An Asymptotic Solution for Washcoat Pore Diffusion in Catalytic Monoliths: Reformulation and Extension to Small Concentrations

A recent publication (Bissett in Emission Control Sci. Technol. 1(1), 3–16, 2015) proposed an alternative to the so-called 1 + 1D modeling of aftertreatment reactors with nontrivial washcoat pore diffusion. Rather than numerically solve the 1D reaction-diffusion problem within the washcoat(s), asymptotic results based on small diffusion resistance give the concentration profiles within the washcoat analytically, and these are integrated within the overall solution for transient reactor performance. The description of the asymptotic solution in the former publication is suitable for the formal derivation and demonstration that all special properties of this solution follow from small diffusion resistance alone, but experience has shown that alternative descriptions and further extensions to accommodate small washcoat concentrations are desirable and perhaps necessary for practical application. In this paper and in a less formal style, we provide the new alternatives and analysis necessary for small concentrations.