Guide to Solving Corrosion Problems in Hydrofinishing Units – A Must-Read for Safe Production!
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In the complex production chain of oil refining, hydrotreating units play a crucial role as “purifiers”. They effectively remove impurities such as sulfur, nitrogen, and oxygen from petroleum products, thereby improving their quality. However, due to prolonged exposure to corrosive media, the equipment is inevitably subject to various corrosion-related problems, which threaten production safety and efficiency. Today, by drawing on the practical experience of a certain petrochemical company’s diesel hydrogenation unit, we will conduct an in-depth analysis of the common corrosion problems in hydrogenation refining units, and provide you with practical protection strategies. I. High-pressure air cooling: A hotspot for corrosion and leaks. High-pressure air cooling serves as the \"heat dissipation hub\" in hydrogen refining units, responsible for cooling the reaction products; however, its unique operating conditions make it a prone area for corrosion and leaks. (1) Plug leakage: The “hidden damage” caused by copper complexes – During the operation of the equipment, high-pressure air-cooled plugs frequently leaked, sometimes even leading to the risk of liquid spraying. Analysis shows that the culprit is the copper-ammonia complex. It turns out that the gasket used for high-pressure air-cooled thread plugs is made of copper; under specific temperatures and pressures, it reacts with NH4+ in water to form dichlorotetraamminecopper. These blue crystals not only cause the precipitation of ammonium salts and the consumption of copper material, but also accumulate in areas with low air cooling flow rates, triggering galvanic reactions that ultimately lead to leaks at the thread plugs and blockages in the tube bundles. The risks cannot be underestimated: blockages in the high-pressure air cooling tube bundles can directly affect the heat exchange efficiency, leading to temperature fluctuations within the high-pressure separator and disrupting oil-gas separation ； In severe cases, it can even force the device to stop operating. At a working pressure of 7.0 MPa, leaks can easily lead to fires and explosions, material loss, and environmental pollution, posing a serious threat to the safety of employees and the stable operation of the device. Solution: Upgrade the materials – use corrosion-resistant materials such as INCLOY825 (a Ni-Fe-Cr alloy). The plug gaskets should preferably be made of the same material as that used for air cooling. Additionally, increase the corrosion resistance of carbon steel by applying proper heat treatment to prevent sulfide stress corrosion cracking. Precise flow rate control: The flow rate is strictly maintained within the optimal range of 4.6–6.1 m/s to reduce the retention of corrosive agents. Scientific control of water injection volume: The water injection volume is maintained at 5.5%–6.5% of the mass of the fresh feed, which ensures sufficient dissolution of ammonium salts while avoiding operational difficulties and increased costs resulting from excessive water injection; simultaneously, it keeps the value of φ(NH3HS) in the wastewater at ≤8%. Optimized design and protection: Ensure symmetrical spatial distribution of air-cooling tubes to prevent flow deviation ； The inlet lining is fitted with titanium tubes and undergoes special polishing; after pipeline welding, solid solution and stabilization heat treatments are carried out strictly ； Add corrosion inhibitors (such as sodium polysulfide), and strictly control the quality of the water used for injection (solid contents < 25μg/g, chloride ions
(II) Tube corrosion: The “erosion storm” caused by NH₄HS solutions – the S and N compounds in the feed oil are converted to H₂S and NH₃ in the hydrogenation reactor, and upon cooling, NH₄HS is formed as a result of these reactions. To prevent crystallization from clogging the tube bundle, water is usually injected before high-pressure air cooling, but this also exposes the tube bundle to the corrosive environment of the NH4HS solution. On the one hand, the FeS protective film formed by the reaction of H₂S with the metal Fe can be eroded and damaged due to changes in the flow rate of the medium ； On the other hand, in a wet environment with high concentrations of NH4HS, FeS forms complexes with it, accelerating the degradation of the protective film and the corrosion process. Furthermore, the deflection and turbulence caused by pollutant blockages, as well as the deposits in areas with extremely low flow speeds, all contribute to increased localized corrosion. The potential risks are enormous: if tube bundle corrosion is not detected in time, it can easily lead to tube failures, resulting in casualties, environmental pollution, and financial losses. Countermeasures: In addition to the 8 solutions mentioned above for plug leakage, it is also necessary to strengthen the monitoring of iron ions in water and materials in order to keep track of corrosion developments in a timely manner.
II. Iron contamination in feed oil: the “invisible killer” of catalysts. As the proportion of catalytic slag used increases and the quality of residue oil used in coking processes declines, the impact of iron content in feed oil on hydrorefining reactions becomes increasingly significant. (1) Sources and hazards of iron: Iron in crude oil exists mainly in the form of suspended inorganic substances, oil-soluble salts (such as iron naphthenate), and complexes (such as iron porphyrin). It can be divided into “inherent iron” originating from the crude oil itself, and “process iron” resulting from the corrosion of pipelines, storage tanks, and other equipment (which usually has a higher concentration). Once these metals enter the device, they react with sulfur and hydrogen to form milky iron sulfide. Although iron sulfide can exert a certain demetallization effect, it also acts as a \"catalyst\" for coking reactions, leading to coking on the catalyst surface, an increase in bed pressure drop, and a reduced efficiency of the catalyst. When the iron deposition reaches saturation, it is also carried by the material to the rear systems, causing blockages in the high-distillation lines and valves, and leading to cascading shutdowns. The economic losses are enormous: the increase in pressure drop across the bed caused by iron deposition often requires shutting down the facility to carry out \"skimming\" operations, with losses that can exceed 10 million yuan per incident; moreover, there is a risk of injuries or deaths due to improper nitrogen protection during these operations. Prevention and control measures: Source filtration: The raw materials are filtered before entering the equipment to remove particles larger than 25μm ； An acoustic filtering device is installed on the backwash filter to further reduce impurity levels. Bed protection: A fouling basket is installed at the top of the catalyst bed, and inert porous ceramic balls are packed in a hierarchical manner. Ring-shaped catalyst carriers or catalysts with high porosity and large surface area are selected and packed densely to effectively capture and prevent iron-containing particulates. Auxiliary purification: Employ electromagnetic fields to remove iron-containing particulates, while also extending the settling time in storage tanks, enhancing monitoring of oil quality, and regularly checking for thinning of wall thicknesses in key areas, thereby establishing a comprehensive protection system.
III. Conclusion: Scientific protection to ensure the long-term operation of the equipment. The corrosion issues associated with hydrogenation and refining units are complex and varied, but by identifying the root causes and taking targeted measures, it is possible to effectively mitigate these risks. Through comprehensive measures such as material upgrades, process optimization, and source control, it is possible to address prominent issues like corrosion and leakage in high-pressure air cooling systems as well as iron contamination in crude oil. Additionally, these measures enhance the operational stability and safety of the facilities, thereby supporting the high-quality development of petroleum refining enterprises. There are no trivial matters when it comes to safe production; every potential corrosion hazard deserves our utmost attention. It is hoped that this corrosion protection guide, based on practical experience from the field, will provide useful guidance for the operation and maintenance of your facilities, helping to keep hydrogenation and refining units in optimal condition so that they can play a greater role in petrochemical production!
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