Fractional distillation of crude oil

Fractional distillation of crude oil is the first step in the production of many of the materials we have come to rely on in modern life.

All our fossil fuels, virtually all our plastics, detergents and commercial alcohols are made from products of this process. Appreciating the process of fractional distillation will further improve our understanding of the sources of fuels and plastics and the limited nature of their availability.

From the Oil Field to the Refinery

The first step is the transport of the crude oil from its natural location to the refinery. Oil drilling occurs both at sea and on land, depending on the size and profitability of the oil deposits located. Fossil fuel formation is the result of environmental conditions in the distant past and the geological processes that have occured since the laying down of the original organic matter from which the oil is formed.

Once obtained from the ground, the oil is transported by ship, truck or pipeline to the refinery.

At the Refinery

Once the oil reaches the refinery the work to separate it into useful products begins. Oil refineries are enormously complex and each part of the distilled oil goes through several stages of processing. However, the very first step is to break up the crude oil.

The crude oil is a mixture of many different chemicals. The majority of these are hydrocarbons, which are molecules made only from the element Carbon and the element Hydrogen. The mixture of hydrocarbons contains both alkane and alkene molecules and the length of the chains vary wildly, from five Carbon atoms long to 60 Carbons or more. Since fuels need to be very specific in terms of the length of the Carbon chain, the different lengths need to be separated. These different length chains are called FRACTIONS.

The boiling point of a Hydrocarbon fraction, which is the temperature at which it evaporates, is dependent on the length of the Carbon chain. Those fractions with shorter chains evaporate more easily than those with longer chains. This explains why petrol, which is mainly made of the 8-Carbon molecule octane, evaporates more easily than engine oil which has carbon chains in the range of 20 or more.

Fractional Distillation Of Crude Oil

In order to separate the different length chains in the crude mix, it is heated to a very high temperature. The temperature is set so that all those fractions with a Carbon chain length of 20 and below are evaporated from the crude mix. The temperature cannot be set higher than this as there is a risk that the lighter fractions will ignite.

The remaining liquid, which is composed of only the heavier fractions, passes to a second location where it is heated to a similar temperature, but at lower pressure. This has the effect of making the heavy Hydrocarbon fractions more likely to evaporate.

How the Distillation Tower Works

The way the Distillation Tower works is by becoming progressively cooler from the base to the top. All the Hydrocarbon fractions start off in gas form, as they have been heated to that point. The gases then rise up the tower.

The gas mixture then encounters a barrier through which there are are only openings into the bubble caps. The gas mixture is then forced to go through a liquid before continuing upwards. The liquid in the first tray is at a cool enough temperature to get the heaviest gas fractions to condense into liquid form, while the lighter fractions stay gaseous.

In this way the heaviest hydrocarbon fractions are separated out from the mixed gas. The remaining gas continues its journey up the tower until it reaches another barrier. Here the bubble cap process is repeated but at a lower temperature than before, which then filters out the next lightest set of fractions.

This process continues until only the very lightest fractions, those of 1 to 4 Carbon atoms, are left. These stay in gas form and are collected at the top of the tower.

The separation of the heavier elements in the second tower follows exactly the same process but at lower pressure.

After the Fractional Distillation Of Crude Oil

The separated fractions still contain a mixture of different hydrocarbons. After their initial separation the fractions require further processing and purification. Treatment of the initial products of the fractional distillation of crude oil also occurs in the refinery. The results of these processes are the products we use in every day life.