Liquefying natural gas requires cooling to extremely low temperatures. On land, this is done in massive, vibration-isolated plants. On an FLNG vessel, the liquefaction plant must operate while the vessel moves in waves. The floating gas liquefaction market has overcome this challenge with specialized equipment and control systems.

The Refrigerant Cascade

Natural gas is a mixture of methane, ethane, propane, and heavier hydrocarbons. It cannot be liquefied by simply cooling it to a single temperature; different components condense at different temperatures. The FLNG market uses a cascade or mixed-refrigerant cycle. In a cascade cycle, multiple refrigerants (propane, ethylene, methane) are used in sequence. In a mixed-refrigerant cycle, a blend of refrigerants is used in a single loop. Mixed-refrigerant is simpler and more compact, making it preferred for FLNG.

Motion-Tolerant Heat Exchangers

The heart of the liquefaction process is the heat exchanger where natural gas gives up its heat to the refrigerant. On land, spiral-wound or plate-fin heat exchangers are used. On an FLNG vessel, these must tolerate vessel motion. The floating gas liquefaction market uses specially designed heat exchangers with internal baffles that prevent liquid maldistribution (refrigerant pooling on one side due to tilt). Coiled tube heat exchangers (which resemble a spring) are more motion-tolerant than straight-tube designs. Some FLNG vessels use printed circuit heat exchangers (PCHEs), which are compact and rugged.

Compressor Vibration and Balancing

Large refrigerant compressors (centrifugal or axial) have rotating impellers. Imbalance causes vibration, which is amplified by vessel motion. The FLNG market mounts compressors on vibration isolators (springs or elastomers) and uses dynamic balancing (adding small weights to the rotor) to minimize vibration. Active vibration control (piezoelectric actuators that cancel vibration) is used for the most sensitive machines. Compressor vibration is continuously monitored; excessive vibration triggers a shutdown to prevent damage.

Gas Pretreatment: Removing Impurities

Before liquefaction, natural gas must be treated to remove water, carbon dioxide, and hydrogen sulfide (acid gas). Water would freeze and block heat exchangers. CO2 would freeze and coat heat exchangers. The floating gas liquefaction market uses absorption (amines for CO2 and H2S removal) and adsorption (molecular sieves for water removal). These processes are sensitive to motion; the amine contactor uses structured packing (rather than random packing) to prevent liquid channeling. Regeneration of molecular sieves (heating to drive off water) is done in swing-bed adsorbers.

Mercury Removal

Some natural gas contains mercury (Hg). Mercury attacks aluminum (used in heat exchangers) and is toxic. The FLNG market includes mercury removal beds (sulfur-impregnated activated carbon) upstream of liquefaction. Mercury is captured on the bed; the spent bed is hazardous waste and must be disposed of appropriately. Mercury content varies by field; not all FLNG vessels require mercury removal. Pre-drilling gas samples determine the need.

Heavy Hydrocarbon Recovery

Natural gas contains heavier hydrocarbons (propane, butane, pentane) that would freeze at LNG temperatures. These must be partially removed before liquefaction. The floating gas liquefaction market uses a scrub column (a distillation tower) to separate heavy hydrocarbons. The heavy hydrocarbons (NGLs, or natural gas liquids) are stored separately and may be offloaded as a separate product (LPG, condensate). The scrub column is sensitive to motion; structured packing and careful liquid level control are used.

The Cold Box Insulation

The cold box (housing the liquefaction heat exchangers) is insulated to minimize heat ingress. The FLNG market uses perlite (expanded volcanic glass) or vacuum insulation panels. Perlite is poured into the annular space between the cold box wall and the heat exchangers. It settles over time, requiring periodic topping-up. Vacuum insulation is more effective but requires maintaining a vacuum over years; a small leak degrades performance. On a moving vessel, vacuum seals are more vulnerable than on land.

Flare System for Emergency Relief

During upsets, gas must be safely vented and burned. The floating gas liquefaction market includes a flare stack (or multiple flares) mounted high on the vessel. The flare is equipped with a pilot flame (continuously burning) and an ignition system (electrical spark for automatic relight). The flare must operate in all weather conditions; wind can blow the flame against the vessel structure, requiring heat shielding. Offshore flares are designed for higher turndown ratios (ability to handle a wide range of flow rates) than onshore flares.

Nitrogen Rejection

Some natural gas fields contain nitrogen (N2). Nitrogen remains gaseous at LNG temperatures (it does not condense). If nitrogen content is high, it accumulates in the LNG as a gas phase, causing tank pressure to rise. The FLNG market uses nitrogen rejection units (NRUs), which are cryogenic distillation columns that separate nitrogen from methane. NRUs are energy-intensive and space-consuming; they are only used for fields with high nitrogen content (above a certain threshold). Nitrogen rejection adds significant complexity to an FLNG vessel.

Process Control in Motion

An onshore LNG plant has stable foundations. An FLNG vessel tilts and moves. The floating gas liquefaction market uses motion-compensated control systems: sensors measure vessel motion (heave, pitch, roll), and control algorithms adjust refrigerant flow, compressor speed, and liquid levels to compensate. For example, if the vessel rolls, liquid levels in scrub columns and tanks will oscillate; the control system must prevent the level from dropping too low (causing gas breakthrough) or rising too high (flooding). Advanced control using model predictive control (MPC) is common. The floating gas liquefaction market has mastered the art of cryogenic engineering on a moving platform. And the FLNG market continues to push the boundaries of what is possible, with new vessels featuring larger capacities and more efficient cycles.

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