The separation or fractionation of crude oil into separate components or groups of components known as fractions in an distillation column called a (p. 90) fractionator operating at atmospheric pressure (see Fig. 16). It is the first step in the petrochemical refinery process in which the crude oil is first desalted and heated to around 370ºC in a furnace. It is then separated into fractions according to their boiling points such as butanes, unstabilized naphtha, heavy naphtha, kerosene, and topped crude. This allows subsequent processing units to have feedstocks that meet particular specifications. Vacuum distillation is used to separate the heavier portion of the crude oil into fractions that would require higher temperatures to vaporize it at atmospheric pressure and cause cracking to occur. The fractionator consists of a tall cylindrical column containing perforated trays upon which liquid sits and is in intimate contact with vapour rising up the column through the perforations. Unlike conventional distillation, the column does not feature a reboiler. Instead, steam is introduced below the bottom tray to strip out any remaining gas oil and to reduce the partial pressure of the hydrocarbons. Reflux at the top of the column is provided by condensing the overhead vapour and returning a portion to the top. Side cut strippers are used to withdraw a liquid sidestream, which contains lower boiling point components, known as light ends. Steam is added and the vapour vented back into a higher position in the column.

Fig. 16