Low-temperature methanol washing system and method suitable for low-pressure low-carbon dioxide raw gas

Abstract

The application discloses a low-temperature methanol washing system and method suitable for low-pressure and low-carbon dioxide raw material gas, and belongs to the field of gas purification. Through high-pressure rich-methanol non-precooling direct pressure reduction flash technology, effective gas is fully desorbed, and the recovery rate is increased to more than 98.5%. A three-gradient desorption low-pressure regeneration tower integrating pressure reduction flash, nitrogen stripping and gradient vacuum flash functions is arranged, CO2 desorption is strengthened, and the desorption rate is increased by 25-40%. External refrigeration is introduced as terminal cold compensation of lean methanol, the temperature of lean methanol is accurately controlled, CO2 in the purified gas is less than 10 ppm, and H2S is less than 0.1 ppm. Compared with the traditional process, the cold energy utilization efficiency is increased by 40-72%, the refrigeration power consumption is reduced by 35-63%, the system is simplified in equipment and heat exchange network, the process flow is reasonable, and the application has the characteristics of large operation flexibility, safety and reliability, high effective gas recovery rate, and significantly reduced investment and operation and maintenance cost.

Description

Technical Field

[0001] This invention relates to the field of gas purification technology, specifically to a low-temperature methanol washing system and method adapted to low-pressure, low-carbon dioxide feed gas, which is particularly suitable for the purification of syngas with low pressure and low carbon dioxide content. Background Technology

[0002] Low-temperature methanol washing is a mature and efficient gas purification technology widely used in coal chemical industry, natural gas processing, and other fields to remove acidic gases such as carbon dioxide and hydrogen sulfide from feed gas. Traditional low-temperature methanol washing processes are typically designed for feed gas with high pressure (e.g., 3.0~8.0 MPa) and high carbon dioxide content (e.g., 16%~53%). Under these typical operating conditions, carbon dioxide has a high partial pressure and high solubility in methanol. The system exhibits a significant self-cooling effect due to the large-scale desorption of carbon dioxide, and the process usually includes steps such as cooling the methanol-rich gas, regeneration using multi-stage flash evaporation, and nitrogen stripping.

[0003] However, when the feed gas is at low pressure (0.5~3.0MPa) and has a low carbon dioxide content (0.5%~16%), its carbon dioxide partial pressure is extremely low, leading to:

[0004] 1. The required methanol circulation volume is much larger than that of conventional processes, resulting in increased equipment size and investment;

[0005] 2. The total amount of carbon dioxide dissolved in methanol is small, and the cooling effect generated during desorption is weak, resulting in a higher overall operating temperature of the system and a deterioration in the absorption effect;

[0006] 3. If the conventional process is used to cool the methanol-rich material, the effective gases (such as hydrogen and carbon monoxide) will be difficult to desorb in the subsequent vacuum flash evaporation due to increased solubility, resulting in the loss of effective gases.

[0007] 4. The system's self-cooling capacity is insufficient, making it difficult to guarantee the purification level.

[0008] In existing technologies, CN111330377A simplifies the process and reduces investment by eliminating the independent H2S concentration tower and integrating equipment, but it cannot solve the problems of a surge in methanol circulation caused by low CO2 partial pressure and system temperature rise caused by weak desorption refrigeration. CN103157311A achieves deep CO2 removal by optimizing the flash evaporation and regeneration sequence, which requires lower operating temperatures or more refrigeration input, contradicting the technical problem of high system temperature and cold energy imbalance that this invention aims to solve. CN102527152A discloses an energy-saving low-temperature methanol washing process that combines multi-stage flash evaporation with vacuum regeneration. Its effectiveness depends on the considerable cold energy recovery brought about by the desorption of a large amount of CO2 during flash evaporation. However, under low pressure and low CO2 conditions, the total amount of desorbable CO2 is scarce, and its refrigeration effect is negligible, failing to solve the fundamental problem of decreased absorption efficiency caused by the overall high system temperature.

[0009] Therefore, existing technologies are mostly designed for energy saving and consumption reduction or equipment integration under conventional or specific high-pressure conditions, without disclosing how to effectively solve the unique technical challenges brought about by the aforementioned "low-pressure, low-CO2" conditions, especially the contradiction between high system temperature, cold energy imbalance, and effective gas recovery. Therefore, there is an urgent need in this field for a customized, low-investment solution for this specific operating condition. Summary of the Invention

[0010] The purpose of this invention is to overcome the shortcomings of the prior art and provide a low-temperature methanol washing system and method adapted to low-pressure, low-carbon dioxide feed gas.

[0011] To achieve the above objectives, the present invention employs the following technical solution: a low-temperature methanol washing system adapted to low-pressure, low-carbon dioxide feed gas, comprising:

[0012] The high-pressure low-temperature absorption unit is used for preliminary cooling, dehydration, low-temperature desulfurization, and decarbonization of the raw process gas. It includes a raw gas heat exchanger, a raw gas cooler, a high-pressure absorption tower, and a methanol cooler arranged in sequence. The high-pressure absorption tower has a decarbonization section in the upper section, a desulfurization section in the middle section, and a methanol-water separator in the lower section.

[0013] The medium-pressure low-temperature desorption unit is connected downstream of the high-pressure low-temperature absorption unit. It includes a sulfur-containing rich methanol pressure reducing valve, a sulfur-free rich methanol pressure reducing valve, a medium-pressure flash tower and a circulating gas compressor arranged in sequence. The upper section of the medium-pressure flash tower is equipped with a sulfur-free rich methanol medium-pressure flash tank and the lower section is equipped with a sulfur-containing rich methanol medium-pressure flash tank, which is used to recover dissolved hydrogen, carbon monoxide and part of carbon dioxide gas in the methanol-rich liquid.

[0014] A low-pressure, low-temperature regeneration unit is connected downstream of the medium-pressure, low-temperature desorption unit and is used for low-temperature desorption and regeneration of methanol-rich liquid. It consists of a first pressure reducing valve for sulfur-containing methanol, a pressure reducing valve for sulfur-free methanol, a low-pressure regeneration tower, a first-stage vacuum pump, a first-stage methanol circulation pump, the tube side of a methanol coiled heat exchanger, a second pressure reducing valve for sulfur-containing methanol, a first-stage vacuum pump, and a third pressure reducing valve for sulfur-containing methanol, arranged sequentially. The low-pressure regeneration tower has a hydrogen sulfide concentration section in the upper section, a first-stage vacuum regeneration section in the middle section, and a second-stage vacuum regeneration section in the lower section.

[0015] The methanol thermal regeneration unit, connected downstream of the low-pressure low-temperature regeneration unit, includes a second methanol circulation pump, a thermal regeneration tower, the shell side of a methanol coiled heat exchanger, and a methanol-water separation tower, used to completely remove residual acidic gases dissolved in methanol and moisture carried into the system by the raw material gas.

[0016] Furthermore, the shell-side inlet of the raw gas heat exchanger is connected to the raw gas pipeline, and the outlet is sequentially connected to the tube side of the raw gas cooler and the methanol-water separator pipeline in the lower section of the high-pressure absorption tower. The liquid phase outlet at the bottom of the methanol-water separator is connected to the feed inlet pipeline in the middle of the methanol-water separator, and the gas phase outlet is connected to the desulfurization section and the decarbonization section sequentially from bottom to top in the high-pressure absorption tower.

[0017] Furthermore, the low-temperature purified gas outlet of the high-pressure absorption tower and the low-temperature exhaust gas outlet at the top of the low-pressure regeneration tower are sequentially connected to the tube side of the raw material gas heat exchanger, as well as the inlet pipes of the purified gas pipeline and the exhaust gas pipeline, respectively.

[0018] Furthermore, the lean methanol liquid phase inlet at the top of the decarbonization section of the high-pressure absorption tower is connected to the tube-side outlet pipe of the methanol cooler, and the sulfur-free rich methanol outlet at the bottom is connected in sequence to the sulfur-free rich methanol pressure reducing valve and the inlet pipe of the sulfur-free rich methanol medium-pressure flash tank, as well as to the sulfur-free rich methanol inlet pipe at the top of the desulfurization section of the high-pressure absorption tower. The sulfur-containing rich methanol outlet at the bottom of the desulfurization section is connected in sequence to the sulfur-containing rich methanol pressure reducing valve and the inlet pipe of the sulfur-containing rich methanol medium-pressure flash tank.

[0019] Furthermore, the gas phase outlet of the sulfur-free rich methanol medium-pressure flash tank is connected to the inlet pipe of the circulating gas compressor, and the liquid phase outlet is connected to the liquid phase inlet pipe at the top of the hydrogen sulfide concentration section of the low-pressure regeneration tower via a sulfur-free methanol pressure reducing valve. The gas phase outlet of the sulfur-containing rich methanol medium-pressure flash tank is connected to the inlet pipe of the circulating gas compressor, and the liquid phase outlet is connected to the inlet pipe in the middle of the hydrogen sulfide concentration section via a sulfur-containing methanol first pressure reducing valve. The outlet of the circulating gas compressor is connected to the raw material gas pipeline via a pipeline.

[0020] Furthermore, the bottom liquid phase outlet of the hydrogen sulfide concentration section of the low-pressure regeneration tower is sequentially connected to the second pressure reducing valve of sulfur-containing methanol and the top liquid phase inlet pipe of the first-stage vacuum regeneration section of the low-pressure regeneration tower. The bottom liquid phase outlet of the first-stage vacuum regeneration section is sequentially connected to the third pressure reducing valve of sulfur-containing methanol and the top liquid phase inlet pipe of the second-stage vacuum regeneration section of the low-pressure regeneration tower. Its top gas phase outlet is sequentially connected to the first-stage vacuum pump and the gas phase inlet at the bottom of the hydrogen sulfide concentration section. A nitrogen pipeline is connected to the gas phase inlet at the bottom of the hydrogen sulfide concentration section via a pipeline. The bottom liquid phase outlet of the second-stage vacuum regeneration section is sequentially connected to the first methanol circulation pump, the tube side of the methanol coiled heat exchanger, and the upper liquid phase inlet of the thermal regeneration tower via a pipeline. Its top gas phase outlet is sequentially connected to the inlet pipes of the second-stage vacuum pump and the first-stage vacuum pump.

[0021] Furthermore, the top gas phase outlet of the thermal regeneration tower is connected to a sulfuric acid-containing gas pipeline via a pipe, and the bottom liquid phase outlet is connected in sequence to the shell side of the methanol coiled tube heat exchanger and the inlet of the second methanol circulating pump. The outlet of the second methanol circulating pump is divided into two paths: one path is connected to the inlet pipeline of the methanol cooler tube side, and the other path is connected to the upper feed inlet of the methanol-water separator and the raw material gas pipeline via pipes respectively. The top gas phase outlet of the methanol-water separator is connected to the gas phase feed inlet pipeline in the middle and lower part of the thermal regeneration tower, and the bottom liquid phase outlet is connected to a water pipeline via a pipe.

[0022] The low-temperature methanol washing method is as follows:

[0023] After the raw gas is cooled down by two stages to -30°C to -45°C, it enters the methanol-water separator in the lower section of the high-pressure absorption tower. Liquid methanol and aqueous solution are condensed and separated at the bottom of the tower.

[0024] The sulfur-free and sulfur-containing rich methanol extracted from the bottom of the decarbonization and desulfurization sections of the high-pressure absorption tower are depressurized to 0.2 to 1.0 MPa directly without passing through any cooler, and then enter the sulfur-free rich methanol medium-pressure flash tank and the sulfur-containing rich methanol medium-pressure flash tank respectively for depressurization flash evaporation to recover effective gas.

[0025] The sulfur-containing methanol collected from the bottom of the hydrogen sulfide concentration section is further depressurized to 0.03 to 0.08 MPa and then enters the top inlet of the first-stage vacuum regeneration section for first-stage vacuum flash evaporation. After that, the sulfur-containing methanol collected from the bottom of the first-stage vacuum regeneration section is further depressurized to 0.01 to 0.03 MPa and then enters the top inlet of the second-stage vacuum regeneration section for second-stage vacuum flash evaporation to further desorb some of the carbon dioxide and a small amount of hydrogen sulfide gas contained in the sulfur-containing methanol.

[0026] The gas phase at the top of the secondary vacuum regeneration section is pressurized to 0.03 to 0.08 MPa by the secondary vacuum pump and then returned to the inlet of the secondary vacuum pump. The gas phase at the top of the primary vacuum regeneration section is pressurized to 0.13 to 0.26 MPa by the primary vacuum pump and then returned to the gas phase inlet at the bottom of the hydrogen sulfide concentration section. It flows from bottom to top and comes into countercurrent contact with the sulfur-free methanol from the top. The hydrogen sulfide in it is then absorbed back into the methanol.

[0027] Using sulfur-containing methanol at the bottom of the secondary vacuum regeneration section, which has a temperature of -42°C to -15°C, as a cold source, its temperature rises to 85°C to 115°C after recovering its cold energy through a methanol coil heat exchanger. At the same time, the temperature of the high-temperature methanol lean liquid from the bottom of the thermal regeneration tower is reduced to -40°C to -13°C.

[0028] The lean methanol solution from the shell-side outlet of the methanol coil heat exchanger is cooled to -30°C to -45°C by the refrigerant in the methanol cooler before being returned to the top of the high-pressure absorption tower.

[0029] Furthermore, the operating pressure of the medium-pressure flash tower is 0.2 to 1.0 MPa, and the temperature is -38°C to -15°C. The liquid phase inlets of the sulfur-free rich methanol medium-pressure flash tank and the sulfur-containing rich methanol medium-pressure flash tank are directly connected to the liquid phase outlets at the bottom of the decarbonization section and the desulfurization section of the high-pressure absorption tower via pipes through the sulfur-free rich methanol pressure reducing valve and the sulfur-containing rich methanol pressure reducing valve, respectively.

[0030] Furthermore, the operating pressure of the primary vacuum regeneration section of the low-pressure regeneration tower is 0.02 to 0.08 MPa, and the temperature is -38°C to -12°C. Its top gas outlet is connected to the gas phase inlet pipe at the bottom of the hydrogen sulfide concentration section of the medium-pressure flash tower via a primary vacuum pump. Its bottom liquid phase outlet is sequentially connected to the third pressure reducing valve for sulfur-containing methanol and the top liquid phase inlet pipe of the secondary vacuum regeneration section of the low-pressure regeneration tower. The operating pressure of the secondary vacuum regeneration section of the low-pressure regeneration tower is 0.01 to 0.03 MPa, and the temperature is -42°C to -15°C. Its bottom liquid phase outlet is sequentially connected to the first methanol circulation pump, the tube side of the methanol coiled heat exchanger, and the upper liquid phase inlet of the thermal regeneration tower via pipes. Its top gas phase outlet is sequentially connected to the inlet pipes of the secondary vacuum pump and the primary vacuum pump.

[0031] Furthermore, in the purified gas at the top of the high-pressure absorption tower, the removal rate of carbon dioxide can be controlled within the range of 40.0% to 99.9% depending on the downstream demand, while the removal rate of hydrogen sulfide is greater than 99.9%.

[0032] Furthermore, the carbon dioxide content in the circulating gas separated from the top of the medium-pressure flash evaporator is 8.0% to 45%, and the effective gas recovery rate is above 98.5%.

[0033] Furthermore, the total investment of the system is reduced by 40% to 60%, the total energy consumption is reduced by 35% to 65%, and the total sulfur content in the exhaust gas is less than 3 ppm.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. To address the problem that pre-cooling of methanol-rich gas in traditional processes leads to increased effective gas solubility and difficulty in desorption during subsequent flash evaporation, this invention eliminates the pre-cooling step and maintains a high flash evaporation temperature (-42℃ to -15℃). By utilizing the direct influence of temperature on gas solubility, it ensures that the effective gas is fully desorbed during the instant of depressurization. The effective gas recovery rate is increased from less than 87.0% in traditional processes to more than 98.5%, and the effective gas loss rate is reduced to below 0.45% under typical operating conditions.

[0036] 2. Under low-pressure, low-carbon dioxide conditions, the desorption driving force of traditional vacuum flash evaporation and nitrogen stripping is insufficient. This invention, based on the traditional hydrogen sulfide concentration tower, integrates vacuum flash evaporation, nitrogen stripping, and staged vacuum flash evaporation into a three-gradient desorption low-pressure regeneration tower. First, vacuum flash evaporation and nitrogen stripping are used to desorb as much dissolved carbon dioxide as possible from methanol, reducing the power consumption of the downstream primary vacuum pump. Then, a secondary vacuum flash evaporation is performed under reduced pressure to reduce the power consumption of the secondary vacuum pump. Finally, a deep vacuum environment (0.01 to 0.03 MPa) is established to significantly reduce the partial pressure of carbon dioxide in the methanol liquid phase, creating a strong desorption driving force. This ensures that even with a low total amount of carbon dioxide, it can still be fully desorbed, increasing the desorption rate by 25% to 40% or more, resulting in a fully low-temperature regenerated methanol stream, providing a stable source of cooling for the system.

[0037] 3. Under low-pressure and low-carbon dioxide conditions, the system's self-cooling capacity is severely insufficient. This invention explicitly introduces external cooling as a supplement to the final cooling capacity of lean methanol. By precisely controlling the final cooling temperature of lean methanol (-30℃ to -45℃), sufficient mass transfer driving force is ensured at the top of the high-pressure absorption tower, overcoming the problem of decreased absorption efficiency caused by the overall high system temperature. A stable cooling supply system independent of the composition of the raw gas is established, significantly enhancing the system's anti-interference ability. The temperature of lean methanol can be stably controlled within the design range, ensuring the product quality of the purified gas (carbon dioxide less than 5ppm and hydrogen sulfide less than 0.1ppm).

[0038] 4. This invention achieves efficient utilization of cooling capacity while ensuring effective gas recovery rate through cascaded refrigeration and precise temperature control strategies. Compared with traditional processes, the cooling capacity utilization efficiency is increased by 40% to 72%, and the cooling power consumption is reduced by 35% to 63% under the same processing capacity.

[0039] 5. This invention is not a simple superposition of technologies, but rather constitutes an organic overall solution that systematically solves the special technical challenges under low-pressure and low-carbon dioxide content conditions. The synergy between "no precooling" and "staged vacuum flash evaporation" achieves precise separation of effective gas and acidic gas. Effective gas is recovered in the first-stage high-temperature flash evaporation, while carbon dioxide is removed in the second-stage staged vacuum flash evaporation. Furthermore, the synergy between "staged vacuum flash evaporation" and "external refrigeration" allows the limited cooling capacity generated by vacuum flash evaporation to be efficiently recovered for precooling lean methanol, which reduces the external refrigeration load while allowing the external refrigeration system to compensate for the insufficient cooling capacity of vacuum flash evaporation.

[0040] 6. The entire system is designed for specific operating conditions, simplifying equipment and heat exchange networks, with a reasonable process and low investment, operation and maintenance costs. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the process flow of the system of the present invention;

[0042] In the diagram, 1. Feed gas heat exchanger; 2. High-pressure absorption tower; 3. Methanol-water separator; 4. Desulfurization section; 5. Decarbonization section; 6. Methanol cooler; 7. Sulfur-rich methanol pressure reducing valve; 8. Sulfur-free rich methanol pressure reducing valve; 9. Sulfur-free rich methanol medium-pressure flash tank; 10. Sulfur-rich methanol medium-pressure flash tank; 11. Medium-pressure flash tower; 12. Circulating gas compressor; 13. Sulfur-containing methanol first pressure reducing valve; 14. Sulfur-free methanol pressure reducing valve; 15. Low-pressure regeneration tower; 16. First-stage vacuum pump; 17. First-stage vacuum regeneration section; 18. Hydrogen sulfide concentration section; 19. First methanol circulation pump; 20. Second methanol circulation pump; 21. Feed gas cooler; 22. Thermal regeneration tower; 23. Methanol coiled tube heat exchanger; 24. Methanol-water separator; 25. Sulfur-containing methanol second pressure reducing valve; 26. Second-stage vacuum pump; 27. Second-stage vacuum regeneration section; 28. Sulfur-containing methanol third pressure reducing valve. Detailed Implementation

[0043] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0044] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0045] The present invention will now be described in further detail with reference to the accompanying drawings:

[0046] See Figure 1 , Figure 1 The diagram illustrates the system structure of this invention. A low-temperature methanol washing system adapted to low-pressure, low-carbon dioxide feed gas mainly includes: a feed gas heat exchanger 1, a high-pressure absorption tower 2, a methanol-water separator 3, a desulfurization section 4, a decarbonization section 5, a methanol cooler 6, a sulfur-containing rich methanol pressure reducing valve 7, a sulfur-free rich methanol pressure reducing valve 8, a sulfur-free rich methanol medium-pressure flash tank 9, a sulfur-containing rich methanol medium-pressure flash tank 10, a medium-pressure flash tower 11, a circulating gas compressor 12, a sulfur-containing methanol first pressure reducing valve 13, a sulfur-free methanol pressure reducing valve 14, a low-pressure regeneration tower 15, a primary vacuum pump 16, a primary vacuum regeneration section 17, a hydrogen sulfide concentration section 18, a first methanol circulation pump 19, a second methanol circulation pump 20, a feed gas cooler 21, a thermal regeneration tower 22, a methanol coiled tube heat exchanger 23, a methanol-water separator 24, a sulfur-containing methanol second pressure reducing valve 25, a secondary vacuum pump 26, a secondary vacuum regeneration section 27, and a sulfur-containing methanol third pressure reducing valve 28.

[0047] After the raw gas is combined with methanol and circulating gas from the second methanol circulation pump 20 and the circulating gas compressor 12, it enters the shell side of the raw gas heat exchanger 1 and the tube side of the raw gas cooler 21 through a common pipeline. Methanol is injected into the raw gas to prevent moisture in the raw gas from freezing when cooled below zero degrees Celsius. The cold source for the raw gas heat exchanger 1 is the low-temperature purified gas from the top of the downstream high-pressure absorption tower 2 and the cold exhaust gas from the top of the low-pressure regeneration tower 15. This gas is reheated in the raw gas heat exchanger 1 and then sent out of the boundary area. The cold source for the raw gas cooler 21 is an external refrigerant. After the above two-stage cooling, the temperature of the raw gas drops to -30°C to -45°C, and partial condensation occurs. Then, it enters the methanol-water separator 3 of the high-pressure absorption tower 2, where liquid methanol and water are separated at the bottom and enter the middle of the methanol-water separator 24 for methanol-water separation. The top gas phase enters the bottom of the desulfurization section 4 of the high-pressure absorption tower 2 and flows upward within the tower, passing through the desulfurization section 4 and the decarbonization section 5 in sequence. It then comes into full counter-current contact with the sulfur-free, rich methanol (temperature -36℃ to -12℃) flowing downward under gravity from the bottom of the decarbonization section 5 and the lean methanol (temperature -30℃ to -45℃) from the methanol cooler 6, respectively. Under the high pressure and low temperature environment of the high-pressure absorption tower 2, this process absorbs acidic gases from the feed gas. The removal rate of carbon dioxide from the feed gas can be controlled within the range of 40.0% to 99.9%, depending on downstream requirements, while the removal rate of hydrogen sulfide is greater than 99.9%. Ultimately, the purified feed gas achieves a carbon dioxide content of less than 5 ppm and a hydrogen sulfide content of less than 0.1 ppm.

[0048] The sulfur-free and sulfur-containing rich methanol extracted from the bottom of the decarbonization section 5 and desulfurization section 4 of the high-pressure absorption tower 2 are directly depressurized to 0.2 to 1.0 MPa and at a temperature of -38°C to -15°C via the sulfur-free rich methanol pressure reducing valve 8 and the sulfur-containing rich methanol pressure reducing valve 7, respectively, without passing through any cooler. They then enter the sulfur-free rich methanol medium-pressure flash tank 9 and the sulfur-containing rich methanol medium-pressure flash tank 10 of the medium-pressure flash tower 11 for flash evaporation. The resulting flash vapor is pressurized by the circulating gas compressor 12 and sent to the raw material gas pipeline to recover hydrogen, carbon monoxide, and a small amount of carbon dioxide dissolved in the methanol-rich liquid. The carbon dioxide content in the separated circulating gas is 42% to 65%, and the effective gas recovery rate is over 98.5%.

[0049] The two methanol-rich liquids after flash evaporation are drawn from the bottom of the sulfur-free methanol-rich medium-pressure flash tank 9 and the sulfur-containing methanol-rich medium-pressure flash tank 10 of the medium-pressure flash tower 11, respectively. After being further depressurized to 0.12 to 0.25 MPa, they enter the top and middle liquid phase inlets of the hydrogen sulfide concentration section 18 of the low-pressure regeneration tower 15 in sequence. They come into countercurrent contact with the nitrogen gas introduced from the bottom of the tower, desorbing some carbon dioxide and a small amount of hydrogen sulfide gas. The small amount of hydrogen sulfide gas comes into countercurrent contact with the sulfur-free methanol from the top of the tower during the upward flow in the tower and is absorbed again. The sulfur content of the gas discharged from the top of the tower can be controlled below 3 ppm.

[0050] The sulfur-containing methanol collected from the bottom of the hydrogen sulfide concentration section 18 is further depressurized to 0.03 to 0.08 MPa by the second sulfur-containing methanol pressure reducing valve 25, and then enters the top inlet of the first-stage vacuum regeneration section 17. There, it undergoes vacuum flash evaporation to further desorb some of the carbon dioxide and a small amount of hydrogen sulfide gas from the sulfur-containing methanol. Afterwards, the gas phase at the top is pressurized to 0.12 to 0.25 MPa by the first-stage vacuum pump 16 and then returns to the gas phase inlet at the bottom of the hydrogen sulfide concentration section 18, flowing upwards in the opposite direction to the sulfur-free methanol from the top of the column. After sufficient contact, the hydrogen sulfide is reabsorbed into the methanol. Then, the sulfur-containing methanol collected from the bottom of the first-stage vacuum regeneration section 17 is further depressurized to 0.01 to 0.03 MPa and enters the top inlet of the second-stage vacuum regeneration section 27, where it undergoes second-stage vacuum flash evaporation to further desorb some of the carbon dioxide and a small amount of hydrogen sulfide gas contained in the sulfur-containing methanol. The gas phase generated by the second-stage vacuum flash evaporation is then pressurized to 0.03 to 0.08 MPa by the second-stage vacuum pump 26 and sent to the inlet of the first-stage vacuum pump 16.

[0051] In the above process, the three-gradient desorption low-pressure regeneration tower, which integrates reduced pressure flash evaporation, nitrogen stripping, and stepped vacuum flash evaporation, firstly desorbs as much dissolved carbon dioxide as possible from methanol through reduced pressure flash evaporation and nitrogen stripping to reduce the power consumption of the downstream primary vacuum pump. Then, it performs secondary vacuum flash evaporation under reduced pressure to reduce the power consumption of the secondary vacuum pump. Finally, it establishes a deep vacuum environment (0.01 to 0.03 MPa) to significantly reduce the partial pressure of carbon dioxide in the methanol liquid phase, creating a strong desorption driving force. This ensures that even when the total amount of carbon dioxide is low, it can still ensure its full desorption. Compared with the traditional process, the carbon dioxide desorption rate can be increased by 25% to 40% or more. The fully regenerated methanol stream also provides a stable source of cooling for the low-temperature methanol washing system.

[0052] The sulfur-containing methanol at the bottom of the secondary vacuum regeneration section 27 is the lowest temperature point of the circulating methanol, with a temperature of -42℃ to -15℃. After being pressurized by the first methanol circulation pump 19, it enters the tube side of the methanol coil heat exchanger 23. After being reheated to 85℃ to 115℃, it enters the upper middle inlet of the thermal regeneration tower 22. Through heating and distillation, the carbon dioxide and hydrogen sulfide gas dissolved in the sulfur-containing methanol are completely desorbed. A high concentration of acid gas can be obtained at the top of the tower, which is then collected into the sulfuric acid gas pipeline and sent out of the boundary area.

[0053] The lean methanol from the bottom of the thermal regeneration tower 22 first enters the shell side of the methanol coil heat exchanger 23 and is cooled to -40°C to -13°C. Then, it is pressurized to 0.8 to 3.5 MPa by the second methanol circulation pump 20 and splits into two streams. The stream with a larger flow rate enters the methanol cooler 6 as the main lean methanol liquid, and is cooled to -30°C to -45°C before returning to the top of the high-pressure absorption tower 2. The stream with a smaller flow rate splits into two streams again. The stream with a larger flow rate enters the upper part of the methanol-water separation tower 24, where methanol and water are separated by heating and distillation to achieve dehydration. The methanol vapor obtained from the top of the tower is sent to the middle and lower part of the thermal regeneration tower 22. The water obtained from the bottom of the methanol-water separation tower 24 is connected by a water pipe and sent out of the boundary area. The lean methanol with a smaller flow rate is used as antifreeze and enters the raw material gas pipeline.

[0054] The above analysis demonstrates the ingenious design of a low-temperature methanol washing system adapted to low-pressure, low-carbon dioxide feedstock gases. Through cascaded refrigeration and precise temperature control strategies, the system achieves efficient utilization of cooling capacity while ensuring effective gas recovery. Compared with traditional processes, cooling capacity utilization efficiency is increased by 40% to 72%, and cooling power consumption is reduced by 35% to 63% under the same processing capacity. Designed for specific operating conditions, the system simplifies equipment and heat exchange networks, has a rational process, and low investment, operation, and maintenance costs. It features simple operation, high load adjustment flexibility, and safe and reliable operation. The system ultimately produces high-purity, high-recovery-rate purified gas and supports continuous operation. This overall technical solution is highly economical and suitable for the field of syngas purification technology with low pressure and low carbon dioxide content.

[0055] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

[0056] Example 1

[0057] The feed gas used in this embodiment has the following volumetric composition: hydrogen 4.48%, carbon monoxide 92.14%, carbon dioxide 2.15%, sulfur-containing compounds 0.41%, and the remainder being nitrogen and other inert gases. The pressure is 1.6 MPa, the temperature is 40°C, and the flow rate is 221215 Nm³. 3 / h.

[0058] A low-temperature methanol washing system and method adapted to low-pressure, low-carbon dioxide feed gas, specifically including the following steps:

[0059] 1. The raw gas is combined with methanol and circulating gas from the second methanol circulation pump 20 and the circulating gas compressor 12, and then enters the shell side of the raw gas heat exchanger 1 and the tube side of the raw gas cooler 21 through a common pipeline. The cold source of the raw gas heat exchanger 1 is the low-temperature purified gas from the top of the high-pressure absorption tower 2 and the cold exhaust gas from the top of the low-pressure regeneration tower 15 downstream. After being reheated to 34°C in the raw gas heat exchanger 1, it is sent out of the boundary area. The cold source of the raw gas cooler 21 is an external refrigerant. After the above two-stage cooling, the temperature of the raw gas drops to -40°C and partial condensation occurs. Then, it enters the methanol-water separator 3 of the high-pressure absorption tower 2. Liquid methanol and water solution are separated at the bottom and enter the middle of the methanol-water separator 24 for methanol-water separation.

[0060] 2. The gas phase from the top of the methanol-water separator 3 sequentially enters the bottom of the desulfurization section 4 of the high-pressure absorption tower 2, and flows upward within the high-pressure absorption tower, passing through the desulfurization section 4 and the decarbonization section 5 in sequence. It comes into full counter-current contact with the sulfur-free rich methanol (temperature -36.2℃) flowing downward from the bottom of the decarbonization section 5 under gravity and the lean methanol (temperature -40℃) flowing from the methanol cooler 6 within the high-pressure, low-temperature environment of the high-pressure absorption tower 2, so as to absorb the acidic gases in the raw gas. Finally, the purified gas at the top of the decarbonization section 5 has a carbon dioxide content of 4.1 ppm and a hydrogen sulfide content of 0.08 ppm.

[0061] 3. The sulfur-free rich methanol and sulfur-containing rich methanol collected from the bottom of the decarbonization section 5 and desulfurization section 4 of the high-pressure absorption tower 2 are directly depressurized to 0.55 MPa through the sulfur-free rich methanol pressure reducing valve 8 and the sulfur-containing rich methanol pressure reducing valve 7 without passing through any cooler. The temperatures are -36.6℃ and -36.9℃, respectively, and they enter the sulfur-free rich methanol medium-pressure flash tank 9 and the sulfur-containing rich methanol medium-pressure flash tank 10 of the medium-pressure flash tower 11 for flash evaporation. The resulting flash vapor is pressurized by the circulating gas compressor 12 and sent to the raw material gas pipeline to recover hydrogen, carbon monoxide and a small amount of carbon dioxide dissolved in the methanol-rich liquid. The carbon dioxide content in the separated circulating gas is 13%, and the effective gas recovery rate is 98.6%.

[0062] 4. The two methanol-rich liquids after flash evaporation are drawn from the bottom of the sulfur-free methanol-rich medium-pressure flash tank 9 and the sulfur-containing methanol-rich medium-pressure flash tank 10 of the medium-pressure flash tower 11, respectively. After being further depressurized to 0.15 MPa, they enter the top and middle liquid phase inlets of the hydrogen sulfide concentration section 18 of the low-pressure regeneration tower 15 in sequence. They come into countercurrent contact with the nitrogen gas introduced from the bottom of the tower, desorbing some carbon dioxide and a small amount of hydrogen sulfide gas. The small amount of hydrogen sulfide gas comes into countercurrent contact with the sulfur-free methanol from the top of the tower during the upward flow in the tower and is absorbed again. The sulfur content of the gas discharged from the top of the tower is 0.29 ppm.

[0063] 5. The sulfur-containing methanol collected from the bottom of the hydrogen sulfide concentration section 18 is further depressurized to 0.06 MPa by the second sulfur-containing methanol pressure reducing valve 25, and then enters the top inlet of the first-stage vacuum regeneration section 17 of the medium-pressure flash distillation tower 11. Vacuum flash distillation is performed there to further desorb some carbon dioxide and a small amount of hydrogen sulfide gas from the sulfur-containing methanol. Afterwards, the gas phase at the top is pressurized to 0.155 MPa by the first-stage vacuum pump 16 and then returns to the gas phase inlet at the bottom of the hydrogen sulfide concentration section 18, flowing upwards and making full counter-current contact with the sulfur-free methanol from the top of the tower. The hydrogen sulfide in the gas phase is absorbed back into the methanol. Then, the sulfur-containing methanol collected from the bottom of the first-stage vacuum regeneration section 17 is further depressurized to 0.03 MPa and enters the top inlet of the second-stage vacuum regeneration section 27, where it undergoes second-stage vacuum flash evaporation to further desorb some of the carbon dioxide and a small amount of hydrogen sulfide gas contained in the sulfur-containing methanol. The gas phase from the second-stage vacuum flash evaporation is then pressurized to 0.03 to 0.08 MPa by the second-stage vacuum pump 26 and sent to the inlet of the first-stage vacuum pump 16. The total power consumption of the two-stage vacuum pumps is 432 KW.

[0064] 6. The sulfur-containing methanol at the bottom of the secondary vacuum regeneration section 27 is the lowest temperature point of the circulating methanol, at -38.5℃. After being pressurized by the first methanol circulation pump 19, it enters the tube side of the methanol coil heat exchanger 23. After being reheated to 97℃, it enters the upper middle inlet of the thermal regeneration tower 22. Through heating and distillation, the carbon dioxide and hydrogen sulfide gas dissolved in the sulfur-containing methanol are completely desorbed. A high concentration of acid gas can be obtained at the top of the tower, which is then collected in the sulfuric acid gas pipeline and sent out of the boundary area.

[0065] 7. The lean methanol from the bottom of the thermal regeneration tower 22 first enters the shell side of the methanol coil heat exchanger 23 and is cooled to -35°C. After being pressurized to 1.8MPa by the second methanol circulation pump 20, it is divided into two streams. The stream with a larger flow rate enters the methanol cooler 6 as lean methanol, which is cooled to -40°C and then returns to the top of the high-pressure absorption tower 2. The lean methanol with a smaller flow rate is divided into two streams again. The stream with a larger flow rate enters the upper part of the methanol-water separation tower 24. Through heating and distillation, methanol and water are separated to achieve dehydration. The methanol vapor obtained from the top of the tower is sent to the middle and lower part of the thermal regeneration tower 22, while the water obtained from the bottom of the tower is sent out of the boundary area via a water pipe. The lean methanol with a smaller flow rate is used as antifreeze and enters the raw gas pipeline.

[0066] In the above process, the three-gradient desorption low-pressure regeneration tower, which integrates reduced pressure flash evaporation, nitrogen stripping and vacuum flash evaporation functions, creates a strong desorption driving force, which ensures that even when the total amount of carbon dioxide is low, it can still ensure that it is fully desorbed and obtain fully low-temperature regenerated methanol stream. It also provides a stable source of cooling for the low-temperature methanol washing system. The final cooling consumption of the system is 645KW and the steam consumption is 21.2t / h.

[0067] Example 2

[0068] A low-temperature methanol washing system and method adapted to low-pressure, low-carbon dioxide feed gas is disclosed in this embodiment. The feed gas volume composition is as follows: hydrogen 3.67%, carbon monoxide 88.79%, carbon dioxide 6.04%, sulfur-containing compounds 0.53%, and the remainder being nitrogen and other inert gases. The pressure is 1.6 MPa, the temperature is 40°C, and the flow rate is 221215 Nm³. 3 / h.

[0069] In addition, the temperature of sulfur-free rich methanol at the bottom of decarbonization section 5 is -34.1℃, the pressure after sulfur-free rich methanol pressure reducing valve 8 and sulfur-containing rich methanol pressure reducing valve 7 is 0.72MPa, and the temperatures are -35.2℃ and -35.7℃, respectively. The carbon dioxide content in the circulating gas is 32%. The temperature of sulfur-containing methanol at the bottom of secondary vacuum regeneration section 27 is -39.3℃, and the temperature of lean methanol at the shell-side outlet of methanol coil heat exchanger 23 is -35.8℃. The remaining conditions are the same as in Example 1.

[0070] In this embodiment, the purified gas has a carbon dioxide content of 4.6 ppm and a hydrogen sulfide content of 0.09 ppm, the emission gas has a sulfur content of 0.23 ppm, the effective gas recovery rate is 98.8%, the total power consumption of the two-stage vacuum pump is 235 KW, the final cooling energy consumption of the system is 302 KW, and the steam consumption is 11.5 t / h.

[0071] Example 3

[0072] A low-temperature methanol washing system and method adapted to low-pressure, low-carbon dioxide feed gas is disclosed in this embodiment. The feed gas volume composition is as follows: hydrogen 5.07%, carbon monoxide 91.09%, carbon dioxide 2.35%, sulfur-containing compounds 0.51%, and the remainder being nitrogen and other inert gases. The pressure is 2.5 MPa, the temperature is 40°C, and the flow rate is 183612 Nm³. 3 / h.

[0073] In addition, the temperature of sulfur-free rich methanol at the bottom of decarbonization section 5 is -35.4℃, the pressure after sulfur-free rich methanol pressure reducing valve 8 and sulfur-containing rich methanol pressure reducing valve 7 is 0.84MPa, and the temperatures are -36.1℃ and -36.5℃, respectively. The carbon dioxide content in the circulating gas is 18%, the primary vacuum pressure is 0.070MPa, the secondary vacuum pressure is 0.035MPa, the temperature of sulfur-containing methanol at the bottom of secondary vacuum regeneration section 27 is -39.7℃, and the temperature of lean methanol at the shell-side outlet of methanol coil heat exchanger 23 is -36.2℃. The remaining conditions are the same as in Example 1.

[0074] In this embodiment, the purified gas has a carbon dioxide content of 3.9 ppm and a hydrogen sulfide content of 0.06 ppm, the emission gas has a sulfur content of 0.20 ppm, the effective gas recovery rate is 99.1%, the total power consumption of the two-stage vacuum pump is 296 KW, the final cooling energy consumption of the system is 413 KW, and the steam consumption is 13.7 t / h.

[0075] Example 4

[0076] A low-temperature methanol washing system and method adapted to low-pressure, low-carbon dioxide feed gas is disclosed in this embodiment. The feed gas volume composition is as follows: hydrogen 3.71%, carbon monoxide 88.91%, carbon dioxide 5.93%, sulfur-containing compounds 0.48%, and the remainder being nitrogen and other inert gases. The pressure is 2.5 MPa, the temperature is 40°C, and the flow rate is 183612 Nm³. 3 / h.

[0077] In addition, the temperature of sulfur-free rich methanol at the bottom of decarbonization section 5 is -34.1℃, the pressure after sulfur-free rich methanol pressure reducing valve 8 and sulfur-containing rich methanol pressure reducing valve 7 is 0.85MPa, and the temperatures are -35.0℃ and -35.5℃, respectively. The carbon dioxide content in the circulating gas is 48%, the primary vacuum pressure is 0.070MPa, the secondary vacuum pressure is 0.035MPa, the temperature of sulfur-containing methanol at the bottom of secondary vacuum regeneration section 27 is -40.2℃, and the temperature of lean methanol at the shell-side outlet of methanol coil heat exchanger 23 is -36.7℃. The remaining conditions are the same as in Example 1.

[0078] In this embodiment, the purified gas has a carbon dioxide content of 4.2 ppm and a hydrogen sulfide content of 0.07 pp, the emission gas has a sulfur content of 0.18 ppm, the effective gas recovery rate is 99.3%, the total power consumption of the two-stage vacuum pump is 128 KW, the final cooling energy consumption of the system is 186 KW, and the steam consumption is 7.1 t / h.

Claims

1. A low-temperature methanol washing system adapted to low-pressure, low-carbon dioxide feed gas, characterized in that, include: The high-pressure low-temperature absorption unit is used for preliminary cooling and dehydration and low-temperature desulfurization and decarbonization of the raw gas. It includes a raw gas heat exchanger (1), a raw gas cooler (21), a high-pressure absorption tower (2) and a methanol cooler (6) arranged in sequence. The high-pressure absorption tower (2) is provided with a decarbonization section (5) in the upper section, a desulfurization section (4) in the middle section and a methanol-water separator (3) in the lower section. The medium-pressure low-temperature desorption unit is connected downstream of the high-pressure low-temperature absorption unit and is used to recover dissolved hydrogen, carbon monoxide and part of carbon dioxide gas in the methanol-rich liquid. It includes a sulfur-containing methanol-rich pressure reducing valve (7), a sulfur-free methanol-rich pressure reducing valve (8), a medium-pressure flash tower (11) and a circulating gas compressor (12) arranged in sequence. The upper section of the medium-pressure flash tower (11) is equipped with a sulfur-free methanol-rich medium-pressure flash tank (9) and the lower section is equipped with a sulfur-containing methanol-rich medium-pressure flash tank (10). The low-pressure low-temperature regeneration unit is connected downstream of the medium-pressure low-temperature desorption unit and is used for low-temperature desorption and regeneration of methanol-rich liquid. It includes a sulfur-containing methanol first pressure reducing valve (13), a sulfur-free methanol pressure reducing valve (14), a low-pressure regeneration tower (15), a first-stage vacuum pump (16), a first-stage methanol circulation pump (19), and the tube side of a methanol coiled heat exchanger (23), a sulfur-containing methanol second pressure reducing valve (25), a second-stage vacuum pump (26), and a sulfur-containing methanol third pressure reducing valve (28), wherein the low-pressure regeneration tower (15) is provided with a hydrogen sulfide concentration section (18) in the upper section, a first-stage vacuum regeneration section (17) in the middle section, and a second-stage vacuum regeneration section (27) in the lower section. The methanol thermal regeneration unit, connected downstream of the low-pressure low-temperature regeneration unit, is used to completely remove the residual acidic gas dissolved in methanol and the moisture brought into the system by the raw material gas. It includes a second methanol circulation pump (20), a thermal regeneration tower (22), the shell side of a methanol coiled heat exchanger (23), and a methanol-water separator (24). The top of the decarbonization section (5) of the high-pressure absorption tower (2) is connected to the liquid inlet of lean methanol and the tube outlet of the methanol cooler (6). The bottom of the sulfur-free rich methanol outlet is connected to the inlet of the sulfur-free rich methanol pressure reducing valve (8) and the medium-pressure flash tank (9) of sulfur-free rich methanol, and to the top of the decarbonization section (5) of the high-pressure absorption tower (2). The bottom of the desulfurization section (4) is connected to the sulfur-containing rich methanol outlet of the sulfur-containing rich methanol pressure reducing valve (7) and the inlet of the medium-pressure flash tank (10) of sulfur-containing rich methanol. The gas phase outlet of the sulfur-free rich methanol medium-pressure flash evaporator (9) is connected to the inlet pipe of the circulating gas compressor (12), and the liquid phase outlet is connected to the top liquid phase inlet pipe of the hydrogen sulfide concentration section (18) of the low-pressure regeneration tower (15) via the sulfur-free methanol pressure reducing valve (14). The gas phase outlet of the sulfur-containing rich methanol medium-pressure flash evaporator (10) is connected to the inlet pipe of the circulating gas compressor (12), and the liquid phase outlet is connected to the middle inlet pipe of the hydrogen sulfide concentration section (18) via the sulfur-containing methanol first pressure reducing valve (13). The outlet of the circulating gas compressor (12) is connected to the raw material gas pipeline through a pipeline. The bottom liquid phase outlet of the hydrogen sulfide concentration section (18) of the low-pressure regeneration tower (15) is sequentially connected to the second pressure reducing valve (25) of sulfur-containing methanol and the top liquid phase inlet pipe of the first-stage vacuum regeneration section (17) of the low-pressure regeneration tower (15). The bottom liquid phase outlet of the first-stage vacuum regeneration section (17) is sequentially connected to the third pressure reducing valve (28) of sulfur-containing methanol and the top liquid phase inlet pipe of the second-stage vacuum regeneration section (27) of the low-pressure regeneration tower (15). Its top gas phase outlet is sequentially connected to the first-stage vacuum pump (16) and the gas phase inlet at the bottom of the hydrogen sulfide concentration section (18). The nitrogen pipeline is connected to the gas phase inlet at the bottom of the hydrogen sulfide concentration section (18) through a pipeline. The bottom liquid phase outlet of the second-stage vacuum regeneration section (27) is sequentially connected to the tube side of the first methanol circulation pump (19), the methanol coiled heat exchanger (23), and the upper liquid phase inlet of the thermal regeneration tower (22) through a pipeline. Its top gas phase outlet is sequentially connected to the inlet pipe of the second-stage vacuum pump (26) and the first-stage vacuum pump (16). After undergoing primary and secondary vacuum flash evaporation, the low-pressure regeneration tower (15) finally establishes a deep vacuum environment of 0.01 to 0.03 MPa.

2. The system according to claim 1, characterized in that: The shell-side inlet of the raw gas heat exchanger (1) is connected to the raw gas pipeline, and the outlet is connected in sequence to the tube side of the raw gas cooler (21) and the methanol-water separator (3) in the lower section of the high-pressure absorption tower (2). The liquid phase outlet at the bottom of the methanol-water separator (3) is connected to the feed inlet pipeline in the middle of the methanol-water separator (24), and the gas phase outlet is connected in sequence to the desulfurization section (4) and the decarbonization section (5) from bottom to top in the high-pressure absorption tower (2).

3. The system according to claim 1, characterized in that: The low-temperature purified gas outlet of the high-pressure absorption tower (2) and the low-temperature exhaust gas outlet at the top of the low-pressure regeneration tower (15) are connected in sequence to the tube side of the raw material gas heat exchanger (1), as well as the inlet pipes of the purified gas pipeline and the exhaust gas pipeline.

4. The system according to claim 1, characterized in that: The top gas phase outlet of the thermal regeneration tower (22) is connected to a sulfuric acid-containing gas pipeline via a pipe, and the bottom liquid phase outlet is connected in sequence to the shell side of the methanol coiled heat exchanger (23) and the inlet of the second methanol circulating pump (20). The outlet of the second methanol circulating pump (20) is divided into two paths: one path is connected to the inlet pipeline of the methanol cooler (6), and the other path is connected to the upper feed port of the methanol water separator (24) and the raw material gas pipeline via pipes. The top gas phase outlet of the methanol water separator (24) is connected to the lower gas phase feed port pipeline of the thermal regeneration tower (22), and the bottom liquid phase outlet is connected to a water pipeline via a pipe.

5. A low-temperature methanol washing method using the system described in any one of claims 1-4, characterized in that, Includes the following steps: After the raw gas is cooled down by two stages, the temperature drops to -30℃ to -45℃ and enters the methanol-water separator (3) in the lower section of the high-pressure absorption tower (2). Liquid methanol and aqueous solution are separated at the bottom of the tower. The sulfur-free rich methanol and sulfur-containing rich methanol extracted from the bottom of the decarbonization section (5) and desulfurization section (4) of the high-pressure absorption tower (2) are depressurized directly to 0.2 to 1.0 MPa without passing through any cooler, and then enter the sulfur-free rich methanol medium-pressure flash tank (9) and the sulfur-containing rich methanol medium-pressure flash tank (10) respectively for depressurization flash evaporation to recover effective gas. The sulfur-containing methanol collected from the bottom of the hydrogen sulfide concentration section (18) is further depressurized to 0.03 to 0.08 MPa and then enters the top inlet of the first-stage vacuum regeneration section (17) for first-stage vacuum flash evaporation. After that, the sulfur-containing methanol collected from the bottom of the first-stage vacuum regeneration section (17) is further depressurized to 0.01 to 0.03 MPa and then enters the top inlet of the second-stage vacuum regeneration section (27) for second-stage vacuum flash evaporation to further desorb some of the carbon dioxide and a small amount of hydrogen sulfide gas contained in the sulfur-containing methanol. The gas phase at the top of the secondary vacuum regeneration section (27) is pressurized to 0.03 to 0.08 MPa by the secondary vacuum pump (26) and then returned to the inlet of the primary vacuum pump (16). The gas phase at the top of the primary vacuum regeneration section (17) is pressurized to 0.13 to 0.26 MPa by the primary vacuum pump (16) and then returned to the gas phase inlet at the bottom of the hydrogen sulfide concentration section (18). It flows from bottom to top and comes into countercurrent contact with the sulfur-free methanol from the top. The hydrogen sulfide in it is then absorbed back into the methanol. Using sulfur-containing methanol with a bottom temperature of -42°C to -15°C in the secondary vacuum regeneration section (27) as a cold source, its temperature rises to 85°C to 115°C after recovering its cold energy through the methanol coil heat exchanger (23). At the same time, the temperature of the high-temperature methanol lean liquid from the bottom of the thermal regeneration tower (22) is reduced to -40°C to -13°C. The lean methanol solution from the shell-side outlet of the methanol coil heat exchanger (23) is cooled to -30°C to -45°C by the refrigerant in the methanol cooler (6) and then returned to the top of the high-pressure absorption tower (2).

6. The low-temperature methanol washing method according to claim 5, characterized in that: The operating pressure of the medium-pressure flash tower (11) is 0.2 to 1.0 MPa. The liquid phase inlets of its sulfur-free rich methanol medium-pressure flash tank (9) and sulfur-containing rich methanol medium-pressure flash tank (10) are directly connected to the liquid phase outlets at the bottom of the decarbonization section (5) and desulfurization section (4) of the high-pressure absorption tower (2) via the sulfur-free rich methanol pressure reducing valve (8) and the sulfur-containing rich methanol pressure reducing valve (7), respectively.

7. The low-temperature methanol washing method according to claim 5, characterized in that: The operating pressure of the first-stage vacuum regeneration section (17) of the low-pressure regeneration tower (15) is 0.02 to 0.08 MPa, and the temperature is -38°C to -12°C. Its top gas outlet is connected to the gas phase inlet pipe at the bottom of the hydrogen sulfide concentration section (18) of the low-pressure regeneration tower (15) via the first-stage vacuum pump (16). Its bottom liquid phase outlet is connected in sequence to the third pressure reducing valve (28) of sulfur-containing methanol and the top liquid phase inlet pipe of the second-stage vacuum regeneration section (27) of the low-pressure regeneration tower (15). The operating pressure of the second-stage vacuum regeneration section (27) of the low-pressure regeneration tower (15) is 0.01 to 0.03 MPa, and the temperature is -42°C to -15°C. Its bottom liquid phase outlet is connected in sequence to the tube side of the first methanol circulation pump (19), the methanol coiled heat exchanger (23), and the upper liquid phase inlet of the thermal regeneration tower (22) via pipes. Its top gas phase outlet is connected in sequence to the inlet pipes of the second-stage vacuum pump (26) and the first-stage vacuum pump (16).

Images