EP4313369A1 - Cyclic adsorptive reactor for upgrade of co2/ch4 mixtures - Google Patents

Abstract

A method for continuous upgrading of methane and carbon dioxide gas mixtures comprising the steps of: feed said gas mixture stream into a first adsorptive reactor for a first carbon dioxide adsorption to obtain a first methane rich gas, wherein said reactor comprises an adsorbent and catalyst filler; when the first adsorptive reactor is fully saturated or partially saturated, feed the gas mixture stream comprising carbon dioxide and methane into a second adsorptive reactor for a second carbon dioxide adsorption in order to obtain a second methane enriched gas, and simultaneously feed a hydrogen-containing gas stream into the first adsorptive reactor for reactive regeneration; periodically switch the gas streams that are fed into the adsorptive reactors in order to prevent the reactors from becoming fully saturated or partially saturated; wherein the first and second adsorptive reactors comprise a CO₂ adsorbent material and a methanation catalyst as a filler.

Description

D E S C R I P T I O N CYCLIC ADSORPTIVE REACTOR FOR UPGRADE OF CO2/CH4 MIXTURES TECHNICAL FIELD [0001] The present disclosure relates to a cyclic adsorptive reactor for upgrading carbon dioxide and methane (CO₂/CH₄) gas mixtures. BACKGROUND [0002] The concept of “Power-to-Gas” (PtG) is based on the conversion of electricity obtained from a renewable source (sun, wind, etc.) into a gas that can be easily transported and stored. Among the existing options, the production of methane (CH₄) benefits from a well-established natural gas infrastructure where CH₄ can be injected, enabling the integration/optimization of power and gas sectors.^1,2 [0003] From a technological viewpoint, the power-to-methane (PtM) process consists of the production of CH₄ from the catalytic reaction between carbon dioxide (CO₂) and hydrogen (H₂) through the methanation (or Sabatier) reaction: CO₂ + 4H₂ ↔ CH₄ + 2H₂O ΔH_r ^{(298 K)} = -165 kJ/mol [0004] Renewable electricity is used for water (H₂O) electrolysis to obtain the “green” H₂ necessary for the methanation. With regard to CO₂, it can be obtained from many different sources (for instance power plants, cement, iron and steel industries), however, the present disclosure relates to the valorisation of CO₂ from CO₂/CH₄ mixtures, such as biogas or raw natural gas streams. [0005] Biogas is an interesting carbon source for PtM processes due to its high CO₂ content (usually between 40%-60%), typically higher than that found in exhaust gases from power plants (5%-15%) or cement and steel production industries (20%-30%).^1,2 [0006] Biogas is the by-product of a biological mediated process, the anaerobic digestion, which consists of the degradation of organic compounds by micro-organisms in the absence of air. The composition of biogas depends on the feedstock origin, substrate composition, as well as the conditions within the anaerobic digestion reactor. Industrially it is produced in sewage treatment plants (sludge fermentation stage), landfills, sites with industrial processing industry and at digestion plants for agricultural organic waste. Raw biogas typically contains 40-60 vol.% of CH₄ and 40-60 vol.% of CO₂, apart from other impurities such as H₂O (up to 10%), N₂ (0-2%), H₂S (0.005-2%), siloxanes (0–0.02%), O₂ (0- 1%), CO (<0.6%), NH₃ (<1%) and others.^3–5 [0007] Another relevant CO₂ source is from the exploitation of unconventional, stranded and contaminated gas reservoirs which were previously considered economically inviable but are now becoming more attractive as the demand for natural gas increases. The composition of raw natural gas depends on the reservoir from which it is extracted and its origin, and results from three major processes: thermogenic (slow decomposition of organic material in sedimentary basils under the influence of temperature and pressure associated with depth), biogenic (by the action of methanogenic bacteria on organic materials) and abiogenic (reduction of carbon dioxide during magma cooling). The composition of the raw natural gas extracted from the conventional reserves (whose exploitation, at this time, is still predominant) can vary, but typically comprises CH₄ (80-99 %), CO₂ (1-5 %), N₂ (0.1-15 %), heavier hydrocarbons such as ethane, (1.7-4.6 %), H₂S (<5 ppm), H₂O, O₂, CO, NH₃ and others.^6–11 [0008] Globally, the volume of sub-quality natural gas reserves is estimated to be rather significant: 43% of natural gas wells are sour (i.e., are rich in acid gases like CO₂ and H₂S) and 25% have high content of CO₂ (i.e., contents exceeding 10% and up to 80%). The high costs of processing gas from these reserves with a high content of impurities have hindered their exploitation.^6–11 [0009] CO₂/CH₄ gas mixture streams, whether they are raw natural gas (sour gas) or raw biogas have roughly the same composition although there are variations according to their origin. As stated, these gas mixtures comprise mainly CH₄, but also CO₂, N₂ and other trace contaminants such as H₂O, H₂S, O₂, CO, heavier hydrocarbons and others. These contaminants must be removed in order to meet pipeline quality specifications, enhance calorific value and minimize environmental pollution.^8–11 [0010] The removal of contaminants from CH₄/CO₂ gas mixtures can be simultaneous with CO₂ separation (upgrading or sweetening stage), but in many cases, there is a need for a pre-upgrade stage to reduce the high concentration of contaminants such as H₂O, H₂S and siloxanes, which may cause damage to downstream equipment due to the formation of harmful corrosive compounds. The removal of moisture that can seriously damage the equipment by corrosion, is essential to every kind of application and is usually attained by condensation (and separation of the condensate with demisters, cyclone separators, moisture traps or water taps), adsorption by silica gel, activated carbon or aluminium oxide or absorption with glycol or hygroscopic salts. The elimination of H₂S, which is both toxic and corrosive, is typically carried out by adsorption using iron oxide or hydroxide, absorption with liquids, membrane separation, through the use of a biological filter or adsorption on activated carbon. In the case of biogas, there is also the possibility of H₂S removal during the digestion via biological treatment through air/O₂ dosing or iron chloride addition to the biogas reactor. The separation of siloxanes can be done by absorption with organic solvents, in strong acids, in strong bases, adsorption in silica gel or activated carbon, or cryogenic separation. The most common method for N₂ removal is by cryogenic distillation.^3–5,8–11 [0011] The current state of the art technologies for sour gas sweetening and biogas upgrade are very similar and include scrubbing processes (based on amines, organic solvents or water), cryogenic distillation, membrane separation and adsorption-based methods (Pressure Swing Adsorption (PSA) and Temperature Swing Adsorption (TSA)).^{3– 5,8–11} [0012] The separation of CO₂ from CO₂/CH₄ gas mixtures through absorption can either be a chemical, physical or hybrid process. These processes, also known as scrubbing, rely on the much higher solubility of CO₂ in a solvent as compared to CH₄. For chemical absorption processes, the most widely used solvents are aqueous amine solutions (such as mono-, di- or tri-ethanolamine) and other aqueous alkaline salts solutions (such as sodium, potassium and calcium hydroxides). In a chemical absorption process, the CH₄/CO₂ gas mixture is typically introduced at the bottom of an absorber tower while the amine solution is fed from the top and the two reactive species interact with each other counter-currently so that the CO₂ is “bound” to the solvent by an exothermic chemical reaction. The solvent is later regenerated in a stripping unit. The main disadvantages of this technology are the toxicity of the used solvents, the significant energy required for regeneration of the chemical solutions and the cost of the solvents (initial investment and loss due to evaporation).^3–5,8–11 [0013] The most common physical absorption techniques for CO₂ separation are high- pressure water scrubbing (with water as the solvent – mainly used for biogas upgrading rather than natural gas sweetening) and organic physical scrubbing (with organic solvents such as methanol, n-methyl pyrrolidone or polyethylene glycol ethers). In these scrubbing processes, the CO₂/CH₄ gas mixture and the solvent are also typically fed counter- currently into a packed column, and CO₂ is physically bounded to the solvent. Since the physical absorption of gases is governed by Henry’s Law, these are particularly interesting techniques when the feed gas is available at high pressure or when the CO₂ fraction is high. In physical absorption, the interactions between the acid gas and the solvent are quite weak, thereby lowering the energy and cost requirement for solvent regeneration. The main weakness of physical absorption remains the relatively low gas absorption capacities of commercially available physical solvents.^3–5,8–11 [0014] Cryogenic distillation is conducted by gradually decreasing the temperature of the CO₂/CH₄ mixture, separating CH₄ from the liquefied CO₂ and other components. This process is conducted by initially drying and compressing the CO₂/CH₄ mixture, followed by a stepwise drop of the temperature until the CO₂ reaches the liquid-state, after which it is removed. Thereafter the remaining gas stream is further cooled until the CO₂ reaches solid-state and is again removed leaving behind purified CH₄. Some drawbacks associated with cryogenic distillation are high energy input, thus demanding a high operating cost, and the corrosion of the process equipment, due to the very low temperatures required. ^3–5,8–11 [0015] CO₂ removal through membrane technology relies upon the selective permeability properties of a membrane (polymeric, inorganic or mixed matrix). In this process, the membrane acts as a permeable barrier that allows a specific compound (e.g., CO₂) to pass through differently by controlling the applied driving force (such as the difference in concentration or pressure between both sides of the membrane). The process and energy requirements of a membrane system typically include pre-treatment and compression of the feed stream (generating the desired driving force). The number of membrane stages required is determined by the membrane selectivity.^3–5,8–11 [0016] The PSA technology for the separation of the components present in CO₂/CH₄ gas mixtures is based on at least two (typically four) columns, filled with a CO₂-selective adsorbent (e.g., activated carbon, carbon molecular sieve, zeolites). When the gas mixture is fed to a column, carbon dioxide is preferentially adsorbed onto the solid while methane goes through relatively untouched. Once the first column reaches its full uptake capacity, the flow of the feed gas mixture is directed toward the second column. Because the adsorption process is reversible, by decreasing the CO₂ partial pressure in the first column, the CO₂-saturated adsorbent is regenerated, therefore being able to capture more CO₂ again (in a later stage). TSA is a similar method, but it relies on the exothermic nature of gas adsorption. Since low temperature favours adsorption and high temperature favours desorption (or regeneration), the adsorption/desorption cycles occur through temperature swing instead of pressure.^3–5,8–11 [0017] EP2009080A1¹² discloses a methane separation method for CO₂/CH₄ mixtures. The disclosed process includes: firstly, mixing the biogas and a CO₂ absorbing liquid in a mixer; then the formed gas-liquid mixed phase is introduced into a gas/liquid separator that separates the methane from the liquid (in which the CO₂ is absorbed); the liquid (and absorbed CO₂) is then forwarded to a membrane module that permeates the CO₂ separating it from the absorbing liquid that is recovered. Additionally, a second gas/liquid separator may be placed after the membrane module, where the absorbing liquid is sent (after CO₂ removal), in order to separate and recover any trace amount of methane that may still be present.¹² [0018] All these technologies are mere separation processes, and therefore CO₂-rich (usually emitted into the atmosphere) and CH₄-rich streams are produced (excluding the units required to deal with impurities such as H₂S, humidity and siloxanes). See Figure 1A for illustration of the state of the art. [0019] Direct methanation of CO₂ is another possibility that may be considered for biogas upgrading. In this approach, the CO₂/CH₄ gas mixture is directly fed into a methanation reactor without prior CO₂ separation. The methanation reactor contains a methanation catalyst (usually Ni or Ru supported on an oxide) that, through the Sabatier reaction (cf. Eq. 1), converts the CO₂ and H₂ (in the PtM framework, produced via H₂O electrolysis using renewable electricity) into CH₄.^3–5 [0020] Dannesboe et al.¹³ disclosed the upgrading of biogas through its direct methanation in a double pass packed bed reactor. This technology is also disclosed in WO2015150420A1¹⁴ which describes a multi-tubing methanation reactor with 2 tubes filled with a methanation catalyst (Ni/Al₂O₃) into which the biogas is fed. Pressurized boiling water is used as a cooling medium for the reactor. In order to increase the low CO₂ conversion, often a limitation to direct hydrogenation of biogas (due to thermodynamic reasons associated to the reversible reaction, i.e., presence of the reaction product – methane – in the reactor feed), after passing the first reactor tube, the water produced in the methanation reaction is removed by condensation before the gas enters the second reactor tube.^13,14 [0021] These technologies are reaction processes, and therefore a CH₄-rich stream containing unconverted CO₂ (and H₂) is obtained. The level of CO₂ obtained is dependent on feed composition, catalyst and operating conditions employed, as well as on the number of reaction stages. Figure 1B illustrates a single reactor stage of the state of the art. [0022] If the intention is to purify CH₄ and valorise the separated CO₂, there is a need for at least another processing unit (reactor) to convert the separated CO₂ into more methane. This approach requires a combination of technologies: a methanation reactor after a separation step (by PSA technology, membranes, scrubbing, or any other method). See Figure 1C that illustrates the state of the art. [0023] CN106554831A¹⁵ describes an apparatus for biogas purification and CO₂ valorisation to CH₄. The technology comprises, after a desulfurization pre-treatment, the purification of biogas through membrane separation (in the membrane module). Thereafter the separated CO₂ proceeds to a preheating device and only then does it proceed to a methanation unit where a layer of methanation catalyst promotes the conversion of the CO₂ into CH₄ (separate process units for permeation and catalysis).¹⁵ [0024] The work developed by Miguel et al. demonstrated a proof-of-concept of a sorptive reactor for CO₂ capture explicitly from flue gas (whose composition is simplified in N₂ and CO₂) and its catalytic conversion to CH₄ via the Sabatier reaction all in the same device. The concept described was tested at lab scale in one reactor filled by a mixture of CO₂ sorbent (commercial K-promoted hydrotalcite) and methanation catalyst (commercial Ni-based material), fed alternately by the simulated flue gas stream (15% CO₂ in N₂) or H₂.^16,17 [0025] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure. [0026] References 1. M. Götz, J. Lefebvre, F. Mörs, A. McDaniel Koch, F. Graf, S. Bajohr, R. Reimert and T. Kolb, Renewable Power-to-Gas: A technological and economic review, Renew. Energy, vol. 85, pp.1371–1390, 2016. 2. K. Ghaib and F. Z. Ben-Fares, Power-to-Methane: A state-of-the-art review, Renew. Sustain. Energy Rev., vol.81, pp.433–446, Jan.2018. 3. I. Ullah Khan, M. Hafiz Dzarfan Othman, H. Hashim, T. Matsuura, A. F. Ismail, M. Rezaei- DashtArzhandi and I. Wan Azelee, Biogas as a renewable energy fuel – A review of biogas upgrading, utilisation and storage, Energy Convers. Manag., vol.150, pp.277–294, 2017. 4. I. Angelidaki, L. Treu, P. Tsapekos, G. Luo, S. Campanaro, H. Wenzel and P. G. Kougias, Biogas upgrading and utilization: Current status and perspectives, Biotechnol. Adv., vol. 36, pp.452–466, 2018. 5. E. Ryckebosch, M. Drouillon and H. Vervaeren, Techniques for transformation of biogas to biomethane, Biomass and Bioenergy, vol.35, pp.1633–1645, May 2011. 6. J. Park, S. Yoon, S. Y. Oh, Y. Kim and J. K. Kim, Improving energy efficiency for a low- temperature CO₂ separation process in natural gas processing, Energy, vol. 214, p. 118844, 2021. 7. W. F. J. Burgers, P. S. Northrop, H. S. Kheshgi and J. A. Valencia, Worldwide development potential for sour gas, Energy Procedia, vol.4, pp.2178–2184, 2011. 8. S. Faramawy, T. Zaki and A. A.-E. Sakr, Natural gas origin, composition, and processing: A review, J. Nat. Gas Sci. Eng., vol.34, pp.34–54, Aug.2016. 9. T. E. Rufford, S. Smart, G. C. Y. Watson, B. F. Graham, J. Boxall, J. C. Diniz da Costa and E. F. May, The removal of CO₂ and N₂ from natural gas: A review of conventional and emerging process technologies, J. Pet. Sci. Eng., vol.94–95, pp.123–154, Sep.2012. 10. D. Saha, H. A. Grappe, A. Chakraborty and G. Orkoulas, Postextraction separation, on- board storage, and catalytic conversion of methane in natural gas: A review, Chem. Rev., vol.116, pp.11436–11499, 2016. 11. S. Mokhatab and W. Poe, Handbook of Natural Gas Transmission and Processing.2012. 12. T. Tomioka, Method for separation of methane, methane separator, and methane utilization system, EP2009080A1, 2007. 13. C. Dannesboe, J. B. Hansen and I. Johannsen, Catalytic methanation of CO₂ in biogas: Experimental results from a reactor at full scale, React. Chem. Eng., vol. 5, pp. 183–189, 2020. 14. C. Wix, M. Boe and A. Helno Hansen, Pseudo-isothermal reactor, WO₂015150420A1, 2015. 15. Z. Li, J. Yin, L. Zheng and X. Zhou, Device and technology for biogas purification and carbon dioxide synchronous methanation transformation, CN106554831A, 2017. 16. C. V. Miguel, M. A. Soria, A. Mendes and L. M. Madeira, A sorptive reactor for CO₂ capture and conversion to renewable methane, Chem. Eng. J., vol.322, pp.590–602, Aug.2017. 17. C. V. Miguel, CO₂ capture and valorisation to chemicals: methane production (Doctoral dissertation), Faculty of Engineering of the University of Porto, 2018. 18. W. J. Lee, C. Li, H. Prajitno, J. Yoo, J. Patel, Y. Yang and S. Lim, Recent trend in thermal catalytic low temperature CO₂ methanation: A critical review, Catal. Today, pp.0–1, 2020. 19. M. Younas, L. Loong Kong, M. J. K. Bashir, H. Nadeem, A. Shehzad and S. Sethupathi, Recent advancements, fundamental challenges, and opportunities in catalytic methanation of CO₂, Energy and Fuels, vol.30, pp.8815–8831, 2016. 20. S. Choi, J. H. Drese and C. W. Jones, Adsorbent materials for carbon dioxide capture from large anthropogenic point sources, ChemSusChem, vol.2, pp.796–854, Sep.2009. 21. J. Wang, L. Huang, R. Yang, Z. Zhang, J. Wu, Y. Gao, Q. Wang, D. O’Hare and Z. Zhong, Recent advances in solid sorbents for CO₂ capture and new development trends, Energy Environ. Sci., vol.7, pp.3478–3518, 2014. 22. Q. Wang, J. Luo, Z. Zhong and A. Borgna, CO₂ capture by solid adsorbents and their applications: current status and new trends, Energy Environ. Sci., vol.4, pp.42–55, 2011. GENERAL DESCRIPTION [0027] The present disclosure relates to a cyclic adsorptive reactor for upgrading carbon dioxide/methane (CO₂/CH₄) gas mixtures. [0028] In an embodiment, the reactor unit of the present disclosure combines, in the same compact unit, the separation of the two components (by CO₂ adsorption) and the valorisation of the CO₂ (through its catalytic conversion to more CH₄). Hence, instead of obtaining two entirely distinct streams (CO₂ rich stream that is often released into the atmosphere, and valuable CH₄ rich stream), the proposed technology produces just CH₄ rich streams. [0029] In an embodiment, the CO₂/CH₄ gas mixture is fed to an adsorptive reactor that first captures the CO₂ (separating it from CH₄) and only then converts it to CH₄ through methanation, instead of being directly fed to a methanation reactor (which, according to the Le Chatelier’s principle inhibits CO₂ conversion due to the presence of the reaction product, CH₄, in the reactor feed). [0030] In an embodiment, the reactor unit of the present disclosure operates in a cyclic mode with at least two adsorptive reactors operating in parallel. While the first adsorptive reactor is capturing CO₂, the second adsorptive reactor is converting the previously captured CO₂ into CH₄, and vice-versa. [0031] In an embodiment, the carbon dioxide conversion of the present disclosure is higher compared to methods in the state of the art. [0032] In an embodiment, in the present disclosure the CO₂ is separated from CH₄ through its adsorption in a solid CO₂-selective adsorbent (that is present in the two adsorptive reactors), while in EP2009080A1¹², CO₂ removal is achieved by mixing the CO₂/CH₄ gas mixture with a CO₂ absorbing liquid (in a mixing unit), followed by gas/liquid separation (in a separation unit from where methane is recovered) and later membrane separation of CO₂ from the absorbing liquid. [0033] In an embodiment, the reactor and method of the present disclosure avoid the formation of hotspots. The present disclosure differs from the document by Dannesboe et al.¹³ and WO2015150420A1¹⁴ in that the present disclosure describes CO₂ being first separated by adsorption, and then subsequently converted into CH₄ in the same compact reactor, i.e., it considers separation and reaction, instead of just reaction in consecutive devices. In the document by Dannesboe et al., the existence of a significant hotspot in the initial section of the reactor tubes increases the temperature of the catalytic bed by ca. 400 °C. In the present disclosure, the endothermic desorption of CO₂ and the exothermic methanation reaction that occur simultaneously prevents the formation of hotspots, thus decreasing possible problems related to materials sintering and safety concerns. [0034] The difference between the present disclosure and CN106554831A¹⁵ is that in the present disclosure, the CO₂/CH₄ separation is achieved through the use of a CO₂ adsorbent (instead of a membrane) and the separation and conversion are carried out in the same compact unit as opposed to using two different units (a membrane module plus a methanation reactor). Further, the present disclosure enables the purification of any CO₂/CH₄ gas mixtures and not just biogas. Although there is no mention of the temperature profile in the catalyst bed in the methanation unit in CN106554831A, the feeding of a concentrated CO₂ stream to the methanation reactor is expected to create a significant hotspot due to the strong exothermicity of the reaction. [0035] The difference between the present disclosure and the document of Miguel et al.¹⁶ is that the present disclosure is for upgrading of CO₂/CH₄ mixtures (such as raw natural gas or biogas) while the document of Miguel et al.¹⁶ is to capture and convert CO₂ explicitly from flue gas. [0036] The operating mode of the present disclosure differs from the state-of-art technologies in that: i) there is no simple separation of the components as shown in Figure 1A, and ii) the user is no longer restricted by thermodynamics which occurs in a methanation reactor (Figure 1B) as it is possible to reach a much higher CO₂ conversion rate or even a complete CO₂ conversion and hence obtain, in a single device, a high-purity CH₄ stream (cf. Figure 1D). [0037] As compared to state of the art separation processes (PSA technology, membranes, scrubbing, etc.) as illustrated in Figure 1A, the present disclosure has the following advantages: - Enables additional CH₄ production by converting the separated CO₂ into more methane instead of simply releasing it; - Potentiates the separated CO₂ instead of letting it go to waste and possibly emitted into the atmosphere. [0038] As compared to a single methanation reactor fed with sour gas or biogas as illustrated in Figure 1B, the present disclosure has the following advantages: - It is not limited, in terms of CO₂ conversion, by the restriction of the reversible methanation reaction, unlike state of the art technologies that are severely affected by the presence of a huge quantity of reaction product (CH₄) in the reactor feed; - Reduces poor heat dissipation risks and safety concerns. [0039] As compared to a separation process followed by a methanation reactor to convert the separated CO₂ as illustrated in Figure 1C, the present disclosure has the following advantages: - Able to reach similar methane production flow rates whilst requiring less process operating units (thus, a more compact technology is provided); - Reduces poor heat dissipation risks and safety concerns (due to the strongly exothermic nature of the methanation reaction that induces thermal limitations in the current state of the art technologies – in particular for concentrated CO₂ streams, which in the case of the cyclic adsorptive reactor, is counterbalanced by the endothermic nature of the simultaneous CO₂ desorption). [0040] In an embodiment, the method for continuous upgrading of methane and carbon dioxide gas mixtures comprises the steps of: - feed a gas mixture stream comprising carbon dioxide and methane into a first adsorptive reactor for a first carbon dioxide adsorption to obtain a first methane rich gas, wherein said reactor comprises an adsorbent and catalyst filler; - when the first adsorptive reactor is fully saturated or partially saturated with CO₂, feed the gas mixture stream comprising carbon dioxide and methane into the second adsorptive reactor for a second carbon dioxide adsorption in order to obtain a second methane enriched gas, and simultaneously feed a hydrogen- containing gas stream into the first adsorptive reactor for reactive regeneration; - when the second adsorptive reactor is saturated or partly saturated, feed the gas mixture comprising carbon dioxide and methane into the first adsorptive reactor for carbon dioxide adsorption, and simultaneously feed a hydrogen-containing gas stream into the second adsorptive reactor for reactive regeneration; - periodically switch the gas streams that are fed into the adsorptive reactors in order to prevent the reactors from becoming fully saturated or partially saturated; wherein the first and the second adsorptive reactors comprise a CO₂ adsorbent material and a methanation catalyst as a filler. [0041] In an embodiment, the adsorptive reactor is fully saturated or partially saturated when the adsorbent becomes fully saturated or partially saturated with CO₂. [0042] In an embodiment, in order to prevent the reactors from becoming fully saturated or partially saturated, the gas streams that are fed into the adsorptive reactors are switched periodically, preferably every 15 mins, more preferably every 10 mins, even more preferably every 5 mins. [0043] In an embodiment, the bed comprises a CO₂ adsorbent material and a methanation catalyst, or a dual-function material with CO₂ adsorbent activity and methanation catalytic activity. [0044] In an embodiment, the CO₂ adsorbent material and the methanation catalyst are arranged in a layered, fluidized, structured or mixed bed configuration. [0045] In an embodiment, the volume ratio of the CO₂ adsorbent material and the catalyst ranges from 1:1 to 30:1 (volume _{CO2 adsorbent} / volume _catalyst), preferably 1:1 to 20:1 (volume _{CO2 adsorbent} / volume _catalyst), even more preferably 1:1 to 10:1 (volume _{CO2 adsorbent} / volume _catalyst). [0046] In an embodiment, the CO₂ adsorbent material is selected from: carbon-based, zeolite-based, silica-based, polymer-based, clay-based, metal-organic framework-based, alkali metal carbonate-based, solid amine-based, hydrotalcite-like, single or mixed oxides, calcium-based, lithium-based, alkali ceramic-based, alkali zirconate-based or alkali silicate-based or mixtures thereof. [0047] In an embodiment, the methanation catalyst is selected from: Ni, Ru, Rh, Fe, Co, Mo, Pd, Ag, W, Os, Ir, Pt or Au, wherein the catalyst is dispersed on a conventional metal oxide support or a structured metal oxide support or a carbon support, or mixtures thereof. [0048] In an embodiment, the conventional metal oxide support is selected from: Al₂O₃, SiO₂, TiO₂, MgO, CaO, ZrO_2, Cr₂O₃, CeO₂, Ce_xZr_1-xO₂, La₂O₃, MnO₂ or ZnO. [0049] In an embodiment, the structured metal oxide support is selected from: mesostructured silica nanoparticles or metal-organic framework materials. [0050] In an embodiment, the carbon support is selected from: activated carbon, carbon nanotubes, carbon nanofibers, biochar or carbon felts. [0051] In an embodiment, the filler is a dual-function material that has CO₂ adsorbent activity and methanation catalytic activity. [0052] In an embodiment, the dual-function material comprises a combination of a CO₂ adsorbent material selected from: carbon-based, zeolite-based, silica-based, polymer- based, clay-based, metal-organic framework-based, alkali metal carbonate-based, solid amine-based, hydrotalcite-like, single or mixed oxides, calcium-based, lithium-based, alkali ceramic-based, alkali zirconate-based or alkali silicate-based or mixtures thereof; and a catalyst selected from: Ni, Ru, Rh, Fe, Co, Mo, Pd, Ag, W, Os, Ir, Pt or Au, wherein the catalyst is dispersed on a conventional metal oxide support or a structured metal oxide support or a carbon support, or mixtures thereof. [0053] In an embodiment, a cyclic reactor unit for continuous upgrading of methane and carbon dioxide gas mixtures according to the method of the present disclosure, comprises: - at least one adsorptive reactor for receiving hydrogen-containing gas stream or methane and carbon dioxide gas mixtures; - at least one moisture trap for separating water produced from the adsorptive reactor; wherein the cyclic reactor unit switches the gas streams that are fed into the adsorptive reactors such that the adsorptive reactors cycle between adsorption stage and reactive regeneration stage. [0054] In an embodiment, the cyclic reactor unit comprises: - a first adsorptive reactor for receiving hydrogen-containing gas stream or methane and carbon dioxide gas mixtures; - a second adsorptive reactor for receiving hydrogen-containing gas stream or methane and carbon dioxide gas mixtures; - a first moisture trap for separating water produced from the first adsorptive reactor; - a second moisture trap for separating water produced from the second adsorptive reactor; and - tubing; wherein the cyclic reactor unit switches the gas streams that are fed into the adsorptive reactors such that the adsorptive reactors cycle between adsorption stage and reactive regeneration stage. [0055] In an embodiment, the cyclic reactor further comprises a 4-way top valve for switching the gas streams. [0056] In an embodiment, the cyclic reactor may further comprise a 4-way bottom valve for switching the gas streams. [0057] In an embodiment, the methanation catalyst is selected from: Ni, Ru, Rh, Fe, Co, Mo, Pd, Ag, W, Os, Ir, Pt or Au dispersed on conventional metal oxide supports (Al₂O₃, SiO₂, TiO₂, MgO, CaO, ZrO₂, Cr₂O₃, CeO₂, Ce_xZr_1-xO₂, La₂O₃, MnO₂, ZnO), structured metal oxide supports (mesostructured silica nanoparticles and metal-organic framework materials), carbon supports (activated carbon, carbon nanotubes, carbon nanofibers, biochar or carbon felts), or mixtures thereof. [0058] In an embodiment, the cyclic reactor unit has a CO₂ conversion rate of at least 70%, more preferably at least 80%, even more preferably at least 90%. BRIEF DESCRIPTION OF THE DRAWINGS [0059] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of the invention. [0060] Figure 1: Scheme of current state of the art technologies addressed for upgrading of CO₂/CH₄ gas mixtures (e.g., sour gas or biogas). Figure 1A shows simple CO₂/CH₄ separation; Figure 1B shows direct CO₂/CH₄ gas mixture methanation in a conventional methanation reactor, Figure 1C shows CO₂/CH₄ separation followed by CO₂ methanation in a conventional methanation reactor. Figure 1D shows CO₂/CH₄ gas mixture purification and methanation in one single cyclic adsorptive reactor as disclosed in the present application. [0061] Figure 2: Scheme of the cyclic adsorptive reactor unit of the present disclosure for CO₂/CH₄ gas mixture upgrading during (A) CO₂ Adsorption Stage in Adsorptive Reactor I and Reactive Regeneration Stage in Adsorptive Reactor II or (B) Reactive Regeneration Stage in Adsorptive Reactor I and CO₂ Adsorption Stage in Adsorptive Reactor II. [0062] Figure 3 shows the reactor’s filling scheme with alternate layers of CO₂ adsorbent and methanation catalyst, and the placement of the 4 thermocouples inside reactor bed. [0063] Figure 4 shows the experimental installation scheme. [0064] Figure 5 shows the flow rate (by component) of the outlet streams of Adsorptive Reactor I and Adsorptive Reactor II . The black frame surrounds the adsorption/reactive regeneration cycles for which the process indicators were calculated. The letters above each stage indicate whether it is an adsorption stage (A) or a reactive regeneration stage (RR). The zoom shows, in further detail, the two adsorption/reactive regeneration cycles performed on Adsorptive Reactor II between the 230^th and 270^th minute of experiment. [0065] Figure 6: shows the process indicators: a) CO₂ adsorption capacity; b) CO₂ conversion; c) CH₄ purity (the bars referring to reactive regeneration stages are marked with the diagonal lines, while the adsorption stages have no pattern) and d) n_H2 ⁱⁿ/n_CH4 ^out (the black horizontal line indicates the stoichiometric ratio). The black frame indicates when the cyclic steady-state had been reached. [0066] Figure 7 shows the temperature variation histories (i.e., with respect to the oven and feed temperature – 350 °C in this example) for all four measuring points in Adsorptive Reactor II. For these operating conditions, the temperature increase remained below 9 °C. DETAILED DESCRIPTION [0067] The present disclosure relates to a cyclic adsorptive reactor for upgrading carbon dioxide/methane (CO₂/CH₄) gas mixtures. Namely, a method for continuous upgrading of methane and carbon dioxide gas mixtures comprising the steps of: feed a gas mixture stream comprising carbon dioxide and methane into a first adsorptive reactor for a first carbon dioxide adsorption to obtain a first methane rich gas, wherein said reactor comprises an adsorbent and catalyst filler; when the first adsorptive reactor is fully saturated or partially saturated, feed the gas mixture stream comprising carbon dioxide and methane into a second adsorptive reactor for a second carbon dioxide adsorption in order to obtain a second methane enriched gas, and simultaneously feed a hydrogen- containing gas stream into the first adsorptive reactor for reactive regeneration; periodically switch the gas streams that are fed into the adsorptive reactors in order to prevent the reactors from becoming fully saturated or partially saturated; wherein the first and second adsorptive reactors comprise a CO₂ adsorbent material and a methanation catalyst as a filler. [0068] In an embodiment, the method of upgrading carbon dioxide/methane (CO₂/CH₄) gas mixtures comprises the following steps: • The CO₂/CH₄ gaseous mixture is fed into Adsorptive Reactor I (CO₂ Adsorption Stage in Adsorptive Reactor I). • Top and bottom valves are actuated. The CO₂/CH₄ gaseous mixture stops being fed into Adsorptive Reactor I and starts being fed into Adsorptive Reactor II (CO₂ Adsorption Stage in Adsorptive Reactor II). Simultaneously, H₂ starts being fed to Adsorptive Reactor I (Reactive Regeneration Stage in Adsorptive Reactor I). • Top and bottom valves are actuated a second time. The inlet streams are switched again. Adsorptive Reactor I returns to the CO₂ Adsorption Stage and Adsorptive Reactor II starts the Reactive Regeneration Stage. • Top and bottom valves are actuated a third time. The inlet streams are switched again. Adsorptive Reactor II returns to the CO₂ Adsorption Stage and Reactor I starts the Reactive Regeneration Stage. • Top and bottom valves are allowed to continue actuating periodically, switching between inlet streams. [0069] In an embodiment, when the CO₂/CH₄ gaseous mixture is fed into Adsorptive Reactor I, CO₂ is captured in the adsorbent material inside the Adsorptive Reactor I and CH₄ passes through, leaving the reactor as a purified CH₄ stream (cf. dashed line in Figure 2A). [0070] In an embodiment, when the top and bottom valves are actuated, CO₂ is now captured in the adsorbent material inside the Adsorptive Reactor II and CH₄ passes through, leaving the reactor as a purified CH₄ stream (cf. dashed line in Figure 2B). In Adsorptive Reactor I, the H₂ that is being fed changes the gas-solid equilibrium inside the bed allowing the desorption of the previously retained CO₂, which then becomes available to react with H₂ at the catalyst material also present in the reactor, through the Sabatier reaction, for producing CH₄ (cf. continuous line in Figure 2B) while regenerating the adsorbent (i.e., making it ready and able to capture more CO₂ in the following step). [0071] In an embodiment, when the top and bottom valves are actuated a second time, CO₂ is being adsorbed again in Adsorptive Reactor I and CH₄ purified (cf. dashed line in Figure 2A). CH₄ is being produced in Adsorptive Reactor II (cf. continuous line in Figure 2A) and the adsorbent regenerated. [0072] In an embodiment, when the top and bottom valves are actuated a third time, CO₂ is being adsorbed in Adsorptive Reactor II and CH₄ purified (cf. dashed line in Figure 2B). CH₄ is being produced in Adsorptive Reactor I (cf. continuous line in Figure 2B) and the adsorbent regenerated. [0073] In an embodiment, when the top and bottom valves are allowed to continue actuating periodically, switching between inlet streams, the CO₂ Adsorption Stage and Reactive Regeneration Stage cycles are perpetuated in both reactors, allowing a continuous process operation with CO₂ capture, CH₄ purification and CH₄ formation from captured CO₂ in the same multifunctional and compact reactor device. [0074] In an embodiment, the reactor of the present disclosure comprises the following components: (1) adsorptive reactor I, (2) adsorptive reactor II, (3) top valve, (4) bottom valve, (5) moisture trap I, (6) moisture trap II, (7) tubing. [0075] In an embodiment, the bed of adsorptive reactor I comprises: a CO₂ adsorbent material (commonly mixed oxides, hydrotalcite-based materials, activated carbons, zeolites and/or other CO₂ adsorbent) and a methanation catalyst (such as unsupported or supported Ni-, Ru- or Rh-based catalysts) or, alternatively, a dual-function material (a CO₂ adsorbent material with methanation catalytic activity). [0076] In an embodiment, the adsorbent and catalyst may be arranged in a layered, structured or mixed bed configuration. [0077] In an embodiment, the adsorbent and the catalyst material that are the filler of the adsorptive reactor may be arranged in a fluidized bed configuration or the deposition of such materials can be done in structured arrangements (e.g., monoliths). [0078] In an embodiment, the mass (or volume) ratio between the catalyst and the adsorbent is adapted to the characteristics and performance of the used materials (e.g., the activity of the catalyst and adsorption capacity of adsorbent). [0079] In an embodiment, the characteristics of Adsorptive Reactor II is similar to Adsorptive Reactor I. [0080] In an embodiment, the top valve is a 4-way valve for switching feeding streams (CO₂/CH₄ mixture or H₂). [0081] In an embodiment, the bottom valve is a 4-way valve for switching the paths of the outlet streams. This valve is optional and must be used if there is a need for separating the purified CH₄ (obtained from the CO₂ Adsorption Stages) from the CH₄ produced (obtained from the Reactive Regeneration Stages). If used, this valve may be actuated simultaneously with the top valve. [0082] In an embodiment, the moisture trap I is a moisture trap that separates the water produced by the Sabatier reaction during the Reactive Regeneration Stage from the outlet stream of Adsorptive Reactor I. This moisture trap I may also be positioned elsewhere downstream in the reactor. [0083] In an embodiment, the moisture trap II is a moisture trap that separates the water produced by the Sabatier reaction during the Reactive Regeneration Stage from the outlet stream of Adsorptive Reactor II. This moisture trap II may also be positioned elsewhere downstream in the reactor. [0084] In an embodiment, the tubing carries gas streams in and out of the 4-way valves and reactors. [0085] In an embodiment, the method disclosed comprises purification of CO₂/CH₄ gas mixtures and conversion of the separated CO₂ into more CH₄ through the use of an adsorptive reactor that combines, in the same unit, CO₂ captured by adsorption and its catalytic conversion into CH₄. [0086] In an embodiment, the reactor’s bed comprises a selective CO₂ adsorbent material to separate CO₂ from other species present in the feed, and a methanation catalyst to accelerate the reaction during the (reactive) desorption stage. [0087] In an embodiment, the bed may further comprise dual-function materials, i.e., adsorbent materials featuring catalytic activity. [0088] In an embodiment, the CO₂ adsorbent material and the methanation catalyst may be placed inside the reactors in different configurations: layered (alternate layers of catalyst and adsorbent particles), fluidized, structured or mixed (a homogeneous mixture of catalyst and adsorbent particles). [0089] In an embodiment, any methanation catalyst (typically based on Ni, Ru, Rh and/or other elements dispersed on a metal oxide support such as Al₂O₃, SiO₂, TiO₂) and CO₂ adsorbent (commonly mixed oxides, hydrotalcite-based materials, activated carbons and/or zeolites) can be used for the method of the present disclosure as the main key issue lies in their compatibility (adsorbent and catalyst) in terms of operating conditions. The methanation reaction is thermodynamically limited at elevated temperatures (because it is exothermal – cf. Eq.1) and kinetically limited at low temperatures, and so catalytic methanation reactors are usually operated at temperatures between 150 °C and 500 °C (for the reactors of the present disclosure it is preferably in the range between 250 °C - 350 °C) and pressures from 1 to 300 bar. The methanation of CO₂ is a highly exothermic reaction and so temperature control is a very important aspect in the design of methanation reactors where poor heat management can lead to the formation of hotspots and thus catalyst deactivation (sintering) and safety issues.^18,19 [0090] In an embodiment, the CO₂ adsorbents materials used can be divided into three categories based on their operating temperature range: low-temperature (< 200°C, for instance, carbon-based, zeolite-based, silica-based, polymer-based, clay-based, metal- organic framework-based, alkali metal carbonate-based, solid amine-based adsorbents); intermediate-temperature solid adsorbents, which are the most compatible with methanation catalysts (200°C to 400°C, such as hydrotalcite-like materials, mixed oxides) and high-temperature solid adsorbents (> 400°C, for example, calcium-based, alkali ceramic-based, alkali zirconate-based, alkali silicate-based adsorbents).^20–22 [0091] In an embodiment, in order for the reactors to operate continuously, the method of the present disclosure requires at least two parallel reactors working under cyclic operation mode and operating 180° out of phase. The 2-column configuration is illustrated in Figure 2. [0092] In an embodiment, the cyclic process occurs by switching a 4-way valve (top valve) that controls and alternates the two possible feeding streams, making each of the reactors periodically switch from the adsorption stage (CO₂/CH₄ mixture feeding) to the reactive regeneration stage (H₂ feeding), and back again, repeatedly. [0093] In an embodiment, during the adsorption stage, the CO₂/CH₄ gas mixture is fed into one of the reactors and so, the adsorbent inside is selectively removing (capturing) the CO₂ in the gas mixture thus producing a purified CH₄ stream that leaves the reactor. [0094] In an embodiment, during the reactive regeneration stage, the inlet stream is composed of H₂ (preferably “green” H₂ – PtM concept), that, by means of the methanation catalyst inside the reactor, reacts with the previously adsorbed CO₂ (captured by the adsorbent in the adsorption stage) through the Sabatier reaction, simultaneously producing methane and regenerating the CO₂ adsorbent. During this stage, two phenomena that, to some extent, counterbalance each other in terms of heat release and consumption, occur at the same time: the endothermic desorption of CO₂ molecules from the adsorbent solid phase to the gas phase and the exothermic methanation reaction at the catalytic active sites between CO₂ and H₂, and so the dangers of hotspots are mitigated. This is also because the reactors are functioning 180° out of phase, while one is in the adsorption stage, the other is in the reactive regeneration stage. [0095] In an embodiment, downstream in the process, after the reactors, a moisture trap is placed so that the water produced in the Sabatier reaction and present in the outlet stream is separated and removed, leaving the CH₄ stream dry, and thus purer. [0096] In an embodiment, the method of removing moisture from the outlet stream is by condensation, adsorption by silica gel, activated carbon, aluminium oxide, glycol or hygroscopic salts. [0097] In an embodiment, the temperature of the adsorptive reactor ranges from 150 °C to 500 °C, preferably from 250 °C to 350 °C. [0098] In an embodiment, the pressure of the adsorptive reactor ranges from 1 to 300 bar, preferably from 1 to 100 bar. [0099] In an embodiment the unit can have 1 or 2 or more than 2 adsorptive reactors. [00100] In an embodiment, the operation conditions are adapted to the materials used (namely temperature and pressure) bearing in mind that higher temperature and pressure are kinetically better for both adsorption and methanation reaction, and so although the use of excessively high temperature and pressure are associated with some disadvantages (such as thermodynamic limitations of the reaction, material resistance, higher costs), they are, to some extent, beneficial to the purity and productivity of the proposed process. [00101] In an embodiment, the unit of the present disclosure may comprise at least 2 adsorptive reactors to run continuously. [00102] In an embodiment, the reactor may comprise different arrangements of the bed, or different positioning of the moisture traps downstream of the adsorptive reactors, other arrangements for the outlet gas streams that might be separated or joined, upflow or downflow, with the use of bottom valve or not. [00103] The experiment described below is an example (and proof-of-concept) of the present invention and so, it is merely illustrative and does not limit its scope. The operating conditions herein used should be regarded as one of many possibilities, leaving room for improvement in CH₄ productivity and purity, as well as in any of the indicators described below (namely changing the pressure, temperature, cycle time, flow rates, materials and their assembly in the bed, etc.). [00104] Two stainless steel reactors (herein called Adsorptive Reactor I and II filled with a CO₂ adsorbent and a methanation catalyst (in a 7.7:1 volumetric ratio) were used in the experiments. The materials were packed inside the columns according to the scheme shown in Figure 3, consisting of 10 alternate layers of adsorbent and catalyst. Inert glass spheres were used to dilute the bed. [00105] Figure 3 shows the reactor’s filling scheme with alternate layers of CO₂ adsorbent and methanation catalyst, and the placement of the 4 thermocouples inside reactor bed. [00106] The reactors were placed inside an oven equipped with forced air convection, ensuring an external homogeneous temperature distribution during all process (Figure 4). The temperature of both reactors was measured through 4 type-K thermocouples placed in contact with the bed, aligned with the first 4 catalyst layers from the top (Figure 3). Mass flow controllers were used to feed CO₂, H₂, and CH₄. The mass flow rate of the outlet streams was measured by two mass flow meters and corrected based on their composition. Two pressure transducers located before and after each reactor measured the pressure. During the experiments, the water produced in the CO₂ methanation reaction and present in the outlet stream was condensed and removed by a Peltier module and a cold trap. The composition of the (dry) outlet streams was recorded along time using an analyzer able to measure CO₂, H₂, CH₄, CO and O₂ (although the latter two components were not observed). Two automated 4-way valves were used to alternate the inlet streams of the reactors (mixture of CO₂ and CH₄ or H₂) (top valve) and to select the outlet stream sent to the analyzer (bottom valve). A scheme of the described experimental installation is presented in Figure 4. Prior to the experiments, the catalyst was activated in-situ by feeding a binary mixture of H₂:N₂ to each reactor. [00107] The experiments were assessed through the following process indicators (calculated for each Adsorption/Reactive Regeneration cycle in each adsorptive reactor): • Adsorption capacity – to assess the amount of CO₂ retained in the adsorbent during the adsorption stage wher are the inlet and outlet molar flow rates of CO₂, respectively, is the time during the adsorption stage, is the duration of the adsorption stage, is the adsorbent mass and is the number of moles of CO₂ adsorbed. [00108] CO₂ conversion – to assess the percentage of the CO₂ adsorbed in the adsorption stage that was converted into CH₄ during the reactive regeneration stage are the inlet and outlet molar flow rates of CH₄, is the time during the reactive regeneration stage, is the duration of the reactive regeneration stage and is the number of moles of CH₄ produced during such stage. [00109] Average CH₄ purity during the adsorption stage where is the outlet total molar flow rate. [00110] Average CH₄ purity during the reactive regeneration stage [00111] Average CH₄ purity during a full cycle (adsorption and reactive regeneration stages) [00112] Moles of H₂ used per mole of CH₄ produced – to assess the amount of H₂ required per mole of CH₄ produced and see how far from the stoichiometric value (4 regeneration stage. [00113] CH₄ productivity – to assess the amount of CH₄ produced per mole of CO2 fed to the reactor where is the number of moles of CO2 that are fed to the reactor (during all the adsorption stages). [00114] In this example, adsorption/reactive regeneration cycles were performed at 350 °C and the atmospheric pressure until cyclic steady-state was reached on both reactors. Each cycle consisted of two stages: a 10-minute adsorption stage during which the inlet composition consisted of a 50%:50% (vol. CO₂:CH₄) mixture, and a 10-minute reactive regeneration stage during which a pure H₂ stream was fed to the adsorptive reactors. The inlet streams (CO₂/CH₄ mixture or H₂) of both adsorptive reactors were switched every stage (i.e., each 10 minutes) by the automated top valve. The bottom valve was not always actuated simultaneously with the top valve, but in a way that allowed the measurement, in particular the measurement of the composition, of consecutive adsorption/reactive regeneration stages from Adsorptive Reactor I or from Adsorptive Reactor II, thus allowing the calculation of process indicators. [00115] Figure 5 shows the flow rate of each component in the outlet streams of Adsorptive Reactor I and Adsorptive Reactor II The adsorption stages are framed in A-boxes and the reactive regeneration stages in RR-boxes, which are constantly being switched in both adsorptive reactors, although only some were followed with the analyzer, which can measure only one stream at a time. [00116] Figure 5 shows the flow rate (by component) of the outlet streams of Adsorptive Reactor I and Adsorptive Reactor II . The black frame surrounds the adsorption/reactive regeneration cycles for which the process indicators were calculated. The letters above each stage indicate whether it is an adsorption stage (A) or a reactive regeneration stage (RR). The zoom-in shows, in further detail, the two adsorption/reactive regeneration cycles performed on Adsorptive Reactor II between the 230^th and 270^th minute of experiment. [00117] The black frames highlighted in Figure 5 correspond to the adsorption/reactive regeneration cycles for which the process indicators described were calculated (cf. Figure 6). [00118] Figure 6 shows the process indicators: Figure 6a shows the CO₂ adsorption capacity; Figure 6b shows the CO₂ conversion; Figure 6c shows the CH₄ purity (the bars referring to reactive regeneration stages are marked with the diagonal lines, while the adsorption stages have no pattern) and Figure 6d shows (the black horizontal line indicates the stoichiometric ratio). The black frame indicates when the cyclic steady- state had been reached. [00119] Table 1 below shows the average value of the process indicators of the 4 adsorption/reactive regeneration cycles when the unit reached a cyclic steady-state (cycles framed in Figure 6). The stream resulting from the (50%/50%) CO₂/CH₄ mixture purification (i.e., during the adsorption stages), presents on average, a CH₄ purity of 80.3 %. During the reactive regeneration stages, 80.6 % of the adsorbed (i.e., captured) CO₂ was converted into more CH₄. This stage produces a stream with 81.9 % of CH₄ on average, resulting in a productivity of (moles of methane produced, by chemical reaction, per mole of CO₂ fed). The overall purity of the outlet streams (i.e., after blending the purified and produced CH₄ streams) is on average 81.1 %. [00120] Table 1 - Process indicators for the system in cyclic steady-state for the considered example. [00121] Figure 7 shows the temperature variation histories (i.e., with respect to the oven and feed temperature – 350°C in this example) for all four measuring points in Adsorptive Reactor II. For these operating conditions, the temperature increase remained below 9 °C. [00122] Figure 7 shows the temperature variation histories of Adsorptive Reactor II (four temperature measuring points along each reactor – T₁ being the nearest to inlet and T₄ the nearest to reactor outlet). The letters above each stage indicate whether it is an adsorption stage (A) or a reactive regeneration stage (RR). T_o stands for the oven and feed temperature (350 °C in this example). [00123] For comparison, if the purification of the CO₂/CH₄ mixture herein considered (50%:50%) was performed with an upgrading technology that separates the two components (e.g., by absorption, PSA, membranes, etc.), the best-case scenario is when full recovery and full purity is obtained, resulting in two pure streams (CO₂-rich and CH₄-rich). However, in the present invention, the two resulting streams are both CH₄-rich after the cyclic pseudo stationary-state is reached: the first is the purified methane resulting from the adsorption stages and contains a CH₄ purity of roughly 80 %; the second is the produced CH₄, resulting from the reactive regeneration stage and contains a CH₄ purity of roughly 82 % (Table 1). This means that the method and reactor of the present disclosure, when compared to the ideal/best case scenario of existing biogas upgrading separation technologies, allows the user to increase the amount of methane produced by 77% (ratio of methane flow rates of 1.77). [00124] In the ideal thermodynamic equilibrium, the direct methanation of the inlet streams used in the example at 350 °C and 1 atm would result in a CO₂ conversion of 81% and originate an outlet stream that, if subsequently dried, would have a CH₄ content of 76% (data obtained by employing the non-stoichiometric Gibbs free energy minimization method with the Aspen Plus V12 software). Comparing these values with the ones obtained in the example (both CO₂ conversion and CH₄ purity of roughly 81%) it is possible to conclude that the present invention poses great advantages as compared to conventional methanation reactors. The equilibrium conversion and CH₄ purity described in the present disclosure (81% and 76%, respectively), which can only be achieved in ideal conditions (namely considering an ideal packed-bed reactor of infinite length), can easily be attained with the method and reactor of the present disclosure. Moreover, the hotspots created by the highly exothermic methanation reaction are not accounted for in these equilibrium determinations, but contribute negatively, further inhibiting the conversion of CO₂ in conventional methanation reactors. With the method and reactor of the present disclosure, the hot-spots are almost completely absent, posing no such restriction and also no major safety concerns. [00125] The example provided is merely illustrative, and any individual with know- how in the art will easily understand that changing one or more of the operating conditions (e.g., feed composition, flow rate/s, pressure, temperature, cycle duration etc.) and/or process configurations (e.g., catalyst-to-adsorbent ratio, bed configuration, materials used, etc.) can easily allow to improve one or more of the above-mentioned process indicators. [00126] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. [00127] Where singular forms of elements or features are used in the specification of the claims, the plural form is also included, and vice versa, if not specifically excluded. For example, the term “a molecule” or “the molecule” also includes the plural forms “molecules” or “the molecules,” and vice versa. In the claims, articles such as “a,” “an,” and “the” may mean one or more than one unless indicated to the contrary or otherwise evident from the context. Claims or descriptions that include “or” between one or more members of a group are considered satisfied if one, more than one, or all of the group members are present in, employed in, or otherwise relevant to a given product or process unless indicated to the contrary or otherwise evident from the context. The invention includes embodiments in which exactly one member of the group is present in, employed in, or otherwise relevant to a given product or process. The invention also includes embodiments in which more than one, or all of the group members are present in, employed in, or otherwise relevant to a given product or process. [00128] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. [00129] The embodiments described above are combinable.

Claims

C L A I M S 1. A method for continuous upgrading of methane and carbon dioxide gas mixtures comprising the steps of: feed a gas mixture stream comprising carbon dioxide and methane into a first adsorptive reactor for a first carbon dioxide adsorption to obtain a first methane rich gas, wherein said reactor comprises an adsorbent and catalyst filler; when the first adsorptive reactor is fully saturated or partially saturated, feed the gas mixture stream comprising carbon dioxide and methane into the second adsorptive reactor for a second carbon dioxide adsorption in order to obtain a second methane enriched gas, and simultaneously feed a hydrogen-containing gas stream into the first adsorptive reactor for reactive regeneration; when the second adsorptive reactor is saturated or partly saturated, feed the gas mixture comprising carbon dioxide and methane into the first adsorptive reactor for carbon dioxide adsorption, and simultaneously feed a hydrogen-containing gas stream into the second adsorptive reactor for reactive regeneration; periodically switch the gas streams that are fed into the adsorptive reactors in order to prevent the reactors from becoming fully saturated or partially saturated; wherein the first and the second adsorptive reactors comprise a CO₂ adsorbent material and a methanation catalyst as a filler; wherein the volume ratio of the CO₂ adsorbent material and the catalyst ranges from 1:1 to 30:1 (volume _{CO2 adsorbent} / volume _catalyst). 2. The method according to the previous claim wherein volume ratio of the CO₂ adsorbent material and the catalyst ranges from 1:1 to 20:1 (volume _{CO2 adsorbent} / volume _catalyst), preferably 1:1 to 10:1 (volume _{CO2 adsorbent} / volume _catalyst). 3. The method according to any of the previous claims, wherein the CO₂ adsorbent material and the methanation catalyst are arranged in a layered, fluidized, structured or mixed bed configuration.

^4. The method according to any of the previous claims, wherein the CO₂ adsorbent material is selected from: carbon-based, zeolite-based, silica-based, polymer-based, clay-based, metal-organic framework-based, alkali metal carbonate-based, solid amine-based, hydrotalcite-like, single or mixed oxides, calcium-based, lithium-based, alkali ceramic-based, alkali zirconate-based or alkali silicate-based or mixtures thereof. 5. The method according to any of the previous claims, wherein the methanation catalyst is selected from: Ni, Ru, Rh, Fe, Co, Mo, Pd, Ag, W, Os, Ir, Pt or Au, wherein the catalyst is dispersed on a conventional metal oxide support or a structured metal oxide support or a carbon support, or mixtures thereof. 6. The method according to claim 5, wherein the conventional metal oxide support is selected from: Al₂O₃, SiO₂, TiO₂, MgO, CaO, ZrO_2, Cr₂O₃, CeO₂, Ce_xZr_1-xO₂, La₂O₃, MnO₂ or ZnO. 7. The method according to claim 5, wherein the carbon support is selected from: activated carbon, carbon nanotubes, carbon nanofibers, biochar or carbon felts. 8. The method according to claim 5, wherein the structured metal oxide support is mesostructured silica nanoparticles or metal-organic framework materials. 9. The method according to the previous claims, wherein the filler is a dual-function material comprising CO₂ adsorbent activity and methanation catalytic activity. 10. The method according to any of the previous claims 4-8, wherein the dual-function material comprises a combination of a CO₂ adsorbent material and a catalyst. 11. The method according to the previous claims, wherein the bed comprises a CO₂ adsorbent material and a methanation catalyst, or a dual-function material with CO₂ adsorbent activity and methanation catalytic activity.

12. The method according to any of the previous claims, wherein the temperature of the adsorptive reactors ranges from 150 °C to 500 °C, preferably from 250 °C to 350 °C. 13. The method according to any of the previous claims, wherein the pressure of the adsorptive reactors ranges from 1 to 300 bar, preferably from 1 to 100 bar. 14. The method according to any of the previous claims, wherein the unit can have 1 or 2 or more than 2 adsorptive reactors. 15. The method according to any of the previous claims, wherein the cyclic reactor unit has a CO₂ conversion rate of at least 70%, more preferably at least 80%, even more preferably at least 90%. 16. A cyclic reactor unit for continuous upgrading of methane and carbon dioxide gas mixtures according to the previous claims 1-14, comprising: at least one adsorptive reactor for receiving hydrogen-containing gas stream or methane and carbon dioxide gas mixtures; at least one moisture trap for separating water produced from the adsorptive reactor; wherein the cyclic reactor unit switches the gas streams that are fed into the adsorptive reactors such that the adsorptive reactors cycle between adsorption stage and reactive regeneration stage. 17. The cyclic reactor unit according to the previous claim comprising: a first adsorptive reactor for receiving hydrogen-containing gas stream or methane and carbon dioxide gas mixtures; a second adsorptive reactor for receiving hydrogen-containing gas stream or methane and carbon dioxide gas mixtures; a first moisture trap for separating water produced from the first adsorptive reactor; a second moisture trap for separating water produced from the second adsorptive reactor; and tubing; wherein the cyclic reactor unit switches the gas streams that are fed into the adsorptive reactors such that the adsorptive reactors cycle between adsorption stage and reactive regeneration stage. 18. The unit according to any of the previous claims 15-16, wherein the adsorptive reactor comprises a bed comprising a CO₂ adsorbent material and a methanation catalyst, or a dual-function material with CO₂ adsorbent activity and methanation catalytic activity. 19. The unit according to any of the previous claims 15-17, wherein the CO₂ adsorbent material is selected from: carbon-based, zeolite-based, silica-based, polymer-based, clay-based, metal-organic framework-based, alkali metal carbonate-based, solid amine-based, hydrotalcite-like, singe or mixed oxides, calcium-based, lithium-based, alkali ceramic-based, alkali zirconate-based or alkali silicate-based or mixtures thereof. 20. The unit according to any of the previous claims 15-18, wherein the methanation catalyst is selected from: Ni, Ru, Rh, Fe, Co, Mo, Pd, Ag, W, Os, Ir, Pt or Au dispersed on conventional metal oxide supports (Al₂O₃, SiO₂, TiO₂, MgO, CaO, ZrO₂, Cr₂O₃, CeO₂, Ce_xZr_1-xO₂, La₂O₃, MnO₂, ZnO), structured metal oxide supports (mesostructured silica nanoparticles and metal-organic framework materials), carbon supports (activated carbon, carbon nanotubes, carbon nanofibers, biochar or carbon felts), or mixtures thereof. 21. The unit according to any of the previous claims 15-19, wherein the CO₂ adsorbent material and the catalyst are arranged in a layered, fluidized, structured or mixed bed configuration.