Dehydrocyclization dehydrogenation

It should be noted that the presence of excess hydrogen is always necessary during hydrocarbon reactions, even in those circumstances when the reaction is hydrogen producing (dehydrogenation, dehydrocyclization, etc.). It appears that the main role of excess hydrogen is to keep the step and kink sites... 

In petroleum refor.ming a metal such as Pt (or Pt / Re) supported on a silica-Alumina or a zeolite catalyst is employed for effecting the hydrocarbon conversions(l-5). The idea is to improve the Research Octane Number (RON) or the Motor Octane Number (MON) of the final reformate. The metal function catalyses several dehydrogenation, dehydrocyclization, aromatization reactions, whereas the acid function effects the skeletal isomerization, mainly through a carbocation mechanism(2-5). 

The sintering leads to a decrease in the activity of the reactions taking place on the metal, such as hydrogenation-dehydrogenation, dehydrocyclization, and hydrogenolysis. Therefore, in an industrial unit it is necessary to redispersed the platinum before the reaction. 

The reforming process consists of exothermic reactions (isomerization, hydrocracking) and endothermic reactions (dehydrogenation, dehydrocyclization). In summary the process is endothermic. Several parallel and sequential reactions with diEFerent rates and equilibrium limitations take place (Table 6.9.1). In addition, coke formation occurs, which leads to a gradual decrease of the activity of the catalyst To minimize coke formation hydrogen is added in a large excess to the feed, that is, H2 formed by dehydrogenation and dehydrocydization is recycled. Nevertheless, coke must be burned off after a certain carbon load has been reached. 

A detailed study of many of the reactions is out of the scope of this work. We refer readers to Fromentet al. [10,11,12] for detailed experimental and mechanistic studies. These studies are very useful in the course of detailed catalyst design and kinetic network generation [15,16,17,18], However, neither of these topics is the subject of the current work. We present these reactions in the context of an integrated process model. As mentioned earlier in this work, the typical reactions in the reforming process are dehydrogenation, dehydrocyclization, isomerization and hydrocracking. Table 5.2 shows examples of these reaction classes. 

Dehydrocyclization refers to the conversion of feed paraffins into alkylcyclohexane and alkylcyclopentane naphthenes. These, in turn, are subsequently converted by isomerization and dehydrogenation into aromatics. Dehydrocyclization is controlled by both acid and platinum functions and is the most sensitive indicator of catalyst selectivity. 

Increasing the octane number of a low-octane naphtha fraction is achieved by changing the molecular structure of the low octane number components. Many reactions are responsible for this change, such as the dehydrogenation of naphthenes and the dehydrocyclization of paraffins to aromatics. Catalytic reforming is considered the key process for obtaining benzene, toluene, and xylenes (BTX). These aromatics are important intermediates for the production of many chemicals. 

Aromatization. The two reactions directly responsible for enriching naphtha with aromatics are the dehydrogenation of naphthenes and the dehydrocyclization of paraffins. The first reaction can he represented hy the dehydrogenation of cyclohexane to benzene. 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

It should be noted that both reactions leading to aromatics (dehydrogenation of naphthenes and dehydrocyclization of paraffins) produce hydrogen and are favored at lower hydrogen partial pressure. 

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. 

The second and third reactors contain more catalyst than the first one to enhance the slow reactions and allow more time in favor of a higher yield of aromatics and branched paraffins. Because the dehydrogenation of naphthenes and the dehydrocyclization of paraffins are highly endothermic, the reactor outlet temperature is lower than the inlet temperature. The effluent from the first and second reactors are reheated to compensate for the heat loss. 

According to this scheme, the catalyst serves primarily to promote dehydrogenation. Cyclization of the hexatriene was shown years ago (JJ.) to occur thermally in the gas phase at temperatures well below these dehydrocyclization conditions. Thus, the overall reaction is projected to be the combination of several catalytic dehydrogenation steps and a non-catalytic cyclization step. This projection implies that the design of the catalytic reactor may be important in order to optimize the ratio of void space for cyclization and catalyst space for dehydrogenation. 

It will be clear from the results so far presented that both C5 and C dehydrocyclization products can be formed, with aromatization proceeding (one would expect) by further dehydrogenation of the initially formed C6 ring-closure species. There is another pathway for the production of aromatics based upon cyclization of a linear triene (133), but this is of relatively small importance, and is only significant at all at quite high temperatures and low hydrogen partial pressures. 

Fries rearrangement, 18 336, 337 isomerization and transalkylation of alky-laromatics, 18 329 epoxide transformations, 18 351-352 hydration and ammonolysis of ethylene oxide, 18 351, 352 isomerization, 18 351 framework composition, 33 226-228 hydrogenation, dehydrogenation, and related reactions, 18 360-365 dehydrocyclization of s-ethylphenyl using zeolites and carbonyl sulfide, 18 364, 365... 

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

The nature of the metal Dehydrogenating properties are important for Cg dehydrocyclization. Platinum, palladium, iridium, and rhodium... 

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. 

---