(458e) Adsorption Models for Treatment of Nuclear Reprocessing Off-Gases

The off-gas stream from reprocessed nuclear fuel contains a number of radioisotopic gases including HTO (³HHO), ¹²⁹I₂, ¹²⁹ICH₃, ¹⁴CO₂, ⁸⁵Kr, and ¹³⁵Xe. Several of these gases are either hazardous to human health or the environment, so there is a great interest in the efficient removal, recovery, and/or disposal of the gases from the off-gas stream. Effectively modeling the separation processes involved in this system will aid both scientists and engineers in the design and implementation of the adsorption systems necessary to perform these tasks. In this work, we have developed a compartmentalized modeling approach for single and multi-component adsorption equilibria and mass transfer kinetics. Each kernel in the model is separated, such that they can all work independently or in conjunction with other kernels. The equilibrium models include software to analyze experimental data and obtain sets of equilibrium parameters, as well as make predictions of the adsorbed amounts attainable under varying conditions of temperature and pressure. Additionally, we have developed a generalized routine for predicting the adsorption equilibrium behavior of a mixed gas system based on the equilibrium parameters of each individual species in the gas phase. The diffusion mass transfer models were developed specifically for spherical, biporous pellets composed of adsorbent crystals held together in a matrix of a binder material. This is the typical configuration of commercial zeolites and mordenites used for water vapor separations. In these models, we consider the mechanisms of film mass transfer, pore diffusion, and surface diffusion to dominate the uptake rates and assume local equilibrium between the gas and solid phases at the boundary of the adsorbent crystals. These models have been examined for water vapor adsorption in a commercial molecular sieve 3A-type adsorbent, and the multi-component equilibrium model has been verified against several sets of literature data. We plan to utilize these models into a macro-scale mass transport model to simulate the capture efficiency of a fixed-bed column for the separation of gas mixtures.