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In the shift process of the ammonia synthesis system, medium-pressure shifting is superior to atmospheric-pressure shifting: it saves 13%–1.5% in energy consumption, increases the reaction rate by about 4 times, and improves the thermal balance of the entire system. For the \"8·13\" renovation project of Jining Fertilizer Plant, medium-voltage conversion was selected. However, due to the increased pressure, the partial pressures of CO2 and H2S rise, resulting in much more severe corrosion in the medium-pressure shift system compared to that in the atmospheric-pressure shift system. Therefore, key equipment such as saturated hot water towers and heat exchangers, as well as the pipelines, are made of stainless steel; at the same time, it is required that the pressure resistance of the catalysts and the steam pressure be increased accordingly. Puncture blockages in heat exchangers, short service life, increased steam consumption, higher electricity usage, and even catalyst poisoning in low-temperature shift catalysts due to oxygen leakage, leading to forced shutdowns for maintenance, are quite common in medium and small nitrogen fertilizer plants. The heat exchanger at Jining Fertilizer Factory became clogged and started leaking, resulting in high system resistance; the pressure difference across the heat exchanger reached over 0.25 MPa at its maximum. Since 1998, the unit has had to stop operating more than 4 times per year for this reason. Repeated water flushing has caused significant thermal stress on the tubes, and dust and impurities present in the gas have further eroded these tubes, leading to severe internal leaks. As a result, electricity and steam consumption has increased dramatically, and the costs associated with starting up and shutting down the unit are high; this has become a major obstacle to long-term operation. 1 Basic information on the heat exchanger in the medium-pressure system: The heat exchanger is of the tube-type design, with dimensions of φ1460×30 mm. It is divided into two sections; the tubes in the upper section are made of stainless steel, with 1550 tubes of diameter φ25×2.5 mm, resulting in a heat exchange area of 240 m2. The tubes in the lower section are made of 20# carbon steel, also with 1550 tubes of diameter φ25×2.5 mm, giving a heat exchange area of 730 m2 ； The inter-tube transition gas is connected by 16 connecting pipes with dimensions of φ108×4.5 mm, and the material of the entire pressure-bearing component is 16MnR. 2 Reasons for blockage and leakage in the heat exchanger: The conversion system was put into operation at the end of 1995, and production stabilized at the designed capacity in 1997. Since 1998, efforts have been made to further increase production, with the output of synthetic ammonia reaching 100 kt. The volume of gas fed into the conversion system amounted to 47,340 m3/h; as a result, the conversion system operated under overload conditions. The saturation tower exhibited significant water carryover even under normal operating conditions. The drain valve of the mixer was kept open, allowing water to flow continuously, and the heat exchangers frequently became clogged or leaked, which led to high resistance in those exchangers, overpressure in the conversion system, high steam consumption, and high power consumption by the compressors. As a result, the conversion system became a bottleneck in the production process. 2.1 Causes of heat exchanger blockage. The blockages occurred at the center of the upper tube sheet, with the contaminants accumulating on the tubes; some of these tubes were completely blocked. These contaminants were yellowish-brown powdery solids mixed with black impurities. Multiple samples of the relevant fluids and the contaminants were taken for analysis, and the results are as follows; details regarding the sulfur content in the contaminants are provided in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0605261421569064.jpg The oil contamination contains 2.4% carbon; the oil content is low. However, due to the sample having been left for an extended period, the lubricating oil has evaporated, and the carbon content can only represent the tar. Therefore, the oil-water separation effect is poor. Water quality: contains 1.7% Na, 0.029% Si, 0.061% P, and 0.017% Ca. The water quality is good, and it is not the main factor contributing to blockages. The corrosion product contains 0.26% Cr, 0.95% Fe, and 0.20% Ni. This indicates mild corrosion. Semi-degassed: contains 14.0% N, 30.0% S, and 50% O; the phase composition is a mixture of (NH4)3H(SO4)2 and NH4HSO4. The sample contains a large amount of SO42-, which is the main contaminant causing blockage. N originates from ammonia brought in by semi-deaeration. Ammonia can raise the pH of water and suppress corrosion. However, it tends to form acidic sulfates; therefore, water in the saturated gas stream and water mist entrainment must be prevented. A comprehensive analysis shows that the sources of these solid substances mainly fall into the following three categories. (1) Intake of water. Blockages often occur in the heat exchangers of many small fertilizer plants as well. One reason for this is that the soft water added to the heating system contains a high amount of salts; these salts enter the heat exchanger along with the water droplets or mist carried by the gas. When heated, the water evaporates, leaving the salts to cause blockages within the tubes of the heat exchanger. What our factory uses to make up the volume is desalinated water, which contains very little salt and will not cause blockages. (2) Corrosion products. It is not excluded that corrosion products formed as a result of corrosion of the equipment and pipelines may enter the heat exchanger along with water droplets or mist in the gas. However, the main equipment and pipeline materials in our factory are all stainless steel, so even if corrosion occurs, it won’t be that severe. (3) Introduce the gas into the transformation system. We believe this is the main cause of the blockage. The gas purification effect is poor; the hot gas coming from the third stage outlet of the compressor at 130°C enters the shift system. Tar, coal dust, sulfur foam, oil residues, ammonia, and other solid impurities carried in this gas end up in the shift system, causing the total solid content in the hot circulating water to reach levels of over 2000×10-6. To solve this problem, in 2000 we added a water cooler to the shift reactor to cool the gas entering the shift system below 35°C, and then separated the oil from the water before allowing it to enter the shift system. However, after the water cooler was put into operation, even though the interval between blockages increased, the problem of blockages in the heat exchanger was not completely resolved. 2.2 Causes of heat exchanger leakage: Based on the numerous inspections carried out on our factory’s heat exchangers, leaks mostly occur at the tube sheet openings. The tube openings located directly above the center of the gas inlet pipe have been eroded to such an extent that their thickness is as thin as paper; the wall thickness of these tubes is less than 0.4 mm. There are multiple holes in the tube walls, and the welds between the tube sheet and the tubes contain pores and cracks visible to the naked eye (the cracks are irregular, interlacing patterns). Regarding its leakage, we analyze it mainly due to the following three reasons. (1) Electrochemical corrosion: Our plant’s conversion system uses superheated steam for supplementation; this steam is introduced at the inlet of the mixer, where it mixes thoroughly with the gas before entering the heat exchanger tubes from the top. However, due to the water present in the saturated tower, the superheated steam added at 2.5 MPa and 420°C must first heat and vaporize part of this water, resulting in significant loss of heat from the superheated steam. The temperature after mixing approaches the dew point temperature; moreover, the unvaporized water mist causes O2, CO2, H2S, and Cl- present in the gas flowing through the tubes on the upper tube sheet to react in the aqueous phase to form highly corrosive substances such as carbonic acid, hydrosulfuric acid, and hydrochloric acid. This creates conditions for electrochemical corrosion and dew point corrosion. Through various phase-state tests (gas phase, liquid phase), it was found that in the liquid film and liquid mist states, even with low levels of H2S in the gas, the corrosion is several dozen times more severe than that caused by high concentrations of H2S in the gas phase. (2) Erosive corrosion: Due to the large amount of impurities in the gas produced by our plant and the poor purity of this gas, there is a high level of dust present. Even after going through various purification processes such as wash towers, desulfurization systems, separators, and saturation towers, this dust still inevitably makes its way into the heat exchangers. This is confirmed by the large amount of fine black coal dust in the wastewater that is discharged during hot water washing after clogging. Dust impurities carried in the gas enter the heat exchanger of the barrier-free device from above, causing the walls of the wear-resistant stainless steel pipes to thin out rapidly, and perforations occur frequently. (3) Stress corrosion: Our factory uses hot water flushing whenever the heat exchangers become clogged. After parking, hot water at 95°C was added from the top to the heated tubes, subjecting them to both scouring and stress changes caused by alternating heat and cold, which exacerbated tube leakage. 3 Renovation and Concepts 3.1 Renovation of the Saturation Tower System After thorough investigation and research, during the major maintenance in the autumn of 2001, our factory collaborated with Hebei Zhengyuan Company to transform the saturation tower into a new type of vertical sieve-plate tower. This transformation increased the production capacity of each tower by 25%, and it eliminated the problem of excessive water carried by the saturation tower. The gas and steam mixture entering the heat exchanger was superheated; even if the gas contained large amounts of H2S, O2, and CO2, the absence of water mist meant there was no environment in which highly corrosive substances could form, thus preventing electrochemical corrosion and dew point corrosion. Due to the high mass transfer efficiency, large mass transfer area, and high processing capacity of vertical sieve plate towers, their height is much lower than that of packed towers. To facilitate the modification of the saturated tower, the liquid inlet pipe of this tower was lowered by 2.3 meters, which increased the separation space in the upper part; a swirl plate separator was installed at the top, and a pipeline separator was also added to the gas outlet pipeline. Thus, three layers of separation are available to remove the tiny droplets carried in the gas phase. Additionally, the mixer was removed, which completely solved the problem of gas containing water entering the heat exchanger. This not only extended the lifespan of the heat exchanger but also protected the medium-temperature catalyst. 3.2 Modification of the heat exchanger system: To address the issue of erosion caused by scouring, a circular baffle was installed at the gas inlet. As the high-speed gas carrying debris enters the heat exchanger at a speed of over 19 m/s, it hits the baffle and then rebounds back into the heat exchanger, thereby avoiding direct scouring of the tubes and preventing erosion due to such direct impact. To prevent electrochemical corrosion and dew point corrosion, three layers of stainless steel insulation plates with a thickness of δ = 1 mm were installed on the tube sheet of the new heat exchanger. Stainless steel protective sleeves with a diameter of φ19×1 mm and a length of 100 mm were placed at each tube entrance. This protection prevents the tubes as well as the welds between the tubes and the tube sheet from being subjected to electrochemical corrosion and dew point corrosion caused by aggressive corrosive agents, thereby **extending the service life of the heat exchanger**. In the next phase of modification, the heat exchanger can be divided into two units; the first heat exchanger is a fully stainless-steel device with a small heat exchange area, and can serve as a pre-corrosion unit. The second heat exchanger is a carbon steel unit with a large heat exchange area. Handling the first small heat exchanger with problems saves time and effort ； The flow pattern in the first heat exchanger can also be changed so that the gas enters from the bottom and exits from the top; even if the gas contains water and impurities, it is possible to remove these substances from the bottom in a timely manner, thereby reducing the erosion and corrosion caused by the gas on the tubes. 4 Effects of the renovation: (1) It prevents 4–5 parking accidents per year, creating conditions for stable production over the long term and saving 250,000 yuan in costs associated with starting up and shutting down the equipment. (2) Steam rejection is significant. The thermal balance of the entire system has been improved: before the renovation, the steam consumption per ton of ammonia was 733–868 kg, while after the renovation it dropped to 501–651 kg per ton of ammonia. This results in a savings of approximately 220 kg of steam per ton of ammonia, amounting to 18 kt of steam saved annually. After the modification, the temperature of the gas at the outlet of the saturation tower increased by about 3°C, while the temperature of the shifted gas at the outlet of the hot water tower dropped by more than 10°C, improving the thermal balance. By saving medium-pressure steam, our plant’s generator sets have reached their designed capacity and are operating well. (3) The pressure difference in the conversion system is reduced, the power consumption of the compressor decreases, and three-stage overpressure operation is avoided. The inherent resistance of the heat exchanger decreased from 0.06–0.20 MPa before the modification to 0–0.02 MPa. (4) It improved gas purity, prevented water from entering the saturated tower, reduced wastewater discharge, saved expensive desalinated water, and increased the compliance rate of total solids in the circulating water to over 85%. In summary, after the renovation, the problem of water carryover in the saturated tower due to overloading was resolved, blockages in the heat exchangers were prevented, and leaks from those heat exchangers were avoided. This ensured the safe and stable operation of the conversion system, maintained optimal operating conditions across all processes within it, saved on maintenance costs as well as consumption of steam and deionized water. It provided a foundation for the long-term stable operation of the ammonia synthesis system and laid the groundwork for subsequent \"medium-low-low\" level renovations of the conversion system.