LNG is made by cooling natural gas to approximately minus 161°C (minus 258°F), the point at which methane condenses into a clear, colorless liquid. This shrinks the gas to roughly 1/600th of its original volume, making it practical to store and ship across oceans. The process involves several distinct stages: cleaning the raw gas, chilling it through massive refrigeration systems, and storing the liquid in specially insulated tanks.
What Raw Natural Gas Contains
Natural gas straight from the well is far from pure methane. It carries water vapor, carbon dioxide, hydrogen sulfide (a toxic, corrosive gas), traces of mercury, and heavier hydrocarbons like butane and pentane. Every one of these contaminants has to be stripped out before liquefaction, because at the extreme cold involved, even tiny amounts of water would freeze into ice crystals that block equipment, and CO2 would solidify into dry ice. Mercury, meanwhile, corrodes the aluminum heat exchangers used downstream.
Pre-Treatment: Cleaning the Gas
The cleaning sequence typically follows a specific order, each step targeting a different category of impurity.
- Solids and liquids removal. Filters and separators strip out dust, sand, and any liquid droplets carried over from the wellhead.
- Acid gas removal. Chemical solvents absorb CO2 and hydrogen sulfide from the gas stream. These “acid gases” are corrosive and would freeze solid during liquefaction.
- Dehydration. Molecular sieve beds (tiny porous granules) adsorb water vapor, drying the gas to levels suitable for cryogenic processing down to minus 110°C and below.
- Mercury removal. Specialized adsorbent beds capture mercury down to extremely low concentrations, protecting aluminum piping and meeting environmental limits.
- Heavy hydrocarbon separation. Heavier compounds like propane and butane are pulled out. These are often sold separately as natural gas liquids.
By the end of pre-treatment, what remains is a stream of nearly pure methane, dry and free of contaminants, ready to enter the coldest part of the plant.
Liquefaction: Chilling Gas Into Liquid
Liquefaction is essentially industrial-scale refrigeration. The cleaned gas passes through a series of heat exchangers where it’s progressively cooled until it condenses. The principle is the same one that keeps your kitchen fridge cold: a refrigerant absorbs heat from the thing you want to cool, then releases that heat elsewhere. At an LNG plant, the scale and temperatures are just vastly greater.
Most large facilities use a “mixed refrigerant” approach. Instead of relying on a single cooling fluid, the system circulates a blend of nitrogen and light hydrocarbons, typically methane, ethane or ethylene, propane, and sometimes butane or pentane. Each component in the mix boils at a different temperature, so the blend can absorb heat efficiently across the entire cooling range, from ambient temperature all the way down to minus 161°C. This staged cooling is what makes the process energy-efficient enough to be commercially viable.
Some plants use a “cascade” design instead, with three separate refrigerant loops running at different temperature levels. Propane handles the first stage of cooling, ethylene takes over for the middle range, and methane brings the gas down to its final liquefaction temperature. Other facilities, particularly smaller ones, use turbo-expander systems that rely on nitrogen or a nitrogen-methane mix. The technology choice depends on plant size, location, and the composition of the incoming gas.
Regardless of the method, the energy cost is significant. Liquefaction consumes roughly 1,000 kilojoules per kilogram of LNG produced, and the compressors that drive the refrigerant loops are the single largest energy users at any LNG facility.
Cryogenic Storage
Once liquefied, the LNG flows into purpose-built storage tanks designed to hold liquid at minus 160°C or colder. These are not ordinary steel tanks. At cryogenic temperatures, standard carbon steel becomes brittle and can crack, so the inner tank that contacts the liquid is built from special low-temperature steel or a nickel-steel alloy. A typical large tank uses a double-wall design: an open-top inner tank for liquid containment, surrounded by a carbon steel outer tank that contains the vapor. Thick insulation fills the gap between the two walls.
Even with heavy insulation, a small amount of heat inevitably leaks in, causing a fraction of the liquid to evaporate. This “boil-off gas” is continuously captured and either re-liquefied, used as fuel for the facility, or compressed back into the gas supply. Tank geometry matters here: wider, shorter tanks have more surface area relative to their volume, which increases boil-off rates compared to taller, narrower designs.
Shipping and Regasification
The 600-to-1 volume reduction is what makes long-distance transport economical. LNG carriers are essentially floating versions of the onshore storage tanks, with insulated cargo holds that keep the liquid cold during voyages that can last weeks. The boil-off gas generated during transit is commonly used to fuel the ship’s engines.
At the destination, the LNG goes through regasification, which is the reverse of liquefaction. The liquid is pumped to higher pressure and then warmed until it returns to its gaseous state. Warming methods vary by location. Coastal terminals often pass the LNG through large rack-style heat exchangers warmed by seawater. Inland or colder-climate facilities may use direct-fired heaters or intermediate fluid systems that transfer heat from water or glycol loops. Some terminals use ambient air vaporizers, essentially large fin-tube structures that absorb heat from the surrounding air. The resulting gas enters the pipeline network at the pressures and temperatures local utilities require.
Why Safety Design Matters
LNG itself is not flammable as a liquid. It has to vaporize first, and even then, the methane vapor only ignites within a narrow concentration range: between 5% and 15% in air. Below 5%, there isn’t enough fuel; above 15%, there isn’t enough oxygen. If a spill occurs, the liquid evaporates quickly into a visible vapor cloud (cold methane vapor is denser than air and hugs the ground initially). If that cloud drifts into an ignition source while still in its flammable range, it can ignite and flash back to the spill point, potentially causing a pool fire on the remaining liquid.
This is why LNG facilities are designed with extensive spacing between equipment, vapor detection systems, and containment basins sized to hold the full contents of a storage tank. The double-wall tank design serves both as thermal insulation and as a secondary containment barrier if the inner tank fails.