Crude oil or Petroleum is the chief source of hydrocarbons. Petroleum which means rock oil in Latin occurs as a dark, sticky, viscous liquid. It is found in huge underground deposits in many parts of the world. Natural gas is usually found together with it. Petroleum is a mixture of gaseous liquid and solid alkanes, alkenes, cycloalkanes, aromatic hydrocarbons and others. Natural gas consists mainly of methane. Crude oil is a mixture of hydrocarbons. It exists in liquid phase in natural underground reservoirs and remains liquid at atmospheric pressure after passing through surface separating facilities.
Crude oil occurs in large quantities in Nigeria, especially in Bayelsa, Edo, Imo, rivers, Delta, Abia, Ondo and Cross river state. It is dark brown in colour though its composition and consistency vary from place to place. In fact, different oil producing areas yield significantly different varieties of crude oil. We have light and heavy crude oil. The light one has low metal and sulphur content, light in colour and flows easily. It is very expensive. The heavy one has high metal and sulphur content and must be heated to become fluid. It is less expensivePetroleum is a naturally occurring, yellow-to-black liquid found in geological formations beneath the Earth’s surface, which is commonly refined into various types of fuels. Components of petroleum are separated using a technique called fractional distillation.
It consists of hydrocarbons of various molecular weights and other organic compounds
Origin of Crude Oil and Natural Gas
They are formed from the remains of marine algae and animals. When these tiny aquatic organisms died, their remains gradually settled on the seabeds. Over the years, the remains became covered by mud, silt and other sediments. As the sediments piled up, their mass exerted a great pressure on the lower layers, changing them to hard sedimentary rocks. During this process, bacterial activity, heat and pressure probably changed the plant and animal remains into crude oil and natural gas.
Refining of Crude Oil
Petroleum or crude oil occurs naturally. it contains many useful products also called fractions. These are separated by the method of fractional distillation. This process of obtaining useful fractions from petroleum is called refining.
The process of dividing petroleum into fractions with different boiling range volatilities and free from impurities is called refining.
The process of turning petroleum into a useful form is done in a crude oil refinery. The steps for making crude oil into oil, petrol or whatsoever are fractional distillation, cracking and reforming.
Petroleum is refined by fractional distillation. The process of separating a mixture into a series of fractions of different volatilities by means of distillation is known as fractional distillation.
In the process of fractional distillation, a mixture of different liquids is evaporated followed by condensation. Different liquids are evaporated according to their boiling point and they are collected in different chambers of distillation tower
Fractional Distillation of Crude Oil
Fractional distillation differs from distillation only in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with low boiling points condense at the top. Like distillation, fractional distillation works because the different substances in the mixture have different boiling points.
Fractions of petroleum from refining are petroleum gases (Methane, butane, etc.), petrol or gasoline, kerosene, diesel, lubricating oil and bitumen (asphalt). Because they have different boiling points, the substances in crude oil can be separated using fractional distillation. The crude oil is evaporated and its vapours allowed to condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.
The mixture is inserted at the bottom, where mostly everything will condense as the temperature is 350°C and more. The condensed crude oil will rise to the next fraction above, which has a very high temperature as well, but a slightly smaller one. Only the part of the mixture, which boiling point is under the temperature of the fraction, will condense and rise to the next fraction. The part of the mixture, whose boiling point is higher than the temperature inside the distillation fraction, will stay there and be pumped out.
Crude oil is heated until it boils and then the hydrocarbon gases are entered into the bottom of the fractionating column. As the gases go up the column the temperature decreases.
The hydrocarbon gases condense back into liquids and the fractions are removed from the sides of the column. The different fractions have different uses.The smaller the hydrocarbon molecule, the further it rises up the column before condensing.
The fractionating column operates continuously. The temperatures shown are approximate. A sample of crude oil may be separated in the laboratory by fractional distillation. The collection vessel is changed as the temperature rises to collect the different fractions.
After the fractional distillation process, the separated mixtures have to be cracked down. This means that a long molecule will be split up in smaller parts.
Firstly, single bonds will be broken down. This results into some lone electrons.
The lone electrons form double bonds. Thus, hydrogen will disconnect from the carbon atom. Hydrogen (H2) remains as a side product. The loss of hydrogen in these smaller organic molecules is logical, because when they are lost, more lone electrons remain with what the previous lone electrons can make a bond.
After cracking, the molecules are ready to undergo the reforming process.
This is given by the octane number. The octane number is very important in petrol. It tells what the percentage of pure heptane (in the earlier days it was octane – that is why it is called octane number) in petrol is. This is of great importance for the chemical behaviour. The quality of petrol is improved by adding mixtures to pure heptane. The chains of heptane are heated up (where platinum is used as a catalyst). So they can change. After the heating process, it shows a higher amount of branched chains. This increases the octane number.
Example: When the petrol you buy has a 98 in the name, then it means that 98% are branched chains and 2% of the mixture is pure heptane (or another pure molecule).
The octane number or octane rating of petrol is a measure of the proportion of branched chain hydrocarbons in a given blend of gasoline (petrol).
In other words, Octane number is a standard which determines the knocking ability and quality of gasoline. Higher is the octane number of a gasoline, lower is the knocking it produces.
Gasoline is composed of C7 – C9 hydrocarbons i.e. heptanes, octane and nonane. These hydrocarbons are present in their straight chain or branched chain isomers. It has been shown that straight chain hydrocarbons burn too rapidly in the car engine thus, causing irregular motion of the pistons which results in rattling noise. This rattling noise is known as “Knocking”.
Knocking is a sharp metallic sound produced in the internal combustion engine. Knocking is caused by the low octane number of gasoline
Synthetic petrol is made from materials such as coal, coke and hydrogen which do not occur in crude petroleum. Synthetic petrol can be gotten from two sources
From coal: When powdered coal is heated with hydrogen in the presence of iron or tin as catalyst at 500oC and 200 atmospheric pressure, it is converted into an oily mixture of hydrocarbons. The mixture is separated by distillation into a petrol fraction boiling at 200oC and a heavy oily residue which can be further treated with fresh coal to obtain more petrol
From Coke: When steam is passed over heated coke at 1000oC, a mixture containing equal volumes of carbon (II) oxide and hydrogen known as water gas is obtained
C(s) + H2O(l) ——–> CO(g) + H2(g)
The water gas can be hydrogenated to a mixture of hydrocarbons by adding hydrogen and passing it over finely divided nickel as catalyst at 200oC. About half of the product is petrol, the less volatile fraction is used as fuel for diesel engines