Reaction mechanisms hydrocracking

The pure compound rate constants were measured with 20-28 mesh catalyst particles and reflect intrinsic rates (—i.e., rates free from diffusion effects). Estimated pore diffusion thresholds are shown for 1/8-inch and 1/16-inch catalyst sizes. These curves show the approximate reaction rate constants above which pore diffusion effects may be observed for these two catalyst sizes. These thresholds were calculated using pore diffusion theory for first-order reactions (18). Effective diffusivities were estimated using the Wilke-Chang correlation (19) and applying a tortuosity of 4.0. The pure compound data were obtained by G. E. Langlois and co-workers in our laboratories. Product yields and suggested reaction mechanisms for hydrocracking many of these compounds have been published elsewhere (20-25). 

In some later work at UOP it was shown that in isomerization and hydrocracking of normal decane the reaction of hydrocracking could be virtually stopped if one operates at very high pressures (up to 1500 atmospheres), however, as the pressure is increased, it is the isomerization reaction which is stopped last. These results reinforce the sequential carbonium ion mechanism, wherein each transformation can be reversed by excessive hydrogen pressure. 

Activity of these catalysts depends on the balance between the hydrogenation and acidic functions. For example, it was found that HZSM-5 was effective for the hydrocracking of HDPE and plastic waste [24]. But the liquid product contained much less n-paraffins and a greater amounts of aromatics (34%) and naphthenes (21.7%) because of a lack of sufficient hydrogenation function. The reaction mechanism over HZSM-5 can be considered as follows ... 

In catalytic cracking many reactions take place simultaneously. Cracking occurs by C-C bond cleavage of paraffins, dealkylation etc. Isomerization and even condensation reactions take place. These reactions occur via positively charged hydrocarbon ions (carbocations). The nature of the carbocations is the subject of debate. For the cracking of paraffinic hydrocarbons it is usually assumed that carbenium ions are the crucial intermediates, which decompose via beta fission into olefins and (smaller) carbenium ions (see Chapter 4, Section 4.4). A typical reaction mechanism for catalytic cracking (and hydrocracking) imder the relatively mild conditions used in FCC is shown overleaf. 

Paraffin isomerization over dual function catalysts based on zeolite Y and mordenite has been reviewedand a reaction mechanism was proposed in which olefin-paraffin equilibrium is established and carbonium ions are formed from both paraffins and olefins. The isomerization of n-hexane and hydrocrack-... 

The reaction mechanisms of paraffin hydrocracking have been studied extensively (2, 7, 15, 16, 22, 30, 34, 40, 44). A number of reactions... 

This section first discusses the reaction mechanism for paraffin hydrocracking and the thus-derived modeling specifications for each reaction family. This is followed by a discussion of the automated model building algorithm and the QSRC/LFERs used to organize the rate parameters. Finally, the thus-developed Cig paraffin mechanistic hydrocracking model diagnostics are presented. 

These mechanistic features were elucidated in detail in the 1960s. Based on the pioneering work of Mills et al. and Weisz ", a carbenium ion mechanism was proposed, similar to catalytic cracking plus additional hydrogenation and skeletal isomerization. More recent studies of paraffin hydrocracking over noble metal-loaded, zeolite based catalysts have concluded that the reaction mechanism is similar to that proposed earlier for amorphous, bifunctional hydrocracking catalysts. ... 

Catalytic and thermal reactions follow different reaction mechanisms therefore, different reaction orders would be expected. The vacuum residue conversion obtained with catalytic hydrocracking has been properly represented by second-order reaction kinetics (Sanchez and Ancheyta, 2007). Regarding the NHDC reaction, the following expressions can be derived as functions of vacuum residue conversion, kinetic parameters (n reaction order, k kinetic constant), the total mass flow, and the inert material volume ... 

It should be noted that many practically important catalytic transformations (such as isomerization of or hydrocracking of paraffins), which are presumed to proceed via consecutive mechanisms, are performed on multifunctional catalysts, with which the coupling of reactions in the sense just discussed may not necessarily occur. The problem of the selectivity of some models of polystep reactions on these catalysts has been discussed in detail by Weisz (56). 

The products obtained from DPM cracking in the present work agree with the results from the literature, mentioned in the Introduction, which indicate that the reaction proceeds via carbocation formation on acidic sites. This implies that the decomposition of DPM does not need the successive intervention of two catalytic sites, like in the "ideal hydrocracking" mechanism. Only acidic sites are sufficient to carry out the reaction. The improved activity of the mixtures when compared to the pure phases must therefore be explained differently. 

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