Methanation Reactions

When carrying out the WGS reaction, methane can be formed in the reactor through the methanation reaction, which is the reverse methane steam reforming reaction and is highly exothermal:

CO + 3H2 <-> CH4 + H2O (AH = -205.8 kJ/mol) (8.21)

Methane is an undesired product in the WGS reaction, and for every CH4 molecule formed, two H2 molecules are taken away from the product stream. Figure 8.14 shows two WGS equilibrium curves for the hydrogen-containing conditions. The upper dashed line is the WGS equilibrium curve including the methanation reaction. The lower dotted line is the WGS equilibrium curve without methanation reactions.

200 400 600 800 1000

Temperature (K)

FIGURE 8.14 Equilibrium curves (dotted lines) for the WGS reaction with/without methanation reaction for feed-gas composition containing 50% H2. The filled squares and circles are CO conversions over G-66 A and Cu0.2Ce0.8O2-y, catalysts, respectively. The solid lines are model fits assuming first-order reversible kinetics.

1.5% H2O \

mcat = 0.150g

■. eq. WGS with methanation eq. WGS \

G-66A

Cu0.2Ce0.8O2-y .

Temperature (K)

Methanation Equilibrium

FIGURE 8.15 Equilibrium curves for the WGS reaction with/without methanation over the Cu0.2Ce0.8O2-y catalyst. Filled squares represent feed-gas composition of 0.5% CO and 1.5% H2O, and the solid line is the model fit assuming first-order reversible kinetics. The other two solid lines are the respective equilibrium curves. Filled triangles represent the feed composition of 50% H2, 0.5% CO, and 1.5% H2O, and the dotted line is the model fit assuming firstorder reversible kinetics. The two other dotted lines are the respective equilibrium curves.

FIGURE 8.15 Equilibrium curves for the WGS reaction with/without methanation over the Cu0.2Ce0.8O2-y catalyst. Filled squares represent feed-gas composition of 0.5% CO and 1.5% H2O, and the solid line is the model fit assuming first-order reversible kinetics. The other two solid lines are the respective equilibrium curves. Filled triangles represent the feed composition of 50% H2, 0.5% CO, and 1.5% H2O, and the dotted line is the model fit assuming firstorder reversible kinetics. The two other dotted lines are the respective equilibrium curves.

The figure shows that even though the methanation reaction is highly exothermal, these reactions do not occur on the catalyst, even at temperatures lower than 600 K. This confirms the selectivity of the copper-containing ceria-supported catalysts for the WGS reaction; otherwise, the experimental data would cross the WGS equilibrium line (lower dotted) calculated without taking into account the methanation reaction. If the methanation reaction had also proceeded, the experimental points would reach the (upper dotted) equilibrium line calculated for complex equilibrium including the methanation reaction. This can also be seen in Figure 8.15, which shows equilibrium curves for the WGS reaction with and without methanation over the Cu0.2Ce0.8O2-y catalyst including the feed-gas composition of 0.5% CO and 1.5% H2O.

For this specific gas composition, the difference between the WGS equilibrium with and without methanation reactions is very small, and in this case it would not be possible to draw the above conclusion about the selectivity of the catalysts. Anyhow, the reactor outlet gases were analyzed by a gas chromatograph, and no methane formation was detected in any experiment.

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  • assunta
    What is methanation reaction?
    7 years ago

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