HYDROCARBONS AND CRUDE OIL

CONTENT
⦁ Hydrocarbon and its Main Classes
⦁ Crude Oil and Natural Gas
⦁ Refining of Crude Oil
⦁ Fractional Distillation of Oil
⦁ Cracking and Reforming of Petroleum Fractions
⦁ Octane Number or Rating
⦁ The Importance of Crude Oil and Petrochemicals

Hydrocarbon and its Main Classes
Hydrocarbons, as the name implies, are compounds of only two elements; hydrogen and carbon. There are many such organic compounds.
Hydrocarbons are usually classified into two main groups namely, saturated and unsaturated hydrocarbons.
⦁ Saturated hydrocarbons (Alkanes). The alkanes have the general molecular formula CnH2n+2. . Members of this group include Methane, CH4, ethane, C2H6, propane, C3H8, butane, C4H10. The structure of butane is given below.
⦁ Unsaturated hydrocarbons (Alkenes and alkynes). The alkenes have the general formula is CnH2n. Members of this group include ethene, C2H4 propene, C3H6 butene, C4H8 etc
Hydrocarbons can be classified into:
(A) Aliphatic compounds
(B) Aromatic compounds

(A) Aliphatic Compounds:
Aliphatic compounds are classified into two sub-classes:
⦁ Acyclic compounds
⦁ Cyclic compounds
1. In acyclic compounds the molecules are made up of straight chain carbon atoms or branch chain e.g.
(i) Butane (a straight chain compound)

(ii) Methyl butane (a branched chain compound)

2. In cyclic compounds the end carbon atoms of an open aliphatic chain can also join together to form a closed system or ring e.g. Cyclopropane

B. Aromatic Compounds:
Aromatic compounds are Benzene or derivative of benzene (that is compounds whose structures are based on the structure of benzene)
Due to resonance exhibit by the structures of benzene, i.e.

The structure of benzene would be (i) or (ii)
Conventionally the structure of benzene is now a hexagon with a ring within it.

Examples of other derivatives of benzene,

Crude Oil and Natural Gas
Another name for crude oil is petroleum. It is the chief source of hydrocarbons. Crude oil occurs as a dark, sticky, viscous liquid. It is believed that crude oil is formed by the gradual decomposition of the remains of marine plants and animals that became incorporated in the sediment and rocks formed at the bottom of the sea.
Crude oil is a mixture of gaseous, liquid and solid alkanes, cycloalkanes, aromatic hydrocarbons etc.
Natural gas; Natural gas is usually found together with crude oil; and water in traps or reservoirs in between some layers of rocks at the bottom of the sea. Natural gas consists mainly of methane. It is used as a domestic and industrial fuel.
In Nigeria crude oil is found in Oloibiri in 1956. In Niger Delta,  Calabar and in Burutu in Delta state.
Refining of Crude Oil
Crude oil must be refined to make useful fuels and chemicals. The crude petroleum taken from the oil fields to refineries is processed in three stages: separation, conversion and purification.
Fractional Distillation of Oil
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 fractions is based on the differences in their boiling points. The amin products or fractions are shown in the table below:

Fraction No. of Carbon Atoms Boiling Points Uses
1 Petroleum gas C1 — C4 Below 40oC Fuel and manufacture of
other organic compounds
2 Petrol or gasoline C5 — C10 40 — 200oC Used as fuel in motor cars
and generators
3 Kerosene or Paraffin oil C11 — C15 200 — 250oC Used as aviation fuels in aircraft
and also as fuel for lantern
4 Diesel or light
gas oil C16 — C20 250 — 350oC Used as fuels in engines or
buses, trains, boats, tractors etc.
5 Lubricating oil C20 — C30 350 — 500oC Used in candle making,
creams, hair care products.
6 Bitumen or Asphalt >C30 Above 500oC For road surfacing

Fractional Distillation of Crude Oil
EVALUATION
⦁ Explain the formation of crude oil.
⦁ Mention four fractions of petroleum and their uses

Cracking of Petroleum Fractions
The quality of petrol obtained from the Fractional distillation of crude oil is about 20% by volume.
Cracking is therefore employed so as to provide more petrol, since its demand is high and as a source of alkenes.
Meaning of Cracking
Cracking is the process whereby large molecules of heavy oils are broken into smaller molecules (light oils) at high temperature, pressure and in the presence of a catalyst. Usually petroleum fractions with more than eleven carbon atoms in their molecules can be cracked.
During cracking, ethene, C2H4 is produced; it is the major raw material in the petrochemical industry.
Cracking Techniques
The two main cracking processes are: thermal cracking and catalyst cracking.
There is also a less common cracking known as hydro cracking.
1. Thermal Cracking:
Thermal cracking involves heating heavy oils, such as lubricating oil, diesel oil and kerosene to about 6000C under pressure; it leads to the decomposition and formation of a mixture of more volatile alkanes, alkenes and hydrogen. The mixture is then separated by fractional distillation. e.g.

C11H28 → C8H18 + C3H6
Kerosene (Alkane)Petrol (Alkene)Propene

2. Catalytic Cracking:
In catalytic cracking, the heavy oil is heated to about 5000C under reduced pressure and in the presence of natural clay (a mixture of silicon (iv) oxide and aluminium oxide) as the catalyst. Under these conditions, a mixture of alkanes and alkenes, which can be separated by fractional distillation, is obtained e.g.
C18H38    →     C6H14 + C8H16 + 2C2H4
Note: Note that the products of cracking depend on the starting material and the reaction conditions, such as temperature, pressure and the nature of the catalyst used.
In hydro cracking, hydrogen is passed into the reaction mixture during the cracking process. Under this condition, only the lower saturated alkanes are obtained.
Reforming of Petroleum Fractions
In reforming the molecules do not crack but are reformed or reshaped. In reforming straight chain alkanes are converted catalytically into branched chain isomers, cycloalkanes and benzene, the purpose is to convert low grade petrol to higher grades, i.e. improve the quality. Reforming techniques includes: isomerization, cyclization and  aromatization.
Differences between Cracking and Reforming
⦁ Cracking involves breaking large molecules of petrol fractions into smaller molecule, while reforming involves rearrangement of atoms in the molecules of petroleum fraction to obtain branched and cylic hydrocarbons.
⦁ Cracking can be achieved thermally or catalytically, while reforming is in the presence of a catalyst.
⦁ Cracking occurs at a temperature higher than that of reforming.
⦁ Cracking is employed to increase the quantity of petrol, while reforming is to improve its quality.
Octane Number or Rating
Each grade of gasoline (petrol) is rated according to the term octane number. The octane rating or number is based on the proportions of the straight chain heptane, C7H16 and the branched- chain 2,2,4 –Trimethylpentane, an isomer of C8H18 called iso-octane in petrol. When petrol with a high proportion of straight chain, heptane is used in the engine of a vehicle, the combustion will not be smooth, and can knock the engine, i.e. damage the engine, while petrol with a high proportion of the branched chain 2,2,4- Trimethylpentane burns smoothly and does not cause knocking in engines. The higher the octane number of petrol, the lower the knocking tendency.
EVALUATION
⦁ What is the advantage of catalytic cracking over thermal cracking.
⦁ Mention the methods of reforming gasoline.
⦁ Mention two ways of improving the octane number of petrol.
The Importance of Crude Oil and Petrochemicals
In Nigeria, natural gas is used as fuel for heavy duty vehicles and engines. The gas power stations Ughelli and Sapele use natural gas to generate electricity.
Economic importance: Nigeria proposes to set up a liquefied natural gas plant. This will enable Nigeria not only to increase its natural gas consumption at home, but also to express the gas.
Petroleum and natural gas are now used in increasing amounts to produce many organic compounds e.g. Ethanol, ethane, propane 1,2,3-triol, benzene, toluene. These are in turn used to make large- molecule organic compounds like plastics, synthetic rubber, insecticides, detergents, synthetic fibres like nylon and dacron. The oil industry has provided employment in various aspects of the industry.

EVALUATION
⦁ Name some places in the Niger Delta in Nigeria where crude oil is found.
⦁ Write short notes on this: IMPORTANCE OF HYDROCARBONS.