Carbon deposition rate temperature effect

Experiments of propane pyrolysis were carried out using a thin tubular CVD reactor as shown in Fig. 1 [4]. The inner diameter and heating length of the tube were 4.8 mm and 30 cm, respectively. Temperature was around 1000°C. Propane pressure was 0.1-6.7 kPa. Total pressure was 6.7 kPa. Helium was used as carrier gas. The product gas was analyzed by gas chromatography and the carbon deposition rate was calculated from the film thickness measured by electron microscopy. The effects of the residence time and the temperature... 

The extent of the hydrocracking is, like the hydrodesulfurization reaction, dependent upon the temperature, and both reaction rates increase with increase in temperature. However, the rate of hydrocracking tends to show more marked increases with temperature than the rate of hydrodesulfurization. The overall effect of the increase in the rate of the hydrocracking reaction is to increase the rate of carbon deposition on the catalyst. This adversely affects the rate of hydro-desulfurization hydrocracking reactions are not usually affected by carbon deposition on the catalyst since they are more dependent upon the noncatalytic scission of covalent bonds brought about by the applied thermal energy. 

The pulse-flow reaction study performed at 298 K with a propyne dihydrogen ratio of 1 3, confirmed many of the infra-red spectroscopy results even though there was a considerable difference in residence time between the systems. No propane was detected in the reactor eluant and, apart from 10 % of the first pulse, only C-3 hydrocarbons were observed in the gas phase, supporting the infra-red results which indicated only C-3 species on the surface. It is also clear from Table 2 that the effect of the higher hydrogen concentration is to reduce the extent of carbon deposition over the first four pulses. The results at the lower temperature and dihydrogen propyne ratio, show that as the surface is covered with retained species the rate of hydrogenation decreases, the amount of carbon deposition decreases, and... 

The inner surface of the stainless steel reactor was deactivated by the following procedure. The tube was cleared by passing air at the rate of 1-2 L/min. for 30 min while maintaining a temperature of 575°C. Hydrogen sulfide was then passed for 30 min at the rate of 250 mL/min. Finally, hydrogen was passed for 30 min at the rate of 3 L/min. To equilibrate the surface a small amount of the liquid hydrocarbon was pumped before starting the experiment. This treatment apparently eliminates any major catalytic effects of the reactor surface, as indicated by the lack of carbon deposits in all runs. 

From the above comparisons it is evident that both structure and composition of the anion may influence the mechanism of decomposition of nickel carboxylates. The crystal structure of the reactant can probably be discounted as a rate controlling parameter because dehydration usually yields amorphous materials. Depending on temperature, carbon deposited on the surface of a germ metallic nucleus may effectively prevent or inhibit growth, it may be accommodated in the structure to yield carbide, or be deposited elsewhere (by carbide decomposition). These mechanistic interpretations are based on the relative reactivities of the nickel salt and of nickel carbide, for which the temperature of decomposition is known, 570 K [150]. 

Figure 2 shows the effect of temperature and benzene concentration on the kinetics of carbon deposition on a nickel foil. The temperature behavior follows a pattern that has been previously observed in the catalytic carbon formation from non-aromatic hydrocarbons (1 ,2). There are three regions in the Arrhenius plot. At low temperatures, the rate increases with increasing temperature and, thus, a negative slope-line is obtained in the Arrhenius plot. This low temperature region is denoted as Region I. 

Figure 1 shows the effect of temperature on the rate of carbon deposition. The rate was found to... 

A model based on the condensation theory was developed to explain the dependence of the rate of carbon deposition on the hydrogen and benzene partial pressure, on the gas phase residence time and on the temperature. The effect of hydrogen was explained assuming that it reacts with the free radicals that form the macromolecules of the condensation theory, decreasing the rate of production of carbon-forming intermediates. 

The activation energy of this reaction is higher than that of the FT reaction and hence the rate of carbon deposition increases more rapidly than that of the FT reaction with an increase in temperature. Thus, catalysts such as Co or Fe operating below about 250°C do not deposit elemental carbon during normal FT operations. When elemental carbon is deposited, the density of the particles is lowered and also catalyst fines are produced. The indirect effect of this on the FT performance is discussed later. 

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