Prof. Linghui gong (Technical Institute of Physics and Chemistry, CAS)Prof. Liqiang Liu (Technical Institute of Physics and Chemistry, CAS)Prof. lianyou xiong (Technical Institute of Physics and Chemistry, CAS)
Helium is an invaluable rare gas as it has a lot application in industrial and research. At present, helium is produced mainly from helium bearing natural gas which contains 1~8% helium. However, most of helium reserves dissolved in natural gas is located in only few countries in the world, and the low concentration of helium in Chinese natural gas field makes extracting helium from the natural gas uneconomical. Accordingly, it is important to explore possible unconventional helium sources especially in China to mitigate the helium shortage. Therefore, the helium bearing boil off gas (BOG) from LNG storage tank in LNG plant, which has a helium concentration of about 1%, has attracted the attention in China as a new helium source. However, as the BOG is usually recondensed to recover methane, it is likely to cause continuous accumulation of nitrogen in the unit, thus a nitrogen removal process must be integrated. This paper describes a conceptional design on a cryogenic separation process of helium extraction and nitrogen removal from LNG boil-off gas. In this process, methane and other few hydrocarbons are firstly separated with N2, He and H2 through cryogenic distillation, and then N2 is separated with He and H2 through cryogenic condensation at a lower temperature, next the H2 content of mixture is removed in a dehydrogenation unit, and finally the helium is extracted and purified in a helium liquefier. As an example, a 3000000m3/day LNG plant in China will produce 4900NM3/h BOG stream with a volumetric concentration of 88.8% CH4, 9.9% N2, 1% He and 0.3% H2. With this method, the helium extracted and liquefied can reach 70 L/hr. Moreover, about 4400NM3/h CH4 and 400NM3/h N2 can be recovered with final purity of 99% and 99.5% respectively.
Prof. lianyou xiong (Technical Institute of Physics and Chemistry, CAS)