Figure 7. 113, 266, 1130 and 10400 ppbv CH3I adsorption behavior prediction using nth order SCM

Rate-Controlling Step

For further analysis and application of the SCM results, it is necessary to understand the rate-controlling step of the process. To determine how the CH3I-Ag0-Aerogel adsorption is controlled, τ1 , τ2 , andτ3 at 4 different concentrations were calculated using Eq 2-4 with the determined parameters shown in Table 1.τ1 , τ2 , andτ3 are the partial adsorption equilibrium time contributed by the gas film diffusion, pore diffusion, and reaction term. More straightforwardly, the equilibrium time,teq , can be expressed as teq = τ1 + τ2 +τ3 . For the SCM, the control term can be represented by overall resistance ratio,τi /teq , where i = 1, 2, or 3.38 Figure 8 shows the resistance contributions of pore diffusion term and reaction term, whereas the gas film diffusion term is less than 1% and not shown in the figure. It was found that the reaction resistance contribution decreases from 25% at 113 ppbv to 6% at 10400 ppbv and the pore diffusion resistance contribution increases accordingly. Generally, for a given adsorption system, the resistance contribution is independent of the concentration of the adsorbate. Therefore, the change shown in Figure 8 is due to the nth order reaction which has been discussed previously.
The adsorption rate is determined by the process with the highest resistance. Visualized from Figure 8, the pore diffusion contributes the most among the gas film diffusion, pore diffusion, and reaction term, especially for high CH3I concentration. Therefore, the CH3I-Ag0-Aerogel adsorption is identified as an ‘overall’ diffusion-controlled process. The overall process is diffusion-controlled, but at certain conditions, the process may not be controlled by diffusion, which will be discussed in the following section. Moreover, as the concentration decreased from 10400 to 113 ppbv, the ratio of reaction resistance contribution increases significantly, indicating that the CH3I-Ag0-Aerogel adsorption may change from a pore diffusion-controlled to a reaction-controlled process if the concentration decreases substantially.