Petroleum is thought to have originated millions of years ago from the remains of aquatic plants and animals that settled on ancient sea floors. As sands swept over the deposits, they underwent chemical transformations, leading to the build up of hydrocarbons and other essential components of crude oil.
Crude oil is extracted with drilling equipment that can go as far as five miles under the ocean floor. The oil frequently comes to the surface with large volumes of gas. The gas is separated from the oil and processed to remove unwanted liquids.
Crude oils are found in a variety of types, ranging from light-colored oils to black, nearly solid asphalts. These are highly complex mixtures containing many hydrocarbons, ranging from methane to compounds containing fifty or more carbon atoms.
The first stage of petroleum refining. Separation is the process of dividing crude oil into some of its fractions. Additional processes may be undertaken for further separation, including Solvent Extraction and Crystallisation.
This is based on the idea that different parts of the oil will boil at different temperatures. For example petrol boils or vaporises at about 24 degrees Celsius, but some heavy oils have boiling points higher than 320 degrees Celsius. Refineries turn the useful fractions into a vapour which is then siphoned from the base oil.
This is a secondary way to separate the parts of the oil and involves the addition of a chemical to dissolve unwanted substances. The main solvents used are benzene, furfural and phenol. Many refineries improve the quality of lubricating oils by solvent extraction.
This is used chiefly to remove wax and other semi-solid substances from heavy fractions. The fractions are cooled to a temperature, which causes them to form crystals or solidify. They are then put through a filter that separates the solid particles.
Although nearly all the elements of crude oil can be converted into useful products, some fractions have more value than others. Petrol, for example, accounts for nearly half the petroleum products used in most countries.
To increase the yield of desirable products from petroleum, including lubricants, scientists have developed several methods to convert less useful fractions from those that are more in demand. These fall into two main groups. (1) Cracking Processes and (2) Combining Processes.
This converts heavy fractions into lighter ones, mainly petrol. These processes not only increase the quantity of petrol obtained from oil but also improve its quality.
This does the opposite of cracking. It combines simple hydrocarbons to form more complex fractions. As a result, many of the gases produced by distillation and cracking are converted into liquid fuels and valuable chemicals.
Nearly all fractions are chemically treated before they are sent to consumers. The method of treatment depends of the type of crude oils and the intended use of the petroleum. These treatments remove impurities such as sulphur compounds, which damage the machinery and pollute the air when burned.
Hydrogen is widely used to remove sulphur compounds. Fractions are mixed with hydrogen, heated and then exposed to a catalyst. The sulphur in the fractions combines with the hydrogen, forming hydrogen sulphide, which is later removed by a solvent.