Carbon-hydrogen-methane equilibrium

Equilibrium. The coal char-hydrogen reaction has been shown to exceed the carbon-hydrogen-methane equilibrium at low conversion and to reach the carbon-hydrogen equilibrium at nearly complete conversion (36, 37). From the equilibrium composition of the hydrogen-char system, a pseudo-equilibrium constant, Kp, is defined as ... 

Thermodynamically, the formation of methane is favored at low temperatures. The equilibrium constant is 10 at 300 K and is 10 ° at 1000 K (113). High temperatures and catalysts ate needed to achieve appreciable rates of carbon gasification, however. This reaction was studied in the range 820—1020 K, and it was found that nickel catalysts speed the reaction by three to four orders of magnitude (114). The Hterature for the carbon-hydrogen reaction has been surveyed (115). 

A change in the amount of any substance that appears in the reaction quotient displaces the system from its equilibrium position. As an example, consider an industrial reactor containing a mixture of methane, hydrogen, steam, and carbon monoxide at equilibrium ... 

For a first approximation to the solution, we will assume that essentially all the methanol condenses, with only trace amounts appearing in the recycle line. We will also assume that most of the water condenses and that very small amounts of carbon monoxide, carbon dioxide, hydrogen, methane, and nitrogen dissolve in the condensate. To account for methanol and water vapor in the recycle gases and the solubility of the gases in the crade methanol, we would have to include phase equilibrium relationships in the analysis. As stated earlier, several condensable byproducts, high and low-boiling compounds in the cmde methanol, are present in small amounts, as shown in Table 3.5.1. We will not consider these compounds in the synthesis-loop analysis. 

In addition to these exchange reactions, a number of alkane/alkane and al-kane/arene exchange reactions could be studied as equilibria (benzene, toluene, cyclopropane, methane, ethane, neopentane, cyclohexane). Determination of equilibrium constants allowed calculation of AG° values and estimation of relative metal-carbon bond energies. Wolczanski concluded that the differences between metal-carbon bond energies and the corresponding carbon-hydrogen bond energies were essentially the same [82]. 

Wolczanski also investigated the chemistry of a tantalum imido system. In this system, elimination of hydrocarbon from the bis-amido imido complex occurs with difficulty at 183°C to give an amido bis-imido complex. The elimination is reversible, with the bis-imido species not being directly observed (Scheme 10). Under methane pressure, the phenyl complex loses benzene and adds methane. Neopentane, benzene, and toluene (benzylic activation) were also found to undergo activation, but not cyclohexane. The authors conclude from their equilibrium studies that the differences in metal-carbon bond strengths are approximately equal to the differences in carbon-hydrogen bond... 

The performance of a CPO reactor is, in the literature, often characterized by the hydrocarbon conversion and selectivities to carbon monoxide and hydrogen. Methane conversion and selectivities are often reported to be more than 80-90%. This corresponds in general to conditions at which the exit gas is close to equilibrium for the shift reaction and the methane steam-reforming reaction with a low value of ATr in Eq. (5). The most likely reaction sequence is total oxidation in the initial part of the catalyst zone followed by other reactions including steam-reforming, shift, and possibly partial oxidation. 

Fig. 16.—Conversion of methane to hydrogen and carbon monoxide at equilibrium by an equimolar amount of steam.

To satisfy the element balance for the compounds involved, that is, methane, water, hydrogen, carbon monoxide, and carbon dioxide in chemical equilibrium, the following amounts of carbon, hydrogen, and oxygen should be found starting from the given initial amounts of methane (1 mol) and water (3.2 mol) ... 

Experimentally determined vapor-liquid equilibrium data for the system hydrogen sulfide-carbon dioxide-methane-water at pressures ranging from atmospheric to 1,014 psia and temperatures from 85° to 115°F have been reported by Froning et al. (1964). These authors found that the equilibrium constants K can be represented by the following equations ... 

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). 

When one of the elements is solid, as in tire case of carbon in the calculation of the partial pressures of tire gaseous species in the reaction between methane and air, CO(g) can be used as a basic element together widr hydrogen and oxygen molecules, and thus the calculation of the final partial pressure of methane must be evaluated using the equilibrium constant for CH4 formation... 

A promoted nickel type catalyst contained in the reactor tubes is used at temperature and pressure ranges of 700-800°C and 30-50 atmospheres, respectively. The reforming reaction is equilibrium limited. It is favored at high temperatures, low pressures, and a high steam to carbon ratio. These conditions minimize methane slip at the reformer outlet and yield an equilibrium mixture that is rich in hydrogen. ... 

Effects of Cold Gas Recycle and Approach to Equilibrium. Product gases resulting from various CGR ratios were analyzed (Table XI). For the experiments tabulated, a decrease in the cold recycle ratio resulted consistently in increases in the product gas concentrations of water vapor, hydrogen, and carbon dioxide and a decrease in methane concentration. These trends may be noted in experiment HGR-12 as the CGR ratio decreased from 8.7 1 to 1.2 1, in experiment HGR-13 as it increased from 1.0 1 to 9.1 1, and in experiment HGR-14 as it decreased from 3.0 1 to 1.0 1. These trends indicate that the water-gas shift reaction (CO + H20 —> C02 + H2) was sustained to some degree. Except for the 462-hr period in experiment HGR-14, the apparent mass action constants for the water-gas shift reaction (based on the product gas compositions in Table XI) remained fairly constant at 0.57-1.6. These values are much lower than the value of 11.7 for equilibrium conversion at 400°C. In... 

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 2.19 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.

Nozaki, T., Kimura, Y., and Okazaki, K., Carbon nanotubes and hydrogen co-production from methane using atmospheric pressure non-equilibrium plasma, Proc. 16th ESCAMPIG and 5th... 

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