Liquefied natural gas (LNG) is produced by cooling natural gas to approximately minus 161 degrees Celsius, the point at which methane becomes a liquid at atmospheric pressure. This shrinks its volume to roughly 1/600th of its gaseous form, making it practical to store and ship across oceans. But reaching that extreme cold requires a multi-step process: cleaning the raw gas, stripping out heavier hydrocarbons, removing moisture and contaminants, then running the purified gas through massive refrigeration systems. Here’s how each stage works.
What’s in Raw Natural Gas
Natural gas straight from the wellhead is far from pure methane. It contains a mix of heavier hydrocarbons like ethane, propane, butane, and pentane, along with impurities such as carbon dioxide, hydrogen sulfide, water vapor, and trace amounts of mercury. The methane content varies significantly depending on the source. Gas from some Canadian wells, for example, can be around 77% methane, with ethane making up about 6.6%, propane 3.1%, and butane and heavier compounds accounting for another 5%. Some wells produce gas with as little as 65% methane.
None of these extra components belong in the final LNG product. Carbon dioxide and water would freeze solid at cryogenic temperatures, plugging equipment. Hydrogen sulfide is toxic and corrosive. Mercury attacks the aluminum heat exchangers used in liquefaction. And the heavier hydrocarbons have their own commercial value when separated out. So before any cooling begins, the gas goes through an extensive purification process.
Removing Acid Gases
The first major cleaning step targets “acid gases,” primarily carbon dioxide and hydrogen sulfide. These are removed using chemical or physical solvents that selectively absorb them from the gas stream. One of the most common approaches in LNG production uses amine-based solvents, a class of chemicals that react with acid gases and can then be heated to release them, regenerating the solvent for reuse. The separated acid gases are routed to further processing: hydrogen sulfide typically goes to a sulfur recovery unit, while carbon dioxide is vented or, increasingly, captured.
The goal is to reduce carbon dioxide to very low concentrations, well below 50 parts per million, to prevent it from freezing and blocking the cryogenic equipment downstream.
Dehydration and Mercury Removal
Even tiny amounts of water in the gas stream would form ice crystals at the temperatures used in liquefaction. To prevent this, the gas passes through dehydration units, most commonly beds of molecular sieves. These are granular materials with microscopic pores that trap water molecules as the gas flows through. The sieves are periodically regenerated by heating them to drive off the captured moisture.
Mercury removal often happens in the same part of the plant, sometimes using a dedicated layer within the molecular sieve beds or a separate adsorption vessel. The adsorbents used include activated carbon (often impregnated with sulfur) or metal sulfide compounds on an alumina base. Removing mercury is critical because even trace quantities cause a phenomenon called liquid metal embrittlement, which can crack the aluminum alloy heat exchangers that are central to the liquefaction process. Protecting these beds from liquid water contamination is essential to keeping them effective over their operating life.
Separating Heavier Hydrocarbons
Before the gas is fully liquefied, the heavier hydrocarbons, ethane, propane, butane, and pentane, are separated out. These compounds are collectively known as natural gas liquids (NGLs), and they’re valuable on their own as petrochemical feedstocks and fuels. Leaving them in the methane stream would also cause problems during liquefaction, since they have different freezing points and could solidify or create unwanted liquid phases.
The separation typically uses cryogenic distillation, which exploits the different boiling points of each compound. As the gas is progressively cooled, the heavier components condense first and can be drawn off at different stages. This technology has evolved through several generations since the 1960s, with modern designs capable of recovering high percentages of propane while selectively rejecting ethane when market conditions make that preferable. In many LNG plants, this NGL extraction is integrated with the liquefaction process itself, since both operate at low temperatures, making the combined system more energy efficient.
The Liquefaction Process
Liquefaction is the heart of any LNG plant and by far the most energy-intensive step. The fundamental principle is the same as a household refrigerator, just scaled up enormously: a refrigerant is compressed, cooled, expanded, and allowed to absorb heat from the natural gas, chilling it further with each cycle. The key difference is that household refrigerators cool to around 4 degrees Celsius, while LNG plants must reach minus 161 degrees.
Three main liquefaction technologies dominate the industry: single mixed refrigerant (SMR), propane pre-cooled mixed refrigerant (C3MR), and double mixed refrigerant (DMR). The C3MR process, originally developed by Air Products and Chemicals, is the most widely used in large-scale LNG plants worldwide.
In C3MR, the process happens in two stages. First, the cleaned natural gas enters a propane refrigeration cycle that pre-cools it to around minus 30 to minus 40 degrees Celsius. Propane works well for this initial stage because it can efficiently remove heat in this temperature range through a series of evaporators at progressively lower pressures. In the second stage, the pre-cooled gas enters a massive coil-wound heat exchanger, where a mixed refrigerant (a carefully tuned blend of nitrogen, methane, ethane, and propane) brings the temperature down to minus 161 degrees. The mixed refrigerant itself is also pre-cooled by the propane cycle before entering this exchanger, which improves overall efficiency.
These coil-wound heat exchangers are among the largest and most specialized pieces of equipment in the plant. They contain thousands of meters of tubing wound into tight coils, providing enormous surface area for heat transfer in a compact footprint. The now-liquefied natural gas exits the bottom of the exchanger and passes through a pressure-reduction valve, where it flashes to atmospheric pressure and cools to its final storage temperature. Any gas that flashes off during this step is captured and recycled.
Cryogenic Storage
Once liquefied, the LNG flows into heavily insulated storage tanks designed to keep it at minus 161 degrees with minimal energy input. Most modern LNG terminals use full-containment tanks with an inner shell of cryogenic nickel steel or aluminum, surrounded by thick insulation, and enclosed within an outer concrete shell that can contain the full liquid volume in case the inner tank fails.
Even with excellent insulation, a small amount of heat inevitably leaks in, causing some LNG to evaporate. This is called boil-off gas, and managing it is a constant operational concern. If left unchecked, boil-off gas raises the pressure inside the tank. Plants control this using boil-off gas compressors that capture the evaporated gas and either re-liquefy it, send it back into the plant’s fuel system to power turbines and compressors, or route it to a pipeline. Tank pressure is typically maintained within a narrow band, with compressors cycling on and off to keep it between set upper and lower limits.
Loading for Transport
The final step at a production facility is transferring LNG from storage tanks to carrier ships. This happens at specialized jetties equipped with cryogenic loading arms, articulated steel pipes that connect the shore-side infrastructure to the ship’s manifold. A typical loading system uses three liquid-phase arms to transfer the LNG and one gas-phase arm to return displaced vapor from the ship’s tanks back to shore, maintaining pressure balance and preventing losses.
Each loading arm consists of a riser pipe, inner and outer articulated arms, and a counterweight system that allows the arm to move freely with the ship as it shifts with waves and tides. At the tip of each arm, a quick-connect/disconnect device uses hydraulically driven fasteners to clamp onto the ship’s flange. In an emergency, if the ship drifts beyond a safe operating range or a hazard is detected, an emergency release system automatically closes valves on both sides of the connection and separates the arm from the ship. These systems use double-ball-valve structures controlled by hydraulic cylinders, ensuring the connection can be safely broken even if electrical systems fail.
Loading a standard LNG carrier with around 170,000 cubic meters of liquid typically takes 12 to 16 hours. Once loaded, the ship maintains the cargo at cryogenic temperatures for the duration of the voyage, with its own boil-off management systems handling the small but continuous evaporation that occurs in transit.