Sulfur and Nitrogen Content
Sulfur content of crude oils is the second most important property of crude oils next to API gravity. Sulfur content is expressed as weight percent of sulfur in oil and typically varies in the range from 0.1 to 5.0%wt. The standard methods that are use to measure the sulfur content are ASTM D129, D1552, and D2622, depending on the sulfur level. Crude oils with more than 0.5%wt sulfur need to be treated extensively during petroleum refining. Using the sulfur content, crude oils can be classified as sweet (<0.5%wt S) and sour (>0.5% %wt S). The distillation process segregates sulfur species in higher concentrations into the higher-boiling fractions and distillation residua. Removing sulfur from petroleum products is one of the most important processes in a refinery to produce fuels compliant with environmental regulations.
Nitrogen content of crude oils is also expressed as weight percent of oil. Basic nitrogen compounds are particularly undesirable in crude oil fractions as they deactivate the acidic sites on catalysts used in conversion processes. Some nitrogen compounds are also corrosive. Crude oils with nitrogen contents greater than 0.25%wt need treatment in refineries for nitrogen removal.