(619c) Determination of Kinetics and Controlling Regimes for Propylene and Methane Oxidation On Pt/Al2O3 Monolithic Catalyst

It is well known that the performance of a catalytic monolith is bounded by two limits: the kinetic regime at low temperatures (or before ignition for the case of exothermic reactions) and the external mass transfer controlled regime at sufficiently high temperatures (or after ignition). The washcoat diffusional resistance can also be significant over an intermediate range of temperatures. The transition temperatures at which the controlling regime changes from kinetic to washcoat diffusion to external mass transfer depend on the various geometric properties of the monolith, flow properties, the catalyst loading and washcoat properties. We determine the intrinsic kinetics of the lean oxidation of propylene and methane on Pt/Al2O3 monolithic catalyst. Further, we quantify the relative contributions of kinetic and transport processes as a function of various design and operating conditions. This study follows a recent study (Chem. Engg. Sci. 65, 1729-1747) in which we developed criteria for characterizing various controlling regimes in catalytic monoliths in terms of various reactor and operating parameters. As is well known, the transition between the controlling regimes (kinetic to pore diffusion to external mass transfer) depends not only on the kinetics but also on catalyst design and operating parameters. A laboratory catalytic monolith reactor is designed to measure conversions over a wide range of operating temperatures. The low temperature (before ignition) data is used to obtain the kinetics whereas, high temperature (after ignition) data is used to determine mass transfer coefficient. Unlike fixed bed reactors containing catalytic pellets or powder, heat and mass transport coefficients can be characterized quite accurately. As a result, the controlling regimes can be unambiguously determined. Specifically, we investigated the effects of Pt loading, space velocity and operating temperature on the regime transition. The analysis reveals that kinetic regime is dominant over a wide range of temperatures for methane oxidation whereas propylene oxidation has a more classical transition between a kinetic and external transport limited regime.