Methanation catalyst composition

Remaining trace quantities of CO (which would poison the iron catalyst during ammonia synthesis) are converted back to CH4 by passing the damp gas from the scmbbers over a Ni methanation catalyst at 325° CO -t- 3H2, CRt -t- H2O. This reaction is the reverse of that occurring in the primary steam reformer. The synthesis gas now emerging has the approximate composition H2 74.3%, N2 24.7%, CH4 0.8%, Ar 0.3%, CO 1 -2ppm. It is compressed in three stages from 25 atm to 200 atm and then passed over a promoted iron catalyst at 380-450°C ... 

For the methanation reaction in the process of converting coal to a high Btu gas, various catalyst compositions were evaluated in order to determine the optimum type catalyst. From this study, a series of catalysts were developed for studying the effect of nickel content on catalyst activity. This series included both silica- and alumina-based catalysts, and the nickel content was varied (Table I). 

Recently, such a temperature oscillation was also observed by Zhang et al (27,28) with nickel foils. Furthermore, Basile et al (29) used IR thermography to monitor the surface temperature of the nickel foil during the methane partial oxidation reaction by following its changes with the residence time and reactant concentration. Their results demonstrate that the surface temperature profile was strongly dependent on the catalyst composition and the tendency of nickel to be oxidized. Simulations of the kinetics (30) indicated that the effective thermal conductivity of the catalyst bed influences the hot-spot temperature. 

In addition to importance of the catalyst composition and temperature, we have shown that methane partial oxidation selectivity is strongly affected by the mass transfer rate. Our experiments show that increasing the linear velocity of the gases or choosing a catalyst geometry that gives thinner boundary layers enhances the selectivity of formation of H2 and CO. Since H2 and CO are essentially intermediate... 

The same catalyst compositions used in the more important methane steam reforming [Eq. (3.1), forward reaction], may be used in methanation, too.222 All Group VIE metals, and molybdenum and silver exhibit methanation activity. Ruthenium is the most active but not very selective since it is a good Fischer-Tropsch catalyst as well. The most widely used metal is nickel usually supported on alumina or in the form of alloys272,276,277 operating in the temperature range of 300-400°C. 

In earlier work, it was found for borides, silicides and nitrides that specific activity, expressed as total rate of methane consumption per unit surface area, plummeted with increasing surface area of the catalyst samples.1718 The same relationship was also found for transition metals carbides (Figure 16.4). It should be noted the dependence of specific activity on surface area rather than catalyst composition is unusual for heterogeneous catalytic reactions. In addition, it can be found that the reaction order in the oxidant is perceptibly in excess of 1 (Tables 16.8 and 16.9). Such an order is hard to explain in terms of common mechanism schemes for heterogeneous catalytic oxidative reactions. 

The two methanation reactions are strongly exothermic. The temperature rise for typical methanator gas compositions in hydrogen plants is about 74°C (133°F) for each 1% of carbon monoxide converted and 60°C (108°F) for each 1% of carbon dioxide converted. At higher temperatures, the intrinsic rates of both methanation reactions can become sufficiently fast for diffusion effects to become important as shown in Figure 5.42. Under these conditions, film diffusion controls the overall rate of reaction. Diffusion limitations can be overcome to some extent by using a catalyst with a smaller particle size (3.1mm diameter by 3.6 mm long compared to regular catalyst dimensions of 5.4 mm by... 

The reaction takes place under fuel-rich conditions to maintain a nonflammable feed mixture. Typical feed composition is 13% to 15% ammonia, 11% to 13% methane and 72% to 76% air on a volumetric basis. Control of feed composition is essential to guard against deflagrations as well as to maximize the yield. The yield from methane is approximately 60% of theoretical. Conversion, yields, and productivity of the HCN synthesis are influenced by the extent of feed gas preheat, purity of the feeds, reactor geometry, feed gas composition, contact time, catalyst composition and purity, converter gas pressure, quench time and materials of construction. 

Rates of deactivation of Ni and Ni bimetallic catalysts as a result of poisoning by 10-ppm H2S during methanation were investigated in a series of studies by Bartholomew and co-workers (23, 113, 161, 194). Effects of catalyst composition and geometry, gas composition and reaction temperature on the rate of deactivation were considered. Deactivation rates were found to be relatively insensitive to temperature and quite sensitive to gas and catalyst composition (194). In fact, the rates of deactivation were 2-3 times more rapid in a H2-rich mixture (H2 /CO = 99), compared to a normal synthesis (H2/CO = 3-4) mixture. 

Methane-coupling reaction conversions and yields less than 25 percent initially were—and still are—below those acceptable for commercial fuel and chemical feedstock production. But worldwide research and development in more recent years continue to suggest that variations in process parameters, reactor design, and catalyst composition and structure may bridge this gap. Lower reaction temperatures—in the 300-400°C range may... 

This leads to a selectivity limitation in the Fischer Tropsch synthesis, as is shown in Figure 8 [42], which clearly demonstrates that it is impossible to develop FT catalysts selectively yielding only one compound, except the Ci cmnpounds methane and methanol, although selectivity tailoring to broader product distributions such as diesel (C9 - 2 ) is viable. It is important to keep in mind that once the progression coefficient a is fixed, the whole product distribution is determined. The constant a depends on both catalyst composition and particle size used and also on rcactitm parameters 43,44],... 

Ceria-based systems showed mixed effects for methane oxidation. Composite catalysts of Ag/Ce02 fall apart, forming large silver metal aggregates and deactivating the catalyst system (38). The only system in which silver-modified ceria found any promise is in solid oxide fuel cells utilizing yttira-stabilized zirconia however, the silver-based system was not the optimum one in this case (39). 

Steam reforming is the reaction of steam with hydrocarbons to make a manufactured gas containing mostly methane with trace amounts of ethylene, ethane, and hydrogen. For the manufacture of this gas, a representative catalyst composition contains 13 wt % Ni, 12.1 wt % U, and 0.3 wt % K it is particularly resistant to poisoning by sulfur. To make hydrogen, the catalyst contains oxides of Ni, Ca, Si, Al, Mg, and K. Specific formulations are given by Satterfield (1980). 

Ni/Mg-Al and Ni-Fe(orCu)/Mg-Al are good catalysts for steam reforming of methane. XRD results showed the presence of defined structures NiO-MgO, NiAl204 or MgAla04. After reaction, the formation of metallic phase Ni° and Ni-Fe or Ni-Cu alloys was detected. The catalyst reducibility increased in the presence of Fe or Cu species, due to the reduction assistance of these elements. TEM-EDX, carried out after reaction on Ni-Fe/Mg-Al catalyst, confirms the formation of Ni-Fe alloy with a ratio Ni/Fe 2. The catalytic reactivity, for all tested catalysts, showed good performance with high CH4 conversion, high H2-yield and low coke formation. The presence of an excess of water in the CH4 -i-H2O gas mixture (H2O/CH4 ratio=3) decreased the coke formation and increased the H2-production. The incorporation of Fe or Cu species in the catalyst composition increased the catalyst stability and decreased the coke formation due probably to the formation of Ni-Fe or Ni-Cu alloys. 

The paper presents data on development of Mn oxide catalysts for selective oxidation of lean methane mixtures with air to produce CO2 and generate heat. To obtain catalysts, new approaches to the synthesis of polyoxide materials based on Mn were adopted. Catalysts were modified by doping with La, Ce, Ba and Sr nitrates which were deposited from solutions onto the stabilized 2%Ce/0-Al2O3 support (of surface area 100 m /g and pellet diameter 4-5 mm). By varying the components of the impregnation mixture, it was possible to optimize the chemical composition and ratio of elements in the multi-component catalysts (at Ba Sr La Ce Mn = 1 1 1 7 10 ratio). The catalyst composition conformed to the oxide stoichiometry in the perovskite structure. 

For an Al/Co ratio of about 1.5, ethane was also the main decomposition product at room temperature But the amount of methane increased significantly with the temperature and represented 50% of the hydrocarbons formed at 180°C in the presence of hydrogen This result could indicate a change of the catalyst composition even if we cannot explain clearly the origin of the methane ethane hydrogenolysis, AlEt acac decomposition,.. . ... 

J. Rasko, P. Pereira, G.A. Somoijai, and H. Heinemann. The Catalytic Low Temperature Oxydehydrogenation of Methane Temperature Dependence, Carbon Balance and Effects of Catalyst Composition. Catal. Lett. 9 395 (1991). 

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