Technology Type

Most products of crude oil atmospheric and vacuum distillation in refineries contain a significant amount of sulfur and nitrogen compounds like mercaptans, sulfides, thiophenes, and pyridines that must be removed prior to further processing or use.  If these are not removed, they will convert into harmful sulfur oxides (SO_x) and nitrogen oxides (NO_x) upon combustion, posing risks to human health and the environment.

Unfortunately, directly separating them out is not easy due to similar boiling points with useful hydrocarbon molecules. For that reason, hydrotreatment or hydrodesulfurization is applied, refering to a set of operations that remove sulfur and other impurities like nitrogen. During hydrotreating, crude oil cuts and other refining fractions are selectively reacted with hydrogen in the presence of a catalyst at relatively high temperatures and moderate pressures.

The process converts undesirable aromatics, olefins, nitrogen, metals, and organosulfur compounds into stabilized products. Some hydrotreated cuts may require additional processing to meet final product specifications.

A typical hydrotreatment process involved the folowing steps:

• Hydrotreater Reactor – Uses hydrogen (H₂) to convert organic sulfur into hydrogen sulfide (H₂S) and nitrogen into ammonia (NH₃).
• Wash Water – Used to dissolve and wash out NH₃.
• 3-Phase Separator – Removes NH₃ via the sour water and H₂S & unused H₂ via the gas, leaving clean hydrocarbons to be sent to the stabilizer to remove any cracked products.
• Recycle Treating Gas – Unused H₂ is recompressed and recycled to the feed. A purge helps avoid buildup of unwanted gases, and fresh H₂ makes up for any consumed in the hydrotreater. H₂ consumption can be lowered if an amine unit is placed on this line.

References

1. Emmanuel Ortega, Oct 2021, An Overview of Hydrotreating, AICHE
2. Joaquim Tsu, 31^st Dec 2024, Hydrotreater-- Basics, Linkedin post.