Nickel cracking, kinetics

Snoeck, J., Froment, G., and Fowles, M., Kinetic study of the carbon filament formation by methane cracking on a nickel catalyst, /. Catal., 169, 250,1997. 

It is appropriate to discuss this pattern of kinetic behavior, and the interpretation offered here, with reference to problems that arise in the elucidation of the mechanisms of cracking reactions of hydrocarbons on nickel. The hydrogenolysis of ethane has been the subject of many studies and it is believed by (inter alia) Sinfelt (74), Tetenyi (87, 123), Shopov (124), and their co-workers that the rate-limiting step is carbon-carbon bond rupture. In... 

TABLE 7.22. Kinetic Parameters for Cracking Reactions Over Nickel Powder (ref. 13") ... 

The mathematical model was constracted on the basis of a three-phase plug-flow reactor model developed by Korsten and Hoffmaim [63]. The model incorporates mass transport at the gas-liquid and liquid-solid interfaces and uses correlations to estimate mass-transfer coefficients and fluid properties at process conditions. The feedstock and products are represented by six chemical lumps (S, N, Ni, V, asphaltenes (Asph), and 538°C-r VR), defined by the overall elemental and physical analyses. Thus, the model accounts for the corresponding reactions HDS, HDN, HDM (nickel (HDNi) and vanadium (HDV) removals), HD As, and HCR of VR. The gas phase is considered to be constituted of hydrogen, hydrogen sulfide, and the cracking product (CH4). The reaction term in the mass balance equations is described by apparent kinetic expressions. The reactor model equations were built under the following assumptions ... 

Grain boundary chemistry due to irradiation-induced chromium depletion and nickel enrichment and impurity (e.g., silicon, phosphorous) segregation, since these can affect the oxidation kinetics at the strained crack tip... 

Residual elements, such as nickel, copper and tin, are normally found in the steels made from or substantially from scrap steels. The presence of these elements in the steels had some effects on their oxidation kinetics but these were not significant [97,98]. The main concern with the presence of copper and tin is surface hot shormess, which is caused by the presence of a molten copper-rich phase on the steel surface as a result of steel oxidation. This molten copper-rich phase can penetrate austenite grain boundaries and cause surface cracking during subsequent hot rolling. Nickel is beneficial in alleviating surface hot shortness but it can cause sticky scale as a result of network-like internal iron oxide formation. Further discussion of surface hot shortness behaviour is beyond the scope of the current review. 

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