Crude oil is a complex mixture of hydrocarbon molecules formed over millions of years when the remains of microscopic sea creatures trapped in sediments were converted by heat and pressure into crude oil

Crude Oil Essay

Crude oil is a complex mixture of hydrocarbon molecules formed over millions of years when the remains of microscopic sea creatures trapped in sediments were converted by heat and pressure into crude oil.

The most abundant of these mixtures are the ones containing hydrogen and carbon only. These hydrocarbons can take on a number of different forms, i.e. alkanes, alkenes and alkynes. They vary in size and shape and viscosity. By manipulating these chains of hydrocarbons you can create any number of useful substances as they are very adaptable. However there are lots of compounds that also contain some of the following; oxygen, sulphur or nitrogen. The purer hydrocarbon compounds are the most required for these produce a higher temperature when they are burned which is the primary use for it as it generates an excess amount of energy when burned and therefore is a useful fuel. The other compounds with sulphur or nitrogen also have a high energy when burned but they have a negative side-effect as they make sulphur dioxide and nitrogen dioxide when burned and these compounds contribute towards acid rain.

Crude oil has to be refined by fractional distillation to separate the short productive hydrocarbon chains from the longer, less useful ones before the compounds can be put to good use.

Fractional distillation is a technique that relies on the difference in boiling points of the chains of hydrocarbons to separate them. It is so specific that it can separate the substances from each other even when there is only a small amount of difference between the boiling points of the substances.

The crude oil is heated and the separate “fractions” of the fractional distillation form as they vaporise and ...

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Crude oil has to be refined by fractional distillation to separate the short productive hydrocarbon chains from the longer, less useful ones before the compounds can be put to good use.

The crude oil is heated and the separate “fractions” of the fractional distillation form as they vaporise and liquefy again. These separate fractions can now be distilled again if necessary to get the useful materials out of the fraction. The fractions are separated as follows:

The crude oil is vaporised by heating it before it reaches the fractionating column
The vaporised crude oil is feed into the bottom of the column.
Steel trays divide the column and the crude oil vapour rises up through holes in the trays.
The temperature gradient between the bottom of the column to the bottom (progressively cooler upwards) means that as the vapour passes through a tray it comes into contact with the cooler liquid on that layer and some of the hydrocarbon chains condense.
This leaves the rest of the vapour to continue rising until they reach their respective levels.

The distillation continues as above until 40 or 50 condenses have taken place then there is enough of each level to remove the separate fractions. In industry there is a careful balance to the amount of crude oil allowed in at the bottom of the tower at a time so that the fractions can be removed repeatedly without interrupting the process.

The fractions are removed in the following order from the bottom (hottest) and the list also shows what can be done with each substance or how it can be used.

Heavy Petroleum Oil

These consist of simple alkanes with no more than 4 carbon atoms in a chain. They are used as a fuel for cooking and heating (for building other molecules). They are more commonly known as methane, ethane, propane and butane.

Naphtha

This is the most important fraction in the petrochemical industry as it can be further processed to make many useful materials. It is made up of between 5 and 10 carbon atom alkanes.

Gasoline

Used for fuelling motor engines. It is a mixture of between 5 and 12 carbon alkanes and also cycloalkanes and is liquid in form.

Kerosine

This liquid fuel is used for jet engines and also tractors. It can also be used to make other useful materials if further processed. Chemically it is made up of alkanes (between 10 and 18 carbons in the chain).

Gas Oil (Diesel)

This is used for diesel fuel and heating oil. It can also be used for making other materials.

It is made up of 12 or more carbon atoms in alkane form.

Lubricating Oil

This is used for motor oil, or grease as it is a thick, viscous liquid which is in the chemical form of 20 to 50 carbon atoms (alkanes, or cycloalkanes).

Lastly come the Residuals which are asphalt, tar and waxes, used for surfacing roads and making other more useful products. They are the compounds with more than 70 atoms in the carbon alkane chains.

The boiling points for these different materials rise the further you go down the list and this is a useful property to be aware of as it help the chemists when making the useful products.

Chemical Processing is what happens to the materials after they have been fractionally distillated. The main way that this is done is by “cracking”. Cracking is defined as the breaking up of larger hydrocarbon chains into smaller pieces / chains.

Then there are two ways in which cracking can be done. The first method is called thermal cracking. It involves heating the large hydrocarbons at high temperatures and high pressures until they break apart. This can be done using high temperature steam at 816°C is used to break ethane, butane and naphtha into ethylene and benzene, which are used to manufacture chemicals.

Or the residual from the distillation tower is heated 482°C, cooled with gas oil and rapidly burned in a distillation tower. This process reduces the viscosity of heavy weight oils and produces tar.

The other method of cracking is called Catalytic Cracking and it uses a catalyst to speed up the cracking reaction. Catalysts include zeolite, aluminium hydrosilicate, and bauxite.

The hydrocarbons (only from the oil fraction) are heated in the same way but the catalyst speeds up the reaction by lowering the boiling point of the substance and allowing high molecular mass alkanes to break into many low molecular mass alkanes and some alkenes. Because the bond breaking happens randomly, many different forms of the substances are made, i.e. cyclo or branched isomers.