Interplay of Kinetics and Thermodynamics in Catalytic Steam Methane Reforming over Ni/MgO-SiO2

The steam methane reforming (SMR) reaction was studied on a Ni/MgO-SiO₂ catalyst at 923 K (650 °C) and 0.40 MPa in a tubular packed-bed reactor. The partial pressures of CH₄ and H₂O 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 CH₄ conversion increased with the inlet partial pressure of H₂O and decreased with the inlet partial pressure of CH₄. However, at low CH₄ 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 CH₄ and H₂O surface coverages. The model was able to capture the increasing selectivity to CO₂ with increasing H₂O inlet partial pressure and methane conversion. The effect of changing the total pressure and H₂O/CH₄ ratio on the CH₄ 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.