Paraffins, hydrocracking reaction

The present modeling approach exploited these notions. The molecules in the paraffin hydrocracking reaction mixture were thus grouped into a few species types (paraffins, olefins, ions, and inhibitors such as NH3), which, in turn, reacted through a limited number of reaction families on the metal (dehydrogenation and hydrogenation) and the acid sites (protonation, hydride-shift, methyl-shift, protonated cyclopropane (PCP) isomerization, 13-scission, and deprotonation). As a result, a small munber of formal reaction operations could be used to generate hundreds of reactions. 

Figure 1. Paraffin Hydrocracking Reaction Families at the Mechanistic Level...

Table 1. Reaction Matrices for Paraffin Hydrocracking Reactions at the Mechanistic Level. Reaction Family Reactant Matrix...

The remainder of this paper describes the division of the paraffin hydrocracking reactions into mechanistic families with a unique reaction matrix operator for each reaction family. The reaction rules and QSRCs used are then discussed for each reaction family. The technical specifications and the iteration process to find the optimum subset of the mechanistic model will also be discussed. 

The mechanistic paraffin hydrocracking model builder has a database of all reaction matrices and rules for all mechanistic paraffin hydrocracking reactions, involving both metal and acid chemistries, with the flexibility of changing them. A detailed library of parafifins, olefins, and ion intermediates consisting of their connectivity and thermod5mamic information was built... 

Figure 6. Mechanism Deduced Reaction Pathways at the Carbon Number and Branch Paraffin Hydrocracking Reaction Number Level...

Ring dealkylation reaction 2 Ring open reaction 3 Aromatic satjration 4. Paraffin hydrocracking reaction P paraffins N naphthenes A aromatics AN ring compound which has naphthene and aromatics... 

The kinetics of hydrocracking reactions has been studied with real feedstocks and apparent kinetic equations have been proposed. First-order kinetics with activation energy close to 50 kcal/gmol was derived for VGO. The reactions declines as metal removal > olefin saturation > sulfur removal > nitrogen removal > saturation of rings > cracking of naphthenes > cracking of paraffins [102],... 

For example in Fig. 12, the n-hexane hydrocracking reaction network—the last which can be drawn entirely by hand—contains 6 paraffins, 14 olefins and 10 carbocations, and involves the following elementary reactions ... 

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

Effect on Gasoline Properties. The hydrocracking reaction is the C—C bond breaking catalyzed by the acid function. In this reaction, either a paraffin molecule is broken into two molecules of lower molecular weight, or a naphthene ring is opened. The aromatics are very difficult to be hydrocracked under normal reforming conditions. 

Kinetics and Mechanism. The hydrocracking reaction is catalyzed by the acid function following the )3-scission mechanism. The paraffin molecule is cracked approximately at a middle position therefore, the production of methane is very low. 

Although olefins are hydrogenated to the corresponding paraffin in the hydrotreater, they rapidly reach low (2-4%), equilibrium concentrations in the reforming unit. Once formed, they undergo reactions similar to paraffins. Hydrocracking and hydroisomerization occur more readily, due to the ease of formation of the carbenium ion intermediate from the olefin. Olefin cyclization also occurs. 

Figure 1 shows the meehanistie reactions for paraffin hydrocracking via the dual site (both metal site and acid site) mechanism. Each reaction family had a single associated reaction matrix, which was used to generate all possible mechanistic reactions. The reaction matrices are summarized in Table 1. 

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. 

The mechanism of paraffin hydrocracking over bifunctional catalysts is, essentially, the carbenium ion chemistry of acid cracking coupled with metal-centered dehydrogenation/hydrogenation reactions. The presence of excess hydrogen and the hydrogenation component of the catalyst result in hydrogenated products and inhibition of some of the secondary reactions and coke formation. 

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. ... 

The thus-deduced Cso paraffin hydrocracking model containing 306 species and 4671 reactions was automatically generated on the computer. This model can thus be further tuned to capture the carbon number distribution and iso-to-normal ratio in the paraffin hydrocracking process. 

Graph theoretic concepts were used to construct a paraffin hydrocracking mechanistic model builder incorporating bi-flinctional catalysis with metal and acid functions. The complete automated approach was successfully demonstrated for a Cm paraffin hydrocracking model with 465 species and 1503 reactions at the mechanistic level. 

The generalized isomorphism algorithm was applied at the carbon and branch number level to reduce the complexity and explosion of modeling heavy paraffin hydrocracking. A thus-deduced Cso paraffin hydrocracking model with 306 species and 4671 reactions at the pathways level was developed from the teachings of the Cie model. 

Sue and Sugiyama, Steinberg et al., and Mohanty et al. have written reviews of kinetic studies on hydrocracking reactions. In most of the cited literature, pure compounds or simple mixtures were processed in small isothermal reactors. For example, a publication by Rappaport reported that the rate of hydrocracking for pure normal paraffins increases as shown in Table 5. 

Dehydrogenation of naphthenes [Eq. (21-3)] occurs very rapidly, and the isomerization of paraffins and naphthenes [Eqs. (21-4) and (21-6)] also occurs rapidly. Thus these reactions predominate, and the slower cyclization and hydrocracking reactions [Eqs. (21-5) and (21-7)] become significant mainly at severe conditions of low space velocity, high pres-... 

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