Reactant mass transfers, representation

I—J Zonfs W 11( 1 iPterjiaMy by fOppl Qrily ICS Figure 4. Representation of reactants mass transfers... 

Schematic representation of reactant concentration profiles in various global rate regimes. I External mass transfer limits rate. II Pore diffusion limits rate. Ill Both mass transfer effects are present. IV Mass transfer has no influence on rate.

A reasonable representation of the concentration profile for a reactant A diffusing to the external smface is shown in Figure 11-3. As illustrated, the change in concentration of A from to Caj takes place in a very narrow fluid layer next to the surface of the sphere. Nearly all of the resistance to mass transfer is foimd in this layer. 

In a heterogeneous reaction sequence, mass transfer of reactants first takes place from the bulk fluid to the external surface of the pellet. The reactants then diffuse from the external surface into and through the pores within the pellet, with reaction taking place only on the catalytic surface of the pores. A schematic representation of this two-step diffusion process is shown in Figures 10-3 and 12-1. 

Figure 1. Schematic representation of the electrochemical cell. First insert concentration profile of the reactant in the stagnant layer in the vicinity of the electrode. Second insert mass transfer black box at the boundary between the stagnant layer and the bulk solution (x = (5).

In Figure 7.2 is a simple representation of gradients for several cases of relative mass transfer/reaction rates. Since these gradients are established when transport rates become finite, the net effect is to reduce the overall rate of reaction due to the lower incident concentration of reactant within the catalyst as compared to external surface (or bulk) concentration. The net activity of the catalyst is diminished, and it is common to define this quantitatively in terms of the catalytic effectiveness factor, given by... 

Since the concentration of reactant at the catalyst surface is highly temperature dependent, the apparent activation energy may not be an accurate representation of the true activation energy and should be used with caution to represent the catalytic activity. If transport resistances are not well accounted (for pore diffusion or film mass transfer), then the observed activation energy will be much lower than the true activation energy of a chemically controlled process since it will include the temperature dependence of the transport processes. 

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