Hydrocarbons are basically chains of carbon and hydrogen atoms. The number of carbon atoms in the chain and the way that chain is arranged will determine the properties of the hydrocarbon. For example, the average hydrocarbon in diesel fuel has 16 carbon atoms, while the hydrocarbons in gasoline have about half as many. Meanwhile, an eight-carbon hydrocarbon may be either naphtha or gasoline, depending on its molecular structure.
The easiest way to tell one kind of hydrocarbon from another is by its boiling point. Just as water goes from liquid to vapor at approximately 212° Fahrenheit, each type of hydrocarbon changes from liquid to vapor within a specific temperature range. In general, the more carbons in a molecule, the higher the boiling point.
The refining process therefore begins by cleaning or desalting the crude oil and then heating it until only waxy residual hydrocarbons remain in liquid form. The mixed hydrocarbon vapor rises through a distilling column, getting cooler as it goes up. When a hydrocarbon cools below its boiling point, it reverts to liquid form. Stacks of trays collect the liquid hydrocarbons, which have now been sorted into several distinct streams.
Surprisingly simple devices called bubble caps are the keys to how a distilling column works. Each collection tray has a network of raised perforations that allow vapor to rise through the tray but prevent the collected liquid from pouring down to the tray below. A bubble cap fits loosely over each perforation forcing the vapor to pass through the hydrocarbon liquid before it continues its upward journey. Contact with the liquid cools the vapor so that the heavier hydrocarbons become liquid, too.»next