Liquefaction: frequently asked questions

What is in natural gas before it is treated?#

Mostly methane, plus varying quantities of ethane, propane, butane and heavier hydrocarbons, and contaminants including water, carbon dioxide, hydrogen sulphide, nitrogen and traces of mercury. The proportions vary enormously between fields.

What is the difference between associated and non-associated gas?#

Associated gas is produced alongside crude oil and its rate is governed by oil production rather than gas demand. Non-associated gas comes from a field produced for gas alone, so output can be matched to the plant.

Why does feed composition matter so much?#

Because everything that is not methane has to be removed, and the size of the treatment section depends on how much of it there is. A plant on lean coal seam gas needs far less treatment than one on rich sour gas.

Does nitrogen in the feed cause problems?#

Yes. Nitrogen does not liquefy at the same temperature as methane, dilutes the heating value, and accumulates in the vapour space. Most LNG specifications hold it to about one per cent, so it has to be rejected during processing.

Can a plant run short of feed gas?#

Yes, and it is common late in a field's life. Reservoirs decline, and a plant sized for early production can find itself unable to fill its trains, which is one reason operating output falls below nameplate capacity.

From Feed gas: where it comes from and what is in it.

Why must carbon dioxide be removed before liquefaction?#

Because it freezes solid at around minus 78 degrees Celsius, far above LNG temperature. Solid CO2 blocks the narrow passages of the main exchanger, so it is removed down to single-digit parts per million.

How is water removed from feed gas?#

By molecular sieve beds, which adsorb water by molecular size down to roughly a tenth of a part per million. Beds work in pairs so one regenerates with hot gas while the other is in service.

Why is mercury a problem?#

Mercury attacks aluminium, and the main cryogenic heat exchanger is aluminium. Even trace quantities cause damage over years, so mercury is adsorbed on guard beds to well below a microgram per cubic metre.

What happens to the heavy hydrocarbons that are removed?#

They are fractionated into ethane, propane, butane and condensate and sold separately. At a plant on rich feed this is a significant revenue line, not a waste stream.

How large is the treatment section compared with liquefaction?#

It varies with the feed. On rich sour gas the treatment plant can occupy more area and cost more than the refrigeration; on lean sweet gas it is comparatively small.

From Gas treatment: acid gas, mercury, water and heavies.

What is an LNG train?#

One complete refrigeration line at a liquefaction plant, including its compressors, heat exchangers and drivers. A plant with six trains runs six of these lines in parallel, which is why capacity is added in discrete steps rather than smoothly.

Why is it called a train?#

Because the equipment is arranged in a fixed sequence that gas passes through end to end, like carriages. The word carries no implication about the technology used.

What is the difference between C3MR and cascade?#

C3MR pre-cools with propane and then condenses against a mixed refrigerant in one large coil-wound exchanger. The cascade process instead uses three separate pure-refrigerant circuits, usually propane, ethylene and methane, with brazed aluminium exchangers.

How long does it take to build a train?#

Roughly four to five years from final investment decision to first cargo for a large onshore train, and longer where the site needs new port works, power or pipeline connections.

Who owns the technology?#

A short list. Air Products, ConocoPhillips, Shell, Chart Industries, Linde and Black & Veatch license the main processes, and an equally short list of contractors builds the plants.

From Liquefaction trains explained.

What is the most widely used LNG liquefaction process?#

Propane pre-cooled mixed refrigerant, usually written C3MR. Propane chills the gas in stages, then a mixed refrigerant does the deep cooling in a coil-wound exchanger.

What is the difference between a mixed refrigerant and a cascade process?#

A mixed refrigerant uses one blend whose components boil at different temperatures, so a single loop covers a wide range. A cascade uses separate loops of pure refrigerants in series. Cascades need more equipment; mixed refrigerants need careful composition control.

Why does AP-X allow larger trains?#

Because it adds a nitrogen expander loop to do the final sub-cooling, removing the bottleneck that limited how much a single mixed refrigerant circuit could handle. It took single-train capacity to roughly 8 Mtpa.

When is a dual mixed refrigerant process chosen?#

Typically in cold climates. Propane pre-cooling loses its advantage where ambient temperatures are already low, and a second mixed refrigerant loop performs better across the pre-cooling range.

Why do floating plants use simpler cycles?#

Space, weight and safety. A single mixed refrigerant or nitrogen expander cycle needs less equipment and, in the nitrogen case, carries no flammable refrigerant inventory, which matters a great deal on a hull.

From The refrigeration cycles: C3MR, DMR, AP-X and SMR.

What is a coil-wound heat exchanger?#

A tall vessel containing many kilometres of small-diameter tubing wound in layers around a central core, with refrigerant boiling on the outside of the tubes and gas condensing inside them. It is the centrepiece of most large liquefaction trains.

How large is a main cryogenic heat exchanger?#

On a large train, several metres in diameter, tens of metres tall and hundreds of tonnes. They are fabricated in a small number of factories and shipped whole, which makes transport a project constraint in itself.

What is a plate-fin heat exchanger?#

A block of brazed aluminium plates separated by corrugated fins, forming many parallel flow passages. Compact and efficient, and the usual choice in cascade processes and nitrogen expander cycles.

Why are so few companies able to build them?#

Because the manufacturing tolerances, the brazing or winding process and the qualification required take decades to accumulate, against a customer base of a few dozen units a year. It is a classic thin supply chain.

Which type is better?#

Neither in general. Coil-wound units tolerate two-phase flow and thermal cycling better and suit very large single duties; plate-fin blocks are more compact and efficient per unit volume but less tolerant of maldistribution and thermal shock.

From Coil-wound and plate-fin heat exchangers.

What drives the compressors in an LNG plant?#

Either gas turbines burning part of the feed gas, or large variable-speed electric motors. A few older plants use steam turbines. The choice sets a large share of the plant's cost, emissions and output profile.

Why does a plant produce less LNG on a hot day?#

Two reasons compound. The refrigeration cycle rejects heat to air or seawater, so it works harder when the surroundings are warm, and a gas turbine's output falls as intake air temperature rises. Both push the same way.

What is an all-electric LNG plant?#

One where the refrigerant compressors are driven by electric motors rather than gas turbines. It removes turbine exhaust from the site and moves the emissions to whatever generates the electricity.

Are aeroderivative or heavy-duty gas turbines better?#

Neither in general. Aeroderivatives are more efficient and can be swapped out quickly for overhaul; heavy-duty machines deliver more power in one unit and cost less per kilowatt. Availability and maintenance philosophy usually decide it.

Why do some trains have starter or helper motors?#

Because a gas turbine cannot start a large compressor from rest on its own, and because a variable-speed motor on the same shaft can add power when ambient conditions rob the turbine of output.

From Drivers: gas turbines, electric motors and the grid.

What is a full containment LNG tank?#

A tank whose outer wall, usually prestressed concrete with a steel liner, can hold the entire liquid contents and contain the vapour if the inner shell fails. It is the standard for new onshore terminals.

Why is 9% nickel steel used?#

Because ordinary carbon steel becomes brittle at cryogenic temperature and cracks rather than deforming. Adding around nine per cent nickel keeps the steel tough at minus 162 degrees.

What is high-manganese steel?#

A newer cryogenic alloy using manganese rather than nickel for low-temperature toughness, developed as a cheaper alternative to 9% nickel plate for tanks and fuel systems.

Are LNG storage tanks pressurised?#

No. They operate at close to atmospheric pressure, typically only a few hundred millibar above it. The liquid stays liquid because it is cold, not because it is compressed.

What is rollover in an LNG tank?#

The sudden mixing of two stratified layers of different density, which can release a large volume of vapour at once. It is prevented by monitoring density and by filling in a way that keeps the contents mixed.

From Storage tanks, 9% nickel and high-manganese steel.

How fast is an LNG cargo loaded?#

Typically 10,000 to 12,000 cubic metres an hour, so roughly twelve hours of pumping for a conventional cargo. The full port call is closer to a day once mooring, cooling the arms, checks and measurement are included.

Why are LNG jetties so long?#

Because the ship needs deep water and the tanks are inland, so a trestle carries the pipework out to where a large vessel can lie safely. Jetties of well over a kilometre exist where the seabed shelves gently.

What sets the spacing between parts of an export terminal?#

Hazard analysis. Thermal radiation from a credible fire and the dispersion distance of a vapour cloud determine how far storage must sit from trains, from the jetty and from the site boundary.

Why do large plants have more than one berth?#

Because a single berth caps how many cargoes can leave. A plant producing tens of millions of tonnes a year cannot clear its production through one jetty, particularly allowing for weather and maintenance.

What is the flare for?#

Safely burning gas that must be released during upsets, start-up and shutdown. A well-run plant flares little in normal operation, and continuous flaring usually indicates something is wrong upstream.

From Loading jetties and the export terminal layout.

What is FLNG?#

A floating liquefaction facility: gas treatment, liquefaction, storage and loading built on a moored hull, so a field can be developed without a pipeline to shore or an onshore plant.

How is FLNG different from an FSRU?#

They are opposites. FLNG turns gas into liquid for export; an FSRU turns liquid back into gas for import. Both float, and that is most of what they have in common.

Why build a plant at sea rather than on land?#

When the field is remote, offshore, or in a country where onshore construction is difficult, floating removes the pipeline to shore and much of the land-based permitting and civil works.

Is FLNG cheaper than an onshore plant?#

Not per tonne of capacity. Its advantage is avoiding an export pipeline and onshore construction, and being redeployable, not a lower headline cost.

What liquefaction process do FLNG units use?#

Usually a simplified one. Single or dual mixed refrigerant cycles need fewer machines and less deck space than a propane pre-cooled train, which is why they dominate offshore.

From FLNG explained.