Requirements for fuel quality are increasing due to continuous improvement of process technology and higher environmental standards. Much attention is paid to the sulfur content of oil products and oil. This parameter shall be indicated in the quality certificate.
Sulfur impurities are present in all types oil and in all oil products, ranging from 0.05 to 6% of the total mass. Sulfur compounds are distributed unevenly in all the fractions, even in deeply purified distillates. High sulfur content in fuel is undesirable for many reasons:
- sulfur is toxic and causes an unpleasant smell of oil products,
- reduces the resistance of gasoline to detonation,
- provokes increased gum formation during cracking,
- increases corrosion activity,
- Pairs of sulfur compounds irritate the human respiratory tract and worsen the condition of plants.
However, it is still impossible to completely eliminate sulfur from fuel. For example, if the sulfur content of diesel fuel is below 0.035%, its lubricity reduces significantly, which accelerates wearing of elements the car fuel system components. To prevent this, additives improving the lubricating properties are added to solar oil. But they are still hardly accessible, since their mass production has not yet been developed. The only way out is to reduce the amount of sulfur in oil products by setting strict standards.
Sulfur classification of oil
GOST R 51858-2002 defines the classes of oil in terms of sulfur content:
- Class 1 – low sulfur – 0.6% of sulfur
- Class 2 – sulfureus – 1.8% of sulfur
- Class 3 – high sulfur – 3.5% of sulfur
- Class 4 – very high sulfur – 3.5% of sulfur
Oil usually contains pure sulfur (a little) and its derivatives. The quality certificate indicates the total share of sulfur (pure sulfur + sulfur-containing impurities). The higher the sulfur content of oil, the lower its cost is.
The main share of sulfur compounds (50-80%) in petroleum products is almost neutral sulphides and disulphides. The most “unpleasant” sulfur derivatives are mercaptans. They cause a sharp smell and more than others provoke corrosion and formation of resins. Mercaptans content is limited to a few hundredths of a percent depending on the type of fuel and is indicated in the quality certificate separately.
Sulfur norms for different fuels
The sulfur content norms are defined for all types of fuels. The most stringent requirements are imposed on automobile gasoline and jet fuel. The permissible sulfur content in them is from 0.02 to 0.1%. The same requirements are applied to gasolines-solvents.
Diesel fuel in terms of sulfur content is divided into emission classes. Today in Russia only Euro-5 diesel engines with sulfur content of less than 10 mg / kg is allowed for production and use.
The sulfur content in marine fuel is also strictly limited in accordance with environmental requirements. The modern sulfur norm of marine low-viscosity fuel is 3.5%, and by 2020 it is planned to be reduced to 0.5%.
How to lower sulfur content
Removal of sulfur from fuel can be done by oil refineries in two ways:
To filter oil to eliminate mechanical impurities. To filter the oil the second time by catalytic hydrogenation method at high temperature. The cost of sweetened oil is approximately double that of the initial raw materials, but in this way it is possible to obtain the oil with a sulfur content up to 1%.
The second method involves removing sulfur from the heavy oil fractions by vacuum distillation. Then these fractions are hydrogenated. The obtained sulfur-free raw material is mixed with the rest of material, and the total sulfur content is reduced by 80-95%.