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Until the early years of the nineteenth century, only the plants and animals were the known sources of organic compounds. With the advent of the Industrial Revolution in Europe, fossil fuels like coal and petroleum gained prominence as the sources of organic compounds, particularly hydrocarbons.

Today, hydrocarbons are mostly obtained from petroleum. In recent years coal has started gaining prominence, probably due to the uncertain conditions in the world oil market.


Origin of Coal and Petroleum

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Coal and petroleum have organic origin, being decomposed plant and animal matter buried deep inside the earth. Coal is mined like any other mineral.

The chief coal producing countries in the world are China, U.S.A., U.S.S.R., U.K., Germany, Poland, Australia and India. In India, coal is mainly mined in Bihar, West Bengal, Madhya Pradesh and to some extent in Andhra Pradesh.
  1. Petroleum occurs deep inside the Earth at depth ranging from a few hundred to few thousand meters.
  2. The name petroleum has been given to the crude oil because it occurs under the Earth's crust entrapped in rocks (petra = rocks oleum = oil). Crude oil (petroleum) is pumped out of Earth by drilling oil wells.
  3. Petroleum is obtained by drilling a hole through the crust till it reaches the oil reserve. The oil gushes out of its own due to high pressure of the gas over its surface. However, when the pressure falls, it is pumped out with the help of pumps.
  4. Large quantity of natural gas is generally associated with petroleum.
    Petroleum is often referred to as liquid gold, due to its importance in transportation, commerce and industry. Prosperity of any country depends upon its petroleum reserves.
The chief petroleum producing countries are, U.S.A., U.S.S.R., Saudi Arabia, Kuwait, Iraq, Iran, Libya, Nigeria, Algeria, Venezuela China and Mexico.

Composition of Coal and Petroleum

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Coal is mainly composed of carbon. Coal, is classified into different varieties depending upon its carbon content. Common varieties of coal are,

Variety of coal: Peat Lignite Bituminous Anthracite Carbon content:10-15% 40% 60-70% 80-90%

Coal also contains arenes or aromatic hydrocarbons like benzene, toluene, xylene, naphthalene and anthracene. It also has some organic compounds of sulfur and nitrogen. Crude oil (or petroleum) is a dark brown viscous liquid sometimes having a greenish tinge. Crude oil is a mixture of about 150 different organic compounds. Crude oil is largely made up of aliphatic hydrocarbons, with much lesser amounts of aromatic compounds and organic compounds of sulfur and nitrogen.

Formation of Coal and Petroleum in Nature

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It is believed that coal in nature was formed from the remains of the trees buried inside the earth some 500 million years ago. Due to the bacterial and chemical action on the wood, it got converted into peat. Then, as a result of high temperature and high pressure inside the earth peat got transformed into coal.

Formation of Coal

It is believed that the petroleum was formed from the partial decomposition of the sea animals and prehistoric forests under high temperature and pressure conditions inside the earth.

Hydrocarbons from Petroleum

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Crude oil or petroleum pumped out of an oil well, is a viscous and complex mixture of several hydrocarbons and small amounts of other compounds. It is not useful in this state.

The 'refining of (crude) petroleum' involves the process of fractional distillation, whereby petroleum is separated into many useful fractions.

Refining of Crude Oil

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The refining of petroleum is done in big refineries. Washing it with acidic or basic solution first neutralizes crude oil. A furnace is used to heat up the crude oil to 650 - 675 K and then the pressure reduced. The resulting crude oil vapors are fed into a fractionating column through an inlet near the bottom of the furnace.

Refining of Crude Oil

  1. The fractionating tower is a tall cylindrical steel structure divided into compartments by horizontal trays fitted into it. Each tray has a number of holes and a short tube with a bubble cap.
  2. Due to a regular temperature gradient along the height of the column, the fractions with lower boiling points rise up and get liquefied at different heights in the trays, depending on the boiling.
  3. These caps allow the lighter vapors to escape up the column while the progressively less volatile and heavier fraction condense and flow into the respective trays (through tray holes and overflow) in the lower section of the tower. They can be tapped out.
  4. The major products obtained from the fractional distillation of crude oil are listed below. Gasoline obtained by this procedure is called straight run gasoline.

Product Chain length Boiling range Uses %
Gas C1 to C5 Below room temperature As fuel: in manufacture of gasoline. rubber, carbon black. ammonia and methane black. 2
Petroleum ether C5toC7 303 - 363 K As a solvent and in dry cleaning. 2
Petrol C7 to C12 343 - 473 K Motor fuel: dry cleaning solvent. 32
Kerosene C1 to C16 440 - 540 K As a fuel and illuminant. 18
Gas oil Fuel oil diesel C15 to C18 523 - 673 K Furnace fuel: diesel engine fuel: in cracking 20

Lubrication oil,greases,petroleum jelly

C16 and up 623 and up As medicines and cosmetics(petrol jelly)
Paraffin wax C20 and up Melts at 325 - 330 K Makinu candles andwater proofinq.
Petroleum coke C30 and up Residue As a fuel for making electrodes.

Liquefied Petroleum Gas

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Petroleum gas a by-product from two sources: natural gas processing and crude oil refining is a mixture of butane, propane and ethane. The main constituent of liquefied petroleum gas is, however, propane, propylene, butane, butylene in various mixtures. These hydrocarbons burn readily, producing a large amount of heat. This makes petroleum gas a very good fuel.

Petroleum gas is supplied in liquid form so that a cylinder of even small volume may contain an appreciable amount of the gas. A domestic gas cylinder whose main constituent is butane, contains about 14 kg of LPG. A strong smelling substance (Ethyl mercaptan C2H5SH) is added to LPG gas cylinders to help in the detection of gas leakage. The gas used for domestic cooking is called Liquefied Petroleum Gas (LPG) because it is present in liquid form in the cylinders and is commonly used for domestic heating purposes.

Owing to demand from industry for butane derivatives, LPG sold as fuel for automobiles is made up largely of propane. This is because,

  • LPG compares favorably in cost per mile.
  • It has a high octane rating making it useful in engines too.
  • LPG leaves little or no engine deposit in the cylinders when it burns, a factor of importance in internal combustion engines.
  • As it enters the engine as a vapor, it cannot wash down the cylinder walls, remove lubricant, and increase cylinder-wall piston and piston-ring wear.

All these factors reduce engine wear, increase engine life, and maintenance costs low.

Compressed Natural Gas and Liquid Natural Gas

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A liquefied form of natural gas usually consists primarily of methane. Its properties are those of liquid methane, slightly modified by minor constituents. One property, which differentiates liquefied natural gas (LNG) from liquefied petroleum gas (LPG), which is principally propane or butane or both, has a much lower critical temperature, about (-73°C).

Unlike LPG, natural gas cannot be liquefied at normal ambient temperature by increasing pressure; natural gas must be cooled to cryogenic temperatures to be liquefied and then stored in well-insulated containers. CNG or LNG is stored in high pressure cylinders or special tanks.

The natural gas is compressed or liquefied for ease of storing and transporting. LNG takes up about 1/600 the space that natural gas does in its gaseous form. LNG technology has made it possible to utilize natural gas from remote areas where it previously had no common use and was burned. CNG or LNG are very clean fuels, which cause very little pollution and have very high calorific value.

Cracking of Hydrocarbons

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The process of breaking higher hydrocarbons with high boiling points into a variety of lower hydrocarbons that are more volatile (low boiling), is called cracking (or pyrolysis). For example, a higher hydrocarbon C10H22 splits according to the reaction.

C10H22 Cracking of hydrocarbons C8H18 + C2H4

The process of cracking, increases the relative amounts of the lower hydrocarbons. During cracking, carbon-carbon bonds get broken in a random manner, leading to various kinds of products being formed.

Thermal Cracking

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Breaking down large molecules by heating at high temperature and pressure is termed as thermal cracking. Thermal cracking is further classified into the following classes.

  • Liquid phase thermal cracking process: The higher boiling fractions e.g., fuel oil, lubricating oil are converted into low boiling fractions by heating the liquids at a temperature of 750 K, under a pressure of about 10 atmosphere.
  • Vapor phase thermal cracking process: Low boiling fraction e.g., kerosene is cracked in the vapor phase at a temperature of about 875 K and under a pressure of 3 atmosphere.

Catalytic Cracking

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Higher hydrocarbons can also be cracked at lower temperature (600 - 650 K) and lower pressure (2 atm) in the presence of a suitable catalyst. Catalytic cracking produces gasoline of higher octane number and therefore this method is used for obtaining better quality gasoline.

A typical catalyst used for this purpose is a mixture of silica (SiO2), 4 parts, alumina (Al2O3), 1 part, and manganese-dioxide (MnO2), 1 part.

Steam Cracking

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Here, higher hydrocarbons are mixed with steam in their vapor phase and heated for a short duration to about 900°C, and cooled rapidly. This process is suitable for obtaining lower unsaturated hydrocarbons.

Cracking Application

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The most important products obtained in straight-run refining are petrol, diesel and kerosene. The demand for these products outstrips that obtained during refining of the petroleum. Conversely, high-boiling fractions find lesser use. So,

  • In petroleum industry, the cracking of less useful high boiling fractions is done to increase the yield of low boiling (lower molecular mass) fractions, such as gasoline.
  • Cracking always yields low boiling alkenes as the by-products. These unsaturated hydrocarbons are called petrochemicals, that form a variety of useful compounds such as, polyethylene etc.

Reforming or Aromatisation

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Reforming or aromatisation involves the conversion of open chain (aliphatic) hydrocarbons and/or cycloalkanes in the presence of a catalyst, into aromatic hydrocarbons (arenes) containing the same number of carbon atoms. Aromatization involves reactions of the type, dehydrogenation, localization, and isomerization.

In reforming (or aromatisation), cyclic and acyclic alkanes containing six to eight carbon atoms are heated at about 670 K in the presence of palladium, platinum or nickel as catalyst. Platinum seems to be the best catalyst and so the process is sometimes called platforming.

For example,

Hexane when passed over Cr2O3 supported over alumina at 670 K benzene is produced.

Formation of Benzene

Under similar conditions, n-heptane yields toluene.

Formation of Toluene

Cyclohexane gives benzene

Cycloheane to Toluene

Applications of Reforming

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  • Production of aromatic hydrocarbons (arenes) like benzene, toluene and xylenes. Earlier the bulk of arenes were obtained from coal tar, but now they are derived from petroleum by the process of reforming.
  • Increasing the octane number of straight-run gasoline by increasing the proportion of aromatic hydrocarbons in gasoline.

Aromatic hydrocarbons have octane numbers higher than 100, while the straight-run gasoline has the octane number of 55. The straight-run gasoline (octane number 55) is first distilled to obtain a fraction containing n-hexane, cyclohexane and methylcyclopentane. This fraction is then heated with a platinum catalyst to convert the cyclohexanes into benzene and toluene (octane number > 100).

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