Atmospheric distillation

Also known as: atmospheric tower, pipe still, crude unit, primary distillation, crude distillation unit, CDU, fractional distillation

Atmospheric distillation is the first and most fundamental step in the the refining process. The primary purpose of the atmospheric distillation tower is to separate crude oil into its components (or distillation cuts, distillation fractions) for further processing by other processing units.

Atmospheric distillation typically sets the capacity limit for the entire refinery. All crude oil processed must first go through atmospheric distillation. Also atmospheric distillation typically provides most of the feed for the other process units in the refinery.

In most refineries the bottoms from the atmospheric distillation tower will be sent to the vacuum tower for further separation.

The design and operation of the atmospheric distillation tower will limit the type of crude that the refinery can process, further limiting the volume and quality of feed to other process units.

How it works

Crude oil is first heated to about 700-750 degrees F (400C). The heated crude is injected into the lower part of the distillation column where much of it vaporizes. As the vapors rise through the tower they pass through a series of perforated trays or structured packing. As the vapors cool, their components will condense back into liquid at different levels in the tower based on their boiling point. A portion of the vapors reaches the top of the column where is it cooled through heat exchangers and air coolers and partly converted back into liquid, a portion of which is fed back into the distillation column as a reflux stream to contact with the rising vapors, helping to cool them. This effect of counter-current flows of rising vapors meeting falling cooler liquids allows equilibrium conditions to be established throughout the column. The lighter (less-dense) hydrocarbons will condense at higher points in the distillation tower, heavier hydrocarbons will condenser lower down. This results in separation of the hydrocarbons based on the different temperatures at which they boil/condense. Hydrocarbons are drawn off of the tower at different heights to get a set of streams of different boiling points. These different streams are called distillation cuts or fractions. These individual streams are then sent to other units for further processing or to finished product blending.

The heaviest fractions of the crude do not vaporize and are drawn off at the bottom of the tower as atmospheric bottoms. These are sent to the vacuum distillation for further fractionation under a vacuum.

Crude with high salt content will typically be processed through a before going to distillation, to remove salts that could cause corrosion in the distillation tower.


The primary input to a atmospheric distillation unit is crude oil. This can be a single crude oil or a mixture of several different crude oil grades.

A refinery may also re-process a mixture of unfinished or off-spec products (called slops) by blending with fresh crude oil and sending the blend to the distillation unit.


The outputs from distillation are distillation cuts. Typically, a crude distillation unit will have the following distillation cuts:

For a specific crude oil grade, the typical volumes and qualities of these distillation cuts are detailed in the crude assay.

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