HYDROCARBONS

Petroleum as a source of hydrocarbons by fractional distillation and cracking.

A knowledge of how these processes are carried out, including the names and nature of the fractions obtained, is expected.(Reforming to benzene derivatives is not required in this module.)

Environmental problems associated with spillage and combustion of hydrocarbons, including global warming and ozone depletion.

HYDROCARBONS
(alkanes, alkenes, benzene.)

Sources

1. Petroleum.

Petroleum deposits were formed by the action of pressure and temperature on marine life sediments, mainly from the fatty acid constituents, under the catalytic action of various rocks and acid clays.

Crude petroleum is a complex mixture of gaseous, liquid and solid hydrocarbons such as alkanes , cycloalkanes, aromatics (benzene) and some alkenes. Also present are some compounds of oxygen, nitrogen and sulphur.

It has no uses in its raw form so to provide useful products its components must be partly separated and if necessary modified. The fundamental process of refining is primary distillation.

Refining

The function of an oil refinery is to manufacture from crude oil those quantities of the oil products required by consumers. This is carried out by the use of various physical and chemical processes.

Refinery gas

( 1 - 2 per cent of crude oil) contains hydrocarbons that are gases at normal temperatures. It includes the alkanes with one to four carbon atoms in their molecules, with methane as a major component. The main use of refinery gas is as a gaseous fuel.

Gasoline

( 15 - 30 per cent) has is a complex liquid mixture of hydrocarbons containing mainly C5 - C10 compounds whose boiling points range from 40oCto 180oC. The major use of gasoline is as a fuel in internal combustion engines. A considerable part proportion of this fraction is used to produce chemicals by cracking.

Kerosene

( 10 - 15 per cent) consists mainly of C4 to C 12 hydrocarbons, with boiling point from 162 to 250 degrees C. It is used as a fuel in jet engines and for domestic heating. It can be cracked to produce extra gasoline.

The diesel oil or gas oil

(15 - 20 per cent) containing C 13 - C25 compounds, boils between to 120 - 350 degrees C. It is used in diesel engines where the fuel is ignited by compression instead of by a spark. And also for industrial heating purposes. It can also be cracked to produce extra gasoline.

Residue

(40 - 50 per cent) boils above 350 degrees C and is a highly complex mixture of non-volatile hydrocarbons. Most of it is used as fuel oil in large furnaces such as those in power stations or big ships. Proportion of it is used to make lubricating oils and waxes. Both these contain C 26 - C 40 carbons

Distillation

Crude oil is fractionally distilled to give 4 main fractions.
C1-C4
refinery gases
methane, ethane, propane, butane
C8-C16
light distillates
petrol, aviation fuel, kerosene, benzene
C17-C20
middle distillates
heating oil, diesel, feedstock for cracking
C21+
residue
paraffin wax, lubricating oil, petroleum jelly, bitumen

2. Coal Tar

When coal is carbonised (burnt in the absence of air) one of the products is a viscous black liquid called coal tar. This is distilled into five main fractions. The light oil fraction (boil pt. up to 170oC) is a source of benzene.

Cracking as a source of alkenes and shorter chain alkanes.

Cracking

In this process larger molecules are broken down into smaller ones, either by high temperature and pressure (thermal cracking) or by a catalyst (catalytic cracking).

Thermal Cracking

When alkanes are heated to high temperatures their molecules vibrate strongly enough to break and form smaller molecules. One of these molecules is usually an alkane. Reducing chain length generally results in unsaturation. Such reactions are known as cracking

e.g. C8H18 C5H12 + CH3CH=CH2

octane pentane propene
Thermal cracking is generally used for cracking residues to middle distillates.

Catalytic Cracking

By using a catalyst, cracking can be made to occur at fairly low temperature. This is known as catalytic cracking.

Catalytic cracking is the most important source of petrol and raw materials for the chemical industry. Heavier fractions can be cracked to produce extra gasoline. Cracking tends to produce branched-chain rather than straight-chain alkanes, so the gasoline produced this way has a high octane rating. Processes similar to cracking can be used to convert low-grade gasoline to high grade fuel.

The catalysts are usually natural clays and synthetic alumina/ silica mixtures (Al2O3/SiO2).

Isomerisation

This involves breaking up straight chainalkanes and reassembling them as branched chain isomers.
Both of these processes are important in the production of unleaded gasoline.



Catalytic Reforming

Reforming involves converting straight chain alkanes into ring molecules such as arenes and cycloalkanes.

Benzene C6H6 and other aromatic compounds can be made by passing petrol vapour over a heated platinum catalyst.
500oC/15 atm
C6H14 -,--- C6H6 + 4H2
hexane Pt catalyst benzene

The u.s.a. obtains about half its benzene in this way.