Carbon oxide methanation

Supporting other oxides, supporting cluster molecules For carbon oxide methanation... 

PimUg). 1 Air. Oxygen. Hydrogen. Carbonic Oxide. Methane. Ethylene. 

Table 6 shows the states of the anthropogenic outputs and the total outputs of the main primary pollutants carbon oxides, methane, nitrogen oxides, sulphur dioxide, chlorofluorocarbons (CFCs) and halons (halogenated hydrocarbons), as well as the resulting concentrations. 

Liguras et al. investigated autothermal reforming of ethanol over ruthenium and nickel catalysts on structured supports such as ceramic foams and monoliths [212,213]. Conditions chosen were an O/C ratio of 0.61 and an S/C ratio of 1.5. The reaction was performed at a very high pre-heating temperature of the monoliths and consequently substantial conversion occurred even upstream of the reactor, which created a hot spot of up to 950 °C in the monoliths. A ceramic monolith coated with 5 wt.% ruthenium formed in addition to carbon oxides methane as the main byproduct, but there were also small amounts of acetaldehyde, ethylene and ethane [212]. When the S/C ratio was increased to 2.0, the by-products could be suppressed. Increasing the O/C ratio had a similar effect and also suppressed the methane formation. The ruthenium catalyst showed stable conversion for a 75-h test duration. Nickel/lanthana catalysts containing 13 wt.% nickel on a lanthana carrier showed similar performances with respect to activity, selectivity and stability [213]. 

Exothermic gas/solid reactions involving large quantities of gas, such as carbon oxide methanation with recycle of cold product gas for temperature control, can be carried out in a parallel passage reactor in which the reactants flow through narrow empty ch2in-nels between shallow beds of solid reactant or catalyst. 

One of the first studies of ethane oxidation under static conditions at pressures of 15—100 atm [22] showed that the oxidation of 9 1 C2H6—O2 mixtures yields, along with carbon oxides, methane, and water, various oxygenates, such as methanol, ethanol, formaldehyde, acetaldehyde, and formic and acetic acids. At higher pressures, the yield of C2 products (ethanol, acetaldehyde, and acetic acid) increases, whereas the yield of methanol and formaldehyde decreases. 

This corrosion phenomenon not only affects the skeleton material but also influences the gas-phase chemistry. Kempen and van Wortel [39] studied the influence of MD on gas reactions using both Fe- and Ni-based alloys. The gas mixtures involved in MD contained CO and Hj, CO2, HjO, and CH4 that were used by these authors to elaborate a kinetic model that considers CO reduction by H2, the water-gas shift reaction, the Boudouard reaction, and carbon oxide methanation. From their experiments it is clear that both the Boudouard reaction and the CO reduction are clearly affected by MD processes. 

The carbon m methane has the lowest oxidation number (—4) of any of the com pounds m Table 2 4 Methane contains carbon m its most reduced form Carbon dioxide and carbonic acid have the highest oxidation numbers (+4) for carbon corresponding to Its most oxidized state When methane or any alkane undergoes combustion to form carbon dioxide carbon is oxidized and oxygen is reduced A useful generalization from Table 2 4 is the following... 

Hydrogenation of the oxides of carbon to methane according to the above reactions is sometimes referred to as the Sabatier reactions. Because of the high exothermicity of the methanization reactions, adequate and precise cooling is necessary in order to avoid catalyst deactivation, sintering, and carbon deposition by thermal cracking. 

The manufacture of the highly pure ketene required for ketenization and acetylation reactions is based on the pyrolysis of diketene, a method which has been employed in industrial manufacture. Conversion of diketene to monomeric ketene is accompHshed on an industrial scale by passing diketene vapor through a tube heated to 350—600°C. Thus, a convenient and technically feasible process for producing ketene uncontaminated by methane, other hydrocarbons, and carbon oxides, is available. Based on the feasibiHty of this process, diketene can be considered a more stable form of the unstable ketene. 

Synthesis Gas Preparation Processes. Synthesis gas for ammonia production consists of hydrogen and nitrogen in about a three to one mole ratio, residual methane, argon introduced with the process air, and traces of carbon oxides. There are several processes available for synthesis gas generation and each is characterized by the specific feedstock used. A typical synthesis gas composition by volume is hydrogen, 73.65% nitrogen, 24.55% methane, <1 ppm-0.8% argon, 100 ppm—0.34% carbon oxides, 2—10 ppm and water vapor, 0.1 ppm. 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Conversion of tlie carbon oxides to methane, at the expense of hydrogen, goes almost to completion and the CO and CO2 content of the treated gas is on the order of a few ppm. A methanator typically operates in the temperature range of 300—400°C. These reactions are strongly exothermic and hence the CO and CO2 at the inlet to the methanator should be carefully monitored, to avoid temperature mnaway. 

Steam Reforming Processes. In the steam reforming process, light hydrocarbon feedstocks (qv), such as natural gas, Hquefied petroleum gas, and naphtha, or in some cases heavier distillate oils are purified of sulfur compounds (see Sulfurremoval and recovery). These then react with steam in the presence of a nickel-containing catalyst to produce a mixture of hydrogen, methane, and carbon oxides. Essentially total decomposition of compounds containing more than one carbon atom per molecule is obtained (see Ammonia Hydrogen Petroleum). 

It is clear that human action can affect seven of eight of the major gi eenhouse forcings carbon dioxide, methane, nitrous oxide, ozone, CFCs, aerosols, and water vapor. As studies of solar variation have shown, it is also clear that human action is not the only factor involved in determining the impact of these forcings. There is still substantial uncertainty regarding the actual climate impact of the climate forcings. 

The rich gas from the absorption operation is usually stripped of the desirable components and recycled back to the absorber (Figure 8-57). The stripping medium may be steam or a dry or inert gas (methane, nitrogen, carbon oxides—hydrogen, etc.). This depends upon the process application of the various components. 

The chapter by Bridger and Woodward deals with methanation as a means for removing carbon oxides from ammonia synthesis gas. This technology, together with earlier pioneer work by Dent and co-workers (I), are the forerunners of all modern methanation developments. The chapter deals with catalyst formulation and characterization and with the performance of these catalysts in commercial plants as a function of time on-stream. 

The chapter by Haynes et al. describes the pilot work using Raney nickel catalysts with gas recycle for reactor temperature control. Gas recycle provides dilution of the carbon oxides in the feed gas to the methanator, hence simulating methanation of dilute CO-containing gases which under adiabatic conditions gives a permissible temperature rise. This and the next two papers basically treat this approach, the hallmark of first-generation methanation processes. 

The synthesis gas for methanation, containing hydrogen and carbon oxides, is produced by gasification of coal by partial oxidation and/or by the reaction with steam. 

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