Thermal Cracking of Larger Molecules

The cracking of naphtha produces most of the world s ethylene. Naphtha is the crude oil fraction boiling Irom about 32°C to 192°C. The composition of naphtha made from crude oil comprises four basic components linear paraffins, branched paraffins, naphthenes (cyclo-paraffins) and aromatics. The relative amount of these in naphtha is dependent on the source crude oil and varies widely. 

Branched paraffins, as illustrated by 2-methylpentane, in a similar manner leads to three Ci fragments, one C2 fragment, two C3 fragments, one branched C4 and two linear C5 fragments. Branched paraffins have relatively high octane (RON is 83 in the case of 3-methylpentane) and are attractive to refiners for the production of gasoline. The lower level of C2 formation means they are less attractive to petrochemical operations. 

Naphthenes, as illustrated by methyl-cyc/o-pentane, leads to the formation of one Ci fragment and a cyclo-Cs fragment. The other possibilities are linear and branched Ce fragments which can ultimately 

Aromatics, as illustrated by toluene, have very strong bonds in the ring. Furthermore, benzene is more thermodynamically stable than ethylene. The main reaction is cracking of the aromatic-aliphatic bonds to produce benzene and a Ci fragment. These molecules are of little use to cracking operations but the high octane (RON 124 for toluene) makes them very attractive for gasoline production. 

Olefins are not present in naphtha made from crude oil. However, some types of naphtha produced as refinery intermediates by thermal or catalytic cracking processes can contain high levels of olefins. Olefins tend to lead to high fouling rates in pyrolysis crackers and are usually avoided as petrochemical feedstock. 

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