Distillation refers to the first step in the refining process. During distillation, crude oil is heated in the base of a distillation tower.
As the temperature increases, the crude's various compounds vaporize in succession at their various boiling points, then rise to prescribed levels within the tower (according to their densities), condense in distillation trays and are drawn off individually for further refining.
Distillation is also used at other points in the refining process to remove impurities.
See also:
"Distillation" is in the HS Code "2707"