Direct decomposition methods for hydrocarbons.

Abstract

DEPCT68 A direct hydrocarbon dissociation method for directly breaking down hydrocarbons into... The carbon and hydrogen in each of the first and second reactors consist of: The process of continuously flowing a catalyst containing an iron-based catalyst element through a kiln. The catalyst is then transferred from the first reactor and then through the second reactor. And the process of flowing the initial gas containing hydrocarbons through one of the reactors or reactors. The second instance continues. And then the starting gas is flowed through another stage of the first or second reactor. The reactant gases and catalyst come into contact in reactor one and reactor two. The device is operating under conditions where the pressure in the second reactor is higher than the pressure in the first reactor;

Description

Direct cracking of hydrocarbons

[0001] This disclosure relates to a method for direct cracking of hydrocarbons. This application claims priority to Japanese Patent Application No. 2022-037899, filed with the Japan Patent Office on March 11, 2022, the contents of which are incorporated herein by reference.

[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 upon 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, a method has been developed in which hydrogen and carbon are produced by directly decomposing methane. The direct decomposition of methane is characterized by the fact that hydrogen fuel can be obtained without emitting carbon dioxide, and that the by-product carbon is solid and can be easily immobilized, and the carbon itself can be effectively used in a wide range of applications such as electrode materials, tire materials, and civil engineering applications. Patent Document 1 describes a method for producing hydrogen and carbon by directly decomposing hydrocarbons in the coexistence of at least one of hydrogen and carbon dioxide, using a supported catalyst in which iron, a catalytic component, is supported on a carrier.

[0005] Patent Document 1 discloses that the activity of the reaction for directly decomposing hydrocarbons into carbon and hydrogen drops sharply within one hour, making maintaining the activity of this reaction a challenge. In contrast, research by the inventors of the present disclosure has revealed that the activity of the direct decomposition reaction of hydrocarbons can be maintained for a long period of time by using an unsupported catalyst consisting of an aggregate of multiple particles made of a metal containing iron (Japanese Patent Application No. 2021-153622 filed by the applicant of the present application).

[0006] Patent No. 4697941

[0007] However, although the research of the inventors of the present disclosure has revealed that the use of an unsupported catalyst consisting of an aggregate of multiple particles made of a metal including iron makes it possible to maintain the activity of the direct cracking reaction of hydrocarbons for a long period of time, the direct cracking of hydrocarbons using this catalyst at a reasonable cost remains a challenge. Note that this challenge is not unique to the unsupported catalyst, but also applies to the direct cracking of hydrocarbons using a supported catalyst in which the catalytic component iron is supported on a carrier.

[0008] In view of the above, at least one embodiment of the present disclosure aims to provide a method for direct cracking of hydrocarbons that can directly crack hydrocarbons at a reasonable cost.

[0009] In order to achieve the above-mentioned object, the method for direct cracking of hydrocarbons according to the present disclosure is a method for direct cracking of hydrocarbons, which directly cracks hydrocarbons into carbon and hydrogen in each of a first reactor and a second reactor, and includes the steps of continuously flowing a catalyst containing iron as a catalytic component through the first reactor and then through the second reactor, and continuously flowing a feed gas containing hydrocarbons through one of the first reactor or the second reactor and then through the other of the first reactor or the second reactor, and the feed gas comes into contact with the catalyst in each of the first reactor and the second reactor under conditions where the pressure in the second reactor is higher than the pressure in the first reactor.

[0010] According to the method for direct cracking of hydrocarbons disclosed herein, after catalyst pretreatment is performed in a first reactor, direct cracking of hydrocarbons is performed in a second reactor, the internal pressure of which is higher than the pressure in the first reactor. This makes it possible to reduce the total volume of the first reactor and the second reactor compared to when catalyst pretreatment and direct cracking of hydrocarbons are performed in a single reactor, thereby enabling direct cracking of hydrocarbons at an appropriate cost.

[0011] FIG. 1 is a schematic diagram of the configuration of an apparatus for carrying out a method for direct cracking of hydrocarbons according to embodiment 1 of the present disclosure. FIG. 2 is a schematic diagram of the configuration of an experimental apparatus for carrying out an experiment for examining the operational effects of the method for direct cracking of hydrocarbons according to embodiment 1 of the present disclosure. FIG. 3 is a graph showing the change over time in methane conversion obtained in an experiment for examining the operational effects of the method for direct cracking of hydrocarbons according to embodiment 1 of the present disclosure. FIG. 4 is a diagram showing the material balance in configuration 1, which is a model for examining the operational effects of the method for direct cracking of hydrocarbons according to embodiment 1 of the present disclosure. FIG. 5 is a diagram showing the material balance in configuration 2, which is a model for examining the operational effects of the method for direct cracking of hydrocarbons according to embodiment 1 of the present disclosure. FIG. 6 is a schematic diagram of the configuration of an apparatus for carrying out a method for direct cracking of hydrocarbons according to embodiment 2 of the present disclosure.

[0012] Hereinafter, a method for direct cracking of hydrocarbons 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.

[0013] (Embodiment 1) <Configuration of an apparatus for carrying out a method for direct cracking of hydrocarbons according to embodiment 1 of the present disclosure> Fig. 1 shows the configuration of an apparatus 1 for carrying out a method for direct cracking of hydrocarbons according to embodiment 1 of the present disclosure. The apparatus 1 includes a first reactor 2 and a second reactor 3. The apparatus 1 is configured to continuously circulate a catalyst 4 having a configuration described below through the first reactor 2 and then through the second reactor 3. To achieve such a configuration, for example, a catalyst supply line 5 for supplying the catalyst 4 to the first reactor 2, a catalyst transfer line 6 for supplying the catalyst 4 flowing out from the first reactor 2 to the second reactor 3, and a catalyst outflow line 7 for flowing out the catalyst 4 from the second reactor 3 may be provided, and these may be configured, for example, by a sealed belt conveyor or a screw feeder.

[0014] The apparatus 1 further includes a gas supply line 8 for supplying a hydrocarbon-containing raw material gas, such as natural gas or a gas obtained by refining natural gas or the like, to the first reactor 2, a gas transfer line 9 for supplying the raw material gas flowing out from the first reactor 2 to the second reactor 3, and a gas outflow line 10 through which the outflow gas flowing out from the second reactor 3 flows. The gas supply line 8 and the gas transfer line 9 are provided with compressors 11 and 12, respectively, for pressurizing the raw material gas.

[0015] Although not an essential component, the apparatus 1 may include a gas recycle line 13 that branches off from the gas outflow line 10 and connects to the gas supply line 8. Furthermore, in the apparatus 1 that includes the gas recycle line 13, a gas separator 14 that separates the outflow gas into a hydrocarbon-rich gas and a hydrogen-rich gas may be provided at the branch point where the gas recycle line 13 branches off from the gas outflow line 10. The configuration of the gas separator 14 is not particularly limited, and it may be, for example, a membrane separator or the like.

[0016] The catalyst 4 comprises a plurality of iron particles. In other words, the catalyst 4 is not a supported catalyst in which iron is supported on a carrier, but an unsupported catalyst that is an aggregate of iron particles. Each particle of the catalyst 4 is not limited to being made of iron alone, and some degree of contamination with components that inevitably become mixed into the iron (unavoidable impurities) and metal elements other than iron is permitted. For this reason, in this application, "made of iron" means made of a metal with an iron purity ranging from the lower limit to 100%.

[0017] Japanese Patent No. 7089235, owned by the applicant of the present disclosure, reveals that the activity of the reaction of directly decomposing hydrocarbons into carbon and hydrogen (hereinafter referred to as the "direct decomposition reaction") can be maintained for a long period of time by bringing a hydrocarbon-containing feed gas into contact with an unsupported catalyst that is an aggregate of multiple metallic particles with an iron purity of 86% or more. Therefore, in the present disclosure, the "lower limit" of the iron purity mentioned above is set to 86%.

[0018] <Method for Direct Cracking of Hydrocarbons According to Embodiment 1 of the Present Disclosure> In the apparatus 1, a catalyst 4 is continuously supplied to a first reactor 2 via a catalyst supply line 5. The catalyst 4 that flows into the first reactor 2 remains in the first reactor 2 for a predetermined residence time, and then sequentially flows out from the first reactor 2. The catalyst 4 that flows out from the first reactor 2 continuously flows into a second reactor 3 via a catalyst transfer line 6. The catalyst 4 that flows into the second reactor 3 also remains in the second reactor 3 for a predetermined residence time, and then sequentially flows out from the second reactor 3 via a catalyst outflow line 7.

[0019] In addition, in the apparatus 1, a raw material gas is continuously supplied to the first reactor 2 via a gas supply line 8. The raw material gas that flows into the first reactor 2 remains in the first reactor 2 for a predetermined residence time, and then sequentially flows out from the first reactor 2. The raw material gas that flows out from the first reactor 2 continuously flows into the second reactor 3 via a gas transfer line 9. The raw material gas that flows into the second reactor 3 also remains in the second reactor 3 for a predetermined residence time, and then sequentially flows out from the second reactor 3 via a gas outlet line 10.

[0020] In each of the first reactor 2 and the second reactor 3, the raw material gas comes into contact with the catalyst 4, whereby the hydrocarbons in the raw material gas are directly decomposed into hydrogen and carbon. This direct decomposition reaction, using methane as an example of the hydrocarbon, occurs in each of the first reactor 2 and the second reactor 3 as shown in the following reaction formula (1): CH ４ →2H ２ + C ... (1)

[0021] As explained in the applicant's prior application, when the above-mentioned unsupported catalyst is used as catalyst 4, the activity of the direct cracking reaction of hydrocarbons is maintained for a long time, but it takes about 5 to 10 hours for the activity of the direct cracking reaction to increase sufficiently. Therefore, the first reactor 2 is primarily used for pre-treating the catalyst 4, and the second reactor 3 is primarily used for the direct cracking of hydrocarbons. In the pre-treatment in the first reactor 2, the hydrocarbons in the feed gas are directly decomposed into hydrogen and carbon, and the iron in the catalyst 4 is broken down into submicron-order iron particles by hydrogen erosion caused by the generated hydrogen, which are highly dispersed in the generated carbon, thereby increasing the activity of the catalyst 4. During this process, a portion of the hydrocarbons in the feed gas are directly decomposed into hydrogen and carbon, so that the feed gas flowing out from the first reactor 2 contains hydrogen, and carbon adheres to the catalyst 4 flowing out from the first reactor 2.

[0022] Due to the pretreatment of the catalyst 4 in the first reactor 2, the catalyst 4 discharged from the first reactor 2 exhibits sufficient activity, so that when it comes into contact with hydrocarbons in the second reactor 3, the hydrocarbons are directly decomposed into hydrogen and carbon with sufficient activity. Hydrogen can be obtained by recovering the effluent gas discharged from the second reactor 3 via the gas outlet line 10, or by purifying the effluent gas depending on the hydrogen concentration in the effluent gas. Since carbon adheres to the catalyst 4 discharged from the second reactor 3 via the catalyst outlet line 7, the carbon can be recovered by removing it from the catalyst 4 by any method, and the carbon-removed catalyst 4 may be reused in the apparatus 1.

[0023] If the apparatus 1 is provided with a gas recycle line 13, a portion of the effluent gas flowing out from the second reactor 3 flows through the gas recycle line 13, merges with the raw material gas flowing through the gas supply line 8, and flows into the first reactor 2. Since the effluent gas contains unreacted hydrocarbons, some of the unreacted hydrocarbons are returned to the first reactor 2, thereby having the opportunity to be directly decomposed again. This can improve the hydrogen yield. Furthermore, since the effluent gas also contains hydrogen, hydrogen is supplied to the first reactor 2. Since hydrogen is required for pretreatment of the catalyst 4 in the first reactor 2, the efficiency of pretreatment can also be improved.

[0024] When the apparatus 1 is provided with a gas separator 14, the hydrocarbons contained in the effluent gas can be supplied to the first reactor 2, thereby improving the yield of hydrogen. In addition, since the hydrocarbons are removed from the effluent gas, the purity of the recovered hydrogen can also be increased.

[0025] The applicant's previous application also revealed that the activity of the direct cracking reaction can be maintained for a long period of time by performing the direct cracking reaction in a temperature range of 600°C to 900°C. Therefore, in the direct cracking method of hydrocarbons disclosed herein, it is preferable to set the temperatures in the first reactor 2 and the second reactor 3 within the above range. Considering that the first reactor 2 mainly performs pretreatment of the catalyst 4, while the second reactor 3 mainly performs direct cracking of hydrocarbons, it is preferable to set the conditions in the first reactor 2 and the second reactor 3 to be suitable for their respective purposes. Considering the purpose of pretreatment of the catalyst 4, it is preferable to set the temperature in the first reactor 2 to 800°C or less. Considering the purpose of direct cracking of hydrocarbons, it is preferable to set the temperature in the second reactor 3 to 700°C or more.

[0026] <Actions and Effects of the Method for Direct Cracking of Hydrocarbons According to Embodiment 1 of the Present Disclosure> The following experiment was conducted to collect basic data necessary for a simulation to examine the actions and effects of the method for direct cracking of hydrocarbons according to Embodiment 1 of the present disclosure. Figure 2 shows a schematic diagram of the configuration of an experimental device 20 used to conduct this experiment.

[0027] The experimental apparatus 20 includes a quartz reactor 23 having an inner diameter of 16 mm and housing a perforated plate 28 on which the above-described catalyst 4 is placed. The reactor 23 is heatable by an electric furnace 24. The reactor 23 is connected to a raw material supply line 25 for supplying methane and argon, respectively, and a reaction gas distribution line 26 through which reaction gas containing hydrogen produced by the direct decomposition reaction of methane flows out of the reactor 23. The reaction gas distribution line 26 is connected to a gas chromatograph 27 for measuring the composition of the reaction gas. Using this experimental apparatus 20, an experiment was conducted to directly decompose methane as a hydrocarbon into hydrogen and carbon. The experimental conditions are summarized in Table 1 below.

[0028]

[0029] After the catalyst 4 was installed in the reactor 23, the atmosphere inside the reactor 23 was replaced with argon. Subsequently, while argon was flowing through the reactor 23, the electric furnace 24 was started and the temperature inside the reactor 23 was raised to 800°C. When the temperature inside the reactor 23 reached 800°C, the gas supplied to the reactor 23 was switched from argon to methane, and methane gas was flowed through the reactor 23. The composition of the reaction gas flowing out from the reactor 23 was periodically measured using a gas chromatograph 27.

[0030] From the composition of the reaction gas measured by the gas chromatography 27, the methane conversion rate CR [%] and the hydrogen production rate PR [Ncc / min] were calculated by the following formulas (2) and (3). The results are summarized in Table 2 below. CR = C Ｍｅ / (C Ｍｅ +C Ｈ２ × 0.5) × 100 ... (2) where C Ｍｅ is the concentration of methane in the reaction gas [vol%], and C Ｈ２ is the hydrogen concentration in the reaction gas [vol%]. PR = F Ｍｅ ×CR / 100×2 (3) where F Ｍｅ is the methane supply rate [Ncc / min].

[0031]

[0032] The change in methane conversion rate over time, summarized in Table 2, is shown in Figure 3. Since the methane conversion rate increases up to about 10 hours after the start of the experiment, this can be considered to be a process in which the activity of catalyst 4 increases. Since the methane conversion rate remains constant between 10 and 20 hours after the start of the experiment, this can be considered to be a process in which the activity of catalyst 4 remains sufficiently high. Since the methane conversion rate decreases after 20 hours has passed since the start of the experiment, this can be considered to be a process in which catalyst 4 deteriorates. In other words, it can be said that the pretreatment of catalyst 4 is mainly carried out up to about 10 hours after the start of the experiment, and that the direct decomposition reaction of methane is mainly carried out after 10 hours has passed since the start of the experiment.

[0033] In the method for direct cracking of hydrocarbons according to Embodiment 1, the catalyst 4 is pretreated in the first reactor 2, and the pretreated catalyst 4 is used to perform direct cracking of hydrocarbons in the second reactor 3. However, by changing the reaction conditions in both reactors, the reactions in both reactors can be carried out more efficiently. Below, two models are compared, namely, a case where one reactor is used (hereinafter referred to as "Configuration 1") and a case where two reactors are used (hereinafter referred to as "Configuration 2"). However, in Configuration 2, the pressures in the two reactors are set under different conditions. The reactor conditions for each of Configurations 1 and 2 are shown in Table 3 below.

[0034]

[0035] In order to compare Configuration 1 and Configuration 2, it is necessary to estimate the methane conversion rate when the pressure is set to 301.3 kPa from the results in Table 2. It is known that the methane decomposition rate r(Me) can be expressed by the following formula (4): r(Me) = kP(Me) (4) where k is the rate constant and P(Me) is the partial pressure of methane. Using this formula (4), if the methane conversion rate is 45% when the pressure is 1 atm (101.3 kPa), the methane conversion rate can be estimated to be 80% when the pressure is 301.3 kPa.

[0036] In Configuration 1, catalyst pretreatment and direct decomposition of hydrocarbons are carried out in one reactor. According to Table 2 and Figure 3, pretreatment takes 10 hours, and sufficient catalytic activity (45% methane conversion) is maintained for the next 10 hours. Therefore, the catalyst residence time in the reactor is set to 20 hours, and the hydrogen production amount in 20 hours is set to 10,000 Nm ３ The mass balance for configuration 1 is shown in Figure 4. In configuration 1, the catalyst feed rate to the reactor is 1.85 m ３ / hr, and the carbon production was 159 m ３ Therefore, the volume of the catalyst and carbon 20 hours after starting to supply the catalyst and methane to the reactor is calculated as follows: 1.85 m ３ +159m ３ / hr×20hr=3182m ３ Therefore, the reactor must be at least 3182 m ３ volume is required.

[0037] In the second configuration, the catalyst is pretreated in the first reactor, and hydrocarbons are directly decomposed in a second reactor separate from the first reactor. The catalyst residence time in the first reactor is 10 hours, the methane conversion rate in the second reactor is 80%, and the hydrogen production amount is 10,000 Nm ３ The mass balance for configuration 2 is shown in Figure 5. In configuration 2, the catalyst feed rate to the first reactor is 1.13 m ３ / hr, and the amount of carbon produced in the first reactor was 24.2 m ３ The catalyst residence time in the second reactor was 6.8 hours, and the amount of carbon produced in the second reactor was 96.9 m ３ Therefore, the volume of the catalyst and carbon 10 hours after starting to supply the catalyst and methane to the first reactor is calculated as follows: 1.13 m ３ +24.2m ３ / hr×10hr=243m ３ Therefore, the first reactor must be at least 243 m ３ The volume of the catalyst and carbon in the second reactor for 6.8 hours is calculated as follows: 1.13 m ３+96.9m ３ / hr x 6.8hr = 660m ３ Therefore, the second reactor must be at least 660 m ３ volume is required.

[0038] Comparing Configuration 1 and Configuration 2, in Configuration 1, the reactor is at least 3182 m ３ In configuration 2, the first and second reactors together require a volume of at least 903 m ３ (243m ３ +660m ３ Therefore, by carrying out catalyst pretreatment in the first reactor and then carrying out direct cracking of hydrocarbons in the second reactor, which has an internal pressure higher than the pressure in the first reactor, the total volume of the first reactor and the second reactor can be made smaller than when catalyst pretreatment and direct cracking of hydrocarbons are carried out in one reactor, and therefore hydrocarbons can be directly cracked at an appropriate cost.

[0039] The catalyst pretreatment is carried out at atmospheric pressure P ０ [kPa], the pressure in the first reactor is approximately atmospheric pressure, i.e., P ０ [kPa] to (P ０ On the other hand, since the direct cracking of hydrocarbons increases the cracking rate as the pressure increases, the pressure in the second reactor is preferably at least higher than the pressure in the first reactor, and is preferably 3 MPa or less in consideration of the equilibrium constraint of the methane conversion rate.

[0040] Here is an example of a method for determining the pressure in the second reactor taking into account the equilibrium constraints of methane conversion. Once the temperature and pressure are determined, the equilibrium methane conversion can be determined. Here, the equilibrium methane conversion refers to the conversion rate at which the conversion rate cannot be further improved even if the catalytic activity is improved due to the constraints of chemical equilibrium in the direct decomposition reaction of methane. For example, the equilibrium methane conversion rate over a certain temperature and pressure range can be calculated using the equilibrium constant data at each temperature listed in the non-commercial catalyst handbook distributed by Clariant AG (Muttenz, Basel-Landschaft, Switzerland), resulting in Table 4 below. Once the target methane conversion rate X in the second reactor is determined, the equilibrium methane conversion rate Y at a certain temperature and pressure can be obtained from Table 4, and the pressure in the second reactor can be determined under the condition that X≦Y. For example, when X is determined to be 45% and the temperature in the second reactor is 800°C, it can be seen from Table 4 that the equilibrium conversion rate of methane at 800°C and 2.0 MPaG is 45.8%, so a pressure of 2.0 MPaG or less can be determined as the pressure in the second reactor.

[0041]

[0042] (Embodiment 2) Next, a method for direct cracking of hydrocarbons according to Embodiment 2 of the present disclosure will be described. The method for direct cracking of hydrocarbons according to Embodiment 2 is modified from Embodiment 1 in that the flow of the catalyst 4 and the flow of the feed gas in the device 1 are reversed. In Embodiment 2, the same components as those in Embodiment 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0043] <Configuration of an Apparatus for Carrying Out a Method for Direct Cracking of Hydrocarbons According to Embodiment 2 of the Present Disclosure> Figure 6 shows the configuration of an apparatus 1 for carrying out a method for direct cracking of hydrocarbons according to Embodiment 2 of the present disclosure. The apparatus 1 includes a gas supply line 8 for supplying a raw material gas to the second reactor 3, a gas transfer line 9 for supplying the raw material gas flowing out from the second reactor 3 to the first reactor 2, and a gas outlet line 10 for discharging the effluent gas from the first reactor 2. The other configurations are the same as those of Embodiment 1, except that the gas transfer line 9 is not provided with a compressor 12 and that there is no need to provide a gas recycle line 13 (see Figure 1) and a gas separator 14 (see Figure 1).

[0044] <Method for direct cracking of hydrocarbons according to embodiment 2 of the present disclosure> In the apparatus 1, a feed gas is continuously supplied to the second reactor 3 via a gas supply line 8. The feed gas that has flowed into the second reactor 3 remains in the second reactor 3 for a pre-designed residence time, and then sequentially flows out from the second reactor 3. The feed gas that has flowed out from the second reactor 3 continuously flows into the first reactor 2 via a gas transfer line 9. The feed gas that has flowed into the first reactor 2 also remains in the first reactor 2 for a pre-designed residence time, and then sequentially flows out from the first reactor 2 via a gas outlet line 10. The flow of the catalyst 4 is the same as in embodiment 1.

[0045] <Action and effect of the method for direct cracking of hydrocarbons according to embodiment 2 of the present disclosure> In embodiment 2, as in embodiment 1, pretreatment of the catalyst 4 is mainly carried out in the first reactor 2, and direct cracking of hydrocarbons is mainly carried out in the second reactor 3. Therefore, in embodiment 2, as in embodiment 1, the total volume of the first reactor 2 and the second reactor 3 can be reduced, and hydrocarbons can be directly cracked at an appropriate cost.

[0046] In the second embodiment, the flow direction of the raw material gas is opposite to that of the first embodiment. That is, in the second embodiment, the raw material gas passes through the second reactor 3 and then the first reactor 2. In this configuration, direct cracking of hydrocarbons is carried out in the second reactor 3, and therefore the gas flowing out from the second reactor 3 contains hydrogen. Therefore, hydrogen flows into the first reactor 2, and the pretreatment of the catalyst 4 is carried out in the presence of hydrogen. The effects of pretreatment of the catalyst 4 being carried out in the presence of hydrogen will be discussed below.

[0047] Experiments for the Example and Comparative Example were carried out using the experimental apparatus 20 shown in Figure 2. In both experiments, catalyst 4 was pretreated for 10 hours, and then the conditions were changed and a direct cracking reaction was carried out for 10 hours using the pretreated catalyst 4. The conditions for the pretreatment and direct cracking reaction were the same as those in Table 1. However, the composition of the gas supplied to the reactor in the pretreatment (volume ratio of methane / hydrogen) was 50 / 50 in the Example, while it was 100 / 0 in the Comparative Example. In other words, the only difference between the two is that catalyst 4 was pretreated in the presence of hydrogen in the Example, while catalyst 4 was pretreated with only methane in the Comparative Example.

[0048] The peak value of methane conversion during the direct cracking reaction was 38.9% in the example and 27.7% in the comparative example. From these results, it can be said that pretreating the catalyst in the presence of hydrogen improves the hydrocarbon conversion in the direct cracking reaction. Therefore, in the hydrocarbon cracking reaction according to the second embodiment, the gas flowing out from the second reactor 3 contains hydrogen, and the gas flowing out from the second reactor 3 is supplied to the first reactor 2, whereby pretreatment of the catalyst 4 is performed in the presence of hydrogen. Therefore, the peak value of hydrocarbon conversion during the direct cracking reaction is also higher than in the first embodiment. As a result, the hydrocarbon conversion can be improved.

[0049] <Modifications to Embodiments 1 and 2> In Embodiments 1 and 2, direct decomposition of hydrocarbons is performed using an unsupported catalyst that is an aggregate of multiple metallic particles with an iron purity of 86% or more, but the present invention is not limited to this catalyst. Any catalyst configuration may be used as long as it contains iron as a catalytic component, and for example, a supported catalyst in which the catalytic component iron is supported on a carrier may be used.

[0050] The contents described in each of the above embodiments can be understood, for example, as follows.

[0051] [1] A method for direct cracking of hydrocarbons according to one embodiment is a method for direct cracking of hydrocarbons, which directly cracks hydrocarbons into carbon and hydrogen in each of a first reactor (2) and a second reactor (3), and includes the steps of: continuously flowing a catalyst (4) containing iron as a catalytic component through the first reactor (2) and then through the second reactor (3); and continuously flowing a feed gas containing hydrocarbons through one of the first reactor (2) or the second reactor (3) and then through the other of the first reactor (2) or the second reactor (3), wherein the feed gas comes into contact with the catalyst (4) in each of the first reactor (2) and the second reactor (3) under conditions where the pressure in the second reactor (3) is higher than the pressure in the first reactor (2).

[0052] According to the method for direct cracking of hydrocarbons disclosed herein, after catalyst pretreatment is performed in a first reactor, direct cracking of hydrocarbons is performed in a second reactor, the internal pressure of which is higher than the pressure in the first reactor. This makes it possible to reduce the total volume of the first reactor and the second reactor compared to when catalyst pretreatment and direct cracking of hydrocarbons are performed in a single reactor, thereby enabling direct cracking of hydrocarbons at an appropriate cost.

[0053] [2] A method for direct cracking of hydrocarbons according to another embodiment is the method for direct cracking of hydrocarbons according to [1], wherein the catalyst is an unsupported catalyst that is an aggregate of a plurality of particles made of a metal having an iron purity of 86% or more.

[0054] According to this method, when an unsupported catalyst consisting of an aggregate of multiple particles made of a metal with an iron purity of 86% or more is used, hydrocarbons can be directly decomposed at a reasonable cost.

[0055] [3] A method for direct cracking of hydrocarbons according to yet another embodiment is the method for direct cracking of hydrocarbons according to [1] or [2], wherein the atmospheric pressure is P ０ kPa, the pressure in the first reactor (2) is P ０ kPa~(P ０ +50) kPa, and the pressure in the second reactor (3) is 3 MPa or less.

[0056] According to this method, the activity of the direct cracking reaction of hydrocarbons in the second reactor can be increased, and therefore hydrocarbons can be directly cracked at a reasonable cost.

[0057] [4] Another aspect of the direct cracking method for hydrocarbons is the method for direct cracking of hydrocarbons according to [3], wherein the pressure in the second reactor (3) is determined by the steps of: determining a target value X (%) of the methane conversion rate in the second reactor (3); obtaining an equilibrium methane conversion rate Y (%) at the temperature in the second reactor (3); and determining the pressure in the second reactor (3) under the condition that X≦Y.

[0058] According to this method, the pressure inside the second reactor can be determined simply and appropriately.

[0059] [5] A method for direct cracking of hydrocarbons according to yet another embodiment is the method for direct cracking of hydrocarbons according to any one of [1] to [4], wherein the raw material gas is brought into contact with the catalyst (4) in a temperature range of 600°C to 900°C in each of the first reactor (2) and the second reactor (3).

[0060] According to this method, the activity of the direct cracking reaction of hydrocarbons can be maintained for a long period of time.

[0061] [6] A method for direct cracking of hydrocarbons according to yet another embodiment is the method for direct cracking of hydrocarbons according to any one of [1] to [5], comprising a step of supplying a part of the effluent gas flowing out from the second reactor (3) to the first reactor (2).

[0062] According to this method, the unreacted hydrocarbons are directly cracked again, thereby improving the yield of hydrogen.

[0063] [7] According to yet another aspect, the method for direct cracking of hydrocarbons is the method for direct cracking of hydrocarbons according to [6], further comprising a step of separating the effluent gas into hydrocarbons and hydrogen before the step of supplying a portion of the effluent gas to the first reactor (2), and supplying the hydrocarbon-containing gas separated from the effluent gas to the first reactor (2).

[0064] According to this method, only the unreacted hydrocarbons are directly cracked again, so that the purity of the recovered hydrogen can be increased and the yield of hydrogen can be improved.

[0065] [8] A method for direct cracking of hydrocarbons according to yet another embodiment is the method for direct cracking of hydrocarbons according to any one of [1] to [5], wherein the raw material gas is continuously passed through the second reactor (3) and then passed through the first reactor (2).

[0066] According to this method, the gas flowing out from the second reactor contains hydrogen, and by supplying the gas flowing out from the second reactor to the first reactor, pretreatment of the catalyst is carried out in the presence of hydrogen in the first reactor, thereby making it possible to improve the conversion rate of hydrocarbons.

[0067] 2 First reactor 3 Second reactor 4 Catalyst

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