Nickel carbide cracking

Fig. 3. Compensation plot for cracking reactions on nickel carbide (see text, Table I, B), line calculated by least squares method (see Appendix II). Points for reactant mixtures containing hydrogen, O line for cracking reactions on nickel metal (from Fig. 2) shown dashed.

Cracking reactions on nickel carbide in the absence of added hydrogen (22), hydrogenation of nickel carbide (162), and the reactions of water and of sulfur dioxide with this solid (163) exhibited a different compensation line (Fig. 3, full line, and Table I, B) from that for cracking reactions on the metal (Fig. 3, dashed line). When data for the reactions of propane on nickel carbide in the presence of some added hydrogen (O on Fig. 3) (22) are included in the calculation, the position of the line is almost unchanged, but the values of a are significantly increased (by the factor x 2, Table I, C). 

We conclude, therefore, that the mechanisms of catalytic cracking reactions on nickel metal and nickel carbide are closely comparable, but that the latter process is subject to an additional constraint, since a mechanism is required for the removal of deposited carbon from the active surfaces of the catalyst. Two phases are present during reactions on the carbide, the relative proportions of which may be influenced by the composition of the gaseous reactant present, but it is not known whether the contribution from reactions on the carbide phase is appreciable. Since reactions involving nickel carbide yielded products other than methane, surface processes involved intermediates other than those mentioned in Scheme I, although there is also the possibility that if cracking reactions were confined to the metal present, entirely different chemical changes may proceed on the surface of nickel carbide. 

In the steam cracking of hydrocarbons, a small portion of the hydrocarbon feed gases decomposes to produce coke that accumulates on the interior walls of the coils in the radiant zone and on the inner surfaces of the transferline exchanger (TLX). Albright et identified three mechanisms for coke formation. Mechanism 1 involves metal-catalyzed reactions in which metal carbides are intermediate compounds and for which iron and nickel are catalysts. The resulting filamentous coke often contains iron or nickel positioned primarily at the tips of the filaments. This filamenteous coke acts as excellent collection sites for coke formed by mechanisms 2 and 3. Mechanism 2 results in the formation of tar droplets in the gas phase, often from aromatics. These aromatics are often produced by trimerization and other reactions involving acetylene. Some, but not all, of these droplets collect... 

Niobium, when added to AUoy 182, stabilizes the carbon content and prevents the precipitation of Cr carbides at the grain boundary [78]. C, P, Si, and S segregate or precipitate at the grain boundary and are critical to hot-cracking resistance of nickel-based weld metals [79]. When Si segregates at the grain boundaries of austenitic stainless steel. 

Recent advances further enhance their commercial potential in metal matrix composites such as aluminum, nickel, and copper ceramic matrix composites, such as alumina, zirconia and silicon nitride and glass ceramic matrix composites such as lithium aluminosilicate. Silicon carbide whiskers increase strength, reduce crack propagation, and add structural reliability in ceramic matrix composites. Structural applications include cutting tool inserts, wear parts, and heat engine parts. They increase strength and stiffness of a metal, and support the design of metal matrix composites with thinner cross sections than those of the metal parts they replace, but with equal properties in applications such as turbine blades, boilers and reactors. 

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