A low temperature methanol wash system and method for treating unconverted gas

Abstract

The application discloses a low-temperature methanol washing system and method for treating unconverted gas, wherein the low-temperature methanol washing system comprises a first lean methanol pipe, a circulating gas pipe, a raw material gas cooler, a raw material gas separation tank, a washing tower, a medium-pressure flash tower, an H2S concentration tower, a semi-lean liquid stripping tower, a normal-temperature stripping tower, a hot regeneration tower, a lean methanol tank, a third lean methanol pipe, a CO2 flash tank, a methanol / water separation tower, a first tail gas pipe, a second tail gas pipe and a tail gas water washing tower. The process flow for separately treating unconverted gas is reasonable in design, meets the purification indexes and environmental protection requirements, and fills the blank of the low-temperature methanol washing for separately treating unconverted gas.

Description

Technical Field

[0001] This invention belongs to the field of gas purification technology for coal chemical plants, and relates to a low-temperature methanol washing system and method for treating unconverted gas, which solves the problem of directly treating unconverted gas with low-temperature methanol washing. Background Technology

[0002] The feed gas from coal chemical plants often contains acidic gases such as CO2 and H2S. These gases need to be removed by low-temperature methanol washing to obtain purified syngas that meets downstream requirements. Low-temperature methanol washing is a physical absorption method that utilizes the high solubility and selectivity of methanol for acidic gases such as CO2 and H2S at low temperatures (-70 to -30°C). Using methanol as a solvent, it removes acidic gases from the feed gas and has advantages such as strong absorption capacity, high purification efficiency, and good selectivity. It is widely used in modern large-scale coal chemical plants.

[0003] Low-temperature methanol washing and absorption to remove acidic media such as CO2, H2S, and COS from feed gas is a physical absorption process. The gas-liquid equilibrium relationship follows Henry's Law, and the content of the absorbed component in the solution is basically proportional to its partial pressure in the gas phase. Therefore, the feed gas commonly treated by low-temperature methanol washing is shift gas obtained from coal gasification process gas through a shift conversion process. For shift gas (CO2 content 35-45% mol), due to the high partial pressure of CO2 in the gas phase, the CO2 concentration in the methanol-rich liquid phase exiting the absorption tower is also high. During low-pressure desorption, the CO2 concentration in methanol will decrease to a low level, resulting in a large CO2 desorption gradient. Due to the endothermic effect of CO2 desorption, the temperature of the methanol-rich liquid can be lowered to -60℃ to -70℃, providing cooling for the system. This low-temperature methanol-rich liquid is then transferred to the lean methanol through heat exchange, resulting in low-temperature lean methanol (-50℃ to -60℃) which is used to wash the top of the tower to absorb acidic media, ensuring purification standards while saving methanol consumption and reducing overall consumption. Therefore, sufficient CO2 content in the feed gas allows the system to be lowered to a sufficiently low temperature during low-pressure desorption, which is an important condition for the normal operation of low-temperature methanol washing.

[0004] Currently, with the increasing demands for downstream products and the need to meet carbon emission reduction requirements, optimized coal chemical processes no longer require shift conversion of feedstock gas. Instead, CO is produced through coal gasification, and coupled with new energy sources, H2 can be supplied by green hydrogen, thus enabling the preparation of feedstock gas with a suitable C / H ratio for methanol synthesis and other applications. However, due to process configuration and downstream needs, the feedstock gas for low-temperature methanol washing is generally shift gas, partially shift gas, or a combination of shift gas and unshifted gas. There is no dedicated low-temperature methanol washing unit for processing unshifted feedstock gas.

[0005] For these purification devices, when the feed gas entering the purification device is unprocessed gas that has not undergone the shift conversion process, the CO2 content of the feed gas is low (e.g., the CO2 content of unprocessed gas in pulverized coal gasification is 6-8% mol, and the CO2 content of unprocessed gas in coal-water slurry gasification is 15-20% mol), resulting in a low partial pressure. If the purification device uses a conventional low-temperature methanol washing process, it cannot obtain a high-concentration rich washing solution. During low-pressure desorption, it cannot provide sufficient cooling capacity and low temperature for the system, making it difficult to achieve the purified gas specifications. Simultaneously, because the temperature cannot be lowered further, the H2S content in the tail gas will exceed the design specifications.

[0006] Therefore, it is necessary to design a reasonable low-temperature methanol washing system and method for separately treating unconverted gas, and to rationally configure the CO2 desorption and heat exchange network to reduce the system temperature in order to meet the purification index and environmental protection requirements, while keeping investment and consumption as low as possible. Summary of the Invention

[0007] In view of the shortcomings of the prior art, the purpose of this invention is to provide a low-temperature methanol washing system and method for treating unconverted gas, so as to meet the purification indicators and environmental protection requirements of unconverted gas after being directly treated by the low-temperature methanol washing system.

[0008] To achieve one aspect of the above-mentioned objectives, the present invention adopts the following technical solution:

[0009] A cryogenic methanol washing system for treating unconverted gas includes:

[0010] The first lean methanol pipe is configured to feed lean methanol into the feed gas to be entered into the feed gas cooler to prevent the feed gas from freezing when it is undergoing heat exchange and cooling in the feed gas cooler.

[0011] The circulating gas pipe is configured to return the first flash vapor and the second flash vapor as circulating gas to the raw gas to be entered into the raw gas cooler;

[0012] The raw material gas cooler is configured to exchange heat and cool the unconverted gas, which is the raw material gas, with the purified gas, CO2 product gas and tail gas, which are the cold source.

[0013] The raw material gas separator is used to separate the raw material gas after it has been cooled by the raw material gas cooler into a liquid phase containing water, methanol.

[0014] The scrubbing tower, from bottom to top, is equipped with a desulfurization section, a first decarbonization section, and a second decarbonization section that are connected by air; wherein...

[0015] The second decarbonization section is configured to use semi-lean methanol from the semi-lean stripper and lean methanol from the lean methanol tank to sequentially wash the first decarbonization gas from the first decarbonization section, so as to further remove CO2 therein and obtain the second decarbonization gas and CO2-containing methanol liquid as the purified gas.

[0016] The first decarbonization section is configured to wash the desulfurized gas from the desulfurization section with the cooled CO2-containing methanol liquid to remove CO2 and obtain the first decarbonized gas and CO2-rich methanol liquid;

[0017] The desulfurization section is configured to use a portion of the CO2-rich methanol liquid to wash the raw gas from the raw gas separator V-001, in order to remove H2S and obtain desulfurized gas and sulfur-containing methanol-rich liquid.

[0018] A medium-pressure flash distillation tower, comprising a first syngas flash distillation section and a second syngas flash distillation section arranged from top to bottom; wherein...

[0019] The first synthesis gas flash evaporation section is configured to flash evaporate the H2 and CO dissolved in the portion of the CO2-rich methanol liquid that has undergone cold energy recovery after exiting the first decarbonization section, so as to obtain the first flash vapor and sulfur-free methanol liquid;

[0020] The second synthesis gas flash evaporation section is configured to flash evaporate the H2 and CO dissolved in the sulfur-containing methanol rich liquid that has undergone cold recovery after exiting the desulfurization section, so as to obtain the second flash vapor and sulfur-containing methanol liquid.

[0021] The H2S concentration tower is provided with a CO2 flash section, a desorption section and a stripping section from top to bottom. The desorption section and the stripping section are connected by gas so that the stripping gas from the lower stripping section can rise into the upper desorption section for washing and desulfurization.

[0022] The CO2 flash section is configured to depressurize and desorb CO2 in the sulfur-free methanol liquid that has been cooled after flowing out of the first synthesis gas flash section, so as to obtain desorbed methanol discharged from the bottom and CO2 product gas discharged from the top.

[0023] The desorption section is configured to desorb CO2 from the sulfur-containing methanol liquid that has been cooled after flowing out of the second synthesis gas flash section under reduced pressure, and to introduce a portion of the desorbed methanol at its upper part for washing and desulfurizing the gas phase in the desorption section, so as to obtain desorbed liquid and sulfur-free desorbed gas discharged from the top.

[0024] The stripping section is configured to strip and desorb the remaining CO2 in the desorbed liquid after it has been cooled and recovered from the desorption section, so as to obtain sulfur-rich methanol at the bottom and stripping gas to be entered into the desorption section at the top.

[0025] The semi-lean stripping tower is configured to use nitrogen to strip the CO2 in the partially desorbed methanol flowing out of the CO2 flash section, so as to obtain sulfur-free stripping gas and the semi-lean methanol. The obtained semi-lean methanol is then fed into the second decarbonization section of the washing tower after cold recovery.

[0026] An ambient temperature stripping tower is configured to use nitrogen to strip the residual CO2 in the sulfur-rich methanol after it has been cooled and recovered, in order to obtain methanol-rich methanol and sulfur-containing stripping gas. The obtained sulfur-containing stripping gas is then fed back into the stripping section to recover sulfur.

[0027] The thermal regeneration tower is configured to thermally regenerate the methanol-rich material that has undergone cold energy recovery after exiting the ambient temperature stripping tower, so as to obtain hydrogen sulfide-containing gas at the top of the tower and hydrogen sulfide-free methanol at the bottom of the tower. The thermal regeneration tower is equipped with a matching thermal regeneration tower reboiler at the bottom of the tower and a matching thermal regeneration tower top condenser and thermal regeneration tower top reflux tank at the top of the tower. The thermal regeneration tower top reflux tank is used to perform gas-liquid separation on the material from the thermal regeneration tower top condenser, and the separated liquid phase is used as the tower top reflux, while the separated gas phase is sent out.

[0028] The lean methanol tank is configured to receive a portion of lean methanol from the thermal regeneration tower, and to deliver lean methanol to the feed gas through the first lean methanol pipe and to deliver lean methanol to the second decarbonization section through the second lean methanol pipe.

[0029] The third lean methanol pipe is used to cool a portion of the lean methanol from the thermal regeneration tower and send it to the methanol / water separator as top reflux.

[0030] The CO2 flash evaporator is configured to flash evaporate the CO2 in the aqueous methanol that has been cooled and recovered after flowing out of the raw gas separator, and send the flashed CO2-containing gas to the stripping section of the H2S concentration tower, while the remaining aqueous methanol is sent to the methanol / water separator.

[0031] The methanol / water separator is configured to distill and separate the methanol containing water from the CO2 flash tank and the methanol containing washing water from the tail gas washing tower, so as to obtain methanol vapor at the top of the tower and wastewater after methanol removal at the bottom of the tower; the bottom of the methanol / water separator is equipped with a matching methanol / water separator reboiler.

[0032] The first tail gas pipe is used to send the sulfur-free desorbed gas and part of the sulfur-free stripping gas as tail gas to the raw material gas cooler for cold energy recovery and then to the tail gas water washing tower.

[0033] The second tail gas pipe is used to send another portion of the sulfur-free stripping gas into the tail gas washing tower after cold recovery.

[0034] The tail gas scrubbing tower is configured to use a portion of the cooled wastewater flowing out of the methanol / water separator and demineralized water from the outside to sequentially scrub the mixture of gases from the first tail gas pipe and the second tail gas pipe, so as to obtain exhaust gas and methanol-containing scrubbing water.

[0035] The present invention also provides a method for treating unchange gas using the above-described low-temperature methanol washing system.

[0036] Compared with the prior art, the present invention has the following advantages:

[0037] 1. Existing low-temperature methanol washing processes are suitable for feed gas that is either shift gas, partially shift gas, or a combination of shift gas and unshift gas. This invention is a novel low-temperature methanol washing system suitable for processing unshift gas alone, which can be implemented in actual equipment.

[0038] 2. The CO2 content in the unconverted gas is low, the temperature rise in the scrubbing tower is small, the scrubbing tower is three-stage, the process is simplified, and investment is reduced.

[0039] 3. A semi-lean liquid gas stripping tower is installed to more effectively desorb the CO2 gas dissolved in the semi-lean methanol liquid. The semi-lean liquid has a strong absorption capacity, which reduces the amount of lean methanol liquid used, ensures the purified gas index, and reduces consumption.

[0040] 4. After the semi-lean methanol liquid is stripped, the temperature drops to -60℃ to -70℃, obtaining a low-temperature cold source comparable to the conventional process. The semi-lean liquid then exchanges heat with the lean methanol liquid, so that the top of the washing tower is low-temperature lean methanol, ensuring the purification index.

[0041] 5. No CO2 desorption tower required, simplifying the process and reducing investment.

[0042] 6. Using propylene evaporated at -45℃ as the cold source for the cryogenic cooler lowers the temperature before methanol-rich desorption, and the temperature after methanol-rich desorption also decreases, thereby lowering the temperature of the entire system and ensuring the purification index and the sulfur content index in the exhaust gas.

[0043] 7. The methanol-rich material is first flashed at medium pressure and then cooled down using a cryostat. This design has a high medium-pressure flashing temperature, low CO solubility in methanol, and a large CO flashing volume, ensuring CO recovery rate. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of one embodiment of the low-temperature methanol washing system of the present invention.

[0045] The annotations in the attached figures are explained as follows:

[0046] T-001 Scrubber; T-002 Medium-Pressure Flash Evaporator; T-003 H2S Concentrator; T-004 Thermal Regeneration Tower; T-005 Methanol / Water Separator; T-006 Tail Gas Scrubber; T-007 Ambient Temperature Stripping Tower; T-008 Semi-Lean Liquid Stripping Tower;

[0047] V-001 Feed gas separator; V-004 Lean methanol tank; V-005 H2S gas separator; V-006 Hot regeneration tower top reflux tank; V-008 CO2 flash tank;

[0048] S-001 lean methanol filter; S-002 #1 rich methanol filter; S-003 #2 rich methanol filter;

[0049] P-001 1# Methanol pump; P-002 2# Methanol pump; P-003 3# Methanol pump; P-004 Lean methanol pump; P-005 Hot regeneration tower bottom pump; P-006 Hot regeneration tower top reflux pump; P-007 Water washing tower bottom pump; P-008 Semi-lean methanol pump.

[0050] C-001 circulating gas compressor;

[0051] E-001 Feed gas cooler; E-002 Circulating gas compressor outlet water cooler; E-003 Sulfur-containing methanol cryocooler; E-004 Sulfur-free methanol cryocooler; E-005 Lean methanol cryocooler; E-006 Scrubber inter-stage cryocooler; E-007 Scrubber rich methanol heat exchanger; E-008 No. 2 lean methanol cooler; E-009 1# Lean methanol cooler; E-010 Hot regeneration tower feed heater; E-011 Hot regeneration tower reboiler; E-012 Hot regeneration tower overhead condenser; E-013 H2S fraction deep cooler; E-014 H2S fraction heat exchanger; E-015 Methanol / water separator reboiler; E-016 Methanol / water separator feed heater; E-017 Purified gas / rich methanol heat exchanger; E-018 Lean methanol water cooler; E-019 Tail gas / nitrogen heat exchanger; E-020 Wastewater heat exchanger; E-021 Lean methanol / semi-lean methanol heat exchanger.

[0052] A. Desulfurization section; B. First decarbonization section; C. Second decarbonization section; D. First syngas flash evaporation section; E. Second syngas flash evaporation section; FCO2 flash evaporation section; G. Desorption section; H. Stripping section;

[0053] 1 First lean methanol pipe; 2 Second lean methanol pipe; 3 Third lean methanol pipe; 4 Circulation pipe; 5 First tail gas pipe; 6 Second tail gas pipe; 7 Fourth lean methanol pipe; 8 Baffle; 9 First spare pipe; 10 Second spare pipe; 11 CO2 emission pipe. Detailed Implementation

[0054] 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. 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.

[0055] 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 for the embodiments of the invention described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover 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.

[0056] Figure 1 A schematic diagram of one embodiment of the low-temperature methanol washing system provided by the present invention is shown. The low-temperature methanol washing system includes:

[0057] The first lean methanol pipe 1 is configured to send lean methanol into the raw gas to be introduced into the raw gas cooler E-001 to prevent the raw gas from freezing when it is cooled down by heat exchange in the raw gas cooler E-001.

[0058] The circulating gas pipe 4 is configured to return the first flash vapor and the second flash vapor as circulating gas to the raw gas to be entered into the raw gas cooler E-001;

[0059] The raw material gas cooler E-001 is configured to exchange heat and cool down the unconverted gas, which is the raw material gas, and the purified gas, CO2 product gas, and tail gas, which are the cold source.

[0060] The raw material gas separator V-001 is used to perform gas-liquid separation on the raw material gas cooled by the raw material gas cooler E-001, so as to separate the liquid phase of water-containing methanol.

[0061] Scrubber T-001, wherein the scrubber T-001 is provided with, from bottom to top, a desulfurization section A, a first decarbonization section B, and a second decarbonization section C, which are connected by air; wherein...

[0062] The second decarbonization section C is configured to use semi-lean methanol from the semi-lean stripping tower T-008 and lean methanol from the lean methanol tank V-004 to sequentially wash the first decarbonization gas from the first decarbonization section B, so as to further remove CO2 therein and obtain the second decarbonization gas and CO2-containing methanol liquid as the purified gas.

[0063] The first decarbonization section B is configured to wash the desulfurization gas from the desulfurization section A with the cooled CO2-containing methanol liquid to remove CO2 and obtain the first decarbonization gas and CO2-rich methanol liquid;

[0064] The desulfurization section A is configured to use a portion of the CO2-rich methanol liquid to wash the raw gas from the raw gas separator V-001, so as to remove H2S and obtain desulfurized gas and sulfur-containing methanol-rich liquid.

[0065] Medium-pressure flash distillation tower T-002, the medium-pressure flash distillation tower includes a first syngas flash distillation section D and a second syngas flash distillation section E arranged from top to bottom; wherein...

[0066] The first synthesis gas flash evaporation section D is configured to flash evaporate the H2 and CO dissolved in the portion of the CO2-rich methanol liquid that has undergone cold recovery after exiting the first decarbonization section B, in order to obtain the first flash vapor and sulfur-free methanol liquid.

[0067] The second synthesis gas flash evaporation section E is configured to flash evaporate the H2 and CO dissolved in the sulfur-containing methanol-rich liquid that has undergone cold recovery after exiting the desulfurization section A, so as to obtain the second flash vapor and sulfur-containing methanol liquid.

[0068] H2S Concentrator T-003, the H2S Concentrator is provided with a CO2 flash section F, a desorption section G and a stripping section H from top to bottom. The desorption section G and the stripping section H are connected by gas so that the stripping gas from the lower stripping section rises into the upper desorption section for washing and desulfurization.

[0069] The CO2 flash section F is configured to depressurize and desorb CO2 in the sulfur-free methanol liquid that has been cooled after flowing out of the first synthesis gas flash section D, so as to obtain desorbed methanol discharged from the bottom and CO2 product gas discharged from the top.

[0070] The desorption section G is configured to desorb CO2 from the sulfur-containing methanol liquid that has been cooled after flowing out of the second synthesis gas flash section E, and to introduce a portion of the desorbed methanol at its upper part for washing and desulfurizing the gas phase in the desorption section G, so as to obtain desorbed liquid and sulfur-free desorbed gas discharged from the top.

[0071] The stripping section H is configured to strip and desorb the remaining CO2 in the desorbed liquid after it has been cooled and recovered from the desorption section G, so as to obtain sulfur-rich methanol at the bottom and stripping gas to be entered into the desorption section G at the top.

[0072] The semi-lean stripper T-008 is configured to use nitrogen to strip the CO2 in the partially desorbed methanol flowing out of the CO2 flash section, so as to obtain sulfur-free stripper gas and the semi-lean methanol. The obtained semi-lean methanol is then fed into the second decarbonization section C of the washing tower T-001 after cold recovery.

[0073] The ambient temperature stripping tower T-007 is configured to use nitrogen to strip the residual CO2 in the sulfur-rich methanol after it has been cooled and recovered, in order to obtain methanol-rich methanol and sulfur-containing stripping gas. The obtained sulfur-containing stripping gas is then sent back to the stripping section H to recover sulfur.

[0074] The thermal regeneration tower T-004 is configured to thermally regenerate the methanol-rich material that has undergone cold energy recovery after exiting the ambient temperature stripping tower T-007, so as to obtain hydrogen sulfide-containing gas at the top of the tower and hydrogen sulfide-free methanol at the bottom of the tower. The thermal regeneration tower T-004 is equipped with a matching thermal regeneration tower reboiler E-011 at the bottom of the tower and a matching thermal regeneration tower top condenser E-012 and a thermal regeneration tower top reflux tank V-006 at the top of the tower. The thermal regeneration tower top reflux tank V-006 is used to perform gas-liquid separation on the material from the thermal regeneration tower top condenser E-012, and the separated liquid phase is used as the top reflux and the separated gas phase is sent out.

[0075] Lean methanol tank V-004 is configured to receive a portion of lean methanol from the thermal regeneration tower T-004, and to deliver lean methanol to the raw material gas through the first lean methanol pipe 1 and to deliver lean methanol to the second decarbonization section C through the second lean methanol pipe 2.

[0076] The third lean methanol pipe 3 is used to cool a portion of the lean methanol from the thermal regeneration tower T-004 and send it to the methanol / water separator T-005 as top reflux.

[0077] CO2 flash evaporator V-008 is configured to flash evaporate the CO2 in the aqueous methanol that has been cooled and recovered after flowing out of the raw material gas separator V-001, and send the flashed CO2-containing gas to the stripping section of the H2S concentration tower T-003, while the remaining aqueous methanol is sent to the methanol / water separator T-005.

[0078] Methanol / water separator T-005 is configured to distill and separate methanol from aqueous methanol from CO2 flash tank V-008 and methanol from methanol-containing wash water from tail gas washing tower T-006, so as to obtain methanol vapor at the top of the tower and methanol-free wastewater at the bottom of the tower; the bottom of methanol / water separator T-005 is equipped with a matching methanol / water separator reboiler E-015;

[0079] The first tail gas pipe 5 is used to send the sulfur-free desorbed gas and part of the sulfur-free stripping gas as tail gas to the raw material gas cooler E-001 for cooling recovery and then to the tail gas washing tower T-006.

[0080] The second tail gas pipe 6 is used to send another part of the sulfur-free stripping gas into the tail gas washing tower T-006 after cold recovery.

[0081] The tail gas scrubbing tower T-006 is configured to use a portion of the cooled wastewater flowing out of the methanol / water separator T-005 and demineralized water from the outside to sequentially scrub the mixture of gases from the first tail gas pipe 5 and the second tail gas pipe 6 to obtain exhaust gas and methanol-containing scrubbing water.

[0082] In some embodiments, the low-temperature methanol washing system further includes an H2S fraction heat exchanger E-014, an H2S fraction cryostat E-013, and an H2S gas separator V-005; wherein the H2S gas separator is configured to perform gas-liquid separation on the gas phase material from the top reflux tank of the thermal regeneration tower and after being cooled sequentially by the H2S fraction heat exchanger E-014 and the H2S fraction cryostat E-013, and send the separated liquid phase to the stripping section and the separated H2S gas to the H2S fraction heat exchanger E-014 to exchange heat with the gas phase material and recover the cooling capacity.

[0083] In some embodiments, the low-temperature methanol washing system further includes a lean methanol pump P-004 and a lean methanol water cooler E-018, for sequentially pressurizing and water-cooling the lean methanol from the lean methanol tank V-004 before it enters the first lean methanol pipe 1 and the second lean methanol pipe 2.

[0084] The second lean methanol pipe 2 is provided with a first lean methanol cooler E-009, a lean methanol deep cooler E-005 for further cooling the lean methanol in the second lean methanol pipe, a second lean methanol cooler E-008, and a lean methanol / semi-lean methanol heat exchanger E-021 along the material flow direction.

[0085] The No. 1 lean methanol cooler E-009 is configured to allow the sulfur-rich methanol from the stripping section H to exchange heat with the lean methanol in the second lean methanol tube 2 before entering the ambient temperature stripping tower T-007.

[0086] The #2 lean methanol cooler E-008 is configured to exchange heat and raise the temperature of the desorbed liquid from the desorption section G with the lean methanol in the second lean methanol tube 2.

[0087] The lean methanol / semi-lean methanol heat exchanger E-021 is configured to allow the semi-lean methanol to enter the second decarbonization section to exchange heat with the lean methanol in the second lean methanol tube in order to cool the lean methanol.

[0088] In some embodiments, the low-temperature methanol washing system further includes a No. 3 methanol liquid pump P-003 and a No. 1 methanol-rich filter S-002, which are used to pressurize and filter the sulfur-rich methanol from the stripping section H before it enters the No. 1 lean methanol cooler E-009.

[0089] Along the flow direction, a methanol pump (P-002), a methanol-rich filter (S-003), and a thermal regeneration tower feed heater (E-010) are sequentially installed on the pipeline that feeds the methanol-rich material from the ambient temperature stripping tower (T-007) into the thermal regeneration tower (T-004). The methanol pump (P-002) and the methanol-rich filter (S-003) are used to pressurize and filter the methanol-rich material before it enters the thermal regeneration tower feed heater (E-010). The thermal regeneration tower feed heater (E-010) is used to exchange heat between the methanol-rich material and the methanol-lean material to be fed into the lean methanol tank (V-004) to raise its temperature.

[0090] In some embodiments, the low-temperature methanol washing system includes a purified gas / rich methanol heat exchanger E-017 on the pipeline that transports the sulfur-containing rich methanol solution from the desulfurization section A to the second synthesis gas flash evaporation section E, for exchanging heat and raising the temperature of the sulfur-containing rich methanol solution with the purified gas to be introduced into the raw material gas cooler E-001.

[0091] A washing tower methanol heat exchanger E-007 is installed on the pipeline that transports the CO2-rich methanol liquid from the first decarbonization section to the first synthesis gas flash section D. The methanol heat exchanger is used to exchange heat and raise the temperature of the CO2-rich methanol liquid with the desorbed liquid to be introduced into the stripping section H.

[0092] A tail gas / nitrogen heat exchanger E-019 is provided on the second tail gas pipe 6 to exchange heat and raise the temperature of the sulfur-free stripping gas to be introduced into the tail gas washing tower T-006 and the nitrogen to be introduced into the semi-lean liquid stripping tower T-008.

[0093] A methanol / water separator feed heater E-016 is provided on the third lean methanol pipe 3 to exchange heat and raise the temperature of the aqueous methanol to be introduced into the CO2 flash tank V-008 and the lean methanol to be introduced into the methanol / water separator T-005 as top reflux.

[0094] A wastewater heat exchanger E-020 is installed on the pipeline that sends the wastewater into the tail gas washing tower T-006, which is used to exchange heat and raise the temperature of the methanol-containing washing water that is to enter the methanol / water separator T-005 and the wastewater that is to enter the tail gas washing tower T-006.

[0095] A washing tower section cryostat E-006 is provided on the pipeline that transports the CO2-containing methanol liquid from the second decarbonization section C to the first decarbonization section B for cooling the CO2-containing methanol liquid.

[0096] A sulfur-containing methanol cryostat E-003 is provided on the pipeline that transports the sulfur-containing methanol from the second synthesis gas flash section E to the desorption section G for cooling the sulfur-containing methanol.

[0097] A sulfur-free methanol cryostat E-004 is provided on the pipeline that transports the sulfur-free methanol from the first synthesis gas flash section D to the CO2 flash section F for cooling the sulfur-free methanol.

[0098] In this invention, each cryostat uses liquid propylene as its cold source.

[0099] In some implementations, such as Figure 1 As shown, the bottom of the thermal regeneration tower T-004 is provided with a baffle 8, thereby dividing the bottom of the thermal regeneration tower T-004 into a first zone with a relatively small volume and a second zone with a relatively large volume. The lean methanol tank V-004 is connected to the first zone via a fourth methanol pipe 7 to receive lean methanol cooled by the thermal regeneration tower feed heater E-010. The third lean methanol pipe 3 is connected to the second zone, and the third lean methanol pipe 3 is also equipped with a thermal regeneration tower bottom pump P-005 and a lean methanol filter S-001, used to sequentially pressurize and filter the lean methanol to be fed into the methanol / water separator feed heater E-016.

[0100] The fourth lean methanol pipe 7 is connected to the lean methanol inlet of the reboiler E-011 of the thermal regeneration tower via the first spare pipe 9, and to the lean methanol inlet of the feed heater E-016 of the methanol / water separator via the second spare pipe 10.

[0101] In some embodiments, the low-temperature methanol washing system further includes:

[0102] A semi-lean methanol liquid pump P-008 is used to pressurize and transport the semi-lean methanol to enter the lean methanol / semi-lean methanol heat exchanger E-021.

[0103] The No. 1 methanol pump P-001 is used to pressurize and transport the desorbed liquid that is to be introduced into the No. 2 lean methanol cooler E-008.

[0104] The heat regeneration tower top reflux pump P-006 is used to pressurize and transport the top reflux liquid from the heat regeneration tower top reflux tank V-006.

[0105] P-007 is a bottom water washing pump used to pressurize and transport methanol-containing wash water entering the wastewater heat exchanger E-020.

[0106] In some embodiments, in the low-temperature methanol washing system, a circulating gas compressor C-001 and a circulating gas compressor outlet cooler E-002 are sequentially provided on the circulating gas pipe along the flow direction for sequentially compressing and water cooling the circulating gas.

[0107] In some embodiments, the low-temperature methanol washing system further includes a CO2 emission pipe 11 for sending CO2 product gas from the raw material gas cooler E-001 as exhaust gas into the exhaust gas washing tower T-006.

[0108] like Figure 1 The operating method of the low-temperature methanol system shown is as follows:

[0109] The feed gas entering the low-temperature methanol washing system is first washed with boiler feedwater to reduce the NH3 content to below 2 ppm. The feed gas entering the low-temperature methanol washing system is mixed with compressed circulating gas and a small amount of lean methanol for anti-icing is injected. After passing through the feed gas cooler E-001, it is cooled by heat exchange with purified gas, tail gas, and CO2 product gas. After the water (water-containing methanol) is separated in the feed gas separator V-001, it enters the desulfurization section of the washing tower T-001.

[0110] In desulfurization section A, the feed gas is washed with a portion of CO2-rich methanol from the first decarbonization section B to remove H2S, COS, and some CO2 before entering the first decarbonization section B. It is then further washed with CO2-containing methanol from the second decarbonization section C, ensuring the gas entering the first decarbonization section B is sulfur-free. In the second decarbonization section C of scrubbing tower T-001, the first decarbonized gas from the first decarbonization section B is washed with lean methanol and semi-lean methanol after gas stripping to meet purification requirements. The purified gas is drawn from the top of the tower, passes through the purified gas / rich methanol heat exchanger E-017 and the feed gas cooler E-001 to recover cooling energy from the rich methanol and feed gas, and is then sent out of the system. The CO2-containing methanol flowing into the first decarbonization section B flows through the inter-section cryogenic cooler E-006 of the scrubbing tower to lower its temperature and improve its absorption capacity.

[0111] After absorbing H2S and CO2, the sulfur-rich methanol liquid from the desulfurization section A of the scrubbing tower T-001 is cooled by heat exchange in the purified gas / rich methanol heat exchanger E-017, and then flashes in the second synthesis gas flash section E of the medium-pressure flash tower T-002, producing dissolved H2, CO, and small amounts of CO2 and H2S as the second flash vapor. Similarly, the CO2-rich methanol liquid from the first decarbonization section B of the scrubbing tower T-001 is cooled by heat exchange in the methanol heat exchanger E-007, and then flashes in the first synthesis gas flash section D of the medium-pressure flash tower T-002, producing dissolved H2, CO, and small amounts of CO2 as the first flash vapor. After the two flash vapors merge, they are pressurized by the recirculating gas compressor C-001 and cooled by the recirculating gas compressor outlet water cooler E-002, and then returned to the feed gas as recirculating gas to recover useful gases.

[0112] The sulfur-containing methanol liquid exiting the second syngas flash section E of the medium-pressure flash distillation tower T-002 is cooled by the sulfur-containing methanol cryocooler E-003 and then sent to the lower part of the desorption section G of the H2S concentration tower T-003, where dissolved CO2 is desorbed under reduced pressure, and some dissolved H2S is also flashed out. The sulfur-free methanol liquid exiting the first syngas flash section D of the medium-pressure flash distillation tower T-002 is cooled by the sulfur-free methanol cryocooler E-004 and then enters the CO2 flash distillation section F at the top of the H2S concentration tower T-003, where CO2 product gas is flashed out under reduced pressure. The CO2 product gas is sent out of the boundary area after heat exchange in the feed gas cooler E-001. When it is not used, it is combined with the tail gas through the CO2 emission pipe 11 and vented after water washing. After flash evaporation in the CO2 flash section F at the top of the H2S concentration tower T-003, part of the desorbed methanol is sent to the semi-lean liquid stripping tower T-008 for stripping to obtain semi-lean methanol, and part is returned to the desorption section G of the H2S concentration tower T-003 to wash and desulfurize the gas phase in the desorption section G, thereby obtaining desorbed liquid and sulfur-free desorbed gas discharged from the top.

[0113] The low-temperature semi-lean methanol obtained from the semi-lean liquid stripping tower T-008 is pressurized and then exchanged with lean methanol in the lean methanol / semi-lean methanol heat exchanger E-021 before being sent to the second decarbonization section C of the scrubbing tower. The sulfur-free desorbed gas obtained from the desorption section of the H2S concentration tower T-003 is heated by heat exchange in the feed gas cooler E-001 before entering the tail gas scrubbing tower T-006 for washing with wastewater and demineralized water. The tail gas, meeting emission standards after scrubbing, is vented through a tall chimney. The methanol-containing scrubbing water at the bottom of the tail gas scrubbing tower T-006, containing a small amount of methanol, is sent to the methanol / water separator T-005 after heat exchange in the wastewater heat exchanger E-020 to recover methanol.

[0114] The sulfur-containing desorbed liquid from the lower part of the desorption section of the H2S concentration tower T-003 serves as a low-temperature cold source. It is heated sequentially by heat exchange in the No. 2 lean methanol cooler E-008 and the rich methanol heat exchanger E-007 of the washing tower before entering the stripping section of the H2S concentration tower T-003. To fully desorb the dissolved CO2 in the rich methanol liquid, low-pressure nitrogen is introduced for stripping. After nitrogen stripping, the bottom of the stripping section of the H2S concentration tower T-003 yields sulfur-rich methanol with a low CO2 content and low temperature. This sulfur-rich methanol contains a small amount of CO2 and virtually all the sulfides in the feed gas. After pressurization, it is filtered through the No. 1 rich methanol filter S-002 and heat exchanged in the No. 1 lean methanol cooler E-009 before entering the ambient temperature stripping tower T-007 for ambient temperature stripping. The methanol rich at the bottom of the ambient temperature stripping tower T-007 is filtered by the No. 2 methanol rich filter S-003, and after exchanging heat with the lean methanol from the thermal regeneration tower T-004 in the feed heater E-010 of the thermal regeneration tower, it enters the thermal regeneration tower T-004 for thermal regeneration.

[0115] Lean methanol is obtained at the bottom of the thermal regeneration tower T-004, and hydrogen sulfide gas rich in H2S is obtained at the top. After exiting the bottom of the thermal regeneration tower T-004, the lean methanol is cooled by the feed heater E-010 and then enters the lean methanol tank V-004. It is then extracted and pressurized by the lean methanol pump P-004 and cooled by heat exchange in the lean methanol cooler E-018, the first lean methanol cooler E-009, the lean methanol cryotherm E-005, the second lean methanol cooler E-008, and the lean methanol / semi-lean methanol heat exchanger E-021 before being sent to the second decarbonization section C of the washing tower T-001, completing the methanol cycle. The hydrogen sulfide gas with a high H2S concentration obtained from the top of the thermal regeneration tower T-004 is cooled by the condenser E-012 at the top of the thermal regeneration tower and then separated into a liquid phase in the reflux tank V-006 at the top of the thermal regeneration tower as top reflux. The separated gaseous material is sent to the H2S fraction heat exchanger E-014 and the H2S fraction cryocooler E-013 for cooling, and then undergoes gas-liquid separation in the H2S gas separator V-006. The separated sulfur-containing methanol liquid is returned to the stripping section of the H2S concentration tower T-003. At the same time, acid gas with a high H2S concentration is separated and sent to the sulfur recovery process as H2S gas product. If necessary, some of the H2S gas can also be recycled back to the H2S concentration tower T-003 to increase the H2S concentration in the acid gas product.

[0116] The aqueous methanol separated from the feed gas separator V-001 also contains CO2. After heat exchange in the methanol / water separator feed heater E-016, it enters the CO2 flash tank V-008 for flash evaporation. The flashed gas phase is sent to the stripping section H of the H2S concentration tower T-003, while the liquid phase is sent to the middle section of the methanol / water separator T-005. The methanol-containing wash water from the bottom of the tail gas washing tower T-006, which contains a small amount of methanol, also enters the middle section of the methanol / water separator T-005. The small amount of lean methanol from the bottom of the thermal regeneration tower T-004 is used as reflux at the top of the methanol / water separator T-005 after heat exchange in the methanol / water separator feed heater E-016. The methanol vapor at the top of the methanol / water separator T-005 returns to the middle section of the thermal regeneration tower T-004, and the wastewater with methanol content meeting the emission standards is obtained from the bottom of the methanol / water separator T-005. After heat exchange and cooling, it is discharged from the system.

[0117] In the low-temperature methanol washing system of the present invention, a heat exchange network consisting of multiple heat exchangers is used to recover cold energy and ensure necessary process conditions, and propylene evaporated at -45°C is used as the cold source for the cryogenic unit; low-pressure nitrogen is introduced from the outside as stripping gas, and demineralized water is introduced to wash the tail gas.

[0118] The present invention will be further described below with reference to the embodiments.

[0119] Example 1

[0120] The unconverted gas used as feedstock is sent into, for example... Figure 1 The low-temperature methanol washing system shown above processes the raw gas according to the above operating method; the composition of the raw gas is shown in Table 1, and the processing results are shown in Tables 2-3.

[0121] Table 1 Composition and Flow Rate of Raw Gas (Unconverted Gas)

[0122]

[0123] Table 2 Gas and Wastewater Emission Indicators

[0124]

[0125]

[0126] Table 3 Guaranteed Consumption Values

[0127]

[0128] As can be seen from the above embodiments, the system designed by the present invention for separately treating unconverted gas has a reasonable configuration of CO2 desorption and heat exchange network, and the temperature of the semi-lean methanol liquid is reduced to -60℃ to -70℃ after gas stripping, and heat exchange with the lean methanol liquid, ensuring that the methanol at the top of the washing tower is low-temperature lean methanol, which meets the purification index and environmental protection requirements, reduces consumption, and fills the gap in low-temperature methanol washing for separately treating unconverted gas.

[0129] All devices or components involved in this invention can be existing processing facilities, devices, or components with corresponding functions in the art, and will not be described in detail. Unless otherwise specified, all matters are understood or known to those skilled in the art based on their prior knowledge, and will not be described in detail. To highlight the concept of this invention, many necessary equipment for industrial applications, such as pumps, instruments, valves, and control components, are omitted from the figures.

[0130] It is readily understood that the above embodiments are merely illustrative examples for clear explanation and do not imply that the invention is limited thereto. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom remain within the scope of protection of this invention.

Claims

1. A low-temperature methanol washing system for treating unprocessed gas, characterized in that, include: The first lean methanol pipe (1) is configured to feed lean methanol into the raw gas to be introduced into the raw gas cooler (E-001) to prevent the raw gas from freezing when it is cooled down by heat exchange in the raw gas cooler (E-001). The circulating gas pipe (4) is configured to return the first flash vapor and the second flash vapor as circulating gas to the raw gas to be entered into the raw gas cooler (E-001); The raw material gas cooler (E-001) is configured to exchange heat and cool the unconverted gas, which is the raw material gas, with the purified gas, CO2 product gas and tail gas, which are the cold source. The raw material gas separator (V-001) is used to perform gas-liquid separation on the raw material gas cooled by the raw material gas cooler (E-001) to separate the liquid phase of water-containing methanol. The scrubbing tower (T-001) is provided with, from bottom to top, a desulfurization section (A), a first decarbonization section (B), and a second decarbonization section (C) that are connected by air; wherein, The second decarbonization section (C) is configured to use semi-lean methanol from the semi-lean stripping tower (T-008) and lean methanol from the lean methanol tank (V-004) to sequentially wash the first decarbonization gas from the first decarbonization section (B) to further remove CO2 therein and obtain the second decarbonization gas and CO2-containing methanol liquid as the purified gas. The first decarbonization section (B) is configured to wash the desulfurized gas from the desulfurization section (A) with the cooled CO2-containing methanol liquid to remove CO2 and obtain the first decarbonized gas and CO2-rich methanol liquid; The desulfurization section (B) is configured to use a portion of the CO2-rich methanol liquid to wash the raw gas from the raw gas separator (V-001) to remove H2S and obtain desulfurized gas and sulfur-containing methanol-rich liquid. The medium-pressure flash distillation tower (T-002) includes a first syngas flash distillation section (D) and a second syngas flash distillation section (E) arranged from top to bottom; wherein, The first synthesis gas flash section (D) is configured to flash evaporate the H2 and CO dissolved in the portion of the CO2-rich methanol liquid that has undergone cold recovery after exiting the first decarbonization section (B) to obtain the first flash vapor and sulfur-free methanol liquid; The second synthesis gas flash evaporation section (E) is configured to flash evaporate the H2 and CO dissolved in the sulfur-containing methanol-rich liquid that has undergone cold recovery after exiting the desulfurization section (A) to obtain the second flash vapor and sulfur-containing methanol liquid. The H2S concentration tower (T-003) is provided with a CO2 flash section (F), a desorption section (G) and a stripping section (H) from top to bottom. The desorption section (G) and the stripping section (H) are connected by gas to allow the stripping gas from the lower stripping section to rise into the upper desorption section for washing and desulfurization. The CO2 flash section (F) is configured to desorb CO2 from the sulfur-free methanol liquid that has been cooled after exiting the first synthesis gas flash section (D) under reduced pressure, so as to obtain desorbed methanol discharged from the bottom and CO2 product gas discharged from the top. The desorption section (G) is configured to desorb CO2 from the sulfur-containing methanol liquid that has been cooled after flowing out of the second synthesis gas flash section (E) under reduced pressure, and to introduce a portion of the desorbed methanol at its upper part for washing and desulfurizing the gas phase in the desorption section (G) to obtain desorbed liquid and sulfur-free desorbed gas discharged from the top. The stripping section (H) is configured to strip and desorb the remaining CO2 in the desorbed liquid after it has been cooled and recovered after exiting the desorption section (G), so as to obtain sulfur-rich methanol at the bottom and stripped gas to be entered into the desorption section (G) at the top. The semi-lean stripping tower (T-008) is configured to use nitrogen to strip the CO2 in the partially desorbed methanol flowing out of the CO2 flash section (F) to obtain sulfur-free stripping gas and the semi-lean methanol. The obtained semi-lean methanol is then fed into the second decarbonization section (B) of the washing tower (T-001) after cold recovery. The ambient temperature stripping tower (T-007) is configured to use nitrogen to strip the residual CO2 in the sulfur-rich methanol after it has been cooled and recovered, so as to obtain methanol-rich methanol and sulfur-containing stripping gas, and send the obtained sulfur-containing stripping gas back into the stripping section (H) to recover sulfur. The thermal regeneration tower (T-004) is configured to thermally regenerate the methanol-rich material that has undergone cold energy recovery after exiting the ambient temperature stripping tower (T-007), so as to obtain hydrogen sulfide-containing gas at the top of the tower and hydrogen sulfide-free methanol at the bottom of the tower. The thermal regeneration tower (T-004) is equipped with a matching thermal regeneration tower reboiler (E-011) at the bottom of the tower and a matching thermal regeneration tower top condenser (E-012) and a thermal regeneration tower top reflux tank (V-006) at the top of the tower. The thermal regeneration tower top reflux tank is used to perform gas-liquid separation on the material from the thermal regeneration tower top condenser, and the separated liquid phase is used as the top reflux and the separated gas phase is sent out. The lean methanol tank (V-004) is configured to receive a portion of the lean methanol from the thermal regeneration tower (T-004), and to deliver lean methanol to the feed gas through the first lean methanol pipe (1) and to deliver lean methanol to the second decarbonization section (C) through the second lean methanol pipe (2). The third lean methanol pipe (3) is used to send a portion of the lean methanol from the thermal regeneration tower (T-004) to the methanol / water separator (T-005) after cooling as top reflux. The CO2 flash evaporator (V-008) is configured to flash evaporate the CO2 in the aqueous methanol that has been cooled and recovered after exiting the raw gas separator (V-001), and send the flashed CO2-containing gas to the stripping section (H) of the H2S concentration tower (T-003), while the remaining aqueous methanol is sent to the methanol / water separator (T-005). The methanol / water separator (T-005) is configured to distill and separate the aqueous methanol from the CO2 flash tank (V-008) and the methanol-containing wash water from the tail gas washing tower (T-006) to obtain methanol vapor at the top of the tower and methanol-free wastewater at the bottom of the tower; the methanol / water separator (T-005) is equipped with a matching methanol / water separator reboiler (E-015) at the bottom of the tower. The first tail gas pipe (5) is used to send the sulfur-free desorbed gas and part of the sulfur-free stripping gas as tail gas to the raw material gas cooler (E-001) for cold energy recovery and then to the tail gas water washing tower (T-006). The second tail gas pipe (6) is used to send another part of the sulfur-free stripping gas into the tail gas washing tower (T-006) after cold recovery. The tail gas scrubbing tower (T-006) is configured to use a portion of the cooled wastewater flowing out of the methanol / water separator (T-005) and demineralized water from the outside to sequentially scrub the mixture of gases from the first tail gas pipe (5) and the second tail gas pipe (6) to obtain exhaust gas and methanol-containing scrubbing water.

2. The low-temperature methanol washing system according to claim 1, characterized in that, The low-temperature methanol washing system further includes an H2S fraction heat exchanger (E-014), an H2S fraction cryostat (E-013), and an H2S gas separator (V-005). The H2S gas separator is configured to perform gas-liquid separation on the gas phase material from the top reflux tank (V-006) of the thermal regeneration tower, after being cooled sequentially by the H2S fraction heat exchanger (E-014) and the H2S fraction cryostat (E-013). The separated liquid phase is sent to the stripping section, and the separated H2S gas is sent to the H2S fraction heat exchanger (E-014) to exchange heat with the gas phase material and recover the cold energy.

3. The low-temperature methanol washing system according to claim 1 or 2, characterized in that, The low-temperature methanol washing system also includes a lean methanol pump (P-004) and a lean methanol water cooler (E-018) for sequentially pressurizing and water-cooling the lean methanol from the lean methanol tank (V-004) before it enters the first lean methanol pipe (1) and the second lean methanol pipe (2). The second lean methanol pipe is provided with a first lean methanol cooler (E-009), a lean methanol deep cooler (E-005) for further cooling the lean methanol in the second lean methanol pipe, a second lean methanol cooler (E-008), and a lean methanol / semi-lean methanol heat exchanger (E-021) in sequence along the material flow direction. The No. 1 lean methanol cooler (E-009) is configured to allow the sulfur-rich methanol from the stripping section (H) to exchange heat with the lean methanol in the second lean methanol tube before entering the ambient temperature stripping tower (T-007). The No. 2 lean methanol cooler (E-008) is configured to exchange heat and raise the temperature of the desorbed liquid from the desorption section (G) with the lean methanol in the second lean methanol tube; The lean methanol / semi-lean methanol heat exchanger (E-021) is configured to allow the semi-lean methanol to enter the second decarbonization section (C) to exchange heat with the lean methanol in the second lean methanol tube in order to cool the lean methanol.

4. The low-temperature methanol washing system according to claim 3, characterized in that, The low-temperature methanol washing system also includes a No. 3 methanol liquid pump (P-003) and a No. 1 methanol-rich filter (S-002), which are used to pressurize, transport and filter the sulfur-rich methanol from the stripping section (H) before it enters the No. 1 lean methanol cooler (E-009). Along the flow direction, a methanol pump #1 (P-002), a methanol-rich filter #2 (S-003), and a thermal regeneration tower feed heater (E-010) are sequentially installed on the pipeline that feeds the methanol-rich material from the ambient temperature stripping tower (T-007) into the thermal regeneration tower (T-004). The methanol pump #1 (P-002) and the methanol-rich filter #2 (S-003) are used to pressurize and filter the methanol-rich material before it enters the thermal regeneration tower feed heater (E-010). The thermal regeneration tower feed heater (E-010) is used to exchange heat between the methanol-rich material and the methanol-lean material to be fed into the lean methanol tank (V-004) to raise its temperature.

5. The low-temperature methanol washing system according to claim 4, characterized in that, A purified gas / rich methanol heat exchanger (E-017) is provided on the pipeline that transports the sulfur-containing rich methanol solution from the desulfurization section (A) to the second synthesis gas flash section (E) to allow the sulfur-containing rich methanol solution to exchange heat with the purified gas to be introduced into the raw material gas cooler (E-001) and increase its temperature. A washing tower methanol heat exchanger (E-007) is installed on the pipeline that transports the CO2-rich methanol liquid from the first decarbonization section (B) to the first synthesis gas flash section (D) to exchange heat and raise the temperature of the CO2-rich methanol liquid with the desorbed liquid to be introduced into the stripping section (H). A tail gas / nitrogen heat exchanger (E-019) is provided on the second tail gas pipe (6) to exchange heat and raise the temperature of the sulfur-free stripping gas to be entered into the tail gas washing tower (T-006) and the nitrogen to be entered into the semi-lean liquid stripping tower (T-008). A methanol / water separator feed heater (E-016) is provided on the third lean methanol pipe (3) to allow the water-containing methanol to enter the CO2 flash tank (V-008) to exchange heat with the lean methanol to enter the methanol / water separator (T-005) as top reflux and raise its temperature. A wastewater heat exchanger (E-020) is installed on the pipeline that sends the wastewater into the tail gas scrubbing tower (T-006) to exchange heat and raise the temperature of the methanol-containing scrubbing water to be entered into the methanol / water separator (T-005) and the wastewater to be entered into the tail gas scrubbing tower (T-006). A deep cooler (E-006) for cooling the CO2-containing methanol liquid is provided on the pipeline that transports the CO2-containing methanol liquid from the second decarbonization section (C) to the first decarbonization section (B). A sulfur-containing methanol cryostat (E-003) is provided on the pipeline that transports the sulfur-containing methanol from the second synthesis gas flash section (E) to the desorption section (G) for cooling the sulfur-containing methanol; A sulfur-free methanol cryostat (E-004) is provided on the pipeline that transports the sulfur-free methanol from the first synthesis gas flash section (D) to the CO2 flash section (F) for cooling the sulfur-free methanol.

6. The low-temperature methanol washing system according to claim 5, characterized in that, The bottom of the thermal regeneration tower (T-004) is provided with a baffle (8), thereby dividing the bottom of the thermal regeneration tower (T-004) into a first zone with a relatively small volume and a second zone with a relatively large volume; wherein, the lean methanol tank (V-004) is connected to the first zone through a fourth lean methanol pipe (7) to receive lean methanol cooled by the feed heater (E-010) of the thermal regeneration tower, the third lean methanol pipe (3) is connected to the second zone, and the third lean methanol pipe is also provided with a thermal regeneration tower bottom pump (P-005) and a lean methanol filter (S-001) for pressurizing, conveying and filtering the lean methanol to be fed into the feed heater (E-016) of the methanol / water separator in sequence; The fourth lean methanol pipe (7) is connected to the lean methanol inlet of the reboiler (E-011) of the thermal regeneration tower via the first spare pipe (9) and to the lean methanol inlet of the feed heater (E-016) of the methanol / water separator via the second spare pipe (10).

7. The low-temperature methanol washing system according to claim 6, characterized in that, The low-temperature methanol washing system also includes: A semi-lean methanol liquid pump (P-008) for pressurizing and conveying the semi-lean methanol to be introduced into the lean methanol / semi-lean methanol heat exchanger (E-021); The No. 1 methanol pump (P-001) is used to pressurize and transport the desorbed liquid entering the No. 2 lean methanol cooler (E-008); A thermal regeneration tower top reflux pump (P-006) is used to pressurize and transport the top reflux liquid from the thermal regeneration tower top reflux tank (V-006); The bottom pump (P-007) is used to pressurize and transport the methanol-containing wash water entering the wastewater heat exchanger (E-020).

8. The low-temperature methanol washing system according to claim 1 or 7, characterized in that, The circulating gas pipe (4) is provided with a circulating gas compressor (C-001) and a circulating gas compressor outlet cooler (E-002) in sequence along the flow direction, for sequentially compressing and water cooling the circulating gas.

9. The low-temperature methanol washing system according to claim 1, characterized in that, The low-temperature methanol washing system also includes a CO2 emission pipe (11) for sending the CO2 product gas from the raw material gas cooler (E-001) as exhaust gas into the exhaust gas washing tower (T-006).

10. The low-temperature methanol washing system according to claim 8, characterized in that, The low-temperature methanol washing system also includes a CO2 emission pipe (11) for sending the CO2 product gas from the raw material gas cooler (E-001) as exhaust gas into the exhaust gas washing tower (T-006).

11. A method for treating unconverted gas using the low-temperature methanol washing system according to any one of claims 1-10.

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