It was illustratively shown that the use of a definition of the effective diffusivity for a multi -component gaseous reaction system easily gives a sufficiently accurate approximate solution for diffusion through a stagnant gas film.
In order to apply the same idea for approximate estimation of the effectiveness factor of the porous catalyst, the effective diffusivity was defined for the conditions of the catalyst surface. The illustratively calculated catalyst effectiveness factors at some particular but isothermal conditions agreed well with the exact values numerically obtained.
It will be remarked that the effect of volume change associated with the chemical reaction in the multi-component system can be easily evaluated by the use of this effective diffusivity.