Several tasks related to the use of xenon (Xe) and neon (Ne) isotopes were formulated at the end of the 20th century [1]. Xe has nine stable isotopes ( 124 Xe, 126 Xe, 128 Xe, 129 Xe, 130 Xe, 131 Xe, 132 Xe, 134 Xe, 136 Xe); Ne, three ( 20 Ne, 21 Ne, 22 Ne) [1, 2].The separation coeffi cient can be used to evaluate technical and economic factors of the process for separating a binary gas mixture [1, 3] and is approximated for diffusion separation of a mixture (through a porous diaphragm) as [1,4] (1)where M h and M l are the molecular masses of the heavy and light components, respectively.Equation (1) is valid for Knudsen fl ow through a porous diaphragm. The separation coeffi cients for the binary mixtures 20 Ne-21 Ne, 20 Ne-22 Ne, and 21 Ne-22 Ne are k s 20-21 ≅ 1.0247, k s 20-22 ≅ 1.0488, k s 21-22 ≅ 1.0235, respectively. The separation in one stage is miniscule; however, it can be increased by combining diffusion cells [5]. The mole ratio 20 Ne: 22 Ne ≈ 9.8:1.0 or y 22 Ne ≈ 0.092 is used for estimating the number of diffusion cells (number of contact stages) for separating the 20 Ne-22 Ne mixture. The separation coeffi cient of the mixture is set to k s 20-22 ≅ 1.0488 = const whereas k s 20-22 ≅ 1.0200 for comparison. Figure 1 shows the required number of diffusion cells (calculated by the literature method [6]). Stable Xe isotopes are produced in Russia mainly by centrifuge technology [1]. The separation coeffi cient for a binary mixture of ideal gases by centrifuge technology (at constant temperature) is [1](2) where ϖ is the angular velocity; a, the rotor radius; T, the temperature; and R, the gas constant.
A schematic diagram of the plant for getting nitrogen-xenon mixture from "dirty" oxygen of air-fractionating plant (AFP) is shown and described. Industrial plant test data are presented in the form of relationship of the adsorber operation time with the temperature of the "dirty" oxygen at the inlet and of the duration of the application stage with the xenon concentration at the inlet.The heavy rare gases krypton and xenon are finding wide use in domestic and global science and technology. In spite of these gases being relatively dear, their industrial production has been going up steadily.Several potential sources for industrial production of krypton and xenon are known, among which are tail streams of ammonia production (nitrous gases), radioactive tail gases of nuclear power plants, etc. Universally recognized and essentially the only source for industrial production of these gases, however, is atmospheric air, which contains 1.14 vol. ppm of krypton and 0.086 vol. ppm of xenon.Because of the small content of these elements, industrial atmospheric air processing for their extraction is inefficient, and at present use is being made of the method of Kr and Xe extraction in the air fractionation process in large air-fractionating plants (AFP).For this purpose, modern AFPs are generally provided with a unit for production of a primary krypton concentrate (PKC) where the krypton and xenon content rises to what is admissible for further treatment (0.2-0.4 vol. % Kr and 0.015-0.03 vol. % Xe).Several industrial methods are known for getting krypton and xenon, of which fractionation and adsorption-fractionation methods have found the widest application. All these methods are based on treatment of PKC [1]. However, because of temporary decline in demand for heavy rare gases in the late eighties and early nineties of the past century, the Kriogenmash OAO, which is the only enterprise producing large-scale AFPs in Russia and exporting them to some foreign countries, discontinued inclusion of the PKC extraction unit in the process lines of their plants (AFPs of the type of etc.).At present, in Russia and in former Soviet Union countries and beyond, there are quite a few AFPs with such modifications of the process lines, which are not suitable for getting the desired (full-valued) PKC. In this situation, as simple
The heavy rare gas xenon is used extensively in science and technology. The industrial production of xenon has been growing constantly despite its relatively high cost. At the same time, the purity requirements for the xenon produced has been increasing. In particular, the total residual content of impurities in it should be less than 1 part per million (ppm).The main source of xenon at present is atmospheric air, which contains about 0.086 ppm xenon by volume. Tail streams from high-tonnage air separation plants (ASPs) are processed to obtain xenon [1].The first stage yields a primary krypton concentrate (PKC), which in the second stage is concentrated to a krypton-xenon mixture (KXM) that contains 6-8 vol. % xenon and 0.5 vol. % impurities, the remainder being krypton. In the third, final stage the KXC is separated into the pure products Kr and Xe.Rectification, the most common method of separating KXC today, entails the problem of purifying xenon to remove the hexafluoroethane (C 2 F 6 ) that concentrates in it. In a Russian-made plant of this kind [2] up to 50 m 3 /yr of dirty xenon fraction is formed with a hexafluoroethane content of 0.5-1.0 vol.%.Although possible in principle, removal of hexafluoroethane from that fraction requires considerable additional capital and operating (primarily, energy) expenditures. It is desirable, therefore, to employ an alternative method of purifying dirty xenon, in particular adsorption purification to remove the hexafluoroethane impurity.To determine the prospects of this treatment we calculated the principal parameters of the process of low-temperature adsorption of C 2 F 6 on an industrial adsorbent of the activated carbon type. Figure 1 shows the xenon vapor saturation curves according to [3,4]. The ternary points on the saturation curves are marked Ter 1 and Ter 2. The hexafluoroethane equilibrium curve lies below the xenon equilibrium curve over the entire range of temperatures considered (accordingly, a higher temperature corresponds to the first component at equal C 2 F 6 and Xe partial pressures). Bearing in mind that the molecular mass of C 2 F 6 is higher than that of Xe (138.0 and 131.3 g/mole, respectively), we can draw this conclusion: adsorption of hexafluoroethane on homogeneous microporous adsorbents such as activated carbon and silica gel is preferential to adsorption of xenon.We know that the purification process must be conducted at the lowest possible temperatures to increase the efficiency.
Results are provided for a study in a laboratory test unit of the adsorption dynamics for krypton, xenon, ethane and nitrogen hemioxide (lower oxide) from air in two-layer equipment simulating an industrial adsorber of a combined purification unit (CPU) of a contemporary air separation unit (ASU). As a result of the studies, it is established that the loss of krypton in a CPU is insignificant (about 0.25%) and it is mainly determined by the presence of this component in the gas phase (air) at the end of the purification stage before the start of switching over adsorbers. The loss of xenon is more marked and it depends on the ratio of the calculated adsorber operating time (before the start of the breakthrough carbon dioxide) to the actual adsorber operating time in the purification stage. According to experimental data, the maximum loss of xenon is about 8%. The data obtained point to a marked reduction in the loss of gases in a CPU compared with regenerators used in old ASU schemes within which the losses were about 10-11% for krypton and 25-30% for xenon. It has been established by experiment that during operation of adsorbers of an industrial CPU about 18% of nitrogen hemioxide and 10% ethane is retained within them, but methane is hardly retained. The data obtained may be used in designing units for primary enrichment of krypton and xenon in an ASU.In the industrial production of heavy rare gases, i.e., krypton and xenon, atmospheric air is processed in air separation units (ASU), as a result of which either a primary krypton concentrate (PKC), or so-called dirty oxygen enriched in these air components is obtained.In order to extract rare gases with the use of an ASU, the firm Khrom has created special units for processing both PKC (if the ASU is equipped with a krypton concentration unit), and also dirty oxygen (if there is no unit for primary concentration in the ASU) [1,2].An important problem in the technology for preparing heavy rare gases is achievement of total extraction of krypton and xenon in the first stage of preparing PKC or dirty oxygen, for which it is necessary to estimate possible losses of krypton and xenon in the main ASU assemblies.
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