IMPROVED PROCESS FOR THE ONE-STEP CONVERSION OF CARBON DIOXIDE AND RENEWABLE HYDROGEN TO LOW-CARBON METHANE

Cross-Reference to Related Application

This application claims the benefit of U.S. Provisional Patent Application No. 63/473,936, filed July 8, 2022, the entire content of which is hereby incorporated by reference into this document.

Field of the Invention

The field of the invention is a process to produce pipe-line quality e-methane from low carbon electricity and captured CO ₂ in one step and at temperatures below about 350 °C.

Background of the Invention

The increase in global atmospheric CO ₂ concentrations has been linked to changes in the earth’s climate. The combustion of fossil fuels in various engines and thermal systems produces atmospheric CO ₂ . Concerns about climate change have led to significant societal changes toward renewable or low carbon electricity. This has also led to increasing activity to decarbonize the economy.

Renewable or low-carbon electric power can be used to produce synthetic methane', kerosine, methanol or other chemicals; which can be gases or liquids. This process is often referred to as Power-to-X (also P2X) which refers to several electricity conversion, energy storage, and reconversion pathways that use renewable power, from wind, solar, nuclear, and other sources. P2X in some cases may also refer to the use of surplus electric power, typically during periods where fluctuating renewable energy generation exceeds load.

Specifically, low-carbon methane produced using renewable electric power is referred to as “e-methane”. Electrical power produced from solar, wind or other sustainable sources is commonly called “decarbonized electricity”. This renewable energy can be used to produce low- carbon H ₂ using electrolysis. The H ₂ when mixed with captured CO ₂ can be converted to low- carbon e-methane. One of the advantages of e-methane is that it can be introduced into the well- established natural gas pipeline distribution system; compressed and transported to another location where the e-methane is converted to H ₂ for the production of low-carbon fuels and chemicals; used as a low-carbon energy source for heating and cooling; compressed and used as a low-carbon fuel for engines; and as a low-carbon alternative to many other natural gas uses (Yugo, 2019).

Currently at least two main steps are required to produce e-methane from low-carbon H ₂ and captured CO ₂ . The first step is the hydrogenation (usually referred to as the reverse water gas shift, or RWGS, reaction) of the CO ₂ to produce CO and H ₂ (syngas) when excess H ₂ is used in the process. The CO ₂ hydrogenation is typically carried out catalytically. The second step is the catalytic conversion of the syngas to e-methane.

In order to convert the CO ₂ efficiently, the CO ₂ hydrogenation reaction requires temperatures of greater than about 700 °C at the inlet. The objective of the e-methane is that is that it allows for a significant reduction in emissions, since it is produced from decarbonized hydrogen and CO ₂ . Due to this objective, it is required that the processes use electrical heaters powered by renewable energy. This represents a large amount of renewable energy input since the CO ₂ hydrogenation is endothermic.

The conversion of the syngas to CH ₄ is highly exothermic - giving off a heat that needs to be removed continuously from the reactor system.

Several processes to convert syngas derived from coal or biomass were developed in the

1970's. Technology development ceased following the drop in oil price, and are now primarily used as purification steps. Currently, methanation of CO and CO ₂ are widely applied in ammonia synthesis plants, as well as refineries and hydrogen plants.

Thus, the state-of-the-art requires the simultaneous provision of input heat in the syngas formation step, and removal of heat in the syngas conversion step (Aryal et al, 2021; Li et al, 2022). Although the latter partially offsets the former, the net requirement is an input of heat to facilitate the integrated process. Furthermore, and of great practical significance, the process management of these two processes present engineering and operational challenges - with resultant deleterious impacts on both process efficiency and capital cost requirements.

There is accordingly a need in the art for novel processes to produce methane.

Brief Summary of the Invention

The invention describes an improved, one-step process for e-methane production from renewable H ₂ and CO ₂ mixtures taking advantage of new and improved catalysts and processes that can perform the CO ₂ C reaction efficiently at temperatures less than 350°C.

Brief Description of the Drawings

FIG. 1 shows the process of converting CO ₂ and H ₂ to e-CH ₄ using a CO ₂ C reactor. For this figure: Stream 1 is low-carbon electric power; Stream, 2 is water; Stream 3 is H ₂ ; Stream 4 is 02; Stream 5 is CO ₂ ; Stream 6 is CO ₂ C Reactor feed; Stream 7 is CO ₂ C Product; Stream 8 is portion of CO ₂ C product; Stream 9 is e-methane; Stream 10 is water; Stream 11 is CO ₂ recovered; Unit 1 is Electrolyzer; Unit 2 is CO ₂ C Reactor; Unit 3 is Product Processing Unit.

FIG. 2 shows the CO ₂ C process as described in Example 1. For this figure: Unit 200 is CO ₂ C Rx; Unit 201 is Water Knockout; Unit 202 is Amine Contactor; Unit 203 is Amine Regenerator; Unit 204 is H ₂ Pressure Swing Adsorption; Stream 101 is Feed to CO ₂ C Rx; Stream 102 is Reactor Effluent; Stream 103 is Water; Stream 104 is Methane, H ₂ , CO ₂ ; Stream 105 is Methane, H ₂ feed to 204; Stream 106 is CO ₂ ; Stream 107 is H ₂ ; Stream 108 is Methane; Stream 109 is C02/Amine Solution; Stream 110 is Amine to Contactor.

FIG. 3 shows a multi-tubular CO ₂ C reactor as described in Example 1. For this figure: 111 is Water/Stream from Steam Drum; 112 is Saturated Steam to Steam Drum.

FIG. 4 shows the CO ₂ C process as described in Examples 2 and 3. For this figure: Unit 200 is CO ₂ C Reactor; Unit 201 is Water Knockout; Unit 202 is Amine Contactor; Unit 203 is Amine Regenerator; Unit 401 is Syngas Methanation Reactor System; Unit 402 is Methanation Reactor Product Separator; Stream 301 is Feed to CO ₂ C Rx; Stream 302 is Reactor Effluent; Stream 303 is Water; Stream 304 is Methane, H ₂ , CO ₂ ; Stream 305 is Methane, H ₂ feed to 401; Stream 306 is CO ₂ ; Stream 307 is Methanation Reactor Effluent; Stream 308 is Methane; Stream 309 is CO ₂ ; Stream 310 is Methanation Produced Water; Stream 311 is CO ₂ /Amine Solution; Stream 312 is Amine to Contactor; Stream 313 is Medium Pressure Steam.

Detailed Description of the Invention

A “direct” one-step process would have the substantial advantage of reducing the requirements of these two (coupled) thermal effects - into a single, slightly exothermic (net) reaction with a required heat management challenge of significantly reduced magnitude. The benefits derived from this reaction engineering improvement, relative to the state-of-the-art, include increased process efficiency, operational simplicity, and reduced capital equipment costs.

The objective of the present invention is to take advantage of new and improved processes and catalysts that can facilitate the efficient, direct CO ₂ conversion (CO ₂ C) reaction to e-methane at temperatures less than about 350°C in one step.

The catalyst described in this document for the conversion of CO ₂ and H ₂ at low > temperatures typically meets the following specifications : 1. The catalyst contains low-cost and readily available constituents and nominal levels (< 0.3%) of precious metals.

2. The catalyst is robust (e.g., Rockwell hardness greater than Mohr 03-04).

3. It is chemically and physical stable up to about 1,100 °C (2,012 F). This means that the physical property measurements such as BET surface area are within 20%, more preferably 10%, of the ambient temperature.

4. It can be loaded readily into tubular catalytic reactors.

5. The pressure drop in the catalytic reactor is acceptable.

6. The catalyst activation (reduction with H ₂ ) can be carried out in-situ at 275-350 °C.

7. The conversion of CO ₂ to CH ₄ is greater than about 85% at 275-350 °C.

8. The selectivity of CH ₄ production is greater than about 99% at 275-350 °C.

9. The catalyst has a long lifetime (2-3 years) and it doesn’t require systematic reactivation (reduction).

Since the catalytic conversion of CO ₂ and H ₂ mixtures to CH ₄ is very exothermic, this reaction should typically be carried out in a tubular reactor with an effective method of removing the heat from the tubes. The maximum, practical operating temperature for a tubular reactor is 275-325 °C (Wikipedia, 2022). Although fluidized and slurry bed reactors can operate at higher temperatures up to 325-350 °C, such reactors have not been developed for distributed plants (e.g., less than about 25,000 barrels/day of product output) (Steynberg, 2004) .

The most common catalysts are nickel-based since this metal is abundant and inexpensive. Table 1 summarizes the performance of several Ni-based catalysts at 275-400 °C. However, the Ni-based catalysts listed in Table 1 suffer from sintering, the creation of gas-phase nickel carbonyls, and formation of carbon deposits (Li et al, 2022; Tan et al, 2022). The only catalyst that was tested for more than 40 hours was reported by Onrubia- Calvo (2022) for which the CO ₂ conversion decreased by about 3% after 72 hours.

Alarcron (2019) described the performance of a 30 wt.% nickel catalyst impregnated on AI2O3 and the 30 wt.% Ni catalyst with a 30 wt.% CeO ₂ 'promoter impregnated on AI2O3. The catalysts were activated at 500 °C resulting in a catalyst surface area of about 5 m ² /g. The 30% Ni on AI ₂ O ₃ converted a 4.0/1.0 mixture of H ₂ /CO ₂ to CH ₄ at 300 °C with a CO ₂ conversion efficiency of 34% and a selectivity of 100%. The addition of 30% CeO ₂ increased the CO ₂ conversion efficiency to 57% with a selectivity of 100%. However, the CO ₂ conversion efficiency rapidly dropped to 10% at 250 °C. The catalyst activation temperature of 500 °C is not feasible to achieve in a commercial scale tubular reactor, and the CO ₂ conversion efficiencies of 57% at 300 °C don’t meet the required minimum CO ₂ conversion efficiency of 85% at this temperature. Although, the CO ₂ conversion efficiency remained constant for the Ni/CeO ₂ catalyst for 110 hours, this is not a long enough test to assess long-term catalyst performance.

Jia et al (2019) described a zirconia (ZrO ₂ ) supported Ni catalyst for CO ₂ methanation which was synthesized by impregnation followed by plasma calcining at 150 °C or thermal calcining at 500 °C. The catalyst was reduced with H ₂ at 500 °C. The efficiency of CO ₂ conversion was 72% and 35% at 300 °C for the plasma and thermally calcined catalyst, respectively. Since this catalyst was tested for less than 40 hours, the long-term durability of this catalyst is not known. In addition, it would not be possible to reduce the catalyst in-situ at 500 °C.

Although Ru, Pd and Rh based catalysts show much better long-term stability than the

Ni-based catalysts, these precious metal catalysts are expensive, and they do not meet the requirements of better than 85% CO ₂ conversion efficiency at 275-350 °F.

Table 1 - Performance of Nickel-Based Catalysts for CO2 Methanation

The improved catalysts for the efficient conversion of CO ₂ and H ₂ to CH ₄ described in this document consist of the solid-solution catalysts NiiMg, Cu ₃ Ni or Cu ₂ Mg. These solid solution catalysts are formed at high temperatures (up to 1,100 °C) when the two metals in the catalysts shown above have similar crystal structures, atomic radii, electronegativities and valences.

The Ni ₂ Mg catalyst was manufactured by mixing two moles of nickel acetate powder with one mole of magnesium acetate powder and producing pellets of the appropriate size (typically 3-10 mm) using a high-pressure, pellet press. The pellets were then calcined at 1,000- 1,100 °C. The Cu ₃ Ni and Cu ₂ Mg catalysts were produced in a similar manner by producing pellets from the metal acetate salts of Cu, Ni and Mg, and then calcining the pellets at 1 ,000- 1,100 °C . The nitrate salts of the Cu, Ni and Mg may also be used to produce the catalyst but NO ₂ and NO emissions are produced during the calcining process which is difficult to control. The calcining of acetates produces primarily CO ₂ with some acetic acid. The acetic acid emissions are relatively easy to control by water scrubbing.

The Ni2Mg, Cu ₃ Ni and Cu ₂ Mg catalysts are robust, and they do not change chemically and physically up to about 1,000 °C. As an example, the conversion efficiency of the Ni ₂ Mg catalyst did not decrease (less than 2%) over a period of 1,750 operating hours in a process development unit (PDU). After the tests were completed, about 500 mg of the catalyst from the PDU was loaded into a laboratory tubular reactor and heated to 650°C under an air flow of 100 standard cubic centimeters per minute (seem) to determine if any carbon (coke) was formed. The exhaust from the reactor was continuously monitored with a mass spectrometer to detect the formation of CO ₂ , which would be consistent with carbon burning off. Since no CO ₂ was detected, it was verified that no coke was formed.

FIG. 1 shows an integrated process to produce methane from CO ₂ and low-carbon electric power. Feed stream 1 is the low-carbon electric power. Low-carbon electric power includes but is not limited to wind power, solar power nuclear power, power generated from biomass or renewable natural gas, and hydropower. Feed stream 2 is water. These two feed streams are used in Unit 1 , the electrolyzer. In the electrolyzer, water and low carbon energy are used to produce H ₂ and O ₂ . H ₂ is produced by electrolysis of water. Electrolyzers consist of an anode and a cathode separated by an electrolyte. Different electrolyzers function in slightly different ways. Different electrolyzer designs that use different electrolysis technology that are used include alkaline electrolysis, polymer electrolyte membrane (PEM) electrolysis, solid oxide electrolysis, high temperature electrolysis and other emerging / types of electrolysis. Different electrolytes that are used including the liquids KOH and NaOH, and with or without activating compounds. Activating compounds are added to the electrolyte to improve the stability of the electrolyte. Most ionic activators for the hydrogen evolution reaction are composed of an ethylenediamine-based metal 'chloride complexes. Different electrocatalysts

I are used on the electrodes including many different combinations of metals and oxides like / , ,

Raney-Nickel-Aluminum, which are enhanced by adding cobalt or molybdenum to the alloy.

The products from the electrolyzer are a stream comprising H ₂ called stream 3 in FIG. 1 and a stream comprising O ₂ called stream 4. Because renewable or low-carbon energy sources are utilized, the electrolyzer produces green H ₂ . Other forms of H ₂ generation that may use renewable sources may also be used including pyrolysis and gasification of biomass; steam reforming of hydrocarbons with carbon capture, steam reforming of CH ₄ in biogas; and other processes. H ₂ may also be sourced from geological sources.

In FIG. 1, stream 5 is a stream that comprises CO ₂ . CO ₂ is obtained from several sources. There are may stationary sources that produce large amounts of CO ₂ including power plants, fertilizer production, ethanol plants, municipal sewage treatment systems, steel and iron manufacturing, oil refining, chemical production, and cement plants.

Typically, an alkylamine is used to remove the CO ₂ from the gas stream. Alkylamines used in the process include monoethanolamine, diethanolamine, methyl diethanolamine, diisopropylamine, aminoethoxy ethanol, or combinations thereof. Metal Organic Framework (MOF) materials have also been used as a means of separating CO ₂ from a dilute stream using chemisorption or physisorption. Other methods to produce concentrated CO ₂ steams include chemical looping combustion where a circulating metal oxide material captures the CO ₂ produced during combustion processes. CO ₂ can also be captured from the atmosphere in what is called direct air capture (DAC).

Captured CO ₂ can be converted into useful low-carbon products such as fuels (e.g., synthetic natural gas, diesel fuel, gasoline blend stocks, jet fuel, and other fuels) and chemicals (e.g., solvents, olefins, alcohols, aromatics, etc.). This is what is meant by low-carbon, very low- carbon, zero carbon, or negative carbon fuels and chemicals.

The CO ₂ that comes from industrial or biological process, or is captured from the atmosphere, or that is available from a commercial CO ₂ pipeline is not generally pure. These CO ₂ streams typically contain sulfur containing compounds up to 2,000 parts per million and hydrocarbons up to 10 vol. %. Purification of the CO ₂ including the removal of sulfur containing compounds and hydrocarbons is necessary to avoid issues with downstream processing. After purification, the purified CO ₂ is suitable for the generation of low-carbon or zero-carbon fuels and chemicals as per this invention.

At least a portion of the stream 3 comprising hydrogen is blended with the stream 5 comprising CO ₂ to produce a stream 6 or CO ₂ C Reactor feed stream. The ratio of H ₂ /CO ₂ is from 2 to 6, or preferably from 3 to 4. The CO ₂ and H ₂ in stream 6 are reacted to products in a CO ₂ C reactor shown as Unit 2 in FIG. 1. The CO ₂ C reactor operates at temperatures of 250 to 425°C, and more preferably from 275 to 350°C. With a nickel-based catalyst, operating temperatures below 220°C should be avoided due to formation of highly toxic nickel carbonyls. The primary reactions that may occur in the CO ₂ C reactor are the following:

The catalytic methanation of CO and CO ₂ has been widely studied since its discovery by Sabatier and Senderens in 1902. The first reaction, Eq. 1, is the Reverse Water Gas Shift reaction. At standard temperature, the reaction is endothermic and in other processes requires much higher temperatures than the preferred CO ₂ C reactor temperatures. The second reaction, Eq. 2, is the methanation reaction where CO and H ₂ are reacted to produce CH ₄ and H ₂ O. Methanation is very exothermic as can be seen with the high negative enthalpy of reaction

The improved catalysts described previously, Ni2Mg, Cu ₃ Ni or Cu2Mg, are used in the CO ₂ C reactor to facilitate the chemical reactions. These catalysts may also be supported on spinels with surface areas greater than about 10 m ² /g)), such as magnesium aluminate at concentrations up to about 25 wt.%.

The product from the CO ₂ C reactor is shown as stream 7 in FIG. 1. At least a portion of stream 7 becomes stream 8 and is fed to the Product Processing unit, Unit 3 in FIG. 1. The product processing unit can be a combination of various processes that allow the separation or additional processing of the CO ₂ C reactor product stream. Various embodiments are done depending on the products, yields, and selectivity observed in the CO ₂ C reactor. Each of the reactions that occur in the CO ₂ C reactor produces water. So, the first step in the product processing unit is to reduce the temperature of reactor product stream. This can be done by many different means including heat exchange with the cooler feed stream, heat exchange with other cooler stream, and with a cooling water heat exchanger. The objective of the cooling is to allow the condensation of water that may be produced in the CO ₂ C. When the CO ₂ conversion in the CO ₂ C reactor is less than about 90%, then the unreacted CO ₂ is removed prior to further processing after the water removal. CO ₂ removal from the stream can be done by any number of available techniques. The removal methods include the use of physical solvents. Physical solvents include refrigerated methanol that is used in the Rectisol process and the Solexol process that uses dimethyl ethers of polyethylene glycol to capture the CO ₂ from the gas stream. Other means include the use of membranes selective for CO ₂ removal. Another means for CO ₂ removal is to use aqueous solutions of alkylamines (referred to as amines) to chemically capture the CO ₂ . The amine contactor captures the CO ₂ in the solution. The overhead of the amine contactor is the CO ₂ free gas and the amine solution with the CO ₂ is heated in an amine regenerator where the CO ₂ is released, and the amine aqueous solution is recycled back to the contactor. Various amines can be used such as diethanolamine (DEA), monoethanolamine (MEA), methyl-diethanolamine (MDEA), and Diisopranolamine (DIPA).

The CO ₂ stream recovered from the CO ₂ capture unit is shown as Stream 11 in FIG. 1. The stream is compressed and recycled and blended with the CO ₂ feed stream, as shown as stream 5 in FIG. 1.

Since the conversion of CO ₂ and H ₂ mixtures to CH ₄ is highly exothermic, the design of the CO ₂ C reactor is critical. Management of the heat is a key element of the reactor design. In one embodiment of the invention, the CO ₂ C reactor is a multi-tubular fixed bed reactor (FIG. 3). The inner reactor tubes are filled with a mixture of steam and water. The reaction heat is removed through the production of steam in the shell of the reactor. The inner diameter of the tubes is between 18 and 50 mm. The tubes are filled with catalyst particles. The catalyst particle size is optimized to minimize the pressure drop across the reactor. This small diameter of the reactor tubes means that as the feed gas reacts on the catalyst, the heat is transferred from no more than 9 to 25 mm to the heat transfer surface. Outside of the tubes, the mixture of steam and water allow heat transfer primarily through the vaporization of the water to steam. The water comes from a steam drum. Boiler Feed Water (BFW) is fed to the steam drum and circulates the system. The pressure of the steam system sets the temperature of the steam/water mixture. The pressure of the steam ranges from 200 to 1600 psig. At 1200 psig (82.7 barg), the temperature of saturated steam is 298°C. At 1100 psig (75.8 barg), the temperature of saturated steam is 292°C. At 1000 psig (69.0 barg), the temperature of saturated steam is 285°C. As such, saturated steam from 1000 to 1200 psig is an ideal cooling fluid for the CO ₂ C reaction that occurs at 300°C. Typically, water is fed from the steam drum to the water entrance to reactor shell and a mixture of about 50% steam and 50% water is at the steam exit of the reactor shell back to the steam drum.

In one embodiment, the CO ₂ C reactor is a multi-tubular reactor. The reactor tubes have an inner diameter of 18 to 50 mm and are filled with catalyst. In this embodiment, heat transfer is done with a hot oil system. Oil as a working fluid is pumped to the reactor shell. Heat is transferred from the tubes to the oil raising the temperature of the oil. The rate of oil circulation is controlled by the pumping rate that sets the heat removal rate. The hot oil leaving the reactor shell is cooled in an external heat exchanger. Cooling water, refrigerated water, or propylene glycol or other suitable materials can be used in the cold side of the external heat exchanger. The cooled hot oil is then pumped back to the reactor shell. Several suitable oils have working temperatures that are useful for this service including Therminol XP, Therminol 55, Therminol SP, Therminol 59, or any equivalent or similar oil working fluid. These high temperature fluids allow the tubular reactor to be operated up to about 350 °C. In one embodiment the CO ₂ C reactor is a multi-tubular reactor is cooled with molten salt.

In one embodiment, the catalyst inside the CO ₂ C reactor can be diluted. Dilution of the catalyst aids in the heat transfer so that the rate of exothermic heat generation per unit volume, is less and as such the local temperature rise is less. Suitable diluents include alumina, or other refractory materials.

In one embodiment, catalyst activity is modified by the amount of active metal loading in formed catalyst. The amount of active metal in the catalyst is defined to optimize the activity and thermal profile across the reactor, and should be between 1- 50 weight percent.

In one embodiment of the invention, the feed gas to the CO ₂ C reactor can be diluted with non-reactive gases. Feed gas dilution aids in heat transfer and temperature control in several ways. It reduces the reaction heat generated per unit time per unit volume. It also adds additional mass flow rate that acts as a heat sink. Various dilution gases can be used. The ideal dilution gas is easily separable from the reactor effluent. As the reactor effluent is cooled, the condensed steam can easily be removed from the effluent by a knockout vessel or separator and since steam is a reaction product, the design change involves just increasing the size of the separator.

In one embodiment, the CO ₂ C reactor is a series of adiabatic reactors or reactor beds. Each reactor bed length or residence time is set such that the maximum conversion that can occur keeps the bed temperature rise to a preset temperature rise of no more than 30 °C, preferably less than 20°C, and more preferably less than 15°C. Catalyst and feed gas dilution can also be used to keep the temperature rise to the preset limit. At the outlet of the reactor bed, the effluent gas is cooled in a heat exchanger that produces high pressure saturated steam in the range of 1000 to 1200 psig. This cools the reactor effluent to a temperature similar to the previous bed inlet temperature. The cooled effluent is then fed to a subsequent bed. The total number of beds is chosen to get the desired overall CO ₂ conversion. Optionally, additional hydrogen or hydrogen and CO ₂ can be added to the subsequent bed feed gas.

In one embodiment, metal partitions can be placed inside the CO ₂ C reactor tubes prior to the addition of the catalyst such that from the center of the tube to the wall of the tube there are metal pathways or connections. This aids in heat transfer since the high thermal conductivity of the metal increases the rate of heat transfer from the, hot center of the tube to the cooled tube walls.

In one embodiment, the reaction is performed in a single adiabatic reactor with at 2-6 distinct reactor zones, or preferably 2-3 reactor zones that are separated by a cooling medium. The inter stage zones are externally cooled by a cooling medium.

In one embodiment, the heat transfer and heat management in the reactor are controlled using sequential H ₂ injection. The initial H ₂ /CO ₂ ratio is below stoichiometric for the net reaction. This keeps the CO ₂ conversion low, and the temperature rise in the reactor bed low. H ₂ at a temperature substantially below the reactor bed temperature is added at the end of the bed. The cold H ₂ lowers the overall gas temperature to about the previous bed inlet temperature. The additional hydrogen with the previous bed CO ₂ reacts in the subsequent bed. The number of reactor beds and cold hydrogen additions sets the overall CO ₂ conversion.

In one embodiment, the CO ₂ C reactor is operated at low CO ₂ conversion but with high overall gas recycle. This allows the temperature in the reactor bed to be controlled and the temperature rise within the acceptable range. The space velocity or residence time in the reactor bed is high such that conversion is low. Gas dilution and catalyst dilution can be used to aid in the control of the temperature. A simpler option is to operate the catalyst at about 225 °C which converts a 4.0/1.0 H ₂ /CO ₂ to CH ₄ with a CO ₂ conversion efficiency of about 40% (Table 3). In this manner, about

3 recycle loops would yield a CO ₂ conversion of about 90%.

The reactor effluent is fed to the Product Processing Unit where the water in the effluent is removed by cooling and separation and the urrreacted CO ₂ and H ₂ are removed and compressed and recycled and blended with the fresh feed to the CO ₂ C reactor.

The e-methane produced by the invention meets specifications for natural gas pipelines in the United States and Europe and can therefore be transported and co-mingled with natural gas. Typical specifications for interstate natural gas pipelines in the United States are shown in AGA Report No. 4 A as shown below.

Trace Components:

• Hydrogen sulfide: 0.25-0.3 g/100 scf (6-7 mg/m ³ )

• Total sulfur: 5-20 g/100 scf (115-460 mg/m ³ )

• Water Vapor: 4.0-7.0 Ib/MM scf (60-110 mg/m ³ )

• Oxygen: 1.0%

• Heating calculated (gross, saturated): 950-1,150 Btu/scf (35,400-42,800 kJ/m ³ )

To be clear, each pipeline can and often does set specifications more stringent than the above for pipeline quality gas. Various other nations have different pipeline quality gas practices and standards. The e-methane product produced as per the invention, meets the representative specification for pipeline quality natural gas and can be distributed in pipelines. In one embodiment, the invention produces no C2+. In one embodiment of the invention, the overall CO ₂ conversion in the CO ₂ C reactor is greater than about 95% and as such there is no need for CO ₂ removal systems downstream of the CO ₂ C reactor.

Example 1

In this example, CO ₂ and H ₂ are converted to CH ₄ using the Ni ₂ Mg, Cu ₃ Ni or Cu ₂ Mg solid-solution catalyst. H ₂ and CO ₂ were fed to the CO ₂ C reactor with a molar H ₂ /CO ₂ ratio of 4.0. Table 3 shows the conversion of CO ₂ conversion to CH ₄ and CO as a function of temperature from laboratory data.

As can be seen in Table 3, substantial conversion of CO ₂ with high selectivity to CH ₄ occurs at temperature from 250-350°C. In this example, the CO ₂ C reactor effluent was sent to the product separator. The amount of CO formed in the CO ₂ C reactor in this example was low at 2% or less. At a temperature of 300°C, the CO ₂ conversion was high at 87%.

Further as an example, the catalyst was used in an integrated process to produce low carbon intensity methane at a CO ₂ C reactor temperature of 300°C. The CO ₂ conversion was 87 mol% with 100% selectivity to CH ₄ .

FIG. 2 shows the process configuration for this example. Hydrogen was produced from the electrolysis of water. CO ₂ was captured from an industrial source and purified. The H ₂ and CO ₂ were mixed and shown as stream 101 in FIG. 2. The molar H ₂ to CO ₂ was 4 to 1. Unit 200 was the CO ₂ C reactor that used the above Ni ₂ Mg catalyst. The reactor effluent was stream 102. The stream was cooled to 40°C and the water product was knocked out in unit 201. The water rich product stream is shown as stream 103. Stream 104 was the stream with a composition of 34 mol% H ₂ , 9 mol% CO ₂ , and 57 mol% methane that feeds unit 202. Unit 202 is an amine contactor. In this example, MDEA was used as the amine. The overhead of Unit 202 is the CO ₂ free gas, stream 105, with a composition of 63 mol% CH ₄ and 37 mol% H ₂ . The CO ₂ -rich amine stream, stream 109, left Unit 202 and was fed to Unit 203, the amine regenerator. The regenerated was heated through a steam reboiler that heats the amine solution and allowed the captured CO ₂ to be released by the amine solution and to leave the top of the amine regenerator. The captured CO ₂ is shown as stream 106. The CO ₂ free amine is shown as stream 110 and was pumped back to the amine contactor.

Table 3 - Catalytic Conversion of CO ₂ to CH ₄ and CO as a function of temperature for the crushed NiiMg catalyst (not impregnated on a substrate) using 5.0 cc of catalyst in 5/16" OD Inconel Tube; H ₂ /CO ₂ = 4.0; 368 psi; 200 seem)

Stream 105 was sent to an H ₂ pressure swing adsorption (PSA) unit, shown as Unit 204.

By use of pressure cycling, the H ₂ was separated from the CH ₄ . Stream 107 is the pure H ₂ product that was recycled back to the front end of the system and blended with the hydrogen in stream 3 of FIG. 1. Stream 108 is the CH ₄ product stream.

The overall chemical reaction for the CO ₂ C reactor in this example is CO ₂ + 4H ₂ = CH ₄ + 2H ₂ O (g) with an enthalpy of reaction of -165 KJ/mol at 25°C. The reaction is very exothermic at the high CO ₂ conversion. As such, the CO ₂ C reactor in this example was a multi-tubular fixed bed reactor. In this example, the saturated steam pressure produced in the shell of the reactor was 1,100 psig with a temperature of 292°C. A portion of the steam produced by the multitubular reactor was used as the steam to heat the amine regenerator reboiler to allow the amine captured CO ₂ to be released.

Example 2

In this example, a 12 wt.% Ni ₂ Mg, Cu ₃ Ni or Cu ₂ Mg catalyst impregnated on a magnesium aluminate spinel substrate was used in the CO ₂ C reactor to convert (CO ₂ ) with (H ₂ ) to a mixture of H ₂ and CH ₄ and CO ₂ at temperatures from 275 to 350°C. Table 2 shows that the CO ₂ conversion efficiency was about 86% with a greater than 99% CH ₄ production selectivity.

The H ₂ and CO ₂ was removed with a 95% efficiency from the product stream to produce a CH ₄ stream with greater than 98% purity. The minor components in the CH ₄ stream include CO ₂ at about 1.5 vol.%, H ₂ at about 0.2%, and H ₂ O at about 0.3%.

The resulting CH ₄ stream met pipeline quality natural gas standards.

References

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