MASS AND HEAT-TRANSFER EFFECTS IN CATALYTIC MONOLITH REACTORS

Numerical simulations bf catalytic oxidation in monolith reactors are performed in order to develop criteria for mass transfer limitation. A two-dimensional finite-element simulator previously developed is used to examine previously reported studies of propane and carbon monoxide combustion in excess oxygen. The Sherwood and Nusselt numbers compute d from the two-dimensional simulation results are compared to numbers derived experimentally. The results from the simulations are much high er than results which have been reported in the literature for experim ental work. Simulation results agree well with numbers obtained analyt ically and experimentally for non-reacting flow in circular tubes, and also with other correlations for reacting flows based on numerical wo rk. The reason for the discrepancy between experimental and simulated results is explained. For first-order reactions, a dimensionless catal ytic reaction number is proposed, which may be used to evaluate whethe r or not the rate is mass transfer controlled. For the oxidation of CO , multiple steady states are possible and the variation in Nusselt and Sherwood numbers under transient conditions is discussed. The influen ce of diffusion in a real monolith washcoat is also examined. In squar e monolith channels of dimension 1 mm, low effectiveness factors are o btained for temperatures above 700 K, and much of the catalyst is not utilised. It is shown that care needs to be taken in the extension of relatively low-temperature kinetic data to the elevated temperatures e ncountered in real operating conditions.