Kinetic carbon deposition

Ivers-Tiffee E, Timmermann H, Leonide A, Mentzer NH, Malzbender J (2009) Methane reforming kinetics, carbon deposition, and redox durability of Ni/8 yttria stabilized zirconia (YSZ) anodes. In Vielstich W, Yokokawa H, Gasteiger HA (eds) Handbook of fuel cells fundamental technology and applications vol 6. Advanced in electrocatalysis, materials, diagnostics and durability Part 2. Wiley, Chichester, pp 933-956... 

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. 

Chemical vapor deposition (CVD) of carbon from propane is the main reaction in the fabrication of the C/C composites [1,2] and the C-SiC functionally graded material [3,4,5]. The carbon deposition rate from propane is high compared with those from other aliphatic hydrocarbons [4]. Propane is rapidly decomposed in the gas phase and various hydrocarbons are formed independently of the film growth in the CVD reactor. The propane concentration distribution is determined by the gas-phase kinetics. The gas-phase reaction model, in addition to the film growth reaction model, is required for the numerical simulation of the CVD reactor for designing and controlling purposes. Therefore, a compact gas-phase reaction model is preferred. The authors proposed the procedure to reduce an elementary reaction model consisting of hundreds of reactions to a compact model objectively [6]. In this study, the procedure is applied to propane pyrolysis for carbon CVD and a compact gas-phase reaction model is built by the proposed procedure and the kinetic parameters are determined from the experimental results. 

There are several factors that may be invoked to explain the discrepancy between predicted and measured results, but the discrepancy highlights the necessity for good pilot plant scale data to properly design these types of reactors. Obviously, the reaction does not involve simple first-order kinetics or equimolal counterdiffusion. The fact that the catalyst activity varies significantly with time on-stream and some carbon deposition is observed indicates that perhaps the coke residues within the catalyst may have effects like those to be discussed in Section 12.3.3. Consult the original article for further discussion of the nonisothermal catalyst pellet problem. 

Derbishire, F. and Trimm, D., Kinetics of deposition of pyrolytic carbon on nickel, Carbon, 13, 189, 1975. 

Many promoters have been used to improve the performance of Ni/Al203 catalysts. The effect of the basic oxides of Na, K, Mg, and Ca on Ni/Al203 was examined by a number of authors (178,203,211 -213). They found that these added oxides markedly decrease the carbon deposition. The kinetics results showed that the added metal oxides changed the reaction order in CH4 from negative to positive and that in C02 from positive to negative. This observation implies that the surface of a nickel catalyst incorporating basic metal oxides is abundant in adsorbed C02, whereas the surfaces devoid of these oxides are abundant in adsorbed CH4 (178). The coverage of nickel with C02 is most likely unfavorable to CH4 decomposition... 

De Deken and his colleagues197 have studied the nature of the carbon deposited on a commercial CCE catalyst (12 wt% Ni on a-Al203) and have concluded that it has diffused into the bulk of the nickel and that some of it is present as carbide. A more applied article from the same group198 presents intrinsic kinetic data from a tubular reactor in the temperature range 823-953K. 

Steam reforming of hydrocarbons has become the most widely used process for producing hydrogen. One of the chief problems In the process Is the deposition of coke on the catalyst. To control coke deposition, high steam to hydrocarbon ratios, n, are used. However, excess steam must be recycled and It Is desirable to minimize the magnitude of the recycle stream for economy. Most of the research on this reaction has focused mainly on kinetic and mechanistic considerations of the steam-methane reaction at high values of n to avoid carbon deposition ( L 4). Therefore, the primary objective of this studyis to determine experimentally the minimum value of n for the coke-free operation at various temperatures for a commercial catalyst. 

In the feed Is sufficiently high so that carbon cannot be present at equilibrium, the equilibrium composition can be calculated from consideration of only Chemical Reactions I and II. Accordingly, the activities of each of the species can be calculated If the equilibrium constants, Kj and KII and value of n are known provided the activity ratio Inequalities given In Equation 1 are met. If the Inequalities of Equation 1 are not met, coke deposition Is possible. Values of the n at which the equalities of Equation 1 are met represent the minimum steam/methane ratio, n., at which no carbon deposition will take place at equilibrium. Figure 1 displays n versus temperature, T. Extending the analysis to Include Chemical Reaction III permits the calculation of the equilibrium coke laydown as a function of n and T shown In Figure 2. These curves will be used later to compare with the corresponding kinetic curves obtained experimentally. 

In the gas phase, atomic hydrogen is necessary to initiate the gas phase chemical kinetics for production of the reactive hydrocarbonaceous radicals required for carbon deposition by the following reactions. 

---