Calculating effectiveness factors in non-uniform washcoat shapes

This paper describes a method for the determination of effectiveness factors in a monolith washcoat of non-uniform thickness. The method is based on the previously reported slice concept, in which the monolith is divided into a series of slices, each of which is analysed using a one-dimensional approach. In each one-dimensional slice, an apparent characteristic length is used to compute the Thiele modulus. In the approach described here, the apparent characteristic length is determined by conducting a single two-dimensional simulation for a first-order reaction in the kinetically controlled region. The characteristic length is then calculated from the flux at the surface using an analytical solution. Results are shown for the idealized circular washcoat in a square channel, as well as for a real washcoat in a square channel and a sinusoidal shape. Results for first-order and LHHW kinetics show good agreement between the one-dimensional approximation and the true solution. Both average and local effectiveness factors can be calculated within acceptable range for monolith reactor simulations.