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How does mercaptan extraction reduce the sulfur content of petroleum products?

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Aerial view of an oil refinery complex near New Orleans, Louisiana, with storage tanks, industrial structures, and smoke rising from the facility, surrounded by open land. — petroleum refining Aerial view of an oil refinery near New Orleans.

Mercaptan extraction is a purification process used to reduce the total sulfur content of petroleum products, distinguishing it from "sweetening" processes that merely convert mercaptans into disulfides without removing them.12

To effectively extract these mercaptans, refineries enhance the solvent properties of caustic soda by adding specific solutizers, such as potassium isobutyrate or sodium cresylate. This addition increases the solubility of higher mercaptans, allowing them to be physically separated and removed from the oil.1

The process typically follows these steps:
* Pretreatment: The oil is first treated with caustic soda in a mixing device or packed column to remove traces of hydrogen sulfide and alkyl phenols.1
* Extraction: The enhanced caustic solution is used to extract the mercaptans from the hydrocarbon stream.1
* Finishing: The mixture is allowed to settle, and the resulting product is water-washed to ensure purity before storage.1

By physically removing these sulfur compounds, refineries can meet strict environmental regulations and prevent issues like engine corrosion or the degradation of exhaust-treatment systems.13