Silver-Exchanged Mordenite for Capture of Water Vapor in Off-Gas Streams: A Study of Adsorption Kinetics

Abstract

Reduced silver-exchanged mordenite (Ag0Z) has been recognized as the benchmark adsorbent for radioactive iodine removal from the nuclear fuel reprocessing off-gases. It has also been shown to have a considerable adsorption capacity for water vapor, which is a major component (tritiated water) in the off-gases of spent nuclear fuel reprocessing facilities. Therefore, understanding the kinetics of water vapor adsorption on Ag0Z is necessary for a better design of off-gas treatment systems. The adsorption kinetics were studied through adsorption experiments of water vapor on Ag0Z pellets and analyses of the kinetic data with adsorption models that describe processes of mass transfer and inter/intracrystalline diffusion. Uptake curves of water vapor on Ag0Z pellets were obtained with a continuous-flow adsorption system at temperatures of 25, 40, 60, 100, 150, and 200 °C and dew points from −53.6 °C to 12.1 °C. The diffusion-controlling factors were determined experimentally and analytically. It was found that the diffusion process of water vapor in Ag0Z pellets was controlled by macropore diffusion. Gas film mass-transfer resistance also contributed to the adsorption process of the 0.9 mm Ag0Z pellets, but it could be minimized with a high gas velocity and small pellet radius. Kinetic models including macropore diffusion (MD), linear driving force (LDF) and shrinking core (SC) were used to fit the uptake curves. The macropore diffusivity for water vapor adsorption on Ag0Z pellets was determined using the three models. It was found that the LDF and SC models could well describe the kinetic process, while the fitting with the MD model was not quite as good due to the existence of external mass-transfer resistance for the 0.9 mm Ag0Z pellets.

Adsorption of methyl iodide on reduced silver‐functionalized silica aerogel: Kinetics and modeling

“…More straightforwardly, the saturation time, t sat , can be expressed as t sat = τ 1 + τ 2 + τ 3 . For the SCM, the control term can be represented by overall resistance ratio, τ i / t sat , where i = 1, 2, or 3 38 . Figure 7 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.…”

“…Therefore, the adsorbate is limited at the reacting surface and no significant concentration gradient exists in the pellet. This model has been widely used in the nuclear waste treatment area including water adsorption on molecular sieves 3A, 40 water adsorption on Ag 0 Z, 16,38 I 2 adsorption on Ag 0 Z, 16,19 CH 3 I adsorption on Ag 0 Z, 18 and so forth.…”

“…The continuous flow adsorption system was used for iodine and water vapor adsorption on multiple materials previously by Nan et al 19,38,39 and Lin et al 40,41 and has been modified for performing the adsorption of CH 3 I on Ag 0 -Aerogel experiments. Figure 1 During the adsorption process, the temperature was controlled by the furnace controller and N 2 was used as the protective gas for the microbalance system.…”

“…Multiple gas-solid adsorption models have been used in explaining adsorption kinetics of I 2 and organic iodides including linear driving force model, SCM, volume reaction model, and pore diffusion model. 16,18,38,[42][43][44][45][46] I 2 adsorption on Ag 0 Z has been identified as the shrinking core process because the core shrinkage was observed by Nan. 16 Shown in Figure 2, a similar process was observed in CH 3 I adsorption on Ag 0 -Aerogel by cutting a partially reacted pellet in half.…”

“…For the SCM, the control term can be represented by overall resistance ratio, τ i /t sat , where i = 1, 2, or 3. 38 Figure 7 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 104 ppbv to 5% at 9,585 ppbv and the pore diffusion resistance contribution increases accordingly.…”