EA041523B1 - METHOD FOR SEPARATING ACID GAS COMPONENTS FROM NATURAL GAS FLOW - Google Patents

Description

Technical field

Natural gas resources vary greatly in composition. Natural gas predominantly contains CH ₄ but usually also contains varying amounts of contaminants (eg CO2, H2S, N2, heavy hydrocarbons and water). The concentration of these contaminants in natural gas must be reduced to a certain level before transportation and distribution. CO2 is the main contaminant in most natural gas resources and can exceed 50% and be under high pressure in some wells. According to the prior art, membrane materials for removing CO ₂ from natural gas are polymers. Non-crosslinked polymers can swell and expand at high concentrations of a highly sorbable penetrant (eg CO2 or heavier hydrocarbons). This phenomenon is called plasticization, and it usually results in increased membrane permeability and a significantly reduced selective membrane permeability.

As a result, non-crosslinked polymers are generally unsuitable for the treatment of high-pressure CO2 natural gas. The problem associated with membrane plasticization is not limited to natural gas processing. In contrast, membrane plasticization can occur in any gas separation application if the gas stream to be treated has a high partial pressure of the selectively sorbed gaseous component or components. Therefore, plasticization can prevent efficient commercial separation of many gases. Plastification, i.e. membrane swelling and resulting reduction in selective permeability has also been observed in some carbon molecular sieve (CMS) membranes, as shown, for example, by Octavio Salinas et al., High-performance carbon molecular sieve membranes for ethylene/ ethane separation derived from intrinsically microporous polyimide, Journal of Membrane Science, vol. 500, p. 115-123 (2006).

In the absence of plasticization, one can expect a significant decrease in the permeability and selective permeability of the membrane in applications related to the separation of corrosive gases, due to the potential saturation of micropores. If the partial pressure of the selectively adsorbed gaseous component(s) is high, then it can be expected that the micropores in the CMS membrane can be saturated with adsorbed gases in a relatively short period of time. For example, at high CO ₂ partial pressures, large amounts of CO ₂ can be expected to be absorbed/adsorbed relatively quickly into the micropores of the CMS membrane, which can reduce pore sizes and reduce the permeability/conductivity of the CMS membrane. Accordingly, a relatively rapid decrease in permeability/conductivity and selective permeability can be expected during treatment of a gas stream in which the gaseous component(s), eg CO2, to be sorbed has a high partial pressure. Severe degradation of membrane performance due to saturation is described, for example, in Cheryl W. Jones & William J. Koros, Carbon molecular sieve gas separation membranes - II. Regeneration following organic exposure, Carbon, vol. 32, issue 8, p. 1427-1432 (1994).

The essence of the invention

One aspect of the present invention relates to a process for separating acid gas components from a natural gas stream by contacting a natural gas stream having a CO ₂ partial pressure of at least 50 bar with a hollow fiber carbon molecular sieve (CMS) membrane obtained by pyrolysis of a polyimide polymeric precursor to produce a retentate stream having a reduced concentration of an acid gas component and a permeate stream having an increased concentration of an acid gas component, wherein the CO2/CH4 selectivity of the carbon molecular sieve membrane during the contact step is at least 80% of CO2/ CH4 selectivity of the carbon molecular sieve membrane when it is contacted with a natural gas stream having a CO2 partial pressure of 10 bar or less under the same conditions.

In one embodiment, the natural gas stream has a CO ₂ partial pressure of at least 60 bar.

In one embodiment, the natural gas stream has a CO ₂ partial pressure of at least 70 bar.

In one embodiment, the CO2/CH4 selectivity of the carbon molecular sieve membrane during the contact step is at least 85% of the CO2/CH4 selectivity of the carbon molecular sieve membrane at a CO ₂ partial pressure of 10 bar or less.

In one embodiment, the _CO2 / _CH4 selectivity of the carbon molecular sieve membrane during the contact step is at least 90% of the CO2/CH4 selectivity of the carbon molecular sieve membrane at a CO2 partial pressure of 10 bar or less.

In one embodiment, the CO2/CH4 selectivity of the carbon molecular sieve membrane during the contact step is at least 95% of the CO2/CH4 selectivity of the carbon molecular sieve membrane at a CO2 partial pressure of 10 bar or less. In some embodiments of the present invention, the natural gas stream may contain ethane, propane, butane isomers, pentane isomers, hexane isomers, heptane isomers, benzene, toluene, ethylbenzene, xylene isomers, or any combination of these substances. In some variants, osu

The contact of the natural gas stream with the carbon molecular sieve membrane may take place at a temperature ranging from 50 to 100°C. In some embodiments of the present invention, both the natural gas stream and the retentate stream are supercritical fluids.

Brief description of graphic materials

A clear understanding of the advantages and features of one or more embodiments of the present invention will become more apparent based on the illustrative and therefore non-limiting embodiments of the present invention illustrated in the drawings.

Fig. 1A is an illustration of the increase in CO2 permeability as a result of plasticization.

Fig. 1B is an illustration of the decrease in CO ₂ /CH ₄ selectivity as a result of plasticization.

Fig. 2A is an illustration showing the slit-like pore structure of carbon molecular sieve membranes.

Fig. 2B is an illustration showing the bimodal pore size distribution in carbon molecular sieve membranes.

Fig. 3 is a schematic representation of a system for separating an equimolar CO ₂ /CH ₄ gas mixture at high pressure (eg, 1500 psig) used in the experiments described in this publication.

Fig. 4 is an illustration showing the effects of test temperature on CO ₂ /CH ₄ gas permeability data in CMS hollow fiber membranes. The total feed gas pressure was 800 psi. inch (abs). The original composition contained 50/50% CO ₂ /CH ₄ at all temperatures except -30 and -50°C (10/90% CO ₂ /CH ₄ ).

Fig. 5 is an illustration of an example phase diagram of a gas mixture containing 50/50% CO ₂ /CH ₄ that contained 250 ppm toluene and 250 ppm heptane. The calculations were performed using the Soave-Redlich-Kwong (SRK) model.

Fig. 6A is a graph showing CO ₂ permeability data for an embodiment of a CMS hollow fiber membrane made according to the present invention at various feed gas pressures and for feed gases containing various amounts of hydrocarbon contaminants.

Fig. 6B is a graph showing CO ₂ /CH ₄ selectivity data of an embodiment of a CMS hollow fiber membrane made according to the present invention at various feed gas pressures and for feed gases containing various amounts of hydrocarbon contaminants.

Information confirming the possibility of carrying out the invention

Membranes are widely used for separating gases and liquids, including, for example, for separating acid gases such as CO ₂ and H2S from natural gas. The transport of gases through such membranes is usually modeled using the sorption-diffusion mechanism. More specifically, gas molecules are sorbed by the inlet membrane and eventually desorbed from the outlet membrane. Two important properties are commonly used to evaluate the performance of a membrane material: throughput and selectivity. The throughput is then defined as a measure of the intrinsic performance of the membrane material; more specifically, it is flow normalized to partial pressure and thickness, usually measured in barrels. The term permeability, which is sometimes used in place of the term permeability, is a measure of the pressure-normalized flow of a particular substance. In this case, selectivity is a measure of the ability of one gas to permeate through a membrane compared to another gas; for example, the permeability of CO2 versus CH4, measured as a dimensionless ratio.

At present, polymeric membranes are well studied and widely available for gas separation due to their manufacturability and low cost. However, CMS membranes have been shown to have attractive separation efficiency properties that are superior to those of polymeric membranes. CMS membranes are typically made by thermal pyrolysis of polymeric precursors. Many polymers have been used to make CMS membranes in fibrous form and in the form of dense films. Polyimides have a high glass transition temperature, are easy to process, and have some of the highest separation efficiencies of any polymeric membrane, even before pyrolysis.

Because carbon molecular sieve (CMS) membranes can be fabricated to selectively sorb the first gaseous component from a gas mixture, they can be used in a wide range of gas separation applications. For example, in various embodiments of the present invention, CMS membranes can be adapted to separate specific gases, including, but not limited to, CO ₂ and CH ₄ , H2S and CH ₄ , CO2/H2S and CH ₄ , CO ₂ and N ₂ , O2 and N ₂ , N2 and CH4, He and CH4, H2 and CH4, H2 and C2H4, ethylene and ethane, propylene and propane and ethylene/propylene and ethane/propane, all of which can be performed in a gas stream containing additional components. One of the many applications related to gas separation, for which there can be

- 2 041523 a particularly suitable CMS membrane is the separation of acidic gaseous components - CO2, H2S or combinations thereof - from a gas stream containing hydrocarbons, such as natural gas. CMS membranes can be made to selectively sorb these acid gases, providing a permeate stream having an increased acid gas concentration and a retentate stream having a reduced acid gas concentration.

At present, gas separation is often performed using polymeric membranes. However, if the gas stream to be treated/separated has a high partial pressure of the selectively sorbed gaseous component or gaseous components, then polymeric membranes tend to plasticize. When a polymer membrane undergoes plasticization, the porous structure of the polymer is destroyed. As a result, the selectivity of the membrane is significantly reduced, since gaseous components other than the desired component can pass through the membrane. This effect is shown in Fig. 1A and 1B.

Plasticization, when used in the context of the present invention, occurs when a highly sorbable penetrant reaches such a concentration that it causes the membrane to swell and expand. In the case of polymers, this usually means increased mobility of the polymer chain segments, which increases the diffusion coefficients of all penetrants and reduces the diffusion selectivity and, accordingly, the selective permeability of the membrane. It is known that high critical temperature gases such as CO ₂ and condensable hydrocarbons normally present in raw natural gas interact strongly with polymers and can potentially cause plasticization. It has also been found that plasticization, i.e. membrane swelling, and a resulting decrease in selective permeability, occurs in some carbon molecular sieve membranes. See, for example, Octavio Salinas et al., High-performance carbon molecular sieve membranes for ethylene/ethane separation derived from intrinsically microporous polyimide, Journal of Membrane Science, vol. 500, p. 115-123 (2006).

Surprisingly, it has been found that CMS membranes made according to the present invention are resistant to plasticization and are therefore suitable for gas separation even under conditions where the sorbed gaseous component (or components) has a high partial pressure in the gas stream to be treated, i.e. separation. It has also surprisingly been found that CMS membranes made according to the present invention do not undergo rapid saturation and therefore there is no significant reduction in their permeability and/or selectivity due to saturation when the sorbed gaseous component(s) has a high partial pressure in the gas stream. to be processed, i.e. separation. Since CMS membranes made according to the present invention do not undergo plasticization or rapid saturation, the selectivity of CMS membranes remains essentially unchanged at high partial pressures of sorbed gas(s) during the gas separation process, i.e. CMS membranes do not undergo a sharp decrease in selectivity such as that shown in FIG. 1B at high partial pressures of adsorbed molecules.

For example, if the CMS membranes disclosed in this publication are used to separate CO2 from natural gas (CH ₄ ) having high CO ₂ partial pressures, then the CO ₂ /CH ₄ selectivity of the CMS membrane is essentially the same as the CO ₂ /CH ₄ selectivity CMS membranes at low partial pressures of CO ₂ . For example, the selectivity of a CMS membrane at high CO ₂ partial pressure (e.g., at least 20 bar, at least 30 bar, at least 40 bar, at least 50 bar, at least 60 bar, at least 70 bar or at least 80 bar) may be at least 80%, more desirably at least 85%, more desirably at least 90%, more desirably at least 95% of the selectivity of the CMS membrane at low CO2 partial pressure (e.g. equal to or less than 10 bar, equal to about 10 bar, equal to about 9 bar, equal to about 8 bar, equal to about 7 bar, equal to about 6 bar, or equal to about 5 bar). For example, the experimental results given in Table. 1 demonstrate that the selectivity values of CMS membranes at a partial pressure of CO ₂ equal to 750 psi. inch (abs.) (or about 50 bar) are in the range from 90 to 95% of the selectivity values of the CMS membrane in the study of the permeability of CO ₂ and CH ₄ in relation to pure gas at a pressure of 100 psi. inch (abs.) (or approximately 7 bar).

CMS membranes can be converted to asymmetric hollow fiber membranes by controlled pyrolysis of the hollow fiber membrane precursor polymer and are able to simultaneously provide attractive throughput and separation efficiency without sacrificing scalability. The pore structure of CMS membranes is formed due to stacking faults in highly disordered and disoriented sp ² hybridized graphene-like layers. An illustration showing the porous structure of CMS membranes is shown in FIG. 2A. As shown in FIG. 2B, the pore structure of a CMS membrane can be represented by a bimodal pore size distribution. Micropores (7A<d<20A) provide most of the surface area for sorption and are responsible for the high permeability of the membrane. On the other hand, ultramicropores (d<7A) connecting the micropores regulate the diffusion capacity and, accordingly, diffusion selectivity.

- 3 041523 fusions.

US Patent No. 8,486,179, US Patent No. 9,211,504, and US Patent Application Publication No. 2015/009445 A1, the contents of which are incorporated herein by reference in their entirety, describe examples of methods for synthesizing CMS membranes.

Asymmetric hollow fiber membranes.

Carbon molecular sieve (CMS) membranes have shown great potential for gas separation, for example in the removal of carbon dioxide (CO ₂ ) and other acid gases from natural gas streams. Asymmetric CMS hollow fiber membranes are preferred for large scale high pressure applications.

Asymmetric hollow fiber membranes can potentially provide the high fluxes required for productive separation by reducing the separation layer to a thin continuous skin layer on the outer surface of the membrane. Morphology of asymmetric hollow fibers, i.e. a thin continuous skin layer supported by a porous substrate or substructure gives the fibers strength and flexibility and makes them able to withstand the large transmembrane pressure differences that drive transport. In addition, asymmetric half-fiber membranes provide a high surface area to volume ratio.

Asymmetric CMS hollow fiber membranes contain a thin and dense skin layer supported by a porous substructure. Asymmetric polymeric hollow fibers, or precursor fibers, are typically formed by the water-quenched dry-jet spinning process, also known as dry-wet phase separation or dry-wet spinning. The precursor fibers are then pyrolyzed at a temperature above the glass transition temperature of the polymer to produce asymmetric CMS hollow fiber membranes.

The polymer solution used to spin an asymmetric hollow fiber is called a dope. During spinning, the dope is surrounded by an internal fluid, which is known as channel forming fluid. The dope and the channel forming liquid are co-extruded through the spinneret into the air gap during the dry jet stage. The spun fiber is then immersed in an aqueous coagulation bath during the water quenching step, which initiates the wet phase separation process. After phase separation, the fibers are collected by means of a take-up drum and subjected to a solvent exchange process.

The solvent exchange process is an extremely important step in the membrane manufacturing process. If the porous precursor fibers contain water at the time when they are exposed to high temperatures, such as during drying or pyrolysis, then the removal of water causes significant changes in the structure and properties of the fiber and the resulting CMS membrane. Large capillary forces associated with the removal of water within small pore radii close to the skin layer cause a compaction of the structure in this area, resulting in a less permeable membrane. To prevent this, the solvent exchange process replaces the water present in the porous substructure of the precursor fiber with a fluid having a lower surface tension.

The standard solvent exchange process involves two or more steps, each step using a different solvent. The first step or series of steps involves contacting the precursor fiber with one or more solvents that effectively remove water from the membrane. It typically involves the use of one or more water-miscible alcohols that are sufficiently inert to the polymer. Aliphatic alcohols containing from 1 to 3 carbon atoms, i.e. methanol, ethanol, propanol, isopropanol, or combinations thereof, are particularly effective as the first solvent. The second step or series of steps involves contacting the precursor fiber with one or more solvents that effectively replace the first solvent with one or more low surface tension volatile organic compounds. Among the organic compounds that can be used as a second solvent are linear or branched aliphatic alkanes containing 5 or more carbon atoms.

The solvent exchange method typically includes soaking the precursor fibers in a first solvent for a first effective time, which may be up to several days, and then soaking the precursor fibers in a second solvent for a second effective time, which may also be up to several days. If the precursor fibers are produced continuously, i. e. in a commercial volume, the long precursor fiber can be continuously drawn through several contact tanks where it contacts each of the solvents. The solvent exchange process is generally carried out at room temperature.

Then, the precursor fibers are dried by heating to a temperature above the boiling point of the last solvent used in the solvent replacement process and subjected to pyrolysis to obtain asymmetric CMS hollow fiber membranes.

The choice of polymeric precursor, the formation and processing of the precursor fiber before pyrolysis, and the conditions of pyrolysis affect the performance properties of the asymmetric CMS hollow fiber membrane.

- 4 041523

precursor fibers.

The asymmetric polymeric precursor fiber may comprise any polymeric material which, when pyrolyzed, provides a CMS membrane that is permeable to the desired gases to be separated, such as carbon dioxide and natural gas, and in which at least one of the desired gases permeates through the CMS fiber with diffusion rate that differs from the diffusion rate of other components.

For example, the polymer may be a rigid glassy polymer (at room temperature) other than a rubbery polymer or a flexible glassy polymer. Glassy polymers differ from rubber-like polymers in the rate of segmental movement of polymer chains. Polymers in the glassy state do not have the rapid molecular motions that give rubbery polymers their fluid-like nature and ability to rapidly adjust segmental configurations over long distances (>0.5 nm). Glassy polymers exist in a non-equilibrium state with entangled molecular chains with immobile molecular frameworks in frozen conformations. The glass transition temperature (Tg) is the dividing point between the rubbery and glassy states. At temperatures above Tg, the polymer is in a rubbery state; at a temperature below Tg, the polymer is in a glassy state. In general, glassy polymers provide a selective medium for gas diffusion and are preferred for gas separation applications. Rigid glassy polymers are polymers with rigid backbones of polymer chains that have limited intramolecular rotational mobility and are characterized by high glass transition temperatures. Preferred rigid glassy polymeric precursors have a glass transition temperature of at least 200°C.

For rigid glassy polymers, the diffusion coefficient generally controls selectivity, and glassy membranes are generally selective for small, low boiling molecules. For example, preferred membranes may be made from rigid, glassy polymeric materials that are better permeable to carbon dioxide, hydrogen sulfide, and nitrogen than methane and other light hydrocarbons. Such polymers are well known in the art and include polyimides, polysulfones and cellulosic polymers.

Polyimides are the preferred polymeric precursor materials. Suitable polyimides include, for example, Ultem® 1000, Matrimid® 5218, 6FDA/BPDA-DAM, 6FDA-6FpDA and 6FDA-IPDA. The polyimide commercially available under the name Matrimid® 5218 is a thermoplastic polyimide based on the specific diamine 5(6)-amino-1-(4'-aminophenyl)-1,3-trimethylindane. Its structure looks like

The Matrimid® 5218 polymers used in the embodiments of the present invention were obtained from Huntsman International LLC. 6FDA/BPDA-DAM is a polymer derived from 2,4,6-trimethyl-1,3-phenylenediamine (DAM), 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA) and 5,5-[2,2 ,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofurandione (6FDA) and having the structure

To obtain these polymers, you can use available sources or synthesize them. For example, such a polymer is described in US patent No. 5234471, the contents of which are fully incorporated into this publication by reference.

Although polyimide polymers are used in the embodiments of the present invention, it is understood that polyimides are merely examples of rigid glassy polymers. Accordingly, the manufacture of CMS membranes from polyimides used in the embodiments of the present invention is an illustrative and representative example of the manufacture of CMS membranes from rigid glassy polymers as a class of materials. Similarly, the use of CMS membranes made from polyimide precursors for gas separation demonstrated in the embodiments of the present invention is an illustrative and representative example of the gas separation efficiency of CMS membranes made from rigid glassy polymers as a class of materials.

Examples of other suitable precursor polymers include polysulfones; polystyrenes,

- 5 041523 including styrene-containing copolymers, such as copolymers of acrylonitrile and styrene, copolymers of styrene and butadiene and styrenevinylbenzyl halide copolymers; polycarbonates; cellulosic polymers such as cellulose acetate butyrate, cellulose propionate, ethylcellulose, methylcellulose, nitrocellulose, and the like; polyamides and polyimides, including arylpolyamides and arylpolyimides; polyethers; polyetherimides; polyetherketones; polyarylene oxides such as polyphenylene oxide and polyxylene oxide; polyesteramide diisocyanate; polyurethanes; polyesters (including polyarylates) such as polyethylene terephthalate, polyalkyl methacrylates, polyacrylates, polyphenylene terephthalate, and the like; polypyrrolones; polysulfides; polymers consisting of monomers having alpha-olefinic unsaturation other than those mentioned above, e.g. polyethylene, polypropylene, poly(butene-1), poly(4-methylpentene-1), polyvinyls, e.g. alcohol, polyvinyl ethers such as polyvinyl acetate and polyvinyl propionate, polyvinyl pyridines, polyvinyl pyrrolidones, polyvinyl ethers, polyvinyl ketones, polyvinyl aldehydes such as polyvinyl formal and polyvinyl butyral, polyvinyl amides, polyvinylamines, polyvinyl urethanes, polyvinyl ureas, polyvinyl phosphates and polyvinyl sulfates; polyallyls; polybenzobenzimidazole; polyhydrazides; polyoxadiazoles; polytriazoles; polybenzimidazole; polycarbodiimides; polyphosphazins and the like; and interpolymers, including block interpolymers containing repeating units from the above, such as terpolymers of acrylonitrile, vinyl bromide and sodium salt of para-sulfophenyl methyl ethers; and graft copolymers and mixtures containing any of the above polymers. Representative substituents forming substituted polymers include halogens such as fluorine, chlorine and bromine; hydroxyl groups; lower alkyl groups; lower alkoxy groups; monocyclic aryl groups; lower acyl groups; and the like.

The asymmetric polymeric precursor fiber may be a composite structure comprising a first polymeric material supported by a porous second polymeric material. Composite structures can be formed using more than one polymeric material as the spinning material during the asymmetrical hollow fiber spinning process.

modifying agents.

The term modifying agent, when used in the context of the present invention, refers to a compound that is capable of reacting within the pores of a polymer precursor fiber to form a morphology stabilizer without any adverse effect on the mechanical properties of the fiber.

Preferred modifying agents are those compounds which undergo a polycondensation reaction to form siloxane bridges. For example, the modifying agent may be a silane having the general formula R1R ² R ³ R ⁴ Si, in which each of the radicals R ¹ , R ² , R ³ and R ⁴ , independently of the other radicals, is a C1-C6 alkyl or alkenyl group, an alkoxy group or a halogen atom; with the proviso that the silane contains at least one C1- _C6 alkyl or alkenyl substituent and at least one alkoxy or halogen substituent. At least one alkoxy or halogen substituent provides a silane having the ability to form a network of chain-type siloxane bonds. The at least one C1- _C6 alkyl or alkenyl substituent ensures that the treatment of the fiber with the modifying agent does not make the fiber brittle. Given this condition, the substituents can be varied to obtain a silane with desired properties. For example, by selecting substituent groups, the porosity of the resulting morphology stabilizer can be changed.

In some preferred embodiments of the present invention, vinyl trimethoxy silane (VTMS; vinyl trimethoxy silane) is used as a modifying agent for precursor treatment, however, other silanes can also be used as a modifying agent. For example, the modifying agent may be a monosilane or an oligosilane such as disiloxane or trisiloxane. For example, in various embodiments of the present invention, the modifying agent may be selected from the group consisting of vinyltrimethoxysilane, vinyltriethoxysilane, vinyldimethoxychlorosilane, vinyldiethoxychlorosilane, vinylmethoxydichlorosilane, vinylethoxydichlorosilane, vinyltrichlorosilane, vinylpentamethoxydisiloxane, divinyltetramethoxydisiloxane, and combinations thereof. In various highly preferred embodiments of the present invention, at least one alkoxy or halogen substituent contains a methoxy or ethoxy group. In various highly preferred embodiments of the present invention, at least one C1-C6 alkyl or alkenyl substituent contains vinyl. Particularly preferred builders include vinyltrimethoxysilane, vinyltriethoxysilane, ethyltrimethoxysilane and methyltrimethoxysilane.

Other modifiers include those that undergo a polycondensation reaction to form metal-oxo and/or metal-oxycarbide bonds. For example, the modifying agent may be a metal alkoxide having the general formula R1R ² R ³ R ⁴ M, where M is a metal and each of R ¹ , R ² , R ³ and R ⁴ , independently of the other radicals, is C 1 -C ₆ alkyl - 6 041523 noah or alkenyl group, alkoxy group or a halogen atom; provided that the metal alkoxide contains at least one C1-C6 alkyl or alkenyl substituent and at least one alkoxy or halogen substituent. At least one alkoxy or halogen substituent gives the metal alkoxide the ability to form a network of chain-type metal-oxo and/or metal-oxycarbide bonds. The at least one C1-C6 alkyl or alkenyl substituent ensures that the metal alkoxide treatment of the fiber does not make the fiber brittle. Given this condition, the substituents can be varied to obtain a metal alkoxide with the desired properties. For example, by selecting substituent groups, the porosity of the resulting morphology stabilizer can be changed. In preferred embodiments of the present invention, the metal M is selected from the group consisting of Ge, B, Al, Ti, V, Fe, and combinations thereof.

Processing and pyrolysis conditions.

When modifying a polymer precursor fiber to produce a substantially non-collapsing asymmetric CMS hollow fiber membrane, the method includes the steps of obtaining a polymer precursor, obtaining a contact solution containing a modifying agent (which is present in solution at a concentration equal to 100 wt.% or less than 100 wt.%), and contacting at least a portion of the polymeric precursor with at least a portion of the contact solution containing the modifying agent to obtain a modified polymeric precursor which upon pyrolysis produces a substantially non-collapsing asymmetric CMS modified hollow fiber membrane. Preferably, the polymeric precursor is soaked in a solution containing the modifying agent at the desired concentration for a period of time sufficient for the modifying agent to penetrate into the pores of the precursor fiber substructure. Preferably, this period of time is in the range of from about 30 minutes to about 24 hours.

The solution containing the modifying agent does not need to come into contact with the end of the hollow precursor fiber in order to penetrate the pores of the precursor fiber substructure. On the contrary, it has been found that the modifying agent can penetrate the outer skin layer of the precursor fiber in the radial direction and thus penetrate into the pores of the fiber substructure.

The contact of the precursor fiber with the modifying agent is preferably carried out at room temperature. However, in some additional embodiments of the present invention, the contact temperature can be maintained within a range selected from the following ranges: from about 20° C. to about the glass transition temperature of the polymeric precursor; from about 100°C to about the glass transition temperature of the polymeric precursor; and from about 100°C to about 250°C.

In various embodiments of the present invention, the reaction of the modifying agent to form a morphology stabilizer may require the addition of a catalyst. For example, if vinyltriethoxysilane is used as the modifying agent, it may be desirable to add a catalyst to promote the sol-gel reaction. This is due to the slow reaction of ethoxy groups compared to methoxy groups such as vinyltrimethoxysilane (VTMS). The sol-gel reaction can be accelerated by the addition of an acid, such as a mineral acid, since it is known in the art that the sol-gel reaction is often greatly accelerated under acidic conditions. Preferred acid catalysts include any readily available mineral acid, such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, boric acid, hydrofluoric acid, hydrobromic acid, perchloric acid, and combinations thereof.

After contacting the precursor fiber with the modifying agent, for example, by soaking in a solution containing the modifying agent at a selected concentration, the treated precursor fiber is brought into contact with moisture, for example, by exposing the fiber to an atmosphere containing moisture. The moisture-containing atmosphere may be an atmosphere having a relative humidity ranging from about 50% to about 100%. Precursor fibers are preferably exposed to an atmosphere containing moisture for a period of time ranging from about 1 hour to about 60 hours.

The treated precursor fibers are then dried and subjected to pyrolysis. The pyrolysis is preferably carried out under an inert atmosphere. The pyrolysis temperature may range from about 500°C to about 900°C; alternatively, the pyrolysis temperature may range from about

500°C to about 800°C; alternatively, the pyrolysis temperature may range from about

500°C to about 700°C; alternatively, the pyrolysis temperature may range from about

500 to 650°C; alternatively, the pyrolysis temperature may range from about 500 to 600°C;

alternatively, the pyrolysis temperature may range from about 500 to 550°C; alternatively, the pyrolysis temperature may range from about 550°C to about 700°C; alternatively, the pyrolysis temperature may range from about 550°C to about 650°C; alternatively, the pyrolysis temperature may range from about 600°C to about 700°C; alternatively, the pyrolysis temperature may range from about 600°C to about 650°C. Typically, the pyrolysis temperature is reached in a process in which the temperature slowly rises

- 7 041523 is raised. For example, when using a pyrolysis temperature of 650°C, the pyrolysis temperature can be obtained by raising the temperature in the range of 50 to 250°C at an increase rate of 13.3°C/min, then raising the temperature in the range of 250 to 635° C at an increase rate of 3.85°C/min, and then increasing the temperature in the range of 635 to 650°C at an increase rate of 0.25°C/min. After the pyrolysis temperature has been reached, the fibers are heated at the pyrolysis temperature for a holding time which may be several hours.

Polymeric precursor fibers can also be bundled and pyrolyzed in bundle form to produce a large amount of CMS-modified hollow fiber membranes in a single pyrolysis cycle. Although pyrolysis in the context of the present invention is generally understood to mean pyrolysis of a precursor fiber, any description of pyrolysis used in this publication should be understood to include pyrolysis of bundled precursor fibers and unbunched precursor fibers.

In various embodiments of the present invention, the processing of the polymeric precursor fiber may be combined with a solvent exchange process. After the precursor fibers have been spun, for example by dry blasting with water quenching, the fibers are subjected to a process known as solvent exchange. To maintain the porosity of the fibers during drying, it is necessary to remove the water contained within the pores of the membrane. Accordingly, in the solvent exchange process, the water present in the porous substructure of the fiber is replaced with an organic compound having a low surface tension. The precursor fiber is subjected to a solvent exchange process for a period of time during which the organic compound displaces the water present in the pores of the fiber.

The solvent change preferably includes two or more steps, each step using a different solvent change material. For example, a standard solvent exchange process involves removing the water present in the membrane with a first solvent and then replacing the alcohol with a second solvent. The first step uses one or more solvent replacement materials that contain a water-miscible alcohol that is sufficiently inert to the polymer. Any compound that effectively displaces water in the membrane may be considered for use as the first solvent. Aliphatic alcohols containing from 1 to 3 carbon atoms, i.e. methanol, ethanol, propanol, isopropanol, and combinations thereof, are particularly effective as a first solvent replacement material.

During the second stage, the alcohol is effectively replaced with one or more volatile organic compounds having a low surface tension. Any organic solvent having a sufficiently low surface tension to prevent damage to the pores in the membrane during heating may be considered for use as a second solvent. Among the organic compounds that are particularly suitable for use as a second solvent are linear or branched aliphatic alkanes containing 5 or more carbon atoms. Toluene has also been proposed for use as a second solvent. n-hexane has been found to be a particularly suitable organic compound for use as a second solvent replacement material. The first and second solvent replacement materials should preferably be sufficiently chemically inactive with respect to the membrane to prevent significant degradation of the membrane properties.

Although the method described in this exemplary embodiment of the present invention includes only two steps, the solvent exchange method may include any number of steps and any number of solvents. Any number of solvent replacement materials may be used in the solvent exchange process, wherein the solvent exchange material in each successive step effectively replaces the solvent exchange material from the previous stage.

It has now been found that by using a solvent replacement material that includes some modifying agent, an asymmetric CMS-modified hollow fiber membrane having increased gas permeability can be obtained. Since the treatment of the precursor fiber with a modifying agent to limit substructure drop can be performed in conjunction with the solvent exchange process, it is possible to manufacture a CMS-modified hollow fiber membrane having increased gas permeability without the need for an additional process step other than those typically carried out during manufacture. asymmetric CMS hollow fiber membrane. The method of treating a precursor fiber with a modifying agent described in this publication as part of the solvent exchange step can be considered as an improved solvent exchange method.

Accordingly, in various embodiments of the present invention, the modifying agent is added to the second solvent during the solvent exchange process. For example, a modifying agent is added to an organic solvent that has a sufficiently low surface tension to maintain pores in the membrane during drying. In a suitable embodiment of the present invention, the organic solvent is n-hexane. The concentration of modifying

- 8 041523 agent in an organic solvent can be selected as described in this publication.

In an improved solvent exchange method, the precursor fiber is soaked in a solution containing an organic compound, such as n-hexane, and a modifying agent, such as VTMS, for a period of time that allows the modifying agent to effectively react with a portion of the water present in the pores of the fiber and replace another part of the water in the pores of the fiber with an organic solvent. Thus, the precursor fibers gain the benefits of treatment with the modifying agent without losing the benefits of the conventional solvent exchange process.

The ability of the modifying agent to penetrate the outer skin layer of the asymmetric precursor fiber makes it particularly suitable for treating the fiber during the solvent exchange process. In the commercial process, the precursor fiber is often transported through the material to replace the solvent in a continuous manner. Accordingly, the ends of the fiber rarely, if ever, come into contact with the solvent exchange material. Therefore, for efficient processing during commercial production of an asymmetric CMS hollow fiber membrane, the modifying agent reaches the precursor fiber substructure through the outer skin layer of the fiber.

experimental procedure.

Preparation of 6FDA/BDPA-DAM precursor hollow fiber membranes.

Monolithic 6FDA/BDPA-DAM precursor hollow fiber membranes were produced using a water-quenched dry jet spinning process.

Preparation of CMS hollow fiber membranes.

CMS hollow fiber membranes were prepared by controlled pyrolysis of 6FDA/BDPADAM precursor hollow fibers using the heating protocol below under continuous purging conditions (20 ^cc /min) with ultra-high-purity (UHP; ultra-high-purity) argon. Details regarding pyrolysis conditions can be found in other publications.

The heating protocol is as follows:

1) from 50 to 250°C (13.3°C/min);

2) from 250 to 535°C (3.85°C/min);

3) from 535 to 550°C (0.25°C/min);

4) heat treatment at 550°C for 120 min;

5) natural cooling.

Assembly of CMS hollow fiber membrane modules.

Assembly of CMS hollow fiber membrane modules using Duralco™ 4525 epoxy.

The resin (100 parts) was mixed with the hardener (8 parts) and stirred vigorously at room temperature for 2 minutes in an ultrasonic bath to ensure degassing. The thick epoxy paste was poured onto one side of the fiber module and allowed to cure at room temperature for 12 hours. The process was repeated on the other side of the fiber module. The epoxy was cured for a further 12 hours at room temperature prior to permeation testing.

Assembly of CMS hollow fiber membrane modules using 3M DP-100 epoxy.

The epoxy resin was first poured onto one side of the fiber module through a mixing nozzle. After curing for 10 minutes at room temperature, the process was repeated on the other side of the fiber module. The epoxy was cured for a further 12 hours at room temperature prior to permeation testing.

Permeability characteristics of CMS hollow fiber membranes.

Permeability to pure gases of a mixture of CO2/CH4 at a pressure of 100 psi. inch (abs.).

Pure CO2/CH4 permeability measurements were made at 35°C using a constant volume permeability system. The feed pressure was about 100 psi. inch (abs.), and the output was a vacuum.

Permeability of an equimolar CO2/CH4 gas mixture at a pressure of 1500 psi. inch (abs.).

Permeability measurements of an equimolar CO2/CH4 gas mixture were taken at 35°C using the constant pressure permeability system shown in FIG. 3. Supply pressure was about 1500 psi. inch (abs.) (approximately 103 bar), and the output was atmospheric pressure. As shown in FIG. 3, a piston pump was used to maintain high pressure at the membrane inlet. The flux permeated through the membrane was kept below 1% by adjusting the volume velocity of the retentate retained by the membrane. A Varian 450 gas chromatograph was used to determine the compositions of permeates passed through the membrane. Membrane separation factors were calculated from at least three gas chromatograph injections.

- 9 041523

Results and discussion.

Permeability of CMS hollow fiber membranes for high pressure CO2/CH4 gas mixtures.

Data on the permeability of CMS hollow fiber membranes subjected to pyrolysis without pre-treatment with 25 wt.% VTMS ⁷ and after such pre-treatment, are presented in table. 1.

Table 1

Initial mixture P( _CO2 )/GPU a (CO ₂ /CH ₄ ) Without VTMS pretreatment Pure gas permeability CO ₂ /CH ₄ mixture (100 psi (abs.)) 138 39 Permeability of an equimolar CO ₂ /CH ₄ gas mixture (1500 psi (abs.)) 81 36 With VTMS pre-treatment (25 wt%) Pure gas permeability CO ₂ /CH ₄ mixture (100 psi (abs.)) 193 47 Permeability of an equimolar CO ₂ /CH ₄ gas mixture (1500 psi (abs.)) 93 49

Both modules were assembled using Duralco™ 4525 epoxy. CO2/CH4 separation factors measured at 1500 psi. inch (abs.), generally consistent with the ideal selectivity for a mixture of CO2/CH4, measured at a pressure of 100 psi. inch (abs.). This indicates that the hollow fiber membranes tested by CMS have excellent plasticization resistance at high pressure natural gas with a high CO ₂ content. The CMS hollow fiber membrane, pyrolyzed after pretreatment with 25 wt% VTMS, showed quite attractive natural gas separation efficiency at 1500 psi. inch (abs.), with a CO2 permeability equal to 93 gas permeance units (GPU; from English: gas permeance units), and a CO2 / CH4 separation factor of 49.

Tab. 1 also demonstrates that the CO ₂ permeability values of the investigated CMS hollow fibers were lower at 1500 psi. inch (abs.). This can be explained by the shape of the Langmuir isotherm for CMS materials, which leads to a decrease in the sorption coefficient with increasing pressure. In addition, it should be noted that the CO2 permeability and CO ₂ /CH ₄ separation factor were higher for CMS hollow fibers pyrolyzed after pre-treatment with 25 wt.% VTMS.

Effect of Epoxy Resin on CMS Permeability Data of High Pressure CO2/CH4 Hollow Fiber Membrane Gas Mixtures

In table. 2 compares the CMS permeability data of hollow fiber membrane modules assembled using Duralco™ 4525 epoxy and 3M DP-100 epoxy.

table 2

Initial mixture P( _CO2 )/GPU a (CO ₂ /CH ₄ ) Duralco™ 4525 Epoxy Pure gas permeability CO ₂ /CH ₄ mixture (100 psi (abs.)) 138 39 Permeability of an equimolar CO ₂ /CH ₄ gas mixture (1500 psi (abs.)) 81 36 Epoxy resin ZM DP-100 Permeability to pure gases of a mixture of CO ₂ /CH ₄ (100 psi (abs.)) 166 34 Permeability of an equimolar CO ₂ /CH ₄ gas mixture (1500 psi (abs.)) 71 23

Both CMS hollow fiber membranes were pyrolyzed without VTMS pre-treatment. While the module assembled with Duralco™ 4525 epoxy exhibited a stable CO2/CH4 separation factor at 1500 psi. in. (abs.) (36 vs. 39), the module assembled using 3M DP-100 epoxy exhibited a significantly lower separation factor at 1500 psi. inch (abs.) (23 vs. 34). These results indicate that 3M DP-100 Epoxy may not be suitable for high pressure natural gas permeation through CMS hollow fiber membranes.

The above examples demonstrate a process for manufacturing CMS hollow fiber membranes with excellent plasticization resistance for processing natural gas under pressure.

- 10 041523 high pressure, high condensable CO2 content. With the penetration of an equimolar gas mixture of CO2 / CH4 under a pressure of 1500 psi. inch (abs.), CMS hollow fiber membranes obtained by pyrolysis of precursor fibers from 6FDA/BDPA-DAM pre-treated with VTMS (25 wt.%) showed an attractive CO2 permeability of 93 gpu and a CO ₂ /CH ₄ separation factor equal to 49. Although the CMS hollow fiber membranes made from the 6FDA/BDPADAM precursor under the above pyrolysis conditions were used in the natural gas processing example, the method can easily be adapted to CMS membranes made from other precursor materials under different pyrolysis conditions for a wide range of applications where plasticization or saturation may be a problem.

Furthermore, while the examples use a test temperature of 35° C., other test temperatures may be used. It should be noted that the transport characteristics of CMS materials depend on the test temperature. Typically, as the test temperature increases, the permeability increases and the selectivity decreases. In FIG. 4 shows an example of the effects of test temperature on CO2/CH4 gas permeability data in CMS hollow fiber membranes.

If the CMS membranes disclosed in this publication are used to separate CO ₂ from natural gas (CH ₄ ) having high partial pressures of CO ₂ , CO ₂ /CH ₄ , the selectivity of the CMS membrane can be substantially the same as the CO ₂ /CH ₄ selectivity of the CMS membranes at low partial pressures of CO ₂ . For example, the selectivity of a CMS membrane at a high partial pressure of CO ₂ (for example, at a pressure of at least 20 bar, at least 30 bar, at least 40 bar, at least 50 bar, at least 60 bar, at least at least 70 bar or at least 80 bar) may be equal to at least 80%, more preferably at least 85%, more preferably at least 90%, more preferably at least 95% of the selectivity of the CMS membrane at low partial pressure CO2 (for example, equal to 10 bar or less, equal to about 10 bar, equal to about 9 bar, equal to about 8 bar, equal to about 7 bar, equal to about 6 bar, or equal to about 5 bar). For example, the experimental results given in Table. 1 demonstrate that the selectivity values of CMS membranes at a partial pressure of CO ₂ equal to 750 psi. inch (abs.) (or about 50 bar) are in the range of 90 to 95% of the selectivity values of the CMS membrane obtained in the study of permeability to pure CO ₂ and CH ₄ gases at a pressure of 100 psi. inch (abs.) (or approximately 7 bar).

Additional test results over a wide range of pressures are shown in FIG. 6A and 6B. In this _CO2 and CO2/CH4 permeability test, the selectivity of a carbon molecular sieve membrane made according to the present invention was determined by applying a range of feed gases containing a 50/50 mixture of CO2 and CH4 at pressures ranging from 100 pounds / sq. in. (abs.) up to 1800 psi inch (abs). The test was carried out at 35°C. As shown in FIG. 6B, the CO2/CH4 selectivity of the carbon molecular sieve membrane was not substantially adversely affected by the high partial pressures of CO2 in the feed gases.

Moreover, the test was carried out using source gases containing varying amounts of hydrocarbon contaminants. Many natural gas sources contain hydrocarbon contaminants such as ethane, propane, butane isomers, pentane isomers, hexane isomers, heptane isomers, benzene, toluene, ethylbenzene, xylene isomers, and combinations thereof. These hydrocarbon contaminants are strong plasticizing agents. For example, even at levels as low as 250 ppm, hydrocarbon contaminants such as toluene and n-heptane can cause plasticization and a severe reduction in selectivity in non-crosslinked polymeric membranes. However, as shown in FIG. 6B, the inclusion of toluene and n-heptane contaminants in the feed gases had little, if any, effect on the CO2/CH4 selectivity of the carbon molecular sieve membranes over the entire pressure range tested.

Methods for the separation of supercritical media.

A supercritical fluid is any substance that is at a temperature and pressure above its critical point where no distinct liquid and gas phases exist. For example, in FIG. 5 shows the critical point for a feed mixture containing 50/50% CO ₂ /CH ₄ and 250 ppm toluene and 250 ppm heptane. The critical point is indicated by a triangle.

It has now been found that carbon molecular sieve membranes can be used to separate supercritical fluids in essentially the same manner that is used to separate gases. In fact, it has been shown that a number of advantages can be obtained by bringing the feed gas to the supercritical state prior to contact of the feed gas with the CMS membrane.

Gas separation using carbon molecular sieve membranes traditionally requires pre-treatment of the feed gas to prevent the formation of liquids on the membrane surface. The formation of liquids on the membrane surface is a problem since the liquid acts as a resistive layer and therefore reduces the flow through the membrane. In addition, the liquid layer typically consists of condensable components such as higher hydrocarbons and water, which

-

Claims (6)

rye can have negative effects on some membranes. Liquids form on the membrane because (1) the composition of the gas changes from the source gas to the retentate as the sorbed gas (or gases) permeates, and (2) the temperature changes due to the Joule-Thompson effect as the gas passes through membrane. By maintaining the feed gas and retentate streams under supercritical conditions, there is no liquid phase during contact of the feed gas stream with the membrane, thereby eliminating the formation of liquids on the membrane surface without the need for pre-treatment. The feed gas can be brought to temperatures and pressures in the supercritical region by heating, compression, or a combination of both. In particular, the feed gas should be brought to a point within the supercritical region that ensures that the retentate stream is also at supercritical conditions, at least in close proximity to the membrane. Because the supercritical point of a gas mixture depends on the relative amounts of each gas in that mixture, the supercritical point of the retentate will differ from that of the feed gas due to sorption of the desired gas component(s). The gaseous permeate does not need to be maintained at supercritical conditions because the low pressure of the permeate (and, at least in the case of separating CO ₂ from natural gas, the limited amount of hydrocarbons in the permeate) reduces the likelihood of liquid formation. Due to the effects of plasticization and saturation, which can be expected during separations of aggressive gases, i.e. gas separations in which the partial pressure(s) of one or more gas species that are sorbed by the membrane in the feed gas is(are) high, supercritical operation can be expected to have adverse effects on the ability membranes for gas separation. However, given the unexpected results disclosed in this publication, membrane separation under supercritical conditions becomes practically possible. In fact, at least in separating CO ₂ from natural gas, there seems to be an additional benefit in that a large amount of CO ₂ limits the adverse effect of highly condensable components such as toluene, which are known to reduce permeability. even in very small amounts, as shown in FIG. 6A. various aspects. It can be seen that the described embodiments of the present invention provide unique and novel methods for corrosive gas separation that have a number of advantages over methods known in the art. Although this publication shows and describes some specific structures that enable the present invention to be carried out, it will be obvious to those skilled in the art that various modifications and permutations of the elements can be made without deviating from the essence and scope of the underlying idea of the invention and that the present invention is not limited to specific forms shown and described in this publication, except as defined in the scope of the claims. CLAIM 1. A process for separating acidic gas components from a natural gas stream, wherein a) taking a hollow fiber carbon molecular sieve membrane obtained by pyrolysis of a polyimide polymer precursor; And b) contacting a natural gas stream having a CO ₂ partial pressure of at least 50 bar with the carbon molecular sieve membrane to obtain i) a retentate stream having a reduced concentration of an acid gas component; and ii) a permeate stream having an increased concentration of an acid gas component; wherein the CO ₂ /CH ₄ selectivity of the carbon molecular sieve membrane during the contact step is at least 80% of the CO ₂ /CH ₄ selectivity of the carbon molecular sieve membrane when it is in contact with a natural gas stream having a CO ₂ partial pressure of 10 bar or less, under the same conditions. 2. Process according to claim 1, wherein the natural gas stream has a CO ₂ partial pressure of at least 60 bar. 3. Process according to claim 1, wherein the natural gas stream has a CO ₂ partial pressure of at least 70 bar. 4. The method according to any one of claims 1 to 3, wherein the _CO2 / _CH4 selectivity of the carbon molecular sieve membrane during the contact step is at least 85% of the CO2/CH4 selectivity of the carbon molecular sieve membrane at a CO2 partial pressure of 10 bar or less. 5. The method according to claim 4, wherein the CO ₂ /CH ₄ selectivity of the carbon molecular sieve membrane during the contact step is at least 90% of the CO ₂ /CH ₄ selectivity of the carbon molecular sieve membrane at a CO ₂ partial pressure of 10 bar or less. 6. The method according to claim 5, wherein the CO ₂ /CH ₄ selectivity of the carbon molecular sieve membrane during the contact step is at least 95% of the CO ₂ /CH ₄ selectivity of the carbon molecular sieve membrane at a CO ₂ partial pressure of 10 bar or less. -