The crude petroleum, obtained by mining, is a dark-coloured viscous liquid having an unpleasant smell due to the presence of sulphur containing compounds. It also contains impurities of sand and brine. Since crude oil is a mixture of hydrocarbons, it cannot be used as such as a fuel or as a base compound for producing other useful substances. The crude oil is subjected to refining by continuous fractional distillation. This continuous fractional distillation of crude oil into different useful fractions and the removal of undesirable impurities is called refining or processing of petroleum, The different fractions, thus, obtained contain hydrocarbons of different chain lengths.
Solids such as sand are separated from the impure crude oil by filtration. The filtrate is allowed to stand in are reservoir for sometime. The oil separates as the upper layer and is then
pumped through pipes to a refinery (such as Tema Oil Refmery in Ghana).
The crude oil is heated to 400°C in coiled pipes in a gas heated furnace and the vapours, thus, obtained are introduced into the fractionating tower. The fractionating tower is made
of steel and is also called bubble tower (shown in Fig. 51.2). The tower is divided into a number of compartments by means of shelves having openings. The openings are covered with
caps known as bubble caps. Each shelf is provided with an overflow pipe which keeps the liquid to a certain level and allows the rest to trickle down to the lower shelf.
The bottom of the fractionating column is at a much higher temperature than the top. As the vapours of the oil rise up the fractionating tower, they become cooler and condense
in different shelves to form different fractions of the liquids which are continuously drawn out. The uncondensed vapours pass out of the tower at the top. The fractions, thus, obtained
consist of mixture of hydrocarbons having close ranges of boiling point. The actual number of fractions and their boiling point ranges depend upon their commercial uses and the
source of petroleum.
Fig. 51.2. Refining of petroleum,
The various components obtained from the fractional distillation of petroleum, from the base upwards are: asphalt, lubricating oil, paraffin wax, fuel oil, diesel, kerosene, gasoline (petrol) and petroleum gas. Except for asphalt, lubricating oil and paraffin wax, all other fractions readily burn and are used as fuels. Asphalt, lubricating oil arid paraffin wax are obtained by further fractionation of residual oil which collects at the bottom of fractionating tower. The major
fractions obtained during fractional distillation along with their uses and composition are given in Table 51.1.
Table 51.1. Typical Fractionation of Petroleum and Uses of Various Fractions
Boiling Range (oC)
1. Petroleum gas
|Below 30°||Gaseous fuel, LPG, production of carbon black, hydrogen and carbon, monoxide (used to make) ammonia and methanol)|
2. Petroleum ether
|30 – 80°||As solvent in dry cleaning|
|80 – 170°||Motor fuel|
|170 – 250°||Household fuel, illuminant fuel, jet engine fuel|
5. Diesel oil
|250 – 350°||Fuel for diesel engines, in cracking|
6. Fuel oil
|350 – 400°||Furnace fuel in industries|
7. Lubricating oils
|above 400°||Lubrication of machinery|
8. Praffin wax
|non – volatile solids||Candles, water-proofing vaseline, fabrics|
||Artificial asphalt, fuel,
Petroleum gas is a mixture of butane, propane and ethane. The main constituent of petroleum gas is butane. Petroleum gas burns readily in the presence of air with a blue flame, producing a large amount of heat. Butane, propane and ethane are gases under ordinary pressure. However, butane can be easily liquefied under pressure. The petroleum gas which has been liquefied under pressure is called Liquefied Petroleum Gas (LPG). Petroleum gas is supplied in liquid form so that a cylinder of even small volume may contain an appreciable amount of the gas. A strong smelling substance (Ethyl mercaptan, C2H5SH is added to LPG gas cylinders to help in the detection of gas leakage. The gas used for domestic cooking is called Liquefied Petroleum Gas (LPG) because it is present in liquid form in the cylinders.
Do You Know? Asphaltenes
The non-volatile tarry residue left after distillation of residue from the fractionating tower contains compounds with large molecules. These are resins and asphaltenes.
Asphaltenes consist primarily of carbon, hydrogen, nitrogen, oxygen and sulphur. Asphaltenes are defined operationally as n-heptane insoluble and toluene soluble component of a carbonaceous material such as crude oil. They constitute the non-volatile, high
molecular mass fraction of petroleum. Moreover, since they are insoluble in heptane, they remain in solid form even in the crude. Asphaltenes are present in dispersed
form in the crude oil. They are stabilised by resins which keep them dispersed in oil.
Asphaltenes create many production problems because of their depositional effect in the production equipment. Asphaltenes in the form of distillation products from
refineries are used as ‘tar-mats’ on roads.
Example 51.1 The boiling points of three components A, B and C of petroleum are 120°C, 70°C and 250°C respectively. if the mixture of three is fractionally distilled, which of the three will be
obtained at the bottom of the distillation column?
Solution. During fractional distillation of a mixture, the component having the highest boiling point is obtained at the bottom of the column. In the given mixture, the component C has the highest boiling point and hence will be obtained at the bottom of the distillation column.
Example 51.2 A mixture contains three components P, Q and R-with boiling points 150°C, 250°C and 350°C respectively. if this mixture is subjected to fractional distillation, in which order would you get these substances?
Solution. During fractional distillation of a mixture, the component having the highest boiling point is obtained at the bottom of the column whereas the component having the least boiling point is obtained at the top of the column. In the given mixture the component R, having the highest boiling point will be obtained at the bottom of the column. The component Q having intermediate boiling point will be obtained in the middle of the column while the component P, having the lowest boiling point, will be obtained at the top of the column. Thus, these substances will be obtained (from bottom to top) in the order R, Q and P.
Activity 51.1 Petroleum fractions are highly inflammable
Collect samples of petroleum ether, gasoline, kerosene oil and diesel oil.
- Study the ignition behaviour of these fractions by burning a little of each fraction.
- Observe the ease with which each fraction ignites and observe the flame.
- Observe the viscous nature of each fraction.
Why are the first fractions such as petroleum ether and gasoline easiest to ignite?
Why do the higher fractions such as kerosene and diesel bum with the smokiest flames?
Suggest a reason why the fractions become increasingly viscous.