Internal and external transport effects during the oxidative reforming of methane on a commercial steam reforming catalyst

In order to establish an isothermal catalyst bed it was necessary to dilute both the catalyst particles (1:10 in α-Al2O3) and the feed mixture (2% reactants in He). Under these conditions, a linear velocity of about 6 cm/s was sufficient to remove the effect of film diffusion at 600°C. Experiments with particle sizes in the range 125–710 μm revealed that internal diffusion did not play a role in methane conversion at a fixed space velocity. No change in the apparent activation energy was found over the range 575–625°C which confirmed the absence of diffusion limitations; at 650°C, however, a slight decline in Ea was observed which may be indicative of the presence of concentration gradients. Correlations taken from the literature and which are normally used to test the absence of internal and external temperature and concentration gradients, were applied to our operating conditions and confirmed our conclusions.