How is petrol for automobiles and kerosene for aircraft engines manufactured? How do we get pure oxygen for use in hospitals? How do you increase alcohol content in fermented beverages*? Distillation is the industrial method of choice for all these products.
When 2 or more molecules in a liquid mixture need separating, their different boiling points are conveniently used as a means of separating the molecules to get pure products. The liquid mixture is vaporized using heat and/or lowering pressure, and subsequently condensed. Separation is achieved due to the difference in vapour pressures of the components at a given temperature. A bi-directional** mass transfer process occurs simultaneously, with the more volatile*** component vaporizing and passing from the liquid to the vapor phase and the less volatile component condensing. The resulting vapour, called the distillate product, is enriched in one component while depleted in the other. The liquid found at the bottom of a distillation column has the highest boiling point, is called the bottom product.
*Fermented Beverages: Examples of fermented beverages consists of wine and beer.
**Bi-directional: Functioning in two different directions.
***Volatile: Easily exaporated at normal temperature.
Figure 1 above shows a typical distillation of crude oil into various grades of fuels for different transportation means. Lighter fractions such petrol is used for automobiles; then comes kerosene for aircraft engines, followed by diesels for automobiles and trains. Heavier fractions are used to make lubricating oils and polishing waxes. This leaves the even more heavier fraction for heating, power generation and ship engines. Lastly, the bitumen residue (bottom product) from crude oil refining is used for road making and roofing.