From Gas to Liquid: The Engineering of Liquefaction

How to make LNG from natural gas: the short version

Making LNG from natural gas is a purification problem followed by a refrigeration problem. Raw gas arrives from a field or a pipeline, everything that would freeze solid at cryogenic temperature is stripped out of it, and what remains is chilled until it condenses. The full sequence, in order:

1. Receive and meter the feed gas. Gas comes in from upstream production or a pipeline grid, and heavier liquids are knocked out first.
2. Remove acid gas. CO₂ and hydrogen sulphide are scrubbed with an amine solution. CO₂ must fall below about 50 ppm or it will freeze into dry ice and plug the exchangers.
3. Dehydrate. Molecular sieve beds take water down to below 1 ppm, because ice and hydrates would do the same thing.
4. Remove mercury. Even trace mercury attacks the aluminium in cryogenic heat exchangers, so it is adsorbed onto sulphur-impregnated carbon.
5. Extract heavy hydrocarbons. Ethane, propane, butane and pentanes-plus are separated — partly to prevent freezing, partly because they are valuable, and partly because they are often reused as refrigerant.
6. Chill to −162 °C. The cleaned gas passes through cryogenic heat exchangers against a refrigerant loop — propane pre-cooling plus mixed refrigerant in the common C3MR design, or a cascade of pure refrigerants. This is the step that consumes 8–10% of the feed gas energy.
7. Flash and store. The liquid is let down to near atmospheric pressure; nitrogen and light vapour flash off, and the LNG goes to insulated tanks at −162 °C.
8. Load. Cryogenic loading arms transfer the cargo to a ship, with vapour returned to the tank to keep pressure balanced.

Each of those steps is examined in detail below. The plant that performs them end to end is called a train, and a modern one produces 4–8 MTPA. For why the target is −162 °C specifically, see LNG temperature.

The Three Stages of Liquefaction

Every LNG liquefaction facility follows a similar process flow:

1. Pre-Treatment (Purification) — Remove impurities that would freeze or damage equipment
2. Liquefaction (Refrigeration) — Cool the gas to -162°C using thermodynamic cycles
3. Storage & Loading — Store LNG at atmospheric pressure in insulated tanks

Stage 1: Pre-Treatment (Purification)

Raw natural gas contains contaminants that must be removed before liquefaction. Failure to remove these can cause:

• Freezing: CO₂, H₂O, and heavy hydrocarbons solidify at cryogenic temperatures
• Corrosion: H₂S (hydrogen sulfide) attacks steel
• Mercury Embrittlement: Mercury amalgamates with aluminum heat exchangers, causing catastrophic failure

Key Pre-Treatment Steps

1. Acid Gas Removal (CO₂ and H₂S)

Method: Amine treating (using MEA, DEA, or MDEA solvents)

• Target: CO₂ < 50 ppm, H₂S < 4 ppm
• Process: Gas contacts amine solution in an absorber column. The amine chemically binds with acid gases, then is regenerated via heating.

2. Dehydration (Water Removal)

Method: Molecular sieve adsorption or glycol dehydration

• Target: H₂O < 0.1 ppm (moisture dew point below -100°C)
• Why Critical: Water forms ice and hydrates at cryogenic temperatures, blocking pipes and heat exchangers

3. Mercury Removal

Method: Fixed-bed adsorption using sulfur-impregnated activated carbon or molecular sieves

• Target: Hg < 0.01 μg/Nm³
• Why Critical: Mercury forms an amalgam with aluminum alloys used in brazed aluminum heat exchangers (BAHX), causing brittleness and potential explosion

4. Heavy Hydrocarbon Removal (NGL Extraction)

Method: Fractionation or turbo-expansion

• Purpose: Remove propane, butane, and heavier hydrocarbons that would freeze during liquefaction
• Economic Benefit: NGLs (natural gas liquids) like propane and butane are valuable co-products sold separately

Stage 2: Refrigeration (Liquefaction)

The refrigeration stage is the heart of the liquefaction process. Several thermodynamic cycles are used in the industry, each with trade-offs between efficiency, complexity, and capital cost.

Common Liquefaction Technologies

1. Propane Pre-cooled Mixed Refrigerant (C3MR) — Air Products/APCI

Market Share: ~50% of global LNG capacity

How it works:

1. Propane Pre-cooling: Natural gas is cooled to approximately -40°C using a propane refrigeration cycle (3 pressure levels)
2. Mixed Refrigerant Cooling: A blend of nitrogen, methane, ethane, and propane further cools the gas to -162°C
3. Final Expansion: The gas is expanded through a Joule-Thomson valve to achieve final liquefaction

Advantages: High efficiency, proven reliability, suitable for large-scale trains (6-8 MTPA)

Disadvantages: Complex equipment (multiple compressors), high capital cost

2. Cascade Cycle — ConocoPhillips (Optimized Cascade)

How it works: Uses three separate refrigerant loops in series:

• Stage 1: Propane (C₃) cools to -40°C
• Stage 2: Ethylene (C₂H₄) cools to -100°C
• Stage 3: Methane (CH₄) cools to -162°C

Advantages: Highest thermodynamic efficiency (~92%), simple control

Disadvantages: Many compressors required, high footprint

3. Dual Mixed Refrigerant (DMR) — Shell

How it works: Two mixed refrigerant cycles in series, no propane pre-cooling

Advantages: Compact design, fewer equipment pieces than C3MR

Disadvantages: Lower efficiency than C3MR, limited to mid-scale trains (3-5 MTPA)

4. Single Mixed Refrigerant (SMR) — Linde/PRICO

How it works: One mixed refrigerant circuit (nitrogen-heavy blend)

Use Case: Small to mid-scale plants (<3 MTPA), offshore FLNG (floating LNG)

Advantages: Simple, low equipment count, suited for remote locations

Disadvantages: Lower efficiency, not economical for large baseload plants

Cryogenic Heat Exchangers

The workhorse of LNG liquefaction is the brazed aluminum heat exchanger (BAHX), also called a "cold box."

Brazed Aluminum Heat Exchanger (BAHX)

• Material: Aluminum alloy (AA 3003, AA 5083) — chosen for excellent thermal conductivity and low-temperature ductility
• Design: Plate-fin construction with multiple parallel flow channels
• Size: A main cryogenic heat exchanger (MCHE) can be 60+ meters tall and weigh 1,000+ tonnes
• Critical Requirement: Zero mercury contamination (mercury causes liquid metal embrittlement of aluminum)

Why Not Steel?

Steel becomes brittle at cryogenic temperatures. Aluminum, nickel alloys (Inconel), and austenitic stainless steel (304, 316) maintain ductility at -162°C. Aluminum is preferred for heat exchangers due to its high thermal conductivity and lower weight.

Energy Consumption & Efficiency

Liquefaction is energy-intensive. The refrigeration compressors and auxiliary equipment consume 8-10% of the feed gas energy.

Example: A 6 MTPA liquefaction train consumes approximately 250-300 MW of power for refrigeration compressors. This is typically provided by gas turbine-driven compressors (GE LM6000, Rolls-Royce Trent) or electric motors powered by on-site gas-fired power plants.

Stage 3: Storage & Loading

Once liquefied, LNG is stored at atmospheric pressure in specially designed cryogenic tanks.

LNG Storage Tank Design

Above-Ground Full Containment Tanks

Construction:

• Inner Tank: 9% nickel steel (maintains ductility at -162°C)
• Insulation: Perlite, fiberglass, or polyurethane foam (typically 1 meter thick)
• Outer Tank: Pre-stressed concrete or carbon steel (provides secondary containment)
• Roof: Suspended deck or self-supporting dome

Typical Dimensions

• Capacity: 160,000 - 200,000 m³ per tank
• Height: 50-60 meters
• Diameter: 80-90 meters
• Insulation Thickness: 1 meter (achieving 0.03-0.05% BOG/day)

Boil-Off Gas (BOG) Management

Even with excellent insulation, heat ingress causes approximately 0.03-0.05% per day of LNG to evaporate in land-based storage.

BOG Handling Options:

1. Fuel Gas: Used as fuel for power generation or liquefaction drivers
2. Reliquefaction: BOG is compressed and re-cooled back to liquid
3. Export as Gas: Sent to a pipeline network (if available)

Loading Arms & Ship Loading

LNG is transferred to ships via articulated loading arms (16-20 inch diameter) at flow rates of 10,000-12,000 m³/hour. A full Q-Max vessel (266,000 m³) can be loaded in approximately 24 hours.

Major Technology Licensors

The LNG liquefaction industry is dominated by a few key technology providers:

Recent Innovations & Future Trends

1. Electric-Driven Liquefaction

Traditionally, refrigeration compressors are driven by gas turbines. New plants are exploring electric motor drives powered by renewable energy or grid electricity to reduce carbon emissions.

Example: Equinor's Hammerfest LNG uses electric drives, reducing CO₂ emissions by ~40%.

2. Carbon Capture and Storage (CCS)

Liquefaction emits CO₂ from burning fuel gas in turbines and heaters. Some facilities are adding CCS to capture up to 90% of emissions.

Example: QatarEnergy's North Field expansion includes CCS targeting 11 MTPA of CO₂ capture by 2035.

3. Floating LNG (FLNG)

Offshore gas fields can now be monetized via ship-based liquefaction facilities.

Example: Shell's Prelude FLNG (Australia) — 3.6 MTPA capacity on a 488-meter vessel, the world's largest floating structure.

4. Modular Small-Scale LNG

Small liquefaction units (0.1-1 MTPA) are being deployed for:

• LNG bunkering (marine fuel)
• Stranded gas monetization
• Peak shaving for utilities

Advantage: Factory-built modules reduce construction time and cost.

Key Takeaways

• Liquefaction requires pre-treatment to remove CO₂, H₂O, H₂S, and mercury
• C3MR (propane pre-cooled mixed refrigerant) is the dominant liquefaction technology
• Energy consumption: 8-10% of feed gas (250-320 kWh/tonne LNG)
• Brazed aluminum heat exchangers are critical — mercury contamination is catastrophic
• Storage tanks use 9% nickel steel inner tanks with 1-meter insulation
• Modern large trains: 6-8 MTPA capacity per train
• Future trends: electric drives, CCS integration, FLNG, and small-scale modular plants