Explanation of the purification process flow 2

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Process description: The process flow of the low-temperature methanol washing purification unit is shown in Figure 1. The feed gas is first washed with boiler water to reduce the NH3 content to below 1 ppm. The feed gas entering the low-temperature methanol wash system is first sprayed with a small amount of anti-freezing methanol; after being cooled by heat exchange with purified gas and exhaust gas in the feed gas cooler E-2201, water is separated out in tank V-2201. After the moisture is removed from the feed gas, it enters the lower section of absorption tower C-2201. Tower C-2201 is divided into four sections; the lowest section is used for desulfurization, while the three upper sections are used for decarburization. In the desulfurization section, the feed gas is washed with CO2-rich methanol to remove components such as H2S and some CO2; thereafter, it enters the decarburization section, where the gas no longer contains sulfur. The top of tower C-2201 is washed with lean methanol, and the purified gas is drawn out from the tower top. Intermediate coolers E-2205 and E-2206 are installed between the decarburization sections of the absorption tower. After absorbing H2S and CO2, the sulfur-containing methanol-rich liquid coming from the bottom of tower C-2201 is subjected to heat exchange, cooling, and depressurization; thereafter, the dissolved gases such as H2, CO, and small amounts of CO2 and H2S are vaporized in flash tank V-2202. Similarly, the sulfur-free methanol stream coming from the decarboxylation section of the absorption tower is subjected to heat exchange, cooling, and depressurization, after which gases such as dissolved H2, CO, and a small amount of CO2 are flashed out in tank V-2203. After the two streams of vaporized gas merge, nitrogen is used for stripping to recover the cooling capacity; the resulting gas is then pressurized by the recycle compressor K-2201 and returned to the feed gas stream in order to recover useful gases. The sulfur-containing methanol coming out of V-2202 is depressurized and sent to the lower part of tower C-2202 and the middle part of tower C-2203, where the dissolved CO2 is flashed off; simultaneously, some of the dissolved H2S is also flashed off. The sulfur-free methanol stream coming out of V-2203 enters the top of tower C-2202, where the dissolved CO2 gas is flashed off; part of the liquid phase is returned to tower C-2202 to wash the sulfur-containing gases. The CO2 product gas is obtained at the top of tower C-2201; this gas leaves the system after being heated by heat exchange with the feed gas in unit E-2201, and is then sent to the CO2 compressor. The liquid coming out from the lower part of C-2202 flows to the lower part of the upper section of tower C-2203, where more dissolved CO2 gas is vaporized. The methanol containing sulfur, which comes from the top of C-2202, enters the top of tower C-2203, where it washes the gas that has been vaporized from the methanol containing sulfur. The overhead gas from tower C-2203 contains hydrogen sulfide at an acceptable level; after heat is recovered in towers E-2203 and E-2201, it is sent to tower C-2206 for washing. After washing, the exhaust gas containing a very small amount of methanol leaves the system, while the wash water containing a slight amount of methanol is sent to the methanol-water separation tower C-2205 for methanol recovery after heat exchange. The methanol liquid coming out from the lower part of the upper section of tower C-2203 represents the coldest heat source in the system. It exchanges heat with lean methanol through E-2208, and after further temperature increase through heat exchange with methanol from between sections via E-2206, some of the dissolved CO2 gas is flashed out in V-2207. The flashed gas is sent to the lower section of tower C-2202, while the liquid is pressurized using pump P-2202; after further temperature increase through E-2207, it is also sent to the lower section of C-2202 to continue the flashing of dissolved gases. The liquid phase at the bottom of tower C-2202 enters the lower section of tower C-2203, where it is stripped with nitrogen to further desorb the dissolved CO2 gas. At the bottom of tower C2203, a methanol stream with a low CO2 content and low temperature is obtained; this methanol stream contains a small amount of CO2 and essentially all of the sulfides present in the feed gas. It is pressurized using pump P-2203, and after exchanging heat with the lean methanol from methanol thermal regeneration tower C-2204 through heat exchangers E-2209 and E-2210, it enters tower C-2204 for thermal regeneration. Lean methanol is obtained at the bottom of the tower, while a gas rich in H2S is obtained at the top. The lean methanol exits at the bottom of tower C-2204 at a high temperature; after being cooled by E-2210, it enters the lean methanol tank V-2204. There, it is pressurized by the lean methanol pump P-2204, and then cooled through heat exchange with E-2218, E-2209, E-2203, and E-2208 before being sent to the top of the absorption tower C-2201 as an absorbent, thus completing the methanol circulation process. The gas with a high H2S concentration obtained from the top of tower C-2204 is cooled to separate the sulfur-containing methanol liquid. During the H2S separation process, the sulfur-containing methanol stream is returned to the bottom of tower C-2203; the acidic gas with a high H2S concentration is separated as an acidic gas product and sent to the sulfur recovery process ； If necessary, some of the H2S gas is recycled back to tower C-2203 to increase the H2S concentration in the acidic gas product. The aqueous methanol separated from tank V-2201 also contains CO2; after heat exchange with a small amount of lean methanol coming from the bottom of tower C-2203 via E-2216, it is sent to the CO2 separation tank V-2208. The gas that flashes off is sent to the lower section of tower C-2203, while the liquid phase enters the middle part of the methanol-water separation tower C-2205. The lean methanol after heat exchange serves as reflux for tower C-2205. The methanol vapor from the top of tower C-2205 is sent back to the middle section of tower C-2204; water with a methanol content that meets the emission standards is obtained at the bottom of the tower, and after heat exchange it is discharged from the system. The heat exchange network in the system, consisting of over twenty heat exchangers, is used to recover cold energy and maintain the necessary process conditions. Overall material balance: Calculation results for material flows after adding a CO2 desorber under design conditions.
Flow number: 101, 102, 103, 104, 106
Temperature, °C: 40.00, 38.72, -26.15, -52.49, -40.00
Pressure, MPa: 2.90, 2.90, 2.86, 2.78, 2.75
Enthalpy, kW: -243345., -249190., -249232., -2282., -2004.
Total flow rate, kmol/h: 4957.42, 5033.18, 4993.54, 2715.46, 2715.46
kg/h: 104612., 107132., 105905., 6404., 6404.
Mole fraction in gas phase: 1.00000, 0.99712, 1.00000, 1.00000, 1.00000
Composition, mole fraction (%): Gas phase, Liquid phase; Gas phase, Liquid phase; etc.
CH3OH: 0.0000, 0.0000, 0.3191, 43.8721, etc.
CO2: 44.7735, 0.0000, 45.0067, 5.8657, etc.
H2: 54.1289, 0.0000, 53.7228, 0.0572, etc.
N2: 0.2175, 0.0000, 0.2179, 0.0000, etc.
H2S: 0.1941, 0.0000, 0.1937, 0.0113, etc.
H2O: 0.2024, 0.0000, 0.0557, 50.1897, etc.
Ar: 0.0967, 0.0000, 0.0969, 0.0001, etc.
CH4: 0.0374, 0.0000, 0.0376, 0.0001, etc.
CO: 0.3495, 0.0000, 0.3494, 0.0038, etc.
COS: 0.0000, 0.0000, 0.0000, 0.0000, etc.
Heat capacity, kJ/kg·K: 1.5883, 0.0000, 1.5800, 3.1204, etc.
Average molecular weight, g/mol: 21.1021, 0.0000, 21.2726, 25.6868, etc.
Density, kg/m³: 24.3383, 0.000, 48.7392, 974.523, etc.
Viscosity, 10⁻⁵ Pa·s: 1.6436, 0.0000, 1.6348, 27.6638, etc.
Thermal conductivity, W/m·K: 0.0697, 0.0000, 0.0706, 0.1913, etc.
Surface tension, mN/m: 0.0000, 0.0000, 0.0000, 41.3147, etc.
Pr, 10⁻⁵: 0.0374, 0.0000, 0.0366, 0.4513, etc.

Flow number: 107, 108, 109, 110, 111
Temperature, °C: 30.00, -52.49, -26.65, -35.45, -19.29
Pressure, MPa: 2.70, 3.43, 2.81, 2.80, 2.82
Enthalpy, kW: -455., -494301., -575166., -576635., -657193.
Total flow rate, kmol/h: 2715.46, 7292.62, 8044.43, 8044.43, 8781.31
kg/h: 6404., 233510., 266174., 266174., 298495.
Mole fraction in gas phase: 1.00000, 0.00000, 0.00000, 0.00000, 0.00000
Composition, mole fraction (%): Gas phase, Liquid phase; etc.
CH3OH: 0.0022, 0.0000, 0.0000, 99.8436, etc.
CO2: 0.0000, 0.0000, 0.0000, 0.0000, etc.
H2: 98.7332, 0.0000, 0.0000, 0.0000, etc.
N2: 0.3917, 0.0000, 0.0000, 0.0000, etc.
H2S: 0.0000, 0.0000, 0.0000, 0.0000, etc.
H2O: 0.0000, 0.0000, 0.0000, 0.1564, etc.
Ar: 0.1747, 0.0000, 0.0000, 0.0000, etc.
CH4: 0.0652, 0.0000, 0.0000, 0.0000, etc.
CO: 0.6330, 0.0000, 0.0000, 0.0000, etc.
COS: 0.0000, 0.0000, 0.0000, 0.0000, etc.
Heat capacity, kJ/kg·K: 12.4234, 0.0000, 0.0000, 2.2312, etc.
Average molecular weight, g/mol: 2.3585, 0.0000, 0.0000, 32.0201, etc.
Density, kg/m³: 2.5308, 0.000, 0.0000, 891.055, etc.
Viscosity, 10⁻⁵ Pa·s: 0.9649, 0.0000, 0.0000, 243.3759, etc.
Thermal conductivity, W/m·K: 0.1621, 0.0000, 0.0000, 0.2107, etc.
Surface tension, mN/m: 0.0000, 0.0000, 0.0000, 30.1361, etc.
Pr, 10⁻⁵: 0.0739, 0.0000, 0.0000, 2.5767, etc