SUPPORTED NANOPOROUS CARBOGENIC GAS SEPARATION MEMBRANE AND PROCESS FOR PREPARATION THEREOF

Abstract: A supported nanoporous carbon membrane (SNPCM) exhibiting improved gas separation performance and a novel method for preparation thereof is described. The supported nanoporous carbon membrane is formed by the ultrasonic atomization and deposition of a thin, uniform layer of furfuryl alcohol resin, in an acetone solvent, onto a porous support, evaporating the acetone, and pyrolyzing the furfuryl alcohol resin.

Description: TITLE SUPPORTED NANOPOROUS CARBOGENIC GAS SEPARATION MEMBRANE AND PROCESS FOR PREPARATION THEREOF FIELD OF THE INVENTION This invention relates to a supported nanoporous carbon membrane (SNPCM) exhibiting improved gas separation performance and a novel method for preparation thereof. The supported nanoporous carbon membrane is formed by: ultrasonically atomizing a solution of poly (furfuryl) alcohol resin in an acetone solvent; depositing a thin, uniform layer of the furfuryl alcohol resin solution onto a porous support; evaporating the acetone; and pyrolyzing the furfuryl alcohol resin. TECHNICAL BACKGROUND OF THE INVENTION Inorganic membranes offer potential for high temperature gas separations and membrane reactor applications. Zeolites, silica, and carbon molecular sieves (CMS) are potential candidates for making these membranes. Zeolites have been difficult to synthesize into crack-free membranes and have been shown to have poor thermal stability. Silica membranes, synthesized in a clean-room environment to minimize defects, have achieved an O 2 N 2 separation factor of 3.9 at 100°C (see De Vos, R.M. and Verweij, H., "High-Selectivity, High-Flux Silica Membranes for Gas Separation" Science, 1998 vol. 279, pp. 1710-1711). Carbon molecular sieves have been prepared by controlled pyrolysis of natural and synthetic precursors, such as wood and coconut shells (see Vyas S.N., Patwardhan, S. R., Vijayalakshmi, S., SriGanesh, K., "Adsorption of gases on Carbon Molecular Sieves" Journal of Colloidal Interface Science, 1994. 168, pp. 275-280) as well as synthesized using a variety of different polymeric resins. Fitzer, E. and Schaefer, W., "The Effect of Crosslinking on the Formation of Glasslike Carbons from Thermosetting Resins" Carbon, 1970. vol. 8, p. 353 reported the pyrolysis of polyfurfuryl alcohol to form a glasslike carbon, but did not address the issue of forming membranes. Walker Jr., P. L., "Carbon - An Old but New Material" Carbon, 1972. vol. 10, p. 369 described a variety of structures and properties for carbon precursors, such as electrode carbon, glassy carbon, molecular sieves, carbon black, pyrolytic graphite, and carbon fibers. Polymers like poly (vinyl chloride) (PVC) are known to form graphitic layers at 1000°C; however, other polymers such as poly(acrylonitrile) (PAN) and poly(furfuryl) alcohol (PFA) form a complex microstructure, with a large concentration of pores in the 3 to 10 angstrom region at much lower temperatures. For gas separation of small molecules this range of microporous diameters is ideal. Foley, H. C, "Carbogenic Molecular Sieves: Synthesis, Properties and Applications" Microporous Materials, 1995. vol. 4, p. 407-433 reported the formation of carbogenic molecular sieves by pyrolyzing a layer of polyfurfuryl alcohol which had been coated (brushed) onto a planar porous metallic support. Carbon molecular sieves can be thought of as consisting of graphite-like planes. The carbon planes are stacked similar to graphite and were first characterized with small-angle X-ray scattering (see Franklin, R. E., "The Interpretation of Diffuse X-ray Diagrams of Carbon" Acta Crystallographica., vol. 3, p. 107, 1950). A hypothetical, ribbon-like structure has been proposed consisting of disordered planes of carbon atoms (see Jenkins, G. M. and Kawamura, K., Polymeric Carbons, Cambridge University Press: Cambridge, MA 1976). This structure hypothesis is also corroborated by high resolution transmission electron microscopy (HRTEM) images combined with fast Fourier transform (FFT) analysis to determine the spacing between the graphitic layers (see Kane, M. S., Goellner, J. F., Foley, H. C, DiFrancesco, R., Billinge, S. J. L., Allard, L. F, "Symmetry Breaking in Nanostructure Development of Carbogenic Molecular Sieves: Effects of Morpohological Pattern Formation on Oxygen and Nitrogen Transport" Chemistry of. Materials., 1996. vol. 8 pp. 2159-2171.). The disordered structure of carbon molecular sieves results in a pore size distribution which can be controlled by varying synthesis parameters such as temperature and time (Lafyatis, D. S., Tung, J., Foley, H. C, "Poly(furfuryl) alcohol-Derived Carbon Molecular Sieves: Dependence of Adsorptive Properties on Carbonization Temperature, Time and Poly(ethylene glycol) Additives" Industrial Engineering for Chemical Research, 1991. vol. 30, pp. 865-873). Due to this pore size distribution, all transport mechanisms are present, including bulk, Knudsen, surface, and configuration diffusion. The challenge is to minimize the number of pores with diameters larger than 20 angstroms, which significantly reduces both bulk and Knudsen diffusion and provides a molecular sieving membrane. Two types of membranes have been synthesized from organic precursors, which includes unsupported or "hollow" fiber and supported or "asymmetric" membranes (see Koresh, J. E. and Soffer, A., "Mechanism of Permeation through Molecular Sieve Carbon Membrane," Journal of the Chemical Society, Faraday Transactions. 1, 1986. vol. 82, pp. 2057-2063). The unsupported membranes have the disadvantage that they lack significant mechanical strength for practical application. Asymmetric membranes have been prepared on a variety of supports, including porous metals, graphite, ceramics and glasses. Both chemical vapor deposition and plasma deposition, as well as conventional coating and dipping techniques, have been reported. Carbon membranes supported on macroporous supports such as porous metals, porous ceramics, porous glasses, or porous composite materials are believed to be advantageous because the supports may be obtained in a variety of sizes, shapes, and porosity, the supports may be readily fabricated and joined, and are relatively inexpensive. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a pictorial view showing an apparatus for coating a porous tubular support. Figure 2 is a pictorial view showing a first apparatus for pyrolyzing the coating on the porous tubular support. Figure 2A is a pictorial view showing a second apparatus for pyrolyzing the coating on the porous tubular support. Figure 3 is a table showing the synthesis temperature and carbon coating results for Examples 1-12. Figure 4 is a table showing the pure component gas permeances and separation factors for Examples 1-12. Figure 5 is a pictorial view, partially in section, showing a membrane testing module. Figure 6 is a plot showing the rise in pressure versus time (at 22°C) for Example 4. Figure 7 is a plot showing the permeance of nitrogen, oxygen, and helium gases through Example 4 as a function of core side pressure. SUMMARY OF THE INVENTION The invention concerns a nanoporous carbogenic membrane, supported on a porous substrate, comprised of a plurality of layers of poly (furfuryl) alcohol resin pyrolyzed in an inert or reactive atmosphere, each layer being no more than 10 microns in thickness and having a weight after pyrolysis of no more than 10 milligrams per square centimeter. The invention further concerns a method of forming a supported nanoporous carbon membrane, comprising the steps of: (a) preparing a solution of poly(furfuryl) alcohol resin in an acetone solvent; (b) ultrasonically atomizing the solution and depositing a thin, uniform layer of less than 25 mg per cm 2 of the poly(furfuryl) alcohol resin/acetone solution onto a porous support; (c) drying the resin/acetone layer by evaporating the acetone; (d) pyrolyzing the poly(furfuryl) alcohol resin in an inert or reactive atmosphere. The invention further concerns a carbogenic molecular sieve membrane, supported on a porous substrate, having improved gas separation performance, wherein the membrane is formed by the steps of: (a) forming a tube assembly by joining one or more segments of porous tubes between two or more segments of nonporous tubes; (b) coating the porous segments of the tube assembly with a solution of poly(furfuryl) alcohol resin in an acetone solvent by ultrasonically atomizing the solution into droplets of substantially uniform size between about 0.1 and 10 microns and depositing a thin, uniform layer no more than about 20 microns in thickness and having a wet weight of no more than about 25 milligrams per square centimeter of the poly(furfuryl) alcohol resin/acetone solution onto the porous support; (c) drying the resin/acetone layer by evaporating the acetone; (d) pyrolyzing the poly(furfuryl) alcohol resin in an inert or reactive atmosphere to form a membrane layer which is no more than about 10 microns in thickness and has a weight after pyrolysis of no more than about 10 milligrams per square centimeter; and (e) repeating steps (b)-(d) one or more times to form a multi-layer membrane. Optionally, step (c) may be performed by either preheating the support or irradiating the coated support with electromagnetic radiation to accelerate the evaporation of the acetone and minimize penetration of the coating into the pores of the support. The pyrolyzing step (d) may be performed either in a furnace or by laser irradiation of the coating. When pyrolyzing the coating in a furnace the temperature of the furnace is first increased at a predetermined rate and then the temperature is held constant within the range from about 150°C to 800°C for a time ranging from 0 minutes to 480 minutes and then is cooled to room temperature. When pyrolyzing the coating with laser irradiation the laser beam may focussed onto a small area of the tube assembly and the tube assembly rotated while the laser beam delivery optics traverse the focussed area along the axial direction. The laser power output, the size of the focussed area, the rotation speed of the tube assembly and the axial traverse rate may be controlled to achieve the desired pyrolysis. DETAILED DESCRIPTION OF THE INVENTION Membrane Preparation The method of preparing the membrane of the present invention comprises a first step of ultrasonic deposition of PFA onto a suitable porous support. Metal, ceramic, glass, or composite material supports have been found suitable for the membrane support. A first embodiment of the membrane uses a metal support, such as sintered stainless steel (SS304) tubing, such as that manufactured by Mott Metallurgical Corp., of Farmington, CT. A preferred porous tubing has an outside diameter of 6.35 mm with a wall thickness of 1.48 mm and a nominal porosity of 0.2 μm. The porous tube was first cut to a length of 25 mm using a lathe to form a porous segment 2. Solid (i.e., nonporous) stainless steel (SS304) tubing 4, 6 was cleaned and then joined by welding to both ends of the porous tube 2 to form a tube assembly 10 (seen in Figure 1). Tube assemblies could comprise one or more porous segments 2 and two or more solid segments 4,6. The tube assembly 10 was typically cleaned for 15 minutes in an ultrasonic bath of 1,1,2-trichlorotrifluoroethane (Freon® TF) to remove any cutting oils and then dried in an oven for at least 2 hours at about 120°C. After cleaning, the tube assembly was handled in a manner to prevent contamination (such as with Nitrile® gloves) and were stored in a dehumidified chamber until coated. Neat PFA resin, such as that available from Monomer Polymer & Dajac Laboratories, Inc. (Lot A-l-143) was diluted with reagent grade acetone, such as that available from J. T. Baker. A solution 20 of about twenty-five (25) weight percent (wt. %) PFA and seventy-five (75) wt. % acetone was prepared gravimetrically. The solution 20 was vigorously shaken before use to ensure it was well mixed. As may be seen in Figure 1 , a 30 cc syringe 30 was filled with the solution 20 and was delivered at a rate of 1 cc/min. using a syringe pump 32 (Sage Instruments, Model 355) to an atomizer comprising an ultrasonic horn 40. Although a custom fabricated ultrasonic horn 40 and a Dukane Corporation of St. Charles, IL ultrasonic generator 42 were used in the examples reported, a commercially available ultrasonic atomizer and generator, such as that sold as model 06-04029 from Sono-Tek Corporation of Milton, NY would also be suitable. An ultrasonic frequency of forty kilohertz (40 kHz) was used, but other frequencies in the range of 20 kHz to 120 kHz are believed suitable. Ultrasonic deposition of the solution 20 onto the porous tube segment 2 was used to overcome the deficiencies of conventional high-pressure gas spraying. Ultrasonic deposition can achieve a factor of 10 2 - 10 3 smaller droplet size than gas spray deposition, and produces a low momentum spray which minimizes penetration of the droplets into the support. With the present arrangement typical droplet sizes from about 10 μm to about 100 μm were achieved. The syringe pump 32 provides precise control of delivery rate of the solution 20 so that wet film thickness can be controlled. The ultrasonic horn 40 was positioned about six millimeters (6 mm) above the porous metal tube assembly 10 and the tube assembly 10 was rotated about its axis 10A while the ultrasonic horn 40 was traversed in a direction parallel to the axis 10A of the tube. Using a commercially available motor controller 50 and motorized rotation/traverse assembly 52 both the rotational speed of the tube assembly 10 and the axial traverse rate of the ultrasonic horn 40 could be varied to control the deposition rate. A rotational speed of one hundred fifty revolutions per minute (150 rpm), and an axial traverse rate ranging from 1 to 10 mm/sec was found suitable. Coatings of from about 0.1 to 25 milligrams per square centimeter were achieved. After applying the "wet" coat, the tube assembly 10 was rotated for about 10 minutes in air to allow the acetone in the coating to evaporate and then the tube assembly 10 was weighed. Depending on the axial traverse rate, from about 0.5 to 125 mg of PF A/acetone could be applied to each porous tube segment 2 using this technique. The coating step may be performed at substantially room temperature or the tube assembly 10 may be preheated to a temperature in the range of 30-300 degrees Celsius (°C) preferably 100-300°C. Preheating the tube assembly 10 causes the acetone to evaporate at a higher rate and limits the penetration of the resin/acetone solution into the pores of the porous segment 2. The rate of evaporation of the acetone may also be increased by irradiating the coated tube assembly 10 during and/or immediately after the coating step with a source of electromagnetic radiation 56, such as an infrared source. The tube assembly 10 would typically be continuously rotated during such a drying step. The pyrolysis of the coating may be performed in a furnace or by laser irradiation of the coating. In a first pyrolyzing method, as may be seen in Figure 2, the tube assembly 10 was placed inside a 57 mm diameter quartz tube 60. The tube 60 was fitted with end caps 62, 64 made of Pyrex-i designed to hold the coated porous tube segment 2 in the center of the quartz tube 60 and to allow the tube assembly 10 to be rotated by a motor drive 66 while being heated. The quartz tube 60 was placed inside a furnace 65 (Lindberg/Blue model HTF55322C) with a temperature controller/timer 65C (Eurotherm model 2416). The quartz tube 60 was purged at a rate of 100 seem with scientific grade helium 70 (total impurities < 1 ppm), such as that available from MG Industries. The quartz tube 60 was typically purged for 15 minutes to ensure all the air had been removed before heating. The rate of temperature increase was controlled to about 5.0°C/min. and both the soak temperature and soak time were controlled to predetermined values. Soak temperatures from 150°C to 800°C and soak times from 0 to 180 minutes were achieved. Soak times of up to 480 minutes are believed possible. After the soak time had elapsed the furnace 65 was turned off and allowed to cool to room temperature. Throughout the heating and cooling cycle the tube assembly 10 was continuously rotated at about 30 revolutions per minute (rpm) to ensure the coating thereon did not flow during pyrolysis. The pyrolysis protocol followed established methods used for synthesis of unsupported carbon molecular sieves such as that described in Lafyatis, D. S., Tung, J., Foley, H. C, "Poly(ftιrfuryl alcohol)-Derived Carbon Molecular Sieves: Dependence of Adsorptive Properties on Carbonization Temperature, Time and Poly(ethylene glycol) Additives" Industrial Engineering for Chemical Research, 1991. vol. 30, pp. 865-873; or Mariwala, R. K., Foley, H. C, "Evolution of Ultramicroporous Adsorptive Structure in Poly(furfuryl alcohol)-Derived Carbogenic Molecular Sieves" Industrial Engineering for Chemical Research, 1994. vol. 33, pp. 607-615. Each tube assembly 10 was weighed after pyrolysis and typical "dry" carbon weights ranged from about 0.1 to 50 mg per porous tube segment 2 and carbon yield ranged from 15-40% depending on the initial "wet" weight and the pyrolysis conditions. This produced a single-coated nanoporous carbon tubular membrane 10M which was formed by cross linking and carbonization of the polymer. Multiple coats were applied and pyrolyzed before the membrane provided significant molecular sieving. In a second pyrolyzing method, as may be seen in Figure 2A, the tube assembly 10 is placed inside a 57 mm diameter quartz tube 60. The tube 60 is fitted with end caps 62, 64 made of Pyrex® designed to hold the coated porous tube segment 2 in the center of the quartz tube 60 and to allow the tube assembly 10 to be rotated by a motor drive 66 while being heated. In this second pyrolyzing method the furnace 65 and associated temperature controller/timer 65C of the first pyrolyzing method are replaced by a continuous wave CO 2 laser 102 and associated laser beam delivery optics 104. The laser 102 produces an output beam 102B. The beam delivery optics 104 comprises a focussing lens 104L and optional beam expanding optics 104B. The beam 102B passes through the delivery optics 104, through the quartz tube 60 and is focussed by the lens 104L to irradiate a small area 102P of the coated porous tube segment 2. The laser power level and the size of irradiated area 102P are controlled to result in rapid pyrolysis of the coating on the tube segment 2. A motorized traverse assembly 152 and associated controller 150 cause the irradiated area 102P to traverse along the axis if the tube assembly 10 while motor 66 rotates the tube assembly 10. The traverse rate and the speed of rotation of the tube assembly 10, and thus tube segment 2, relative to the laser beam 102B, and thus the irradiated area 102P, are controlled to effect complete pyrolysis of the coating. A second embodiment of the membrane uses a ceramic support. Porous alpha alumina supports, coated with gamma alumina, zirconia or titania, such at that manufactured by U.S. Filter Corporation of Deland, Florida and sold under the trademark Membralox® have been found suitable for the membrane support. For this second embodiment a preferred porous tubing has an outside diameter of 8 mm with a wall thickness of 1 mm and a nominal porosity over the range of 5 μm to 200 Angstroms. The nominal porosity depends upon on the coating. A porosity of 5 μm is typical for the gamma alumina coating, while a porosity of 200 Angstroms is typical for the zirconia coating. The porous tubes are 250 mm long and no cleaning was required before coating. The tubes were handled in such a manner to prevent contamination (such as with Nitrile ® gloves) and were stored in a dehumidified chamber until coated. A third embodiment of the membrane uses a porous glass support. Porous glass, such at that manufactured by Corning, Inc. of Corning, NY under the trademark Vycor® has been found suitable for the membrane support. For this embodiment a preferred porous tubing has an outside diamter of 6 mm with a wall thickness of 1 mm and a nominal porosity of 20 to 40 angstroms. Porous tubes 25 mm long which are attached to 15-20 cm long quartz tubes using a proprietary heat treatment joining technique are commercially available from Corning. No cleaning was required before coating. The tubes were kept in deionized water to prevent contamination and handled with with Nitrile® gloves. A fourth embodiment of the membrane uses a carbon composite material as a support. Porous carbon supports composed of carbon fibers and coated with a proprietary carbon coating, such as that manufactured by KOCH membrane systems of New York, NY under the trade name Carbo-Cor™ are believed suitable for the membrane support. A preferred porous tubing has an outside diameter of 8 mm with a wall thickness of 1 mm and a nominal porosity of 0.01 μm. The porous tubes are 25 to 250 mm long. No cleaning is required before coating. The tubes are handled in a manner to prevent contamination (such as with Nitrile® gloves) and are stored in a dehumidified chamber until coated. A desirable property of the support material is to have porosity less than about 5 μm. Finer porosity of the support generally results in thinner nanoporous carbon membranes that have corresponding improved flux and selectivity for separation of small molecules such as gases. When supports are employed that have a porosity of greater than about 0.1 μm, one or more intermediate layers may be desirable to further reduce the average pore size of the support before coating with polyfurfuryl alcohol. Materials such as titanium dioxide, silica, and colloidal graphite suspended in isopropyl alcohol have been applied to the exterior of the porous supports to form intermediate layers and thus reduce the average pore size (also called support macroporosity) before coating with polyfurfuryl alcohol. Other materials such as silicon dioxide may also be used as an intermediate layer. This intermediate layer minimizes penetration of the polyfurfuryl alcohol into the pores and reduces the thickness of the resulting carbon molecular sieving layer. This has the added benefit of increasing the permeance of the resulting membrane without sacrificing small molecule separation selectivity. Typically these intermediate coatings would be applied by an ultrasonic atomization technique, similar to that described in conjunction with the first embodiment. Optionally, small amounts of polyfurfuryl alcohol may be added to this intermediate layer material as a binder and then this intermediate layer may be fired in a furnace at temperatures between 150 to 800°C. Subsequent coatings of polyfurfuryl alcohol in acetone would then be applied and fired to produce the molecular sieving layer, as previously described in conjunction with the first embodiment. Improved uniformity of membrane layer thickness is believed important. Such improved uniformity has been achieved by the following method: (a) preparing a solution of poly(furfuryl) alcohol resin in an acetone solvent; (b) as shown in Figure 1, positioning an ultrasonic atomizer comprising the ultrasonic horn 40 at a predetermined distance from a tube assembly 10 having an axis 10A, the tube assembly 10 comprising at least one porous tubular support; (c) rotating the tube assembly 10 at a predetermined rotation speed and traversing the ultrasonic horn 40 in a direction parallel with the axis 10A of the tube assembly 10 at a predetermined speed, so that the motion of the ultrasonic horn 40 defines a helical path along the tube assembly, the helical path being defined by an axial distance along the tube assembly and a rotational phase angle with respect to the tube assembly; the rotational phase angle and the axial position of the ultrasonic horn 40 relative to the tube assembly 10 are controlled by motor controller 50 and motorized traverse assembly 52; (d) ultrasonically atomizing the solution 20 and depositing a thin, uniform layer of no more than about 25 milligrams per square centimeter of the poly(furfuryl) alcohol resin/acetone solution onto the porous segments 2 of the tube assembly 10; (e) drying the resin/acetone layer by evaporating the acetone; (f) pyrolyzing the poly(furfuryl) alcohol resin; (g) repeating steps (c)-(f) to form a membrane having a plurality of successive layers, the rotational phase angle of the ultrasonic horn 40 relative to the tube assembly 10 being selected to be different for each successive coating step (c), causing the helical path of the ultrasonic horn 40 for each successive repetition of step (c) to be offset from the previous helical path, so that a membrane of more uniform thickness is achieved. It has been found that minimizing the thickness of each layer of the membrane improves the membrane permeance and minimizes the occurrence of defects in the membrane. It is believed that defects in the membrane are related to areas in the coating which exceed a critical film thickness. The critical film thickness has been determined empirically to be about 20 +/- 3 microns on porous metal supports. It is therefore desirable to employ coating methods that reduce the deposited coating thickness and pyrolysis methods that reduce the thickness of the pyrolyzed layer. It has been found that preheating the porous support (tube assembly 10) to a temperature above ambient before application of the polyfurfuryl alcohol coating is desirable. Preheating the support to a temperature above 30°C, preferably from 100 to 300°C, before application of the polyfurfuryl alcohol coating facilitates faster evaporation of the acetone and is believed to produce a thinner film on the surface of the support with less penetration into the pores of the porous support. It is also believed desirable to pyrolyze the coating as rapidly as possible. Heating the support and the applied coating with a continuous wave (C W) CO 2 laser is believed to produce an almost instantaneous pyrolysis of the coating. It is believed that this rapid pyrolysis results in a thinner film on the surface of the support and minimizes penetration of the coating into the pores of the support. Although inert atmospheres have typically been employed in prior art pyrolysis methods, pyrolysis in a reactive atmosphere is believed to result in different structural forms of nanoporous carbon having desirable ranges of pore dimensions for separation of certain molecular size ranges. Heating carbon particles in a reducing atmosphere (such as hydrogen) or an oxidizing atmosphere (such as oxygen, carbon dioxide, and carbon monoxide) is known to result in different surface chemistries on carbon particles. The use of such reactive atmospheres in the pyrolysis step of the present method is believed to produce different forms of nanoporous carbon structure within the membrane layers. It is believed that control of the range of pore sizes in the membrane may be thus achieved. Permeation Experiments and Analysis The tubular membranes 10M were inserted in a module 70 for testing. As seen in Figure 5, the module 70 consisted of a cylindrical membrane holder 72 with knife-edge flanges 72F on either side and was sealed with copper gaskets 74. The flanged ends 72F were welded to compression fittings 76 (such as Swagelok®) each having a ferrule 78. Metal ferrules were not used since the tube assemblies 10 would be impossible to remove; therefore, for low temperature conditions polymer ferrules 78, such as Nylon® or Teflon® ferrules, were used and for high temperature conditions graphite ferrules 78 were used. With external compression applied to the outside of the fittings, pressures up to 7000 kPa could be maintained with no measurable leakage at 22°C. Pressure rise time experiments were performed on twelve tubular membranes (identified as SNPCM 1, 2, ..., 12) which were synthesized at 150, 300, 450 and 600°C. Each were held at temperature for 0 to 180 minutes and coated two to six times, see Figure 3. Initially, both the core side C and outer shell sides S of the membranes were at atmospheric pressure. A probe gas 80 was introduced on the core side of the supported membrane 10M at a pressure of 300 kPa while the shell side pressure rise was continuously monitored using National Instruments data acquisition hardware 90 (model AT-AO-6/10 and a model AIO-16) and Lab View® data acquisition software. The core side and outer shell side pressures were monitored by MKS Instruments, Inc. model 122BA05000BB pressure transmitters 92 which were accurate to +/- 0.5% of reading over the range of 0 to 667 kPa. The membrane module 70 was evacuated and returned to atmospheric pressure (with air) on both the core and outer shell side before the introduction of the next probe gas. Typically, the probe gases were tested in the following order: nitrogen, oxygen, helium, and hydrogen. All experiments were conducted at 22°C. Figure 6 is a plot of the data measured for example SNPCM 4. Notice the pressure rise time curves are arranged in order of increasing molecular size. The high sensitivity of the membrane to gas molecular size is evident from the fact that there is noticeable separation between nitrogen and oxygen, which differ by only 0.2 Angstroms in size. A simple model was used to describe the unsteady state experiments and calculate the pure gas permeabilities, (see Rao, M. B. and Sircar, S., "Nanoporous carbon membranes for separation of gas mixtures by selective surface flow" Journal of Membrane Science, 1993. vol. 85, pp. 253-264; or Acharya, M., Raich, B. A., Foley, H. C, Harold, M. P., Lerou, J. J., "Metal-Supported Carbogenic Molecular Sieve Membranes: Synthesis and Applications" Industrial Engineering of Industrial Research, 1997. vol. 36, p. 2924-2930). A mass balance for the permeating species from the core to the shell side of the SNCPM can be written: ^ = J . M W . A (1) where m (gm) is the mass of the gas, J is the molar flux (mol/m 2 .sec), M w (gm/gmol) is the gas molecular weight, A (m 2 ) is the membrane area, and t (sec) is time. The flux across the membrane was defined by: J = * (Pcs - Pss) (2) where π' is the gas permeability (mol/m.sec.Pa), Λ is the membrane thickness (m), and P cs and P ss are the pressures (Pa) on the core side and outer shell side of the tubular membrane, respectively. Using the ideal gas law to rewrite the mass of the gas in terms of the shell side pressure, P ss the final expression is obtained: dP ss A - R - T π dt V ss A A * (Pcs ~ Pss) (3) where R (m 3 .Pa/gmol.K) is the gas constant, T (K) is the temperature, and V ss (m 3 ) is the shell side volume. Integrating from the initial shell side pressure, P ss0 at time t = 0 to the final shell side pressure, P ss at t = t provides our final expression: v ss , l p cs - p ssOi π' A - R - T * !" ι Ppcs _ " p p ss il = T Λ * t ( 4 ) A plot of the left-hand-side of equation (4) versus time should give a straight line with a slope of π'/Λ = π 0 which is called the gas permeance. The permeances are provided in Figure 4 along with the ratio of the permeance with respect to N2 which provides the ideal separation factors. It was assumed in the integration of equation 3 that πo was independent of pressure. The permeances for SNPCM 4 were measured as a function of pressure from 300 to 7000 kPa and determined to be substantially independent of pressure, see Figure 7. Scanning electron microscope (Hitachi S4000 SEM) images taken both of the exterior surface as well as cross and axial sections of a similarly prepared SNPCM at 573 K reveal a defect-free membrane with a uniform radial thickness of about 16 +/- 3 μm. The thickness along the axis varied in some areas possibly due to the manual movement of the ultrasonic horn during coating. Finally, a steady state gas separation experiment was run with SNPCM 4 with air (MG Industries, scientific grade) fed to the core side at a pressure of 7000 kPa and a flow rate of 150 seem. A 5 seem helium purge (MG Industries, scientific grade) was used as a sweep gas on the shell side of the supported membrane. Samples were taken manually using a gas tight syringe from the feed and permeate sides of the membrane and analyzed using a Hewlett Packard gas chromatograph (model HP5890), a Supelco molecular sieve column (60/80 mesh, molecular sieve 5A, 10' x 1/8", column 256398-10), and a thermal conductivity detector (TCD). The feed contained 21.0-21.1 vol. % oxygen while the permeate contained between 41.5-44.0 vol. % oxygen. The supported nonoporous carbon membrane described herein exhibited improved gas separation performance. Permeation measurements with pure gases such as nitrogen (N 2 ), oxygen (O2), helium (He), and hydrogen (H 2 ) reveal a molecular sieving behavior with permeation decreasing with increasing molecular size. Gas separation factors ranging up to about 30 for O2/N2; up to about 178 for He/N2; and up to about 330 for H2/N2 were measured in single gas permeation experiments at 22°C. The separation factors and permeation values, which ranged from 2.7 x 10" 14 to 4.1 x 10 ~8 mol/m 2 .Pa.sec, were found to depend on the amount of carbon deposited, the pyrolysis temperature, and the pyrolysis soak time. The pure component permeance values were found to be independent of pressure from 300 to 7000 kPa indicating shape and size selective effects dominate the separation. Scanning electron microscope (SEM) images of the surface reveal a defect-free membrane. A high pressure air feed was continuously separated with a permeate composition containing 41.5 to 44 volume percent (vol. %) oxygen.

Claims: What is claimed is: 1. A nanoporous carbogenic membrane, supported on a porous substrate, comprised of a plurality of layers of poly(furfuryl) alcohol resin pyrolyzed in an inert atmosphere, each layer being no more than 10 microns in thickness and having a weight after pyrolysis of no more than 10 milligrams per square centimeter. 2. The membrane of Claim 1, exhibiting a gas separation factor greater than 8 for O2 N2; a separation factor greater than 29 for He/N ; and a separation factor greater than 58 for H2 N2 at a temperature of 22°C. 3. The membrane of Claim 1 wherein the membrane has a permeation value between 10 -14 and lO -8 mol/m 2 .Pa.sec. 4. The membrane of Claim 3 wherein the membrane has a permeation value between 2.7 x 10 -14 and 4.1 xlO -8 mol/m 2 .Pa.sec. 5. The membrane of Claim 1 wherein the membrane is substantially free of cracks and wherein there is essentially no convective flow across the membrane. 6. The membrane of Claim 1 , wherein the membrane is formed on a support selected from the group consisting of a porous metallic support, a porous ceramic support, a porous glass support, and a porous carbon composite support. 7. The membrane of Claim 1 wherein the support is tubular and the membrane is formed on the outer surface of the support. 8. A nanoporous carbogenic membrane, supported on a porous substrate, comprised of: one or more intermediate layers of a material selected from the group consisting of titanium dioxide, silica, colloidal graphite and silicon dioxide; and a plurality of layers of poly(furfuryl) alcohol resin pyrolyzed in an inert atmosphere, each of ρoly(furfuryl) alcohol resin layers being no more than 10 microns in thickness and having a weight after pyrolysis of no more than 10 milligrams per square centimeter. 9. A carbogenic molecular sieve membrane, supported on a porous substrate, having improved gas separation performance, wherein the membrane is formed by the steps of: (a) forming a tube assembly by joining one or more segments of porous tubes between two or more segments of nonporous tubes; (b) coating the porous segments of the tube assembly with a solution of poly(furfuryl) alcohol resin in an acetone solvent by ultrasonically atomizing the solution into droplets of substantially uniform size between about 0.1 and 10 microns and depositing a thin, uniform layer no more than about 20 microns in thickness and having a wet weight of no more than about 25 milligrams per square centimeter of the poly(furfuryl) alcohol resin/acetone solution onto the porous support; (c) drying the resin/acetone layer by evaporating the acetone ; (d) pyrolyzing the poly(furfuryl) alcohol resin in an atmosphere to form a membrane layer which is no more than about 10 microns in thickness and has a weight after pyrolysis of no more than about 10 milligrams per square centimeter; (e) repeating steps (b)-(d) one or more times to form a multi-layer membrane. 10. The membrane of Claim 9 wherein the coating step (b) is performed with the support preheated to a temperature in the range of 30-300°C. 11. The membrane of Claim 9 wherein steps (b) and (c) are performed in air at substantially room temperature. 12. The membrane of Claim 9 wherein the pyrolyzing step (d) is performed in an atmosphere selected from the group consisting of: an inert atmosphere, a reactive atmosphere, a reducing atmosphere, and an oxidizing atmosphere. 13. The membrane of Claim 9 wherein step (d) is performed in a furnace. 14. The membrane of Claim 13 where step (d) is performed by first increasing the temperature of the furnace at a predetermined rate and holding the temperature constant within the range from about 150°C to 800°C for a time ranging from 0 minutes to 480 minutes and then cooling the furnace to room temperature. 15. The membrane of Claim 9 wherein the pyrolyzing step (d) is performed by irradiating the resin layer with focussed laser radiation. 16. The membrane of Claim 13 where step (d) is performed by first increasing the temperature of the furnace at a predetermined rate and holding the temperature constant within the range from about 150 to 600°C for a time ranging from 0 minutes to 180 minutes. 17. The membrane of Claim 9 where in the pyrolyzing step (d) the tube assembly is continuously rotated. 18. The membrane of Claim 9 wherein step (b) the ultrasonic atomization is performed at a frequency in the range of 20-120 kHz. 19. The membrane of Claim 18 wherein step (b) the ultrasonic atomization is performed at a frequency in the range of 40-60 kHz. 20. A method of forming a supported nanoporous carbon membrane, comprising the steps of: (a) preparing a solution of poly(furfuryl) alcohol resin in an acetone solvent; (b) ultrasonically atomizing the solution and depositing a thin, uniform layer of no more than about 25 milligrams per square centimeter of the poly(furfuryl) alcohol resin/acetone solution onto a porous support; (c) drying the resin/acetone layer by evaporating the acetone; (d) pyrolyzing the poly(furfuryl) alcohol resin. 21. The method of Claim 20 wherein the step (d) further comprises: first increasing the temperature from room temperature to the pyrolyzing temperature at a predetermined rate. 22. The method of Claim 20 wherein the steps (b)-(d) are repeated to form a membrane having a plurality of layers. 23. The method of Claim 20 wherein step (b) the ultrasonic atomization is performed at a frequency in the range of 20-120 kHz. 24. The method of Claim 20 wherein step (b) the ultrasonic atomization is performed at a frequency in the range of 40-60 kHz. 25. A method of forming a supported nanoporous carbon membrane, comprising the steps of: (a) preparing a solution of poly(furfuryl) alcohol resin in an acetone solvent; (b) ultrasonically atomizing the solution and depositing a thin, uniform layer of no more than about 25 milligrams per square centimeter of the poly(furfuryl) alcohol resin/acetone solution onto a porous support; (c) drying the resin/acetone layer by evaporating the acetone in air at substantially room temperature; (d) pyrolyzing the poly(furfuryl) alcohol resin in an inert atmosphere by irradiating the resin layer with focussed laser radiation. 26. A method of forming a supported nanoporous carbon membrane, comprising the steps of: (a) preparing a solution of poly (furfuryl) alcohol resin in an acetone solvent; (b) positioning an ultrasonic atomizer at a predetermined distance from a tube assembly having an axis, the tube assembly comprising at least one porous tubular support; (c) rotating the tube assembly at a predetermined rotation speed and traversing the atomizer in a direction parallel with the axis of the tube assembly at a predetermined speed, so that the motion of the atomizer defines a helical path along the tube assembly, the helical path being defined by an axial distance along the tube assembly and a rotational phase angle with respect to the tube assembly; (d) ultrasonically atomizing the solution and depositing a thin, uniform layer of no more than about 25 milligrams per square centimeter of the poly(furfuryl) alcohol resin/acetone solution onto the porous support; (e) drying the resin/acetone layer by evaporating the acetone; (f) pyrolyzing the poly(furfuryl) alcohol resin; (g) repeating steps (c)-(f) to form a membrane having a plurality of successive layers, the rotational phase angle of the atomizer relative to the tube assembly being selected to be different for each successive coating step (c), so that the helical path of the atomizer for each successive repetition of step (c) is offset from the previous helical path, so that a membrane of more uniform thickness is achieved.

Inventors: COOPER THOMAS BETHEA IV, FOLEY HENRY CHARLES, SHIFLETT MARK BRANDON, STAUNTON HAROLD FRANCIS, COOPER, THOMAS, BETHEA, IV, FOLEY, HENRY, CHARLES, SHIFLETT, MARK, BRANDON, STAUNTON, HAROLD, FRANCIS
Applicants: COOPER THOMAS BETHEA IV, DU PONT, FOLEY HENRY CHARLES, SHIFLETT MARK BRANDON, STAUNTON HAROLD FRANCIS, COOPER, THOMAS, BETHEA, IV, E.I. DU PONT DE NEMOURS AND COMPANY, FOLEY, HENRY, CHARLES, SHIFLETT, MARK, BRANDON, STAUNTON, HAROLD, FRANCIS
Application year: 2000
Main classification: B01D71
IPC codes: B01D53, B01D71, C03C17, C04B38