A coupling type purification method and device for a green methanol synthesis system

Abstract

The application discloses a kind of green methanol synthesis system's coupling type purification method, comprising: raw material gas is compressed and mixed with circulating gas, then methanol synthesis reaction is carried out;Reaction product is cooled to T1 after first stage, and first stage flash evaporation is carried out, to obtain crude methanol A and first stage flash gas;First stage flash gas is cooled to T2 after second stage, and second stage flash evaporation is carried out, to obtain crude methanol B and second stage flash gas, and second stage flash gas returns and is recycled in circulating gas compression unit;Crude methanol A is separated by membrane at temperature T1 and reaction pressure, to obtain crude methanol C and waste water;Crude methanol B and crude methanol C are sent into methanol rectification system and are rectified, to obtain refined methanol.The application also discloses a kind of green methanol synthesis system's coupling type purification device.The application is combined by "gradual cooling section separation" and "membrane separation+rectification coupling", and the comprehensive energy consumption of methanol separation and purification process is greatly reduced, reaction heat is fully recycled, so that the device can meet the energy consumption demand of separation by reaction heat.

Description

Technical Field

[0001] This invention relates to a coupled purification method and apparatus for a green methanol synthesis system, belonging to the field of green methanol production technology. Background Technology

[0002] Methanol is an important organic chemical raw material, solvent, and high-quality fuel, widely used in industries such as organic synthesis, pharmaceuticals, pesticides, coatings, fuels, plastics, automobiles, and defense. Supported by low-carbon policies, green methanol has a huge potential market in automotive fuel, fuel cells, marine fuel, and organic additives. The core definition of green methanol is low-carbon raw materials and renewable energy. Compared with traditional fossil-based methanol (coal / natural gas-based methanol), its life-cycle carbon emissions are reduced by 60%-95%, making it one of the clean energy sources with the greatest industrialization potential under the current carbon neutrality context. Its main processes include: biomass gasification to methanol, green hydrogen + air capture CO2 hydrogenation to methanol, and industrial tail gas coupled with green hydrogen to methanol, among which green hydrogen + air capture CO2 hydrogenation to methanol is the most highly regarded.

[0003] However, the production technology of green hydrogen + air capture CO2 hydrogenation to methanol faces many problems that urgently need to be solved. Compared with the traditional methanol synthesis process (which mainly uses CO and H2 to synthesize methanol), the reaction heat released in the CO2 and H2 reaction to synthesize methanol is halved, while the reaction produces more water. This leads to greater energy consumption in the subsequent methanol separation and purification process, and the reaction heat in the reaction section cannot meet such a large energy demand, requiring additional external heating. This results in excessively high energy consumption in the green methanol synthesis process, which seriously restricts the large-scale development of the green methanol industry. Summary of the Invention

[0004] The purpose of this invention is to provide a coupled purification method for a green methanol synthesis system, and also to provide a coupled purification device for a green methanol synthesis system. This invention significantly reduces the energy consumption of the green methanol synthesis process by combining a "stage-by-stage cooling and segmented separation" with a "multi-method coupled methanol separation and purification" process, achieving efficient recovery and utilization of reaction heat and reducing the operating cost of the device.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a coupled purification method for a green methanol synthesis system, comprising the following steps: a. The feed gas is compressed to the reaction pressure and mixed with the compressed circulating gas to form a mixed synthesis gas; b. The mixed synthesis gas is reacted in a methanol synthesis reactor to produce reaction products containing hydrogen, carbon dioxide, water, methanol and impurities; c. The reaction product is cooled to temperature T1 in the first stage, and then separated by flash evaporation in the first stage to obtain liquid crude methanol A and gaseous first-stage flash gas. d. The primary flash gas is cooled to temperature T2 in two stages, and then subjected to secondary flash separation to obtain liquid crude methanol B and secondary flash gas. The secondary flash gas is pressurized by the circulating gas compression unit and returned to step a as circulating gas. e. The crude methanol A is fed into a membrane separation system, and membrane separation is performed using the temperature T1 and pressure of the crude methanol A to obtain methanol-rich crude methanol C and wastewater. f. The crude methanol B and crude methanol C are fed into a methanol distillation system for distillation to obtain refined methanol.

[0006] Furthermore, the temperature T1 is 100–130°C, and the temperature T2 is 30–60°C.

[0007] Further, in step c, the first-stage cooling is achieved by sequentially exchanging heat between the reaction products and the mixed synthesis gas, and by generating steam through a steam generator. The steam generated by the steam generator is used to heat the reboiler of the methanol distillation system.

[0008] Furthermore, the membrane separation in step e is carried out at a temperature T1 and a reaction pressure of crude methanol A, without the need for vacuuming on the permeate side, and the resulting crude methanol C has a methanol content ≥95%.

[0009] Furthermore, in step f, the methanol distillation system includes a pressurized column 11 and an atmospheric column, and the reboiler of the atmospheric column is heated by the hot gas at the top of the pressurized column 11.

[0010] Furthermore, the methanol distillation system also includes a primary distillation column, the reboiler of the primary distillation column and the reboiler of the pressurized column are heated by the steam generated in step c, and the reboiler of the atmospheric column is heated by the hot gas at the top of the pressurized column.

[0011] The present invention also provides a coupled purification apparatus for a green methanol synthesis system for implementing the above method, comprising: The feed gas compression unit is used to compress the feed gas to the reaction pressure; The circulating gas compression unit is used to compress the circulating gas to the reaction pressure; The methanol synthesis reaction unit is connected to the feed gas compression unit and the recycle gas compression unit, respectively, and is used to receive the mixed synthesis gas and react to generate reaction products. The primary cooling unit is connected to the methanol synthesis reaction unit and is used to cool the reaction products to temperature T1. The primary flash unit is connected to the primary cooling unit and is used to separate the cooled reaction products into liquid crude methanol A and gaseous primary flash gas. The secondary cooling unit is connected to the gas phase outlet of the primary flash unit and is used to cool the primary flash gas to temperature T2. A secondary flash evaporation unit, connected to a secondary cooling unit, is used to separate the cooled primary flash gas into liquid crude methanol and secondary flash gas. The outlet of the secondary flash gas is connected to a circulating gas compression unit. The membrane separation unit is connected to the liquid phase outlet of the primary flash evaporation unit and is used to separate crude methanol A into methanol-rich crude methanol C and wastewater. The methanol distillation unit is connected to the crude methanol C outlet of the membrane separation unit and the liquid phase outlet of the secondary flash evaporation unit, respectively, and is used to distill crude methanol B and crude methanol C to obtain refined methanol. The primary cooling unit includes a steam generator, which generates steam using the heat from the reaction products. The steam outlet is connected to a steam pipeline and the steam is used to heat the reboiler of the methanol distillation unit. The methanol distillation unit includes a pressurized column and an atmospheric column. The hot gas outlet at the top of the pressurized column is connected to the reboiler of the atmospheric column.

[0012] Furthermore, the primary cooling unit also includes a heat exchanger for exchanging heat between the reaction products and the mixed syngas.

[0013] Furthermore, the membrane separation unit includes one or more membrane separation units, the inlet of crude methanol A is connected to the liquid phase outlet of the first-stage flash evaporation unit, and the outlet of crude methanol C of the membrane separation unit is connected to the methanol distillation unit.

[0014] Furthermore, the methanol distillation unit also includes a primary distillation column, the reboiler of the primary distillation column and the reboiler of the pressurized column are both heated by the steam, and the reboiler of the atmospheric column is connected to the hot gas outlet at the top of the pressurized column.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention employs a step-by-step cooling and segmented separation method, utilizing the boiling point difference between water and methanol to achieve the early condensation and separation of a large amount of water in the first-stage flash evaporation section, reducing the separation energy consumption of the subsequent methanol purification process by approximately 40%.

[0016] The crude methanol A obtained by the first-stage flash evaporation of this invention contains a large amount of water and is in a high temperature T1 and high pressure state, making it suitable for membrane separation. The temperature T1 of crude methanol A makes it easier for methanol to vaporize, and its pressure energy provides power for methanol permeation. There is no need to set up a vacuum step on the permeation side to obtain high-concentration crude methanol C, thus saving separation energy consumption.

[0017] The crude methanol C of this invention has a high temperature T1 and is directly fed into the methanol distillation system, utilizing its sensible heat and saving heating energy consumption in the distillation process.

[0018] This invention employs a methanol purification process that couples membrane separation with methanol distillation. The distribution relationship between primary and secondary condensation can be adjusted according to the composition of the reaction products, aiming for the most energy-efficient combination of membrane separation and distillation.

[0019] This invention utilizes the heat of reaction to generate low-pressure steam, which is used to heat the reboiler in the methanol distillation section, thus realizing the recovery and utilization of the heat of reaction. At the same time, it uses the hot gas from the top of the pressurized tower to heat the reboiler in the atmospheric tower, which saves heating energy consumption and reduces the amount of cooling water used at the top of the pressurized tower, achieving bidirectional energy saving. Attached Figure Description

[0020] Figure 1 Overall process flow diagram of the present invention; Figure 2 The process flow diagrams are for Embodiments 1 and 2 of the present invention; Figure 3 The process flow diagrams are for Embodiments 3 and 4 of the present invention; Figure Descriptions: 1-Feed gas compression unit, 2-Recycle gas compression unit, 3-Methanol synthesis reactor, 4-First-stage cooling, 5-First-stage flash evaporation, 6-Second-stage cooling, 7-Second-stage flash evaporation, 8-First-stage membrane separation, 9-Second-stage membrane separation, 10-Preliminary distillation column, 11-Pressure column, 12-Recovery column, 10U-Methanol rectification unit, 101-Feed gas, 102-Compressed feed gas, 201-Second-stage flash gas, 202-Recycle gas, 203-Mixed synthesis gas, 301-Heat synthesis gas, 302-Recycle gas, 203-Mixed synthesis gas, 301-Heat synthesis gas, 302-Recycle gas, 204-Methanol synthesis unit, 205-Methanol synthesis unit, 206-Methanol synthesis unit, 207-Methanol synthesis unit, 208-Methanol synthesis unit, 209-Methanol synthesis unit, 2000-Methanol synthesis unit, 2000-Methanol synthesis unit, 201-Methanol synthesis unit, 202 ... The products are as follows: 401 - Deoxygenated water, 402 - Low-pressure steam, 501 - Primary flash gas, 502 - Crude methanol A, 701 - Crude methanol B, 801 - Crude methanol C, 802 - Wastewater, 901 - Refined methanol C, 1001 - Light components, 1002 - Crude methanol D, 1003 - Condensate, 1101 - Refined methanol A, 1102 - Refined methanol B, 1103 - Pressurized tower overhead condensate, 1104 - Methanol-containing wastewater, 1105 - Condensate, 1201 - Refined methanol D, 1202 - Wastewater Detailed Implementation

[0021] The coupled purification method and apparatus of the green methanol synthesis system of the present invention will be described in detail below with reference to the accompanying drawings.

[0022] Example 1 of the present invention: A coupled purification method for a green methanol synthesis system of the present invention includes the following steps: a. The raw material gas 101 is compressed to the reaction pressure and mixed with the compressed circulating gas 202 to form a mixed synthesis gas 203; b. The mixed synthesis gas 203 is reacted in the methanol synthesis reactor 3 to generate reaction product 302 containing hydrogen, carbon dioxide, water, methanol and impurities. c. The reaction product 302 is cooled to temperature T1 in the first stage, and then separated by flash evaporation in the first stage to obtain liquid crude methanol A502 and gaseous first-stage flash gas. d. The primary flash gas is cooled to temperature T2 in two stages, and then subjected to secondary flash separation to obtain liquid crude methanol B701 and gaseous secondary flash gas. The secondary flash gas is returned to step a as circulating gas 202 after being pressurized by the circulating gas compressor. e. The crude methanol A502 is fed into a membrane separation system, and membrane separation is performed using the temperature T1 and pressure of the crude methanol A502 to obtain methanol-rich crude methanol C801 and wastewater. f. The crude methanol B701 and crude methanol C801 are fed into a methanol distillation unit for distillation to obtain refined methanol.

[0023] Specifically, the temperature T1 is 100-130°C, and the temperature T2 is 30-60°C.

[0024] Specifically, in step c, the first-stage cooling process is achieved by sequentially exchanging heat between the reaction product 302 and the mixed synthesis gas 203, and by generating steam through a steam generator. The steam generated by the steam generator is used to heat the reboiler of the methanol distillation system.

[0025] Specifically, the membrane separation in step e is carried out at a temperature T1 and a reaction pressure of crude methanol A502, without the need for vacuuming on the permeate side, and the resulting crude methanol C801 has a methanol content of ≥95%.

[0026] Specifically, in step f, the methanol distillation system includes a pressurized column 11 and an atmospheric column 12, and the reboiler of the atmospheric column 12 is heated by the hot gas at the top of the pressurized column 11.

[0027] Specifically, the methanol distillation system further includes a primary distillation column 10, the reboiler of the primary distillation column 10 and the reboiler of the pressurized column 11 are heated by the steam generated in step c, and the reboiler of the atmospheric column is heated by the hot gas at the top of the pressurized column 11.

[0028] Embodiment 2 of the present invention: A coupled purification device for a green methanol synthesis system, comprising: The raw material gas compression unit 1 is used to compress the raw material gas 101 to the reaction pressure; The circulating gas compression unit 2 is used to compress the secondary flash gas 201 to the reaction pressure; The methanol synthesis reaction unit is connected to the feed gas compression unit 1 and the circulating gas compression unit 2 respectively, and is used to receive the mixed synthesis gas 203 and react to generate reaction product 302. The primary cooling unit 4 is connected to the methanol synthesis reaction unit and is used to cool the reaction product 302 to temperature T1. The primary flash evaporation unit 5 is connected to the primary cooling unit and is used to separate the cooled reaction product 302 into liquid crude methanol A502 and gaseous primary flash gas. The secondary cooling unit 6 is connected to the gas phase outlet of the primary flash unit and is used to cool the primary flash gas to temperature T2. The secondary flash evaporation unit 7 is connected to the secondary cooling unit and is used to separate the cooled primary flash gas into liquid crude methanol B701 and secondary flash gas 201. The outlet of the secondary flash gas 201 is connected to the circulating gas compression unit 2. The membrane separation unit is connected to the liquid phase outlet of the primary flash evaporation unit and is used to separate crude methanol A502 into methanol-rich crude methanol C801 and wastewater. The methanol distillation unit is connected to the crude methanol C801 outlet of the membrane separation unit and the liquid phase outlet of the secondary flash evaporation unit, respectively, and is used to distill crude methanol B701 and crude methanol C801 to obtain refined methanol. The primary cooling unit includes a steam generator, which uses the heat from the reaction product 302 to generate steam. The steam outlet is connected to a steam pipeline and the steam is used to heat the reboiler of the methanol distillation unit. The methanol distillation unit includes a pressurized column 11 and an atmospheric column 12. The hot gas outlet at the top of the pressurized column 11 is connected to the reboiler of the atmospheric column 12.

[0029] Specifically, the primary cooling unit also includes a heat exchanger for exchanging heat between the reaction product 302 and the mixed synthesis gas 203.

[0030] Specifically, the membrane separation unit includes one or more membrane separation units, the inlet of the crude methanol A502 is connected to the liquid phase outlet of the first-stage flash evaporation unit, and the outlet of the crude methanol C801 of the membrane separation unit is connected to the methanol distillation unit.

[0031] Specifically, the methanol distillation unit further includes a primary distillation column 10, the reboiler of the primary distillation column 10 and the reboiler of the pressurized column 11 are both heated by the steam, and the reboiler of the atmospheric column is connected to the hot gas outlet at the top of the pressurized column. This embodiment employs a combination of "staged cooling and separation" and "membrane separation and methanol distillation separation" to reduce the water content in the distillation feed, thereby reducing distillation energy consumption. Simultaneously, membrane separation consumes no additional energy, achieving the goal of reducing overall energy consumption. The CO2 hydrogenation to methanol unit constructed using the process scheme of Embodiment 2 of this patent can meet the methanol separation energy consumption solely through the reaction heat of the reaction section, requiring no external energy input and even enabling the external supply of low-pressure steam.

[0032] Table 1: Comparison of Example 2 with Traditional Solution

[0033] Example 3 of the present invention: A coupled purification method for a green methanol synthesis system, comprising the following steps: feed gas compression, recycle gas compression, methanol synthesis reaction, first-stage cooling 4, first-stage flash evaporation 5, second-stage cooling 6, second-stage flash evaporation 7, first-stage membrane separation 8, second-stage membrane separation 9, and methanol distillation. The specific details are as follows: Raw material gas 101 is compressed to 8.0-10 MPa by a raw material gas compressor and mixed with circulating gas 202 from a circulating gas compressor. Then, it is exchanged with reaction product 302 to the required temperature (200-230℃) and enters methanol synthesis reactor 3. The reaction product 302 first exchanges heat with the hot mixed raw material, then generates steam through a steam generator, and exchanges heat with the mixed raw material again before its temperature drops to T1 (130-100℃), completing the first-stage cooling. It is then sent to the first-stage flash evaporation stage to separate crude methanol A502 (methanol content 40-45%) and first-stage flash gas. Crude methanol A502 is sent to the first-stage membrane separation section to separate crude methanol C and wastewater. The crude methanol C is then separated by a second-stage membrane separation stage to separate refined methanol C and wastewater. Refined methanol C (methanol content 98.5-99.5%) is directly sent out as a product.

[0034] The primary flash vapor phase is further cooled to T2 (30-60℃) and sent to the secondary flash phase, where it is separated into crude methanol B701 (methanol content 65-75%) and secondary flash vapor. The secondary flash vapor 201 is sent to the circulating gas compression unit for compression and reuse, while the crude methanol B701 is depressurized and flashed to be sent to the methanol distillation section.

[0035] The methanol distillation section consists of three columns: a primary distillation column 10, a pressurized column 11, and an atmospheric distillation column 12. The primary distillation column 10 removes gases and impurities with boiling points lower than methanol from the top. The bottom liquid is pumped to the pressurized column 11, where it distills refined methanol A1101 at the top. The bottom wastewater is sent to the atmospheric distillation column, where it recovers residual methanol from the wastewater at the top to obtain refined methanol B1102. The bottom wastewater is discharged from the system. All three columns are equipped with reboilers. The heat source for the reboilers in the primary distillation column 10 and the pressurized column 11 is low-pressure steam generated by the heat of reaction. The heat source for the reboiler in the atmospheric distillation column is the hot gas from the top of the pressurized column.

[0036] Table 2: Comparison of Example 3 and Traditional Solution

[0037] Example 4 of the present invention: A coupled purification device for a green methanol synthesis system, comprising: The raw material gas compression unit 1 is used to compress the raw material gas 101 to the reaction pressure; The circulating gas compression unit 2 is used to compress the secondary flash gas 201 to the reaction pressure; The methanol synthesis reaction unit is connected to the feed gas compression unit 1 and the circulating gas compression unit 2 respectively, and is used to receive the mixed synthesis gas 203 and react to generate reaction product 302. The primary cooling unit 4 is connected to the methanol synthesis reaction unit and is used to cool the reaction product 302 to temperature T1. The primary flash evaporation unit 5 is connected to the primary cooling unit and is used to separate the cooled reaction product 302 into liquid crude methanol A502 and gaseous primary flash gas. The secondary cooling unit 6 is connected to the gas phase outlet of the primary flash unit and is used to cool the primary flash gas to temperature T2. The secondary flash evaporation unit 7 is connected to the secondary cooling unit and is used to separate the cooled primary flash gas into liquid crude methanol B701 and secondary flash gas 201. The outlet of the secondary flash gas 201 is connected to the circulating gas compression unit 2. The primary membrane separation unit is connected to the liquid phase outlet of the primary flash evaporation unit and is used to separate crude methanol A502 into methanol-rich crude methanol C801 and wastewater. The secondary membrane separation unit is connected to the crude methanol C outlet of the primary membrane separation unit and is used to separate crude methanol C into refined methanol C and wastewater. The primary distillation column is connected to the liquid phase outlet of the secondary flash distillation unit and is used to remove gases and impurities with boiling points lower than methanol from the top of the column. The crude methanol D outlet at the bottom of the column is connected to the pressurization column. A pressurized tower, connected to crude methanol D, is used to separate crude methanol D into refined methanol A at the top of the tower and methanol-containing wastewater 1104 at the bottom of the tower. The outlet of the methanol-containing wastewater is connected to an atmospheric pressure tower. An atmospheric pressure tower, connected to the methanol-containing wastewater 1104, is used to recover methanol from the methanol-containing wastewater and produce refined methanol B at the top of the tower, while the wastewater at the bottom of the tower is sent out.

[0038] The primary cooling unit includes a steam generator, which is used to generate steam using the heat of the reaction product 302. The steam outlet is connected to a steam pipeline and the steam is used to heat the reboiler of the methanol distillation unit. The atmospheric column is equipped with a bottom reboiler, and the hot gas outlet at the top of the pressurized column 11 is connected to the reboiler of the atmospheric column 12.

[0039] Specifically, the primary cooling unit also includes a heat exchanger for exchanging heat between the reaction product 302 and the mixed synthesis gas 203.

[0040] Specifically, the inlet of crude methanol A502 is connected to the liquid phase outlet of the first-stage flash evaporation unit, the outlet of crude methanol C801 of the first-stage membrane separation unit is connected to the second-stage membrane separation unit, and the refined methanol C from the second-stage membrane separation unit is directly sent out as a product.

[0041] Specifically, the primary distillation column, the pressurized column, and the atmospheric column are all equipped with bottom reboilers. The reboilers of the primary distillation column 10 and the pressurized column 11 are heated by the steam. The reboiler of the atmospheric column is connected to the hot gas outlet at the top of the pressurized column.

[0042] This embodiment employs a combination of "staged cooling and separation" and "membrane separation and methanol distillation" to obtain refined methanol products. First, a portion of the methanol product, refined methanol C901 (30-40% of the total methanol in the refined methanol C901 station), is obtained in the membrane separation section. This reduces the processing volume in the distillation section and lowers the water content in the distillation feed, thereby reducing distillation energy consumption. Simultaneously, membrane separation consumes almost no additional energy, achieving the goal of reducing overall energy consumption. Using the process scheme of this patent embodiment, the CO2 hydrogenation to methanol plant can meet the methanol separation energy consumption solely through the reaction heat of the reaction section, requiring no external energy input and even allowing the external supply of low-pressure steam 402.

[0043] Table 3: Comparison of Example 4 with Traditional Solution

[0044] Compared with existing technologies, this invention achieves a significant reduction in separation energy consumption and the cascaded utilization of energy by combining a "step-by-step cooling and segmented separation" and a "membrane separation + distillation coupling" process. It utilizes the boiling point difference between water and methanol to achieve pre-separation of water, utilizes the temperature and pressure of crude methanol A502 to achieve energy-free membrane separation, utilizes the sensible heat of crude methanol C801 to reduce the distillation heating load, utilizes the reaction heat to generate steam for distillation heating, and utilizes the hot gas from the top of pressurized tower 11 to heat the reboiler of the atmospheric tower.

[0045] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A coupled purification method for a green methanol synthesis system, characterized in that, Includes the following steps: a. The feed gas is compressed to the reaction pressure and mixed with the compressed circulating gas to form a mixed synthesis gas; b. The mixed synthesis gas is reacted in a methanol synthesis reactor to produce reaction products containing hydrogen, carbon dioxide, water, methanol and impurities; c. The reaction product is cooled to temperature T1 in the first stage, and then separated by flash evaporation in the first stage to obtain liquid crude methanol A and gaseous first-stage flash gas. d. The primary flash gas is cooled to temperature T2 in two stages, and then subjected to secondary flash separation to obtain liquid crude methanol B and secondary flash gas. The secondary flash gas is pressurized by the circulating gas compression unit and returned to step a as circulating gas. e. The crude methanol A is fed into a membrane separation system, and membrane separation is performed using the temperature T1 and pressure of the crude methanol A to obtain methanol-rich crude methanol C and wastewater. f. The crude methanol B and crude methanol C are fed into a methanol distillation system for distillation to obtain refined methanol.

2. The method according to claim 1, characterized in that, The temperature T1 is 100-130℃, and the temperature T2 is 30-60℃.

3. The method according to claim 1, characterized in that, In step c, the first-stage cooling is achieved by sequentially exchanging heat between the reaction products and the mixed synthesis gas, and by generating steam through a steam generator. The steam generated by the steam generator is used to heat the reboiler of the methanol distillation system.

4. The method according to claim 1, characterized in that, The membrane separation described in step e is carried out at the temperature T1 and reaction pressure of crude methanol A, and no vacuum is required on the permeate side.

5. The method according to claim 1, characterized in that, In step f, the methanol distillation system includes a pressurized column and an atmospheric column, and the reboiler of the atmospheric column is heated by the hot gas at the top of the pressurized column.

6. The method according to claim 5, characterized in that, The methanol distillation system also includes a primary distillation column, the reboiler of the primary distillation column and the reboiler of the pressurized column are heated by the steam generated in step c, and the reboiler of the atmospheric column is heated by the hot gas at the top of the pressurized column.

7. A coupled purification apparatus for implementing the method of any one of claims 1-6 in a green methanol synthesis system, characterized in that, include: The feed gas compression unit is used to compress the feed gas to the reaction pressure; The circulating gas compression unit is used to compress the circulating gas to the reaction pressure; The methanol synthesis reaction unit is connected to the feed gas compression unit and the recycle gas compression unit, respectively, and is used to receive the mixed synthesis gas and react to generate reaction products. The primary cooling unit is connected to the methanol synthesis reaction unit and is used to cool the reaction products to temperature T1. The primary flash unit is connected to the primary cooling unit and is used to separate the cooled reaction products into liquid crude methanol A and gaseous primary flash gas. The secondary cooling unit is connected to the gas phase outlet of the primary flash unit and is used to cool the primary flash gas to temperature T2. A secondary flash evaporation unit, connected to a secondary cooling unit, is used to separate the cooled primary flash gas into liquid crude methanol B and secondary flash gas. The outlet of the secondary flash gas is connected to a circulating gas compression unit. The membrane separation unit is connected to the liquid phase outlet of the primary flash evaporation unit and is used to separate crude methanol A into methanol-rich crude methanol C and wastewater. The methanol distillation unit is connected to the crude methanol C outlet of the membrane separation unit and the liquid phase outlet of the secondary flash evaporation unit, respectively, and is used to distill crude methanol B and crude methanol C to obtain refined methanol. The primary cooling unit includes a steam generator, which generates steam using the heat from the reaction products. The steam outlet is connected to a steam pipeline and the steam is used to heat the reboiler of the methanol distillation unit. The methanol distillation unit includes a pressurized column and an atmospheric column. The hot gas outlet at the top of the pressurized column is connected to the reboiler of the atmospheric column.

8. The apparatus according to claim 7, characterized in that, The primary cooling unit also includes a heat exchanger for exchanging heat between the reaction products and the mixed syngas.

9. The apparatus according to claim 7, characterized in that, The membrane separation unit includes one or more membrane separation units. The inlet of crude methanol A is connected to the liquid phase outlet of the first-stage flash evaporation unit, and the outlet of crude methanol C of the membrane separation unit is connected to the methanol distillation unit.

10. The apparatus according to claim 7, characterized in that, The methanol distillation unit also includes a primary distillation column, the reboiler of the primary distillation column and the reboiler of the pressurized column are both heated by the steam, and the reboiler of the atmospheric column is connected to the hot gas outlet at the top of the pressurized column.

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