INTRAPHASE DIFFUSION AND INTERPHASE MASS-TRANSFER EFFECTS DURING THE CATALYTIC-OXIDATION OF CO IN A TUBE WALL REACTOR

Chemical kinetics of catalytic reactions are often obscured by intraphase diffusion and interphase mass transfer effects. Such complexities are especially true of catalytic combustion reactions effected within multichannel monoliths whose channel walls are coated with a catalyst layer. Assessment of the extent of intraphase and interphase resistances to the catalytic conversion of low concentrations of carbon monoxide in air were achieved by conducting experiments in a tube wall reactor, the walls of which were coated with a platinum-alumina deposit. Results indicated that, for a 1.34% CO in air mixture, kinetics below 610 K were less than first order with an activation energy of 30.4 kJ mol-1. Above 610 K there was strong evidence of both intraphase and interphase resistances to catalytic conversion, the overall kinetics displaying an apparent activation energy of 11.7 kJ mol-1. Near to the reactor tube entrance where the temperature was about 650 K, the mass transfer resistance from fluid to tube wall was only one-sixth that of the diffusive resistance within the thin catalyst washcoat, increasing to one half of the diffusive resistance at the tube exit where the temperature was about 820 K. Computer estimations of the performance of the tube wall reactor, using measured kinetic data for a small element of reactor containing catalyst deposited on the wall and interphase heat and mass transfer data estimated from first principles assuming laminar flow, are in satisfactory agreement with the measured performance of the whole tube wall reactor.