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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