How is crude oil refined? Crude oil is a complex mixture of hydrocarbons and its high value has made it to be referred to as black gold. For crude oil to become useful, it needs to be separated into its various hydrocarbon component via a process known as industrial fractionation or more commonly known as fractional distillation which is based on the relative boiling point of these fractions and each of them can be identified by their odor, color, volatility, texture and flammability.
In industrial fractionation, the distillation process is carried out in huge fractionating columns or towers, each standing some 40 to 50 meters high with a diameter of 7meters. The crude oil is preheated to about 400C in an electric furnace where about 75% of the crude oil is vaporized and the mixture of hot vapor and liquid flows through pipes to the fractionating column. The fractionating column is divided into several compartments by perforated plates called trays, each of which is maintained within a specific range of temperature. Each tray is a little cooler than the one below, so that towards the bottom of the column, the temperature is around 400C, while the temperature at the top part of the column is only 40C.
As the vapor containing a mixture of substances of substances ascends the column, it cools, condenses and separates out into several fractions according to the volatility of the substances in the mixture. This means that substances with higher boiling points separate out in trays on the lower part of the column, while those with lower boiling points separate out on the trays in the upper part of the column. The different fractions are drawn off at different heights on the column, redistilled to improve purity and then further treated to obtain different fuels and petrochemicals.
Treatment of Crude Oil Fractions
Hydrocarbons obtained from the fractional distillation of crude oil range from those with one carbon atom like methane to those with more than sixty carbon atoms. It is only the low molecular mass and low boiling fractions, which make up 49% of the fractions that are useful as fuels and raw materials in the chemical industry. It is economical to convert some of the higher boiling hydrocarbon fractions to lower boiling ones. This can be achieved by a process known as cracking. Cracking is the breaking up of bigger hydrocarbons into smaller hydrocarbons. The main methods used are thermal cracking and catalytic cracking.