7.3.3 Extension for Stoichiometries with Multiple Gas Phase Reactants

When more than one gas phase reactant takes part in the reaction, the reaction stoichiometry must be extended to account for this case:

To describe the rate of reaction $r$ of a particle surface species $j$ in the presence of $n_{\rm max}$ gas phase species $n$, it is necessary to define the diffusion-limited species for each solid particle reaction, i.e., the species for which the concentration gradient between the bulk and the particle surface is the largest. For the rest of the species, the surface and the bulk concentrations are assumed to be equal. The concentration of the diffusion-limited species is shown as $C_{d,b}$ and $C_{d,s}$ in Figure  7.3.1, and the concentrations of all other species are denoted as $C_k$. For stoichiometries with multiple gas phase reactants, the bulk partial pressure $p_n$ in Equations  7.3-4 and 7.3-8 is the bulk partial pressure of the diffusion-limited species, $p_{r,d}$ for reaction $r$.

The kinetic rate of reaction $r$ is then defined as

where

$p_n$	=	bulk partial pressure of gas species $n$
$N_{r,n}$	=	reaction order in species $n$

When this model is enabled, the constant $C_{1,r}$ (Equation  7.3-6) and the effectiveness factor $\eta_r$ (Equation  7.3-4) are entered in the Reactions dialog box (see this section in the separate User's Guide).