distillation (fractionation) - the primary refining step, in which crude oil is separated into fractions, or components, in a distillation tower, or pipe still. Heat, usually applied at the bottom of the tower, causes the oil vapors to rise through progressively cooler levels of the tower, where they condense onto plates and are drawn off in order of their respective condensation temperatures, or boiling points - the lighter-weight, lower boiling-point fractions, exiting higher in the tower. The primary fractions, from low to high boiling point, are: hydrocarbon gases (e.g., ethane, propane); naphtha (e.g., gasoline); kerosene, diesel fuel (heating oil); and heavy gas oil for cracking. Heavy materials remaining at the bottom are called the bottoms, or residuum, and include such components as heavy fuel oil (see fuel oil) and asphaltic substances (see asphalt). Those fractions taken in liquid form from any level other than the very top or bottom are called side-stream products; a product, such as propane, removed in vapor form from the top of the distillation tower is called overhead product. Distillation may take place in two stages: first, the lighter fractions - gases, naphtha, and kerosene - are recovered at essentially atmospheric pressure; next, the remaining crude is distilled at reduced pressure in a vacuum tower, causing the heavy lube fractions to distill at much lower temperatures than possible at atmospheric pressure, thus permitting more lube oil to be distilled without the molecular cracking that can occur at excessively high temperatures. See hydro-cracking.