Despite being abundant throughout the universe, helium is actually rather uncommon here on earth. Trace amounts exist in the atmosphere and it can be extracted through liquefaction (cooling air to extremely low temperatures,) but this approach is too expensive for commercial use. The more economical method of obtaining helium is to distill it from natural gas. Even then, only in some areas, such as Kansas, is the concentration of helium in natural gas high enough to make distillation attractive.
Distillation is the process of separating out the various compounds in a mixture. It entails boiling a liquid and cooling the steam that comes off. As different compounds have different boiling points, they condense (change from vapor to liquid) at different temperatures. So distillation involves allowing steam to condense in a tall chamber or tower and capturing the liquids that result.
Helium however presents a couple of challenges. First, it boils at very low temperatures, around -200 degrees Celsius, meaning that it’s difficult to get it into a liquid form, and second, its boiling point is quite close to that of the other gases extracted in natural gas, such as neon, hydrogen and nitrogen. As a result, the distillation process had to be modified to produce pure helium.
The process used to distill helium is known as fractional distillation. The principle is the same as for regular distillation, but with some modifications.
First, the natural gas is cooled and cleansed of as many impurities as possible (this actually improves the properties of the natural gas itself,) before being pressurized. The gas is then forced through an expansion nozzle into a high pressure fractionating column. As gas expands, it cools, (the same process is used in your refrigerator,) and the natural gas becomes a part liquid, part gas mixture.
The liquid, being heavier, sinks to the bottom of the column while the lighter gases such as nitrogen rise up. As the gases move higher they cool and start to condense. This leaves a gas which is 50 to 70% helium at the top. (The balance is made up of nitrogen, hydrogen, neon and some other gases.)
The next step is to purify this crude helium. This is done by first cooling the crude to nearly -200 degrees Celsius, at which temperature most of the other gases liquefy and are separated out. Next, hydrogen is passed over the helium. This attracts any oxygen in the crude helium, which combines to form water, and again, is separated off. A final filtration process then removes any remaining impurities, resulting in a gas that is 99.9% pure helium.
Helium distillation is carried out in large industrial plants configured to run as a continuous process. This means a steady flow of natural gas is introduced, cleansed, pressurized and then cooled. The resulting liquids and gasses, not only helium but neon, nitrogen and of course, natural gas, are captured continuously, purified and then stored ready for transportation and use.
