Date of Award

December 2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Lawrence L. Tavlarides

Keywords

Adsorption, co-adsorption, iodine removal, kinetics, zeolites

Subject Categories

Engineering

Abstract

The reprocessing of used nuclear fuel to recover the fissile materials generates off gases including radioactive nuclides namely 3H (tritiated water, H3HO), 129I, 85Kr, 135Xe, and 14C. Due to its high quantity in off-gas streams and long half-life (1.59 million years), 129I removal from the off-gas streams is a significant objective of off-gas treatment.

129I retention by solid adsorbents was recognized a better strategy in comparison to liquid scrubbers due to the simplicity of system design and low cost. Among the studied adsorbents, reduced silver-exchanged mordenite (Ag0Z) was widely recognized as the state-of-art adsorbent for iodine removal. It has been selected to be used in the Hanford Treatment and Immobilization Plant in Washington State, US, to control releases of the radioactive iodine in off-gases of spent nuclear fuel reprocessing facilities.

Previous studies have shown the outstanding performance of Ag0Z for adsorption of both molecular iodine and organic iodide. However, the detailed adsorption kinetics of I2 on Ag0Z was never reported. Therefore, this study included determining the adsorption dynamics of I2 adsorption on Ag0Z through continuous -flow experiments and data analyses by kinetic models. Mass transfer, diffusion and reaction processes involved in the adsorption process were evaluated.

In addition, one issue that has to be addressed when using Ag0Z for iodine removal is the potential co-adsorption of other gas species, among which is H2O vapor. Mordenites have been shown to have a considerable adsorption capacity for H2O vapor, which is also a major component in the off-gases of spent nuclear fuel reprocessing facilities. Therefore, understanding the kinetics of H2O vapor adsorption on Ag0Z is necessary for a better design of off-gas treatment systems. The kinetics and equilibrium of H2O adsorption on Ag0Z were studied. Uptake curves and isotherms were obtained at temperatures from 25 to 200 oC. Data were analyzed by kinetic and isotherm models, and parameters related to the adsorption kinetics and thermodynamics were determined.

Moreover, the co-adsorption on I2 and H2O on Ag0Z were studied. Co-adsorption uptake curves were obtained to determine the performance of Ag0Z in humid gas streams and effect of H2O concentration on the capacity of Ag0Z for I2 adsorption. It was found that H2O vapor in the gas stream deactivated the iodine adsorption sites (Ag particles) in Ag0Z. The deactivation mechanism was determined by chemical analyses with XRD and SEM-EDX.

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