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.
The low concentration methyl iodide (CH3I) adsorption process on reduced silver‐functionalized silica aerogel (Ag0‐Aerogel) was studied. The kinetic data were acquired using a continuous flow adsorption system. Because the corresponding physical process was observed, the shrinking core model (SCM) was modified and applied. An average CH3I pore diffusivity was calculated, the CH3I‐Ag0‐Aerogel reaction was identified as a 1.40 order reaction instead of first order reaction, and the nth order reaction rate constant was determined. This modified SCM significantly increases the accuracy of adsorption behavior prediction at low adsorbate concentration. Modeling results indicate that the overall adsorption process is controlled by the pore diffusion. However, at low adsorbate concentration (ppbv level), the CH3I adsorption is limited to the surface reaction due to the low uptake rate in a predictable time period.
The low concentration methyl iodides (CH 3 I) adsorption process on reduced silver-functionalized silica aerogel (Ag 0 -Aerogel) was studied. The kinetic data were acquired using a continuous flow adsorption system. Because the corresponding physical process was observed, the shrinking core model (SCM) was modified and applied. An average CH 3 I pore diffusivity was calculated, the CH 3 I-Ag 0 -Aerogel reaction was identified as a 1.37 order reaction instead of first order reaction, and the n th order reaction rate constant was determined. This modified SCM significantly increases the accuracy of adsorption behavior prediction at low adsorbate concentration. Modeling results indicate that the overall adsorption process is controlled by the pore diffusion. However, at low adsorbate concentration (<100 ppbv), the CH 3 I adsorption is limited to the surface reaction due to the low uptake rate in a predictable time period.
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