Figure 8. Resistance contributions of pore diffusion and reaction terms for CH3I-Ag0-Aerogel adsorption at 113, 266, 1130 and 10400 ppbv.
Although the overall process is controlled by the pore diffusion of CH3I, at VOG conditions, the rate determining step may vary because of the actual adsorption process. As mentioned above, in the VOG stream, the CH3I concentration is lower than 100 ppbv, more dilute than the low boundary of the current work, and the adsorption rate is extremely low. For example, at 113 ppbv, the prediction shows that the adsorption may not reach equilibrium in approximately 50 years if the capacity loss is neglected. Therefore, the actual active regions are only the initial parts of the adsorption curves. These regions correspond to the surface reaction between CH3I and Ag0-Aerogel, which are highly reaction-controlled. In this process, CH3I reacts with Ag on the surface of Ag0-Aerogel and only a limited amount of CH­3I diffuses into the pellets. Quantitatively speaking, at VOG conditions, theq/qe term in Eq. 1 is much smaller than 1 and by specifying the q/qe values, the real-time contributions of diffusion term and reaction term can be calculated. For example, as Figure 9 shows, at 113 ppbv, the reaction term contributes approximately 62% at q/qe = 0.2 or q = 7.4 wt%, which approximately 600 days are required to reach this point. Additionally, within one year, the reaction term contribution is higher than 93% and the mass uptake is below 0.74 wt%. Therefore, it is important to notice that although the overall adsorption process is controlled by the pore diffusion, at actual VOG conditions, the effect of pore diffusion to the uptake rate is minor in at least 1-2 years. To determine the CH3I-Ag0-Aerogel adsorption behavior at low concentration conditions, the analysis should focus on the reaction rate instead of pore diffusivity.