CH4 Separation Using Carbon Molecular Sieve

Increasing the final pyrolysis temperature was shown to dramatically increase the CO2/CH4 selectivity (>600) of the CMS membranes but was detrimental ...
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Ind. Eng. Chem. Res. 2002, 41, 367-380

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High Pressure CO2/CH4 Separation Using Carbon Molecular Sieve Hollow Fiber Membranes De Q. Vu and William J. Koros* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1062

Stephen J. Miller Chevron Research & Technology Center, 100 Chevron Way, Richmond, California 94802-0627

Carbon molecular sieve (CMS) hollow fiber membranes have been investigated for CO2/CH4 separation. High-pressure (up to 1000 psia), mixed-gas feeds of 10% CO2/90% CH4 on the shell side were examined for three different temperatures (24, 35, and 50 °C). The mechanical, permeance, and selectivity stabilities of the CMS membranes under high pressure were encouraging and could be industrially relevant for many high-pressure applications, such as CO2 removal from natural gas. Two asymmetric polyimide precursor fibers, 6FDA/BPDA-DAM and Matrimid 5218, were pyrolyzed under vacuum to form the CMS membrane fibers. When pyrolyzed under identical protocols, the two types of CMS fibers had different permeation properties and physical characteristics. Modifications of the pyrolysis protocol and conditions were explored. Increasing the final pyrolysis temperature was shown to dramatically increase the CO2/CH4 selectivity (>600) of the CMS membranes but was detrimental to the CO2 permeance. On the other hand, using a helium purge gas instead of a vacuum environment during pyrolysis did increase CO2 permeance but resulted in a significant loss of CO2/CH4 selectivity. Shortening the thermal soak time at the final pyrolysis temperature was the most effective approach to increasing the CO2 permeance while maintaining the CO2/CH4 selectivity. 1. Introduction Many membrane-based gas separation industries (natural gas processing, landfill gas recovery, olefin/ paraffin separation, air separation, etc.) continue to seek robust membrane materials with higher selectivities and permeabilities. Current polymeric membrane materials have seemingly reached a limit in the productivity-selectivity tradeoff despite concentrated efforts to tailor polymer structures to change separation properties.1,2 Carbon molecular sieve (CMS) membrane materials are attractive alternatives because they offer very high selectivities and productivities and have advantages in high-temperature and high-pressure applications. Because of their high internal surface areas and adsorptive capacities, carbon materials are not new to gas separation applications and have been used successfully in adsorption processes where kinetic separation can be achieved through diffusivity differences of gases.3-6 However, there is ongoing research into the use of carbon molecular sieves as size- and shapeselective membrane materials that can capitalize on separations of differently sized gas molecules. CO2/CH4 separation, which is industrially important in natural gas processing, is one such application and is the focus of this work. Previous work in the literature has examined various gas separations using carbon molecular sieve membranes, such as O2/N2, H2/N2, CO2/N2, and more recently CO2/CH4 separation. However, most of the CO2/CH4 studies deal with pure-gas permeation experiments, use * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 512-471-5866. Fax: 512-4719643.

dense, flat membranes, and/or are predominantly performed at low pressures (generally less than 200 psia). For industrial applications, it is important to examine the mixed-gas environment including interactions with other components in natural gas streams and to evaluate performance at relevant elevated pressures (up to 1000 psia). With these objectives, this work considers the high-pressure separation of CO2/CH4 mixtures using asymmetric carbon molecular sieve hollow fiber membranes synthesized from the pyrolysis of two polyimide precursor fibers, namely, 6FDA/BPDA-DAM and Matrimid 5218. Additionally, the asymmetric fiber geometry of the CMS fibers enables sufficiently high fluxes to evaluate performance in aggressive environments that could potentially decrease permeation. 2. Background Carbon molecular sieves (CMSs) are highly porous materials and possess a distribution of pore sizes with constricted, ultramicroporous pore openings having dimensions that are of the same order of magnitude as the molecular sizes of gas molecules. As a consequence, the porous nature of carbon molecular sieves provides an explanation for their capacity for high gas permeabilities, yet their molecular-sieving morphology permits precise discrimination of gas penetrants to yield highly selective membranes. As expected, permeation through CMS membranes is accomplished by the adsorption of gas molecules and activated transport through the selective pore openings.7 Mass transfer through CMS membranes has been analyzed in the literature with various models (e.g., the surface barrier and pore resistance models8-10) to correlate experimental singlecomponent and multicomponent diffusitivies.5,11

10.1021/ie010119w CCC: $22.00 © 2002 American Chemical Society Published on Web 08/30/2001

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Recently, nonequilibrium molecular dynamics simulations of pure components and gas mixtures have provided additional insight into the importance of both the pore width and length on the morphology of CMS membranes.12-14 Also, in comparing gas transport in CMS membranes with that in polymers, further elucidation has been provided by separating diffusion selectivity of gas penetrants through these materials into enthalpic and entropic contributions. With this fundamental framework, the role of enhanced entropic selectivity in CMS membranes has been quantified and shown to provide a significant advantage over polymers because of the ability of CMS membranes to restrict the degrees of freedom of gas penetrants.15,16 In conjunction with the theoretical and modeling work, numerous permeation studies with CMS materials from both natural and synthetic sources have been performed.17 However, recent studies have shown good separation and mechanical properties with CMS membranes prepared through the pyrolysis of polymeric precursors, such as polyimides and polypyrrolones. Early work with polymer precursor-derived CMS membranes demonstrated high selectivities for O2/N2 separation.18-23 Subsequent work by various researchers revealed a range of CMS morphologies and permeation properties, depending on the starting polymer precursor material [polyimide, polypyrrolone, poly(furfuryl alcohol), polyetherimide, etc.], the precursor geometry, and the pyrolysis conditions.11,23-26 With these varying factors, CMS membranes derived by pyrolyzing polymeric precursors offer a very desirable feature in that the final CMS morphology and microstructure can be tailored to result in the desired permeation properties.27 Generally, pyrolysis results in an amorphous carbon material that exhibits a distribution of micropore dimensions with only short-range order of specific pore sizes and also contains pores larger than the ultramicropores required for the molecular-sieving process. These larger pores connect the ultramicropores that perform the molecular-sieving process and allow for high productivities. The pore sizes of the final CMS membrane tend to decrease with increasing final pyrolysis temperature. Higher final pyrolysis temperatures also tend to produce more selective membranes, whereas lower final pyrolysis temperatures yield membranes with higher fluxes.28 Moreover, recent work has demonstrated that CMS membranes with specific pore sizes designed for a particular separation might not be applicable for another, but can be retailored with a different heating cycle to obtain the desired pore sizes for the required separation.27 In general, most CMS membranes have also been pyrolyzed in inert atmospheres, such as argon and nitrogen. However, vacuum pyrolysis was found to produce more selective, although less productive, CMS membranes.28 Although there are numerous studies on CMS membranes with air separation,29-33 CO2/CH4 separation is becoming increasingly important. Kita et al.24 found high permeabilities and CO2/CH4 selectivities (>40) for unsupported pyrolyzed polypyrrolone films (under N2 atmosphere) tested at 35 °C and 1 atm. Centeno and Fuertes34 prepared flat composite CMS membranes via vacuum pyrolysis (700 °C) of a thin phenolic resin on a disk support. With 10% CO2/90% CH4 gas mixtures at 1 bar, they report CO2/CH4 permselectivities of 140160 with CO2 permeance of around 7 GPU [1 GPU ) 10-6 cm3 (STP)/(s cmm2 cmHg)] at 25 °C. They also

report a higher permselectivity for the mixture (a factor of 2) than for pure-gas experiments. Hayashi et al.35 pyrolyzed 3,3′,4,4′-biphenyl tetracarboxylic dianhydride4,4′-oxydianiline polyimide films on porous alumina supports under nitrogen gas for temperatures up to 500-900 °C. They cite CO2/CH4 selectivities of about 100 with CO2 permeances of about 6 GPU at 30 °C and 15 psi feed pressure. Sedigh et al.14 also formed supported CMS membranes by pyrolyzing poly(furfuryl alcohol) films on flat and tubular ceramic supports to 600 °C under an argon atmosphere. They found CO2/CH4 selectivities in the 34-37 range and CO2 permeances of about 130 GPU with single-gas experiments and feed mixtures under a 30 psi pressure gradient at 20 °C. Later, they examined polyetherimide tubular supports as the precursor and found CO2/CH4 selectivities as high as 150 with equimolar gas mixtures with transmembrane pressure differences of 20 psi at 20 °C.36 They found that the CO2/CH4 selectivity was higher in the binary CO2/ CH4 mixture than in pure-gas experiments, attributing the enhancement to pore blocking by adsorbed CO2 molecules. In addition to the work with supported and unsupported CMS films previously discussed, some work has also been done with asymmetric CMS fibers in CO2/CH4 separation. After pyrolyzing asymmetric aromatic polyimide fibers under vacuum, Jones and Koros23 performed limited studies showing CO2/CH4 permselectivities of around 140 with CO2 permeances of 50 GPU for an equimolar feed mixture at 150 psig and 25 °C. Kusuki et al.37 pyrolyzed polyimide fibers prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and various aromatic diamines to about 600-1000 °C under nitrogen. They found CO2/CH4 selectivies of about 60 and CO2 permeances of about 100 GPU with puregas permeation at about 200 psia feed pressure and 50 °C. Ogawa and Nakano,38 also using CMS fibers at 25 °C and a pressure of 23 psia, reported CO2/CH4 permselectivities of between 40 (with a CO2 permeance of 95 GPU) and 200 (with a CO2 permeance of 4 GPU). They also found that the CO2/CH4 permselectivity was slightly greater in mixed-gas feeds than pure-gas studies because of the decreased CH4 permeance. 3. Experimental Section As previously mentioned, carbon molecular sieves (CMSs) can be obtained from a variety of natural and synthetic sources, with pyrolysis of polymeric precursors being one of the most attractive options for forming selective gas-separating membranes. This section describes our procedure for producing CMS hollow fibers via pyrolysis and making minimodules for permeation testing, as well as our permeation and characterization techniques. 3.1. Preparation of Carbon Molecular Sieve Hollow Fibers. The CMS hollow fibers used in this study were formed by the pyrolysis of integrally skinned asymmetric polyimide fibers. Polyimides are rigid, highmelting, high-Tg, thermally stable polymers formed by the condensation reactions of dianhydrides with diamines. Because of their high thermal stabilities, polyimides are used in machined devices and other hightemperature applications.39 Two different aromatic polyimide fibers were used, namely, 6FDA/BPDA-DAM and Matrimid 5218. Their chemical structures and properties are shown in Table 1. The structure of the

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Figure 1. Scanning electron micrographs of polymer precursors: (a) 6FDA/BPDA-DAM fiber and (b) Matrimid 5218 fiber. Table 1. Chemical Structures and Properties of Aromatic Polyimide Hollow Fiber Precursors Used for Pyrolysis

6FDA/BPDA-DAM polyimide consists of one diamine (DAM) for each dianhydride (6FDA or BPDA) in alternating fashion with a diamine-to-dianhydride ratio of 1:1. The 6FDA/BPDA-DAM polyimide was spun into asymmetric hollow fiber form by Medal, L. P. (Newport, DE). The asymmetric structure describes a structure consisting of a thin (1000-2000 Å) selective skin layer surrounding a porous core layer. The other polyimide precursor, Matrimid 5218, is a commercially available polyimide from Vantico, Inc. (Brewster, NY) that is used in structural composites and adhesives. The Matrimid

5218 polyimide was spun into asymmetric hollow fiber form by a fellow researcher in our group using a dryjet, wet-quench spinning apparatus.40,41 Scanning electron micrographs with cross sections of these precursor fibers are shown in Figure 1. The inside and outside diameters of the Matrimid 5218 fibers were approximately 135 and 250 µm, respectively, while those of the 6FDA/BPDA-DAM fibers were approximately 160 and 250 µm, respectively. The polymeric precursors (6FDA/BPDA-DAM and Matrimid 5218) were pyrolyzed in a quartz tube furnace.

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Figure 2. Apparatus for pyrolyzing polymer precursor fibers: (a) pyrolysis furnace oven with quartz tube and (b) stainless steel mesh support and copper bus wire used to secure fibers during pyrolysis.

A diagram of the apparatus is shown in Figure 2a. The quartz tube measured 50-mm i.d. × 54-mm o.d. × 45in. length with a stopcock connection at one end and a flat O-ring joint at the other end. The polymer precursor fibers (typically 6-8 fibers and approximately 30 cm in length) were supported on a stainless steel (316) wire mesh (size 14, MacNichols, Houston, TX) and loosely bound lengthwise with bus wire (24 AWG, soft bare copper), as shown in Figure 2b. After the precursor fibers were placed inside the quartz tube, the quartz tube was connected with a Pyrex glass tube attachment (50-mm i.d. × 57-mm o.d. × 12-in. length) with a stopcock connection at one end and a flat O-ring joint at the other end. The Pyrex glass tube was connected to the quartz tube by mating the flat O-ring joints with a Viton O-ring and a pinch-type clamp. Both the quartz tube and the Pyrex glass tube were custom-made by Heraeus Amersil (Austin, TX). The heating zone of the quartz tube (approximately 18 in.) was completely enclosed by a hinged split tube furnace (Thermcraft, Inc., model 23-24-17H, WinstonSalem, NC), which had a 3-in. i.d., a 24-in. length, and a maximum temperature rating of 1204 °C (2200 °F). The pyrolysis can be generally effected over a wide range of temperatures between the decomposition temperature of the polymer material and the graphitization temperature (about 3000 °C). Typically, pyrolysis temperatures can range from 500 to 1000 °C in inert or vacuum environments. The specific structural morphology and carbon composition can be controlled with three critical variables: the temperature set points, the rate at which these temperature set points are reached (“ramp”), and the amount of time maintained at these set points (“soak”). The heating protocol was implemented by a programmable temperature controller

(Omega Engineering, Inc., model CN-2010 series, Stamford, CT) using a type K thermocouple sensor to the tube furnace. For the CMS membranes reported in this paper, two different pyrolysis protocols were used: protocol A (start at 50 °C, heat to 250 °C at a rate of 13.3 °C/min, then heat to 535 °C at a rate of 3.85 °C/ min, and then heat to 550 °C at a rate of 0.25 °C/min; maintain the 550 °C temperature for 2 h) or protocol B (start at 50 °C, heat to 250 °C at a rate of 13.3 °C/min, then heat to 785 °C at a rate of 3.57 °C/min, and then heat to 800 °C at a rate of 0.25 °C/min; maintain the 800 °C temperature for 2 h). After the heating cycle was completed, the system was typically allowed to cool under ambient conditions over a 16-24-h period. The CMS membranes were removed once the system temperature was below about 40 °C. The pyrolysis system can function in two different modes of operation: vacuum service or inert purge gas service. For vacuum pyrolysis, the quartz tube chamber was evacuated to less than 0.05 mmHg with vacuum being maintained throughout the pyrolysis. A McLeod mercury vacuum gauge (model 8726-06, Ace Glass, Inc., Vineland, NJ) was connected to the stopcock end of the quartz tube and was used to measure the pressure (see Figure 2). Vacuum was pulled and maintained on the system with a rotary high-vacuum pump (model E2M5, BOC Edwards, Wilmington, MA) equipped with a liquid nitrogen trap and an alumina-filled foreline trap (model FL20K, BOC Edwards) to prevent mechanical pump oil backstreaming. For inert gas pyrolysis, ultrahigh purity gas (O2 < 1 ppm) was supplied by commercial compressed gas cylinders with flow rates controlled by gasspecific mass flow controllers (model 1159A, MKS Instruments, Inc., Burlington, MA) connected to a digital readout and mainframe controller (MKS Instru-

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Figure 3. Module construction for single CMS fiber.

ments, Inc., model 247C). Before an inert gas pyrolysis was performed, the quartz tube chamber and the gas feed line were first evacuated in the same manner as for vacuum pyrolysis. With the Pyrex glass tube stopcock closed, the tubing to the vacuum pump was disconnected, and the quartz tube chamber was filled with the inert purge gas to create positive pressure in the chamber. After the stopcock was opened, the flow rate was measured by a bubble flow meter. The flow rate was adjusted by changing the settings on the mass flow controller. 3.2. Construction of Membrane Testing Module and Permeation Testing. After pyrolysis, the CMS fibers were removed and potted into modules for permeation testing. For laboratory-scale permeation experiments, double-ended hollow-fiber modules were constructed. This design has been used in a number of prior studies by our group but only recently reported in the open literature.42 The module consisted of brass and 316 stainless steel (316 SS) Swagelok and NPT fittings, including two Swagelok 1/4-in. union tees (316 SS), one 1/ -in. port connector (316 SS), two adapters for 1/ -in. 4 4 NPT female to 1/4-in. tubing (brass), two adapters for 1/ -in. NPT male to 1/ -in. tubing (brass), four 1/ -in. nuts 4 4 4 with ferrules (brass), and two 1/4-in. nuts with ferrules (316 SS). The material selection was based on the reusability and cost of the parts. Both the port connector and the two union tees were reusable for future module housings; hence, 316 stainless steel was chosen for its durability. However, the NPT fittings at the ends were not reusable, so inexpensive brass fittings were used. The module housing was first constructed with the two Swagelok 1/4-in. union tees (316 SS), 1/4-in. port connector (316 SS), two adapters for 1/4-in. NPT female to 1/4-in. tubing (brass), two 1/4-in. nuts with ferrules (brass), and two 1/4-in. nuts with ferrules (316 SS), as shown in Figure 3. Modules can be formed in either a multifiber bundle or a single-fiber form. Because of the limited supply of CMS fibers, single-fiber modules were constructed. The potting (or sealing) compound was a general-purpose epoxy encapsulant (Stycast 2651 from Emerson & Cuming, Billerica, MA) that has excellent adhesion to a wide range of substrates and adequately

low viscosity to flow between fibers for potting. In addition, the epoxy material after application had high tensile strength (>6500 psi) for high-pressure applications and a relatively high upper temperature tolerance (130 °C). The hollow fiber carbon membrane (approximately 30 cm in length) was threaded through the minimodule housing, so that a length (approximately 7-8 cm) of carbon fiber extended on each end. Before addition of the epoxy, the fiber was secured into the module by use of plastic holders. The plastic holders were obtained by cutting small hemispherical sections from the spherocylindrical bulbs of standard 6-in. plastic, disposable transfer pipets (Fisherbrand). Next, a needle was used to pierce a small hole in the center of each hemispherical plastic section. The carbon fiber was threaded through the small aperture of the plastic section. The aperture should be small enough such that it can shrink (i.e., “heal”) to conform to the diameter of the fiber after threading. The plastic holder should prevent the epoxy from dripping down the fiber. The carbon fiber with the plastic section attached at one end was slipped through the module housing until the hemispherical plastic section (concave up) rests outside on top of the 1/4-in. NPT female fitting. Forceps were used to gently push the plastic section down so that it rested firmly and conformed tightly to the bottom curvature of the 1/4-in. NPT female fitting. Care was taken to avoid breaking the carbon fiber as the plastic section was pushed down the NPT female fitting. The end of the fiber with the plastic section extended approximately 7-8 cm beyond the fitting. On the other end of the module housing, the carbon fiber was threaded through another hemispherical plastic section. Forceps were again used to secure the plastic section to the bottom curvature of the 1/4-in. NPT female fitting. The potting procedure (i.e., application of the epoxy) is shown in Figure 3. The Stycast 2651 epoxy encapsulant (resin) was mixed with Catalyst 9 (Emerson & Cuming) [7 parts Catalyst 9 per 100 parts Stycast 2651 (by weight)]. After being mixed for approximately 3-5 min, the epoxy was poured into the cavity of the 1/4-in. NPT female fitting at one end of the module. Next, the

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Figure 4. High-pressure permeation system. 1/ -in. 4

NPT male to 1/4-in. tubing (brass) adapter was used. A 1/4-in. brass nut and a 3-cm piece of 3/16-in. i.d. Tygon tubing was attached to the brass tubing end of the NPT adapter. The NPT adapter was threaded through the fibers and tightened into the 1/4-in. NPT female fitting. The adapter was tightened to the desired level with the epoxy extruding upward but contained by the Tygon tubing. A period of 1 day was necessary for the mix to cure at room temperature. The other end was potted in the same manner. To hasten the curing process, the epoxy was dried in an oven at 65 °C for 2 h after epoxy was applied to each end. Finally, the epoxy was cured more extensively at 65 °C under vacuum for at least 12 h. After the epoxy hardened, the Tygon tubing and the ends of the carbon fiber membrane were snapped off by hand by applying a small transverse force, resulting in a level epoxy surface. For the permeation experiments discussed in this paper, the CMS modules were tested in a pressure-rise permeation system with shell-side feeding. A diagram of the permeation testing apparatus is shown in Figure 4. The system was constructed to permit high-pressure testing of mixed feed gas and sampling of gas streams with a gas chromatograph. The module and ballast volumes are placed in a circulating water bath to control and maintain a constant temperature. The downstream (bore-side) of the CMS hollow fiber membrane module was maintained under vacuum. The permeation rate was measured from the pressure rise of a Baratron pressure transducer connected to digital readout display (MKS Instruments, Burlington, MA) using the known downstream (permeate) volume. The pressure rise was plotted on a chart recorder. The compositions of all of the streams were determined by a gas chromatograph (Hewlett-Packard, model 5890A) with a thermal conductivity detector. The plumbing of the system consisted of stainless steel Swagelok and VCR fittings, Whitey and Nupro valves, and stainless steel tubing with welded elements. The apparatus was rated for over 1500 psia pressure. The CMS hollow fiber membrane performance was quantified by the separation factor (or selectivity) and

the permeance of the components. The separation factor (or selectivity) was determined from the compositions of feed and permeate streams (obtained from gas chromatography analysis)

RA/B )

( )( ) yA xB yB xA

(1)

where y and x denote the mole fractions of components A and B in the permeate side and feed side, respectively. With mixed-gas feeds, permeances were typically calculated using the component fluxes and the partial pressure (or fugacity) driving force

permeance t

Pi Ni ) l ∆pi

(2)

where Pi is the permeability of component i, l is the thickness, Ni is component i’s flux, and ∆Pi is the partial pressure (or fugacity) driving force of component i. Permeance is typically expressed in gas permeation units [1 GPU ) 10-6 cm3 (STP)/(cm2 s cmHg)]. Component fluxes were calculated from the pressure rise measurements using the ideal gas law and known permeate compositions, permeate volume, and membrane area. However, the pressure driving force can vary along the length of the fiber. On the shell side (feed) of the membrane module, the flow rate of the retentate stream was regulated by a metering valve, which was set to obtain less than a 1% stage cut (ratio of permeate flow rate to feed flow rate) during steadystate operation. The 1% stage cut was a reasonable criterion to ensure that the feed composition did not vary along the membrane length. Because of the small permeate fluxes of the single-fiber membrane module, the retentate flow rate was very small to meet the 1% stage cut criterion, and it can be assumed that shellside pressure drop was negligible. The pressure drop for a single fiber in the tube side of a permeable tube can be reasonably approximated for the laminar flow regime by the expression43-45

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d(pT2) 25.6RTµ QT ) dz πd 4 22 400

(3)

i

Table 2. Dimension Measurements ((5 µM) of Precursor Fiber and Resulting CMS Fiber of 6FDA/BPDA-DAM and Matrimid 5218. 6FDA/BPDA-DAM

where pi is the pressure in the tube side, R is the ideal gas constant (8.3145 J mol-1 K-1), T is the absolute temperature (in K), µ is the viscosity (in µP), di is the inner diameter of the membrane fiber, and QT is the flow rate in the tube side [in cm3 (STP)/s]. To calculate the permeance of each component, a finite element analysis was performed on a spreadsheet, where each discrete axial element along the fiber had a separate pressure driving force and flow rate. From eq 3, the pressure drop for element j along the single fiber was calculated using

pT(j) )

x

pT2(j - 1) -

25.6RTQT(j) ∆z 22 400πdi4

(4)

where ∆z is the differential length of each element. Because the exit permeate pressure was known, an iterative technique (e.g., Solver in Excel) was used to determine the permeate pressure profile by varying the midpoint axial permeate pressure until the calculated exit permeate pressure agreed with the measured exit permeate pressure within the specified error tolerance (