Oil and gas production handbook An introduction to oil ... - ABB Group

LNG carriers are used when the transport distance does not justify the cost
of a pipeline. The main drawback is the cost of the liquefaction, calculated as
how much of the total energy content of the gas is used for liquefaction.
About 6% of energy content is used to produce LNG in a large modern plant,
due to overall thermal efficiency. More than 10% could be consumed with
smaller, gas turbine-driven trains. This compares to losses of about 0.9% per
1,000 km of transport distance for large pipeline systems.
Melkøya LNG Plant with LNG Carrier Arctic Princess Photo: Statoil
The LNG feedstock comes from a gas plant as outlined above. It must
satisfy sales gas specifications. Ethane, propane and butane all have
freezing points of less than -180 °C and can be part of the LNG, but the
concentration of methane is generally above 90%. Some NGLs are also
needed as refrigerant for the cryogenic process.
5.5.1 LNG liquefaction
LNG processes are generally patented by large engineering, oil and gas
companies, but are generally based on a one- two- or three-stage cooling
process with pure or mixed refrigerants. The three main process types of
LNG process with some examples of process licensors are:
• Cascade cycle:
o Separate refrigerant cycles with propane, ethylene and
methane (ConocoPhillips)
• Mixed refrigerant cycle:
o Single mixed refrigerant (SMR) (PRICO)
o Single mixed refrigerant (LIMUM ® ) (Linde)
o Propane pre-cooled mixed refrigerant: C3MR (sometimes
referred to as APCI: Air Products & Chemicals, Inc.)
o Shell dual-mixed process (DMR) (Shell)
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