Petroleum Fractional Distillation: The Topping Process Explained

The Topping Process in Petroleum Refining

The oil is heated to 350°C and sent to a fractionating tower, a metal structure 50 meters high, containing many bubbling trays. A bubbling tray is a perforated plate mounted horizontally; each hole features a small tube with a cap. This design forces hot gases within the tower to pass through the cooler liquid retained by the plates. Once the liquid level rises, it spills over and falls to the plate below.

The temperature in the fractionating tower is progressively graded from 350°C at its base to less than 100°C at its head. During continuous operation, hot oil enters the tower while specific fractions are extracted at convenient heights. These fractions correspond to distinct characteristics, though their relative proportions depend on the quality of the crude oil, the size of the tower, and other technical specifications.

Distillery Gases and Liquid Fractions

Gases emerge from the top of the tower. This distillery gas is treated similarly to dry reservoir gas and is attached to natural gas, while the liquid is sold as supergas or in canisters. The three most important liquid fractions, ordered from top to bottom (low to high distillation temperature), are:

• Naphtha: Very light fractions (density = 0.75 g/ml) with a low distillation temperature of less than 175°C. They are composed of hydrocarbons with 5 to 12 carbon atoms.
• Kerosene: Distilled between 175°C and 275°C, with medium density (density = 0.8 g/ml). Its components are hydrocarbons with 12 to 18 carbon atoms.
• Gas Oil: A thick, oily liquid (density = 0.9 g/ml) that distills between 275°C and 325°C. These oils contain more than 18 carbon atoms.

Residue Processing and Vacuum Distillation

A residue remains after distillation: fuel oil, extracted from the base of the tower. It is a viscous black liquid with an excellent calorific value of 10,000 cal/g. It is commonly used as fuel in thermal power plants, ships, and cement or glass manufacturing.

Alternatively, this residue undergoes a second fractional distillation known as conservative distillation or vacuum distillation, performed at very low pressure (a few millimeters of mercury). Using similar fractionation towers, new fractions are separated, including light, medium, and heavy lubricating oils based on their density and distillation temperature. The final residue is bitumen, which cannot be further split.