The Effect of Catalytic Washcoat Geometry on Light-off in Monolith Reactors

Abstract

This paper presents the results of a series of simulations of the light-off of carbon monoxide in excess oxygen in a single channel monolith reactor. The catalytic washcoat geometry is tested for its effect on the light-off position and shape. The simulations used a combination of two and three-dimensional modelling with non-linear oxidation kinetics. Different temperature ramp rates were employed. It is seen that the shape of the washcoat has an influence on the shape of the light-off curve, especially for high values of gas hourly space velocity and larger temperature ramp rates. The washcoat geometry that gives the fastest rise to complete conversion is the one that is the most non-uniform in shape; that is, the one that has thin sections. Nomenclature: a i : Species dependent constant in the Fuller diffusion equation; C i : Molar concentration of species i, mol/m 3 ; C p : Heat capacity, J/(kg K); d p : Pore diameter, m; D eff : effective diffusion coefficient in washcoat, m 2 /s; D i : Bulk diffusion coefficient, m 2 /s; DH R : Enthalpy change of reaction, J/mol; k f : Thermal conductivity of fluid, W/(m K); k eff : Effective thermal conductivity of the solid, W/(m K); k CO : Reaction rate constant, s )1 ; K CO : Adsorption equilibrium constant, m 3 /mol; M i : Molecular mass of species i; P: Total pressure, Pa; ()R i ): Reaction rate based on washcoat volume, mol/(m 3 s)

Flux response technology applied in zero length column diffusivity measurements

“…To this effect, understanding the diffusion that occurs within the washcoat and substrate material which strongly affects reactor performance is essential in the design of monolith supported catalysts (Zhang et al 2004;Koci et al 2007; More et al 2006). With these factors in mind, it must be ac- knowledged that the diffusion coefficients reported in literature for microporous materials often vary with differences as large as five orders of magnitude being reported for different techniques (Kärger 2012).…”

Multi‐scale modelling of monolith reactors: A 30‐year perspective from 1990 to 2020

“…We have shown that taking the real channel geometry into consideration can influence the predicted light-off curve significantly under some conditions. [73] As a final point, it is obviously essential to have good values for the effective diffusion coefficient. Typically, this must be done experimentally, but to make the measurements on real washcoats is quite tricky.…”

Effect of washcoat diffusion resistance in foam based catalytic reactors

“…In the beginning of the light-off region during heating the conversion with finite rate washcoat diffusion is larger than in the case with instantaneous washcoat diffusion. This increase in conversion due to finite rate washcoat diffusion is a known phenomena and is due to the negative reaction order of the overall reaction with respect to CO [26].…”