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The main corrosion issues in aromatic extraction units depend on the different process flows of such units. After the sulfolane solvent deteriorates, the key areas where corrosion occurs include the trays and internal components of the extraction distillation tower, as well as the bottom reboiler; the heat exchangers for lean/rich solvents; the trays, internal components, bottom reboiler, and top air coolers of the solvent recovery tower and stripping tower; the trays and bottom reboiler of the regeneration tower; as well as the corresponding high-temperature bottom pumps and lean/rich solvent pipelines. The main areas affected by the macromolecular degradation polymers resulting from sulfolane degradation in aromatic extraction units include: the trays of the extraction distillation column, the reboiler, the lower parts of the stripping column and recovery column as well as the reboiler, the regeneration column and reboiler, and the poor/rich solvent heat exchangers. This is manifested as tray blockage, which reduces extraction efficiency; blockage of the heat exchanger tubes and pump filters; or coking on the outer surface of the reboiler tubes, which lowers heat and mass transfer efficiency. Causes and influencing factors of corrosion 01 Degradation mechanism of sulfolane and its influencing factors. The main mechanisms for the degradation of the sulfolane solvent in aromatic extraction units are high-temperature oxidative decomposition and hydrolysis. High-temperature oxidative decomposition refers to the breakdown of sulfolane in a high-temperature, oxygen-containing environment, resulting in the formation of SO2 and butadiene; butadiene then polymerizes to form large molecular organic polymers, which cause blockages in equipment and pipelines ； SO2 further reacts with water and free oxygen to form sulfuric acid, causing corrosion of equipment and pipelines. Under normal operating temperatures and in an anaerobic environment, the decomposition rate of sulfolane is very slow; decomposition occurs when the temperature exceeds 180°C. The decomposition rate increases gradually between 180°C and 220°C, and it accelerates significantly above 220°C ； The presence of oxygen **accelerates the decomposition rate of sulfolane and oxidizes SO2 to SO3, thereby creating a highly acidic corrosive environment.** The hydrolysis reaction of sulfolane occurs under the conditions of aromatic extraction processes; sulfolane hydrolyzes to form sulfonic acid compounds, and acidic substances act as catalysts for this hydrolysis reaction, accelerating it. Degradation tests were conducted on sulfolane with different water contents at 180°C, and it was found that as the water content increased, the pH value of the sulfolane solution decreased. When the mass fraction of water exceeded 3%, sulfolane degraded rapidly, and the corrosivity of the sulfolane solution increased significantly. At the same time, water entering the aromatic extraction system can also accelerate the degradation of sulfolane due to dissolved oxygen. 02 Corrosion by sulfolane solvent: Sulfolane degrades and decomposes to produce acidic substances, which include inorganic acids such as sulfuric acid and sulfurous acid, as well as organic acids such as sulfonic acids and small amounts of carboxybutyric acid. Furthermore, oxygen increases the corrosivity of the sulfolane solvent. ①The corrosive sulfone solvent, when exposed to high temperatures, oxygen, water content, and cyclobutenesulfone, decomposes to produce acidic corrosive substances such as sulfurous acid, sulfuric acid, sulfonic acid, and carboxybutyric acid. This results in a significant decrease in the pH value of the solvent and an increase in its acidity; these acidic substances cause corrosion and thinning of carbon steel equipment and pipelines. Generally, the more acidic substances present in the sulfolane solvent, the lower its pH value and the greater its corrosiveness ； At the same time, the corrosivity of the sulfolane solvent increases as the temperature rises. ②Chloride ions: Chloride ions in sulfolane solvents tend to accumulate within the system, causing corrosion. On one hand, chloride ions may promote the degradation of sulfolane, exacerbating the formation of acidic substances and further lowering the pH value of the solution ； On the other hand, chloride ions accelerate localized metal corrosion, leading to pitting and stress corrosion cracking in austenitic stainless steels. The chloride ions in the aromatic extraction unit mainly come from the extraction feedstock, system water, and water leaking into the reboiler and water cooler. Due to the continuous accumulation of chloride ions in the sulfolane solvent, the mass fraction of chloride ions in the solvent is usually high, exceeding 100 μg/g in severe cases. The impact of 03 cyclobutanesulfoxide impurity: Cyclobutanesulfoxide is the raw material used in the production of cyclobutanesulfoxide, and a certain amount of it is present in cyclobutanesulfoxide solvents. Compared to sulfolane, cyclobutenesulphone has unstable properties and tends to decompose upon heating to produce SO2. The higher the content of cyclobutenesulphone, the more SO2 is generated, resulting in a stronger acidity of the solvent. An acidic environment readily promotes the high-temperature oxidative decomposition and hydrolysis of sulfolane, further accelerating its degradation. Corrosion control strategy 01: Raw material quality control – The chlorine content in aromatic raw materials shall not exceed 1 μg/g. Before fresh sulfolane solvent is introduced into the system, it is necessary to control the technical parameters of the sulfolane, especially its content of sulfolene and its water content; generally, the mass fraction of sulfolene should not exceed 0.2%, while the mass fraction of water should not exceed 0.5%. At the same time, while ensuring the quality of the fresh sulfolane solvent, the equipment should use sulfolane solvents of the same brand as much as possible, in order to avoid mixing sulfolane solvents from different brands. 02 Material selection for equipment: For the aromatic extraction unit, carbon steel is used as the material for most of the equipment and pipelines. Taking into account the key corrosion areas of the equipment as well as the risk factors for stress corrosion cracking in austenitic stainless steels in chloride-containing environments, carbon steel + 0Cr13/0Cr13Al was selected as the material for the cylinders of the stripping tower, solvent recovery tower, and regeneration tower. The trays and internal components are made of 0Cr13, 022Cr19Ni10, or 06Cr18Ni11Ti. The tube bundle material for reboilers and heat exchangers is 06Cr18Ni11Ti or 022Cr17Ni12Mo2. The material for the inlet and outlet pipelines of the reboiler and heat exchanger, as well as the pipeline at the top of the regeneration tower, is 022Cr19Ni10 or 06Cr18Ni11Ti. 03 Process corrosion prevention: The temperature at the bottom of the recovery tower and the stripping tower is kept below 180°C. The bottom reboiler uses steam as a heat source, with the temperature of the superheated steam being controlled to stay below 220°C, in order to prevent local overheating that could lead to the decomposition of sulfolane. Some companies use the following methods to improve solvent recovery efficiency: (1) Injecting an appropriate amount of stripping steam at the bottom of the recovery tower reduces the vapor pressure of oil and gas within the tower, which facilitates the separation of aromatics from the sulfolane solvent. This prevents the solvent from decomposing due to high temperatures, thus protecting it. The amount of water used for stripping is approximately 1% of the mass fraction of the feed material. (2) The recovery tower and regeneration tower are operated under negative pressure to ensure that sulfolane is separated from the aromatics at lower temperatures and regenerated, thereby preventing it from escaping along with the aromatics due to vaporization or from decomposing at high temperatures. To prevent oxygen from entering the extraction system, the main measures include: (1) ensuring the sealing of equipment such as the feed system, turbine water system, and solvents, in order to prevent active oxygen from entering via steam ； (2) Ensure that the flanges, valves, and instrument connections are properly sealed ； (3) Ensure the sealing of the negative pressure operating system to prevent air from entering it ； (4) Add an external oxygen supply steam stripper, and establish analysis of active oxygen and carbonyl groups in the feed materials; materials with excessive oxygen content should be subjected to steaming first before being fed into the extraction unit. Control the pH of the sulfolane solvent. Refining and chemical processing enterprises typically use the addition of mon ethanolamine (MEA) to control the pH value of the sulfolane solvent; when the pH value of the solvent falls below 6.5, an appropriate amount of mon ethanolamine should be added to the system in a continuous manner. At the same time, it is necessary to control the temperatures of the recovery tower and the regeneration tower to prevent significant temperature fluctuations from causing the decomposition of monoethanolamine and initiating new polymerization reactions ； Excessive monoethanolamine will react chemically with the sulfonic acids produced by the degradation of sulfolane, forming amine salt precipitates that can cause blockages in the equipment. Water content in the control system. In daily production operations, strict adherence to the process dehydration procedures is necessary. Especially during winter when temperatures are low, it is important to closely monitor the liquid levels in the reflux and regeneration solvent tower dehydration vessels, and to carry out dehydration promptly. Leaks in the reboiler at the bottom of the tower are also one of the major causes of water contamination in the system. During equipment maintenance, it is necessary to strengthen the monitoring of heat exchangers, especially during the later stages of operation; leaks should be detected promptly and sealed to prevent water from entering the solvent system and affecting the quality of the sulfolane solvent. 04 Corrosion monitoring and inspection: Conduct corrosion risk assessments on the sulfolane solvent-related equipment and pipelines in the aromatic extraction unit; establish inspection plans and procedures for areas prone to corrosion. Use pulse eddy current and ultrasonic thickness measurement techniques to assess the corrosion status in these key areas, thereby helping to avoid corrosion risks in advance. Regularly analyze the chloride ions in aromatic raw materials, as well as the cyclosulfone and water content in the cyclosulfolane solvent ； Regularly analyze key parameters in the sulfolane solvent such as pH value, chloride ions, iron ions, water content, and solid content ； Monitor the bottom temperatures of the recovery tower and stripping tower, the system’s sealing performance, as well as the liquid level and dehydration status of the water pockets in the top reflux tanks of each distillation column. 05 Purification of sulfolane solvent: Current sulfolane purification techniques mainly include vacuum extraction, desulfurization and dechlorination agents, membrane separation, activated carbon purification, and anion exchange resin methods. Among them, the anion exchange resin method yields the best purification effect, enabling the effective removal of chlorine, sulfur, and acidic substances from sulfolane. The anion exchange resin method is primarily used to remove the corrosive anions from sulfolane. The mechanism involves an exchange reaction between the anions in the sulfolane solvent and the basic groups on the ion exchange resin, or a salt formation reaction with the amine groups on the resin. Once the exchange groups on the resin are converted into salts and become inactive, they are regenerated using a NaOH solution with a mass fraction of 2% to 4%. After washing with deionized water until the solution reaches neutrality, the resin can be reused. Industrial applications have shown that large-pore, weakly basic anion exchange resins possess good thermal stability and strong resistance to organic contamination. They are capable of effectively removing anions such as sulfonate, sulfate, and chloride ions from sulfolane solvents, thereby raising the pH value of these solvents and significantly reducing corrosion, which in turn ensures the stable operation of the equipment.