The cold section of a liquefaction plant is the part that gets photographed. The treatment section is the part that determines whether the cold section survives its first year.
Everything below exists because of one fact: at minus 162 degrees, almost every substance other than methane is a solid.
Acid gas removal
Carbon dioxide freezes at around minus 78 degrees Celsius. That is more than eighty degrees warmer than the cargo, so any CO2 still present when the gas reaches the cold section will come out as a solid, in the narrowest passages of the most expensive equipment on site.
It is removed by washing the gas with an amine solution in an absorber column. The amine picks up the acid gases; the loaded solution goes to a regenerator where heat drives them back off; the stripped amine returns. Hydrogen sulphide, where present, comes out in the same step.
The target is single-digit parts per million of CO2 — a specification that would be considered extreme in most process industries and is routine here.
The amine unit is one of the largest and most energy-hungry parts of the plant, because regenerating the solvent means boiling it, continuously, at scale. On a sour feed it can dominate the site.
Dehydration
Water freezes at a temperature everyone can visualise, and the same logic applies. Feed gas leaves the amine unit water-saturated, because it was just washed with an aqueous solution, so dehydration follows acid gas removal rather than preceding it.
Molecular sieves do the work: beds of porous crystalline adsorbent that trap water molecules by size while letting methane through. The target is roughly a tenth of a part per million, which is dry beyond any ordinary meaning of the word.
Beds are installed in pairs or threes. One is in service while another regenerates with hot gas, and they swap on a timed cycle, because an adsorbent bed saturates and has to be driven off rather than continuously renewed.
Mercury removal
Natural gas carries trace mercury — micrograms per cubic metre, sometimes less. It sounds negligible.
Mercury amalgamates with aluminium, and the main cryogenic heat exchanger is made of aluminium, with tube walls measured in fractions of a millimetre. Over years, trace quantities accumulate at exactly the places where a failure is most expensive and least repairable.
So the gas passes through guard beds, typically sulphur-impregnated activated carbon, which fix the mercury chemically. The target is well below a microgram per cubic metre, and the beds are placed before anything aluminium.
This is a small, cheap unit protecting the single most expensive item on the site, which is a reasonable summary of why it exists.
Heavy hydrocarbon removal
Ethane, propane, butane and heavier hydrocarbons have to come out for two independent reasons, and the plant would do it even if only one applied.
They freeze. Benzene and cyclohexane in particular solidify well above cargo temperature, and aromatics in the feed are a recognised freeze-out risk that has to be designed around explicitly.
They are worth more separately. A scrub column and a fractionation train separate them into saleable products. On a rich feed the natural gas liquids revenue is substantial, which is why plants on rich gas carry equipment plants on lean gas do not.
There is also a product-specification reason. Heavier components raise the heating value of the LNG, and a receiving market with a narrow Wobbe index range will not accept a cargo outside it. How much of the heavies to leave in is therefore partly a commercial decision about which markets the plant can serve.
Nitrogen rejection
Nitrogen is the awkward one because it does not freeze. It simply refuses to liquefy at the same conditions as methane, dilutes the heating value, and concentrates in the vapour space of tanks and ships.
Rejection happens at the cold end rather than in treatment — typically by flashing the liquefied product and separating the nitrogen-rich vapour, which is then used as plant fuel. Specifications generally hold nitrogen near one per cent in the product.
Why this section decides project cost
Two plants with the same nameplate capacity can differ enormously in cost, and treatment is usually why. The refrigeration equipment for 5 Mtpa is roughly the refrigeration equipment for 5 Mtpa wherever it is built. The treatment section is sized entirely by what came out of the ground.
When comparing cost per tonne across projects, that difference is doing a lot of the work, and it is not visible in the capacity figure at all.