The optimization and full understanding of chemical reactions is aided by the construction of an adequate kinetic model. The development of such a kinetic model remains a challenging task. To tackle this challenge in the most efficient way, an iterative, systematic methodology, originally demonstrated for n‐hexane hydroisomerization, is now extended aiming at finding the balance between the envisaged model detail and available information, often originating from time‐consuming and expensive experiments. Steam methane reforming on the Ni/MgO‐SiO2 case study is used for this purpose, that is, the construction of a kinetic model that embeds a maximum amount of information contained in the dataset. The kinetic model is expanded stepwise from a power law model over a model with reactant adsorption toward a Langmuir–Hinshelwood–Hougen–Watson model. The performance of the initially underparameterized model improved significantly by adding reactant adsorption, yet including product adsorption led to overparameterization rather than enhanced model performance. © 2019 American Institute of Chemical Engineers AIChE J, 65: 1222–1233, 2019
The 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.
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