The Methanation Reaction

An example where two desalptors are necessary is the CO methanation reaction  

Another distinct difference from anunonia synthesis is the fact the CO is a strongly adsorbed precursor as compared to N, which only binds weakly to most transition metal surfaces. Hence, there is need for including an extra reaction step, as described in Chapter 5. The rate is given by 

The activity map for CO hydrogenation shows a single maximum for (AE, AE ) = (0.5 eV, -3.0 eV), and it can be seen that Ru and Co the elemental metals are closest to the maximum. This is also what is found experimentally. Ni is used industrially simply because it is cheaper than the other metals. 

FIGURE 7.8 Turnover frequency (TOP) of ammonia synthesis as a function of the dissociative chemisorption energy of nitrogen. Top panel Experimental data from Aika et al. (1973). Middle panel Result of the microkinetic model for stepped metal surfaces (blue Une). Reaction conditions are 673 K, 100 bar, Hj N2 ratio of 3 1, and y = 0.1. The effect of potassium promotion has been included (red Une). Effects of promotion will be discussed in Chapter 12. Lower panel Microkinetic model using a two-site model for the adsorption of intermediates. Adapted from Vojvodic et al. (2014). 

Knowing the optimum value for (AE ) allows a search for other catalysts for this process. In such a computational search, it was predicted that Ni-Fe alloys should be closer to the maximum than Ni and Fe alone. This was confirmed in subsequent experiments (see Fig. 7.10). 

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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). 

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. 

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. 

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

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... 

Stevenson and Schissler (37) have shown the wide divergence which exists between two repeller studies on the methane reaction (B). A similar situation exists for the perdeutero reaction... 

In microkinetics, overall rate expressions are deduced from the rates of elementary rate constants within a molecular mechanistic scheme of the reaction. We will use the methanation reaction as an example to illustrate the... 

Oxygen-containing molecules cannot be tolerated in the ammonia synthesis, primarily because they form iron oxide that blocks the active surface. First the CO2 is removed, through a scrubber, by reaction with a strong base. The remaining CO (and CO2) is then removed by the methanation reaction, converting the CO into methane and water. Finally the water is removed by, for example, molecular sieves. Methane does not present problems because it interacts weakly with the catalyst surface. The gas mixture (Tab. 8.6) is compressed to the roughly 200 bar needed for ammonia synthesis and admitted to the reactor. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Reaction (9), the methanation reaction, proceeds very slowly at low temperatures in the absence of catalysts. 

Reaction between carbon monoxide and dihydrogen. The catalysts used were the Pd/Si02 samples described earlier in this paper. The steady-state reaction was first studied at atmospheric pressure in a flow system (Table II). Under the conditions of this work, selectivity was 100% to methane with all catalysts. The site time yield for methanation, STY, is defined as the number of CH molecules produced per second per site where the total number of sites is measured by dihydrogen chemisorption at RT before use, assuming H/Pd = 1. The values of STY increased almost three times as the particle size decreased. The data obtained by Vannice et al. (11,12) are included in Table II and we can see that the methanation reaction on palladium is structure-sensitive. It must also be noted that no increase of STY occurred by adding methanol to the feed stream which indicates that methane did not come from methanol. 

Methanation Reactions While carrying out the WGS reaction, methane can be formed in the reactor through the methanation reaction, which is the reverse methane SR reaction and is highly exothermal. 

As yet, the only work that has been reported on a single crystalline surface has been done by Hoffmann and Robbins . They used the same approach as Burrows eta/. to study the methanation reaction (CO-I-3H2- CH4 + HjO) on a Ru(001) surface. Figure 21 shows the spectra of the C—O stretch mode taken at 50 torr of a mixture of CO and Hj at temperatures up to 600 K. The decrease in peak height at SOO K indicates the buildup of a passivating carbon layer. The negative absorption around 2140 cm is due to imperfect canceling of the gas phase CO band. 

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

The reactions occurring in the POX of liquid hydrocarbons are extremely complex. However, the overall catalytic POX reaction can be described by the simple reaction (1). This is followed by the WGS reaction (reaction (3)) and the methanation reaction. 

The reverse reaction, steam cracking of methane, involves the same elementary steps as the methanation reaction. The kinetics for that reaction have been developed for a single direct mechanism by Snagovskii and Ostrovskii (39). 

We have chosen to concentrate on a specific system throughout the chapter, the methanation reaction system. Thus, although our development is intended to be generally applicable to packed bed reactor modeling, all numerical results will be obtained for the methanation system. As a result, some approximations that we will find to apply in the methanation system may not in other reaction systems, and, where possible, we will point this out. The methanation system was chosen in part due to its industrial importance, to the existence of multiple reactions, and to its high exothermicity. 

To retain consistency throughout this presentation, we will consider a general nonadiabatic, packed bed reactor, as shown in Fig. 1, with a central axial thermal well and countercurrent flow of cooling fluid in an exterior jacket.1 We focus on the methanation reaction since methanation is a reaction of industrial importance and since methanation exhibits many common difficulties such as high exothermicity and undesirable side reactions. 

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