Preparation method of high-purity helium

High-purity helium refers to the gas whose component content is greater than 99.999%, that is, the impurity content is less than 10×10-6 (volume fraction). The total amount of impurities in the pure gas provided by industry is generally less than 100×10-6 (volume fraction). Therefore, further purification must be used to remove impurities to less than 10×10-6 (volume fraction).

The main impurities of industrial pure helium gas are nitrogen, oxygen, moisture, carbon monoxide, carbon dioxide and hydrocarbons, which are removed by the following methods.

1、Nitrogen removal

The removal of nitrogen is sometimes accompanied by the removal of oxygen. Mainly use metal getter for absorption. Commonly used metal getters are calcium, titanium, uranium and zirconium-aluminum 16 and so on. When metallic calcium is used as a getter, nitrogen and oxygen are absorbed at the same time. The reaction temperature is 650~680℃. When the impurity concentration of the inlet gas is equivalent to 240×10-6 (volume fraction) air, the outlet impurity is less than 50×10-6 ( Volume fraction). The utilization rate of metallic calcium is 50%~60%. Using sponge-like metal titanium or zirconium-aluminum 16 as a getter can simultaneously absorb impurities such as oxygen, nitrogen, hydrogen, water vapor, carbon monoxide, carbon dioxide, and hydrocarbons.

2、Deoxygenation

Chemical methods are generally used to remove oxygen. Commonly used deoxidizers include manganese oxide or Ag-X molecular sieves. Manganese oxide deoxidizer absorbs oxygen. The total reaction formula is: 2MnO+O2→2MnO2. When the reaction temperature is 150℃ and the space velocity is 1000h-1, 0.1% (volume fraction) of oxygen can be removed to 2×10-6 ( Volume fraction). Deoxidize with Ag-X molecular sieve at room temperature. The impurity oxygen content can be reduced from 44×10-6 to 3×10-6. Both manganese oxide and Ag-X molecular sieve deoxidizer need to be reduced with hydrogen after use, and the reduced deoxidizer can be reused.

3、Dehydrogenation

Copper oxide or Pd-X molecular sieve is used for hydrogen removal. For copper oxide dehydrogenation, the reaction temperature is 350~400℃, the space velocity is 200h-1, and the volume fraction of hydrogen in the gas can be reduced from 1% to 1×10-6. After the copper oxide absorbs hydrogen, it needs to be regenerated with an inert gas containing 2% to 5% oxygen for reuse.

When using Pd-X molecular sieve, the volume fraction of hydrogen and carbon monoxide in neon gas can be removed from 11×10-6 to 0.1×10-6 at room temperature.

4、Removal of carbon compounds

Carbon-containing compounds are mainly carbon oxygen compounds, mainly carbon monoxide, carbon dioxide and methane. As mentioned earlier, the use of metal getters (such as zirconium-aluminum-16) can remove impurities such as carbon monoxide, carbon dioxide and hydrocarbons at one time while removing nitrogen. The removal depth can reach 1×10-6.

5、Cryogenic adsorption

Since helium has a very low boiling point relative to impurity components such as neon, oxygen, nitrogen, carbon monoxide, carbon dioxide, and methane, low-temperature adsorption can be used to remove all these impurities at once. The adsorption temperature has negative pressure liquid nitrogen temperature. The adsorbent can be fine-pored silica gel, alumina or molecular sieve, and the removal depth can reach 1×10-6 (volume fraction) or more.