【Weekly Topic】How many types are there of corrosion in atmospheric and vacuum distillation unit equipment?

Chlorides and sulfides in crude oil undergo thermal decomposition or hydrolysis during the distillation process, producing hydrogen chloride and hydrogen sulfide, as well as organic acids and other corrosive substances that cause corrosion of equipment and pipelines. During combustion in the heating furnace, fuel oil or fuel gas generates high-temperature flue gases containing SO2 and SO3. In the cooler sections of the heating furnace, these gases condense together with the moisture in the air at the dew point, resulting in sulfuric acid dew point corrosion. Anti-corrosion measures: 1. Mixing 2. Controlling flow rate and flow pattern 3. Paying attention to anti-corrosion in the manufacturing process 4. Selecting appropriate materials

1. HCl-H2O type and HCl-H2S-H2O type corrosion in low-temperature areas (1) Cause: Salts present in crude oil; hydrolysis of these salts produces HCl. (2) Locations where it occurs: The tops of atmospheric and vacuum distillation columns, as well as the air coolers and water coolers in the overhead condensation and cooling systems, in areas where liquid water is present at low temperatures. (3) The process anti-corrosion measures are: “one removal and three injections”: 1) Electrodesalination and dehydration of crude oil; 2) Injection of ammonia into the overhead distillate stream; 3) Injection of a corrosion inhibitor into the overhead distillate stream; 4) Injection of alkaline water into the overhead distillate stream. 2. Sulfur corrosion in high-temperature areas: (1) Cause: Crude oil contains sulfur, and reactive sulfur compounds (elemental sulfur, hydrogen sulfide, thiols) can react with iron. Sulfur corrosion begins around 250°C and intensifies as the temperature rises, with the most severe corrosion occurring between 340°C and 430°C. (2) Locations where it occurs: the outlet tubes of the atmospheric furnace and the oil transfer lines, the upper and lower trays at the feed section of the atmospheric tower, the oil transfer lines from the vacuum furnace to the vacuum tower, the tower walls and internal components in the feed section, as well as the bottom of the vacuum tower; the oil transfer lines for vacuum residue, and the heat exchangers for vacuum residue. (3) Anti-corrosion measures: mainly include material upgrades and corrosion monitoring of the system. 3. Naphthenic acid corrosion in high-temperature areas (1) Causes: Crude oil contains acids (naphthenic acids), and at certain temperatures, naphthenic acids react with iron. At temperatures between 230°C and 300°C, the corrosion reaction between naphthenic acids and iron is as follows: 2RCOOH + Fe → Fe(RCOO)2 + H2. At temperatures between 330°C and 400°C, naphthenic acids react with sulfur to form FeS, resulting in the following reaction: 2RCOOH + FeS → Fe(RCOO)2 + H2S; Fe + H2S → FeS + H2. (2) Locations where it occurs: atmospheric oil transfer lines, vacuum oil transfer lines, the outlets of the atmospheric furnace and vacuum furnace, the tower walls in the feed section of both atmospheric and vacuum towers, etc. (3) Anti-corrosion measures: 1) Blending different types of crude oil, 2) Alkaline neutralization, 3) Material upgrades, 4) Corrosion inhibitor technologies, 5) Corrosion monitoring and prediction techniques

Sulfur corrosion, electrocorrosion, alkaline corrosion, naphthenic acid corrosion.

Mainly electrochemical corrosion and sulfur corrosion

It is related to processed oil products. Low-sulfur, low-acid crude oil is basically non-corrosive ； In the case of high sulfur and high acidity, naphthenic acid corrosion and sulfur corrosion occur.

Corrosion prevention measures can be divided into two main categories: material upgrades and process-based corrosion protection. Process corrosion prevention involves proper implementation of the \"one removal and four injections\" approach: ensuring effective electrodialysis, as well as the injection of water, ammonia, corrosion inhibitors, and alkali (with selective use) ； Regarding the equipment itself, it is necessary to upgrade its materials, replacing the equipment and pipes with materials that are more resistant to corrosion.

The salts contained in crude oil cause equipment corrosion, primarily due to magnesium chloride and calcium chloride. Under certain conditions, they readily hydrolyze to produce hydrogen chloride. Hydrogen chloride is highly volatile and rises to the top of the distillation column along with the light fractions and water vapor. When the temperature drops below the dew point of water, hydrogen chloride dissolves in large quantities in water to form hydrochloric acid, causing severe corrosion in areas such as the top plate of the atmospheric pressure column, the condensation equipment for the products at the column top, as well as the primary and secondary vacuum systems of the vacuum column.

The hydrolysis of salts produces inorganic acid corrosion and organic cycloaliphatic acid corrosion; when fuel oil used in heating furnaces is sourced from unrefined side-stream oil from distillation towers, the flue gases emitted after combustion can cause dew point corrosion.

As far as I know, there are several types of severe corrosion associated with atmospheric and vacuum distillation: 1. Low-temperature corrosion at the tower top, which occurs in areas such as the tower top trays, tower walls, and the cooling system for the gas at the tower top. 2. Erosive corrosion in areas with high-temperature heavy oil, especially where the equipment operates under high load for extended periods of time, such as at the elbows in the furnace feed pipelines. 3. Stress corrosion: In the areas where the tower top gas passes through, stress corrosion cracking of the air-cooled welds is quite evident. 4. Flue gas dew point corrosion. Anticorrosion measures: 1. Optimize the desalination efficiency of electrodialysis; 2. Inject water and corrosion inhibitors at the tower top; 3. Control the flue gas temperature properly; 4. Select more suitable equipment materials

Based on several years of observation, in simpler terms, the main types are as follows: 1. Low-temperature S-HCl corrosion, which occurs primarily at the tower top; the main control measures involve improving desalination (to address the root cause) and adding neutralizing agents or ammonia to the tower top to control the pH value. 2. High-temperature S-corrosion of naphthenic acids occurs mainly in the middle section of the reflux stream; this can be addressed by injecting high-temperature corrosion inhibitors, although the effect may not be significant. Another option is to replace the pipes and equipment with 316L material. 3. Hydrogen sulfide corrosion, especially that of copper, is primarily manifested in the wiring of control columns and grounding systems. 4. For erosion corrosion at the furnace tubes, use high-quality tubes and try to reduce the flow rate of the medium inside the tubes through process adjustments. 5. The corrosion in the convection section of the heating furnace is mainly caused by sulfur in the flue gas; it is important to control the temperature of the flue gas as well as the sulfur content in the fuel. In particular, the flue gas temperature should not be too low, otherwise the furnace tubes can easily be damaged. 6. S-corrosion of the bottom pump: When processing high-sulfur crude oil, corrosion of the bottom pump is relatively severe, and erosion is also a factor, especially in the flow-through areas. I only have a superficial understanding; please forgive any mistakes I may make