Biogas upgrading apparatus and process

Abstract

A biogas purification system can include a multi-stage membrane system configured to remove oxygen such that the biogas is purified to have a higher concentration of methane, a preselected oxygen (02) concentration (e.g., less than or equal to 0.2 mole percent, etc.), and a preselected carbon dioxide (C02) concentration (e.g., less than or equal to 5 mole percent, etc.). The membrane system can be configured to reject 02 by utilizing a low C02 / 02 selectivity within a predetermined range (e.g., less than 5 or less than 4.5). In some embodiments, the purified biogas output from the system can consist entirely of methane, carbon dioxide, and oxygen. In other embodiments, the biogas can consist almost entirely of these components, as well as small amounts of nitrogen and trace amounts (e.g., less than or equal to 0.2-0.1 percent, etc.) of other components.

Description

Technical Field

[0001] This innovation relates to biogas purification systems, membrane arrangements and equipment that can be used in such systems, and processes for manufacturing and using them. Background Technology

[0002] Raw biogas can be purified into a higher purity methane stream for downstream use (e.g., injection into pipelines for use in combined heat and power (CHP) plants, etc.). Biogas is often purified so that its main component is methane.

[0003] Biogas is generally defined as a byproduct of the anaerobic digestion of organic waste. Biogas produced from the anaerobic digestion of organic waste often contains methane, carbon dioxide, nitrogen, oxygen, hydrogen sulfide, and siloxanes. Capturing and purifying biogas can help reduce greenhouse gas emissions and create marketable renewable energy.

[0004] Examples of systems that can be used to process raw biogas to obtain high-purity methane can be found in U.S. Patent Nos. 9,050,553 and 8,999,038, U.S. Patent Application Publication No. 2011 / 0094378, International Publication No. WO2006 / 069868 and EP 1,634,946.

[0005] Traditional purification methods used to aid in biogas refining include water washing, amine washing, catalytic systems, or pressure swing adsorption (PSA) to remove carbon dioxide and other contaminants. PSA processes are expensive to build and operate. Catalytic systems are also often expensive to operate because they utilize consumable elements (e.g., potentially poisoned catalyst materials) and produce unwanted byproducts (e.g., water) that must be removed downstream. Summary of the Invention

[0006] New systems are needed to purify biogas so that it can be more easily used for downstream operations (e.g., for pipeline supply, use in CHP systems, etc.). In some embodiments, biogas may be referred to as biomethane gas, landfill gas, or biofuel. In some embodiments, a multi-stage membrane system may be used to treat biogas from a biogas system to increase the methane concentration while also removing oxygen and carbon dioxide from the biogas, such that the oxygen and carbon dioxide in the purified biogas output from the membrane system are within a preselected range, wherein the oxygen concentration is less than or equal to 0.2% of the purified biogas output stream. In such embodiments, the preselected carbon dioxide concentration in the purified biogas output stream may be less than 2%. Preferably, embodiments of the system may be configured to avoid the use of PSA and also avoid the use of a catalytic system to remove oxygen from the purified biogas. Oxygen may also be referred to herein as O2, carbon dioxide may also be referred to herein as CO2, and methane may also be referred to herein as CH4.

[0007] For example, in some embodiments, the biogas purification system may include a multi-stage membrane system configured to remove oxygen, such that the biogas is purified to have a higher methane concentration at a preselected methane concentration (e.g., 94%, greater than or equal to 94%, 96%, greater than or equal to 96%, greater than 97%, greater than or equal to 97%, greater than or equal to 99.5%, etc.), a preselected oxygen concentration (e.g., less than 1%, less than 0.2%, or less than or equal to 0.2%), and a preselected carbon dioxide concentration (e.g., less than or equal to 5%, less than or equal to 2%, less than or equal to 0.5%, etc.). The concentration percentages discussed herein may be volume fractions (volume%) or mole fractions (mole%).

[0008] In some embodiments, the membrane system may include multiple stages, each stage comprising at least one membrane configured to repel O2 by utilizing low CO2 / O2 selectivity within a predetermined range (e.g., less than 5.0 or less than 4.5, etc.) to form a purified methane output stream. In some embodiments, the purified biogas output from the system may consist entirely of methane, CO2, and O2. In other embodiments, the biogas may consist almost entirely of these components along with small amounts (e.g., less than 5%-1%) of nitrogen and / or trace amounts (e.g., less than 0.2%-0.1%, etc.) of other components (e.g., hydrogen sulfide, etc.).

[0009] For example, some embodiments of a biogas purification system may include a multi-stage membrane system configured to remove oxygen and carbon dioxide, such that the biogas is purified to have a higher concentration of methane (e.g., at least 95% methane, at least 97% methane, at least 99.5% methane, etc.) and an oxygen level at or below a preselected oxygen concentration (e.g., less than 3%, less than 1%, at most 0.2%, or less than 0.2%) while maintaining a high level of methane recovery (e.g., greater than 90% methane recovery, greater than 95% methane recovery, methane recovery between 95% and 98%, methane recovery between 98% and 99.5%, etc.), where the recovery rate is defined as the ratio of methane in the product stream to methane in the feed stream. The biogas output from the purification system may also contain carbon dioxide within a preselected composition range (e.g., at most 5%, less than 2%, less than or equal to 2%, 0.5%, less than or equal to 0.5%, etc.). One or more membranes used in each stage of the membrane system can be configured to have a CO2 / O2 selectivity of less than 5 or less than 4.5.

[0010] In some embodiments, the membrane system can be configured such that the oxygen rejection factor (e.g., the ratio of the molar concentration of O2 in the feed to the molar concentration of O2 in the purified biogas output from the membrane system, mol%O2 feed / mol%O2 output) is within a predetermined range (e.g., greater than 5, greater than 1.0, greater than 0.75, between 1.0 and 1.25, between 0.75 and 1.25, in the range of 0.77 to 1.22, between 0.8 and 1.2, in the range of 0.7 to 1.3, in the range of 0.9 to 1.5, or in the range of 0.9 to 2.2, etc.).

[0011] In a first aspect, a process for purifying a biogas feed stream by removing oxygen (O2) and carbon dioxide (CO2) may include supplying the biogas feed stream to a first stage of a membrane system to generate a first-stage non-permeable stream and a first-stage permeable stream. The first-stage non-permeable stream may consist of methane, O2, and CO2, such that the first-stage non-permeable stream has a lower CO2 concentration than the first-stage permeable stream. The first-stage permeable stream may consist of methane, O2, and CO2, and may have a lower methane concentration than the first-stage non-permeable stream. The process may further include supplying the first-stage non-permeable stream to a second stage of the membrane system to generate a second-stage non-permeable stream and a second-stage permeable stream. The second-stage non-permeable stream may contain at least 90 mol% methane and less than 1 mol% O2. The CO2 content of the second-stage permeable stream may be greater than the CO2 content of the second-stage non-permeable stream.

[0012] Embodiments of the process may further include supplying a first-stage permeate stream to a third stage of the membrane system to generate a third-stage non-permeate stream and a third-stage permeate stream, and providing the third-stage non-permeate stream to form a recirculation stream for feeding the third-stage non-permeate stream to the first stage or a compressor system. For example, the recirculation may include mixing the third-stage non-permeate stream with the second-stage permeate stream to form the recirculation stream before feeding the recirculation stream to the compressor system to undergo compression to a preselected pressure. In some aspects, the process may also include mixing the recirculation stream with the biogas feed stream upstream of the membrane system before feeding the biogas feed stream to the compressor system.

[0013] One aspect of the process for purifying the biogas feed stream may further include supplying a first-stage permeate stream to a third stage of the membrane system to produce a third-stage non-permeate stream and a third-stage permeate stream. At least one membrane in the first stage may have a CO2 / O2 selectivity value less than 5.0, less than 4.5, less than 4.0, less than or equal to 3.5, between 3.0 and 5.0, or between 3.4 and 4.5, and at least one membrane in the third stage may have a CO2 / O2 selectivity value less than 5.0, less than 4.5, less than 4.0, less than or equal to 3.5, between 3.0 and 5.0, or between 3.4 and 4.5. In some embodiments, at least one membrane in the first stage may have a CO2 / O2 selectivity value less than 5.0, less than 4.5, less than 4.0, less than or equal to 3.5, between 3.0 and 5.0, or between 3.4 and 4.5. In some aspects of the process, each membrane in the first stage, each membrane in the second stage, and / or each membrane in the third stage has a CO2 / O2 selectivity value of less than 5, less than 4.5, between 3.0 and 5.0, or between 3.4 and 4.5.

[0014] Examples of this process can be implemented such that the oxygen reduction factor is greater than 1.0 or between 1.0 and 1.25. The oxygen reduction factor can be the ratio of the molar concentration of O2 in the biogas feed stream to the molar concentration of O2 in the second-stage non-permeable stream.

[0015] An embodiment of the process can be implemented such that the second-stage non-permeable stream is a purified biogas output stream having a methane content of at least 95 mol% methane (e.g., 95 mol%-100 mol%), at least 98 mol% methane (e.g., 98 mol%-100 mol%), or at least 99.5 mol% methane (e.g., 99.5 mol%-100 mol%).

[0016] In some embodiments of this process, the second-stage non-permeable stream has less than 0.2 mol% O2. In other embodiments of this process, the second-stage non-permeable stream has less than or equal to 0.2 mol% O2 and greater than or equal to 0.05 mol% O2. In still other embodiments of this process, the second-stage non-permeable stream has less than or equal to 0.2 mol% O2 and greater than or equal to 0.1 mol% O2. In such embodiments, the CO2 concentration of the second-stage non-permeable stream may be less than or equal to 2 mol% CO2, less than or equal to 5 mol% CO2, or within different preselected CO2 concentration ranges.

[0017] Embodiments of a biogas purification system are also provided. These embodiments can be arranged and configured to perform a process for purifying a biogas feed stream. The embodiments of the biogas purification system may include a membrane system configured to receive a biogas feed stream output from the biogas system. The membrane system may include a first stage positioned and configured to receive the biogas feed stream to produce a first-stage non-permeable stream and a first-stage permeable stream. The first-stage non-permeable stream may consist of methane, O2, and CO2, such that the first-stage non-permeable stream has a lower CO2 concentration than the first-stage permeable stream. The first-stage permeable stream may consist of methane, O2, and CO2, and has a lower methane concentration than the first-stage non-permeable stream. A second stage may be connected to the first stage to receive the first-stage non-permeable stream to produce a second-stage non-permeable stream and a second-stage permeable stream. The second-stage non-permeable stream may contain at least 90 mol% methane and less than 1 mol% O2. The CO2 content of the second-stage permeable stream may be greater than the CO2 content of the second-stage non-permeable stream.

[0018] Embodiments of the biogas purification system can be configured such that the membrane system further includes a third stage connected to the first stage to receive the first stage permeate flow and generate a third stage non-permeate flow and a third stage permeate flow. The third stage can be connected to a recirculation conduit such that the third stage non-permeate flow can reach an upstream location of the first stage of the membrane system via the recirculation conduit.

[0019] For example, some embodiments of a biogas purification system may include a compressor system located upstream of a first stage to compress the biogas feed stream before it is fed into the first stage. A third stage may be connected to a recirculation duct such that the third-stage non-permeable stream can reach the compressor system via the recirculation duct without using another compressor system. The recirculation duct is configured to mix the third-stage non-permeable stream with the second-stage permeable stream to form the recirculation stream before it is fed into the compressor system to undergo compression to a preselected pressure.

[0020] In embodiments of the biogas purification system, the second-stage non-permeable stream may have less than 0.2 mol% O2, or may have less than or equal to 0.2 mol% O2 and greater than or equal to 0.05 mol% O2. In other embodiments of the system, the second-stage non-permeable stream may have less than or equal to 0.2 mol% O2 and greater than or equal to 0.1 mol% O2, or have another preselected oxygen content within different ranges involving oxygen concentrations of less than 1 mol% O2 or less than or equal to 0.2 mol% O2. In such embodiments, the CO2 concentration of the second-stage non-permeable stream may be less than or equal to 2 mol% CO2, less than or equal to 5 mol% CO2, or within different preselected CO2 concentration ranges.

[0021] In some embodiments, at least one membrane in the first stage, at least one membrane in the second stage, and / or at least one membrane in the third stage have a CO2 / O2 selectivity value less than 5.0, less than 4.5, less than 4.0, less than or equal to 3.5, between 3.0 and 5.0, or between 3.4 and 4.5. For example, embodiments of a biogas purification system may be configured such that at least one membrane in the first stage has a CO2 / O2 selectivity value less than 5.0, less than 4.5, less than 4.0, less than or equal to 3.5, between 3.0 and 5.0, or between 3.4 and 4.5. As another example, each membrane in the first stage, each membrane in the second stage, and / or each membrane in the third stage may have a CO2 / O2 selectivity value less than 5.0, less than 4.5, less than 4.0, less than or equal to 3.5, between 3.0 and 5.0, or between 3.4 and 4.5.

[0022] Examples of biogas purification systems can be configured such that the oxygen reduction factor is greater than 1.0 or between 1.0 and 1.25. The oxygen reduction factor can be the ratio of the molar concentration of O2 in the biogas feed stream to the molar concentration of O2 in the second-stage non-permeable stream.

[0023] Embodiments of the biogas purification system can be configured such that the second-stage non-permeable flow is the purified biogas output flow, having a methane content of at least 95 mol% (e.g., 95 mol%-100 mol%), at least 98 mol% (e.g., 98 mol%-100 mol%), or at least 99.5 mol% (e.g., 99.5 mol%-100 mol%). Some embodiments can be designed to provide methane recovery rates greater than 95%, greater than 98%, or greater than 99%.

[0024] Embodiments of a biogas purification system may include a biogas system. The biogas system may include at least one digester (e.g., an anaerobic digester) forming a biogas feed stream and at least one biogas cleaning device positioned to treat the biogas feed stream before feeding it into a first stage of a membrane system. The biogas system may be positioned to feed the biogas feed stream into a compressor system to feed the compressed biogas feed stream into the first stage of the membrane system. The membrane system may also include second and third stages as discussed herein.

[0025] Further details, purposes, and advantages of biogas purification systems, membrane arrangements and equipment that can be used in such systems, and processes for manufacturing and using them will become apparent as some exemplary embodiments of the invention are described below. Attached Figure Description

[0026] Exemplary embodiments of biogas purification systems, membrane arrangements and equipment that can be used in such systems, and processes for manufacturing and using them are shown in the accompanying figures. It should be understood that the same reference numerals used in the figures can identify the same components.

[0027] Figure 1 This is a block diagram of a first exemplary embodiment of a biogas purification system.

[0028] Figure 2 This is a schematic diagram of a first exemplary embodiment of an exemplary multi-stage membrane system that can be used in a first exemplary embodiment of a biogas purification system. It should be understood that the compressor system 11 and the compressor feed cooler HX, shown in dashed lines, may be included in the embodiment of the membrane system 3, or may be separate components located between the biogas system 2 and the membrane system 3, or may be part of the biogas system 2 arranged to compress the biogas feed stream 2o before feeding it into the membrane system 3 and subsequently cool the compressed feed stream.

[0029] Figure 3 This is a graph illustrating the relationship between methane recovery level (x-axis, expressed as percentage %) and oxygen rejection factor (e.g., molar % O2 in feed / molar % O2 in biogas output) using systems with single-stage, two-stage, and three-stage membrane arrangements.

[0030] Figure 4 This is a graph illustrating the relationship between the oxygen rejection factor (molar % O2 in the feed / molar % O2 in the biogas output) and the CO2 / O2 selectivity when an embodiment of the exemplary biogas purification system 1 is used for purified biogas output stream 3o, the purified biogas output stream 3o having methane within a first predetermined methane concentration, CO2 within a first predetermined CO2 concentration of less than or equal to 0.5%, and O2 within a first predetermined O2 concentration of less than or equal to 0.2%. Detailed Implementation

[0031] refer to Figures 1 to 2 The biogas purification system 1 can be configured to receive a biogas feed stream 2o from the biogas system 2. The biogas system may include a digester 2a that processes farm waste, energy crops, or other biogas production materials (e.g., biodegradable waste, sewage sludge, food waste, etc.) to form the biogas feed stream 2o. For example, the digester 2a may be an anaerobic digester.

[0032] The biogas feed stream 2o may include methane and other components such as, for example, nitrogen, carbon dioxide, oxygen, siloxanes, and hydrogen sulfide. The biogas feed stream 2o may be fed into the membrane system 3 to improve the purity of the methane within the biogas, allowing the purified biogas output stream 3o to be fed into the pipeline 4 or other downstream devices (e.g., CHP). At least one feed conduit may be positioned to guide the biogas feed stream 2o from the biogas system 2 to the membrane system 3. The membrane system 3 may be a multi-stage membrane system 3.

[0033] In some embodiments, the biogas system 2 may be configured to purge the biogas from the digester 2a into the membrane system 3 to form a biogas feed stream 2o, such that the biogas feed stream 2o into the membrane system consists of or is substantially composed of methane, oxygen, and carbon dioxide, while also containing trace amounts of nitrogen (e.g., up to 5% nitrogen, up to 2% nitrogen, etc.) and trace amounts of other components (e.g., one or more of water, hydrogen sulfide, siloxanes, etc.). In such embodiments, the trace amounts of other components may be less than 0.5%, less than 0.2%, or less than 0.1% of the total composition of the biogas feed stream in some embodiments having such trace components.

[0034] For example, biogas output from digester 2a can be fed to at least one biogas cleaning device 2b, such that the biogas is cleaned to remove all or substantially all siloxanes, hydrogen sulfide, and / or other components to form a biogas feed stream 2o. The at least one biogas cleaning device 2b may include an arrangement of one or more biogas cleaning devices 2b (e.g., coalescing filters, one or more adsorption systems, or other biogas conditioning devices, etc.) positioned between membrane system 3 and digester 2a to clean the biogas before feeding the biogas feed stream 2o into membrane system 3, which is configured to increase the methane concentration of the biogas to a preselected higher concentration.

[0035] The biogas cleaning conduit can be positioned to feed biogas from digester 2a to at least one biogas cleaning device 2b and between different biogas cleaning devices 2b. A biogas feed conduit can exist between the downstream biogas cleaning device and the compressor system 11. The biogas membrane system feed conduit can be positioned to feed compressed biogas feed stream 11o from the compressor system 11 to the first stage 13 of the membrane system 3.

[0036] The biogas feed stream 2o output from the biogas system 2 may include methane at a first primary biogas feed methane concentration, carbon dioxide at a first primary biogas carbon dioxide feed concentration, and oxygen at a first primary biogas feed oxygen concentration. In some embodiments, the first primary biogas feed methane concentration may be in the range of 30%-80% methane (e.g., 40%, 50%, 60%, 70%, or 75% methane, etc.), the first primary biogas feed carbon dioxide concentration may be in the range of 20%-70% (e.g., 25%, 30%, 40%, 50%, 60%, 70%, etc.), and the first primary biogas feed oxygen concentration may be in the range of less than 0.2% to less than or equal to 2% (e.g., less than 0.3%, less than 0.4%, less than 1%, less than 1.5%, up to 2%, etc.). As mentioned above, it should be understood that the concentration percentages discussed herein may be volume fractions (volume%) or mole fractions (mole%).

[0037] As discussed above, the biogas feed stream 2o output from biogas system 2 can be fed into membrane system 3. Membrane system 3 can be a membrane arrangement constructed as a multi-stage membrane system configured to remove oxygen and carbon dioxide, such that the biogas is purified to form a purified biogas output stream 3o, which has a higher concentration of methane (CH4) and an oxygen (O2) level at or below a preselected O2 concentration and a carbon dioxide (CO2) concentration at or below a preselected CO2 concentration. For example, the purified biogas output may include CO2 within a preselected composition range (e.g., up to 5%, less than 2%, less than or equal to 2%, less than 1%, less than 0.5%, less than or equal to 0.5%, less than 0.1%, etc.) and O2 within a preselected composition range (e.g., up to 1%, less than 1%, up to 0.3%, less than 0.3%, up to 0.2%, or less than 0.2%, etc.). In some embodiments, the purified biogas output stream 3o may consist of or substantially consist of: CH4 within a preselected CH4 composition range (e.g., greater than or equal to 94% CH4, greater than or equal to 95% CH4, greater than or equal to 98% CH4, greater than or equal to 99.5% CH4, etc.), CO2 within a preselected composition range (e.g., up to 5%, less than 2%, less than or equal to 2%, 0.5%, less than or equal to 0.5%, etc.), and O2 within a preselected composition range (e.g., 0.2%, less than 0.2%, or less than or equal to 0.2%, etc.). As mentioned above, it should be understood that the concentration percentages discussed herein may be volume fractions (volume%) or mole fractions (mole%).

[0038] At least one membrane may be used in each stage of membrane system 3, which operates selectively for CO2 / O2 within a predetermined range (e.g., less than 5, less than 4.5, between 5.0 and 3.0, between 4.5 and 3.4, etc.). For example, each stage of membrane system 3 may include a single membrane or multiple membranes that may be arranged in series or in parallel. Membrane system 3 may be configured such that the oxygen rejection factor (e.g., the ratio of O2 concentration in biogas feed 2o to O2 concentration in purified biogas output 3o from the membrane system, %O2 feed / %O2 output) is within a predetermined range (e.g., greater than 5, greater than 1.0, greater than 0.75, between 0.75 and 1.25, in the range of 0.77 to 1.22, between 0.8 and 1.2, in the range of 0.7 to 1.3, in the range of 0.9 to 1.5, or in the range of 0.9 to 2.2, etc.).

[0039] from Figure 2 An example of membrane system 3 can be understood. It should be understood that... Figure 2 The compressor system 11 and the feed cooler HX, shown by the dashed lines, can be included in embodiments of the membrane system 3, and may be separate components located between the biogas system 2 and the membrane system 3, or part of the biogas system 2. Other components may also be present upstream of the membrane system 3. For example, a liquid removal device and a reheater may be positioned between the feed cooler HX and the membrane system 3 to remove liquid from the cooled feed before it is fed into the membrane system 3, and subsequently reheat the dried feed. These components may be part of the biogas system 2 or the membrane system 3.

[0040] When the compressor system 11 and the feed cooler HX are included in the biogas system 2, arranged between the biogas system 2 and the membrane system 3, or included in the membrane system 3, these components can be arranged such that the compressor system 11 compresses the biogas feed stream 2o before feeding the feed stream into the membrane system 3 and subsequently cools the compressed feed stream. A duct can be positioned between the compressor system 11 and the feed cooler HX and between the feed cooler HX and the first stage 13 of the membrane system 3 to guide the compressed biogas from the compressor system 11 to the first stage 13 of the membrane system 3 after it has been cooled to a desired preselected temperature within a preselected temperature range.

[0041] For example, compressor system 11 may be positioned to receive a biogas feed stream 2o from at least one biogas cleaning device 2b to compress the biogas feed stream 2o to a preselected pressure within a preselected pressure range, thereby outputting a compressed biogas feed stream 11o. The preselected pressure may be, for example, within the ranges of 14 barg (barometer pressure), 10 barg, 15 barg, or 10-15 barg, 10-16 barg, 5-15 barg, 5-20 barg, or 5-25 barg, and the preselected pressure range may be a pressure range of 10-15 bar, 5-15 bar, or 5-20 bar, or other preselected pressure ranges. In some embodiments, compression system 11 may be configured as a single-stage compressor, a multi-stage compressor, or a series of compressors.

[0042] The compressed biogas feed stream 11o can then be cooled to a preselected temperature within a preselected temperature range by passing it through at least one compressed feed cooler HX. The preselected temperature can be, for example, 20°C, 25°C, 30°C, or a temperature within the range of 15°C-25°C, 10°C-30°C, or 0°C-65°C, and the preselected temperature range can be the following temperature ranges: 15°C-25°C, 10°C-30°C, 5°C-40°C, 0°C-50°C, 0°C-65°C, or other preselected temperature ranges. In other embodiments, the feed cooler HX may not be used (e.g., embodiments where the compressed biogas feed stream 2o may be in a desired preselected temperature range without cooling or after cooling). In such embodiments, the compressed biogas feed stream 11o can be fed directly into the first stage 13 of the membrane system 3 via a compressed biogas feed duct.

[0043] The first stage 13 of the membrane system 3 may include at least one membrane vessel in which one or more membranes are held for fluid separation, the membrane vessel comprising a single membrane vessel or multiple membrane vessels arranged in parallel or series. The first stage 13 may be fluidly connected to the second stage 15 and the third stage 17 of the membrane system 3. Stage conduits may be positioned to connect the first stage 13 to the second stage 15 and the third stage 17 to guide fluid between these stages.

[0044] The second stage 15 may include at least one membrane vessel in which one or more membranes are held for fluid separation, the membrane vessel comprising a single membrane vessel or multiple membrane vessels arranged in parallel or series. A stage conduit may be positioned to connect the second stage 15 to the first stage 13 and the third stage 17 to guide fluid between these stages.

[0045] The third stage 17 may include at least one membrane vessel in which one or more membranes are held for fluid separation, the membrane vessel comprising a single membrane vessel or multiple membrane vessels arranged in parallel or series. A recirculation stage conduit may be positioned to connect the third stage 17 to the compressor system 11 (e.g., upstream of the compressor system 11 for feeding into the compressor system 11, to a specific compressor stage of the compressor system, etc.). An output conduit may also be connected to the third stage 17 to guide fluid to be discharged from the third stage as exhaust or to deliver the fluid to another unit within the plant for use therein (e.g., as a cooling or heating medium in a heat exchanger, etc.).

[0046] The first stage 13 may include at least one membrane designed to allow compressed biogas feed to enter the first stage 13 and contact the membrane structure within the container of the first stage. The first stage 13 of the membrane system 3 may include a single membrane or multiple membranes arranged in series or in parallel. Each membrane may be configured such that the CO2 component of the biogas feed passes through the membrane structure rapidly as a first stage permeate stream 13p or a first stage fast stream output, while the methane component of the biogas feed passes through the membrane structure at a significantly slower rate, allowing most of the CO2 to be separated from the methane in the biogas fed into the first stage 13, and allowing most of the methane in the biogas stream fed into the first stage 13 to be output as a first stage non-permeate stream 13np. The membranes of the first stage 13 may be positioned and configured such that O2 in the biogas fed into the first stage 13 also substantially passes through the membrane structure under the operating conditions of the first stage 13 (e.g., within the first stage permeate stream 13p, which also includes most of the CO2 from the biogas).

[0047] For example, the first-stage permeate gas stream 13p output from the first stage 13 may consist primarily of CO2 and may also contain some methane and some O2. For example, the first-stage permeate gas stream 13p may contain CO2 at a first-stage CO2 permeate gas stream concentration within the ranges of 60%-99%, 60%-95%, 60%-80%, 80%-90%, 70%-99%, 70%-97%, 75%-95%, 80%-95%, or 85%-95% CO2. The first-stage permeate gas stream 13p may include a first-stage O2 permeate gas stream concentration within the ranges of 0.2%-6%, 0.1%-5%, 0.2%-4%, 0.2%-2%, 0.2%-0.4%, or 0.3%-0.35% O2. The first-stage permeate gas flow 13p may also include a first-stage methane permeate gas flow concentration as the remainder of the permeate gas flow. For example, the first-stage permeate gas flow 13p may also include a first-stage methane permeate gas flow concentration in the ranges of 1%-40% CH4, 3%-30% CH4, 5%-25% CH4, 5%-20% CH4, 5%-40% CH4, 10%-20% CH4, 5%-15% CH4, or 5%-30% CH4. As previously mentioned, it should be understood that these exemplary concentration percentages discussed herein can be volume fractions (volume %) or mole fractions (mole %). The first-stage permeate pressure can be maintained at a feed-to-permeate pressure ratio in the range of 1.5 to 15, 1.5 to 10, 2 to 10, or 3 to 7. The feed-to-permeate pressure ratio can be the feed pressure (P... f ) / Osmotic pressure (P) p The ratio P) f / P p , where P f and P p These are the feed pressure and osmotic pressure on either side of the membrane, expressed as absolute pressure rather than gauge pressure (e.g., P in stage 13). f / P p The ratio can be [pressure of compressed biogas feed stream 11o] / [pressure of first-stage infiltration flow stream 13p]).

[0048] The first-stage non-permeable gas stream 13np may consist primarily of methane and may also include some CO2 and some O2. For example, the first-stage non-permeable gas stream 13np may include methane at a concentration of 50%-95% CH4, 80%-90% CH4, 60%-80% CH4, 60%-90% CH4, 65%-90% CH4, 70%-85% CH4, 75%-85% CH4, or 70%-95% CH4. The first-stage non-permeable gas flow 13np may include a first-stage O2 non-permeable gas flow concentration within the ranges of 0.2%-5%, 0.1%-5%, 0.1%-4%, 0.2%-4%, 0.2%-2%, 0.1%-0.4%, 0.2%-0.5%, 0.3%-0.35%, or 0.2%-0.4% O2. The first-stage non-permeable gas flow 13np may also include a first-stage CO2 non-permeable gas flow concentration as the remaining portion of the non-permeable gas flow. For example, the first-stage non-permeable gas flow 13np may also include a first-stage CO2 non-permeable gas flow concentration within the ranges of 5%-50% CO2, 5%-40% CO2, 10%-40% CO2, 10%-35% CO2, 15%-20% CO2, 0%-25% CO2, 10%-30% CO2, 10%-20% CO2, or 15%-30% CO2. As previously stated, it should be understood that these exemplary concentration percentages discussed herein can be volume fractions (volume%) or mole fractions (mole%). The pressure of the first-stage non-permeable gas flow 13np output from the first stage 13 may be nearly identical to the feed pressure of the feed into the first stage 13 (e.g., the pressure of the compressed biogas feed flow 11o).

[0049] The first-stage non-permeable flow 13np can be fed into the second stage 15 of the membrane system 3 via a first-stage non-permeable flow conduit positioned between the first and second stages. The second stage 15 may include at least one membrane (e.g., it may include a single membrane or multiple membranes arranged in series or parallel). The membrane of the second stage 15 can be configured such that the first-stage non-permeable flow 13np enters the second stage 15 and contacts the membrane structure within the container of the second stage 15. The membrane can be configured such that the CO2 component of the first-stage non-permeable flow 13p passes rapidly through the membrane structure as the second-stage permeable flow 15p or a rapid flow output, while the methane component of the first-stage non-permeable flow 13np fed into the second stage 15 passes through the membrane structure at a significantly slower rate, such that most of the CO2 can be separated from the methane in the gas fed into the second stage 15, and most of the methane in the first-stage non-permeable flow 13np fed into the second stage 15 can be output as the second-stage non-permeable flow 15o.

[0050] One or more membranes in the second stage 15 can be configured such that O2 fed into the first stage non-permeable flow 13np of the second stage 15 also passes through the membrane structure (e.g., in a "fast gas" flow, or in a second stage permeable flow 15p that also includes most of the CO2 in the first stage non-permeable flow 13np). The second stage permeation pressure can be maintained at a feed-to-permeation pressure ratio in the range of 1.5 to 100, 1.5 to 50, 1.5 to 30, or 5 to 20. As mentioned above, the feed-to-permeation pressure ratio can be the feed pressure (P... f ) / Osmotic pressure (P) p The ratio P) f / P p , where P f and P p These are the feed pressure and osmotic pressure on either side of the membrane, expressed as absolute pressure rather than gauge pressure (e.g., P in stage 2, 15). f / P p The ratio could be [pressure of 13np for the first stage non-permeable flow] / [pressure of 15p for the second stage permeable flow]).

[0051] The second-stage permeate gas stream 15p output from the second stage 15 may consist primarily of CO2, but may also include some methane and O2. For example, the second-stage permeate gas stream 15p may include CO2 at a second-stage CO2 permeate gas stream concentration of 30%-80%, 40%-70%, 45%-65%, 50%-85%, 55%-75%, or 60%-80%. The second-stage permeate gas stream 15p may include a second-stage O2 permeate gas stream concentration in the range of up to 6%, up to 5%, 0.2-1.0%, 0.2-5.0%, 0.5-3.0%, or 0.45%-0.85%. The second-stage permeate gas stream 15p may also include a second-stage methane permeate gas stream concentration as the remainder of the permeate gas stream. For example, the second-stage permeate gas stream 15p may also include a second-stage methane permeate gas stream concentration within the ranges of 20%-70% CH4, 30%-60% CH4, 35%-55% CH4, 15%-50% CH4, 20%-40% CH4, 25%-45% CH4, or 15%-20% CH4. As previously mentioned, it should be understood that these exemplary concentration percentages discussed herein may be volume fractions (volume %) or mole fractions (mole %). The second-stage permeate gas stream 15p may be output from the second stage 15 such that it may be included in a recirculation stream 19, which is recirculated back to the compressor system 11 to be combined with the biogas feed stream 2o and returned through the membrane system 3 to enhance the recovery of methane output from the second stage 15. The second-stage permeate gas stream 15p may be recovered at pressures above, near, or below atmospheric pressure and may be recovered via a vacuum pump or compressor.

[0052] The second-stage non-permeable gas stream 15o output from the second stage 15 may consist primarily of methane and may also contain some CO2 and O2. For example, the second-stage non-permeable gas stream 15o may contain methane at a second-stage methane non-permeable gas stream concentration of 90%-99.8%, 95%-99.5%, 95%-99.8%, or 97%-99.8%. As other examples, the second-stage non-permeable gas stream 15o may contain methane at a second-stage methane non-permeable gas stream concentration greater than 94%, greater than 95%, greater than 98%, greater than 99%, or greater than 99.5%. The second-stage non-permeable gas stream 15o may include a maximum of 1% O2 (e.g., O2 between 0% and 1.0%) or a second-stage O2 non-permeable gas stream concentration in the range of less than or equal to 0.2% O2, 0.1-0.2% O2, 0.05%-0.20% O2, or less than 1% O2. The second-stage non-permeable gas flow 15o may also include a second-stage CO2 non-permeable gas flow concentration as the remainder of the non-permeable gas flow 15o. For example, the second-stage non-permeable gas flow 15o may also include a second-stage CO2 non-permeable gas flow concentration in the range of up to 10% CO2, up to 5% CO2, up to 2% CO2, less than 2% CO2, less than 1% CO2, up to 0.5% CO2, less than 0.5% CO2, less than 0.1% CO2, or up to 0.1% CO2. As previously mentioned, it should be understood that these exemplary concentration percentages discussed herein may be volume fractions (volume%) or mole fractions (mole%). The non-permeable gas flow 15o output from the second stage 15 may be substantially the same as the feed pressure of the feed flow into the second stage 15 (e.g., the pressure of the first-stage non-permeable flow 13np fed into the second stage 15).

[0053] The second-stage non-permeable gas flow 15o can be a membrane system output flow 3o, which provides a product gas consisting primarily of methane and suitable for injection into a methane gas pipeline 4 (e.g., a natural gas pipeline). The second-stage non-permeable gas flow 15o can also be a membrane system output flow 3o, which may also be used to output to CHP and / or methane storage containers for subsequent transportation, sale, and / or use.

[0054] The third stage 17 of membrane system 3 can receive the permeate flow 13p output from the first stage 13 via a first-stage permeate feed conduit positioned between the first stage 13 and the third stage 17. Feed of the first-stage permeate flow 13p to the third stage 17 can be provided such that the first-stage permeate flow does not undergo any compression or other pressure increase before being fed into the third stage 17. The third-stage permeate pressure can be maintained at a feed-to-permeate pressure ratio within the range of 1.5 to 15, 1.5 to 10, 2 to 10, or 3 to 7. As mentioned above, the permeate pressure ratio can be the feed pressure (P...f ) / Osmotic pressure (P) p The ratio P) f / P p , where P f and P p These are the feed pressure and osmotic pressure on either side of the membrane, expressed as absolute pressure rather than gauge pressure (e.g., P in stage 3, 17). f / P p The ratio can be [pressure of the first-stage permeate flow at 13p] / [pressure of the third-stage permeate flow at 17o]).

[0055] The third stage 17 may include at least one membrane (e.g., the third stage 17 may include a single membrane or multiple membranes arranged in series or in parallel). At least one membrane in the third stage 17 may be configured such that the first stage permeate stream 13p enters the third stage 17 and contacts the membrane structure within the container of the third stage 17. The membrane may be configured such that the CO2 component of the first stage permeate stream 13p passes rapidly through the membrane structure as the third stage permeate stream 17o or a rapid gas output, while the methane component of the first stage permeate stream 13p fed into the third stage 17 passes through the membrane structure at a significantly slower rate, such that most of the CO2 can be separated from the methane in the gas fed into the third stage 17, and most of the methane in the first stage permeate stream 13p fed into the third stage 17 can be output as the third stage non-permeate stream 17np, thereby being recycled back to the compressor system 11 by incorporating it into the recirculation stream 19 and feeding it into the compressor system 11 via the recirculation conduit 19a. The recirculation stream 19 can be mixed with the biogas feed stream 2o before undergoing compression, or it can be fed separately into the compressor system 11 via the compressor recirculation feed stream 19b of the recirculation stream duct 19a, as shown below. Figure 2 As shown by the dashed line.

[0056] In some embodiments, the second-stage permeate flow 15p may be mixed with the third-stage non-permeate flow 17np to form a recirculation flow 19, which is recirculated to the compressor system 11 via a recirculation duct 19a. In other embodiments, these flows may be individually recirculated back to the compressor system 11 via separate recirculation flow ducts (e.g., directly to the compressor system 11 or recirculated to a location upstream of the compressor system for feeding into the compressor system 11).

[0057] The membrane of the third stage 17 can be configured such that O2 fed into the first stage permeate stream 13p of the third stage 15 also passes through the membrane structure (e.g., in the "fast gas" stream or in the third stage permeate stream 17o, which also includes most of the CO2 in the first stage permeate stream 13p).

[0058] The third-stage permeate gas stream 17o output from the third stage 17 may consist primarily of CO2, and may also include some methane and O2. For example, the third-stage permeate gas stream 17o may contain CO2 at a third-stage CO2 permeate gas stream concentration of 85%-99.9%, 85%-99.5%, 90%-99.9%, 95%-99.5%, or 95%-99.9%. The third-stage permeate gas stream 17o may include a third-stage O2 permeate gas stream concentration in the range of less than or equal to 5%, less than or equal to 4%, less than or equal to 2%, 0.1%-0.4%, or 0.1%-1.0% O2. The third-stage permeate gas stream 17o may also include a third-stage methane permeate gas stream concentration as a remainder of the permeate gas stream fed into the third stage 17. For example, the third-stage permeate gas flow 17o may also include a third-stage methane permeate gas flow concentration in the range of less than or equal to 15% CH4, less than or equal to 6% CH4, less than or equal to 5% CH4, or less than or equal to 1.0% CH4. As previously stated, it should be understood that these exemplary concentration percentages discussed herein may be volume fractions (volume%) or mole fractions (mole%). The third-stage permeate gas flow 17o can be recovered at pressures above, near, or below atmospheric pressure and can be pumped by a vacuum pump or compressor.

[0059] The third-stage permeate gas flow 17o can be output from the third stage 17, allowing it to be released into the environment (e.g., as exhaust gas). In other embodiments, the third-stage permeate gas flow 17o can be fed into a carbon capture system or output to provide a CO2 gas source or process flow for another purpose in the plant (e.g., as flue gas, as a process flow for one or more heat exchangers before being output from the plant or stored in a container, etc.).

[0060] The third-stage non-permeable gas stream 17np output from the third stage 17 may consist primarily of methane and may also contain some CO2 and O2. For example, the third-stage non-permeable gas stream 17np may contain methane at a third-stage methane non-permeable gas stream concentration of 25%-95% CH4, 30%-90% CH4, 30%-80% CH4, or 30%-50% CH4. The third-stage non-permeable gas stream 17np may include a third-stage O2 non-permeable gas stream concentration in the ranges of 0.2%-7% O2, 0.5%-5% O2, 0.5%-4% O2, 0.7%-3% O2, up to 7% O2, up to 5% O2, less than or equal to 2% O2, 0.2%-2.0% O2, 0.4%-1.0% O2, 0.5%-0.9% O2, or 0.4%-0.9% O2. The third-stage non-permeable gas flow 17np may also include a third-stage CO2 non-permeable gas flow concentration as the remainder of the non-permeable gas flow 17np. For example, the third-stage non-permeable gas flow 17np may also include a third-stage CO2 non-permeable gas flow concentration in the ranges of 5%-75% CO2, 5%-70% CO2, 10%-70% CO2, 20%-70% CO2, 50%-70% CO2, 5%-50% CO2, or 5%-15% CO2. As previously mentioned, it should be understood that these exemplary concentration percentages discussed herein may be volume fractions (volume%) or mole fractions (mole%). The pressure of the third-stage non-permeable gas flow 17np output from the third stage may be nearly the same as the feed pressure into the third stage 17 (e.g., the pressure of the first-stage permeable gas flow 13p fed into the third stage).

[0061] In embodiments where nitrogen is present in a small amount of feed, the methane concentrations of different permeate and nonpermeate streams in different stages of the membrane system (e.g., stage 13, stage 15, and / or stage 17) can be determined in such a way that it also includes the nitrogen concentration within the determined methane concentration of the stream, which may exaggerate the methane concentration value (e.g., when a small amount of nitrogen is present in the feed, in embodiments where the nitrogen and methane concentrations are combined as a methane concentration due to the presence of nitrogen, the methane concentration in different membrane output streams can be exaggerated by up to 10%). This approach can be utilized in some embodiments because nitrogen is present in relatively small amounts and, due to its properties, acts similarly to methane when contacting the membrane structure (e.g., neither nitrogen nor methane readily permeates the membrane structure). When this approach is used, the presence of nitrogen slightly affects some methane concentration, causing that concentration to be exaggerated because the nitrogen concentration is included within the determined methane concentration. In other embodiments, the methane concentration can be determined without combining any nitrogen in the stream with the methane, thus eliminating this slight exaggeration of the methane concentration. While the presence of nitrogen may slightly overstate some methane concentration values ​​in some embodiments, this has no effect on any calculations of methane recovery rate using the embodiment utilizing membrane system 3.

[0062] An embodiment of membrane system 3 can be configured to recover 99.5%-90% of the methane fed into membrane system 3 via biogas system 2. The oxygen reduction factor generated by the embodiment of membrane system 3 can be greater than 5.0, greater than 1.0, greater than 0.75, between 0.9 and 1.5, or between 0.9 and 2.2. The oxygen rejection factor can be defined as the ratio of the molar concentration of oxygen in the biogas feed stream 2o to the molar concentration of oxygen in the second-stage non-permeable gas stream 15o, which can be the purified biogas output stream 3o (e.g., [molar concentration of oxygen in biogas feed stream 2o] / [molar concentration of oxygen in second-stage non-permeable gas stream 15o]).

[0063] The membranes of the first, second, and third stages 13, 15, and 17 of membrane system 3 can each be configured to have a relatively low CO2 preselectivity relative to O2 (also known as CO2 / O2 selectivity). This relatively low preselectivity CO2 / O2 selectivity can be achieved by selecting membrane materials having said selectivity characteristics. For example, at a temperature of 30°C, the selected membrane's CO2 preselectivity relative to O2 can be less than 5, less than 4.5, between 3.0 and 5.0, or between 3.4 and 4.5. A relatively low preselectivity CO2 / O2 selectivity can also be achieved by selecting membrane materials with a selectivity greater than 4.5 at 30°C, but operating the membrane system at higher temperatures above 30°C, where the membrane selectivity will decrease to below 4.5 at higher temperatures (e.g., between 3.4 and 4.5, less than or equal to 3.5, etc.). Relatively low preselective CO2 / O2 selectivity can also be achieved by selecting membrane materials with a selectivity of less than 3.0 at 30°C, but operating the membrane system at lower temperatures below 30°C, where the membrane selectivity will increase to a number between 3.0 and 5.0 or between 3.4 and 4.5 at the lower temperatures.

[0064] The membranes in stages 13, 15, and 17 of the first, second, and third stages can also utilize relatively low CO2 / CH4 selectivity values ​​relative to methane (also known as CO2 / CH4 selectivity values). For example, the embodiments may utilize CO2 / CH4 selectivity values ​​between 25 and 45, between 30 and 40, or less than 45 and greater than 16. For example, the CO2 / CH4 selectivity value of the membrane may be a value corresponding to the CO2 / O2 selectivity value, which is less than 5, less than 4.5, between 3.0 and 5.0, or between 3.4 and 4.5.

[0065] Some embodiments can be used such that all membranes in the first, second, and / or third stages of membrane system 3 utilize such CO2 / O2 selectivity and CO2 / CH4 selectivity values. Other embodiments can be designed and utilized such that the first stage 13 and the third stage 17 utilize membranes having such CO2 / O2 selectivity and CO2 / CH4 selectivity values, while the second stage 15 can utilize membranes with different selectivity values ​​(e.g., much higher CO2 / O2 selectivity values). Other embodiments can also be configured such that only each membrane in the second stage 15, only in the third stage 17, or only in the first stage 13 has such CO2 / O2 selectivity values. Other embodiments can also be provided such that only some membranes in some of these stages utilize such CO2 / O2 selectivity values. Surprisingly, it has been found that using low CO2 selectivity relative to O2 provides improved production results, as it is generally desirable to utilize high CO2 selectivity relative to O2 to obtain sufficiently high concentrations of methane in the output product gas.

[0066] Traditionally, the removal of small or trace amounts of gases, such as oxygen in this case, is accomplished by using highly permeable membranes that permeate CO2 and O2 at very high rates, but have low selectivity for slower-moving gases such as methane and nitrogen. This conventional approach typically targets the highest possible CO2 / CH4 selectivity, which results in the use of very high CO2 / O2 selectivity because CO2 / CH4 and CO2 / O2 selectivity cannot be controlled independently.

[0067] Compared to this conventional method, the membranes used in the embodiments of the biogas purification systems described herein, the membrane arrangements and equipment used in such systems, and the methods for manufacturing and using them, surprisingly and unexpectedly balance a sufficiently high CO2 / CH4 selectivity to enrich biomethane while the circulation flow rate is not too high but low enough that the CO2 / O2 selectivity is not too high, allowing the multi-stage process to adequately exclude both O2 and CO2, thus allowing for a relatively low oxygen concentration in the enriched methane output from the multi-stage membrane system 3, while also providing a high methane recovery rate.

[0068] The membranes that can be used in the first, second, and third stages 13, 15, and 17 can be manufactured as hollow fibers and packaged as membrane bundles or wound membrane tubes, or as flat sheets packaged as spiral wound or plate-and-frame units. For example, some preferred embodiments can utilize hollow fiber membrane units containing hollow fiber membranes made of polymers. Examples of polymers used to manufacture membranes that provide preselective CO2 / O2 selectivity for a preselective operating temperature include, but are not limited to, polystyrene, polysulfone, polyethersulfone, polyvinylidene fluoride, polyvinylidene fluoride, polyetheretherketone, polycarbonate, polyphenylene ether, polyethylene, polypropylene, cellulose acetate, polyimide (such as Matrimid 5218 or P-84), polyamide, polyvinyl alcohol, polyvinyl acetate, polyethylene oxide, polydimethylsiloxane, copolymers, block copolymers, or polymer blends. The hollow fiber membrane can be asymmetrical with a non-porous layer, or it can contain a porous carrier with a non-porous coating. The coating can be applied to the inner or outer surface of the hollow fiber.

[0069] In some embodiments of the membrane stage, multiple hollow fiber membranes can form a membrane bundle, which can be held together at one or both ends by a tube sheet made of a thermoplastic or thermosetting material. Examples of tube sheet materials include cured epoxy resins or polyurethane-based formulations. In some embodiments, the hollow fiber membrane bundle can be housed in a container with a diameter ranging from 25.4 mm to 254 mm (1 inch to 10 inches) and a length ranging from 305 mm to 3050 mm (1 foot to 10 feet). The container can be made of plastic, metal, or other suitable materials. The container can have at least one fluid port in fluid flow communication with the shell side of the hollow fiber membrane. The container can also have at least one port in fluid flow communication with the pore side of the hollow fiber membrane, and can have another port in fluid flow communication with the opposite end of the hollow fiber membrane. The tube sheet can be used to separate the pore-side fluid from the shell-side fluid. Feed gas can be introduced into the pore side or shell side of the membrane.

[0070] Figure 3 Simulation results illustrate how embodiments of membrane system 3, utilizing one-stage, two-stage, and three-stage examples of the membrane system, are expected to perform when operating at a pressure of 14 barg, a temperature of 20°C, a flow rate of 1000 nmch (standard cubic meters per hour), and a biogas feed composition of 59.88 mol% CH4, 39.93 mol% CO2, and 0.2% O2. This membrane system has a membrane with low CO2 selectivity relative to O2. Figure 3 As shown, for both one-stage and two-stage membrane systems, there is a significant balance between achieving high recovery rates and high oxygen reduction, with optimal methane recovery often expected when the O2 reduction factor is below approximately 1.2.

[0071] Surprisingly, however, it was found that oxygen reduction could be improved when using membrane materials with reduced CO2 / O2 selectivity, with only a modest sacrifice in recovery. The lower selectivity resulted in more oxygen permeating with CO2 into each stage of the membrane system 3, which helped to facilitate the removal of oxygen from the methane in the purified biogas output stream 3o.

[0072] Table 1 below illustrates the properties of [the product / service] at a reference temperature of 30°C. Figure 2 Examples of membrane system 3 with configurations similar to those shown in the exemplary embodiments are demonstrated using membranes with different CO2 / O2 selectivity values ​​ranging from 5 to 3.4 in the first, second, and third stages 13, 15, and 17. Table 1 below provides simulation results for an embodiment of membrane system 3, operating the system to obtain a second non-permeable flow 15° containing 0.5 mol% CO2, a methane recovery rate of 99.5%, and operating at 14 barg and 20°C using a membrane made of polymeric hollow fibers with the listed variable CO2 / O2 selectivity values:

[0073] Table 1

[0074]

[0075]

[0076] As can be understood from the above, it has been found that using a membrane with a low selectivity value provides an oxygen concentration in the second-stage non-permeable gas stream 15o (which could be a purified biogas output stream 3o) at or below 0.2 mol%. In the examples above, using such a selectivity of less than or equal to 4.0 or less than or equal to 3.5 can provide such a reduced oxygen level. Such a result could allow output stream 3o to meet highly stringent methane pipeline oxygen specifications, which may require oxygen at or below 0.2 mol% to help avoid hazardous conditions (e.g., accelerated corrosion of pipeline materials).

[0077] Figure 4 The results of the above simulation were also explained. Figure 4 This illustrates how the CO2 / O2 selectivity values ​​of the membranes in stages 13, 15, and 17 of the first, second, and third stages affect the oxygen rejection factor for obtaining the second-stage non-permeable gas flow 15° listed in Table 1. Surprisingly, it was found that using lower CO2 / O2 selectivity values ​​of the membranes provides a higher oxygen rejection factor while still producing a high-methane purity product suitable for meeting highly stringent pipeline oxygen content specifications and providing high recovery rates. From Figure 4 It is understood that in the embodiments of the membrane system, an oxygen rejection factor of up to 1.2-1.0 can be generated while still providing a highly purified methane content in the purified biogas output stream 3o and providing a methane recovery rate of 99.5% (e.g., 99.5% of the methane in the feed into the membrane system 3 is recovered in the product stream output by the system).

[0078] This is a surprising and unexpected result. Traditionally, it has been believed that low oxygen levels in the output stream cannot be achieved using membranes without significantly reducing methane recovery (see, for example, paragraph 9 of U.S. Patent Application Publication 2011 / 0094378). Contrary to this conventional view, embodiments of the biogas purification system 1 and / or membrane system 3 can provide high levels of methane recovery (e.g., 99.5%) while also providing very low oxygen concentrations in the purified biogas output stream 3o.

[0079] Embodiments of the biogas purification system 1 and / or membrane system 3 can be configured to provide process flexibility. If oxygen is found to be concentrated to undesirable levels in the purified biogas output stream 3o or to otherwise deplete methane within the biogas output stream 3o, this situation can be addressed by operating the membrane system 3 to provide improved oxygen rejection within the membrane stage of the membrane system 3 to remove more oxygen. For example, more oxygen can be output as a third-stage permeate stream 17o. This operational flexibility can be enhanced by utilizing membranes with low CO2 / O2 selectivity values, as discussed herein, within the stages of the membrane system. Furthermore, embodiments can be provided that can provide purified biogas output stream 3o without the need for a PSA system and / or a catalytic system to further remove oxygen from the stream before it is fed into the pipeline 4.

[0080] It should be understood that modifications can be made to the embodiments explicitly shown and discussed herein to meet a specific set of design objectives or a specific set of design criteria. For example, the arrangement of valves, pipes, and other conduit elements (e.g., conduit connections, tubing, seals, etc.) for flow communication between different units of the biogas purification system and / or membrane system for fluid flow between the different units can be arranged to meet a specific plant layout design that takes into account the available area of ​​the plant, the size of the plant's equipment, and other design considerations. As another example, the flow rate, pressure, and temperature of fluids through different stages of the membrane system and through other system elements can be varied to account for different plant design configurations and other design criteria. As yet another example, the number of membrane stages and their arrangement can be adjusted to meet a specific set of design criteria. As yet another example, the material composition of the different structural components used for the membrane, compressor system, and biogas system can be any type of suitable material that may be required to meet a specific set of design criteria.

[0081] It should be understood that embodiments of the biogas purification system 1 or membrane system 3 can be configured as a stand-alone plant or incorporated into another type of plant (e.g., a biogas plant with at least one biogas system 2), wherein at least one membrane system 3 can be utilized. The plant biogas purification system 1 and membrane system 3 can each be configured to include process control elements positioned and configured to monitor and control operations (e.g., temperature and pressure sensors, flow sensors, automated process control systems having at least one workstation including a processor, non-transient memory, and at least one transceiver for communication with the sensor elements, valves, and controllers, etc., for providing a user interface for the automated process control system that can operate at the workstation and / or another computer device in the plant).

[0082] As another example, specific features described individually or as part of an embodiment are intended to be combined with other separately described features or portions of other embodiments. Therefore, elements and actions of the various embodiments described herein can be combined to provide further embodiments. Thus, while certain exemplary embodiments of biogas purification systems, membrane arrangements and devices suitable for such systems, and methods of manufacturing and using them have been shown and described above, it should be clearly understood that the invention is not limited thereto, but may be embodied and practiced differently in other ways within the scope of the following claims.

Claims

1. A process for purifying a biogas feed stream by removing oxygen (O2) and carbon dioxide (CO2), the process comprising: The biogas feed stream is provided to the first stage of the membrane system to generate a first stage non-permeable stream and a first stage permeable stream. The biogas feed stream contains 30 mol% to 80 mol% methane, 20 mol% to 70 mol% CO2, and 0.2 mol% to 2 mol% O2. The first stage non-permeable stream consists of methane, O2, and CO2, such that the first stage non-permeable stream has a lower CO2 concentration than the first stage permeable stream. The first stage permeable stream consists of methane, O2, and CO2, and has a lower methane concentration than the first stage non-permeable stream. as well as The first-stage non-permeable flow is supplied to the second stage of the membrane system to generate a second-stage non-permeable flow and a second-stage permeable flow. The second-stage non-permeable flow contains at least 90 mol% methane and less than 1 mol% O2, and the second-stage permeable flow has a higher CO2 content than the second-stage non-permeable flow. At least one membrane in the first stage has a CO2 / O2 selectivity value between 3.4 and 4.

0.

2. The process according to claim 1, comprising: The first-stage permeate flow is supplied to the third stage of the membrane system to generate a third-stage non-permeate flow and a third-stage permeate flow. as well as The third-stage non-permeable flow is provided to form a recirculation flow for feeding the third-stage non-permeable flow into the first-stage or compressor system.

3. The process according to claim 2, wherein the recycling comprises: The third-stage non-permeable flow is mixed with the second-stage permeable flow to form the recirculation flow before the recirculation flow is fed into the compressor system to undergo compression to a preselected pressure.

4. The process according to claim 3, comprising: The recirculated stream is mixed with the biogas feed stream before the biogas feed stream is fed into the compressor system, which is upstream of the membrane system.

5. The process according to claim 1, comprising: The first-stage permeate flow is supplied to the third stage of the membrane system to generate a third-stage non-permeate flow and a third-stage permeate flow. At least one membrane in the first stage has a CO2 / O2 selectivity value between 3.4 and 4.0; and The third stage membrane has a CO2 / O2 selectivity value between 3.4 and 4.

0.

6. The process according to claim 2, wherein each membrane of the first stage, each membrane of the second stage, and / or each membrane of the third stage has a CO2 / O2 selectivity value of less than 5, less than 4.5, between 3.0 and 5.0, or between 3.4 and 4.

5.

7. The process according to claim 1, wherein the process is performed such that the oxygen reduction factor is greater than 1.0 or between 1.0 and 1.25, the oxygen reduction factor being the ratio of the molar concentration of O2 in the biogas feed stream to the molar concentration of O2 in the second-stage non-permeable stream.

8. The process according to claim 1, wherein the second-stage non-permeable stream is a purified biogas output stream having a methane content of at least 95 mol% methane.

9. The process according to claim 1, wherein the second-stage non-permeable flow has less than or equal to 0.2 mol% O2.

10. A biogas purification system, comprising: A membrane system configured to receive a biogas feed stream output from a biogas system, the membrane system comprising: The first stage is positioned and configured to receive the biogas feed stream to produce a first-stage non-permeable stream and a first-stage permeable stream, the biogas feed stream comprising 30 mol% to 80 mol% methane, 20 mol% to 70 mol% CO2, and 0.2 mol% to 2 mol% O2, the first-stage non-permeable stream consisting of methane, O2, and CO2, such that the first-stage non-permeable stream has a lower CO2 concentration than the first-stage permeable stream, and the first-stage permeable stream consisting of methane, O2, and CO2, and having a lower methane concentration than the first-stage non-permeable stream; and The second stage is connected to the first stage to receive the non-permeable flow from the first stage, thereby generating a second-stage non-permeable flow and a second-stage permeable flow. The second-stage non-permeable flow contains at least 90 mol% methane and less than 1 mol% O2, and the CO2 content of the second-stage permeable flow is greater than that of the second-stage non-permeable flow. At least one membrane in the first stage has a CO2 / O2 selectivity value between 3.4 and 4.

0.

11. The biogas purification system according to claim 10, comprising: The third stage is connected to the first stage to receive the first stage permeate flow and generate a third stage non-permeate flow and a third stage permeate flow. The third stage is connected to a recirculation conduit so that the third stage non-permeate flow can reach a location upstream of the first stage of the membrane system through the recirculation conduit.

12. The biogas purification system according to claim 11, comprising: A compressor system is positioned upstream of the first stage to compress the biogas feed stream before it is fed into the first stage. The third stage is connected to the recirculation duct such that the non-permeable flow of the third stage can reach the compressor system through the recirculation duct without using another compressor system.

13. The biogas purification system of claim 12, wherein the recirculation conduit is configured to mix the third-stage non-permeable flow with the second-stage permeable flow to form a recirculation flow before feeding the recirculation flow into the compressor system to undergo compression to a preselected pressure.

14. The biogas purification system according to claim 10, wherein the second stage non-permeable flow has less than or equal to 0.2 mol% O2.

15. The biogas purification system according to claim 11, wherein: At least one membrane in the first stage, at least one membrane in the second stage, and / or at least one membrane in the third stage have a CO2 / O2 selectivity value between 3.4 and 4.0; or Each membrane in the first stage, each membrane in the second stage, and / or each membrane in the third stage has a CO2 / O2 selectivity value between 3.4 and 4.

0.

16. The biogas purification system of claim 10, wherein the biogas purification system is configured such that the oxygen reduction factor is greater than 1.0 or between 1.0 and 1.25, the oxygen reduction factor being the ratio of the molar concentration of O2 in the biogas feed stream to the molar concentration of O2 in the second-stage non-permeable stream.

17. The biogas purification system according to claim 10, wherein the second-stage non-permeable flow is a purified biogas output flow having a methane content of at least 95 mol% methane.

18. The biogas purification system according to claim 10, comprising: The biogas system includes at least one digester forming the biogas feed stream and at least one biogas cleaning device positioned to process the biogas feed stream before feeding it into the first stage of the membrane system.

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