Refining
Petroleum contains hydrocarbons. After distillation, it still contains chemical compounds with sulfur, oxygen and  Nitrogen as well as other unwanted compounds, such as Alkalenes. Processed petroleum is used for lubricants; the substances mentione are unwanted in lubricants as they can speed up the aging process. For this reason, they are extracted using a process called hydrogenation. In hydrogenation, Hydrogen sulfide and ammonia gases are produced. These gases require special handling, as they are very toxic.