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The Chemistry of Petroleum Products

The term petroleum comes from the Latin stems petra , "rock," and oleum , "oil." It isused 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 gasand crude oil.

Natural gas is a mixture of lightweight alkanes. A typical sample of natural gas whenit is collected at its source contains 80% methane (CH 4), 7% ethane (C 2H6), 6%

propane (C 3H8), 4% butane and isobutane (C 4H10), and 3% pentanes (C 5H12). The C 3,C4, and C 5 hydrocarbons are removed before the gas is sold. The commercial naturalgas 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 asan important source of energy in the United States until after World War II, when anetwork of gas pipelines was constructed. By 1980, annual consumption of natural gashad 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 producedup 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 barrelsof oil were produced by the petroleum industry between 1859 and 1970. Another 200

billion barrels were produced between 1970 and 1980. The total proven worldreserves of crude oil in 1970 were estimated at 546 billion barrels, with perhapsanother 800 to 900 billion barrels of oil that remained to be found. It took 500 millionyears 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 200thanniversary 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 differenthydrocarbon fractions by distillation. A typical set of petroleum fractions is given inthe 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 500different hydrocarbons have been identified in the gasoline fraction, for example.

Petroleum Fractions

Fraction Boiling Range ( oC) Number of Carbon Atoms

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natural gas < 20 C 1 to C 4 petroleum ether 20 - 60 C 5 to C 6 gasoline 40 - 200 C 5 to C 12, but mostly C 6 to C 8 kerosene 150 - 260 mostly C 12 to C 13

fuel oils > 260 C 14 and higher lubricants > 400 C 20 and aboveasphalt or coke residue polycyclic

About 10% of the product of the distillation of crude oil is a fraction knownas 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 mademore powerful by increasing the compression ratio. Modern cars run at compressionratios 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 yieldof gasoline from each barrel of crude oil and decreasing the tendency of gasoline toknock when it burned.

The relationship between knocking and the structure of the hydrocarbons in gasolineis summarized in the following general rules.

Branched alkanes and cycloalkanes burn more evenly than straight-chainalkanes.

Short alkanes (C 4H10) burn more evenly than long alkanes (C 7H16). Alkenes burn more evenly than alkanes. Aromatic hydrocarbons burn more evenly than cycloalkanes.

The most commonly used measure of a gasoline's ability to burn without knocking isits octane number . Octane numbers compare a gasoline's tendency to knock againstthe tendency of a blend of two hydrocarbons heptane and 2,2,4-trimethylpentane,or isooctane to knock. Heptane (C 7H16) is a long, straight-chain alkane, which

burns unevenly and produces a great deal of knocking. Highly branched alkanes suchas 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 speedand high temperature are reported as motor octane numbers . Measurements takenunder relatively mild engine conditions are known as research octane numbers .The road-index octane numbers reported on gasoline pumps are an average of these

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two. Road-index octane numbers for a few pure hydrocarbons are given in the table below.

Hydrocarbon Octane Numbers

Hydrocarbon Road Index Octane Number Heptane 02-Methylheptane 23Hexane 252-Methylhexane 441-Heptene 60Pentane 621-Pentene 84Butane 91

Cyclohexane 972,2,4-Trimethylpentane (isooctane) 100Benzene 101Toluene 112

By 1922 a number of compounds had been discovered that could increase the octanenumber 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 to20 units. This discovery gave rise to the first "ethyl" gasoline, and enabled the

petroleum industry to produce aviation gasolines with octane numbers greater than100.

Another way to increase the octane number is thermal reforming . At hightemperatures (500-600C) and high pressures (25-50 atm), straight-chain alkanesisomerize to form branched alkanes and cycloalkanes, thereby increasing the octanenumber of the gasoline. Running this reaction in the presence of hydrogen and acatalyst such as a mixture of silica (SiO 2) and alumina (Al 2O3) results in catalyticreforming , which can produce a gasoline with even higher octane numbers. Thermalor catalytic reforming and gasoline additives such as tetraethyllead increase the octane

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number of the straight-run gasoline obtained from the distillation of crude oil, butneither 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 oilfractions 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 C 12 hydrocarbon in kerosene, for example,might break into two C 6 fragments. Because the total number of carbon and hydrogenatoms remains constant, one of the products of this reaction must contain a C=Cdouble 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-crackedgasoline less stable for long-term storage. Thermal cracking has therefore beenreplaced 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 asgasoline, kerosene, and fuel oil. The remainder went for nonfuel uses, such as

petroleum solvents, industrial greases and waxes, or as starting materials for thesynthesis of petro-chemicals . Petroleum products are used to produce synthetic fiberssuch as nylon, orlon, and dacron, and other polymers such as polystyrene,

polyethylene and synthetic rubber. They also serve as raw materials in the productionof refrigerants, aerosols, antifreeze, detergents, dyes, adhesives, alcohols, explosives,weed killers, insecticides, and insect repellents. The H 2 given off when alkanes areconverted to alkenes or when cycloalkanes are converted to aromatic hydrocarbonscan 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 thedecomposition of plant matter, and it is a complex substance that can be found in

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many forms. Coal is divided into four classes: anthracite, bituminous, sub-bituminous,and lignite. Elemental analysis gives empirical formulas such as C 137H97O9 NS for

bituminous coal and C 240H90O4 NS for high-grade anthracite.

Anthracite coal is a dense, hard rock with a jet-black color and a metallic luster. Itcontains 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-bituminouscoal 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 byweight.

The total energy consumption in the United States for 1990 was 86 x 10 15 kJ. Of thistotal, 41% came from oil, 24% from natural gas, and 23% from coal. Coal is unique asa 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 wecan 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. Thecost of coal delivered to the Purdue University physical plant was $1.41 per million kJof heating energy. The equivalent cost for natural gas would have been $5.22 and #2fuel oil would have cost $7.34. Although coal is cheaper than natural gas and oil, it ismore difficult to handle. As a result, there has been a long history of efforts to turncoal into either a gaseous or a liquid fuel.

Coal Gasif ication

As early as 1800, coal gas was made by heating coal in the absence of air. Coal gas isrich in CH 4 and gives off up to 20.5 kJ per liter of gas burned. Coal gas or towngas , 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 wereadjusted to burn a fuel that produced 20.5 kJ/L. Gas lanterns, of course, wereeventually replaced by electric lights. But coal gas was still used for cooking andheating 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 carbonin coal with steam.

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C( s) + H 2O( g ) CO( g ) + H 2( g ) H o = 131.3 kJ/mol rxn

Water gas burns to give CO 2 and H 2O, releasing roughly 11.2 kJ per liter of gasconsumed. 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 oxygenthrough a bed of white-hot coal. The exothermic reactions between coal and oxygen to

produce CO and CO 2 provide enough energy to drive the reaction between steam andcoal.

Water gas formed by the reaction of coal with oxygen and steam is a mixture of CO,CO2, and H 2. The ratio of H 2 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 ) + H 2O( g ) CO 2( g ) + H 2( g ) H o = -41.2 kJ/mol rxn

The concentration of CO 2 can be decreased by reacting the CO 2 with coal at hightemperatures to form CO.

C( s) + CO 2( g ) 2 CO( g ) H o = 172.5 kJ/mol rxn

Water gas from which the CO 2 has been removed is called synthesis gas because itcan be used as a starting material for a variety of organic and inorganic compounds. Itcan be used as the source of H 2 for the synthesis of ammonia, for example.

N2( g ) + 3 H 2( g ) 2 NH 3( g )

It can also be used to make methyl alcohol, or methanol.

CO( g ) + 2 H 2( g ) CH 3OH( l )

Methanol can then be used as a starting material for the synthesis of alkenes, aromaticcompounds, acetic acid, formaldehyde, and ethyl alcohol (ethanol). Synthesis gas canalso be used to produce methane, or synthetic natural gas (SNG).

CO( g ) + 3 H 2( g ) CH 4( g ) + H 2O( g )

2 CO( g ) + 2 H 2( g ) CH 4( g ) + CO 2( g )

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Coal L iquefaction

The first step toward making liquid fuels from coal involves the manufacture of synthesis gas (CO and H 2) from coal. In 1925, Franz Fischer and Hans Tropschdeveloped a catalyst that converted CO and H 2 at 1 atm and 250 to 300C into liquidhydrocarbons. 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 H 2 toreduce CO to CH 2 groups linked to form long-chain hydrocarbons.

CO( g ) + 2 H 2( g ) (CH 2)n(l ) + H 2O( g ) H o = -165 kJ/mol rxn

The water produced in this reaction combines with CO in the water-gas shift reactionto form H 2 and CO 2.

CO( g ) + H 2O( g ) CO 2( g ) + H 2( g ) H o = -41.2 kJ/mol rxn

The overall Fischer-Tropsch reaction is therefore described by the following equation.

2 CO( g ) + H 2( g ) (-CH 2-)n(l ) + CO 2( g ) H o = -206 kJ/mol rxn

At the end of World War II, Fischer-Tropsch technology was under study in mostindustrial nations. The low cost and high availability of crude oil, however, led to adecline in interest in liquid fuels made from coal. The only commercial plants usingthis technology today are in the Sasol complex in South Africa, which uses 30.3million tons of coal per year.

Another approach to liquid fuels is based on the reaction between CO and H 2 to formmethanol, CH 3OH.

CO( g ) + 2 H 2( g ) CH 3OH( l )

Methanol can be used directly as a fuel, or it can be converted into gasoline withcatalysts 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 gradualshift may be observed toward liquid fuels made from coal. Whether this takes theform of a return to a modified Fischer-Tropsch technology, the conversion of methanol to gasoline, or other alternatives, only time will tell.