A flue gas carbon dioxide two-stage membrane capture process and system based on inter-stage negative pressure coupling

Abstract

The application discloses a flue gas carbon dioxide two-stage membrane capture process and system based on inter-stage negative pressure coupling and belongs to the technical field of greenhouse gas emission reduction and gas membrane separation. CO2-containing raw gas is mixed with second retentate gas of a second-stage membrane assembly, pressurized by a first-stage compressor and then sent to the first-stage membrane assembly for separation. The core lies in that an inter-stage vacuum pump is arranged in series between the permeation side of the first-stage membrane assembly and the gas inlet end of a second-stage compressor. First permeation gas is led out under the action of vacuum pumping, pressurized and cooled and then sent to the second-stage membrane assembly for deep separation. The application reconstructs a multi-stage membrane topology structure, the introduction of the inter-stage vacuum pump greatly strengthens the transmembrane driving force of the first-stage membrane, greatly reduces the total membrane area required for separation and significantly reduces the circulating gas flow rate returned to the source, thereby greatly reducing the processing load of the first-stage compressor. The application breaks through the energy consumption bottleneck of the traditional reflux process and can stably achieve the target of CO2 purity ≥ 95% and recovery rate ≥ 90% even if low-selectivity membrane materials are used.

Description

Technical Field

[0001] This invention belongs to the field of greenhouse gas emission reduction and gas membrane separation technology, specifically relating to a multi-stage membrane separation process and system for capturing carbon dioxide (CO2) in flue gas emitted after combustion. Background Technology

[0002] With the increasing severity of global warming, reducing CO2 emissions has become a top priority. Industrial flue gas from the combustion of fossil fuels (such as coal and natural gas) is a significant source of CO2 emissions. Among the many carbon capture and storage (CCS) technologies, membrane separation technology is considered a green separation technology with great development potential due to its advantages such as small system size, low operating cost, no chemical solvent pollution, and high modularity.

[0003] To achieve industrial application, the U.S. Department of Energy (DOE) has set strict commercial standards for CO2 capture in post-combustion flue gas: the purity of the CO2 product gas must reach more than 95%, and the CO2 recovery rate must be no less than 90%. However, existing single-stage membrane separation processes are limited by the trade-off effect that is common in membrane materials, making it extremely difficult to meet the dual standards of high purity and high recovery rate on their own.

[0004] To address this issue, existing technologies typically employ a two-stage membrane separation process. Common two-stage processes include direct-flow structures without reflux and structures with tail gas recirculation. When the selectivity (α) of the membrane material is low or moderate, the no-reflux process cannot achieve the purity standard; therefore, traditional technologies generally adopt a two-stage process with tail gas recirculation, where the CO2-rich tail gas from the permeate side of the second stage is directly recycled to the feed end of the first stage to mix with the feed gas and separate again. However, this traditional recirculation process has a fatal technical bottleneck: the addition of recirculated gas greatly increases the total gas volume entering the first-stage membrane module. This not only requires an extremely large membrane area to handle the large mixed gas, but also leads to a dramatic increase in the throughput and energy consumption of the first-stage feed compressor, severely weakening the economic feasibility of the membrane separation process and hindering its industrialization.

[0005] Therefore, there is an urgent need in this field to develop a new process topology that can achieve extremely high separation targets of purity ≥95% and recovery rate ≥90% using existing industrial-grade low-selectivity membrane materials, while breaking through the bottlenecks of large circulating gas volume and high compression energy consumption in traditional reflux processes, and effectively reducing the total membrane area and comprehensive capture energy consumption of the system. Summary of the Invention

[0006] To address the technical shortcomings of existing two-stage membrane separation reflux processes, such as large circulation flow rates, high energy consumption of the feed compressor, large membrane area requirements, and poor adaptability to industrial environments, this invention provides a two-stage membrane capture process and system for flue gas carbon dioxide based on interstage negative pressure coupling. This invention, by coupling a vacuum pump between stages, fundamentally alters the system's mass flow balance, achieving an exponential reduction in the front-end compressor's processing capacity and energy consumption with only a small amount of vacuum pump energy consumption, significantly lowering the overall capture cost of the system.

[0007] A two-stage membrane capture process for carbon dioxide in flue gas based on interstage negative pressure coupling includes the following steps:

[0008] (a) The carbon dioxide-containing feed gas is mixed with the recycle gas from step (d) to obtain a mixed feed gas;

[0009] (b) The mixed feed gas is pressurized by the first stage compressor and then sent to the first stage membrane module for separation to obtain the first permeate gas rich in non-permeable components and the first permeate gas rich in carbon dioxide.

[0010] (c) The first permeate gas is pressurized by the second stage compressor and then sent to the second stage membrane module for separation, and carbon dioxide product gas is obtained on the permeate side of the second stage membrane module.

[0011] (d) The second permeate gas discharged from the permeate side of the second-stage membrane module is used as circulating gas and returned to step (a) to be mixed with the feed gas; and, between the permeate outlet of the first-stage membrane module and the air inlet of the second-stage compressor, the first permeate gas is drawn by an interstage vacuum pump located thereto, so as to form a negative pressure state on the permeate side of the first-stage membrane module.

[0012] The absolute pressure on the permeate side of the first-stage membrane module is maintained at 0.02-0.05 MPa by the suction action of the interstage vacuum pump.

[0013] The absolute pressure of the mixed feed gas entering the first-stage membrane module and the absolute pressure of the first permeate gas entering the second-stage membrane module are both independently controlled within the range of 0.3-0.7 MPa.

[0014] The absolute pressure of the mixed feed gas entering the first-stage membrane module and the absolute pressure of the first permeate gas entering the second-stage membrane module are both set to 0.4-0.6 MPa.

[0015] In step (c), the permeate side pressure of the second-stage membrane module is maintained at or near atmospheric pressure to obtain carbon dioxide product gas while suppressing the permeation of non-permeable components and ensuring the purity of the product gas.

[0016] It also includes step (e): introducing the first residual gas with residual pressure discharged in step (b) into a turbine expander to expand and do work, so as to recover its pressure energy;

[0017] The first permeate gas is cooled after leaving the interstage vacuum pump and before entering the second-stage compressor, and after leaving the second-stage compressor and before entering the second-stage membrane module.

[0018] Before step (a), there is also a step of pre-treating the raw gas, which includes at least one of cooling, dust removal, desulfurization and dehydration.

[0019] The process is operated such that the purity of the carbon dioxide product gas is not less than 95% and the total recovery rate of carbon dioxide is not less than 90%.

[0020] The feed gas is the flue gas produced after the combustion of fossil fuels, in which the volume fraction of carbon dioxide is between 10-20%.

[0021] A two-stage membrane capture system for flue gas carbon dioxide based on interstage negative pressure coupling for implementing the aforementioned process, characterized in that it comprises:

[0022] First-stage membrane module;

[0023] Second-stage membrane module;

[0024] The first-stage compressor is used to pressurize the feed gas entering the first-stage membrane module;

[0025] A second-stage compressor is used to pressurize the feed gas entering the second-stage membrane module;

[0026] The return line is used to connect the permeate outlet of the second-stage membrane module to the inlet of the first-stage compressor.

[0027] And an interstage vacuum pump, whose intake port is connected to the permeate outlet of the first-stage membrane module, and whose exhaust port is connected to the intake port of the second-stage compressor.

[0028] It also includes a mixer, whose first inlet is connected to the raw material gas supply line, the second inlet is connected to the return line, and its outlet is connected to the air inlet of the first-stage compressor.

[0029] It also includes a turbine expander, which is installed on the permeate outlet pipeline of the first-stage membrane module to recover the pressure energy of the first permeate, and the power output shaft of the turbine expander is mechanically or electrically coupled to the power input end of the first-stage compressor or the second-stage compressor.

[0030] The gas separation membranes packed in the first-stage and second-stage membrane modules have an ideal selectivity of not less than 20 for carbon dioxide and nitrogen.

[0031] The permeate outlet of the second-stage membrane module is connected directly or via a pressure regulating valve to the product gas collection line, and no vacuum pump is installed on the outlet line to keep the outlet pressure below 0.1 MPa.

[0032] The beneficial effects of this invention are:

[0033] This invention achieves significant technical advantages by reconstructing the process topology of a two-stage membrane separation system and innovatively introducing an interstage vacuum pump between the permeate side of the first-stage membrane module and the second-stage compressor. This design creates a powerful driving force field in the first-stage membrane separation unit, combining positive pressure pushing from the front end with negative pressure suction from the permeate side, greatly enhancing the transmembrane mass transfer efficiency of carbon dioxide. This enhanced driving force allows the first-stage membrane module to pre-enrich carbon dioxide with higher efficiency. Its most direct effect is a significant reduction in the amount of recirculated gas that does not permeate from the second-stage membrane module and needs to be returned to the system front end; this recirculated gas volume can be reduced to less than one-quarter of that in traditional reflux processes.

[0034] Since the first-stage compressor is the main energy-consuming unit in traditional processes, its load is mainly composed of the raw gas to be treated and a large amount of recirculated gas. This invention significantly reduces the total gas volume that the first-stage compressor needs to process by drastically reducing the flow rate of the recirculated gas. This directly leads to a significant reduction in the required specifications of the first-stage compressor, equipment investment costs, and operating power consumption, fundamentally solving the energy consumption bottleneck problem caused by excessive recirculation load in traditional reflux processes, and significantly improving the economic efficiency of the entire carbon capture system.

[0035] The enhanced transmembrane driving force of this invention also means that a higher gas permeation flux can be achieved per unit membrane area. Therefore, to achieve the same separation target—a carbon dioxide product purity of not less than 95% and a recovery rate of not less than 90%—the total membrane area required by this invention is significantly reduced compared to conventional processes. As one of the core equipment investments in the system, the reduction in membrane module area directly lowers the initial construction cost and equipment footprint, further enhancing the economic competitiveness of the technology.

[0036] Furthermore, the robust mass transfer mechanism constructed in this invention significantly enhances the robustness of the process and relaxes the stringent requirements on membrane material performance. Low-selectivity membrane materials that cannot achieve separation targets in traditional processes can have their performance limitations compensated for by the powerful physical driving force when applied in this invention, still achieving stable separation goals with high purity and high recovery rates. This allows the system to use lower-cost, more technologically mature industrial-grade membrane materials, reducing reliance on expensive high-performance membranes and greatly improving the applicability and feasibility of this technology in real-world industrial environments.

[0037] This invention employs an asymmetric pressure configuration, using a vacuum pump only in the first-stage enhanced separation phase and omitting it in the second-stage refining and purification phase, demonstrating ingenious system optimization. Compared to designs that incorporate vacuum pumps on both permeation sides, this invention avoids the problem of increased permeation of impurity gases such as nitrogen due to excessive suction in the second stage, thereby reducing product purity. This design achieves global optimization of separation performance and operating costs with minimal equipment increments, ensuring high purity and high recovery rates while minimizing overall capture costs, demonstrating outstanding industrial application value. Attached Figure Description

[0038] Figure 1 Schematic diagram of a gas membrane separation device;

[0039] Figure 2 Schematic diagram of a two-stage membrane separation process;

[0040] Figure 3 Flowcharts of three design concepts for two-stage membrane separation processes under different operating conditions Figure labeling: Compressor (front end) - First-stage compressor, used to pressurize the CO2-containing flue gas and recirculated mixture, providing the driving force for transmembrane mass transfer in the first-stage membrane separation; MB-I - First-stage CO2 separation membrane module, used for pre-enrichment and separation of CO2 in flue gas, preferentially achieving initial separation from N2 through CO2; Vacuum pump - Interstage vacuum pump, connected in series between the permeate side of the first-stage membrane module and the second-stage compressor, used to maintain the negative pressure state on the permeate side of the first-stage membrane, enhancing the driving force for transmembrane mass transfer; Compressor (back end) - Second-stage compressor, used for secondary pressurization of the CO2-rich permeate gas from the first stage, providing the driving force for mass transfer in the second-stage membrane for deep separation; MB-II - Second-stage CO2 separation membrane module, used for deep purification and separation of CO2-rich gas, producing high-purity CO2 product; Expander - Turbine expander, used to recover the pressure energy of the residual gas from the first-stage membrane, the recovered energy can be fed back to the compressor to reduce the total energy consumption of the system. Flue gas - The feed gas containing CO2 to be treated, typically industrial post-combustion flue gas, with CO2 and N2 as the core components; Residue gas (1st) - The permeate gas from the first-stage membrane module (first permeate gas), which is the tail gas that has not permeated through the membrane and is mainly composed of N2. It is pressurized and enters the expander to recover energy; Permeate gas (1st) - The permeate gas from the first-stage membrane module (first permeate gas), which is CO2-rich gas after pre-enrichment by the first-stage membrane; Residue gas (2nd) - The permeate gas from the second-stage membrane module (second permeate gas), which is CO2-rich gas that has not permeated through the second-stage membrane. It is recycled back to the feed gas inlet for separation; Product gas - High-purity CO2 product gas, obtained through deep separation by the second-stage membrane, meeting the industrial separation target of CO2 purity ≥95% and recovery rate ≥90%. Detailed Implementation

[0041] This invention discloses a two-stage membrane capture process and system for flue gas carbon dioxide based on interstage negative pressure coupling, belonging to the field of greenhouse gas emission reduction and gas membrane separation technology. The process mixes CO2-containing feed gas with the second permeate gas from the second-stage membrane module, pressurizes it by the first-stage compressor, and then sends it to the first-stage membrane module for separation. The core of this invention lies in the series connection of an interstage vacuum pump between the permeate side of the first-stage membrane module and the inlet of the second-stage compressor. The first permeate gas is drawn out under vacuum suction, pressurized and cooled, and then sent to the second-stage membrane module for deep separation. This invention reconstructs the multi-stage membrane topology. The introduction of the interstage vacuum pump greatly enhances the transmembrane driving force of the first-stage membrane, significantly reduces the total membrane area required for separation, and significantly reduces the recirculated gas flow rate back to the source, thereby greatly reducing the throughput load of the first-stage compressor. This process overcomes the energy consumption bottleneck of traditional recirculation processes, and can stably achieve CO2 purity ≥95% and recovery rate ≥90% even when using low-selectivity membrane materials, demonstrating extremely high industrial applicability and economic benefits.

[0042] In some embodiments, this patent includes the following technical solutions:

[0043] A two-stage membrane capture process for flue gas carbon dioxide based on interstage negative pressure coupling includes the following steps: Step (1): Mixing CO2-containing flue gas feed gas with recirculated gas to obtain mixed feed gas; Step (2): The mixed feed gas is pressurized by a first-stage compressor and sent to a first-stage membrane module. First permeate gas is discharged from the permeate side of the first-stage membrane module, and CO2-rich first permeate gas is obtained on the permeate side; Step (3): The first permeate gas is pressurized by a second-stage compressor and sent to a second-stage membrane module; Step (4): CO2 product gas is drawn out from the permeate side of the second-stage membrane module; second permeate gas is obtained from the permeate side of the second-stage membrane module, and the second permeate gas is returned to step (1) as the recirculated gas; An interstage vacuum pump is connected in series between the permeate outlet of the first-stage membrane module and the inlet of the second-stage compressor; The first permeate gas is drawn out under the suction action of the interstage vacuum pump, so that the absolute pressure on the permeate side of the first-stage membrane module is maintained at 0.02 MPa ~ 0.05 MPa. The negative pressure vacuum state; In steps (2) and (3), the absolute pressure of the mixed feed gas entering the first stage membrane module and the absolute pressure of the first permeate gas entering the second stage membrane module are independently controlled between 0.3 MPa and 0.7 MPa.

[0044] Before step (1), the CO2-containing flue gas feedstock enters the flue gas pretreatment unit and undergoes cooling, dust removal and desulfurization and dehydration treatment in sequence.

[0045] The first permeate gas needs to be cooled by heat exchange through a cooler after leaving the interstage vacuum pump and before entering the second stage compressor, and after leaving the second stage compressor and before entering the second stage membrane module.

[0046] The absolute operating pressure of the feed for both the first-stage membrane module and the second-stage membrane module is set and controlled at 0.4 MPa.

[0047] In step (4), the absolute pressure on the permeate side of the second-stage membrane module is maintained between 0.02 MPa and 0.1 MPa by connecting a product-side vacuum pump.

[0048] The gas separation membranes packed in the first-stage and second-stage membrane modules have a CO2 permeation flux of not less than 300 GPUs under operating conditions, and an ideal CO2 / N2 selectivity of not less than 20.

[0049] The residual gas with residual pressure discharged in step (2) is introduced into the turbine expander to expand and do work, which is used to recover energy and output power.

[0050] A two-stage membrane capture system for flue gas carbon dioxide based on interstage negative pressure coupling, implementing any of the processes described above, includes: a mixer, a first-stage compressor, a first-stage membrane module, an interstage vacuum pump, a second-stage compressor, and a second-stage membrane module; the feed gas inlet of the mixer is connected to an external CO2-containing flue gas supply pipeline, and the mixed gas outlet of the mixer is sequentially connected to the feed inlets of the first-stage compressor, a first cooler, and the first-stage membrane module; the permeate outlet of the first-stage membrane module is connected to an exhaust pipeline; the permeate outlet of the first-stage membrane module is connected to the suction port of the interstage vacuum pump; the exhaust port of the interstage vacuum pump is sequentially connected to the feed inlets of the second cooler, the second-stage compressor, a third cooler, and the second-stage membrane module; the permeate outlet of the second-stage membrane module is connected to a CO2 product gas collection pipeline; and the permeate outlet of the second-stage membrane module is connected to the return gas inlet of the mixer via a return pipeline.

[0051] A turbine expander is connected in series between the permeate outlet of the first-stage membrane module and the venting pipeline. The power output shaft of the turbine expander is mechanically or electrically coupled to the power input end of the first-stage compressor or the second-stage compressor.

[0052] A product-side vacuum pump is connected in series between the permeate outlet of the second-stage membrane module and the CO2 product gas collection pipeline.

[0053] Example 1

[0054] This embodiment deals with a typical industrial post-combustion flue gas with a total flow rate of 8.1 Nm³.3 The flue gas has a flow rate of approximately 30°C per second and an initial temperature of approximately 30°C. After pretreatment for dust removal, desulfurization, and dehydration, the volume fraction of CO2 is 15%, and the volume fraction of N2 is 85%. The separation targets are: CO2 product purity ≥ 95% and CO2 recovery rate ≥ 90%. The membrane material used in this embodiment is a hollow fiber membrane module with a CO2 permeation flux of 1000 GPU and a CO2 / N2 selectivity of 50.

[0055] The specific process flow is as follows: Step 1 (Material Mixing): 15% CO2 industrial flue gas at atmospheric pressure is combined with the second permeate gas (recirculated gas) from the back end of the system in a mixer. Step 2 (First-Stage Pressurization and Separation): The mixed gas enters the first-stage compressor and is pressurized to the optimal operating pressure of 0.4 MPa, then sent to the feed side of the first-stage membrane module (MB-I). Driven by the pressure difference, a large amount of CO2 preferentially permeates through the membrane. Step 3 (First-Stage Venting and Energy Recovery): The first permeate gas (mainly composed of enriched N2) that has not permeated through the first-stage membrane module is maintained at a high pressure and discharged from the permeate side, directly sent to the expander for expansion to atmospheric pressure. The shaft work recovered by the expander is fed back to the first-stage compressor, and then the exhaust gas is vented. Step 4 (Interstage Vacuum Coupling): An interstage vacuum pump is installed on the permeate side of the first-stage membrane module to draw in and maintain the absolute pressure on the permeate side of the first-stage membrane module at 0.02 MPa. The CO2-rich first permeate gas is extracted. The presence of the interstage vacuum pump significantly enhances the mass transfer driving force of the first-stage membrane, reducing the amount of gas that must circulate in the system (i.e., the second permeate gas flow rate) to less than a quarter of that in conventional processes. Step 5 (Secondary Pressurization and Deep Separation): The first permeate gas discharged from the interstage vacuum pump enters the second-stage compressor, is pressurized again to 0.4 MPa, and then sent to the second-stage membrane module (MB-II). Step 6 (Product Gas and Recirculation): The permeate side of the second-stage membrane module is connected to the product-side vacuum pump, maintaining an absolute pressure of approximately 0.1 MPa. The high-purity gas permeating the membrane is collected, with a CO2 concentration reaching 95.0% (product gas), and a total recovery rate of 90%. The unpermeated second permeate gas (containing a certain concentration of unseparated CO2) is recirculated back to Step 1 through a pipeline.

[0056] In this embodiment, the total membrane area requirement is only 7067m². 2 The system's specific energy consumption is only 0.88 MJ / kgCO2, and the total CO2 capture cost is only $27.49 / tonCO2. In contrast, if a conventional exhaust gas recirculation method is used instead of an interstage vacuum pump, the system's specific energy consumption will rise to 1.01 MJ / kgCO2, and the capture cost will rise to $31.59 / tonCO2. The process of this invention demonstrates superior economic advantages.

[0057] Example 2

[0058] This embodiment aims to verify the superior compatibility of the process with membrane materials of ordinary performance. For specific extreme conditions, a low-cost, low-selectivity membrane material (CO2 permeation flux 1000 GPU, ideal CO2 / N2 selectivity only 20) is selected. In traditional direct-flow processes or conventional two-stage processes with recirculation, when the selectivity drops to 20, the separation limit prevents the achievement of a 95% purity standard (the product is generally between 80% and 88%), leading to process failure. The process flow of this embodiment is as follows: the absolute pressure of the first and second stage feeds is set to 0.6 MPa, and the interstage vacuum pump is maintained at a negative pressure of 0.02 MPa. Under this topology, the gas recirculated from the permeate side of the second-stage membrane module (containing a relatively high concentration of leaked CO2, close to 39.8%) returns to the feed end, greatly increasing the absolute CO2 partial pressure on the first-stage feed side. Simultaneously, the strong suction of the interstage vacuum pump at 0.02 MPa compensates for the originally lacking mass transfer power of the low-selectivity membrane. The system successfully outputs CO2 product gas with a purity of 95% and meets the 90% total recovery rate. The total energy consumption is 1.48 MJ / kgCO2, demonstrating that the underlying process architecture of this invention can significantly relax the stringent requirements for membrane material selectivity in industrial applications.

[0059] Example 3

[0060] Maintaining a feed CO2 concentration of 15%, membrane flux of 1000 GPU, and selectivity of 50. Sub-example 3A (low-pressure end): The absolute pressure of the feed for both the first and second stages was set to the lower limit of 0.3 MPa as defined in this invention, and the interstage vacuum was maintained at 0.02 MPa. Results showed that although the power of a single compressor unit decreased, the required total membrane area soared to approximately 23,500 m² due to the smaller overall pressure difference, but the technical specifications of 95% purity and 90% recovery rate were still achieved. This proves that 0.3 MPa is a reasonable lower limit that can achieve the purpose of the invention while taking into account the limitations of membrane module space arrangement. Sub-example 3B (high-pressure end): The absolute pressure of the feed was set to the upper limit of 0.7 MPa. At this point, the transmembrane pressure difference was extremely large, and the required total membrane area was reduced to a very small 5,800 m². 2 However, the energy consumption of the first and second stage compressors increases sharply with the pressure ratio, and the system specific energy consumption rises to about 1.49 MJ / kgCO2. This proves that when the pressure exceeds 0.7 MPa, the equipment depreciation benefits caused by the reduction in membrane area will be completely offset by the soaring electricity costs. Therefore, 0.7 MPa is the upper limit of the economically feasible operating range.

[0061] Example 4: Quantitative comparison and techno-economic analysis of three two-stage membrane separation process configurations

[0062] The following comparison examines the impact of three different separation component setups on separation performance. The test conditions and economic evaluation criteria are as follows:

[0063]

[0064]

[0065] Concept A (Stepless Vacuum Coupling Technology)

[0066] Concept A relies entirely on the feed-side compressor for transmembrane mass transfer, requiring both feed pressures to be increased to 1.2 MPa, while the permeate side maintains an atmospheric pressure of 0.1 MPa. Results show that, without the negative pressure traction on the permeate side, the CO2 purity of the system product gas is only 85.93%, and the total recovery rate is 78.65%, both the lowest among the three configurations. Furthermore, the high feed pressure of 1.2 MPa significantly increases the load and operating energy consumption of the front-end compressor, resulting in a system CO2 capture cost as high as 1103 CNY / ton. This indicates that simply increasing the feed pressure to enhance mass transfer not only fails to achieve efficient separation but also leads to a sharp deterioration in system economics.

[0067] Concept B (Two-stage vacuum coupling process)

[0068] Concept B incorporates vacuum pumps (operating pressure 0.02 MPa) on the permeate side of both membrane units, effectively reducing the pressure requirement on the feed side (down to 0.9 MPa) by increasing the transmembrane pressure differential. Under this strong negative pressure, the system achieves a maximum total CO2 recovery rate of 89.11%. However, the extremely low absolute pressure of 0.02 MPa on the second-stage permeate side leads to an excessively large transmembrane pressure differential, intensifying competitive permeation of N2 molecules and causing the final product gas CO2 purity to deteriorate to 90.23%. In terms of techno-economics, the addition of the second-stage vacuum pump to achieve the maximum recovery rate increases the system's fixed equipment investment and operating power consumption. This additional cost completely offsets the economic benefits of additional CO2 capture, resulting in a final capture cost of 983 CNY / ton, failing to achieve further cost optimization.

[0069] Concept C (Single-stage vacuum coupling process)

[0070] Concept C employs an asymmetric pressure field configuration, applying a negative pressure of 0.02 MPa only on the permeate side of the first-stage membrane unit, while maintaining a normal pressure of 0.1 MPa on the permeate side of the second-stage membrane, and keeping the feed pressure at 0.9 MPa. This configuration achieves efficient CO2 pre-enrichment through the strong negative pressure in the first stage; simultaneously, the vacuum environment is removed in the second stage, avoiding excessive transmembrane pressure differential and effectively suppressing excessive N2 permeation, resulting in a maximum CO2 purity of 90.86% in the product gas. While ensuring a high recovery rate of 85.96%, this process avoids redundant investment and energy consumption in the second-stage vacuum pump, minimizing capture costs to 982 CNY / ton.

[0071] Comprehensive technical and economic evaluation

[0072] Comparing the three process topologies, the introduction of interstage vacuum suction (Concept C), compared to pure positive pressure drive (Concept A), resulted in an absolute improvement of CO2 purity and recovery rate of 4.93% and 7.31%, respectively, with a significant reduction in capture cost of approximately 10.97%, under a 25% decrease in feed pressure (from 1.2 MPa to 0.9 MPa). This quantitatively demonstrates the significant advantage of interstage negative pressure coupling in reconstructing the distribution of mass transfer driving force in the flow field. Furthermore, compared to two-stage vacuum coupling (Concept B), Concept C clarifies the necessity of a moderate negative pressure configuration, successfully avoiding the purity degradation and diminishing marginal utility problems caused by excessive suction. Therefore, Concept C achieves globally optimal separation performance and techno-economic efficiency with minimal equipment increment, making it a two-stage membrane capture process with significant potential for industrial application.

[0073]

Claims

1. A two-stage membrane capture process for carbon dioxide in flue gas based on interstage negative pressure coupling, characterized in that, Includes the following steps: (a) The carbon dioxide-containing feed gas is mixed with the recycle gas from step (d) to obtain a mixed feed gas; (b) The mixed feed gas is pressurized by the first stage compressor and then sent to the first stage membrane module for separation to obtain the first permeate gas rich in non-permeable components and the first permeate gas rich in carbon dioxide. (c) The first permeate gas is pressurized by the second stage compressor and then sent to the second stage membrane module for separation, and carbon dioxide product gas is obtained on the permeate side of the second stage membrane module. (d) The second permeate gas discharged from the permeate side of the second-stage membrane module is used as circulating gas and returned to step (a) to be mixed with the feed gas; and, between the permeate outlet of the first-stage membrane module and the air inlet of the second-stage compressor, the first permeate gas is drawn by an interstage vacuum pump located thereto, so as to form a negative pressure state on the permeate side of the first-stage membrane module.

2. The process according to claim 1, characterized in that, The absolute pressure on the permeate side of the first-stage membrane module is maintained at 0.02-0.05 MPa by the suction action of the interstage vacuum pump. The absolute pressure of the mixed feed gas entering the first-stage membrane module and the absolute pressure of the first permeate gas entering the second-stage membrane module are both independently controlled within the range of 0.3-0.7 MPa.

3. The process according to claim 3, characterized in that, The absolute pressure of the mixed feed gas entering the first-stage membrane module and the absolute pressure of the first permeate gas entering the second-stage membrane module are both set to 0.4-0.6 MPa. In step (c), the permeate side pressure of the second-stage membrane module is maintained at or near atmospheric pressure to obtain carbon dioxide product gas while suppressing the permeation of non-permeable components and ensuring the purity of the product gas.

4. The process according to claim 1, characterized in that, It also includes step (e): introducing the first residual gas with residual pressure discharged in step (b) into a turbine expander to expand and do work, so as to recover its pressure energy; The first permeate gas is cooled after leaving the interstage vacuum pump and before entering the second-stage compressor, and after leaving the second-stage compressor and before entering the second-stage membrane module.

5. The process according to claim 1, characterized in that, Before step (a), there is also a step of pre-treating the raw gas, which includes at least one of cooling, dust removal, desulfurization and dehydration.

6. The process according to claim 1, characterized in that, The process is operated such that the purity of the carbon dioxide product gas is not less than 95% and the total recovery rate of carbon dioxide is not less than 90%. The feed gas is the flue gas produced after the combustion of fossil fuels, in which the volume fraction of carbon dioxide is between 10-20%.

7. A two-stage membrane capture system for flue gas carbon dioxide based on interstage negative pressure coupling for implementing the process described in any one of claims 1-6, characterized in that, include: First-stage membrane module; Second-stage membrane module; The first-stage compressor is used to pressurize the feed gas entering the first-stage membrane module; A second-stage compressor is used to pressurize the feed gas entering the second-stage membrane module; The return line is used to connect the permeate outlet of the second-stage membrane module to the inlet of the first-stage compressor. And an interstage vacuum pump, whose intake port is connected to the permeate outlet of the first-stage membrane module, and whose exhaust port is connected to the intake port of the second-stage compressor.

8. The system according to claim 7, characterized in that, It also includes a mixer, whose first inlet is connected to the raw material gas supply line, the second inlet is connected to the return line, and its outlet is connected to the air inlet of the first-stage compressor.

9. The system according to claim 7, characterized in that, It also includes a turbine expander, which is installed on the permeate outlet pipeline of the first-stage membrane module to recover the pressure energy of the first permeate, and the power output shaft of the turbine expander is mechanically or electrically coupled to the power input end of the first-stage compressor or the second-stage compressor.

10. The system according to claim 7, characterized in that, The gas separation membranes packed in the first-stage and second-stage membrane modules have an ideal selectivity of not less than 20 for carbon dioxide and nitrogen. The permeate outlet of the second-stage membrane module is connected directly or via a pressure regulating valve to the product gas collection line, and no vacuum pump is installed on the outlet line to keep the outlet pressure below 0.1 MPa.

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