Stages of Oil Refining - Separation

By: Bob Jent

When crude oil is produced from oil reservoirs deep within the earth, it contains a range of molecules that must be separated for production of the many petroleum products we use today. Once oil is extracted from underground oil reservoirs by companies such as , it is transported to refineries, where it undergoes the processes of separation, conversion and finally treatment. Here we examine the basic process of separation, which involves heating crude oil inside a distillation tower to separate components with different properties so that they may be used in various energy sources and products.

Petroleum is composed of hydrocarbon chains of various lengths. Hydrocarbon chains which are longer in length and contain more carbon atoms have higher boiling points, while smaller chains with less carbon atoms have lower boiling points. Separating the components of crude oil involves the process of fractional distillation, which divides molecules according to their different weights and boiling points. Once crude oil is prepared for separation, it is put into the base of a distillation tower with controlled pressure, where it is subjected to heat. Distillation towers are the tall, columnar structures which give refineries their characteristic appearances. As the temperature within a tower increases, the oil begins to physically rise inside the column. The temperature within the tower is lower as the height increases, causing molecules with higher boiling points to remain near the bottom, and those with lower boiling points to separate and rise to various levels along the column. The lightest substances, those with the shortest hydrocarbon chains and fewest carbon atoms, reach their boiling points and rise to the highest, coolest part of the column. As gaseous compounds rise and cool back below their boiling points, they return to liquid form and are collected at various points along the tower.

The result of fractional distillation is the division of molecules according to their unique properties. Gasoline, which is the most demanded petroleum product, rises to the top in gaseous form during separation, where it then cools below its boiling point and returns to a liquid state. Medium weight fractions, which include diesel oil distillates and kerosene, are collected near the center of the tower. The heaviest components, which have the highest boiling points and are thus less affected by the heat inside the column, remain at the bottom of the column.

After crude oil is separated into its individual components, refineries use various conversion methods to add value to the petroleum products. During this process, heavy hydrocarbons can be "cracked" into lighter ones, for which there is greater demand. Finally, the components are treated and refined so that they may be distributed to meet the high demand for petroleum products throughout the world.

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