Dehydrogenation cycloparaffins

Catalytic Reforming. Worldwide, approximately 30% of commercial benzene is produced by catalytic reforming, a process ia which aromatic molecules are produced from the dehydrogenation of cycloparaffins, dehydroisomerization of alkyl cyclopentanes, and the cycHzation and subsequent dehydrogenation of paraffins (36). The feed to the catalytic reformer may be a straight-mn, hydrocracked, or thermally cracked naphtha fraction ia the... 

Selecting the naphtha type can be an important processing procedure. For example, a paraffinic-base naphtha is a better feedstock for steam cracking units because paraffins are cracked at relatively lower temperatures than cycloparaffins. Alternately, a naphtha rich in cycloparaffins would be a better feedstock to catalytic reforming units because cyclo-paraffins are easily dehydrogenated to aromatic compounds. Table 2-5 is a typical analysis of naphtha from two crude oil types. 

The latest catalyst development is the contact DeH-9, which in terms of activity and stability is comparable with DeH-7 but with improved selectivity (fewer iso- and cycloparaffins and aromatics). This contact has been produced since 1990 and probably used commercially since 1992 [59]. In Table 7 the composition of the dehydrogenation products in relation to the catalyst and the application of the DeFine step is summarized. Table 8 shows the performance data for various catalysts [10] in relation to LAB production. 

Attention will be focussed on three typical chemical reaction schemes. For the first illustration, two parallel competing reactions are considered. For instance, it may sometimes be necessaru to convert into a desired product only one component in a mixture. The dehydrogenation of six-membered cycloparaffins in the presence of five-membered cycloparaffins without affecting the latter is one such example of a selectivity problem in petroleum reforming reactions. In this case, it is desirable for the catalyst to favour a reaction depicted as... 

See FIGURE 2-8 for an example of dehydrogenation of cycloparaffins to yield aromatics + hydrogen... 

Production of carbonium ions gives the possibility to produce different hydrocarbons molecules, e.g. cycloparaffins and aromatics by cyclization and dehydrogenation reactions (Figure 4.2). In the first step presumably intramolecular reactions between carbonium ions and double bonds take place. 

The dehydrogenation of hexane to hexene or cyclohexane (reactions 5 and 6) only becomes appreciable at temperatures approaching 800 °K. The dehydrogenation to methylcyclopentane however appears to be thermodynamically feasible at temperatures as low as 350 °K. One cannot place too much reliance on this particular result since the affinity of formation of methylcyclopentane is known less accurately than the others. These three reactions, however, scarcely affect the synthesis of aromatic compounds in the reaction since the ethylenes and cycloparaffins are thermodynamically unstable relative to aromatic hydrocarbons above 550 °K, and they decompose spontaneously to form aromatics at this temperature. They can therefore only appear as intermediates in reaction (9) above 550 °K. 

In the first group, the production of aromatics is a complementary objective to the refinery processing of gasoline fractions to raise the aromatic content, which evidently links these refining functions. Catalytic reforming processes are used to convert paraffins to naphthenes (cycloparaffins) to be followed by dehydrogenation of naphthenes to aromatics (Chap. 18). Since aromatization of naphthenes is an easier process to accomplish than cycloalkylation, the emphasis in refinery operations is on maximization of the second step in this sequence, when there is an adequate supply of naphthenes. The demand for the aromatics component of gasoline will compete with the feedstock aromatic need from this source. 

It follows that, as the experiments with the radioactive carbon show, the mechanisms considered above can coexist, and in certain instances one or the other of them will prevail. Dehydrogenation of six-mem-bered cycloparaffins to aromatic compounds (except their geminal forms) is very favorable thermodynamically as aromatic compounds are especially stable, possessing a conjugation energy of 36 kcal/mole for benzene. Therefore the dehydrogenation of six-membered rings, as compared to the other hydrocarbons, can take place at lower temperatures (about 300°C). 

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