There are too many larger hydrocarbons to be used as petrol or diesel, they are mainly alkanes and there are no alkenes for polymer production and other important products derived from unsaturated hydrocarbons. Therefore the secondary processes in the petrochemical industry address See basic discussion for the need of cracking see CRACKING - a problem of supply and demand, other products As well as cracking to produce alkenes and lower alkanes, this advanced discussion below does include isomerisation and reforming which are described in detail below.
There are two main types of CRACKING Cracking is essentially the thermal decomposition of hydrocarbons such as alkanes into molecules of smaller carbon number, namely lower alkanes as chemical feedstock for other processes and alkenes for polymer production. Cracking does involve breaking a strong C-C bond in the alkane to produce smaller molecules.
You therefore need to employ high reaction temperatures and catalysts to effect the decomposition efficiently. Cracking and reforming reactions are quite varied in their products e. Hydrogen is also produced in some reactions. Branched and cyclic alkane and aromatic hydrocarbon compounds are important components in petrol mixtures designed to produce the cleanest most efficient burning, with good antiknock properties, particularly as lead tetraethyl is now banned.
The hydrogen can be used in other chemical processes e. Haber synthesis of ammonia and hydrogenating vegetable oils to make margarine - nothing wasted, all helps the economics of the petrochemical industry. Naphtha C 6 -C 10 and kerosene C 10 - C 16 are the chemical feedstocks and the vaporised hydrocarbons are exposed to the high temperature for just a short time. Steam is added as a diluent to prevent 'coking' carbon deposit on reactor surface. Reaction conditions can be set to maximise alkene production - remember, alkenes are NOT found in oil but one of the most important chemical feedstocks for polymer production and other organic industrial products.
The alkanes are only heated for a few seconds at these high temperatures, otherwise the hydrocarbons will break down into carbon soot and hydrogen.
Typical products are ethene, ethane, propene, propane and C 4 - C 5 alkanes and alkenes. The products of thermal cracking depend on conditions i. At lower temperatures the alkane carbon atom chain breaks nearer the middle of the molecule. This produces a higher proportion of medium sized straight chain alkanes and alkenes, all of which are important raw chemicals feedstock for the chemical industry.
This gives less ethene and more high grade petrol. At higher temperatures the alkane carbon atom chain breaks nearer the end of the molecule.
This produces a higher proportion of smaller alkenes like ethene and propene. The products from thermal cracking are separated by fractional distillation. During catalytic reforming, gasoline fractions obtained from the distillation of crude oil are being passed at ca. Therefore, the process is also called plate-reforming.
Three processes mainly increase the octane number. Fragmentation into shorter alkanes cracking and dehydrogenation to olefins are side reactions observed during catalytic reforming. The formation of olefins, especially of polyolefins , is undesirable because they reduce the stability of fuels towards oxidation and have a tendency to polymerize.
The formation of undesirable olefinic compounds is prevented by adding hydrogen to the mixture of alkanes thereby re-hydrogenating the olefins to the corresponding alkanes.
The feedstock is a mixture of the naphtha or gasoline fractions and hydrogen. The hydrogen is there to help prevent the formation of carbon by decomposition of the hydrocarbons at the high temperatures used. The carbon would otherwise contaminate the catalyst. A typical catalyst is a mixture of platinum and aluminium oxide. With a platinum catalyst, the process is sometimes described as "platforming". Methylbenzene is much less commercially valuable than benzene.
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