is 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 sidestream 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.
Also see hydrocracking.
Application
Notes:
