How an oil refinery works
By Marshall Brain
http://www.JewishWorldReview.com | (MCT) Every time you pump gas into your gas tank, you are benefiting from one of the most amazing chemical factories know to humans: the oil refinery. An oil refinery produces gasoline as well as many other substances - everything from dry cleaning fluid to petroleum jelly to road tar comes from an oil refinery. Let's go behind the scenes and see how an oil refinery works.
An oil refinery is made possible by the amazing nature of crude oil. When an oil company pumps crude oil out of the ground, it is pumping a mixture of many different substances. All those substances have one thing in common: They are all made of hydrocarbon chains of different lengths. The goal of an oil refinery is to sort out all the different chains by length.
A hydrocarbon chain is simply a molecule made of hydrogen and carbon. Carbon atoms link together to form the backbone of the chain, and then hydrogen atoms attach to the carbons. The simplest hydrocarbon chain has just one carbon atom along with four hydrogen atoms. This hydrocarbon chain is called methane. Methane gas is so light it floats, like helium. When you use natural gas, you are using methane. Ethane is next, with two carbons in the chain.
Once you get five carbon atoms in a chain, you have a liquid, and the liquids get thicker and thicker as the chains get longer. Gasoline, kerosene, diesel fuel and motor oil are all made from hydrocarbon chains. Gasoline has between seven and 11 carbons in the chain. Kerosene has between 12 and 15 carbons, and so on. Once you get beyond 20 atoms in a chain, you move from liquids to solids: things like petroleum jelly, paraffin wax and finally tar.
An oil refinery sorts out all these different substances using heat. If you heat crude oil up to 600 degrees centigrade, it boils and all the crude oil evaporates. This crude oil steam then flows into the bottom of a tall distillation column. The column is set up so that, at different heights inside the column, there are different temperature zones. As the crude oil steam rises in the column, it cools, and the different hydrocarbon chains condense out into liquids. There are trays at different levels that collect the different liquids and send them through pipes to storage tanks.
If you were to go back 100 years, an oil refinery was this simple. You had a tank that boiled crude oil and a distillation column to collect the different kinds of hydrocarbons. The big problem with this approach is that only 40 percent of a barrel of oil is naturally gasoline. The rest of the barrel naturally contains longer or shorter chains. Many oil refineries today want to make mostly gasoline, so there needs to be a way to change longer and shorter chains into gasoline, either by putting them together or splitting them apart.
Cracking is the process of breaking longer chains apart. Using heat and catalysts, long hydrocarbon chains break down into shorter, gasoline-length chains. So the refinery might take the paraffin wax from a barrel of crude oil and run it through a cracker. Then it can distill the output of the cracker and extract a lot more gasoline.
The other side of the coin is unification. Here, short hydrocarbon chains link together to form longer chains. A catalyst breaks hydrogen atoms off the end of the chain and links chains together.
Using these techniques, an oil refinery can turn almost everything in a barrel of oil into gasoline-length chains. The chains are blended together to get gasoline that performs perfectly in a car engine.
The final step at a refinery is to purify the gasoline. For example, there might be a step that removes sulfur from the fuel. Another step removes any water and nitrogen compounds.
What comes out of the refinery is gasoline ready to go into a pipeline or tanker truck so that it ends up at your local gas station. From there, you pump it into your tank.
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© 2007, How Stuff Works Inc. Distributed by McClatchy-Tribune Information Services.