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.
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.
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.
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.
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.
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.
|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 lubricant.in 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.|
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,
All these factors reduce engine wear, increase engine life, and maintenance costs low.
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 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 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.
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.
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.
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.
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,
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.
Hexane when passed over Cr2O3 supported over alumina at 670 K benzene is produced.
Under similar conditions, n-heptane yields toluene.
Cyclohexane gives benzene
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