A simplified kinetic model for isothermal catalytic ignition

The ignition of catalytic combustion of the stoichiometric propane/air mixture on an isothermally heated platinum wire in different experimental conditions of total pressure and wire temperature is studied and discussed on the basis of a simplified kinetic model. The platinum wire is heated electrically with a specially designed power supply, which ensures a quasi-rectangular profile of its temperature. The ignition process is monitored by measuring the input power required to maintain a constant temperature of the wire during an exothermic catalytic reaction. The difference between the input powers recorded in air and in a fuel/air mixture, for the same wire temperature and gas total pressure, allows the elimination of the heat transferred to surroundings and conversion of the results into the catalytic reaction rate r (R) versus time curves of S-shaped form, illustrating the transition from kinetic to diffusion regime. The curve can be used to evaluate the ignition delay, as reported previously and also to fit different models to the data. The quasi-exponential increase of the isothermal reaction rate during the early stages of the process can be rationalized on the basis of a simplified kinetic model implying the multiplication of the adsorbed active intermediates. The adopted hypotheses allow the derivation of an analytical solution for the catalytic reaction rate before and during the ignition process, without diffusion limitations.