The Chemistry of Life: Oil's Many Uses

Editor's Note: This occasional series of articles looks at the vital things in our lives and the chemistry they are made of.

Besides water, there's no liquid that humans rely on more than petroleum. It fuels our vehicles, heats our homes, paves our roads and fills our shopping shelves with innumerable consumer products.

About 85 percent of the petroleum, or crude oil, consumed in the United States goes into fuels, such as gasoline, jet fuel, and home-heating oil, according the most recent data from the Energy Information Administration.

"The liquid fuels are what everyone knows about, but a very large number of other products come from petroleum," said Jean-François Larivé, technical coordinator for Concawe, a European oil company association.

The list of non-fuel compounds that can be extracted from oil is exhaustive: lubricants for cars, asphalt for roads, tars for roofing, waxes for food wrapping, as well as solvents for paints, cosmetics, and dry cleaning products. Petrochemicals also provide the building blocks for a vast panoply of plastics and foams.

Early uses

Petroleum has been used for thousands of years. The ancient Babylonians built walls and towers with asphalt, the Chinese drilled for "rock oil" to provide heating and lighting, and the Byzantines sprayed "Greek fire" as an incendiary weapon.

Many cultures have also employed petroleum as a medicinal cure, giving it names such as "St. Quirinus oil," "Barbados tar" and "Seneca oil." Our modern society continues to use petroleum jelly as a skin ointment.

Petroleum became a significantly valuable commodity in the mid-19th century, when modern techniques of chemical distillation were developed to separate kerosene from crude oil. The refined kerosene could be used in lamps, replacing the more expensive whale oil.

In fact, the first oil boom in the United States — which began exactly 150 years ago in Western Pennsylvania — was sparked by the kerosene market.

However, in 1919, kerosene production was surpassed by that of gasoline, another derivative of petroleum. The fortunes of the petroleum industry have since been propelled largely by the popularity of the automobile. Almost 90 percent of transportation energy comes from petroleum-derived fuels.

Chemical potpourri

Petroleum is not a single molecule but a mix of thousands of molecules, the most important of which are hydrocarbons. These are chains or rings of carbons atoms surrounded by hydrogen atoms.

The bonds between the carbon and hydrogen atoms store a great deal of chemical energy. When a hydrocarbon is burned, these bonds break apart in order to form carbon dioxide and water.

"With hydrocarbons, you can carry a lot of energy in a small volume, which makes them attractive for transportation fuels," Larivé told LiveScience.

Although gasoline comprises nearly half of all petroleum production in the United States, a wide range of fuels and specialty oils come out of a modern-day oil refinery. The petroleum is first heated in a boiler to separate the smaller hydrocarbons with low boiling points from the larger hydrocarbons with high boiling points.

The various distillates can be roughly characterized by the number of carbon atoms in their constituent hydrocarbons:

* Petroleum gas is made up of molecules with 1 to 4 carbons, including methane, ethane, propane and butane. It is used for heating, vehicle fuel (as so-called liquefied petroleum gas), and as chemical feedstock.

* Naphtha is a mixture of molecules in the range of 5 to 9 carbons. It is employed as a solvent in products such as paint and wood polish, or it can be blended into gasoline.

* Gasoline is a combination of molecules with mainly 6 to 11 carbons. Of these, heptane with 7 carbons causes undesirable engine knocking, so refiners try to blend in more smooth-burning octane with 8 carbons.

* Kerosene, with around 10 to 18 carbons per molecule, is now largely used to make jet fuel.

* Diesel is made up of hydrocarbons with roughly 12 to 24 carbons. It can power cars, trains and heavy machinery.

* Lubricating oil comes from heavier molecules with 20 to 50 carbons.

* Fuel oil has roughly 20 to 70 carbons per molecule and finds uses in power stations and in ship boilers.

* Residuals are room-temperature solids made up of molecules with 30 or more carbons. These comprise asphalt, tar and waxes. Another leftover is carbon-rich "coke," which finds uses as fuel or in heat-resistant electrodes.

Refineries do not have to simply content themselves with the mixture of hydrocarbons that Nature provides in petroleum. There are several chemical processes that can change one hydrocarbon into another. For example, "cracking" involves breaking larger hydrocarbons into smaller ones with either heat or catalysts.

Petrochemistry

Besides having a high energy density, the long chains of hydrocarbons are also good building blocks for constructing more complex molecules, such as plastics and pharmaceutical drugs.

"Oil is a large source of ready-made and semi-ready-made molecules," Larivé said.

The two main classes of petrochemicals are olefins and aromatics. These molecules have "unsaturated" carbon bonds that allow them to be hooked together in long chains, called polymers.

The olefins, such as ethylene and propylene, are formed from petroleum gas or from cracking of larger hydrocarbons. Ethylene is used to make polyethylene (e.g. shopping bags) and vinyl (e.g. seats, siding, records). Propylene is used to make polypropylene (e.g. stiff packaging, furniture) and isopropyl (rubbing) alcohol.

The aromatics are ringed hydrocarbons, such as benzene, toluene and xylene. Benzene is used to make polystyrenes (e.g. plastic utensils, Styrofoam). Toluene is used to make polyurethanes (e.g. furniture foams) and nylon (e.g. ropes, stockings). Xylene is used in polyesters (e.g. fabrics).

Oil's future use

For all its usefulness, there is a strong desire to limit our dependence on petroleum. It has a destabilizing influence on global politics, as well as a destabilizing influence on the global climate through the associated carbon dioxide emissions.

Biofuels, such as ethanol and biodiesel, have been attracting a lot of attention, and researchers are working on making plastics from plants and agricultural wastes.

However, alternatives will have to compete against oil's relatively low price and the high-energy density of petroleum-derived fuels. Moreover, current farming practices require a great deal of petroleum to drive tractors and delivery trucks, as well as make some pesticides and fertilizers.

"In all likelihood we'll need oil for quite a long time," Larivé said.