistent with the modeling calculations (see below), which indicate that CO2 preferentially adsorbs within the membrane structure. Increasing the temperature significantly impacts on the amount adsorbed. The H2 permeance remains practically unchanged, as the temperature increases from 20 to 150 °C. As a result, the ideal H2/CH4 separation factor also decreases, although not as significantly as the corresponding CO2/CH4 ideal separation factor. The fact that the effect of temperature rise on the permeance of H2 is not as pronounced as that on the permeances of Ar and CH4 suggests that the average pore size of the selective layer is probably larger than the kinetic diameter of H2. Hydrogen diffusion is not as strong of an activated process as is CH4 and Ar diffusion. We have also studied the effect of transmembrane pressure difference on the membrane permeance and ideal separation factors. Although the permeance of CO2 remained almost unchanged while increasing the transmembrane pressure difference from 20 to 40 psi, the CH4 permeance almost doubled. These results are again

Figure 3. (a) Effect of transmembrane pressure difference on permeance and separation factor of the equimolar CO2/CH4 mixture: T ) 20 °C. (b) Effect of temperature on permeance and separation factor of the equimolar CO2/CH4 mixture: ∆P ) 30 psi.

consistent with the notion that CO2 adsorbs preferentially within the membrane structure and that pressure changes, beyond a certain level, have a small effect on the mechanism of transport. That is not the case, though, for a more sparingly adsorbed species such as CH4. That the transport of two different species is affected differently by increasing the transmembrane pressure difference is indicative of the absence of convective flow through the selective layer, implying that the selective layer has no significant cracks or pinholes. The CO2/CH4 Binary Mixture. The effect of the transmembrane pressure difference, temperature, and composition on the permeation behavior of the CO2/CH4 binary mixture was also studied. The effect of transmembrane pressure difference is shown in Figure 3a for an equimolar CO2/CH4 mixture using the original membrane 1. The results indicate that low transmembrane pressure differences are in favor of separation of CO2 from the CO2/CH4 mixture. These results are consistent with those obtained for the CO2/CH4 ideal separation factor from the single gas permeation experiments. A CO2/CH4 pair separation factor as high as 262 was obtained at a transmembrane pressure difference of 20 psi. The CO2/CH4 separation factor in the binary mixture is higher than the ideal separation factor measured from the single gas permeation experiments. This is due to the “blocking of the pores” effect on the methane permeation brought upon by the adsorbed CO2


Figure 4. Effect of feed composition on the permeance and separation factor of the CO2/CH4 mixture: T ) 20 °C and ∆P ) 30 psi.

molecules, which enhances separation of CO2 from its mixture with CH4 (see the discussion below). Figure 3b presents both the effect of the temperature on the CO2 and CH4 permeances, and the CO2/CH4 mixture separation factor. The results show the same trends as those observed with the single gas permeation tests. The separation factor decreases from 127 to 27 as the temperature increases from 20 to 150 °C. Figure 4 depicts the effect of feed composition on the permeances and separation factor of the CO2/CH4 mixture for both the original membrane 1 and the same membrane after it had been repaired (membrane 2). The separation factor goes through a minimum at a CH4 mole fraction of about 0.5. The effect is more pronounced for the most permselective membrane 2. A similar composition effect was not observed with the PFFAbased CMSMs,1 which had a lower permselectivity than the PEI-based CMSMs. One may conclude, therefore, from these observations that the more permselective the membrane is, the stronger is the dependence of its separation factor on feed composition. The highest separation factor obtained in this study for the more permselective membrane was about 285 for a mixture containing 70% CH4 at a transmembrane pressure difference of 30 psi and a temperature of 20 °C. The H2/CH4 Binary Mixture. The permeances and separation factor of the H2/CH4 binary mixture were also studied as a function of the transmembrane pressure difference, temperature, and composition. Figure 5 shows the effect of the transmembrane pressure difference on the permeation behavior for membrane 1. The results indicate that the higher transmembrane pressure differences result in higher H2 permeances, and they only slightly increase the CH4 permeance. As a result the H2/CH4 separation factor increases from 27 to 43 as the transmembrane pressure difference increases from 20 to 40 psi. This result is different from what was observed for CH4 as a single gas. This difference can be attributed to the presence of H2 molecules in the mixture. Figure 6 shows the effect of the temperature and mixture composition on the permeances and separation factor of the H2/CH4 gas pair for membrane 2. A temperature increase results in enhanced CH4 permeation through the membrane. Only a slight corresponding increase in H2 permeance is observed over the same range of temperatures. As a result, as can be seen in Figure 6a, the separation factor decreases from 68 to

Figure 5. Effect of transmembrane pressure difference on the permeance and separation factor of the equimolar H2/CH4 mixture: T ) 20 °C.

Figure 6. (a) Effect of temperature on the permeance and separation factor of the equimolar H2/CH4 mixture: ∆P ) 30 psi. (b) Effect of feed composition on the permeance and separation factor of the H2/CH4 mixture: T ) 20 °C and ∆P ) 30 psi.

40, when the temperature is increased from 20 to 150 °C. This result is consistent with the behavior observed with the single gases. Figure 6b presents the effect of the feed composition on the permeances and separation factor. The results indicate that increasing the hydrogen concentration in the mixture increases the separation factor, a result qualitatively similar to that observed by increasing the transmembrane pressure difference.


Figure 8. Effect of temperature on the permeance and separation factor of equimolar H2/CO2 mixture: ∆P ) 30 psi.

Figure 7. (a) Effect of the transmembrane pressure difference on the permeance and separation factor of the equimolar H2/CO2 mixture: T ) 20 °C. (b) Effect of feed composition on the permeance and separation factor of the H2/CO2 mixture: T ) 20 °C and ∆P ) 30 psi.

These experiments indicate that the presence of CH4 molecules not only lowers the permeance of H2 through the membrane but also results in a smaller separation factor than the ideal H2/CH4 separation factor. The results are consistent with the picture in which the larger CH4 molecules block passage through the pores of the smaller H2 molecules, which, as a result, do not transport through the membrane as readily as they would have had in the absence of any CH4 molecules. The CO2/H2 Binary Mixture. The CO2/H2 binary mixture shows interesting behavior. Figure 7a shows the effect of changing the transmembrane pressure difference on the permeances and separation factor of the CO2/H2 mixture at 20 °C. Although the H2 permeance does not change considerably with increasing the pressure, the CO2 permeance decreases. The CO2/ H2 separation factor, therefore, decreases from 4.1 to 3.4 as the transmembrane pressure difference increases from 20 to 40 psi. Figure 7b shows the effect of the feed composition on the permeances and separation factor. The higher the H2 mole fraction and partial pressure are, the lower is the CO2/H2 separation factor. This effect is similar to that observed by increasing the transmembrane pressure difference. Figure 8 shows the effect of the temperature on the permeances and separation factor of the CO2/H2 mixture for membrane 2. Higher temperatures decrease the CO2 permeance. The

Figure 9. Effect of transmembrane pressure difference on the separation factors of the gas pairs in the equimolar ternary mixture of CO2/CH4/H2: T ) 20 °C.

H2 permeance, on the other hand, increases considerably. As a result, the membrane, which is CO2-selective at low temperatures, becomes more selective toward H2 at the higher temperatures with the separation factor decreasing from 2.6 to 0.4 as the temperature increases from 20 to 150 °C. The CO2/CH4/H2 Ternary Mixture. The effect of the transmembrane pressure difference on the pair separation factors in an equimolar ternary mixture of CO2/ CH4/H2 is presented in Figure 9 for membrane 1. For the CO2/CH4 and CO2/H2 gas pairs, the results are in qualitative agreement with those obtained for each individual binary mixture as discussed above. The separation factors for the CO2/CH4 and CO2/H2 gas pairs, for example, decrease from 156 and 6.6 to 75 and 2.8, respectively, as the transmembrane pressure difference increases from 20 to 40 psi. The H2/CH4 separation factor, on the other hand, first increases from 23.6 to 33.3 and then drops to 27.4 as the transmembrane pressure difference increases from 20 to 30 and then to 40 psi. This is a different behavior than that observed for the corresponding binary mixture. The effect of the temperature on the pair separation factors is illustrated in Figure 10. The CO2/CH4 and CO2/H2 separation factors show the same trends as they did during their respective binary mixtures experiments. They decrease as the temperature increases. The behavior of the H2/CH4 separation factor, on the other hand, is again somewhat different than what was


Figure 10. Effect of temperature on the separation factors of gas pairs in the equimolar mixture of CO2/CH4/H2: ∆P ) 30 psi.

Figure 11. Effect of feed composition on the separation factors of the gas pairs in the ternary CO2/CH4/H2 mixture: T ) 20 °C and ∆P ) 30 psi.

observed with the binary mixture experiments. It passes through a maximum at about 50 °C. This difference in the behavior is an indication of the effect that the third species in the system, i.e., CO2, has on the separation characteristics of the membrane. It indicates, furthermore, that single gas or binary mixture permeation data are not always reliable enough to be used for predicting the behavior of a multicomponent gas mixture. Figure 11 shows the effect that the ternary mixture feed composition has on the pair separation factors. In the experiments reported in Figure 11, the CH4 mole fraction was varied, while keeping equal the mole fractions of CO2 and H2. The CO2/CH4 separation factor remains constant around 147, in a range around the equimolar mixture, but it increases at both the high and low CH4 mole fraction regions. This effect is similar to what was observed for the effect of feed composition on the separation factor of the CO2/CH4 binary mixture. A separation factor as high as 452 was obtained for a mixture containing 60% CH4, 20% CO2, and 20% H2. The H2/CH4 separation factor reaches a maximum value of 120 for the 10% CH4, 45% CO2, and 45% H2 mixture. This value decreases to 20 for the 60% CH4, and 20% CO2, and 20% H2 mixture. The CO2/H2 separation factor decreases from 21 for the 60% CH4, 20% CO2, and 20% H2 mixture (a value considerably higher than what we obtained for CO2/H2 binary mixture) to 2.5 for the 10% CH4, 45% CO2, and 45% H2 mixture.

Morphological Characterization. Studying the transport characteristics of microporous membranes provides useful information regarding the separation mechanisms and the overall behavior of the membrane under different operating conditions. However, to complete the understanding provided by the transport studies and to gain a thorough understanding of the separation and permeation mechanisms at a fundamental level, it is also important that one studies in detail the structure and the morphology of these membranes. As part of our research, we have also undertaken a number of complementary morphological characterizations. Electron microscopy is a useful technique for such characterizations, since it provides details of the position, geometry, and gross (50-100 Å) features of these membranes. Figure 12, for example, shows the SEM image of a cross section of a PEI CMSM, prepared according to the technique previously described, after the first coating/carbonization cycle. There are four distinct sections in this image. The upper part, which has a coarse granular structure, is the macroporous R-alumina layer with a mean pore diameter ≈ 15 µm that provides the mechanical strength for the membrane. The support layer is followed by two other R-alumina layers, which have finer granular structures, with mean pore diameters of 0.8 and 0.2 µm, respectively. At the bottom of the cross section, a thin layer with an average thickness of ∼2-3 µm is seen. This layer, which at this resolution looks very smooth and nonporous, is the 50 Å γ-alumina layer after being impregnated by the polymer solution and carbonized. Figure 13 shows the layer for the same membrane at a higher magnification. Figure 14 shows the layer for the same membrane after a second coating/carbonization cycle. From these images, it appears that the layer is free of any cracks or pinholes larger than the resolution of the SEM (∼50-100 Å). The film, furthermore, appears to have a more compact and homogeneous structure after the second coating/carbonization cycle. Gas adsorption experiments in the micro- and mesoporous regions also provide useful information regarding the structure of the supported CMSMs. Using this technique, one is able to detect pores with dimensions much smaller than those that can be detected by most conventional electron microscopy techniques. One can also generate the pore size distribution (PSD) of the microporous region. The average pore size and PSD are helpful for understanding and explaining the permeation mechanisms of various species through the membrane. They are also of value in modifying the preparation technique for optimizing the performance of the membrane in terms of both the separation factor and the permeance of the desired species. The gas adsorption experiments were performed in our laboratories using an ASAP 2010 micropore analyzer, from Micromeritics, Inc. Figure 15 presents the PSD obtained by N2 adsorption at 77 K for the mesoporous region (20-500 Å) of a PEI CMSM prepared on a γ-alumina tube. The ideal CO2/CH4 separation factor of the membrane at 20 °C was 56.4, close to the ideal separation factor of the membrane 1 (∼64) used in the transport characterization experiments. The average pore size in the mesoporous region has decreased from ∼50 Å for the original γ-alumina layer to ∼38 Å. Figure 16 shows the results of the experiments of N2 adsorption at 77 K, analyzed by the Horvath-Kavazoe technique for the pores that are smaller than 20 Å. Obvious from the figure is the presence of a sharp peak at 3.6 Å,


Figure 12. SEM image of the cross section of a supported CMSM membrane.

Figure 13. Higher magnification SEM image of the cross section of the supported CMSM membrane after the first carbonization cycle.

indicating the presence of a microporous region within the membrane. Taken together, Figures 15 and 16 indicate that the mesoporous structure of the original γ-alumina layer was modified as a result of the coating and carbonization processes. As was also obvious from the SEM images, the PEI has penetrated into the mesoporous region and the selective carbogenic layer is in fact formed inside the γ-alumina layer. The presence of such small pores in the selective amorphous carbon layer of the membrane is consistent with the molecular sieving properties of such membranes and the relatively high CO2/CH4 and H2/CH4 separation factors, as previously discussed. In addition to the pore structure, the surface chemistry of the resulting membrane also has a significant

impact on the adsorptive properties of the final membrane and strongly affects the separation characteristics of the membrane. In our ongoing studies of these membranes, we are utilizing a variety of conventional techniques for looking at their surface characteristics. Due to the nature of these materials, interpretation of the data is rather difficult. A highly sensitive measure of the bulk characteristics of these materials is provided by elemental analysis. Table 2, for example, shows the results of elemental analysis of the resulting carbogenic material prepared from the PEI carbonization, and compares the results with the initial elemental composition of the polymer itself. The results in this Table show that although about 90% of the final structure is carbon,


Figure 14. Higher magnification SEM image of the cross section of the supported CMSM membrane after the second carbonization cycle.

Figure 15. Pore size distribution of the supported membrane obtained by N2 adsorption at T ) 77 K, for pores bigger than 20 Å.

there are still nonnegligible amounts of nitrogen, oxygen, and hydrogen present in the structure. Similar observations were reported by Morooka and co-workers for CMSMs prepared by polyimides.36 How the presence of the various heteroatoms affects the separation behavior of the resulting membrane is currently under investigation. The conclusions of these studies will be the key to determining the direction of future efforts for modeling the transport characteristics of CMSMs, since all such efforts (including those by our group), so far, have assumed that the membrane has a pure graphitic structure. Modeling of Transport In a parallel effort, a nonequilibrium molecular dynamics (NEMD) technique is being used to model

Figure 16. Pore size distribution of the supported membrane obtained by N2 adsorption at T ) 77 K, for pores smaller than 20 Å. Table 2. Weight Percent of Polyetherimide before and after Pyrolysis at T ) 600 °C element

wt % before pyrolysis

wt % after pyrolysis

oxygen carbon hydrogen nitrogen

16.18% 74.74 4.38% 4.70%

3.20% 89.55% 2.00% 4.30%

transport and sorption phenomena in CMSMs. Molecular simulation techniques, which are based on either equilibrium MD or NEMD simulations, can provide a deeper understanding of the mechanisms of adsorption and transport in porous materials. In particular, NEMD is ideally suited for the practical situation encountered here, in which a large external driving force (e.g., a chemical potential gradient) is imposed across a thin porous film. Among such methods, the Grand-Canonical molecular dynamics (GCMD) method,37-47 in which


Figure 17. Separation factor versus pore size H* ) H/σCH4 for the three equimolar binary mixtures of CO2/H2, CO2/CH4, and H2/ CH4: ∆P ) 3 atm and T ) 20 °C.

Figure 18. Effect of temperature (in °C) on the separation factor for three equimolar binary mixtures of CO2/H2, CO2/CH4, and H2/ CH4: ∆P ) 3 atm and H* ) 1.67.

Monte Carlo (MC) and MD simulations are combined in a dual control-volume configuration, has been used for calculating the diffusivity of a single gas through a slit pore. Here we present the results of GCMD simulation of transport of three binary mixtures of CO2/H2, CO2/CH4, and H2/CH4 in a slitlike carbon nanopore to compare qualitatively with our experimental results. In our simulations, we have utilized smooth pore walls and the classical 10-4-3 potential of Steele to calculate the interaction between a fluid atom and the wall.48 The fluid-fluid interactions were modeled with the cut-andshifted Lennard-Jones (LJ) 6-12 potential. Details of the simulation method are discussed elsewhere.35,46,47 As shown in Figure 17, our simulations indicate that the transport and sorption of binary mixtures are strongly affected by H*, the dimensionless pore size defined as H* ) H/σCH4. Here H is the pore width and σCH4 is the kinetic diameter of CH4 molecule, which is modeled as a LJ hard sphere. The simulation shows that for pore sizes less than 1.65σCH4 the H2 molecules are transported more readily through the pore, followed in turn by CO2 and CH4. This can be attributed mainly to the size exclusion and molecular sieving effects, which do not allow larger molecules to enter the pore. In the pore size range of 1.66-1.68σCH4, however, CO2 molecules are the most readily transported, followed in turn by H2 and CH4. This is because of enhanced adsorption properties of the CO2 molecules, as determined by their LJ energy parameters (i.e., CO2 is higher than its counterparts for CH4 and H2). It is obvious that in this region of pore sizes both the energy and size parameters play an important role in determining the overall transport. In this pore size range, the simulation results are in qualitative agreement with the experimental data that are reported here. For the pore sizes larger than 1.68σCH4, CO2 is still the most readily permeated species, but followed now by CH4 and H2. Obviously, under such conditions, the energy parameters have a dominant effect on the permeation. This is because the size of the pore is much larger than the size of the molecules. As a result, the transport of the larger molecules cannot effectively be hindered by size exclusion and molecular sieving effects. In fact, larger pore sizes allow the more strongly adsorbed CO2 molecules to enter the pore more

readily, adsorb on its surface, and permeate through the pore along its surface. Figure 18 shows the variation of the separation factor as a function of the temperature for the three binary mixtures at H* ) 1.67. The separation factors of CO2/ H2 and CO2/CH4 decrease as the temperature increases, exhibiting the same qualitative trend as that of the experimental results. However, for the H2/CH4 mixture, the simulation results indicate that the H2/CH4 separation factor increases slightly from 2.75 to 3.5 as the temperature increases, whereas the experimental results indicate a slight decrease as the temperature increases. The simulation result for the H2/CH4 mixture is, in fact, in agreement with our experimental results for the less selective PFFA-based CMSM.1 This implies that the more selective the membrane is, the more important are the effects of the pore connectivity, surface heterogeneity, and chemistry. The CMSMs are, of course, far from being pure graphitic layers (see the discussion of the results of the elemental analysis) with a smooth surface (as assumed in the simulation). Given that the simulation is done with a single pore instead of a set of interconnected pores with different sizes (which is likely the case with our membranes; see Figure 16) and keeping in mind that we have neglected the effect of the chemical nature of the surface, the qualitative agreement between the simulation results and the experimental data (albeit in a narrow region of pore sizes) is, in our opinion, promising. Our current modeling investigations aim at improving the agreement between the simulation results and experiments by utilizing (1) more realistic descriptions of the pore structure (e.g., a network of interconnected pores), rather than a single pore, (2) more realistic molecular representation of the gas molecules (accounting, for example, for the electrical charges on the CO2 molecule), and (3) a method of accounting for the entrance effects that are present in the experiments and may hinder motion of one of the components into the membrane more strongly than the others. As we develop a better understanding of the surface characteristics of these membranes, more realistic membrane molecular structures will supplant the currently utilized graphitic structure.


Conclusions Polyetherimides have excellent mechanical and thermal properties and have been used in the past for the preparation of dense polymeric membranes with good gas separation characteristics. PEIs have been used in this study to prepare a new class of CMSMs. To prepare these membranes it is important that one first prepares an initial pinhole- and crack-free polymeric film prior to carbonization. The separation properties of the initial carbon film can subsequently be improved by additional coatings and carbonizations with less concentrated PEI solutions. Single gas permeation tests, after adding each additional carbon layer, indicated that the separation properties of the membranes were enhanced, but this came at a cost of diminished permeances. It still remains a challenge in improving the PEI-CMSM preparation technique to avoid this tradeoff between permeance and separation factor. Although in our transport studies all the gases have a 99.95% or better purity and gas purifiers are used for all of them, results of our prior investigations with PFFA-based CMSMs indicated a change in the membrane performance with time on stream.1 We attributed this behavior to small amounts of impurities that still remain in these gases, such as heavier hydrocarbons or VOCs, which may potentially adsorb on the membrane. PEI-based CMSMs, however, have shown a much better operational stability during single and mixed gas permeation tests than the PFFA-based CMSMs. This is an important observation, since membrane stability is one of the key factors, along with satisfactory separation properties, mechanical strength, and a cost-effective preparation technique, that determines the likelihood of industrial applications. High separation factors for the CO2/CH4 pair combined with high CO2 permeances, in comparison with other supported CMSMs, make the PEI-based CMSMs promising systems for landfill gas, biogas, and reformate mixture gas separation applications. In this study, the transport and separation characteristics of these membranes were tested with gas permeation tests, using single gases and binary and ternary gas mixtures relevant to the aforementioned applications. The membranes exhibited reasonable separation factors for the CO2/CH4 and H2/CH4 pairs. For the CO2/H2 mixture, the PEI-based CMSMs are relatively selective toward CO2 at 20 °C but show selectivity toward H2 at higher temperatures. The effect of feed composition on separation factor and permeances of the various species in their binary and ternary mixture has also provided useful information about the separation characteristic of the membrane. The CO2/CH4 separation factor, both during the binary as well as the CO2/H2/CH4 ternary mixture experiments, exhibited a minimum around the equimolar feed composition. This effect had not previously been observed with the less selective PFFA-based membranes.1 The CO2/CH4 and the CO2/H2 pair separation factors, measured with the ternary mixture, followed the same general trends as those measured in their binary mixture. However, this was not the case with the H2/CH4 separation factor. The behavior observed with the ternary mixture is different than that observed with the binary mixture. This indicates, of course, that one should not rely only on single and binary mixture separation data for characterizing the behavior of multicomponent mixtures in CMSMs.

Morphological characterization of the membranes using SEM and gas adsorption has shown that the selective carbogenic layer is likely formed mostly within the pore network of the γ-alumina layer. Gas adsorption clearly indicates the presence of a microporous permselective layer. Elemental analysis of the resulting carbogenic material has shown that, though carbon is its main constituent, there are also other elements (nitrogen, oxygen, and hydrogen) present that can affect the adsorption and separation properties of the membrane. We have also carried out nonequilibrium molecular dynamics simulation of transport and separation of the binary mixtures of interest in this paper. The simulation results are in qualitative agreement with the trends observed during the experiments in terms of the behavior of separation factor and permeance at different conditions. It is encouraging to see that a simplified transport model, as the one used in this study, can predict the qualitative trends fairly well. It is clear, however, that for a more quantitative agreement a more sophisticated model is needed, which will more faithfully reflect the complex structure and surface chemistry of the membranes. Such a model must, of course, be coupled with more detailed structural characterization studies to obtain better insight into the structure of the PEI-based CMSMs. These investigations are currently in progress in our group, the results of which will be published in due course. Acknowledgment The authors thank Media and Process Technology, Inc., PA, for supplying the support substrates, and Drs. P. K. T. Liu and R. J. Ciora, Jr., for their useful discussions in the early stages of this work. The support of the National Science Foundation is also gratefully acknowledged. Literature Cited (1) Sedigh, M. G.; Onstot, W. J.; Xu, L.; Peng, W. L.; Tsotsis, T. T.; Sahimi, M. Experiments and Simulation of Transport and Separation of Gas Mixtures in Carbon Molecular Sieve Membranes. J. Phys. Chem. A 1998, 102, 8580. (2) Bourgerette, C.; Oberlin, A.; Inagaki, M. Structural and Textural Changes from Polyimide Kapton to Graphite: Part I. Optical Microscopy and Transmission Electron Microscopy. J. Mater. Res. 1992, 7, 1158. (3) Hishiyama, Y.; Yoshida, A.; Kaburagi, Y.; Inagaki, M. Graphite Films Prepared from Carbonized Polyimide Films. Carbon 1992, 30, 333. (4) Hatori, H.; Yamada, Y.; Shiraishi, M. In-plane Orientation and Graphitizability of Polyimide Films. Carbon 1992, 30, 763. (5) Hatori, H.; Yamada, Y.; Shiraishi, M. In-plane Orientation and Graphitizability of Polyimide Films: II. Film Thickness Dependence. Carbon 1993, 31, 1307. (6) Mariawala, R. K.; Foley, H. C. Evolution of Ultramicroporous Adsorptive Structure in Poly(furfuryl alcohol)-derived Carbogenic Molecular Sieves. Ind. Eng. Chem. Res. 1994, 33, 607. (7) Emmerich, F. G. Evolution with Heat Treatment of Crystallinity in Carbons. Carbon 1995, 33, 1709. (8) Koresh, J. E.; Sofer, A. Molecular Sieve Carbon Permselective Membrane. Part I. Presentation of a New Device for Gas Mixture Separation. Sepn. Sci. Technol. 1983, 18, 723. (9) Jones, C. W.; Koros, W. J. Carbon Molecular Sieve Gas Separation Membranes. 1: Preparation and Characterization Based on Polyimide Precursors. Carbon 1994, 32, 1419. (10) Jones, C. W.; Koros, W. J. Carbon Molecular Sieve Gas Separation Membranes. 2: Regeneration Following Organic Exposure. Carbon 1994, 32, 1427.

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Received for review October 16, 1998 Revised manuscript received June 15, 1999 Accepted June 21, 1999 IE9806592

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