Method for producing catalyst for direct cracking of hydrocarbons by regenerating catalytic function

Abstract

Converting carbon into methane increases carbon utilization options. [Solution] A method for converting carbon to methane includes the steps of preparing an aggregate of a plurality of particles containing 4.3 mass% to 54.3 mass% iron and 45.7 mass% to 95.7 mass% carbon, the plurality of particles including cementite, and contacting the aggregate with hydrogen.

Description

[Technical Field]

[0001] The present disclosure provides a catalyst for the direct cracking of hydrocarbons. By regenerating catalytic function It relates to a manufacturing method. [Background technology]

[0002] Currently, the production of various types of energy is heavily dependent on fossil fuels such as oil, coal, and natural gas, but from the perspective of global environmental conservation, the increase in carbon dioxide emissions released by the combustion of fossil fuels is seen as a problem. The Paris Agreement, agreed in 2015, calls for a reduction in carbon dioxide emissions in order to address the issue of climate change, and reducing carbon dioxide emissions from the combustion of fossil fuels is a key issue for thermal power plants and other facilities. While processes for separating and capturing emitted carbon dioxide are being actively studied, technologies for producing energy without emitting carbon dioxide by using alternative fuels to fossil fuels are also being considered.

[0003] Therefore, hydrogen, a clean fuel that does not emit carbon dioxide when burned, has been attracting attention as an alternative to fossil fuels. Hydrogen can be produced, for example, by steam reforming methane contained in natural gas. However, this production method produces carbon monoxide as a by-product, which is ultimately oxidized and emitted as carbon dioxide. Meanwhile, methods such as water electrolysis and photocatalysis have been investigated as methods for producing hydrogen from water without using fossil fuels, but these methods require a large amount of energy and are therefore economically problematic.

[0004] In response to this, methods for producing hydrogen and carbon by directly decomposing hydrocarbons (hereinafter referred to as "direct hydrocarbon decomposition methods") have been developed. For example, the applicant of the present disclosure has developed a direct hydrocarbon decomposition method using an unsupported catalyst that is an aggregate of multiple iron particles, as described in Patent Document 1. The direct hydrocarbon decomposition method is characterized by its potential for obtaining hydrogen fuel without emitting carbon dioxide, and its ability to recover carbon in the hydrocarbon feed gas as solid by-product carbon after hydrogen purification, which means that from the perspective of decarbonization, it has the potential for superior handling and lower costs compared to recovering and storing gaseous carbon dioxide. There is a demand for even more effective use of this by-product carbon. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 7089235 Summary of the Invention [Problem to be solved by the invention]

[0006] When a direct hydrocarbon cracking method is performed, the carbon produced adheres to the catalyst, resulting in a graphite-coated catalyst with reduced activity. If carbon could be efficiently separated from this graphite-coated catalyst, it would be possible to more effectively utilize unsupported catalysts and carbon. In this regard, the inventors of the present disclosure have demonstrated that the graphite-coated catalyst after performing the direct hydrocarbon cracking method using the catalyst of Patent Document 1 has methanation activity.

[0007] In general, the synthesis of methane from hydrogen and carbon dioxide is often called methanation, but in this disclosure, the reaction of converting carbon into methane by reacting carbon with hydrogen, as shown in the following reaction formula (1), is defined as "methanation." C + 2H2 → CH4 (1) If the carbon on the graphite-coated catalyst is converted to methane by methanation, the carbon can be effectively utilized as methane, and if the carbon is removed from the graphite-coated catalyst, the unsupported catalyst obtained by carbon removal can be reused in a method for direct cracking of hydrocarbons.

[0008] In view of the above, at least one embodiment of the present disclosure provides a catalyst for direct cracking of hydrocarbons using a method for converting carbon to methane. By regenerating catalytic function The object is to provide a method for manufacturing the same. [Means for solving the problem]

[0009] In order to achieve the above object, the catalyst for direct cracking of hydrocarbons according to the present disclosure is By regenerating catalytic function The manufacturing method includes the steps of preparing an aggregate of a plurality of particles, the aggregate including 6.8 mass % to 54.3 mass % iron and 45.7 mass % to 93.2 mass % carbon, the aggregate including iron fine particles of submicron order with carbon attached to the iron fine particles, and cementite, the aggregate having a ratio I1 / (I1+I2) of 0.17 or more and 1.0 or less, when the quantitative values ​​of the peaks attributable to cementite and alpha iron obtained by X-ray diffraction analysis are determined using an RIR method, and the quantitative values ​​of the peaks attributable to cementite and alpha iron are I1 and I2, respectively; removing carbon from the aggregates by contacting the aggregates with hydrogen to convert the carbon to methane; and the set Remove carbon from The step is carried out in the temperature range of 500°C to 680°C. [Effects of the Invention]

[0011] of the present disclosure Manufacture of catalysts for direct cracking of hydrocarbons According to this method, carbon contained in an aggregate of a plurality of particles containing iron and carbon is converted into methane. Carbon can be removed. When hydrocarbons are directly decomposed into hydrogen and carbon using an unsupported catalyst containing multiple iron particles, the carbon produced coats the unsupported catalyst, reducing its activity. but, Carbon contained in unsupported catalysts with reduced activity Except The unsupported catalyst obtained by the removal can be reused in the direct cracking of hydrocarbons. do It is possible. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram illustrating the configuration of a hydrogen generation device for implementing the carbon-to-methane conversion method of the present disclosure. [Figure 2] FIG. 1 is a schematic diagram illustrating the configuration of an experimental device for verifying the effectiveness of the method for converting carbon into methane disclosed herein. [Figure 3] 1 is a graph showing the experimental results of Examples 1 to 3. [Figure 4] 1 shows X-ray diffraction patterns of methanation catalysts before the experiments in Examples 1 and 2 and Comparative Example 1. [Figure 5] 1 is a graph showing the relationship between temperature and methanation activity in Example 2. [Figure 6] This is a scanning electron microscope photograph of the surface of a particle aggregate after methanation at a pressure of 1 ata and a temperature of 500°C. [Figure 7] This is a scanning electron microscope photograph of the surface of a particle aggregate after methanation at a pressure of 1 ata and a temperature of 600°C. [Figure 8] This is a scanning electron microscope photograph of the surface of a particle aggregate after methanation at a pressure of 1 ata and a temperature of 680°C. [Figure 9] This is a scanning electron microscope photograph of the surface of a particle aggregate after methanation at a pressure of 1 ata and a temperature of 700°C. [Figure 10] This is a scanning electron microscope photograph of the surface of a particle aggregate after methanation at a pressure of 1 ata and a temperature of 800°C. [Figure 11]This is a scanning electron microscope photograph of the surface of a particle aggregate after methanation at a pressure of 1 ata and a temperature of 900°C. [Figure 12] 1 is a graph showing the relationship between the specific surface area of ​​particle aggregates after methanation versus the methanation temperature. [Figure 13] 1 is a graph showing the experimental results of Examples 2 and 4. [Figure 14] 1 is a graph showing the change over time in the cumulative amount of hydrogen produced by direct decomposition of methane using a particle aggregate after methanation at a pressure of 1 ata and a temperature of 500°C. [Figure 15] 1 is a graph showing the change over time in the cumulative amount of hydrogen produced by direct decomposition of methane using a particle aggregate after methanation at a pressure of 1 ata and temperatures of 600°C and 680°C. [Figure 16] 1 is a graph showing the change over time in the cumulative amount of hydrogen produced by direct decomposition of methane using a particle aggregate after methanation at a pressure of 1 ata and temperatures of 800°C and 900°C. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, a method for converting carbon to methane according to an embodiment of the present disclosure will be described with reference to the drawings. The embodiment described below shows one aspect of the present disclosure, but does not limit the present disclosure and can be modified as desired within the scope of the technical concept of the present disclosure.

[0014] <Configuration of an apparatus for carrying out a method for converting carbon into methane> As shown in FIG. 1, a methanation apparatus 10 for carrying out a method for converting carbon to methane includes a reactor 11 in which methanation occurs. The reactor 11 contains an aggregate 12 of a plurality of particles containing iron and carbon. The plurality of particles constituting the aggregate 12 contain 4.3 mass % to 54.3 mass % iron and 45.7 mass % to 95.7 mass % carbon, with a portion of the iron and carbon constituting cementite. That is, the plurality of particles contain cementite. Regarding the amount of cementite contained in the plurality of particles, the quantitative values ​​of the peaks attributable to cementite and alpha-iron (αFe) obtained by X-ray diffraction analysis of the aggregate are determined using the RIR (reference intensity ratio) method. When the quantitative values ​​of the peaks attributable to cementite and αFe are I1 and I2, respectively, the ratio I1 / (I1 + I2) is preferably 0.17 or more and 1.0 or less.

[0015] In order to supply hydrogen for methanation into the reactor 11, a hydrogen-containing gas line 13 through which a gas containing at least hydrogen, i.e., a hydrogen-containing gas, flows is connected to the reactor 11. The hydrogen-containing gas is not limited to a gas containing only hydrogen, and may contain methane or other hydrocarbons, rare gases, nitrogen, or the like in addition to hydrogen. In addition, a methane-containing gas line 14 is connected to the reactor 11 so that a methane-containing gas, which is a mixed gas of methane produced by methanation (described later) and unreacted hydrogen (including components other than hydrogen if present in the hydrogen-containing gas), flows out of the reactor 11. A compressor 15 may be provided in the methane-containing gas line 14.

[0016] An example of the assembly 12 may be, but is not limited to, an unsupported catalyst (graphite-coated catalyst) coated with carbon produced by directly decomposing hydrocarbons into hydrogen and carbon. A reactor 2 for directly decomposing hydrocarbons into hydrogen and carbon contains an unsupported catalyst 3 containing a plurality of iron or iron oxide particles. The reactor 2 is equipped with a heating device 4 (e.g., a jacket through which steam flows, an electric furnace, etc.) for heating the interior of the reactor 2, particularly the unsupported catalyst 3. The reactor 2 is connected to a feed gas line 5 through which a hydrocarbon-containing gas (feed gas) flows. Examples of feed gases that can be used include natural gas, compressed natural gas (CNG), city gas, liquefied petroleum gas, and naphtha. A product gas distribution line 6 is connected to the reactor 2 at a position opposite the position where the feed gas line 5 connects to the reactor 2 relative to the unsupported catalyst 3.

[0017] The reactor 2 and the reactor 11 may be connected by a catalyst transfer line 7 so that the unsupported catalyst 3 (i.e., graphite-coated catalyst) obtained after the direct decomposition of hydrocarbons into hydrogen and carbon in the reactor 2 can be transferred to the methanation device 10. In addition, a methane-containing gas line 14 may be connected to the raw gas line 5 so that the methane-containing gas can be used as part of the raw gas supplied to the reactor 2.

[0018] As described above, since the methane-containing gas typically contains hydrogen that was not consumed during methanation, a portion of the methane-containing gas may be used as part of the hydrogen-containing gas. That is, a recycle line connecting the methane-containing gas line 14 and the hydrogen-containing gas line 13 may be provided. Furthermore, as will be described later, since the product gas flowing through the product gas distribution line 6 also contains hydrogen, a portion of the product gas may be used as part of the hydrogen-containing gas. That is, a recycle line connecting the product gas distribution line 6 and the hydrogen-containing gas line 13 may be provided. Furthermore, considering the concept of a direct decomposition method for hydrocarbons without carbon dioxide emissions, it is preferable to use hydrogen produced using renewable energy, for example, hydrogen produced by electrolysis of water using electricity generated using solar energy, as at least part of the hydrogen-containing gas.

[0019] The methanation device 10 may be installed in a factory separate from the factory in which the reactor 2 is installed. In this case, the catalyst transfer line 7 may be connected to a container or the like for transporting the graphite-coated catalyst to the reactor 11, instead of the reactor 11, and the methane-containing gas line 14 may be connected to a tank for storing the methane-containing gas or a consumption facility for consuming the methane-containing gas, instead of the raw material gas line 5.

[0020] <Method of converting carbon into methane> Next, the operation of the methanation device 10 (a method for converting carbon into methane) will be described. After the assemblies 12 are placed in the reactor 11, a hydrogen-containing gas flows into the reactor 11 via the hydrogen-containing gas line 13. The hydrogen-containing gas that has flowed into the reactor 11 passes through the assemblies 12 while coming into contact with them. At this time, the hydrogen in the hydrogen-containing gas reacts with the carbon contained in the assemblies 12 due to the catalytic action of the assemblies 12, and is converted into methane. The existence of such a methanation catalytic action in the assemblies 12 will be demonstrated in the examples described later. In this way, the carbon contained in the assemblies of a plurality of particles containing iron and carbon can be converted into methane and used, thereby increasing the options for using carbon.

[0021] The methane-containing gas containing methane produced by methanation flows out of the reactor 11 and flows through a methane-containing gas line 14, and is transported to a storage facility or a consumption facility. In this way, the carbon contained in the aggregate of a plurality of particles containing iron and carbon can be converted into methane and used, thereby increasing the options for using carbon.

[0022] When the methane-containing gas line 14 is connected to the feed gas line 5, as in the configuration shown in FIG. 1, a methane-containing gas can be used as part of the feed gas supplied to the reactor 2. Furthermore, when the reactor 2 and the reactor 11 are connected via a catalyst transfer line 7, as in the configuration shown in FIG. 1, the graphite-coated catalyst can be transferred as an assembly 12 from the reactor 2 to the reactor 11. After transferring the graphite-coated catalyst to the reactor 11, a new unsupported catalyst 3 can be loaded into the reactor 2, or the catalyst after methanation in the reactor 11 (i.e., the regenerated catalyst) can be loaded into the reactor 2, and the direct cracking of hydrocarbons in the reactor 2 can be performed. The graphite-coated catalyst is an unsupported catalyst coated with carbon produced by the direct cracking of hydrocarbons, and therefore has reduced activity for the direct cracking of hydrocarbons. Converting the carbon contained in the graphite-coated catalyst into methane and reusing the resulting methane in the direct cracking of hydrocarbons can increase the carbon utilization options, while reusing the unsupported catalyst obtained by removing the carbon in the direct cracking of hydrocarbons can reduce the amount of unsupported catalyst discarded.

[0023] In the examples described below, it is verified that the catalyst after methanation can be used as a catalyst for the direct decomposition of hydrocarbons. Therefore, although the operation of regenerating the graphite-coated catalyst to obtain a catalyst for the direct decomposition of hydrocarbons is an operation for regenerating the graphite-coated catalyst, it can also be said to be an operation for producing a catalyst for the direct decomposition of hydrocarbons (unsupported catalyst 3). The specific surface area of ​​the iron particles contained in the catalyst produced by such an operation (catalyst production method), i.e., the plurality of particles constituting the aggregate 12, is 0.52 m 2The specific surface area is preferably 1 / g or more. A detailed method for calculating this specific surface area will be explained in detail in the examples below.

[0024] If the pressure is 0.098 MPa (1 ata) or more, it is preferable to bring hydrogen into contact with the assemblies 12 in a temperature range of 450° C. to 680° C., and it is more preferable to bring hydrogen into contact with the assemblies 12 in a temperature range of 500° C. to 680° C. The preferable temperature range for methanation will be verified in the examples described later.

[0025] <Direct decomposition of hydrocarbons> Next, the operation of the hydrogen generator 1 will be described. The raw material gas flows into the reactor 2 via the raw material gas line 5. The raw material gas that has flowed into the reactor 2 passes through the unsupported catalyst 3 while coming into contact with the catalyst. During this process, hydrocarbons in the raw material gas are directly decomposed into hydrogen and carbon. Taking methane as an example of the hydrocarbon in this direct decomposition reaction, the reaction represented by the following reaction formula (2) occurs in the reactor 2. The mechanism by which the direct decomposition reaction of hydrocarbons occurs using the unsupported catalyst 3 is explained in detail in Patent Document 1 by the applicant of the present disclosure. CH4 → 2H2 + C (2)

[0026] Carbon produced by the direct cracking of hydrocarbons adheres to the unsupported catalyst 3, and the produced hydrogen flows out of the reactor 2 as a product gas together with unreacted hydrocarbons and circulates through the product gas distribution line 6. Carbon can be recovered by stopping the supply of raw material gas to the reactor 2, recovering the graphite-coated catalyst from the reactor 2, and, if necessary, removing the carbon adhered to the graphite-coated catalyst using a carbon remover. Hydrogen is recovered by recovering the product gas circulating through the product gas distribution line 6. If the hydrogen concentration in the product gas is low, a hydrogen generator may be provided to purify the product gas to increase the hydrogen concentration.

[0027] The carbon produced by the direct decomposition of hydrocarbons can be recovered not only by the above-mentioned methods, but also by transferring at least a portion of the graphite-coated catalyst in reactor 2 to reactor 11 via catalyst transfer line 7 and converting it to methane by methanation in methanation device 10. Alternatively, the graphite-coated catalyst may be temporarily stored in a storage container connected to catalyst transfer line 7, the container may be transferred to methanation device 10, and the graphite-coated catalyst in the container may be loaded into reactor 11 of methanation device 10 to perform methanation. [Example]

[0028] <Experimental apparatus for performing the disclosed carbon-to-methane conversion method> The configuration of an experimental apparatus for verifying the effectiveness of the disclosed method for converting carbon to methane is shown in Figure 2. The experimental apparatus 20 includes a quartz glass reactor 23 with an inner diameter of 22 mm and a length of 700 mm, which houses a methanation catalyst 22. The reactor 23 is heatable by an electric furnace 24. The reactor 23 is connected to a hydrogen supply line 25 for supplying hydrogen and a reaction gas distribution line 26 through which the reaction gas containing methane produced by methanation flows after leaving the reactor 23. The reaction gas distribution line 26 is connected to a gas chromatograph 27 for measuring the composition of the reaction gas.

[0029] <Preparation of methanation catalyst of Example 1> A reduced iron particle aggregate (K-100T, particle size 100 μm to 180 μm) obtained from JFE Steel Corporation was weighed out to 3.5 g (bulk specific gravity 3, volume 1.2 cc) and packed into a quartz glass reactor with an inner diameter of 22 mm and a length of 700 mm. The reduced iron particle aggregate was kept at 800°C and a gas containing 99 vol% or more of methane was pumped for 10,000 h. -1The reduced iron particle aggregate was contacted with the reduced iron particle aggregate at a high space velocity (GHSV) of 1000 kJ / s (methane gas flow rate of 12 L / h) for 14 hours. This contact directly decomposed part of the methane into hydrogen and carbon, resulting in an aggregate of reduced iron particles with carbon attached. The aggregate was purged with nitrogen and then cooled to room temperature. The composition of the aggregate was calculated from the mass of the aggregate before the experiment (containing only reduced iron) and the mass of the aggregate after the experiment (containing reduced iron and carbon). It was confirmed that the aggregate contained 54.3 mass% iron and 45.7 mass% carbon. The aggregate obtained in this manner was designated methanation catalyst 22 of Example 1.

[0030] <Preparation of methanation catalysts of Examples 2 and 3> The methanation catalyst 22 of Example 2 was prepared under the same conditions as the preparation method of the methanation catalyst 22 of Example 1, except that the contact time between the methane-containing gas and the aggregates of reduced iron particles was changed to 90 hours. The methanation catalyst 22 of Example 3 was prepared under the same conditions as the preparation method of the methanation catalyst 22 of Example 1, except that the contact time between the methane-containing gas and the aggregates of reduced iron particles was changed to 329 hours. The composition of the methanation catalyst 22 of Example 2 was confirmed to contain 6.8 mass% iron and 93.2 mass% carbon. The composition of the methanation catalyst 22 of Example 3 was confirmed to contain 4.3 mass% iron and 95.7 mass% carbon.

[0031] <Preparation of methanation catalyst of Comparative Example 1> A mixture of graphite (MG-1) obtained from Nippon Graphite Industries Co., Ltd. and the reduced iron used in Examples 1 to 3 was used as methanation catalyst 22 of Comparative Example 1.

[0032] <Experimental conditions for Examples 1 to 3 and Comparative Example 1> The methanation catalysts 22 thus obtained for Examples 1 to 3 and Comparative Example 1 were placed in the reactor 23 of the experimental apparatus 20, and methanation was carried out by adjusting the temperature and pressure in the reactor 23 to 600°C and 1 ata, respectively, and supplying hydrogen into the reactor 23 at a rate of 83 Ncc / min. The compositions of the catalysts for Examples 1 to 3 and Comparative Example 1 are summarized in Table 1 below.

[0033] [Table 1]

[0034] <Experimental Results of Examples 1 to 3 and Comparative Example 1> The experimental results of Examples 1 to 3 are shown in Figure 3. Figure 3 shows the change over time in the conversion rate of carbon to methane in methanation using the methanation catalyst 22 of each of Examples 1 to 3. The conversion rate of carbon to methane is defined by the following formula (3). Note that the "amount of methane in the reaction gas" in the following formula (3) can be calculated from the measurement results of gas chromatography 27. Conversion rate = (amount of methane in the reaction gas) × 2 / hydrogen supply amount × 100 (3)

[0035] As can be seen from FIG. 3 , in Examples 1 and 2, the conversion rate increased immediately after the start of hydrogen supply, indicating that methanation occurred immediately. Approximately 4 hours after the start of hydrogen supply, the conversion rate reached a maximum value of 22.1% in Example 1 and 30.4% in Example 2. The conversion rate then began to decrease, reaching nearly 0% approximately 15 hours after the start of hydrogen supply in Example 1 and nearly 0% approximately 18 hours after the start of hydrogen supply in Example 2. In Example 3, the conversion rate remained at 0% and no methanation occurred until approximately 12 hours after the start of hydrogen supply. However, the conversion rate then increased, indicating that methanation began approximately 12 hours after the start of hydrogen supply. In Example 3, the conversion rate reached a maximum value of 18.5% approximately 22 hours after the start of hydrogen supply, then began to decrease, reaching nearly 0% approximately 45 hours after the start of hydrogen supply.

[0036] Observation of the appearance of the methanation catalyst 22 of Example 1 before and after the experiment revealed that, in the methanation catalyst 22 after the experiment, almost all of the carbon attached to the reduced iron particles had been removed by methanation, and only the iron (reduced iron) remained. This suggests that almost all of the carbon attached to the reduced iron particles had been converted to methane.

[0037] Observation of the appearance of the methanation catalyst 22 of Example 2 before and after the experiment revealed that although some carbon remained on the reduced iron particles in the methanation catalyst 22 after the experiment, most of the carbon had been removed from the reduced iron particles. This suggests that most of the carbon attached to the reduced iron particles had been converted to methane.

[0038] Although not shown in Figure 3, when methanation catalyst 22 of Comparative Example 1 was used, the conversion rate remained at 0%. In other words, almost no methanation occurred even when methanation catalyst 22 of Comparative Example 1 was used. Although methanation catalyst 22 of Comparative Example 1 and methanation catalyst 22 of Example 2 have almost the same iron concentration, almost no methanation occurred with the former. This suggests that the bonding and contact state between iron and carbon in the catalyst may be an important factor in methanation activity.

[0039] A comparison of the experimental results of Examples 1 to 3 with those of Comparative Example 1 confirmed that when hydrogen is brought into contact with an aggregate of multiple particles obtained by contacting a hydrocarbon-containing gas with an unsupported catalyst that is an aggregate of multiple iron particles, the aggregate containing 4.3 mass% to 54.3 mass% iron and 45.7 mass% to 95.7 mass% carbon, methanation occurs and carbon can be converted to methane. From the viewpoint of rapidly generating methanation activity, it is preferable to bring hydrogen into contact with an aggregate of multiple particles containing 6.8 mass% to 54.3 mass% iron and 45.7 mass% to 93.2 mass% carbon.

[0040] Next, we present the results of examining the physical properties of the methanation catalysts 22 of Example 2 and Comparative Example 1, which showed completely different results in terms of methanation activity despite having nearly the same iron concentration. X-ray diffraction analysis was performed on both catalysts before the experiment. X-ray diffraction analysis was performed using a benchtop X-ray diffractometer (MiniFlex 600) manufactured by Rigaku Corporation in accordance with JIS K 0131. Figure 4 shows the X-ray diffraction patterns of the methanation catalysts 22 of Examples 1 and 2 and Comparative Example 1 before the experiment. The X-ray diffraction patterns of the methanation catalysts 22 of Examples 1 and 2 before the experiment confirmed that they contained αFe and cementite. In contrast, the X-ray diffraction pattern of the methanation catalyst 22 of Comparative Example 1 before the experiment confirmed that it mainly consisted of αFe and graphite.

[0041] In the X-ray diffraction pattern of FIG. 4 , quantitative values ​​I1 and I2 of the peaks attributed to cementite and αFe were determined using the RIR (reference intensity ratio) method. The cementite and αFe peaks in Example 1 were determined at 2θ = 47.6° and 2θ = 77.2°, respectively, and the cementite peak in Example 2 was determined at 2θ = 53.8°. No αFe peak was detected in Example 2. When the cementite content in the particles of methanation catalyst 22 was evaluated using the ratio I1 / (I1 + I2), this ratio was 0 in Comparative Example 1, whereas this ratio was 0.17 in Example 1 and 1.0 in Example 2. While methanation activity was observed in the methanation catalysts 22 of Examples 1 and 2, methanation activity was not observed in the methanation catalyst 22 of Comparative Example 1. This is thought to be because the former contained cementite while the latter did not. Therefore, in order to exhibit methanation activity, the methanation catalyst 22 must contain cementite, and it was found that the cementite content ratio, the above-mentioned ratio I1 / (I1+I2), is preferably 0.17 or more and 1.0 or less.

[0042] The reason why methanation catalysts 22 of Examples 1 to 3 have higher performance than the methanation catalyst of Comparative Example 1 in terms of methanation activity is that carbon generated by the direct decomposition of hydrocarbons adheres to the iron surface of methanation catalysts 22 of Examples 1 to 3 and then grows, the main active metal (iron) is in solid-state contact with a large amount of carbon in a fine and highly dispersed state, and further, some of the iron becomes iron carbide (forming cementite), and hydrogen easily reacts with the carbon in the iron and the carbon adhered to the iron, which is thought to promote reaction (1). Furthermore, from a microscopic perspective, methanation is thought to be promoted by the sequential occurrence of reactions (5) and (6) below, which repeat the oxidation-reduction behavior between carbide and iron. Fe3C+2H2→CH4+3Fe (5) 3Fe+C→Fe3C (6)

[0043] <Verification of the relationship between temperature and pressure and methanation activity> Next, using the methanation catalyst 22 of Example 2, methanation was performed under the conditions of a pressure in the reactor 23 of 1 ata and temperatures in the reactor 23 of 400°C, 450°C, 500°C, 550°C, 600°C, 680°C, 700°C, 800°C, and 900°C, and it was examined how the methanation activity changed. In addition, methanation was performed under the conditions of a pressure in the reactor 23 of 0.59 MPa (6 ata) and temperatures in the reactor 23 of 400°C, 450°C, 500°C, 550°C, 600°C, 680°C, 700°C, and 900°C, and it was also examined how the methanation activity changed.

[0044] During methanation, the reaction gas composition was analyzed by gas chromatography. The methane conversion rate at each temperature setting was calculated from the composition analysis results. The maximum conversion rate was determined from the time-dependent change in conversion rate under each condition. Figure 5 shows the relationship between methanation temperature and maximum conversion rate at both 1 ata and 6 ata pressures. As can be seen from Figure 5, methanation activity was observed at temperatures above 450°C for both 1 ata and 6 ata pressures. At 1 ata, methanation activity increased with increasing temperature up to 550°C, and at 6 ata, it increased up to 600°C. At 1 ata, methanation activity decreased with increasing temperature from 550°C to 900°C, and at 6 ata, it decreased from 600°C to 900°C. At 900°C, a conversion rate of approximately 3% was observed at 1 ata pressure and approximately 17% at 6 ata pressure. This confirmed that methanation activity is achieved in the temperature range of 450°C to 900°C. In Figure 5, the equilibrium conversion rate to methane at 1 ata is shown by a dashed line, and the equilibrium conversion rate to methane at 6 ata is shown by a dotted line. It was found that at a pressure of 1 ata, the conversion rate to methane was nearly equilibrium in the temperature range of 700°C to 900°C, and at a pressure of 6 ata, the conversion rate to methane was nearly equilibrium in the temperature range of 800°C to 900°C.

[0045] As described above, methanation activity was confirmed in the pressure range from 1 ata to 6 ata and the temperature range from 450°C to 900°C. Next, we consider the preferred temperature range for methanation. If the preferred range for methanation activity is one in which the maximum conversion rate is approximately 10% or higher, then, based on Figure 5, a temperature range from approximately 470°C to approximately 770°C at 1 ata pressure is preferred. Furthermore, a temperature range from 500°C to 900°C at 6 ata pressure is preferred. Note that in Figure 5, the regression curve for the 6 ata condition is located above the regression curve for the 1 ata condition in the temperature range above approximately 510°C. Therefore, it is believed that the curve representing the relationship between methanation temperature and conversion rate shifts upward as the pressure increases. Therefore, at pressures above 1 ata, the preferred temperature range for methanation is at least the preferred temperature range at 6 ata, i.e., 500°C to 680°C.

[0046] As explained in Patent Document 1, a prior application of the applicant of the present disclosure, when hydrocarbons such as methane come into contact with iron particles, the hydrocarbons are directly decomposed into hydrogen and carbon, and the iron constituting the iron particles is decomposed into submicron-order fine particles. It has been revealed that when hydrogen is brought into contact with iron particles in this state, with carbon attached to the iron, methanation occurs as described above, and the carbon attached to the iron is removed from the iron as methane, and iron particles with a fine mesh-like structure are obtained. Therefore, next, we consider a more preferable temperature range for methanation based on the structure of such iron particles obtained after methanation.

[0047] 6 to 11 show photographs taken with a scanning electron microscope (SEM) at 3000x magnification of the surface of particle aggregates after methanation (removal of adhering carbon by direct decomposition of methane) using the methanation catalyst 22 of Example 2 at a pressure of 1 ata and temperatures of 500°C, 600°C, 680°C, 700°C, 800°C, and 900°C. These photographs confirm that methanation resulted in particle aggregates with a fine mesh-like structure. From these photographs, the specific surface area of ​​the iron particles contained in the particles after methanation was calculated using the method described below.

[0048] First, the diameter of each iron mesh structure is measured from a photograph of the surface of the particle aggregate after methanation, and the average value d [μm] of these is calculated. Next, the cross section of each mesh structure is considered to be circular, and the area CSA [μm] of the circular cross section with the average value d as the diameter is calculated. 2 ] is calculated using the following formula (7). CSA = π × (d 2 / 4) ···(7) Next, we assume that the network structure is a single cylindrical structure, and calculate the particle mass W [g] and the iron density ρ [g / cm 3 ] and the cross-sectional area CSA, the length L [m] of one cylindrical structure is calculated by the following formula (8). L=W / (CSA·ρ)×10 -6 ···(8) Next, the surface area SA [m 2 ] is calculated using the following formula (9). SA = π × d × L × 10 -6 ···(9) Finally, the specific surface area SSA [m2 / g] is calculated from the surface area SA and the mass W of the particles using the following formula (10). SSA=SA / W (10)

[0049] Figure 12 shows a graph plotting the calculated specific surface area (SSA) versus methanation temperature. In Figure 12, the slope of the line obtained by linear regression of the three plots (500°C, 600°C, and 680°C) is almost the same as the slope of the line obtained by linear regression of the three plots (700°C, 800°C, and 900°C). However, the two lines appear to be discontinuous between 680°C and 700°C, and they do not form a single line. When both lines are extrapolated to a temperature of 400°C, the intercept of the vertical axis of the former line is larger than that of the latter line. Regarding the relationship between methanation temperature and specific surface area, the three data points from 500°C to 680°C regressed by the former line and the three data points from 700°C to 900°C regressed by the latter line indicate that the specific surface area estimated from the methanation temperature is smaller for the latter line than for the former line. This is thought to be because, as the methanation temperature increases, the carbon is removed and some of the exposed iron particles sinter. In Figures 6 to 8, no sintering of the iron particles is observed, and all have a fine mesh-like structure. However, in Figures 9 to 11, the iron particles are relatively larger than in Figures 6 to 8, and the surface of the mesh-like structure is smooth, which confirms that sintering has occurred.

[0050] Therefore, from the viewpoint of the specific surface area of ​​the particle aggregate after methanation, the upper limit of the preferred temperature range for the methanation temperature is less than 700°C, preferably 680°C or less. The lower limit of the methanation temperature is 450°C, at which methanation activity is expressed, and 500°C, at which the maximum conversion rate to methane is approximately 10%, is the preferred lower limit. Therefore, the methanation temperature range is 450°C to 680°C, preferably 500°C to 680°C.

[0051] <Preparation of methanation catalyst of Example 4> Methanation catalyst 22 of Example 4 was prepared in the same manner as in Example 1, except that 1.47 g of Australian hematite (JSS806-1, particle size 100 μm to 180 μm) was sampled, the temperature of the hematite was maintained at 800°C, and a gas containing 99 vol% or more of methane was contacted with the hematite at a flow rate of 83 cc / min for 25 hours. The iron concentration of methanation catalyst 22 of Example 4 was 9.8 mass% (iron mass: 0.37 g, total mass: 3.78 g).

[0052] <Experimental conditions for Example 4> The methanation catalyst 22 of Example 4 thus obtained was placed in the reactor 23 of the experimental apparatus 20. Hydrogen was supplied to the reactor 23 at a rate of 83 Ncc / min under the following conditions: a pressure of 1 ata within the reactor 23; and temperatures of 400°C, 500°C, 550°C, 600°C, 680°C, 700°C, and 900°C. During methanation, the reaction gas was analyzed using gas chromatography 27. The conversion rate to methane at each temperature was calculated from the composition analysis results. The maximum conversion rate was determined from the change in conversion rate over time under each condition. The relationship between methanation temperature and maximum conversion rate is shown in Figure 13. Figure 13 also shows the relationship between methanation temperature and maximum conversion rate obtained by methanation using the catalyst 22 of Example 2.

[0053] 13, in the temperature range of 600°C or less, the methanation activity performed using the methanation catalyst 22 of Example 4 was slightly lower than the methanation activity performed using the methanation catalyst 22 of Example 2. However, it was confirmed that methanation activity was also exhibited in the methanation performed using the methanation catalyst 22 of Example 4 in the temperature range of from above 400°C to 900°C. Therefore, it became clear that when preparing an aggregate of a plurality of particles containing iron and carbon, it is possible to employ not only a configuration in which a hydrocarbon-containing gas is brought into contact with an unsupported catalyst containing a plurality of particles made of iron (e.g., reduced iron), but also a configuration in which a hydrocarbon-containing gas is brought into contact with an unsupported catalyst containing a plurality of particles of iron oxide (e.g., iron ore). Even when a configuration is adopted in which a hydrocarbon-containing gas is brought into contact with an unsupported catalyst containing a plurality of iron oxide particles, there is no difference from a configuration in which a hydrocarbon-containing gas is brought into contact with an unsupported catalyst containing a plurality of iron particles in that the particles obtained after methanation are iron particles. Therefore, in the former case, the temperature range for methanation is 450°C to 680°C, preferably 500°C to 680°C.

[0054] <Use of catalyst after methanation in direct cracking of hydrocarbons> We investigated whether the iron particles obtained after methanation using the methanation catalyst 22 of Example 2 under conditions of a pressure of 1 ata and temperatures of 500°C, 600°C, 680°C, 800°C, and 900°C have catalytic activity for the direct decomposition of hydrocarbons. The experiment for direct decomposition of hydrocarbons used the same apparatus used to prepare the methanation catalyst 22 of Examples 1 to 3. 0.27 g of each iron particle was packed into the reactor of the apparatus, and the pressure in the reactor was maintained at 1 ata and the temperature at 800°C. Gas containing 99 vol% or more of methane was decomposed for 0.307 min. -1 ·g -1 A weight hourly space velocity (WHSV) of 1000 kJ / s (gas flow rate of 83 Ncc / min) was supplied to the reactor, and the gas was allowed to come into contact with each iron particle for 20 hours. The amount of hydrogen produced in the experiment under each condition was measured, and Figures 14 to 16 show the cumulative change over time in the amount of hydrogen produced in the experiment under each condition.

[0055] 14 to 16, hydrogen was generated in the experiments using each iron particle, confirming that each iron particle has activity for the direct decomposition of hydrocarbons, i.e., that each iron particle can be used as a catalyst for the direct decomposition of hydrocarbons. From this, it can be said that a catalyst for the direct decomposition of hydrocarbons can be produced by removing carbon from the above-mentioned aggregate of multiple particles containing iron and carbon using methanation (a method of converting carbon to methane as disclosed herein).

[0056] 14 and 15, in experiments using iron particles obtained after methanation using the methanation catalyst 22 of Example 2 at a pressure of 1 ata and temperatures of 500°C, 600°C, and 680°C (hereinafter referred to as "particles obtained under low-temperature conditions"), hydrogen was produced well immediately after the start of the experiments, i.e., the activity for direct decomposition of hydrocarbons was well exhibited. On the other hand, as can be seen from FIG. 16, in experiments using iron particles obtained after methanation using the methanation catalyst 22 of Example 2 at a pressure of 1 ata and temperatures of 800°C and 900°C (hereinafter referred to as "particles obtained under high-temperature conditions"), hydrogen was produced more slowly than in experiments using particles obtained under low-temperature conditions, i.e., the activity for direct decomposition of hydrocarbons was exhibited more slowly. From these results, it is preferable to use methanation to remove carbon from an aggregate of multiple particles containing iron and carbon in a temperature range of 450°C to 680°C, preferably in a temperature range of 500°C to 680°C, also as a method for producing a catalyst for direct decomposition of hydrocarbons.

[0057] According to Figure 12, the specific surface area of ​​iron particles obtained by methanation at a temperature of 680°C or less increases as the methanation temperature decreases. Therefore, the specific surface area of ​​iron particles contained in a catalyst for direct decomposition of hydrocarbons produced by removing carbon from an aggregate of multiple particles containing iron and carbon using methanation is 0.52 m, which is the specific surface area of ​​iron particles obtained by methanation at a temperature of 680°C. 2 / g or more is preferable.

[0058] The contents described in each of the above embodiments can be understood, for example, as follows.

[0059] [1] A method for converting carbon to methane according to one embodiment includes the steps of: preparing an aggregate (12) of a plurality of particles containing 4.3 mass % to 54.3 mass % iron and 45.7 mass % to 95.7 mass % carbon, the aggregate (12) being an aggregate of the plurality of particles containing cementite; contacting the assembly (12) with hydrogen; Includes:

[0060] According to the method for converting carbon to methane disclosed herein, the carbon contained in an aggregate of a plurality of particles containing iron and carbon can be converted into methane and utilized, thereby increasing the options for utilizing the carbon. Furthermore, when hydrocarbons are directly decomposed into hydrogen and carbon using an unsupported catalyst containing a plurality of iron particles, the carbon produced coats the unsupported catalyst, reducing the activity of the unsupported catalyst. By converting the carbon contained in the unsupported catalyst with reduced activity into methane and reusing the resulting methane in a method for directly decomposing hydrocarbons, the options for utilizing the carbon can be increased, and by reusing the unsupported catalyst obtained by removing the carbon in a method for directly decomposing hydrocarbons, the amount of unsupported catalyst that is discarded can be reduced.

[0061] [2] A method for converting carbon into methane according to another embodiment is the method for converting carbon into methane according to [1], The quantitative values ​​of the peaks attributable to cementite and the peaks attributable to alpha iron obtained by X-ray diffraction analysis of the aggregate (12) are determined using the RIR method. When the quantitative values ​​of the peaks attributable to cementite and alpha iron are I1 and I2, respectively, the ratio I1 / (I1+I2) is 0.17 or more and 1.0 or less.

[0062] According to this configuration, the carbon contained in the aggregate of the plurality of particles containing iron and carbon can be converted into methane and used, thereby increasing the options for using the carbon.

[0063] [3] A method for converting carbon into methane according to yet another embodiment is the method for converting carbon into methane according to [1] or [2], The step of preparing the assembly (12) includes: Providing an unsupported catalyst (3) in the form of a cluster of iron particles; contacting a hydrocarbon-containing gas with the unsupported catalyst (3); Includes:

[0064] When hydrocarbons are directly decomposed into hydrogen and carbon using an unsupported catalyst consisting of an aggregate of multiple iron particles, the carbon produced adheres to the unsupported catalyst, forming a graphite-coated catalyst, which reduces its activity in the direct decomposition of hydrocarbons. Converting the carbon contained in the decomposed unsupported catalyst into methane and reusing the resulting methane in the direct decomposition of hydrocarbons increases the options for utilizing carbon, while reusing the unsupported catalyst obtained by removing the carbon in the direct decomposition of hydrocarbons reduces the amount of unsupported catalyst that is discarded.

[0065] [4] Another embodiment of a method for converting carbon into methane comprises the steps of: A step of preparing an aggregate (12) of a plurality of particles containing 4.3 mass % to 54.3 mass % iron and 45.7 mass % to 95.7 mass % carbon; contacting the assembly (12) with hydrogen; Including, The step of preparing the assembly (12) includes: Providing an unsupported catalyst (3) comprising a plurality of particles of iron or iron oxide; contacting a hydrocarbon-containing gas with the unsupported catalyst (3); Includes:

[0066] When hydrocarbons are directly decomposed into hydrogen and carbon using an unsupported catalyst containing multiple particles of iron or iron oxide, the carbon produced adheres to the unsupported catalyst, forming a graphite-coated catalyst, which reduces its activity for direct decomposition of hydrocarbons. Converting the carbon contained in the decomposed unsupported catalyst into methane and reusing the resulting methane in the direct decomposition method for hydrocarbons can increase the options for utilizing carbon, and reusing the unsupported catalyst obtained by removing the carbon in the direct decomposition method for hydrocarbons can reduce the amount of unsupported catalyst that is discarded.

[0067] [5] A method for producing a catalyst for direct cracking of hydrocarbons according to one embodiment includes the steps of: The method includes a step of removing carbon from the assembly (12) using any one of the methods [1] to [4] for converting carbon into methane.

[0068] When direct cracking of hydrocarbons is carried out using a catalyst obtained by the method for producing a catalyst for direct cracking of hydrocarbons according to the present disclosure, good activity can be exhibited immediately after the start of direct cracking.

[0069] [6] A method for producing a catalyst for direct cracking of hydrocarbons according to another embodiment is a method for producing a catalyst for direct cracking of hydrocarbons according to [5], comprising the steps of: The specific surface area of ​​the iron particles contained in the plurality of particles is 0.52 m 2 / g or more.

[0070] According to this configuration, the activity can be satisfactorily exhibited immediately after the start of direct decomposition. [Explanation of symbols]

[0071] 3. Unsupported catalysts 12 aggregates

Claims

1. 1. A method for producing a catalyst for direct decomposition of hydrocarbons, which directly decomposes hydrocarbons into carbon and hydrogen, by regenerating catalytic function, comprising: The sample contains 6.8% by mass to 54.3% by mass of iron and 45.7% by mass to 93.2% by mass of carbon, and contains iron particles of submicron order with carbon attached to the iron particles. The sample contains cementite, and the quantitative values ​​of the peaks attributable to cementite and alpha iron obtained by X-ray diffraction analysis are determined using the RIR method. The quantitative values ​​of the peaks attributable to cementite and alpha iron are respectively determined by the RIR method. １ and I ２ In this case, the ratio I １ / (I １ +I ２ providing a collection of a plurality of particles, wherein the σ is greater than or equal to 0.17 and less than or equal to 1.0; removing carbon from the aggregates by contacting the aggregates with hydrogen to convert the carbon to methane; Including, The method for producing a catalyst for direct cracking of hydrocarbons by regenerating catalytic function, wherein the step of removing carbon from the aggregate is carried out at a temperature in the range of 500°C to 680°C.

2. The specific surface area of ​​the iron particles contained in the plurality of particles after removing carbon from the aggregate is 0.52 m ２ 2. A method for producing a catalyst for direct cracking of hydrocarbons according to claim 1, wherein the catalyst has a catalytic activity of 1 / g or more.

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