Carbon formation methane

Two other components, methanol and benzene, were included in this study. Methanol is important in processes using Rectisol Systems for C02 removal prior to methanation. Benzene was considered in order to determine the effect of aromatics on catalyst activity and potential carbon formation. 

The methanation of synthesis gas occurs by Reactions 1 and 2 in the absence of carbon formation. With the given hydrogen carbon monoxide... 

Various design and operating problems have been experienced by most developers of methanation systems. Specifically, carbon formation and catalyst sintering are two of the more common problems in methanation processes. Carbon formation refers to the potential production of carbon from carbon oxides and methane by the following reactions. 

Feed gases to most, if not all, methanation systems for substitute natural gas (SNG) production are theoretically capable of forming carbon. This potential also exists for feed gases to all first-stage shift converters operating in ammonia plants and in hydrogen production plants. However, it has been demonstrated commercially over a period of many years that carbon formation at inlet temperatures in shift converters is a relatively slow reaction and that, once shifted, the gas loses its potential for carbon formation. Carbon formation has not been a common problem at the inlet to shift converters. It has been no problem at all in our bench-scale work, and it is not expected to be a problem in our pilot plant operations. 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

The addition of steam to the CH4/C02 feedstock to avoid excessive carbon formation is a widely used technique in practical systems [3]. The resulting C02-steam gasification of methane process can be described by the following chemical equation ... 

Snoeck, J.-W., Froment, G. F., and Fowles, M. 2002. Steam/C02 reforming of methane. Carbon formation and gasification on catalysts with various potassium contents. Ind. Eng. Chem. Res. 41 3548-56. 

In the C02 reforming of methane, carbon formation can occur via two possible pathways CH4 decomposition and CO disproportionation (the Boudouard reaction). Carbon formation by CH4 decomposition is a structure-sensitive reaction (158,159). Specifically, the Ni(100) and Ni(110) surfaces are more active in the decomposition of CH4 to carbon than the Ni(lll) surface (158). The CO disproportionation,... 

Concerning the reduction step of the redox reaction, the heterotrophic microorganisms may use different electron acceptors. If oxygen is available, it is the terminal electron acceptor, and the process proceeds under aerobic conditions. In the absence of oxygen, and if nitrates are available, nitrate becomes the electron acceptor. The redox process then takes place under anoxic conditions. If neither oxygen nor nitrates are available, strictly anaerobic conditions occur, and sulfates or carbon dioxide (methane formation) are potential electron acceptors. Table 1.1 gives an overview of these process conditions related to sewer systems. 

For dry reforming, carbon formation is very likely, especially when carried out in a membrane reactor [24]. For this application noble metals are used, which are intrinsically less prone to carbon formation because, unlike nickel, they do not dissolve carbon. Irusta et al. [24] have shown above-equilibrium methane conversion in a reactor equipped with a self-supported Pd-Ag tube. Small amounts of coke were formed on their Rh/La203/Si02 catalyst, but this is reported not to have any effect on activity. 

Which has been studied on supported Ni catalysts and on Ni films . Studies such as those described here show that methane can be catalytically synthesized over Ni by an active (carbidic) carbon formed via the Boudouard reaction and its subsequent hydrogenation to methane. However, to demonstrate that this surface carbon route is the major reaction pathway, kinetic measurements of both carbon formation from CO and its removal by H2 were carried out . 

In the first set of measurements the rate of carbon build-up on a Ni(lOO) surface was measured at various temperatures as follows (1) surface cleanliness was established by AES (2) the sample was retracted into the reaction chamber and exposed to several torr of CO for various times at a given temperature (3) after evacuation the sample was transferred to the analysis chamber and (4) the AES spectra of C and Ni were measured. Two features of this study are noteworthy. First, two kinds of carbon forms are evident - a carbidic type which occurs at temperatures < 650 K and a graphite type at temperatures > 650 K. The carbide form saturates at 0.5 monolayers. Second, the carbon formation data from CO disproportionation indicates a rate equivalent to that observed for methane formation in a H2/CO mixture. Therefore, the surface carbon route to product is sufficiently rapid to account for methane production with the assumption that kinetic limitations are not imposed by the hydrogenation of this surface carbon. 

A second set of experiments further supported the surface carbon route to methane. In these experiments a Ni(lOO) surface was precarbided by exposure to CO and then treated with hydrogen in the reaction chamber for various times. Steps (3) and (4) above were then followed to measure the carbide level This study showed that the rate of carbon removal in hydrogen compared favorably to the carbide formation rate in CO and to the overall methanation rate in H2/CO mixtures. Thus in a H2-CO atmosphere the reaction rate is determined by a delicate balance of the carbon formation and removal steps and neither of these is rate determining in the usual sense. 

Barnett and co-workers recently reported that it might be possible to utilize hydrocarbons directly in SOFC with Ni-based anodes. " ° First, with methane. they observed that there is a narrow temperature window, between 550 and 650 °C. in which carbon is not as stable. The equilibrium constant for methane dissociation to carbon and Hz is strongly shifted to methane below 650 °C. and the equilibrium constant for the Boudouard reaction, the disproportionation of CO to carbon and COz, is shifted to CO above 550 °C. Therefore, in this temperature range, they reported that it is possible to operate the cell in a stable manner. (However, a subsequent report by this group showed that there is no stable operating window for ethane due to the fact that carbon formation from ethane is shifted to lower temperatures. ) In more recent work, this group has suggested that, even when carbon does form on Ni-based anodes, it may be possible to remove this carbon as fast as it forms if the flux from the electrolyte is sufficient to remove carbon faster than it is formed.Observations by Weber et al. have confirmed the possibility of stable operation in methane. Similarly, Kendall et al. showed that dilution of methane with COz caused a shift in the reaction mechanism that allowed for more stable operation. 

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