Corrosion and clogging in the low-temperature methanol washing system of ammonia synthesis plants and countermeasures
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Corrosion and blockage in the low-temperature methanol washing system of ammonia synthesis plants and countermeasures: The causes of corrosion and blockage in this system within ammonia synthesis plants are discussed, the impact of such corrosion and blockage on the proper operation of the system is analyzed, and measures to address these problems are proposed. Synthetic ammonia ； Methanol washing ； corrosion ； blockage ； Impact ； Measure 1 Introduction: In the large-scale ammonia synthesis plants introduced in our country, which use residue oil and coal as raw materials, low-temperature methanol washing is generally employed to remove acidic gases such as CO2 and H2S from the process gas. Many plants that are already in operation, especially those using Linde’s Rectlsol low-temperature methanol wash process, suffer from corrosion and blockage to varying degrees, which makes it difficult to ensure long-term full-load operation of the ammonia synthesis plants. As a result, the low-temperature methanol wash system becomes a bottleneck for the entire ammonia synthesis plant, severely affecting its economic efficiency. This paper analyzes the causes of corrosion and blockage in low-temperature methanol washing systems, and proposes corresponding solutions. 2 Causes of corrosion and blockage 2.1 Causes of corrosion 2.1.1 Formation of iron carbonyls The areas where corrosion occurs in low-temperature methanol washing systems are usually equipment such as heat exchangers in the gas flow path. The main cause of corrosion is the formation of iron carbonyls, particularly Fe(CO)5 and sulfur-containing iron carbonyls; the latter are intermediate products in the formation of Fe(CO)5. The presence of H2S significantly accelerates the reaction between CO and Fe. 2.1 2 Electrochemical corrosion: In pure methanol, H2S and CO2 do not cause corrosion to equipment and pipelines. In the presence of water, H2S and CO2 in the methanogenic solution will render the equipment and pipelines through which the liquid flows acidic, leading to electrochemical corrosion; the main product formed is ferrous sulfide. The design requirement is that the water content in recycled methanol should be kept below 0.5%, but most manufacturers often experience water levels that exceed this limit; in severe cases, these levels can reach 3% to 5%. Moreover, it is difficult to determine the exact water content in recycled methanol, which inevitably leads to electrochemical corrosion in the system. 2.1 3 Influence of air: After operating for a period of time, the low-temperature methanol washing system generates a certain amount of sulfides due to corrosion. Although the system is usually washed with water before shutdown for maintenance in order to remove most of these sulfides, it is inevitable that some sulfides remain in the system ； When the equipment is opened for maintenance, air enters, and sulfides react with water in the presence of air to form polysulfates, which cause stress corrosion to the equipment, especially in austenitic materials containing ferrite. Additionally, oxygen in the air also undergoes an oxidation reaction with metals to form oxides. Rust forms on the inner walls of equipment and pipes. 2.2 Causes of blockage 2.2.1 Decomposition of carbonyl compounds The carbonyl compounds dissolved in the methanol solution within the low-temperature methanol washing system do not decompose in the low-temperature sections of this system. However, they do decompose when the methanol solution is thermally regenerated, resulting in the formation of solid precipitates such as elemental sulfur and iron sulfide. These particles are very small, and they can easily deposit on the inner surfaces of pipes and equipment, causing fouling over time, which in turn leads to blockages in heat exchangers, pipes, and instruments. The solids removed from the filters in the system are a black powdery substance, and sampling analysis confirmed that its main component is ferrous sulfide. 2.2 Solid impurities from the previous system 2.2.1 Catalyst powder: During the CO conversion process, due to factors such as gas erosion and the mechanical strength of the catalyst, the catalyst is abraded to form powder; these solid impurities are carried out of the conversion reactor along with the process gas. Although there is a water washing process before entering the low-temperature methanol wash system, some solid impurities still make their way into this system and then into the methanol solution, where they deposit on the inner surfaces of pipes and equipment, causing blockages in heat exchangers, pipes, and instruments. 2.2.2 2-Carbonyl compounds: Metal carbonyl compounds present in the feed gas from the vaporization unit, such as Ni(CO)4 and Fe(CO)5, are mostly decomposed during the CO conversion process. However, a small amount of these carbonyl compounds still end up in the low-temperature methanol washing unit along with the process gas; they dissolve in the methanol solution and are subsequently decomposed into solid precipitates during the thermal regeneration of the methanol solution. 2.2.3 Poor filtration efficiency within the system: In the low-temperature methanol washing system, solid substances coming from upstream processes as well as those resulting from corrosion within the system are removed by filtration. Two types of filters have been installed: one is the inlet filter screens for each pump in the system. The second is an independent filter. However, practice has shown that the filtering efficiency of the filters is not very satisfactory. There are mainly two reasons: first, the solid particles within the system are very small, and the pore size of the filter’s mesh is relatively large ； Second, the particles in the methanol solution in the low-temperature zone of the system are difficult to remove by filtration. 3 Effects of corrosion and blockage 3.1 Equipment damage Corrosion in the low-temperature methanol washing system occurs mainly in the carbon steel equipment within the system, and severe corrosion can lead to damage to such equipment. In one plant, internal leakage occurred in the heat exchange tubes of E15, the reboiler at the bottom of the C5 tower, due to corrosion. Causes significant loss of methanol in the system ； Moreover, during a subsequent major maintenance period, an internal leakage in E9 occurred, forcing it to be opened for repair (see Figure 1 for the low-temperature methanol washing process flow). It is evident that the severe corrosion of the system poses a very serious threat.
3.2 Reduced heat transfer efficiency: The corrosion products within the low-temperature methanol washing system, along with the solid impurities brought in from previous systems, are very small in size; as a result, it is difficult for the filters installed in the equipment to remove them completely. Moreover, these solid particles tend to deposit easily at normal temperatures. The boundary devices E9 and El0 that separate the cold zone from the hot zone in the system typically use coiled heat exchangers, whose inner diameter of the heat exchange tubes and tube spacing are much smaller than those of conventional shell-and-tube heat exchangers. As a result, the inter-tube passage between the E9 shell side and the E10 tube side often becomes fouled and blocked, leading to a decrease in heat exchange efficiency, an increase in heat loss from the system, and even making it difficult to maintain the system’s operation. As operating time increases, both the temperature difference at the cold ends and those at the hot ends of the two heat exchangers keep rising, resulting in an increasing cold loss that severely affects the cooling balance of the system. During the severe crisis in 1999, when El0 was opened for cleaning, it was found that more than 70% of the internal heat exchange tubes were blocked, demonstrating how serious the blockages in the equipment are for daily production. 3.3 Clogged filters require frequent cleaning of the solid particles resulting from corrosion and those carried in from the previous system. Some of these particles are trapped by the filters within the system as well as by the filter screens at the pump inlet, which leads to clogging of these screens. This increases the resistance in the system, reduces the pressure at the pump inlet, and raises the pump’s rotation speed; as a result, it becomes necessary to carry out cleaning operations. When the system is severely corroded, frequent cleaning is required, which not only increases the labor intensity but also results in a large amount of methanol being wasted each time the filter is cleaned, thereby increasing the methanol loss in the system. Also, during driving after a major repair, the cleaning of the filters is required very frequently; on the most intense days, this can occur up to 22 times, resulting in a considerable amount of physical strain. 3.4 Increased energy consumption of moving equipment: When the filters or heat exchangers in the system become clogged, it leads to an increase in the system’s resistance, which reduces the pumping capacity of the pumps and increases their energy consumption; as a result, operating the system becomes very difficult. This phenomenon is most likely to occur at P3; when E9 and El0 become blocked, the liquid level at the bottom of tower C3 often remains high, affecting normal production control. 4 Countermeasures 4.1 Corrosion prevention measures 4.1.1 Strict control of water content in circulating methanol To reduce the likelihood of electrochemical corrosion, it is necessary to keep the water content in circulating methanol strictly within the design value (i.e., below 0.5%). This can be achieved from the following aspects: (1) Under normal conditions, the water in the circulating methanol mainly comes from the upstream carbon monoxide conversion process, through careful operation of that process. Reduce the amount of water that enters the low-temperature methanol washing system along with the transformed gas ； (2) Strengthen the operation of the methanol-water mixture separator V1 and the methanol/water separation tower C5 within the system to ensure effective separation of methanol and water ； (3) Strengthen the monitoring of the water content in the methanol circulating within the system; if the water content increases, adjust the operation of the methanol/water separation tower C5 promptly ； (4) Monitor the water content of the fresh methanol added to the system; if the water content is high, it should be sent to the C5 unit for dehydration before being added back to the circulating methanol. 4.1 Control the pH value of the circulating methanol in a weakly alkaline environment (pH=7～8). The corrosion of metal equipment caused by acidic gases such as CO2 and H2S is completely suppressed or reduced to an insignificant level. Linde Company suggests that alkaline compounds (such as sodium hydroxide, ammonia, amines, etc.) can be added to prevent the corrosion of carbon steel equipment. The Rectisol process introduced in China in the early days was equipped with a device for adding sodium hydroxide, used to adjust the pH value of the circulating methanol and the wastewater discharged. Since it was not very effective, it was never put into use, and all later introduced devices eliminated this feature. According to the operational experience of some manufacturers and the data provided by Linde, keeping the ammonia content in the circulating methanol at 30–60 mmol/L can effectively suppress corrosion in the system. It also improves the absorption capacity of the methanol solution, reduces the flow rate of circulating methanol in the system, and lowers the system’s energy consumption. However, too high an ammonia content can affect the proper operation of the C4 unit in the methanol thermal regeneration tower, resulting in excessive H2S levels in both the purified gas and the CO2 product gas. Generally, when the ammonia content in circulating methanol does not exceed 100×10-6, it will have no adverse effect on the system. The ammonia content in the circulating methanol can be controlled by adjusting the amount of wash water for the shift gas in the carbon monoxide process. 4.1.3 Preventing internal system exposure to air: During the shutdown of the low-temperature methanol washing system, it is necessary to prevent air from entering the system as much as possible; if conditions permit, nitrogen purging can be used to maintain a slight positive pressure within the system. To carry out equipment maintenance, the part that needs repair must be isolated from the other parts to prevent air from entering those other devices. Once the maintenance is complete, the system should be sealed immediately and protected with nitrogen, so as to avoid exposing the equipment to air for long periods of time; this prevents corrosion of the equipment due to contact with air. 4.2 Measures to address clogging 4.2.1 Improving the filtration efficiency of filters To prevent clogging in the low-temperature methanol washing system. Solid particles in the circulating methanol can be removed in a timely manner using filtration, thereby preventing their deposition and scaling on the surface of the heat exchanger. The following measures can be taken to improve filtration efficiency: (1) Reduce the pore size of the filters at the inlets of each pump to enhance the filtration effect ； (2) A filter with a pore size equal to or smaller than that of Sl shall be installed on the methanol-poor pipeline from the C4 cold zone of the heat-exchange regeneration tower to El0, in order to filter the methanol-poor stream exiting the C4 cold zone and prevent solid impurities from entering El0. 4.2 Blowing nitrogen into the shell side of the heat exchanger: For heat exchangers in the system where blockages are likely to occur, nitrogen blowing can be used to prevent solid impurities from depositing and forming scale on the surface of the heat exchange tubes. Taking E9 as an example, nitrogen can be introduced from its bottom drain, and nitrogen is periodically filled into the shell side. When nitrogen is introduced, disturbances occur in the shell side of the heat exchanger. The backflow of nitrogen helps to dislodge any solid substances that are not firmly attached to the outer surface of the heat exchange tubes, thereby preventing the formation of a solid scale layer. When using this method, it is necessary to pay attention to controlling the pressure and flow rate of nitrogen to prevent the formation of air blocks in the heat exchangers and pipes, which could impair the proper operation of the system. If a firm scale layer has formed on the surface of the heat exchange tubes, this method can also help restore the heat exchange efficiency of the heat exchanger to a certain extent, but it cannot resolve the problem fundamentally. 4.2 3 Chemical cleaning: A thick layer of scale that forms on the heat exchange surface of a heat exchanger can significantly reduce its heat exchange efficiency. And it’s difficult to remove. In severe cases, chemical cleaning can be used to remove it during parking for maintenance. However, since the cleaning agents used for chemical cleaning are acids and bases, which can cause corrosion to the equipment being cleaned, care must be taken and all operational parameters must be strictly controlled. 5 Conclusion To prevent corrosion and blockage in the low-temperature methanol washing system and ensure long-term full-load operation of the ammonia synthesis plant, a preventive approach should be adopted, using methods to control the pH value and water content of the circulating methanol in order to reduce or completely eliminate corrosion. Enhance the filtration of recycled methanol to remove solid impurities promptly, preventing the formation of scale layers within the system. References: Operating Procedures for Ammonia Synthesis in the Chemical Engineering Department of Zhenhai Refining & Chemical Co., Yu Zunhong et al. Process Analysis of Large-Scale Ammonia Synthesis Plants. Beijing: Sinopec Press, 1993, p. 319. Tan Tiannen, Mai Benxi, Ding Huihua. Principles of Chemical Engineering. Beijing: Chemical Industry Press, 1990