Overview of the methanol process flow

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(1) Coke oven gas compression: The coke oven gas coming from the gas holder, with a temperature of 25°C and a pressure of 200 mmH2O (gauge), enters the compressor. After three stages of compression, the pressure rises to 2.5 MPa (gauge) and the temperature rises to 140°C. After being stabilized by three-stage outlet buffers, the gas enters the cooler, where it is cooled to 40°C before being sent via the main pipeline to the desulfurization unit. (2) Advanced desulfurization: The coke oven gas from the coke oven gas compression section is transported via pipelines to this section. After oil mist is removed and inorganic sulfur is eliminated using filters and pre-desulfurization tanks, the gas is heated to 300°C in a conversion unit; within this unit, the organic sulfur in the gas is converted into inorganic sulfur, while the oxygen present in the gas reacts with hydrogen to form water. The gas after hydrogenation conversion contains approximately 245 mg/Nm3 of inorganic sulfur, which is removed in a medium-temperature desulfurization tank to eliminate the vast majority of it. Subsequently, the residual organic sulfur is converted using a cobalt-molybdenum converter, and then passed through a medium-temperature zinc oxide desulfurization tank to reduce the total sulfur content in the gas to 0.1 ppm. The gas exiting the zinc oxide desulfurization tank has a pressure of approximately 2.3 MPa and a temperature of around 350°C, and it is sent to the conversion unit. (3) Conversion: The desulfurized coke oven gas is mixed with saturated steam, preheated to 660°C in a coke oven gas preheater and a preheating furnace, and then introduced into the upper part of the conversion furnace. There it mixes thoroughly with the preheated oxygen and flows from top to bottom through the catalytic bed where an oxidation reaction takes place, releasing heat. It quickly enters the catalytic bed where the following reactions occur: 2H2 + O2 = H2O + 115.48 kcal (1); 2CH4 + O2 = 2CO + 4H2 + 17.0 kcal (2); CH4 + H2O = CO + 3H2 – 49.3 kcal (3); CH4 + CO2 = 2CO + 2H2 – 59.1 kcal (4); CO + H2O = CO2 + H2 + 9.8 kcal (5). The reaction eventually reaches equilibrium according to equation (5). The gas resulting from this conversion process is drawn out from the bottom of the conversion furnace, where heat is recovered in a waste heat boiler to produce steam; as a result, the temperature of the gas drops to 540°C. It is then cooled to 370°C using a coke oven gas preheater; after heat is recovered through the coke oven gas primary preheater, boiler feedwater preheater, reboiler, and desalinated water preheater, it is further cooled in a water cooler, and the process condensate is separated before being sent to the syngas compression section. (4) Syngas compression: The fresh gas from the conversion unit, with a temperature of 40°C and a pressure of 2.1 MPa (A), enters the first stage of the syngas compressor where it is compressed; thereafter it goes to the recycle stage where it mixes with the recycle gas from methanol synthesis, and is compressed to 6.0 MPa (A). The syngas from the compressor outlet is sent to methanol synthesis. (5) Methanol synthesis: The syngas obtained from the compression of syngas is preheated in a gas-to-gas heat exchanger before entering the methanol synthesis reactor, where the methanol synthesis reactions take place under the action of a catalyst: CO + 2H2 → CH3OH + Q; CO2 + 3H2 → CH3OH + H2O + Q, as well as side reactions such as 4CO + 8H2 → C4H9OH + 3H2O and 8CO + 17H2 → C8H18 + 8H2O. The gas exiting the synthesis reactor passes through a gas-to-gas heat exchanger and a water cooler, after which its temperature is reduced to 40°C; it then enters a methanol separator for gas-liquid separation. Most of the gas exiting the methanol separator is used as recycle gas to pressurize the syngas compressor and supplement fresh gas; a small portion is sent to the alcohol washing tower to have the methanol removed from it, another small portion is sent to the conversion unit as fuel gas, and the rest goes into the fuel gas system. The crude methanol coming out of the bottom of the separator is depressurized and sent to a flash tank; most of the dissolved gases in the crude methanol are released and, after mixing with the vent gas, are used as fuel gas. The crude methanol is mixed with the dilute methanol-water stream from the bottom of the alcohol washing tower and sent for methanol distillation. (6) Methanol distillation: Methanol distillation is carried out using a three-column system. The crude methanol produced via methanol synthesis is preheated in a crude methanol preheater before entering the pre-distillation column. The methanol obtained after light components have been removed from the bottom of this column is sent to the pressurized distillation column, where it is then condensed using a atmospheric pressure column reboiler. Part of the methanol condensate at the outlet of the atmospheric pressure column reboiler is pumped back to the top of the pressurized column by a pressurized column reflux pump for reflux ； The other part serves as finished methanol. The finished methanol is cooled by the crude methanol preheater and the refined methanol cooler before being sent to the refined methanol intermediate tank. After the methanol product in the refined methanol intermediate tank is found to meet the quality standards through analysis, it is pumped into the finished product storage area using a refined methanol pump. (7) Air separation: The air separation process utilizes a molecular sieve purification and pressurization system. After being filtered, compressed, and cooled, the air enters the molecular sieve purifier, where impurities such as moisture, carbon dioxide, and acetylene are removed, resulting in purified air. The purified air is then divided into two streams and fed into a distillation tower to produce oxygen and nitrogen as the final products. Part of the contaminated nitrogen is used as regenerated molecular sieve, while nitrogen is used to cool the water in the cooling tower; part of the oxygen emerging from the tower is compressed by a medium-pressure oxygen compressor and then sent to the conversion unit for use. A portion of the air drawn from behind the purifier is taken as instrument air and sent to the users. (8) Gas boiler: The qualified coke oven gas produced by the coking unit is sent to the gas boiler via the gas main for combustion. The heat generated by the gas boiler is used to heat the boiler feed water, converting it into medium-pressure steam, which is then supplied to users such as process conversion units and methanol distillation units. .