Hydrocracking selectivity

The development of zeolite-containing catalysts has led to the development of binders. Modem catalyst technology (especially for fluidized-bed catalytic cracking and hydrocracking) selects binders which may have a variety of properties of their own (catalytic, trapping of poisons, etc.)... 

Catalyst selectivity is a measure of the rate of formation of a desired product relative to the rate of conversion of the feed (or formation of other products). Hydrocracking selectivity is expressed as the yield of desired product at a specific conversion. Yield is determined by the rate of formation of the desired product relative to the feed rate. At 100% conversion, catalyst yield equals catalyst selectivity. Hydrocracking selectivity is affected by operating conditions. In general, more severe operating conditions cause higher selectivity for secondary products. 

In two stages with recycle to the second stage, the conversion per pass is approximately 50 wt. % and the selectivity to middle distillates is maximal 75 to 80 wt. %. However, the investment is clearly higher and is justified only when feedstocks are difficult to convert and that their content in nitrogen is high. Figure 10.11 represents two variants of the hydrocracking process. 

This form of limited-conversion hydrocracking is a process that selectively prepares high quality residues for the special manufacture of base oils of high viscosity index or treating residues having low BMCl for the conversion of heavy fractions to ethylene, propylene, butadiene and aromatics. 

The question then lies in the selection of more appropriate feedstocks for these two processes. The cost of hydrocracking leads to selecting feedstocks that are the easiest to convert as for catalytic cracking, its flexibility and extensive capabilities lead to selection of heavier feedstocks. 

All of the above reactions are reversible, with the exception of hydrocracking, so that thermodynamic equilibrium limitations are important considerations. To the extent possible, therefore, operating conditions are selected which will minimize equilibrium restrictions on conversion to aromatics. This conversion is favored at higher temperatures and lower operating pressures. 

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). 

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

A single-event microkinetic description of complex feedstock conversion allows a fundamental understanding of the occurring phenomena. The limited munber of reaction families results in a tractable number of feedstock independent kinetic parameters. The catalyst dependence of these parameters can be filtered out from these parameters using catalyst descriptors such as the total number of acid sites and the alkene standard protonation enthalpy or by accounting for the shape-selective effects. Relumped single-event microkinetics account for the full reaction network on molecular level and allow to adequately describe typical industrial hydrocracking data. 

Hydrodewaxing Selective hydrocracking of paraffins from oil fractions or products to prevent wax precipitation (negative influence on cold flow properties) ... 

As a result, selectivity is not optimal and excessive hydrocracking results (10). Catalyst aging is also excessive. 

Leckel, D. O. 2007. Selectivity effect of oxygenates in hydrocracking of Fischer-Tropsch waxes. Energy Fuels 21 662-67. 

Bifunctional catalysis is one of the most important routes to green (more economical and more environmentally friendly) processes. Indeed, the deactivation of bifunctional catalysts by coking is much slower than that of monofunctional catalysts and their selectivity generally higher (e.g., hydrocracking compared to... 

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