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Research and application of filtering technology for fluoride impurities in krypton and xenon
2017-01-05589
Through the exploration, research and practice in recent years, it is first found and confirmed that there are three kinds of fluoride impurities in krypton and xenon produced by air separation with cryogenic method in China. In order to remove these harmful impurities, a special purification material and a complete set of filtration device have been developed. This device can effectively remove these three fluorides which are difficult to be removed by other methods. Compared with other methods abroad, the advantages of this method are outstanding.
1. Confirmation of impurity gas
In 1992, the krypton gas exported to Japan was comprehensively analyzed, and all impurity contents were within the index range specified by the national standard superior products; however, a new impurity peak appeared before krypton peak on helium ionization chromatograph. Later, we used mass spectrometer qualitative identification and helium ionization chromatograph quantitative detection to confirm that the impurity is carbon tetrafluoride (CF4).
2. Development of special purification materials
It can be seen from the above experiments that the clearance of zr-al16 to CF4 is not ideal. According to the analysis with a researcher in Beijing, there may be some defects in the existing alloy, which does not have the function of removing impurities completely. According to the data, if the catalysts such as aluminum, hafnium, nickel, titanium and lanthanide rare earth elements are added to the zirconium based alloy, the crystal lattice of various intermetallic compounds can be changed obviously, the channel of gas impurity diffusion from the surface of zirconium alloy to the interior can be increased, and the surface shape of the powder will also be changed obviously, becoming porous, increasing the specific surface area of zirconium alloy. According to these theories, through experiments with a Beijing Technology Co., Ltd., a proper amount of aluminum, hafnium, nickel, titanium and other metal catalysts and a small amount of lanthanide rare earth elements (similar to the Chinese medicine introducer) were added to the zirconium base to form a multi-element intermetallic compound; at the same time, the composition formula of the original zirconium aluminum alloy and the lattice shape of the material were changed, and the new material was characterized in the manufacturing process Finally, a special getter for zirconium based multi-component alloy was developed. This kind of getter not only can effectively remove the three fluorides in Kr or Xe which are difficult to be removed by other methods, but also can partially remove the conventional impurities such as O2, N2, H2, Co, CO2 in the gas.
4. Development and application of filtration device
In order to further improve the filtering effect of impurities, we adopted the secondary purification process in the process design of the filtering device. Among them, a high-efficiency catalyst filler is installed in the first stage purification furnace, whose main function is to filter most of the impurity gas by catalysis; the second stage purification furnace is installed with our newly developed special purification material, whose main function is to absorb, absorb and catalyze the gas and residual fluoride decomposed by the first stage catalytic furnace. Krypton or xenon product gas is removed from the purification furnace, cooled to normal temperature by water cooler, and finally sent to the product freezing and bottle filling system. Item 1 and item 2 in Figure 6 constitute a complete set of terminal filtration device, which is also equipped with corresponding flowmeter, thermometer, pressure gauge, safety valve (bursting disc) and regulating valve. Meanwhile, two sets of purification devices are made, which are respectively used for purification of krypton gas and xenon gas. The gas treatment capacity is 2m3 / h and 1m3 / h respectively, the working pressure is 0.3MPa, and the working temperature is 700-800 ℃. Around January 2002, it was put into production successively. Since the main impurity in Kr is CF4, the main impurity in Xe is C2F6.
The purity of krypton and xenon products after purification is more than 99.9995%, and the content of three kinds of fluoride impurities in the products is less than 0.1 × 10-6, becoming ultra-high purity gas. The net removal rate of impurities is more than 99.5%. The purified products of ultra-high purity krypton and ultra-high purity xenon have been used by several users in Shanghai and Beijing who have high purity requirements, and fully meet the requirements.
Through more than two years of production practice and observation, it is found that CF4 impurities are mainly concentrated in pure krypton, while C2F6 and SF6 impurities are mainly concentrated in pure xenon (due to their different boiling points). Of course, if the operation condition of krypton xenon filter separator is not stable or the operation procedure is not proper, this rule will be broken.
The author talks about the net removal effect of CF4 and C2F6 in detail, but how about the net removal of SF6? Here, it involves the use of analytical instruments and methods. We use 2700 helium ionization chromatograph to detect fluoride impurity in gas. Especially in the detection of C2F6 and SF6 in Xe gas, there is a mixed peak of C2F6 and SF6 in front of H2 peak when using 5 μ molecular sieve column, and the peak of SF6 is covered by Xe peak when using PQ column (but C2F6 peak is very clear). That is to say, it is difficult to determine it independently no matter which adsorption column is used
SF6 content. However, before gas purification, the mixed peak of C2F6 and SF6 is obvious, and after purification, there is no mixed peak. This phenomenon shows that the fluoride in the gas has been removed after purification by the purifier. Aiming at the problem of high SF6 content in Xe of a gas plant in China, a mixture with SF6 content of 36.3 × 10-6 has been added into Ar gas. The content of SF6 is less than 0.1 × 10-6 after purification by a purifier. This further proves that the removal effect of SF6 is very obvious. It should be noted that the service life of the special material is related to the quantity of purified gas, the content of various impurities and the use method.
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