Method and apparatus for treating methane-containing gas

Abstract

The present invention relates to a method for treating a methane-containing gas comprising CO2 and at least one compound from the group of volatile organic compounds (VOCs), the latter being selected from the group consisting of ketones, sulfur-containing hydrocarbons and terpenes, and the method is such that the VOC concentration in the gas mixture is 10 to 10,000 ppm. The methane-containing gas mixture is compressed, cooled and then fed to an absorption apparatus comprising a liquid reversible VOC-absorbing absorption means. In the absorption apparatus, at least a part of the VOCs and up to 55% by volume of the CO2 are absorbed from the methane-containing gas mixture, thereby obtaining a methane-containing VOC- and CO2-reduced gas mixture and a VOC- and CO2-loaded absorption means. Thereafter, the VOC- and CO2-loaded absorption means are transported from the absorption apparatus to a desorption apparatus. Further, the methane-containing VOC- and CO2-reduced gas mixture is fed from the absorption apparatus to a separation apparatus where the CO2 is removed. A regeneration gas stream containing at least a part of the CO2-enriched gas stream is used in the desorption apparatus for the regeneration of the VOC- and CO2-loaded absorption means. Thereafter, an exhaust gas stream is discharged from the desorption apparatus and at least partially regenerated absorption means are transported from the desorption apparatus to the absorption apparatus.

Description

Technical Field

[0001] The present invention relates to a method for treating a methane-containing gas according to claim 1 and an apparatus for carrying out the method.

Background Art

[0002] Considering the ongoing global warming, CO2-neutral energy carriers are becoming increasingly important for modern society. One proven method of obtaining such CO2-neutral energy carriers is the fermentation of food waste in fermenters for biogas production. Foods such as fruits or vegetables bind CO2 from the atmosphere as they grow, so the biogas formed in the fermentation of these foods and the methane present therein are neutral with respect to CO2 or the climate. The biogas in the present application means a methane-containing gas mixture of natural origin. In addition to biogas produced by fermentation, for example, from fermenters or landfills, biogas may also be in the form of non-fermentation origin, such as natural gas, pit gas, or coal seam gas.

[0003] In order to make such a methane-containing gas mixture or biogas usable for energy generation, the methane concentration in the gas mixture must be increased by removing substantially all other compounds (referred to as impurities) or at least reducing their level. Therefore, biogas produced especially by fermentation is usually purified to increase the methane content rate.

[0004] The use of suitable adsorbents for the purification of gas mixtures is generally known, for example, in industrial off-gas purification or in domestic ventilation, for example in extractor hoods. By adsorption onto the inner surface of the porous adsorbent material, air pollutants, especially hydrocarbons, are removed from the gas mixture to be purified. A distinction can be made between fixed-bed adsorbents, moving-bed adsorbents, rotor adsorbents, fluidized-bed adsorbents, and entrained-flow adsorbents. In multi-stage off-gas purification systems, the adsorbent is often also used as a "polishing filter" as the final purification stage.

[0005] Today, it is possible to separate impurities in biogas by various biological, chemical, and physical processes, which is generally referred to as "(bio)gas treatment" and constitutes an important field of use for gas purification. Compounds with high boiling points can be separated, for example, by adsorption on activated carbon filters, while compounds with low boiling points can be removed, for example, by condensation, and water-soluble compounds can be removed by scrubbers. Impurities typically present in biogas are, for example, carbon dioxide (CO2), hydrogen sulfide (H2S), ammonia (NH3), and volatile organic compounds (abbreviated as VOC).

[0006] VOC is a general term for organic substances, i.e., carbonaceous substances, that migrate into the gas phase at room temperature or higher temperatures by evaporation, i.e., are volatile. Examples of VOCs are terpenes, ketones, amines, aldehydes, sulfur-containing hydrocarbons (S-HC), etc. Methane (CH4) is not a component of the group of VOCs in the context of this application.

[0007] In off-gas purification, the removal of VOCs is becoming increasingly important because the reduction of VOC emissions in the industrial and commercial sectors is important for two reasons: First, from a health perspective, VOCs in ambient air can cause certain symptoms in humans, such as headaches, allergic reactions, fatigue, reduced physical ability, sleep disorders, and inflammation of the respiratory tract, which are also collectively called by the term "sick building syndrome". Second, the presence of VOCs in industrially utilized gas mixtures can lead to malfunctions, damage, and reduced efficiency in processing or supply plants, which rapidly has a financial impact. In membrane plants used for gas purification, the filter capacity can be impaired by condensed terpenes, while ketones damage the seals, which means that maintenance intervals are shortened and the membranes have to be replaced earlier and more frequently. Therefore, specific methods for separating VOCs from gas mixtures at an early stage have been developed in both the field of exhaust gas purification and biogas treatment.

[0008] U.S. Patent Application Publication No. 2019 / 0001263 (A1) discloses a method for removing CO2, O2, N2, and VOCs from biogas. In a first step, the gas is compressed, and then the VOCs are continuously removed by a first adsorbent (preferably "pressure temperature swing adsorption", abbreviated as "PTSA"). Subsequently, the CO2 is removed by a cryogenic membrane, and finally, the O2 and N2 are separated by a second adsorbent (preferably PTSA) to obtain purified biogas. This method is relatively complex in that it has three separate separation means (two PTSAs and one membrane), and thus is relatively expensive and difficult to maintain. The two parallel-flowing PTSAs, in particular, have to be heated for the regeneration of the adsorption medium and then cooled again to the operating temperature to ensure a continuous process.

[0009] International Publication No. 2017 / 099581 (A2) discloses a method for purifying a supply gas. This includes compressing the supply gas and removing water from the compressed gas. Subsequently, an absorber removes VOC, carbon dioxide, and any water from the gas. One objective of the method in International Publication No. 2017 / 099581 (A2) is to extract the maximum amount of carbon dioxide from the supply gas and further use this carbon dioxide. This method is optimized such that the absorber is reprocessed by two absorption apparatuses connected in series, and the components extracted from the supply gas are partially returned to the purification process to minimize the loss of carbon dioxide. One drawback of the described method is that it cannot remove VOCs having a low boiling point, which remain in the circuit due to recirculation and thus accumulate in the plant. In this context, VOCs having a low boiling point mean compounds having a boiling point below 50 °C.

Summary of the Invention

Problems to be Solved by the Invention

[0010] Accordingly, an object of the present invention is to provide an improved method for removing impurities, particularly CO2 and VOC, from a methane-containing gas mixture, which solves the above-mentioned drawbacks of the prior art and enables an efficient and effective reduction in the contents of CO2 and VOC.

Means for Solving the Problems

[0011] This object is achieved according to the present invention by the method according to claim 1 and the apparatus according to claim 16. Preferred embodiments of the present invention are reflected in the dependent claims.

[0012] In the method of the present invention, in a first step a), a methane-containing gas mixture is provided which comprises CO2 and at least one compound from the group of volatile organic compounds (VOCs). The expression "compound from the group of volatile organic compounds (VOCs)" is also optionally used hereinafter as the expression "VOC". According to the present invention, at least one VOC is selected from the group consisting of ketones, sulfur-containing hydrocarbons, and terpenes. Further, according to the present invention, the VOC concentration in the methane-containing gas mixture is 10 to 10,000 ppm. In a second step b), the methane-containing gas mixture is compressed and cooled. In a third step c), the compressed and cooled methane-containing gas mixture is fed to an absorption device containing an absorbent that reversibly absorbs VOCs in liquid form. In a fourth step d), in the absorption device, at least a part of the VOCs from the methane-containing gas mixture and up to 5 volume % of the CO2 are absorbed by the absorbent to obtain a methane-containing gas mixture with a reduced level of VOCs and CO2 and an absorbent carrying the VOCs and CO2. In a fifth step e), the absorbent carrying the VOCs and CO2 from the absorption device is fed to a desorption device. In a sixth step f), the methane-containing gas mixture with a reduced level of VOCs and CO2 is fed from the absorption device to a separation device containing a membrane, and in the separation device, the methane-containing gas mixture with a reduced level of VOCs and CO2 is separated into a CO2-enriched gas stream at reduced pressure and a methane-enriched gas stream at isobaric pressure. In a seventh step g), for the regeneration of the absorbent carrying the VOCs and CO2, a regeneration gas stream containing at least a part of the CO2-enriched gas stream from step f) is fed to the desorption device to obtain an off-gas stream containing CO2 and at least one VOC and an at least partially regenerated absorbent. In an eighth step h), the off-gas stream from the seventh step is removed from the desorption device to a regenerative afterburner, and the at least partially regenerated absorbent from the desorption device is fed to the absorption device.

[0013] In the context of the present invention, the term "liquid" means the liquid state of a substance under standard conditions, i.e., ambient pressure of 1 bar and 20 °C.

[0014] In the context of the present invention, the expression "reversibly absorb" means a reversible interaction between the absorbent and the compound to be absorbed, by which the compound can reversibly bind to the absorbent and can also be separated from the absorbent again. Here, for example, it is not excluded that an absorbent that reversibly absorbs VOCs can also reversibly absorb further compounds such as CO2.

[0015] In the context of the present invention, the term "absorbent" means a material that can bind at least temporarily the compound to be absorbed.

[0016] In the method of the present invention, it is preferred to use a droppable absorbent, i.e., an absorbent that can form drops or small droplets. For example, in the case of water, a drop volume of about 50 μL (i.e., the volume of a typical drop) is assumed. It will be apparent to those skilled in the art that the drop volume has to be determined individually for each absorbent. The example of water is here only useful for orientation. The formation of drops increases the surface area of the absorbent and thus its efficiency. It is preferred to generate the drops using a conventional liquid disperser. Furthermore, it is also possible to use random packings or structured packings, preferably in order to distribute the gas better and thereby optimize the contact between the gas and the absorbent. Here, the design of the absorption device is selected to operate below the flooding point of the packing or absorbent in order to prevent the packing or absorbent from being pushed out of the absorption device by the gas flow. It is also possible to install a physical barrier to prevent this entrainment. Drop traps and meshes are conceivable.

[0017] In the context of the present invention, the expression "reduced pressure" means that the pressure of the gas is reduced, for example, by expanding the gas.

[0018] Furthermore, in the context of the present invention, the term "isobaric" means that, in terms of pressure conditions, there is only a slight change, if any. A slight change means a change of up to ±5% at most.

[0019] Although the steps in the method of the present invention are numbered, it will be apparent to those skilled in the art that certain steps may proceed in parallel. For example, steps e) and f) may be performed in parallel.

[0020] An advantage of the method of the present invention over known prior art methods is that, as a result of the local separation of VOC absorption and desorption, the containers within the apparatus, particularly the apparatus in which the above-described absorption and desorption take place, are not subject to any fluctuating stresses and thus their lifespan is extended.

[0021] Replacing the supported absorbent with a lower-loading absorbent takes only a short time and enables the continuous purification of the methane-containing gas mixture. Furthermore, the method of the present invention reduces the production cost of purified biogas since the method requires only one absorption apparatus. Additionally, the absorbent is regenerated, for example, in the case of activated carbon as the absorbent, and since it is not consumed, the absorption apparatus can be operated in a more cost-effective manner.

[0022] One further advantage of the method of the present invention over the prior art is that a liquid absorbent is used. Thus, the liquid absorbent can be pumped from a first container to a second container and is easier to handle than the adsorbents disclosed in the prior art, which typically take the form of bulk materials.

[0023] The use of the membrane in the method of the present invention further has the advantage that CO2 can be continuously separated and the maintenance cost of the membrane is lower compared to other separation apparatuses, for example PTSA.

[0024] The use of CO2 coming from the membrane as the regeneration gas has the advantages that it is at substantially ambient pressure and is substantially VOC-free. Therefore, the cost incurred in providing individual regeneration gas can be reduced because it is constantly consumed. In the desorption of VOC from the absorbent, CO2 is carried on the absorbent in the desorption device, and exits the desorption device as the absorbent carrying CO2. By cooling the absorbent before it enters the absorption device, additional absorption binding sites in the absorbent become vacant, and as a result, the absorbent can absorb CO2 again in the absorption device.

[0025] However, it will be apparent to those skilled in the art that, for example, if the CO2 gas stream from the membrane is used in some other way, it is also possible to provide the regeneration gas separately, or it is preferable to purify the absorbent with a different regeneration gas, such as N2.

[0026] In a preferred embodiment of the present method, a single absorption device is used in step d). One advantage of using only one absorption device instead of two serially connected absorption devices as disclosed in the prior art is that the process using only one absorption device incurs less construction and maintenance costs than a plant using two or more absorption devices.

[0027] In a further preferred embodiment of the present method, the absorption device in step d) includes at least four stages. The stage in the context of this application means an intermediate plate in the absorption device that divides the absorption process into individual sub-processes. The use of at least four stages in the absorption device enables the absorption of the maximum concentration of VOC, as will be shown later in FIGS. 3 to 11.

[0028] Preferably, in step h), the off-gas stream from the desorption device generated in step g) is fed to a regenerative afterburner for oxidation. This can oxidize both the VOC and any methane component present, allowing the off-gas stream to be released into the environment. The destruction of the VOC saves additional disposal costs and increases the overall viability of the plant for biogas treatment. By using the CO2-enriched permeate as the regenerative gas to be oxidized after use, the so-called methane slip in the CO2-enriched permeate is also simultaneously oxidized, preventing emissions to the atmosphere.

[0029] A regenerative afterburner generally consists of a mixing device where the gas mixture to be burned is mixed with air as an oxygen source, a heat exchange device where the gas mixture to be burned mixed with air is heated by the heat of the combustion gas mixture and the combustion gas mixture is cooled, a combustion chamber where the organic components of the gas mixture are completely oxidized, and a chimney where the combustion gas mixture is released into the atmosphere.

[0030] In a preferred embodiment, at least steps f) and h), more preferably steps c) to h), are carried out continuously. This has the advantage that the method of treating the methane-containing gas is carried out more efficiently and economically than when the gas is simply treated batchwise. Furthermore, continuous operation is less prone to failure because, in contrast to the periodic switching usually proposed in the prior art, no fixtures that are prone to failure are required to monitor the switching.

[0031] In a preferred embodiment, in step c), 2 to 10 liters of absorbent per Bm 3 of the methane-containing gas mixture 1Bm from step b) is used in the absorption device.

[0032] In the context of this application, the unit Bm 3 representing cubic meters per hour of operation defines the actual volume of the gas present in the operating state. The operating volume of the gas can be confirmed with sufficient accuracy by a first-order approximation using the ideal gas state equation.

[0033] Preferably, in step g), 1 to 3 liters of the absorbent carrying VOC from step d) is regenerated by the CO2-enriched gas of 1Bm from step f) in the desorption device. 3 is regenerated by the CO2-enriched gas of 1Bm from step f).

[0034] In a preferred embodiment, the absorbent for absorbing VOC has a boiling point of > 250 °C at 1013.25 mbar and is a compound selected from the group consisting of polyethylene glycol (PEG), mineral oil, ester, or a combination thereof, more preferably containing polyethylene glycol.

[0035] The absorbents having the above-described properties have particularly good compatibility with the method of the present invention because they efficiently absorb and desorb VOC under clearly distinguishable conditions without any special costs and inconveniences for achieving those conditions. This means that these absorbents can ensure a good and efficient treatment method.

[0036] The absorbent that reversibly absorbs VOC preferably has the formula (I): I) R1-O-(CH2CH2O) n -R2 and contains a compound of wherein n = 3 to 8, R1 and R2 are independently selected from linear C1-C 10 alkyl.

[0037] Based on formula (I), the absorbent that reversibly absorbs VOC more preferably has the compound (Ia) R1-O-(CH2CH2O) n -R2 where n = 11 and R1 and R2 are CH3.

[0038] Therefore, the preferred compound (Ia) is CH3-O-(CH2CH2O) 11 -CH3 where

[0039] A more preferred absorbent is known under the name Genosorb 300 with CAS No. 24991-55-7.

[0040] Surprisingly, the absorbent of formula (I) has been found to be particularly well-suited for the process of the present invention for the reasons given above.

[0041] A more preferred absorbent is a stabilized reaction product of 5,8,11,14-tetraoxaoctadecane and 5,8,11,14,17-pentaoxaheneicosane, also known as Genosorb 1843.

[0042] In a preferred embodiment, at least one VOC has a vapor pressure of at least 0.1 mbar at 20 °C and / or a boiling point of up to 240 °C at 1013.25 mbar.

[0043] In a preferred embodiment, at least one VOC is selected from the group consisting of acetone, 2-butanone, 3-methyl-2-butanone, 2-pentanone, 3-pentanone, 3,3-dimethyl-2-butanone, 2-methyl-3-pentanone, 4-methyl-2-pentanone, 3-methyl-2-pentanone, 3-hexanone, 2-hexanone, 5-methyl-3-hexanone, 3-methyl-2-hexanone, 2-heptanone, 4-octanone, 3-octanone, 2-octanone, 2,9-decanedione, α-thujene, α-pinene, camphene, sabinene, β-pinene, myrcene, 3-carene, thujanon, thujopsene, thymol, α-terpinene, β-caryophyllene, 1,4-cineole, eucalyptol, fenchone, γ-terpinene, terpinolene, limonene, tricyclene, linalool, menthone, nopinion, p-menthan-2-one, p-menthan-2-ol, camphor, carvomentone, 3,3-dimethyl-2-boronanone, carbonyl sulfide, methyl mercaptan, ethyl mercaptan, dimethyl sulfide, carbon disulfide, 2-propanethiol, 2-methyl-2-propanethiol, 1-propanethiol, thiophene, 2-butanethiol, isobutyl mercaptan, methyl allyl sulfide, methyl propyl sulfide, butanethiol, dimethyl disulfide, 2-methylthiophene, 3-methylthiophene, tetrahydrothiophene, 1-pentanethiol, thiophenol, dimethyl trisulfide, diisopropyl disulfide, dimethyl tetrasulfide, methyl propyl disulfide, and methyl isopropyl disulfide. It will be apparent that it is also possible to use mixtures of the compounds mentioned.

[0044] The method of the present invention has been found to be also suitable for treating gas mixtures containing at least two VOCs. The gas mixture preferably contains at least one further VOC selected from the group consisting of aldehydes, alcohols, amines, BTEX, esters, ethers, CFCs, and siloxanes.

[0045] In a preferred embodiment, the at least one additional VOC is selected from the group consisting of linear or branched C1-C5 alcohols, sulfides, and terpenes.

[0046] Said at least one further VOC is preferably 2-methylbutyraldehyde, acetaldehyde, decanal, formaldehyde, hexanal, isobutyraldehyde, isopentanal, n-butyraldehyde, propionaldehyde, 1-butanol, 1-propanol, 2-butanol, 2-ethylhexanol, 2-methyl-1-butanol, 2-methyl-1-propanol, 2-methyl-2-butanol, 2-methyl-3-pentanol, 3-methyl-1-butanol, 3-pentanol, ethanol, isopropanol, methanol, 4,4-dimethyloxazolidine, 1,4-dimethylpiperazine, 1-methylpiperazine, 1-methylpyrrole, 4-methylpiperazin-1-ethanol, dimethylaminoethanol, dimethylpiperazine, methyldiethanolamine, N-aminoethylpiperazine, piperazine, pyridine, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, benzene, ethylbenzene, ethyltoluene, m-xylene, o-xylene, propylbenzene, p-xylene, styrene, toluene, 6,6-dimethyl-tetrahydropyran-2-one, ethyl butyrate, methyl butyrate, propyl butyrate, dimethyl carbonate, ethyl acetate, ethyl heptanoate, methyl hexanoate, ethyl isobutyrate, methyl isobutyrate, isopropyl acetate, methyl acetate, methyl formate, n-butyl acetate, n-propyl acetate, n-propyl propionate, ethyl pentanoate, methyl pentanoate, ethyl propionate, methyl propionate, sec-butyl acetate, 1,3-dioxane, 1,3-dioxolane, 2,4,5-trimethyl-1,3-dioxolane, 2-butylfuran, 2-ethyl-2-methyl-1,3-dioxolane, 2-ethyl-4-methyl-1,3-dioxolane, 2-ethyl-5-methylfuran, 2-ethylfuran, 5,6-dihydro-2-methyl-2H-pyran, 2-methylfuran, 2-propylfuran, 3-methylfuran, diethyl ether, dimethylfuran, methyl-1,3-dioxane, pentylfuran, tetrahydrofuran, trimethyldioxolane, 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, 1,2-dibromoethane, 1,2-dichlorobenzene, 1,2-dichloroethane, 1,2-dichloropropane, 1,3-dichlorobenzene, 1,4-dichlorobenzene, benzyl chloride, chloroethane, chloromethane, chloroform, dichloromethane, tetrachloroethene, trichlorofluoromethane, trichlorofluoromethane, vinyl chloride, 2,3-butanedione, 2,4-dimethylpentan-3-one, 2-methylhexan-3-one, 3,3-dimethyl-2-butanone, 3-ethylcyclopentanone, 3-heptanone, 4-ethylcyclohexanone, 4-methylhexan-3-one, 6-methylheptan-2-one, cyclohexanone, cyclopentanone, 2-methylcyclopentanone, 3-methylcyclopentanone, hydroxybutanone, 1,4-pentadiene, 2-methyl-1-butene, 1,1-dimethylcyclopropane, 1,3-butadiene, 1-nonene, 1-octene, 1-pentene, 2,2,4-trimethylpentane, 2,2-dimethylbutane, 2,3-dimethyloctane, 2,4-hexadiene, 2,6-dimethyloctane, 2-methyl-2-butene, 2-methylbutane, 2-methylhexane, 2-methylpentane, 2-nonene, 2-octene, 3-methyl-1-butene, 3-methylhexane, 3-methyloctane, 3-methylpentane, 4-methyloctane, 4-octene, butane, butene, cumene, cyclohexane, cyclopentene, trans-1,Selected from the group consisting of 2-dimethylcyclopropane, decane, dimethyloctene, heptane, hexane, isobutane, methylcyclohexane, methylcyclohexene, methylcyclopentane, naphthalene, n-dodecane, nonane, nonene, n-tetradecane, n-tetradecane, n-tridecane, n-tridecane, octane, pentane, propane, propene, tetramethylbenzene, undecane, decamethylcyclopentasiloxane, decamethyltetrasiloxane, dodecamethylcyclohexasiloxane, dodecamethylpentasiloxane, ethoxytrimethylsilane, fluorotrimethylsilane, hexamethylcyclotrisiloxane, hexamethyldisiloxane, hydroxytrimethylsilane, octamethylcyclotetrasiloxane, octamethyltrisiloxane, 1-ethyl-1H-pyrrole, acetonitrile, isobutyric acid, methylpropanenitrile, 1-(methylthio)-1-propene, 2-ethyl-5-methylthiazole, 2-ethylthiophene, 2-propylthiophene, 3-pentanethiol, dipropyldisulfide, dipropyltrisulfide, methylethyldisulfide, methylpropyldisulfide, methylthirane, methylthiopropane, 1-(methylthio)pentane, sec-butylpropyldisulfide, 2,6-dimethyl-2,6-octadiene, 2,6-dimethyl-4-octene, 2-carene, 3,3,5-trimethyl-1,5-heptadiene, 3,6,6-trimethylbicyclo-[3.1.1]-heptan-2-one, 3,7-dimethyl-2,4-octadiene, alloaromadendrene, α-farnesene, α-terpineol, α-copaene, α-cubebene, α-guaiene, α-pinocarbon, α-terpineol acetate, β-elemene, carene, dihydromyrcene, dihydroumbellulone, isomenthol, myrtenol, p,α,α-trimethylbenzyl alcohol, p-cymene, farnesene, pinocamphone, p-mentha-3-ene, terpinene-4-ol, α-caryophyllene, and γ-cadinene.,

[0047] Preferably, the methane-containing gas mixture is compressed to 6 to 24 bar(g), preferably 10 to 20 bar(g), more preferably 14 to 18 bar(g) in step b). This is because the methane-containing gas can be treated particularly efficiently within these pressure ranges.

[0048] In a preferred embodiment, the methane-containing gas mixture is cooled to 0°C to 20°C, preferably 2°C to 10°C in step b). This is because within these temperature ranges, VOCs are efficiently absorbed by the absorbent.

[0049] Preferably, the absorbent carrying VOCs from step d) is heated to 30°C to 90°C, more preferably 50°C to 70°C before being transported to the desorption device. Heating the absorbent enables the absorbent to be regenerated more easily and efficiently in the desorption device.

[0050] In a preferred embodiment, the regenerated absorbent from step g) is preferably cooled to 0°C to 20°C, more preferably 2°C to 10°C before being recirculated to the absorption device.

[0051] In a preferred embodiment, in step c), the absorbent that reversibly absorbs VOCs in liquid form is at least 6 kg per 1 Bm of the gas mixture 3 preferably at least 7 kg per 1 Bm of the gas mixture 3 more preferably at least 7.8 kg per 1 Bm of the gas mixture 3 and is supplied to the absorption device at a mass flow rate. For the absorbent that reversibly absorbs VOCs in liquid form, at least 6 kg per 1 Bm of the gas mixture 3 preferably at least 7 kg per 1 Bm of the gas mixture 3 more preferably at least 7 kg per 1 Bm of the gas mixture 3A mass flow rate of at least 7.8 kg per hour also enables the absorption of VOCs having a low boiling point (less than 50 °C) and thus enables their removal from the gas mixture. For example, in order to remove the VOC substance dimethyl sulfide having a boiling point of 37 °C at standard pressure, a volume flow rate of the absorbent of 7.8 kg per cubic meter during operation of the gas mixture has been found to be ideal. The density of the absorbent in this embodiment is approximately 0.9 to 1.1 kg / liter.

[0052] A further aspect of the present invention relates to an apparatus for treating a methane-containing gas by the method of the present invention. The apparatus comprises the following components: A gas source, A cooling device, A compressor, An absorption device having an upper end and a lower end, A desorption device having an upper end and a lower end, A separation device, A first connecting pipe connecting the lower end of the desorption device to the upper end of the absorption device, A second connecting pipe connecting the lower end of the absorption device to the upper end of the desorption device, A third connecting pipe connecting the upper end of the absorption device to the separation device, and A fourth connecting pipe connecting the separation device to the lower end of the desorption device.

[0053] The present invention will be described in more detail with reference to the following embodiments. The drawings are purely illustrative.

Brief Description of the Drawings

[0054]

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DETAILED DESCRIPTION OF THE INVENTION

[0055] In a preferred embodiment of the method of the present invention schematically shown in FIG. 1, in the first step, a methane-containing gas mixture is provided from a gas source 101. The gas mixture 103 contains CO2 and at least one compound from the group of volatile organic compounds (VOCs). The methane-containing gas mixture 103 is compressed to 14 to 18 bar(g) by a compressor 105 and then cooled to 2 to 10 °C by a cooler 107.

[0056] Subsequently, the methane-containing gas mixture 103 is supplied to an absorber 109. Here, the methane-containing gas mixture 103 is introduced from the lower end 111 of the absorber 109 and rises to the upper end 113 of the absorber 109.

[0057] The terms "upper" and "bottom" relate to the orientation of each device shown in the figure.

[0058] Furthermore, a liquid absorbent 117 is supplied to the absorption device 109 through a first connecting pipe 115 that communicates with the absorption device 109 in the region of the upper end 113. This liquid absorbent 117 flows in the form of drops from the upper end 113 to the lower end 111 of the absorption device 109 and comes into contact with the countercurrent of the methane-containing gas mixture 103. The contact between the absorbent 117 and the methane-containing gas mixture 103 results in the reversible binding of VOCs present in the gas mixture in the absorbent 117. As a result, a methane-containing gas mixture 119 having a reduced VOC level is obtained at the upper end 113 of the absorption device 109, and the absorbent 121 carrying the VOC is collected at the lower end 111 of the absorption device 109. The methane-containing gas mixture 119 having a reduced VOC level is sent from the upper end 113 of the absorption device 109 to the separation device 125 through the second connecting pipe 123. The absorbent 121 carrying the VOC is sent from the lower end 111 of the absorption device 109 to the heat exchange device 129 through the third connecting pipe 127 by a pump (not shown) and is heated to 50 - 70 °C there. Subsequently, the absorbent 121 carrying the VOC is further fed into the upper end 133 of the desorption device 135 and supplied to the desorption device 135. In the separation device 125, the methane-containing gas mixture 119 having a reduced VOC level is separated into a reduced-pressure CO2-enriched gas stream 137 (pressure -0.5 to +0.5 bar(g)) and an isobaric methane-enriched gas stream 139 (pressure 14 - 18 bar(g)). Next, the CO2-enriched gas stream 137 is introduced at the lower end 141 of the desorption device 135 through the fourth connecting pipe 143 and rises to the upper end 133 of the desorption device 135. This liquid VOC-carrying absorbent 121 flows in the form of drops from the upper end 133 to the lower end 141 of the desorption device 135 and comes into contact with the countercurrent of the CO2-enriched gas stream 137. As a result of heating the liquid VOC-carrying absorbent 121, the bound VOCs are reversibly released and flow to the upper end 133 of the desorption device 135 together with the CO2-enriched gas stream 137. Thereby, a purified absorbent 145 is produced, and the purified absorbent 145 collects at the lower end 141 of the desorption device 135. The fifth connecting pipe 147 transports the purified absorbent 145 to the heat exchange device 129 by a pump (not shown), the purified absorbent 145 is cooled to 2 - 10 °C, and then is pumped into the upper end 113 of the absorption device 109 through the supply pipe 115.The heat exchanger 129 may comprise a plurality of separate heat exchangers 131, 131', 131''. By this method, in order to reduce environmental pollution, at the upper end 133 of the desorption device 135, the CO2 and VOC-enriched gas stream 149 is removed and sent to the regenerative afterburner 151.

[0059] In order to utilize a methane-containing gas as a natural gas substitute, the sulfur-containing components in the product gas must be reduced to less than the limit of sulfur equivalent of 6 mg / m 3 This limit is described, for example, in the rule book G260 of DVGW (German Gas and Water Technology Scientific Association).

[0060] For example, for other VOCs, restrictions can be applied to the use of gas permeable membranes. In this case, for example, in order to meet the usage conditions of Evonik (manufacturer of gas permeable membranes), the total VOC concentration needs to be less than 10 ppm.

[0061] A further reason for the discussion on the limitation of VOCs in biogas is the heterogeneous odors that can be caused by VOCs.

[0062] Figure 2 shows an in silico simulation model of a preferred embodiment of the method of the present invention. This simulation model includes an absorber 201 and a desorber 203. The absorbent, here PEG, enters the absorber 201 as a liquid 205. The absorbent exits the absorber 201 as a VOC-carrying liquid 207 and is heated in a heat exchanger 209. The heated VOC-carrying absorbent 211 thus obtained is transported from the heat exchanger 209 to a further heat exchanger 213, where it is further heated. Here, external heat 215 is supplied to the heat exchanger 213. The significantly heated VOC-carrying absorbent 217 exits the heat exchanger 213 and expands at a control valve 219. The heated and expanded VOC-carrying absorbent 221 enters the desorber 203, where it releases VOC. The heated, expanded, and mostly regenerated absorbent 223 is compressed by a pump 225 and then further transported to the heat exchanger 209 as a heated, mostly regenerated, and compressed absorbent 227. In the heat exchanger 209, the absorbent 227 releases part of its heat to the VOC-carrying absorbent 207. Then, the cooler, mostly regenerated, and compressed absorbent 229 is transported to a further heat exchanger 231, where it is further cooled. For this purpose, external cooling 233 is supplied to the heat exchanger 231. The mostly regenerated, compressed, and cooled absorbent 205 re-enters the absorber 201, thus completing the absorbent circuit. The compressed biogas 235 to be purified is cooled by external cooling 239 in a heat exchanger 237 and then condensate is removed in a condensate separator 241. The resulting cooled compressed biogas 243 is supplied to the absorber 201, where it contacts the mostly regenerated, compressed, and cooled absorbent 205 and releases its VOC to the absorbent 205. The purified biogas 245 exits the absorber 201 and can then be used for further treatment 247. The regeneration gas 249, for example lean gas from a biogas treatment 251, is supplied to a heat exchanger 253, where it is heated. The heated regeneration gas 255 enters the desorber 203, where it causes the VOC to be released from the heated and expanded VOC-carrying absorbent 221.Next, the VOC-containing regeneration gas 257 exits the desorption device 203, and its heat is released to the regeneration gas 249 in the heat exchanger 253. The cooled VOC-containing regeneration gas 259 is conveyed to the post-treatment unit 263 by the suction blower 261.

[0063] In the above computer simulation, the solubility of a specific VOC in the absorbent, here PEG, was simulated according to Henry's law. In the first step, an equilibrium line where the gas phase and the liquid phase are in equilibrium is simulated. The slope of the equilibrium line corresponds to the Henry's coefficient of the compound at each temperature. In the second step, a mass balance line resulting from the mass balance of the VOC in the absorption device is simulated. The starting point of the mass balance line is the VOC concentration (purified biogas 245) at the upper end of the absorption device and the concentration of the liquid phase (regenerated PEG 205) at the upper end of the absorption device. The end point of the mass balance line is found from the gas phase concentration (VOC-containing biogas 243) at the lower end of the absorption device and the concentration of the liquid phase (VOC-supported PEG 207) at the lower end of the absorption device. The starting point and the end point of the mass balance line are connected by a straight line (mass balance line). This is only applicable under the assumption that the temperature in the absorption device is constant. In the case of a temperature gradient, both the mass balance line and the equilibrium line are shown as a mass balance curve and an equilibrium curve. A staircase structure can be drawn between the mass balance line and the equilibrium line. Each step represents one theoretical stage of the column. The number of steps is a measure of the required height of the absorption device. In absorption, the mass balance line is always above the equilibrium line. If the mass balance line is below the equilibrium line, the operation is desorption.

[0064] Figure 3a shows a similar process diagram for VOC methyl mercaptan (CAS 74-93-1) in the absorption device and PEG as the absorbent. The mass balance line (dashed line) is arranged above the equilibrium line (dash-dotted line), and the staircase structure (solid line) is shown between them.

[0065] Figure 3b shows a process diagram similar to the above-mentioned process diagram for VOC methyl mercaptan (CAS 74-93-1) and PEG as an absorbent in the desorption device. The equilibrium line (dashed-dotted line) is located above the material balance line (dashed line), and the staircase structure (solid line) is shown in between. From Figure 3a, it is clear that when the flow of the absorbent is too small (in the simulation model), a large amount of residue of methyl mercaptan remains in the purified biogas, so the staircase structure is not feasible for methyl mercaptan. From Figure 3b, it is clear that the absorbent is substantially completely regenerated in the desorption device and methyl mercaptan is removed.

[0066] Figures 4a and 4b show process diagrams for VOC dimethyl sulfide (CAS 75-18-3) and PEG as an absorbent in the absorption device (Figure 4a) and the desorption device (Figure 4b), similar to the above-mentioned process diagrams. From Figure 4a, it is clear that although a large number of steps are required for the purification of biogas, residues of dimethyl sulfide remain in the biogas. From Figure 4b, it is clear that the absorbent is substantially completely regenerated in the desorption device and dimethyl sulfide is removed.

[0067] Figures 5a and 5b show such exact process diagrams for VOC acetone (CAS 67-64-1) and PEG as an absorbent in the absorption device (Figure 5a) and the desorption device (Figure 5b), similar to the above-mentioned process diagrams. From Figure 5a, it is clear that although a large number of steps are required for the purification of biogas, a very small amount of residue of acetone remains in the biogas. From Figure 5b, it is clear that the absorbent is substantially completely regenerated in the desorption device and acetone is removed.

[0068] Figures 6a and 6b show such exact process diagrams for VOC 2-butanone (CAS 78-93-3) and PEG as an absorbent in the absorption device (Figure 6a) and the desorption device (Figure 6b), similar to the process diagrams described above. From Figure 6a, it is clear that only a few steps are required for the purification of biogas and no residue of 2-butanone remains in the biogas. From Figure 6b, it is clear that the absorbent is regenerated in the desorption device and 2-butanone is removed by a number of steps, but a small amount of 2-butanone remains in the regenerated absorbent.

[0069] Figures 7a and 7b show process diagrams for VOC 1-propanol (CAS 71-23-8) and PEG as an absorbent in the absorption device (Figure 7a) and the desorption device (Figure 7b), similar to the above-mentioned figures. From Figure 7a, it is clear that only a few steps are required for the purification of biogas and a very small residue of 1-propanol remains in the biogas. From Figure 7b, it is clear that despite a number of steps, the absorbent for removing 1-propanol is not completely regenerated in the desorption device.

[0070] Figures 8a and 8b show process diagrams prepared in the same way as the above-mentioned figures for VOC toluene (CAS 108-88-3) and PEG as an absorbent in the absorption device (Figure 8a) and the desorption device (Figure 8b). From Figure 8a, it is clear that only a few steps are required for the purification of biogas, but due to incomplete regeneration of the biogas, residues of toluene remain in the biogas. From Figure 8b, it is clear that despite a number of steps, the absorbent for removing toluene is not completely regenerated in the desorption device. Since the material balance line and the equilibrium line extend parallel to each other, complete regeneration is impossible.

[0071] Figures 9a and 9b show process diagrams prepared in the same manner for VOC limonene (CAS 7705-14-8) and PEG as an absorbent in the absorption device (Figure 9a) and the desorption device (Figure 9b). From Figure 9a, it is clear that only a few steps are required for the purification of biogas, but due to the incomplete regeneration of the biogas, residues of limonene remain in the biogas. From Figure 9b, it is clear that despite a large number of steps, the mass flow rate of the absorbent is too high (in the simulation model), so the absorbent for removing limonene is not completely regenerated in the desorption device.

[0072] Based on the simulations according to Figures 2 to 9, it can be said that VOCs having a boiling point of 35°C to 100°C can be substantially completely released from the biogas in the absorption device and can be substantially completely released from the absorbent in the desorption device. VOCs having a boiling point higher than 100°C are substantially completely removed from the biogas. However, the regeneration of the absorbent is incomplete. VOCs having a boiling point below 35°C are incompletely removed from the biogas. The gas components CO2 and methane remain almost completely in the gas phase.

[0073] Figure 10 shows a diagram based on 48 measurements of VOCs in a biogas sample, and the biogas was produced from renewable raw materials. The concentration of VOCs in the biogas is 5 to 20 ppm. Individual values are higher. VOCs in biogas from renewable raw materials do not pose any problems in further processing.

[0074] Figure 11 shows a diagram based on 138 measurements of VOCs in a biogas sample. Here, the biogas was produced from waste. The concentration of VOCs in the biogas is 10 to 250 ppm. In many cases, higher concentrations were also achieved. VOCs in biogas from waste often pose problems in further processing. Therefore, controlled removal of VOCs is necessary.

Example

[0075] Experimental data Figure 12 shows an in silico simulation model of a preferred embodiment of the method of the present invention described in Figure 2. To clarify how much CO2 and VOC are absorbed, various parameters for biogas, absorbent, and regeneration gas were added to the simulation model. This simulation model includes three gas streams at different stages. The compressed methane-containing gas BG1 is supplied to the heat exchanger 1201 and exits the heat exchanger as the compressed and cooled methane-containing gas BG2. The methane-containing gas BG2 is supplied to the absorber 1203, and the CO2 and VOC components present in the methane-containing gas BG2 are absorbed. The methane-containing gas BG3 with reduced levels of CO2 and VOC exits the absorber 1203. In the absorber 1203, the methane-containing gas BG2 contacts the regenerated and cooled absorbent AB1. The regenerated and cooled absorbent AB1 absorbs CO2 and VOC in the absorber 1203 and exits the absorber 1203 as the absorbent AB2 carrying CO2 and VOC. The absorbent AB2 carrying CO2 and VOC is heated in the heat exchanger 1205 and exits the heat exchanger as the heated absorbent AB3 carrying CO2 and VOC. The heated absorbent AB3 carrying CO2 and VOC is further heated in the additional heat exchanger 1207 and exits the heat exchanger as the heated absorbent AB4 carrying CO2 and VOC. The heated absorbent AB4 carrying CO2 and VOC is supplied to the control valve 1209 and expands, exiting the control valve as the heated and expanded absorbent AB5 carrying CO2 and VOC. The heated and expanded absorbent AB5 carrying CO2 and VOC is supplied to the desorber 1211.

[0076] The regeneration gas RG1 is heated in the heat exchanger 1213 and exits the heat exchanger as the heated regeneration gas RG2. The regeneration gas RG1 is a gas stream having CO2 as its main component. The heated regeneration gas RG2 is supplied to the desorption device 1211, where it contacts the absorbent AB5 carrying CO2 and VOC that has been heated and expanded. The heated regeneration gas RG2 releases CO2 and VOC from the absorbent AB5 to obtain a regeneration gas RG3 containing CO2 and VOC and an absorbent AB6 that has been heated, expanded, and mostly regenerated. The absorbent AB6 that has been heated, expanded, and mostly regenerated is compressed in the pump 1215 and then supplied to the heat exchanger 1205 as the absorbent AB7 that has been heated, mostly regenerated, and compressed. In the heat exchanger 1205, the absorbent AB7 that has been heated, mostly regenerated, and compressed releases its heat to the absorbent AB2 carrying CO2 and VOC and exits the heat exchanger 1205 as the absorbent AB8 that has been mostly regenerated, compressed, and cooled. The absorbent AB8 that has been mostly regenerated, compressed, and cooled is supplied to the heat exchanger 1217 and cooled, and exits the heat exchanger as the cooled regenerated absorbent AB1. The regeneration gas RG3 containing CO2 and VOC is supplied to the heat exchanger 1213, releases its heat to the regeneration gas RG1, and exits the heat exchanger 1213 as the cooled regeneration gas RG4 containing CO2 and VOC. The cooled regeneration gas RG4 containing CO2 and VOC is transported by the suction blower 1219 to the post-treatment unit 1221 as the cooled regeneration gas RG5 containing CO2 and VOC.

[0077] Table 1 below shows the parameters of the simulation model using the absorbent PEG1843. This absorbent absorbs only 2.5 kg / h (0.4%) of methane, but absorbs 36.6 kg / h of CO2 (3.6%) and more than 98% of VOC (dimethyl sulfide (DMS), acetone, 2-butanone, and terpene). Here, it should be noted that VOCs having low boiling points, such as DMS (boiling point 37°C), etc., VOCs having medium boiling points, such as acetone (boiling point 56°C) and 2-butanone (boiling point 79.6°C), etc., and VOCs having high boiling points, such as terpene (boiling point 140 - 230°C), etc., are absorbed.

[0078] This is evident from the difference in mass between BG2 and BG3. 36.6 kg / h of CO2 and VOC are released from the methane-containing gas BG2 to the absorbent AB1. This is manifested in the increase in the CO2 and VOC concentrations from AB1 to AB5. The VOC- and CO2-containing absorbent then passes 36.6 kg / h of CO2 and VOC to the regeneration gas RG1, which is manifested in the increase in the CO2 and VOC concentrations from RG1 to RG3.

[0079] [Table 1]

[0080] Tables 2 to 4 show the same parameter configurations as Table 1, and the only differences from Table 1 are the points where the temperatures of the regeneration gases RG1 and RG3, the temperature of the absorbent AB5, and the mass flow rates of RG1, RG3, AB1, and AB5 are changed.

[0081] Tables 5 to 8 show the same parameter configurations as Tables 1 to 4, and the only change in the simulation model was the change of the absorbent from PEG1843 (Tables 1 to 4) to PEG300 (Tables 5 to 8).

[0082] [Table 2]

[0083] [Table 3]

[0084] [Table 4]

[0085] [Table 5]

[0086]

Table 6

[0087]

Table 7

[0088]

Table 8

[0089] In all parameter configurations in the above simulation model, it is clear that only about 3.6% by volume of the CO2 present in BG1 is absorbed by the absorbent. It should be noted that in Tables 2 and 6, the concentrations of terpenes in AB1 and AB5 are higher than those in Tables 1, 3 to 5, and 7 to 8. In all parameter configurations, the terpenes could not be completely removed from the absorbent (see AB1). Nevertheless, surprisingly, it is possible to largely remove the terpenes present in the methane-containing biogas BG2 therefrom. See BG3.

Claims

1. A method for treating a methane-containing gas, comprising: a) 20 to 60% by volume of CO ２ providing a methane-containing gas mixture (103) comprising at least one compound from the group of volatile organic compounds (VOCs), wherein the at least one VOC is selected from the group consisting of ketones, sulfur-containing hydrocarbons, and terpenes; a step in which the VOC concentration in the gas mixture is 10 to 10,000 ppm; b) compressing and cooling the methane-containing gas mixture (103) from step a); c) supplying the compressed and cooled methane-containing gas mixture to an absorber (109), wherein the absorber (109) contains an absorbent (117) that reversibly absorbs VOCs in a liquid state; d) at least a part of the VOC and 5% by volume or less of the CO by the absorbent (117) ２ is absorbed to obtain a methane-containing gas mixture (119) having a reduced level of VOC and CO ２ and an absorbent (121) carrying VOC and CO ２ and obtaining a step e) supplying the absorbent (121) carrying the VOC and CO from the absorption device (109) to the desorption device (135); ２ ​ f) The methane-containing gas mixture (119) having the reduced levels of VOC and CO ２ is supplied from the absorption device (109) to a separation device (125) including a membrane, and in the separation device (125), the methane-containing gas mixture (119) having the reduced levels of VOC and CO ２ is separated into a CO ２ enriched gas stream (137) at reduced pressure and a methane-enriched gas stream (139) at isobaric pressure; g) said VOC and CO ２ For the regeneration of the absorbent (121) carrying said CO from step f), ２ A regeneration gas stream (138) comprising at least a portion of the CO-rich gas stream (137) is supplied to the desorption device (135) to obtain an off-gas stream (149) containing CO ２ and a step of obtaining at least partially regenerated absorbent (145) and said VOC h) removing the off-gas stream (149) from the desorber (135) and recycling at least a partially regenerated absorbent (145) from the desorber (135) to the absorber (109); A method comprising the steps of:

2. The method according to claim 1, characterized in that in step h), the off-gas stream (149) is supplied to a regenerative afterburner (151) for oxidation.

3. The method according to claim 1 or 2, characterized in that at least steps c) to h) are performed continuously.

4. In step c), 2 to 10 liters of absorbent per 1 Bm of the methane-containing gas mixture from step b) is used in the absorption device, characterized in that the method according to any one of claims 1 to 3. ３ The method according to any one of claims 1 to 3, characterized in that 2 to 10 liters of absorbent per 1 Bm of the methane-containing gas mixture from step b) is used in the absorption device.

5. In step g), 1 to 3 liters of the VOC-supported absorbent from step d) is regenerated in the desorption device with the CO-rich gas from step f) at 1 Bm ３ ３ of CO ２ ２ The method according to any one of claims 1 to 4, characterized in that it is regenerated with the CO-rich gas of 1 Bm ３

6. The absorbent (117, 145) for absorbing VOCs has a boiling point exceeding 250 °C at 1013.25 mbar and is selected from the group consisting of polyethylene glycol (PEG), mineral oil, esters, or combinations thereof, preferably containing polyethylene glycol. The method according to any one of claims 1 to 5.

7. The absorbent (117, 145) for absorbing VOCs has the formula (I): I) R １ -O-(CH ２ CH ２ O) ｎ -R ２ where n = 3 to 11; R １ and R ２ are independently selected from linear C １ ~C １０ alkyl. ) The method according to any one of claims 1 to 6, characterized in that it contains a compound of the formula:

8. The method according to any one of claims 1 to 7, characterized in that the VOC has a vapor pressure of at least 0.1 mbar at 20 °C and / or a boiling point of at most 240 °C at 1013.25 mbar.

9. The at least one VOC is selected from the group consisting of acetone, 2-butanone, 3-methyl-2-butanone, 2-pentanone, 3-pentanone, 3,3-dimethyl-2-butanone, 2-methyl-3-pentanone, 4-methyl-2-pentanone, 3-methyl-2-pentanone, 3-hexanone, 2-hexanone, 5-methyl-3-hexanone, 3-methyl-2-hexanone, 2-heptanone, 4-octanone, 3-octanone, 2-octanone, 2,9-decanedione, α-thujene, α-pinene, camphene, sabinene, β-pinene, myrcene, 3-carene, thujanon, thujopsene, thymol, α-terpinene, β-caryophyllene, 1,4-cineole, eucalyptol, fenchone, γ-terpinene, terpinolene, limonene, tricyclene, linalool, menthone, nopinion, p-menthan-2-one, p-menthan-2-ol, camphor, carvone, 3,3-dimethyl-2-boronanone, carbonyl sulfide, methyl mercaptan, ethyl mercaptan, dimethyl sulfide, carbon disulfide, 2-propanethiol, 2-methyl-2-propanethiol, 1-propanethiol, thiophene, 2-butanethiol, isobutyl mercaptan, methyl allyl sulfide, methyl propyl sulfide, butanethiol, dimethyl disulfide, 2-methylthiophene, 3-methylthiophene, tetrahydrothiophene, 1-pentanethiol, thiophenol, dimethyl trisulfide, diisopropyl disulfide, dimethyl tetrasulfide, methyl propyl disulfide, and methyl isopropyl disulfide, the method according to any one of claims 1 to 8.

10. The method according to any one of claims 1 to 9, characterized in that the methane-containing gas mixture (103) is compressed to 6 to 24 bar (g), preferably 10 to 20 bar (g), more preferably 14 to 18 bar (g) in step b).

11. The method according to any one of claims 1 to 9, characterized in that the methane-containing gas mixture (103) is cooled to 0°C to 20°C, preferably 2°C to 10°C in step b).

12. The method according to any one of claims 1 to 10, characterized in that the absorbent (121) carrying VOC from step d) is heated, preferably to 30°C to 90°C, more preferably to 50°C to 70°C, before being transported to the desorption device (135).

13. The method according to any one of claims 1 to 11, characterized in that the regenerated absorbent (145) from step g) is cooled, preferably to 0°C to 20°C, more preferably to 2°C to 10°C, before being recirculated to the absorption device (109).

14. In step c), the absorbent (117) that reversibly absorbs VOCs from the liquid is supplied to the absorption device (109) at a volume flow rate of at least 7.8 kg per 1 Bm of the gas mixture (103). ３ A method according to any one of claims 1 to 13, characterized in that it is supplied to the absorption device (109) at a volume flow rate of at least 7.8 kg per 1 Bm of the gas mixture (103).

15. An apparatus (100) for treating a methane-containing gas (103) by the method according to any one of claims 1 to 14, comprising: a gas source (101); a cooling device (107); a compressor (105); an absorption device (109) having an upper end (113) and a lower end (111); a desorption device (135) having an upper end (133) and a lower end (141); a separation device (125) including a membrane; a first connecting pipe (115, 147) connecting the lower end (141) of the desorption device (135) to the upper end (113) of the absorption device (109); a second connecting pipe (127) connecting the lower end (111) of the absorption device (109) to the upper end (133) of the desorption device (135); a third connecting pipe (123) connecting the upper end of the absorption device (109) to the separation device (125); a fourth connecting pipe (143) connecting the separation device (125) to the lower end (141) of the desorption device (135); and the apparatus (100) is provided with these components.

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