DE102024102040A1 - Sorption-assisted process for the production of methane gas from carbon dioxide - Google Patents

Abstract

The present invention relates to a process for producing methane from carbon dioxide, in which a gas mixture of carbon dioxide and a proportion of gases inert with respect to the conversion of carbon dioxide to methane is converted to methane in a fixed-bed reactor filled with a metal-modified zeolite with the addition of hydrogen gas, and to the use of metal-modified zeolites for producing methane from corresponding gas mixtures. Compared to a conventional process for producing methane, the elimination of gas separation and the adsorption of the water of reaction in the reactor significantly increases the degree of conversion of _CO2 to methane and thus the efficiency of the process. The present invention further relates to a system comprising a device, during the intended operation of which a gas mixture of carbon dioxide and gases inert with respect to the conversion of carbon dioxide to methane is formed, and a fixed bed reactor which is filled with a metal-modified zeolite, and wherein the fixed bed reactor is connected to the device in such a way that gases formed during operation of the device are introduced into the fixed bed reactor via the feed line for carbon dioxide-containing gases.

Description

The present invention relates to a process for producing methane from carbon dioxide, in which a gas mixture of carbon dioxide and a proportion of gases inert with respect to the conversion of carbon dioxide to methane is converted to methane in a fixed-bed reactor filled with a metal-modified zeolite with the addition of hydrogen gas, and to the use of metal-modified zeolites for producing methane from corresponding gas mixtures. The present invention further relates to a system comprising a device, during the intended operation of which a gas mixture of carbon dioxide and gases inert with respect to the conversion of carbon dioxide to methane is formed, and a fixed-bed reactor filled with a metal-modified zeolite, wherein the fixed-bed reactor is connected to the device in such a way that gases formed during operation of the device are introduced into the fixed-bed reactor via a supply line containing carbon dioxide.

State of the art

The production of methane from carbon dioxide has long been known and was first described by Paul Sabatier and Jean-Baptiste Senderens in 1897. Named after Sabatier, the reaction involves the conversion of carbon dioxide and hydrogen to form methane and water. Since the reaction usually does not proceed to completion, especially at atmospheric operating pressures, the reactants and reaction products of methane must be separated in a subsequent step to obtain pure methane. CO ₂ + 4 H ₂ ⇄ CH ₄ + 2 H ₂ O ΔH = -165.0 kJ/mol

The negative reaction enthalpy also shows that this is a strongly exothermic reaction.

This reaction, also known as methanation, represents an important opportunity for the production of non-fossil methane or as a replacement for natural gas, which will become increasingly important in the context of the ongoing defossilization process in the coming years. In particular, this method can produce a carbon-containing product that can then be "refined" through further reactions and converted into compounds with more than one carbon atom, for example, to produce synthetic kerosene or chemical starting materials that cannot be obtained from plant-based raw materials and thus serve as a carbon sink.

However, one disadvantage of producing methane from carbon dioxide is that producing one unit of methane requires four units of hydrogen, which, for a " _CO2 -free" economy, must be produced via water electrolysis using electricity (the process is therefore also referred to as a "power-to-gas" process). Although the efficiency of the water electrolysis process has improved significantly in recent years, so-called "green hydrogen" is still an extremely scarce and expensive commodity.

An additional problem is the fact that the Sabatier reaction is reversible (depending on the system temperature). Therefore, under normal reaction conditions, the product obtained does not contain only methane and water, but rather a product with residual amounts of carbon dioxide and hydrogen starting materials, which, depending on the process pressure and temperature, can amount to up to 30%. This obviously represents a significant impediment to the economic viability of the process, because the methane must subsequently be separated from the residual amounts of carbon dioxide and hydrogen, which is an energy-intensive process.

A further problem is the carbon dioxide itself, which must be introduced into the process with a certain degree of purity (i.e., preferably free of oxygen) in order to efficiently convert it into methane. Although _CO2 product gas generated in combustion processes can be processed with relatively little effort, for example, through an extraction process with amines, even such an additional step has a negative impact on the economic viability of the process.

An alternative to methane production is through the anaerobic digestion of biogenic raw materials; the resulting product is also known as biogas. The gas produced in this process consists primarily of carbon dioxide and around 60% methane (as well as smaller amounts of other gases such as nitrogen). This lean gas can then be used directly on-site, for example, for combined heat and power generation using a CHP plant. This gas mixture is not suitable for supplying gas consumers more distantly via municipal gas networks, so a gas purification step is usually required to separate the carbon dioxide from the gas mixture. Obviously, such a step also leads to a reduced efficiency of the overall process.

Against this background, there is a need for a process for the production of methane that offers improved efficiency and conversion compared to the established power-to-gas processes or processes for the production of biogas by fermentation of biogenic raw materials.

For the production of methane from carbon dioxide, it is also known that the equilibrium of a reaction can be shifted further toward the product side by reducing the product content in the resulting mixture (based on LeChatelier's principle). For the reaction equation given above, this means that by removing water from the product mixture, the reaction equilibrium can be shifted further from the carbon dioxide/hydrogen side toward methane. This can be exploited, for example, by absorbing the water in a system such as a molecular sieve.

In Delmelle et al. Catalysts 2018, 8, 341 (https://doi.org/10.3390/catal8090341 ) describes a gas mixture of _CO2 and hydrogen in a ratio of 1:4, which is passed over a zeolite loaded with a catalytic amount of nickel, which absorbs the reaction water formed during the production of methane. For such a reaction, it has been shown at 300°C and 1 bar that, as long as the catalyst is capable of absorbing water, a product gas consisting of 90% methane is obtained.

Borgschulte et al. in Phys. Chem. Chem. Phys. 2016 (DOI: 10.1039/c5cp07686b ) describe a “sorption reactor” for the production of methane from carbon dioxide and hydrogen, in which the starting gas mixture is passed over a bed of Ni-modified zeolite to obtain a product gas with a high methane content.

In all these studies, a gas mixture of one equivalent of carbon dioxide and four equivalents of hydrogen was used as the starting product.

Description of the invention

In the investigations underlying this invention, it was surprisingly discovered that the efficiency of the conversion of carbon dioxide with hydrogen using metal-modified zeolites is not impaired by the fact that the gas mixture contains other gases besides carbon dioxide and hydrogen from which no methane can be formed during the conversion. Such gases are primarily nitrogen, but also methane gas itself, which, in a conventional reaction, would be expected to have a detrimental effect on the equilibrium of the reaction and thus on the amount of methane gas product obtained. Based on this finding, the complex separation of gases to obtain the purest possible carbon dioxide can be dispensed with, which significantly improves the efficiency of the process. Based on this finding, it is accordingly possible to add a corresponding 4-fold excess of hydrogen to a given _CO2 -containing gas mixture (which does not contain any relevant amounts of oxygen), depending on the _CO2 content, and to process the resulting mixture as part of the process so that the _CO2 contained in the gas mixture is converted to methane. Accordingly, an enrichment of the CO ₂ in the starting gas mixture is not necessary, since the efficiency of the conversion is not impaired even if the CO ₂ is present in the gas mixture only in small amounts.

According to a first aspect, the present invention accordingly relates to a process for producing methane from carbon dioxide and/or carbon monoxide, in which a gas mixture with a proportion of carbon dioxide and/or carbon monoxide and a proportion of gases inert with respect to the conversion of carbon dioxide to methane of at least 23 vol.% is converted to methane in a fixed-bed reactor filled with a metal-modified zeolite with the addition of hydrogen gas, wherein a methane content of at least 80%, based on the non-inert gases, is achieved in the product gas. In this way, the process according to the invention is clearly superior to conventional processes in which a maximum methane content of approximately 82% can be achieved under the same conditions.

Gases that are inert with regard to the conversion of carbon dioxide to methane include, for example, noble gases such as argon, but in particular the more relevant nitrogen, as well as methane gas itself, and other gases that do not react under the reaction conditions of the conversion of carbon dioxide with hydrogen. On the other hand, gases that are not inert are oxygen (which can react with hydrogen to form water) and halogens, which form hydrogen halides upon reaction with hydrogen. Water is regarded as an inert gas in the context of the specified process, although it is detrimental to the efficiency of the process according to the invention if the gas mixture contains relevant amounts of water. It is therefore preferred if the gas mixture used in the process has a residual moisture content of less than 2% and in particular less than 1%.

The requirement “inert gases of at least 23 vol.%” indicates that that, in the context of the specified process, a gas mixture should be used that contains a relevant proportion of gas that is non-reactive in the reaction of hydrogen with carbon dioxide (i.e., not just residual amounts of such a gas that can also be obtained in conventional carbon dioxide purification). The specification "23 vol.%" here refers to the gas mixture included in the reaction without hydrogen.

The requirement that "a methane content of at least 80%, based on the non-inert gases, is achieved" expresses that the process does not necessarily have to be designed in such a way that maximum conversion of the hydrogen and carbon dioxide used is achieved. This may, for example, be desirable because certain residual amounts of hydrogen and carbon dioxide can be tolerated, and the sorption-effective conversion can be used for a longer period with a corresponding process control (because less water must be absorbed) than with a process control geared to maximum conversion of hydrogen and carbon dioxide. This requirement therefore allows a certain optimization of the process control depending on the desired product composition.

The main advantage of this process is that product gases from other processes can be used directly for the conversion without the carbon dioxide having to be laboriously separated from other gases beforehand.

• i) etwa 40% Kohlendioxid, etwa 60% Methan und etwa 5% Stickstoff (solche Gemische entstehen beispielsweise bei der Herstellung von Biogas durch anaerobe Vergärung von biogenen Stoffen),
• ii) etwa 25% Kohlendioxid und etwa 70% Stickstoff (solche Gemische entstehen beispielsweise bei der Herstellung von Zement),
• iii) etwa 35% Kohlendioxid und etwa 60% Methan (solche Gemische entstehen beispielsweise bei der Abwasseraufbereitung in Kläranlagen),
• iv) etwa 35% Kohlendioxid und etwa 65% Stickstoff (solche Gemische entstehen beispielsweise bei der Verbrennung von Restmüll), und
• v) etwa 10% Kohlendioxid, etwa 20% Wasserstoff, etwa 45% Stickstoff und etwa 25% Kohlenmonoxid (solche Gemische entstehen beispielsweise bei der Vergasung von Holz)
• vi) etwa 5% Kohlendioxid, etwa 20% Wasserstoff und etwa 75% Methan (solche Gemische entstehen beispielsweise bei power-to-gas Verfahren)

With regard to the proportion of carbon dioxide in the gas mixture included in the process, the present invention is not subject to any relevant restrictions, provided that the carbon dioxide provides part of the 100% missing portion. In most cases, however, the gas mixture will contain a proportion of carbon dioxide in the range of 5 to 60 vol.%. In a preferred embodiment, the gas mixture is selected from the group comprising the following gas mixtures:

• i) about 40% carbon dioxide, about 60% methane and about 5% nitrogen (such mixtures are produced, for example, in the production of biogas by anaerobic fermentation of biogenic substances),
• ii) about 25% carbon dioxide and about 70% nitrogen (such mixtures are formed, for example, in the production of cement),
• iii) about 35% carbon dioxide and about 60% methane (such mixtures are formed, for example, during wastewater treatment in sewage treatment plants),
• iv) about 35% carbon dioxide and about 65% nitrogen (such mixtures are produced, for example, by the incineration of residual waste), and
• v) about 10% carbon dioxide, about 20% hydrogen, about 45% nitrogen and about 25% carbon monoxide (such mixtures are formed, for example, during the gasification of wood)
• vi) about 5% carbon dioxide, about 20% hydrogen and about 75% methane (such mixtures are produced, for example, in power-to-gas processes)

The 100% missing component in such gas mixtures is generally provided by water, oxygen, and nitrogen oxides, which in most cases, however, make up a maximum of 1 vol.% and preferably a maximum of 0.5 vol.% of the gas mixture. For inclusion in the process according to the invention, it is also preferred if the oxygen content in the gas mixture is as low as possible, preferably no more than 3 vol.%, more preferably no more than 1 vol.%, and in particular no more than 0.1 vol.%.

In one embodiment, the gas mixture fed into the fixed bed reactor contains at least 5 vol% and preferably at least 10 vol% methane.

As mentioned above, the inert gases with respect to the conversion of _CO2 to methane can be gases selected from one or more of nitrogen, noble gas, water, hydrocarbons, ammonia, or methane. It is also possible for the gas mixture involved in the process to contain carbon monoxide, which, like carbon dioxide, can be converted to methane within the process (only three units of hydrogen are required for such a conversion).

The zeolite used in the water absorption process can be any type of zeolite capable of absorbing water when contacted with a gas containing water or moisture. Preferred zeolites for this purpose are 13X zeolite, 3A, 4A zeolite, or 5A zeolite.

The zeolites used in the process can have any pellet shape (e.g. cylindrical), but preferably have a spherical shape with a diameter in the range of 0.5 to 5 mm, whereby a diameter in the range of 1.0 to 2.5 mm can be stated as particularly preferred.

For use in the process according to the invention, the zeolite is modified with a metal that is catalytically active in the conversion of carbon dioxide with hydrogen to methane. Preferred metals for zeolite modification include nickel, cobalt, molybdenum, and ruthenium. Nickel is preferred due to its cost-effectiveness combined with its activity. It is also possible for the zeolite to contain several of the aforementioned metals.

The metal is contained in the zeolite in an amount sufficient to ensure sufficient catalytic activity. A suitable metal amount can be specified within a broad range, e.g., 0.1 to 10 wt.%, although a lower proportion, e.g., in the range of 0.5 to 8 wt.% and in particular 1.0 to 5 wt.%, can be specified as particularly advantageous. The modification of the zeolite with the metal can be achieved, for example, by treating the zeolite with a solution of the metal salt in water, subsequently removing the water by evaporation, and subsequently heating the zeolite to high temperatures, e.g., approximately 500°C, and reducing the metal content using a reducing gas mixture. Zeolites that can be modified in this way are available, for example, from Zeochem.

The actual process is preferably carried out in a fixed-bed reactor having a cylindrical reactor chamber containing the metal-modified zeolite pellets. This reactor has an inlet at one end of the cylinder and an outlet at the other. The gas mixture and hydrogen are fed onto the zeolite bed through the inlet and flow through it until it is discharged from the reactor via the outlet. The hydrogen can be mixed with the gas mixture before being fed into the reactor, or it can be fed into the reactor separately. To ensure good conversion, homogeneous mixing of the gases fed into the reactor should be ensured. The person skilled in the art is well aware of the measures by which this can be achieved, so they require no further discussion here.

In a preferred embodiment, the inlet and outlet of the reactor are conical, with the larger diameter side of the cone facing the fixed bed in the reactor. To ensure that the fixed bed is held in place during filling into the reactor, a permeable plate, a sieve, or other porous fixture can be provided at the outlet to prevent the filled fixed bed material from falling out of the reactor through the outlet. The fixed bed is preferably formed exclusively from the metal-modified zeolite.

The gas mixture is preferably passed through at a flow rate of the reactive gas (i.e., the gas formed from the hydrogen and the gas mixture) that corresponds to 30 to 250 times the bulk volume of the catalyst per hour (also referred to as the gas hourly space velocity (GHSV)). This gas quantity refers to the amount of gas fed to the reactor. The process is particularly preferably carried out at a GHSV in the range of 50 to 150. The person skilled in the art will recognize that at a slower flow rate, a higher proportion of methane is formed than at higher flow rates, which means that absorption by the zeolite may not be fast enough to completely remove the water from the gas mixture.

When adjusting the process parameters, care must be taken to keep the mass transfer zone of gas adsorption as short as possible in the longitudinal direction of the flow, as homogeneous as possible in the transverse direction, and achieve a locally limited equilibrium between the product formation rate and adsorption rate. Parameters such as flow velocity, reactant concentration, adsorption capacity, and diffusion rate of water into the zeolite pellets are relevant for this, as are the zeolite type, composition, and porosity, and especially the adsorption temperature and its distribution in the reactor bed.

It is also advantageous if the reactor for the conversion of carbon dioxide and hydrogen is kept at an elevated temperature, in particular at a temperature of more than 100°C. A particularly suitable fixed bed temperature (i.e. temperature which prevails while the gas mixture is passed through the zeolite bed) can be specified as a temperature in the range from 150°C to 400°C, preferably in the range from 200°C to 350°C and more preferably at about 300°C. If the reaction temperature is too high, the catalytic activity of the catalyst metal increases (so that the catalysis time is shortened), but the water absorption capacity of the zeolite is impaired, so that the reaction temperature should not exceed 400°C in most cases. Since the conversion reaction itself is exothermic, this temperature can be adjusted, for example, by setting a suitable flow rate. Alternatively or additionally, the temperature can be controlled by means of a heating jacket around the reactor or by heating elements mounted inside the reactor or by appropriate devices for cooling the reactor. For such control, it is expedient if the reactor is equipped with a sensor that measures the temperature inside the fixed-bed reactor and a control unit that adjusts heating or cooling devices depending on the internal temperature and the desired reaction temperature. Furthermore, it is advantageous and preferred according to the invention if the reactor wall is made of a material with high thermal conductivity in order to effectively dissipate heat generated in the reactor.

The process according to the invention generally does not require the application of elevated pressure, which is used in conventional processes for producing methane from carbon dioxide and water to shift the reaction equilibrium more toward methane and water. Accordingly, in the context of the process of the present invention, it is preferred if the process is carried out at a pressure (absolute) in the range of up to a maximum of 2 bar, in particular in the range of 0.8 to 1.8 bar, and even more preferably at about 1 bar.

For certain catalyst compositions, pressure can have a supportive effect on methane formation or can realize process-related advantages. For example, hydrogen is often present at 30 to 35 bar due to the electrolyzer pressure for hydrogen production. To avoid process efficiency losses due to pressure reduction and downstream compression, for example, for grid feed-in, it is preferred in one embodiment of the process according to the invention if the process is carried out at a pressure in the range of 5-35 bar (absolute), and in particular 8-16 bar.

As already mentioned above, the process can be controlled, e.g., by adjusting the flow rate through a zeolite bed containing zeolites modified according to the invention, such that not a maximum conversion of carbon dioxide to methane is achieved, but rather only a substantial portion of this carbon dioxide is converted, so that a methane content of at least 80%, based on the non-inert gases, is achieved. In one embodiment, however, it is preferred if the process is controlled by adjusting the reactive gas flow rate and temperature such that a methane content of at least 85%, more preferably at least 90%, even more preferably at least 95%, even more preferably at least 98%, and even more preferably in the range from 98% to 99.9% (in each case based on the non-inert gases) is achieved. This ensures that the product gas obtained has a suitable calorific value or a quality that allows it to be used for synthetic purposes.

Obviously, the water absorption capacity of the zeolite is exhausted after a certain time after passing through the reaction of carbon dioxide and hydrogen, which can be seen from the decrease in the methane concentration in the gas leaving the reactor. In this case, zeolite can be regenerated by a treatment in which water is removed from the zeolite. For regeneration, the zeolite can be heated (temperature swing), exposed to a negative pressure (pressure swing), or it is possible to flush the zeolite with a dry gas to expel the water. For this purpose, flushing with hydrogen gas, in particular flushing with methane gas, has proven particularly useful within the scope of the process specified here. Accordingly, in a preferred embodiment, the process specified here also comprises a step of stopping the flow of carbon dioxide-containing gas and a step of passing hydrogen gas, preferably methane gas, through the reactor to regenerate the zeolite (i.e., to expel the water adsorbed in the zeolite).

The quality of the resulting product gas can be further improved with respect to the methane it contains by subjecting it to downstream gas separation. Accordingly, in a preferred embodiment, the process according to the invention comprises a step of separating the resulting product gas into methane and gases inert with respect to the conversion of carbon dioxide to methane. Processes that can be used for such treatment include, for example, separations using membranes, which are available, for example, under the name SEPURAN from Evonik.

A further aspect of the present invention relates to an apparatus for carrying out a process with a downstream gas separation as described above, wherein the apparatus comprises a fixed bed reactor filled with a metal-modified zeolite, feed lines for hydrogen and carbon dioxide-containing gases into the fixed bed reactor and an outlet of the fixed bed reactor, as well as a gas separation unit which is fluidly connected to the outlet of the fixed bed reactor and is designed to separate methane from other gases.

The specified device preferably comprises a unit for determining the composition of the gas leaving the outlet of the fixed bed reactor, which at least determines the concentration of methane gas and/or water in the gas This allows the reaction to be monitored and stopped when the zeolite's water absorption capacity decreases, which can be detected by a decrease in the methane content or an increase in the water content in the product gas.

As already mentioned above, a key advantage of the described process is that exhaust gases or biogas, such as that obtained from biogas plants, can be directly incorporated into the process without the need for extensive prior gas treatment to isolate the carbon dioxide. To effectively utilize this potential, it is clearly advantageous to combine a plant that produces exhaust gases or biogas with a plant that can convert the carbon dioxide contained in the exhaust gases or biogas with hydrogen to produce methane.

Accordingly, a further aspect of the present invention relates to a system comprising a device, during the intended operation of which a gas mixture of at least 10 vol.% carbon dioxide and at least 40 vol.% gases inert with respect to the conversion of carbon dioxide to methane is formed, a fixed-bed reactor filled with a metal-modified zeolite and having feed lines for hydrogen and carbon dioxide-containing gases into the fixed-bed reactor and an outlet, wherein the device is fluidly connected to the fixed-bed reactor such that gases formed during operation of the device are introduced into the fixed-bed reactor via the feed line containing carbon dioxide. This means that there are no devices for separating the carbon dioxide from other gases between the device in which the carbon dioxide-containing gases are generated and the fixed-bed reactor. The device, during the operation of which a gas mixture of at least 10 vol.% carbon dioxide and at least 40 vol.% gases inert with regard to the conversion of carbon dioxide to methane is formed, is preferably a plant for the anaerobic fermentation of biogenic substances, a power-to-gas or power-to-X plant, a plant for the production of cement, a wastewater treatment plant, a landfill, a waste incineration plant, a wood gasification plant, or a gas or coal-fired power plant.

To provide the hydrogen required for the conversion of carbon dioxide to methane, it is also advantageous for the system to further comprise a hydrogen production device, preferably a water electrolysis device, which is fluidly connected to the supply line of the fixed-bed hydrogen reactor. This allows for the creation of a highly integrated methane production system in which the exhaust gases or product gases can be upgraded to methane gas at the same location.

For the aforementioned device or the described system, it is further advantageous if the fixed-bed reactor is designed as a double-tube reactor with an outer tube and at least one, preferably several, smaller-diameter tubes arranged within this tube, which are adapted for the passage of a cooling or heating medium, and wherein the space formed by the outer tube and the smaller-diameter tubes arranged therein is filled with the metal-modified zeolites. In a particularly preferred embodiment, the device or system comprises a plurality of such fixed-bed reactors, e.g., at least two, at least three, or 2 to 10 fixed-bed reactors. In this way, it is possible to operate the device or system continuously; if the zeolite catalyst in one of the fixed-bed reactors needs to be regenerated, it can be disconnected from the system while the reaction continues in other fixed-bed reactors.

In yet another aspect, the present invention relates to a method for operating the device or system described above.

In yet another aspect, the present invention relates to the use of metal-modified zeolites for producing methane from gas mixtures containing carbon dioxide and at least 10 vol.% of gases inert to the conversion of carbon dioxide to methane. Embodiments indicated above as preferred for the process are also considered preferred for the use, provided that this does not result in an obvious contradiction for the use.

In the following, the present invention is further illustrated by means of some embodiments, which, however, should not be considered in any way as limiting the interpretation or scope of the present application:

Examples:

Example 1:

Nickel catalysts were applied to a commercial zeolite LTX (13X) (pellets, 2 mm, Zeochem) by a wet impregnation process from a solution of nickel nitrate hexahydrate (Ni(NO ₃ ) ₂ 6H ₂ O, Sigma Aldrich) in water. Zeolite was dried at 140 °C for 12 hours prior to synthesis. The mixture of zeolite spheres and Ni solution was stirred for 1 h at room temperature (RT), and then the solvent was removed at 70 °C in a rotary evaporator. The samples were dried again at 140 °C for another 12 hours and then calcined in a muffle furnace at 500 °C for 4 hours to remove residual nitrates. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of the catalysts showed that all samples contained approximately 4 wt% nickel before calcination and approximately 5 wt% nickel after calcination.

The catalytic screening of material systems for _CO2 methanation was carried out in a 1.5 L plate-type fixed-bed reactor with dimensions of 25 x 150 x 400 mm, coupled to a Pfeiffer OmniStar GSD 320 O1 mass spectrometer (MS). The reactor contained a catalyst mass of 1100 g. The reactions were carried out under atmospheric pressure. Prior to reaction, the samples were first reduced for 1 hour at 500 °C in 800 ml/min of a 1:1 molar ratio of _H2 /Ar. After cooling to 300 °C under this reducing atmosphere, CO ₂ methanation was carried out with a GHSV (gas hourly space velocity) of 92 h ^-1 , using as test gas a gas mixture of 30% CO ₂ , 70% argon and an addition of hydrogen gas to adjust a CO ₂ to H ₂ molar ratio of 1:4.05 (5% excess hydrogen to avoid coking).

The product gas discharged from the reactor was measured by mass spectrometry and as a function of the reaction time to determine the conversion of _CO2 to methane. The results of the measurements are shown in 1 where the relative proportions of methane, CO ₂ and CO are shown (the argon content of the product gas is not shown).

1 shows that the relative methane content in the product gas increases to more than 90% after a short time, while the proportion of _CO2 (as starting gas) and CO (as by-product of the reaction) decreases to almost 0%. After just over 200 minutes, the methane content decreases, while the proportion of _CO2 and CO in the product gas increases. This indicates that the zeolite's water absorption capacity is exhausted.

QUOTES CONTAINED IN THE DESCRIPTION

This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Delmelle et al. Catalysts 2018, 8, 341 (https://doi.org/10.3390/catal8090341 [0011]
• Phys. Chem. Chem. Phys. 2016 (DOI: 10.1039/c5cp07686b [0012]

Claims (15)

A process for producing methane from carbon dioxide, characterized in that a gas mixture with a proportion of carbon dioxide and a proportion of gases inert with respect to the conversion of carbon dioxide to methane of at least 40 vol.% is converted to methane in a fixed-bed reactor filled with a metal-modified zeolite with the addition of hydrogen gas, wherein a methane proportion of at least 80%, based on the non-inert gases, is achieved in the product gas. Procedure according to Claim 1 , wherein the gas mixture introduced into the fixed bed reactor has a CO ₂ content in the range of about 5 to about 60% and is preferably selected from the group of gas mixtures comprising about 40% carbon dioxide/about 60% methane/about 5% nitrogen, about 25% carbon dioxide/about 70% nitrogen, about 35% carbon dioxide/about 60% methane, about 35% carbon dioxide/about 65% nitrogen, about 10% carbon dioxide/about 20% hydrogen, about 45% nitrogen/about 25% carbon monoxide, and about 5% carbon dioxide/20% hydrogen/75% methane, preferably wherein the 100% missing portion is provided by water, oxygen and nitrogen oxides. Procedure according to Claim 1 or 2 , wherein the gases inert with respect to the conversion of CO ₂ to methane comprise one or more of nitrogen, noble gas, water, hydrocarbons, ammonia or methane. Method according to at least one of the Claims 1 until 3 , wherein the zeolite is selected from 13X zeolite, 3A, 4A zeolite or 5A zeolite. Process according to at least one of the preceding claims, wherein the metal of the metal-modified zeolite is selected from the group comprising nickel, cobalt, _[MB1] molybdenum and ruthenium, and is preferably based on nickel. Process according to at least one of the preceding claims, wherein the amount of metal in the metal-modified zeolite is set in the range of 0.1 to 10 wt.%, preferably in the range of 0.5 to 8 wt.%, and more preferably in the range of 1 to 5 wt.%. A process according to at least one of the preceding claims, wherein the process is carried out at a reactive gas flow rate of 30 to 250 times the reactor volume per hour, based on the total amount of gas fed to the reactor. Process according to at least one of the preceding claims, wherein the process is carried out at a fixed bed temperature in the range of 150°C to 400°C and preferably in the range of 200°C to 350°C. Process according to at least one of the preceding claims, wherein the process is controlled by adjusting the reactive gas flow rate and temperature so that a conversion of the carbon dioxide fed into the reactor into methane of at least 96%, preferably at least 98% and even more preferably in the range from 98% to 99.9% is achieved. Process according to at least one of the preceding claims, characterized in that the product gas obtained from the process is subjected to a step of separating methane and gases inert with respect to the conversion of carbon dioxide to methane. Device for carrying out a method according to Claim 10 , wherein the device comprises a fixed bed reactor filled with a metal-modified zeolite, feed lines for hydrogen and carbon dioxide-containing gases into the fixed bed reactor and an outlet of the fixed bed reactor, and a gas separation unit which is fluidly connected to the outlet of the fixed bed reactor and is designed to separate methane from other gases. System comprising a device, during the intended operation of which a gas mixture of at least 10 vol.% carbon dioxide and at least 40 vol.% gases inert with regard to the conversion of carbon dioxide to methane is formed, wherein the device is preferably selected from a plant for the anaerobic fermentation of biogenic substances, a plant for the production of cement, a wastewater treatment plant, a landfill, a waste incineration plant, a wood gasification plant, or a gas or coal-fired power plant, a fixed bed reactor which is filled with a metal-modified zeolite and has feed lines for hydrogen and carbon dioxide-containing gases into the fixed bed reactor and an outlet, wherein the device is fluidly connected to the fixed bed reactor such that gases formed during operation of the device are introduced into the fixed bed reactor via the feed line for carbon dioxide-containing gases. System according to Claim 12 , further comprising a device for producing hydrogen, preferably a water electrolysis device, which is fluidly connected to the supply line of the fixed bed reactor for hydrogen. Device or system according to one of the Claims 11 until 13 , wherein the fixed bed reactor is designed as a double tube reactor with an outer tube and at least one, preferably several, tubes of smaller diameter arranged in this tube, which are adapted for the passage of a cooling or heating medium, and wherein the space formed by the outer tube and the tubes of smaller diameter arranged therein is filled with the metal-modified zeolites, preferably, wherein the device or the system has a plurality of such fixed bed reactors. Use of metal-modified zeolites for the production of methane from gas mixtures containing carbon dioxide and at least 10 vol.% of gases inert with respect to the conversion of carbon dioxide to methane.