Methanation thermodynamic equilibrium

Direct thermal decomposition of methane was carried out, using a thermal plasma system which is an environmentally favorable process. For comparison, thermodynamic equilibrium compositions were calculated by software program for the steam reforming and thermal decomposition. In case of thermal decomposition, high purity of the hydrogen and solidified carbon can be achieved without any contaminant. 

Figure 2.10 provides a thermodynamic equilibrium molar fraction of the products of CPO of methane as a function of temperature. It is evident that at temperatures above 800°C, hydrogen and CO (in molar ratio of 2 1) are two major products of the reaction. The oxidant (oxygen or air) and the hydrocarbon feedstock (e.g., methane) are premixed in a mixer... 

Figure 2.14 depicts the thermodynamic equilibrium data related to C02 reforming of methane at atmospheric pressure. It is noteworthy that at temperatures below 800°C, elemental... 

Thermodynamic equilibrium data for methane decomposition reaction at atmospheric pressure. 

Figure 2.19 provides the thermodynamic equilibrium data for methane decomposition reaction. At temperatures above 800°C, molar fractions of hydrogen and carbon products approach their maximum equilibrium value. The effect of pressure on the molar fraction of H2 at different temperatures is shown in Figure 2.20. It is evident that the H2 production yield is favored by low pressure. The energy requirement per mole of hydrogen produced (37.8 kj/mol H2) is significantly less than that for the SMR reaction (68.7 kj/mol H2). Owing to a relatively low endothermicity of the process, <10% of the heat of methane combustion is needed to drive the process. In addition to hydrogen as a major product, the process produces a very important by-product clean carbon. Because no CO is formed in the reaction, there is no need for the WGS reaction and energy-intensive gas separation stages.

In addition to small amounts of methane, acetaldehyde or acetic anhydride can be generated in substantial quantities depending on conditions. However, they are not present simultaneously in any appreciable quantity. Acetic anhydride and acetaldehyde must be competitively formed (equation 6), and subsequently react with each other to form EDA (step C). This reaction (step C) is generally catalyzed by protic acids (2-4). The reaction solution for reductive carbonylation is quite acidic HI is an intermediate generated under reaction conditions of high alkyl iodide concentration and hydrogen pressure. The thermodynamic equilibrium of this condensation is quite favorable for diester formation existence of an abundance of either anhydride or aldehyde in the presence of the other is not found. Yields of stoichiometric preparations are in excess of 95%... 

For CO, reforming of methane. KIT-1 performed better than Al20, or La,0 as support. Ni/KIT-1 co-impregnated with 3 wt% Ca lasted 20 h without deactivation, and CO, and methane conversions close to the thermodynamic equilibrium were obtained. According to TG/DTA. coke formed during a given reaction increased in the order of Ni/Ca/KIT-1 < Ni/K1T-1 < Ni/Al,0, < ICI 46-1. Methane combustion study showed the activity pattern of Pd/KIT-1 > Pd/MCM-41. Pd/HMS > Pd/Al,0, > Pd/SiO,. MIBK combustion experiment demonstrated that catalyst ignition temperature can be lowered by ca. 30-35 °C when Pt was supported on KIT-1. MCM-41. MCM-48 and HMS produced similar results. 

We also made thermodynamic equilibrium calculations to see how much methane could be formed by methanation of carbon oxides. These calculations show that, for all conditions within the reactor tube, it is possible (thermodynamically) to form methane in quantities even greater than those observed. 

Steam reformers are used industrially to produce syngas, i.e., synthetic gas formed of CO, CO2, 11-2, and/or hydrogen. In this section we present models for both top-fired and side-fired industrial steam reformers by using three different diffusion-reaction models for the catalyst pellet. The dusty gas model gives the simplest effective method to describe the intermediate region of diffusion and reaction in the reformer, where all modes of transport are significant. This model can predict the behavior of the catalyst pellet in difficult circumstances. Two simplified models (A) and (B) can also be used, as well as a kinetic model for both steam reforming and methanation. The results obtained for these models are compared with industrial results near the thermodynamic equilibrium as well as far from it. 

Note that all three models give almost the same exit conversion and yield for methane and carbon dioxide and that the second unit (2) is also operating relatively closely to its thermodynamic equilibrium, though further away from it when compared to Plant (1). The close agreement between the industrial performance data and the simulated data for the reformers (1) and (2) that was obtained by three different diffusion-reaction models validates the models that we have used, at least for plants operating near their thermodynamic equilibria. 

For Plant (3) the exit conversions and yields of methane and carbon dioxide obtained by all three models are much lower than the equilibrium values. Therefore, Plant (3) is run far from its thermodynamic equilibrium. Large differences between the predictions of the three models exist in the data the exit conversion simulations of methane differ by 16 to 23% that of the carbon dioxide yield by 12 to 18%. Since the dusty gas model is the more rigorous one, we can use its simulation output as a base for comparison in place of experimental or industrial data which is unavailable in this case. 

Catalyst testing was performed in packed beds at a S/C ratio of three and reaction temperatures between 527 and 750 °C. The feed was composed of 12.5 vol.% methane and 37.5 vol.% steam, balance argon. At 700 °C reaction temperature and a space velocity of 32 h-1, conversion rates close to the thermodynamic equilibrium could be achieved. With increasing WHSV, the point of equal carbon dioxide to carbon monoxide selectivity was shifted to higher temperatures (Figure 2.17). In other words, C02/CO ratio of one was always achieved at about 90% conversion. During 96 h of operation, the catalyst showed no detectable deactivation, in contrast to its commercial nickel-based counterpart. 

Figure 2.19 Conversion and selectivity vs. pressure for partial oxidation of methane at 1200 °C reaction temperature and an O/C ratio of 1 half-filled symbols indicate calculated thermodynamic equilibrium values [44] (by courtesy of ACS).

A feed of 100 Ncm3 min-1 methane and 50 Ncm3 min-1 oxygen was introduced into the reactor at a pressure loss of < 2.5 mbar. The residence time of the reaction was 50 ms. 60% conversion was achieved along with a high carbon monoxide selectivity of 70% at 700 °C reaction temperature. Owing to the short residence times applied, no coke formation was observed and carbon monoxide selectivity was higher than expected from the thermodynamic equilibrium [46],... 

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... 

A simple model of the chemical processes governing the rate of heat release during methane oxidation will be presented below. There are simple models for the induction period of methane oxidation (1,2.>.3) and the partial equilibrium hypothesis (4) is applicable as the reaction approaches thermodynamic equilibrium. However, there are apparently no previous successful models for the portion of the reaction where fuel is consumed rapidly and heat is released. There are empirical rate constants which, due to experimental limitations, are generally determined in a range of pressures or concentrations which are far removed from those of practical combustion devices. To calculate a practical device these must be recalibrated to experiments at the appropriate conditions, so they have little predictive value and give little insight into the controlling physical and chemical processes. 

A table listing the thermodynamic equilibrium methane concentration in the outlet of the primary and secondary reformer over a wide range of operating pressures, outlet temperatures and S/C ratios can be found in [417] (see also [424]). 

Zirconia sensors have been used primarily in the exhaust system of automobiles to control the air-to-fuel ratio for meeting the federal requirements on such noxious gases as carbon monoxide, methane and nitrogen oxides. The applicability of zirconia sensors for this particular application is based on the assumption that, under thermodynamic equilibrium, the partial pressure of oxygen in the exhaust gas depends primarily on the air-to-fuel ratio. To compensate for the fact that in reality equilibrium is not reached, catalytic platinum electrics are incorporated in the zirconia sensor design [Stevens, 1986]. In the zirconia sensor, the outside of the zirconia tube is exposed to the exhaust gas while the inside is exposed to the ambient air as a reference atmosphere. 

Figure 2 shows the TPH results on NidojMg O and 3.0 mol% Ni/MgO after the reaction at 773 K, and the activity was listed in Table 1. Under this reaction condition, methane conversion is far from the thermodynamic equilibrium level. Two peaks were observed in the TPH profiles. One appeared at 550 K-700 K (a-carbon), and the other above 873 K ( 6-carbon). It is found that the peak intensity of -carbon was almost constant, while that of )8-carbon increased linearly with the time on stream. From the behavior and reactivity, )S-carbon is ascribed to deposited carbon. /3formation rate and selectivity were also in Table 1. Selectivity to carbon is much related to the dispersion of Ni metal particles. This suggested that carbon formation tended to proceed on the larger Ni particles. And carbon was formed on solid solution catalysts with higher Ni content. 

Measurements of the isobaric thermal expansivity of methane in the pressure range from 50 to 165 MPa and along four isotherms (303, 333,363, and 393 K) have been performed in decreasing the pressure at a low constant rate, a=0.02 MPas , in such a way as to remain at thermodynamic equilibrium. Several hundreds of data points were collected and fitted, as a function of p along each isotherm, to the following empirical equation ... 

Horn, R., Williams, K.A., Degenstein, N.J., Bitsch-Larsen, A., Dalle Nogare, D., Tupy, S.A., and Schmidt, L.D. Methane catalytic partial oxidation on autothermal Rh and Pt foam catalysts Oxidation and reforming zones, transport effects, and approach to thermodynamic equilibrium. Journal of Catalysis, 2007, 249 (2), 380. 

---