Experimental and Theoretical Investigation of Controlling Regimes during Lean Oxidation of Methane and Propylene on Pt/Al2O3 Monolithic Reactors

Abstract

The performance of a catalytic reactor is bounded by the kinetic regime at low temperature (or before light-off) and the mass-transfer-controlled regime at high temperature. Pore diffusion may also be significant at intermediate temperature. We utilize the recently developed criteria (Joshi, S. Y.; Harold, M. P.; Balakotaiah, V. Chem. Eng. Sci.2010, 65, 1729–1747) to characterize the controlling regimes during lean oxidation of CH4 and C3H6 in Pt/Al2O3 monolithic reactors. First, we determine the global kinetics of lean oxidation of CH4 and C3H6 in a Pt/Al2O3 monolithic catalyst. We also present a method for estimation of the effective diffusivity of the limiting reactant and the external mass-transfer coefficient under reacting conditions. Further, we characterize the relative contributions of chemical kinetics, washcoat diffusion, and external mass transfer as a function of the various catalyst design and operating parameters. The analysis reveals that methane oxidation is kinetically controlled over a wide range of temperatures (350–600 °C) whereas propylene oxidation has a more classical transition between the kinetic and mass-transfer-controlled regimes. We use the bench-scale results to analyze the impact of various design and operating variables on transitions between the controlling regimes.