Mass Transfer in Heterogeneous Systems

Components have to be transported to the catalyst and once they have reached the outer surface of the particle, and possibly into and through the pores of the particle to reach the catalytically active material. This chapter we discusses transport to the outer surface of the particle transport inside the particle is discussed in Chapter 3. 

Several cases can be distinguished. In the case of a hydrogenation, for example, the components to be hydrogenated may be in the liquid phase, in which small catalyst particles are suspended. Here the hydrogen has to pass through the gas to the interface with the liquid, dissolve in the liquid and eventually through the liquid to the catalyst particle. This is a three-phase system with a gas, a liquid and a solid. 

In the case that the reactants are already in the phase surrounding the catalyst pellet, the components only have to be transported through this single phase, being either liquid or gas. Reactant A is assumed to be converted according to a first-order reaction. For convenience a chemical reaction rate R is defined per unit of external surface  

Elimination of the unknown concentration CAj gives the overall conversion rate  

The fast reaction regime for k rlkg 1. The rate of mass transfer completely limits the conversion rate, which follows from 

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E.U. Schlunder, On the mechanism of mass transfer in heterogeneous systems—in particular in fixed beds, fluidized beds and on bubble trays, Chem. Eng. ScL 32 845 (1977). 

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