Isomerization of naphthenes

The reactions of major importance in the octafining process are isomerization of naphthenes and aromatics, hydrogenation of aromatics, dehydrogenation of naphthenes, disproportionation of aromatics, dealkylation of aromatics, and hydrocracking of saturates (Figure 3). The last three reactions, of course, result in loss of product xylenes. These reactions, like the desired isomerization reactions, are carbonium-ion catalyzed. 

Radical decomposition is one of the most important types of reactions. In this case, a larger radical decomposes to an olefin and a smaller radical. Radicals usually decompose at the beta position of the radical center where the C—C bond is the weakest. In the case of naphthenes and aromatics this may not be the case, and C—H bond may be the weakest. Radical isomerization frequently occurs for large radicals, and explains to a large extent the observed product distribution. 

Powerforming is one tecnique used for aromatics chemical production. Powerforming uses a platinum catalyst to reform virgin naphthas. The principal reaction is the conversion of naphthenes in virgin naphthas to aromatics e.g., isomerization and dehydrocyclization reactions also occur in catalytic reforming. 

Catalytic reformers are normally designed to have a series of catalyst beds (typically three beds). The first bed usually contains less catalyst than the other beds. This arrangement is important because the dehydrogenation of naphthenes to aromatics can reach equilibrium faster than the other reforming reactions. Dehydrocyclization is a slower reaction and may only reach equilibrium at the exit of the third reactor. Isomerization and hydrocracking reactions are slow. They have low equilibrium constants and may not reach equilibrium before exiting the reactor. 

Catalytic reforming rearranging hydrocarbon molecules in a gasoline-boiling-range feedstock to produce other hydrocarbons having a higher antiknock quality isomerization of paraffins, cyclization of paraffins to naphthenes (g.v.), dehy-drocyclization of paraffins to aromatics (g.v.). 

Ethylbenzene Isomerization Isomerization of EB requires both metal and acid function. Hydrogenation results in an intermediate naphthene. The acid function is required to isomerize the naphthene to a methyl-ethyl-substituted five-mem-bered ring species that can further convert to a dimethyl-substituted six-membered ring naphthene. This can be dehydrogenated by the metal function to a xylene isomer, OX in the example shown in Figure 14.9. 

The process for isomerization of EB requires that some fraction of the feed be maintained in a saturated state, as described in Sections 14.4.1.1 and 14.4.1.2. The ability to isomerize the EB is affected by the naphthene concentration and constrained by the equilibrium ratio of EB/xylenes at the reaction temperature. Use of a pore-restricted molecular sieve can be used to eliminate more sterically demanding species from the reaction network and can effectively remove these from consideration. In this maimer, one can achieve higher levels of a desired species, such as PX from EB, than could ordinarily be obtained by isomerization over a larger-pore zeolite. 

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 predominant reaction during reforming is dehydrogenation of naphthenes. Important secondary reactions are isomerization and dehydrocyclization of paraffins. All three reactions result in high-octane products. 

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. 

Several years ago, one of the authors found that nickel, platinum, and some other hydrogenating agents, when deposited on fresh synthetic silica-alumina cracking catalyst, made a new catalyst that would isomerize paraffin and naphthene hydrocarbons in the presence of hydrogen at elevated pressures and nominal temperatures. Table I shows some early typical results calculated from mass spectrometer analyses of the products obtained by passing methyl cyclopentane, cyclohexane, and n-hexane over a catalyst composed of 5% nickel in silica-alumina at the indicated reaction conditions. Isomerization of a number of other hydrocarbons has also been studied and reported elsewhere (2). 

Besides dehydrogenation of naphthenes and the isomerization of both naphthenes and... 

A fourth type of petroleum isomerization, which was commercialized on a small scale, involves the rearrangement of naphthenes. In the manufacture of toluene by dehydrogenation of methylcyclohexane, the toluene yield can be increased by isomerizing to methylcyclohexane the dimethylcyclopentanes also present in the naphtha feed. This type of isomerization is also of interest in connection with the manufacture of benzene from petroleum sources. 

Isomerization of paraffins and naphthenes is a reversible first-order reaction limited by thermodynamic equilibria. It is slightly exothermic in nature and does not take place to any appreciable extent without a catalyst (4). Although the mechanism of the reaction... 

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

Naphthene Isomerization. In addition to the paraffin isomerization processes, naphthene isomerization also proved useful during the war in connection with the manufacture of toluene. In the Shell dehydrogenation process for the manufacture of toluene, good yields depend upon increasing the methylcyclohexane content of the feed by isomerization of dimethylcyclopentanes. This process was employed commercially at one refinery in the Midwest and one on the Pacific Coast. 

Like the paraffins, naphthenes do not appear to isomerize before cracking. However, the naphthenic hydrocarbons (from C9 upward) produce considerable amounts of aromatic hydrocarbons during catalytic cracking. Reaction schemes similar to that outlined here (page 131) provide possible routes for the conversion of naphthenes to aromatics. 

The wide ranges of temperature and pressure employed for the hydrodesulfurization process virtually dictate that many other reactions will proceed concurrently with the desulfurization reaction. Thus, the isomerization of paraffins and naphthenes may occur and hydrocracking will increase as the temperature and pressure increase. Furthermore, at the higher temperatures (but low pressures) naphthenes may dehydrogenate to aromatics and paraffins dehydrocyclize to naphthenes, while at lower temperature (high pressures) some of the aromatics may be hydrogenated. 

The rest of the cyclic terpenoid sulfides are complex mixtures of partially degraded and isomerized derivatives of the terpenoid sulfides which elute on the capillary GC column as a broad, unresolved hump. On Raney nickel reduction this fraction yields a complex mixture of naphthenic hydrocarbons which cannot be resolved further by GC analysis. 

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. 

Catalytic Reforming A catalytic reaction of heavy naphtha(1) used to produce high-octane gasoline. The byproducts are hydrogen and light hydrocarbons the primary reaction is dehydrogenation of naphthenes to produce aromatics. Some reshaping of paraffins to produce aromatics and some isomerization of paraffins to produce isoparaffins also occur. 

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