Non-hydrocarbons that occur in crude oils and petroleum products may be small in quantity but some of them have considerable influence on product quality. In many cases they have noxious or harmful effects and must be removed or converted to less harmful compounds during the refining process. The most common occurring non-hydrocarbons are sulphur, nitrogen, and oxygen. There may also be small amounts of vanadium, nickel, sodium, and potassium.
Sulphur is common in crude oils but it varies from 0.2% by weight to 6% by weight in crude oils. There are corrosive and non-corrosive sulphur compounds. One corrosive compound is Hydrogen Sulphide, the deadly H2S which can kill an operator in 10 seconds. Extensive training and safety precautions are taken in the oil patch to reduce the risk from H2S, whose molecule appears below:
Fortunately sulphides have a highly obnoxious smell which gives some warning of their danger. H2S is often produced in Canadian natural gas wells, the so called "sour gas" wells. Production of H2S may be as high as 30% by volume of the well flow.
If one of the hydrogen atoms is replaced by a hydrocarbon group, the compound is called a mercaptan or thiol. Such compounds are formed during the distillation of crude oils. They can cause severe corrosion of the processing units and the addition of chemicals, proper temperature control and the use of special alloys in refinery equipment are required to control them.
If both of the two hydrogen atoms are replaced by hydrocarbon groups, the compound is called a sulfide or thioether. An example, thiophene (C4H4S) is shown below.
Thiophenes, shown above, have a relatively pleasant odour, comparable to benzene, are relatively stable, and may even be beneficial. Other sulphur compounds are decidedly not. They cause serious corrosion of engines and furnaces, reduce the effect of anti-knock additives in gasoline, cause charring and deposits when burned, for example in a kerosene lamp, give a bad odour to dry cleaning solvents and may dis-colour paints.
Nitrogen compounds in crude oils are complex and distillation may give rise to nitrogen compounds. These may cause discoloration in gasoline and kerosene, or may produce a "lacquer" quality, reducing the effectiveness of lubricating oils.
Some crude oils contain oxygen compounds. Their structure has not yet been established but on distillation of the crudes, the oxygen compounds decompose to form ring compounds with a carboxylic acid group in the side chain. These compounds are know as "naphthenic acids" having first been found in large quantities in the distillation of Russian naphthenic crudes.
Phenolic compounds occur in some crudes and are formed during cracking. Derived from aromatic hydrocarbons, the simplest member is the phenol, shown below.
Several other organic and inorganic compounds occur in crude oils. One example is the metal vanadium which is sometimes present. Vanadium is an important component in the manufacture of specialty steels and can be recovered from the residue of the refining process.