AIChE Journal, Vol.50, No.11, 2939-2955, 2004
Mass-transfer coefficients in washcoated monoliths
The asymptotic mass-transfer coefficients are determined in washcoated monoliths of various geometric shapes by solving the convection-diffusion equation in the fluid phase coupled with the diffusion-reaction equation in the washcoat. The dependency of the asymptotic Sherwood number (Sh(infinity)) on washcoat properties (relative effective washcoat thickness A, ratio of diffusivity of reactant in the fluid phase to that in the washcoat 5, and the catalyst loading phi(s)(2)) is examined. It is found that in the kinetic regime (phi(s)(2) --> 0), Sh(infinity) approaches a new asymptote (Sh(w),(infinity)) which depends on the flow as well as washcoat geometries. For delta --> 0, Sh(w),(infinity) approaches Sh(H1),(infinity) corresponding to flow geometry, whereas for delta --> infinity it approaches Sh(H2),(infinity) of flow geometry. As can be expected, in the mass transfer controlled regime, Sh(infinity) approaches Sh(T),(infinity) (flow geometry), which is independent of washcoat properties. It is also found that the variation of Sh(infinity) with catalyst loading is not always monotonic when washcoat distribution around the channel perimeter is nonuniform. Numerical results, describing the variation of Sh(infinity) with washcoat properties, are presented for some commonly used channel geometries. (C) 2004 American Institute of Chemical Engineers.
Keywords:catalytic monolith;washcoat diffusion;asymptotic mass transfer coefficients;convection-diffusion-reaction equation;Sherwood number