With a seemingly endless list of commodity shortages roaming the newsfeeds daily, you’d be forgiven for not noticing one shortage in particular. But amid shortages of everything from eggs to fertilizer to sriracha sauce, there’s a growing awareness that we may be missing something so fundamental that it could have repercussions that will be felt in all aspects of our technological society: helium.
It’s hard to overstate how central helium is to almost every aspect of daily life. The unique properties of helium, such as the fact that it remains liquid at a few degrees above absolute zero, contribute to its use in countless industrial processes. From leak detection and soldering to producing silicon wafers and cooling the superconducting magnets that make magnetic resonance imaging possible, helium has entrenched itself in technology in a way that belies its relative rarity. .
But where does helium come from? As we will see, the second lightest element in the periodic table is not easy to find, and considerable effort is required to extract and purify it sufficiently for industrial use. While great strides are being made towards improving extraction methods and discovering new deposits, for all intents and purposes helium is a non-renewable resource for which there is no substitute. So it pays to know a thing or two about how we get our hands on it.
A product of decomposition
Despite being the second most abundant element in the visible universe, helium is surprisingly rare on Earth. While it was first discovered in spectrographs of the sun and other stars in the 1860s, getting enough helium to study and determine it to be an element would take another 30 years, when a gas with the same spectral signature was released by dissolving a sample of uranium ore in acid.
The discovery of helium on Earth came at an opportune time in the history of chemistry. The late 1800s and early 1900s saw interest in chemistry expand from reactions involving atoms as a whole to the subatomic realm, at the level of the electrons, protons, and neutrons that make up atoms. Radioactivity had just begun to be explored and the existence of alpha, beta and gamma rays was already known when helium was first isolated. So when Rutherford and Boyd discovered that alpha rays are actually particles made up of two protons and two neutrons, which is identical to the nucleus of a helium atom, they immediately suggested a mechanism for how the helium managed to get trapped in the uranium ore.
Like all heavy radioactive elements, uranium decays along a specific series of elements. The Uranium series begins with the isotope 238U, the natural and relatively abundant isotope of uranium. 238U has a half-life of about 4 billion years, and when it decays it does so by releasing an alpha particle. The loss of a pair of protons and a pair of neutrons transforms the 238U in 234Th, or thorium-234. The liberated alpha particle, which is actually a helium nucleus, readily absorbs two electrons when absorbed by just about any matter it is in, creating a helium atom.
This perfectly explains why the helium was inside this sample of uranium ore – over time the decay of the uranium released alpha particles which were absorbed by the rock, gaining the necessary electrons to become helium atoms. The helium built up over time, collecting in the pores of the rock, only to be released when the minerals in the rock were finally dissolved. And this same process, albeit on a geological scale, is the key to the industrial production of helium.
A gas within a gas
Unlike most industrial gases, helium is not present in the atmosphere in any significant concentration. Any helium that is not somehow sequestered after it is produced will end up in the atmosphere and be quickly lost, rising rapidly into the upper atmosphere and eventually into space. It is therefore impractical to isolate helium from air as we do with oxygen, nitrogen, argon and other gases. Instead, we have to search beneath our feet for large reservoirs of helium.
Fortunately, the same geological conditions that tend to trap natural gas in underground reservoirs also tend to trap helium, and natural gas wells are therefore the greatest source of helium. Historically, the United States has been the main supplier of helium to world markets, with most coming from natural gas wells in Oklahoma, Kansas and Texas. Here, the gas coming out of the ground contains up to 7% helium, which is more than enough for profitable extraction.
Natural gas is a mixture of methane, nitrogen, carbon dioxide and higher gaseous alkanes like ethane and propane. When a sufficient amount of helium is mixed – anything above 0.4% is considered profitable – the extraction and purification of helium is carried out by fractional distillation. Helium has the lowest boiling point of all the elements, which means all other gases can be isolated by lowering the temperature and controlling the pressure.
The first step in producing helium is to clean up any CO2 and hydrogen sulfide (H2S) natural gas. This is done in an amine treatment, where the chemical monoethanolamine (MEA) is sprayed into the gas stream inside a reaction vessel. MEA ionizes acidic compounds and makes them water soluble, allowing them to be scrubbed from natural gas. The scrubbed gas is then pretreated by passing it through a molecular sieve, such as zeolite, and a bed of activated carbon, to remove water vapor and any heavier hydrocarbons.
What remains after these pretreatment steps is mainly methane and nitrogen, but also neon and helium. The gas is cooled by passing it through a heat exchanger and then through an expansion valve into a baffled fractionator. The sudden drop in pressure lowers the temperature of the gas enough for the methane, which boils at -161.5°C, to condense into a liquid and flow to the bottom of the column.
The remaining gas, now mostly nitrogen and helium, passes through a condenser which further cools the stream. When the temperature of the mixture drops below -195.8°C, the nitrogen condenses in liquid form. Along with liquid methane, liquid nitrogen is routed to heat exchangers that were originally used to cool the incoming pretreated process gas. The now gaseous nitrogen and methane, both valuable, are routed to storage tanks.
About half of the remaining process gas is helium, the rest being a mixture of contaminating methane and nitrogen, with some hydrogen and neon. This mixture is called cold raw helium and must now undergo further purification to reach the level of purity required for industrial use. Purification begins with another heat exchanger that drops the raw helium mixture below the boiling point of nitrogen, to condense the remaining nitrogen and methane contaminants. This step brings the raw helium to about 90% purity.
To get rid of hydrogen, oxygen is introduced and the mixture is heated in the presence of a catalyst. Hydrogen and oxygen form water, which can be separated from the process gas stream before it is directed to final purification by pressure swing adsorption, or PSA. Pressure swing adsorption is the same process used in oxygen concentrators, including many DIY versions we’ve seen in response to COVID-19. PSA uses the ability of materials known as molecular sieves to selectively adsorb gas. In helium purification, the 90% pure gas is pumped into a pressure vessel containing a molecular sieve, usually zeolite. The contaminating gases are preferentially adsorbed in the zeolite, leaving the helium exit stream almost pure. When the first column is saturated with contaminants, the flow is switched to a second column which had been previously regenerated by flushing it with pure helium. The gas flow alternates between the two columns, one purifying the helium while the other is regenerated. The result is Grade A helium gas at 99.995% purity.
The process described here is by no means the only way to extract helium from natural gas, but it does represent the most common way to produce the gas, primarily because most of the pretreatment and initial purification steps are already used to process natural gas as a fuel. and as a raw material for the chemical industry. Other methods include a fully PSA process, which can use natural gas with a helium concentration of only 0.06%, and membrane separation, which relies on the fact that helium can penetrate a semi-permeable membrane much more easily than methane molecules and much larger nitrogen. Membrane separation technology can be much more energy efficient than traditional fractional distillation because it does not require phase changes and the energy they require.
But are we short?
Knowing the abundance of uranium 238 in the terrestrial lithosphere as well as its half-life, it is possible to estimate the quantity of helium produced by the radiogenic process. It turns out that’s not much – only about 3,000 metric tons per year. And almost all of that escapes into the atmosphere and into space. So, similar to the natural gas in which it is usually found, helium is effectively a non-renewable resource.
But does that mean we’re running out of it? Yes, like any other finite resource, we will eventually extract whatever there is to extract. But that doesn’t necessarily mean we’ve found all the helium there is to find. Exploration has led to new deposits in the United States and massive discoveries of helium in places like Algeria, which became the world’s second largest helium producer in the early 2000s. Qatar has also made a huge discovery of helium in 2013, which propelled it to second place in the world. These discoveries, along with the recent discovery of natural gas wells in South Africa containing up to 12% helium, promise to address some of the concerns about the loss of access to this irreplaceable gas.
But in the end, these new discoveries only push back the clock and prevent the inevitable day when helium will finally run out. We might pause if ever commercial-scale fusion becomes a thing, but that breakthrough has only been “twenty years away” in the last 80 years.