Naphthene isomerization

In addition to the paraffin isomerization processes, naphthene isomerization also proved useful during the war in the manufacture of toluene. In making toluene by the Shell dehydrogenation process, good yields depend upon raising the methylcyclohexane content of the feed by isomerizing the dimethylcyclopentanes. This isomerization reaction was used commercially at one refinery in the Midwest and one on the Pacific Coast. 

Naphthene isomerization has been applied also to the conversion of methylcyclopentane to cyclohexane for subsequent dehydrogenation to benzene (24). 

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The catalysts used for isomerization of Cg aromatics contain an acidic function to perform xylene isomerization and naphthene isomerization for EB conversion to xylenes. Relatively high metal activity is needed to maintain the naphthene/ aromatic equilibrium that allows isomerization of EB. For conversion of EB by dealkylation, an acidic function is required along with metal activity capable of capturing and hydrogenating the ethylene by-product before it can re-alkylate another aromatic ring. 

The octane number improvement obtained by isomerization of paraffin hydrocarbons is not great since the amounts of the more highly branched paraffins formed at equilibrium are small at the temperatures employed in catalytic reforming (5). Naphthene isomerization, on the other hand, plays a more important role in reforming. In most naphthas about 50% of the naphthene hydrocarbons are of the cyclopentane type (4) so that in order to obtain the maximum aromatic formation, isomerization of these rings to cyclohexane rings must be promoted by the catalyst. 

In the case of naphthenes, isomerization takes place under such mild conditions that side reactions do not interfere. 

The naphthene isomerization process has been applied also to the conversion of meth-ylcyclopentane to cyclohexane for subsequent dehydrogenation to benzene. Shell s Wilmington, Calif., refinery has been operating commercial equipment on this basis since March 1950 (18). 

At the present time, the national shortage of aromatics is also reviving interest in naphthene isomerization. 

Turova-Polyak and co-workers have carried out extensive studies of naphthene isomerization with AICI3, particularly of the substituted cyclopentanes. The conversion of mono- and disubstituted cyclopentanes to cyclohexanes was reported as an analytical technique for the determination of cyclopentanes in mixture with paraffins (411). Ethyl-cyclopentane at room temperature gave an 18-20% yield of cyclohexane derivatives (412). At 140-145°, an 85% yield of 1,3,5-trimethyl-cyclohexane was obtained. This work was also extended to 1,1-dimethyl-cyclopentane (410), up to 95% of which was converted to methyl-cyclohexane at 115°. Similar conversions of alkylated cyclopentanes were also reported by Shulkin and Plate (375). These researches parallel similar work done in the United States. 

Essential features of the Shell naphthene isomerization process (24) are outlined in Figure 26. Although the contactor principle employed in the other liquid-phase Shell processes is used, the catalyst is handled in the form of hydrocarbon complex. A carefully fractionated and dried concentrate of dimethylcyclopentanes is preheated to 200°F., and about 0.1% of anhydrous hydrogen chloride is added. The feed is joined by a stream of catalyst complex and charged to the reactor under a gauge... 

Petroleum is fast becoming a major source of aromatics for the growing chemical industry. This trend should bring about revived interest in naphthene isomerization as a means of supplying maximum production to meet expanding needs. 

Tsai, K.-Y., Wang, L, Tsai, T.-C. (2011). Zeolite Supported Platinum Catalysts for Benzene Hydrogenation and Naphthene Isomerization. Catalysis Today, Vol.166, No.l, (May 2011), pp. 73-78, ISSN 0920-5861... 

Naphthene Dehydrogenation > Paraffin Dehydrogenation > Naphthene Isomerization... 

The kinetics of paraffin and naphthene isomerization or cracking can be described by a mathematical equation in which the hydrogen pressure is in the denominator. This would mean that the rate decreases when hydrogen pressure increases. However, the reverse effect is observed for heavy feedstocks. This can be explained by considering all the reactions taking place simultaneously, especially the HDN and HDA reactions, and by knowing that the acidic sites of the catalyst are very sensitive to nitrogen and heavy aromatic compounds. 

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