A steady-state, mass-transfer-limited monolith model has been formulated to investigate analytically the effects of gas-phase phenomena on converter performance. Gas-phase reaction kinetics are included to demonstrate the significance of noncatalytic processes to overall conversion, especially at temperatures above 850°K. Developing boundary layers in the monolith channels are shown to increase the calculated conversions relative to conditions for fully developed, laminar flow. Substantially different conversion efficiencies are predicted for various hydrocarbon species because of the effect of component diffusivity on convective mass-transfer rates.
The expression for the gas-phase oxidation of hydrocarbons indicates that carbon monoxide forms as an intermediate product. Experimental evidence of carbon monoxide formation in conjunction with high concentrations of inlet hydrocarbons has been modeled with this partial oxidation reaction. Comparisons of the model calculations with other experimental results reveal important trends which are related to the gas-phase reactions. Over-prediction of the magnitude of the hydrocarbon conversion occurs because the mass-transfer-limited assumption is probably inapplicable to some hydrocarbon species.
In addition to quantifying the effects of gas-phase phenomena, areas for additional research have been identified: improvement of expressions for gas-phase kinetics, measurement of hydrocarbon species distributions, and quantification of kinetic limitations for catalytic conversion in monoliths.