The lighter compounds boil and vaporize at lower temperatures, while the heavier ones do so at higher temperatures. This separation of petroleum into its various fractions takes place in a crude distillation tower. Crude oil is first heated in a furnace. The resulting mixture of hot vapors and liquid is pumped into the closed, vertical distillation tower, which is sometimes as high as 100 feet. As the vapors rise, they cool and condense at various levels where they are caught by a number of horizontal trays.
Middle trays collect components such as light heating oil and diesel fuel.
Heavy fuel oils, asphalt and pitch fractions settle on lower trays. Those vapors that do not condense in the distillation tower are removed from the top as light gases. At each condensation level, the separated fractions are removed from the trays through pipes known as side draws. The heaviest liquid residue is drawn off at the bottom of the tower as reduced crude. The fractions obtained by this distillation process are known as straight-run products.