Colleen Murphy, age 11, of Spokane, Washington, for her question:
How do they make gasoline?
Gasoline was made many millions of years ago, and blended with a large assortment of other ingredients. Nowadays, we extract the small percentage of gasoline from the oily mixture called petroleum. Some of the other ingredients have large molecules that may be cracked, or broken up to yield more gasoline. Others have small molecules that can be joined together to yield still more gasoline.
Petroleum, or crude oil, was not made by human hands. It is the fossil remains of tiny organisms who thrived in the ancient seas, millions of years ago. While they lived, they synthesized a vast assortment of hydrocarbon chemicals from hydrogen and carbon. Their remains fossilized underground, under heat and'pressure. The buried petroleum is a complex blend of hydrocarbons, made from molecules of various shapes and sizes. Some have as many as SO atoms. Some are linked in long chains, other are branched and some are rings.
Gasoline is a mixture of several hydrocarbons. Their different molecules contain from 12 to 20 hydrogen atoms and from six to ten atoms of carbon. The chemicals in this range boil and vaporize near normal temperatures. Those in kerosene have higher boiling points. Heavier lubricating oils require more heat to vaporize. Groups of similar hydrocarbons can be vaporized at different temperatures and fractioned, or separated from petroleum. This is done in a tall fractioning tower with a furnace that pipes in air as hot as 700 degrees Fahrenheit.
Most of the gasoline fraction is a mixture of hexane, heptane and octane. These compounds vaporize quickly and rise to the top of the tower. There they are cooled, liquified and piped away. Fractions of kerosene and heavier oils vaporize at higher temperatures. They are piped off at lower levels. Greases and tacky tars collect in the residue of crude oil at the bottom of the tower.
Some compounds in the gasoline fraction are not suitable for autos. Heptane molecules are straight chains that cause the engine to knock. The best anti knock hydrocarbon is octane. So the gasoline mixture is processed to increase its octane rating. In the hydroforming process, hydrogen is added and the mixture heated with a catalyst. This forms straight molecules into rings. Other processes add branches to straight molecules. The alkylation process molds suitable gasoline molecules from simpler hydrocarbons.
Other highly complex processes are used to improve the quality of the gasoline and to increase the yield. But even with all this, not very much of the crude oil ends up as gasoline. Most of its hydrocarbons are processed for a long list of diversified duties.
For example, the benzene is used in making dyes and detergents, aspirin and glassy resins, synthetic rubber and dozens of other items. Ethelene is used in making rayon, dacron, vinyl and explosives. Propylene .gives us epoxy glues, plexiglass, cellophane and more detergents. The list of products from petroleum is a long one = without counting all the different grades of gasoline.