Permeability of fluorine and some fluorine-containing gases through nonporous fluorine-stable polymers

Abstract

The experimental coefficients of permeability of fluorine, hydrogen fluoride, and uranium and tungsten hexafluorides through nonporous films consisting of certain fluorine resistant polymer materials are presented. It is shown that the membrane method of removing hydrogen fluoride from fluorine and uranium and tungsten hexafluorides is attractive.Membrane technology for separating gas mixtures is undergoing intensive development. The attention being given to this technology is largely due to the development of the chemistry and technology of polymer materials, which make it possible to synthesize polymer materials with prescribed properties and to make different articles from these materials, including films and fibers with an extremely thin nonporous diffusion layer, which have been termed asymmetric polymer gas membranes. As a result of the absence of pores in the diffusion layer, a polymer membrane retains the separation properties characteristic of the polymer and the thinness of the layer (several microns) determines its high specific capacity.The permeability coefficients of a large number of polymers and block-copolymers for different gases are presented in handbooks, for example, [1]. However, the information on the permeability of fluorine-stable polymer materials, which are conventional for the nuclear industry, refer only to noncorrosive gases such as nitrogen, oxygen, helium, water vapor, and certain hydrocarbons [2]. For this reason, it seemed interesting and helpful to investigate the permeability of the fluoropolymers widely used in the nuclear industry for gases in the main process flows in sublimate production and in uranium isotope separation plants.One modification of fluoroplastic-4 (F-4NA) and the best known fluorine-stable block-copolymers based on vinylidene fluoride F-42, -23, and -26, produced by the Polimer plant at the Kirovo-Chepetskii Chemical Works were chosen as the object of investigation. The film samples made of these materials were first evacuated for a period of one day at pressure ∼10 Pa to remove any volatile organic substances that could be present and checked for the absence of pores by measuring the rate of helium permeation.Fluorine, hydrogen fluoride, and uranium and tungsten hexafluorides were chosen as the gases. Hydrogen fluoride was of special interest, since the removal of fluorine obtained by electrolysis in sublimate production and uranium hexafluoride from it requires, once again, finding the optimal solution. All gases were first subjected to deep purification by rectification. Fluorine was purified on a cryogenic rectification setup [3]. All other substances were rectified at 80-100 °C in a packed column filled with a fine spiral pakcing. The purity of the rectified substances was checked by the spectral method and hydrogen fluoride and tungsten and molybdenum hexafluorides were additionally checked by measuring the saturated vapor pressure at different temperatures.The gas permeability coefficients of fluorine and fluorine-containing gases were determined by t...

Solubility of fluorine in liquid uranium, tungsten, and molybdenum hexafluorides

“…Equilibrium was considered to be attained when the solubility with decreasing gas pressure was the same as with increasing gas pressure. Methods for purifying the initial substances and checking their purity as well as the technique for performing chromatographic analysis of extracted samples are described in [2]. The computed error in determining the fluorine concentration did not exceed 0.1%.…”