Corrosion Control Strategies for Aromatics Extraction Units
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. Aromatics extraction is an important link in the production of aromatics. It uses the reformed oil from the catalytic reforming unit and the by-product pyrolysis gasoline from the ethylene unit as raw materials to produce benzene, toluene, and p-xylene. and o-xylene and other aromatic products.
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 aromatics extraction equipment 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.
Corrosion Control Strategies:
01. Raw material quality control.
Control the mass fraction of chlorine in aromatic hydrocarbon raw materials to not exceed 1 μg/g.
Before the fresh sulfolane solvent enters the device, control the quality technical indicators of sulfolane, especially the content of sulfolane and water content, usually requiring that the mass fraction of sulfolane should not exceed 0.2%, and the mass fraction of water should not exceed 0.5%.
At the same time, while ensuring the quality of fresh sulfolane solvent, try to use the same brand of sulfolane solvent in the device, and avoid mixing multiple brands of sulfolane solvents.
02. Equipment selection.
Most of the equipment and pipelines of the aromatics extraction unit are made of carbon steel.
Comprehensively considering the key corrosion parts of the device and the risk factors of stress corrosion cracking of austenitic stainless steel in the chlorine-containing environment, carbon steel +0Cr13/0Cr13Al is selected for the material of the stripper, solvent recovery tower and regeneration tower cylinder. 0Cr13, 022Cr19Ni10 or 06Cr18Ni11Ti are selected for trays and internals.
The tube bundle material of reboiler and heat exchanger is 06Cr18Ni11Ti or 022Cr17Ni12Mo2. The inlet and outlet pipelines of the reboiler and heat exchanger, and the regeneration tower top pipeline are made of 022Cr19Ni10 or 06Cr18Ni11Ti.
03. Process anticorrosion.
Control the temperature at the bottom of the recovery tower and stripping tower not to exceed 180°C, and the reboiler at the bottom of the tower uses steam as the heat source, and control the temperature of the superheated steam to not exceed 220°C to prevent the decomposition of sulfolane caused by local overheating.
The following options are available to increase solvent recovery efficiency:
(1) An appropriate amount of stripping steam is injected into the bottom of the recovery tower, which can reduce the partial pressure of oil and gas in the tower, which is beneficial to the separation of aromatics and sulfolane solvent, and prevents the solvent from being decomposed by high temperature, thereby protecting the solvent. The amount of stripping water is about 1% of the feed mass fraction.
(2) The recovery tower and regeneration tower maintain negative pressure operation to ensure that sulfolane is separated and regenerated from aromatics at a lower temperature, and avoid loss or pyrolysis with aromatics after vaporization.
To prevent oxygen from entering the extraction system, the main measures include:
(1) Ensure the tightness of feed, turbine water, solvent and other related equipment to prevent active oxygen from entering the steam;
(2) Ensure that flanges, valves, and instrument joints are well sealed;
(3) Ensure the tightness of the negative pressure operating system to prevent air from entering the operating system;
(4) Add an oxygen stripper for externally supplied raw materials, and establish the data analysis of active oxygen and carbonyl in raw materials. Materials with excessive oxygen content should be stripped before entering the extraction device.
Control the water content in the system. In daily production operations, strictly implement the process dehydration operation, especially in winter when the temperature is low, pay close attention to the liquid level of the reflux of each tower and the dehydration bag of the regenerated solvent tower, and dehydrate in time.
The leakage of the reboiler at the bottom of the tower is also one of the important reasons for the system to carry water. During the equipment maintenance process, the monitoring and detection of the heat exchanger should be strengthened, especially in the later stage of equipment operation. It is especially important to find the leak in time and stop it to prevent water from entering. The solvent system affects the quality of sulfolane solvent.
04. Corrosion Monitoring and Detection.
Carry out corrosion risk assessment for the sulfolane solvent equipment and pipelines of the aromatics extraction unit, establish inspection plans and schemes for key corrosion areas, use pulsed eddy current and ultrasonic thickness measurement methods to detect and grasp the corrosion conditions of key areas, and avoid corrosion risks in advance.
Regularly analyze the chlorine ions in the aromatic raw materials, sulfolane and water content in the sulfolane solvent; regularly analyze key indicators such as pH value, chloride ion, iron ion, water content, and solid content in the sulfolane solvent; monitor the recovery tower and stripping The temperature at the bottom of the tower, the tightness of the system, the liquid level of the water bag in the return tank at the top of each fractionation tower and the dehydration situation.
05. Purification treatment of sulfolane solvent.
At present, sulfolane purification technology mainly includes vacuum extraction method, desulfurization and dechlorination agent method, membrane separation method, activated carbon purification method, and anion exchange resin method.
Among them, the anion exchange resin method has the best purification effect, and can effectively remove chlorine, sulfur and acidic substances in sulfolane.
The anion exchange resin method is mainly to remove corrosive anions in sulfolane. The mechanism is that the anions in the sulfolane solvent exchange with the basic groups on the ion exchange resin, or react with the amine groups on the resin to form a salt. When the resin After the exchange group on the battery is converted into a salt and loses its activity, it can be regenerated with a NaOH solution with a mass fraction of 2% to 4%, washed with deionized water to neutrality, and then reused.
Industrial application shows that macroporous weak base anion exchange resin has good thermal stability and strong anti-organic pollution ability, and can effectively remove anions such as sulfonate, sulfate and chloride ions in sulfolane solvent, and the pH value of sulfolane solvent has risen , significantly reducing the occurrence of corrosion and ensuring the stable operation of the device.
