Effect of Transport Phenomena on Heterogeneous Catalysis

The first step in heterogeneous catalytic processes is the transfer of the reactant from the bulk phase to the external surface of the catalyst pellet. If a nonporous catalyst is used, only external mass and heat transfer can influence the effective rate of reaction. The same situation will occur for very fast reactions, where the reactants are completely exhausted at the external catalyst surface. As no internal mass and heat transfer resistances are considered, the overall catalyst effectiveness factor corresponds to the external effectiveness factor, 

The characteristic time for the external mass transfer between the bulk fluid and the surface (t ) depends on the mass transfer coefficient in the fluid and the outer surface-to-particle volume ratio commonly called specific outer surface area a (Eq. (11.3)). 

The importance of the external mass transfer on the transformation rate is characterized by the ratio between and the characteristic reaction time the second Damkohler number is Dall = 

The influence of internal mass transfer on the transformation rate is commonly characterized by the Thiele modulus fi, defined as the ratio between the reaction time tj and the characteristic diffusion time in the porous catalyst particle or layer tp. 

As an example of the decreasing efficiency caused by external and internal mass transfers, we consider an irreversible first-order reaction and a porous catalyst layer. The situation corresponds to a catalytic wall reactor. The relative importance between external and internal mass transfers is characterized by the ratio of the diffusion time in the porous layer tp and the characteristic time for external mass transfer called the Biot number, Bi = t /t - (L /DJk a for mass transfer. 

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