The Continuing Need for Hydrocarbons and their Products

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Besides still providing the bulk of our energy needs, fossil fuels also are the sources for our hydrocarbon fuels and derived products. Hydrocarbons are the compounds of carbon and hydrogen. In methane (CH4), the simplest saturated hydrocarbon (alkane) and the main component of natural gas, a single carbon atom is bonded to four hydrogen atoms. The higher homologues of methane, ethane, propane, butane and so on, have the general formula CnH2n+2, displaying the tendency of carbon to form chains involving C-C bonds. These can be either straight-chain or branched. Carbon can also form multiple bonds with other carbon atoms, resulting in unsaturated hydrocarbons with double or triple, C=C or CaC bonds. Carbon atoms are also able to form rings. Cyclic ring compounds of carbons involving both saturated and unsaturated systems are abundant and involve aromatic hydrocarbons, a class of hydrocarbons of which benzene is the parent (Fig. 6.1).

All fossil fuels, natural gas, petroleum and coal, are basically hydrocarbons, but they deviate significantly in their hydrogen to carbon ratio and composition. Natural gas, depending on its origin, contains besides methane (usually in concentrations above 80-90%), some of the higher homologous alkanes (ethane, propane, butane). In "wet" natural gases the amount of C2-C6 alkanes is more significant. These so-called natural gas liquids, which were generally only considered for their thermal value, are increasingly perceived as feedstocks for more valuable products such as gasoline. Methane itself, though used mainly as a fuel, is also today's primary source of hydrogen and can be transformed (albeit at a considerable energy cost, via syn-gas) to products otherwise obtained from petroleum. Petroleum or crude oil is a remarkably varied substance, both in its composition and uses. Depending on the source, its color can range from almost transparent clear to amber, brown, black or even green, and it may flow like water or be a semisolid viscous liquid. Crude oil contains hundreds, if not thousands, of individual hydrocarbons but is predominantly constituted of saturated straight-chain compounds (alkanes) and small amounts of branched alkanes, cycloalkanes and aro-matics. Petroleum is the most versatile of our three primary fossil fuels, and can be transformed economically and easily to a vast palette of useful products. Coal, on the other hand, is more hydrogen-deficient and contains large, complex hydrocarbon systems composed mainly of aromatic cycles (Fig. 6.2). Its transformation

Hydrocarbon Pool Mto
Figure 6.1 Examples of hydrocarbons.
Coal Liquefaction

Figure 6.2 Schematic representation of structural groups and connecting bridges in bituminous coal.

Figure 6.2 Schematic representation of structural groups and connecting bridges in bituminous coal.

to liquid fuels as well as other petrochemicals (coal liquefaction) is feasible and has been demonstrated, involving cleavage and hydrogenation processes, which are very energy-intensive. Coal is thus presently almost exclusively used as a fuel.

In all hydrocarbons, chemical energy is stored in the C-H and C-C bonds. When hydrocarbons react with oxygen, present in the air, in a combustion reaction, they form CO2 and water. At the same time, energy in the form of heat is released because there is more chemical energy stored in the hydrocarbons and oxygen than in the resulting CO2 and water. This energy difference is the basis for the utilization of fossil fuels as a source for energy. The burning of hydrocarbons for energy generation is their main use today. A great variety of petrochemicals and chemical products derived from fossil fuels are also made for numerous other applications.

Because of its versatility, our civilization is today especially dependent on oil, and it is so embedded in our daily lives that we hardly think about it. Mankind's use of petroleum is as old as recorded history. As eluded to in earlier chapters, ancient cultures such as the Sumerians and Mesopotamians used asphalt and bi tumen from natural pools to seal joints in wooden boats, line water canals or inlay mosaics in walls and floors. At that time, liquid oil was also the fuel of choice for oil lamps. The Egyptians embalmed mummies with asphalt, while the Romans used flaming containers filled with oil as weapons. Native Americans used crude oil for medicinal ointments. As mentioned earlier, the modern petroleum industry however, was only born in the middle of the 19th century in America, with the invention of the kerosene lamp leading to the formation of the first oil companies in Pennsylvania. Commercial production was first aimed at fulfilling the growing demand for kerosene used in lamps. At that time, the lighter gasoline was mainly a wasted byproduct of the distillation of kerosene from crude oil, until the early 1900s when automobiles with gasoline and diesel engines became commonplace. Farm equipment powered by gasoline and diesel fuels soon also became popular, dramatically increasing agricultural productivity. During the 1930s and 1940s, a substantial market for heating oil also developed. Today, our civilization is utterly dependent on oil products that we can find in every area of our lives. The most common are the gasoline used to fuel our automobiles, and the heating oil to warm our homes and offices. Gasoline, diesel and jet fuel provide more than 95% of all the energy consumed in the transportation field by automobiles, trucks, farm and industrial machinery, trains, ships and aircraft. Transportation fuels alone account for about 60% of the petroleum consumed worldwide [14]. Oil is an essential raw material (with natural gas) for the synthesis of fertilizers on which agriculture depends, and it also provides us with the chemicals, dyes, cosmetics, pharmaceuticals, plastics and an innumerable host of other products that are essential for everyday life. As an indicator of our enormous demand for petroleum products, we can take the example of the United States which uses on average more than 20 million barrels of oil per day, representing about 12 L per day for each person in the country.

Oil in its raw state has limited uses, and the processing of crude oil via refining is first necessary to unlock the full potential of this resource. The earliest refineries in the mid-1800s used distillation to produce mainly kerosene, and distillation remains the starting point for oil refining to this day, though many more complex processes such as cracking, reforming and alkylation have been added to convert crude oil into a wide array of desired products.

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