WO2025069650A1 - Methane gas conversion method and methane gas conversion device - Google Patents

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

METHOD FOR CONVERSION OF METHANE GAS AND APPARATUS FOR CONVERSION OF METHANE GAS

This invention relates to a methane gas conversion method and a methane gas conversion device.

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, so it is not desirable to release methane gas directly into the atmosphere. Therefore, the recovery of methane gas released into the atmosphere is being considered. Methane gas can be decomposed by using the recovered methane gas to run an engine or extracting thermal energy in a boiler. However, methane gas is a gas at room temperature and pressure, and transporting it in a gaseous state to the combustion site is inefficient and difficult to store.

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 curbing global warming. Furthermore, by selling the organic matter produced, the cost of converting methane gas can be recovered. Furthermore, converting methane gas into organic matter is advantageous in terms of the efficiency of transporting methane gas and ease of storage.

Patent Document 1 describes how light is irradiated onto an environment in which hydrocarbons such as methane and chlorine dioxide exist to generate chlorine radicals, which are then bonded to the hydrocarbons to generate organic substances such as methanol.

Patent No. 6080281

The technology described in Patent Document 1 requires chlorine dioxide. To prepare chlorine dioxide, it is necessary to prepare raw materials for chlorine dioxide, such as _NaClO2 and HCl, and cause a chemical reaction.
Then, energy is required for distillation and evaporation to separate methanol and formic acid from the aqueous solution containing NaCl and HCl, which are salts produced after the synthesis. In other words, monetary and energy costs are incurred for preparing chlorine dioxide and separating the product. Regarding energy costs, it is not desirable from the viewpoint of carbon neutrality to input a large amount of energy for the conversion of methane gas, which is a greenhouse gas. Furthermore, chlorine-based materials such as chlorine dioxide have problems such as being prone to corrode equipment such as reaction vessels.

Therefore, we provide a method for converting methane gas without using chlorine-based materials such as chlorine dioxide.

The methane gas conversion method of the present invention is characterized in that a mixed fluid containing an ozone-containing fluid and a methane-containing fluid 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. Although details will be described 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( ^1D ), is generated from ozone, and a large amount of an intermediate called methoxy is generated in the methane gas conversion process. The presence of methoxy causes a cycle reaction that leads to the production of organic substances such as methanol or formic acid. This makes it possible to convert a large amount of methane gas into industrially useful organic substances such as methanol or formic acid without using chlorine-based materials.

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. In the conversion process, methane gas may be converted to a compound containing carbon other than methanol or formic acid (e.g., methyl formate) after producing methanol or formic acid as an intermediate.

The ozone-containing fluid may be generated by passing ozone gas through an ozone-permeable membrane immersed in a polar or non-polar 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 gas-containing fluid may be a fluid (gas) containing only methane gas, or may contain gases or liquids other than methane gas.

The mixed fluid may be heated in addition to the irradiation of light. Heating the mixed fluid promotes the chemical reaction.

A catalyst may be contacted with the mixed fluid. The catalyst aids in exceeding the activation energy required to cause a chemical reaction, particularly a cyclic reaction, and thus accelerates the chemical reaction.

The ozone-containing fluid may be ozone water.

The light may be irradiated onto the liquid layer formed in the mixed fluid without passing through the gas layer formed in the mixed fluid. Since the chemical reaction caused by the light takes place in the liquid layer and is less likely to take place in the gas layer, the risk of explosion is reduced.

The mixed fluid may be stirred during or before the light irradiation.
Stirring can accelerate the chemical reaction.

The present invention provides a methane gas conversion device for converting methane gas into organic matter, comprising:
a reaction vessel including 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 that irradiates the inside of the reaction vessel with light having a wavelength of 200 nm or more and 411 nm or less.

The reaction vessel may be provided with a stirrer. The mixed fluid is in the reaction vessel, and the stirrer stirs the mixed fluid. Stirring can promote the chemical reaction.

The bottom of the reaction vessel may be tilted relative to the horizontal plane so that the stirrer is positioned across the liquid and gas layers. This allows the methane gas contained in the gas layer to dissolve in the liquid layer.

The light source may be positioned so that the light is irradiated onto the liquid layer formed in the mixed fluid without passing through the gas layer formed in the reaction vessel in the mixed fluid. Since the chemical reaction caused by the light takes place in the liquid layer and is less likely to take place in the gas layer, the risk of explosion is reduced.

As a result, methane gas can 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, there are no problems with the generation of chlorine gas, equipment corrosion due to chlorine-based materials, or waste liquid treatment of chlorine-based materials. In particular, the distillation tank for separating organic matter from chlorine-based solutions is large, and the costs of introducing and maintaining the equipment are high, so if the distillation layer is not necessary, it will greatly contribute to reducing such space and costs.

Converting methane gas into organic matter is expected to be a technology that contributes to global warming countermeasures by suppressing the release of methane gas into the atmosphere. In addition, converting methane gas into organic matter such as methanol and formic acid and then transporting or storing it is also expected to be a technology that contributes to global warming countermeasures by contributing to improving the efficiency of methane gas transportation and the storage properties of methane gas. Converting methane gas to produce methanol and formic acid contributes to Goal 13 of the United Nations-led Sustainable Development Goals (SDGs), which is to "take urgent action to mitigate climate change and its impacts."

FIG. 1 is a diagram showing an overview of a methane gas conversion process. FIG. 1 illustrates a first embodiment of a methane gas conversion apparatus. 2B is a cross-sectional view taken along line 2B-2B in FIG. 2A. FIG. 2 illustrates a second embodiment of a methane gas conversion apparatus. FIG. 13 illustrates a third embodiment of a methane gas conversion apparatus. FIG. 13 illustrates a fourth embodiment of a methane gas conversion apparatus. FIG. 1 is a diagram showing an experimental setup for a methane gas conversion experiment.

The following explanations will be given with reference to the drawings as appropriate. Note that all drawings are schematic illustrations, and the dimensional ratios in the drawings do not necessarily match the actual dimensional ratios, and the dimensional ratios between the different drawings do not necessarily match.

<Methane gas conversion process>
First, the methane gas conversion process will be described. FIG. 1 is a diagram showing an overall view of the methane gas conversion process. With reference to FIG. 1, a process of decomposing methane to produce methanol (CH ₃ OH) and formic acid (HCOOH) will be described. In FIG. 1, CH ₄ and O ₃ enclosed in a square represent methane and ozone input into the reaction field. Methane gas and ozone input into the reaction field are mainly decomposed according to the methane gas conversion process shown in FIG. 1. Underlined CH ₃ OH and HCOOH represent methanol and formic acid to be produced. Each major chemical reaction in the methane gas conversion process is given a symbol combining R and a number. Each major intermediate produced in the methane gas conversion process is given a symbol combining P and a number. In this specification, an intermediate refers to a substance that exists temporarily during the methane gas conversion process. The overall view of the methane gas conversion process will be described while explaining each chemical reaction and each intermediate.

[Photodecomposition of ozone]
The mixed fluid in the reaction field includes a fluid containing ozone and a fluid containing methane gas. The ozone may be present in a state 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. In the following, the case where the fluid is water, that is, the case where the "fluid containing ozone" is ozone water, will be described. However, the type of fluid to contain ozone is not particularly limited. In addition, even when ozone water is used as the "fluid containing ozone", the ozone water may contain other materials (e.g., additives). In the "methane gas conversion process" section, the "fluid containing methane gas" is treated as being composed only of methane gas.

Chemical reaction R1 represents a decomposition reaction of ozone by light L1 including a wavelength of 200 nm or more and 411 nm or less. When light L1 is irradiated to ozone water and energy hν of light L1 is given to ozone molecules, atomic oxygen O( ^1D ) or O( ^3P ) is extracted from the ozone molecules. O( ^1D ) is atomic oxygen in an active state, and O( ^3P ) is atomic oxygen in a ground state. Chemical reaction R1 is expressed by formula (1) or formula (2). In FIG. 1, chemical reaction R1 that irradiates light L1 to ozone to generate O appears in two places, but "O" that appears in the two chemical reactions R1 is a concept that includes both O( ^1D ) and O( ^3P ).
That is, "O" in FIG. 1 may include at least one of O( ^1D ) and O( ^3P ).

O ₃ +hν → O( ¹ D) + O ₂ ...(1)
O ₃ +hν → O( ³ P) + O ₂ ...(2)

When light having 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( ^1D ) is generated. When light having a wavelength of more than 411 nm and 1180 nm or less is used, the reaction of formula (1) does not occur, and the reaction of formula (2) occurs. As will be described in detail later, since O( ^3P ) is less reactive than O( ^1D ), it is preferable to generate a large amount of O( ^1D ). For this reason, in this embodiment, light having a wavelength of 200 nm or more and 411 nm or less is used.

[Production of methyl (CH ₃ )]
A 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). _CH3 generated in formulas (3) and (4) is methyl as intermediate P1. The chemical reaction of formula (3) proceeds even if the temperature of the reaction field is room temperature. In contrast, the chemical reaction of formula (4) does not proceed easily unless the temperature of the reaction field is about 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( ^1D )+ _CH4 → _CH3 +OH...(3)
O( ^3P )+ _CH4 → _CH3 +OH...(4)

The OH (hydroxyl radical) generated in formulas (3) and (4) (particularly formula (3)) undergoes chemical reaction R3 in which it bonds with _CH4 . Chemical reaction R3 is shown in formula (5). Formula (5) also generates methyl as an intermediate P1.

OH+ _CH4 → _CH3 + _H2O ...(5)

The places where OH is generated are not limited to formulas (3) and (4) (chemical reaction R2). Although not shown in FIG. 1, when light L1 containing a wavelength of 200 nm or more and 411 nm or less is used, a large amount of O( ^1D ) is obtained from ozone, and O( ^1D ) reacts with water ( _H2O ) to generate a large amount of OH. This is shown in formula (6). The large amount of OH generated promotes chemical reaction R3 (formula (5)).

O( ¹ D) + H ₂ O → 2OH...(6)

In this way, a large amount of intermediate P1 (methyl) is produced by chemical reaction R2 and chemical reaction R3. However, not all O( ^1D ) produced in chemical reaction R1 leads to the reaction of formula (3). O( ^1D ) is deactivated to become O( ^3P ), and O( ^3P ) often combines with itself to produce oxygen molecules, or O( ^1D ) combines with oxygen molecules to produce ozone again. Also, as shown in chemical reaction R4, O( ^1D ) may combine with methane to directly produce methanol. Chemical reaction R4 is shown in formula (7).

O( ^1D )+ _CH4 → _CH3OH ...(7)

Chemical reaction R4 is a favorable reaction that converts methane directly 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.

[Methoxy (CH ₃ O) production and cycle reaction]
Methoxy (CH ₃ O) is produced by chemical reaction R5 from intermediate P1 (CH ₃ ) and atomic oxygen obtained by irradiating ozone with light L1. Chemical reaction R5 is shown in formula (8). In formula (8), an example of O( ¹ D) is shown as the atomic oxygen, but a reaction similar to formula (8) occurs even if O( ³ P) is used.

_CH3 +O( ^1D ) → _CH3O ...(8)

The intermediate P2, methoxy, undergoes chemical reaction R6 in which it combines with methane. Chemical reaction R6 is shown in formula (9).

_CH3O + _CH4 → _CH3OH + _CH3 ...(9)

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. A large amount of methane is decomposed by Loop A, producing a large amount of methanol.

[Formation of formic acid by reaction of methoxy with molecular oxygen]
The intermediate P2, methoxy, can undergo chemical reaction R7 in which it bonds with an oxygen molecule. Chemical reaction R7 is shown in formula (10).

_CH3O + _O2 → HCHO+ _HO2 ...(10)

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. Since 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 FIG. 1, the notation "[O]" represents "oxygen atoms contained in oxygen molecules." Note that the hydroperoxide (HO ₂ ) obtained by formula (10) is used as an oxidizing agent for other chemical reactions, and as a result, it is converted into water.

[Production of formic acid starting from CH ₂ (OH)]
The intermediate P2, methoxy, may be converted to intermediate P3 (CH ₂ (OH)). Intermediate P3 (CH ₂ (OH)) undergoes chemical reaction R8 in which it combines with an oxygen molecule to produce intermediate P4 (CH ₂ (OH)OO). Chemical reaction R8 is shown in formula (11).

_CH2 (OH)+ _O2 → _CH2 (OH)OO...(11)

The intermediate P4 (CH ₂ (OH) OO) undergoes a chemical reaction R9 in which it combines with methane. Chemical reaction R9 is shown in formula (12).

_CH2 (OH)OO+ _CH4 → _CH2 (OH)+ _CH3OOH ...(12)

Methyl hydroperoxide (CH ₃ OOH) produced by chemical reaction R9 is converted into formaldehyde and water. As described above, formaldehyde gradually combines with oxygen atoms contained in oxygen molecules in the reaction vessel to produce formic acid through an oxidation reaction.

Intermediate P3 (CH ₂ (OH)) produced by chemical reaction R9 undergoes chemical reaction R8 again to obtain intermediate P4 (CH ₂ (OH)OO). Thus, a cyclic reaction (Loop B) occurs in which chemical reaction R8 and chemical reaction R9 are repeated. A large amount of formic acid is produced from intermediate P3 (CH ₂ (OH)), which is converted from methoxy (intermediate P2) by Loop B.

Not all intermediates P4 lead to 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 oxygen molecules. As described above, formaldehyde gradually combines with oxygen atoms contained in oxygen molecules in the reaction vessel in an oxidation reaction to produce the target product, formic acid.

[Direct generation of methanol and formic acid starting from methyl]
Methyl peroxide (CH ₃ O ₂ ), which is an intermediate P5, is generated by chemical reaction R10 in which methyl, which is an intermediate P1, is combined with an oxygen molecule. Chemical reaction R10 is shown in formula (13). M represents a third body.

_CH3 + _O2 +M→ _CH3O2 + _M ...(13)

Chemical reaction R11 in which two molecules of intermediate P5 (CH ₃ O ₂ ) combine to produce methanol, formaldehyde, and oxygen is shown in formula (14).

_2CH3O2 → _CH3OH +HCHO+ _O2 ...(14 ₎

As described 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 methyl, which is the intermediate P1, without going through methoxy. In addition, methoxy can be produced from two methyl peroxide (CH ₃ O ₂ ) molecules through chemical reaction R12. Chemical reaction R12 is shown in formula (15).

_2CH3O2 → _2CH3O + _{O2 ...} (15)

As described above, the methoxy obtained by formula (15) is a starting material for generating methanol and formic acid through various reaction routes. From this point of view, the chemical reaction R10 constituting the methoxy generation route can also be said to be an important chemical reaction. However, the amount of methoxy generated through chemical reactions R10 and R12 is smaller than the amount of methoxy generated through chemical reaction R5. This is because the chemical reaction R10 without atomic oxygen is unlikely to occur, and the concentration of methyl peroxide (CH ₃ O ₂ ) is low. When the concentration of methyl peroxide is low, the probability that two methyl peroxide molecules (CH ₃ O ₂ ) will come into contact with each other in the liquid is low. As a result, the chemical reaction R12 is unlikely to occur. Similarly, the 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-limiting step. In contrast, a large amount of methoxy is produced from intermediate P1 (methyl) via chemical reaction R5, which combines with atomic oxygen. In addition, the chemical reaction is accelerated by the above-mentioned cycle reaction. As a result, a large amount of methanol and formic acid is produced. This is evidence that atomic oxygen, especially O( ^1D ), is important in the methane gas conversion process.

[Summary of 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 obtained as a result of oxidative decomposition of methane gas serves as the starting point to generate two cycle reactions (Loop A and Loop B). The two cycle reactions generate large amounts of methanol (CH ₃ OH) and formic acid (HCOOH).

First Embodiment
[Outline of methane gas conversion device]
2A is a diagram showing a first embodiment of a methane gas conversion apparatus. The methane gas conversion apparatus includes a reaction vessel 10, a light source 4 for irradiating the inside of the reaction vessel 10 with light L1, a stirrer 5, and a power source 6 for the stirrer 5.

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

There is a liquid layer S1 in the reaction vessel 10. The liquid layer S1 is a fluid containing ozone and is mainly composed of ozone water. The supply source 12 is either a generator that stores ozone water, or that produces ozone water, or a gaseous ozone supply source. Ozone water or gaseous ozone is supplied from the first supply port 11. The ozone can be dissolved in the liquid layer by bubbling the gaseous ozone into the liquid layer in the reaction vessel while stirring with a stirrer. Details of ozone water will be described later.

Pump Pa sends a predetermined amount of ozone water or gaseous ozone from the supply source 12 into the liquid layer S1 in the reaction vessel 10. 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 pump Pb. Methane gas is stored in the supply source 22. Such methane gas is, for example, gas recovered from sewage, livestock waste, or food waste. Pump Pb sends a predetermined amount of methane gas from the supply source 22 into the liquid layer S1 of the reaction vessel 10. Pump Pb is controlled by a control unit (not shown). The methane gas sent to 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.

In this embodiment, the main component of the liquid layer S1 is ozone water. The main component of the gas layer S2 is methane gas. However, because the contents of the reaction vessel 10 are stirred by a stirrer, which will be described later, bubbles 25 of methane gas are contained in the liquid layer S1. In addition, droplets of ozone water, oxygen molecules, and ozone molecules may be contained in the gas layer S2.

The internal space of the reaction vessel 10 has a cylindrical shape. There is no particular limit to the volume of the internal space, but it is preferable that the volume is, for example, 1 liter (0.001 m ³ ) or more and 1000 liters (1 m ³ ) or less.

[Ozone water]
Details of ozone water will be described. Ozone water is water ( _H2O ) in which _O3 molecules are dissolved. Ozone water also includes substances other than _O3 molecules. For example, ozone water also includes _O2 molecules (oxygen molecules). Ozone is an unstable substance, so it changes into _O2 molecules (oxygen molecules) by itself over time, exposure to light, or heat. Ozone 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).

Ozone water can be produced in a variety of ways, including passing ozone gas through an ozone-permeable membrane immersed in a polar or non-polar solvent, or by placing electrodes in water to sandwich an electrolyte membrane and applying a voltage between the two electrodes to cause electrolysis. By passing ozone through an ozone-permeable membrane, large amounts of highly concentrated ozone water can be produced, in which the ozone dissolves as ultra-fine nano-sized bubbles that are smaller than microbubbles.

[light source]
In this embodiment, a light source 4 that generates light L1 is located outside the reaction vessel 10. The reaction vessel 10 includes a light guiding section 3. The light L1 from the light source 4 is guided into the reaction vessel 10 through the light guiding section 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 to generate atomic oxygen (O( ^1D ) or O( ^3P )). As described above, the atomic oxygen generated by the light L1 near the grain boundary combines with methane molecules at the grain boundary to cause a chemical reaction shown in chemical reaction R2 or chemical reaction R4. As described above, the atomic oxygen combines with the intermediate P1 (methyl) generated by the chemical reaction R2 to generate the intermediate P2 (methoxy). These reactions mainly occur around the bubbles 25 of methane gas present in the liquid layer S1, but also occur near the interface between the liquid layer and the gas layer.

Light L1 is light having a wavelength of 200 nm or more and 411 nm or less. As described above, light having a wavelength of 200 nm or more and 411 nm or less generates a large amount of O ( ¹ D) through the reaction of formula (1), and the large amount of OH obtained through the reaction of formula (6) in addition to the reaction of formula (3) promotes the reaction of formula (5). As a result, the methane gas conversion process is accelerated by the above-mentioned cycle reactions (Loop A and Loop B). Formulas (1), (3), (5), and (6) are shown again below.

O ₃ +hν → O( ¹ D) + O ₂ ...(1)
O( ^1D )+ _CH4 → _CH3 +OH...(3)
OH+ _CH4 → _CH3 + _H2O ...(5)
O( ¹ D) + H ₂ O → 2OH...(6)

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.

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. The interval between turning on and off the light source 4 may be, for example, repeated at regular intervals. The time for turning on may be 5 to 60 seconds, or 10 to 20 seconds. The time for turning off may be 5 to 60 seconds, or 10 to 20 seconds. The time for turning on and off may be the same. Intermittent lighting allows the overall reaction rate of the chemical reaction to be controlled.

[Reaction vessel]
It is preferable to use a material that transmits light having a wavelength of 200 nm or more and 411 nm or less well for the light guiding section 3 of the reaction vessel 10. A light diffusing element such as a lens may be disposed at the tip of the light guiding section 3 to diffuse the light L1.

The light guide 3, which is a long rod in one direction or is made of optical fiber, may be inserted deep into the reaction vessel 10. This reduces attenuation of light in the liquid and allows 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 optical fiber, it has the advantage that it is more resistant to mechanical distortion than general glass and is less likely to break even if the weight of the liquid is applied to the optical fiber. As a variation, a reaction vessel 10 in which the light source 4 is embedded in the wall may be used without using the light guide 3.

Since methane gas is flammable, there is a risk of explosion if the methane gas is present in the gas layer S2 at a concentration above the lower explosion limit. If methane gas is present in high concentration in the gas layer S2 inside the reaction vessel 10, the passage of light L1 through the gas layer S2 increases the risk of explosion.

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

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 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 they may be arranged so that they form unequal angles. In order to reduce the risk of an 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.

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 the light L1 from the light source 4. By preventing the light from leaking out, photons are utilized without being wasted, and the photoreaction efficiency can be increased. In addition, in order to reduce the risk of an explosion of methane gas in the gas layer S2, the reflective film 7 may be provided away from the upper part of the reaction vessel 10 which is likely to come into contact with the gas layer S2.

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 in 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 by glass. Note that if the inner wall of the reaction vessel 10 itself reflects the light L1, it is not necessary to form the reflective film 7. In this way, it is preferable for the inner wall of the reaction vessel 10 to have a reflective surface that reflects the light L1, but the reflective surface is not a required component.

In this embodiment, the liquid layer S1 is mainly composed of one liquid layer, which is 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 mainly composed of ozone water and a second liquid layer composed of a non-polar solvent in which methane is easily dissolved. In this case, when the liquid layers are left to stand in the reaction vessel, the two liquid layers do not dissolve in each other, but are separated in the direction of gravity due to the difference in specific gravity between the two liquid layers. In the case of separation, particularly when the liquid layer located at the top is a solution that absorbs light, it becomes difficult for light to reach the gas layer S2, so even if light L1 is irradiated toward the upper part of the reaction vessel 10, the risk of explosion is unlikely to increase.
If the light L1 irradiated upward is also reflected by the reflective film 7, the efficiency of use of the light L1 is further improved, and the reaction efficiency is further increased.

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 will further promote the chemical reaction. The fluid containing methane gas in the gas layer S2 may be collected and sent back to the liquid layer S1 for bubbling.

[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 stirrer 5. The stirrer 5 is disposed inside the reaction vessel 10, particularly near the bottom of the reaction vessel 10. The stirrer 5, which is not in physical contact with the power source 6, is rotated using a plurality of electromagnetic coils. As the stirrer 5 rotates, the interface BS flows, and bubbles of methane gas are generated in the liquid layer S1, promoting the conversion reaction of methane gas. In this embodiment, the stirrer 5 and the power source 6 are a non-contact type power transmission system that is not physically connected, but a contact type power transmission system in which the stirrer 5 and the power source 6 are physically connected, as shown in the second embodiment and thereafter, may be adopted.

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.

The light source 4 may be constantly on to continuously irradiate the light L1 while the stirrer 5 is intermittently rotated and stopped. The interval between repeated rotation and stopping of the stirrer 5 may be 5 to 60 seconds, preferably 10 to 20 seconds. The stop time of the stirrer 5 may be 5 to 60 seconds, preferably 10 to 20 seconds. The rotation time and stop time may be set to the same time and repeated.

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 moved continuously. "Continuously moving" the reaction vessel 10 includes, for example, rotating, rocking, and vibrating the reaction vessel 10. Note that relocating the installation location of the reaction vessel 10 does not fall under "continuously moving". The methane gas conversion device may have a power source for continuously moving the reaction vessel 10. If there is a protruding piece protruding from the inner wall of the reaction vessel 10 toward the inside, it is easier to stir the contents when the reaction vessel 10 is moved continuously. Also, instead of vibrating the reaction vessel 10, the contents of the reaction vessel 10 may be stirred by introducing or discharging the contents (liquid containing ozone and methane gas) of the reaction vessel 10, or by vibrating the contents themselves. Also, stirring may be performed by bubbling methane gas into the liquid layer S1. Therefore, the stirrer 5 and its power source 6 are not essential components.

Second Embodiment
A second embodiment of the methane gas conversion apparatus will be described with reference to Fig. 3. The second embodiment will be described with a focus on features that are different from the previously described embodiments, 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.

The second embodiment will be described with reference to FIG. 3. The reaction vessel 60 is arranged in a tilted, vertically elongated cylindrical shape. The bottom surface 61 of the cylindrical reaction vessel 60 is inclined with respect to the horizontal plane. Inside the reaction vessel 60, there is a liquid layer S1 that is mainly filled with ozone water, and a gas layer S2 that mainly contains methane gas. By tilting the vertically elongated reaction vessel 60, the area of the interface BS between the liquid layer S1 and the gas layer S2 increases, and the stirrer 5 is rotated within it, making it easier for methane to dissolve in the liquid layer S1.

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 tilted, 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 back into the liquid layer S1 for bubbling.

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 the light source 4 is located below the reaction vessel 60.

In this embodiment, the stirrer 5 is physically connected to the power source 6 arranged 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, and more preferably 35 degrees or less. This makes it easy 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 mixed more and the contact area of each component increases. As can be seen in FIG. 3, many bubbles 25 of methane gas appear in the liquid layer S1, and many droplets 26 of ozone water appear in the gas layer S2. The increase in the bubbles 25 and droplets 26 leads to an increase in the contact area between the ozone water and methane gas, promoting the chemical reaction.

Note that placing the stirrer 5 across the liquid layer S1 and the gas layer S2 is one way to increase the contact area, and is not a requirement when tilting the reaction vessel 60. Even when tilting the reaction vessel 60 so that the stirrer 5 is in the liquid layer S1, the contact area is expected to increase, promoting the conversion of methane gas.

Third Embodiment
The third embodiment will be described with reference to Fig. 4. In the reaction vessel 70, the 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 directly contacts the liquid layer S1. This allows the heat generated by the light source 4 to be directly transferred to the liquid layer S1. In the above-mentioned 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 (especially a cycle reaction), and the reactivity becomes higher.

Furthermore, in the reaction vessel 70 of this embodiment, the power source 6 is surrounded by the heat transfer section 32. The heat transfer section 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.

The heat transfer section (31, 32) may incorporate, for example, a heat pipe, and may be made of a material with high thermal conductivity. 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 liquid containing ozone and the methane gas may be heated in advance and supplied to the reaction vessel 70. In order 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 set to 30°C or more and 60°C or less.

<Fourth embodiment>
The fourth embodiment will be described with reference to Fig. 5. A catalyst 9 (hatched granules 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 to 1 mm.
Examples of the catalyst 9 include _SnO2 particles, _TiO2 particles, or _ZrO2 particles. The catalyst 9 is selected according to the wavelength of the light L1 to be used. Furthermore, platinum, which is resistant to oxidation, may be used as the co-catalyst. Specifically, a co-catalyst in which platinum is supported on a _TiO2 catalyst may be used. Furthermore, silver nanoparticles, which are resistant to oxidation, may be used as the catalyst 9. Silver nanoparticles have the property of being easily adsorbed to the interface BS, and are particularly effective in promoting the interface reaction. While there were limitations on the use of the catalyst 9 in chlorine-based solutions as in the past, various catalysts 9 can be used without significant limitations in liquids containing ozone.

A number of embodiments and their variations have been described above. The above-described embodiments and their variations are merely examples of the present invention, and the present invention is in no way limited to the above-described embodiments and their variations. Various modifications or improvements can be made to the above-described embodiments and their variations, and the above-described embodiments or variations can be combined, without departing from the spirit of the present invention.

The following experiment was conducted to confirm whether the above-mentioned 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 assumed that the experimental results will show the same tendency regardless of the scale of the experiment.

[Experiment details]
The experimental apparatus is shown in FIG. 6. The experimental apparatus includes a methane gas supply source 22 (gas cylinder), a flow rate controller 27, a reaction vessel 80, a light source 4, and a recovery 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 plug 81, and has a maximum capacity of about 50 ^cm3 . The liquid layer is composed of a first liquid layer S11 of 15 ^cm3 of ozone water ( _O3 amount: 1.5 ppm) and a second liquid layer S12 of 15 ^cm3 of perfluorohexane (hereinafter referred to as "PFH"). PFH is a non-polar solvent that easily dissolves methane gas. In the above-mentioned embodiment, PFH may be introduced as the second liquid layer, as in the experimental apparatus.

A glass tube for supplying methane gas is inserted into the reaction vessel 80. The liquid layers (S11, S12) are bubbled while adjusting the amount of methane gas introduced from the supply source 22 with 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 uses two 250W U-shaped constant pressure mercury lamps (dominant wavelength showing maximum intensity: 253.7 nm, irradiation energy: 61 J/cm ² ). In this experimental form, a stirrer is not used because the liquid layers (S11, S12) are stirred by the bubbling of methane gas. The chemical reaction caused by the irradiation of light 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.

A tube 87 for collecting samples is inserted into the reaction vessel 80. The tube 87 is connected to a collection vessel 85. The collection vessel 85 has stored pure water 86 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 generated and evaporated in the reaction vessel 80.

In the above-mentioned experimental apparatus, methane gas was introduced at 500 ^cm3 /min for 60 minutes, while the light source 4 was turned on during that time. 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 amount of formic acid was measured by electrophoresis. The amount of methanol was measured by GCMS (gas chromatograph mass spectrometer).

[Experimental Results]
The measurement results of formic acid and methanol are shown as the content ratio by mass per 1 L (1000 cm ³ ) in Table 1. Furthermore, Table 1 shows the total amount in the container calculated based on the content ratio and the amount of liquid in the container.

[Discussion of experimental results]
The mass of _O3 contained in 15 ^cm3 of ozone water containing 1.5 ppm _O3 is 22.5 μg. The total amount of formic acid and methanol produced, which are the products, calculated from Table 1, is 29.2 μg. From this, it was found that the reaction efficiency was about 130% (≒29.2/22.5×100). This reaction efficiency value is considered to be a high value from the viewpoint of energy balance with carbon neutrality in mind, and also from the economic viewpoint. This experiment was carried out in an environment where the reaction vessel 80 was at approximately atmospheric pressure and approximately room temperature.

The reason why the product could be obtained with high reaction efficiency even when the reaction vessel 80 was in the range of room temperature and normal pressure is believed to be due to the use of light including a wavelength of 200 nm or more and 411 nm or less. When the atomic oxygen generated from ozone is O( ^3P ), a temperature of 300°C or more is required to generate the intermediate P1. However, by using light including a wavelength of 200 nm or more and 411 nm or less, a large amount of O( ^1D ) was generated from ozone, and it is believed that the methane gas conversion process was promoted even at room temperature and normal pressure.

3: Light guide section 4: Light source 5: Stirrer 6: (Stirrer) power source 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: Splashes 27: Flow rate control meter 31, 32: Heat transfer section 61: (Reaction vessel) bottom surface 81: (Reaction vessel) plug 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 layers Pa, Pb: Pump

Claims (10)

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. The methane gas conversion method according to claim 1, characterized in that the ozone-containing fluid is generated by passing ozone gas through an ozone-permeable membrane immersed in a polar solvent or a non-polar solvent. The methane gas conversion method according to claim 1 or 2, characterized in that the mixed fluid is heated while the light is irradiated. The method for converting methane gas according to claim 1 or 2, characterized in that the mixed fluid is brought into contact with a catalyst. The methane gas conversion method according to claim 1 or 2, characterized in that the ozone-containing fluid is ozone water. The methane gas conversion method according to 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. The 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. A methane gas conversion device for converting methane gas into organic matter,
a reaction vessel including a first supply port for supplying a fluid containing ozone and a second supply port for supplying the fluid containing methane gas;
A methane gas conversion apparatus comprising: a light source that irradiates the inside of the reaction vessel with light having a wavelength of 200 nm or more and 411 nm or less. The methane gas conversion device according to claim 8, characterized in that a stirrer is provided in the reaction vessel. 10. The methane gas conversion apparatus according to claim 9, wherein the bottom of the reaction vessel is inclined with respect to a horizontal plane so that the stirrer is disposed across the liquid layer and the gas layer.