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.