Low-temperature methanol washing process

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For desulfurization and carbon dioxide removal in this project, the low-temperature methanol washing purification method is employed. Its advantages include a simple process, mature technology, inexpensive and readily available solvents, as well as low operating costs. The shift gas from the shift unit, with a pressure of 3.2 MPa and a temperature of 40°C, enters the feed gas cooler. Methanol is injected into the shift gas before it enters the feed gas cooler, in order to prevent water and hydrates present in the shift gas from crystallizing and blocking the pipes in the feed gas cooler. The methanol-spiked shift gas enters the raw material cooler, where it exchanges heat with the purified gas coming from the CO2 scrubber and the flue gas rich in H2S, thereby reducing the temperature of the shift gas. The methanol-water solution is separated out in a crude gas separator and sent to the methanol-water tower, while the dried shift gas enters the lower part of the scrubber. The CO2 scrubber is divided into an upper tower and a lower tower; the lower tower is primarily used for desulfurization. Since the solubility and dissolution rate of CO2 in methanol are much lower than those of H2S and COS gases, the lower tower requires only a portion of the scrubbing agent used in the upper tower to absorb CO2. The methanol solution containing all the sulfur is taken out from the bottom of the wash tower, cooled in the bottom cooler and bottom cryocooler of the wash tower, expanded to 2.2 MPa, and fed into the 2# sulfur-rich methanol flash tank to recover most of the H2 dissolved in the methanol solution. The upper section of the CO2 scrubber tower is divided into three stages: the top stage is the final purification stage, where the washing liquid – consisting of methanol at -50°C – is used to absorb any remaining amounts of CO2 and H2S gases in the gas stream. This ensures that the CO2 content in the gas sent to the synthesis section is ≤3–4% (mol), and the total sulfur content is ≤0.1 ppm. The gas exiting from the top of the tower is then sent to the synthesis section. The middle section of the upper tower in the wash tower serves as the CO2 absorption section. The wash liquid coming from the fine washing section enters the main washing section after being cooled through heat exchange, where it absorbs CO2 from the gas. The wash liquid coming from the main washing section then enters the initial washing section after being cooled through heat exchange, to absorb CO2 from the gas there as well. Another portion of methanol, free of H2S and COS, drawn from the bottom of the tower is cooled in the 1# Methanol-Enriched Cooler and the 2# Methanol-Enriched Cryocooler, then expanded to 2.2 MPa and fed into the 1# CO2-Enriched Methanol Flash Tank to recover most of the H2 gas dissolved in the methanol. The flashed gas is pressurized by a recovered gas compressor and then returned to the inlet of the desulfurization and decarburization unit. Methanol containing CO2 but no sulfur is throttled and expanded into the top of the H2S concentration tower, where H2S and COS in the gas phase are removed from the upper portion of this tower; the off-gas from the top of the tower is discharged with a maximum sulfur content of 100 ppm (v). The sulfur-containing methanol enters the H2S concentration tower after throttling expansion. To increase the H2S concentration in the gas and reduce regeneration costs, N2 is used to remove CO2 from the gas stream at the lower part of the H2S concentration tower. The off-gases discharged from the H2S concentration tower are released after having their heat recovered, with the H2S and COS content being below 100 ppm (V). The sulfur-rich methanol liquid coming out of the bottom of the H2S concentration tower is heated before entering the regeneration tower. The methanol vapor containing H2S that emerges from the top is cooled; after methanol is separated, the non-condensable gases are sent to the sulfur recovery unit. The separated methanol solution is partially used as a concentration reflux liquid, and partially sent to the H2S concentration tower. The regenerated lean methanol taken from the bottom of the thermal regeneration tower is cooled and then sent to the methanol collection tank. Thereafter, it is pressurized by a lean methanol pump and cooled again; aside from a portion that is used as spray methanol, the remaining lean methanol is sent to the washing tower as an absorption liquid. To remove the moisture brought in from the feed gas and reduce corrosion, this unit is equipped with a methanol/water separation tower to eliminate water from the system. The methanol-aqueous solution coming out of the water separator is heated by methanol coming from the bottom of the thermal regeneration tower in the methanol/water separation column reflux cooler, and then enters the methanol/water separation column. During the production process, some heavy metals, impurities, and water accumulate in the system. To maintain the balance of the entire system, a portion of the lean methanol is drawn from the bottom of the thermal regeneration tower; it is filtered through a lean methanol filter under pressure provided by a pump at the bottom of the tower, undergoes heat exchange in a heat exchanger, and then enters the top of the methanol/water separation tower. The heat source for the separation tower comes from medium-pressure steam. Wastewater containing various impurities is discharged from the bottom of the tower; the methanol content in this wastewater is kept at <3000 ppm for use in coal slurry preparation.