Hydrocarbons decomposition

Deposition of coke, tar formed by glycol overheating and/or hydrocarbon decomposition. 

The CVD of graphite is theoretically simple and is based on the thermal decomposition (pyrolysis) of a hydrocarbon gas. The actual mechanism of decomposition, however, is complex and still not completely understood. This may be due, in part, to the fact that most of the studies on the subject of hydrocarbon decomposition are focused on the improvement of fuel efficiency and the prevention of carbon formation (e.g., soot), rather than the deposition of a coating. 

Extensive catalyst poisoning results from the deposition of carbon in or on the catalyst surface. In technological parlance, this is usually referred to as the formation of coke. At temperatures in excess of those normally used for catalysis, hydrocarbon decomposition at metal surfaces is known... 

Guemter, S. et al., Photocatalytic composite element for cleavage of hydrogen containing compounds, especially, for hydrocarbon decomposition and wastewater treatment, German Patent No. DE 10210465,2003. 

The main purpose of this chapter is to survi atmospheric concentrations of photochemical oxidants, with emphasis on surface concentrations and the distribution patterns associated with them. The reason for that em> phasis is that the photochemical oxidants that affect public health and welfare are largely concentrated in this region. The whole subject of stratospheric ozone (and its filtering of ultraviolet light and interactions with supersonic-transport exhaust products), nuclear weapon reaction products, and halogenated hydrocarbon decomposition pr ucts is not treated here. 

The pyrolytic reforming reactor was a packed bed in a quartz tube reactor. Quartz was selected to reduce the effect of the reactor construction material on the hydrocarbon decomposition rate. ° The reactor was packed with 5.0 0.1 g of AC (Darco KB-B) or CB (BP2000) carbon-based catalyst. The reactor was heated electrically and operated at 850—950 °C, and the reactants had a residence time of 20—50 s, depending on the fuel. The reactor was tested with propane, natural gas, and gasoline as the fuels. Experiments showed that a flow of 80% hydrogen, with the remainder being methane, was produced for over 180 min of continuous operation.The carbon produced was fine particles that could be blown out... 

In the preceding expression, log(FJ is related to the depression of the fall-off curve at the center relative to the L-H expression in a og k/k ) vs. log(2f/(l -I- X)) plot. The values for F<. can then be related to the properties of specific species and reaction and temperature using methods discussed in Gardiner and Troe (1984). In Fig. 19, values of F for a variety of hydrocarbon decompositions are presented. As evident from this figure, in the limit of zero or infinite temperatures and pressures, all reactions exhibit Lindemann-Hinshelwood behavior and F approaches unity. From this figure, it is clear that L-H analysis generally does an adequate job in... 

This requires sufficient energy inserted into the relevant bond vibration for the bond to break or for bonding locations to move. C-C and C-H bond energies in stable alkanes are greater than 80 kcal/molc, and these processes are very infrequent. As we wiU see later, hydrocarbon decomposition, isomerization, and oxidation reactions occur primarily by chain reactions initiated by bond breaking but are propagated by much faster abstraction reactions of molecules with parent molecules. 

With these parameters, the half-life of a typical hydroperoxide is about 1 second at 330°C. and about 10 seconds at 290°C. These short lifetimes permit the hydroperoxides to act as secondary initiation sources to increase the rate of hydrocarbon decomposition. This is the effect that has been described by Semenov and his co-workers as degenerate chain branching. 

In the case of carbon nanotubes (CNTs), numerous syntheses methods have been developed during the last years, for example, the discharge between two graphite electrodes, laser ablation, hydrocarbon decomposition, and catalytic chemical vapor decomposition (CCVD) however, the most applied methods are arc discharge, laser ablation, and CVD [177-179]. 

Thermochemistry. Profiles of potential energy surfaces which are representative of hydrocarbon decomposition reactions and the associ-... 

Figure 5.2 A schematic of the mechanism of the nanofilaments growth at catalytic pyrolysis of hydrocarbons. Hydrocarbon decomposition on the metal nanoparticle (the dark area) surface produces chemisorbed atomic carbon Cg species with a high chemical potential. In the (pseudo)fluidized catalyst particle, the atomic carbon is capable of diffusing through the metal nanoparticles toward the interphase boundary between the active component and the growing face of the carbon nanofiber (the light areas).

Figure 21 Carbon nanotube production, (a) carbon arc method for multi-walled carbon nanotubes (b) the carbon arc method with metal impregnated rods for single wall nanotube production (c) laser irradiation of metal impregnated graphite for single wall nanotube production (d) hydrocarbon decomposition in presence of a catalyst for multiwall nanotube production (e) fullerene oven to produce graphitic nanoparticles, including short tubes, by heating fullerene soot to high temperatures in vacuum.

In a complementary series of experiments the cobalt/molybdenum disulfide samples were initially treated in hydrogen at S00°C for O.S hours. Under these conditions metal particles which had accumulated at edges were observed to catalyze the removal of material from these regions. When such specimens were subsequently heated in the presence of acetylene there was no evidence for the formation of carbon filaments. It was apparent that when specimens were treated in the hydrocarbon for extended periods at 6S0°C then many of the surface features were obscured by the build up of carbon deposits resulting from uncatalyzcd hydrocarbon decomposition reactions. 

The effect of the addition of a potassium promoter to a nickel steam reforming catalyst has been probed in terms of the propensity of the catalyst to resist carbon formation. It has been found that potassium facilitates a reduced accumulation of carbon by decreasing the rate of hydrocarbon decomposition on the catalyst and by increasing the rate of steam gasification of filamentary carbon from the catalyst. The effect of the promoter on the carbon removal reaction is evident in an enhancement of the pre-exponential factor in the rate equation by promotion of water adsorption on the catalyst surface. 

A.D.Stepukhovich and E.E.Nikitin, Kinetics and mechnism of hydrocarbon decomposition initiation of the butane cracking by traces of azomethane, Zhum.Fiz.Khim. 30, 1291 (1956)... 

---