Radioactive Rare Gas Separation Using a Separation Cell with Two Kinds of Membrane Differing in Gas Permeability Tendency

Abstract

A separation cell embodying two kinds of membrane—porous and nonporous, i.e. differing in gas permeability—has a separation factor higher than possible with a conventional separation cell with a single kind of membrane. The performance of such separation cells and of cascades constituted thereof are analyzed theoretically and measured experimentally for different conditions of operation, to determine the applicability of the concept to the separation of rare gases from gaseous waste out of nuclear plants. Theoretical considerations indicate that, in a cascade composed of symmetric separation cells, the separation performance can be improved by recycling part of the effluent from a cell back through the same cell (recycling cascade). It is shown that its performance is better than with the arrangement of diverting another effluent several stages upstream. With the recycling cascade, the symmetric separation recycling rate is determined by the depletion separation and enrichment separation factors relevant to the respective membranes. The separation performance of a 9-stage recycling cascade composed of separation cells with silicone rubber tubular membranes and cellulose acetate tubular membranes is derived for a case of Kr separation from N2-Kr mixture. The experimental data coincide well with the analytical results. From both the experimental and the analytical results, it is found that the attainable separation coefficient per stage of the cascade comes to average approximately 0.97.