Gasoline

The United States is the world's leading consumer of petroleum. In 2001, Americans consumed over nineteen million barrels of oil per day.

Gasoline is a volatile (evaporates quickly), flammable (readily burns) liquid obtained from the refining (purifying) of petroleum, or crude oil. Almost all gasoline produced is used to fuel automobiles. A small percentage is used to power farm equipment and airplanes.

Underground treasure

Scientists are not sure how petroleum is formed, but most believe that it is formed from the remains of tiny plants and animals called plankton. According to this theory, millions of years ago these plankton lived on the surfaces of the oceans that covered a large part of the earth. When they died, they sank to the bottom of the ocean where they were covered by mud and sediments, or materials that had settled to the ocean bottom. Over the years, this process was repeated until the extreme pressure and heat from the sediments turned the plankton remains into petroleum and natural gas. It is believed that the crude oil and gas seeped upward into porous rocks (rocks with pores, or small holes). Solid rocks eventually formed around the porous rocks, trapping the oil and gas within them.

Some scientists do not believe that crude oil deposits come from formerly living things subjected to great heat and pressure for millions of years. They believe that crude oil deposits were created when the planet Earth was being formed and that many of these reservoirs are as yet untapped.

Since the beginning of civilization, people have recovered the oil that seeped through the earth's surface and put it to use. They used it for water-proofing their clothing and for caulking (patching up cracks) ships and artificial waterways. They also used it as a lubricant and medicine.

The first oil well

On August 27, 1859, Edwin L. Drake (1819–1880) struck oil near Titusville, Pennsylvania, at a depth of 69.5 feet (21.2 meters). This accomplishment could not have come at a better time. Whale oil that was used for lighting was getting more expensive due to scarcity, and people were looking for a cheaper lamp fuel. Prior to Drake's oil discovery, several scientists had already found ways of producing kerosene from the crude oil that seeped out of certain rock formations.

Soon, oil wells were being drilled in the Titus region, as well as in other states, including Ohio, California, Texas, and Oklahoma. One of the biggest oil wells drilled occurred in Spindletop, Texas, on January 10, 1901. Observers claimed that the well spewed oil about 100 feet (30 meters) higher than the 64-foot (19.5-meter) derrick (the support structure surrounding the drilling equipment). The oil continued to flow out of the well for nine days. That tremendous oil find was the start of the American oil industry.

Gasoline and automobiles

When crude oil was originally refined to make kerosene, gasoline was also produced. At first, people tried to use gasoline for lamp fuel, but the explosion that followed such use quickly discouraged the practice. Gasoline was considered of no value and was therefore thrown away. However, the invention of gasoline-powered engines to run the newly invented automobile made gasoline a very valuable product.

In 1883, German engineer Gottlieb Daimler (1834–1900) invented an engine that ran on gasoline. Four years later, he used this gasoline engine in a four-wheeled vehicle. His automobile traveled at a speed of eleven miles per hour. In 1899, Daimler produced the first Mercedes automobile, named after his daughter. Daimler's engine has been the model for almost all automobiles manufactured since the late nineteenth century. The first American car powered by gasoline was built in Chicopee, Massachusetts, by Charles Duryea (1861–1938) and his brother, J. Frank (1869–1967), in 1893.

How much petroleum do we use?

Petroleum supplies more energy to the world today than any other source. The United States is the world's leading consumer of petroleum. In 2001, Americans consumed over nineteen million barrels of oil per day. That same year, the United States had an estimated oil supply of just nine million barrels per day. That means that the nation had to import additional oil to meet the public demand.

Raw Materials

Gasoline is one of the products derived from the distillation and refining of crude oil. Gasoline first has to be separated from the other components through a process called fractional distillation (see below) and then refined and treated with additives to improve its quality. Other chemicals are also added to gasoline to further stabilize it and improve its color and smell in a process called "sweetening."

GASOLINE FOR DIFFERENT SEASONS

Oil refineries manufacture different blends of gasoline for different seasons and climates. In the winter, the gasoline blend has to be such that it vaporizes (changes to gas) faster so that cars will start easily when it is cold outside. In the summer, the gasoline blend must not vaporize quickly, or else the vapor bubbles would block the gasoline supply to the engine.

The Manufacturing Process

The manufacture of gasoline is a complex process. Separating the gasoline fraction (part) from crude oil is just the first step in the process. To produce gasoline of good quality, manufacturers experiment with different combinations of ingredients and additives.

Exploration

1 The first step in the manufacture of gasoline is to find its parent ingredient, crude oil. Sometimes, the presence of oil leakages or of certain sedimentary rocks indicates the possible presence of oil. If obvious signs of oil reservoirs are absent, oil exploration is undertaken.

Oil prospectors, people who explore an area for the presence of oil, may examine the surface characteristics of an area and take pictures of these features. They may set off explosives underground and then analyze the resulting shock waves to determine the rock type and depth. Some explorers use devices to measure the gravity and magnetic field of rock formations, which tend to differ depending on the presence of oil deposits. Others study sound waves that bounce off rocks below the earth's surface, providing information about the types of rocks underground.

2 After a possible oil reservoir is found, core samples are taken from test wells using a drill. These samples of the underground layers of the earth are analyzed to confirm the presence of oil. Chemical tests are also performed to determine if further drilling should be done.

Drilling

3 Crude oil is obtained through wells that can be as deep as 1,000 feet (305 meters). Rotary drilling is the method most commonly used to make holes in the ground. A tower-like steel framework called a derrick is constructed over the site where the well will be dug. This framework holds the drilling equipment. The drill bit, a large circular metal cutting tool fitted with teeth, is attached to a hollow pipe. As the pipe turns, the drill bit spins, cutting through the subsoil (soil below the earth's surface) and hard rock layers. More pipes are added to the equipment top as the bit digs deeper.

A heavy liquid mixture called "mud" is poured down the pipe to keep the drill bit from overheating. The mud also carries crushed rock pieces to the surface, keeps the well wall from collapsing, and prevents oil from "gushing" should the bit reach an oil reservoir. When the oil reservoir is reached, the mud's weight keeps the oil from escaping uncontrollably. The drill is removed and the well is sealed with a contraption called a "Christmas tree," a pyramid-like structure consisting of valves, nozzles, and pressure gauges that control the flow of oil.

Recovery

4 To remove the oil from the well, a complicated system of pipes and valves is installed directly into the drilling well. The natural pressure in the reservoir rock forces the oil out of the well and into the pipes. The pipes are connected to a recovery system, which consists of a series of larger pipes that transport the oil to the refinery by first passing through an oil (liquid) and gas (nonliquid) separator. The refinery is the industrial factory where crude oil is separated into its various parts and converted to usable products.

OFFSHORE DRILLING

In addition to drilling on land, oil companies have also drilled for oil at sea. The offshore drilling techniques are the same as those used for land drilling, using derricks and rotary motors. In addition, a sturdy, raised platform is built to accommodate the equipment and provide living spaces for the crew.

5 Eventually, the natural pressure in the rock reservoir decreases. However, great quantities of oil may still remain in the rock formation. To recover the rest of the oil, pressure is restored using water. Holes are drilled around the perimeter of the well, and water is then added, flooding the well. This causes the crude oil to float on the water. Another recovery method involves injecting carbon dioxide gas into the pocket above the oil deposit, thereby pushing out the oil. Some drillers use chemicals or steam to force more oil from the reservoir rock.

Fractional distillation

6 Crude oil that comes out of the ground cannot be used in its natural form. It has to be separated into its different parts, or fractions, by a process called distillation, which is performed in a fractional distillation column. During distillation, the crude oil components are separated according to molecular weight.

The crude oil is first pumped into a furnace and heated to over 600 degrees Fahrenheit (316 degrees Centigrade), causing it to evaporate (change from liquid to vapor). The vapors enter the bottom of a fractional distillation column (a huge tower fitted with a series of horizontal trays) and rise through the column. The lightest vapors, which rise to the top of the column, condense (change back to liquid) and settle on the different levels of trays throughout the length of the tower. Gasoline, having a low molecular weight, condenses at the top of the column and is one of the first fractions drawn off.

Fractional distillation itself does not produce gasoline from crude oil. It is just the first step in separating the crude oil fractions. Further refining processes are later used to improve the quality of the raw gasoline.

Refining crude oil fractions

7 In order to increase the amount of gasoline produced from crude oil, thermal cracking is used. Thermal cracking cracks, or breaks down, the heavier parts of crude oil by subjecting them to intense heat and high pressure. In the past, only 10 percent of crude oil produced gasoline. With thermal cracking, this proportion has been increased more than four times.

Other refining processes include catalytic cracking and polymerization. In catalytic cracking, the combination of heat and a catalyst (a substance that causes or speeds up a chemical reaction without itself being changed), such as aluminum, platinum, processed clay, and acids, breaks down the larger molecules of crude oil into gasoline. Polymerization is the opposite of cracking. In this process, the smaller molecules of crude oil are combined to form gasoline.

Additives

8 After gasoline is refined, antiknock additives are added to react with the chemicals in gasoline to prevent "engine knock." Knocking is the sound and the damage caused by the premature burning of gasoline in the combustion chamber of the internal-combustion engine. Antiknock compounds added include tertiary-butyl alcohol and methyl tertiary-butyl ether.

Other additives include antioxidants, which prevent the formation of gum in the engine. Gum is a substance formed in gasoline that can coat the internal engine parts, causing damage.

Rating gasoline

9 Gasoline is primarily a mixture of two volatile liquids, heptane and isooctane. Pure heptane, a lighter fuel, burns so quickly that it produces a great amount of knocking in the engine. Pure isooctane burns slowly and produces almost no knocking. The higher the percentage of octane in gasoline, the less knocking occurs. Octane ratings measure the ability of a certain gasoline to prevent knocking. For example, an octane rating of 87 means that the gasoline mixture contains 87 percent isooctane and 13 percent heptane. The higher the octane rating or number, the less likely it is for the gasoline to cause knocking.

Quality Control

Drilling for crude oil is a complex process. Quality control involves using the latest technology that not only preserves the environment but also reduces the drilling time, considered the most expensive part of oil exploration.

Refineries have to meet U.S. Environmental Protection Agency (EPA) regulations in managing the various processes involved in gasoline manufacture. These include the storage of crude oil, intermediate products, and finished products. They also include the discharge of chemical pollutants into the air and the disposal of waste products, such as wastewater, incinerator ash, and used filters and catalysts.

The Future

Petroleum, or crude oil, the source of gasoline, is a nonrenewable resource. Once it is used, it cannot be replaced. Today, crude oil provides about 97 percent of transportation fuels in the United States. Because gasoline is derived from a limited supply of petroleum, scientists are researching other sources of energy that could power cars.

One area of research involves different types of fuel cells that would replace the car engine. A fuel cell is basically a device that converts hydrogen fuel into electricity. Essentially, a fuel cell is a battery with an external fuel source—hydrogen—that continually recharges it. Scientists point out that fuel cells produce no pollution since their byproduct is water. Vehicles powered by fuel cells are currently being tested in different countries.

Scientists have also been looking for other sources of energy, including steam power used in steamboats of the past. Electric cars have been developed, and wind energy is also powering cars.

TRANSPORTING PETROLEUM

Crude oil may be transported from the drilling well to the refinery through a long system of steel pipes called a pipeline. Pipelines can run on the ground, underground, and even underwater. Other methods of transport include tankers (large ships), trucks, and railroad tank cars.

In the meantime, the petroleum industry continues to develop advanced technology in drilling for oil. In the Gulf of Mexico, computers are being used to direct drilling operations almost two miles beneath the surface of the water. Drills are fitted with computers that transmit information about wells. Advanced diagnostic and imaging technology allows one to "see" oil and gas reservoir features to a certain point from the earth's surface. Scientists are also looking into using high-intensity lasers for drilling.

bit:

A metal tool attached to a rotary drill and used for breaking up rock formations under the earth's surface.

Christmas tree:

A pyramid-like structure consisting of control valves, nozzles, and pressure gauges and installed at the top of a well to control the flow of oil at the completion of the drilling.

core sample:

A sample of underground mud that is drilled and analyzed for the presence of oil.

gusher:

An oil well that flows freely and abundantly as a result of natural gas pressure.

kerosene:

A fuel produced during the breaking down of crude oil into its parts. It is used for cooking, heating, and lighting.

knocking:

The sound and the damage caused by the premature burning of gasoline in the engine cylinder.

natural gas:

A gas that occurs in nature, usually with petroleum, and believed to have been formed by the actions of extreme pressure and heat on buried plankton millions of years ago.

octane number/rating:

A measure of the ability of a gasoline to prevent engine knocking. Gasolines with a higher octane rating are less likely to cause knocking.

plankton:

Tiny plants and animals found in bodies of water.

refining:

The process by which petroleum is purified and converted into a useful product, such as gasoline.

reservoir rock:

A porous rock formation containing a significant amount of oil and/or natural gas and typically enclosed by nonporous rocks that have prevented the oil from escaping.

For More Information

Books

Aaseng, Nathan. Business Builders in Oil. Minneapolis, MN: The Oliver Press, 2000.

Bredeson, Carmen. The Spindletop Gusher: The Story of the Texas Oil Boom. Brook-field, CT: The Mill Press, 1996.

Periodicals

Hart, David. "Fueling the Future." New Scientist. (June 16, 2001): pp. 1–5.

Peters, Eric. "Premium Gas." Consumers' Research Magazine. (September 1998): pp. 1–2.

Web Sites

"Fuel Cells." U.S. Department of Energy.http://www.fueleconomy.gov/feg/fuelcell.shtml (accessed on July 22, 2002).

Gasoline

In 1859 Edwin Drake and E. B. Bowditch of the Seneca Oil Company drilled the first commercial oil well in the United States in Titusville, Pennsylvania. The well produced about 400 gallons of crude oil, less than ten barrels a day. Soon, similar wells all over western Pennsylvania were providing crude oil for kerosene production that was needed to fuel the nation's streetlights and house lamps. The lighter boiling component, gasoline, was discarded, since it had no market. There are historical reports that "waste" gasoline, which had been dumped into rivers, sometimes caught fire. In 1892 the first gasoline-powered engines, for both car and tractor, were developed: This soon provided a market for the once useless substance, gasoline.

Today gasoline is the most important product of a typical oil refinery: The entire refinery process is designed to maximize its production. Gasoline is a complex mixture of molecules with a boiling range of 40–200°C (104–392°F). To produce various grades, there is a blending of many refinery components, each of which promotes specific fuel qualities such as desired octane rating, volatility, and minimization of engine deposits.

Octane Quality

The most important quality parameter for gasoline is the octane quality. Octane number is a measure of the antiknock properties of the fuel. Knocking in a gasoline engine is a metallic clattering noise (pinging), which indicates excessive intensity in preflame reactions. Severe knocking can damage the engine.

Preflame reactions occur in the engine cylinders when portions of the fuel self-initiate combustion prior to the advancing flame from the spark plug. This additional combustion causes an excessive rate of energy release, which is knock. The tendency of a fuel to engage in preflame reactions is dependent upon the structure of its component molecules (see Figure 1);

the tendency for preflame reactions is high for straight chain hydrocarbons, medium for branched hydrocarbons, and low for aromatics.

The octane number for a test gasoline represents the percentage by volume of isooctane (2,2,4-trimethylpentane) in a reference fuel consisting of the mixture of isooctane and heptane that would be necessary to match the test fuel's knocking tendency. Isooctane burns with a minimal knocking and is given an octane rating of 100. This is in contrast to heptane, which burns with much knocking and is given an octane rating of 0. Thus, a gasoline that burns with the same amount of knocking as a mixture of 92 percent isooctane and 8 percent heptane is classified as a 92 octane gasoline.

The octane ratings of gasoline can be increased by the addition of small amounts of antiknock agents. The first commercially successful antiknock agent, tetraethyllead (TEL), was developed in the 1920s. TEL was used to promote the development of higher efficiency, higher compression engines. However, TEL is highly toxic and poisons catalytic converters. Since 1974 all new U.S. automobile engines have used catalytic converters in order to reduce exhaust emissions.

Methyl t -butyl ether (MTBE) has been the antiknock agent of choice for unleaded gasoline. MTBE provides high-octane quality along with low volatility and is readily soluble in gasoline. However, leakage of gasoline from underground storage tanks has resulted in the detection of MTBE in the drinking water of several urban areas. This prompted the state of California to order the removal of MTBE from California gasoline by 2003.

Alcohols also have found use as octane enhancers. At higher concentration alcohols can be used as gasoline extenders, thus decreasing our dependency upon imported crude oil. A significant portion of all U.S. marketed gasoline is believed to contain ethanol.

Gasoline Additives

Trace amounts of olefins and diolefins found in gasoline are prone to reaction with oxygen dissolved in the gasoline. This process is referred to as autoxidation and involves a radical chain reaction that can incorporate oxygen

into the olefin and also can promote a molecular size increase via polymerization reactions. The end result of this complex process is the formation of deposits and gums that can block fuel filters and interfere with the metering of fuel and air in the carburetor. This can result in adverse engine performance. Additives are frequently added to gasoline to address oxidative stability and other issues; they include antioxidants, metal deactivators, and detergents.

Antioxidants are additives that minimize autoxidation reactions. They function as hydrogen atom donors that stop the chain oxidation process of the olefins. The two different types of antioxidants used in gasoline are phenylenediamines (PDA) and hindered phenols (such as BHT).

Trace levels of soluble metal compounds, particularly copper, catalyze the oxidative degradation of gasoline by promoting the formation of gums and deposits. Metal deactivators overcome this problem by chelating the metal and rendering it inactive. The most widely used metal deactivator is N, N′-disalicylidene-1,2-propanediamine, the copper complex of which is shown in Figure 3.

Detergents minimize fuel system deposits at low concentrations, and at high concentrations can remove deposits that have already formed. Detergents are molecules that have a highly polar end group and a nonpolar hydrocarbon tail. A conventional amino amide type detergent is shown in Figure 4.

Presumably, the polar groups in the detergent attach themselves to metal surfaces and to polar deposits on these surfaces. The nonpolar tails of these molecules "stick out" into the fuel in such a way that a monomolecular film is formed on the metal surface, preventing deposition and particle aggregation. This process is also believed to solubilize any deposits already on the metal surface. The detergent monolayer is also believed to prevent the buildup of ice on carburetor surfaces during winter. Thus, detergents can also function as anti-icing additives.

Gasoline Production

The production of gasoline begins with desalting the viscous crude oil. Salts and metals in the crude oil promote corrosion and poison processing catalysts. Thus, the crude oil is heated (to decrease viscosity) and extracted with water to remove the salts and metals. Frequently this process results in the formation of an oil/water mixture referred to as an emulsion (suspension). This emulsion is typically broken by the addition of a chemical surfactant (demulsifier) that promotes the separation of discrete oil and water layers. After separation of the aqueous layer, the oil is heated to about 400 ^oC (752^oF): This converts the oil into gaseous products and increases the fluidity of the remaining liquid. In this form, the gaseous mixture enters the fractionating column, where the process of atmospheric fractional distillation separates the crude oil into different components based upon boiling point.

The lightest boiling fractions are molecules that are gases under ambient conditions: methane, ethane, propane, butane, and olefins derived from these compounds. Uses for this distillate stream include burning as a fuel at the refinery; as petrochemical feed stocks; or processing into liquefied petroleum gas (LPG). There are three other major distillate streams collected during atmospheric distillation: the naptha fraction, which has a boiling range of 30 to 180°C (86–356°F); the kerosene fraction, which distills at between 180 and 240°C (356–464°F); and the gas oil fraction, which distills at between 240 and 355°C (464–671°F).

In order to meet current environmental regulations for the sulfur content in fuel products, the three-distillate streams are subjected to the process of hydrodesulfurization. In the presence of a catalyst , distillates are heated in the presence of hydrogen to reduce various organosulfur compounds to simple organic compounds and H₂S. The hydrogen needed for this process is a by-product of the catalytic reforming process. The H₂S product can be readily removed. In this process the refiner can control the octane number of the gasoline blending stock. By heating the naphtha fraction in the presence of an especially designed platinum catalyst, straight-chain hydrocarbons are cyclized, and saturated cyclic hydrocarbons are converted into aromatic compounds. In addition, this process converts straight-chain hydrocarbons into branched hydrocarbons. Catalytic reforming facilitates the production of gasoline blending stocks with octane ratings of from 90 to 100+.

Redistilling the atmospheric residue at a temperature of less than 400°C (752°F) under vacuum produces a vacuum gas oil. Typically, the vacuum gas oil is subjected to fluid catalytic cracking (FCC) to produce lower-boiling liquids that can be blended to make gasoline. This is achieved by breaking large molecules of the vacuum gas oil into smaller, lower-boiling molecules. An important gasoline blending component that can be produced in this manner is alkylate. It is a mixture of highly branched hydrocarbons produced by the acid-catalyzed reaction of isobutene and light olefinic hydrocarbons. Alkylate is a valuable blending component because of its high-octane quality and the absence of aromatics or olefins, which can lead to environmental and oxidative stability problems.

The 1990 Clean Air Act required the Environmental Protection Agency (EPA) to issue regulations that required gasoline to be "reformulated," resulting in significant reductions in vehicle emissions of ozone-forming and toxic air pollutants. This cleaner gasoline is called reformulated gasoline (RFG). RFG is required in the nine major metropolitan areas in the United States having the worst ozone problems. In addition, several other areas with ozone levels exceeding the public health standard have voluntarily chosen to use RFG.

Use of RFG decreases the amounts of volatile organic compounds (VOCs) and oxides of nitrogen (NO_x ) in the atmosphere that react in the presence of sunlight to produce ozone, a major component of smog. Vehicles also release toxic emissions, one of which (benzene) is a known carcinogen.

RFG contains 2 percent by weight oxygen additives (oxygenates), such as MTBE or ethanol. Oxygenates increase the combustion efficiency of gasoline, reducing vehicle emissions of carbon monoxide, a serious public health threat. The appearance of MTBE in some urban water supplies has resulted in legislation pending in the U.S. Congress to phase out the use of MTBE in RFG. Ethanol would then most likely become the primary oxygenate for future RFG.

Gasoline is the most important product of the oil refinery. The most important quality parameter for gasoline is its octane number. Additional quality characteristics for gasoline are controlled by the use of additives, for example, antioxidants, metal deactivators, and detergents. By blending various refinery streams and additives a gasoline formulation can be achieved that minimizes environmental degradation. Such a fuel is called reformulated gasoline.

see also Detergents; Energy Sources and Production; Fossil Fuels; Petroleum.

Bruce Beaver

Bibliography

Marshall, E. L., and Owen, K., eds. (1995). Critical Reports on Applied Chemistry, Vol. 34:Motor Gasoline. Cambridge, U.K.: Royal Society of Chemistry.

Gasoline

Background

Gasoline is a volatile, flammable liquid obtained from the refinement of petroleum, or crude oil. It was originally discarded as a byproduct of kerosene production, but its ability to vaporize at low temperatures made it a useful fuel for many machines. The first oil well in the United States was struck by Edwin L. Drake near Titusville, Pennsylvania, in 1859 at a depth of almost 70 feet (21 m). With the development of the four-stroke internal combustion engine by Nikolaus Otto in 1876, gasoline became essential to the automotive industry. Today, almost all gasoline is used to fuel automobiles, with a very small percentage used to power agricultural equipment and aircraft.

Petroleum, a fossil fuel, supplies more energy to the world today than any other source. The United States is the world's leading consumer of petroleum; in 1994, Americans used 7,587,000 barrels of oil per day. Petroleum is formed from the remains of plants and animals that have been held under tremendous pressure for millions of years. Ordinarily, this organic matter would decompose completely with the help of scavengers and aerobic bacteria, but petroleum is created in an anaerobic environment, without the presence of oxygen. Over half of the world's known crude oil is concentrated in the Persian Gulf basin. Other major areas include the coasts of Alaska and the Gulf of Mexico.

Petroleum products, including gasoline, are primarily a mixture of hydrocarbons (molecules containing hydrogen and carbon molecules) with small amounts of other substances. Crude oil is comprised of different lengths of hydrocarbon chains, with some short chains and some very long chains. Depending on how much the oil is broken down, or refined, it may become any number of products. In general, the smaller the molecule, the lower the boiling point. Therefore, gas, with very small chains of one to five carbons, boils at a very low temperature. Gasoline, with 6-10 carbons, boils at a slightly higher temperature. The heaviest oils may contain up to 25 carbon atoms and not reach their boiling point until 761°F (405°C).

Raw Materials

Gasoline is one of the products derived from distilling and refining petroleum. Compounds of organic lead were added to gasoline in the past to reduce knocking in engines, but due to environmental concerns this is no longer common. Other chemicals are also added to gasoline to further stabilize it and improve its color and smell in a process called "sweetening."

The Manufacturing
Process

Exploration

• 1 The first step in the manufacture of gasoline is to find its parent ingredient, petroleum. Crude oil is trapped in areas of porous rock, or reservoir rock, after it has migrated there from the area of its origin. Possible areas of oil concentration may be pinpointed by looking for rock types that are commonly found in those areas. Explorers may examine the surface features of the land, analyze how sound waves bounce off the rock, or use a gravity meter to detect slight differences in rock formations.
• 2 After a possible oil reservoir is found, the area must be test drilled. Core samples are taken from test wells to confirm rock formations, and the samples are chemically analyzed in order to determine if more drilling is justified. Although the methods used today are more advanced than any of the past, there is still no certainty in oil exploration.

Drilling

• 3 Crude oil is recovered through wells that can reach over 1,000 feet (305 m) into the rock. The holes are made by rotary drillers, which use a bit to bore a hole in the ground as water is added. The water and soil create a thick mud that helps hold back the oil and prevent it from "gushing" due to the internal pressure contained in the reservoir rock. When the reservoir is reached, the mud continues to hold back the oil while the drill is removed and a pipe is inserted.

Recovery

• 4 To recover the oil, a complicated system of pipes and valving is installed directly into the drilling well. The natural pressure of the reservoir rock brings the oil out of the well and into the pipes. These are connected to a recovery system, which consists of a series of larger pipes taking the crude oil to the refinery via an oil (liquid) and gas (non-liquid) separator. This method allows the oil to be recovered with a minimum of waste.
• 5 Eventually, the natural pressure of the well is expended, though great quantities of oil may still remain in the rock. Secondary recovery methods are now required to obtain a greater percentage of the oil. The pressure is restored by either injecting gas into the pocket above the oil or by flooding water into the well, which is far more common. In this process, four holes are drilled around the perimeter of the well and water is added. The petroleum will float on the water and come to the surface.

Fractional distillation

• 6 Crude oil is not a good fuel, since it is not fluid and requires a very high temperature to burn. The long chains of molecules in crude oil must be separated from the smaller chains of refined fuels, including gasoline, in a petroleum refinery. This process is called fractional distillation.

  A fractional distillation tower is a huge unit that may hold up to 200,000 barrels of crude oil. The oil is first pumped into a furnace and heated to over 600°F (316°C), causing all but the largest molecules to evaporate. The vapors rise into the fractionating column, which may be as tall as 150 feet (46 m). The vapors cool as they rise through the column. Since the boiling points of all the compounds differ, the larger, heavier molecules will condense first lower in the tower and the shorter, lighter molecules will condense higher in the tower. Natural gases, gasoline, and kerosene are released near the top. Heavier compounds used in the manufacture of plastics and lubricants are removed lower in the tower.

  Fractional distillation itself does not produce gasoline from crude oil, it just removes the gasoline from other compounds in crude oil. Further refining processes are now used to improve the quality of the fuel.

Refining petroleum

• 7 Catalytic cracking is one of the most important processes in oil refining. This process uses a catalyst, high temperature, and increased pressure to affect chemical changes in petroleum. Catalysts such as aluminum, platinum, processed clay, and acids are added to petroleum to break down larger molecules so that it will possess the desired compounds of gasoline.

  Another refining process is polymerization. This is the opposite of cracking in that it combines the smaller molecules of lighter gases into larger ones that can be used as liquid fuels.

Additives

• 8 Once gasoline is refined, chemicals are added. Some are anti-knock compounds, which react with the chemicals in gasoline that burn too quickly, to prevent "engine knock." In leaded gasoline, tetraethyl lead is the anti-knock additive. (Unleaded gasoline is refined further so the need for anti-knock additives is minimal.) Other additives (antioxidants) are added to prevent the formation of gum in the engine. Gum is a resin formed in gasoline that can coat the internal parts of the engine and increase wear.

Rating gasoline

• 9 Gasoline is primarily a mixture of two volatile liquids, heptane and isooctane. Pure heptane, a lighter fuel, burns so quickly that it produces a great amount of knocking in an engine. Pure isooctane evaporates slowly and produces virtually no knocking. The ratio of heptane to isooctane is measured by the octane rating. The greater the percentage of isooctane, the less knocking and the higher the octane rating. For example, an octane rating of 87 is comparable to a mixture of 87% isooctane and 13% heptane.

Byproducts/Waste

On average, 44.4% of petroleum becomes gasoline. There really are no waste products from petroleum. The lighter chemicals are natural gas, liquified petroleum gas (LPG), jet fuel, and kerosene. The heavier products are used for the manufacture of lubricants, plastics, and asphalt. In addition, many less valuable products can be chemically converted into more saleable compounds.

The Future

Gasoline, though widely used in many applications today, is destined to become a fuel of the past because petroleum is a nonrenewable resource. Current technology centers on making the most of the remaining petroleum reservoirs and exploring alternative energy sources. New methods to accurately determine the extent of oil reservoirs, automated systems to control oil recovery, and ways of enabling workers to recover more oil from known reservoirs are all being investigated to fully utilize the oil stores available today.

The newest methods in oil field exploration measure the physical size of the reservoir and its volume of oil. Frequently, the pressure inside the well is measured over a period of time as the oil is recovered. Using this data, scientists can determine the size of the reservoir and its permeability. An echo meter, which bounces sound waves off the sides of the reservoir, can also be used to discover the well's characteristics.

Modern oil recovery methods are most often controlled, at least in part, by computerized systems. SCADA (Systems for Supervisory Control of Data Acquisitions) use specialized software to monitor operations through one or more master terminals and several remote terminals. These systems increase efficiency, help prevent mishaps that could harm the environment, and reduce the number of laborers with increased safety.

Enhanced oil recovery methods increase the percentage of oil that can be obtained from a reservoir. In the past, workers were able to extract less than half of the oil contained in a reservoir. New methods involve injecting gases or foams into the well to force out the oil, drilling horizontally into the well, and using more geophysical information to accurately predict the characteristics of the reservoir.

Because gasoline is produced from a limited supply of petroleum, scientists are looking for clean, renewable sources of energy to power machines of the future. Steam power, used in the steamboats of the past, is an energy source that is receiving renewed attention. Electric vehicles have been developed, and solar and wind energies are also powering cars and homes.

Where To Learn More

Book

Shilstone, Beatrice. The First Book of Oil. Franklin Watts, Inc., 1969.

Periodicals

Gibbs, L.M. "The Changing Nature of Gasoline." Automotive Engineering, January 1994, pp. 99-102.

Langreth, Robert. "Less Smog, More Buildup?" Popular Science, April 1995, p. 36.

"Getting the Lead Out." Motor Trend, April 1992, pp. 106-107.

—Barry Marton /

Kristine M. Krapp

Gasoline

Crude oil in its natural state has very few practical uses. However, when it is separated into its component parts by the process of fractionation, or refining, those parts have an almost unlimited number of applications.

In the first 60 years after the process of petroleum refining was invented, the most important fraction produced was kerosene, widely used as a home heating product. The petroleum fraction slightly lighter than kerosene — gasoline — was regarded as a waste product and discarded. Not until the 1920s, when the automobile became popular in the United States, did manufacturers find any significant use for gasoline. From then on, however, the importance of gasoline has increased with automobile use.

The term gasoline refers to a complex mixture of liquid hydrocarbons that condense in a fractionating tower at temperatures between 100° and 400°F (40° and 205°C). The hydrocarbons in this mixture are primarily single- and double-bonded compounds containing five to 12 carbon atoms.

Gasoline that comes directly from a refining tower, known as naphtha or "straight-run" gasoline, was an adequate fuel for the earliest motor vehicles. But as improvements in internal combustion engines were made, problems began to arise. The most serious problem was "knocking."

If a fuel burns too rapidly in an internal combustion engine, it generates a shock wave that makes a "knocking" or "pinging" sound. The shock wave will, over time, also cause damage to the engine. The hydrocarbons that make up straight-run gasolines proved to burn too rapidly for automotive engines developed after 1920.

Early in the development of automotive fuels, engineers adopted a standard for the amount of knocking caused by a fuel and, hence, for the fuel's efficiency. That standard was known as "octane number." To establish a fuel's octane number, it is compared with a very poor fuel (n-heptane), assigned an octane number of zero, and a very good fuel (isooctane), assigned an octane number of 100. The octane number of straight-run gasoline is anywhere from 50 to 70.

As engineers made more improvements in automotive engines after the 1920s, chemists tried to keep pace by developing better fuels. One approach they used was to subject straight-run gasoline (as well as other crude oil fractions) to various treatments that changed the shape of hydrocarbon molecules in the gasoline mixture. One such method, called cracking, involves the heating of straight-run gasoline or another petroleum fraction to high temperatures. The process results in a better fuel from newly-formed hydrocarbon molecules.

Another method for improving the quality of gasoline is catalytic reforming. In this case, the cracking reaction takes place over a catalyst such as copper , platinum, rhodium, or other "noble" metal, or a form of clay known as zeolite. Again, hydrocarbon molecules formed in the fraction are better fuels than straight-run gasoline. Gasoline produced by catalytic cracking or reforming has an octane number of at least eighty.

A very different approach to improving gasoline quality is the use of additives, chemicals added to gasoline to improve the fuel's efficiency. Automotive engineers learned more than 50 years ago that adding as little as two grams of tetraethyl lead , the best-known additive, to one gallon of gasoline raises its octane number by as much as ten points.

Until the 1970s, most gasolines contained tetraethyl lead . Then, concerns began to grow about the release of lead to the environment during the combustion of gasoline. Lead concentrations in urban air had reached a level five to 10 times that of rural air. Residents of countries with few automobiles, such as Nepal, had only one-fifth the lead in their bodies as did residents of nations such as the United States, with many automotive vehicles.

The toxic effects of lead on the human body have been known for centuries, and risks posed by leaded gasoline became a major concern. In addition, leaded gasoline became a problem because it damaged a car's catalytic converter , which reduced air pollutants in exhaust.

Finally, in 1973, the Environmental Protection Agency (EPA) acted on the problem and set a time-scale for the gradual elimination of leaded fuels. According to this schedule, the amount of lead was to be reduced from 2 to 3 grams per gallon (the 1973 average) to 0.5 g/gal by 1979. Ultimately, the additive was to be totally eliminated from all gasoline.

The elimination of leaded fuels has been made possible by the invention of new and safer additives. One of the most popular is methyl-t-butyl ether (MTBE). By 1988 MTBE had become so popular that it was among the 40 most widely produced chemicals in the United States. In 2001, MTBE was asked to be phased out of production in California by 2003.

Yet another approach to improving fuel efficiency is the mixing of gasoline and ethyl or methanol . This product, known as gasohol , has the advantage of high octane rating , lower cost, and reduced emission of pollutants, compared to normal gasoline.

See also Air pollution; Alternative fuels

[David E. Newton ]

RESOURCES

BOOKS

Joesten, M. D., et al. World of Chemistry. Philadelphia: Saunders, 1991.

Lapedes, D. N., ed. McGraw-Hill Encyclopedia of Energy. New York: McGraw-Hill, 1976.

PERIODICALS

"MBTE Growth Limited Despite Lead Phasedown in Gasoline." Chemical & Engineering News (July 15, 1985): 12.

Williams, R. "On the Octane Trail." Technology Illustrated (May 1983): 52–53.

gas·o·line / ˌgasəˈlēn; ˈgasəlēn/ (dated also gas·o·lene) • n. refined petroleum used as fuel for internal combustion engines.