Brief description of the low-temperature methanol washing process flow

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Brief description of the low-temperature methanol washing process (1): The raw gas coming from the gas cooling section is fed into the low-temperature methanol washing unit, where it exchanges heat with purified gas, CO2 flash vapor, and other substances in a series of heat exchangers, thereby being cooled. During the cooling process, a small amount of methanol needs to be injected into the gas pipeline to prevent water vapor from freezing and blocking the pipeline once it cools below the freezing point. After that, the raw gas enters the pre-washing section at the bottom of the H2S absorption tower. Here, the sulfur-free methanol-rich liquid is used for washing in order to remove high-molecular-weight hydrocarbons (naphtha) from the raw gas, as well as other trace components such as organic sulfur, HCN, and NH3. The pre-washed methanol-rich liquid exits the H2S absorption tower and is sent to the pre-washed methanol regeneration system for treatment. The pre-washed raw gas enters the desulfurization section of the H2S absorption tower, where it is sprayed and washed with sulfur-free methanol rich liquid from the bottom of the CO2 absorption tower, to remove sulfides such as H2S and COS from the raw gas. The desulfurized gas exits the top of the H2S absorption tower and enters the bottom of the CO2 absorption tower. Washing is carried out in two stages within the CO2 absorption tower: the lower stage serves as the main washing section, where the methanol semi-poor solution from the CO2 flash tower and the methanol solution from the upper stage are combined to wash the gas, and most of the CO2 in this gas is absorbed. Since the endothermic CO2 absorption process raises the system temperature to a certain level at which absorption ceases, it is necessary to extract the methanol solution from appropriate intermediate trays; after some of the heat of dissolution of CO2 is removed using a methanol circulation pump and a methanol circulation cooler, the solution is returned to the absorption tower as the absorbing liquid in the lower section of the CO2 absorption tower. The upper section of the CO2 absorption tower is the polishing section, where low-temperature purified methanol from the thermal regeneration tower is used to further remove residual CO2 as well as trace amounts of sulfides such as H2S and COS, ensuring that the CO2 content in the gas is ≤1.5% and the total sulfur content is ≤0.1 ppm before it is sent to the methane synthesis unit. A portion of the methanol-rich liquid from the bottom of the CO2 absorption tower is cooled in a methanol cryocooler using the feed pump of the H2S absorption tower, and then sent to the top of the H2S absorption tower as desulfurization liquid. The remaining methanol-rich liquid is flashed and regenerated in the CO2 flash tower. (2) The regeneration of the main washing liquid involves feeding the sulfur-free methanol-rich liquid from the CO2 absorption tower into Section I of the CO2 flash tower for flashing. The vapor produced in Section I contains, in addition to CO2, some other useful components such as CH4, CO, H2, etc. This vapor is then heated again using methanol semi-poor liquid from Section III of the flash tower, after which some of the CO2 has been removed; it is subsequently sent to the coal lock gas holder. The methanol flash liquid from Stage I is cooled in a methanol after-cooler, and then proceeds to Stages II and III. The flashing pressure is gradually reduced to dissolve most of the dissolved CO2. To further reduce the CO2 content in the methanol semi-poor liquid, a nitrogen stripping process is employed. The flash vapor from stage II of the CO2 flash tower is CO2 of high purity, which can be recovered when needed. The flash vapor from stage III is a mixed gas; after the heat is recovered in a series of heat exchangers, this vapor is sent to the tail gas methanol scrubber, where it is washed with deionized water to remove any methanol contained within it. Once the emission standards are met, the resulting gas is released. The methanol semi-poor liquid, which has been regenerated through flash evaporation in stages I, II, and III of the CO2 flash tower, is pressurized by a stripping stage pump and then divided into four streams, which are sent respectively to the middle section of the CO2 absorption tower, the first flash stage of the H2S flash tower, the reabsorption stage, and the first flash stage of the CO2 flash tower. (3) Regeneration of sulfur-containing methanol rich liquid and concentration of hydrogen sulfide: The sulfur-containing methanol rich liquid from the H2S absorption tower. First, it enters Section I of the H2S flash tower for flashing. Similarly, apart from CO2 gas in that portion of the vapor ； There are also some active components such as CH4, CO, H2, etc. The methanol semi-poor stream from stage III of the CO2 flash tower, after reabsorbing some CO2, is combined with the vapor from stage I of the CO2 flash tower. After being reheated in the heat exchanger, it is sent as fuel gas out of the area to the sulfur recovery unit. The methanol solution then enters the second flash section to remove CO2 gas. The sulfur-containing methanol liquid from this section goes to the reabsorption section of the H2S flash tower, where cold nitrogen gas from the nitrogen cooler is used to extract CO2 gas. The gas thus extracted comes into countercurrent contact with a partially methanol-depleted liquid from section III of the CO2 flash tower, in order to reabsorb H2S and COS from the gas. After that, the gas leaves the H2S flash tower, its heat being recovered through a heat exchanger, before being sent to the tail gas methanol scrubber. The H2S-rich methanol liquid from the bottom of the reabsorption section of the H2S flash tower is pressurized by a thermal flash feed pump; after being heated in the rich/lean methanol heat exchanger, it enters the thermal flash section at the top of the thermal regeneration tower for flashing. The thermal flash gas, containing large amounts of CO2 and methanol, undergoes cooling through a series of heat exchangers before entering the reabsorption section of the H2S flash tower. The methanol condensate in the heat exchanger is discharged into the thermal regeneration loop tank. The methanol liquid from the thermal flash section enters the thermal regeneration section of the thermal regeneration tower, where it is completely regenerated through stripping with methanol vapor. Methanol vapor is generated by heating with low-pressure vapor in the reboiler of the thermal regeneration tower. The mixture of H2S, CO2, and methanol vapor from the top of the thermal regeneration section has the methanol condensed through a series of coolers. The liquid enters the reflux tank, and the H2S-enriched gas is finally reheated in a heat exchanger before being sent out of the area to the sulfur recovery unit. The condensed methanol in each section is collected in the heat regeneration column reflux tank, and then sent back to the heat regeneration column as reflux liquid using a heat regeneration column reflux pump. The methanol that has been fully regenerated in the bottom of the thermal regeneration tower is pressurized by a lean methanol pump; after being cooled in the rich/lean methanol heat exchanger, it is sent to the top of the CO2 absorption tower as purified methanol liquid. (4) The regeneration of the pre-washed methanol comes from the methanol liquid in the pre-washing section of the H2S absorption tower, which contains compounds such as CO2, H2S, and naphtha. This liquid is heated in the pre-washed methanol heater and then sent to the pre-washing flash tower for flashing; the flashed gas is returned to the absorption section of the H2S flash tower. The methanol-naphtha mixture leaving the pre-washing flash tower enters the feed buffer chamber of the extractor; at the same time, a certain amount of water (coming from the upper part of the pre-washing flash tower) is added to this buffer chamber in order to extract methanol from the naphtha. Another stream from the top of the azeotropic tower is also added to the buffer chamber of the extractor. The mixture of methanol, water, and naphtha is pumped from the feed buffer chamber to the extraction chamber of the extractor by an extractor pump. Here, the mixture separates into two layers. The upper layer is naphtha, while the lower layer is a methanol-water mixture; the naphtha overflows into the naphtha chamber and is pumped out of the area by a pump. The methanol-water mixture coming out of the extraction chamber of the extractor, which still contains some naphtha, HCN, CO2, COS, and organic sulfur compounds, is fed into the azeotropic tower via the azeotropic tower feed pump and the feed heater, where it undergoes stripping of substances such as naphtha. The overhead product of the azeotropic tower is a mixture of methanol, water, and naphtha, containing CHN, CO2, and organic sulfur. The product is condensed by the azeotropic tower condenser located at the top of the tower; part of the condensed liquid is sent to the feed buffer chamber of the extractor, while another part returns to the tower via gravity as reflux. The non-condensable gas is sent to the upper section of the pre-washing flash tower; after methanol is recovered through washing, it is sent to the sulfur recovery unit. The methanol-water mixture from the bottom of the azeotropic tower is sent to the middle of the methanol-water tower by a feed pump; within this tower, methanol and water are separated through distillation. The heat required is supplied by the reboiler of the methanol-water tower, with low-pressure steam serving as the heat source. The bottom product wastewater from the methanol water column is cooled in the azeotropic column feed heater before being sent to the wastewater treatment facility. To prevent pipe corrosion in the methanol-water system caused by the accumulation of acidic substances such as hydrocyanic acid, it is necessary to spray NaOH solution into the methanol-water feed line of the reboiler in order to adjust the pH value. (5) Exhaust gas washing is carried out to minimize methanol loss from the system; the vapor from the CO2 flash tower and the gas released from the reabsorption section of the H2S flash tower are combined and sent together to the exhaust gas methanol washing tower. Here, the gas comes into counter-current contact with water to wash away the residual methanol in the gas, and the washing water is chemically softened water. Then this washing water is pumped to the upper section of the pre-washing flash tower by an extraction pump, and finally enters the extractor as water for methanol extraction; the washed gas is released into the atmosphere at a high point.