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The low-temperature methanol washing unit at the Inner Mongolia Fertilizer Factory came online in April 1997. Half a year later, severe corrosion was detected in the No. 2 main methanol heat exchanger and its pipelines, with leaks occurring on multiple occasions. Frequent shutdowns for maintenance disrupted normal production and resulted in significant economic losses. By analyzing the leakage, the cause of corrosion was identified, and corresponding protective measures were taken; since then, the production has been operating well. 1 Corrosion conditions and causes 1.1 Corrosion conditions The original material specified for the tube bundle of the 2# main methanol heat exchanger was carbon steel. Half a year after it came online, severe corrosion was detected at the bottom ends of the tubes, as well as on the tube sheets and end caps, resulting in leaks that were difficult to repair. The tube bundle inside the heat exchanger was replaced, and the material of the heat exchange tubes was upgraded to 1Crl8Ni9Ti. However, corrosion still occurred; later, aluminum-doped steel was used to manufacture the heat exchanger tubes, which provided excellent corrosion resistance. The 2# main methanol heat exchanger and its associated pipelines are severely corroded, causing the heat exchange tubes and related pipelines to thin out or even perforate, resulting in methanol leakage. A macroscopic examination of the corrosion on the tubes of carbon steel heat exchangers reveals that corrosion primarily occurs inside the U-shaped sections of the tube steel, on the tube sheets that hold the tubes in place, and on the heat exchanger’s end caps. A reddish-brown layer of rust covers the surface; the corrosion is of a localized nature, with many pitting and holes present. The holes caused by corrosion are generally circular in shape, and there are no transgranular or intergranular cracks around or at the bottom of these pitted areas. 1.2 Causes of corrosion: Based on the analysis of the corrosion conditions, in addition to local corrosion caused by the medium, the erosion and wear resulting from the flow of the material also contribute to the corrosion of the No. 2 main methanol heat exchanger. 1) Chemical corrosion of the medium. The 6 unit 2# main methanol heat exchangers in the methanol washing unit have the same structure; they are all fixed-tube-sheet U-tube heat exchangers. The tube specifications are Φ19mm×2mm. The fluid flowing through the tube side is methanol solution rich in hydrogen sulfide, resulting from the washing of the raw gas, with an exit temperature of 96°C and an inlet temperature of –24°C. The fluid flowing through the shell side is methanol depleted after hydrogen sulfide regeneration, with an exit temperature of –12°C and an inlet temperature of 92°C. Due to the large temperature difference at the inlet and outlet of the pipe train, corrosion is severe, resulting in significant material leakage. This indicates that the degree of corrosion is related to the properties of the medium and the temperature. The methanol in the tube side of the 2# main methanol heat exchanger contains large amounts of H2S, and H2S can react directly with iron or iron oxides to form sulfates, thereby causing corrosion to the equipment. In actual production, due to the large amount of moisture present in the methanol system, hydrogen sulfide dissolves in water to form hydrosulfuric acid under certain temperature conditions. Hydrosulfuric acid reacts with iron to produce ferrous sulfide, which can also cause corrosion to the equipment. When a medium corrodes the tubes of a heat exchanger and the pipes connected to them, the external dimensions of the metal appear almost unchanged on the surface, but the metal’s strength and ductility decrease, as does its elastic range. After CO is flashed out of the H2S-rich methanol in the hydrogen sulfide absorption tower, the carbonyl compounds and a small amount of CO present in the feed gas dissolve in methanol; upon contact with metal equipment, they form iron sulfide, which adheres to the metal surfaces. To ensure proper heat transfer, a stream of feed gas with a flow rate of 200 m3/h was introduced at the outlet of the methanol pump in the hydrogen sulfide flash tower. This resulted in the methanol in Main Methanol Exchanger No. 2 containing high levels of CO. CO reacts chemically with iron metal to form iron carbonyl, and the presence of hydrogen sulfide further accelerates this reaction between CO and iron metal, speeding up the corrosion of the equipment caused by iron carbonyl. Although methanol rich in H2S passes through the hydrogen sulfide absorption tower and is pressurized by a methanol pump after going through the hydrogen sulfide flash tower, as the temperature rises, hydrogen sulfide gas continues to flash out, resulting in a process of dissolution, flashing, and re-dissolution. This leads to uneven stress on the surface of the equipment, thereby accelerating the corrosion of iron metal by H2S. 2) Erosive corrosion. Methanol rich in H2S flows over the surface of the tubes; especially when vortices occur or the flow direction of the liquid methanol changes suddenly, the mechanical erosion caused by the methanol fluid damages the protective film on the metal surface of the tubes. As a result, the H2S in the methanol undergoes chemical or electrochemical reactions with the metal surface of the tubes, leading to erosion corrosion. The characteristic of erosion corrosion is the appearance on the metal surface of directional corrosion patterns such as grooves, channels, ripples, and circular holes. 3) Wear corrosion. The series of tubes suffer from both wear and corrosion as forms of degradation on their metal surfaces. It occurs at the points where the flow direction of liquid methanol changes, such as the U-shaped sections of heat exchanger tubes and bends in pipelines. In these areas, the flow of liquid methanol is turbulent, resulting in significant changes in the momentum of the vapor and liquid; this leads to severe wear and erosion of the heat exchanger, accelerating the corrosion of its tubes and pipes. 2 Protection measures 1) Improve production management and change operating conditions. To prevent H2S from dissolving in water to form hydrosulfuric acid, which could cause severe corrosion to the No. 2 main methanol heat exchanger and the pipelines connected to it, measures were taken first to adjust the operating conditions of the methanol-water distillation tower and to strictly control the rate of pressure surges and releases, in order to reduce the moisture content in methanol. 2) Choose materials with better corrosion resistance. Given the poor resistance of 1Crl8Nl 9Ti material to H2S corrosion and stress corrosion, it is being considered to replace it with other materials with better corrosion resistance, such as aluminum-coated carbon steel. Aluminized carbon steel is obtained by alloying the surface of carbon steel, resulting in the formation of a dense aluminum-iron alloy on that surface. The bonded strength between this coated layer and the base material is high, and it does not affect the heat transfer properties of the carbon steel. Aluminized steel possesses excellent corrosion resistance; its resistance to media such as H2S, SO2, SO3, NH3, CO2, and seawater is 4 to 40 times that of carbon steel, and its ability to resist high-temperature oxidation exceeds that of 1Crl8Ni9Ti. It is currently one of the best materials used in China to resist high-temperature H2S corrosion. Increasing the wall thickness of the tubes, by using thick-walled tubes for the heat exchanger tubes, can increase the wall thickness by 1–3.5 mm. When selecting elbows and bends, increasing the outer wall thickness of these components by 1–2 mm helps to increase the radius of curvature of the pipelines and reduce turbulence. This helps to reduce the scouring and wear corrosion caused by liquid methanol on equipment and pipelines, extends their service life, and ensures the smooth progress of production. 3) Follow procedures strictly. The ammonia synthesis production process has a long sequence of steps and numerous chemical units; any mistake in operation can have adverse effects on the equipment and pipelines. Strict operation is extremely crucial for reducing equipment corrosion. 4) Strengthen equipment corrosion investigations and monitoring of corrosion-prone areas. Corrosion detection and standardized treatment: introducing and developing a range of mature online detection methods and implementing computer-based standardized management. 3 Conclusion The corrosion of the 2# main methanol heat exchanger is a common problem in methanol washing units; the main causes are corrosion caused by H2S in methanol, erosion corrosion due to the flow of materials, and wear corrosion. Therefore, corrosion control for the No. 2 main methanol heat exchanger is a systematic project that involves many aspects such as the design, manufacturing, and operational procedures of the heat exchanger. Therefore, in production practice, comprehensive protective measures must be taken to fully control the formation of various corrosive environments, which can effectively extend the service life of heat exchangers and ensure the safe and stable operation of the entire methanol washing unit.