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technology

expanded gas to provide the main refrigera-
tion duty for natural gas cooling; the natural According to the above-mentioned characteristics,
recirculation is ensured by a gas compressor this liquefaction technology allows the following
that has also the function to maintain the LNG advantages:
liquefaction exchanger at the optimal pressure • Compact arrangement due to a limited num-
for an effective liquefaction ber of equipment
• one closed nitrogen turbo-expander cycle for • Versatile process for a wide range of feed gas
natural gas cold end refrigeration; the refrige- composition
rant recirculation is performed by a nitrogen • Intrinsic safety as liquid hydrocarbon inventori-
compressor es are minimized, and cryogenic spillages are
limited
The compression train driver is selected according • Low depressurization flowrates
to the liquefaction capacity required and the overall • Simplified operation as the refrigerant compo-
plant energy configuration. sition adjustment is not required
• Fast and smooth transient management as no
NGL is formed at the exhaust of the warm gas ex- sudden temperature variation will occur due to
pander and is separated from the residue gas in liquid boiling
a dedicated drum, equipped with a reboiler that • High efficiency due to an optimized thermal
ensures that methane is fully rejected in the resi- balancing of the liquefaction cold box
due gas. The separation is performed at a redu- • High availability, as the plant can continue to
ced pressure; thus it is very selective and a limited produce (at reduced capacity) even if an ex-
amount of methane is being condensed together pander is shut down.
with the NGL. According to the gas composition
and the project needs, the drum can be replaced According to the feed gas quality, if the content of
with a contactor to enhance the NGL extraction. C + is already acceptable for the cryogenic facili-
5
ties, an NGL extraction system is not required. To
The main equipment of the liquefaction process scrub traces of aromatics that can affect the proper
is the Cold Box, that assembles plate fin heat operation of the liquefaction unit, additional replace
exchangers, a drum, connecting piping and mani- with “adsorbent bed” may be integrated into the
folds and valves for a maximum compactness. This gas dehydration unit.
also allows to confine inside the cold box most of
the cryogenic liquids. The optimization of the hot For richer pipeline gases with an excessive heavies
and cold composite curves profile along the whole content, an NGL extraction drum will be included
temperature range of the cold box allows the maxi- and the extracted NGL is used as fuel or exported.
mization of the energy efficiency. The low-pressure The opportunity of LPG reinjection is to be evalua-
end flash drum is included in the cold box. The ted according to the Methane number specifica-
flashed gas is partially warmed up in the Cold Box tion, that may need to be adjusted for bunkering
to utilize the thermal duty of this gas stream for re- and gas engine applications.
Modelling of LNG conversion frigeration at the Cold Box cold end.

30 30 Impiantistica Italiana - Gennaio-Febbraio 2020