Modern separation involves piping crude oil through hot furnaces. The resulting liquids and vapors are passed into distillation tower so that hydrocarbons can be separated via their boiling point. In the first step separation tower the lightest fractions, including gasoline and liquid petroleum gas (LPG), vaporize and rise to the top of the tower, where they condense back to liquids. Medium weight liquids, including kerosene and diesel oil distillates, stay in the middle. Heavier liquids, called gas oils, separate lower down, while the heaviest fractions with the highest boiling points settle at the bottom.
In the secondary separation processing unit consists of vacuum distillation column. Vacuum distillation is a part of the refining process that helps to produce petroleum products out of the heavier oils left over from atmospheric distillation. Vacuum distillation produces several types of gas oil. These are slightly heavier than middle distillates such as jet fuel, kerosene, and diesel.