Coke formation acetylene

Filament-type coke was produced from acetylene in the range 400°-600°C this coke contained nickel atoms or particles. Apparently nickel granules were lifted from the surface as a result of the coke formation. 

Thermal reactions of acetylene, butadiene, and benzene result in the production of coke, liquid products, and various gaseous products at temperatures varying from 4500 to 800°C. The relative ratios of these products and the conversions of the feed hydrocarbon were significantly affected in many cases by the materials of construction and by the past history of the tubular reactor used. Higher conversions of acetylene and benzene occurred in the Incoloy 800 reactor than in either the aluminized Incoloy 800 or the Vycor glass reactor. Butadiene conversions were similar in all reactors. The coke that formed on Incoloy 800 from acetylene catalyzed additional coke formation. Methods are suggested for decreasing the rates of coke production in commercial pyrolysis furnaces. 

An important finding of this investigation was that cokes formed from acetylene on Incoloy 800 surfaces caused what appears to be an ever increasing rate of coke formation, as indicated by the results of Run 15 and especially of Run 18. In other words, this coke resulted in an autoacceleration phenomenon. Yet the coke formed from butadiene (as indicated by the results of Run 14) seemed to deactivate the surface so that a slow and rather steady rate of coke formation occurred. The reason for this difference in the rates of coke formation will be discussed later in this chapter. 

The influence of the reduction temperature, preparation method and type of support on the textural properties and on the activity, selectivity and coking formation on Ni-Ti02-Al20j catalysts, using acetylene hydrogenation as the test reaction, is investigated. 

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... 

In just the same way two or more mechanisms of coke formation were observed in the decomposition of the naphthenes, of the paraffin hydrocarbons, olefins, acetylene, carboxylic acids, etc. In all cases the mechanisms of coke and tar formation could be represented by a cyclic sequence of a number of elementary stages, which involve the addition of every new molecule of the coke-forming material. Depending on whether the initial organic substance when the temperature is rising can give one, two, or more kinds of molecules of coke-forming material capable of... 

The effects have been studied of the total or partial substitution of Ni by Cu or Co on the physicochemical properties and catalytic activity of Ni (Co,Cu)-Zn-Al catalysts used for the selective hydrogenation of acetylene. In addition, the influence of the hydrogen concentration in the feed on the activity, selectivity and coke formation on the catalysts has been investigated. The results obtained in this work could be explained by the formation, to different degrees, of bimetallic clusters of Ni-Cu and Ni-Co in the quaternary Ni-Co-Zn-Al and Ni-Cu-Zn-Al catalysts. The formation of these bimetallic clusters would be responsible for the diminution of methane and ethane, both undesirable products. 

When comparing the activity of the ternary catalysts in the acetylene hydrogenation (Fig. 3), the following pattern is found Ni-Zn-Al > Cu-Zn-Al Co-Zn-Al. The opposite trend is obtained when the initial activity for coke formation is compared. The activity of the Cu-Zn-Al catalyst, in spite of being initially of the same order as that of the Ni-Zn-Al sample, decreases over the reaction time, so that after three hours the conversion does not rise 800 above 1%. The coke deposition on this catalyst has a drastic effect on its activity, provoking its almost complete deactivation. 

Coke formation during acetylene hydrogenation over palladium catalyst could promote hydrogen transfer to acetylene and ethylene adsorbed molecules leading to a decrease in ethane selectivity and an increase in ethylene loss. 

Extremely high rates of temperature change (10 K/s) show that it is possible to intermpt the methane cracking chain before solid carbon is produced from acetylene. Thus the problem of coke formation can be eliminated. In any case possible formation of coke in liquids is not so detrimental problem as in the conventional hydrocarbon crackers - in view of low wettability in many liquids coke could be relatively easier separated. 

It is, of course, recognized that both improved techniques for correcting experimental data and improved mechanistic (or kinetic) models can be developed in the future. Considering first the correction technique, other olefins (besides ethylene), di-olefins, acetylene, and aromatics which were formed in small amounts (4) all probably contributed to some extent to coke formation in the present investigation. Hence the ethylene yields based on the correction technique used are thought to be too high. 

Rives et al. (427,428) studied the hydrogenation of acetylene to ethylene with multimetallic oxide catalysts prepared by calcining ZnNiAlCr-LDHs at 500°C for 3 h with an H2/N2 (50 50 vol) gas mixture. The redox property of Ni is essential for the activity and selectivity of the catalysts and the presence of ZnO decreases the coke formation. 

S. Asplund, 1996, Coke formation and its effect on internal mass transfer and selectivity in Pd-catalysed acetylene hydrogenation. Journal of Catalysis, 158,267-278. 

Polymerization and crosslinking by reactions of this type can explain the formation of the explosive coke . The compn of the gases evolved was found to vary with the extent of reaction and with the temp (Ref 64). The complicated nature of the process is also shown by the fact that the evolved gases contain not only water, corresponding to the formation of the products cited above, but also CO, N2)N0,N20, and even acetylene (Ref 64)... 

Acetylene is a reactive molecnle with a low C H stoichiometry that can be used to evaluate the resistance of metal-based catalysts to the formation of carbonaceous residue (coking). Pt is very reactive, and the chemisorption of on Pt(lll) is irreversible under UHV conditions, with complete conversion of into surface carbon during heating in TPD. Alloying with Sn strongly reduces the amount of carbon formed during heating [49]. This is consistent with observations of increased lifetimes for commercial, supported Pt-Sn bimetallic catalysts compared to Pt catalysts used for hydrocarbon conversion reactions. 

Small electrically heated tungsten coils have also been used experimentally for the formation of acetylene by cracking methaue.80 At 3000° C. and with a time of contact of 0.0004 second a conversion of 86 per cent has been reported as obtainable from coke-oven gas containing 25.5 per cent methane. The results of this experimental work indicate that higher yidds are possible by the electric arc method. 

The catalyst may be poisoned permanently by components such as AsH3, PH3, H>S. etc., or temporarily by the formation of deposits of tars or polymers, or ultimately coke. This is corrected by prior scrubbing of the reactants (especially acetylene) by caustic soda, and by periodic regeneration with steam. The average gaseous VHSV, at standard temperature and pressure, is about 200 to 500 h l. Catalyst life is no longer than a few months (2000 h). 

Formation and Removal oi Coke Deposited on Stainless Steel and Vycor Surfaces from Acetylene and Ethylene... 

Calcium carbide is produced by reacting quicklime and coke at 1800 to 2100 °C (see section 31.4). The carbide is then reacted with water to form acetylene and calcium hydroxide as a by-product. A few acetylene producers use a dry generation process (otherwise known as the water to carbide process), which results in a powdered hydrated lime, commonly called carbide lime . A high level of control is exercised over the water addition and the calcium carbide/hydrate mixture is agitated continuously to prevent localised over-heating and the formation of undesirable polymerised by-products. 

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