sulphur doesn’t just sit there in crude oil. It shows up in many forms—from a highly toxic gas to corrosive liquids. That’s what makes the process so challenging.

Crude oil contains a certain amount of sulphur

Crude oil contains a certain amount of sulphur, and to a lesser extend nitrogen and oxygen atoms; some crude oils more than others, depending on the origin of the crude oil. The resulting oil products refined from crude oil will therefore also contain these elements, were sulphur is the most dominating one. 

Sulphur in crude oil is not elementary sulphur

Sulphur is not present in crude oils as pure sulphur – however connected to hydrogen and/or carbon: as a toxic and corrosive gas called hydrogen sulphide (or: H2S), corrosive mercaptan sulphur (mostly liquids) or (di)sulphides.

Sulphur causes health and safety issues, next to emission

When the total sulphur content in a particular product is too high then catalytic desulphurisation (called: hydrodesulphurisation HDS or hydrotreating HT) will be applied, enabling the reduction of sulphur content to even (well) below 10 mg/kg (or: ppm). 

Sweetening entails the removal of corrosive and smelly sulphur compounds

In case the total sulphur content is within the sulphur specification of a lighter fuel a more simplistic processes could be applied, such as the Merox sweetening. Merox or mercaptan oxidation, just converts (corrosive and smelly) mercaptan sulphur into a (non-corrosive and non-smelly) disulphide, without actually removing it from the fuel.