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Behind the barrel: fueling a century

As the wildly swinging price of a barrel of crude oil grabs headlines and stirs fears of higher inflation, some folks might wonder: Just what is a barrel of oil? Physically, it's the unit by which oil is sold, a relic of early days in the industry when drillers filled 42-gallon barrels with crude oil. Nowadays, however, oil is bought and sold in amounts equivalent to thousands of barrels.

Crude oil is as it sounds, a raw, unrefined version of the century's most important fossil fuel.

Before oil is used by businesses or consumers as gasoline or other products, it is refined. That is a process of cooking and distilling that actually turns the 42 gallons of crude into slightly more than 44 gallons of petroleum products, even though nothing is added.

"You do get more product than the crude," said Jim Williams of the American Petroleum Institute, the oil industry's trade group in Washington.

"After you break it up into various gasoline and diesel and stuff, you've rearranged the molecules. It's just rearranged differently. You're just changing the chemical makeup of the crude oil to make it more usable."

The 2.03 gallons that appear through the molecular shuffling is known as the "processing gain."

The refining is done in stages, with the first step occurring as crude oil is heated at the bottom of a "distillation tower."

The oil turns to gas and rises, with various portions of the barrel condensing into liquids at different heights of the

tower. At certain points of the tower, the refinery equipment collects the liquids that can be blended into gasoline. At other levels of the tower come portions of the oil that can be used in diesel and kerosene.

What's left over at the bottom is then put through a similar tower that has a vacuum at the top and takes out portions of the oil that were too heavy to be collected the first time out.

After that stage, the refiner is left with even heavier portions of the oil barrel that need to be broken up before they are used.

"You keep working it down until you can get it into gasoline," Williams said. "The heavier the crude, the more of this you have to do. Not all refiners can do all of these things."

The different steps that must be taken with different barrels of oil are necessary because all oil is not created equal.

Crudes range from light sweet, the most desirable blend, to heavy sour, the least desirable.

A crude's "sweetness" measures its sulfur content. Low-sulfur or "sweet" oil is easier to convert into fuels that meet U.S. environmental standards. Heavy "sour" has a lot of sulfur and takes more refining before it can be used as fuel.

When the refining is done, a little less than half of the barrel, 19.5 gallons, has been converted into finished motor gasoline.

Some of the other products from a barrel are: distillate fuel oil, 8.61 gallons; jet fuel, 4.2 gallons; residual fuel oil, 2.77 gallons; asphalt and road oil, 1.39 gallons; liquified gases, 1.43 gallons; kerosene, 0.25 gallon; and aviation gasoline, 0.08 gallon.