Prediction of a Breakthrough Curve of Iodine on a Reduced Silver-Loaded Adsorbent Bed

An adsorption process of iodine using Ag⁰-loaded adsorbents was studied for the removal of radioactive iodine in the process off-gas from a spent nuclear fuel reprocessing plant. A mathematical model to predict a breakthrough curve of I₂ on the adsorbent bed was proposed. This model consists of the mass balance equation of I₂ in the adsorbent bed, the mass transfer equation of I₂ through the boundary layer surrounding the adsorbent particle, the intraparticle diffusion equation of I₂, and the kinetic equation for the gas-solid reaction between I₂ and loaded Ag⁰. Two unknown parameters in the model, the intraparticle diffusivity D_e and the apparent rate constant for the gas-solid reaction k_r were determined simultaneously from the adsorption data measured by a thermogravimetric analyzer. The breakthrough curves predicted by the model using these parameters were in good agreement with the experimental ones. The rate-controlling step was evaluated by the effectiveness factor calculated from the k_r value and the concentration gradient of I₂ in the adsorbent particles, which was estimated by the model. From these results, the adsorbent structure required to improve the process performance is discussed. The proposed model is available as a calculation tool to support the design of the adsorption process.