JP2024119779A - Catalyst for dehydration and hydrogenation of alcohol and method for producing same - Google Patents

Abstract

To provide a dehydration hydrogenation catalyst for alcohols that does not require reactions under extremely high pressure conditions or installation of expensive decarboxylation equipment when producing a first fuel gas primarily composed of methane through methanation of hydrogen and carbon oxide in the presence of a methanation catalyst, and is essential for preparing a second fuel gas with reduced hydrogen levels by a simple and economically beneficial method.SOLUTION: A dehydration hydrogenation catalyst for alcohols includes an inorganic oxide with palladium supported thereon, and a solid acid catalyst with no palladium supported thereon.SELECTED DRAWING: Figure 1

Description

The present invention relates to a catalyst for producing a fuel gas with a high calorific value and mainly composed of methane by methanation reaction of hydrogen and carbon oxides in the presence of a methanation catalyst, and more specifically to a catalyst for producing a fuel gas with a high hydrocarbon concentration and a reduced hydrogen concentration, and a method for producing the same.

In recent years, as a measure to combat global warming, attention has been focused on carbon-neutral fuels, which do not substantially increase the concentration of carbon dioxide in the atmosphere even when burned.

Methane can be obtained by recovering carbon dioxide from exhaust gases generated during industrial processes and thermal power generation, and reacting it with hydrogen obtained through electrolysis using electricity generated by renewable energy sources such as solar and wind power. Methane obtained in this way can be considered a carbon-neutral fuel that does not contribute to global warming, as no additional carbon dioxide is produced when it is burned.

The methanation reaction (Equation 1), in which carbon dioxide reacts with hydrogen to produce methane, is well known.

CO ₂ +4H ₂ → CH ₄ +2H ₂ O (Formula 1)
In Patent Document 1, when a gas containing CO and _H2 is methanated, a methanation reactor having a Cu-Zn-based low-temperature shift catalyst arranged on the upstream side and a methanation catalyst arranged on the downstream side is used. A method for methanating a gas containing CO and _H2 is disclosed, characterized in that the CO shift reaction (Equation 2) proceeds in the upstream low-temperature shift reactor, and the amount of carbon monoxide contained in the raw gas is reduced. Most of the carbon dioxide reacts with water vapor and is converted to carbon dioxide, and it is believed that the methanation reaction of carbon dioxide proceeds on the downstream methanation catalyst.

CO+H ₂ O → CO ₂ +H ₂ (Formula 2)
Methanation has long been used to remove carbon monoxide and carbon dioxide from hydrogen for ammonia synthesis, and it is known that catalysts carrying Ni or Ru exhibit high activity (non- Patent documents 1 and 2).

The methanation reaction, in which carbon oxides (carbon monoxide and carbon dioxide) are reacted with hydrogen to obtain methane, is an industrially established technology (e.g., Non-Patent Document 3), but there are still challenges to be overcome in obtaining fuel gas of a quality that can be used as a raw material for city gas.

Natural gas is commonly used as a raw material for city gas, and is composed mainly of methane, with small amounts of ethane, propane, and butane. Natural gas does not normally contain hydrogen or carbon monoxide, and carbon dioxide is removed during the natural gas refining process. In particular, in the case of city gas produced from liquefied natural gas, hydrogen, carbon monoxide, and carbon dioxide are almost completely removed during the liquefaction refining process, so it contains virtually no hydrogen, carbon monoxide, or carbon dioxide.

The concentration of hydrocarbons other than methane (ethane, propane, and butane) contained in natural gas varies depending on the natural gas's production area and refining method, so when producing city gas, propane or butane is usually added to adjust the calorific value to within a certain range, and an odorant is added to ensure safety before it is sent to consumers through city gas pipelines.

When hydrogen, carbon monoxide and carbon dioxide are present in city gas, they can cause the following problems:

Firstly, carbon monoxide is highly toxic, so if the gas leaks, there is a risk of poisoning. The permissible concentration is 200 ppm, and from a safety standpoint, it is desirable to keep the concentration in fuel gas below this level. Even taking into account dilution with air, it needs to be below 1000 ppm.

Secondly, carbon dioxide is not only non-flammable, but also acts to suppress combustion. Therefore, if it is mixed into fuel gas in high concentrations, it not only reduces the efficiency of gas transport in the ducts, which reduces the heating value of the fuel gas, but it may also cause a decrease in the efficiency of combustion equipment.

Finally, although hydrogen is a fuel gas, its calorific value per unit volume is only about one-third that of methane, the main component of city gas. Therefore, when hydrogen is mixed into a fuel gas whose main component is methane, the calorific value per unit volume decreases. Furthermore, hydrogen is known to have a large impact on combustion equipment due to its fast combustion speed.

As described above, when hydrogen, carbon monoxide and carbon dioxide are mixed into city gas, they have various effects at each stage of gas supply and consumption, so the quality standards for gas accepted into city gas pipeline networks generally set restrictions on the concentrations of hydrogen, carbon monoxide and carbon dioxide.

In pipeline networks with fuel filling stations for natural gas vehicles, there are known examples where the upper limit of hydrogen concentration is set at 2.0% by volume (Non-Patent Document 4). There are also known examples where the hydrogen concentration is set at 4.0% or less by volume, the carbon dioxide concentration at 0.50% or less by volume, and the carbon monoxide concentration at 0.050% or less by volume (Non-Patent Document 5), as well as examples where the total concentration of methane and ethane is set at 93% or more by volume, and the total concentration of components other than hydrocarbons at 4.0% or less by volume (Non-Patent Document 6).

The contamination of city gas with hydrogen, carbon monoxide, and carbon dioxide poses problems beyond the quality of the city gas. As mentioned above, city gas is supplied after being adjusted to a certain calorific value range by adding propane or butane to ensure stable use in combustion appliances. Therefore, if high concentrations of hydrogen or carbon dioxide are present, it is necessary to mix in large quantities of propane and butane, which are necessary to adjust the calorific value. This not only increases the cost of gas production, but also undermines the carbon neutrality of the produced gas if fossil fuel-derived propane and butane are used to adjust the calorific value.

The methanation reaction of carbon dioxide (Equation 1) is an equilibrium reaction, and under normal industrial operating conditions, carbon dioxide and hydrogen cannot be completely converted to methane. When a mixed gas with a stoichiometric ratio (hydrogen:carbon dioxide = 4:1) is reacted at normal pressure (0.10 MPa), the equilibrium conversion rate of carbon dioxide to methane is 95.0% when the reaction temperature is 300°C, 97.5% when the reaction temperature is 250°C, and 98.9% when the reaction temperature is 200°C.

Thus, at normal pressure, only fuel gas containing a large amount of hydrogen can be obtained. Because the methanation reaction is an exothermic reaction, the lower the temperature, the higher the equilibrium conversion rate; however, in the case of a catalytic reaction, the lower the temperature, the lower the catalytic activity. For this reason, there is a lower limit to the reaction temperature, and with a typical methanation catalyst, a temperature of 250°C or higher is required to achieve a practical reaction rate (Non-Patent Document 4, Patent Documents 2 and 3).

The methanation reaction of carbon dioxide (Equation 1) is a reaction in which the amount of substance (molar number) decreases, so the higher the pressure, the higher the equilibrium conversion rate. Comparing at 250°C, the equilibrium conversion rate of carbon dioxide to methane (when reacted at a stoichiometric ratio of hydrogen:carbon dioxide = 4:1) is 97.5% at normal pressure (0.10 MPa), but improves to 98.9% at 0.70 MPa and 99.5% at 5.0 MPa. However, even when a mixed gas of hydrogen:carbon dioxide = 4:1 is reacted at 250°C and 5.0 MPa, the equilibrium composition (composition by volume after dehydration excluding the water produced) is 97.45% methane, 2.04% hydrogen, and 0.51% carbon dioxide. Therefore, in order to obtain a fuel gas with 2.0% hydrogen (by volume) or less, the reaction must be carried out under high-pressure conditions exceeding 5.0 MPa.

There are several known methods for reducing the hydrogen concentration in fuel gas obtained by methanation reactions. For example, if methanation reactions are carried out under conditions where the amount of hydrogen is less than the stoichiometric ratio, and excess carbon dioxide is removed by decarbonation treatment, both the hydrogen content and the carbon dioxide content can be reduced (Patent Document 4). There is an example of this method being adopted in the methanation of coke oven gas (Non-Patent Document 3). However, decarbonation equipment generally requires high equipment costs and consumes a large amount of energy to operate, which reduces its economic viability.

As an alternative method, a method is known in which the methanation reaction is carried out in multiple stages, the gas produced in the intermediate stages is cooled, and the water is condensed and separated (Non-Patent Document 4, Patent Documents 2 and 3). By removing the produced water, the methanation reaction can be allowed to proceed further to the production side, improving the conversion rate of carbon dioxide to methane. However, this method requires heat exchange equipment, which increases the equipment costs, and there are also issues with the fact that if too much produced water is removed, the composition will enter the region where equilibrium carbon deposition occurs, making it difficult to control the water separation process.

Furthermore, in both the method of carrying out the methanation reaction under conditions of less hydrogen than the stoichiometric ratio and removing excess carbon dioxide by decarbonation treatment, and the method of carrying out the methanation reaction in multiple stages and cooling the gas produced in the intermediate stages to condense and separate the water, the conditions are favorable for the production of carbon monoxide in equilibrium, resulting in a problem of high carbon monoxide concentrations in the produced fuel gas.

Patent Document 5 and Patent Document 6 describe a method for producing a gas with reduced hydrogen by adding alcohol to a hydrogen-containing gas after a methanation reaction, and then reacting the alcohol with hydrogen in the hydrogen-containing gas in the presence of a dehydration hydrogenation catalyst. As an example of the catalyst, Patent Document 5 describes a catalyst in which tungstosilicic acid and palladium are supported on a silica carrier (Pd/SiW/silica catalyst), and Patent Document 6 describes a catalyst in which palladium is supported on tungsten oxide zirconia (Pd/WO ₃ /ZrO ₂ catalyst). It is described that the Pd/SiW/silica catalyst and the Pd/WO ₃ /ZrO ₂ catalyst exemplified in Patent Document 5 and Patent Document 6 decompose a part of the alcohol by a steam reforming reaction and convert it into carbon monoxide, carbon dioxide, and hydrogen, but this can be a disadvantage in using the generated fuel gas as city gas.

Japanese Patent Application Publication No. 60-235893 JP 2015-124217 A JP 2018-135283 A JP 2019-26595 A JP 2021-138927 A JP 2022-133257 A

Chemical Engineering Society, ed., Chemical Process Collection, 1970, p. 153 Catalysis Society of Japan, Catalyst Handbook, 2008, p. 535 Kawagoe, Matsuda, Matsushima and Uematsu, Hitachi Review, Vol. 68, No. 10, 1986, p. 73 E. I. Koytsoumpa and S. Karellas, Renewable and Sustainable Energy Reviews, Vol. 94, 2018, p. 536 Biogas Purchasing Guidelines, Osaka Gas Co., Ltd., 2008 Biogas Purchasing Guidelines, Tokyo Gas Co., Ltd., 2008

In view of the above problems, the problem that the present invention aims to solve is to provide a catalyst and a method for producing the catalyst necessary to obtain fuel gas with a reduced hydrogen concentration in a simple and economical manner, without requiring reactions under extremely high pressure conditions or the installation of expensive decarbonation equipment, when producing fuel gas mainly composed of methane by a methanation reaction between hydrogen and carbon oxides in the presence of a methanation catalyst.

The alcohol dehydration and hydrogenation catalyst according to the present invention is characterized in that it is a catalyst for dehydrating and hydrogenating alcohol, which is used to convert a first fuel gas containing 2.0% to 20% hydrogen by volume after dehydration, obtained by subjecting hydrogen and carbon oxides to a methanation reaction in the presence of a methanation catalyst, into a second fuel gas containing alkanes having 2 to 4 carbon atoms and having a reduced hydrogen concentration, by treating the first fuel gas in the presence of an alcohol having 2 to 4 carbon atoms, and the first fuel gas contains an inorganic oxide carrying palladium and a solid acid catalyst not carrying palladium.

This characteristic configuration allows the reaction between hydrogen and alcohols having 2 to 4 carbon atoms to proceed at a sufficient rate while suppressing the steam reforming reaction that produces hydrogen and carbon dioxide, effectively producing a second fuel gas that has a lower hydrogen concentration on a volumetric basis after dehydration than the first fuel gas. In addition, this catalyst has high resistance to high temperatures and can be used stably for long periods of time even under high temperature conditions.

The characteristic configuration of the alcohol dehydration hydrogenation catalyst according to the present invention is that the amount of palladium supported on the inorganic oxide is 0.2% by mass or more and 15% by mass or less, and the solid acid catalyst on which palladium is not supported is contained in a mass ratio of 1.0 to 10, with the mass of the inorganic oxide on which palladium is supported being taken as 1.

According to this characteristic configuration, an appropriate amount of palladium is supported on the inorganic oxide, and a solid acid catalyst not supporting palladium is included in the above mass ratio, resulting in an alcohol dehydration/hydrogenation catalyst with high ethane selectivity.

The characteristic feature of the alcohol dehydration hydrogenation catalyst according to the present invention is that the solid acid catalyst is a SiW/silica catalyst or tungsten oxide zirconia.

This characteristic configuration allows the reaction between hydrogen and alcohol to occur at a sufficient rate, making it possible to produce a catalyst that can effectively produce fuel gas with a reduced hydrogen concentration.

The characteristic configuration of the method for producing an alcohol dehydration hydrogenation catalyst according to the present invention is that the method includes a mixing step of mixing an inorganic oxide carrying palladium with a solid acid catalyst not carrying palladium, and a mixing step of mixing a first fuel gas containing 2.0% to 20% hydrogen by volume after dehydration, which is obtained by subjecting hydrogen and carbon oxides to a methanation reaction in the presence of a methanation catalyst, in the presence of an alcohol having a carbon number of 2 to 4, to convert the first fuel gas into a second fuel gas containing alkanes having a carbon number of 2 to 4 and having a reduced hydrogen concentration.

According to this characteristic configuration, the reaction between hydrogen and alcohol having 2 to 4 carbon atoms is carried out at a sufficient rate while the steam reforming reaction that produces hydrogen and carbon dioxide is suppressed, so that it is possible to effectively obtain a second fuel gas having a lower hydrogen concentration on a volumetric basis after dehydration than the first fuel gas, and it is possible to produce an alcohol dehydration and hydrogenation catalyst that has high high-temperature resistance and can be used stably for a long period of time even under high-temperature conditions. In addition, since it is not necessary to bake a solid acid catalyst that does not support palladium at high temperatures, a solid acid catalyst with low heat resistance can be used. In addition, when a commercially available catalyst in which palladium is supported on an inorganic oxide carrier is used as the inorganic oxide on which palladium is supported, there is no need to perform an impregnation process to impregnate the carrier with palladium, making it possible to economically produce the catalyst.

The characteristic configuration of the method for producing an alcohol dehydration hydrogenation catalyst according to the present invention is that in the mixing step, the inorganic oxide carrying 0.2% by mass or more and 15% by mass or less of palladium and the solid acid catalyst not carrying palladium are mixed in a mass ratio of 1.0 to 10, where the mass of the inorganic oxide carrying palladium is taken as 1.

According to this characteristic configuration, an appropriate amount of palladium is supported on the inorganic oxide, and by mixing a solid acid catalyst on which palladium is not supported in the above mass ratio, an alcohol dehydration and hydrogenation catalyst with high ethane selectivity can be obtained.

A further characteristic feature of the method for producing an alcohol dehydration hydrogenation catalyst according to the present invention is that it includes a calcination step, prior to the mixing step, in which the catalyst precursor containing palladium is calcined at 400°C or higher to obtain the inorganic oxide on which palladium is supported.

This characteristic configuration makes it possible to produce an alcohol dehydration hydrogenation catalyst with high ethane selectivity. In addition, since it is not necessary to calcinate the solid acid catalyst that does not support palladium, which is mixed in later, at high temperatures, a solid acid catalyst with low heat resistance can be used.

The characteristic feature of the method for producing alcohol dehydration and hydrogenation according to the present invention is that the solid acid catalyst is a SiW/silica catalyst or tungsten oxide zirconia.

This characteristic configuration allows the reaction between hydrogen and alcohol to occur at a sufficient rate, making it possible to produce a catalyst that can effectively produce fuel gas with a reduced hydrogen concentration.

FIG. 1 is a block flow diagram showing a method for producing fuel gas using the alcohol dehydration/hydrogenation catalyst of the present invention. FIG. 1 shows the effect of temperature and pressure on the equilibrium conversion of the methanation reaction of carbon dioxide. FIG. 3 is an expanded view of the region of equilibrium conversion of 80-100% in FIG. 2. 1 is an example of a reactor configuration for a methanation reaction (fixed-bed adiabatic multi-stage reactor). FIG. 2 is a diagram showing the conversion rate of carbon dioxide to methane and the gas temperature in each reaction stage when a methanation reaction is carried out using a fixed-bed adiabatic multi-stage reactor. This is an example of the configuration of a reactor for a methanation reaction (a fixed-bed adiabatic multi-stage reactor having a recycle line that returns the first-stage outlet gas to the first-stage inlet). This is an example of the structure of a reactor for a methanation reaction (a heat exchange reactor). This is an example of the configuration of a reactor for a methanation reaction (the first stage is a fixed-bed adiabatic reactor with a recycle line, and the second stage is a heat exchange type reactor). FIG. 13 is a diagram showing the effect of the inlet hydrogen/carbon dioxide ratio on the hydrogen and carbon dioxide concentrations in the equilibrium composition (after dehydration) of the carbon dioxide methanation reaction (reaction pressure 0.70 MPa). FIG. 13 is a diagram showing the effect of the inlet hydrogen/carbon dioxide ratio on the hydrogen and carbon dioxide concentrations in the equilibrium composition (after dehydration) of the carbon dioxide methanation reaction (reaction pressure 5.0 MPa). FIG. 1 is a diagram showing the effect of the inlet hydrogen/carbon dioxide ratio on the hydrogen and carbon dioxide concentrations in the equilibrium composition (after dehydration) of fuel gas obtained by a fuel gas production method using an alcohol dehydration hydrogenation catalyst of the present invention (ethanol/hydrogen molar ratio: 0.90). FIG. 1 is a diagram showing the effect of the inlet hydrogen/carbon dioxide ratio on the hydrogen and carbon dioxide concentrations in the equilibrium composition (after dehydration) of fuel gas obtained by a fuel gas production method using an alcohol dehydration hydrogenation catalyst of the present invention (ethanol/hydrogen molar ratio: 0.50).

[Embodiment]
Hereinafter, embodiments of the alcohol dehydration/hydrogenation catalyst for producing a fuel gas and the method for producing the alcohol dehydration/hydrogenation catalyst according to the present invention will be described.

Figure 1 is a block flow diagram showing a method for producing fuel gas using the alcohol dehydration/hydrogenation catalyst of the present invention. The method for producing fuel gas using the alcohol dehydration/hydrogenation catalyst of this embodiment has a methanation reaction process (methanation reactor 10) and a dehydration/hydrogenation reaction process (dehydration/hydrogenation reactor 20).

In the methanation reaction process, the mixed gas 1 of hydrogen and carbon oxides is brought into contact with a methanation catalyst packed in a methanation reactor 10 to carry out a methanation reaction. This methanation reaction produces a fuel gas 2 (an example of a first fuel gas, the same applies below) that is mainly composed of methane and contains 2.0% to 20% hydrogen by volume after dehydration.

In the dehydration hydrogenation reaction process, first, alcohol 3 having 2 to 4 carbon atoms is added to the fuel gas 2 obtained in the methanation reaction process via a flow rate control valve 30. Then, the fuel gas 2 to which alcohol 3 having 2 to 4 carbon atoms is added is fed to the dehydration hydrogenation reactor 20, and the fuel gas 2 to which alcohol 3 having 2 to 4 carbon atoms is added is brought into contact with a dehydration hydrogenation catalyst filled in the dehydration hydrogenation reactor 20, and an alkane having 2 to 4 carbon atoms is produced by a reaction between the alcohol having 2 to 4 carbon atoms and hydrogen. This reaction consumes the hydrogen contained in the fuel gas 2, and a fuel gas 4 (second fuel gas) having a reduced hydrogen concentration is obtained. The obtained fuel gas 4 contains methane as a main component and an alkane having 2 to 4 carbon atoms. The alcohol 3 is one or more alcohols selected from alcohols having 2 to 4 carbon atoms, and the carbon number of the alkane contained in the second fuel gas 4 is the same as that of the alcohol 3 used.

[Methanation reaction process conditions]
The hydrogen and carbon oxides used in the mixed gas 1 in the methanation reaction step may be produced by any method as long as they have a purity and properties that do not interfere with the methanation reaction in the presence of a methanation catalyst. The hydrogen may be electrolytic hydrogen obtained by electrolyzing water, for example. The carbon oxides are carbon monoxide or carbon dioxide, or a mixture thereof. The carbon dioxide may be carbon dioxide recovered from a combustion exhaust gas by a known carbon dioxide recovery method such as an amine absorption method, or may be carbon dioxide contained in a biogas obtained by methane fermentation of organic matter. Biogas is usually obtained as a mixed gas of methane and carbon dioxide, and carbon dioxide may be separated from this mixed gas and used in the methanation reaction, or methane may be supplied to the methanation reaction without being separated.

If the hydrogen and carbon oxides contain sulfur, halogen compounds, siloxane compounds, heavy hydrocarbons, etc., these may cause deterioration of the methanation reaction catalyst, so it is preferable to remove these, if necessary, before subjecting the mixture to the methanation reaction.

The methanation catalyst used in the methanation reaction process can be a known methanation catalyst containing Ni, Ru, or the like.

The type of the methanation reactor 10 used in the methanation reaction step is not particularly limited as long as the hydrogen concentration of the fuel gas 2 after the methanation reaction is 2.0% or more and 20% or less on a volumetric basis after dehydration.
However, the methanation reaction is an equilibrium reaction that generates a relatively large amount of heat, and the equilibrium conversion rate varies greatly with temperature and pressure, becoming higher at lower temperatures and higher pressures, as shown in Figures 2 and 3. On the other hand, there are problems in that it is difficult to ensure the activity of the methanation catalyst at low temperatures, and that the equipment costs are high at high pressures.

In addition, since the methanation reaction generates a relatively large amount of heat, the gas temperature rises as the reaction progresses, and the equilibrium conversion rate decreases accordingly, which cannot be ignored. For these reasons, when using a fixed-bed adiabatic reactor, which is the most commonly used in chemical processes, as the methanation reactor 10, it is usually difficult to obtain the desired conversion rate in one stage, so it is better to connect the reactors in multiple stages and cool the outlet gas, whose temperature has risen due to the heat of reaction, before introducing it into the next stage reactor.

Figure 4 shows an example of the configuration of the methanation reactor 10 when it is set as a fixed-bed adiabatic multi-stage reactor. In this reactor configuration, five sets of reactors (11a-15a) in which the methanation reaction proceeds and heat exchangers (11b-15b) are provided. In the configuration of the methanation reactor 10 shown in Figure 4, when the raw material gas with a stoichiometric ratio of hydrogen:carbon dioxide = 4:1 (molar ratio) is reacted at an inlet temperature of 250°C and a reaction pressure of 0.70 MPa, the temperatures at the inlet and outlet of each stage and the conversion rate of carbon dioxide to methane are as shown in Figure 5. At the outlet of the fourth stage (14a), the conversion rate of carbon dioxide to methane is 94.3%, and at this stage, the hydrogen concentration in the gas (based on the volume after dehydration with moisture removed; the same applies below) is 18.3%, which can be reduced to 20% or less. At the outlet of the fifth stage (15a), the conversion rate of carbon dioxide to methane is 97.7%, and the hydrogen concentration in the gas drops to 8.3%, making it possible to reduce it to 10% or less. Therefore, when methanation is performed by a simple adiabatic multi-stage reaction, under the above temperature and pressure conditions, a four-stage reaction is required to reduce hydrogen to 20% or less by volume, and a five-stage reaction is required to reduce it to 10% or less. In addition, the large heat generated by methanation causes the outlet temperature of the first stage to reach approximately 700°C, which is problematic in terms of catalyst durability.

Figure 6 shows an example of the configuration when the methanation reactor 10 is provided as a fixed-bed adiabatic multi-stage reactor with a recycle line. In this reactor configuration, three sets of reactors (11a-13a) in which the methanation reaction proceeds and heat exchangers (11b-13b) are provided, and the first set of these is provided with a recycle line 11c that returns the product after heat exchange to the reactor 11a. When this reactor configuration is used, the durability of the catalyst is improved because the dilution effect suppresses temperature rise. In addition, the lower reactor outlet temperature has the effect of increasing the equilibrium conversion rate, so that it may be possible to reduce the number of reactor stages compared to a simple multi-stage reactor configuration.

Figure 7 shows an example of the configuration when the methanation reactor 10 is provided as a heat exchange type reactor. In this reactor configuration, the reaction can be carried out while maintaining the catalyst layer temperature at a specified temperature, which makes it possible to obtain a high conversion rate in a single-stage reactor. However, heat exchange type reactors have a complex structure and may require high equipment and maintenance costs. In addition, if the balance between the heat generated by the reaction and the heat removed by heat exchange is lost, localized high-temperature areas may occur, causing the catalyst to deteriorate in a short period of time.

Figure 8 shows an example of a methanation reactor 10 configured by combining a fixed-bed adiabatic reactor with a recycle line and a heat exchange reactor. In this reactor configuration, the set of reactors (12a, 13a) and heat exchangers (12b, 13b) from the second stage onwards in the example of Figure 6 is replaced with a single heat exchange reactor. With this reactor configuration, it is possible to reduce the number of reactor stages and suppress equipment costs while ensuring the durability of the catalyst.

[Example of methanation reaction process]
In the following description, it is assumed that the methanation reactor 10 has the reactor configuration shown in FIG. 8, a sufficient amount of methanation catalyst is used, and an equilibrium composition for the methanation reaction and the CO shift reaction is obtained under specified conditions.

When the molar ratio of hydrogen to carbon dioxide at the inlet (hydrogen/carbon dioxide molar ratio) is in the range of 3.8 to 4.2 and the second-stage reaction temperature is 250°C, the concentrations of hydrogen and carbon dioxide contained in the gas after the methanation reaction are as shown in Figure 9 when the reaction pressure is 0.70 MPa. When the hydrogen/carbon dioxide molar ratio is 4.0, the hydrogen concentration is 4.4% and the carbon dioxide concentration is 1.1%. Reducing the hydrogen/carbon dioxide molar ratio reduces the hydrogen concentration, but even if the molar ratio is reduced to 3.8, the hydrogen concentration does not fall below 2.0% and the carbon dioxide concentration rises to 5.6%. In other words, when the reaction pressure is 0.70 MPa, it is not possible to obtain fuel gas with sufficiently reduced hydrogen and carbon dioxide concentrations.

When the reaction pressure is 5.0 MPa, the hydrogen and carbon dioxide contained in the gas after the methanation reaction are as shown in Figure 10. When the hydrogen/carbon dioxide molar ratio is 4.0, the hydrogen concentration is 2.04% and the carbon dioxide concentration is 0.51%. Under these conditions, a fuel gas can be obtained that generally meets the quality standard of a hydrogen concentration of 2% or less and a carbon dioxide concentration of 0.50% or less (this standard is the strictest among the generally known standards). However, under these conditions, even a slight decrease in the hydrogen/carbon dioxide molar ratio increases the carbon dioxide concentration, and conversely, a slight increase in the hydrogen/carbon dioxide molar ratio increases the hydrogen concentration. In this way, even a slight change in the hydrogen/carbon dioxide molar ratio greatly affects the purity of the resulting fuel gas.

In other words, when obtaining fuel gas whose main component is methane only through the methanation reaction between hydrogen and carbon dioxide, in order to obtain fuel gas that meets the quality standards of a hydrogen concentration of 2.0% or less and a carbon dioxide concentration of 0.50% or less, a high-pressure reaction facility with an operating pressure sufficiently higher than 5.0 MPa is required.

[Conditions for the dehydration/hydrogenation reaction step]
As the alcohol 3 having 2 to 4 carbon atoms used in the dehydration hydrogenation reaction step, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, and 2-methyl-2-propanol can be used. These may be used alone or in combination.

The alcohol 3 having 2 to 4 carbon atoms may be produced by any method as long as it has a purity and properties that do not interfere with the dehydration hydrogenation reaction. For example, ethanol produced by fermentation using sugar cane or corn as a raw material can be suitably used. In the fuel gas production method according to this embodiment, the alcohol 3 having 2 to 4 carbon atoms may contain a normal amount of water. However, if the water content is too high, evaporation heat is required to vaporize the water, which may reduce the efficiency of fuel gas production. In consideration of this, the water content of the alcohol 3 having 2 to 4 carbon atoms is preferably 50 mass% or less.

The reaction in the dehydration hydrogenation reactor 20 proceeds as follows.
C _n H _{2n + 1} OH → C _n H _2n + H ₂ O (Formula 3)
C _n H _2n + H ₂ → C _n H _2n+2 (Formula 4)
When the amount of the alcohol 3 having 2 to 4 carbon atoms added is small, the reduction in the hydrogen concentration in the fuel gas 4 is insufficient. On the other hand, when the amount of the alcohol 3 having 2 to 4 carbon atoms added is large, the hydrogen concentration becomes extremely low, so that the hydrogenation reaction (formula 4) of the alkene (ethylene when ethanol is used as the alcohol) generated by the dehydration of the alcohol 3 having 2 to 4 carbon atoms does not proceed sufficiently, and the alkene may remain in the fuel gas 4. In addition, since the alkene concentration during the reaction is high, the alkene may polymerize on the catalyst, causing deterioration of the catalyst due to carbon deposition. Therefore, the amount of the alcohol 3 having 2 to 4 carbon atoms added is preferably such that the molar ratio (alcohol/hydrogen) of the alcohol 3 having 2 to 4 carbon atoms to the hydrogen contained in the fuel gas 2 obtained in the methanation reaction step is 0.45 to 0.90.

The reaction temperature when converting alcohol to alkanes by reaction with hydrogen using a dehydration hydrogenation catalyst is preferably 200°C or higher and 400°C or lower. Since dehydration hydrogenation catalysts generally exhibit good activity at 200°C or higher, the reaction between alcohol 3 having 2 to 4 carbon atoms and hydrogen (Equation 3 and Equation 4) is likely to proceed when the reaction temperature is 200°C or higher. In addition, the steam reforming reaction of methane, ethane, propane, and butane is easily suppressed when the reaction temperature is 400°C or lower. When the steam reforming reaction of methane, ethane, propane, and butane proceeds, even if hydrogen is reduced by the hydrogenation reaction of alkenes, new hydrogen is generated by the steam reforming reaction of methane, ethane, propane, or butane, so that the hydrogen concentration in the fuel gas may not be reduced. The reaction temperature is more preferably 250°C or higher and 350°C or lower.

The type of dehydration/hydrogenation reactor 20 used in the dehydration/hydrogenation reaction process is not particularly limited, and may be, for example, a fixed-bed adiabatic reactor, a fixed-bed adiabatic reactor with a recycle line, a heat exchange reactor, etc.

[Alcohol dehydration hydrogenation catalyst]
The catalyst for dehydration and hydrogenation of alcohols of the present invention includes an inorganic oxide on which palladium is supported and a solid acid catalyst on which palladium is not supported.
In the present invention, the palladium supported on the carrier is usually considered to be supported in the form of a salt such as a nitrate or chloride before calcination, in the form of palladium oxide after calcination, and in the metallic state after reduction treatment. In the present invention, these various forms of palladium are collectively referred to as palladium.

As described in Patent Documents 5 and 6, when producing an alcohol dehydration and hydrogenation catalyst containing palladium and a solid acid catalyst, a method of directly impregnating and supporting palladium on the solid acid catalyst is usually adopted. As a result, palladium is highly dispersed and uniformly supported in the form of fine particles, and it is presumed that when this catalyst is used, an alkene produced by dehydrating alcohol on the solid acid catalyst is rapidly hydrogenated by the palladium to produce an alkane.

However, the inventors have discovered that an alcohol dehydration/hydrogenation catalyst that contains an inorganic oxide on which palladium is supported and a solid acid catalyst on which palladium is not supported advances the alcohol dehydration/hydrogenation reaction with high selectivity.

The reason why the alcohol dehydration/hydrogenation catalyst of the present invention is able to promote the alcohol dehydration/hydrogenation reaction with high selectivity is not entirely clear, but the inventors speculate that it has two effects.

The first effect is that the alcohol dehydration/hydrogenation catalyst of the present invention is less likely to cause a steam reforming reaction of alcohol due to the large particle size of the palladium, resulting in an improved selectivity for the alcohol dehydration/hydrogenation reaction.

As described above, the dehydration and hydrogenation reaction of an alcohol using a catalyst containing palladium and a solid acid catalyst proceeds as follows.
C _n H _{2n + 1} OH → C _n H _2n + H ₂ O (Formula 3)
C _n H _2n + H ₂ → C _n H _2n+2 (Formula 4)

At the same time, the steam reforming reaction of alcohol proceeds as follows.
C _n H _2n+1 OH + (n-1)H ₂ O → nCO + 2nH ₂ (Formula 5)
C _n H _2n+1 OH + (2n-1)H ₂ O → nCO ₂ + 3nH ₂ (Formula 6)

Here, the palladium contained in the alcohol dehydration hydrogenation catalyst is active in both the hydrogenation reaction of alkene shown in formula 4 and the steam reforming reaction of alcohol shown in formulas 5 and 6. Here, since it is known that the hydrogenation reaction of alkene shown in formula 4 proceeds at a sufficiently high speed, it is considered that when the particle size of palladium contained in the catalyst becomes large, only the steam reforming reaction of alcohol shown in formulas 5 and 6 is suppressed, and as a result, the selectivity of the dehydration hydrogenation reaction of alcohol is improved. Since the catalyst of the present invention contains an inorganic oxide on which palladium is supported and a solid acid catalyst on which palladium is not supported, the concentration of palladium supported on the inorganic oxide is high. For example, in a catalyst in which silica supporting 10% by mass of palladium is mixed with a SiW/silica catalyst having a mass 9 times that of the silica, the palladium contained in the catalyst is 1.0% by mass, but in the supporting process (impregnation process), 10% by mass of palladium is supported on the silica. When a high concentration of palladium is supported, palladium is likely to aggregate and the palladium particle size is likely to become large.

The second effect is that the alcohol dehydration hydrogenation catalyst of the present invention has high acid strength because it contains a solid acid catalyst that does not support palladium. When the acid strength of the catalyst is high, the dehydration of alcohol shown in formula 3 proceeds more quickly. Since the steam reforming of alcohol shown in formulas 5 and 6 is in a competitive relationship with the dehydration of alcohol, when the acid strength is high, the steam reforming of alcohol does not occur easily, and the selectivity of the alcohol dehydration hydrogenation reaction is improved.

The carrier that supports palladium may or may not be a solid acid. Examples include silica, alumina, titania, zirconia, zeolites such as high silica zeolites, heteropolyacids such as tungstosilicic acid, and composite oxides such as tungsten oxide zirconia, with tungstosilicic acid and tungsten oxide zirconia being preferred.

When using only an alcohol dehydration/hydrogenation catalyst in which palladium is directly supported on a solid acid catalyst, there is a risk that the strong acid sites on the solid acid catalyst will be covered by palladium, making it difficult for the alcohol dehydration/hydrogenation reaction to proceed. On the other hand, the alcohol dehydration/hydrogenation catalyst of the present invention contains an inorganic oxide on which palladium is supported and a solid acid catalyst on which palladium is not supported, and therefore the acid strength of the catalyst is high.

In the present invention, there are no particular limitations on the solid acid catalyst and inorganic oxide used, but in order to cause dehydration and hydrogenation of alcohol at temperatures between 250°C and 350°C, those with high acid strength are suitable. From this perspective, high-silica zeolites, heteropolyacids, and composite oxides are preferred, with tungstosilicic acid and tungsten oxide zirconia being the most suitable.

The content (concentration) of palladium in the alcohol dehydration hydrogenation catalyst is not particularly limited, but is preferably 0.040% by mass or more and 5.0% by mass or less, more preferably 0.050% by mass or more and 5.0% by mass or less, even more preferably 0.060% by mass or more and 2.0% by mass or less, even more preferably 0.080% by mass or more and 1% by mass or less, and most preferably 0.10% by mass or more and 0.50% by mass or less, based on the total weight of the alcohol dehydration hydrogenation catalyst. If the amount of palladium supported is less than 0.040% by mass, the catalytic activity will be low, whereas if it is more than 5.0% by mass, the amount of palladium used will increase, resulting in poor economic efficiency.

The amount of palladium supported (support concentration) in the inorganic oxide is not particularly limited, but is preferably 0.2 mass% or more and 15 mass% or less, more preferably 1.0 mass% or more and 15 mass% or less, even more preferably 2.0 mass% or more and 10 mass% or less, and most preferably 5.0 mass% or more and 8.0 mass% or less, based on the weight of the inorganic oxide. By making the amount of palladium supported 0.2 mass% or more, the particle size of palladium becomes large, and the proportion of solid acid catalyst not supporting palladium in the catalyst increases, and the acid strength of the catalyst increases. If the amount of palladium supported is less than 0.2 mass%, there is a risk that sufficient performance as a dehydration hydrogenation catalyst for alcohol cannot be exhibited. If the amount of palladium supported is greater than 15 mass%, the amount of palladium used increases, resulting in reduced economic efficiency.

The mass ratio of the inorganic oxide carrying palladium to the solid acid catalyst not carrying palladium is preferably 1.0 to 10, more preferably 2.0 to 8.0, and even more preferably 3.0 to 6.0, with the inorganic oxide carrying palladium being taken as 1.0. If the mass ratio of the solid acid catalyst not carrying palladium to the inorganic oxide carrying palladium is less than 1.0, there is a risk that the amount of solid acid catalyst will be insufficient and the dehydration and hydrogenation of alcohol will not proceed smoothly. If the mass ratio of the solid acid catalyst not carrying palladium to the inorganic oxide carrying palladium is greater than 10, there is a risk that the amount of palladium carried will be insufficient and the dehydration and hydrogenation of alcohol will not proceed smoothly.

It is preferable that the dehydration/hydrogenation catalyst used in the dehydration/hydrogenation reaction step is active in the dehydration reaction of alcohols (Formula 3) and the hydrogenation reaction of alkenes (Formula 4), and is substantially inactive in the steam reforming reaction of methane, ethane, propane, butane, and alcohol. It is also preferable that the dehydration/hydrogenation catalyst can be used stably even at high temperatures. If the catalyst has low high-temperature resistance, the dehydration/hydrogenation reaction cannot proceed stably for a long period of time, and the catalyst needs to be replaced more frequently, resulting in high operating costs for the entire process.

[Method for producing an alcohol dehydration/hydrogenation catalyst]
The method for producing a catalyst for dehydration and hydrogenation of alcohols of the present invention includes an impregnation step of immersing an inorganic oxide in an aqueous solution containing palladium to obtain a catalyst precursor containing palladium, and a calcination step of calcining the catalyst precursor at a predetermined temperature.

[Preparation of Support]
First, a support for supporting the catalyst is prepared. The support may be the above-mentioned commercially available inorganic oxide, or may be prepared by calcining a precursor of the inorganic oxide to be used.

[Impregnation process]
In the impregnation step, a carrier such as an inorganic oxide is usually impregnated with an aqueous solution containing palladium and dried at a predetermined temperature to obtain a catalyst precursor containing palladium. This catalyst precursor is dried to fix palladium to the carrier. There is no particular restriction on the method of supporting palladium in the impregnation step, as long as the palladium is in the form of fine particles having sufficient activity for hydrogenating alkenes in the subsequent calcination step. There is no particular restriction on the concentration of the aqueous solution containing palladium, but in order to support an appropriate amount of palladium on the catalyst, it is preferably 0.10% by mass or more and 20% by mass or less, more preferably 0.10% by mass or more and 10% by mass or less.
When preparing the aqueous solution containing palladium, palladium nitrate, palladium chloride, etc. can be used. The amount of the aqueous solution containing palladium is appropriately determined depending on the water absorption amount of the carrier such as an inorganic oxide.

[Firing process]
In the subsequent calcination step, the catalyst precursor containing palladium is calcined at a predetermined temperature to obtain an inorganic oxide carrying palladium. By calcining the inorganic oxide impregnated with an aqueous solution containing palladium at a high temperature, the particle size of palladium can be increased, and ethane selectivity is improved. From the viewpoint of dispersibility of palladium on the carrier, the calcination temperature is preferably 300° C. or higher, and more preferably 400° C. or higher and 900° C. or lower. By setting the temperature range, palladium particles are supported on the inorganic oxide with appropriate particle size and dispersibility, so that an alcohol dehydration hydrogenation catalyst having excellent ethane selectivity can be produced. In particular, when palladium is supported on an inorganic oxide such as alumina, which is heat-resistant as an inorganic oxide, a solid acid catalyst that is not suitable for high-temperature calcination, such as SiW/silica, which is mixed later, does not need to be calcined at high temperatures, so that an alcohol dehydration hydrogenation catalyst having high ethane selectivity can be obtained.
If the calcination temperature is less than 300° C., the palladium impregnated in the inorganic oxide may not be formed into particles of sufficient size. If the calcination temperature is more than 900° C., the structure of the inorganic oxide may be destroyed, the inorganic oxide may be sintered to significantly reduce the surface area, and the aggregation of palladium may significantly progress. If these phenomena occur, the activity of the catalyst may be significantly reduced, and the dehydration and hydrogenation reaction of alcohol may not proceed sufficiently.
The amount of palladium supported on the inorganic oxide obtained through the calcination step is preferably 0.2% by mass or more and 15% by mass or less, more preferably 2.0% by mass or more and 10% by mass or less, and even more preferably 5.0% by mass or more and 8.0% by mass or less.

[Mixing process]
In the mixing step, the inorganic oxide carrying palladium and the solid acid catalyst not carrying palladium are mixed in a container. With this configuration, it is not necessary to calcinate the solid acid catalyst not carrying palladium at a high temperature, which is to be mixed later, so that a solid acid catalyst with low heat resistance can be used. The mass ratio of the inorganic oxide carrying palladium to the solid acid catalyst not carrying palladium is preferably 1.0 to 10, more preferably 2.0 to 8.0, and even more preferably 3.0 to 6.0, with the inorganic oxide carrying palladium being 1. If the mass ratio of the solid acid catalyst not carrying palladium to the inorganic oxide carrying palladium is less than 1.0, there is a risk that the solid acid catalyst will be insufficient and dehydration of alcohol will not proceed easily. If the mass ratio of the solid acid catalyst not supporting palladium to the inorganic oxide supporting palladium is greater than 10, the amount of palladium supported (supporting concentration) may be insufficient, and the dehydration and hydrogenation of alcohol may not proceed smoothly.
When only an alcohol dehydration/hydrogenation catalyst in which palladium is directly supported on a solid acid catalyst is used, there is a risk that the strong acid sites on the solid acid catalyst are covered with palladium in the palladium supporting step, making it difficult for the alcohol dehydration/hydrogenation reaction to proceed. On the other hand, in the alcohol dehydration/hydrogenation catalyst of the present invention, after the inorganic oxide on which palladium is supported is synthesized, a mixing step is performed in which a solid acid catalyst on which palladium is not supported is mixed, and therefore the acid strength of the catalyst is increased.

[Example of dehydration hydrogenation reaction process]
8 is used as the methanation reactor 10, and a dehydration/hydrogenation reactor 20 (not shown) configured as a fixed-bed adiabatic reactor is provided downstream of the methanation reactor 10. In this example, it is assumed that the methanation reaction and the CO shift reaction reach equilibrium in the methanation reactor 10, ethanol is used as the alcohol 3 having a carbon number of 2 to 4, and the dehydration reaction of ethanol and the hydrogenation reaction of ethylene reach equilibrium in the dehydration/hydrogenation reactor 20.

In the following example, the reaction temperature of the heat exchange reactor 12d in the methanation reactor 10 was set to 250°C. The inlet temperature of the dehydration hydrogenation reactor 20 was set to 250°C so that the reaction proceeded under adiabatic conditions. The reaction pressure was set to 0.70 MPa in both the methanation reactor 10 and the dehydration hydrogenation reactor 20. The hydrogen and carbon dioxide concentrations of the fuel gas 4 obtained in this case are shown in Figures 11 and 12 relative to the hydrogen/carbon dioxide molar ratio of the mixed gas 1 supplied to the methanation reactor 10.

Figure 11 shows the relationship between the composition of mixed gas 1 and the concentrations of hydrogen and carbon dioxide in fuel gas 4 when the molar ratio of ethanol to hydrogen contained in fuel gas 2 is 0.90. When the hydrogen/carbon dioxide molar ratio of mixed gas 1 is 4.0, the hydrogen concentration is 0.44% and the carbon dioxide concentration is 1.09%. When only the methanation reaction process is performed under similar conditions, the hydrogen concentration is 4.4% and the carbon dioxide concentration is 1.1% (Figure 9), so it can be seen that the hydrogen concentration of fuel gas 2 obtained by the methanation process is significantly reduced by the dehydration reaction process. Also, when the hydrogen/carbon dioxide molar ratio is 4.04, the hydrogen concentration is 0.55%, the carbon dioxide concentration is 0.43%, the methane concentration is 94.1%, and the ethane concentration is 4.9%, and the concentration of hydrocarbons (methane + ethane) exceeds 99%. When only the methanation reaction process was performed, the hydrogen concentration was 5.5% and the carbon dioxide concentration was 0.43%, so even in this case, it can be seen that the hydrogen concentration was significantly reduced by the dehydration reaction process.

Although both the methanation reaction process and the dehydration/hydrogenation reaction process are carried out at the same inlet temperature (250°C), hydrogen remains in the methanation reaction process and hydrogen is reduced in the dehydration/hydrogenation reaction process for the following reasons. The methanation reaction of carbon dioxide is an exothermic reaction that generates 41 kJ per mole of hydrogen. On the other hand, the hydrogenation reaction of ethylene is a reaction that generates a slightly large amount of heat (136.0 kJ per mole of ethylene), and even when combined with the dehydration reaction of ethanol (endothermic heat of 45.0 kJ per mole of ethanol), it is an exothermic reaction of about 90.0 kJ per mole of hydrogen. Therefore, the equilibrium between the production of ethylene by dehydration of ethanol and the production of ethane by the subsequent hydrogenation of ethylene is significantly biased toward the production of ethane. For this reason, the dehydration/hydrogenation reaction proceeds sufficiently even under temperature conditions where hydrogen remains in the methanation reaction process.

When 1-propanol is used instead of ethanol as the alcohol 3 having 2 to 4 carbon atoms, the endothermic heat generated by the dehydration reaction is 33.0 kJ per mole of 1-propanol, and the hydrogenation reaction of propylene generates heat of 125.0 kJ per mole of propylene, resulting in an exothermic reaction of approximately 90.0 kJ per mole of hydrogen for the entire dehydration/hydrogenation reaction. When other alcohols having 2 to 4 carbon atoms are used, the entire dehydration/hydrogenation reaction generates heat of approximately 60.0 to 90.0 kJ per mole of hydrogen, and the dehydration/hydrogenation reaction similarly proceeds satisfactorily.

In the dehydration hydrogenation reaction process, the carbon dioxide cannot be reduced. If the hydrogen/carbon dioxide molar ratio used in the methanation reaction is slightly higher than 4.0 and the methanation reaction process is performed under conditions of a slight excess of hydrogen, the carbon dioxide concentration in the fuel gas 2 after the methanation reaction process can be reduced. Here, the hydrogen concentration in the fuel gas 2 increases due to the excess addition of hydrogen, but this hydrogen can be reduced in the dehydration hydrogenation reaction process. If the hydrogen/carbon dioxide molar ratio of the mixed gas 1 used in the methanation reaction process is 4.04, the hydrogen concentration in the resulting fuel gas will be 0.55% and the carbon dioxide concentration will be 0.43%. In other words, the quality standards of a hydrogen concentration of 2.0% or less and a carbon dioxide concentration of 0.50% or less, which cannot be achieved even under high-pressure reaction conditions of 5.0 MPa by the methanation process alone, can be achieved by a reaction at a low pressure of 0.70 MPa by using the method according to this embodiment.

If the hydrogen/carbon dioxide molar ratio of the mixed gas 1 to be subjected to the methanation reaction step is further increased, the carbon dioxide concentration in the resulting fuel gas can be further reduced. For example, if the hydrogen/carbon dioxide molar ratio of the mixed gas 1 to be subjected to the methanation reaction step is 4.2, the hydrogen concentration in the resulting fuel gas will be 1.7% and the carbon dioxide concentration will be 0.10% or less. In addition, the methane concentration will be 83.3%, the ethane concentration will be 15.0%, and the calorific value per ^{1 m3} of fuel gas will be 44.0 MJ, which is a gas with almost the same properties as general natural gas-based city gas.

On the other hand, in addition to the need to add more alcohol 3 having a carbon number of 2 to 4, when the dehydration hydrogenation reaction is performed under adiabatic conditions, the outlet temperature of the dehydration hydrogenation reaction increases, and the alkene concentration increases, which may lead to deterioration of the catalyst due to carbon deposition and an increase in the hydrogen and carbon dioxide concentrations due to the simultaneous occurrence of a steam reforming reaction. From this perspective, it is preferable that the hydrogen/carbon dioxide molar ratio of the mixed gas 1 to be subjected to the methanation reaction process does not exceed 4.08. If the hydrogen/carbon dioxide molar ratio is in the range of 4.04 to 4.08, it is easy to achieve the quality standards of a hydrogen concentration of 2% or less and a carbon dioxide concentration of 0.50% or less.

Figure 12 shows the relationship between the composition of the mixed gas 1 and the concentrations of hydrogen and carbon dioxide in the fuel gas 4 when the amount of ethanol added is 0.50 in molar ratio to the hydrogen contained in the fuel gas 2. In the example of Figure 12, the amount of ethanol added is less than that of the example of Figure 11, so the hydrogen concentration in the resulting fuel gas 4 is higher than that of Figure 11, but it can be understood that the hydrogen concentration in the fuel gas 4 is reduced compared to Figure 9. If the hydrogen/carbon dioxide molar ratio of the mixed gas 1 is in the range of 4.04 to 4.08, all of the quality standards of hydrogen concentration 4.0% or less, carbon dioxide concentration 0.50% or less, and components other than hydrocarbons 4% or less can be achieved. This condition cannot be achieved in a methanation reaction at 0.70 MPa, no matter how the hydrogen/carbon dioxide molar ratio of the mixed gas to be subjected to the methanation reaction is set, so it is clear that the method of the present invention is useful for producing high-quality fuel gas without using high-pressure reaction equipment. Even in this case, as in the above, when at least one of carbon monoxide and carbon dioxide is contained as carbon oxides, if the ratio of hydrogen to carbon oxides supplied to the methanation reaction process is a ratio that satisfies the value of {(hydrogen) + (carbon monoxide)} / {(carbon monoxide) + (carbon dioxide)} calculated on a substance mass basis of 4.04 or more and 4.08 or less, the above quality standard is easily achieved.

Other embodiments
Finally, other embodiments of the method for producing fuel gas according to the present invention will be described. Note that the configurations disclosed in the following embodiments can be combined with the configurations disclosed in other embodiments as long as no contradiction occurs.

In this embodiment, a catalyst precursor containing palladium is produced by immersing a carrier in an aqueous solution containing palladium, and the catalyst precursor is calcined at a predetermined temperature. However, there are no particular limitations on the method of supporting (impregnating) palladium on the carrier, as long as the supported (impregnated) palladium is in the form of fine particles that have sufficient activity for hydrogenating alkenes.

Another embodiment of the method for producing an alcohol dehydration hydrogenation catalyst of the present invention is a method for producing an alcohol dehydration hydrogenation catalyst, which is obtained by subjecting hydrogen and carbon oxides to a methanation reaction in the presence of a methanation catalyst and containing 2.0% to 20% hydrogen on a volume basis after dehydration, and which is converted into a second fuel gas containing alkanes having 2 to 4 carbon atoms and having a reduced hydrogen concentration by treating the first fuel gas in the presence of an alcohol having 2 to 4 carbon atoms, and which includes a calcination step in which a catalyst precursor in which a support is impregnated with an aqueous solution containing palladium and a solid acid catalyst not carrying palladium are mixed and then calcined. With this configuration, in cases in which the catalyst precursor is impregnated with a relatively high concentration of palladium before mixing, the alcohol dehydration hydrogenation catalyst produced as a whole has a low concentration of palladium, and an economically excellent catalyst can be produced.

In addition, instead of a catalyst precursor in which a support is impregnated with an aqueous solution containing palladium, a commercially available catalyst in which palladium is supported on an inorganic oxide support may be used. In such a case, there is no need to perform an impregnation step, making it possible to produce the catalyst economically.

In addition, in this embodiment, the calcination step is included before the mixing step, in which the catalyst precursor containing palladium is calcined at 400°C or higher to obtain the inorganic oxide on which palladium is supported. However, as another embodiment of the method for producing an alcohol dehydration hydrogenation catalyst of the present invention, instead of these steps, a mixing step may be included in which a commercially available catalyst in which palladium is supported on a carrier is used as the inorganic oxide on which palladium is supported, and the solid acid catalyst on which palladium is not supported is mixed.

In such cases, there is no need to perform the impregnation and calcination steps, making it possible to produce the catalyst economically. In addition, there is no need to calcinate a commercially available catalyst in which palladium is supported on a carrier and a solid acid catalyst in which palladium is not supported at high temperatures, so inorganic oxides and solid acid catalysts with low heat resistance can be used.

Furthermore, another embodiment of the method for producing an alcohol dehydration hydrogenation catalyst of the present invention is a method for producing an alcohol dehydration hydrogenation catalyst, which involves treating a first fuel gas containing 2.0% to 20% hydrogen by volume after dehydration, obtained by subjecting hydrogen and carbon oxides to a methanation reaction in the presence of a methanation catalyst, in the presence of an alcohol having a carbon number of 2 to 4, to convert the first fuel gas into a second fuel gas containing alkanes having a carbon number of 2 to 4 and having a reduced hydrogen concentration, and includes a calcination step of calcining a mixture containing an inorganic oxide supporting palladium and a precursor of a solid acid catalyst.

As the inorganic oxide carrying palladium, for example, a commercially available catalyst carrying palladium or a catalyst precursor impregnated with an aqueous solution containing palladium can be used. As the precursor of a solid acid catalyst not carrying palladium, for example, _NH4 type zeolite, which includes a calcination process to function as a catalyst, can be used. As a result, the _NH4 type zeolite becomes a proton type zeolite and exhibits strong solid acidity.

By adopting this configuration, if the inorganic oxide supporting palladium has a relatively high concentration of palladium supported on it before mixing, mixing it with a precursor of a solid acid catalyst that does not support palladium and calcining it will result in a low concentration of palladium in the overall alcohol dehydration hydrogenation catalyst produced, making it possible to produce an economical catalyst.

The inorganic oxide carrier may be the same type of solid acid catalyst as the solid acid catalyst not supporting palladium, may be different from the solid acid catalyst not supporting palladium, or may not be a solid acid catalyst. When producing the catalyst in this manner, an inorganic oxide can be freely selected as a carrier for supporting palladium, and therefore, by selecting an appropriate inorganic oxide, the dispersion degree and supporting concentration of palladium can be widely controlled.
The carrier to be used may be a commercially available product, or may be prepared by, for example, calcining a precursor of silica, alumina, titania, zirconia, tungsten oxide zirconia, zeolite, or the like.

In this embodiment, the methanation reactor 10 used in the methanation reaction process is a combination of a fixed-bed adiabatic reactor having a recycle line and a heat exchange reactor, as shown in FIG. 8. However, the reactor used in the methanation reaction process according to the present invention is not particularly limited as long as it can produce a fuel gas containing 2.0% by volume or more and 20% by volume or less of hydrogen. Examples of such reactors include a fixed-bed adiabatic multi-stage reactor and a single-stage or multi-stage heat exchange reactor.

In this embodiment, an example in which the pressure in the methanation reaction process is 0.70 MPa has been particularly described. However, the pressure in the methanation reaction process according to the present invention is not particularly limited as long as a fuel gas containing 2.0 volume % or more and 20 volume % or less of hydrogen is obtained. However, if the pressure is 0.50 MPa or more, the hydrogen and carbon dioxide contents in the fuel gas obtained in the methanation reaction process are reduced, and therefore the hydrogen and carbon dioxide contents in the final product fuel gas are also likely to be reduced. In addition, since it is easy to reduce the cost of the equipment used in the methanation reaction process, the pressure is preferably 3.0 MPa or less, and more preferably 1.0 MPa or less.

In this embodiment, the dehydration/hydrogenation reactor 20 used in the dehydration/hydrogenation reaction process is configured as a fixed-bed adiabatic reactor. However, the reactor used in the dehydration/hydrogenation reaction process according to the present invention is not particularly limited as long as the reactions represented by formulas 3 and 4 proceed. Examples of such reactors include a fixed-bed adiabatic reactor having a recycle line and a heat exchange reactor.

In this embodiment, an example in which the pressure in the dehydration hydrogenation reaction process is 0.70 MPa has been described in particular. However, the pressure in the dehydration hydrogenation reaction process according to the present invention is not particularly limited as long as the reactions represented by formulas 3 and 4 proceed. However, if the pressure is 0.50 MPa or more, it is easy to reduce the hydrogen and carbon dioxide contents in the resulting fuel gas without using a large amount of dehydration hydrogenation catalyst. In addition, since it is easy to reduce the cost of the equipment used in the dehydration hydrogenation reaction process, the pressure is preferably 3.0 MPa or less, and more preferably 1.0 MPa or less.

In this embodiment, an example in which the pressure in the methanation reaction process and the dehydration/hydrogenation reaction process are the same (0.70 MPa) has been described in particular. However, in the methanation reaction process and the dehydration/hydrogenation reaction process according to the present invention, the pressure may be the same or different.

The present invention does not exclude the use of conventionally known means for improving methane conversion. For example, the reaction gas may be cooled in the middle of a multi-stage reaction to condense and separate a portion of the water vapor. Even in this case, the method of the present invention can be used to reduce the hydrogen and carbon dioxide concentrations in the fuel gas without excessively removing water vapor.

The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

Example 1
78 g of zirconium hydroxide (Taiyo Koko Co., Ltd.; Z-999-D) and 21 g of an aqueous solution of ammonium metatungstate (Nippon Inorganic Chemical Industry Co., Ltd.; MW-2, containing 50% by mass as WO ₃ ) were mixed, and 50 g of pure water was added and wet kneaded. The mixture was evaporated to dryness in a hot water bath, dried overnight in a dryer at 120°C, and then calcined at 800°C for 4 hours to obtain WO ₃ /ZrO _2. 35 g of the obtained WO ₃ /ZrO ₂ was impregnated with an aqueous solution of palladium nitrate containing 0.35 g as Pd, evaporated to dryness in a hot water bath, dried overnight in a dryer at 120°C, and then calcined at 350°C in air for 4 hours to obtain catalyst A' (1.0% Pd/WO ₃ /ZrO ₂ catalyst) in which 1% by mass of palladium was supported on WO ₃ /ZrO ₂ . This was mixed with four times the mass of _WO3 / _ZrO2 not carrying palladium to prepare catalyst A (1.0% Pd/ _WO3 / _ZrO2 + _WO3 / _ZrO2 (20:80)). The palladium content in the entire catalyst A was 0.2 mass%.

Catalyst A was tableted and classified into 1.0-2.5 mm tablets. 6.0 mL of the tablets were filled into a SUS reaction tube (inner diameter 16 mm). The tube was heated in an electric furnace while passing 2.0% hydrogen, the balance nitrogen gas, until the temperature reached 240°C. After that, 10% hydrogen, the balance nitrogen gas was passed through and the temperature was maintained for 1 hour, and then 20% hydrogen, the balance nitrogen gas was passed through and the temperature was maintained for 2 hours, thereby reducing the catalyst. After the reduction process was completed, the temperature of the reaction tube was raised and maintained at 270°C.

Ethanol was added to 400 mL per minute (volume at standard conditions of 0°C and 101.325 kPa) of fuel gas (first fuel gas) consisting of 32.6 vol% methane, 65.3 vol% water vapor, 1.9 vol% hydrogen, and 0.15 vol% carbon dioxide, so that the molar ratio of ethanol to hydrogen in the fuel gas was 0.70. This fuel gas with added ethanol was passed through the catalyst at a reaction tube temperature of 270°C and a pressure of 0.70 MPa (absolute pressure).

The composition of 32.6 vol% methane, 65.3 vol% water vapor, 1.9 vol% hydrogen, and 0.15 vol% carbon dioxide corresponds to the equilibrium composition when a gas with a (hydrogen)/(carbon dioxide) value of 4.04 calculated on a mass of substance basis is subjected to a methanation reaction at a temperature of 250°C and a pressure of 0.70 MPa (absolute pressure), and simulates the fuel gas (first fuel gas) obtained in the first process (methanation process) that contains approximately 5.5% hydrogen on a volumetric basis after dehydration.

After the reaction, the second fuel gas was cooled to separate the water, and the concentrations of methane, ethane, hydrogen, carbon monoxide, and carbon dioxide were quantified using a gas chromatograph.

The concentrations of each component in the second fuel gas (by volume after dehydration) were: hydrogen 2.51%, carbon dioxide 0.45%, carbon monoxide 0.30%, ethane 3.44%, and the remainder methane.

(Comparative Example 1)
Catalyst A' (1.0% Pd/WO ₃ /ZrO ₂ catalyst) was used as catalyst B as it is.

A test similar to that in Example 1 was conducted, except that catalyst B was used as the catalyst.

The concentrations of each component in the second fuel gas (by volume after dehydration) were: hydrogen 5.76%, carbon dioxide 0.56%, carbon monoxide 1.70%, ethane 1.68%, and the remainder methane.

(Comparative Example 2)
Catalyst C (0.20% Pd/WO ₃ /ZrO 2 catalyst) was prepared in the same manner as in Comparative Example 1, except that 25 g of WO ₃ /ZrO ₂ obtained in Example 1 was impregnated with an aqueous palladium nitrate solution containing 0.050 _g of Pd.

A test similar to that in Example 1 was carried out, except that catalyst C was used as the catalyst.

The concentrations of each component in the second fuel gas (by volume after dehydration) were: hydrogen 6.14%, carbon dioxide 0.49%, carbon monoxide 2.00%, ethane 1.41%, and the remainder methane.

Example 2
11.6 g of zirconium hydroxide (Taiyo Koko Co., Ltd.; Z-999-D) was impregnated in an aqueous palladium nitrate solution containing 0.02 g of Pd, evaporated to dryness in a water bath, dried overnight in a 110°C dryer, and then calcined in air at 800°C for 4 hours to obtain catalyst D carrying 0.2% by mass of palladium. Catalyst D (0.20% Pd/ _ZrO2 catalyst) was mixed with 4 times the mass of _WO3 / _ZrO2 not carrying palladium to obtain catalyst E (0.2% Pd/ _ZrO2 + _WO3 / _ZrO2 (20:80)). The palladium content in the entire catalyst E was 0.04% by mass.
The same test as in Example 1 was carried out except that Catalyst E was used as the catalyst, the reduction treatment temperature was set to 250°C, and the temperature of the reaction tube thereafter was set to 300°C.
The concentrations of each component contained in the second fuel gas (based on the volume after dehydration) were as follows: hydrogen 3.30%, carbon dioxide 1.13%, carbon monoxide 0.0018%, ethane 3.32%, and the balance methane.

Example 3
10.0 g of aluminum oxide (AKP-G15 manufactured by Sumika Alchem) was impregnated with an aqueous palladium nitrate solution containing 0.02 g of Pd, evaporated to dryness in a water bath, dried overnight in a 110°C dryer, and then calcined in air at 800°C for 3 hours to obtain catalyst F (0.2% Pd/Al ₂ O ₃ catalyst) in which 0.2% by mass of palladium was supported. Catalyst F was mixed with 4 times the amount by mass of WO ₃ /ZrO ₂ in which no palladium was supported, to obtain catalyst G (0.2% Pd/Al ₂ O ₃ +WO ₃ /ZrO ₂ (20:80)). The palladium content in the entire catalyst G was 0.04% by mass.
The same test as in Example 2 was carried out except that Catalyst G was used as the catalyst.
The concentrations of the components contained in the second fuel gas (based on the volume after dehydration) were as follows: hydrogen 2.12%, carbon dioxide 0.59%, carbon monoxide 0.16%, ethane 3.69%, and the balance methane.

(Comparative Example 3)
The same test as in Example 2 was carried out except that Catalyst C was used as the catalyst.
The concentrations of each component contained in the second fuel gas (based on the volume after dehydration) were as follows: hydrogen 3.83%, carbon dioxide 0.64%, carbon monoxide 0.75%, ethane 2.84%, and the balance methane.

Example 4
10.0 g of silicon oxide (manufactured by Fuji Silysia; CARiACT G-6) was impregnated with an aqueous palladium nitrate solution containing 0.10 g of Pd, evaporated to dryness in a water bath, dried overnight in a 110°C dryer, and then calcined in air at 500°C for 4 hours to obtain catalyst H (1% Pd/ _SiO2 catalyst) in which 1% by mass of palladium was supported. Catalyst H was mixed with 4 times the amount by mass of _WO3 / _ZrO2 not supporting palladium to obtain catalyst I (1% Pd/ _SiO2 + _WO3 / _ZrO2 (20:80)). The palladium content in the entire catalyst I was 0.2% by mass.
The same test as in Example 2 was carried out except that Catalyst I was used as the catalyst.
The concentrations of each component contained in the second fuel gas (based on the volume after dehydration) were as follows: hydrogen 1.58%, carbon dioxide 0.47%, carbon monoxide 0.0048%, ethane 3.85%, and the balance methane.

Example 5
26.7 g of silicotungstic acid (Kishida Chemical; silicotungstic acid (26 water)) was dissolved in 300 mL of water, and the solution was impregnated into 80.0 g of silica (Fuji Silysia; CARiACT G-6), evaporated to dryness in a water bath, dried overnight in a dryer at 120°C, and then calcined in air at 350°C for 4 hours to prepare catalyst J (25% SiW/ _SiO2 catalyst) in which 25% by mass of silicotungstic acid was supported.
20.1 g of aluminum oxide (KC501, manufactured by Sumika Alchem Co., Ltd.) was impregnated with an aqueous solution of palladium nitrate containing 2.23 g of Pd, evaporated to dryness in a water bath, dried overnight in a dryer at 120° C., and then calcined in air at 650° C. for 4 hours to prepare catalyst K (10% Pd/ _Al2O3 catalyst) carrying 10% by mass of palladium.
Catalyst K was mixed with 4 times the amount of catalyst J by mass to prepare catalyst L (10% Pd/Al ₂ O ₃ + 25% SiW/SiO ₂ (20:80)). The palladium content in the entire catalyst L was 2.0 mass%.
The same test as in Example 1 was carried out except that Catalyst L was used as the catalyst and the temperature of the reaction tube was set to 240°C.
The concentrations of each component contained in the second fuel gas (based on the volume after dehydration) were as follows: hydrogen 2.10%, carbon dioxide 0.52%, carbon monoxide 0.063%, ethane 3.80%, and the balance methane.

(Comparative Example 4)
7.69 g of tungstosilicic acid (Kishida Chemical; tungstosilicic acid (26 water)) and an aqueous palladium nitrate solution containing 0.77 g of Pd were dissolved in 50 mL of water, and the solution was impregnated into 30.0 g of silica (Fuji Silysia; CARiACT G-6), evaporated to dryness in a water bath, dried overnight in a dryer at 120°C, and then calcined in air at 350°C for 4 hours to prepare catalyst M (2.0%Pd/20%SiW/ _SiO2 catalyst) in which 2% by mass of palladium and 20% by mass of tungstosilicic acid were supported.
The same test as in Example 4 was carried out except that Catalyst M was used as the catalyst.
The concentrations of each component contained in the second fuel gas (based on the volume after dehydration) were as follows: hydrogen 2.81%, carbon dioxide 0.44%, carbon monoxide 0.37%, ethane 3.30%, and the balance methane.

For the examples and comparative examples, the ethane selectivity was calculated according to the following formula, assuming that the carbon-containing products produced using ethanol as a raw material were only carbon dioxide, carbon monoxide, and ethane.
Ethane selectivity (%)=ethane concentration (%)×2/[ethane concentration (%)×2+carbon monoxide concentration (%)+carbon dioxide concentration (%)]×100

The conditions and results of the examples and comparative examples are summarized in Table 1.

Comparing Example 1, Comparative Example 1 and Comparative Example 2, which were carried out under the same reaction conditions (reaction temperature 270°C), when catalyst A, in which palladium-loaded _WO3 / _ZrO2 was mixed with WO3/ _ZrO2 on which palladium was not loaded, was used, the ethane concentration in the second fuel gas was high and the concentrations of carbon dioxide and carbon monoxide were low, resulting in a high ethane selectivity. On the other hand, when catalysts _B and C, in which palladium was directly loaded on _WO3 / _ZrO2 and did not contain _WO3 / _ZrO2 on which palladium was not loaded, were used, the ethane concentration in the second fuel gas was low, and instead the concentrations of carbon dioxide and carbon monoxide were high, resulting in a low ethane selectivity.
Similarly, in comparison of Example 2, Example 3, Example 4, and Comparative Example 3 under the same reaction conditions (reaction temperature 300° C.), the ethane selectivity was high when catalysts E, G, and I containing WO ₃ /ZrO ₂ not supported by palladium were used, while the ethane selectivity was low when catalyst C not containing WO ₃ /ZrO ₂ not supported by palladium was used. Also, in comparison of Example 5 and Comparative Example 4 under the same reaction conditions (reaction temperature 240° C.), the ethane selectivity was high when catalyst L in which palladium-supported Al ₂ O ₃ was mixed with palladium-supported SiW/SiO ₂ was used, while the ethane selectivity was low when catalyst M in which palladium and SiW were directly supported on SiO ₂ was used.

These results demonstrate that a catalyst containing an inorganic oxide carrying palladium and a solid acid catalyst not carrying palladium can promote the dehydration and hydrogenation reaction of alcohols with high selectivity.

The present invention can be used, for example, as a method for producing fuel gas to be supplied as city gas.

Reference Signs List 1: mixed gas 2: first fuel gas 3: alcohol having 2 to 4 carbon atoms 4: second fuel gas 10: methanation reactor 11a to 15a: reactor 11b to 15b: heat exchanger 11c: recycle line 12d: heat exchange reactor 20: dehydration hydrogenation reactor 30: flow control valve

Claims (7)

A catalyst for dehydration and hydrogenation of alcohol, for treating a first fuel gas, which is obtained by subjecting hydrogen and carbon oxides to a methanation reaction in the presence of a methanation catalyst and contains 2.0% or more and 20% or less hydrogen on a volume basis after dehydration, in the presence of an alcohol having a carbon number of 2 to 4, to convert the first fuel gas into a second fuel gas which contains an alkane having a carbon number of 2 to 4 and has a reduced hydrogen concentration,
A catalyst for dehydration and hydrogenation of alcohols, comprising an inorganic oxide on which palladium is supported, and a solid acid catalyst on which palladium is not supported. The amount of palladium supported on the inorganic oxide is 0.2% by mass or more and 15% by mass or less,
2. The catalyst for dehydration and hydrogenation of alcohols according to claim 1, wherein the solid acid catalyst not supporting palladium is contained in a mass ratio of 1.0 to 10, based on a mass of the inorganic oxide supporting palladium being 1. The alcohol dehydration and hydrogenation catalyst according to claim 1 or 2, wherein the solid acid catalyst is a SiW/silica catalyst or tungsten oxide zirconia. A method for producing an alcohol dehydration/hydrogenation catalyst for converting a first fuel gas, which is obtained by subjecting hydrogen and carbon oxides to a methanation reaction in the presence of a methanation catalyst and contains 2.0% to 20% hydrogen on a volume basis after dehydration, into a second fuel gas which contains an alkane having 2 to 4 carbon atoms and has a reduced hydrogen concentration, by treating the first fuel gas in the presence of an alcohol having 2 to 4 carbon atoms.
A method for producing a catalyst for dehydration and hydrogenation of alcohols, comprising a mixing step of mixing an inorganic oxide carrying palladium with a solid acid catalyst not carrying palladium. The method for producing an alcohol dehydration hydrogenation catalyst according to claim 4, wherein in the mixing step, the inorganic oxide carrying 0.2% by mass or more and 15% by mass or less of palladium and the solid acid catalyst not carrying palladium are mixed in a mass ratio of 1.0 to 10, where the mass of the inorganic oxide carrying palladium is taken as 1. The method for producing an alcohol dehydration hydrogenation catalyst according to claim 4 or 5, further comprising a calcination step of calcining a catalyst precursor containing palladium at 400°C or higher to obtain the inorganic oxide carrying palladium, prior to the mixing step. The method for producing an alcohol dehydration and hydrogenation catalyst according to claim 4 or 5, wherein the solid acid catalyst is a SiW/silica catalyst or tungsten oxide zirconia.

Images