Fractional distillation is the process in which the alkanes in petroleum can be partly separated. This is due to the petroleum having different boiling points. The process starts off with the heating of crude oil for it to vaporize. The vaporized crude oil is then fed into the bottom of the distillation tower. The resulting vapor rises through a vertical column. As the gases rises through the distillation tower, its temperature decreases. Thus, certain hydrocarbons begin to condense and run off at varying levels. Each level-specific condensed fraction contains hydrocarbon molecules that has a similar number of carbon atoms. These "cuts" of boiling point allow numerous hydrocarbons to be separated out in a single process. The cooling of the tower height allows for the separation. Subsequent to the refinement, individual fuels may undergo more refinement to remove existing contaminants or substances so as to improve the quality of the fuel through cracking.