Chemical reaction influenced by intraparticle diffusion

The porous structure of either a catalyst or a solid reactant may have a considerable influence on the measured reaction rate, especially if a large proportion of the available surface area is only accessible through narrow pores. The problem of chemical reaction within porous solids was first considered quantitatively by Thiele [1] who developed mathematical models describing chemical reaction and intraparticle diffusion. Wheeler [2] later extended Thiele s work and identified model parameters which could be measured experimentally and used to predict reaction rates in 

Thin edges (neglect diffusion through these) 

The mathematical properties of the above function r 4 ) indicate that, when 0 0.2, t is close to unity. Under such conditions, there would be no diffusional resistance to reaction. On the other hand, when 0 5, 7 = 1/0 is a good approximation and for such conditions, diffusion is the rate-limiting process. 

No practical catalyst pellet can be described by the geometry of the wafer, yet it will be shown that eqn. (6), albeit with a slightly different interpretation of the Thiele modulus, 0, is of practical value. For a cylindrical catalyst pellet (a shape often used in practice) of radius r and sealed at its flat ends 

The Thiele modulus for the cylinder and sphere differ from that for the slab. However, if, for a first-order reaction, this modulus is redefined in 

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