Cracking reaction extent

The presence of metal may catalyze demethylation and can occur to some extent in catalysts where the metal function is under-passivated, as by incomplete sulfiding. This would convert valuable xylenes to toluene. The demethylation reaction is usually a small contributor to xylene loss. Metal also catalyzes aromatics saturation reactions. While this is a major and necessary function to facilitate EB isomerization, any aromatics saturation is undesirable for the process in which xylene isomerization and EB dealkylation are combined. Naphthenes can also be ring-opened and cracked, leading to light gas by-products. The zeolitic portion of the catalyst participates in the naphthene cracking reactions. Cracked by-products can be more prevalent over smaller pore zeolite catalysts. 

An approximation of the extent of hydrogen transfer reactions occurring compared to cracking reactions and the net effect on product distribution can be initially seen by a consideration of the zeolite properties of the catalysts tested in the present study ... 

While the definitions of the various hydroprocesses are (as has been noted above) quite arbitrary, it may be difficult, if not impossible, to limit the process to any one particular reaction in a commercial operation. The prevailing conditions may, to a certain extent, minimize, cracking reactions during a hydrotreating operation. However, with respect to the heavier feedstocks, the ultimate aim of the operation is to produce as much low-sulfur liquid products as possible from the feedstock. Any hydrodesulfurization process that has been designed for application to the heavier oils and residua may require that hydrocracking and hydrodesulfurization occur simultaneously. 

However, the precise location of the decomposition and glass transition lines may depend upon pyrolysis conditions because kinetic factors such as gas flow rate, stirring conditions, heating rate, etc., all affect the relative extents to which evaporation and cracking reactions contribute to the change in composition. 

Steam reforming was the primary reaction over these nickel catalysts. The presence of hydrocarbons (G2 to G5) which would indicate cracking reactions occurred to the extent of less than 10% in the reaction products. The presence of methane, which would indicate partial reforming, did not exceed 5% in the reaction products. There does not appear to be any significant difference in product selectivity for the n-hexane steam reforming reaction over nickel on the 2 quite different supports—zeolite vs. alumina. Carbonaceous residues accumulated in the case of all the nickel catalysts where reforming activity was sustained and the carbon deposition on the zeolite catalysts compared favorably with G56. 

The relative extents of the three types of cracking reactions are shown on p. 64. To compute the distribution shown, all paraffins were assumed to be formed from Reaction 1. After deduction of an equivalent quantity... 

The resulting equation was found empirically by E. B. Burk (3) and M. D. Tilicheev (22). The above-studied mechanism of the cracking of n-paraffins explains the absence of dependence of the cracking rate on reaction extent. This differs from the case of the low molecular weight paraffins experimentally determined by Kasanskaya (6) and Panchenkov and Baranov (19) for n-octane and n-hexadecane. 

Hydrotreating of feeds intended for further processing is desirable to the extent that the principal contributors to poor product quality or refining characteristics are eliminated this assures protection of the expensive catalysts employed in reforming and cracking reactions. Undesirable effects produced by certain types of contaminates are itemized below ... 

Another reaction that occurs and illustrates the complementary operation of the hydrogenation and cracking reactions is the initial hydrogenation of a condensed aromatic compound to a cycloparaffin. This allows subsequent cracking to proceed to a greater extent and thus converts a low-value component of catalytic cycle oils to a more valuable product. 

Originally, the idea was to mechanistically separate ethylene formation from the formation of higher alkenes [93]. It was found that ethylene was formed from the (lower) methylbenzenes (ie, from the aromatics carbon pool) whereas propylene and higher alkenes were to a considerable extent formed from alkene methylations and interconversions (eg, cracking reactions). 

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