Cracking of hydrocarbons
The process of breaking higher hydrocarbons with high boiling points into a variety of lower hydrocarbons that are more volatile (low boiling), is called cracking (or pyrolysis). For example, a higher hydrocarbon C10H22 splits according to the reaction.
C8H18 + C2H4
higher hydrocarbon lower hydrocarbons
The process of cracking, increases the relative amounts of the lower hydrocarbons. During cracking, carbon-carbon bonds get broken in a random manner, leading to various kinds of products being formed.
Cracking is generally carried out in the following ways.
Types of cracking
Breaking down large molecules by heating at high temperature and pressure is termed as thermal cracking. Thermal cracking is further classified into the following classes.
- Liquid phase thermal cracking process: The higher boiling fractions e.g., fuel oil, lubricating oil are converted into low boiling fractions by heating the liquids at a temperature of 750 K, under a pressure of about 10 atmosphere.
- Vapor phase thermal cracking process: Low boiling fraction e.g., kerosene is cracked in the vapor phase at a temperature of about 875 K and under a pressure of 3 atmosphere.
Higher hydrocarbons can also be cracked at lower temperature (600 - 650 K) and lower pressure (2 atm) in the presence of a suitable catalyst. Catalytic cracking produces gasoline of higher octane number and therefore this method is used for obtaining better quality gasoline. A typical catalyst used for this purpose is a mixture of silica (SiO2), 4 parts; alumina (Al2O3), 1 part, and manganese-dioxide (MnO2), 1 part.
Here, higher hydrocarbons are mixed with steam in their vapor phase and heated for a short duration to about 900°C, and cooled rapidly. This process is suitable for obtaining lower unsaturated hydrocarbons.
Applications of cracking
The most important products obtained in straight-run refining are petrol, diesel and kerosene. The demand for these products outstrips that obtained during refining of the petroleum. Conversely, high-boiling fractions find lesser use. So,
- In petroleum industry, the cracking of less useful high boiling fractions is done to increase the yield of low boiling (lower molecular mass) fractions, such as gasoline.
- Cracking always yields low boiling alkenes as the by-products. These unsaturated hydrocarbons are called petrochemicals, that form a variety of useful compounds such as, polyethylene etc.
Reforming or aromatisation
Reforming or aromatisation involves the conversion of open chain (aliphatic) hydrocarbons and/or cycloalkanes in the presence of a catalyst, into aromatic hydrocarbons (arenes) containing the same number of carbon atoms. Aromatisation involves reactions of the type, dehydrogenation, cyclisation, and isomerisation.
In reforming (or aromatisation), cyclic and acyclic alkanes containing six to eight carbon atoms are heated at about 670 K in the presence of palladium, platinum or nickel as catalyst. Platinum seems to be the best catalyst and so the process is sometimes called platforming.
Hexane when passed over Cr2O3 supported over alumina at 670 K benzene is produced.
Under similar conditions, n-heptane yields toluene.
Cyclohexane gives benzene
Applications of reforming
- Production of aromatic hydrocarbons (arenes) like benzene, toluene and xylenes. Earlier the bulk of arenes were obtained from coal tar, but now they are derived from petroleum by the process of reforming.
- Increasing the octane number of straight-run gasoline by increasing the proportion of aromatic hydrocarbons in gasoline.
Aromatic hydrocarbons have octane numbers higher than 100, while the straight-run gasoline has the octane number of 55.The straight-run gasoline (octane number 55) is first distilled to obtain a fraction containing n-hexane, cyclohexane and methylcyclopentane. This fraction is then heated with a platinum catalyst to convert the cyclohexanes into benzene and toluene (octane number > 100).