TWI903492B - Direct decomposition method of hydrocarbons - Google Patents

Abstract

The direct decomposition method for directly decomposing hydrocarbons into carbon and hydrogen hydrocarbons comprises the following steps: bringing a feed gas containing methane and hydrocarbons having two or more carbon atoms into contact with a non-supporting catalyst made of iron, which is an aggregate of multiple particles, and the concentration of hydrocarbons having two or more carbon atoms in the feed gas is 0.02 to 10 vol.

Description

Direct decomposition method of hydrocarbons

This disclosure relates to a method for the direct decomposition of hydrocarbons. This application claims priority based on Japanese Patent Application No. 2023-083207 filed with the Japanese Patent Office on May 19, 2023, the contents of which are incorporated herein by reference.

Currently, the production of various energy sources relies heavily on fossil fuels such as oil, coal, and natural gas. However, from the perspective of global environmental protection, the increasing carbon dioxide emissions from the combustion of fossil fuels are considered a problem. The Paris Agreement, signed in 2015, calls for a reduction in carbon dioxide emissions to address climate change. Reducing carbon dioxide emissions from the combustion of fossil fuels, particularly in thermal power plants, has become a crucial issue. This study carefully explores processes for separating and recovering emitted carbon dioxide, and also examines technologies for producing energy using alternative fuels to fossil fuels without emitting carbon dioxide.

Therefore, hydrogen, as a clean fuel that does not emit carbon dioxide upon combustion, has attracted attention as an alternative to fossil fuels. Hydrogen can be produced, for example, by steam reforming methane contained in natural gas. However, this method generates carbon monoxide as a byproduct, which is ultimately oxidized and emitted as carbon dioxide. Additionally, methods for producing hydrogen from water without using fossil fuels, such as water electrolysis and photocatalytic oxidation, have been considered, but these methods require extremely high energy consumption, posing economic challenges.

In contrast, methods have been developed for the direct decomposition of methane to produce hydrogen and carbon. The characteristics of direct methane decomposition are: the production of hydrogen fuel without carbon dioxide emissions, and the ease with which the byproduct carbon can be immobilized due to its solid nature, allowing for efficient utilization in a wide range of applications such as electrode materials, tire materials, and building materials. Currently, methods have been developed for the direct decomposition of hydrocarbons into hydrogen and carbon by bringing a catalyst into contact with the hydrocarbon gas. However, the carbon products of the direct hydrocarbon decomposition reaction are attached to the catalyst, leading to a short-term decrease in catalyst activity.

In contrast, the applicant disclosed herein, as described in Patent 1, has developed a method for directly decomposing hydrocarbons into carbon and hydrogen using a non-supporting catalyst made of an aggregate of iron particles. According to this method, even if the carbon from the products of the direct decomposition reaction of hydrocarbons adheres to the catalyst, it exhibits new active sites and maintains activity, thus sustaining the reaction activity for a long period. [Prior Art Documents] [Patent Documents]

[Patent Document 1] Japanese Invention Patent No. 7089235

[Problem to be Solved by the Invention] The applicant has discovered a method for further developing the direct decomposition method of the hydrocarbons described in Patent Document 1, thereby increasing the hydrocarbon conversion rate.

In light of the foregoing, at least one embodiment disclosed herein aims to provide a direct decomposition method for hydrocarbons that increases their conversion rate. [Means for Solving the Problem]

To achieve the above objectives, the disclosed direct hydrocarbon decomposition method is a direct decomposition method that directly decomposes hydrocarbons into carbon and hydrogen hydrocarbons, comprising the step of contacting a feed gas containing methane and hydrocarbons having two or more carbon atoms with a non-supporting catalyst, which is an aggregate of iron particles, wherein the concentration of the aforementioned hydrocarbons having two or more carbon atoms in the feed gas is 0.02 to 10 vol%. [Effects of the Invention]

The direct decomposition method for hydrocarbons disclosed herein can improve hydrocarbon conversion rates compared to using feed gas containing only methane.

[Forms of the Invention] The following description, based on drawings, explains the direct decomposition method of the hydrocarbon in the embodiments of this disclosure. The embodiments described below represent one aspect of this disclosure and are not limiting; they can be arbitrarily modified within the scope of the technical concept of this disclosure.

<Configuration of an Apparatus for Implementing a Direct Decomposition Method for Hydrogen of an Embodiment of the Presently Disclosed> As shown in Figure 1, the apparatus 1 for implementing a direct decomposition method for hydrocarbon of an embodiment of the presently disclosed includes a reactor 3 housing a catalyst 2 as an essential component. The reactor 3 is equipped with a heating device 4 (e.g., a steam jacket) for heating the interior of the reactor 3, particularly the catalyst 2. Connected to the reactor 3 are: a feedstock supply line 5 for supplying feedstock gas to the reactor 3; and a reaction gas flow line 6 for the hydrogen-containing reaction gas generated by the reaction of hydrocarbons in the feedstock gas with the catalyst 2, which flows out of the reactor 3 and into the reactor 3.

The hydrocarbons in the feed gas include methane and hydrocarbons having two or more carbon atoms, such as ethane, propane, or butane. The concentration of hydrocarbons having two or more carbon atoms in the feed gas is 0.02 to 10 vol%, preferably 5 to 10 vol%, and more preferably 5 to 9.7 vol%. Examples of such feed gases include natural gas (town gas) with a concentration of approximately 10 vol% of hydrocarbons having two or more carbon atoms, but it is not limited to natural gas; any feed gas prepared in a manner that allows the concentration of hydrocarbons having two or more carbon atoms to fall within the aforementioned range can be used.

Catalyst 2 is a non-supported catalyst consisting of a plurality of iron particles. Within reactor 3, the particles of catalyst 2 can be in a static state or in a fluidized bed state, suspended within the raw material gas by upward ejection of the raw material gas. Although the carbon generated from the reaction of hydrocarbons in the raw material gas by catalyst 2 adheres to the particles of catalyst 2, when catalyst 2 forms a fluidized bed, the particles of catalyst 2 rub against each other, physically removing the carbon adhering to the particles. Furthermore, if, for example, a fixed-bed reactor is used as reactor 3, a carbon removal device can be provided externally to reactor 3 to remove the carbon adhering to catalyst 2.

A solid-gas separation device 7, such as a bag filter or cyclone separator, can be installed in the reaction gas flow line 6. Furthermore, although it also depends on the concentration of hydrogen in the reaction gas, a hydrogen purification device 11 for purifying the hydrogen in the reaction gas, i.e., for increasing the hydrogen concentration, can be installed in the reaction gas flow line 6 if necessary. The configuration of the hydrogen purification device 11 is not particularly limited; for example, a pressure variation adsorption (PSA) device can be used.

As shown in Figure 2, a circulation line 8 can also be installed to connect the raw material supply line 5 to the reaction gas flow line 6. In the circulation line 8, a flow regulating valve 9 can be installed to regulate the flow rate of the gas (a part of the reaction gas) flowing in the circulation line 8.

<A Direct Decomposition Method for Hydrocarbons in One Embodiment Disclosed> Next, the operation of the apparatus 1 shown in FIG. 1 (the direct decomposition method for hydrocarbons) will be explained. A feed gas containing hydrocarbons such as methane is supplied to the reactor 3 through the feed supply line 5. The feed gas flowing into the reactor 3 is in contact with the catalyst 2 while passing through the catalyst 2. At this time, the hydrocarbons in the feed gas are directly decomposed into hydrogen and carbon (hereinafter referred to as the "direct decomposition reaction"). When methane is used as an example of hydrocarbons in the direct decomposition reaction, the reaction shown in the following reaction formula (1) occurs in the reactor 3.

The principle of maintaining the activity of the direct decomposition reaction over a long period of time has been explained in detail by the applicant in Patent Document 1. Furthermore, in order to promote the direct decomposition reaction, it is also preferable to maintain the temperature of the catalyst 2 in the range of 600°C to 900°C by the heating device 4.

The carbon generated by the direct decomposition method adheres to catalyst 2. The generated hydrogen, along with unreacted hydrocarbons (and inert gases), flows out of reactor 3 as reactant gases and circulates within reactant gas flow line 6. Carbon recovery can be achieved by recovering catalyst 2 from reactor 3 after the supply of reactant gases to reactor 3 is stopped. If necessary, the carbon adhering to catalyst 2 can be removed using a carbon removal device. Hydrogen recovery can be achieved by recovering the reactant gases flowing within reactant gas flow line 6. When a hydrogen purification device 11 is installed in reactant gas flow line 6, the hydrogen is purified. When the hydrocarbon conversion rate is low, the hydrogen concentration in the reaction gas becomes low, but the hydrogen concentration in the final product can be increased by using the hydrogen purification device 11.

In the apparatus 1 shown in Figure 2, a portion of the reaction gas flows into the feedstock 5 through the circulation line 8, and then flows into the reactor 3 as a mixed gas that mixes with the feedstock gas flowing in the feedstock 5. In this apparatus 1, the mixed gas formed by the mixture of a portion of the reaction gas and the feedstock gas comes into contact with the catalyst 2, resulting in a direct decomposition reaction. Because the reaction gas contains hydrogen generated from the direct decomposition of hydrocarbons, the concentration of hydrocarbons with two or more carbon atoms in the reaction gas is lower than the concentration of hydrocarbons with two or more carbon atoms in the feedstock gas. Therefore, the concentration of hydrocarbons with two or more carbon atoms in the mixed gas is lower than the concentration of hydrocarbons with two or more carbon atoms in the feedstock gas. For example, when natural gas is used as the feed gas, the concentration of hydrocarbons with two or more carbon atoms in the natural gas is about 10 vol%, but the concentration of hydrocarbons with two or more carbon atoms in the mixed gas in contact with the catalyst in reactor 3 is lower than 10 vol%. Moreover, the more the flow rate of the reactant gas flowing in the circulation pipeline 8 is increased by the flow regulating valve 9, the lower the concentration of hydrocarbons with two or more carbon atoms in the mixed gas can be.

Although verified by the embodiments described later, the conversion rate of methane is improved by directly decomposing a gas with a concentration of 0.02 to 10 vol%, preferably 5 to 10 vol%, of a gas containing two or more carbon atoms, compared to the conversion rate of methane by directly decomposing a gas containing only methane. However, when the concentration of a gas containing two or more carbon atoms is 10 vol% (for example, in the case of using natural gas as the feed gas), the effect of improving the conversion rate is slightly lower than that when the concentration is 5 to 9.7 vol%. However, even when using natural gas as the feedstock, if a direct decomposition reaction occurs in apparatus 1 shown in Figure 2, the hydrocarbon conversion rate can be improved compared to the case where natural gas is used as the feedstock in apparatus 1 shown in Figure 1, because the concentration of hydrocarbons with two or more carbon atoms in the mixed gas is lower than 10 vol%. Furthermore, feedstock gases with a concentration of hydrocarbons with two or more carbon atoms lower than 10 vol% must be prepared separately, for example, by mixing natural gas and methane gas. However, if natural gas is used, this preparation of feedstock gases is unnecessary, thus improving the operability of the direct hydrocarbon decomposition method. [Example]

The inventors of this disclosure demonstrated the effectiveness of the direct decomposition method for hydrocarbons of this disclosure by comparing Examples 1-4, which are equivalent to the direct decomposition method of hydrocarbons of this disclosure, with Comparative Example 1, which is not equivalent to the direct decomposition method of hydrocarbons of this disclosure. Figure 3 shows the configuration of the experimental apparatus used for comparing Examples 1-4 and Comparative Example 1. The experimental apparatus 20 has a quartz reactor 23 with an inner diameter of 16 mm, which houses the catalyst 22 of each of Examples 1-4 and Comparative Example 1. The reactor 23 can be heated by an electric furnace 24. As the catalyst 22, particles of electrolytic iron obtained from NILACO Co., Ltd., with particle sizes ranging from 32 μm to 40 μm, are used.

Connected to reactor 23 are: feed lines 25 for supplying methane and natural gas respectively; and a reaction gas flow line 26 containing the reaction gas generated by the direct decomposition reaction of hydrocarbons, which flows out of reactor 23 and then through the reaction gas. That is, the feed gas supplied to reactor 23 can be one of three types: methane, natural gas, or a mixture of methane and natural gas. The reaction gas flow line 26 is connected to a gas chromatograph 27 for determining the composition of the reaction gas. Table 1 below summarizes the experimental conditions of Examples 1-4 and Comparative Example 1. Furthermore, in Comparative Example 1, only natural gas is supplied to reactor 23, and in Embodiment 3, only methane is supplied to reactor 23. In Embodiments 1 and 2, the concentration of hydrocarbons with two or more carbon atoms in the feed gas is adjusted to achieve the values shown in Table 1, thereby regulating the supply of methane and natural gas to reactor 23.

[Table 1] Implementation examples/comparative examples Implementation Example 1 Implementation Example 2 Implementation Example 3 Implementation Example 4 Comparative example 1 Reaction temperature (°C) 800 Pressure (MPa) 0.1 Flow rate of raw material gas (cc/min) 28 Concentration of hydrocarbons with two or more carbon atoms in the feed gas (vo1%) 0.02 5 9.7 10 0 Space velocity ( ^h⁻¹ ) 10000 Catalyst volume (cc) 0.2

Figure 4 shows the experimental results of Examples 1-4 and Comparative Example 1. In the experiments of Examples 1-4 and Comparative Example 1, the change of methane conversion rate over time was measured, and the peak value of methane conversion rate was defined. Figure 4 shows the relationship between the concentration of hydrocarbons with two or more carbon atoms in the feed gas and the peak value of methane conversion rate. Furthermore, the methane conversion rate is defined by the following formula (2).

As shown in Figure 4, the conversion rate of hydrocarbons with two or more carbon atoms in the feed gas is higher when the concentration is 0.02–10 vol%, especially when the concentration is 5–10 vol%, than the conversion rate of methane when using feed gas containing only methane.

Furthermore, as shown in Figure 4, when the concentration of hydrocarbons with two or more carbon atoms is 10 vol%, the improvement in hydrocarbon conversion rate is slightly lower compared to concentrations of 5–9.7 vol%. Since the concentration of hydrocarbons with two or more carbon atoms in natural gas is approximately 10 vol%, the improvement in hydrocarbon conversion rate is expected to be lower when using natural gas as a feedstock gas compared to using a feedstock gas containing only methane.

In contrast, in the apparatus 1 shown in Figure 2, if the direct decomposition method of the disclosed hydrocarbons is implemented using natural gas as the feed gas, a portion of the reaction gas flowing out of reactor 3 is mixed with the feed gas and flows into reactor 3, where it comes into contact with catalyst 2. As mentioned above, the reaction gas contains hydrogen generated from the direct decomposition of hydrocarbons, therefore the concentration of hydrocarbons with two or more carbon atoms in the reaction gas is lower than the concentration of hydrocarbons with two or more carbon atoms in the feed gas. Therefore, the concentration of hydrocarbons with two or more carbon atoms in the mixed gas is lower than the concentration of hydrocarbons with two or more carbon atoms in the feed gas. That is, even when using natural gas with a hydrocarbon concentration of 10 vol% containing two or more carbon atoms as the feed gas, the concentration of hydrocarbons with two or more carbon atoms in the mixed gas in contact with catalyst 2 within reactor 3 can be in the range of 5–9.7 vol%. Thus, as shown in Figure 4, even when using natural gas as the feed gas, compared to using a feed gas containing only methane, the desired hydrocarbon conversion rate can be improved.

The contents recorded in the above embodiments can be understood, for example, as follows.

[1] The direct decomposition method for a single-state hydrocarbon is a direct decomposition method that directly decomposes a hydrocarbon into carbon and hydrogen, comprising: a step of bringing a feed gas containing methane and a hydrocarbon having two or more carbon atoms into contact with a non-supporting catalyst (2) of an aggregate of iron particles, wherein the concentration of the aforementioned hydrocarbon having two or more carbon atoms in the feed gas is 0.02 to 10 vol%.

The direct decomposition method for hydrocarbons disclosed herein can improve hydrocarbon conversion rates compared to using feed gas containing only methane.

[2] Another direct decomposition method for a hydrocarbon is the direct decomposition method for a hydrocarbon as described in [1], wherein the concentration of the aforementioned hydrocarbon having two or more carbon atoms in the aforementioned raw material gas is 5 to 9.7 vol.

Based on this composition, the hydrocarbon conversion rate can be increased compared to using feed gas containing only methane.

[3] Another direct decomposition method for hydrocarbons of another state is the direct decomposition method for hydrocarbons as in [1] or [2], wherein the particle size of the aforementioned plurality of particles ranges from 32 to 180 μm.

Based on this composition, the hydrocarbon conversion rate can be increased compared to using feed gas containing only methane.

[4] Another direct decomposition method for a hydrocarbon is the direct decomposition method for any of the hydrocarbons in [1] to [3], wherein the step of bringing the aforementioned raw material gas into contact with the aforementioned catalyst (2) is carried out in a temperature range of 600 to 900°C.

Based on this composition, the hydrocarbon conversion rate can be increased compared to using feed gas containing only methane.

[5] Another direct decomposition method of a hydrocarbon is the direct decomposition method of any of the hydrocarbons in [1] to [4], wherein a portion of the gas after the aforementioned raw material gas comes into contact with the aforementioned catalyst (2) is mixed with the aforementioned raw material gas and thus comes into contact with the aforementioned catalyst (2).

By setting the concentration of hydrocarbons with two or more carbon atoms in the feed gas to 0.02–10 vol%, the hydrocarbon conversion rate can be improved compared to using a feed gas containing only methane. However, when the concentration of hydrocarbons with two or more carbon atoms is 10 vol%, the improvement in hydrocarbon conversion rate is slightly lower compared to a concentration of 5 vol%. Since the gas after the raw material gas comes into contact with the catalyst contains hydrogen generated by the direct decomposition of hydrocarbons, even if the concentration of hydrocarbons with two or more carbon atoms in the raw material gas is 10 vol%, the concentration of hydrocarbons with two or more carbon atoms in the gas after the raw material gas comes into contact with the catalyst is lower than 10 vol%. Therefore, based on this composition [5], even if a raw material gas with a concentration of 10 vol% of hydrocarbons with two or more carbon atoms is used, Furthermore, by mixing the raw material gas with the catalyst and then allowing the gas to come into contact with the catalyst, and by allowing the raw material gas to come into contact with the catalyst when the concentration of hydrocarbons with two or more carbon atoms is lower than 10 vol%, the result is that even when using a raw material gas with a concentration of 10 vol% of hydrocarbons with two or more carbon atoms, the effect of improving the hydrocarbon conversion rate is close to that obtained when the concentration is 5 vol%.

[6] Another direct decomposition method for a hydrocarbon in another state is the direct decomposition method for any of the hydrocarbons in [5], wherein the aforementioned raw material gas is natural gas.

Since the concentration of hydrocarbons with two or more carbon atoms in natural gas is about 10 vol%, the effect of increasing hydrocarbon conversion rate is slightly lower compared to using a feedstock gas with a concentration of 5 vol% of hydrocarbons with two or more carbon atoms. However, for the same reasons as those given by the above-mentioned composition [5], even when using natural gas, an effect close to the hydrocarbon conversion rate increase obtained with a concentration of 5 vol% can be achieved. Furthermore, feedstock gases with a concentration of hydrocarbons with two or more carbon atoms lower than 10 vol% must be prepared separately, but if natural gas is used, the preparation of feedstock gases is not required, thus improving the operability of the direct hydrocarbon decomposition method.

1: Apparatus 2: Catalyst 3: Reactor 4: Heating Device 5: Raw Material Supply Pipeline 6: Reactant Gas Flow Pipeline 7: Solid-Gas Separator 8: Circulation Pipeline 9: Flow Control Valve 11: Hydrogen Purification Device 20: Experimental Apparatus 22: Catalyst 23: Reactor 24: Electric Furnace 25: Raw Material Supply Pipeline 26: Reactant Gas Flow Pipeline 27: Gas Chromatography (GC) System

[Figure 1] is a schematic diagram of the apparatus used to implement the direct decomposition method for a hydrocarbon of one embodiment disclosed herein. [Figure 2] is a schematic diagram of a modified example of the apparatus used to implement the direct decomposition method for a hydrocarbon of one embodiment disclosed herein. [Figure 3] is a schematic diagram of an experimental apparatus used to verify the effectiveness of the direct decomposition method for a hydrocarbon of one embodiment disclosed herein. [Figure 4] is a graph showing the experimental results of Examples 1-3 and Comparative Example 1 (the relationship between the concentration of hydrocarbons with two or more carbon atoms in the feed gas and the peak value of methane conversion rate).

Claims (4)

A method for the direct decomposition of hydrocarbons, comprising the steps of: a step of contacting a feed gas containing methane and hydrocarbons having two or more carbon atoms with a non-supporting catalyst, which is an aggregate of iron particles; a step of mixing a portion of the gas after the feed gas has contacted the catalyst with the feed gas; and a step of adjusting the flow rate of the portion of the gas after the feed gas has contacted the catalyst, wherein the adjustment step involves reducing the concentration of the hydrocarbons having two or more carbon atoms in the feed gas to 5–9.7 vol%. For example, the direct decomposition method of hydrocarbon in claim 1, wherein the particle size of the aforementioned plurality of particles ranges from 32 to 180 μm. For example, the direct decomposition method of the hydrocarbon in claim 1 or 2, wherein the step of bringing the aforementioned raw material gas into contact with the aforementioned catalyst is carried out in a temperature range of 600 to 900°C. For example, the direct decomposition method of the hydrocarbon in claim 1 or 2, wherein the aforementioned raw material gas is natural gas.