Separation of krypton and xenon by selective permeation

Abstract

The objective of this investigation was to study the separation of krypton and xenon from nuclear reactor atmospheres by selective permeation through silicone rubber capillaries. Effective permeability coefficients for pure krypton xenon, nitrogen, and oxygen were determined between 0 and 40°C and at pressure differences across the capillary walls (Ap) of up to 3.45 x lo5 N/m2 (50 psi). The silicone rubber capillaries had an 0. D. of 635 p m (0.025 in.) and an I. D. of 305 pm (0.012 in.), and were pressurized externally. The effective permeability coefficients decreased with increasing Ap due to the elastic deformation of the capillaries, in general agreement with a deformation analysis of thick-walled elastic tubes.Gas separation studies were made with a Kt--Xe-N2-02 mixture in a permeator containing a bundle of silicone rubber capillaries. The permeator had an effective permeation area of 0.480 m2 (5.165 ft') at a packing density of 4132 m2/rn3 permeator volume (1260 ftYft3), and was operated in a countercurrent mode. The separation studies were conducted at -10 and 20°C and at three Ap values. The separation achieved in the permeator at A p ' s of 1.38 x lo5 N/m2 (20 lb/in.') and 2.07 x lo5 N/m2 (30 lbIin.2) was in good agreement with that predicted from a theoretical model of a permeation stage with countercurrent flow. At 3.45 x lo5 N/m2 (50 lb/in. ' ), the separation approached that predicted from a "perfect mixing" model. This behavior probably was due to local collapses of the capillaries at weak spots in their walls, as was evidenced also by a sharp increase in the axial pressure drop inside the capillaries. SCOPEThe increasing use of nuclear energy to augment the conventional sources of energy has resulted in a strong demand for improved safety in the operation of nuclear reactors. One of the possible dangers in the operation of a nuclear reactor is the accidental release of radioactive 85Kr and '33135Xe to the atmosphere within a reactor containment dome. The radioactive krypton and xenon must then be separated and safely disposed of before the reactor can be further used (Belter, 1963;Blumkin et al., 1966). In 1963, Belter stated that this problem "could be the controlling factor in building large power reactors in populated areas." In the aftermath of the accident at the Three Mile Island nuclear power plant near Harrisburg, Pa., Belter's statement shows remarkable foresight.Several techniques of separating radioactive krypton and xenon from mixtures with air and other gases have been studied in recent years. These studies were aimed at the development of a separation plant that is sufficiently compact to be easily transported to the scene of an accident. The removal of radioactive krypton and xenon is also of interest for the decontamination of the off-gas from plants that process spent reactor fuels and of the cover gas of a liquid-metal-cooled fast breeder reactor.

A new liquid membrane technique for gas separation

“…Pan and Habgood (1974) analyzed binary mixture permeation in a flat membrane permeator with an impermeable sweep gas and no pressure drop in either gas stream. Stern and Leone (1980) treated the permeation separation of a quaternary mixture without any sweep gas or flow pressure drop. Further, although we have two sets of hollow fibers in the permeator, their function is radically different from multimembrane gas permeators studied by Perrin andStern (1985, 1986) for binary separation and by Sengupta andSirkar (1984, 1987) for ternary separations.…”

Separation of a helium‐methane mixture in permeators with two types of polymer membranes

“…(Stern, et al, 1977;Stern and Leone, 1980 Young's modulus E of 3.09 x lo6 N/m2 (448 psi) for the silicone capillaries (Stern et al, 1977). Figure 4 shows that the permeability coefficients for H e and CH, in the asymmetric hollow fibers of cellulose triacetate are also independent of Ap under the conditions of this study.…”

“…(Stern, et al, 1977;Stern and Leone, 1980). This behavior was attributed to the elastic deformation of the capillaries under external pressure, which resulted in an unequal decrease in their outer and inner diameters with an attendant increase in wall thickness.…”

“…The permeability coefficients for He and CH4 used in these calculations (7 or F / 6 ) were those reported for the pure gases in Figures 3 and 4, since these agreed closely with the values obtained from permeability measurements with He-CH, mixtures. The axial pressure drop inside the silicone rubber capillaries was neglected, having been found to be insignificant for Ap values of less than 3.45 x lo5 N/m2 (50 psi) (Stern and Leone, 1980). The axial pressure drop inside the hollow fibers of cellulose triacetate was estimated to be less than 2% of the applied Ap under the experimental conditions (Pan and Habgood, 1978a), and was therefore also neglected.…”

“…The pressurepv, (<pu,) of the permeated gas stream, or permeate, was always close to atmospheric pressure a t the permeator outlet. [The pressures po and pv, have been designated po and p , respectively, in previous publications (Stern et al, 1977;Stern and Leone, 1980). The present nomenclature is more suitable for the subject matter under consideration.]…”