JP2012140382A - Method of synthesizing methane from carbon dioxane and hydrogen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for synthesizing methane finally through reaction steps separated to two-stages elementary reactions.SOLUTION: A methane synthesis reaction using carbon dioxide and hydrogen as starting material presented by equation (1) actually relies on a thermal equilibrium relation of a first reaction step presented by equation (2) and a second reaction step presented by equation (3). CO2+4H2→CH4+2H2O ΔH=-39.4 kcal/mol (1), CO2+H2→CO+H2O ΔH=+9.8 kcal/mol (2), CO+3H2→CH4+H2O ΔH=-49.3 kcal/mol (3). It is found out that, irrespective of composition ratios before the second reaction step, concentrations of remaining CO and CO2 after reaction can be controlled by means of such a parameter as 1/((3CO+4CO2)/H2). By utilizing this knowledge, the reaction can be controlled in accordance with the degree of CO and CO2 allowance required for application of synthetic methane.

Description

  The present invention relates to a method of synthesizing methane using carbon dioxide and hydrogen as raw materials, and more particularly to a method of finally synthesizing methane through a reaction process separated into two-stage elementary reactions.

Conventionally, various techniques for synthesizing methane by reacting carbon dioxide and hydrogen have been proposed for the purpose of fixing carbon dioxide in order to solve the global warming problem (for example, Patent Documents 1 to 5).
Reference 1 is characterized in that rhodium (Rh) is used as a catalyst, and methane can be selectively synthesized. In this case, the reaction molar ratio of hydrogen and carbon dioxide is preferably 2-4.
Document 2 is characterized in that an intermetallic compound containing a rare earth metal is used as a catalyst, and methane can be synthesized in an effective yield even under conditions of relatively low reaction temperature and pressure. The reaction molar ratio of hydrogen to carbon dioxide in this case is not particularly limited, but 2-8, preferably 3-6, particularly preferably 4 is recommended.

Document 3 uses a supported metal catalyst prepared by spray decomposition from a solution containing nickel nitrate, sodium chloride and zirconyl nitrate. The reaction molar ratio of hydrogen and carbon dioxide is 0.1-40, preferably 1.0-20 is recommended.
Document 4 uses a catalyst in which a metal oxide mixed oxide coating is provided on the surface of a deteriorated iron group transition element powder due to wear even when used in a fluidized bed reactor. Although there is no description about the molar ratio, in the examples, CO: 20%, CO2: 50%, H2: 60% raw material gas, CO: 1%, CO2: 65%, H2: 2%, CH _4: 32% of the production results are shown.

JP-A-6-142513 JP-A-6-340557 JP-A-7-76528 JP 2009-34654 A JP 2009-34650 A

However, each of the above documents relates to the selection of a reaction catalyst, and is a technique related to a method for directly synthesizing methane from carbon dioxide and hydrogen (hereinafter referred to as a direct synthesis method). It is not a study that focuses on.
The methane synthesis reaction using carbon dioxide and hydrogen as raw materials is actually due to the complicated thermal equilibrium relationship between elementary reaction A (formula (2) below) and elementary reaction B (formula (3) below) that have completely different reaction equilibrium characteristics. It is established. Conventionally, there have been many disclosures regarding the optimization of elementary reactions A and B, respectively, but we have not been able to find any documents that have been studied for optimization by positioning the methane synthesis reaction as an elementary reaction.
On the other hand, in recent years, high-purity hydrogen is generated using renewable energy, and methane synthesis using this as a raw material is becoming realistic. However, when such synthetic methane is used as, for example, a city gas raw material, It is necessary to reduce the cost of separation and recovery of reaction residual components (H2, CO2, CO), and setting of optimum reaction conditions in the methane synthesis reaction is an urgent issue.
As a result of intensive studies on the optimum conditions for each elementary reaction and the overall reaction, the inventors of the present application have completed a high-purity methane synthesis method capable of significantly reducing the carbon dioxide and carbon monoxide concentrations after the reaction.

The gist of the present invention is as follows. That is, the method of synthesizing methane from carbon dioxide and hydrogen according to the present invention is as follows.
(1) A method of synthesizing methane from carbon dioxide and hydrogen, in which a carbon dioxide and hydrogen are reacted to obtain carbon monoxide, and carbon monoxide and hydrogen produced by the first reaction step are combined. A second reaction step of reacting to obtain methane.
(2) In the above invention, in the second reaction step, the composition ratio parameter (Pcr) = 1 with respect to the composition ratio (in terms of mole) of hydrogen and carbon monoxide as raw materials and carbon dioxide remaining in the first reaction step. / ((3CO + 4CO2) / H2) so that the raw material hydrogen and carbon monoxide supply ratio and the carbon dioxide hydrogen and hydrogen separation / recycling ratio are 1.0 to 1.1. It is characterized by controlling.
(3) In the above invention, in the second reaction step, hydrogen as a raw material so that the value of the composition ratio parameter (Pcr) = 1 / ((3CO + 4CO2) / H2) is 1.0 to 1.02. And the carbon monoxide supply ratio and the carbon dioxide hydrogen and hydrogen separation / recycling ratio are controlled.
(4) In each of the above inventions, a step of separating only water without separating and collecting carbon dioxide after the first reaction step, and a step of separating only water without separating and collecting hydrogen after the second reaction step; , Further included.
(5) In the said invention, the reaction temperature in said 1st reaction process is set to 460-550 degreeC, and the reaction temperature in said 2nd reaction process is set to 250-450 degreeC, It is characterized by the above-mentioned.
(6) In each of the above inventions, the reaction pressure in the second reaction step is set to 1.0 to 5.0 MPa.

The methane synthesis reaction using carbon dioxide and hydrogen as raw materials is represented by the formula (1), but actually, from the thermal equilibrium relationship between the elementary reaction A represented by the formula (2) and the elementary reaction B represented by the formula (3). It is made up.
CO2 + 4H2 → CH4 + 2H2O ΔH = −39.4 kcal / mol (1)
CO2 + H2 → CO + H2O ΔH = + 9.8 kacl / mol (2)
CO + 3H2 → CH4 + H2O ΔH = −49.3 kcal / mol (3)
In the present invention, in synthesizing methane from carbon dioxide and hydrogen, carbon monoxide is first synthesized by elementary reaction A (first reaction step), and then methane is synthesized by elementary reaction B (second reaction step). ) It consists of two reaction steps.

The first reaction step is the reverse reaction of the carbon monoxide shift reaction. As apparent from the reaction formula, there is no change in the number of moles before and after the reaction, and there is almost no pressure dependence. However, in consideration of separation of generated water and unreacted carbon dioxide and pressurization conditions in the second reaction step, it is desirable to employ pressure conditions that are matched to the second reaction step in the subsequent stage.
The molar ratio of the raw material hydrogen to carbon dioxide may be H2 / CO2 ≧ 1.1 which is slightly larger than the stoichiometric ratio in order to promote efficient reaction progress. Further, the amount of hydrogen can be adjusted to the molar ratio of the second reaction step.
As for the reaction temperature, the higher the temperature, the better the reaction rate. However, considering that the catalyst poisoning by carbon monoxide and that the reaction rate of carbon dioxide does not increase significantly even if the temperature is raised are considered. A temperature around 460-550 ° C, more preferably around 500 ° C is suitable.
In this reaction step, unreacted carbon dioxide can be separated and recycled on the upstream side of the second reaction step. In addition, it is desirable to discharge the generated water out of the system.

Next, methane synthesis using carbon monoxide and hydrogen, which is the second reaction step, is an industrially established technique. In this reaction, the temperature dependence of the reaction equilibrium constant is large, and it is known that the lower the temperature, the more it shifts to the positive reaction side (for example, see Hidehiro Kumazawa, Chemical Engineering, Vol. 138, No. 2 (1993), P22). ). From a simulation result of reaction temperature-equilibrium conversion using software described later (see FIG. 8), it is sufficient if the reaction pressure is 1.0 MPa and about 700 K (about 427 ° C.). Furthermore, since the reaction rate becomes slow at 200 ° C. or lower, the lower limit of the reaction temperature is 200 ° C., preferably 250 ° C.
As is clear from the reaction formula, the higher the reaction pressure, the more advantageous. Below 0.5 MPa, the effect on increasing the reaction rate is small, and above 7.0 MPa, not only the pressurizing effect is small, but also side reactions such as methanol formation are not preferable. In consideration of these, it is appropriate to set the pressure to 0.5-7.0 MPa, more preferably 1.0-5.0 MPa.

Next, the optimum control of the raw material supply amount in the second reaction step is as follows. In the first stage of the second reaction step, CO2 is present in a considerable concentration in addition to CO as a raw material. Therefore, when considering the H2 balance, it is necessary to handle CO and CO2 as a total. In this case, the theoretical H2 amount required for CO methanation is 3 mol, while the theoretical H2 amount required for direct methanation of CO2 is 4 mol. When converted based on the stoichiometric ratio, H2 / 3CO = 1.0 (H2: CO = 3: 1) for CO and H2 / 4CO2 = 1.0 (H2: CO2 = 4: 1) for CO2. Become.
From this, it is surmised that the parameter H2 / (3CO + 4CO2) or 1 / ((3CO + 4CO2) / H2) (hereinafter may be abbreviated as Pcr) can be a management index for the above purpose. The

As shown in the examples described later, as a result of variously changing the raw material supply ratio (H2 / CO2), CO2, and the recycling ratio of H2, and examining the relationship with the composition ratio parameter Pcr, CO before the second reaction step, It has been found that the composition after reaction can be controlled by the above parameters regardless of the composition of CO2, CH4, H2, and H2O. It was also found that an inflection point exists in the vicinity of the Pcr value of 1.0, and the reaction behavior changes greatly with this value as a boundary. By utilizing this characteristic, the carbon dioxide concentration after the reaction is set to 1000 ppm or less by controlling the composition ratio Pcr related to CO, CO2, and H2 before the second reaction step to be 1.0 to 1.1. be able to.
Further, in order to increase the CH4 purity by lowering the H2 and CO2 concentrations in the final composition, it is preferable to control the parameter in the vicinity of 1.00 to 1.02.

According to the present invention, it is possible to obtain high-purity methane in which the concentration of residual carbon dioxide or carbon monoxide is further reduced as compared with a direct synthesis method in which methane is directly synthesized from carbon dioxide and hydrogen.
In addition, although the catalyst suitable for each elementary reaction is different, according to the present invention, the optimum catalyst can be selected for each elementary reaction, so the flexibility of catalyst selection is increased, the yield is improved, and the cost is reduced. Contribute to
Moreover, since the heat generated in the second reaction step, which is an exothermic reaction, can be used for the first reaction step (endothermic reaction), the heat balance can be improved.
For example, when hydrogen obtained by solar power generation in a desert or the like is applied to the system according to the present invention, the water separated in the first reaction step can be effectively used for other purposes.
Moreover, in the invention including the step of separating and removing the water produced in the first reaction step before the second reaction step, in order to promote the elementary reaction A to the production side, the yield of the first reaction step Further improvements are possible.

It is a figure which shows the relationship between the composition ratio parameter Pcr in a 2nd reaction process, CO2 density | concentration after reaction, and H2 density | concentration (setting condition 1-3). It is a figure which shows the relationship between the composition ratio parameter Pcr in a 2nd reaction process, CO density | concentration after reaction, and H2 density | concentration (setting condition 1-3). It is a figure which shows the relationship between the raw material supply ratio in a 1st reaction process, the CO2 density | concentration after a 2nd reaction process, and H2 density | concentration (setting condition 2). It is a figure which shows the relationship between the raw material supply ratio in a 1st reaction process, CO density | concentration after a 2nd reaction process, and H2 density | concentration (setting condition 2). It is a figure which shows the relationship between CH4 density | concentration after a 2nd reaction process, and CO2 density | concentration (setting condition 3). It is a figure which shows the relationship between CH4 density | concentration after a 2nd reaction process, and H2 density | concentration (setting condition 3). It is a figure which shows the relationship between the reaction temperature in an elementary reaction A, and an equilibrium composition ratio. It is a figure which shows the relationship between the reaction temperature in the elementary reaction B, and an equilibrium conversion rate. It is a figure showing methane synthesizer 1 concerning a first embodiment. It is a figure which shows the methane synthesizer 20 which concerns on 2nd embodiment.

(First embodiment)
Hereinafter, with reference to FIG. 9, the methane synthesizer 1 which concerns on one Embodiment of this invention is demonstrated. In the present embodiment, H2 and CO2 as raw materials are supplied at a molar ratio of 4.1-4.2 in consideration of optimization of the H2 and CO molar ratio in the second reaction step in the subsequent stage. As the raw material H2, for example, pure H2 obtained by electrolysis of water by photovoltaic power generation can be used. Moreover, about CO2, CO2 discharged | emitted by a city gas consumer's destination can be collect | recovered and used, for example.
The pressures of H2 and CO2 are increased to 2.0 to 5.0 MPa by the compressors 2a and 2b, respectively, mixed, and then introduced into the reactor 3, where the elementary reaction A, which is the first reaction step, is performed to generate CO and H2O. . As the compressor for boosting, any of an axial flow type, a reciprocating type, a screw type, a rotary type, a scroll type and the like can be used.
As the reaction catalyst, metal oxides such as ZnO, Cr2O3, FeO, and CuO supported on an alumina carrier can be used.

The reaction temperature is adjusted so that the reactor outlet temperature is about 500 ° C., and is controlled to be maintained at 460-550 ° C. by an adiabatic reaction or (adiabatic + isothermal) reaction with intermediate heat exchange. The reaction pressure is adjusted to about 3.0 MPa.
The gas produced by the first reaction step contains CO and H2O as main components and contains H2 and CO2 as unreacted residual components. Among these, H2O is separated and removed using a flash distillation column 6a (or a separation membrane or the like). The CO2 is separated by the CO2 regeneration unit 4 (absorption by a separation membrane or chemicals (amine system, potassium carbonate system)), recovered by heat dissipation, and pressurized by the compressor 2c via the CO2 recovery line 10 , Resupply.

A gas mainly containing H2 and CO from which CO2 and H2O have been removed in the first reaction step is guided to the reactor 5, and elementary reaction B as the second reaction step is performed to generate CH4. As the reaction catalyst, a Ni-based catalyst can be used, and a noble metal such as Ru-based or Pd-Rh-based can also be used depending on conditions.
The reaction temperature condition is adjusted by a heat exchanger so that the reactor outlet temperature is about 250 ° C., and is controlled to be maintained at 250-450 ° C. by an adiabatic reaction or (adiabatic + isothermal) reaction with intermediate heat exchange. To do. The reaction pressure is set to about 3.0 MPa.
Gas after the reaction is mainly composed of CH4, _{H 2} O, containing as an unreacted residual components H2, CO2, CO. Of these, H ₂ O
Is separated and removed by the flash distillation column 6b as in the first reaction step. For H2, PSA (Pressure Swing
Absorption) 7 is separated and recovered, and is pressurized by the compressor 2d through the H2 recovery line 9, and then returned to the raw material line 8.

  Through the above steps, the supply gas line 11 is supplied with CH4 and a small amount of CO2 and CO (CO2: 404 ppm, CO: 0.717 ppm in the representative example of setting condition 1 in Table 5 described later). The necessity of CO2 and CO removal can be determined in consideration of the supply application and the cost required for removal. When the generated gas of the supply gas line 11 is supplied as a city gas raw material, the generated gas may be supplied as it is by adjusting the molar ratio according to the required gas specification.

(Second embodiment)
Furthermore, with reference to FIG. 10, the methane synthesizer 20 which concerns on other embodiment of this invention is demonstrated. The present embodiment relates to a form in which the residual CO2 after the first reaction step and the residual H2 after the second reaction step are controlled to a level not requiring recycling by managing the composition ratio parameter Pcr.
The structure of the methane synthesizer 20 is different from the methane synthesizer 1 described above in that the methane synthesizer 1 includes a regeneration unit 4 for CO2 separation, a recycling pipe 10, a compressor 2c, and an H2 separation. The PSA 7, the recycling pipe 9, and the compressor 2d are not provided. Since the other structure is the same as that of the methane synthesizer 1, redundant description is omitted.
Next, the methane synthesis process in this embodiment will be described. In the same manner as in the first embodiment, H2 and CO2 after pressurization and mixing are guided to the reactor 3, and elementary reaction A which is the first reaction step is performed to generate CO and H2O. Here, the supply amounts of H 2 and CO 2 are managed so that the composition ratio of the gas introduced into the second reaction step described later falls within the range of the parameter value Pcr = 1.00−1.02. Other reaction conditions are the same as in the first embodiment (the same applies to the following steps).
The gas produced by the first reaction step contains CO and H2O as main components and contains H2 and CO2 as unreacted residual components. Of these, only H2O is separated and removed using the flash distillation column 6a. CO2 is not separated and supplied to the second reaction step as it is.

  H2O is removed, and a gas containing H2 and CO as main components and residual CO2 is introduced to the reactor 5 to perform an elementary reaction B as a second reaction step to generate CH4. In this case, as described above, the H2, CO, and CO2 composition ratios are controlled so that the parameter value Pcr is within the range of 1.00 to 1.02. Thereby, the gas after the reaction contains CH4 and H2O as main components and includes H2, CO2 and CO as unreacted residual components. Among these, only H2O is separated and removed by the flash distillation column 6b as in the first reaction step.

  Through the above steps, CH4, H2 and a small amount of CO2 and CO are supplied to the supply gas line 21 (CH4: 93.6%, H2: 5 in the condition 3 representative example of Table 5 described later). 3% as a main component, including CO2: 952 ppm and CO: 0.757 ppm). When the generated gas of the supply gas line 21 is supplied as a city gas raw material, it can be considered that the generated gas is supplied as it is by adjusting the molar ratio according to the required gas specification.

Hereinafter, the results of calculation of elementary reactions A and B in accordance with the process flow of the above-described embodiment using process simulator software (Aspen Plus (registered trademark)) will be described.
(A) Simulation model The simulation model used in the calculation is as follows.
The composition components were nitrogen, hydrogen, carbon dioxide, carbon monoxide, methane, and water. As the physical property value of each component, a database DB of Aspen Plus (registered trademark) was used as a pure substance. Moreover, the thermal equilibrium model utilized 'PSRK' (Predictive Redlich-Kwrong-SoaveEOS) equation of state. Since the reaction between carbon dioxide and hydrogen is carried out under pressure, it was judged that “PSRK”, which shows high accuracy especially under pressure, was appropriate.

(B) Examination of reaction conditions (b-1) Temperature dependence of reaction equilibrium composition in elementary reaction A FIG. 75 shows the composition ratio (%) of each component after the reaction when raw material CO2: H2 = 1: 1. Indicates. It can be seen that the composition ratio of CO and H 2 O increases as the reaction temperature increases, but not significantly. Further, considering the problem of catalyst poisoning by carbon monoxide due to temperature rise, it can be determined that 460-550 ° C., preferably around 500 ° C. is appropriate.
(B-2) Temperature Dependence of Reaction Equilibrium Composition in Elementary Reaction B FIG. 86 is a diagram showing the temperature dependence of the CO pass-through rate after the reaction when the raw material is supplied with CO: H2 = 1: 3. From the figure, it was found that when the reaction pressure was 1.0 MPa, the conversion rate could be maintained up to about 700 K (427 ° C.).
In consideration of the relationship between the endothermic reaction characteristics and the reaction rate, the lower limit of the reaction temperature should be 200 ° C., preferably 250 ° C. as described above.

  Using the same software as in Example 1, the concentration of each component after completion of the second reaction step when the raw material H2 / CO2 molar ratio change or the recycle ratio after each reaction step is changed is calculated. Examinations were conducted to find out the presence or absence of inflection points and the point at which the CO2 concentration was 1000 ppm or less. The process flow conforms to the above-described embodiment.

(Raw material supply condition setting)
The raw material supply ratio and the recycle ratio are set as shown in (a)-(c), the mass balance and heat balance calculation in the first reaction step and the second reaction step are performed, and CO2, CO and H2 after the second reaction step The behavior of concentration was studied.
(A) Setting condition 1
The CO2 and H2 supply amounts were substantially constant (H2 / 4CO2 = 0.85 to 1.01), and the respective recycling ratios of CO2 after the first reaction step and H2 after the second reaction step were changed.
(B) Setting condition 2
Recycle ratios of CO2 after the first reaction step and H2 after the second reaction step were made substantially constant (about 90%), and the supplied CO2 / H2 molar ratio was changed.
(C) Setting condition 3
Especially in the above setting condition 2, the recycle ratio of CO2 after the first reaction step and H2 after the second reaction step to the respective reaction systems is particularly zero, the purge amount of CO2 and H2 is 0.1%, and the H2 supply amount And the range of change in CO2 supply is increased.
(H2 / 4CO2 = 0.85 to 1.018), The behavior of CO2, CO and H2 concentration in methane after the reaction in the second reaction step and in the supply line was examined.
The above is summarized in Table 1.

  Tables 2 to 4 show the relationship between the raw material supply ratio, the CO 2 / H 2 recycle ratio, and the composition ratio parameter Pcr change with respect to the setting conditions 1-3. The reaction conditions are all the first reaction: 500 ° C., the second reaction: 300 ° C., and the reaction pressure: 3 MPa (however, only Case 37 in Table 4 is the second reaction: 282 ° C.).

（設定条件２）

（設定条件３）

(Setting condition 1)

(Setting condition 2)

(Setting condition 3)

(simulation result)
In FIG. 1, the horizontal axis is Pcr before the second reaction step (= 1 / ((3CO + 4CO2 +) / H2), and the vertical axis is the CO2 concentration and H2 concentration after the reaction. The result of condition 3 is shown in Fig. 2. Similarly, the results are shown in which the vertical axis represents CO concentration and H2 concentration.
1 and 2, the CO2 concentration and the CO concentration after the second reaction step rapidly decrease in the vicinity of Pcr≈1.0 with respect to the change in Pcr before the second reaction step under any set condition, and the logarithm It is clear that it has an inflection point with respect to the axis.

(Representative examples of gas composition near the inflection point)
Table 5 shows typical examples of gas composition near the inflection point.

(Relationship between raw material supply ratio and component concentration)
Regarding the setting condition 2, FIG. 3 shows the relationship between the CO2 concentration and the H2 concentration after the second reaction step, and FIG. 4 shows the relationship between the CO concentration and the H2 concentration. From these figures, it can be seen that, based on the stoichiometric ratio H2 / CO2 = 4, a significant decrease in CO2 and CO and an increase in residual H2 are observed.

(Optimum value of composition ratio parameter value without recycling)
For setting condition 3 (recycling ratio: 0), FIG. 5 shows the relationship between the CO2 concentration and the H2 concentration in the supply line after the second reaction step, and FIG. 6 shows the relationship between the CO concentration and the H2 concentration. Each concentration is a value after removal of H2O. The reactor temperature condition is 300 ° C. (some of the values for the 282 ° C. condition are also included).
5 and 6, the methane purity shows the maximum value (about 94%) in the vicinity of the composition ratio parameter Pcr value = 1.00 before the second reaction step, and the purity decreases at 1.00 or more. The reason is that, as is apparent from FIG. 6, the residual H2 concentration becomes extremely large when Pcr is 1.00 or more. On the other hand, the residual CO2 concentration significantly decreases with an increase in Pcr, and is particularly remarkable at 1.00 or higher.
From the above results, it is determined that Pcr = 1.00-1.02 is optimal for reducing the H2 and CO2 concentrations in the supply line and increasing the methane purity.
5 and 6 show the results of lowering the temperature condition of the second reaction step to 282 ° C. (indicated by white symbols). This examination condition corresponds to Case 37 in Table 4, and is the same as Case 35 except for the temperature condition. From this, the methane purity (93.0 vol% → 93.6 vol%) in the supply line, the absolute concentration of CO2 (2,229 molppm → 952 molppm) and H2 (58,262 molppm → 53,342 molppm) as impurities are shown to be improved. .

  The methane synthesized by the present invention can be widely applied not only to city gas supply applications but also to applications and fields where high-purity methane supply is required, such as power generation fuel and NGV (natural gas vehicle).

DESCRIPTION OF SYMBOLS 1 ... Methane synthesis apparatus 2a-2d ... Compressor 3, 5 ... Reactor 4 ... CO2 regeneration unit 6a, 6b ... Flash distillation column 7 ... PSA
8 ... Raw material line 9 ... H2 recovery line 10 ... CO2 recovery line 11, 21 ... Supply gas lines 12a, 12b, 22a, 22b ... H2O removal line

Claims (6)

A method of synthesizing methane from carbon dioxide and hydrogen,
A first reaction step of reacting carbon dioxide and hydrogen to obtain carbon monoxide;
A second reaction step of reacting carbon monoxide produced in the first reaction step with hydrogen to obtain methane;
A method for synthesizing methane from carbon dioxide and hydrogen, characterized by comprising: In the second reaction step, the composition ratio (molar conversion) of hydrogen and carbon monoxide as raw materials and carbon dioxide remaining in the first reaction step,
Composition ratio parameter (Pcr) = 1 / ((3CO + 4CO2) / H2) The feed ratio of hydrogen and carbon monoxide as raw materials, and carbon dioxide hydrogen and hydrogen so that the value is 1.0 to 1.1 Controlling the separation / recycling ratio of
The method for synthesizing methane from carbon dioxide and hydrogen according to claim 1.   The method for synthesizing methane from carbon dioxide and hydrogen according to claim 1 or 2, wherein the parameter value is controlled to 1.00 to 1.02. A step of separating only water without separating carbon dioxide after the first reaction step;
A step of separating only water without separating hydrogen after the second reaction step;
The method for synthesizing methane from carbon dioxide and hydrogen according to any one of claims 1 to 3, further comprising: Setting the reaction temperature in the first reaction step to 460-550 ° C .;
Setting the reaction temperature in the second reaction step to 250-450 ° C;
The method for synthesizing methane from carbon dioxide and hydrogen according to any one of claims 1 to 4.   The method for synthesizing methane from carbon dioxide and hydrogen according to any one of claims 1 to 5, wherein the reaction pressure in the second reaction step is set to 1.0 to 5.0 MPa.

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