Methane gas conversion method and methane gas conversion device

Abstract

Provided are a methane gas conversion method and a methane gas conversion device for efficiently converting methane gas into organic matter.　In the methane gas conversion method, a mixed fluid, in which an ozone-containing fluid is mixed with a methane gas-containing fluid, is irradiated with light having a wavelength of 200 to 411 nm, inclusive, to convert the methane gas into organic matter. The methane gas conversion device comprises: a reaction vessel including a first supply port for supplying the ozone-containing fluid and a second supply port for supplying the methane gas-containing fluid; and a light source for irradiating the inside of the reaction vessel with light having a wavelength of 200 to 411 nm, inclusive.

Description

Methane gas conversion method and methane gas conversion device

[0001] The present invention relates to a method and an apparatus for converting methane gas.

[0002] Methane gas is released into the atmosphere from sewage, livestock waste, and food waste. Methane gas is known as a greenhouse gas that causes global warming. The global warming potential of methane gas is said to be 25 times that of carbon dioxide, making it undesirable to release methane gas directly into the atmosphere. Therefore, efforts are being made to capture methane gas released into the atmosphere. Methane gas can be decomposed by using the captured methane gas to power engines or extracting thermal energy in boilers. However, methane gas is a gas at room temperature and pressure, and transporting it in gaseous form to the combustion site is inefficient and difficult to store.

[0003] A method is known in which recovered methane gas is converted to produce organic matter. Using recovered methane gas as a raw material to produce organic matter not only reduces the amount of methane gas released, but also fixes the carbon contained in the methane gas, further contributing to the prevention of global warming. Furthermore, the cost of converting methane gas can be recovered by selling the organic matter produced. Furthermore, converting methane gas to organic matter is advantageous in terms of the efficiency of transporting methane gas and ease of storage.

[0004] Patent Document 1 describes a method of generating chlorine radicals by irradiating an environment in which hydrocarbons such as methane and chlorine dioxide exist with light, and then bonding the chlorine radicals to the hydrocarbons to generate organic substances such as methanol.

[0005] Patent No. 6080281

[0006] The technology described in Patent Document 1 requires chlorine dioxide. To prepare chlorine dioxide, a raw material of chlorine dioxide, for example, NaClO ２It is necessary to prepare and chemically react NaCl and HCl. Then, to separate methanol and formic acid from the resulting salt, NaCl, and from the aqueous solution containing HCl, energy is required for distillation and evaporation. In other words, financial and energy costs are incurred for preparing chlorine dioxide and separating the products. Regarding energy costs, investing a large amount of energy to convert methane gas, a greenhouse gas, is undesirable from the perspective of carbon neutrality. Furthermore, chlorine-based materials such as chlorine dioxide have the problem of easily corroding equipment such as reaction vessels.

[0007] Therefore, a method for converting methane gas without using chlorine-based materials such as chlorine dioxide is provided.

[0008] The methane gas conversion method of the present invention is characterized in that a mixed fluid obtained by mixing a fluid containing ozone and a fluid containing methane gas is irradiated with light having a wavelength of 200 nm or more and 411 nm or less to convert the methane gas into an organic substance. As will be described in detail later, by irradiating the mixed fluid with light having a wavelength of 200 nm or more and 411 nm or less, a large amount of atomic oxygen, particularly O( １ This produces a large amount of methyl methyl ether (D), which produces a large amount of an intermediate called methoxy in the methane gas conversion process. The presence of methoxy causes a cycle reaction that leads to the production of the organic compounds methanol or formic acid. This makes it possible to convert large amounts of methane gas into industrially useful organic compounds such as methanol or formic acid without using chlorine-based materials.

[0009] In this specification, the organic substances obtained by converting methane gas are methanol and formic acid. However, the "organic substance" obtained by converting methane gas may be an organic substance other than methanol and formic acid. In this specification, the "organic substance" refers to a compound containing carbon, excluding carbon monoxide, carbon dioxide, and methane. The "organic substance" may be, for example, an alcohol other than methanol, a carboxylic acid other than formic acid, or an ester obtained by dehydration condensation of an alcohol and a carboxylic acid. Methane gas may be converted into another carbon-containing compound (e.g., methyl formate) other than methanol or formic acid after producing methanol or formic acid as an intermediate in the conversion process.

[0010] The ozone-containing fluid may be generated by passing ozone gas through an ozone-permeable membrane immersed in a polar or nonpolar solvent. This generates a liquid in which ozone is dissolved as bubbles less than 1 mm in diameter (including microbubbles or nano-sized ultrafine bubbles). When the bubbles are less than 1000 μm, a state in which ozone is dissolved in the liquid at a high concentration for a long period of time (a state in which ozone is not discharged from the liquid) can be maintained. The methane-containing fluid may be a fluid (gas) containing only methane gas, or may contain gases or liquids other than methane gas.

[0011] The mixed fluid may be heated in addition to the light irradiation, and the chemical reaction is accelerated by heating the mixed fluid.

[0012] The fluid mixture may be contacted with a catalyst, which accelerates chemical reactions by helping to overcome the activation energy required to cause the chemical reactions, particularly cyclic reactions.

[0013] The ozone-containing fluid may be ozone water.

[0014] The light may be irradiated onto a liquid layer formed in the mixed fluid without passing through a gas layer formed in the mixed fluid, since the light-induced chemical reaction occurs in the liquid layer and is less likely to occur in the gas layer, thereby reducing the risk of explosion.

[0015] The mixed fluid may be stirred during or before the light irradiation, which can promote the chemical reaction.

[0016] The present invention provides a methane gas conversion device for converting methane gas into organic matter, comprising: a reaction vessel having a first supply port for supplying a fluid containing ozone and a second supply port for supplying the fluid containing methane gas; and a light source for irradiating the inside of the reaction vessel with light having a wavelength of 200 nm or more and 411 nm or less.

[0017] The reaction vessel may be provided with a stirrer. The mixed fluid is contained in the reaction vessel and stirred by the stirrer. Stirring can promote the chemical reaction.

[0018] The bottom of the reaction vessel may be inclined relative to the horizontal plane so that the stirrer is positioned across the liquid and gas layers, thereby allowing methane gas contained in the gas layer to dissolve in the liquid layer.

[0019] The light source may be arranged so that the light irradiates a liquid layer formed in the mixed fluid without passing through a gas layer formed in the reaction vessel in the mixed fluid, and the chemical reaction caused by the light occurs in the liquid layer and is less likely to occur in the gas layer, thereby reducing the risk of explosion.

[0020] This allows methane gas to be converted into organic matter without using chlorine-based materials such as chlorine dioxide. Because no chlorine-based materials are used, no energy is required to separate the organic matter obtained by converting methane gas from the chlorine-based materials. Furthermore, problems such as the generation of chlorine gas, corrosion of equipment due to chlorine-based materials, and waste liquid treatment of chlorine-based materials do not arise. In particular, the distillation tank for separating organic matter from a chlorine-based solution is large, and the costs of introducing and maintaining the equipment are high. Therefore, eliminating the need for the distillation tank significantly contributes to reducing the space and costs involved.

[0021] Converting methane gas into organic compounds is expected to contribute to global warming countermeasures by reducing the atmospheric release of methane gas. Furthermore, converting methane gas into organic compounds such as methanol and formic acid before transporting or storing it is also expected to contribute to global warming countermeasures by improving the efficiency of methane gas transportation and its storage properties. Converting methane gas to produce methanol and formic acid contributes to Goal 13 of the United Nations' Sustainable Development Goals (SDGs), which states, "Take urgent action to combat climate change and its impacts."

[0022] 2B is a diagram showing an overall view of a methane gas conversion process; FIG. 2C is a diagram showing a first embodiment of a methane gas conversion apparatus; FIG. 2D is a cross-sectional view taken along the line 2B-2B in FIG. 2A; FIG. 2E is a diagram showing a second embodiment of a methane gas conversion apparatus; FIG. 2F is a diagram showing a third embodiment of a methane gas conversion apparatus; FIG. 2G is a diagram showing a fourth embodiment of a methane gas conversion apparatus; FIG. 2H is a diagram showing an experimental apparatus for a methane gas conversion experiment;

[0023] The following description will be made with reference to the drawings where appropriate. Note that the drawings are all schematic illustrations, and the dimensional ratios in the drawings do not necessarily correspond to the actual dimensional ratios, and the dimensional ratios between the drawings do not necessarily correspond to the actual dimensional ratios.

[0024] <Methane Gas Conversion Process> First, the methane gas conversion process will be described. FIG. 1 is a diagram showing an overview of the methane gas conversion process. With reference to FIG. 1, the process of decomposing methane into methanol (CH ３ The process for producing hydroxybenzoates (HCOOH) and formic acid (HCOOH) is described. ４ and O ３ represents methane and ozone introduced into the reaction field. The methane gas and ozone introduced into the reaction field are mainly decomposed according to the methane gas conversion process shown in Figure 1. The underlined CH ３OH and HCOOH represent the products, methanol and formic acid. Each major chemical reaction in the methane gas conversion process is designated by a symbol consisting of an R and a number. Each major intermediate produced in the methane gas conversion process is designated by a symbol consisting of a P and a number. In this specification, intermediates refer to substances that exist temporarily during the methane gas conversion process. An overview of the methane gas conversion process will be explained while explaining each chemical reaction and each intermediate.

[0025] [Photolysis of Ozone] The mixed fluid in the reaction field contains a fluid containing ozone and a fluid containing methane gas. The ozone may be dissolved (dispersed) in the liquid. The liquid may be water or a nonpolar solvent. Examples of nonpolar solvents include hexane, cyclohexane, n-decane, n-octane, n-nonane, octane, isooctane, perfluorohexane, tetradecafluorohexane, perfluoroheptane, perfluorooctane, octadecafluorooctane, perfluorononane, eicosafluorononane, and perfluorodecalin. The following describes the case where the fluid is water, i.e., the "ozone-containing fluid" is ozone water. However, the type of fluid containing ozone is not particularly limited. Furthermore, even when ozone water is used as the "ozone-containing fluid," the ozone water may contain other materials (e.g., additives). In the "Methane Gas Conversion Process" section, the "methane gas-containing fluid" is considered to consist solely of methane gas.

[0026] Chemical reaction R1 represents the decomposition reaction of ozone by light L1 having a wavelength of 200 nm or more and 411 nm or less. When light L1 is irradiated onto ozone water and the energy hν of light L1 is imparted to ozone molecules, atomic oxygen O( １ D) or O( ３ P) is taken out. O( １ D) is atomic oxygen in an activated state, and O( ３P) is atomic oxygen in the ground state. Chemical reaction R1 is expressed by formula (1) or formula (2). In Figure 1, there are two chemical reactions R1 in which ozone is irradiated with light L1 to produce O. The "O" that appears in both chemical reactions R1 is O( １ D) and O( ３ In other words, "O" in Figure 1 is a concept that includes both O( １ D) and O( ３ P) may contain at least one of the following: ３ +hν → O( １ D) + O ２ ... (1) O ３ +hν → O( ３ P) + O ２ ... (2)

[0027] When light with a wavelength of 200 nm or more and 411 nm or less is used, the reaction of formula (1) is particularly likely to occur, and a large amount of O( １ When light having a wavelength longer than 411 nm and equal to or shorter than 1180 nm is used, the reaction of formula (1) does not occur, but the reaction of formula (2) occurs. As will be described in detail later, O( ３ P) is O( １ D) is less reactive than O( １ For this purpose, in this embodiment, light having a wavelength of 200 nm or more and 411 nm or less is used.

[0028] [Methyl (CH ３ Chemical reaction R2 occurs in which atomic oxygen generated by light L1 combines with methane. Chemical reaction R2 is shown in formulas (3) and (4). CH generated by formulas (3) and (4) ３ is methyl as intermediate P1. The chemical reaction of formula (3) proceeds even when the temperature of the reaction field is room temperature. In contrast, the chemical reaction of formula (4) is unlikely to proceed unless the temperature of the reaction field is approximately 300°C or higher. Therefore, in this embodiment, light with a wavelength of 200 nm or more and 411 nm or less is used so that the reaction proceeds even at room temperature. O( １ D) + CH ４ → CH ３+OH...(3) O( ３ P) + CH ４ → CH ３ +OH...(4)

[0029] The OH (hydroxyl radical) generated in the formulas (3) and (4) (especially the formula (3)) is CH ４ Chemical reaction R3 occurs, combining with OH + CH. Chemical reaction R3 is shown in equation (5). Equation (5) also produces methyl as intermediate P1. ４ → CH ３ +H ２ O... (5)

[0030] OH is generated not only in the chemical reaction R2 (reactions (3) and (4)). Although not shown in FIG. 1, when light L1 containing wavelengths of 200 nm or more and 411 nm or less is used, a large amount of O( １ D) is obtained, but O( １ D) is water (H ２ O) and produces a large amount of OH. This is shown in formula (6). The large amount of OH produced promotes chemical reaction R3 (formula (5)). O( １ D) + H ２ O → 2OH...(6)

[0031] In this way, a large amount of intermediate P1 (methyl) is produced by chemical reaction R2 and chemical reaction R3. However, all of the O( １ D) does not lead to the reaction of formula (3). １ D) is inactivated and O( ３ P) and O( ３ P) bond with each other to generate oxygen molecules, or O( １ D) often combines with oxygen molecules to produce ozone again. Also, as shown in chemical reaction R4, O( １ D) can also combine with methane to produce methanol directly. Chemical reaction R4 is shown in equation (7). O( １ D) + CH ４ → CH ３ OH... (7)

[0032] Chemical reaction R4 is a preferred reaction that directly converts methane to methanol with minimal chemical reaction processes. However, it is a minor chemical reaction, and the amount of methanol directly produced by chemical reaction R4 is small. The major chemical reactions that convert methane to methanol or formic acid are the various chemical reactions R5 to R9 that start with methoxy, as described below.

[0033] [Methoxy(CH ３ O) and Cyclic Reaction of Intermediate P1(CH ３ ) and atomic oxygen obtained by irradiating ozone with light L1, a chemical reaction R5 produces methoxy (CH ３ Chemical reaction R5 is shown in formula (8). In formula (8), O( １ D) example, but O( ３ P), a reaction similar to that in equation (8) occurs. ３ +O( １ D) → CH ３ O... (8)

[0034] The intermediate P2, methoxy, undergoes chemical reaction R6, combining with methane. Chemical reaction R6 is shown in formula (9). CH ３ O + CH ４ → CH ３ OH + CH ３ ... (9)

[0035] As shown in equation (9), chemical reaction R6 produces methyl and the target product, methanol. Methyl combines with atomic oxygen in chemical reaction R5 to produce methoxy again. Thus, a cyclic reaction (Loop A) occurs in which chemical reactions R5 and R6 are repeated. Loop A decomposes a large amount of methane, producing a large amount of methanol.

[0036] [Formation of formic acid from methoxy and oxygen molecules] Methoxy, which is intermediate P2, can undergo chemical reaction R7, bonding with oxygen molecules. Chemical reaction R7 is shown in formula (10). CH ３ O+O ２ → HCHO+HO ２ ...(10)

[0037] Formaldehyde HCHO obtained by formula (10) is a stable substance and exists for a relatively long time. However, an oxidation reaction in which it gradually bonds with oxygen atoms contained in oxygen molecules in the reaction vessel proceeds, and as this progresses, formic acid is obtained. Because ozone is used, atomic oxygen and oxygen molecules that have not contributed to the reaction up to this point are utilized in chemical reaction R7, which is efficient. In Figure 1, the notation "[O]" represents "oxygen atoms contained in oxygen molecules." Note that the hydroperoxide (HO) obtained by formula (10) ２ ) is used as an oxidizing agent in other chemical reactions, resulting in its conversion to water.

[0038] [CH ２ (OH) to form formic acid] Intermediate P2, methoxy, is converted to intermediate P3 (CH ２ (OH)) to intermediate P3 (CH ２ (OH)) undergoes chemical reaction R8, combining with an oxygen molecule to form intermediate P4 (CH ２ (OH)OO) is produced. Chemical reaction R8 is shown in formula (11). CH ２ (OH) + O ２ → CH ２ (OH)OO...(11)

[0039] Intermediate P4(CH ２ (OH)OO) undergoes chemical reaction R9, combining with methane. Chemical reaction R9 is shown in equation (12). CH ２ (OH)OO+CH ４ → CH ２ (OH) + CH ３ OOH...(12)

[0040] Methyl hydroperoxide (CH ３ OOH) is converted into formaldehyde and water. As mentioned above, formaldehyde gradually combines with oxygen atoms contained in oxygen molecules in the reaction vessel through an oxidation reaction to produce formic acid.

[0041] Intermediate P3 (CH ２ (OH)) undergoes chemical reaction R8 again to form intermediate P4 (CH２ Thus, a cyclic reaction (Loop B) occurs in which chemical reaction R8 and chemical reaction R9 are repeated. Intermediate P3 (CH ２ (OH)) to produce a large amount of formic acid.

[0042] Not all intermediates P4 reach the cycle reaction shown in Loop B. As shown in Figure 1, intermediates P4 may combine with each other to produce formic acid, formaldehyde, water, and molecular oxygen. As described above, formaldehyde undergoes an oxidation reaction in which it gradually combines with oxygen atoms contained in molecular oxygen in the reaction vessel to produce formic acid, the target product.

[0043] [Direct production of methanol and formic acid starting from methyl] The chemical reaction R10 in which methyl, which is intermediate P1, combines with an oxygen molecule produces methyl peroxide (CH ３ O ２ ) is produced. Chemical reaction R10 is shown in formula (13). M represents a third body. CH ３ +O ２ +M → CH ３ O ２ + M ... (13)

[0044] Two intermediates P5 (CH ３ O ２ ) molecules combine to produce methanol, formaldehyde, and oxygen through chemical reaction R11. Chemical reaction R11 is shown in equation (14). 2CH ３ O ２ → CH ３ OH + HCHO + O ２ ...(14)

[0045] As mentioned above, formaldehyde gradually combines with oxygen atoms contained in oxygen molecules in the reaction vessel to produce formic acid through an oxidation reaction. In this way, methanol and formic acid can be produced directly from the methyl intermediate P1 without going through methoxy. In addition, two methyl peroxides (CH ３ O ２) molecule through chemical reaction R12 to produce methoxy. Chemical reaction R12 is shown in formula (15). 2CH ３ O ２ → 2CH ３ O+O ２ ...(15)

[0046] As mentioned above, the methoxy obtained in formula (15) is the starting material for generating methanol and formic acid via various reaction pathways. From this point of view, chemical reaction R10, which constitutes the methoxy generation route, can also be considered an important chemical reaction. However, the amount of methoxy generated via chemical reactions R10 and R12 is smaller than the amount of methoxy generated via chemical reaction R5. The reason for this is that chemical reaction R10, which does not involve atomic oxygen, is unlikely to occur, and methyl peroxide (CH ３ O ２ When the concentration of methyl peroxide is low, two methyl peroxide molecules (CH ３ O ２ ) come into contact with each other. As a result, chemical reaction R12 is unlikely to occur. Similarly, chemical reaction R11 is unlikely to occur. In other words, the direct production of methanol and formic acid starting from methyl (intermediate P1) (chemical reaction R10-chemical reaction R11) and the production of methoxy via chemical reaction R10-chemical reaction R12 are both not expected because chemical reaction R10 is the rate-determining step. In contrast, methoxy is produced in large quantities via chemical reaction R5, which combines intermediate P1 (methyl) with atomic oxygen. In addition, the chemical reaction is accelerated by the cycle reaction described above. As a result, large quantities of methanol and formic acid are produced. This is why atomic oxygen, especially O( １ This is evidence that D) is important.

[0047] [Summary of the methane gas conversion process] In the methane gas conversion process, atomic oxygen generated by irradiating ozone with light L1 participates in chemical reactions at various points, oxidizing methane gas and converting it into organic matter. In particular, methoxy, which is obtained as a result of the oxidative decomposition of methane gas, acts as a starting point, generating two cycle reactions (Loop A and Loop B). The two cycle reactions produce a large amount of methanol (CH ３ OH) and formic acid (HCOOH).

[0048] 2A is a diagram showing a first embodiment of a methane gas conversion apparatus, which includes a reaction vessel 10, a light source 4 that irradiates the inside of the reaction vessel 10 with light L1, a stirring bar 5, and a power source 6 for the stirring bar 5.

[0049] The reaction vessel 10 has a first supply port 11 for supplying an ozone-containing fluid and a second supply port 21 for supplying a methane gas-containing fluid. The first supply port 11 is connected to a supply source 12 of the ozone-containing fluid via a pump Pa.

[0050] A liquid layer S1 is present in the reaction vessel 10. The liquid layer S1 is composed mainly of ozone water as a fluid containing ozone. The supply source 12 is a source of ozone water, in which ozone water is stored, a generator that generates ozone water, or a gaseous ozone supply source. Ozone water or gaseous ozone is supplied from a first supply port 11. By bubbling the gaseous ozone into the liquid layer in the reaction vessel and stirring with a stirrer, the ozone can be dissolved in the liquid layer. Details of ozone water will be described later.

[0051] The pump Pa feeds a predetermined amount of ozone water or gaseous ozone from the supply source 12 into the liquid layer S1 in the reaction vessel 10. The pump Pa is controlled by a control unit (not shown). The second supply port 21 is connected to a methane gas supply source 22 via a pump Pb. Methane gas is stored in the supply source 22. Such methane gas is, for example, gas recovered from sewage, livestock excrement, or food waste. The pump Pb feeds a predetermined amount of methane gas from the supply source 22 into the liquid layer S1 of the reaction vessel 10. The pump Pb is controlled by a control unit (not shown). The methane gas fed into the liquid layer S1 is dissolved in the liquid layer S1 by bubbling and stirring with a stirrer. The methane gas that is not completely dissolved in the liquid layer S1 forms a gas layer S2.

[0052] In this embodiment, the liquid layer S1 is mainly composed of ozone water. The gas layer S2 is mainly composed of methane gas. However, since the contents of the reaction vessel 10 are stirred by a stirrer (described later), bubbles 25 of methane gas are contained in the liquid layer S1. Furthermore, droplets of ozone water, oxygen molecules, and ozone molecules may be contained in the gas layer S2.

[0053] The internal space of the reaction vessel 10 has a cylindrical shape. The volume of the internal space is not particularly limited, but for example, it is 1 liter (0.001 m ３ ) or more, and 1000 liters (1 m ３ ) or less.

[0054] [Ozone Water] Details of ozone water will be described. ２ O) in ３ Ozone water is a water in which the molecules are dissolved. ３ It also includes substances other than molecules. For example, ozone water is ２ Ozone is an unstable substance and can be decomposed by the passage of time, exposure to light, or heat. ２Ozone water may be a liquid in which ozone exists as bubbles with a diameter of less than 1000 μm. Ozone water in which ozone is dissolved in the form of bubbles with a diameter of less than 1000 μm can maintain ozone in water for a long period of time. The bubbles may be microbubbles (bubbles with a diameter of 1 μm or more and less than 100 μm) or nano-sized ultrafine bubbles (bubbles with a diameter of less than 1 μm).

[0055] There are various methods for producing ozone water, such as passing ozone gas through an ozone-permeable membrane immersed in a polar or non-polar solvent, or placing electrodes in water to sandwich an electrolyte membrane and applying a voltage between the electrodes to cause electrolysis. By passing ozone through an ozone-permeable membrane, ozone dissolves in the water as ultrafine nano-sized bubbles, which are smaller than microbubbles, and large amounts of highly concentrated ozone water can be produced.

[0056] [Light Source] In this embodiment, the light source 4 that generates the light L1 is located outside the reaction vessel 10. The reaction vessel 10 includes a light guide 3. The light L1 from the light source 4 is guided into the reaction vessel 10 through the light guide 3. The light L1 passes through the liquid layer S1 of the ozone water and is irradiated toward the interface between the granular methane gas and the ozone. The light L1 activates the ozone dissolved in the ozone water and converts it into atomic oxygen (O( １ D) or O( ３ As described above, atomic oxygen near the grain boundary surface generated by light L1 combines with methane molecules at the grain boundary surface, causing a chemical reaction shown as chemical reaction R2 or chemical reaction R4. As described above, atomic oxygen also combines with intermediate P1 (methyl) generated by chemical reaction R2 to generate intermediate P2 (methoxy). These reactions occur mainly around bubbles 25 of methane gas present in liquid layer S1, but also occur near the interface between the liquid layer and the gas layer.

[0057] The light L1 is light having a wavelength of 200 nm or more and 411 nm or less. As described above, the light having a wavelength of 200 nm or more and 411 nm or less converts a large amount of O( １The reaction of (5) is accelerated by the large amount of OH obtained by the reaction of (3) and (6). As a result, the methane gas conversion process is accelerated by the cycle reactions (Loop A and Loop B) described above. Equations (1), (3), (5), and (6) are shown below again. ３ +hν → O( １ D) + O ２ ... (1) O ( １ D) + CH ４ → CH ３ +OH...(3) OH+CH ４ → CH ３ +H ２ O...(5) O( １ D) + H ２ O → 2OH...(6)

[0058] As the light source 4 that emits light having a wavelength of 200 nm or more and 411 nm or less, it is preferable to use a solid-state light source such as an LED or LD.

[0059] The light source 4 may be constantly lit, or may be intermittently lit by repeatedly turning on (ON) and off (OFF) to irradiate light intermittently. Regarding the interval between turning on and off the light source 4, for example, the light may be turned on and off repeatedly at regular intervals. The light-on time may be 5 to 60 seconds, or 10 to 20 seconds. The light-off time may be 5 to 60 seconds, or 10 to 20 seconds. The light-on time and the light-off time may be the same. Intermittent lighting allows the overall reaction rate of the chemical reaction to be controlled.

[0060] [Reaction vessel] It is preferable to use a material that is highly transparent to light having wavelengths of 200 nm or more and 411 nm or less for the light guide 3 of the reaction vessel 10. A light diffusion element such as a lens may be disposed at the tip of the light guide 3 to diffuse the light L1.

[0061] The light guide 3 may be a rod-shaped light guide that is long in one direction or may be made of an optical fiber, and may be inserted deep into the reaction vessel 10. This reduces the attenuation of light in the liquid, allowing the light L1 to reach every corner of the reaction vessel 10. It is particularly preferable to use quartz glass for the light guide 3. When the light guide 3 is made of an optical fiber, it has the advantage that it is more resistant to mechanical strain than general glass, and the optical fiber is less likely to break even when the weight of the liquid is applied to it. As a variant, a reaction vessel 10 may be used in which the light source 4 is embedded in the wall without using the light guide 3.

[0062] Since methane gas is a flammable gas, there is a risk of explosion if the methane gas is present in the gas layer S2 at a concentration equal to or higher than the lower explosion limit. When methane gas is present in the gas layer S2 in the reaction vessel 10 at a high concentration, the passage of light L1 through the gas layer S2 increases the risk of explosion.

[0063] As shown in Figure 1, in this embodiment, light L1 is irradiated toward the liquid layer S1 without passing through the gas layer S2. Although methane gas bubbles 25 are also contained in the liquid layer S1, the risk of explosion of methane gas bubbles 25 is low, so the risk of explosion is low when light L1 is irradiated toward the liquid layer S1. Furthermore, the emission direction and diffusion angle of light L1 may be adjusted by the light guide unit 3 so that light L1 is not irradiated toward the gas layer S2. If the gas layer S2 does not exist or if the methane gas concentration in the gas layer S2 is low, light L1 may be directed toward the gas layer S2.

[0064] The number of light guiding units 3 may be one or more. The number of light guiding units 3 may be set depending on the size of the reaction vessel 10, etc. When multiple light guiding units 3 are arranged on the side surface of the reaction vessel 10, the light guiding units 3 may be arranged so that they form equal angles with respect to the central axis of the reaction vessel 10, or may be arranged so that they form unequal angles. In order to reduce the risk of explosion of methane gas in the gas layer S2, the emission direction and light diffusion angle of each light guiding unit 3 may be adjusted individually.

[0065] As shown in FIG. 2A , the reaction vessel 10 is provided with a reflective film 7 in contact with the inner wall of the reaction vessel 10. The reflective film 7 reflects light L1 from the light source 4. By preventing light from leaking out, photons are utilized without being wasted, thereby increasing the photoreaction efficiency. Furthermore, in order to reduce the risk of methane gas explosion in the gas layer S2, the reflective film 7 may be provided in a location away from the upper part of the reaction vessel 10, which is likely to come into contact with the gas layer S2.

[0066] The reflective film 7 is preferably made of a material that is highly corrosion-resistant to the liquid in the reaction vessel 10. If a highly corrosion-resistant material is not used, a highly corrosion-resistant light-transmitting material may be provided on the surface of the reflective film 7. For example, the reflective film 7 of this embodiment is formed by vapor-depositing an aluminum-based material, but the surface on which the aluminum-based material is vapor-deposited may be protected with glass. Note that if the inner wall of the reaction vessel 10 itself reflects the light L1, the reflective film 7 does not need to be formed. In this way, it is preferable that the inner wall of the reaction vessel 10 has a reflective surface that reflects the light L1, but the reflective surface is not a required component.

[0067] In this embodiment, the liquid layer S1 is primarily composed of a single liquid layer of ozone water. However, the liquid layer S1 may be composed of multiple liquid layers. For example, the liquid layer S1 may be composed of a first liquid layer primarily composed of ozone water and a second liquid layer composed of a nonpolar solvent in which methane is easily dissolved. In this case, when the reaction vessel is left to stand, the two liquid layers do not dissolve and are separated by gravity due to the difference in their specific gravities. In the case of separation, particularly when the upper liquid layer is a light-absorbing solution, light is less likely to reach the gas layer S2. Therefore, even if light L1 is irradiated upward in the reaction vessel 10, the risk of explosion is less likely to increase. If the light L1 irradiated upward is also reflected by the reflective film 7, the utilization efficiency of light L1 is further improved, further increasing the reaction efficiency.

[0068] Both the liquid layer S1 and the gas layer S2 may contain other substances. The internal pressure of the reaction vessel 10 may be increased using pumps (Pa, Pb). Increasing the internal pressure of the reaction vessel 10 further promotes the chemical reaction. The fluid containing methane gas in the gas layer S2 may be recovered and sent back to the liquid layer S1 to bubble.

[0069] [Stir Bar] FIG. 2B is a cross-sectional view taken along the line 2B-2B in FIG. 2A. As shown in FIGS. 2A and 2B, in this embodiment, an electromagnetic coil is used as the power source 6, and a magnetic material is used as the stir bar 5. The stir bar 5 is disposed inside the reaction vessel 10, particularly near the bottom of the reaction vessel 10. Multiple electromagnetic coils are used to rotate the stir bar 5, which is not in physical contact with the power source 6. The rotation of the stir bar 5 causes the interface BS to flow, and bubbles of methane gas are generated in the liquid layer S1, promoting the methane gas conversion reaction. In this embodiment, the stir bar 5 and the power source 6 are not physically connected to each other through a non-contact power transmission system. However, a contact power transmission system in which the stir bar 5 and the power source 6 are physically connected, as shown in the second embodiment and subsequent embodiments, may also be employed.

[0070] The stirrer 5 may be rotated continuously or intermittently. When the stirrer 5 is rotated intermittently, irradiation with light L1 and stirring with the stirrer 5 may be performed alternately, or a period during which irradiation with light L1 and stirring with the stirrer 5 are performed simultaneously may be included.

[0071] The stirring bar 5 may be rotated and stopped intermittently while the light source 4 is constantly turned on to continuously irradiate the light L1. The interval between the repeated rotation and stopping of the stirring bar 5 may be 5 seconds or more and 60 seconds or less, and preferably 10 seconds or more and 20 seconds or less. The stopping time of the stirring bar 5 may be 5 seconds or more and 60 seconds or less, and preferably 10 seconds or more and 20 seconds or less. The rotation time and the stopping time may be set to the same time and repeated.

[0072] The contents of the reaction vessel 10 may be stirred by a method other than using the stirrer 5. For example, the reaction vessel 10 may be continuously moved. "Continuously moving" the reaction vessel 10 includes, for example, rotating, rocking, or vibrating the reaction vessel 10. Note that relocating the reaction vessel 10 does not constitute "continuous movement." The methane gas conversion device may have a power source for continuously moving the reaction vessel 10. Having a protruding piece protruding from the inner wall of the reaction vessel 10 makes it easier to stir the contents when the reaction vessel 10 is continuously moved. Furthermore, rather than vibrating the reaction vessel 10, the contents of the reaction vessel 10 may be stirred by introducing or discharging the contents (ozone-containing liquid and methane gas) into the reaction vessel 10, or by vibrating the contents themselves. Alternatively, stirring may be achieved by bubbling methane gas through the liquid layer S1. Therefore, the stirrer 5 and its power source 6 are not essential components.

[0073] Second Embodiment A second embodiment of a methane gas conversion apparatus will be described with reference to Figure 3. Features that differ from the previously described embodiments will be mainly described, and features that are common to the previously described embodiments will not be described in principle. The same applies to the third and subsequent embodiments.

[0074] A second embodiment will be described with reference to Figure 3. The reaction vessel 60 is a vertically elongated cylindrical vessel arranged at an angle. The bottom surface 61 of the cylindrical reaction vessel 60 is inclined relative to the horizontal plane. Inside the reaction vessel 60, there is a liquid layer S1 filled mainly with ozone water and a gas layer S2 containing mainly methane gas. By arranging the vertically elongated reaction vessel 60 at an angle, the area of ​​the interface BS between the liquid layer S1 and the gas layer S2 increases, and the stirrer 5 is rotated within the interface BS, making it easier for methane to dissolve in the liquid layer S1.

[0075] In the first embodiment (see FIG. 1 ), the second supply port 21 was designed to be located in the liquid layer S1 so that methane could be bubbled into the liquid layer S1, but in the present embodiment, where methane can be dissolved in the liquid layer S1 without bubbling, the second supply port 21 may be designed to be located in the gas layer S2. Of course, even in the present embodiment in which the reaction vessel 60 is disposed at an angle, the position of the second supply port 21 may be adjusted so that methane gas can be drawn directly into the liquid layer S1. The fluid in the gas layer S2 may be recovered and sent again to the liquid layer S1 to bubble it.

[0076] In this embodiment, the light source 4 is disposed near the side wall of the reaction vessel 60. In this embodiment, the reaction vessel 60 is disposed at an angle, so that the light source 4 is located below the reaction vessel 60.

[0077] In this embodiment, the stirrer 5 is physically connected to the power source 6 located outside the reaction vessel 30 by a rod 35 that transmits rotation. The extension direction of the rod 35 coincides with the axial direction of the rotation axis x1 of the stirrer 5. The angle t1 between the axial direction of the rotation axis x1 and any direction including the horizontal plane h1 is preferably 45 degrees or less, more preferably 35 degrees or less. This makes it easier for the stirrer 5 to be positioned across the liquid layer S1 and the gas layer S2. As a result, when the stirrer 5 is rotated using the power source 6, the components in the mixed fluid are more thoroughly mixed, increasing the contact area of ​​each component. As shown in FIG. 3 , many methane gas bubbles 25 appear in the liquid layer S1, and many ozone water droplets 26 appear in the gas layer S2. The increase in bubbles 25 and droplets 26 leads to an increase in the contact area between the ozone water and methane gas, accelerating the chemical reaction.

[0078] Note that placing the stirring bar 5 across the liquid layer S1 and the gas layer S2 is one method for increasing the contact area, and is not a requirement when tilting the reaction vessel 60. Even when the reaction vessel 60 is tilted so that the stirring bar 5 is present in the liquid layer S1, the contact area is expected to increase, and the conversion of methane gas is promoted.

[0079] Third Embodiment A third embodiment will be described with reference to Figure 4. In a reaction vessel 70, a light source 4 is surrounded by a heat transfer unit 31. The heat transfer unit 31 is fitted into the opening of the reaction vessel 70 and is in direct contact with the liquid layer S1. This allows heat generated by the light source 4 to be directly transferred to the liquid layer S1. In the above-described methane gas conversion process, as the temperature of the mixed fluid increases, it becomes easier to secure the activation energy required to cause a reaction (particularly a cycle reaction), thereby increasing reactivity.

[0080] Furthermore, in the reaction vessel 70 of this embodiment, the power source 6 is surrounded by the heat transfer unit 32. The heat transfer unit 32 is fitted into the opening of the reaction vessel 70 and is in direct contact with the liquid layer S1. This allows the heat generated by the power source 6 to be directly transferred to the liquid layer S1.

[0081] The heat transfer units (31, 32) may incorporate, for example, a heat pipe, or may be made of a material with high thermal conductivity. Furthermore, in addition to utilizing waste heat from the light source 4 and the power source 6, heat from outside the methane gas conversion device may be introduced. A heater for heating the mixed fluid may be disposed in the reaction vessel 70. Furthermore, at least one of the ozone-containing liquid and methane gas may be preheated and supplied into the reaction vessel 70. To assist in the activation energy of the cycle reaction, the temperature rise range is preferably set to 10°C or more and 80°C or less, and more preferably 30°C or more and 60°C or less.

[0082] <Fourth embodiment> A fourth embodiment will be described with reference to Fig. 5. A catalyst 9 (hatched particles in S1 in Fig. 5) is placed inside a reaction vessel 60. The catalyst 9 helps to exceed the activation energy required to cause a reaction (particularly a cycle reaction). The catalyst 9 is preferably in powder form. The average particle size of the catalyst 9 is preferably, for example, 10 µm or more and 1 mm or less. The catalyst 9 is, for example, SnO ２ particles, TiO ２ particles, or ZrO ２ The catalyst 9 is selected depending on the wavelength of the light L1 to be used. Furthermore, platinum, which is resistant to oxidation, may be used as a promoter. Specifically, TiO ２A promoter in which platinum is supported on a catalyst may also be used. Silver nanoparticles, which are resistant to oxidation, may also be used as catalyst 9. Silver nanoparticles have the property of being easily adsorbed to the interface BS, and are particularly effective in promoting interfacial reactions. While there were limitations on the use of catalyst 9 in conventional chlorine-based solutions, a variety of catalysts 9 can be used without significant limitations in liquids containing ozone.

[0083] A number of embodiments and their modifications have been described above. The above-described embodiments and their modifications merely illustrate examples of the present invention, and the present invention is not limited to the above-described embodiments and their modifications. Various changes or modifications can be made to the above-described embodiments and their modifications, and the above-described embodiments or their modifications can be combined, without departing from the spirit of the present invention.

[0084] The following experiment was conducted to confirm whether the above-described methane gas conversion method and methane gas conversion device are effective. The scale of the experiment was relatively small, as shown in the experimental conditions below, but it is theoretically expected that the experimental results will have the same tendency regardless of the scale of the experiment.

[0085] [Experimental Contents] The experimental equipment is shown in Figure 6. The experimental equipment includes a methane gas supply source 22 (gas cylinder), a flow rate controller 27, a reaction vessel 80, a light source 4, and a collection vessel 85. The reaction vessel 80 has a liquid layer S1 and a gas layer S2. The reaction vessel 80 is a synthetic quartz glass beaker. The reaction vessel 80 is sealed using a silicone rubber stopper 81, and has a maximum capacity of approximately 50 cm3. ３ The liquid layer is 15 cm ３ Ozone water (O ３ A first liquid layer S11 having a volume of 1.5 ppm and a volume of 15 cm ３ and a second liquid layer S12 of perfluorohexane (hereinafter referred to as "PFH"). PFH is a non-polar solvent that easily dissolves methane gas. In the above-described embodiment, PFH may be introduced as the second liquid layer, as in the experimental apparatus.

[0086] A glass tube for supplying methane gas is inserted into the reaction vessel 80. The liquid layers (S11, S12) are bubbled while the amount of methane gas introduced from the supply source 22 is adjusted by the flow rate controller 27. The light source 4 is disposed below the reaction vessel 80 and irradiates light from the bottom of the reaction vessel 80. The light source 4 is a 250 W U-shaped constant pressure mercury lamp (dominant wavelength showing maximum intensity: 253.7 nm, irradiation energy: 61 J / cm ２ In this experimental configuration, a stirrer was not used because the liquid layers (S11, S12) were stirred by the bubbling of methane gas. Chemical reactions caused by light irradiation can occur not only between the bubbled methane gas and the first liquid layer S11, but also between the second liquid layer S12 and the first liquid layer S11, and between the gas layer S2 and the first liquid layer S11.

[0087] A pipe 87 for collecting samples is inserted into the reaction vessel 80. The pipe 87 is connected to a collection vessel 85. The collection vessel 85 has a storage container 86 for pure water and an exhaust line for gas G1. Gas G1, such as methane gas, ozone, and PFH, is exhausted from the exhaust line. On the other hand, the pure water 86 traps formic acid and methanol that are produced in the reaction vessel 80 and evaporate.

[0088] In the above-mentioned experimental apparatus, methane gas was introduced at a rate of 500 cm ３ / min for 60 minutes, with the light source 4 turned on. After 60 minutes had elapsed, the amounts of methanol and formic acid contained in the reaction vessel 80 and the recovery vessel 85 were measured. The formic acid content was measured by electrophoresis. The methanol content was measured by GCMS (gas chromatograph mass spectrometer).

[0089] [Experimental Results] Table 1 shows the measurement results for formic acid and methanol at 1 L (1000 cm ３ Furthermore, Table 1 shows the total amount in the container calculated based on the content ratio and the amount of liquid in the container.

[0090]

[0091] [Discussion of Experimental Results] 1.5 ppm O ３ Including 15cm ３ O contained in ozonated water ３The mass is 22.5 μg. The total amount of formic acid and methanol produced, as determined from Table 1, is 29.2 μg. This indicates that the reaction efficiency is approximately 130% (≒29.2 / 22.5 × 100). This reaction efficiency value is considered to be high from the perspective of energy balance with carbon neutrality in mind, as well as from an economical perspective. This experiment was carried out in an environment where the reaction vessel 80 was at approximately atmospheric pressure and at approximately room temperature.

[0092] The reason why the product could be obtained with high reaction efficiency even when the reaction vessel 80 was kept at room temperature and pressure is thought to be that light containing wavelengths of 200 nm or more and 411 nm or less was used. The atomic oxygen generated from ozone was converted into O( ３ In the case of P), 300°C or higher is required to generate intermediate P1, but a large amount of O( １ D), which is thought to have promoted the methane gas conversion process even at room temperature and pressure.

[0093] 3: Light guide section 4: Light source 5: Stirring bar 6: Power source (of stirring bar) 7: Reflecting film 9: Catalyst 10, 30, 60, 70, 80: Reaction vessel 11: First supply port 12: Ozone supply source 21: Second supply port 22: Methane gas supply source 25: Bubbles 26: Droplets 27: Flow rate controller 31, 32: Heat transfer section 61: Bottom surface (of reaction vessel) 81: Plug (of reaction vessel) 85: Liquid recovery vessel 86: Pure water 87: Tube BS: Interface L1: Light S1: Liquid layer S11: First liquid layer S12: Second liquid layer S2: Gas layer Pa, Pb: Pump

Claims

1. A method for converting methane gas, comprising irradiating a mixed fluid, which is a mixture of a fluid containing ozone and a fluid containing methane gas, with light having a wavelength of 200 nm or more and 411 nm or less, thereby converting the methane gas into organic matter.

2. The method for converting methane gas as described in claim 1, wherein the ozone-containing fluid is generated by passing ozone gas through an ozone-permeable membrane immersed in a polar or non-polar solvent.

3. A method for converting methane gas according to claim 1 or 2, characterized in that the mixed fluid is heated simultaneously with the irradiation of light.

4. A method for converting methane gas according to claim 1 or 2, characterized in that a catalyst is brought into contact with the mixed fluid.

5. A method for converting methane gas as described in claim 1 or 2, characterized in that the fluid containing ozone is ozone water.

6. A method for converting methane gas as described in claim 1 or 2, characterized in that the light is irradiated onto a liquid layer formed in the mixed fluid without passing through a gas layer formed in the mixed fluid.

7. A method for converting methane gas according to claim 1 or 2, characterized in that the mixed fluid is stirred during or before the light irradiation.

8. A methane gas conversion apparatus for converting methane gas into organic matter, comprising: a reaction vessel having a first supply port for supplying a fluid containing ozone, and a second supply port for supplying the fluid containing methane gas; and a light source for irradiating the inside of the reaction vessel with light having a wavelength of 200 nm or more and 411 nm or less.

9. The methane gas conversion apparatus according to claim 8, further comprising a stirrer in the reaction vessel.

10. The methane gas conversion apparatus according to claim 9, wherein the bottom of the reaction vessel is inclined relative to a horizontal plane so that the stirrer is positioned across the liquid layer and the gas layer.

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