Prediction of effective reaction rates in monoliths presenting channels of non-uniform coating

Abstract

A two-zone model (1D-2Z) for the estimation of the effective reaction rates in monoliths with non-uniform coating is presented in this contribution. The model relies on the idea of splitting the cross section of the coating in two zones that can be modelled as independent zones with diffusion (and simultaneous reaction) in a single direction (1D). One of the zones is considered a slab, while the variable diffusivity model (1D-VD), whose predictive capabilities has been largely tested in previous works for a great variety of kinetic expressions and catalyst geometries, is applied to the other. The performance of the 1D-2Z model, as well as the results from approximate methods from literature are assessed for three kinetic expressions: a first order reaction, as an elementary kinetics, and two more challenging cases defined by Langmuir-Hinshelwood-Hougen-Watson expressions: one of them with a moderate inhibition effect, and the other with such an effect on the verge of the onset of multiple steady states. Square and triangular channel cross-sections presenting different degrees of non-uniform coating are considered. It was found that the 1D-2Z model is highly accurate, leading to errors in the estimation of the effectiveness factors below 8.5 % in all the cases analysed, for the entire range of Thiele modulus. Simultaneously, a noticeable decrease in computational demand can be achieved from the use of the 1D-2Z model, in comparison to the previous simplifying approaches.