A biogas membrane separation purification system with switchable double working conditions and a control method

Abstract

The application discloses a biogas membrane separation and purification system capable of switching between two working conditions and a control method, and belongs to the technical field of biogas resource gas separation. The system realizes flexible switching between two operation modes through cooperation of a set of membrane separation devices and an automatic control valve group. The first working condition is a biogas natural gas mode, and high-purity natural gas is obtained through two-stage membrane separation; the second working condition is a biogas methanol raw gas mode, and mixed gas with a specific proportion of methane and carbon dioxide is obtained through specific bypass adjustment. The application comprises two core architectures, namely, a bypass shunting architecture before the membrane and a bypass architecture between the membranes, and the conversion of product schemes can be realized without changing the main hardware, a set of equipment can produce two types of products, repeated investment is avoided, system operation energy consumption is reduced, market price and downstream equipment linkage adjustment requirements are adapted, and the economic efficiency and operation flexibility of biogas resource utilization are improved.

Description

Technical Field

[0001] This invention belongs to the field of biogas resource utilization and gas membrane separation technology, specifically involving a biogas membrane separation and purification system and control method with switchable dual operating conditions, which is particularly suitable for integrated purification process scenarios where biogas is simultaneously adapted to biomethane production and biomethanol steam reforming feedstock gas preparation. Background Technology

[0002] Biogas is a biomass renewable and clean energy source with great application potential. Membrane separation technology is currently the mainstream process for high-value utilization of biogas, separating and purifying methane and carbon dioxide from it. Currently, biogas resource utilization mainly focuses on two application directions: one is the production of high-quality biomethane that meets the GB 17820-2018 standard, which is then integrated into natural gas pipelines or used as vehicle fuel; the other is the preparation of feedstock gas with specific components, which is fed into a steam methane reforming (SMR) unit to produce syngas, and then catalytically synthesized into biomethanol. Studies have shown that when the volume ratio of methane to carbon dioxide in the reforming feed gas is 3:1 (75% methane, 25% carbon dioxide), the carbon-hydrogen ratio of the reforming reaction is optimal, significantly improving syngas yield and methanol product selectivity.

[0003] Currently, various biogas membrane separation and purification processes and devices have been disclosed both domestically and internationally, but all of them have significant technical shortcomings. Based on the publicly available patent literature, the current state of the technology is as follows:

[0004] The first category is traditional two-stage membrane separation biogas purification technology. For example, CN109593582A discloses an integrated biogas membrane purification device, which uses a pretreatment + two-stage membrane series process to achieve deep decarbonization of biogas and stably produce high-purity biomethane. CN205676433U discloses a standardized two-stage membrane biogas purification system that relies on fixed pipelines to remove carbon dioxide. The above two categories of existing technologies, which are closest to this invention, both adopt mature two-stage membrane separation hardware architectures. However, the system pipelines and operating modes are fixed, and they only have a single biomethane production mode. They lack bypass diversion structures, cannot adjust biogas components, and are completely unsuitable for the preparation of biomethanol feedstock.

[0005] The second category is the combined biogas and methanol process technology. For example, CN120885138A discloses an integrated system for biomass biogas purification coupled with carbon capture to produce methanol, which realizes the series connection of biogas purification and methanol synthesis processes. However, its biogas purification unit has a fixed operating process and does not have a membrane separation bypass adjustment structure. It cannot switch the product type according to production needs, nor can it accurately match the optimal intake ratio for methanol reforming.

[0006] The third category is biogas membrane purification technology with bypass regulation. For example, CN115960643A discloses a biogas purification process with auxiliary regulation pipeline, which can slightly adjust the purity of the product gas. However, its regulation purpose is only to optimize the quality of biogas, and it does not design a precise ratio logic for methanol reforming conditions, nor does it have a fully automatic operating condition switching control program.

[0007] In summary, existing biogas membrane purification systems operate under single conditions and cannot achieve the dual production of biogas and biomethanol feedstock gas from a single system. If the industry needs to simultaneously deploy two production lines, two separate membrane separation units must be built, resulting in high redundant investment, large footprint, high ineffective energy consumption, and poor operational flexibility. Furthermore, existing technologies lack two differentiated bypass gas distribution schemes—pre-membrane diversion and inter-membrane diversion—and also lack a corresponding PID feedforward closed-loop automatic switching control system. Based on these technological gaps and industry pain points, this invention was developed. Summary of the Invention

[0008] To address the industry pain points of existing two-stage membrane separation biogas purification systems, such as limited operating conditions, inability to simultaneously produce biogas and biomethanol feedstock, lack of precise gas matching in biogas-methanol coupling processes, limited bypass regulation for minor optimization of fuel gas purity, absence of fully automated operating mode switching logic, redundant investment in dual-production line construction, high energy consumption, and insufficient operational flexibility, this invention provides a biogas membrane separation purification system and control method with switchable dual operating modes. This invention relies on a single two-stage membrane separation unit, coupled with two types of bypass regulation pipelines and a PLC fully automated control system, to achieve one-click switching between biogas and biomethanol feedstock production modes. It eliminates the need for two independent purification units and precisely matches the optimal inlet C-H ratio for steam methane reforming, reducing equipment investment and operating energy consumption, and improving the economic benefits and operating mode adaptability of biogas resource utilization.

[0009] To achieve the above objectives, the core technical solution of this invention is a biogas membrane separation and purification system with switchable dual operating conditions, specifically as follows: It includes a feed gas pretreatment unit, a compression unit, a membrane separation unit, an automatic control valve group, and a PLC automatic control unit; the membrane separation unit includes a primary membrane separator and a secondary membrane separator; the automatic control valve group includes a pre-membrane bypass regulating valve group located at the front end of the membrane separation unit and an inter-membrane bypass regulating valve group located between the primary and secondary membrane separators; the PLC automatic control unit is configured to control the automatic control valve group, enabling the system to switch between the following two operating conditions: First operating condition: all feed gas is processed in series by the primary and secondary membrane separators, outputting biogas with a methane concentration ≥97%; Second operating condition: by activating the pre-membrane bypass regulating valve group or the inter-membrane bypass regulating valve group, the ratio of the gas flow entering the membrane separation unit to the gas flow not entering the membrane separation unit is adjusted, outputting biomethanol synthesis feed gas with a methane concentration of 70%~80%.

[0010] Furthermore, when the second operating condition is operated using a pre-membrane bypass diversion method: at the inlet of the membrane separation unit, the raw biogas is divided into a membrane treatment gas stream and a bypass gas stream; the membrane treatment gas stream enters the first-stage membrane separator for deep decarbonization, and the bypass gas stream bypasses the membrane separation unit; the two gas streams mix at the outlet of the membrane separation unit to form the bio-methanol synthesis raw gas; wherein, the flow ratio of the membrane treatment gas stream to the bypass gas stream is configured such that the volume ratio of methane to carbon dioxide in the mixed gas is close to 3:1.

[0011] Furthermore, when the second operating condition is operated in the inter-membrane bypass mode: all feed gas enters the primary membrane separator for separation; the inter-membrane bypass regulating valve group cuts off the secondary membrane separator, so that the gas produced by the primary membrane separator is directly output as product gas, and no longer needs to be processed by the secondary membrane separator; by utilizing the natural methane enrichment characteristics of the gas produced by the primary membrane separator, feed gas that meets the C-H ratio requirements for bio-methanol synthesis can be directly obtained.

[0012] Furthermore, during the first operating condition, both the pre-membrane bypass regulating valve group and the inter-membrane bypass regulating valve group are in the closed state, and the system operates in a two-stage series mode with a methane recovery rate ≥99%.

[0013] Furthermore, the flow rate of the membrane treatment gas stream accounts for 34.78% of the total flow rate of the raw material gas, and the flow rate of the bypass gas stream accounts for 65.22% of the total flow rate of the raw material gas.

[0014] Furthermore, the methane concentration in the product gas after the membrane treatment gas flow is separated by two stages of membrane separation is 97.5%, the bypass gas flow maintains the original composition of the feed gas, and the two gas flows are mixed to obtain a target mixed gas with a methane concentration of 75% and a carbon dioxide concentration of 25%.

[0015] Furthermore, the methane concentration in the gas produced by the primary membrane separator is 75.83%, and the carbon dioxide concentration is 24.17%, and this component is directly output as the raw material gas for methanol synthesis.

[0016] Furthermore, the biogas used as raw material is designed with the following composition: methane volume concentration of 63% and carbon dioxide volume concentration of 37%.

[0017] Furthermore, the membrane separation unit adopts a polyimide hollow fiber membrane module, with an operating pressure of 12~15 barg and an operating temperature of 35~45℃.

[0018] This invention also provides a switchable dual-mode biogas membrane separation and purification control method, specifically including the following steps: S1. Receiving a mode switching command; S2. The PLC calls the corresponding valve control logic subroutine according to the command type; S3. If switching to the second mode, determining and opening the corresponding pre-membrane bypass or inter-membrane bypass path; S4. Monitoring the product gas concentration in real time through an online gas analyzer and adjusting the valve opening according to feedback until the concentration stabilizes within the target range.

[0019] Furthermore, in step S4, a PID control algorithm with feedforward compensation is used to adjust the opening of the bypass regulating valve based on the real-time data from the online analyzer, so that the product gas concentration control accuracy reaches ±0.5%.

[0020] Compared with existing patents and conventional biogas purification technologies, this invention has the following five core beneficial effects:

[0021] Reduce redundant investment and reduce land occupation: Relying on a single set of two-stage membrane separation equipment to achieve the co-production of two products, replacing the traditional two sets of independent purification units, the equipment investment is reduced by more than 30%, the system land area is reduced by 40%, and the on-site pipeline layout and civil construction costs are simplified.

[0022] It features flexible operation modes and strong resistance to market risks: it can quickly switch production conditions with one click according to fluctuations in natural gas and methanol market prices, prioritizing the production of high-priced products; at the same time, it supports automatic switching of downstream equipment to adapt to different production scheduling needs, solving the shortcomings of traditional fixed processes that cannot flexibly adjust production.

[0023] Graded energy saving and extended membrane module life: The membrane bypass scheme can directly cut off the secondary membrane module under methanol feed gas production conditions, reducing the overall system energy consumption by 25%~40%, avoiding ineffective energy consumption and carbon dioxide resource waste caused by deep decarbonization; at the same time, it reduces the running time of the secondary membrane, effectively delays membrane module aging, and reduces membrane element replacement costs.

[0024] High gas mixing precision improves methanol synthesis efficiency: PID + feedforward compensation closed-loop control is adopted, and the gas ratio control precision can reach ±0.5%. The optimal 3:1 C / H ratio feedforward gas is accurately output, matching the best process conditions for SMR reforming reaction, and ultimately improving the synthesis gas yield and methanol product selectivity by 5%~8%.

[0025] High degree of automation and low operation and maintenance cost: The entire system operates automatically by PLC, eliminating the need for manual adjustment of valve parameters. It has full-dimensional safety interlock protection for pressure, temperature, concentration, and valve failure, enabling unattended operation, reducing on-site manual operation and maintenance costs, and providing stronger operational stability. Attached Figure Description

[0026] Figure 1This is a flow chart of the pre-membrane bypass diversion process for the biogas model of this invention;

[0027] Figure 2 This is a flow chart of the pre-membrane bypass diversion process in the bio-methanol mode of this invention;

[0028] Figure 3 This is a flow chart of the membrane bypass diversion process in the biogas model of the present invention;

[0029] Figure 4 This is a flow chart of the membrane bypass diversion process in the bio-methanol mode of the present invention.

[0030] Figure 5 This is a logic block diagram of the PLC automatic control system of the present invention;

[0031] Explanation of reference numerals in the attached diagram: 1. Biogas buffer tank; 2. Biogas compressor; 3. Pretreatment unit; 4. Primary membrane separator; 5. Secondary membrane separator; 6. Pre-membrane three-way bypass valve; 7. Inter-membrane bypass valve; 8. Online gas analyzer; 9. PLC control cabinet; 10. Product gas buffer tank; 11. Carbon dioxide-rich permeate collection tank. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0033] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0034] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the use of "and / or" or "and / or" throughout the text includes three parallel solutions. For example, "A and / or B" includes solution A, solution B, or a solution where both A and B are satisfied simultaneously. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0035] The dual-condition operation logic and material balance of the present invention will be described in detail below with reference to the accompanying drawings.

[0036] Figure 1 This is a flow chart of the pre-membrane bypass diversion process for the biogas model of this invention. The raw biogas first enters the raw biogas buffer tank 1 for pressure stabilization and buffering, resulting in raw biogas with a flow rate of 1200 Nm³ / h, CH4 62%, and CO2 38%, which is then stably transported to the downstream pretreatment unit 3. Within the pretreatment unit 3, four processes are sequentially completed: dust removal, desulfurization, dehydration, and deep activated carbon filtration. These processes remove dust, hydrogen sulfide, water vapor, and trace organic impurities from the biogas, preventing corrosion and contamination of subsequent membrane modules. The purified biogas is then sent to the biogas compressor 2. The biogas compressor 2 pressurizes the purified biogas to 15 barg and controls the temperature at 45°C. The high-pressure biogas is then transported to the pre-membrane three-way bypass valve 6. The three-way bypass valve 6 before the membrane is closed, and all high-pressure biogas enters the primary membrane separator 4 (5 6-inch membranes). Preliminary separation is achieved by relying on the difference in CO2 and CH4 permeation rates between the membranes: on the permeate side, a large amount of CO2 permeates the membrane, forming CO2-rich permeate gas at a pressure of 0.05 barg, which is sent to the carbon dioxide-rich permeate gas collection tank 11 for unified collection; on the retrieval side, the crude biogas with increased methane concentration is transported to the secondary membrane separator 5. The crude biogas enters the secondary membrane separator 5 (3 6-inch membranes) for deep decarbonization. The retrieval side produces biomethane with a methane content >97%, which is sent to the product gas buffer tank 10 for pressure stabilization and then distributed to the pipeline network; a small amount of CO2-rich permeate gas is also produced on the permeate side of the secondary membrane, which is collected in the carbon dioxide-rich permeate gas collection tank 11 for unified recovery. A network gas analyzer 8 is installed at key process nodes to monitor the biogas CH4 and CO2 components, pressure, and temperature in real time; the entire system is centrally controlled by a PLC control cabinet 9, automatically adjusting the compressor load, valve status, and membrane system operating parameters.

[0037] The entire process integrates pretreatment, compression, two-stage membrane separation, automatic monitoring, and dual bypass switching. One set of equipment can switch between high and low load operating conditions, taking into account both full-load export of refined natural gas and low-load simple gas production for plant use. It is also equipped with impurity pretreatment and protective membrane elements, and online instruments + PLC realize fully automatic and stable operation.

[0038] Figure 2 This is a flow chart of the pre-membrane bypass diversion process in the biomethanol mode of this invention. The total intake of biogas is 1200 Nm³ / h, with a gas composition of 62% methane (CH4) and 38% carbon dioxide (CO2). After being pressurized by biogas compressor 2, the biogas operating conditions are 15 barg and 45°C. The pre-membrane three-way bypass valve 6 adopts a diversion operation mode, with the total gas flow divided into two paths: 34.78% flow (417.36 Nm³ / h) enters the membrane separation system, and 65.22% flow (782.64 Nm³ / h) flows through the bypass pipeline. The biogas first enters the biogas buffer tank 1 for pressure and flow stabilization, outputting stable biogas with a flow rate of 1200 Nm³ / h, 62% CH4, and 38% CO2, which is then transported to the downstream pretreatment unit 3. Biogas enters pretreatment unit 3, where it undergoes dust removal, desulfurization, dehydration, and deep filtration with activated carbon in sequence. This process removes dust, hydrogen sulfide, water vapor, and trace organic impurities from the biogas, preventing contamination and damage to subsequent membrane elements. The purified biogas is then fed into biogas compressor 2. Biogas compressor 2 pressurizes the purified biogas to 15 barg and controls the temperature at 45°C. The pressurized high-pressure biogas is then delivered to the pre-membrane three-way bypass valve 6. Under this condition, the valve switches to a split-flow operation mode, dividing the high-pressure biogas into two independent airflows.

[0039] The pre-membrane three-way bypass valve 6 handles two separate flows. Flow branch one: 417.36 Nm³ / h (34.78%) enters the two-stage membrane separation unit. All gas flow from this branch enters the first-stage membrane separator 4, utilizing the different permeation performance of the membranes for CO2 and CH4 to achieve initial decarbonization separation. The permeate side of the first-stage membrane separator 4 generates a large amount of CO2-rich gas, which is uniformly transported to the carbon dioxide-rich permeate gas collection tank 11 for centralized collection. The retrieval side of the first-stage membrane separator 4 obtains crude biogas with increased methane concentration, which is sent to the second-stage membrane separator 5 for further purification. The crude biogas enters the second-stage membrane separator 5 for further removal of residual CO2. The retrieval side of the second-stage membrane separator 5 produces membrane product gas with a methane content >99%. A small amount of CO2-rich tail gas generated on the permeate side of the second-stage membrane separator 5 is collected in the carbon dioxide-rich permeate gas collection tank 11 for unified recovery. Under this operating condition, the inter-membrane bypass valve 7 remains closed, and there is no cross-stage bypass gas flow. Branch 2: 782.64 Nm³ / h (65.22%) enters the bypass pipeline. The high-pressure biogas in this branch bypasses the primary membrane separator 4 and the secondary membrane separator 5, flowing directly through the bypass pipeline. The biogas composition remains unchanged at 62% CH4 and 38% CO2, and is directly delivered to the gas flow junction. The gas flow is mixed to achieve the target composition. The high-purity membrane product gas produced by the secondary membrane separator 5 is fully mixed with the unseparated raw biogas from the bypass pipeline at the gas flow junction, resulting in a stable mixed gas with 75% CH4 and 25% CO2, meeting the feed gas requirements for downstream units. The mixed gas is then delivered to the downstream reaction unit, and the component-adjusted mixed gas is delivered to the SMR reforming unit as the feed gas source for the reforming reaction.

[0040] Online gas analyzers 8 are installed at key nodes in the process flow to monitor biogas flow rate, CH4 and CO2 composition, pressure, temperature and other operating parameters in real time. PLC control cabinet 9 provides centralized linkage control for the entire system, which can automatically adjust the load of biogas compressor 2 and the flow ratio of membrane three-way bypass valve 6, dynamically match the gas load of downstream SMR reforming unit, and stabilize the target composition of mixed gas of 75% CH4 and 25% CO2. All CO2-rich permeate generated by the system is collected in carbon dioxide-rich permeate collection tank 11, which can be used for subsequent carbon dioxide resource utilization.

[0041] A large proportion of the gas flow is bypassed through the pre-membrane three-way bypass valve 6, allowing only a small flow of biogas to enter the primary membrane separator 4 and the secondary membrane separator 5 for processing. This significantly reduces the load on the membrane modules, decreases membrane loss, and lowers system compression energy consumption. The methane concentration is precisely adjusted by online mixing of the bypass crude biogas and high-purity membrane product gas, eliminating the need for additional external gas to adjust the components. Fully automatic flow control is achieved through the online gas analyzer 8 and PLC control cabinet 9, which can adjust the ratio of the two gas flows in real time according to the load of downstream units, ensuring a continuous and stable mixture composition. The CO2-rich permeate is collected uniformly into the carbon dioxide-rich permeate collection tank 11, realizing the resource recovery of carbon dioxide and improving the economic benefits of comprehensive biogas utilization.

[0042] Table 1 is... Figure 1 and Figure 2 Material balance diagram under operating conditions (taking 1200 Nm3 / h biogas as an example, corresponding to a methanol production capacity of 10,000 tons / year).

[0043]

[0044] Figure 3 This is a flow chart of the membrane bypass diversion process for biogas in this invention. The total feed biogas volume is 1200 Nm³ / h, with a gas composition of 62% CH4 and 38% CO2. The biogas compressor 2 operates at 15 barg and 45°C. The primary membrane separator 4 is equipped with 5 6-inch membrane modules, and the secondary membrane separator 5 is equipped with 3 6-inch membrane modules. The produced biogas has a methane content >99% and can be sent to the pipeline network. The CO2-rich permeate gas generated by the two-stage membrane separation has a pressure of 0.05 barg and is collected uniformly. The feed biogas is first sent to the feed biogas buffer tank 1 for pressure and flow stabilization, outputting stable biogas of 1200 Nm³ / h with 62% CH4 and 38% CO2, which is then transported to the pretreatment unit 3. The biogas enters the pretreatment unit 3 and undergoes dust removal, desulfurization, dehydration, and activated carbon filtration in sequence to remove dust, hydrogen sulfide, water vapor, and trace organic impurities, preventing impurities from contaminating the membrane elements. The purified biogas is then sent to the biogas compressor 2. The biogas compressor 2 pressurizes the purified biogas to 15 barg and controls the temperature at 45℃. Under this condition, the three-way bypass valve 6 before the membrane is fully closed, and all the high-pressure biogas is directly delivered to the first-stage membrane separator 4.

[0045] All high-pressure biogas is fed into the primary membrane separator 4, where initial decarbonization is achieved by relying on the membrane's differentiated permeability to CO2 and CH4. The permeate side of the primary membrane separator 4 generates CO2-rich permeate gas, which is then collected in the carbon dioxide-rich permeate gas collection tank 11. The retrieval side of the primary membrane separator 4 yields crude biogas with increased methane concentration, which is then transported to the inter-membrane bypass valve 7. Under this condition, the inter-membrane bypass valve 7 is closed, and all crude biogas enters the secondary membrane separator 5. The crude biogas enters the secondary membrane separator 5 for further CO2 removal. The retrieval side of the secondary membrane separator 5 produces high-purity biomethane with a methane content >99%, which is sent to the product gas buffer tank 10 for pressure stabilization before being distributed to the pipeline network. The small amount of CO2-rich tail gas generated on the permeate side of the secondary membrane separator 5 is collected in the carbon dioxide-rich permeate gas collection tank 11 for unified recovery.

[0046] This mode is a full-load biogas refining operation. The three-way bypass valve 6 before the membrane and the bypass valve 7 between the membranes are both closed. All raw biogas passes through the first-stage membrane separator 4 and the second-stage membrane separator 5 for two-stage deep decarbonization to remove carbon dioxide to the maximum extent, producing qualified pipeline biogas with methane purity of over 99%. The CO2-rich permeate produced by separation is collected in the carbon dioxide-rich permeate collection tank 11 for subsequent resource utilization.

[0047] Figure 4 This is a flow chart of the membrane bypass diversion process in the biomethanol mode of this invention. The feed biogas flow rate is 1200 Nm³ / h, with initial components of CH4 62% and CO2 38%. The biogas compressor 2 operates at 15 barg and 45°C. The primary membrane separator 4 is equipped with five 6-inch membrane modules. The product gas after primary membrane separation has CH4 75.83% and CO2 24.17% components, which is directly supplied to the downstream SMR reforming unit. The CO2-rich permeate gas produced by separation has a pressure of 0.05 barg and is collected in the carbon dioxide-rich permeate gas collection tank 11. Under this condition, the membrane bypass valve 7 is fully open, and the secondary membrane separator 5 is disconnected from the system and not in use. The feed biogas first enters the feed biogas buffer tank 1 for pressure and flow stabilization, outputting stable feed biogas of 1200 Nm³ / h with CH4 62% and CO2 38%, which is then transported to the pretreatment unit 3. Biogas is fed into pretreatment unit 3, where it undergoes dust removal, desulfurization, dehydration, and activated carbon filtration processes in sequence. This removes dust, hydrogen sulfide, water vapor, and trace organic impurities from the biogas, preventing contamination and damage to the membrane elements. The purified biogas is then sent to biogas compressor 2. Biogas compressor 2 pressurizes the purified biogas to 15 barg and controls the temperature at 45°C. Under these conditions, the three-way bypass valve 6 before the membrane is completely closed, and all high-pressure biogas is directly delivered to the primary membrane separator 4.

[0048] All high-pressure biogas enters the primary membrane separator 4 for single-stage membrane separation: The permeate side of the primary membrane separator 4 produces CO2-rich permeate gas at a pressure of 0.05 barg, which is then collected in the carbon dioxide-rich permeate gas collection tank 11. The retrieval side of the primary membrane separator 4 produces crude biogas. At this point, the inter-membrane bypass valve 7 is fully open, allowing the gas to bypass directly through the valve, completely cutting off the secondary membrane separator 5 and preventing further purification. The retrieval crude biogas output through the inter-membrane bypass valve 7 is the target product gas, with a stable composition of CH4 75.83% and CO2 24.17%, which is directly fed to the SMR reforming unit as the feedstock for bio-methanol production.

[0049] Online gas analyzers 8 are installed at key nodes in the process to monitor biogas flow, CH4 and CO2 composition, pressure, temperature and other operating parameters in real time. The PLC control cabinet 9 centrally manages the entire system and can automatically switch the on / off state of the membrane bypass valve 7 to realize the switching of the secondary membrane separator 5. At the same time, it adjusts the operating load of the biogas compressor 2 to stably supply the feed gas composition to the SMR reforming unit. All CO2-rich permeate produced by the system is collected in the carbon dioxide-rich permeate collection tank 11 for subsequent carbon dioxide resource recovery and utilization.

[0050] The system operates with the membrane bypass valve 7 fully open, activating only the primary membrane separator 4 for single-stage decarbonization while completely shutting down the secondary membrane separator 5, thus reducing membrane module load and operating energy consumption. After single-stage membrane separation, the system directly yields feed gas components suitable for SMR reforming units and used to produce biomethanol, eliminating the need for additional gas mixing and simplifying the process. The system leverages the linkage between the PLC control cabinet 9 and the online gas analyzer 8 to control valve switching, allowing for flexible switching between biogas and biomethanol modes, enabling a single system to meet the production needs of both products. The removed CO2-rich permeate gas is centrally recovered, achieving comprehensive utilization of carbon resources and improving the economic efficiency of biogas resource utilization.

[0051] Table 2 is... Figure 3 and Figure 4 Material balance diagram under operating conditions (taking 1200 Nm3 / h biogas as an example, corresponding to a methanol production capacity of 10,000 tons / year).

[0052]

[0053] like Figure 5 As shown, the PLC control cabinet 9 of this invention includes a concentration closed-loop control circuit. During bio-methanol operation, the online gas analyzer 8 monitors the CH4 and CO2 concentrations of the mixed product gas in real time and feeds the signals back to the PLC control cabinet 9. The PLC control cabinet 9 dynamically fine-tunes the opening of the pre-membrane three-way bypass valve 6 or confirms the fully open state of the inter-membrane bypass valve 7 using a PID algorithm, ensuring that the product gas concentration is stably controlled within the set value ±0.5%.

[0054] The switching process adopts a smooth transition strategy: when switching from biogas mode to biomethanol mode, the PLC first opens the bypass valve and gradually reduces the flow rate into the secondary membrane until the system stabilizes at the new target ratio. No shutdown is required throughout the process, effectively avoiding pipeline pressure fluctuations.

[0055] The embodiments described in this specification are merely preferred embodiments of the present invention, and are only used to illustrate the technical solutions of the present invention and not to limit the present invention. Any technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation should be within the scope of the present invention.

Claims

1. A biogas membrane separation and purification system with switchable dual operating modes, characterized in that, It includes a raw gas pretreatment unit, a compression unit, a membrane separation unit, an automatic control valve group, and a PLC automatic control unit; The membrane separation unit includes a primary membrane separator and a secondary membrane separator; the automatic control valve group includes a pre-membrane bypass regulating valve group located at the front end of the membrane separation unit and an inter-membrane bypass regulating valve group located between the primary and secondary membrane separators; the PLC automatic control unit is configured to control the automatic control valve group, enabling the system to switch between the following two operating conditions: First operating condition: All feed gas is processed in series by the first-stage membrane separator and the second-stage membrane separator to output biogas with a methane concentration ≥97%; Second operating condition: By activating the pre-membrane bypass regulating valve group or the inter-membrane bypass regulating valve group, the ratio of the gas flow entering the membrane separation unit to the gas flow not entering the membrane separation unit is adjusted, and bio-methanol synthesis feed gas with a methane concentration of 70%~80% is output.

2. The system according to claim 1, characterized in that, When the second operating condition is operated using a pre-membrane bypass diversion method: at the inlet of the membrane separation unit, the biogas feedstock is divided into a membrane treatment gas stream and a bypass gas stream; the membrane treatment gas stream enters the first-stage membrane separator for deep decarbonization, and the bypass gas stream bypasses the membrane separation unit; the two gas streams mix at the outlet of the membrane separation unit to form the bio-methanol synthesis feedstock gas; wherein, the flow ratio of the membrane treatment gas stream to the bypass gas stream is configured such that the volume ratio of methane to carbon dioxide in the mixed gas is close to 3:

1.

3. The system according to claim 1, characterized in that, When the second operating condition is operated in the inter-membrane bypass mode: all feed gas enters the primary membrane separator for separation; the inter-membrane bypass regulating valve group cuts off the secondary membrane separator, so that the gas produced by the primary membrane separator is directly output as product gas and no longer needs to be processed by the secondary membrane separator; by utilizing the natural methane enrichment characteristics of the gas produced by the primary membrane separator, feed gas that meets the C-H ratio requirements for bio-methanol synthesis can be directly obtained.

4. The system according to claim 1, characterized in that, When the first operating condition is in operation, both the pre-membrane bypass regulating valve group and the inter-membrane bypass regulating valve group are in the closed state, and the system operates in a two-stage series mode with a methane recovery rate of ≥99%.

5. The system according to claim 2, characterized in that, The flow rate of the membrane treatment gas stream accounts for 34.78% of the total feed gas flow rate, and the flow rate of the bypass gas stream accounts for 65.22% of the total feed gas flow rate.

6. The system according to claim 5, characterized in that, The methane concentration in the product gas after the membrane treatment gas flow is separated by two-stage membrane separation is 97.5%. The bypass gas flow maintains the original composition of the feed gas. After the two gas flows are mixed, a target mixed gas with a methane concentration of 75% and a carbon dioxide concentration of 25% is obtained.

7. The system according to claim 3, characterized in that, The methane concentration in the gas produced by the primary membrane separator is 75.83%, and the carbon dioxide concentration is 24.17%. This component is directly output as the raw material gas for methanol synthesis.

8. The system according to claim 1, characterized in that, The biogas used as raw material is designed with a methane volume concentration of 63% and a carbon dioxide volume concentration of 37%.

9. The system according to claim 1, characterized in that, The membrane separation unit uses a polyimide hollow fiber membrane module, with an operating pressure of 12~15 barg and an operating temperature of 35~45℃.

10. A control method for the system as described in claim 1, characterized in that, Includes the following steps: S1. Receive operating condition switching command; S2. The PLC calls the corresponding valve control logic subroutine according to the instruction type; S3. If switching to the second operating condition, determine and open the corresponding pre-membrane bypass or inter-membrane bypass pathway; S4. Monitor the product gas concentration in real time using an online gas analyzer and adjust the valve opening accordingly until the concentration stabilizes within the target range.

11. The control method according to claim 10, characterized in that, In step S4, a PID control algorithm with feedforward compensation is used to adjust the opening of the bypass regulating valve based on the real-time data from the online analyzer, so that the product gas concentration control accuracy reaches ±0.5%.

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