Fractional Distillation

Alkanes and alkenes are two homologous series of hydrocarbons, made up of hydrogen and carbon only. Alkanes are mainly found in crude oil and natural gas, found in underground reservoirs.

Crude oil was formed over millions of years from the remains of tiny aquatic plants and animals that lived in ancient seas. Crude oil, or petroleum, owes its existence largely to one-celled marine organisms. As these organisms died, they sank to the sea bed. Usually buried with sand and mud, they formed an organic-rich layer that eventually turned to sedimentary rock. The process repeated itself, one layer covering another.

Then, over millions of years, the seas withdrew. In lakes and inland seas, a similar process took place with deposits formed of non-marine vegetation.

In some cases, the deposits that formed sedimentary rock didn't contain enough oxygen to completely decompose the organic material. Bacteria broke down the trapped and preserved residue, molecule by molecule, into substances rich in hydrogen and carbon. Increased pressure and heat from the weight of the layers above then caused a partial distillation of the organic remnants, transforming them, ever so slowly, into crude oil and natural gas.

Although various types of hydrocarbons - molecules made of hydrogen and carbon atoms - form the basis of all petroleum, they differ in their configurations. The carbon atoms may be linked in a ring or a chain, each with a full or partial complement of hydrogen atoms. Some hydrocarbons combine easily with other materials, and some resist such bonding.

The number of carbon atoms determines the oil's relative "weight" or density. Gases generally have one to four carbon atoms, while heavy oils and waxes may have 50, and asphalts, hundreds.

Hydrocarbons also differ ...

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The number of carbon atoms determines the oil's relative "weight" or density. Gases generally have one to four carbon atoms, while heavy oils and waxes may have 50, and asphalts, hundreds.

Hydrocarbons also differ in their boiling temperatures - a key fact for refiners who separate the different components of crude oil by weight and boiling point. Gases, the lightest hydrocarbons, boil below atmospheric temperature. Crude oil components used to make gasoline boil in the range of 55 to 400 degrees Fahrenheit. Those used for jet fuel boil in the range of 300 to 550 degrees, and those for diesel, at about 700 degrees.

The bigger the hydrocarbon chain, the less flammable it becomes. Provided the combustion is complete, all the hydrocarbons will burn with a blue flame. However, combustion tends to be less complete as the number of carbon atoms in the molecules rises. That means that the bigger the hydrocarbon, the more likely you are to get a yellow, smoky flame.

Crude oil is a mixture of hundreds of different hydrocarbons, most of which are alkanes. It is a complex mixture of different fractions, each consisting of a mixture of hydrocarbons boiling within a temperature range. Since crude oil is a mixture, each component boils at different temperatures and hence a boiling temperature range emerges for each fraction. The boiling point of a fraction increases with the number of carbon atoms, due to stronger dispersion forces.

At a fixed temperature, the total vapour pressure above a liquid mixture depends on the vapour pressure of each liquid component and the proportion of each liquid within the mixture. The total vapour pressure of a mixture of two liquids varies linearly with the composition of the mixture. The liquid boils when the total vapour pressure equals the external atmospheric pressure.

If you have a liquid mixture of 1 mol of 2 substances, the vapour pressure of both liquids increase with temperature. The vapour pressure of the more volatile component will increase more. The vapour above the boiling liquid mixture becomes richer in the more volatile component. If you condense the vapour, a liquid richer in the more volatile component is produced. If the evaporation and condensation process is repeated, then the liquid mixture can be separate into its two intial components. This occurs in a fractionating column.

In fractional distillation in a lab, the rising vapour and the falling liquid meet and mix on the surface of the glass beads that are present in the fractionating column.

However, in an industrial-scale fractionating column, glass beads are not used and bubble caps are using instead. They allow the falling liquid to equilibrate fully with the rising vapour. The similarity between both columns is that they are cooler at the top and hotter at the bottom, so a temperature gradient is present. The cooler temperature at the top ensures that the vapour can condense.