Why is hexane reformed and isomerised

The isomerisation of these compounds to C 6 structures and the aromatisation of the latter are the most important reactions contributing to an increase in octane number of a feedstock. Under the conditions of platinum reforming, cracking competes with dehydrogenation reactions. Various oil companies have developed a number of commercial reforming processes employing platinum catalysts.

The most important of these are described below. The Universal Oil Products Company introduced in the first platinum reforming process, termed Platforming.

Today, although several other processes exist, Platforming still accounts for the greater part of platinum reforming operations. It employs a catalyst consisting of platinum on alumina with a small but critical percentage of halogen added to maintain correct balance between conversion of paraffins by dehydrocyclisation, hydrocracking and isomerisation.

Platforming raises the research octane numbers of feed napthas from 30 to 50 to over 90 with 85 to 95 per cent yields. Catalyst life may exceed bbl. Ultraforming was developed in by the Standard Oil Company Indiana and is a fixed-bed cyclic regenerative process. It employs a 0. The relatively high rate of decrease in catalyst activity necessitates frequent regeneration—up to 30 to 40 regenerations are possible with a total catalyst life of to bbl.

It is a fixed-bed regenerative system producing a reformate of octane number and over. The conversion of paraffins is more complete at low pressures and high temperatures, which are conditions also favourable to the formation of aromatics. Houdriforming was introduced by the Houdry Process Corporation, Philadelphia, in and is a fixed-bed reforming process employing a platinum-alumina catalyst. A desulphurised feed is generally used and high conversion efficiencies are obtained yielding a reformate of research octane number exceeding The Penex process was introduced by Universal Oil Products as a means of isomerising the unchanged n-pentane and n-hexane obtained from platformates.

By isomerisation the octane number of an n-pentane feed may be raised from 62 to 93, that of n-hexane from 25 to Complete isomerisation is not possible, but the high activity of the catalyst allows conversion at lower temperatures which favour the equilibrium. This process, developed by M. Kellogg Company, is also specifically for the isomerisation of n -pentanes and n -hexanes. The Butamer process was developed by Universal Oil Products for the isomerisation of n -butane to iso-butane over a platinum-containing catalyst.

Over 40 per cent of the feed n -butane is converted to iso-butane in a single pass. Aromatics for the chemical industry had been produced during the first half of this century primarily from coal. 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.

The reactor pressure is slightly above normal - very different from thermal cracking. Zeolites are minerals composed of alumino-silicates Al, Si, O , found naturally or manufactured to have specific catalytic properties. The zeolite is employed as a fluidised bed of solid particles fine particles can flow like a liquid mixed in with the vapourised hydrocarbons that pass up the 'catalytic cracker', a tall reaction vessel called a riser. The zeolite gets coated in carbon from hydrocarbon decomposition, so it is collected and the carbon burned off, this regenerates the zeolite catalyst and the heat is used to heat up the catalyst which then heats up the incoming hydrocarbon feedstock, so reducing thermal energy costs.

Catalytic cracking is more efficient than thermal cracking and doesn't need as higher temperature or pressure. Catalytic cracking produces more branched alkanes, cyclic alkanes and cyclic aromatic compounds with a benzene ring as well as some alkenes too.

You can minimise the yield of unsaturated alkenes, but some will still be produced. Extra note on the zeolite catalysts Zeolites can also act as molecular sieves - which tells you exactly how small the pores can be! Unbranched hydrocarbon molecules can enter the pores but branched molecules cannot - they have a more 'bulky' cross-section! Therefore zeolites allow the separation of branched and unbranched hydrocarbons - two sub-types of raw materials.

This double explosion produces knocking in the engine. Octane ratings are based on a scale on which heptane is given a rating of 0, and 2,2,4-trimethylpentane an isomer of octane a rating of In order to raise the octane rating of the molecules found in petrol gasoline and so make the petrol burn better in modern engines, the oil industry rearranges straight chain molecules into their isomers with branched chains. It is used particularly to change straight chains containing 5 or 6 carbon atoms into their branched isomers.

Reforming is another process used to improve the octane rating of hydrocarbons to be used in petrol, but is also a useful source of aromatic compounds for the chemical industry. Aromatic compounds are ones based on a benzene ring. Reforming uses a platinum catalyst suspended on aluminium oxide together with various promoters to make the catalyst more efficient.

Isomerisation reactions occur as above but, in addition, chain molecules get converted into rings with the loss of hydrogen.

Hexane, for example, gets converted into benzene, and heptane into methylbenzene.