Oil is divided into fractions to obtain petroleum products in two stages, that is, the distillation of oil passes through the primary and secondary processing.
Primary refining process
At this stage of the distillation, crude oil is dehydrated and desalted using special equipment to separate salts and other impurities, which can cause corrosion of the equipment and reduce the quality of oil products. After that, the oil contains only 3-4 mg of salts per liter and not more than 0.1% of water. The prepared product is ready for distillation.
Due to the fact that liquid hydrocarbons boil at different temperatures, this property is used in the distillation of oil to separate fractions from it at different boiling phases. The distillation of oil at the first refineries made it possible to isolate the following fractions depending on temperature: gasoline (boils off at 180 ° C and below), jet fuel (boils off at 180-240 ° C) and diesel fuel (boils off at 240-350 ° C). From the distillation of oil remains fuel oil.
In the process of distillation, oil is separated according to boiling points into fractions (components). The result is marketable petroleum products or their components. Distillation of oil is the initial stage of its refining at specialized plants.
When heated, a vapor phase forms, the composition of which is different from the liquid. The fractions obtained by the distillation of oil are usually not a pure product, but a mixture of hydrocarbons. Separate hydrocarbons can be isolated only due to the multiple distillation of oil fractions.
Direct oil distillation is performed
- by the method of single evaporation (the so-called equilibrium distillation) or simple distillation (fractional distillation);
- with the use of rectification and without it;
- using an evaporating agent;
- under vacuum and at atmospheric pressure.
Equilibrium distillation separates oil into fractions less clearly than simple distillation. Moreover, in the first case, more oil passes into the vapor state at the same temperature in the first case than in the second.
Fractional distillation of oil makes it possible to obtain various fuels (gasoline, fuel for diesel and jet engines), as well as raw materials (benzene, xylenes, ethylbenzene, ethylene, butadiene, propylene), solvents and other products.
Next, a secondary distillation of oil is already carried out from the products obtained, the so-called cracking.
Oil refining process
Secondary distillation of oil is carried out by chemical or thermal catalytic decomposition of those products that are extracted from it as a result of primary oil distillation. This results in a greater number of gasoline fractions, as well as raw materials for the production of aromatic hydrocarbons (toluene, benzene and others). The most common oil refining technology is cracking.
Cracking is the process of high-temperature processing of oil and separated fractions to produce (mainly) products with a lower molecular weight. These include motor fuel, lubricating oils, etc., raw materials for the petrochemical and chemical industries. Cracking proceeds with the breaking of C – C bonds and the formation of carbanions or free radicals. The breaking of C – C bonds is carried out simultaneously with dehydrogenation, isomerization, polymerization and condensation of intermediate and starting materials. The last two processes form a cracking residue, i.e. fraction with a boiling point above 350 ° C and coke.
The distillation of oil by cracking was patented in 1891 by V. G. Shukhov and S. Gavrilov, then these engineering decisions were repeated by W. Barton during the construction of the first industrial unit in the USA.
Cracking is carried out by heating raw materials or exposure to catalysts and heat.
Cracking allows you to highlight more useful components from fuel oil.