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Analysis of corrosion failure mechanism of elbow in return tower of liquefied gas desulfurization unit

DataTime:[3/3/2019]
Liquefied gas is mainly produced in oilfields and refining and chemical enterprises. It has the characteristics of less pollution, high calorific value, easy transportation and simple storage, so it is widely used in civil, commercial services, industrial production and other fields. Table 1 shows the global supply of liquefied petroleum gas and forecast data from 2015 to 2021. As can be seen from the table, in recent years, global LPG production has continued to increase, with annual growth rates of more than 2%. In addition, the average annual compound growth rate of LPG consumption and output in China has reached 15.69% and 9.95%, respectively. In 2016, China's liquefied gas production was 35.539 million tons, an increase of 19.41% year-on-year. LPG has become an important part of China's energy structure.
Table 1 Global liquefied gas supply and forecast for 2015-2021
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Liquefied gas is a by-product of catalytic cracking and thermal cracking of crude oil. Therefore, sulfides such as H2S and SO2 contained in crude oil will enter the liquefied gas along with the refining process. Sulfide can cause poisoning of catalysts in subsequent production processes, corrode pipes and equipment, and affect product quality. When sulfur-containing liquefied gas is used as a fuel, it will cause serious pollution to the surrounding environment. In recent years, with the world's light and low-sulfur crude oil reserves decreasing, domestic refinery raw materials are heavy and high vulcanization trend is obvious, so liquefied gas desulfurization has a strong practical significance.
At present, the methods for desulfurization of liquefied gas (mainly H2S) include wet desulfurization and dry desulfurization. In the industry, wet desulfurization is mainly used, and the alcoholamine method is most used. The alcoholamine method uses an amine solution as a desulfurizing agent to achieve a desulfurization effect by chemical reaction of the amine solution with H2S. Commonly used desulfurizing agents are monoethanolamine (MEA), diisopropanolamine (DIPA) and N-methyldiethanolamine (MDEA), among which MDEA is most widely used in liquefied gas desulfurization. The main reaction of alcohol removal by H2S is
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Wherein RNH2 represents an alcoholamine.
Both reaction (1) and reaction (2) are reversible exothermic reactions. Cooling and pressurization are beneficial to the reaction to the right, producing sulfide and acid sulfide, which is the absorption reaction of H2S; heating and depressurization are beneficial to the reaction to the left, and the sulfide of the amine is decomposed to release H2S. This is the regeneration process of the amine solution.
Through the above two reactions, the absorption of H2S and the regeneration of the amine liquid are realized, and the purification effect is good and the energy consumption is low, so the alcohol amine method has become the main method for desulfurization of the liquefied gas. However, according to recent reports, since the desulfurization device is mostly made of carbon steel, H2S in the liquefied gas brings serious corrosion problems. Corrosion will not only lead to thinning and perforation of equipment and pipelines, but also cause leakage of materials. Unplanned shutdown of the equipment will seriously affect the normal operation of the production, and corrosion products will cause foaming and degradation of the desulfurizer, resulting in increased desulfurization loss. Process energy consumption increases.
Li Feng et al. conducted a comprehensive study on the corrosion behavior of the alcohol-amine desulfurization device. The regeneration tower, reboiler, lean-rich liquid heat exchanger and high-temperature rich liquid pipeline are all serious corrosion sites, and the regeneration tower is a desulfurization device. The most severely corroded equipment.
Shengli refinery hydrogenation unit acid gas desulfurization system amine liquid regeneration tower top air cooler exit pipeline severe corrosion, a total of 13 corrosion leaks occurred in this section of the pipeline within six months. After cutting the old pipeline, it was found that the top of the outer arc of the three elbows (f108 mm × 6 mm) had been corroded and perforated to form a fist-sized hole, and the outer top of the two elbows was thinned to less than 1 Mm. After analysis, it is concluded that the root cause of pipeline corrosion is that hydrogenation reaction generates a large amount of H2S, and the water vapor in the regeneration tower forms a corrosive environment of H2S-NH3-H2O-CO2. In addition, fluid scour and dissolved oxygen are also important causes of corrosion. .
Combining array electrode technology with CFD, the relationship between corrosion behavior of pipe elbow and fluid flow was determined. The results show that the corrosion rate distribution is basically consistent with the flow velocity and shear stress distribution in the tube. The maximum corrosion rate appears on the innermost side of the elbow, and the minimum corrosion rate appears on the outermost side of the elbow.
Zeng and other independent designs set up an elbow erosion corrosion experimental device. The total corrosion rate was measured using the weight loss method, and the array electrode technique was used to measure the pure corrosion rate at different positions of the elbow, the pure scouring rate, the rate of erosion promotion by scouring, and the rate of promotion of erosion versus scouring. By quantifying the corrosion rate of each part, the dominant factors of erosion corrosion at different positions of the elbow are determined, and the inherent causes of this difference are revealed.
In summary, there have been many reports on the corrosion of desulfurization devices, and some anti-corrosion recommendations have been proposed for specific conditions. However, most studies have only started from the chemical point of view, and have not dealt with the physical laws of corrosion. For the failure elbow, this paper will start from the two perspectives of physics (thinning law, pore size distribution) and chemistry (XRD, EDS) to explore the corrosion failure mechanism of the elbow.
In conclusion
(1) The entire elbow is subject to varying degrees of corrosion, with a localized honeycomb corrosion morphology. After measuring the pore size and wall thickness in the severely corroded area, it is found that the pore size is trapezoidal in the flow direction, the pore diameter of the complete destruction zone is infinite, the wall thickness is gradually thinned, and the wall thickness of the complete failure zone is zero.
(2) The corrosion products mainly contain four elements of Fe, C, S and O. The corrosion products are mainly determined by XRD and FeS2 and FeS.
(3) The cause of corrosion of elbow corrosion is mainly caused by electrochemical corrosion caused by H2S, erosion corrosion and corrosion caused by hot steady salt, which promote each other and eventually lead to corrosion failure of elbow.
1 elbow failure analysis
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 Figure 1 elbow repair welding photos
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  Figure 2 Elbow physical map photo
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Figure 3 Desulfurization process flow and failure location
Table 2 Composition of raw material gas
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Table 3 Partial parameters of the amine solution

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Source: China Stainless Steel Elbows Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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