Craig Freudenrich from HowStuffWorks explains that diesel fuel is made from fractional distillation of crude oil in oil refineries. The crucial elements of the oil refining process are a distillation column with several condensers, a heat source and a purification unit.
Heat crude oil to 600 C
Crude oil contains various substances that have different boiling points. These variations in boiling points allow for separation when heated. When crude oil is heated to 600 C, it evaporates into a tall distillation column that is fitted above the heating tank.
Condense diesel in the column
The distillation column is hot at the bottom and cool at the top. As crude oil evaporates, the various substances condense within the distillation column at different temperatures. Diesel has a boiling range of 200 C to 350 C. When it reaches the height within the column where the temperature is equal to its boiling point, it will condense and form a liquid.
The diesel is purified through hydrodesulfurization: the sulfur is removed from diesel fuel to lower engine emissions. Such diesel is known as ultra-low-sulfur diesel (ULSD). According to new emissions regulations from 2006, all 2007 or later model vehicles in United States must run on ULSD.