Corrosion and Protection of Ammonia Vaporization Towers

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Our company’s ammonia recovery system was originally designed to produce ammonia water through atmospheric-pressure absorption; the ammonia absorption tower used in this system is a packed tower. The ammonia water produced is sent to an ammonia water station, where it is mainly used by farmers. During the off-seasons, large amounts of ammonia water are discharged, which not only pollutes the environment but also results in waste. In 1994, modifications were made to the ammonia recovery process: the ammonia absorption section was changed to pressurized absorption, the ammonia absorption tower was replaced with an isothermal plate-type bubble column, and an ammonia vaporization system was added. The liquid ammonia produced was sent for use in urea production, while the exhaust gases were directed to the combustion gas network, thereby completely eliminating pollution. Although the ammonia recovery retrofitting has achieved good economic and social benefits, the resulting corrosion problem in the ammonia evaporation tower has become a major factor affecting the stable operation of ammonia recovery. 1 Technical parameters and corrosion conditions of the ammonia recovery process 1.1 Ammonia recovery process The gas from the liquid ammonia storage tank, combined with the purge gas from the synthesis process, enters at the bottom of the ammonia absorption tower, where it comes into countercurrent contact with the dilute ammonia solution sprayed from the top of the tower; the ammonia in the gas is absorbed, and the exhaust gas is sent to subsequent processes via a gas-liquid separator. The ammonia in the gas is absorbed by dilute ammonia water to produce concentrated ammonia water with a concentration of 10%–12%. At a temperature of 50–55°C, it enters the ammonia water heat exchanger due to pressure differences and is heated to 150–160°C, after which it enters the ammonia vaporization tower from its middle to upper part. In the ammonia vaporization tower, the ammonia in the concentrated ammonia solution is vaporized and rises to the top of the tower, where it is condensed into liquid ammonia by fresh water at 14°C in the ammonia condenser. A portion returns to the upper part of the ammonia vaporization tower, while the rest enters the liquid ammonia storage tank as a product. The dilute ammonia solution after ammonia vaporization is sent to the ammonia heat exchanger where it exchanges heat with the concentrated ammonia solution coming from the ammonia absorption tower. The dilute ammonia solution is cooled to 60–80°C and then sent to the ammonia cooler where it is further cooled to 35°C. After being pressurized by the ammonia circulation pump, it is fed into the ammonia absorption tower for cyclic absorption within the system. The heat required in the ammonia vaporization tower is provided by the steam fed to the reboiler. To maintain the proper liquid level, steam condensate must be added to the system regularly. The ammonia recovery process is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0512291411204922.jpg 1.2 Main technical parameters of the ammonia vaporization tower: (1) Design parameters: Diameter = ￠600 mm, H = 15,640 mm ； The column is equipped with 23 floating valve trays, each tray having 10 floating valves ； Tray spacing: 400 mm ； Operating temperature 200～180℃ ； Operating pressure: 1.3 MPa. (2) Material: Simplified 16MnR ； Plate Q235A-F, ； Spacing tube, 10# carbon steel ； Floating valve 1Crl8Ni9Ti. (3) Medium and process parameters for various sections: Operating pressure of the ammonia vaporization tower is 1.3–1.4 MPa ； The temperature at the bottom of the ammonia vaporization tower is 180°C; the medium used is 0.5%–1% ammonia water ； The feed temperature to the ammonia vaporization tower is 150–160°C, with the medium being 10%–12% ammonia water ； The top temperature of the ammonia vaporization tower is 55–65°C, with the medium being 99.5% liquid ammonia. 1.3 Corrosion condition of the ammonia vaporization tower: Since the ammonia recovery system was put into operation in October 1994, it has been operating well. During the major maintenance in May 1997, it was found that the ammonia vaporization tower was severely corroded: the thinnest part of the tower, with a thickness of 8 mm, had only 3.6 mm left ； Most of the floating valves on the trays have fallen off due to corrosion, and there are numerous penetrations in the welds on the trays ； On the trays of the 12th, 13th, and 14th floors, the 3-mm arc-shaped downcomer channels were penetrated due to severe corrosion, resulting in holes approximately 15 mm in length. To this end, Section 4 in simplified format, 6 trays, all spacing tubes, and all floating valves were replaced. In February 1998, a leak was detected at the 5th section of the shell at the top of the ammonia vaporization tower (at the thermometer location), and severe corrosion was observed overall. In just 9 months, the wall thickness of the shell decreased from 7.8 mm to 2.4 mm, with a corrosion rate of 7.2 mm per year. Based on the distribution of corrosion, the feed section of the ammonia vaporization tower is severely corroded; there is also a certain degree of corrosion in the lower part, while the top section shows less corrosion. 2 Analysis of corrosion causes Based on the pattern of corrosion, it can be divided into uniform corrosion and local corrosion; the former occurs evenly across the entire surface, while the latter takes place only in certain areas. Based on the mechanism of corrosion, it can be divided into chemical corrosion and electrochemical corrosion. The degree of corrosion is expressed by the corrosion rate, and the criteria for judgment are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0512291413469820.jpg According to available information, about 60% of the damage to chemical processing equipment is caused by corrosion. Of the cases of corrosion-induced damage, 30% are due to uniform corrosion, while 70% involve dangerous forms of localized corrosion, with stress corrosion cracking being the most common type. Based on the corrosion data analysis, 16MnR exhibits excellent corrosion resistance to both ammonia and liquid ammonia, with a corrosion rate of less than 0.1 mm/a. It also shows a low corrosion rate in the presence of wet ammonia and ammonia solutions, around 1 mm/a or less. The reasons for the corrosion of the ammonia vaporization tower are analyzed as follows. 2.1 Chemical corrosion: From the perspectives of process and medium, the temperatures at the top of the tower, at the feed point, and in the lower part of the tower are different, as are the concentrations of the media. The ammonia concentration at the top is high (99.5%), while the temperature is lower (34–40°C) ； The ammonia concentration at the feed inlet is high (10%–12%), and the temperature is also high (150–160°C) ； The ammonia concentration in the lower part is low (<1%), and the temperature is high (180°C). Since the medium inside the tower is alkaline, and oxygen in alkaline solutions has a significant impact on the corrosion of steel, impurities such as Cl‑ also have a considerable effect on steel corrosion. In the ammonia recovery system, O2 and Cl‑ originate from the chemical soft water added during startup and the condensate added on a regular basis. The chemical soft water used was deoxygenated soft water, with dissolved O2 levels of <15 μg/L and Cl– levels of 6.0 mg/L ； The dissolved O2 in the added condensate is <15 μg/L, and C1– is 3.0 mg/L. As for Cl－, depending on production requirements, the more water is lost through evaporation under normal conditions, the more condensed liquid needs to be added (with replenishment occurring once per day). However, very little Cl－ is carried away by the evaporated water; the majority remains in the circulating fluid, causing its concentration to increase over time within the circulation system. According to the analysis of ammonia by the quality inspection center, the Cl‑ concentration can sometimes exceed 300 mg/L. According to available information, in solutions containing stress-corrosion-sensitive ions such as Cl‑ and OH‑, stress corrosion cracking may occur in the stressed areas (such as near welds), and intergranular corrosion is also likely to arise in the sensitized zones on both sides of the welds. Carbon steel is only slightly corroded in dilute ammonia solution, but the corrosion rate increases in hot and concentrated ammonia solution. Trace amounts of Cl－ and O2 have a significant impact on equipment corrosion; just a few ×10－6 of Cl－ can cause stress cracking in 18-8 stainless steel. Austenitic stainless steels are highly susceptible to stress corrosion cracking in high-temperature environments with chloride-containing aqueous solutions. Type 18-8 stainless steel has a greater tendency to pitting in water containing Cl— compared to ordinary aqueous solutions. 2.2 Electrochemical corrosion: Ammonia is alkaline, and materials undergo electrochemical corrosion in alkaline solutions. Before starting up ammonia recovery, chemical soft water must be added to the system to establish a liquid level. During production, steam condensate is added to the system intermittently to compensate for water losses; an electrochemical reaction occurs when there is trace oxygen present in the soft water. The reactions are as follows: Fe → Fe + 2e-, O2 + 2H2O + 4e- → 4OH-. According to available information, factors such as stirring, aeration, and increasing temperature can all increase the corrosion rate. During normal operation of the ammonia vaporization tower, the ammonia solution is in a boiling state, with the highest temperature at the bottom reaching 180°C; corrosion is quite severe. This corrosion manifests as complete corrosion throughout the area, with less severe corrosion in the areas at lower temperatures compared to those at higher temperatures. 2.3 Erosive corrosion: Overall, the corrosion is most severe at the feed inlet; as shown by the corrosion data, high flow rates can increase the corrosion rate by several times. Here, the feed temperature is above 130°C, and the ammonia concentration ranges from 10% to 12%. At such high temperatures, corrosion caused by the scouring effect of concentrated ammonia is the most severe. The temperature distribution in the ammonia vaporization tower increases gradually from top to bottom, with the temperature at the top being below 40°C and reaching 180°C at the bottom. The ammonia concentration is 99.5% at the top of the tower, 10%–12% at the feed inlet in the middle, and 1% at the bottom. The temperature at the bottom of the ammonia vaporization tower is high, but the concentration of ammonia solution is low ； The ammonia concentration at the top is high, but the temperature is low; meanwhile, pure liquid ammonia causes less corrosion ； At the feed inlet, the ammonia concentration is high and the temperature is also high, resulting in the most severe corrosion. Corrosion in other areas is relatively mild, manifesting as localized corrosion. As can be seen from the above analysis, the corrosion of the ammonia vaporization tower made of 16MnR, along with its stainless steel floating valves, tray plates, etc., is the result of various types of corrosion such as chemical corrosion, electrochemical corrosion, erosion corrosion, intergranular corrosion, and pitting corrosion. The main factors causing corrosion in the ammonia vaporization tower are Cl‑ and dissolved O2 in the alkaline ammonia solution. The top corrosion is mild, with NH3 being the main form of existence ； The concentration is low, while corrosion is severe in the middle section where the temperature and concentration are higher ； The lower part, with higher temperature and lower concentration, is next. 3 Protection Measures and Effects 3.1 Removal of Cl– Adding a shallow desalination unit can reduce the amount of Cl– in soft water by 1/2. To address the accumulation of Cl— in the system due to the addition of condensate, the solution is to enhance monitoring and carry out regular thorough cleaning of the ammonia recovery system, keeping the Cl— level below 10 mg/L. 3.2 Control of oxygen content: The soft water used is deoxygenated soft water delivered under pressure, having undergone vacuum deoxidation and atmospheric deoxidation. The improvement approach is to enhance the O2 removal efficiency of the vacuum O2 removal device, keeping the O2 content below 10 μg/L, thereby achieving a level of 10‑9. 3.3 Material replacement: Under the current conditions, the C1 content is generally above 1×10‑6. To prevent corrosion, special materials must be used. In May 1999, the material of the steam ammonia tower shell was changed to 1Crl8Ni9Ti, and the materials of the tower trays and spacers in the steam ammonia tower were all replaced with 1Crl8Ni9Ti. After taking the above measures, the corrosion problem in the ammonia vaporization tower was completely eliminated. During the major maintenance in May 2003, the tower was opened for inspection; all the floating valves and trays were in good condition, and there was no thinning of the tower shell’s wall thickness.