Hydrocracking paraffin

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

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

As a demonstration of the unsuitability of optimised crude oil refinery-based maximum distillate yield hydrocracking catalysts for F-T wax duty, a simple study is presented below wherein such a catalyst was applied to n-paraffin hydrocracking in the absence of sulphur. 

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. 

MECHANISTIC KINETIC MODELING OF HEAVY PARAFFIN HYDROCRACKING... 

Catalytic hydrocracking is a flexible process for the conversion of heavy, hydrogen-deficient oils into lighter and more-valuable produets. Part of its flexibility is its ability to handle a wide range of feeds, from heavy aromatics to paraffinic crudes and cycle stocks. The ability to extend this flexibility to customized product slates depends on the ability to control and manipulate the process chemistry, which is, in turn, enhanced by a rigorous representation of fte process chemistry. This motivates the present interest in a heavy paraffin hydrocracking kinetics model. 

Mechanistic Kinetic Modeling of Heavy Paraffin Hydrocracking... 

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 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. 

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. 

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. ... 

As shown in Figure 1, the elementary steps used to model the mechanism of paraffin hydrocracking over a bifunctional catalyst are as follows ... 

Figure 3. Algorithm for Automated Paraffin Hydrocracking Mechanistic Modeling...

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... 

The model was optimized with the lumped data from experiments (the unconverted Cie, the mono-branched and multi-branched Cie, and all the cracked products). Then the model was used to predict the carbon number distribution and iso-to-normal ratio for the various carbon numbers. Figures 5a and 5b show the parity plots for the Cie paraffin hydrocracking for both (a) the lumped observations and (b) the carbon number distribution at various conditions (T, P, LHSV and NH3). The agreement between the predicted and experimental data sets for all the high and low yield compounds is good, as all predictions are within the experimental error. The good prediction validates the fundamental nature of the model. 

Figure 5. Parity Plots for Cig Paraffin Hydrocracking Model at Various Operation Conditions (T, P, LHSV, and Inhibition)...

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. 

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

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. 

Dehydrogenation to form aromatics Isomerization to form different naphthenes Dehydrocyclization to form aromatics Isomerization to form other paraffins Hydrocracking to form C4-minus... 

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