Experimental Studies of Intrinsic Kinetics and Diffusion during Methane Steam Reforming

The intrinsic kinetics and diffusion behaviour of methane steam reforming have been investigated in this work. Measurements of effective diffusivities of the gases present in methane steam reforming have been carried out by using the steady-state technique over wide ranges of temperature and pressure in a modified Wicke-Kallenbach (W-K) type diffusion apparatus. The effects of diffusion limitation on the reactions were examined at atmospheric pressure in a pellet reactor; and the intrinsic kinetics of methane steam reforming have been studied on a commercial nickel/alumina catalyst (ICI 57-4) in an integral reactor. A simulation study has been carried out to determine the effects of hydrogen removal on the performance of a membrane reactor and the catalyst activity for methane steam reforming. For the measurements of effective diffusivities, the temperature and pressure dependencies of effective diffusivities of gases measured have been obtained, and the tortuosities of pellets used for the gases measured have been estimated by using the parallel path pore model. At ambient pressure, the temperature exponent values range from 1.0 to 1.25. This indicates that the diffusion occurs in the transition region. At pressures up to 1MPa, the diffusion lies mainly in the bulk diffusion region. The pressure exponent was generally less than 1.0 with values lying between 0.4 and 0.85 except for gas-water vapour pairs where it is close to 1.0. The tortuosities estimated for the pellets varied from 1.84 to 2.51 for different gases at ambient pressure, but decreased with increase in pressure. Using the pellet reactor that couples the diffusion and reaction for methane steam reforming, experimental results, which were obtained over catalyst pellets without holes, show that the diffusion rate of methane into the catalyst pellets almost totally controlled the reaction rate.