Liquefied Natural Gas (LNG) is the process of converting gaseous natural gas into liquid form by cooling it to about -162℃, reducing its volume by about 600 times, making it easier to store and transport. The following are typical liquefaction processes and key steps:

Natural gas needs to remove impurities before liquefaction to avoid freezing and blocking equipment or corroding pipelines

-Deacidification: Use amine absorption methods (such as MEA, DEA) or physical solvents (such as Selexol) to remove CO₂ H₂S.

-Dehydration: Molecular sieve adsorption or triethylene glycol (TEG) dehydration is used to reduce the water content to below 1ppm.

-Mercury removal: Activated carbon adsorbs mercury to prevent aluminum equipment from becoming brittle.

-Heavy hydrocarbon separation: Remove ethane and above components (C ₅+) through low-temperature separation or distillation to avoid freezing during liquefaction.

The core is cooling through a refrigeration cycle, and mainstream technologies include:

(1) Cascade Process

-Principle: Three stage independent refrigeration cycle, gradually cooling down:

-Characteristics: High efficiency but complex equipment, suitable for large-scale LNG projects.

(2) Mixed Refrigerant Process (MRC)

-Principle: Using a mixture of refrigerants (methane, ethane, propane, nitrogen, etc.) to achieve wide temperature range cooling in a single cycle.

-Features: Simplified equipment, low energy consumption, such as AP-X ™ Process (combining MRC and nitrogen expansion).

(3) Nitrogen Expansion refrigeration process

-Principle: Nitrogen expansion refrigeration, suitable for small and medium-sized liquefaction units (such as peak shaving LNG).

-Characteristics: The process is simple, but the efficiency is low.

-Storage tank: Double layer vacuum insulation design (such as 9% nickel steel or aluminum alloy) to maintain low temperature.

-Transportation: LNG ship (membrane or spherical tank type) or tanker transportation.

-Large scale base type: such as the North Field project in Qatar, which adopts MRC technology and has a production capacity of 7.8 million tons per year.

-Floating liquefaction (FLNG): Direct liquefaction at sea, such as Shell Prelude FLNG.

Key Technologies and Challenges

-Energy consumption optimization: Liquefaction energy consumption accounts for 70% of the entire process, and new processes such as DMR dual mixed refrigerants can reduce energy consumption.

-Cold energy utilization: The cold energy released during LNG regasification can be used for power generation or cold storage.

Simplified flowchart

Natural gas → pretreatment (deacidification, dehydration, mercury removal) → heavy hydrocarbon separation → liquefaction (cascade/MRC/expansion) → LNG storage tank → transportation

Liquefaction projects of different scales will choose appropriate process combinations to balance efficiency and cost.