Causes of corrosion in the circulating water cooler of methanol plants and preventive measures

1 Corrosion condition of the circulating water cooler: The circulating water system was put into operation at the end of 2013, and the circulating water station as well as the coolers that use circulating water as a cooling medium have been operating stably. In June 2015, a leak occurred in the gas processing process wastewater cooler E1405; 3 tubes were sealed. This was the first circulating water cooler to experience a leak, and it was also the only cooler in which there was a leak in the flow path between the inlet and outlet. Half a year later, a leak occurred in the boiler feed water cooler E1622 in the purification process. To avoid disrupting the continuous operation of the entire system, it was isolated, and it was not until February 2016 that the opportunity during a shutdown was taken to locate and repair the leak. During the major system maintenance in March 2016, leaks were successively detected in 5 ordinary carbon steel coolers located in the inlet and outlet tubes of the circulating water system. Among them, the Propylene gas cooler E1703 in the purification process suffers the most severe corrosion and has the highest number of leaking tubes. Special clamps are used to apply pressure to it, and this process is repeated at least 10 times; a total of 139 tubes were sealed, accounting for 11.0% of the total number of tubes in this cooler, not including those tubes with cracks that required rewelding. Additionally, the 3 lockhopper wash water coolers E1302 corresponding to the 3 gasification furnaces all experienced leaks, with E1302C having 12 leaking pipes and a plugging rate of 2.17%.

2 Analysis of the causes of corrosion and leakage (1) In June 2015, the pH of the circulating water dropped to around 6 for some time, and the reason for this was unknown. Even using large amounts of raw water with a higher pH (pH=8.1~8.5) for replacement did not help; therefore, it was necessary to add large quantities of caustic soda (solid NaOH) to the water intake tank at the circulating water station in order to neutralize the acidity, eventually bringing the pH back within the normal operational range of 8.0~9.2. This is a sign of corrosion in the circulating water cooler. (2) Also during this period, a leak was detected in the wastewater cooler of the gasification unit, causing large amounts of ammonia-containing wastewater to enter the circulating water. In this way, as the algicidal and bactericidal agents are added gradually, bacteria multiply at an exponential rate, converting ammonia nitrogen in the circulating water into nitrate nitrogen, which leads to a decrease in pH and causes biochemical corrosion. (3) Regardless of the reason, when the pH of the circulating water is around 6, it can be said that the circulating water system is in a state of corrosion, and this type of corrosion is usually chemical corrosion. Once the pH returns to normal levels, it is necessary to replace the circulating water in the entire system promptly, and to carry out online pre-coating and re-coating to prevent further corrosion of the circulating water system. This is also the underlying reason for the increased corrosion of the cooler. In other words, chemical corrosion and the failure to apply a protective film in a timely manner were the main causes of this corrosion. (4) Under normal conditions, the main type of corrosion associated with circulating water is oxygen corrosion (also known as electrochemical corrosion). Since, during the cooling process in a cooling tower, circulating water relies mainly on fans and air for heat exchange, the dissolved oxygen in this water reaches near-saturation levels; as a result, the equipment and pipes in contact with it are subject to oxygen corrosion. In fact, the true purpose of chemical cleaning and pre-coating in a circulating water system is to prevent oxygen corrosion and biochemical corrosion; it is completely ineffective against preventing chemical corrosion. (5) Most people believe that the main reason for corrosion in the company’s circulating water coolers is the poor effectiveness of the previous chemical cleaning and film-forming treatments, but I disagree with this view. Even without chemical cleaning or pre-coating, as long as the circulating water system is operating properly, the cooler will not experience such significant leakage after one year of use. (6) In coolers whose tube bundles are made of stainless steel tubes or copper alloys, no leaks were observed; whereas the tubes that leaked were all made of ordinary carbon steel, with thin-walled tubes having a higher likelihood of leaking. For example, in the purification process, the propylene gas cooler E1703 has a carbon steel wall thickness of only 1.5 mm. This is another reason for the leakage to occur.

3 Chemical cleaning – pre-coating: March 20, 2016 – March 31, 2016. The circulating water system was subjected to chemical cleaning and pre-coating in order to improve process conditions, enhance heat exchange efficiency, prevent equipment corrosion, extend its service life, and ensure long-term operation. In fact, pre-coating involves forming a protective layer, such as zinc iron phosphate or complexed zinc iron phosphate, on the metal contact surfaces of equipment and pipes in the circulating water system, in order to prevent oxygen-induced corrosion and biochemical corrosion by the circulating water. Chemical cleaning – pre-coating mainly consists of three steps: sludge removal, chemical cleaning, and pre-coating. Corresponding chemicals are added in each step, and water quality parameters in the circulation system are monitored regularly to determine whether the conditions are suitable for proceeding to the next stage. After each step is completed, the circulating water is replaced until the replacement meets the requirements after pre-film formation, after which it is put into operation. In principle, all equipment and pipes in the circulating water system should be involved in chemical cleaning—pre-coating. In practice, to prevent the sludge separated by the system from adhering to the plastic fillers, 7 cooling towers were excluded from the chemical cleaning process. For the same reason, 12 coolers that were severely corroded or not suitable for chemical cleaning were isolated to prevent further corrosion. The operation and isolation status of the circulating water coolers are detailed in Table 2.

During chemical cleaning, 10 tons of non-oxidizing biocides, 8 tons of sludge removers, 5 tons of chemical cleaners, and 2 tons of corrosion inhibitors are added to the circulating water system. The test results for various parameters in the circulating water are shown in Table 3.

To achieve a better chemical cleaning effect, 3 t of sulfamic acid was added to adjust the pH within the specified range, and the chemical cleaning process was carried out for 36 hours. The test results of various parameters during the chemical cleaning of circulating water are shown in Table 4. Table 4 Test results of various parameters during chemical cleaning of circulating water. Sample number, Parameter: 03-241, 03-242, 03-251, 03-252, 03-253, 03-254. pH: 6.88, 6.65, 6.41, 6.55, 5.92, 6.14; Calcium ions/mg·L-1: 210.3, 120.2, 81.5, 103.0; Total iron/mg·L-1: 2.28, 3.00, 3.00, 3.84, 5.04, 4.52; Turbidity/NTU: 48.8, 49.5, 47.2, 44.4, 44.4, 43.9; Total alkalinity/mg·L-1: 212.1, 157.5, 146.4, 151.4, 80.7, 85.8

On March 27, the turbidity of the circulating water dropped to 17 NTU, and the total iron concentration fell below 1.0 mg/L. A pre-coating agent was started to be added to the water intake tank for pre-coating; a total of 10 tons of pre-coating agent, 1 ton of sulfamic acid, 1.2 tons of concentrated sulfuric acid, and 1 ton of corrosion inhibitor were added, with the pre-coating process lasting 48 hours. The test results for various parameters during the pre-film formation of circulating water are shown in Table 5. Table 5 Test results of various parameters during the pre-coating process of circulating water. Sample number, Parameter: 03-271, 03-281, 03-291, 03-301, 03-311. pH: 7.91, 6.85, 7.77, 7.59, /. Calcium ions/mg·L-1: 111.6, 111.6, 115.9, 128.8. Total iron/mg·L-1: 2.72, 3.04, 2.60, 2.20. Total phosphorus/mg·L-1: 9.40, 98.0, 92.0, 71.0, 47.0. Positive phosphorus/mg·L-1: 1.60, 2.44, 3.64, 3.36. Zinc ions/mg·L-1: 2.30, 3.70, 2.30, 1.80. Turbidity/NTU: 35.4, 20.6, 20.3, 15.6. Total alkalinity/mg·L-1: 186.8, 52.5, 70.7, 287.6, 131.2. Meanwhile, two carbon steel specimens, one copper specimen, and one stainless steel specimen were suspended in the circulating water intake tank to conduct a hanging specimen test, in order to evaluate the effectiveness of chemical cleaning and pre-coating. Visual inspection shows that the surfaces of the two carbon steel pre-coated test pieces exhibit a blue hue; the coating is uniform, the hue is distinct, and there are no signs of corrosion deposits or pitting deposits. The copper sulfate colorimetry test showed that the coloring time for both pre-coated test pieces exceeded 13 s, at 22 s and 16 s respectively, which is above the required 10 s threshold, indicating that the pre-coating effect was achieved.

4 Corrosion prevention measures: (1) Strictly control the pH of the circulating water at 8.0–9.52; provided that scaling in the coolers is not present, it is advisable to keep it at the upper limit of this range. (2) When adjusting the pH with sulfuric acid, the addition amount should be uniform, continuous, and steady; concentrated or intermittent addition is prohibited to prevent corrosion by sulfuric acid. (3) To prevent corrosion caused by Cl- and PO43-, it is necessary to strictly control ρ(Cl-) ≤ 100 mg/L and ρ(PO43-) ≤ 15 mg/L. (4) Carefully carry out regular coupon testing for the circulating water system; periodically remove the coupons to measure their corrosion rate, with the requirement that this rate be ≤0.075 mm/year. (5) When the total iron mass concentration is ≥1.0 mg/L, it indicates that corrosion is occurring in the circulating water system; the cause of this corrosion must be identified, and the pH of the circulating water should be adjusted promptly. (6) When the pH level is normal, chemical corrosion generally does not occur in circulating water systems; the main form of corrosion is oxygen corrosion. The primary measures to reduce oxygen corrosion are to lower the oxygen content in the circulating water and to ensure that the pre-coating on the equipment remains intact. (7) When chemical corrosion occurs or leaks appear in the circulating water system, the corrosion should be eliminated or the leak points fixed promptly, and online pre-coating or re-coating should be carried out to prevent subsequent oxygen corrosion. (8) Strictly add bactericides, algaecides, corrosion inhibitors, scale inhibitors, etc. at specified times, in designated locations, and by designated personnel as required. Monitor various indicators of the circulating water at the prescribed frequency, and adjust the dosage of these chemicals promptly based on the test results. (9) Maintain the flow rate of the circulating water in the equipment and pipes at 1.2~1.5 m/s; special attention should be paid to this when the circulating water enters and exits the shell side of the cooler, to prevent scaling and under-scale corrosion caused by too low a flow rate. (10) Strictly control the ammonia nitrogen concentration in the circulating water to ≤10 mg/L and the COD concentration to ≤5 mg/L, in order to prevent excessive bacterial growth that could lead to biochemical corrosion. Any leaks must be eliminated, the dosage of chemicals should be increased to eliminate algae and bacteria, and bypass filtration should be properly operated to remove sludge.

【Ten Years of Rapid Development in Chemical Engineering Equipment】The world’s first carbon dioxide + flywheel energy storage demonstration project was completed in Deyang, Sichuan, from 2003 to 2022. https://bbs.hcbbs.com/thread-5704593-1-1.html (Source: Haichuan Chemical Industry Forum)