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Advan'd A/AS Level Organic Chemistry: Oil fractions - cracking, isomerisation, reforming
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Part 1. ALKANES and the PETROCHEMICAL INDUSTRY 1.3 Modification of alkanes by cracking, isomerisation and reforming How to make better use of the chemical feedstock from the fractional distillation of crude oil, thermal/catalytic cracking, use of zeolite catalysts Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry What is cracking? Why is cracking important? What are the products of cracking? What are the products of cracking used for? What is reforming? Why is reforming important? What are the products of reforming? What are the products of reforming used for? See also 1.5 Modification of hydrocarbon fuel mixtures and alternative fuels Alkanes and Petrochemical Industry INDEX All Advanced Organic Chemistry Notes Index of GCSE/IGCSE Oil - Useful Products Chemistry Revision Notes A basic introduction to the chemistry of alkanes Introduction to CRACKING - a problem of supply and demand and other products. Introduction
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.
(a) Thermal cracking (steam cracking) Thermal cracking is done at high temperatures e.g. 450oC to 900oC and high pressure (up to 7000 kPa, 70 atm = 70 x normal atmospheric pressure), with no catalyst and the absence of air.
(b) Catalytic cracking Catalytic cracking involves passing a fraction like gas oil (C14 - C20 alkanes) over a zeolite catalyst at 450oC to 500oC. The reactor pressure is slightly above normal - very different from thermal cracking.
CRACKING continued Equations and examples of three ways in which hydrocarbons (mainly alkanes) are modified to make them more useful PLEASE READ my basic notes on CRACKING - a problem of supply and demand, other products (I'm NOT repeating them here) Included are several structural formula equation examples of cracking alkanes into alkenes, lower alkanes and hydrogen. I won't repeat all the structural formula equations here, but add I've added more above on the reaction conditions above, and showing the products using skeletal formula equations below. e.g. cracking to produce alkenes (for polymer production) and lower alkanes for fuels to reduce the octane number of petrol, and note the 'theoretical' variety of products possible!
(i) cracking hexane ===> products
(ii) cracking octane ===> products
(iii) Cracking a large alkane molecule to give mainly alkenes (propene and ethene) and hydrogen
(iv) The production of more branched alkanes from decane C10H22
ISOMERISATION (isomerization), in some cases this is the first stage of reforming described in the next section) Converting linear alkane molecules into branched alkane molecules is an example of an isomerisation reaction because the reactant and products have the same molecular formula. Branched alkanes increase the octane number of petrol fuels. The chemical feedstock is usually C4 to C6 alkanes. Typical reaction conditions: Pt/Al2O3 at 150oC Examples of isomerisation reactions - good for making branched isomers. (i) butane ===> methylpropane (all C4H10)
(ii) pentane ===> methylbutane (2-methylbutane) or dimethylpropane (2,2-dimethylpropane, numbers not strictly needed)
(iii) hexane ===> 2-methylpentane or 3-methylpentane
(v) heptane ===> 3-ethylpentane or 2,2-dimethylpentane or 2,3-dimethylpentane
By changing the reaction conditions you can vary, and to some extent change, the ratio of products to select those most desired from an isomerisation reaction.
REFORMING Converting linear alkane molecules into cyclic hydrocarbons (cycloalkanes and aromatic molecules - have a benzene ring). The products have the same number of carbon atoms as the reactants, but less hydrogen atoms and a ring structure. Naphtha is the chemical feedstock (C6 - C10 alkanes) and passed over a Pt/Al2O3 catalyst at 500oC. The hydrogen formed is recycled to minimise 'coking' (thermal decomposition to give a carbon deposit on the catalyst). e.g (i) hexane ===> 1st cyclohexane ===> 2nd benzene
1st
then 2nd
This reforming reaction probably goes through the dehydrogenation sequence ? hexane ==> cyclohexane ==> cyclohexene ==> cyclohexa-1,3-diene ==> benzene
(ii) heptane or 2-methylhexane or 3-methylhexane ===> methylcyclohexane ===> methylbenzene
Extra note on an example of an aromatic disproportionation reaction Disproportionation is said to occur, when a reactant is transformed into two or more dissimilar products. 2 This is not a redox disproportionation reaction, but it does involve two identical molecules reacting together to give two different products with no other reactant or product involved. The reaction is a way of producing a mixture of 1,2-dimethylbenzene, 1,3-dimethylbenzene and 1,4-dimethylbenzene from methylbenzene. If you need less methylbenzene and more benzene or dimethylbenzenes, this is the way to do it.
Alkanes and Petrochemical Industry INDEX All Advanced Organic Chemistry Notes * Index of GCSE/IGCSE Oil - Useful Products Chemistry Revision Notes
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Doc Brown's Chemistry |
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