The number that matters is not minus 162. It is minus 82.6.
The critical temperature is the whole reason
Every gas has a critical temperature above which it cannot exist as a liquid, no matter how hard it is squeezed. For methane that temperature is about minus 82.6 degrees Celsius. Above it, compressing the gas just makes denser gas.
This is why there is no ambient-temperature route to a liquid cargo of natural gas, and why the comparison people reach for — propane in a cylinder, liquefied by modest pressure at room temperature — does not transfer. Propane’s critical temperature is about 96 degrees Celsius, comfortably above anything the weather does, so pressure alone works. Methane’s is a hundred and eighty degrees lower.
Compressed natural gas is the alternative that does exist, and it is genuinely useful for vehicles and short-haul marine transport. But compressing gas to 200 or 250 bar buys an expansion ratio of a couple of hundred to one, in steel vessels heavy enough that the containers weigh far more than the fuel. Chilling buys 600 to one in a thin-walled insulated tank. Over an ocean, the cold route wins easily.
So the gas has to be refrigerated below its boiling point
Once you accept that the gas must be cooled, the target follows automatically. At atmospheric pressure, methane condenses at about minus 161.5 degrees Celsius. Cool the treated gas to roughly that and it becomes a liquid you can pour into an ordinary, unpressurised tank.
The alternative would be to liquefy at elevated pressure and a warmer temperature — somewhere between the boiling point and the critical point, where liquid methane can exist under pressure. It is thermodynamically possible and it is done at very small scale. At cargo scale it is not, because it puts you straight back into building pressure vessels big enough to hold a hundred thousand cubic metres, which nobody wants to do.
Atmospheric pressure is therefore a design choice as much as the temperature is, and the two are locked together. Type C pressure tanks appear on small vessels and bunker barges precisely because at those volumes a pressure vessel is affordable.
What the cold costs
Removing that much heat is expensive. A modern liquefaction plant consumes somewhere between 8 and 12 per cent of the energy in its feed gas to run the refrigeration, almost all of it turning the refrigerant compressors.
Three things move the figure within that band. The process matters: a cascade or a well-optimised mixed-refrigerant cycle beats a nitrogen expander loop substantially, which is why the efficient cycles dominate onshore and the simple ones appear offshore where space and safety count for more. The ambient temperature matters, because the cycle rejects heat to air or seawater and a hot climate makes that harder. And the drivers matter, because a gas turbine burns feed gas directly while an electric motor moves the emissions to whatever generates the power.
That energy cost is the reason debottlenecking is such a persistent theme, and why plant capacity figures drift upward between tracker releases. Small efficiency gains in the cold section translate directly into more tonnes from the same equipment.
The cost does not stop at the plant
Liquefaction is the largest energy loss in the chain but not the only one. A laden voyage loses cargo to boil-off at roughly 0.1 per cent a day on a modern ship, and the vessel burns fuel — often the boil-off itself — to move. Over a long Atlantic-to-Asia run that is a few more per cent. Regasification costs another one or two, depending on whether the terminal burns gas to warm the liquid or uses seawater.
Add it up and delivering a molecule of gas as LNG costs somewhere between a tenth and a fifth of the molecule’s energy. That is the price of the 600-to-one expansion ratio, and it is why pipelines beat LNG whenever a pipeline is actually possible.
Why the number never changes
Plants do not choose a temperature. Methane boils where it boils, and the design target is set by physics rather than optimisation. What plants choose is how efficiently to get there, how much of the cooling to do in each stage, and what to do with the small amounts of nitrogen and heavier hydrocarbons that behave differently on the way down.
That is why every liquefaction process in commercial use, however different in layout, ends at the same place: treated gas leaving the main exchanger at around minus 160 degrees, dropping through a valve or an expander to atmospheric pressure, and running into a tank.