Cracking dehydrocyclization

Figure 2. Dehydrocyclization/Cracking Selectivity Ratio plotted as a function of n-hexane conversion A SAP0-11,...

In hydrocarbon reforming processes the vapour of an alkane is passed over a supported metal catalyst such as platinum on silica or alumina. Dehydrocyclization, isomerization and cracking reactions all take place to... 

The first reaction is the isomerization from a zero-octane molecule to an alkane with 100 octane the second is the dehydrocyclization of heptane to toluene with 120 octane, while the third is the rmdesired formation of coke. To reduce the rate of cracking and coke formation, the reactor is run with a high partial pressure of H2 that promotes the reverse reactions, especially the coke removal reaction. Modem catalytic reforming reactors operate at 500 to 550°C in typically a 20 1 mole excess of H2 at pressures of 20-50 atm. These reactions are fairly endothermic, and interstage heating between fixed-bed reactors or periodic withdrawal and heating of feed are used to maintain the desired temperatures as reaction proceeds. These reactors are sketched in Figure 2-16. 

Other reactions may also occur. These include carbon formation, hydrocracking or thermal cracking, dehydrocyclization of paraffins to naphthenes, and dehydrogenation of naphthenes to aromatics. These have been discussed in the deactivation of reforming catalysts, in Section 2. 

Paraffin dehydrogenation, isomerization, and dehydrocyclization Paraffin cracking and isomerization Alcohol -> olefin + H20 Paraffin hydrocracking Olefin polymerization,... 

In contrast, a literature model [16] contains 14 reactions, since it allows almost all possible (reversible) reactions between lumps. By feeding toluene and hydrogen to the microbalance, we find that toluene is virtually unreactive, indicating that there are no reverse reactions from toluene to nCv and iC . Consider another literature model in which there is neither direct nC dehydrocyclization to toluene (k2 = 0) nor direct nC7 cracking [5]. Using this literature model, we can find a set of k s that would fit our fixed-bed data over a limited range of conditions. However, the new ke and do not even come close to describing the ECP-feed differential microbalance reactor data. 

On the other hand, the selective dehydrocyclization, which does not allow the formation of secondary-primary C-C bonds, must involve only two methylic carbon atoms in the 1 and 5 positions. Although the reverse reaction (selective hydrogenolysis of methylcyclopentane) could be observed on platinum catalysts of low dispersion at 220°C (86), the selective dehydrocyclization of methylpentanes on these catalysts is detectable only at higher temperatures (280°-300°C), where it competes with another process, ascribed to Mechanism C (33). Fortunately, it was found recently that iridium supported on AI2O3 or SiOj selectively catalyzes at 150°C the cyclic type interconversion of 2-methyl- and 3-methylpentanes (88). n-Hexane under the same conditions yields only cracked products (702) (Scheme 52). Similarly,... 

Data on rates of dehydrocyclization rD and cracking rc of n-heptane at 495°C and 14.6 atm are given in Table 5.2 for platinum-iridium on alumina and platinum-rhenium on alumina catalysts, and also for catalysts containing platinum or iridium alone on alumina (33). The rate rD refers to the rate of production of toluene and C7 cycloalkanes, the latter consisting primarily of methylcyclohexane and dimethylcyclopentanes. The rate of cracking is the rate of conversion of n-heptane to C6 and lower carbon number alkanes. 

The rates of dehydrocyclization of n-heptane for the iridium and platinum-iridium catalysts are more than twice as high as the rate for the platinum catalyst, and almost twice as high as the rate for the platinum-rhenium catalyst. The rates of cracking are also higher for the iridium and the platinum-iridium catalysts. 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

The objectives of the catalytic reforming of naphtha are to increase the naphtha octane number (petroleum refination) or to produce aromatic hydrocarbons (petrochemistry). Bifunctional catalysts that promote hydrocarbon dehydrogenation, isomerization, cracking and dehydrocyclization are used to accomplish such purposes. Together with these reactions, a carbon deposition which deactivates the catalyst takes place. This deactivation limits the industrial operation to a time which depends on the operational conditions. As this time may be very long, to study catalyst stability in laboratory, accelerated deactivation tests are required. The knowledge of the influence of operational conditions on coke deposition and on its nature, may help in the efforts to avoid its formation. 

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