What is fractional distillation used to separate

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The basic process of oil refurbishment is distillation. The oil is broken down into its various components depending on its boiling behavior. This process is known as "fractionating" or "cutting". The sub-areas are accordingly referred to as "fractions" or "cuts".

The following distillation systems are usually used in a refinery:

  • Crude oil distillation (atmospheric distillation and vacuum distillation)
  • Liquefied gas separation
  • De-isopentanization

Crude Oil Distillation Plant

Fractionation (fractional distillation) takes place in the crude oil distillation plant.

The crude oil is pumped from a storage tank via a desalinator and heat exchanger into a tube furnace. It is heated to 350-400 in the heat exchanger. The vapor-liquid mixture is then distilled into the first tower at atmospheric pressure. The vapor-liquid mixture rises in the distillation tower and cools down. Depending on the boiling point, the various parts of the oil condense on the soil and are then discharged. At temperatures above 400, many of the hydrocarbons in the crude oil break down. In order to avoid this undesirable effect here, only about 350-400 is heated. The unevaporated, liquid residue at the bottom of the atmospheric distillation tower is therefore passed into a second column which operates under reduced pressure (vacuum). Under reduced pressure, liquids start to boil even at lower temperatures. The liquid residue of the atmospheric distillation is evaporated and rises while cooling in the vacuum tower. Here, too, fractions are derived.

Deepening: rectification

Deepening: Desalinator

Illustrative: Typical output of a crude oil distillation plant

Atmospheric crude oil distillation

The atmospheric crude oil distillation takes place at normal pressure. The following fractions are generally obtained:

  • Gases and liquefied gases (approx. Up to 35): The lightest proportions of crude oil (methane, ethane) and the liquefied gases (propane, butane) flow through the column as a gas and are drawn off at the top of the column. They are then separated by liquid gas separation systems (see deepening).
  • Raw gasoline / naphtha (approx. 35-180): The gasoline is taken from the upper part of the column. The gasoline is divided into light gasoline (approx. 35-100) and heavy gasoline (approx. 100-180).
  • Middle distillates: The middle distillates are taken from the side of the column. They consist of petroleum / kerosene (180-250), the light gas oils (250-350) and the heavy gas oils (350-400).
  • Atmospheric residue (> ~ 350): The portion of the product that is not distilled under normal pressure is called the atmospheric residue. This is the feedstock for vacuum distillation.

Specialization: Liquid gas separation systems

Deepening: De-Isopentaniser

Vacuum distillation

The residue from the atmospheric distillation is distilled at about 50 mbar. The following fractions are created:

  • Vacuum gas oil (350-450): Light product of vacuum distillation. The properties are similar to those of the heavy gas oil from atmospheric distillation. It is generally mixed with heavy gas oil.
  • Wax distillates (450-550): These fractions are the starting product for the production of lubricants or feedstock for conversion systems.
  • Vacuum residue: preliminary product for the production of bitumen (road construction, roofing felt, insulation, etc.) or components of the heavy fuel oil.