Petroleum and Coal
The Chemistry of Petroleum Products
The term petroleum comes from the Latin stems petra, "rock," and oleum, "oil." It is used to describe a broad range of hydrocarbons that are found as gases, liquids, or solids beneath the surface of the earth. The two most common forms are natural gas and crude oil.
Natural gas is a mixture of lightweight alkanes. A typical sample of natural gas when it is collected at its source contains 80% methane (CH4), 7% ethane (C2H6), 6% propane (C3H8), 4% butane and isobutane (C4H10), and 3% pentanes (C5H12). The C3, C4, and C5 hydrocarbons are removed before the gas is sold. The commercial natural gas delivered to the customer is therefore primarily a mixture of methane and ethane. The propane and butanes removed from natural gas are usually liquefied under pressure and sold as liquefied petroleum gases (LPG).
Natural gas was known in England as early as 1659. But it didn't replace coal gas as an important source of energy in the United States until after World War II, when a network of gas pipelines was constructed. By 1980, annual consumption of natural gas had grown to more than 55,000 billion cubic feet, which represented almost 30% of total U.S. energy consumption.
The first oil well was drilled by Edwin Drake in 1859, in Titusville, PA. It produced up to 800 gallons per day, which far exceeded the demand for this material. By 1980, consumption of oil had reached 2.5 billion gallons per day. About 225 billion barrels of oil were produced by the petroleum industry between 1859 and 1970. Another 200 billion barrels were produced between 1970 and 1980. The total proven world reserves of crude oil in 1970 were estimated at 546 billion barrels, with perhaps another 800 to 900 billion barrels of oil that remained to be found. It took 500 million years for the petroleum beneath the earth's crust to accumulate. At the present rate of consumption, we might exhaust the world's supply of petroleum by the 200th anniversary of the first oil well.
Crude oil is a complex mixture that is between 50 and 95% hydrocarbon by weight. The first step in refining crude oil involves separating the oil into different hydrocarbon fractions by distillation. A typical set of petroleum fractions is given in the table below. Since there are a number of factors that influence the boiling point of a hydrocarbon, these petroleum fractions are complex mixtures. More than 500 different hydrocarbons have been identified in the gasoline fraction, for example.
|Fraction||Boiling Range (oC)||Number of Carbon Atoms|
|natural gas||< 20||C1 to C4|
|petroleum ether||20 - 60||C5 to C6|
|gasoline||40 - 200||C5 to C12, but mostly C6 to C8|
|kerosene||150 - 260||mostly C12 to C13|
|fuel oils||> 260||C14 and higher|
|lubricants||> 400||C20 and above|
|asphalt or coke||residue||polycyclic|
About 10% of the product of the distillation of crude oil is a fraction known as straight-run gasoline, which served as a satisfactory fuel during the early days of the internal combustion engine. As the automobile engine developed, it was made more powerful by increasing the compression ratio. Modern cars run at compression ratios of about 9:1, which means the gasoline-air mixure in the cylinder is compressed by a factor of nine before it is ignited. Straight-run gasoline burns unevenly in high-compression engines, producing a shock wave that causes the engine to "knock,"or "ping." As the petroleum industry matured, it faced two problems: increasing the yield of gasoline from each barrel of crude oil and decreasing the tendency of gasoline to knock when it burned.
The relationship between knocking and the structure of the hydrocarbons in gasoline is summarized in the following general rules.
The most commonly used measure of a gasoline's ability to burn without knocking is its octane number. Octane numbers compare a gasoline's tendency to knock against the tendency of a blend of two hydrocarbons heptane and 2,2,4-trimethylpentane, or isooctane to knock. Heptane (C7H16) is a long, straight-chain alkane, which burns unevenly and produces a great deal of knocking. Highly branched alkanes such as 2,2,4-trimethylpentane are more resistant to knocking. Gasolines that match a blend of 87% isooctane and 13% heptane are given an octane number of 87.
There are three ways of reporting octane numbers. Measurements made at high speed and high temperature are reported as motor octane numbers. Measurements taken under relatively mild engine conditions are known as research octane numbers. The road-index octane numbers reported on gasoline pumps are an average of these two. Road-index octane numbers for a few pure hydrocarbons are given in the table below.
Hydrocarbon Octane Numbers
|Hydrocarbon||Road Index Octane Number|
By 1922 a number of compounds had been discovered that could increase the octane number of gasoline. Adding as little as 6 mL of tetraethyllead (shown in the figure below) to a gallon of gasoline, for example, can increase the octane number by 15 to 20 units. This discovery gave rise to the first "ethyl" gasoline, and enabled the petroleum industry to produce aviation gasolines with octane numbers greater than 100.
Another way to increase the octane number is thermal reforming. At high temperatures (500-600C) and high pressures (25-50 atm), straight-chain alkanes isomerize to form branched alkanes and cycloalkanes, thereby increasing the octane number of the gasoline. Running this reaction in the presence of hydrogen and a catalyst such as a mixture of silica (SiO2) and alumina (Al2O3) results in catalytic reforming, which can produce a gasoline with even higher octane numbers. Thermal or catalytic reforming and gasoline additives such as tetraethyllead increase the octane number of the straight-run gasoline obtained from the distillation of crude oil, but neither process increases the yield of gasoline from a barrel of oil.
The data in the table of petroluem fractions suggest that we could increase the yield of gasoline by "cracking" the hydrocarbons that end up in the kerosene or fuel oil fractions into smaller pieces. Thermal cracking was discovered as early as the 1860s. At high temperatures (500C) and high pressures (25 atm), long-chain hydrocarbons break into smaller pieces. A saturated C12 hydrocarbon in kerosene, for example, might break into two C6 fragments. Because the total number of carbon and hydrogen atoms remains constant, one of the products of this reaction must contain a C=C double bond.
The presence of alkenes in thermally cracked gasolines increases the octane number (70) relative to that of straight-run gasoline (60), but it also makes thermally-cracked gasoline less stable for long-term storage. Thermal cracking has therefore been replaced by catalytic cracking, which uses catalysts instead of high temperatures and pressures to crack long-chain hydrocarbons into smaller fragments for use in gasoline.
About 87% of the crude oil refined in 1980 went into the production of fuels such as gasoline, kerosene, and fuel oil. The remainder went for nonfuel uses, such as petroleum solvents, industrial greases and waxes, or as starting materials for the synthesis of petro-chemicals. Petroleum products are used to produce synthetic fibers such as nylon, orlon, and dacron, and other polymers such as polystyrene, polyethylene and synthetic rubber. They also serve as raw materials in the production of refrigerants, aerosols, antifreeze, detergents, dyes, adhesives, alcohols, explosives, weed killers, insecticides, and insect repellents. The H2 given off when alkanes are converted to alkenes or when cycloalkanes are converted to aromatic hydrocarbons can be used to produce a number of inorganic petrochemicals, such as ammonia, ammonium nitrate, and nitric acid. As a result, most fertilizers as well as other agricultural chemicals are also petrochemicals.
The Chemistry of Coal
Coal can be defined as a sedimentary rock that burns. It was formed by the decomposition of plant matter, and it is a complex substance that can be found in many forms. Coal is divided into four classes: anthracite, bituminous, sub-bituminous, and lignite. Elemental analysis gives empirical formulas such as C137H97O9NS for bituminous coal and C240H90O4NS for high-grade anthracite.
Anthracite coal is a dense, hard rock with a jet-black color and a metallic luster. It contains between 86% and 98% carbon by weight, and it burns slowly, with a pale blue flame and very little smoke. Bituminous coal, or soft coal, contains between 69% and 86% carbon by weight and is the most abundant form of coal. Sub-bituminous coal contains less carbon and more water, and is therefore a less efficient source of heat. Lignite coal, or brown coal, is a very soft coal that contains up to 70% water by weight.
The total energy consumption in the United States for 1990 was 86 x 1015 kJ. Of this total, 41% came from oil, 24% from natural gas, and 23% from coal. Coal is unique as a source of energy in the United States, however, because none of the 2118 billion pounds used in 1990 was imported. Furthermore, the proven reserves are so large we can continue using coal at this level of consumption for at least 2000 years.
At the time this text was written, coal was the most cost-efficient fuel for heating. The cost of coal delivered to the Purdue University physical plant was $1.41 per million kJ of heating energy. The equivalent cost for natural gas would have been $5.22 and #2 fuel oil would have cost $7.34. Although coal is cheaper than natural gas and oil, it is more difficult to handle. As a result, there has been a long history of efforts to turn coal into either a gaseous or a liquid fuel.
As early as 1800, coal gas was made by heating coal in the absence of air. Coal gas is rich in CH4 and gives off up to 20.5 kJ per liter of gas burned. Coal gas or town gas, as it was also known became so popular that most major cities and many small towns had a local gas house in which it was generated, and gas burners were adjusted to burn a fuel that produced 20.5 kJ/L. Gas lanterns, of course, were eventually replaced by electric lights. But coal gas was still used for cooking and heating until the more efficient natural gas (38.3 kJ/L) became readily available.
A slightly less efficient fuel known as water gas can be made by reacting the carbon in coal with steam.
|C(s) + H2O(g) CO(g) + H2(g)||Ho = 131.3 kJ/molrxn|
Water gas burns to give CO2 and H2O, releasing roughly 11.2 kJ per liter of gas consumed. Note that the enthalpy of reaction for the preparation of water gas is positive, which means that this reaction is endothermic. As a result, the preparation of water gas typically involves alternating blasts of steam and either air or oxygen through a bed of white-hot coal. The exothermic reactions between coal and oxygen to produce CO and CO2 provide enough energy to drive the reaction between steam and coal.
Water gas formed by the reaction of coal with oxygen and steam is a mixture of CO, CO2, and H2. The ratio of H2 to CO can be increased by adding water to this mixture, to take advantage of a reaction known as the water-gas shift reaction.
|CO(g) + H2O(g) CO2(g) + H2(g)||Ho = -41.2 kJ/molrxn|
The concentration of CO2 can be decreased by reacting the CO2 with coal at high temperatures to form CO.
|C(s) + CO2(g) 2 CO(g)||Ho = 172.5 kJ/molrxn|
Water gas from which the CO2 has been removed is called synthesis gas because it can be used as a starting material for a variety of organic and inorganic compounds. It can be used as the source of H2 for the synthesis of ammonia, for example.
N2(g) + 3 H2(g) 2 NH3(g)
It can also be used to make methyl alcohol, or methanol.
CO(g) + 2 H2(g) CH3OH(l)
Methanol can then be used as a starting material for the synthesis of alkenes, aromatic compounds, acetic acid, formaldehyde, and ethyl alcohol (ethanol). Synthesis gas can also be used to produce methane, or synthetic natural gas (SNG).
CO(g) + 3 H2(g) CH4(g) + H2O(g)
2 CO(g) + 2 H2(g) CH4(g) + CO2(g)
The first step toward making liquid fuels from coal involves the manufacture of synthesis gas (CO and H2) from coal. In 1925, Franz Fischer and Hans Tropsch developed a catalyst that converted CO and H2 at 1 atm and 250 to 300C into liquid hydrocarbons. By 1941, Fischer-Tropsch plants produced 740,000 tons of petroleum products per year in Germany.
Fischer-Tropsch technology is based on a complex series of reactions that use H2 to reduce CO to CH2 groups linked to form long-chain hydrocarbons.
|CO(g) + 2 H2(g) (CH2)n(l) + H2O(g)||Ho = -165 kJ/molrxn|
The water produced in this reaction combines with CO in the water-gas shift reaction to form H2 and CO2.
|CO(g) + H2O(g) CO2(g) + H2(g)||Ho = -41.2 kJ/molrxn|
The overall Fischer-Tropsch reaction is therefore described by the following equation.
|2 CO(g) + H2(g) (-CH2-)n(l) + CO2(g)||Ho = -206 kJ/molrxn|
At the end of World War II, Fischer-Tropsch technology was under study in most industrial nations. The low cost and high availability of crude oil, however, led to a decline in interest in liquid fuels made from coal. The only commercial plants using this technology today are in the Sasol complex in South Africa, which uses 30.3 million tons of coal per year.
Another approach to liquid fuels is based on the reaction between CO and H2 to form methanol, CH3OH.
CO(g) + 2 H2(g) CH3OH(l)
Methanol can be used directly as a fuel, or it can be converted into gasoline with catalysts such as the ZSM-5 zeolite catalyst developed by Mobil Oil Company.
As the supply of petroleum becomes smaller and its cost continues to rise, a gradual shift may be observed toward liquid fuels made from coal. Whether this takes the form of a return to a modified Fischer-Tropsch technology, the conversion of methanol to gasoline, or other alternatives, only time will tell.
Organic Chemistry: Structure and Nomenclature of Hydrocarbons
Structure and Nomenclature of Hydrocarbons | Isomers | The Reactions of Alkanes, Alkenes, and Alkynes | Hydrocarbons | Petroleum and Coal | Chirality and Optical Activity
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