Separation membrane, coating liquid for forming separation function layer, and separation membrane module

Abstract

This separation membrane comprises a separation function layer and a gas permeation layer that supports the separation function layer. The separation function layer contains a hydrophilic polymer, a hydrophobic polymer, and a carbon dioxide carrier. The hydrophilic polymer and the hydrophobic polymer are mingled in the separation function layer.

Description

Separation membrane, coating liquid for forming separation functional layer, and separation membrane module Cross-reference of related applications

[0001] This disclosure claims priority with respect to Japanese Patent Application No. 2025-42793, filed with the Japan Patent Office on 17 March 2025, and the entirety of the said patent application is incorporated herein by reference.

[0002] The present invention relates to a separation membrane, a coating liquid for forming a separation functional layer, and a separation membrane module for efficiently separating and recovering carbon dioxide from a gas containing carbon dioxide.

[0003] In recent years, methods for selectively separating carbon dioxide from mixed gases containing carbon dioxide have been investigated. One such method involves separating carbon dioxide using a separation membrane with a hydrophilic separation layer. Furthermore, to improve the selectivity and permeation rate of carbon dioxide, as well as the stability of the separation membrane, separation membranes using hydrophilic and hydrophobic components have been developed.

[0004] For example, Patent Document 1 describes a CO2-12000- ２ A facilitated transport membrane is disclosed.

[0005] Patent Document 2 discloses a composite for carbon dioxide separation comprising a gas-permeable support, a carbon dioxide separation layer containing a water-absorbing polymer and a carbon dioxide carrier, and a porous protective layer having water vapor permeability. It is proposed that the porous protective layer and at least the surface of the gas-permeable support that is in contact with the carbon dioxide separation layer be made hydrophobic.

[0006] Patent Document 3 discloses a separation membrane for separating carbon dioxide, comprising a separation functional layer having a polymer matrix such as polyvinyl alcohol and containing an amine compound and a crack inhibitor, and hydrophobic layers laminated on both sides of the separation functional layer.

[0007] Patent Document 4 discloses a separation membrane comprising a separation functional layer disposed on a porous support layer for separating carbon dioxide, a hydrophilic porous membrane disposed on the gas supply side of the separation functional layer, and a hydrophobic porous membrane disposed on the gas supply side of the hydrophilic porous membrane.

[0008] Patent Document 5 discloses a separation membrane comprising a separation functional layer containing a polyether block amide resin or the like, a porous support supporting the separation functional layer, and an intermediate layer disposed between the separation functional layer and the porous support, the intermediate layer containing a matrix and nanoparticles dispersed in the matrix.

[0009] Patent Document 6 discloses a separation membrane comprising a separation functional layer containing a polymer such as polyacrylamide, a porous support supporting the separation functional layer, and an intermediate layer disposed between the separation functional layer and the porous support, formed from an emulsion resin composition of a silicone polymer and a hydrophilic polymer.

[0010] International Publication No. 2009 / 093666, Japanese Patent Publication No. 2014-79751, Japanese Patent Publication No. 2014-46305, Japanese Patent Publication No. 2018-130699, Japanese Patent Publication No. 2020-163375, International Publication No. 2022 / 190612

[0011] The separation of carbon dioxide from a mixed gas containing carbon dioxide is often carried out under pressure. Therefore, carbon dioxide separation membranes with a separation functional layer are required to have sufficiently high selectivity and permeability of carbon dioxide even under pressure. Furthermore, there is a desire for separation membranes that can be formed with a simpler manufacturing process and at a lower manufacturing cost.

[0012] One aspect of the present invention relates to a separation membrane comprising a separation functional layer and a gas permeable layer supporting the separation functional layer, wherein the separation functional layer comprises a hydrophilic polymer, a hydrophobic polymer, and a carbon dioxide carrier, and the hydrophilic polymer and the hydrophobic polymer are mixed in the separation functional layer.

[0013] Another aspect of the present invention relates to a coating liquid for forming a separation functional layer, comprising an aqueous solution of a hydrophilic polymer, a hydrophobic polymer dispersed in the aqueous solution, and a carbon dioxide carrier dissolved in the aqueous solution.

[0014] A further aspect of the present invention relates to a separation membrane module comprising the separation membrane and a porous support for supporting the gas permeable layer.

[0015] According to the present invention, a separation membrane having excellent carbon dioxide selectivity and carbon dioxide permeation rate stably, and a coating liquid for forming a separation functional layer used in the manufacture of the separation membrane can be provided simply and at low cost.

[0016] This is a schematic diagram showing the separation membrane according to the present invention. This is an image obtained by measuring the cross-section of the separation membrane obtained in Example 1 with a transmission electron microscope (TEM). This is a phase image obtained by measuring the surface of the separation membrane obtained in Example 1 with a scanning probe microscope (SPM).

[0017] Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0018] Embodiments of the present invention will be described below, but the present invention is not limited to the embodiments described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined as long as the lower limit is not greater than or equal to the upper limit. When multiple materials are given as examples, one of them may be selected and used alone, or two or more may be used in combination.

[0019] <Separation Membrane> The separation membrane according to the present invention (hereinafter also referred to as "separation membrane (M)") will be described with reference to Figure 1. Figure 1 is a schematic diagram showing the separation membrane (M).

[0020] The separation membrane (M) 1 is a separation membrane for separating and recovering carbon dioxide from a carbon dioxide-containing gas, and comprises a separation functional layer 2 and a gas permeable layer 3 that supports the separation functional layer 2. The separation functional layer 2 contains a hydrophilic polymer, a hydrophobic polymer, and a carbon dioxide carrier, wherein the hydrophilic polymer and the hydrophobic polymer are mixed in the separation functional layer 2. In other words, the separation functional layer 2 is composed of a mixture of a hydrophilic polymer, a hydrophobic polymer, and a carbon dioxide carrier.

[0021] The separation functional layer 2 is a layer that selectively permeates carbon dioxide from a carbon dioxide-containing gas. Carbon dioxide in the gas taken into the separation membrane (M) 1 reacts with the carbon dioxide carrier contained in the separation functional layer 2 to generate carbamate and bicarbonate ions. Thereby, carbon dioxide in the gas permeates the separation functional layer 2 as bicarbonate ions and moves to the gas permeable layer 3. On the other hand, a pseudo-crosslinked structure is formed in the separation functional layer 2 by carbamate, and permeation of molecules other than carbon dioxide in the gas is suppressed. As a result, carbon dioxide is selectively separated from the gas by the separation functional layer 2.

[0022] The hydrophilic polymer in the separation functional layer 2 has an action of immobilizing the carbon dioxide carrier in the separation functional layer 2. Further, the hydrophilic polymer in the separation functional layer 2 forms a network structure so that the separation membrane (M) 1 can maintain its membrane structure even under pressure. The hydrophobic polymer in the separation functional layer 2 is more permeable to gas than the hydrophilic polymer, and improves the permeation rate of carbon dioxide. In addition, due to the action of the hydrophobic polymer, cracks are less likely to occur in the separation membrane (M) 1. That is, the separation membrane (M) 1 of the present invention, in which a hydrophilic polymer and a hydrophobic polymer are mixed in the separation functional layer 2, maintains strength and performance even under pressure, and exhibits excellent carbon dioxide selectivity and carbon dioxide permeation rate.

[0023] In an arbitrary cross-section of the separation functional layer 2, the hydrophilic polymer and the hydrophobic polymer may exhibit a sea-island phase separation structure. The sea 2a is composed of the hydrophilic polymer and the carbon dioxide carrier, and the islands 2b are composed of the hydrophobic polymer. Since the hydrophilic polymer phase contributing to the generation of carbamate and bicarbonate ions and the hydrophobic polymer phase with high gas permeability are dispersedly present in the separation functional layer 2, the permeation rate of carbon dioxide in the thickness direction of the membrane is further improved.

[0024] Furthermore, when the surface of the separation functional layer 2 has a sea-island phase separation structure, irregularities are formed on the surface of the separation functional layer 2, which increases the surface area and the contact area with the carbon dioxide-containing gas. In addition, since the hydrophobic polymer with high gas permeability exists on the surface of the separation functional layer 2, the gas permeation rate is also increased. Therefore, by using the separation membrane (M) 1 in which the surface of the separation functional layer 2 has a sea-island phase separation structure, it becomes possible to further improve the carbon dioxide selectivity and permeation rate.

[0025] The separation membrane (M) 1 contains a hydrophilic polymer and a hydrophobic polymer in the separation functional layer 2, and it is not essential to separately provide a layer containing the hydrophilic polymer and a layer containing the hydrophobic polymer. Therefore, the number of layers required for the configuration of the separation membrane is small, and the separation membrane can be produced easily at low cost.

[0026] The hydrophilic polymer contained in the separation functional layer is not particularly limited as long as it can immobilize a carbon dioxide carrier in the separation functional layer 2 and form a network structure. For example, polyvinyl alcohol-based polymers, polyhydric alcohols, polyethylene glycol having epoxy groups at both terminals, etc., can be used.

[0027] Examples of the polyvinyl alcohol-based polymer include polyvinyl alcohol, copolymers of vinyl alcohol and other hydrophilic monomers, and the like. The polyvinyl alcohol-based polymer may have a linear structure or a branched structure. Examples of other hydrophilic monomers that form a copolymer with vinyl alcohol include (meth)acrylic acid and the like. By introducing (meth)acrylic acid, the water absorption capacity and water retention capacity of the hydrophilic polymer are increased, and high water absorption and water retention are maintained even when the separation membrane (M) is used under pressurized conditions. In order to improve the water absorption capacity and water retention capacity, all or part of the carboxy groups of (meth)acrylic acid may be neutralized with an alkaline compound. Examples of the alkaline compound used for neutralization include sodium hydroxide. In a copolymer of vinyl alcohol and another hydrophilic monomer, the proportion of vinyl alcohol units is preferably 20 mol% or more, and may be 50 mol% or more and 99 mol% or less.

[0028] Examples of polyhydric alcohols include glycerin and glyceryl glycosides. Since polyhydric alcohols also act as plasticizers, they may be used in combination with other hydrophilic polymers such as polyvinyl alcohol-based polymers.

[0029] Examples of polyethylene glycol having epoxy groups at both ends include Epolite manufactured by Kyoei Chemical Industry Co., Ltd. The type of Epolite is not particularly limited and may be "Epolite 400E" (epoxy equivalent 264 to 290 g / eq).

[0030] The hydrophilic polymer may have a crosslinked structure formed by a crosslinking agent. Having a crosslinked structure improves the strength and durability of the separation functional layer. Since polyethylene glycol having epoxy groups at both ends also acts as a crosslinking agent, it may be used in combination with other hydrophilic polymers such as polyvinyl alcohol-based polymers. Furthermore, crosslinking agents other than polyethylene glycol having epoxy groups at both ends may be used. For example, epoxy compounds, isocyanate compounds, aldehyde compounds, UV-crosslinkable compounds, leaving group-containing compounds, carboxylic acid compounds, urea compounds, organometallic compounds, etc., can be used.

[0031] In order for the separation functional layer to maintain its water absorption and water retention properties even under pressure, it is preferable to use a mixture of two or more hydrophilic polymers. For example, a mixture may be used that includes at least one polyvinyl alcohol-based polymer, at least one polyhydric alcohol, and at least one polyethylene glycol having epoxy groups at both ends. Alternatively, a mixture may be used that includes two or more polyvinyl alcohol-based polymers.

[0032] The carbon dioxide carrier is not particularly limited as long as it is a compound that has an affinity for carbon dioxide and is water-soluble. For example, compounds containing amino groups that can react with carbon dioxide to produce carbamates or bicarbonate ions can be used. Examples of compounds containing amino groups include ethanolamine, diethanolamine, triethanolamine, propanolamine, dipropanolamine, trippropanolamine, glycine, polyallylamine, and polyamines. Among these, polyamines having primary amino groups are preferred. Polyamines may have a linear structure or a branched structure. The number of primary amino groups in one polyamine molecule is not particularly limited, but may be 2 to 4. Polyamines may also be dendrimers such as polyamidoamine dendrimers.

[0033] In addition to compounds containing amino groups, alkali metal salts, alkali metal hydroxides, ammonia, ammonium salts, etc., can also be used as carbon dioxide carriers. Specifically, examples include sodium carbonate, potassium carbonate, cesium carbonate, sodium bicarbonate, potassium bicarbonate, lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. In the case of alkali metal salts, polydentate ligands that form complexes with alkali metal ions, sodium arsenite, carbonate anhydrase, boric acid, etc., may be used as auxiliary additives. One type of carbon dioxide carrier may be used alone, or two or more types may be used in combination.

[0034] The hydrophobic polymer is not particularly limited, and for example, silicone (polysiloxane) can be used. One type of silicone may be used as the hydrophobic polymer, a mixture of two or more types of silicone may be used, or a mixture of silicone and alkoxysilane, which is a raw material for silicone, may be used.

[0035] If the total amount of hydrophilic polymer and carbon dioxide carrier is 100% by mass, the proportion of carbon dioxide carrier is usually, for example, 5% to 50% by mass, and preferably 10% to 40% by mass.

[0036] When the hydrophilic polymer includes, for example, a polyvinyl alcohol-based polymer, a polyhydric alcohol, and polyethylene glycol having epoxy groups at both ends, and the carbon dioxide carrier includes a polyamine, the polyvinyl alcohol-based polymer, polyhydric alcohol, polyethylene glycol having epoxy groups at both ends, and polyamine may each be present in amounts of 1 to 95 parts by mass.

[0037] Preferably, the mixture contains 5 to 50 parts by mass of polyvinyl alcohol polymer, 5 to 50 parts by mass of polyhydric alcohol, 3 to 30 parts by mass of polyethylene glycol having epoxy groups at both ends, and 5 to 50 parts by mass of polyamine, totaling 100 parts by mass. More preferably, the mixture contains 10 to 40 parts by mass of polyvinyl alcohol polymer, 10 to 40 parts by mass of polyhydric alcohol, 5 to 20 parts by mass of polyethylene glycol having epoxy groups at both ends, and 10 to 40 parts by mass of polyamine, totaling 100 parts by mass.

[0038] The mass ratio of each component is not particularly limited, but it is preferable that, per 100 parts by mass of polyvinyl alcohol polymer, there are 50 to 200 parts by mass of polyhydric alcohol, 10 to 100 parts by mass of polyethylene glycol having epoxy groups at both ends, and 50 to 200 parts by mass of polyamine.

[0039] The hydrophobic polymer content may be 1 to 50 parts by mass, preferably 5 to 20 parts by mass, per 100 parts by mass of the total amount of hydrophilic polymer and carbon dioxide carrier. Within this range, a sea-island phase separation structure is more easily formed in the separation functional layer, with the hydrophilic polymer as the sea and the hydrophobic polymer as the islands, resulting in increased carbon dioxide selectivity and permeation rate.

[0040] In addition to hydrophilic polymers, hydrophobic polymers, and carbon dioxide carriers, the separation functional layer may also contain neutralizing agents, crosslinking agents, plasticizers, water-retaining agents, etc., for hydrophilic polymers.

[0041] When any cross-section of the separation functional layer has a sea-island phase separation structure, the islands may be formed from hydrophobic polymers and the sea from hydrophilic polymers. The presence of a sea-island phase separation structure in the cross-section can be observed by transmission electron microscopy (TEM). The islands may be distributed in a portion of the cross-section or throughout, but it is preferable that they be uniformly distributed throughout to increase carbon dioxide selectivity and permeability. The islands may exist independently or in a chain-like manner.

[0042] The total area of ​​the islands in the cross-section may be 5% to 60% or 7% to 60% of the total area of ​​the cross-section. When within this range, the hydrophilic polymer phase that contributes to the generation of carbamates and bicarbonate ions and the hydrophobic polymer phase with high gas permeability tend to be dispersed within the separation functional layer, thereby improving the selectivity and permeation rate of carbon dioxide in the thickness direction of the membrane. The shape of the islands is not particularly limited, but is observed to be circular or elliptical due to the surface tension at the interface between the sea and the islands. The maximum diameter of the islands is preferably 0.4 μm to 10 μm, more preferably 0.4 μm to 5 μm, and even more preferably 0.4 μm to 3 μm, from the viewpoint of the strength of the separation functional layer and the selectivity and permeability of carbon dioxide. The area of ​​the islands and the maximum diameter of the islands in the cross-section of the separation functional layer can be adjusted, for example, by using a hydrophobic polymer emulsion to form the separation functional layer and changing the concentration and particle size of the hydrophobic polymer.

[0043] Even when the surface of the separation functional layer has a sea-island phase separation structure, the islands may be formed from hydrophobic polymers and the sea from hydrophilic polymers. The presence of a sea-island phase separation structure on the surface can be confirmed by scanning probe microscopy (SPM). In SPM, the island portions with low elastic modulus formed by hydrophobic polymers appear as darker images, allowing observation of the sea-island phase separation structure. The islands may be distributed in only a portion of the surface or throughout, but it is preferable that they be uniformly distributed throughout the surface to increase carbon dioxide selectivity and permeability. The islands may exist individually or in a chain-like manner.

[0044] The total area of ​​islands on the surface may be 5% to 60% or 7% to 60% of the total surface area. When within this range, a highly gas-permeable hydrophobic polymer is present on the surface of the separation functional layer, and the sea-island structure creates irregularities on the surface of the separation functional layer, increasing the surface area, and thus increasing the contact area between the surface and the gas containing carbon dioxide. Consequently, the selectivity for carbon dioxide increases, and the gas permeation rate also increases. In this respect, the surface roughness Rq (root mean square height) of the surface of the separation functional layer is preferably 40 μm or more, and more preferably 42 μm to 80 μm. The surface roughness Rq can also be adjusted, for example, by pressing a substrate with irregularities on its surface onto it and transferring the result, or by adding inorganic or organic fine particles. Furthermore, the maximum diameter of the islands may be 0.2 μm to 10 μm or 0.5 to 5 μm from the viewpoint of the strength of the separation functional layer, the selectivity for carbon dioxide, and the permeability. The total area of ​​islands on the surface of the separation functional layer and the maximum diameter of the islands can be adjusted, for example, by using a hydrophobic polymer emulsion to form the separation functional layer and varying the concentration and particle size of the hydrophobic polymer, the coating thickness of the coating solution, and the drying time.

[0045] The thickness of the separation functional layer is measured, for example, by a scanning electron microscope (SEM). To achieve both carbon dioxide permeability and membrane strength, the thickness is preferably 0.1 μm to 100 μm, more preferably 0.1 μm to 50 μm, and even more preferably 0.1 μm to 20 μm.

[0046] The gas permeable layer contained in the separation membrane (M) supports the separation functional layer and improves the membrane strength. The molecular weight cutoff of the gas permeable layer is usually 50,000 to 1,000,000, preferably 100,000 to 500,000. The material of the gas permeable layer is not particularly limited, but it is preferably hydrophilic since the coating solution used to form the separation functional layer uses water as a solvent. For example, it may be a polyethersulfone membrane (PES membrane). Furthermore, in order to suppress the penetration of components that form the separation functional layer into the gas permeable layer under pressure, the average pore diameter of the pores on the surface of the gas permeable layer is preferably 10 nm to 500 nm.

[0047] As described above, the separation membrane (M) of the present invention is a separation membrane comprising a separation functional layer and a gas permeable layer supporting the separation functional layer, wherein the separation functional layer comprises a hydrophilic polymer, a hydrophobic polymer, and a carbon dioxide carrier, and any cross-section of the separation functional layer may have a sea-island phase separation structure in which the hydrophilic polymer and the hydrophobic polymer are in phase separation from each other.

[0048] Furthermore, the separation membrane (M) of the present invention is a separation membrane comprising a separation functional layer and a gas permeable layer supporting the separation functional layer, wherein the separation functional layer comprises a hydrophilic polymer, a hydrophobic polymer, and a carbon dioxide carrier, and the surface of the separation functional layer has a sea-island phase separation structure in which the hydrophilic polymer and the hydrophobic polymer are in phase separation from each other.

[0049] Furthermore, the separation membrane (M) of the present invention is a separation membrane comprising a separation functional layer and a gas permeable layer supporting the separation functional layer, wherein the separation functional layer comprises a hydrophilic polymer, a hydrophobic polymer, and a carbon dioxide carrier, and the surface roughness Rq of the surface of the separation functional layer is 40 μm or more.

[0050] The separation membrane (M) may be used as a separation membrane module by supporting the gas permeable layer on a porous support. While there are no particular limitations on the porous support that supports the gas permeable layer, nonwoven fabrics are usually used because they are economically readily available. Examples of nonwoven fabric materials include polyolefins, polyphenylene sulfide, and polyethylene terephthalate, but polyolefins or polyphenylene sulfide may be used from the viewpoint of inhibiting hydrolysis. Polyolefins may also be used from the viewpoint of economic efficiency.

[0051] The separation performance of the separation membrane (M) is evaluated by the carbon dioxide permeation flux, selectivity, etc. Separation performance evaluation can be performed, for example, by heat-treating the separation membrane (M) to promote the crosslinking reaction of the hydrophilic polymer, and then spraying it with cesium carbonate, which is a carbon dioxide carrier. The surface on which the cesium carbonate is sprayed can be either side of the separation membrane (M). It may also be sprayed from the separation functional layer side. Typically, cesium carbonate is sprayed as an aqueous solution. For every 100 parts by mass of the total amount of hydrophilic polymer and carbon dioxide carrier, typically 10 to 1500 parts by mass, preferably 200 to 1000 parts by mass, and more preferably 500 to 750 parts by mass of cesium carbonate is sprayed. The concentration of the cesium carbonate aqueous solution to be sprayed is typically 2% to 90% by mass, preferably 7% to 45% by mass, and more preferably 10% to 30% by mass. In order to allow the cesium carbonate to permeate the separation membrane (M), it is usually aged after spraying. The aging temperature is not particularly limited. The maturation time is not particularly limited, but is usually 0.1 days to 1 year, preferably 0.5 days to 1 month, and more preferably 1 day to 1 week. The separation performance of the separation membrane (M) produced in this way is evaluated using a carbon dioxide separation performance evaluation device.

[0052] The carbon dioxide separation performance evaluation device includes a section for supplying gas containing carbon dioxide to the separation membrane, a section for controlling and maintaining the pressure and temperature on the gas supply side of the separation membrane, a section for controlling and maintaining the pressure and temperature on the gas permeation side, a section for analyzing gas that does not permeate the separation membrane, and a section for analyzing gas that has permeated the separation membrane.

[0053] The temperature of the carbon dioxide-containing gas supplied to the separation membrane is typically 15°C to 120°C, preferably 40°C to 110°C, and more preferably 60°C to 100°C. The humidity of the supplied gas is typically 0% RH to 95% RH, preferably 40% RH to 90% RH, and more preferably 50% RH to 80% RH. The gas supply rate is per 1 m of separation membrane. ２The gas flow rate is typically 5 NL / min to 500 NL / min, preferably 50 NL / min to 300 NL / min, and more preferably 50 NL / min to 150 NL / min. The pressure on the gas supply side of the separation membrane is typically atmospheric pressure to 3000 kPa (absolute pressure), preferably atmospheric pressure to 2400 kPa (absolute pressure), and more preferably atmospheric pressure to 1000 kPa (absolute pressure). The temperature on the gas supply side of the separation membrane is typically 25°C to 110°C, preferably 40°C to 110°C, and more preferably 60°C to 110°C. The humidity on the gas supply side of the separation membrane is typically 0% RH to 100% RH, preferably 40% RH to 90% RH, and more preferably 40% RH to 80% RH.

[0054] The pressure on the gas permeation side of the separation membrane is typically 1 kPa to 200 kPa (absolute pressure), preferably 10 kPa to 150 kPa (absolute pressure), and more preferably 10 kPa to 100 kPa (absolute pressure). The temperature on the gas permeation side of the separation membrane is typically 25°C to 110°C, preferably 40°C to 110°C, and more preferably 60°C to 110°C. The humidity on the gas permeation side of the separation membrane is typically 0% RH to 100% RH, preferably 40% RH to 90% RH, and more preferably 40% RH to 80% RH.

[0055] Analysis of gases that do not permeate the separation membrane and those that do is usually performed using gas chromatography. The permeation flux and selectivity of carbon dioxide can be calculated from the gas chromatography analysis values. The selectivity of carbon dioxide is determined by the ratio of the permeation rate coefficient of carbon dioxide to the permeation rate coefficient of the gas to be separated.

[0056] <Coating liquid for forming a separation function layer> The coating liquid for forming a separation function layer according to the present invention comprises an aqueous solution of a hydrophilic polymer, a hydrophobic polymer dispersed in the aqueous solution, and a carbon dioxide carrier dissolved in the aqueous solution. By allowing the hydrophilic polymer and the hydrophobic polymer to coexist in the coating liquid and simultaneously coating it onto a gas permeable layer, a separation function layer in which the hydrophilic polymer and the hydrophobic polymer are mixed is formed.

[0057] The solvent for the coating solution for forming the separation function layer is water, and the aqueous solvent contains a hydrophilic polymer, a hydrophobic polymer, and a carbon dioxide carrier. The hydrophilic polymer, hydrophobic polymer, and carbon dioxide carrier only need to be dispersed throughout the coating solution, and may or may not be dissolved in water. It is preferable that the hydrophilic polymer and carbon dioxide carrier are dissolved in water and the hydrophobic polymer is in an emulsion state, as this facilitates the formation of a sea-island phase separation structure in which the hydrophilic polymer is the sea and the hydrophobic polymer is the island, at any cross-section or surface of the separation function layer. It is more preferable that the hydrophobic polymer is dispersed in an aqueous solution of the hydrophilic polymer and carbon dioxide carrier in an emulsion state. Furthermore, it is even more preferable that the emulsion state is stable and the hydrophobic polymer does not elute.

[0058] The above-mentioned compounds can be used as the hydrophilic polymer, hydrophobic polymer, and carbon dioxide carrier, respectively. For example, the hydrophilic polymer may include polyvinyl alcohol-based polymers such as copolymers of vinyl alcohol and (meth)acrylic acid, and the hydrophobic polymer may include silicone. The carbon dioxide carrier may include compounds containing amino groups.

[0059] The proportions of hydrophilic polymers, hydrophobic polymers, and carbon dioxide carriers contained in the coating liquid for forming the separation function layer are the same as the proportions of hydrophilic polymers, hydrophobic polymers, and carbon dioxide carriers contained in the separation function layer described above.

[0060] The method for producing the coating liquid for forming a separation functional layer according to the present invention is not particularly limited, but for example, it can be produced by the following steps (1) to (3): (1) a step of preparing a first aqueous emulsion in which a hydrophobic polymer is dispersed in water; (2) a step of preparing a second aqueous emulsion by dissolving a carbon dioxide carrier in the water in the first aqueous emulsion; (3) a step of preparing a third aqueous emulsion by dissolving a hydrophilic polymer in the water in the second aqueous emulsion.

[0061] The content of the hydrophobic polymer in the first aqueous emulsion prepared in step (1) is preferably 0.05 parts by mass to 30.0 parts by mass, more preferably 0.1 parts by mass to 20.0 parts by mass, per 100 parts by mass of the first aqueous emulsion.

[0062] The carbon dioxide carrier added in step (2) may be dissolved in water in the first aqueous emulsion, or it may be added to the first aqueous emulsion in aqueous solution form.

[0063] As the hydrophilic polymer added in step (3), for example, a copolymer of vinyl alcohol and (meth)acrylic acid may be used. The hydrophilic polymer may be dissolved in water in the second aqueous emulsion, or it may be added to the second aqueous emulsion in aqueous solution form. If the hydrophilic polymer contains units of (meth)acrylic acid and neutralizes the carboxyl group, for example, an aqueous sodium hydroxide solution may be added after step (2).

[0064] For example, if the separation functional layer contains hydrophilic polymers such as polyvinyl alcohol, polyhydric alcohol, or polyethylene glycol with epoxy groups at both ends, these can be added to the third aqueous emulsion after step (3). Furthermore, if other compounds are to be included in the separation functional layer, they can be added as appropriate before or after each of steps (1) to (3).

[0065] In the process of manufacturing a coating liquid for forming a separation functional layer, when preparing an aqueous solution of a hydrophilic polymer or carbon dioxide carrier, the content of the hydrophilic polymer or carbon dioxide in the water is 0.1 to 100 parts by mass, preferably 1 to 20 parts by mass, per 100 parts by mass of water. More preferably, the viscosity of the liquid containing the hydrophilic polymer or carbon dioxide carrier and water is adjusted to be in the range of 100 Pa·m to 2000 Pa·m.

[0066] In a separation functional layer formed using the coating liquid for forming a separation functional layer manufactured as described above, a sea-island phase separation structure, in which hydrophilic polymers are the sea and hydrophobic polymers are the islands, is easily formed on any cross-section or surface of the separation functional layer, and the drying resistance of the separation membrane is also improved. For example, if the coating liquid is manufactured in the order of process (1), process (3), and process (2), the sea-island boundaries of the separation functional layer formed from the coating liquid will have a different structure from the sea-island boundaries of a separation functional layer formed from a coating liquid manufactured in the order of process (1), process (2), and process (3), resulting in lower drying resistance of the separation membrane.

[0067] The method for applying the coating liquid for forming a separation functional layer according to the present invention to a gas permeable layer is not particularly limited. When applying a small amount of coating liquid, the coating liquid is applied to the gas permeable layer and spread thinly with a bar coater. When applying a large amount of coating liquid, a continuous film forming apparatus can be used. The amount of coating liquid applied to one section of the gas permeable layer is 1 m of film. ２ The amount per unit is typically 0.2 g to 800 g, preferably 2 g to 400 g. The coating solution may be filtered before coating in order to form a separation functional layer having the desired sea-island phase separation structure. The pore size of the filter used for filtration is typically 1 μm to 100 μm, preferably 5 μm to 50 μm.

[0068] A separation membrane (M) is produced by coating a coating liquid onto a gas permeable layer and then drying the coating liquid in a drying process. The drying method may be room temperature ventilation, heated ventilation, or heated no-air method. Unwanted water is removed by drying. If it is desired to promote the crosslinking reaction of polyethylene glycol having epoxy groups at both ends after heating, further heat treatment may be performed. The temperature, humidity, and time of the heat treatment are not particularly limited, but the temperature is preferably 40°C to 140°C, more preferably 60°C to 90°C, and the humidity is preferably 0%RH to 90%RH, more preferably 40%RH to 80%RH. The heat treatment time is preferably 5 minutes to 48 hours, more preferably 8 minutes to 24 hours.

[0069] <Separation Membrane Module> From the viewpoint of industrial use, it is preferable to process the separation membrane (M) into a separation membrane module for use. The separation membrane module according to the present invention comprises a separation membrane (M) and a porous support that supports the gas permeable layer of the separation membrane (M), and separates carbon dioxide by permeating carbon dioxide in the gas supplied from the separation functional layer side to the porous support side. The module can take the form of a spiral type, cylindrical type, flat plate type, etc., but is not particularly limited. Preferably, it is a spiral type or a flat membrane type. Because the separation membrane module according to the present invention has high carbon dioxide permeability, the required membrane area (total module volume) can be reduced.

[0070] The separation membrane module according to the present invention is preferably used under high humidity conditions from the viewpoint of improving carbon dioxide permeability. Therefore, the supplied gas may be any gas containing carbon dioxide, but it is preferably a gas containing carbon dioxide and water vapor. For example, it may be reformed gas generated in a hydrogen production plant, extraction gas in a natural gas plant, pre-combustion gas in an integrated gasification combined cycle (IGCC) power plant, combustion exhaust gas generated in a thermal power generation system, fuel cell system, etc., off-gas generated from a fuel cell system, exhaust gas generated in a chemical plant, etc.

[0071] The separation membrane module according to the present invention may be incorporated into a carbon dioxide separation system that separates carbon dioxide from a gas containing carbon dioxide generated inside or outside the system. The carbon dioxide separation system is used to separate carbon dioxide from a gas containing carbon dioxide.

[0072] A carbon dioxide separation system may include a supply gas flow path through which a gas containing carbon dioxide flows, and a separation membrane module that separates carbon dioxide by permeating the carbon dioxide in the gas supplied to the gas supply side to the gas permeate side. In addition, means for depressurizing or suctioning the gas permeate side may be provided to promote carbon dioxide separation, and a sweep gas supply path for supplying sweep gas to the gas permeate side may be provided.

[0073] [Examples] The present invention will now be described in more detail using examples, but the present invention is not limited in any way to these examples.

[0074] Example 1 1. Preparation of coating solution for forming separation functional layer The following (1) to (9) were prepared and added to the reaction vessel in this order while stirring. Stirring was carried out at room temperature until all components were uniformly mixed to prepare 250 g of coating solution. (1) 115 g of deionized water (2) 2.8 g of silicone emulsion containing 48% by mass of silicone and silicone precursor (hydrophobic polymer) (3) 18.7 g of aqueous solution containing 15% by mass of polyallylamine (carbon dioxide carrier) (4) 4.9 g of aqueous solution containing 50% by mass of polyamidoamine (PAMAM) dendrimer molecules (with amino surface groups and ethylenediamine core (generation 0, MW=516)) (carbon dioxide carrier) (5) 19.7 g of 1N sodium hydroxide aqueous solution (neutralizing agent) (6) 21.1 g of aqueous solution containing 7.5% by mass of a copolymer of vinyl alcohol and acrylic acid (hydrophilic polymer) (7) 43.4 g of aqueous solution containing 8% by mass of polyvinyl alcohol (hydrophilic polymer) (8) 5 g of glycerin (hydrophilic polymer) (9) 21 g of aqueous solution containing 10% by mass of polyethylene glycol with epoxy groups at both ends (product name "Epolite 400E", manufactured by Kyoeisha Chemical Co., Ltd., epoxy equivalent 264-290 g / eq) (hydrophilic polymer)

[0075] 2. Formation of the separation membrane A polyethersulfone (PES) membrane, which will serve as the gas permeable layer, was formed on a nonwoven fabric, which is a porous support, and attached to a flat plate in a size of 8 cm x 18 cm. The coating solution prepared in "1. Preparation of coating solution for forming the separation functional layer" was placed on the upper edge of the PES membrane, and the coating solution was uniformly applied to the PES membrane using a No. 32 bar coater. After coating, the separation functional layer was formed by drying at 80°C, and a separation membrane was fabricated.

[0076] 3. Evaluation of the Separation Membrane <Scanning Electron Microscope (SEM) Observation> The separation membrane obtained in "2. Formation of the Separation Membrane" was observed using an SEM. A separation functional layer formed by the coating solution was observed in the upper layer, a gas permeable PES layer in the middle layer, and a porous support nonwoven fabric layer in the lower layer. The thickness of the separation functional layer was 2.5 μm.

[0077] <Transmission Electron Microscopy (TEM) Observation> When observing the cross-section of the separation functional layer, a sea-island phase separation structure was confirmed as shown in Figure 2. Furthermore, since the Si analysis results obtained by energy dispersive X-ray spectroscopy (EDX) matched the analysis results of the island portions, it was also confirmed that the islands contain silicone. The ratio of the total area of the islands to the total area of the cross-section was 15%.

[0078] <Scanning Probe Microscopy (SPM) Observation> When observing the phase image of the surface of the separation functional layer, a sea-island phase separation structure was observed as shown in Figure 3. Binarized image processing was performed to obtain the ratio of the total area of the islands to the total surface area, the maximum diameter of the islands, and the surface roughness Rq. The ratio of the islands was 16%, the maximum diameter of the islands was 2.0 μm, and the surface roughness Rq was 58 nm.

[0079] <Carbon Dioxide Separation Performance Evaluation> The separation membrane obtained in the above "2. Formation of Separation Membrane" was cut into a circular shape with a diameter of 47 mm, and an aqueous cesium carbonate solution (concentration: 14% by mass) was sprayed from the separation functional layer side. Thereafter, the volume ratio of carbon dioxide to nitrogen (CO ２ / N ２ ) of 20 / 80 was supplied as a mixed gas from the separation functional layer side. The temperature of the supply gas was set to 85°C, the humidity was 60% RH, and the total pressure was 0.85 MPa. By analyzing the gas compositions of the non-permeated gas passing over the separation functional layer and the permeated gas permeating through the porous support, the CO ２ permeation flux and CO ２ selectivity were obtained. CO ２ permeation flux was 7.5×10 －５ m ３ (STP) / m ２ ·s. CO ２ permeation rate and N ２ permeation rate were obtained, and when selectivity was calculated from the ratio thereof, the CO ２ / N ２ selectivity was 3090. The above evaluation results are shown in Table 1 together with the results of Examples 2 to 4 and Comparative Example 1.

[0080] Example 2 A separation film was prepared and evaluated in the same manner as in Example 1, except that 1.4 g of silicone emulsion containing silicone and a silicone precursor was added when preparing the coating solution, and a No. 56 bar coater was used when applying the coating solution onto the PES film. The thickness of the separation functional layer was 4.0 μm, the ratio of the total island area to the total surface area was 10%, the maximum island diameter was 1.7 μm, and the surface roughness Rq was 62 μm. ２ The transmitted flux is 5.6 × 10⁻⁶. －５ I understand ３ (STP) / m ２ ・s, CO ２ / N ２ The selectivity was 2070.

[0081] Example 3 A separation film was prepared and evaluated in the same manner as in Example 1, except that a No. 64 bar coater was used when applying the coating solution onto the polyethersulfone film. The thickness of the separation functional layer was 5.3 μm, the ratio of the total island area to the total surface area was 55%, the surface roughness Rq was 70 μm, and the islands were connected. CO ２ The permeation flux is 4.9 × 10⁻⁶. －５ I understand ３ (STP) / m ２ ・s, CO ２ / N ２ The selectivity was 3300.

[0082] Example 4 A separation membrane was prepared and evaluated in the same manner as in Example 1, except that the amount of silicone emulsion containing silicone and a silicone precursor added when preparing the coating solution was 0.7 g. The thickness of the separation functional layer was 4.0 μm, the ratio of the total island area to the total surface area was 11%, the maximum island diameter was 1.3 μm, and the surface roughness Rq was 43 μm. CO ２ The transmitted flux is 4.8 × 10⁻⁶. －５ I understand ３ (STP) / m ２ ・s, CO ２ / N ２ The selectivity was 2410.

[0083] Comparative Example 1 A separation membrane was prepared and evaluated in the same manner as in Example 1, except that a silicone emulsion containing silicone and a silicone precursor was not added when preparing the coating solution. The thickness of the separation functional layer was 2.5 μm, CO ２ The transmitted flux is 3.9 × 10⁻⁶. －５ I understand ３ (STP) / m ２ ・s, CO ２ / N ２ The selectivity was 400.

[0084]

[0085] The separation membranes of the present invention obtained in Examples 1 to 4 have a sea-island phase separation structure in the cross-section and surface of the separation functional layer, and have high CO2 ２ Permeation flux and CO ２ / N ２ Selectivity was demonstrated. On the other hand, the separation membrane of Comparative Example 1, which does not contain a hydrophobic polymer in the separation functional layer, showed CO2 levels compared to the separation membranes of Examples 1 to 4. ２ Low permeation flux, CO ２ / N ２ The selectivity was also significantly reduced. It is believed that the hydrophilic polymer phase, which contributes to the formation of carbamates and bicarbonate ions, and the hydrophobic polymer phase, which has high gas permeability, are dispersed inside and on the surface of the separation functional layer, and that the surface roughness Rq also increases, resulting in higher carbon dioxide permeability and selectivity. Furthermore, as shown in Example 3, it was confirmed that the separation membrane of the present invention can maintain high carbon dioxide permeability and selectivity even when the film thickness increases.

[0086] The separation membrane according to the present invention stably exhibits excellent carbon dioxide selectivity and carbon dioxide permeation rate in the separation of carbon dioxide from gases containing carbon dioxide, and can therefore be used in processes for separating carbon dioxide from gases containing carbon dioxide emitted from various sources. Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be construed as restrictive. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the present invention pertains by reading the above disclosure. Accordingly, the appended claims should be construed as encompassing all modifications and alterations without departing from the true spirit and scope of the present invention.

[0087] 1. Separation membrane 2. Separation functional layer 2a. Sea with a phase-separated structure of a sea-island 2b. Island with a phase-separated structure of a sea-island 3. Gas permeable layer

Claims

1. A separation membrane comprising a separation functional layer and a gas permeable layer supporting the separation functional layer, wherein the separation functional layer comprises a hydrophilic polymer, a hydrophobic polymer, and a carbon dioxide carrier, and the hydrophilic polymer and the hydrophobic polymer are mixed within the separation functional layer.

2. The separation membrane according to claim 1, wherein the carbon dioxide carrier comprises a compound containing an amino group.

3. The separation membrane according to claim 1 or 2, wherein the hydrophilic polymer includes a polyvinyl alcohol-based polymer.

4. The separation membrane according to claim 1 or 2, wherein the hydrophobic polymer comprises silicone.

5. The separation membrane according to claim 1 or 2, wherein any cross-section of the separation functional layer has a sea-island phase separation structure.

6. The separation membrane according to claim 5, wherein the proportion of islands (total area) in the cross-section is 5% to 60% of the total area of ​​the cross-section.

7. The separation membrane according to claim 5, wherein the islands of the sea-island phase separation structure are formed of the hydrophobic polymer.

8. The separation membrane according to claim 5, wherein the surface of the separation functional layer has a sea-island phase separation structure.

9. The separation membrane according to claim 8, wherein the proportion of islands (total area) on the surface is 5% to 60% of the total surface area.

10. The separation membrane according to claim 8, wherein the maximum diameter of the islands in the phase separation structure of the sea-island is 0.2 μm to 10 μm.

11. The separation membrane according to claim 8, wherein the surface roughness Rq of the surface of the separation functional layer is 40 μm or more.

12. A coating solution for forming a separation functional layer, comprising an aqueous solution of a hydrophilic polymer, a hydrophobic polymer dispersed in the aqueous solution, and a carbon dioxide carrier dissolved in the aqueous solution.

13. The coating liquid for forming a separation functional layer according to claim 12, wherein the carbon dioxide carrier comprises a compound containing an amino group.

14. The coating liquid for forming a separation functional layer according to claim 12 or 13, wherein the hydrophilic polymer comprises a polyvinyl alcohol-based polymer, and the hydrophobic polymer comprises silicone.

15. A separation membrane module comprising a separation membrane according to claim 1 or 2, and a porous support for supporting the gas permeable layer of the separation membrane.

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