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 CHÂ3I 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.