Equipment and membrane process for separating methane and carbon dioxide from a gas stream

Abstract

An equipment and process having four membrane separation units, wherein a second separation unit separates the retentate of the first unit, a third separation unit separates the permeate of the first unit, a fourth separation unit separates the retentate of the third unit, the permeate of the second unit and the retentate of the fourth unit are recycled to the feed to the first unit, the permeate of the fourth unit is sent to a methane oxidation unit, and the permeate of the third unit is discharged to the atmosphere, thereby enabling separation of methane and carbon dioxide from a gas stream, providing a methane-enriched stream with the retentate of the second unit with a high methane yield, and complying with low limits on methane emissions to the atmosphere with a small size methane oxidation unit.

Description

Technical Field

[0001] The present invention is directed to membrane processes and equipment for separating methane and carbon dioxide from a gas stream and providing a methane stream suitable for injection into a natural gas grid, which can achieve low emissions of methane to the atmosphere with little additional equipment and energy consumption.

Background Art

[0002] Biogas obtained from anaerobic fermentation, such as biogas from an anaerobic digester or landfill gas, contains methane and carbon dioxide as main components. Separating methane from biogas with a quality suitable for supplying it to a gas distribution grid is commercially important. Membrane processes are advantageous for separating methane from carbon dioxide because they do not require an absorbent for carbon dioxide and can be operated with low energy consumption. Since methane is a more potent greenhouse gas than carbon dioxide, the carbon dioxide-enriched stream obtained by the membrane separation process can be discharged to the atmosphere only if it is separated with a low methane content or has undergone additional treatment for methane removal. Such additional treatment for methane removal consumes energy and requires additional equipment.

[0003] WO 2012 / 000727 pamphlet discloses a membrane process having three membrane units that can separate biogas into a biomethane stream containing more than 98% by volume of methane and a carbon dioxide-enriched stream containing about 0.5% of methane with a low recycle rate of less than 60%, which improves the energy efficiency of the process.

[0004] International Publication No. WO 2015 / 036709 pamphlet aims to further reduce the energy required to compress recycled gas, and discloses a membrane process having four membrane units with a lower methane recovery rate compared to the process of International Publication No. WO 2012 / 000727 pamphlet. This process provides two carbon dioxide enrichment streams from the third and fourth membrane units. International Publication No. WO 2015 / 036709 pamphlet suggests that these two streams can be treated separately or together by thermal oxidation, used to upgrade carbon dioxide, or discharged to the atmosphere.

[0005] On September 24, 2018, the Oil and Gas Climate Initiative (OGCI) announced the first methane emission target for its member companies. A baseline for methane lost when producing up to 0.32% of oil and gas and a target of 0.25% methane loss in 2025 were set.

[0006] Strengthened regulations regarding greenhouse gas emissions, for example, §36 of the German "42. Verordnung über den Zugang zu Gasversorgungsnetzen (Gasnetzzugangsverordnung - GasNZV)", require even more aggressive targets for reducing methane emissions from biogas upgrading or natural gas purification (up to 0.2%). Conventional technology membrane processes can achieve such targets only by a significantly high recycle rate or by an additional step of removing methane from the carbon dioxide enrichment stream before discharging to the atmosphere. Both means increase costs and reduce the efficiency of the conventional technology process.

[0007] Therefore, an efficient process for separating methane and carbon dioxide from gas streams is still strongly needed, which meets the requirements of strict regulations regarding greenhouse gas emissions with little additional equipment and little additional energy consumption.

Prior Art Documents

Patent Document

[0008]

Patent Document 1

Patent Document 2

Non-Patent Document

[0009]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] The subject of the present invention was to provide new equipment and a new process that do not have the drawbacks of the prior art processes and equipment to a reduced extent, respectively.

[0011] A specific problem of the present invention is to provide new equipment and a new process for separating methane and carbon dioxide from a gas stream, which, in particular, for a gas stream that is discharged into the atmosphere and must have a methane content of 0.3% by volume or less, preferably 0.2% by volume or less, meets the requirements of strengthened regulations regarding greenhouse gas emissions.

[0012] Another specific problem of the present invention was to provide new equipment and a new process for separating methane and carbon dioxide from a gas stream such that at least one carbon dioxide-enriched stream discharged into the atmosphere has a methane content of 0.3% by volume or less, preferably 0.2% by volume or less, without an oxidative methane removal post-treatment step.

[0013] In another specific problem of the present invention, new equipment and a new process for upgrading a gas containing methane and carbon dioxide are provided, in which a methane product stream having a methane content of 97% by volume or more can be obtained, and at the same time, a higher methane yield than that disclosed in WO 2015 / 036709 can be achieved.

[0014] In another specific problem of the present invention, new equipment and a new process for upgrading a gas containing methane and carbon dioxide are provided, which are very efficient from the viewpoints of operating costs and / or investment costs. Preferably, the investment and / or operating costs for gas recompression and / or post-treatment of the off-gas stream for reducing the methane content should be minimized.

[0015] In another specific problem of the present invention, new equipment and a new process for upgrading a gas containing methane and carbon dioxide are provided, which enables continuous compliance with the regulatory requirements regarding methane emissions to the atmosphere even if the composition and / or flow rate of the raw gas stream change.

Means for Solving the Problems

[0016] Further problems solved by the present invention but not described so far can be derived from the following description, examples, figures and claims.

[0017] The inventor has surprisingly found that the above problems can be solved by using a membrane separation facility having four membrane units as known from WO 2015 / 036709, and this facility a. Connecting only the permeate outlet of the fourth membrane unit to the methane oxidation unit and discharging the permeate directly to the atmosphere from the third membrane unit, b. Configuring and operating the facility to provide a carbon dioxide concentration in the first permeate stream of 90 to 99% by volume, c. In the first membrane separation unit, it is improved by using a membrane having a pure gas selectivity of carbon dioxide to methane of at least 30, measured at 20 °C and 5 bar.

[0018] The facility and process of the present invention enable compliance with strict regulatory requirements for methane emissions to the atmosphere for both the third and fourth permeate streams, even if the third permeate stream does not undergo methane removal post-treatment and is not directly discharged to the atmosphere. As shown in Comparative Examples 1a and 1b below, the process of WO 2015 / 036709 pamphlet does not disclose a facility or process that provides a third permeate stream with a methane content of 0.3% by volume without oxidative post-treatment.

[0019] The merit of providing a third permeate stream having a methane content of 0.3% by volume or less after membrane separation makes it possible to reduce the investment cost of the facility for oxidative methane removal in the facility and process of the present invention. Also, the operating cost for methane removal can be reduced compared to the prior art. In a preferred embodiment of the present invention, additionally, minimizing the volumetric flow rate of the fourth permeate stream is achieved, which further reduces the volume for oxidative post-treatment and makes it possible to further reduce the investment cost and the operating cost.

[0020] Compared with the prior art process, the facility and process of the present invention can operate at a minimum cost for recompression even if strict requirements for methane emissions to the atmosphere are met.

[0021] Preferably, the installation and process of the present invention include means for direct or indirect measurement and / or means for controlling the methane concentration in the third permeate stream. In a preferred embodiment, the operating conditions of the first membrane unit of the installation are adjusted based on the direct or indirect measurement of the methane concentration in the third permeate stream. Thereby, even if the composition and / or flow rate of the feed gas stream change, a third permeate stream having a methane concentration of 0.3% by volume or less can be continuously provided. Therefore, the installation and process of the present invention can be flexibly used for various feed gas sources and feed gas sources including various amounts and / or various compositions of feed gas.

[0022] The process and installation of the present invention provide a methane product stream having a very high methane content and a very high methane yield.

[0023] Further advantages of the installation and process of the present invention are revealed in the following description, examples, drawings and claims.

[0024] Therefore, the subject matter of the present invention is an installation for separating methane and carbon dioxide from a gas stream, the installation comprising a compressor (1), four membrane separation units (2) to (5), each membrane separation unit having a gas separation membrane with a higher permeability to carbon dioxide than to methane, a gas inlet, a retentate outlet and a permeate outlet four membrane separation units (2) to (5), a methane oxidation unit (6), a feed gas conduit (7) connected to the inlet of the compressor (1), a supply conduit (8) connecting the outlet of the compressor (1) to the gas inlet of the first membrane separation unit (2), a first retentate conduit (9) connecting the retentate outlet of the first membrane separation unit (2) to the gas inlet of the second membrane separation unit (3), a second retentate conduit (10) connected to the retentate outlet of the second membrane separation unit (3), A first permeate conduit (11) connecting the permeate outlet of the first membrane separation unit (2) to the gas inlet of the third membrane separation unit (4); A third retentate conduit (12) connecting the retentate outlet of the third membrane separation unit (4) to the gas inlet of the fourth membrane separation unit (5); A fourth retentate conduit (13) connecting the retentate outlet of the fourth membrane separation unit (5) to the inlet of the compressor (1); A second permeate conduit (14) connecting the permeate outlet of the second membrane separation unit (3) to the inlet of the compressor (1); A third permeate conduit (15) connected to the permeate outlet of the third membrane separation unit (4); A fourth permeate conduit (16) connected to the permeate outlet of the fourth membrane separation unit (5), comprising: The third permeate conduit (15) is configured to discharge the third permeate to the ambient atmosphere; The fourth permeate conduit (16) connects the permeate outlet of the fourth membrane separation unit (5) and the methane oxidation unit (6); The first membrane separation unit (2) comprises a membrane having a pure gas selectivity of carbon dioxide with respect to methane of at least 30, preferably 40 to 120, more preferably 50 to 100, determined at 20 ° C and 5 bar; The facility is characterized in that it is configured to provide a carbon dioxide concentration in the range of 90 to 99% by volume in the gas stream in the first permeate conduit (11), which is the first permeate stream.

[0025] A further subject of the present invention is a membrane process for separating methane and carbon dioxide from a gas stream, the process comprising: a) providing the facility of the present invention; b) introducing a raw gas stream containing 20 to 60% by volume, preferably 20 to 50% by volume of carbon dioxide and having a total content of methane and carbon dioxide of at least 95% by volume into the raw gas conduit (7) of the facility; c) Compressing the raw gas stream combined with the recirculation flow from the fourth holding liquid conduit (13) and the second permeate conduit (14) with a compressor (1) to provide a feed stream with a supply pressure of 7 to 25 bar and a temperature of 15 to 50 °C; d) Using a membrane with a mixed gas selectivity of carbon dioxide to methane of at least 30, preferably 40 to 100, at the supply pressure and the temperature of the supply stream, separating the supply stream in the first membrane separation unit (2) into a first permeate stream and a first holding liquid stream, and selecting the permeate side pressure in the first membrane separation unit and the separation capacity in the four membrane separation units so as to provide a carbon dioxide concentration of 90 to 99% by volume in the first permeate stream, wherein the separation capacity of the membrane separation unit is the product of the membrane area and the membrane permeability of carbon dioxide at a temperature of 25 °C and a supply side pressure of 5 bar; e) Separating the first holding liquid stream in the second membrane separation unit (3) into a second holding liquid stream and a second permeate stream, further processing the second holding liquid stream or taking out the second holding liquid stream as a product stream rich in methane, and recirculating the second permeate stream through the second permeate conduit (14); f) Separating the first permeate stream in the third membrane separation unit (4) into a third holding liquid stream and a third permeate stream, and discharging the third permeate stream to the ambient atmosphere without further methane removal; g) Separating the third holding liquid stream in the fourth membrane separation unit (5) into a fourth holding liquid stream and a fourth permeate stream, and recirculating the fourth holding liquid stream through the holding liquid conduit (13); h) Oxidizing the fourth permeate stream in the methane oxidation unit (6) to provide an offgas stream containing less than 0.3% by volume of methane, and discharging the offgas stream to the ambient atmosphere.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Figure 3

[0027] The installation of the present invention for separating methane and carbon dioxide from a gas stream comprises a compressor (1) and a feed gas conduit (7) connected to the inlet of the compressor (1). Any gas compressor known to be suitable for compressing a mixture containing methane and carbon dioxide can be used, such as a turbo compressor, a piston compressor or preferably a screw compressor. The screw compressor may be a dry-running compressor or a fluid-cooled compressor cooled with water or oil. When an oil-cooled compressor is used, the installation preferably also includes a droplet separator downstream of the compressor to prevent oil droplets from entering the membrane separation stage.

[0028] The facility of the present invention comprises four membrane separation units (2) to (5). Each membrane separation unit comprises a gas separation membrane having a higher permeability to carbon dioxide than to methane, as well as a gas inlet, a retained liquid outlet, and a permeate outlet. Here, the term "permeate" refers to a gas stream containing the gas components of the gas stream supplied to the membrane separation unit that has passed through the gas separation membrane due to the difference in partial pressure across the membrane. The term "retained liquid" refers to the gas stream remaining after the gas components have passed through the gas separation membrane. Since the gas separation membrane has a higher permeability to carbon dioxide than to methane, the permeate will have a higher molar ratio of carbon dioxide to methane than the gas stream supplied to the membrane separation unit, i.e., it will contain more carbon dioxide, and the retained liquid will have a higher molar ratio of methane to carbon dioxide than the gas stream supplied to the membrane separation unit, i.e., it will contain more methane.

[0029] Suitable membranes having a higher permeability to carbon dioxide than to methane are known from the prior art. Generally, membranes comprising a separation layer of a glassy polymer, i.e., a polymer having a glass transition temperature above the operating temperature of the membrane separation stage, provide a higher permeability to carbon dioxide than to methane. The glassy polymer may be a polyetherimide, a polycarbonate, a polyamide, a polybenzoxazole, a polybenzimidazole, a polysulfone, or a polyimide, and the gas separation membrane preferably comprises at least 80% by weight of a polyimide or a mixture of polyimides.

[0030] In a preferred embodiment, the gas separation membrane comprises at least 50% by weight of a polyimide prepared by reacting a dianhydride selected from 3,4,3',4'-benzophenonetetracarboxylic dianhydride, 1,2,4,5-benzenetetracarboxylic dianhydride, 3,4,3',4'-biphenyltetracarboxylic dianhydride, oxydiphthalic dianhydride, sulfonyldiphthalic dianhydride, 1,1,1,3,3,3-hexafluoro-2,2-propylidenediphthalic dianhydride and mixtures thereof with a diisocyanate selected from 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-methylenediphenyl diisocyanate, 2,4,6-trimethyl-1,3-phenylene diisocyanate, 2,3,5,6-tetramethyl-1,4-phenylene diisocyanate and mixtures thereof. The dianhydride is preferably 3,4,3',4'-benzophenonetetracarboxylic dianhydride, or a mixture of 3,4,3',4'-benzophenonetetracarboxylic dianhydride and 1,2,4,5-benzenetetracarboxylic dianhydride. The diisocyanate is preferably a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, or a mixture of 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate and 4,4'-methylenediphenyl diisocyanate. A suitable polyimide of this type is commercially available from Evonik Fibres GmbH under the trade name P84® type 70, which has the CAS number 9046-51-9 and is a polyimide prepared from a mixture of 3,4,3',4'-benzophenonetetracarboxylic dianhydride and 64 mol% of 2,4-tolylene diisocyanate, 16 mol% of 2,6-tolylene diisocyanate and 20 mol% of 4,4'-methylenediphenyl diisocyanate, and the trade name P84® HT, which has the CAS number 134119-41-8 and is a polyimide prepared from a mixture of 60 mol% of 3,4,3',4'-benzophenonetetracarboxylic dianhydride and 40 mol% of 1,2,4,5-benzenetetracarboxylic dianhydride and 80 mol% of 2,4-tolylene diisocyanate and 20 mol% of 2,6-tolylene diisocyanate.The gas separation membrane of the present embodiment is preferably heat-treated in an inert atmosphere as described in WO 2014 / 202324 pamphlet in order to improve the long-term stability in the process of the present invention.

[0031] In another preferred embodiment, the gas separation membrane contains at least 50 wt% of block copolyimide as described on page 6, line 20 to page 16, line 4 of WO 2015 / 091122 pamphlet. The block copolyimide preferably contains at least 90 wt% of polyimide blocks having a block length of 5 to 1000, preferably 5 to 200.

[0032] The gas separation membrane may be a flat membrane or a hollow fiber membrane, and is preferably an asymmetric hollow fiber membrane having a dense polyimide layer on a porous support. The term "dense layer" in the present specification refers to a layer essentially free of macropores extending through the layer, and the term "porous support" in the present specification refers to a support material having macropores extending through the support. The asymmetric hollow fiber membrane can be produced by coating a porous hollow fiber with polyimide to form a dense polyimide layer on the support. In a preferred embodiment, the asymmetric hollow fiber membrane is a membrane produced by a phase inversion process by spinning using an annular two-component spinning nozzle, passing a solution of polyimide through the annular opening, and passing a liquid containing a non-solvent of polyimide through the central opening.

[0033] The gas separation membrane preferably includes a dense separation layer of a glassy polymer coated with a dense layer of a rubbery polymer having higher gas permeability than the glassy polymer. A preferred gas separation membrane containing a polyimide separation layer is preferably coated with a polydimethylsiloxane elastomer.

[0034] When the gas separation membrane is a flat membrane, the membrane separation unit preferably comprises one or more spiral membrane modules including the flat membrane. When the gas separation membrane is a hollow fiber membrane, the membrane separation unit preferably comprises one or more membrane modules including a bundle of hollow fiber membranes. Each of the membrane separation units may comprise several membrane modules arranged in parallel or several membrane modules arranged in series. In a series of membrane modules, the retentate provided by the membrane module is sent as a feed to the membrane module after the series of membrane modules. The last membrane module in the series provides the retentate of the membrane separation stage, and the permeates of all the membrane modules in the series are combined to provide the permeate of the membrane separation unit. When the membrane separation unit comprises several membrane modules arranged in series, the membrane modules are preferably removable membrane cartridges arranged in series as a chain of cartridges in a common pressure vessel and connected to each other by a central permeate collection tube, as described in detail in WO 2016 / 198450 pamphlet. A membrane separation unit comprising several membrane modules arranged in parallel is preferred.

[0035] The facility of the present invention comprises a supply conduit (8) connecting the outlet of the compressor (1) to the gas inlet of the first membrane separation unit (2). The supply conduit (8) preferably comprises a heat exchanger (19) arranged in the supply conduit to adjust the temperature of the compressed gas to the operating temperature of the first membrane separation unit (2).

[0036] A dehumidifier may be arranged in the supply conduit. Such a dehumidifier is preferably configured to cool the compressed gas, condense water from the cooled gas in a condenser, and reheat the gas. The reheating can be done by the compressed gas in a countercurrent heat exchanger.

[0037] The equipment of the present invention includes a first holding liquid conduit (9) connecting the holding liquid outlet of the first membrane separation unit (2) to the gas inlet of the second membrane separation unit (3), and a second holding liquid conduit (10) connected to the holding liquid outlet of the second membrane separation unit (3). The second holding liquid conduit (10) preferably includes a pressure regulating valve for adjusting or controlling the supply side pressures of the first membrane separation unit (2) and the second membrane separation unit (3).

[0038] The first permeate conduit (11) connects the permeate outlet of the first membrane separation unit (2) to the gas inlet of the third membrane separation unit (4). This first permeate conduit (11) preferably connects the permeate outlet of the first membrane separation unit (2) to the gas inlet of the third membrane separation unit (4) without an intermediate compressor or pump.

[0039] The third holding liquid conduit (12) connects the holding liquid outlet of the third membrane separation unit (4) to the gas inlet of the fourth membrane separation unit (5), and the fourth holding liquid conduit (13) connects the holding liquid outlet of the fourth membrane separation unit (5) to the inlet of the compressor (1). Preferably, a pressure regulating valve (17) is arranged in the fourth holding liquid conduit (13) to adjust or control the supply side pressures of the third membrane separation unit (4) and the fourth membrane separation unit (5) and the permeate side pressure of the first membrane separation unit (2). When a multi-stage compressor is used, the fourth holding liquid conduit (13) is connected to the inlet between the stages of the compressor, and the energy consumption for recompression can be reduced.

[0040] The second permeate conduit (14) connects the permeate outlet of the second membrane separation unit (3) to the inlet of the compressor (1).

[0041] The equipment of the present invention includes a third permeate conduit (15) connected to the permeate outlet of the third membrane separation unit (4). The third permeate conduit (15) is configured to discharge the third permeate to the ambient atmosphere.

[0042] In a preferred embodiment, the facility of the present invention comprises means for direct or indirect measurement and / or means for controlling the gas flow in the third permeate conduit (15), i.e., the methane concentration in the third permeate flow. "Direct measurement" means an analytical method for analyzing the gas composition of the third permeate flow. "Indirect measurement" means determining another process parameter of the gas flow that can be correlated with the methane concentration in the third permeate flow, preferably. A preferred means for direct measurement is a methane concentration sensor (18) connected to the third permeate conduit (15) for monitoring the methane concentration in the third permeate flow. Any device known in the prior art to be suitable for determining the methane concentration in a gas mixture containing methane and carbon dioxide may be used as the methane concentration sensor (18). As the methane concentration sensor (18), it is preferable to use a commercially available gas analyzer that measures the methane concentration by infrared absorption or a process gas chromatograph. Means suitable for indirect measurement are devices for measuring other components such as CO 2 and / or O 2 and N 2 and assuming that the remainder is methane. In addition, the means can measure the heating or calorific value of the gas. Examples are calorimeters such as thermopiles, micro combustion and residual oxygen combustion calorimeters.

[0043] The facility of the present invention further comprises a methane oxidation unit (6) and a fourth permeate conduit (16) connecting the permeate outlet of the fourth membrane separation unit (5) to the methane oxidation unit (6). Any device known in the prior art to be suitable for oxidizing methane in a gas flow containing carbon dioxide as the main component may be used for the methane oxidation unit (6). The methane oxidation unit (6) preferably includes a catalytic oxidation device, a regenerative thermal oxidation device or a biofilter.

[0044] The four membrane separation units (2) to (5) may all contain the same membrane or different membranes. The membrane used in the first membrane separation unit (2) preferably has a pure gas selectivity of carbon dioxide to methane of at least 30, preferably 40 to 120, more preferably 50 to 100, measured at 20 °C and 5 bar. More preferably, all the membrane separation units contain membranes having such a high selectivity of carbon dioxide to methane. Suitable membrane modules and membrane cartridges containing hollow fiber polyimide membranes having such a high pure gas selectivity are commercially available from Evonik Fibres GmbH under the trade name SEPURAN® Green.

[0045] In a preferred embodiment, all the membrane separation units contain the same membrane in the form of membrane modules of the same size arranged in parallel within the membrane separation unit. At this time, by installing different numbers of membrane modules in the membrane separation unit, different membrane areas are provided within the membrane separation unit. This embodiment has the advantage that when only one type of membrane module or a module having a membrane cartridge is used, it is necessary to stock and maintain one type of membrane cartridge for replacing defective membranes in the facility.

[0046] In another preferred embodiment, the fourth membrane separation unit (5) comprises a membrane having a higher carbon dioxide permeability than the membrane used in the first membrane separation unit (2). In this embodiment, the membrane in the fourth membrane separation unit (5) may also have a lower pure gas selectivity of carbon dioxide with respect to methane than the membranes used in other membrane separation units. By using a more permeable membrane type having a lower selectivity in the fourth membrane separation unit (5), a considerably smaller membrane area and only a slight increase in the recycle rate can achieve the desired methane content and the desired methane yield in the second permeate stream as compared to the case of using the same membrane as in the first membrane separation unit (2). Membranes having a higher permeability and a lower selectivity for carbon dioxide may also be used in the second membrane separation unit (3) and / or the third membrane separation unit (4) if using a smaller membrane area for separation is prioritized over achieving a low recycle rate for low operating costs. In a preferred embodiment, the second membrane separation unit (3) comprises a membrane having a lower pure gas selectivity of carbon dioxide with respect to methane as compared to the first membrane separation unit (2), or as compared to the first membrane separation unit (2), the third membrane separation unit (4) and the fourth membrane separation unit (5).

[0047] Preferably, the membrane areas of the second membrane separation unit (3) and the fourth membrane separation unit (5) are selected such that the separation capacity of the second membrane separation unit (3) is greater than that of the fourth membrane separation unit (5), and the separation capacity of the membrane separation unit is the product of the membrane area of the membrane separation unit and the membrane permeability of carbon dioxide at 25 °C and a supply-side pressure of 5 bar. Such a selection of the membrane separation capacity provides a lower flow rate of the fourth permeate stream that has to be processed in the methane oxidation unit when producing the third permeate stream with the targeted low methane concentration.

[0048] The second membrane separation unit (3) is preferably configured to provide a countercurrent flow from the supply side of the membrane towards the permeate side. Preferably, all the membrane separation units of the installation according to the invention are configured to provide such a countercurrent flow. Suitable membrane modules or cartridges having such a countercurrent flow are known from the prior art, for example from WO 2016 / 198450 or WO 2017 / 016913. The countercurrent flow within the membrane module or cartridge increases the purity of the retentate generated by the membrane separation unit and provides better separation.

[0049] The installation according to the invention is configured to provide a carbon dioxide concentration in the range of 90 to 99% by volume in the gas flow in the first permeate conduit (11), i.e. in the first permeate stream. Preferably, the installation comprises means for controlling the permeate side pressure of the first membrane separation unit (2) and / or the separation capacity of the four membrane separation units (2) to (5) in order to provide a carbon dioxide concentration of 90 to 99% by volume in the first permeate stream. More preferably, the separation capacity, which is the product of the permeate side pressure and the membrane area of the first membrane separation unit (2) and the membrane permeability of carbon dioxide at a temperature of 25 °C and a supply side pressure of 5 bar in the four membrane separation units (2) to (5), is configured to provide a carbon dioxide concentration of 90 to 99% by volume in the first permeate stream.

[0050] In a preferred embodiment, the installation according to the invention further comprises a controller connected to a methane concentration sensor (18) for controlling at least one process parameter in order to keep the concentration of methane in the third permeate stream below a target value. By adjusting the operating conditions of the installation based on the measurement of the methane concentration in the third permeate stream, it is possible to comply with the limits of methane emissions when the composition or flow rate of the raw gas stream changes.

[0051] In the first alternative form, the process parameter is the permeate-side pressure of the first membrane separation unit (2). At this time, the facility of the present invention includes a pressure regulating valve (17) disposed in the fourth holding liquid conduit (13), and a controller controls the pressure regulating valve (17) based on data measured by the methane concentration sensor (18). When the concentration of methane in the third permeate stream rises until it exceeds the target value, the controller controls the pressure regulating valve (17) to reduce the permeate-side pressure of the first membrane separation unit (2). Disposing the pressure regulating valve (17) in the fourth holding liquid conduit (13) is advantageous compared to disposing the pressure regulating valve (17) in the third holding liquid conduit (12) or the first permeate conduit (11), because the membrane area required in the third membrane separation unit (4) and the fourth membrane separation unit (5) is smaller than in the alternative forms for disposing the pressure regulating valve.

[0052] In a second alternative form, the process parameter is the supply stream temperature. At this time, the facility of the present invention includes a heat exchanger (19) in a supply conduit (8) and a flow control valve (20) for controlling the flow of heating fluid or cooling fluid to the heat exchanger (19), and a controller controls this flow control valve (20) based on data measured by a methane concentration sensor (18). The controller preferably controls the heat exchanger (19) via the control valve (20) to lower the temperature of the supply stream when the concentration of methane in the third permeate stream rises above a target value. This embodiment is advantageous for operating the facility at a low load because the recycle ratio is lower at a low load compared to a facility that adjusts the permeation pressure of the first membrane separation unit (2). The flow control valve (20) may be disposed in a conduit that sends heating fluid or cooling fluid to the heat exchanger (19). When the facility includes a dehumidifier in the supply conduit (8), the heat exchanger (19) may be part of the dehumidifier or may exist in addition to the dehumidifier. In a preferred embodiment, the second retentate conduit (10) is connected to the cooling fluid inlet of the heat exchanger (19), and the flow control valve is disposed in a bypass conduit connected to the second retentate conduit (10). Thereby, it becomes possible to control the temperature of the supply stream by cooling the supply stream with the second retentate stream and controlling the ratio of the second retentate stream passing through the heat exchanger (19). This alternative form has the advantage that no additional energy is required to cool the supply stream.

[0053] In a third alternative form, the process parameter is the membrane area used in the third membrane separation unit (4). At this time, the facility of the present invention includes a number of membrane modules arranged in parallel within the third membrane separation unit (4), and at least one of these membrane modules is provided with a shut-off valve for shutting off the flow through the membrane module. Next, the controller controls the shut-off valve based on the data measured by the methane concentration sensor (18) to close the shut-off valve of the membrane module when the concentration of methane in the third permeate stream rises until it exceeds the target value. The flow through the membrane module can be shut off by shut-off valves at at least two of the gas inlet, the holding liquid outlet, and the permeate outlet of the membrane module, and the shut-off valves at the gas inlet and the permeate outlet are preferred. In order to prevent pressure surges that may cause membrane damage, it is preferable to close the shut-off valve slowly. This embodiment is typically advantageous when the flow rate or composition of the gas stream exhibits large fluctuations over time, such as in the case of landfill gas or fermentation processes using various feedstocks.

[0054] In a fourth alternative form, the process parameter is the operating mode of the modules within the first membrane separation unit (2). At this time, the facility of the present invention is a pore-side feed hollow fiber membrane module within the first membrane separation unit (2), where the gas inlet is at the first end of the module, the holding liquid outlet is at the second end opposite the first end of the module, the first permeate outlet is adjacent to the first end of the module and is connected to the first permeate conduit (11), and an additional permeate outlet is adjacent to the second end of the module. The facility further includes an additional conduit (21) connecting the additional permeate outlet to the gas inlet of the fourth membrane separation unit (5), and a flow rate regulating valve (22) disposed within the additional conduit (21). The controller controls this flow rate regulating valve (22) based on the data measured by the methane concentration sensor (18) to reduce the flow rate through the additional conduit (21) when the concentration of methane in the third permeate stream rises until it exceeds the target value.

[0055] The process of the present invention is implemented in the facility of the present invention as described above.

[0056] A raw gas stream containing 20 to 60% by volume, preferably 20 to 50% by volume of carbon dioxide, with the total content of methane and carbon dioxide being at least 95% by volume, is introduced into the raw gas conduit (7) of the facility. The raw gas may be natural gas or landfill gas, or preferably biogas from an anaerobic digester. The raw gas preferably contains 30 to 50% by volume of carbon dioxide. The raw gas is preferably desulfurized biogas from an anaerobic digester. By desulfurizing the raw gas stream, corrosion of the compressor and the gas conduits of the facility is prevented. The biogas may be pretreated by drying and / or adsorption onto an adsorbent for volatile organic compounds such as volatile siloxanes. When the raw gas is biogas from an anaerobic digester that is operated with the addition of controlled air to reduce hydrogen sulfide formation in the digester, the raw gas typically contains small amounts of oxygen and nitrogen.

[0057] The raw gas stream is combined with the recirculation streams from the fourth holding liquid conduit (13) and the second permeate conduit (14), compressed by the compressor (1) to provide a feed stream with a supply pressure of 7 to 25 bar and a temperature of 15 to 50 °C. Compression typically raises the temperature of the gas to a value higher than that desired to operate the first membrane separation unit (2), and thus the compressed gas is typically cooled to provide the feed stream at the required temperature. The compressed gas may be cooled to a temperature lower than the temperature required to operate the first membrane separation unit (2), water condensed from the compressed gas at this low temperature, and the gas after separating the condensed water reheated to the required temperature to be dehumidified. The compressed gas is preferably dehumidified by a dehumidifier disposed in the supply conduit as described above. By dehumidifying the compressed gas, condensation of water in the membrane separation unit is prevented, reducing the separation capacity of the membrane separation unit.

[0058] Next, the feed stream is separated in a first membrane separation unit (2) into a first permeate stream and a first retentate stream using a membrane having a mixed gas selectivity for carbon dioxide to methane of at least 30, preferably 40 to 100, more preferably 40 to 80, at the feed pressure and temperature of the feed stream. Suitable membrane modules and membrane cartridges comprising hollow fiber polyimide membranes having such high mixed gas selectivity are commercially available from Evonik Fibres GmbH under the trade name SEPURAN® Green. The permeate side pressure in the first membrane separation unit and the separation capacity in the four membrane separation units are selected to provide a carbon dioxide concentration of 90 to 99 volume % in the first permeate stream. The separation capacity of the membrane separation unit is, as further defined above, the product of the membrane area and the membrane permeability of carbon dioxide at a temperature of 25 °C and a feed side pressure of 5 bar. The selection of appropriate values for the permeate side pressure in the first membrane separation unit and the separation capacity in the four membrane separation units can be carried out using process simulation software that calculates the mass transfer of the gas components through the membrane by numerical integration of known differential equations for mass transfer through the membrane according to the solution-diffusion process based on experimental data of the membrane permeabilities for methane and carbon dioxide. Such calculations are preferably carried out using boundary conditions set for the methane concentration in the third permeate stream, the carbon dioxide concentration in the second retentate stream, and the target value of methane recovery by the second retentate stream. The temperature dependence of the permeation can be explained by applying the equations known from M. Scholz et al., Ind. Eng. Chem. Res. 52 (2013) 1079 - 1088.

[0059] The first retentate stream is separated in a second membrane separation unit (3) into a second retentate stream and a second permeate stream. The second retentate stream is further processed or removed as a methane-rich product stream, preferably removed as a methane-rich product stream. A non-limiting list of examples for further processing includes odorization, calorific value adjustment, pressure adjustment, processing into compressed natural gas or liquefied natural gas, grid injection, polishing (removing components below 0.5% to ppm levels), power generation, or using at least the split stream, and a process according to one of the options mentioned above. The second retentate stream is removed or advanced for further processing through a second retentate conduit (10), which preferably includes a pressure regulating valve in the conduit and maintains a constant retentate pressure by said valve. The second permeate stream is recycled through a second permeate conduit (14). An additional pressure regulating valve may be disposed in the second permeate conduit (14) to adjust or control the permeate pressure of the second membrane separation unit (3). The separation capacity of the second membrane separation unit (3) is preferably selected to provide a carbon dioxide concentration in the second retentate stream of 0.5 to 4.0% by volume. It is also preferred to select the separation capacity of the second membrane separation unit (3) to provide a carbon dioxide concentration in the second permeate stream of 81 to 89% by volume carbon dioxide. Such selection may be made by process simulation as described above, using target values ​​within these ranges for the carbon dioxide concentration in the second retentate stream and / or the second permeate stream as boundary conditions for the process simulation.

[0060] The first permeate stream is separated in the third membrane separation unit (4) into a third retentate stream and a third permeate stream, and the third permeate stream is discharged to the ambient atmosphere without further methane removal. The separation capacity of the third membrane separation unit (4) is preferably selected to provide a carbon dioxide concentration in the third permeate stream of 0.3% by volume or less, preferably from 0.1 to 0.2% by volume. Such a selection can be made by the above-described process simulation with the target value within this range of the carbon dioxide concentration in the third permeate stream as a boundary condition of the process simulation. The third permeate stream is preferably discharged through a third permeate conduit (15) having a methane concentration sensor (18) connected to the third permeate conduit (15), and the carbon dioxide concentration in the third permeate stream is monitored.

[0061] The third retentate stream is separated in the fourth membrane separation unit (5) into a fourth retentate stream and a fourth permeate stream, and the fourth retentate stream is recycled through a retentate conduit (13). The separation capacity of the fourth membrane separation unit (5) is preferably selected to provide methane recovery in combination with a carbon dioxide concentration in the second retentate stream of from 98.0 to 99.9% of the second retentate stream, preferably from 0.5 to 4.0% by volume. Such a selection can be made by the process simulation as described above with the target value of methane recovery within this range as a boundary condition of the process simulation. Preferably, the separation capacities of the second membrane separation unit (3) and the fourth membrane separation unit (5) are selected such that the separation capacity of the second membrane separation unit (3) is 1.2 to 8 times that of the fourth membrane separation unit (5). Such a selection of the membrane separation capacity provides a lower flow rate of the fourth permeate stream that has to be processed in the methane oxidation unit when producing the third permeate stream with a targeted low methane concentration.

[0062] The fourth permeate stream is sent to the methane oxidation unit (6), where it is oxidized to provide an off-gas stream containing less than 0.3% by volume of methane, and this off-gas stream is discharged to the ambient atmosphere. Methane is preferably oxidized in the methane oxidation unit (6) using an oxygen-containing gas as the oxidant, preferably air. The oxygen-containing gas can be mixed with the fourth permeate stream before being introduced into the methane oxidation unit (6), or can be supplied separately to the methane oxidation unit (6). Methane is preferably oxidized in a catalytic oxidizer, a regenerative thermal oxidizer or a biofilter. In a preferred embodiment, the methane oxidation unit (6) comprises a catalytic oxidizer or a regenerative thermal oxidizer, and the separation capacity of the fourth membrane separation unit is selected to provide a methane concentration in the fourth permeate stream that enables autothermal operation of the oxidizer.

[0063] The process of the present invention enables compliance with strict limits on methane emissions to the atmosphere with only a small methane oxidation unit because the flow rate of the fourth permeate stream treated in the methane oxidation unit is typically lower than the flow rate of the third permeate stream that can be discharged without treatment. This process can provide a high methane recovery based on the feed gas even when operating the methane oxidation unit as an autothermal catalytic oxidizer or a regenerative thermal oxidizer without supplying additional fuel.

[0064] By using a membrane having at least 30 mixed gas selectivities in the first membrane separation unit (2) and adjusting the separation capacity to provide a carbon dioxide concentration in the first permeate stream of 90 to 99% by volume, a greater proportion of the carbon dioxide contained in the feed gas stream having a third permeate stream at a low methane concentration of 0.3% by volume can be separated, thereby reducing the flow rate of the fourth permeate stream and as a result reducing the size of the methane oxidation unit (6).

[0065] Selecting the separation capacity of the second membrane separation unit (3) to provide a carbon dioxide concentration of 0.5 to 4.0 volume % in the second holding liquid stream and 81 to 89 volume % in the second permeate stream increases the proportion of carbon dioxide removed in the third permeate stream and reduces the overall recycle rate in the process.

[0066] In a preferred embodiment of the process of the present invention, the feed pressure and the permeate side pressure of the first membrane separation unit (2) are selected to provide a pressure ratio of a third membrane separation unit (4) that is 0.4 to 1.2 times, preferably 0.4 to 1.0 times, the pressure ratio of the first membrane separation unit (2). Here, the pressure ratio within the membrane unit is defined as the ratio of the feed side pressure to the permeate side pressure within the membrane unit. Selection of such a pressure ratio enables the process to be operated at a lower overall recycle rate.

[0067] In another preferred embodiment of the process of the present invention, the concentration of methane in the third permeate stream is measured using a methane concentration sensor (18), and the operating parameters of the separation process are adjusted based on the measured value to maintain the concentration of methane in the third permeate stream below a target value, preferably within the range of 0.1 to 0.3 volume %. Preferably, the operating parameters of the first membrane separation unit (2) are adjusted. This makes it possible to maintain the methane concentration in the third permeate stream below the regulatory limit for methane emissions even if the composition or flow rate of the raw material gas stream changes.

[0068] Preferably, the permeate-side pressure of the first membrane separation unit (2) is adjusted based on the measured concentration of methane in the third permeate stream, and the permeate-side pressure is decreased when the concentration of methane in the third permeate stream rises until it exceeds the target value. This is typically the case when the flow rate of the raw gas stream decreases or the methane content of the raw gas stream increases (see Example 10 compared to Example 6). The permeate-side pressure of the first membrane separation unit (2) is preferably controlled by a pressure regulating valve (17) disposed in the fourth retentate conduit (13). The permeate-side pressure is preferably controlled to maintain the concentration of methane in the third permeate stream essentially constant, such that the variation in methane concentration is 0.03% by volume or less.

[0069] In another preferred embodiment, the temperature of the feed stream is adjusted based on the measured concentration of methane in the third permeate stream, and the temperature of the feed stream is decreased when the concentration of methane in the third permeate stream rises until it exceeds the target value. The temperature of the feed stream can be adjusted by adjusting the cooling of the gas stream exiting the compressor. When dehumidifying the compressed gas by cooling and condensing water as further described above, the temperature of the feed stream can also be adjusted by adjusting the reheating of the compressed gas after the condensation step. Alternatively, the temperature of the first permeate stream is adjusted based on the measured concentration of methane in the third permeate stream, and the temperature of the first permeate stream is decreased when the concentration of methane in the third permeate stream rises until it exceeds the target value. Both of these options have the advantage of less increase in the recycle rate compared to the option of adjusting the permeate side pressure of the first membrane separation unit (2) by operating the process with a reduced flow rate of the raw gas stream. For both embodiments, the temperature preferably maintains the concentration of methane in the third permeate stream essentially constant, and the variation is controlled such that the variation in methane concentration is 0.03 volume % or less. In both embodiments, the temperature can be decreased by heat exchange with the second hold-up stream, and the temperature can be adjusted by controlling the proportion of the second hold-up stream used for this heat exchange. Using the second hold-up stream to cool the feed stream or the first permeate stream has the advantage of not requiring extra energy to adjust the temperature.

[0070] In yet another preferred embodiment, the process is carried out in a facility comprising a number of membrane modules arranged in parallel within the third membrane separation unit (4), at least one of these membrane modules comprising a shut-off valve for shutting off the flow through the membrane module, the shut-off valve of the membrane module being closed when the measured concentration of methane in the third permeate stream rises until it exceeds the target value.

[0071] In yet another preferred embodiment, the process is carried out in a facility comprising a pore-side feed hollow fiber membrane module in which, as further described above, a first membrane separation unit (2) has a first permeate outlet adjacent to one end of the module and an additional permeate outlet adjacent to the opposite end of the module and is connected by an additional conduit (21) to the gas inlet of a fourth membrane separation unit (5). The flow through the additional conduit (21) is then controlled by a flow control valve (22) disposed within the additional conduit (21) based on the measured concentration of methane in the third permeate stream and the flow through the additional conduit (21) is reduced when the concentration of methane in the third permeate stream rises above a target value.

[0072] These different alternative ways of adjusting the operating parameters of the separation process based on the measured concentration of methane in the third permeate stream may also be combined with each other in order to maintain an essentially constant concentration of methane in the third permeate stream over a wider range of feed gas compositions and flow rates of the feed gas stream. Combinations are preferred which are combined with blocking the flow through one or several membrane modules arranged in parallel within the third membrane separation unit (4), which allow adjustment only in discrete steps over a wide range, in particular in a narrow range which only fills the gaps between the operations of the third membrane separation unit (4) using different numbers of membrane modules in use, in combination with adjusting the permeate-side pressure, the temperature of the feed stream or the temperature of the first permeate stream.

[0073] The following examples demonstrate the invention and its advantages.

Example

[0074] Based on the experimental data of the permeation rates of the methane and carbon dioxide membranes, process simulation software for calculating the mass transfer of gas components through the membrane by numerical integration of known differential equations for mass transfer through the membrane by the solution-diffusion method was used to perform calculations of gas separation in a facility as shown in FIG. 1. All pressures are given as absolute pressures.

[0075] The simulation on which the examples are based was conducted on the premise that the methane concentration in the third permeate stream was set to 0.2% by volume and measured and controlled to be 0.3% by volume. Specific values are shown in the examples.

[0076] Comparative Example 1 International Publication No. WO 2015 / 036709 pamphlet provides equipment and methods that can be used to purify biogas. According to the sixth paragraph on the first page of International Publication No. ’709 pamphlet, biogas typically contains 30 to 75% methane, 15 to 60% CO 2 , 0 to 15% N 2 and 0 to 5% O 2 . International Publication No. ’709 pamphlet further discloses in the last paragraph on page 3 that this method should enable the production of a gas containing more than 85%, preferably more than 95%, more preferably more than 97.5% methane. Page 7 of International Publication No. ’709 pamphlet provides a table showing methane recovery and recycle rates for membrane separation processes with two units, three units, four units, and five units. However, International Publication No. ’709 pamphlet does not disclose - how these yields and recycle rates were achieved, - which raw gas mixtures were used, - which membranes were used, - which process pressures and temperatures were used.

[0077] Since International Publication No. ’709 pamphlet does not include examples that can be reproduced to compare the method and equipment with the present invention, Comparative Examples 1a and 1b were based on the basic information summarized above. As shown for the four-unit process in the table on page 7 of International Publication No. ’709, 99.09% CH 4Process simulations were carried out in Comparative Examples 1a and 1b with the goal of matching the revenue and a recycle rate of 1.42. Since it is unclear exactly what "revenue" means, it may mean "content" or "yield", so Comparative Example 1a was prepared with a 99.09% CH 4 content in the methane-enriched product stream as a boundary condition, and Comparative Example 1b was prepared with a 99.09% CH 4 yield in the methane-enriched stream as a boundary condition.

[0078] Comparative Example 1a The raw gas stream was supplied at a flow rate of 5,420 Nm 3 / h at a pressure of 1.01 bar, which contained 50 vol% methane, 49.7 vol% carbon dioxide, 0.2 vol% nitrogen and 0.1 vol% oxygen. The raw gas stream was subjected to a membrane separation process in a facility according to Figure 3 of WO '709 pamphlet, which included 367 SEPULAN® Green membrane modules, each module having a mixed gas selectivity of 50 for carbon dioxide with respect to methane, 5.0 for carbon dioxide with respect to oxygen, 31 for carbon dioxide with respect to nitrogen, and a separation capacity of 2.101 mols -1 MPa -1 membrane. The feed temperature was set at 25 °C and the feed pressure was set at 16 bar. Calculations were performed for isothermal separation assuming a pressure drop of 70 mbar on the retentate side of the module. The simulation was carried out with the boundary conditions that the methane content in the second retentate stream was 99.09 vol% and the total recycle rate was 42% for all recycle gas streams. 137 membrane modules of the first membrane separation unit, 83 membrane modules of the second membrane separation unit, 62 membrane modules of the third membrane separation unit, and 85 membrane modules of the fourth membrane separation unit were used. The calculated flow rates and compositions of the process streams are provided in Table 1.

[0079]

Table 1

[0080] The feed gas stream used in Comparative Example 1a meets the "Biogas Specifications" of WO '709 pamphlet, and as required in WO '709 pamphlet, the methane content in the second retentate stream exceeds 97.5%. A recycle ratio of 1.42 (7713 Nm 3 / h (feed stream) / 5420 Nm 3 / h (feed gas stream) = 1.42) and a methane content of 99.09% in the second retentate stream both correspond to the disclosure in the table on page 7 of WO '709 pamphlet when "profit" means yield.

[0081] Table 1 shows that the CO content of the first permeate stream is 88.75%, thus indicating that it is outside the scope of the claims of the present invention. The methane content in the third permeate stream is 0.48%. As a result, the process of WO '709 pamphlet cannot be used without subjecting both the third and fourth permeate streams to a methane reduction post-treatment step in locations with strong regulatory requirements for methane emissions, i.e., for the methane content in the off-gas stream. 2

[0082] Comparative Example 1b Comparative Example 1a was reproduced with the same feed gas stream, membrane type, feed temperature and feed pressure. Calculations were performed for isothermal separation assuming a pressure drop of 70 mbar on the retentate side of the module. The simulation was carried out with boundary conditions providing a total methane yield of 99.09% and a recycle ratio of 42% for all recycle gas streams. 137 membrane modules of the first membrane separation unit, 83 membrane modules of the second membrane separation unit, 62 membrane modules of the third membrane separation unit, and 85 membrane modules of the fourth membrane separation unit were used. The calculated flow rates and compositions of the process streams are provided in Table 2.

[0083]

Table 2

[0084] The feed gas stream used in Comparative Example 1a meets the "Biogas Specification" of WO '709 pamphlet, and the methane content in the second hold-up liquid stream exceeds 97.5% as required in WO '709 pamphlet. A recycle ratio of 1.42 (6900 Nm 3 / h (feed stream) / 4870 Nm 3 / h (feed gas stream) = 1.42) and a methane yield of 99.09% in the second hold-up liquid stream both correspond to the disclosure in the table on page 7 of WO '709 pamphlet when "profit" means yield.

[0085] Table 2 shows that the CO 2 content in the first permeate stream is 87.15%, thus being outside the scope of the claims of the present invention. The methane content in the third permeate stream is 0.59%. As a result, the process of WO '709 pamphlet cannot be used without subjecting both the third and fourth permeate streams to a methane reduction post-treatment step in places with strong regulatory requirements for methane emissions, i.e., for the methane content in the off-gas stream.

[0086] Example 1 In a facility comprising 330 SEPURAN® Green membrane modules, each module having a mixed gas selectivity of carbon dioxide over methane of 50, a mixed gas selectivity of carbon dioxide over oxygen of 5.0, a mixed gas selectivity of carbon dioxide over nitrogen of 31, and a separation capacity of 2.101 mols -1 MPa -1 , 10,000 Nm 3A gas separation for separating a raw gas stream supplied at a flow rate of / h at 1.01 bar and containing 49.9 vol% methane, 50 vol% carbon dioxide and 0.1 vol% oxygen was calculated. The feed temperature was set at 25 °C and the feed pressure was set at 16 bar. Calculations were performed for isothermal separation assuming a pressure drop of 70 mbar on the retentate side of the module. The methane content of the second retentate stream was 97.0 vol%, the methane content of the third permeate stream was 0.2 vol%, the methane recovery of the second retentate stream was 99.8%, and the flow rate of the fourth permeate stream was 550 Nm 3 / h and optimization was carried out with the boundary conditions. The permeate side pressure of the first membrane separation unit and the distribution of membrane modules to the four membrane separation units were changed to provide a minimum recycle rate (the combined second permeate stream and the fourth retentate stream with respect to the raw gas stream). By optimization, the permeate side pressure of the first membrane separation unit was 3.48 bar, and for the distribution of 59.8 membrane modules in the first membrane separation unit, 126.6 membrane modules in the second membrane separation unit, 118.1 membrane modules in the third membrane separation unit, and 25.4 membrane modules in the fourth membrane separation unit, the minimum value of the recycle rate was calculated to be 46.0%. The calculated flow rates and compositions of the process streams are provided in Table 3.

[0087] The calculations show that the process of the present invention can upgrade typical biogas to biomethane with a methane content of 97 vol% at a methane recovery of 99.8% with a recycle rate of only 46%. The process of the present invention separates most of the carbon dioxide in a gas stream containing only 0.2 vol% methane that can be directly discharged to the atmosphere. Only a small off-gas stream with a flow rate of 6% with respect to the biogas has to be treated in a methane oxidation unit. This methane oxidation unit can operate as a self-thermal catalytic oxidizer or a regenerative thermal oxidizer without additional fuel supply since the off-gas stream contains 1.7 vol% methane.

[0088]

Table 3

[0089] Comparative Example 2 The calculations of Example 1 were repeated with the following modifications. The mixed gas selectivity of carbon dioxide with respect to methane was 20, the mixed gas selectivity of carbon dioxide with respect to oxygen was 5, the mixed gas selectivity of carbon dioxide with respect to nitrogen was 56, and the separation capacity was 2.101 mols -1 MPa -1 A membrane with such properties was used in the first separation unit (2), and 108 modules were used in the third separation unit (4) instead of 118 modules.

[0090] At 1.01 bar and a flow rate of 10,000 Nm 3 / h, the gas separation for separating a feed gas stream containing 49.9 vol% methane, 50 vol% carbon dioxide, and 0.1 vol% oxygen was calculated. The modules were SEPURAN (registered trademark) Green membrane modules, and each module had a mixed gas selectivity of carbon dioxide with respect to methane of 50, a mixed gas selectivity of carbon dioxide with respect to oxygen of 5.0, a mixed gas selectivity of carbon dioxide with respect to nitrogen of 31, and a separation capacity of 2.101 mol s -1 MPa -1 Membrane-containing modules were used in the second separation unit (3), the third separation unit (4), and the fourth separation unit (5). The feed temperature was set at 25 °C and the feed pressure was set at 16 bar. Calculations were performed for isothermal separation assuming a pressure drop of 70 mbar on the retentate side of the modules. 60 membrane modules of the first membrane separation unit, 127 membrane modules of the second membrane separation unit, 108 membrane modules of the third membrane separation unit, and 25 membrane modules of the fourth membrane separation unit were used. The calculated flow rates and compositions of the process streams are provided in Table 4.

[0091]

Table 4

[0092] Table 4 shows that even by using a less selective membrane in the first separation unit, a methane content of 0.21% in the third permeate stream can be obtained, but the process efficiency is much lower. The recycle rate of 85.6% in Comparative Example 2 is almost twice that of 46% in Example 1, and the methane content in the second retentate stream decreases to 95.35%.

[0093] Example 2 The calculations of Example 1 were repeated with the following modifications. In the second membrane separation unit (3), the mixed gas selectivity of carbon dioxide with respect to methane is 20, the mixed gas selectivity of carbon dioxide with respect to oxygen is 15, and the mixed gas selectivity of carbon dioxide with respect to nitrogen is 169, and the separation capacity is 6.303 mols -1 MPa -1 of membrane was used. In the second membrane separation unit (3), 42 modules were used instead of 127 modules.

[0094] As in Example 1, gas separation was calculated for separating a raw gas stream supplied at a flow rate of 10,000 Nm 3 / h at 1.01 bar and containing 49.9 vol% methane, 50 vol% carbon dioxide, and 0.1 vol% oxygen. Modules containing membranes that are SEPURAN (registered trademark) Green membrane modules, and each module has a mixed gas selectivity of carbon dioxide with respect to methane of 50, a mixed gas selectivity of carbon dioxide with respect to oxygen of 5.0, and a mixed gas selectivity of carbon dioxide with respect to nitrogen of 31, and the separation capacity is 2.101 mols -1 MPa -1 were used in the first separation unit (2), the third separation unit (4), and the fourth separation unit (5). The feed temperature was set at 25 °C and the feed pressure was set at 16 bar. Calculations were performed for isothermal separation assuming a pressure drop of 70 mbar on the retentate side of the module. 60 membrane modules of the first membrane separation unit, 42 membrane modules of the second membrane separation unit, 108 membrane modules of the third membrane separation unit, and 25 membrane modules of the fourth membrane separation unit were used. The calculated flow rates and compositions of the process streams are provided in Table 5.

[0095]

Table 5

[0096] Table 5 shows that, in contrast to using such a membrane in the first separation unit (2) as in Comparative Example 2, when a less selective membrane is used in the second separation unit (3), a significant increase in the volumetric flow rate of the fourth permeate stream can be avoided compared to Example 1. Also, the target methane content of 97% in the second retentate and 0.21% in the third permeate stream can be achieved as in Example 1.

[0097] Example 3 The boundary condition of the flow rate of the fourth permeate stream was changed to 1000 Nm 3 / h, and the calculations of Example 1 were repeated. By optimization, the permeate-side pressure of the first membrane separation unit is 3.51 bar, and for the distribution of 69.6 membrane modules in the first membrane separation unit, 118.2 membrane modules in the second membrane separation unit, 104.5 membrane modules in the third membrane separation unit, and 34.5 membrane modules in the fourth membrane separation unit, the minimum recycle rate was calculated to be 39.1%. The calculated flow rates and compositions of the process streams are provided in Table 6.

[0098] This calculation shows that there is a trade-off between providing a low recycle rate and reducing the size of the off-gas stream that must be processed in the methane oxidation unit.

[0099]

Table 6

[0100] Example 4 The calculations of Example 1 were repeated for a feed gas containing 69.9 vol% methane, 30.0 vol% carbon dioxide, and 0.1 vol% oxygen. By optimization, the permeate-side pressure of the first membrane separation unit was 3.10 bar, and for the distribution of 34.3 membrane modules in the first membrane separation unit, 183.7 membrane modules in the second membrane separation unit, 73.2 membrane modules in the third membrane separation unit, and 38.8 membrane modules in the fourth membrane separation unit, the minimum value of the recycle ratio was calculated to be 69.3%. The calculated flow rates and compositions of the process streams are provided in Table 7.

[0101] The calculations show that the process of the present invention can separate most of the carbon dioxide at a methane content suitable for direct emission of methane-rich biogas to the atmosphere, although at a higher recycle ratio.

[0102] [Table 7]

[0103] Comparative Example 3 The calculations of Example 1 were repeated for a feed gas containing 84.9 vol% methane, 15.0 vol% carbon dioxide, and 0.1 vol% oxygen. By optimization, the permeate-side pressure of the first membrane separation unit was 3.45 bar, and for the distribution of 19 membrane modules in the first membrane separation unit, 226 membrane modules in the second membrane separation unit, 21 membrane modules in the third membrane separation unit, and 33 membrane modules in the fourth membrane separation unit, the minimum value of the recycle ratio was calculated to be 79.7%. The calculated flow rates and compositions of the process streams are provided in Table 8.

[0104] The calculations show that when the CO 2 content in the feed stream decreases, the recycle ratio increases. Also, when the methane content in the fourth permeate stream increases, the cost of oxidative post-treatment increases.

[0105] [Table 8]

[0106] Example 5 The calculations of Example 1 were repeated for a feed gas containing 39.9 vol% methane, 60.0 vol% carbon dioxide and 0.1 vol% oxygen. By optimization, the permeate side pressure of the first membrane separation unit was 3.45 bar, and for the distribution of 87 membrane modules in the first membrane separation unit, 92 membrane modules in the second membrane separation unit, 147 membrane modules in the third membrane separation unit, and 17 membrane modules in the fourth membrane separation unit, the minimum recycle rate was calculated to be 35.4%. The calculated flow rates and compositions of the process streams are provided in Table 9.

[0107] The calculations show that the process of the present invention can separate most of the carbon dioxide with a low methane content in the third permeate stream, which is suitable for direct emission of biogas with a high methane content to the atmosphere. The recycle rate is very low.

[0108] [Table 9]

[0109] Example 6 Using the same membranes as in Example 1 and SEPURAN® Green membrane modules with a separation capacity of 2.460 mols -1 MPa -1 for 10,000 Nm 3The gas separation for separating a raw gas stream supplied at 1.01 bar with a flow rate of / h and containing 50.0 vol% methane, 49.7 vol% carbon dioxide, 0.2 vol% nitrogen and 0.1 vol% was calculated. The separation was calculated for a facility with 137 membrane modules in the first membrane separation unit, 83 membrane modules in the second membrane separation unit, 62 membrane modules in the third membrane separation unit, and 85 membrane modules in the fourth membrane separation unit. The temperature dependence of permeation and the pressure drop within the module were accounted for by applying the equations known from Ind. Eng. Chem. Res. 52 (2013) 1079 - 1088 by M. Scholz et al. The feed temperature was set to 25 °C, the pressure on the retentate side of the second membrane separation unit was set to 16.0 bar, and the pressure on the retentate side of the fourth membrane separation unit was set to 3.20 bar. The calculated flow rate, pressure, temperature and composition of the process stream are shown in Table 10.

[0110] The calculation shows that almost half of the carbon dioxide contained in the raw gas can be separated as a gas stream containing only 0.3 vol% methane with a recycle rate of only 28%.

[0111]

Table 10

[0112] Example 7 The calculation of Example 6 was repeated for a raw gas stream with a lower flow rate of 5% of 9500 Nm 3 / h, and the pressure on the retentate side of the fourth membrane separation unit was reduced to maintain the same methane concentration of 0.3% in the third permeate stream. However, this required reducing the pressure on the retentate side of the fourth membrane separation unit from 3.20 bar to 3.05 bar. The calculated flow rate, pressure, temperature and composition of the process stream are shown in Table 11.

[0113] The calculation shows that when the flow rate of the raw gas stream decreases, the pressure on the retentate side of the fourth membrane separation unit can be reduced to maintain the methane concentration in the third permeate stream at the target value. However, the recycle rate will increase from 28% to 30%.

[0114]

Table 11

[0115] Example 8 The calculations of Example 6 were repeated for a feed gas stream with a flow rate 5% lower than 9500 Nm 3 / h, reducing the temperature of the feed stream to maintain the same methane concentration of 0.3 vol% in the third permeate stream, which required reducing the temperature of the feed stream from 25 °C to 22.8 °C. The calculated flow rates, pressures, temperatures and compositions of the process streams are shown in Table 12.

[0116] The calculations show that when the flow rate of the feed gas stream decreases, the methane concentration in the third permeate stream can be maintained at the target value by reducing the temperature of the feed stream. The recycle rate decreases from 28% to 26%.

[0117]

Table 12

[0118] Example 9 The calculations of Example 6 were repeated by reducing the temperature of the first permeate stream instead of reducing the temperature of the feed stream. The temperature of the first permeate stream had to be reduced from 20.8 °C to 17.5 °C before being fed to the third membrane separation unit to maintain the same methane concentration of 0.3 vol% in the third permeate stream. The calculated flow rates, pressures, temperatures and compositions of the process streams are shown in Table 13.

[0119] The calculations show that by reducing the temperature of the first permeate stream, the methane concentration in the third permeate stream can be maintained at the target value without changing the recycle rate when the flow rate of the feed gas stream decreases.

[0120]

Table 13

[0121] Example 10 The calculations of Example 6 were repeated for a feed gas stream having a higher methane concentration of 51.0% by volume and a lower carbon dioxide concentration of 48.7% by volume, and the pressure on the retentate side of the fourth membrane separation unit was reduced to maintain the same methane concentration of 0.3% by volume in the third permeate stream, which required reducing the pressure on the retentate side of the fourth membrane separation unit from 3.20 bar to 3.12 bar, and this measure was caused by reducing the pressure on the permeate side of the first membrane separation unit (2) from 3.6 bar in Example 6 to 3.54 bar in Example 10. The calculated flow rates, pressures, temperatures and compositions of the process streams are shown in Table 14.

[0122] Based on Example 6, without adjusting the permeate side pressure of the first membrane separation unit (2), when the CH 4 concentration in the feed gas was increased by 1 percentage point, the CH 4 concentration in the permeate of the third membrane separation unit (4) increased from 0.30% to 0.32%. In this example, a stable methane concentration of 0.30% in the third permeate stream can be achieved by reducing the pressure on the permeate side of the first membrane separation unit (2) through reducing the pressure on the retentate side of the fourth membrane separation unit.

[0123] The calculations show that when the methane concentration in the feed gas stream increases, the pressure on the retentate side of the fourth membrane separation unit can be reduced to maintain the methane concentration in the third permeate stream at the target value. However, the recycle ratio will increase from 28% to 29%.

[0124] [Table 14]

[0125] Example 11 The calculations of Example 6 were repeated for a feed gas stream having a higher methane concentration of 51.0% by volume and a lower carbon dioxide concentration of 48.7% by volume, reducing the temperature of the feed stream to maintain the same methane concentration of 0.3% by volume in the third permeate stream, which required reducing the temperature of the feed stream from 25 °C to 23.8 °C. The calculated flow rates, pressures, temperatures and compositions of the process streams are shown in Table 15.

[0126] The calculations show that when the methane concentration in the feed gas stream increases, the temperature of the feed stream can be reduced to maintain the methane concentration in the third permeate stream at the target value. The recycle ratio decreases from 28% to 27%.

[0127]

Table 15

Explanation of symbols

[0128] 1 Compressor 2 First membrane separation unit 3 Second membrane separation unit 4 Third membrane separation unit 5 Fourth membrane separation unit 6 Methane oxidation unit 7 Feed gas conduit 8 Feed conduit 9 First retentate conduit 10 Second retentate conduit 11 First permeate conduit 12 Third retentate conduit 13 Fourth retentate conduit 14 Second permeate conduit 15 Third permeate conduit 16 Fourth permeate conduit 17 Pressure regulating valve 18 Methane concentration sensor 19 Heat exchanger 20 Flow regulating valve 21 Additional conduit 22 Flow regulating valve

Claims

1. A compressor (1), Four membrane separation units (2) to (5), each membrane separation unit having a gas separation membrane with a higher permeability to carbon dioxide than methane, a gas inlet, a holding liquid outlet, and a permeate outlet, four membrane separation units (2) to (5), A methane oxidation unit (6), A raw material gas conduit (7) connected to the inlet of the compressor (1), A supply conduit (8) connecting the outlet of the compressor (1) to the gas inlet of the first membrane separation unit (2), A first holding liquid conduit (9) connecting the holding liquid outlet of the first membrane separation unit (2) to the gas inlet of the second membrane separation unit (3), A second holding liquid conduit (10) connected to the holding liquid outlet of the second membrane separation unit (3), A first permeate conduit (11) connecting the permeate outlet of the first membrane separation unit (2) to the gas inlet of the third membrane separation unit (4), A third holding liquid conduit (12) connecting the holding liquid outlet of the third membrane separation unit (4) to the gas inlet of the fourth membrane separation unit (5), A fourth holding liquid conduit (13) connecting the holding liquid outlet of the fourth membrane separation unit (5) to the inlet of the compressor (1), A second permeate conduit (14) connecting the permeate outlet of the second membrane separation unit (3) to the inlet of the compressor (1), A third permeate conduit (15) connected to the permeate outlet of the third membrane separation unit (4), A fourth permeate conduit (16) connected to the permeate outlet of the fourth membrane separation unit (5), An apparatus for separating methane and carbon dioxide from a gas stream, comprising: The third permeate conduit (15) is configured to discharge a third permeate to the ambient atmosphere, The fourth permeate conduit (16) connects the permeate outlet of the fourth membrane separation unit (5) to the methane oxidation unit (6), The first membrane separation unit (2) comprises a membrane having a pure gas selectivity of carbon dioxide to methane of at least 30, determined at 20 °C and 5 bar, The apparatus is configured to provide a carbon dioxide concentration in the range of 90 to 99% by volume in the gas stream in the first permeate conduit (11), which is a first permeate stream. An apparatus characterized by this.

2. In the four membrane separation units (2) to (5), the permeate-side pressure in the first membrane separation unit (2), the membrane area at a temperature of 25°C and a feed-side pressure of 5 bar, and the membrane permeability of carbon dioxide, the separation ability which is the product of these, is configured to provide a carbon dioxide concentration of 90 to 99% by volume in the first permeate stream. And / or The facility includes means for controlling the permeate-side pressure in the first membrane separation unit (2) and / or the separation ability of the four membrane separation units (2) to (5) to provide a carbon dioxide concentration of 90 to 99% by volume in the first permeate stream. The facility according to claim 1.

3. The facility according to claim 1, wherein the methane oxidation unit (6) includes a catalytic oxidizer, a regenerative thermal oxidizer, or a biofilter.

4. The facility according to claim 1, wherein the first permeate conduit (11) connects the permeate outlet of the first membrane separation unit (2) to the gas inlet of the third membrane separation unit (4) without an intermediate compressor or pump.

5. The separation ability of the second membrane separation unit (3) is greater than the separation ability of the fourth membrane separation unit (5), and the separation ability of the membrane separation unit is the product of the membrane area of the membrane separation unit and the membrane permeability of carbon dioxide at 25°C and a feed-side pressure of 5 bar. The facility according to claim 1.

6. The facility according to claim 1, wherein a pressure regulating valve (17) is arranged in the fourth holding liquid conduit (13).

7. The facility according to claim 1, wherein a methane concentration sensor (18) is connected to the third permeate conduit (15).

8. The facility according to claim 7, comprising a pressure regulating valve (17) arranged in the fourth holding liquid conduit (13) and a controller for controlling the pressure regulating valve (17) based on data measured by the methane concentration sensor (18).

9. The facility according to claim 7, comprising a heat exchanger (19) in the supply conduit (8), a flow regulating valve (20) for controlling the flow of heating fluid or cooling fluid to the heat exchanger (19), and a controller for controlling the flow regulating valve (20) based on data measured by the methane concentration sensor (18).

10. The third membrane separation unit (4) comprises a plurality of membrane modules arranged in parallel, and at least one of the membrane modules comprises a shut-off valve for shutting off the flow through the membrane module and a controller for controlling the shut-off valve based on data measured by the methane concentration sensor (18). The facility according to claim 7.

11. The first membrane separation unit (2) has the gas inlet at a first end of the module, the holding liquid outlet at a second end of the module opposite the first end, the permeate outlet adjacent to the first end of the module and connected to the first permeate conduit (11), and an additional permeate outlet adjacent to the second end of the module. The facility comprises a pore-side feed hollow fiber membrane module, an additional conduit (21) connecting the additional permeate outlet to the gas inlet of the fourth membrane separation unit (5), a flow rate regulating valve (22) disposed within the additional conduit (21), and a controller for controlling the flow rate regulating valve (22) based on data measured by the methane concentration sensor (18). The facility according to claim 7.

12. A membrane process for separating methane and carbon dioxide from a gas stream, a) providing the facility according to claim 1; b) introducing a feed gas stream containing 20 to 60% by volume of carbon dioxide and having a total methane and carbon dioxide content of at least 95% by volume into the feed gas conduit (7) of the facility; c) compressing the feed gas stream combined with the recirculation streams from the fourth holding liquid conduit (13) and the second permeate conduit (14) in a compressor (1) to provide a feed stream at a supply pressure of 7 to 25 bar and a temperature of 15 to 50 °C; d) at the supply pressure and temperature of the feed stream, using a membrane having a mixed gas selectivity of carbon dioxide with respect to methane of at least 30, separating the feed stream within the first membrane separation unit (2) into a first permeate stream and a first holding liquid stream, and selecting the permeate-side pressure within the first membrane separation unit and the separation capabilities in the four membrane separation units so as to provide a carbon dioxide concentration of 90 to 99% by volume in the first permeate stream, wherein the separation capability of the membrane separation unit is the product of the membrane area and the membrane permeability of carbon dioxide at a temperature of 25 °C and a supply-side pressure of 5 bar. e) separating the first retained liquid stream within the second membrane separation unit (3) into a second retained liquid stream and a second permeate stream, further processing the second retained liquid stream, or taking the second retained liquid stream as a product stream rich in methane, and recycling the second permeate stream through the second permeate conduit (14); f) separating the first permeate stream within the third membrane separation unit (4) into a third retained liquid stream and a third permeate stream, and discharging the third permeate stream to the ambient atmosphere without further methane removal; g) separating the third retained liquid stream within the fourth membrane separation unit (5) into a fourth retained liquid stream and a fourth permeate stream, and recycling the fourth retained liquid stream through the retained liquid conduit (13); h) oxidizing the fourth permeate stream within the methane oxidation unit (6) to provide an off-gas stream containing less than 0.3% by volume of methane, the off-gas stream being discharged to the ambient atmosphere, a membrane process.

13. The concentration of methane in the third permeate stream is measured using a methane concentration sensor (18), and the operating parameters of the first membrane separation unit (2) are adjusted based on the measured value to maintain the concentration of methane in the third permeate stream below a target value. The process according to claim 12.

14. The permeate-side pressure of the first membrane separation unit (2) is adjusted based on the measured concentration of methane in the third permeate stream, and the permeate-side pressure is decreased when the concentration of methane in the third permeate stream rises above the target value. The process according to claim 13.

15. The permeate-side pressure of the first membrane separation unit (2) is controlled by a pressure regulating valve (17) disposed within the fourth retained liquid conduit (13). The process according to claim 14.

16. The temperature of the feed stream is adjusted based on the measured concentration of methane in the third permeate stream, and the temperature of the feed stream is decreased when the concentration of methane in the third permeate stream rises above the target value. The process according to claim 13.

17. The temperature of the first permeate stream is adjusted based on the measured concentration of methane in the third permeate stream, and the temperature of the first permeate stream is decreased when the concentration of methane in the third permeate stream rises above the target value. The process according to claim 12.

18. The process according to claim 16, wherein the temperature is reduced by heat exchange with the second holding liquid stream.

19. The process according to claim 12, wherein the equipment according to claim 10 is used, and the shut-off valve of the membrane module is closed when the concentration of methane in the third permeate stream rises until it exceeds the target value.

20. The process according to claim 12, wherein the equipment according to claim 11 is used, the flow through the additional conduit (21) is controlled by the flow rate regulating valve (22) arranged in the additional conduit (21) based on the measured concentration of methane in the third permeate stream, and the flow through the additional conduit (21) is decreased when the concentration of methane in the third permeate stream rises until it exceeds the target value.

21. The process according to claim 13, wherein the target value of the concentration of methane in the third permeate stream is in the range of 0.1 to 0.3% by volume.

22. The process according to claim 12, wherein the separation capacity of the second membrane separation unit (3) is selected to provide a carbon dioxide concentration of 0.5 to 4.0% by volume in the second holding liquid stream, and the separation capacity of the fourth membrane separation unit (5) is selected such that the methane recovery rate is 98.0 to 99.9% with the second holding liquid stream.

23. The process according to claim 22, wherein the separation capacity of the second membrane separation unit (3) is 1.2 to 8 times the separation capacity of the fourth membrane separation unit (5).

24. The process according to claim 22, wherein the separation capacity of the second membrane separation unit (3) is selected to provide a carbon dioxide concentration of 81 to 89% by volume in the second permeate stream.

25. The supply pressure and the permeate side pressure of the first membrane separation unit (2) are selected such that the pressure ratio in the third membrane separation unit (4) is 0.4 to 1.0 times the pressure ratio in the first membrane separation unit (2), and the pressure ratio in the membrane unit is the ratio of the supply side pressure and the permeate side pressure in the membrane unit. The process according to claim 12.

26. The methane oxidation unit (6) comprises a catalytic oxidation device or a regenerative thermal oxidation device, and the separation capacity of the fourth membrane separation unit is selected to provide the methane concentration in the fourth permeate stream that enables self-thermal operation of the oxidation device. The process according to claim 12.

Images