in the refining of crude oil , hydroprocessing units such as hydrotreaters and hydrocrackers are used to remove impurities such as sulfur , nitrogen , and metals from the crude oil . they are also used to convert the feed into valuable products such as naphtha , jet fuel , kerosene and diesel . the current invention provides very high to total conversion of heavy oils to products in a single high - pressure loop , using multiple reaction stages . a hot high pressure separator is located between the first and second reaction stages . overhead from the separator is treated in a distillate upgrader , which may operate in co - current or countercurrent mode .