Methanation reaction system

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. 

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 scheme of commercial methane synthesis includes a multistage reaction system and recycle of product gas. Adiabatic reactors connected with waste heat boilers are used to remove the heat in the form of high pressure steam. In designing the pilot plants, major emphasis was placed on the design of the catalytic reactor system. Thermodynamic parameters (composition of feed gas, temperature, temperature rise, pressure, etc.) as well as hydrodynamic parameters (bed depth, linear velocity, catalyst pellet size, etc.) are identical to those in a commercial methana-tion plant. This permits direct upscaling of test results to commercial size reactors because radial gradients are not present in an adiabatic shift reactor. 

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). 

The SASOL plant was operated with a surplus of C02 during a long term test of 4000 hrs. Of the C02 in the synthesis gas, 33.4% was metha-nated while the remaining 66.6% left the reaction system unconverted. Product gas from final methanation yielded specification grade SNG containing residual hydrogen of 0.7 vol % and residual CO of less than 0.1 vol %. The heating value was 973 Btu/standard cubic foot (scf) after C02 removal to 0.5 vol % (calc.). 

Liquid-solid reaction systems, gas- 152 (LPM), liquid-phase methanation 149 LPM process, development of. .. 151... 

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. 

In order to determine the errors that may be introduced by the Zeldovich model, Miller and Bowman [6] calculated the maximum (initial) NO formation rates from the model and compared them with the maximum NO formation rates calculated from a detailed kinetics model for a fuel-rich (isothermal system was assumed and the type of prompt NO reactions to be discussed next were omitted. Thus, the observed differences in NO formation rates are due entirely to the nonequilibrium radical concentrations that exist during the combustion process. Their results are shown in Fig. 8.1, which indicates... 

Di-TT-methane reactions are not restricted to 1,4-dienes. Other 1,4-unsatu-rated systems, such as p,y-unsaturated ketones, undergo similar rearrangements yielding cyclopropyl ketones in the oxa-di-TT-methane (ODPM) version of the reaction. The first example of the ODPM reaction was reported in 1966, in which irradiation of ketone 3 affords the cyclopropyl ketone 4 [13] (Scheme 3). 

Another interesting observation in this study is the boron trifluoride etherate-catalyzed rearrangement of tri-ir-methane systems 141 that afford the corresponding cyclopentenes 142. These reactions can be considered as the first examples of tri-ir-methane rearrangements in the ground state. Interestingly, compounds 141 only undergo conventional di-TT-methane reactions on irradiation. The mechanism shown in Scheme 25 is proposed to account for this novel reaction [79]. 

In the first part of the chapter, a state-of-the-art review and also a thermodynamic analysis of the autothermal reforming reaction are reported. The former, relevant to both chemical and engineering aspects, refers to the reaction system and the relevant catalysts investigated. The latter discusses the effect of the operating conditions on methane conversion and hydrogen yield. 

Much recent research (7-5) has been devoted to converting methane to products that are more easily transported and more valuable. Such more valuable products include higher hydrocarbons and the partial oxidation products of methane which are formed by either direct routes such as oxidative coupling reactions or indirect methods via synthesis gas as an intermediate. The topic of syngas formation by oxidation of CH4 has been considered primarily from an engineering perspective (7-5). Most fundamental studies of the direct oxidation of CH4 have dealt with the CH4 + O2 reaction system in excess O2 and at lower temperatures (6-10). 

The difference in H2 selectivity between Pt and Rh can be explained by the relative instability of the OH species on Rh surfaces. For the H2-O2-H2O reaction system on both and Rh, the elementary reaction steps have been identified and reaction rate parameters have been determined using laser induced fluorescence (LIF) to monitor the formation of OH radicals during hydrogen oxidation and water decomposition at high surface temperatures. These results have been fit to a model based on the mechanism (22). From these LIF experiments, it has been demonstrated that the formation of OH by reaction 10b is much less favorable on Rh than on Pt. This explains why Rh catalysts give significantly higher H2 selectivities than Pt catalysts in our methane oxidation experiments. 

The methanation reaction is a highly exothermic process (AH = —49.2 kcal/ mol). The high reaction heat does not cause problems in the purification of hydrogen for ammonia synthesis since only low amounts of residual CO is involved. In methanation of synthesis gas, however, specially designed reactors, cooling systems and highly diluted reactants must be applied. In adiabatic operation less than 3% of CO is allowed in the feed.214 Temperature control is also important to prevent carbon deposition and catalyst sintering. The mechanism of methanation is believed to follow the same pathway as that of Fischer-Tropsch synthesis. 

The reactions of alkenes and related compounds are grouped here into nine sections. The first five deal essentially with photoisomeriza-lion processes—geometrical isomerization about a carbon-carbon double bond, concerted (electrocydic) cyclization, concerted shifts (usually of hydrogen) along the ir-system, the di-jr-methane reaction. 

In combustion systems it is generally desirable to minimize the concentration of intermediates, since it is important to obtain complete oxidation of the fuel. Figure 13.5 shows modeling predictions for oxidation of methane in a batch reactor maintained at constant temperature and pressure. After an induction time the rate of CH4 consumption increases as a radical pool develops. The formaldehyde intermediate builds up at reaction times below 100 ms, but then reaches a pseudo-steady state, where CH2O formed is rapidly oxidized further to CO. Carbon monoxide oxidation is slow as long as CH4 is still present in the reaction system once CH4 is depleted, CO (and the remaining CH2O) is rapidly oxidized to CO2. 

The initiating step in the oxidation of methane is the first abstraction of a hydrogen atom. However, because of the tetrahedral molecular structure with comparatively high C-H bond energies, the methane molecule is extremely stable, and at lower temperatures the initiation step may be rate limiting for the overall conversion. In methane-oxygen systems, the chemistry is generally initiated by reaction of CH4 with O2,... 

A final possibility for a type of reaction resulting in acetylene formation which must be considered is that of combination of radicals. This is not likely to be significant in most systems, but in the case of methane reactions it becomes of particular interest. The production of ethylene, and acetylene in the thermal decomposition of methane requires some sort of combination reaction (40, 65). Kassel suggested a mechanism involving methylene radicals ... 

The barriers for all CH3Y/CH2=Y- systems are lower than for the CH4/ CHj" system. This means that the stabilization of the transition states by the Y-group is greater than that of the respective anions. The situation is illustrated in Fig. 4 for the case of CH3N02 it shows that the transition state for the CH3NO2 reaction is more stable than the transition state for the methane reaction by 73.3 kcal mol-1 while CH2=N02 is more stable than Cl I3 by only 59.1 kcal mol-1. 

All hydrocarbons in the feed higher than methane are assumed to be instantly cracked into CH4, CO2, 112, and CO. Consequently the reaction system inside a reformer tube is described by the rate expressions of the kinetics of steam reforming in the methane reactions I, II, and III. 

The catalyst intraparticle reaction-diffusion process of parallel, equilibrium-restrained reactions for the methanation system was studied. The non-isothermal one-dimensional and two-dimensional reaction-diffusion models for the key components have been established, and solved using an orthogonal collocation method. The simulation values of the effectiveness factors for methanation reaction Ch4 and shift reaction Co2 are fairly in agreement with the experimental values. Ch4 is large, while Co2 is very small. The shift reaction takes place as direct and reverse reaction inside the catalyst pellet because of the interaction of methanation and shift reaction. For parallel, equilibrium-restrained reactions, effectiveness factors are not able to predict the catalyst internal-surface utilization accurately. Therefore, the intraparticle distributions of the temperature, the concentrations of species and so on should be taken into account. 

Promotion of Hydrogen Exchange The radicals derived from the coal molecules by simple thermal homolyses and by molecule-induced homolyses initiate exchange reactions between tetralin-d, and the donors in the reaction system—principally diphenyl-methane and the hydroaromatic compounds in the macerals (2. ... 

However, most fuel cell systems can tolerate methane concentrations up to at least 1% in the reformate, no special purification reactions are required. In contrast, hence, removing small residual amounts of carbon monoxide from pre-purifled reformate applying the methanation reaction may be considered as an alternative to the preferential oxidation of carbon monoxide, provided that the CO concentration is low enough to have no significant impact on the hydrogen yield. However, no applications of methanation for CO clean-up in micro structured devices appear to have been reported, hence the issue is not discussed in depth. Finally, during hydrocarbon reforming all hydrocarbon species (saturated and unsaturated) smaller than the feed molecule may be formed. 

The gold catalyst showed only negligible conversion at this reaction temperature. The value of 96% conversion found by the authors exceeds the thermodynamic equilibrium (Figure 2.51), which might be due to the formation of methane disturbing the reaction system. However, the authors claim to have detected no... 

Aqueous hydrogen peroxide of a given concentration was poured into a vessel with a piston pump, which continuously injected it into the reactor. Methane was supplied from a gas cylinder with pressure sensors at the output. It was purified and dried, then heated and injected into the reactor. The reaction system is of a homogeneous (non-catalytic) flow type and operates in plug flow mode. 

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