An experimental and theoretical study of a catalytic monolith to control automobile exhaust emissions

Abstract

Experimental investigations of automobile exhaust emissions were examined by combusting a mixture of propane and air within a multi-channel monolith. Chemical kinetics, mass transfer and heat transfer effects were studied using appropriate temperature and flow conditions to separate the effects. The results were used to construct both a one- and two-dimensional mathematical model. Simulations of monolith behaviour were then compared with observed performance. First-order chemical kinetics were observed for the low hydrocarbon concentrations examined in the temperature range 557–648 K, while mass transfer limitation was apparent at temperatures between 736 K and 769 K. Perturbations to inlet concentration and temperature were effected while studying monolith performance, and the responses recorded. Computer simulations using the two mathematical models predicted correct trends, but did not agree quantitatively with the experimental results. The one-dimensional model predicts both concentration and temperature responses to a change in inlet conditions better than the more comprehensive two-dimensional model, even when heat losses are taken into account. This is because experimentally determined heat and mass transfer coefficients are used for computations relating to the one-dimensional model, whereas these parameters were calculated theoretically in the two-dimensional model. Further computer simulations revealed discontinuities in the values of Nusselt numbers, values depending on elapsed time following a step change in inlet conditions and axial position along the monolith channel. This unusual feature is accounted for by a reversal in heat transfer between wall and bulk fluid as the reaction develops along the monolith channel.