Figure 9. Real-time contribution
of reaction term and pore diffusion term in
CH3I-Ag0-Aerogel adsorption at 113
ppbv (percentage represents reaction contribution).
Because the adsorption behavior at low concentration minorly depends on
gas film diffusion and pore diffusion term, the SCM in Eq. 1 can be
reduced to Eq. 13 and the mass uptake rate is given in Eq. 14.
Since at the initial region,t <<τ3 , Eq. 14 can be
written as Eq. 15 for nth order reaction.
Furthermore, by replacing Ra andρp , Eq. 15 becomes,
where A is the specific surface area (cm2/g) of
the material. This result indicates that at VOG conditions, the initial
part, the only region need be considered, of nth order
SCM reduces to a simple nth order surface reaction
with a constant uptake rate, which can be demonstrated by 113 and 266
ppbv adsorption curves in Figure 3 and Figure 4. However, to increase
the adsorption efficiency, simply increasing the surface area by
reducing the diameter may not be applicable. The surface reaction
condition may not hold due to the change of flow regime caused by fine
pellets.
Conclusion
The kinetic data of CH3I adsorption on
Ag0-Aerogel at 150 ℃ were obtained using the
continuous flow adsorption system. The CH3I
concentrations were 113, 266, 1130 and 10400 ppbv. Because the
corresponding shrinking core process was observed, the shrinking core
model was applied to determine the gas film diffusivity, pore
diffusivity and reaction rate constant. The 1st order
reaction was originally assumed. The well-agreed pore diffusivities were
determined in three of the total four trails. The average value was 4.59
± 0.102 ×10-4 cm2/s. Orderly
increasing reaction rate constants were observed and, therefore, the
modified nth order SCM was selected for analysis.
The reaction order of CH3I-Ag0-Aerogel
adsorption was calculated to be approximately 1.37 and the reaction rate
constant was approximately 1287
(cm/s)∙(mol/cm3)1-n. This
nth order SCM effectively increases the accuracy of
adsorption behavior prediction. Using nth order SCM
instead of 1st order SCM, the AARD of 113 ppbv
adsorption behavior prediction decreases from 200.3% to 24.13%.
Furthermore, the overall adsorption behaviors at 113, 266, 1130 and
10400 ppbv were predicted. It requires more than 50 years for
Ag0-Aerogel reach to equilibrium at 113 ppbv condition
if the capacity loss due to dry air aging effects is not considered.
The rate-controlling step of
CH3-Ag0-Aerogel adsorption was
identified by plotting the resistance of different rate-dependent terms.
Although the overall adsorption process is controlled by pore diffusion,
the surface reaction between CH3I and Ag is more crucial
at VOG conditions. The nature of low concentration in VOG streams
(Cb <100 ppbv) limits the adsorption
from a full nth order SCM to a surface reaction. To
increase the adsorption efficiency, decreasing the size of pellets is a
theoretically applicable method. However, the detailed solution still
requires further studies in deep-bed adsorption. As replacing the
1st order SCM by nth order SCM, the
accuracy of adsorption behavior prediction at VOG conditions was
increased significantly. The parameters determined can be widely applied
to the deep-bed adsorption system design of the off-gas treatment in the
nuclear fuel reprocessing process.
Acknowledgement
This research was funded by the Nuclear Energy University Program of the
U.S. Department of Energy, Office of Nuclear Energy (Grant No.
DE-NE0008761)
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