Helium is a colorless, odorless, and nontoxic gas distinguished by its remarkable properties, including being the lightest noble gas and possessing the lowest boiling point of any element, at approximately -269°C (4 Kelvin). This makes it indispensable for applications like cooling superconducting magnets in MRI machines and specialized welding processes. The element is a finite, non-renewable resource on Earth because its atoms are so light that they escape the planet’s gravitational pull once released into the atmosphere. Industrial extraction from underground reserves is therefore required to secure a reliable supply for technological and medical uses.
The Geological Source of Helium
The helium found on Earth is primarily a byproduct of natural radioactive decay occurring deep within the planet’s crust. Elements such as uranium and thorium slowly decay over geological timescales, emitting alpha particles, which are essentially helium nuclei. These particles combine with electrons from the surrounding rock to form stable helium atoms.
This newly formed helium gas migrates upward through faults and fissures, often becoming trapped within the same geological structures that hold natural gas. Commercially viable concentrations are almost exclusively found mixed with hydrocarbons in natural gas reservoirs. These reservoirs require dense, non-porous cap rocks acting as a seal to prevent the tiny helium atoms from diffusing away.
While helium exists in the atmosphere, its concentration is extremely dilute, around 5.2 parts per million. Extracting it from the air would require an immense and uneconomical amount of energy. Recovery from natural gas, where concentrations can reach up to 7% by volume, is the only practical method, directly resulting from this geological concentration mechanism.
Pre-Treatment of the Natural Gas Stream
Before the gas stream can be subjected to the extreme cold necessary for separation, it must undergo extensive purification, known as pre-treatment. The raw natural gas mixture contains various impurities that would solidify at cryogenic temperatures, potentially fouling and damaging the processing equipment. This initial cleaning step is mandatory for the entire process to function reliably.
A primary step involves removing heavy hydrocarbons, which are compounds larger than methane that would easily freeze. Water vapor and acidic components, specifically carbon dioxide and hydrogen sulfide, must also be scrubbed from the gas. These contaminants are typically removed using chemical solvents, specialized molecular sieves, or adsorption-based processes.
Carbon dioxide is often removed via amine scrubbing, while water vapor is adsorbed onto desiccants or molecular sieves. Should any impurities remain, they would form solid blockages, known as “freezing out,” when the gas stream is cooled below approximately -73°C, halting the cryogenic operation. This preparation ensures the gas stream entering the main separation unit is clean and ready for deep cooling.
Cryogenic Separation: The Core Process
The separation of helium from the natural gas stream is performed using cryogenic fractional distillation. This complex, energy-intensive technique exploits the significant differences in the boiling points of the components in the mixture. The prepared gas stream is first compressed and then cooled in stages to progressively lower temperatures.
Methane, the primary component of natural gas, liquefies at about -162°C, allowing it to be separated first. Further cooling is applied to remove other components, often integrating the process with a Nitrogen Rejection Unit (NRU) in a liquefied natural gas (LNG) plant. Nitrogen, which is also present in many helium-rich reserves, has a boiling point of -196°C, and it is separated next.
Achieving these extremely low temperatures is accomplished through a combination of heat exchangers, compression, and expansion cycles, which utilize the Joule-Thomson effect. As the gas expands rapidly from a high-pressure state, its temperature drops significantly, providing the necessary refrigeration. This progressive cooling and liquefaction process leaves behind a gaseous mixture highly enriched in helium.
The resulting product from this cryogenic stage is not pure helium but a “crude helium” stream, typically containing between 50% and 90% helium, with the remainder being primarily nitrogen and trace amounts of other noble gases. This crude product is the feedstock for the final purification steps.
Refining and Liquefaction
Once the crude helium stream is recovered from the cryogenic separation unit, it must undergo a final refining process to meet commercial grade purity requirements, which is typically 99.999% or better. The remaining impurities, chiefly nitrogen and any trace hydrogen, must be removed to achieve this high standard. Hydrogen is often eliminated using catalytic oxidation, where the gas is warmed and reacted with oxygen to form water, which is then condensed and removed.
The final trace amounts of nitrogen and other gases are removed using specialized adsorption technologies, such as Pressure Swing Adsorption (PSA). This method involves cycling the gas stream through beds of adsorbent material, like activated carbon or molecular sieves, at varying pressures. These materials selectively trap the larger impurity molecules, allowing the helium atoms to pass through and achieve the desired ultra-high purity.
The highly purified helium is then cooled once more for liquefaction, a necessary step for efficient storage and transport. Since helium’s boiling point is only 4.2 Kelvin (-269°C), this final stage is the most technologically challenging. The purified gas is progressively cooled using liquid nitrogen as a pre-coolant, followed by a series of specialized turboexpanders and heat exchangers to reach the final cryogenic temperature, converting the gas into liquid helium for global distribution.