Dual layer-coated membranes for gas separations

Abstract

The invention discloses dual layer-coated membranes and methods for making and using these membranes. The dual layer-coated membranes have a relatively porous and substantial void-containing selective asymmetric membrane support, a first coating layer comprising a hydrogel, and a second coating layer comprising a hydrophobic fluoropolymer. The membrane support has low selectivity and high permeance. The dual layer coating improves the selectivity of the membrane support and maintains the membrane performance with time. The dual layer-coated membranes are suitable for a variety of liquid, gas, and vapor separations such as water purification, non-aqueous liquid separation such as deep desulfurization of gasoline and diesel fuels, ethanol/water separations, pervaporation dehydration of aqueous/organic mixtures, fuel gas conditioning, CO.sub.2/CH.sub.4, He/CH.sub.4, CO.sub.2/N.sub.2, H.sub.2/CH.sub.4, O.sub.2/N.sub.2, olefin/paraffin, iso/normal paraffins separations, and other light gas mixture separations. The dual layer-coated membranes are especially useful for natural gas liquid (NGL) recovery and CO.sub.2 removal from natural gas.

Claims

1. A membrane comprising a relatively porous and substantial void-containing selective asymmetric membrane support comprising a high performance glassy polymer and two coating layers, a first coating layer comprising a hydrogel on the top surface of said membrane support and a second coating layer comprising a fluoropolymer on top of said first coating layer, wherein said hydrogel is selected from the group consisting of gelatin and sodium alginate.

2. The membrane of claim 1 wherein said high performance glassy polymer is selected from the group consisting of polysulfones, sulfonated polysulfones, polyethersulfones, sulfonated polyethersulfones, polyetherimides, cellulosic polymers, polyimides, polyamide/imides, polyether ether ketones, poly(benzobenzimidazole)s, polybenzoxazoles, polymers of intrinsic microporosity, and mixtures of thereof.

3. The membrane of claim 1 wherein said high performance glassy polymer is selected from the group consisting of cellulose acetate, cellulose triacetate, cellulose acetate-butyrate, cellulose propionate, ethyl cellulose, methyl cellulose, nitrocellulose, and mixtures of thereof.

4. The membrane of claim 1 wherein said fluoropolymer is an amorphous fluoropolymer.

5. The membrane of claim 1 wherein said fluoropolymer is selected from the group consisting of a homopolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD), an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) with a complementary amount of another fluorine-containing monomer selected from the group consisting of tetrafluoroethylene (TFE), perfluoro(alkyl vinyl ether)s, hexafluoropropylene, vinylidene fluoride, and chlorotrifluoroethylene, an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE), an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE) with 65 mol-% of dioxole and an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE), a copolymer of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) and tetrafluoroethylene (TFE) and a copolymer of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) and tetrafluoroethylene (TFE) with 80 mol-% of TTD and 20 mol-% of TFE, a fluoro-silane fluorinated copolymer with silane functional groups and a fluoro-epoxide fluorinated oligomer with epoxide functional groups.

6. A method of preparing a membrane comprising making a relatively porous and substantial void-containing selective asymmetric membrane support comprising a high performance glassy polymer, applying a hydrogel coating to the top surface of said membrane support and then applying a fluoropolymer coating on said hydrogel coating.

7. The method of claim 6 wherein said high performance glassy polymer is selected from the group consisting of polysulfones, sulfonated polysulfones, polyethersulfones, sulfonated polyethersulfones, polyetherimides, cellulosic polymers, polyimides, polyamide/imides, polyether ether ketones, poly(benzobenzimidazole)s, polybenzoxazoles, polymers of intrinsic microporosity, and mixtures of thereof.

8. The method of claim 6 wherein said high performance glassy polymer is selected from the group consisting of cellulose acetate, cellulose triacetate, cellulose acetate-butyrate, cellulose propionate, ethyl cellulose, methyl cellulose, nitrocellulose, and mixtures of thereof.

9. The method of claim 6 wherein said hydrogel is selected from the group consisting of gelatin and sodium alginate.

10. The method of claim 6 wherein said fluoropolymer is selected from the group consisting of a homopolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD), an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) with a complementary amount of another fluorine-containing monomer selected from the group consisting of tetrafluoroethylene (TFE), perfluoro(alkyl vinyl ether)s, hexafluoropropylene, vinylidene fluoride, and chlorotrifluoroethylene, an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE), an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE) with 65 mol-% of dioxole and an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE), a copolymer of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) and tetrafluoroethylene (TFE) and a copolymer of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) and tetrafluoroethylene (TFE) with 80 mol-% of TTD and 20 mol-% of TFE, a fluoro-silane fluorinated copolymer with silane functional groups and a fluoro-epoxide fluorinated oligomer with epoxide functional groups.

11. A process for separating at least one gas from a mixture of gases using a membrane comprising a relatively porous and substantial void-containing selective asymmetric membrane support comprising a high performance glassy polymer and two coating layers, a first coating layer comprising a hydrogel on the top surface of said membrane support and a second coating layer comprising a fluoropolymer on top of said first coating layer, the process comprising: (a) providing a membrane comprising a relatively porous and substantial void-containing selective asymmetric membrane support comprising a high performance glassy polymer and two coating layers, a first coating layer comprising a hydrogel on the top surface of said membrane support and a second coating layer comprising a fluoropolymer on top of said first coating layer, wherein said membrane is permeable to the at least one gas; (b) contacting the mixture on one side of said membrane to cause the at least one gas to permeate said membranes; and (c) removing from the opposite side of said membrane a permeate gas composition comprising a portion of the at least one gas which permeated said membrane.

12. The process of claim 11 wherein said mixture of gases comprises a mixture of volatile organic compounds in an atmospheric gas.

13. The process of claim 11 wherein said mixture of gases comprises nitrogen or oxygen in air.

14. The process of claim 11 wherein said mixture of gases comprises CO.sub.2 in natural gas.

15. The process of claim 11 wherein said mixture of gases comprises H.sub.2 from a mixture of N.sub.2, CH.sub.4, and Ar.

16. The process of claim 11 wherein said mixture of gases is a mixture of olefins and paraffins or a mixture of iso/normal paraffins.

17. The process of claim 11 wherein said mixture of gases is a fuel gas comprising methane, ethane, propane, and C3+ heavier hydrocarbons.

18. The process of claim 11 wherein said mixture of gases is at least two gases selected from the group consisting of nitrogen and oxygen, carbon dioxide and methane, hydrogen and methane or carbon monoxide, helium and methane.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The use of membranes for separation of both gases and liquids is a growing technological area with potentially high economic reward due to the low energy requirements and the potential for scaling up of modular membrane designs. Advances in membrane technology, with the continuing development of new membrane materials will make this technology even more competitive with traditional, high-energy intensive and costly processes such as distillation. Among the applications for large scale gas separation membrane systems are nitrogen enrichment, oxygen enrichment, hydrogen recovery, removal of hydrogen sulfide and carbon dioxide from natural gas and dehydration of air and natural gas. Also, various hydrocarbon separations are potential applications for the appropriate membrane system. The materials that are used in these applications must have high selectivity, durability, and productivity in processing large volumes of gas or liquid in order to be economically successful. Membranes for gas separations have evolved rapidly in the past 25 years due to their easy processability for scale-up and low energy requirements. More than 90% of the membrane gas separation applications involve the separation of noncondensable gases such as carbon dioxide from methane, nitrogen from air, and hydrogen from nitrogen, argon or methane. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications of membrane gas separation have achieved commercial success, including carbon dioxide removal from natural gas and biogas and in enhanced oil recovery.

(2) The membranes most commonly used in commercial gas separation applications are asymmetric polymeric membranes characterized by a thin, dense, selectively semipermeable surface skin and a less dense porous, void-containing, non-selective support region. Gas separation by these membranes is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. This mechanism assumes that each component is sorbed by the membrane at one interface, transported by diffusion across the membrane through the voids between the polymeric chains (or called free volume), and desorbed at the other interface. According to the solution-diffusion model, the membrane performance for a given pair of gases (e.g., CO.sub.2/CH.sub.4, O.sub.2/N.sub.2, H.sub.2/CH.sub.4) is determined by two parameters: permeability coefficient (P.sub.A) and the selectivity (.sub.A/B). The P.sub.A is the product of the gas flux and the membrane skin thickness, divided by the pressure difference across the membrane. The .sub.A/B is the ratio of the permeability coefficients of the two gases (.sub.A/B=P.sub.A/P.sub.B) where P.sub.A is the permeability of the more permeable gas and P.sub.B is the permeability of the less permeable gas. Gases can have high permeability coefficient because of a high solubility coefficient, a high diffusion coefficient, or both. The diffusion coefficient decreases and the solubility coefficient increases with the increase in the molecular size of the gas. For high-performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given amount of gas, thereby decreasing the capital cost of membrane units, and because higher selectivity results in a higher purity product gas with increased efficiency. However, some of the high-performance polymeric gas separation membrane materials still have the issues of high cost, poor hydrocarbon contaminant resistance, poor plasticization resistance, low chemical and thermal stability, unstable permeance (or flux) and selectivity over time, and poor processability to form a defect-free thin selective skin layer.

(3) U.S. Pat. No. 6,368,382 by Chiou claimed a method of making an epoxysilicone coated membrane by coating a porous asymmetric membrane layer with a UV-curable controlled release epoxysilicone coating. A mixture of the epoxysilicone resin and an onium photocatalyst are applied to the porous asymmetric membrane layer and cured by UV or electron beam radiation to produce a dry epoxysilicone coated membrane. The coating of such coated membranes comprising siloxane or silicone segments, however, is subject to swelling by solvents, poor performance durability, low resistance to hydrocarbon contaminants, and low resistance to plasticization by the sorbed penetrant molecules such as CO2 or C3H6.

(4) US 20090277837 A1 by Liu et al. provided a fluoropolymer coated membrane where the porous asymmetric membrane layer was coated directly by a thin layer of a hydrophobic fluoropolymer to improve the selectivity of the gas separation membrane. The coating of such coated membranes comprising hydrophobic fluoropolymer segments, however, is subject to delamination by liquid hydrocarbon contaminants in the natural gas feed such as BTEX for natural gas upgrading. Delamination will result in poor performance durability, reduced resistance to hydrocarbon contaminants and plasticization.

(5) This invention relates to a dual layer-coated asymmetric membrane comprising a relatively porous and substantial void-containing selective asymmetric membrane support, a first coating layer comprising a hydrogel and a second coating layer comprising a hydrophobic fluoropolymer. In addition, this invention relates to a method for making the dual layer-coated asymmetric membrane as well as the application of these membranes not only for a variety of gas separations such as separations of CO.sub.2/CH.sub.4, He/CH.sub.4, olefin/paraffin separations (e.g. propylene/propane separation), fuel gas conditioning, H.sub.2/CH.sub.4, O.sub.2/N.sub.2, iso/normal paraffins, polar molecules such as H.sub.2O, H.sub.2S, and NH3/mixtures with CH.sub.4, N.sub.2, H.sub.2, and other light gases separations, but also for NGL recovery and CO.sub.2 removal from natural gas in a single step.

(6) The dual layer-coated asymmetric membrane described in the current invention comprises a relatively porous and substantial void-containing selective asymmetric membrane support which is first coated with a hydrogel such as gelatin and then coated with a thin layer of a hydrophobic fluoropolymer. The dual layer coating provides improved selectivity for the relatively porous and substantial void-containing selective asymmetric membrane support. The dual layer coating also provides essentially no loss in selectivity or no loss in flux rates over a typical operating period in the presence of high concentrations of CO.sub.2 and/or in the presence of liquid hydrocarbon contaminants such as BTEX (benzene, toluene, ethylbenzene, and xylenes). The term essentially no loss in flux rates means that the flux declines less than about 30%, and more particularly the flux rate declines less than 20% over a typical operating period of about 3 years.

(7) The relatively porous and substantial void-containing selective asymmetric membrane support with a low selectivity and high flux described in the current invention can be formed by phase inversion followed by direct air drying or it can also be formed by phase inversion followed by solvent exchange methods (see U.S. Pat. No. 3,133,132). Selection of the relatively porous and substantial void-containing selective asymmetric membrane support may be made on the basis of the heat resistance, solvent resistance, and mechanical strength of the porous asymmetric membrane layer, as well as other factors dictated by the operating conditions for selective permeation. The hydrogel coating, fluoropolymer coating, and the relatively porous and substantial void-containing selective asymmetric membrane support need to have the prerequisite relative separation factors in accordance with the invention for at least one pair of gases or liquids. The relatively porous and substantial void-containing selective asymmetric membrane support is preferably at least partially self-supporting, and in some instances may be essentially self-supporting. The relatively porous and substantial void-containing selective asymmetric membrane support may provide essentially all of the structural support for the membrane, or the double coated membrane may include a structural support member which can provide little, if any, resistance to the passage of gases or liquids.

(8) Generally, the relatively porous and substantial void-containing selective asymmetric membrane support is prepared from cellulosic polymers such as cellulose acetate and cellulose triacetate, other polymers such as polysulfone, polyethersulfone, polyimide, polyetherimide, and polybenzoxazole. These polymers provide a range of properties such as low cost, high permeance, good mechanical stability, and ease of processability that are important for gas and liquid separations. Typical polymers that are used can be substituted or unsubstituted polymers and may be selected from but is not limited to, polysulfones; sulfonated polysulfones; polyethersulfones; sulfonated PESs; polyethers; polyetherimides; polycarbonates; cellulosic polymers such as cellulose acetate, cellulose triacetate, cellulose acetate-butyrate, cellulose propionate, ethyl cellulose, methyl cellulose, nitrocellulose; polyamides; polyimides; polyamide/imides; polyketones, polyether ketones; poly(arylene oxides) such as poly(phenylene oxide) and poly(xylene oxide); polyurethanes; poly(benzobenzimidazole)s; polybenzoxazoles; polymers of intrinsic microporosity; and mixtures of thereof.

(9) Some preferred polymers that are suitable for the preparation of the relatively porous and substantial void-containing selective asymmetric membrane support include, but are not limited to polyetherimides, cellulosic polymers such as cellulose acetate and cellulose triacetate, polyamides, polyimides, and mixtures thereof.

(10) The solvents used for dissolving the polymer material for the preparation of the relatively porous and substantial void-containing selective asymmetric membrane support are chosen primarily for their ability to completely dissolve the polymers and for ease of solvent removal in the membrane formation steps. Other considerations in the selection of solvents include low toxicity, low corrosive activity, low environmental hazard potential, availability and cost. Representative solvents include most amide solvents that are typically used for the formation of the relatively porous and substantial void-containing selective asymmetric membrane support, such as N-methylpyrrolidone (NMP) and N,N-dimethyl acetamide (DMAc), methylene chloride, tetrahydrofuran (THF), acetone, methyl acetate, isopropanol, n-octane, n-hexane, n-decane, methanol, ethanol, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), lactic acid, citric acid, dioxanes, 1,3-dioxolane, mixtures thereof, others known to those skilled in the art and mixtures thereof.

(11) The hydrogel coating on the relatively porous and substantial void-containing selective asymmetric membrane support of the new dual layer-coated asymmetric membrane described in the present invention can be formed from a water-soluble polymeric species capable of forming a hydrogel such as gelatin and sodium alginate. Gelatin is a heterogeneous mixture of water-soluble proteins. The proteins are extracted by boiling skin, tendons, ligaments, bones, etc. in water. There are two types of gelatins, type A and type B. Type A gelatin is derived from acid-cured tissue and Type B gelatin is derived from lime-cured tissue. Gelatin has a combination of high molecular weight, which is typically advantageous for a membrane coating material, and the ability to readily form a hydrogel, even without a metal species being present.

(12) The layer of hydrophobic fluoropolymer coating on top of the layer of hydrogel coating on the relatively porous and substantial void-containing selective asymmetric membrane support of the new dual layer-coated asymmetric membrane can be formed from an organic solvent-soluble hydrophobic fluoropolymer with high gas permeability. The fluoropolymers have high thermal, chemical, mechanical and electrical stability, as well as high gas permeability. The fluoropolymer may be an amorphous fluoropolymer selected from the DuPont Teflon AF family of amorphous fluoropolymers including Teflon AF1600 and Teflon AF2400, FluoroPel PFC 504A CoE5 and FluoroPel PFC 504A CoFS fluoropolymers from Cytonix Corporation. Teflon AF fluoropolymers include a fluoropolymer that is a homopolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD), and a fluoropolymer that is an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) with a complementary amount of another fluorine-containing monomer selected from the group consisting of tetrafluoroethylene (TFE), perfluoro(alkyl vinyl ether)s, hexafluoropropylene, vinylidene fluoride, and chlorotrifluoroethylene. Other fluoropolymers include a fluoropolymer that is an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE), a fluoropolymer that is an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE) with 65 mol-% of dioxole and a glass transition temperature of 160 C. (DuPont Teflon AF1600) and a fluoropolymer that is an amorphous copolymer of 2,2-bistrifluoro-methyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE) with 87 mol-% of dioxole and a glass transition temperature of 240 C. (DuPont Teflon AF2400). Another type of fluoropolymers used in the present invention is Hyflon AD fluoropolymers from Solvay Solexis including a fluoropolymer that is a copolymer of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) and tetrafluoroethylene (TFE) and a fluoropolymer that is a copolymer of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) and tetrafluoroethylene (TFE) with 80 mol-% of TTD and 20 mol-% of TFE. Fluoropolymers from Cytonix Corporation that can also be used in the present invention include a fluoropolymer that is a fluoro-silane fluorinated copolymer with silane functional groups and a fluoropolymer that is a fluoro-epoxide fluorinated oligomer with epoxide functional groups.

(13) The organic solvents that can be used for dissolving the fluoropolymer in the present invention are essentially perfluorinated solvents and mixtures thereof such as Fluorinert FC-75 (perfluoro(n-butyltetrahydrofuran)), Fluorinert FC-72, Fluorinert FC-770, and Fluorinert FC-40 (perfluoro(alkyl amine)). It is preferred that the fluoropolymer be diluted in the perfluorinated organic solvent or mixtures thereof in a concentration of from about 0.01 to about 10 wt-% to provide an effective coating.

(14) The new dual layer-coated asymmetric membrane can be either a flat sheet membrane or a hollow fiber membrane.

(15) The present invention also provides a method of making the dual layer-coated asymmetric membrane without delamination of the hydrophobic fluoropolymer coating layer from the membrane. The method involves the use of a rough micro/nano structured surface of a first coating layer comprising a hydrogel coating on the relatively porous and substantial void-containing selective asymmetric membrane support to improve the adhesion of a second coating layer comprising a hydrophobic fluoropolymer on the membrane. The coating of the hydrogel on the relatively porous and substantial void-containing selective asymmetric membrane support results in the formation of a rough membrane surface in micro/nano scale that is critical to improve the adhesion between the membrane and the hydrophobic fluoropolymer coating and prevents delamination of the hydrophobic fluoropolymer coating layer from the membrane. The rough micro/nano structured surface described in the current invention means an uneven rough membrane surface covered with spherical, needle-like, or other types of tiny bumps of less than about 10 micrometers in height.

(16) The method to form the relatively porous and substantial void-containing selective asymmetric membrane support either in a flat sheet form or a hollow fiber form comprises casting or spinning a membrane casting or spinning dope to form a wet relatively porous and substantial void-containing selective asymmetric membrane, and then drying the relatively porous and substantial void-containing selective asymmetric membrane through a direct air drying method (see U.S. Pat. No. 4,855,048) or through a solvent exchange method (see U.S. Pat. No. 3,133,132) to form a dried relatively porous and substantial void-containing selective asymmetric flat sheet membrane support or asymmetric hollow fiber membrane support. The membrane casting or spinning dope comprises a polymer dissolved in a mixture of organic solvents or two or more blend polymers dissolved in a mixture of organic solvents. For example, the relatively porous and substantial void-containing selective asymmetric membrane support can be a thin relatively porous and substantial void-containing asymmetric cellulosic membrane support having a skin thickness of less than about 10,000 angstroms. Preferably, the thin relatively porous and substantial void-containing asymmetric cellulosic membrane support has a skin thickness between about 200 and about 1000 angstroms, and more preferably, the thin relatively porous and substantial void-containing asymmetric cellulosic membrane support has a skin thickness between about 300 and about 500 angstroms. The membrane performance of the relatively porous and substantial void-containing selective asymmetric membrane support for a given pair of gases (e.g., CO.sub.2/CH.sub.4, O.sub.2/N.sub.2, H.sub.2/CH.sub.4) is determined by two parameters: permeability coefficient (or called permeability, P.sub.A) and the selectivity (.sub.A/B). Generally, in order to separate one gaseous component from another, the ratio of the permeability of the more permeable component to the other component, which is the selectivity of the more permeable component over the other component should be at least five. The term relatively porous and substantial void-containing selective asymmetric cellulosic membrane support in the context of the current invention includes cellulose ester membranes such as cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose cyanoethylate, cellulose methacrylate, and mixtures thereof. A particularly preferred relatively porous and substantial void-containing selective asymmetric cellulosic membrane support comprises cellulose acetate or/and cellulose triacetate. The relatively porous and substantial void-containing selective asymmetric cellulosic membrane support can be made to any degree of initial porosity as characterized by its initial selectivity, which may range from 1.1 to about 8. The relatively porous and substantial void-containing selective asymmetric cellulosic membrane support of the present invention is porous and is characterized as having an initial selectivity of higher than 1.1 and less than about 8, and more preferably having a selectivity of higher than 2 and less than about 5.

(17) An aqueous hydrogel-forming polymer solution such as an aqueous gelatin solution is applied to the relatively porous and substantial void-containing selective asymmetric membrane support by nipping, dip-coating, spin coating, casting, soaking, spraying, painting, and other known conventional solution coating technologies. The resulting hydrogel coating such as gelatin coating on the surface of the relatively porous and substantial void-containing selective asymmetric membrane support provides a membrane with an uneven rough membrane surface covered with spherical, needle-like, or other types of tiny bumps of less than about 10 micrometer in height.

(18) The concentration of the hydrogel-forming polymer in the aqueous hydrogel-forming polymer solution is dependent upon the initial porosity of the relatively porous and substantial void-containing selective asymmetric membrane support.

(19) A thin hydrophobic fluoropolymer coating layer is formed on the top surface of the hydrogel coating layer on the relatively porous and substantial void-containing selective asymmetric membrane support by applying a dilute fluoropolymer solution to the top surface of the hydrogel coating layer on the relatively porous and substantial void-containing selective asymmetric membrane support by nipping, dip-coating, spin coating, casting, soaking, spraying, painting, and other known conventional solution coating technologies. The second thin hydrophobic fluoropolymer coating layer is formed after evaporating the perfluorinated organic solvent(s).

(20) The concentration of the fluoropolymer in the fluoropolymer coating solution is dependent upon the initial porosity of the relatively porous and substantial void-containing selective asymmetric membrane support and the performance of the hydrogel-coated asymmetric membrane.

(21) The combination of the rough micro/nano structured surface created from the hydrogel coating with the low surface energy created from the hydrophobic fluoropolymer coating provided the new dual layer-coated asymmetric membranes described in the present invention with high resistance to delamination and liquid contaminants. The new dual layer-coated asymmetric membranes have the advantages of low cost, high permeance (or flux), as well as stable permeance (or flux) and sustained selectivity over time by resistance to solvent swelling, plasticization and liquid hydrocarbon contaminants for gas separation applications. The dual layer coating improves the selectivity of the relatively porous and substantial void-containing selective asymmetric membrane support and exhibits essentially no loss in selectivity or no loss in flux rates over a typical operating period. The term essentially no loss in flux rates means that the flux declines less than about 30%, and more particularly the flux rate often declines less than 20% over a typical operating period.

(22) The invention provides a process for separating at least one gas from a mixture of gases using the dual layer-coated membranes described in the present invention, the process comprising: (a) providing a dual layer-coated membrane which is permeable to the at least one gas; (b) contacting the mixture on one side of the dual layer-coated membrane to cause the at least one gas to permeate the dual layer-coated membranes; and (c) removing from the opposite side of the membrane a permeate gas composition comprising a portion of the at least one gas which permeated said membrane.

(23) The dual layer-coated membranes of the present invention are especially useful in the purification, separation or adsorption of a particular species in the liquid or gas phase. In addition to separation of pairs of gases, these dual layer-coated membranes may, for example, be used for natural gas liquid (NGL) recovery and CO.sub.2 removal from natural gas in one-step, fuel gas conditioning to separate methane from C2 and C2+ hydrocarbons, as well as natural gas upgrading applications such as FLNG and FPSO applications. The dual layer-coated membranes may also be used for the purification of water or for the separation of proteins or other thermally unstable compounds, e.g. in the pharmaceutical and biotechnology industries. The dual layer-coated membranes may also be used in fermenters and bioreactors to transport gases into the reaction vessel and transfer cell culture medium out of the vessel. Additionally, the dual layer-coated membranes may be used for the removal of microorganisms from air or water streams, water purification, ethanol production in a continuous fermentation/membrane pervaporation system, and in detection or removal of trace compounds or metal salts in air or water streams.

(24) The dual layer-coated membranes of the present invention are especially useful in gas separation processes in air purification, petrochemical, refinery, and natural gas industries. Examples of such separations include separation of volatile organic compounds (such as toluene, xylene, and acetone) from an atmospheric gas, such as nitrogen or oxygen and nitrogen recovery from air. Further examples of such separations are for the separation of CO.sub.2 from natural gas, H.sub.2 from N.sub.2, CH.sub.4, and Ar in ammonia purge gas streams, H.sub.2 recovery in refineries, fuel gas conditioning, olefin/paraffin separations such as propylene/propane separation, and iso/normal paraffin separations. Any given pair or group of gases that differ in molecular size, for example nitrogen and oxygen, carbon dioxide and methane, hydrogen and methane or carbon monoxide, helium and methane, can be separated using the dual layer-coated membranes described herein. More than two gases can be removed from a third gas. For example, some of the gas components which can be selectively removed from a raw natural gas using the membrane described herein include carbon dioxide, oxygen, nitrogen, water vapor, hydrogen sulfide, helium, and other trace gases. Some of the gas components that can be selectively retained include hydrocarbon gases. When permeable components are acid components selected from the group consisting of carbon dioxide, hydrogen sulfide, and mixtures thereof and are removed from a hydrocarbon mixture such as natural gas, one module, or at least two in parallel service, or a series of modules may be utilized to remove the acid components. For example, when one module is utilized, the pressure of the feed gas may vary from 275 kPa to about 2.6 MPa (25 to 4000 psi). The differential pressure across the membrane can be as low as about 0.7 bar or as high as 145 bar (about 10 psi or as high as about 2100 psi) depending on many factors such as the particular membrane used, the flow rate of the inlet stream and the availability of a compressor to compress the permeate stream if such compression is desired. Differential pressure greater than about 145 bar (2100 psi) may rupture the membrane. A differential pressure of at least 7 bar (100 psi) is preferred since lower differential pressures may require more modules, more time and compression of intermediate product streams. The operating temperature of the process may vary depending upon the temperature of the feed stream and upon ambient temperature conditions. Preferably, the effective operating temperature of the dual layer-coated membranes of the present invention will range from about 50 to about 100 C. More preferably, the effective operating temperature of the dual layer-coated membranes of the present invention will range from about 20 to about 70 C., and most preferably, the effective operating temperature of the dual layer-coated membranes of the present invention will be less than about 70 C.

(25) The dual layer-coated membranes described in the current invention are also especially useful in gas/vapor separation processes in chemical, petrochemical, pharmaceutical and allied industries for removing organic vapor or liquid from gas streams, e.g. in off-gas treatment for recovery of volatile organic compounds to meet clean air regulations, or within process streams in production plants so that valuable compounds (e.g., vinylchloride monomer, propylene) may be recovered. Further examples of gas/vapor separation processes in which these dual layer-coated membranes may be used are hydrocarbon vapor separation from hydrogen in oil and gas refineries, for hydrocarbon dew pointing of natural gas (i.e. to decrease the hydrocarbon dew point to below the lowest possible export pipeline temperature so that liquid hydrocarbons do not separate in the pipeline), for control of methane number in fuel gas for gas engines and gas turbines, and for gasoline recovery. The dual layer-coated membranes may incorporate a species that adsorbs strongly to certain gases (e.g. cobalt porphyrins or phthalocyanines for O.sub.2 or silver(I) for ethane) to facilitate their transport across the membrane.

(26) These dual layer-coated membranes may also be used in the separation of liquid mixtures by pervaporation, such as in the removal of organic compounds (e.g., alcohols, phenols, chlorinated hydrocarbons, pyridines, ketones) from water such as aqueous effluents or process fluids. A dual layer-coated membrane which is ethanol-selective would be used to increase the ethanol concentration in relatively dilute ethanol solutions (5-10% ethanol) obtained by fermentation processes. Another liquid phase separation example using these dual layer-coated membranes is the deep desulfurization of gasoline and diesel fuels by a pervaporation membrane process similar to the process described in U.S. Pat. No. 7,048,846, incorporated by reference herein in its entirety. The dual layer-coated membranes that are selective to sulfur-containing molecules would be used to selectively remove sulfur-containing molecules from fluid catalytic cracking (FCC) and other naphtha hydrocarbon streams. Further liquid phase examples include the separation of one organic component from another organic component, e.g. to separate isomers of organic compounds. Mixtures of organic compounds which may be separated using the dual layer-coated membranes include: ethylacetate-ethanol, diethylether-ethanol, acetic acid-ethanol, benzene-ethanol, chloroform-ethanol, chloroform-methanol, acetone-isopropylether, allylalcohol-allylether, allylalcohol-cyclohexane, butanol-butylacetate, butanol-1-butylether, ethanol-ethylbutylether, propylacetate-propanol, isopropylether-isopropanol, methanol-ethanol-isopropanol, and ethylacetate-ethanol-acetic acid.

(27) The dual layer-coated membranes may be used for separation of organic molecules from water (e.g. ethanol and/or phenol from water by pervaporation) and removal of metal and other organic compounds from water.

(28) An additional application of the dual layer-coated membranes is in chemical reactors to enhance the yield of equilibrium-limited reactions by selective removal of a specific product in an analogous fashion to the use of dual layer-coated membranes to enhance esterification yield by the removal of water.

(29) The dual layer-coated membranes have immediate applications for the separation of gas mixtures including carbon dioxide removal from natural gas. The dual layer-coated membranes permit carbon dioxide to diffuse through at a faster rate than the methane in the natural gas. Carbon dioxide has a higher permeation rate than methane because of higher solubility, higher diffusivity, or both. Thus, carbon dioxide enriches on the permeate side of the membrane, and methane enriches on the feed (or reject) side of the membrane.

(30) Any given pair of gases that differ in size, for example, nitrogen and oxygen, carbon dioxide and methane, carbon dioxide and nitrogen, hydrogen and methane or carbon monoxide, helium and methane, can be separated using the dual layer-coated membranes described herein. More than two gases can be removed from a third gas. For example, some of the components which can be selectively removed from a raw natural gas using the dual layer-coated membranes described herein include carbon dioxide, oxygen, nitrogen, water vapor, hydrogen sulfide, helium, and other trace gases. Some of the components that can be selectively retained include hydrocarbon gases.

EXAMPLES

(31) The following examples are provided to illustrate one or more preferred embodiments of the invention, but are not limited embodiments thereof. Numerous variations can be made to the following examples that lie within the scope of the invention.

Example 1

Preparation of High Flux Gelatin-Coated and then AF2400-Coated Asymmetric Cellulose Acetate-Cellulose Triacetate Membrane (Abbreviated as HF-AF-Gelatin/CA-CTA)

(32) A relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate membrane support having a CO.sub.2/CH.sub.4 selectivity of about 1.5-2 and CO.sub.2 permeance of about 400 GPU (50 C., 1000 psig, 10% CO.sub.2/90% CH.sub.4) was prepared in a conventional manner from a casting dope comprising, by approximate weight percentages, 8.2% cellulose triacetate, 8.2% cellulose acetate, 18.4% N-methyl pyrrolidone, 26.4% 1,3-dioxolane, 12.3% acetone, and 26.5% of non-solvents. A film was cast on a nylon fabric then gelled by immersion in a 1 C. water bath for about 10 minutes, and then annealed in a hot water bath at 80 to 90 C. for about 15 minutes to form a relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane. An aqueous solution of gelatin coating material was dripped onto the surface of the relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane to form a gelatin-coated asymmetric cellulose acetate-cellulose triacetate wet membrane. The gelatin-coated asymmetric cellulose acetate-cellulose triacetate wet membrane was dried with a continuous drying machine at 70 C. at 2.0 fpm. The dried gelatin-coated asymmetric cellulose acetate-cellulose triacetate membrane was dip coated with a AF2400 polymer solution in Fluorinert FC-770 solvent and dried at 85 C. to form the dried high flux gelatin-coated and then AF2400-coated asymmetric cellulose acetate-cellulose triacetate membrane (abbreviated as HF-AF-Gelatin/CA-CTA).

Example 2

Preparation of High Selectivity Gelatin-Coated and then AF2400-Coated Asymmetric Cellulose Acetate-Cellulose Triacetate Membrane (Abbreviated as HS-AF-Gelatin/CA-CTA)

(33) A relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate membrane support having a CO.sub.2/CH.sub.4 selectivity of about 3-4 and CO.sub.2 permeance of about 290-350 GPU (50 C., 1000 psig, 10% CO.sub.2/90% CH.sub.4) was prepared in a conventional manner from a casting dope comprising, by approximate weight percentages, 8.2% cellulose triacetate, 8.2% cellulose acetate, 12.3% N-methyl pyrrolidone, 34.8% 1,3-dioxolane, 10.2% acetone, and 26.3% of non-solvents. A film was cast on a nylon fabric then gelled by immersion in a 1 C. water bath for about 10 minutes, and then annealed in a hot water bath at 80 to 90 C. for about 15 minutes to form a relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane. An aqueous solution of gelatin coating material was dripped onto the surface of the relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane to form a gelatin-coated asymmetric cellulose acetate-cellulose triacetate wet membrane. The gelatin-coated asymmetric cellulose acetate-cellulose triacetate wet membrane was dried with a continuous drying machine at 70 C. at 2.0 fpm. The dried gelatin-coated asymmetric cellulose acetate-cellulose triacetate membrane was dip coated with a AF2400 polymer solution in Fluorinert FC-770 solvent and dried at 85 C. to form the dried high selectivity gelatin-coated and then AF2400-coated asymmetric cellulose acetate-cellulose triacetate membrane (abbreviated as HS-AF-Gelatin/CA-CTA).

Comparative Example 2

Preparation of Gelatin-Coated Asymmetric Cellulose Acetate-Cellulose Triacetate Membrane (Abbreviated as Gelatin/CA-CTA)

(34) A relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate membrane support having a CO.sub.2/CH.sub.4 selectivity of about 3-4 and CO.sub.2 permeance of about 290-350 GPU (50 C., 1000 psig, 10% CO.sub.2/90% CH.sub.4) was prepared in a conventional manner from a casting dope comprising, by approximate weight percentages, 8.2% cellulose triacetate, 8.2% cellulose acetate, 12.3% N-methyl pyrrolidone, 34.8% 1,3-dioxolane, 10.2% acetone, and 26.3% of non-solvents. A film was cast on a nylon fabric then gelled by immersion in a 1 C. water bath for about 10 minutes, and then annealed in a hot water bath at 80 to 90 C. for about 15 minutes to form a relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane. An aqueous solution of gelatin coating material was dripped onto the surface of the relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane to form a gelatin-coated asymmetric cellulose acetate-cellulose triacetate wet membrane. The gelatin-coated asymmetric cellulose acetate-cellulose triacetate wet membrane was dried with a continuous drying machine at 70 C. at 2.0 fpm to form the dried gelatin-coated asymmetric cellulose acetate-cellulose triacetate membrane (abbreviated as Gelatin/CA-CTA).

Example 3

CO2/CH4 Separation Performance of HF-AF-Gelatin/CA-CTA and HS-AF-Gelatin/CA-CTA Membranes

(35) A 76 mm (3 inch) diameter circle of HF-AF-Gelatin/CA-CTA membrane of Example 1 and a 76 mm (3 inch) diameter circle of HS-AF-Gelatin/CA-CTA of Example 2 were evaluated for gas transport properties using a natural gas feed containing 10% CO.sub.2 and 90% CH.sub.4 at a feed pressure of 6996 kPa (1000 psig) at 50 C. Table 1 shows a comparison of CO.sub.2 permeance (P.sub.CO2/L) and CO.sub.2/CH.sub.4 selectivity (.sub.CO2/CH4) of the HF-AF-Gelatin/CA-CTA and HS-AF-Gelatin/CA-CTA membranes of the present invention. The results in Table 1 show that the high selectivity HS-AF-Gelatin/CA-CTA membrane showed higher CO.sub.2/CH.sub.4 selectivity and lower CO.sub.2 permeance than the high flux HF-AF-Gelatin/CA-CTA membrane.

(36) TABLE-US-00001 TABLE 1 CO.sub.2/CH.sub.4 separation performance of HF-AF-Gelatin/CA-CTA and HS-AF-Gelatin/CA-CTA membranes .sup.a P.sub.CO2/L Membrane (GPU) .sup.b .sub.CO2/CH4 HF-AF-Gelatin/CA-CTA 262 12.6 HS-AF-Gelatin/CA-CTA 190 15.1 .sup.a Tested at 50 C. under 6996 kPa (1000 psig), 10% CO.sub.2/90% CH.sub.4 mixed gas pressure. .sup.b 1 GPU = 2.7 10.sup.5 m.sup.3 (STP)/m.sup.2 .Math.h .Math. kPa.

Example 4

CO2/CH4 Separation Performance of HF-AF-Gelatin/CA-CTA Membrane with High CO2 Concentration Feed Gas

(37) A 76 mm (3 inch) diameter circle of HF-AF-Gelatin/CA-CTA membrane of Example 1 was evaluated for gas transport properties for 22 h of continuous testing using a natural gas feed containing high CO.sub.2 concentration of 50% CO.sub.2 and 50% CH.sub.4 at a feed pressure of 3549 kPa (500 psig) at 50 C. Table 2 shows P.sub.CO2/L and .sub.CO2/CH4 of the HF-AF-Gelatin/CA-CTA membrane of the present invention after 1 h, 2 h, 4 h, and 6 h of permeation. It can be seen from Table 2 that the HF-AF-Gelatin/CA-CTA membrane has P.sub.CO2/L of 336 GPU and .sub.CO2/CH4 of 16.0 after 1 h of permeation in the presence of 50% CO.sub.2/50% CH.sub.4 feed under 500 psig feed pressure. The membrane showed no drop in CO.sub.2 permeance and CO.sub.2/CH.sub.4 selectivity after 6 h of permeation in the presence of 50% CO.sub.2/50% CH.sub.4 feed under 500 psig feed pressure.

(38) TABLE-US-00002 TABLE 2 CO.sub.2/CH.sub.4 separation performance of HF-AF-Gelatin/CA-CTA membrane in the presence of high CO.sub.2 concentration natural gas feed .sup.a HF-AF-Gelatin/ P.sub.CO2/L CA-CTA Membrane (GPU) .sup.b .sub.CO2/CH4 1 h performance 335.3 16.0 2 h performance 334.9 16.1 4 h performance 335.9 16.1 6 h performance 335.9 16.1 .sup.a Tested at 50 C. under 3549 kPa (500 psig), 50% CO.sub.2/50% CH.sub.4 mixed gas pressure. .sup.b 1 GPU = 2.7 10.sup.5 m.sup.3 (STP)/m.sup.2 .Math.h .Math. kPa.

Example 5

CO2/CH4 Separation Performance of HS-AF-Gelatin/CA-CTA Membrane after Exposure to Liquid Toluene

(39) A 76 mm (3 inch) diameter circle of HS-AF-Gelatin/CA-CTA membrane of Example 2 and a 76 mm (3 inch) diameter circle of Gelatin/CA-CTA membrane of Comparative Example 2 were evaluated for CO.sub.2/CH.sub.4 separation properties using a natural gas feed containing 10% CO.sub.2 and 90% CH.sub.4 at a feed pressure of 6996 kPa (1000 psig) at 50 C. before and after the membrane coating layer surface was soaked with liquid toluene for 10 min. Experimental results in Table 3 demonstrated that the HS-AF-Gelatin/CA-CTA membrane showed 2.6% decrease in CO.sub.2 permeance and 5.3% drop in CO.sub.2/CH.sub.4 selectivity after the membrane coating layer surface was soaked in liquid toluene for 10 min. However, the Gelatin/CA-CTA membrane without fluoropolymer coating showed 51.6% drop in CO.sub.2 permeance after the membrane coating layer surface was soaked in liquid toluene for 10 min. The hydrophobic fluoropolymer coating of the HS-AF-Gelatin/CA-CTA membrane of Example 2 significantly reduced the wetting and membrane structure collapsing of the gelatin coated CA-CTA membrane underneath the fluoropolymer coating layer in the presence of liquid toluene. Therefore, the HS-AF-Gelatin/CA-CTA membrane of Example 2 with a hydrophobic fluoropolymer coating did not show significant CO.sub.2 permeance drop compared to the gelatin-coated CA-CTA membrane without fluoropolymer coating. These results demonstrated that the gelatin and then fluoropolymer double-coated CA-CTA membrane has high resistance to hydrocarbons and can be used for high hydrocarbon resistant applications and natural gas condensing service.

(40) TABLE-US-00003 TABLE 3 CO.sub.2/CH.sub.4 separation performance of HS-AF-Gelatin/CA-CTA membrane before and after exposure to liquid toluene .sup.a P.sub.CO2/L Membrane (GPU) .sup.b .sub.CO2/CH4 HS-AF-Gelatin/CA-CTA before 189.9 15.1 exposure to liquid toluene HS-AF-Gelatin/CA-CTA after 185.0 14.3 exposure to liquid toluene Gelatin/CA-CTA before 252.1 10.3 exposure to liquid toluene Gelatin/CA-CTA after 122.0 14.8 exposure to liquid toluene .sup.a Tested at 50 C. under 6996 kPa (1000 psig), 10% CO.sub.2/90% CH.sub.4 mixed gas pressure. .sup.b 1 GPU = 2.7 10.sup.5 m.sup.3 (STP)/m.sup.2 .Math.h .Math. kPa.

Example 6

Preparation of High Flux Gelatin-Coated and then AF1600-Coated Asymmetric Cellulose Acetate-Cellulose Triacetate Membrane (Abbreviated as HF-AF1600-Gelatin/CA-CTA-1-2)

(41) A relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate membrane support having a CO.sub.2/CH.sub.4 selectivity of about 1.4 and CO.sub.2 permeance of about 800 GPU (50 C., 250 psig, 10% CO.sub.2/90% CH.sub.4) was prepared in a conventional manner from a casting dope comprising, by approximate weight percentages, 14.4% cellulose triacetate, 5.1% cellulose acetate, 18.7% N-methyl pyrrolidone, 27.0% 1,3-dioxolane, 12.4% acetone, and 26.5% of non-solvents. A film was cast on a nylon fabric then gelled by immersion in a 1 C. water bath for about 10 minutes, and then annealed in a hot water bath at 80 to 90 C. for about 15 minutes to form a relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane. An aqueous solution of gelatin coating material was dripped onto the surface of the relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane to form a gelatin-coated asymmetric cellulose acetate-cellulose triacetate wet membrane. The gelatin-coated asymmetric cellulose acetate-cellulose triacetate wet membrane was dried with a continuous drying machine at 70 C. at 2.0 fpm. The dried gelatin-coated asymmetric cellulose acetate-cellulose triacetate membrane was dip coated with a AF1600 polymer solution in Fluorinert FC-770 solvent and dried at 85 C. to form the dried high flux gelatin-coated and then AF1600-coated asymmetric cellulose acetate-cellulose triacetate membrane (abbreviated as HF-AF1600-Gelatin/CA-CTA-1-2).

Example 7

Preparation of High Flux Chitosan-Coated and then AF1600-Coated Asymmetric Cellulose Acetate-Cellulose Triacetate Membrane (Abbreviated as HF-AF1600-Chitosan/CA-CTA-1-2)

(42) A relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate membrane support having a CO.sub.2/CH.sub.4 selectivity of about 1.4 and CO.sub.2 permeance of about 800 GPU (50 C., 250 psig, 10% CO.sub.2/90% CH.sub.4) was prepared in a conventional manner from a casting dope comprising, by approximate weight percentages, 14.4% cellulose triacetate, 5.1% cellulose acetate, 18.7% N-methyl pyrrolidone, 27.0% 1,3-dioxolane, 12.4% acetone, and 26.5% of non-solvents. A film was cast on a nylon fabric then gelled by immersion in a 1 C. water bath for about 10 minutes, and then annealed in a hot water bath at 80 to 90 C. for about 15 minutes to form a relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane. An aqueous acetic acid solution of chitosan coating material was dripped onto the surface of the relatively porous and substantial void-containing selective asymmetric cellulose acetate-cellulose triacetate wet membrane to form a chitosan-coated asymmetric cellulose acetate-cellulose triacetate wet membrane. The chitosan-coated asymmetric cellulose acetate-cellulose triacetate wet membrane was dried with a continuous drying machine at 70 C. at 2.0 fpm. The dried chitosan-coated asymmetric cellulose acetate-cellulose triacetate membrane was dip coated with a AF1600 polymer solution in Fluorinert FC-770 solvent and dried at 85 C. to form the dried high flux chitosan-coated and then AF1600-coated asymmetric cellulose acetate-cellulose triacetate membrane (abbreviated as HF-AF1600-Chitosan/CA-CTA-1-2).

Example 8

HF-AF1600-Gelatin/CA-CTA-1-2 and HF-AF1600-Chitosan/CA-CTA-1-2 Membranes for Fuel Gas Conditioning Application

(43) A 49.5 mm diameter circle of HF-AF1600-Gelatin/CA-CTA-1-2 membrane of Example 6 and a 49.5 mm diameter circle of HF-AF1600-Chitosan/CA-CTA-1-2 of Example 7 were evaluated for fuel gas conditioning application using a fuel gas feed containing 5% CO.sub.2, 70% CH.sub.4, 15% C.sub.2H.sub.6, and 10% C.sub.3H.sub.8 at a feed pressure of 3549 kPa (500 psig) at 50 C. The results in Table 4 show that both HF-AF1600-Gelatin/CA-CTA-1-2 membrane and HF-AF1600-Chitosan/CA-CTA-1-2 membrane permeate methane (CH.sub.4) faster than ethane (C.sub.2H.sub.6) and propane (C.sub.3H.sub.8) and have high CH.sub.4 permeability, good CH.sub.4/C.sub.2H.sub.6 selectivity and high CH.sub.4/C.sub.3H.sub.8 selectivity to produce high CH.sub.4 content fuel gas and C2+ enriched tail gas.

(44) TABLE-US-00004 TABLE 4 HF-AF1600-Gelatin/CA-CTA-1-2 and HF-AF1600-Chitosan/CA-CTA-1-2 membranes for fuel gas conditioning application .sup.a P.sub.CH4/L .sub.CH4/ .sub.CH4/ Membrane (GPU) .sup.b .sub.C2H6 .sub.C3H8 HF-AF1600-Gelatin/CA-CTA-1-2 22.9 3.20 20.1 HF-AF1600-Chitosan/CA-CTA-1-2 24.6 3.03 16.6 .sup.a Tested at 50 C. under 3549 kPa (500 psig) feed pressure with a fuel gas feed containing 5% CO.sub.2, 70% CH.sub.4, 15% C.sub.2H.sub.6, and 10% C.sub.3H.sub.8. .sup.b 1 GPU = 2.7 10.sup.5 m.sup.3 (STP)/m.sup.2 .Math.h .Math. kPa.