Sweetening of distillates is accomplished by the conversion of mercaptans to alkyl disulfides in the presence of a catalyst. Conversion may be followed by an extraction step for removal of the alkyl disulfides. In the conversion process, sulfur is added to the sour distillate with a small amount of caustic and air. The mixture is then passed upward through a fixed-bed catalyst, counter to a flow of caustic soda entering at the top of the vessel.
In the conversion and extraction process, the sour distillate is washed with caustic and then is contacted in the extractor with caustic soda. The extracted distillate is then contacted with air to convert mercaptans to disulfides. After oxidation, the distillate is settled, inhibitors are added, and the distillate is sent to storage. Regeneration is accomplished by mixing caustic from the bottom of the extractor with air and then separating the disulfides and excess air.
In gas processing units, amines such as MEA, DEA, and MDEA are used for sulfur removal, while anti-foam agents are applied to prevent foaming.
Amines have a functional group that contains nitrogen. Primary amines arise when one of the three hydrogen atoms in ammonia is replaced by an organic substituent. Secondary amines have two organic substituents bound to N together with one H. The most commonly used amines in gas treating are:
- Primary monoethanolamine (MEA)
- Secondary diethanolamine (DEA)
- Tertiary methyldiethanolamine (MDEA)
Activated carbon is used mainly for adsorption of sulfur compounds and trace contaminants in gas streams either before or after the main amine sweetening unit.
It is not the primary agent for bulk H₂S/CO₂ removal (that’s done by amine solvents like MEA, DEA, or MDEA), but it plays critical support roles, especially for polishing and protection.
