Liquefying natural gas means cooling it to approximately -162°C (-259°F) at atmospheric pressure, which shrinks its volume by a factor of roughly 600 to 1. That dramatic reduction is what makes it practical to ship natural gas across oceans on tanker vessels rather than through pipelines. The process combines rigorous purification, staged refrigeration, and specialized storage, and it consumes a meaningful amount of energy to pull off.
Why Natural Gas Is Liquefied
In its gaseous state, natural gas takes up an enormous amount of space relative to the energy it contains. Cooling it into a liquid compresses 600 cubic meters of gas into a single cubic meter of liquid, making long-distance transport economically viable. Without liquefaction, the only way to move natural gas from a producing region to a distant market is a pipeline, which is impractical across oceans or politically unstable territory. Once liquefied, the fuel can be loaded onto purpose-built ships, delivered to an import terminal, and warmed back into gas for use in power plants, heating systems, and industrial facilities.
Cleaning the Gas First
Raw natural gas straight from a well contains impurities that would freeze solid at cryogenic temperatures and clog equipment. Before any cooling begins, operators run the gas through a pretreatment stage that strips out carbon dioxide, hydrogen sulfide, water vapor, and mercury. Carbon dioxide and water are the most critical targets: CO₂ freezes at -78°C, and water turns to ice well before the gas reaches -162°C. Even trace amounts of mercury can corrode the aluminum heat exchangers used later in the process. Removing these contaminants protects the equipment and ensures the final liquid product is nearly pure methane with small amounts of ethane and propane.
The Core Cooling Process
Once clean, the gas enters the liquefaction section, where both its sensible heat (ordinary warmth) and latent heat (the energy holding it in gas form) are removed. The fundamental physics at work is the Joule-Thomson effect: when a gas expands, the distance between its molecules increases, and the molecules must work against their natural attraction to one another. That work draws energy from the gas itself, lowering its temperature. If the gas is already cold enough before expansion, this temperature drop is sufficient to push part of it into a liquid state.
In practice, no single expansion step is enough to go from ambient temperature to -162°C. Instead, liquefaction plants use refrigeration cycles that compress a refrigerant, cool it, expand it to absorb heat from the natural gas, and repeat. The most widely used approach is the mixed refrigerant (MR) process, which blends several hydrocarbons and nitrogen into a custom refrigerant cocktail. This mixture boils across a range of temperatures rather than at a single point, so it can absorb heat efficiently across the entire cooling curve from ambient down to cryogenic.
Plants vary in complexity. A basic single mixed refrigerant system uses one large multi-stream heat exchanger and consumes around 995 kilojoules of energy per kilogram of LNG produced. More advanced configurations use two or three separate refrigerant loops cascading in sequence, each handling a different temperature range. A triple mixed refrigerant cascade can cut energy consumption by about 19% compared to the single-loop design, bringing it down to roughly 806 kilojoules per kilogram. The tradeoff is added equipment: each loop needs its own heat exchangers and compressors, which increases capital cost and physical footprint.
How Much Energy Liquefaction Requires
Cooling gas to -162°C is energy-intensive. A typical plant consumes around 0.56 to 0.62 kilowatt-hours per kilogram of LNG, meaning that producing one tonne of LNG requires roughly 560 to 620 kilowatt-hours of electricity or its mechanical equivalent. Most of that energy drives the massive compressors that circulate refrigerants. In many facilities, a portion of the incoming natural gas is burned in gas turbines to generate the power needed, so the plant effectively “uses up” some of its own feedstock during production. Optimizing compressor efficiency, heat exchanger design, and refrigerant composition can shave close to 10% off energy costs, translating to hundreds of thousands of dollars in annual savings at a single facility.
Storing LNG at Cryogenic Temperatures
Once liquefied, the product must be kept at or below -162°C, or it boils back into gas. LNG storage tanks are heavily engineered for this job. The most common design is a full-containment tank: an inner vessel made of metals rated for service at -162°C or colder, surrounded by thick insulation, and enclosed in an outer shell of reinforced concrete. The inner tank holds the liquid and cold vapor, while the outer shell provides structural support and acts as a secondary barrier in case of a leak.
The metal components that contact the liquid are typically nickel steel or aluminum alloys, chosen because ordinary carbon steel becomes brittle and fractures at cryogenic temperatures. Even with heavy insulation, a small amount of LNG continuously evaporates inside the tank, producing what’s called boil-off gas. Operators either re-liquefy this gas, pipe it back into the facility’s fuel supply, or use it to help power on-site equipment. Keeping boil-off rates low is a constant engineering challenge, and modern tanks are designed to limit heat leakage to a fraction of a percent of the tank’s contents per day.
Turning LNG Back Into Gas
At the destination terminal, LNG is warmed and converted back into gas through regasification. The simplest method pumps the liquid through pipes heated by seawater, which works well in warmer climates like those of India, Spain, and South America, where ocean temperatures provide enough thermal energy year-round. In colder regions where seawater alone is insufficient, terminals use direct-fired heaters or hot-water systems to supply the necessary warmth. Some facilities use ambient air vaporizers, essentially large radiator-like structures that let the surrounding air do the heating.
Each method has tradeoffs. Seawater systems are cheap to operate but require environmental permits because they discharge cooled water back into the ocean. Gas-fired heaters are reliable in any climate but consume a portion of the LNG itself as fuel, reducing the net amount delivered to customers. Air-based systems avoid both issues but work best only in consistently warm locations. Most large terminals combine methods to balance cost, reliability, and local conditions.
The Full Sequence, Step by Step
- Pretreatment: Raw gas is scrubbed of CO₂, hydrogen sulfide, water, mercury, and other impurities that would freeze or corrode equipment at low temperatures.
- Precooling: The clean gas is chilled partway down, often using a propane refrigeration loop or the warm end of a mixed refrigerant cycle, dropping the temperature well below 0°C.
- Liquefaction: One or more mixed refrigerant loops progressively extract remaining heat through multi-stream heat exchangers, bringing the gas to approximately -162°C.
- Expansion: The deeply cooled gas passes through an expansion valve, where a sudden pressure drop triggers the Joule-Thomson effect and converts it to liquid.
- Storage: The LNG flows into insulated, full-containment cryogenic tanks for holding until it’s loaded onto a carrier ship or consumed.
- Regasification: At the receiving terminal, the liquid is warmed back to a gas using seawater, heated pipes, or ambient air, then fed into the local pipeline network.