Corrosion Causes of Aromatics Extraction Unit in Refinery
Aromatic hydrocarbons BTX (benzene, toluene, xylene) are basic raw materials for organic chemical products such as synthetic fibers, resins, rubber, detergents and medicines. Petroleum-based aromatics are the main source of aromatics raw materials, and aromatics extraction is an important part of aromatics production.
It uses the reformed oil from the catalytic reforming unit and the pyrolysis gasoline by-product from the ethylene unit as raw materials to produce a variety of aromatic products such as benzene, toluene, p-xylene and o-xylene. Aromatics extraction technology is mainly divided into extractive distillation and liquid-liquid extraction. The key to aromatics extraction is to choose a suitable extraction solvent. Among them, the application ratio of sulfolane or sulfolane composite solvent in aromatics extraction devices exceeds 80%.
At present, sulfolane or sulfolane composite solvent is mainly used as the extraction solvent of the aromatics extraction device in China, and the problems of corrosion and scaling have always existed.
The deterioration and degradation of sulfolane lead to polymer deposition to block trays and heat exchangers, and the decomposed acidic substances cause corrosion and leakage of heat exchangers and pipelines, which have become an important reason restricting the long-term safe operation of aromatics extraction equipment.
Based on the different technological processes of the aromatics extraction unit, the main parts that cause corrosion of the aromatics extraction unit after the deterioration of the sulfolane solvent include:
1. Extractive distillation column trays;
2. Tower internals and bottom reboiler;
3. Lean/rich solvent heat exchanger;
4. Trays for solvent recovery towers and strippers;
5. Tower internals;
6. Bottom reboiler;
7. Overhead air cooler.
8. Regenerating tower trays and bottom reboilers and corresponding high-temperature tower bottom pumps, lean/rich solvent pipelines and other parts.
The main parts affected by the macromolecular degradation polymer produced by the degradation of sulfolane on the aromatics extraction device include: extraction and distillation tower trays, reboilers, lower parts of stripping towers and recovery towers and reboilers, regeneration towers and reboilers , Lean/rich solvent heat exchanger and other parts. The performance is that the tray is clogged, which reduces the extraction efficiency, the heat exchanger tube bundle and the pump filter are clogged, or the outer surface of the reboiler tube bundle is coked, which reduces the heat transfer and mass transfer efficiency.
Corrosion causes and influencing factors：
01. Degradation mechanism and influencing factors of sulfolane.
The degradation mechanism of sulfolane solvent in aromatics extraction unit mainly includes high temperature oxidation decomposition and hydrolysis.
High-temperature oxidative decomposition refers to the decomposition of sulfolane in a high-temperature oxygen-containing environment to generate SO2 and butadiene, and the polymerization of butadiene to form a macromolecular organic polymer, causing equipment and pipeline blockage; SO2 reacts with water and free oxygen to generate sulfuric acid. Corrosion of equipment and pipelines.
In the normal operating temperature and anaerobic environment, the decomposition rate of sulfolane is very slow. When the temperature exceeds 180°C, it will decompose. When the temperature is 180~220°C, the decomposition rate will gradually increase. When the temperature exceeds 220°C, the decomposition rate will be significantly accelerated; the presence of oxygen The decomposition rate of sulfolane is greatly accelerated, and SO2 is oxidized to SO3 to form a strong acid corrosion environment.
02. Corrosion of sulfolane solvent.
Sulfolane degrades and decomposes to produce acidic substances, which include inorganic acids such as sulfuric acid and sulfurous acid, organic acids such as sulfonic acid, and a small amount of carboxybutyric acid.
In addition, oxygen exacerbates the corrosiveness of the sulfolane solvent.
① Corrosion of acidic substances
Sulfolane solvent decomposes under the influence of high temperature, oxygen, water content, sulfolene and other factors to produce acidic corrosive substances such as sulfurous acid, sulfuric acid, sulfonic acid, carboxybutyric acid, etc., which directly manifests as a significant decrease in the pH value of the solvent and an increase in acidity.
These acids cause corrosion and thinning of carbon steel equipment and piping.
②Corrosion of chloride ions
Chloride ions in sulfolane solvent tend to accumulate in the system, causing corrosion.
On the one hand, chloride ions may promote the deterioration of sulfolane, aggravate the formation of acidic substances, and further reduce the pH value of the solution; on the other hand, chloride ions accelerate local corrosion of metals, and are prone to pitting corrosion and stress corrosion cracking of austenitic stainless steel .
Chloride ions in the aromatics extraction unit mainly come from the extraction raw materials, system water, reboiler and water cooler leakage. Due to the continuous accumulation of chloride ions in the sulfolane solvent, the mass fraction of chloride ions in the solvent is usually high, exceeding 100 μg/g in severe cases.
03. Effect of sulfolene impurity.
Sulfolane is the raw material for producing sulfolane, and the sulfolane solvent contains a certain amount of sulfolane.
Compared with sulfolane, sulfolane is unstable in nature and easily decomposes to generate SO2 when heated. The higher the content of sulfolene, the more SO2 will be decomposed and the stronger the acidity of the solvent. The acidic environment can easily promote the high-temperature oxidation decomposition and hydrolysis of sulfolane, which further aggravates the deterioration of sulfolane.
