This process is carried out in a fractionating column, which has a series of trays as shown in the figure. The effect of the superheated steam on the heated crude petroleum is to cause the lighter fractions to rise up the column. When rising, the vapour cools and a certain amount condenses on each tray until the tray is full of liquid to the overflow. Thus, each tray is a little cooler than the one below it, and therefore, lighter and lighter fractions will be present on each tray, as the vapours pass up the column. The temperature is controlled at the bottom of the column by the temperature of the crude oil, and at the top of the column by taking a certain amount of the product as it leaves, condensing it and pumping it back into the top of the column. This is known as the reflux.
A certain amount of material will condense, which has a lower boiling point than the bulk of the liquid on a particular tray. To enable separation of these fractions, the liquid from a selected tray is drawn into a smaller auxiliary column, called a ‘side-stripper'. Here it is treated with steam that causes the lightest fractions to vaporise and pass along with the steam into the main column.
The use of these side-strippers enables kerosene and gas oil to be obtained direct from the plant. Lubricating oil distillate, if such is present, can usually be drawn direct from a tray without the use of a side-stripper, while gasoline leaves the top of the column as a vapour and must be cooled to condense it to liquid gasoline.