Methane , reaction

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. 

By-product water formed in the methanation reactions is condensed by either refrigeration or compression and cooling. The remaining product gas, principally methane, is compressed to desired pipeline pressures of 3.4—6.9 MPa (500—1000 psi). Einal traces of water are absorbed on siHca gel or molecular sieves, or removed by a drying agent such as sulfuric acid, H2SO4. Other desiccants maybe used, such as activated alumina, diethylene glycol, or concentrated solutions of calcium chloride (see Desiccants). 

Methanation. The methanation reactions are the reverse of the reforming reactions... 

Methanation. Since 1902, when Sabatier discovered that carbon monoxide could be hydrogenated to methane [74-82-8] the methanation reaction (eq. 12) has been the subject of intense investigation (47,48) (see Hydrocarbons, C —C ). 

The methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

The methanation reaction is currently used to remove the last traces (<1%) of carbon monoxide and carbon dioxide from hydrogen to prevent poisoning of catalysts employed for subsequent hydrogenation reactions. Processes for conversion of synthesis gas containing large quantities of carbon monoxide (up to 25%) into synthetic natural gas have been investigated to serve plants based on coal-suppHed synthesis gas. 

Hydrogen and carbon monoxide are produced by the gasification reaction, and they react with each other and with carbon. The reaction of hydrogen with carbon as shown in reaction (27-15) is exothermic and can contribute heat energy. Similarly, the methanation reaction (27-19) can contribute heat energy to the gasification. These equations are interrelated by the water-gas-shift reaction (27-18), the equilibrium of which controls the extent of reactions (27-16) and (27-17). 

It is not possible, however, to calculate accurately actual gas composition by using the relationships of reactions (27-14) to (27-19) in Table 27-12. Since the gasification of coal always takes place at elevated temperatures, thermal decomposition (pyrolysis) takes place as coal enters the gasification reactor. Reaction (27-15) treats coal as a compound of carbon and hydrogen and postulates its thermal disintegration to produce carbon (coke) ana methane. Reaction (27-21) assumes the stoichiometiy of hydrogasifying part of the carbon to produce methane and carbon. 

The groups at the termini of the 1,4-pentadiene system also affect the efficiency and direction of the the di-7c-methane reaction. The general trend is that cyclization oceurs at the diene terminus that best stabilizes radical character. Thus, a terminus substituted with aryl groups will cyclize in preference to an unsubstituted or alkyl-substituted terminus ... 

The reaction produces additional hydrogen for ammonia synthesis. The shift reactor effluent is cooled and tlie condensed water is separated. The gas is purified by removing carbon dioxide from the synthesis gas by absorption with hot carbonate, Selexol, or methyl ethyl amine (MEA). After purification, the remaining traces of carbon monoxide and carbon dioxide are removed in the methanation reactions. 

Catalytic methanation is the reverse of the steam reforming reaction. Hydrogen reacts with carbon monoxide and carbon dioxide, converting them to methane. Methanation reactions are exothermic, and methane yield is favored at lower temperatures ... 

Figure 7. Effect of temperature on methanation reaction constants (10)...

Undoubtedly our understanding of the methanation reaction is unsatisfactory. Fortunately, the application of newer techniques (9) of vibrational and electronic spectroscopy to the study of the chemisorbed layer on single crystals will soon lead to greater insights and ultimately to better catalysts and better reactor design and operation. 

Steam-Moderated Process. The basic idea behind this approach is to limit the extent of conversion of the methanation reaction, Reaction 1, by adding steam to the feed gases. This process simultaneously provides for (46) elimination of the CO shift, Reaction 2, to get a 3 1 H2 CO ratio from the make-up gas ratio of about 1.5 1 and avoidance of carbon laydown by operation under conditions in which carbon is not a thermodynamically stable phase (see Chemistry and Thermodynamics section above). 

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). 

The catalysts were reduced with 100% H2 at 371 °C and an inlet space velocity of 1000/hr. Because of the carbon-forming potential of a dry gas recycle composition and the cost of reheating the recycle if the water produced by the methanation reaction is removed, a wet gas recycle composition was used. The catalyst loading, gas composition, and test conditions for these tests are listed in Table II, and the effects of nickel content are compared in Table III. 

An examination of some laboratory runs with diluted C150-1-02 catalyst can illustrate this problem. In one run with 304°C at inlet, 314 °C at exit, and 97,297 outlet dry gas space velocity, the following results were obtained after minor corrections for analytical errors. Of the CO present (out of an inlet 2.04 mole % ), 99.9885% disappeared in reaction while the C02 present (from an initial 1.96%) increased by over 30%. Equilibrium carbon oxides for both methanation reactions were essentially zero whereas the equilibrium CO based on the water-gas shift reaction at the exit composition was about one-third the actual CO exit of 0.03 mole %. From these data, activities for the various reactions may be estimated on the basis of various assumptions (see Table XIX for the effect of two different assumptions). 

The methanation process commonly operates at pressures up to 30 atm, and, with the nickel catalyst which is almost universally used for the process, the inlet temperature is about 300°C ( 570°F). Almost complete conversion of the oxides of carbon occurs giving a product synthesis gas containing less than 5 ppm CO + C02. The temperature rise for the exothermic methanation reactions is typically 35 °C (63°F). 

For SNG manufacture, it is necessary to reduce the residual hydrogen to a minimum in order to achieve a high calorific value. This is best realized if the synthesis gas, instead of having a stoichiometric composition, contains a surplus of C02 which can be utilized to reduce the H2 content by the C02 methanation reaction to less than 1% according to equilibrium conditions. The surplus C02 must be removed at the end of the process sequence. It is, of course, also possible to operate a methanation plant with synthesis gas of stoichiometric composition then there is no need for a final C02 removal system. The residual H2 content will be higher, and therefore the heating value will be lower (cf. the two long term runs in Table II). 

This transformation is only favorable at high temperatures, and at lower temperatures, e.g. 600°C, the methanation reaction occurs. 

Bis(K-butylthio)benzene, 42,24 4,4 -Bis(diethy amino)benzil, 41, 3 4,4 -Bis(dimethylamino)benzil, 41, 1 Bis(dimethylamino)methane, reaction... 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

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