## Interplay of Kinetics and Thermodynamics in Catalytic
Steam Methane Reforming over Ni/MgO-SiO_{2}

journal contribution

posted on 10.01.2017, 00:00 by Naoki Kageyama, Brigitte R. Devocht, Atsushi Takagaki, Kenneth Toch, Joris W. Thybaut, Guy B. Marin, S. Ted OyamaThe steam methane reforming (SMR)
reaction was studied on a Ni/MgO-SiO

_{2}catalyst at 923 K (650 °C) and 0.40 MPa in a tubular packed-bed reactor. The partial pressures of CH_{4}and H_{2}O were varied between 20 and 140 kPa and 80 and 320 kPa, respectively. Measurements were carried out without mass and heat transport limitations, as verified by the Weisz–Prater and Mears criteria. Experimentally, the CH_{4}conversion increased with the inlet partial pressure of H_{2}O and decreased with the inlet partial pressure of CH_{4}. However, at low CH_{4}inlet partial pressures, i.e., at 40 and 60 kPa, the conversion passed through a maximum. Rate expressions were derived based on a simple two-step sequence. A statistical analysis led to a globally significant, weighted regression and resulted in a good agreement between the model and the experimental data, as indicated by a low*F*value of model adequacy of 2.84. The rate and equilibrium coefficient parameters were statistically significant as indicated by narrow confidence intervals. The model was able to correctly describe the experimentally observed maximum in the methane conversion and allowed relating this behavior to CH_{4}and H_{2}O surface coverages. The model was able to capture the increasing selectivity to CO_{2}with increasing H_{2}O inlet partial pressure and methane conversion. The effect of changing the total pressure and H_{2}O/CH_{4}ratio on the CH_{4}conversion as a function of the space velocity was simulated and corresponded to both the experimental and literature data. A major finding of the modeling was that as flow rate was increased there was a crossover in the order of conversion with pressure due to a transition from thermodynamic to kinetic control. Although the SMR equilibrium conversion decreased with pressure, away from equilibrium at high flow rates, conversion was higher at higher pressures because of enhanced adsorption rates.