Petroleum or crude oil occurs naturally as a dark brown viscous liquid with an unpleasant smell. Products of crude oil are being used in our everyday life even without noticing it. Some of the products of crude oil include fuel, petrol, air gas, and vaseline. Crude oil has been formed millions of years ago by the decay and decomposition of marine organisms[plants and animals] which have been subjected to high pressure and temperatures in the absence of air.
Petroleum can be reached by sinking boreholes into the oil beds. Often, the pressure of the natural gas, which is associated with petroleum, usually forces the crude oil to the surface with high velocities. Petroleum is a complex mixture of hydrocarbons with various chain lengths, from one to about forty carbon atoms.
The three homologous series of hydrocarbons in petroleum are chiefly alkanes, cycloalkanes and aromatics, while the small number of impurities are compounds of sulphur, nitrogen, oxygen, phosphorus, chlorine, and some metals. The natural gas associated with petroleum is a mixture of alkanes with one to six carbon atoms, and small quantities of gases.
Refining Of Petroleum
The process of refining petroleum is quite an interesting one. Petroleum(Crude Oil) is separated into various fractions by the physical process of fictional distillation on a large scale in a refinery by the use of a fractionating tower. The crude oil is heated to about 600oC in a furnace, and the vapor is introduced into the tower from the bottom. The tower consists of various sections where condensation occurs. Usually, the high boiling fractions condense first, followed by the medium gas oil, and finally, the low-boiling fractions and gases are collected at the top.
The high-boiling fractions are usually subjected to redistillation under reduced pressure. Then, all the fractions are treated with concentrated tetraoxosulphate(VI) acid H2SO4 in order to remove sulphur and other impurities; hence obtain pure petroleum products.
Petroleum Products(Factions)
Petroleum products are the fractions obtained directly from the fractionating tower where the crude oil has been refined by fractional distillation. The separation of petroleum fraction is based on the differences in their boiling points and the main products are outlined in the table below.
| Fraction | No Of Carbons | Boiling Point |
| Refinery Gas | C1-C5 | < 30oC |
| Gasoline(Petrol and naphtha) | C5-C10 | 40-180oC |
| Paraffin Oil(Kerosene) | C11-C15 | 180-250oC |
| Light Gas Oil(Diesel) | C16-C20 | 250-300oC |
| Heavy Gas Oil(Lubricating Oil) | C20-C30 | 300-350oC |
| Asphalt(Bitumen) | >C30 | >350oC |
Cracking Of Petroleum Products
We used the word cracking in the introduction to hydrocarbons and very few know what it means or denotes. The quantity of petrol obtained from the fractional distillation of crude oil is about 20% by volume. Cracking is employed as a source of alkenes and to provide more petrol since its demand is high. Cracking is a process whereby large molecules of heavy oils are broken into smaller molecules(light oils) at higher temperatures and pressures, and in the presence of catalysts. Usually, petroleum fractions with more than eleven carbon atoms in their molecules can be cracked.
The three cracking processes are thermal cracking, catalytic cracking, and hydrocracking. Thermal cracking involves heating heavy oils, at a high temperature and pressure for decomposition, and the mixture is separated by fractional distillation.
Catalytic cracking involves heating the heavy oil at a high temperature and under reduced pressure in the presence of natural clay. Natural clay is a mixture of SiO2 and AlO3. The products of cracking depend on the starting materials, and the reaction condition.
Hydrocracking is when hydrogen is passed into the mixture during the cracking process. The disadvantage of this process is that only lower saturated alkanes are obtained.