Carbon deposition mechanisms

Figure 3.33 Generalised carbon deposition mechanism as proposed by Sone et al. [120].

Many explosives, which have solid carbon as a detonation product, exhibit behavior that is not described adequately without including some time-dependent phenomenon, such as diffusion-controlled carbon deposition or some other kinetic behavior of the detonation products. A time-dependent carbon deposition is the only process known that could account for the large energy deficits required by the build-up model. The observed velocity constancy and large C-J pressure variations can be reproduced by the time-dependent carbon deposition mechanism. 

Thermal cracking tends to deposit carbon on the catalyst surface which can be removed by steaming. Carbon deposition by this mechanism tends to occur near the entrance of the catalyst tubes before sufficient hydrogen has been produced by the reforming reactions to suppress the right hand side of the reaction. Promoters, such as potash, are used to help suppress cracking in natural gas feedstocks containing heavier hydrocarbons. Carbon may also be formed by both the disproportionation and the reduction of carbon monoxide... 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

Polyphosphate, often with sodium chloride. This is a very low-tech approach, relying primarily on the threshold mechanism of polyphosphate to prevent calcium carbonate deposition at the membrane-water interface. Products based on this simple technology are subject to many limitations and probably are inappropriate to most industrial RO situations. 

Ni3C decomposition is included in this class on the basis of Doremieux s conclusion [669] that the slow step is the combination of carbon atoms on reactant surfaces. The reaction (543—613 K) obeyed first-order [eqn. (15)] kinetics. The rate was not significantly different in nitrogen and, unlike the hydrides and nitrides, the mobile lattice constituent was not volatilized but deposited as amorphous carbon. The mechanism suggested is that carbon diffuses from within the structure to a surface where combination occurs. When carbon concentration within the crystal has been decreased sufficiently, nuclei of nickel metal are formed and thereafter reaction proceeds through boundary displacement. 

Research on plasma-deposited a-C(N) H films has been frequently included in the general discussion of carbon nitride solids [2, 3]. However, the presence of hydrogen in its composition, and the complexity of the deposition process, which introduces the nitrogen species in the already intricate hydrocarbon plasma-deposition mechanism, make a-C(N) H films deserve special consideration. This is the aim of the present work to review and to discuss the main results on the growth, structure, and properties of plasma-deposited a-C(N) H films. As this subject is closely related to a-C H films, a summary of the main aspects relative to the plasma deposition of a-C H films, their structure, and the relationship between the main process parameters governing film structure and properties is presented... 

On the basis of the analysis presented in Tables II, III, and IV and measurements of the mass of C02 evolved during oxidation, Figure 1 was constructed to display the fraction of original carbon mobilized by heating, the fraction of the remaining (available) carbon mobilized as incompletely oxidized hydrocarbon by oxidation, and the fraction of available carbon deposited as coke by oxidation. The distribution of available carbon between the mobile and non-mobile products of oxidation lends additional support to our proposed "two-reactions" mechanism. 

Although the FTS is considered a carbon in-sensitive reaction,30 deactivation of the cobalt active phase by carbon deposition during FTS has been widely postulated.31-38 This mechanism, however, is hard to prove during realistic synthesis conditions due to the presence of heavy hydrocarbon wax product and the potential spillover and buildup of inert carbon on the catalyst support. Also, studies on supported cobalt catalysts have been conducted that suggest deactivation by pore plugging of narrow catalyst pores by the heavy (> 40) wax product.39,40 Very often, regeneration treatments that remove these carbonaceous phases from the catalyst result in reactivation of the catalyst.32 Many of the companies with experience in cobalt-based FTS research report that these catalysts are negatively influenced by carbon (Table 4.1). 

Moodley, D. J., van de Loosdrecht, J., Saib, A. M., Overett, M. J., Datye, A. K., and Niemantsverdriet, J. W. 2009. Carbon deposition as a deactivation mechanism of cobalt-based Fischer-Tropsch synthesis catalysts under reahstic conditions. Appl. Catal. A, 354 102-10. 

The most satisfactory mechanism used to date to convert the acetylene gas into graphite-like carbon has been electrical dissociation. The apparatus is shown in figure 9. The efficiency of the mechanism of dissociation is strongly dependent upon gas pressure, and after experimentation a pressure of about 10 torr has been selected as optimum. At this pressure, a hard black carbon deposit is observed on the substrate with a thickness of more than 0.1 mm. In the current source, samples produce several microamperes after about 1 hour of running, and this current remains very steady for several hours of operation. 

The history of the development of methane conversion to synthesis gas is summarized as an introduction to the partial oxidation of methane, which is reviewed with emphasis on hot spots in reactors, major developments in the reduction of O2 separation costs, and reaction mechanisms. The various catalysts employed in CO2 reforming are examined, with emphasis on inhibition of carbon deposition. 2004 Elsevier Inc. 

Bartholomew and coworkers32 described deactivation of cobalt catalysts supported on fumed silica and on silica gel. Rapid deactivation was linked with high conversions, and the activity was not recovered by oxidation and re-reduction of the catalysts, indicating that carbon deposition was not responsible for the loss of activity. Based on characterization of catalysts used in the FTS and steam-treated catalysts and supports the authors propose that the deactivation is due to support sintering in steam (loss of surface area and increased pore diameter) as well as loss of cobalt metal surface area. The mechanism of the latter is suggested to be due to the formation of cobalt silicates or encapsulation of the cobalt metal by the collapsing support. 

C. M. Chun and T. A. Ramanarayanan, Mechanism and Control of Carbon Deposition on High Temperature Alloys, J. Electrochem. Soc., 154 9 C465-C471 (2007). 

OZIPR contains two comprehensive chemical mechanisms that use two different approaches to lumping organics. The two mechanisms used in these models, the RADM (Regional Acid Deposition Model) and the Carbon Bond Mechanism (CBM), are discussed in Chapter 16.A.3b and in detail by Stockwell et al. [J. Geophys. Res., 95, 16343 (1990) and J. Geophys. Res., 102, 25847 (1997)] and by Gery et al. [J. Geophys. Res., 94, 12925 (1989)]. 

Deactivation is due primarily to two mechanisms formation of carbon-containing deposits and sulfur poisoning. Carbon deposition may be minimized by the addition of alkali metals, optimization of metal cluster size, and use of oxygen ion-conducting supports. Sulfur poisoning is usually irreversible and there are few reports of catalysts that are tolerant of sulfur levels typical of commercial fuels. 

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