Catalytic partial oxidation of methane over a 4% Rh/α-Al2O3 catalyst: Part I: Kinetic study in annular reactor

Abstract The catalytic partial oxidation (CPO) of CH4 to synthesis gas over a 4 wt% Rh/α-Al2O3 catalyst was investigated by means of a short contact time annular reactor, specifically designed for testing very fast and exothermic reactions. Data were collected by feeding CH4/O2/inert gas mixtures, at varying temperature (from 350 to 850 ◦C), GHSV (up to 4.5 × 106 N l/Kgcat/h), O2/CH4 ratio (from 0.56 to 1.3), reactant dilution (1 to 27% CH4 v/v) and adding CO2 (1%) and H2O (1 and 2%) to the standard feed. Steam reforming, CO2 reforming, water gas shift (WGS), reverse-WGS, H2 and CO combustion tests were also carried out to refine the study. A quantitative analysis of the experimental data was performed by a 1D mathematical model of the reactor, wherein a molecular kinetic scheme of the process was incorporated. The scheme consists of CH4 total oxidation and reforming, the water gas shift and reverse water gas shift reactions, and H2 and CO post-combustion reactions. On the basis of experimental data and numerical analysis, it was found that, under the CPO conditions: (1) the kinetic role of CO2 reforming is negligible, so that steam reforming and CH4 total combustion alone can account for the consumption of CH4; (2) oxidation and steam reforming of methane have comparable intrinsic kinetics under differential conditions, but surface coverages differently influence the reaction rates under integral conditions; (3) the direct and the reverse water gas shift reactions (WGS and RWGS), when far from the chemical equilibrium, have independent kinetics; (4) the process kinetics is significantly affected by the dilution of the reacting mixture: since the global reaction order is lower than 1, conversion and selectivity decrease at decreasing dilution. Part I of the work deals with the development and the validation of the proposed kinetic scheme; Part II deals with the analysis of CO2 reforming and RWGS experiments and supports the assumption that CO2 reforming can be excluded form the CPO kinetic scheme.