Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
  • B01D67/0039—Inorganic membrane manufacture
  • B01D67/0067—Inorganic membrane manufacture by carbonisation or pyrolysis
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/22—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
  • B01D53/228—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion characterised by specific membranes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
  • B01D67/0039—Inorganic membrane manufacture
  • B01D67/0069—Inorganic membrane manufacture by deposition from the liquid phase, e.g. electrochemical deposition
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
  • B01D67/0039—Inorganic membrane manufacture
  • B01D67/0072—Inorganic membrane manufacture by deposition from the gaseous phase, e.g. sputtering, CVD, PVD
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
  • B01D69/10—Supported membranes; Membrane supports
  • B01D69/108—Inorganic support material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
  • B01D71/02—Inorganic material
  • B01D71/021—Carbon
  • B01D71/0212—Carbon nanotubes
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
  • C03C17/22—Surface treatment of glass, not in the form of fibres or filaments, by coating with other inorganic material
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
  • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
  • C03C17/3411—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions with at least two coatings of inorganic materials
  • C03C17/3429—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions with at least two coatings of inorganic materials at least one of the coatings being a non-oxide coating
  • C03C17/3441—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions with at least two coatings of inorganic materials at least one of the coatings being a non-oxide coating comprising carbon, a carbide or oxycarbide
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B38/00—Porous mortars, concrete, artificial stone or ceramic ware; Preparation thereof
  • C04B38/0022—Porous mortars, concrete, artificial stone or ceramic ware; Preparation thereof obtained by a chemical conversion or reaction other than those relating to the setting or hardening of cement-like material or to the formation of a sol or a gel, e.g. by carbonising or pyrolysing preformed cellular materials based on polymers, organo-metallic or organo-silicon precursors
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/009—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone characterised by the material treated
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
  • C04B41/50—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials
  • C04B41/5001—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials with carbon or carbonisable materials
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2323/00—Details relating to membrane preparation
  • B01D2323/34—Use of radiation
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/02—Details relating to pores or porosity of the membranes
  • B01D2325/027—Nonporous membranes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2325/00—Details relating to properties of membranes
  • B01D2325/04—Characteristic thickness
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2217/00—Coatings on glass
  • C03C2217/20—Materials for coating a single layer on glass
  • C03C2217/28—Other inorganic materials
  • C03C2217/282—Carbides, silicides
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
  • C04B2111/00474—Uses not provided for elsewhere in C04B2111/00
  • C04B2111/00612—Uses not provided for elsewhere in C04B2111/00 as one or more layers of a layered structure
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
  • C04B2111/00474—Uses not provided for elsewhere in C04B2111/00
  • C04B2111/00793—Uses not provided for elsewhere in C04B2111/00 as filters or diaphragms
  • C04B2111/00801—Membranes; Diaphragms

Definitions

• 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.
• 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.
• 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.
• 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).
• 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.
• 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.
• 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:
• 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:
• 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.
• 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.
• the laser beam 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.
• 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.
• SS304 sintered stainless steel
• 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 ⁇ m.
• 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.
• Freon® TF 1,1,2-trichlorotrifluoroethane
• 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.
• the solution 20 was vigorously shaken before use to ensure it was well mixed.
• a 30 cc syringe 30 was filled with the solution 20 and was delivered at a rate of 1 cc/min.
• a syringe pump 32 (Sage Instruments, Model 355) to an atomizer comprising an ultrasonic horn 40.
• 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 ⁇ m to about 100 ⁇ m 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 ⁇ m to 200 Angstroms. The nominal porosity depends upon on the coating. A porosity of 5 ⁇ m 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.
• 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 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-CorTM 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 ⁇ m.
• 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 ⁇ m.
• 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.
• these intermediate coatings would be applied by an ultrasonic atomization technique, similar to that described in conjunction with the first embodiment.
• 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.
• step (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.
• 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.
• 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.
• 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.
• a reducing atmosphere such as hydrogen
• an oxidizing atmosphere such as oxygen, carbon dioxide, and carbon monoxide
• 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.
• the tubular membranes 10M were inserted in a module 70 for testing.
• 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.
• 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.
• 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.
• 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
• t (sec) is time.
• the flux across the membrane was defined by:
• ⁇ ' is the gas permeability (mol/m.sec.Pa)
• ⁇ is the membrane thickness (m)
• P cs and P ss are the pressures (Pa) on the core side and outer shell side of the tubular membrane, respectively.
• R (m 3 .Pa/gmol.K) is the gas constant
• T (K) is the temperature
• the permeances are provided in Figure 4 along with the ratio of the permeance with respect to N2 which provides the ideal separation factors.
• 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.

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