At present, the processes for helium extraction from natural gas mainly include cryogenic method, adsorption method, membrane separation method, or a combination of two or three of the three methods. Product helium purity can meet 99.9999%
(1) Cryogenic method
Cryogenic distillation is a commonly used method in most medium to large natural gas recovery helium plants. The principle is to use liquid nitrogen as a refrigerant (about -190 ℃) to gradually condense hydrocarbons and other components in natural gas. Due to the extremely low boiling point of helium, it is ultimately separated from other components in natural gas.
(2) Pressure Swing Adsorption Method
The pressure swing adsorption (PSA) process uses different adsorbents to adsorb components other than helium in natural gas, thereby separating helium products. Generally, PSA adsorption towers are equipped with separation beds of different adsorption materials to deal with different components in natural gas, such as silica gel or alumina beds for removing water, activated carbon beds for removing carbon dioxide, zeolite beds for removing methane and nitrogen, and so on. After the adsorbent saturates the impurity gas, methods such as depressurization blowback or vacuum pumping can be used to remove the impurity gas and restore the activity of the adsorbent.
(3) Membrane Separation Method
Gas separation membrane is a relatively novel separation technology. Gas components undergo mass transfer on polymer films (such as acetate fibers, polypropylene, poly-sulfone, polyimide, etc.) based on the principle of dissolution diffusion. The separation of each component is achieved according to the different solubility and diffusion rates of different gas components in the membrane material. When two or more gas mixtures pass through a polymer film, the permeation rate of components such as helium and hydrogen is much higher than that of nitrogen, methane, and other hydrocarbons.