Note: Descriptions are shown in the official language in which they were submitted.
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LIQUID NATURAL GAS PLANT AND METHOD OF OPERATING
THEREOF
FIELD OF THE INVENTION
The present invention is directed to liquid natural gas plant comprising a
plurality of treatment and liquefaction trains. The invention is further
related to a
method of retrofitting and/or operating such a liquid natural gas plant.
BACKGROUND TO THE INVENTION
Natural gas can be liquefied for purposes of storage and transportation, as it
is
occupying a smaller volume in liquid state than in gaseous state. Typically,
before
being liquefied, the natural gas is treated to remove contaminants (such as
H2O, CO2,
H2S and the like) and heavy hydrocarbon molecules, which may freeze out during
the
liquefaction process.
Liquefaction of natural gas is an energy consuming process. Designing and
operating liquid natural gas plants in the most efficient manner is therefore
a constant
focus area.
W02006/120127 describes an LNG plant having a single liquefaction train for
providing LNG in liquid or "pseudo-liquid" form. The LNG in liquid or "pseudo-
liquid" form is sent into a separation unit for providing purified LNG and a
nitrogen-
enriched stream.
W0201576975 describes a method of retrofitting a full-scale LNG plant to
enhance the LNG production capacity of the LNG plant and a method for
operating
such a retrofit plant. A small scale LNG plant having a capacity less than 2
MTPA
can be integrated with a main LNG plant having a capacity of at least 4 MTPA
such
that end flash gas and boil off gas from the main LNG plant can be liquefied
by the
small scale LNG plant as incremental LNG. According to W0201576975 the
production capacity of the integrated system can be improved by increasing the
temperature of the gas stream exiting the main cryogenic heat exchanger of the
main
LNG plant between 5 C and 30 C as compared with the design temperature.
W02006009646 is related to hydrocarbon fluid processing plants, methods of
designing hydrocarbon fluid processing plants, methods of operating
hydrocarbon
fluid processing plants, and methods of producing hydrocarbon fluids using
1
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hydrocarbon fluid processing plants. More particularly, some embodiments of
the
invention are related to natural gas liquefaction plants, methods of designing
natural
gas liquefaction plants, methods of operating natural gas liquefaction plants
and
methods of producing LNG using natural gas liquefaction plants. One embodiment
of
the invention includes a hydrocarbon fluid processing plant including a
plurality of
process unit module types, the plurality of process unit module types
including at
least a first process unit module type including one or more first process
unit
modules and a second process unit module type including two or more integrated
second process unit modules wherein at least one of the first process unit
modules
and at least one of the second process unit modules are sized at their
respective
substantially maximum processing efficiency.
SUMMARY OF THE INVENTION
It is an aim to provide a more efficient LNG plant.
In one aspect the present invention is directed to a liquid natural gas plant
for
producing liquefied natural gas from a contaminated natural gas feed stream,
the
liquid natural gas plant comprising two or more parallel treatment and
liquefaction
trains arranged in process portions of the contaminated natural gas feed
stream in
parallel, the treatment and liquefaction trains each comprising:
- an inlet for receiving a portion of the contaminated natural gas feed
stream,
- a gas treatment stage for removing contaminants from the respective
portion
of the contaminated natural gas feed stream) thereby generating a cleaned
natural
gas stream,
- a cooling stage arranged to receive the cleaned natural gas stream from
the
gas treatment stage for cooling at least part of the cleaned natural gas
stream,
wherein the cooling stage comprises an NGL-extraction unit for extracting
natural
gas liquids , thereby generating a light natural gas stream to be at least
partially
further cooled by the cooling stage to be at least partially liquefied and
- an outlet for discharging liquefied natural gas,
wherein the liquid natural gas plant comprises at least one additional
liquefaction train, the additional liquefaction train comprising:
- an inlet for receiving an additional feed stream,
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- an additional cooling stage arranged to receive the additional feed
stream for
cooling the additional feed stream thereby generating additional liquefied
natural gas,
and
- an outlet for discharging the additional liquefied natural gas,
wherein the additional feed stream comprises two or more side streams taken
from the respective light natural gas streams of the two or more parallel
treatment
and liquefaction trains.
The liquid natural gas plant as defined above also encompasses liquid natural
gas plants comprising more than one additional liquefaction train.
NGL-extraction unit for extracting natural gas liquids may be performed on
any suitable natural gas stream in the cooling stage. According to an
embodiment,
the cooling stage comprises a first cooling unit, arranged to generate a pre-
cooled
cleaned natural gas stream and a second cooling unit arranged to generate a
further
cooled stream. NGL-extraction unit for extracting natural gas liquids may be
performed on the pre-cooled cleaned natural gas stream.
The inlet of the additional liquefaction train is directly or indirectly in
fluid
connection with conduits in the respective treatment and liquefaction trains
which in
use carry the respective light natural gas streams. Furthermore, the inlet of
the
additional liquefaction train is directly or indirectly in fluid connection
with conduits
in the respective treatment and liquefaction trains carrying any further
streams to be
comprised by the additional feed stream.
The additional feed stream may comprise further streams, including one or
more side streams of natural gas taken downstream from the inlet of the
respective
treatment and liquefaction trains, such as an end flash stream, a boil-off gas
stream
(taken from one or more LNG storage tanks), the cleaned natural gas stream.
The gas
treatment stage may comprise one or more gas treatment units (as explained in
more
detail below). The additional feed stream may further optionally comprise one
or
more side streams taken from intermediate partially cleaned natural gas
streams in
between respective gas treatment units.
Additionally, or alternatively, the additional feed stream may comprise
further
streams obtained from a fractionation unit provided to receive and fractionate
the
natural gas liquids obtained from the NGL-extraction unit. The further stream
may in
particular be at least a portion of one or more methane enriched streams
generated by
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respective fractionation units and/or at least a portion of one or more ethane
enriched
streams generated by respective fractionation units.
Depending on the composition of the additional feed stream, the additional
liquefaction train may comprise some gas treatment units and may comprise a
NGL-
extraction unit.
However, according to an embodiment, at least 30 mol% of the additional feed
stream is formed from the respective light natural gas streams generated by
the NGL-
extraction units. According to a preferred embodiment at least 50 mol%, or at
least
75 mol% of the additional feed stream, e.g. 100 mol%, is formed from the
respective
light natural gas streams generated by the NGL-extraction units. The
additional feed
stream may only be formed form the respective light natural gas streams . The
light
natural gas stream generated by the NGL-extraction unit are clean, lean and at
a
relatively high pressure, so do not require any further gas treatment units
and need no
or relatively little compression. The more of the additional feed stream is
formed by
the respective light natural gas streams, the less compression and gas
treatment is
needed with respect to the additional liquefaction train.
The additional liquefaction train preferably doesn't comprise a NGL-extraction
unit, as the additional feed stream, at least partially and preferably for at
least 30
mol%, at least 50 mol% or at least 75mo1% already passed through a NGL-
extraction
unit. According to an embodiment, the additional liquefaction train comprises
a
relatively small NHL-extraction unit only. The additional liquefaction train
requires a
relatively small gas treatment stage with only a subset of the gas treatment
units of
the treatment and liquefaction train and preferably doesn't comprise a gas
treatment
stage as the additional feed stream is already cleaned. Therefore, the
additional
liquefaction train requires less hardware and is relatively cheap, both in
terms of
capital investment costs as well as in operational costs.
By combining streams from two or more parallel treatment and liquefaction
trains, the additional liquefaction train can be given a considerable size to
benefit
from economy of scale.
The respective light natural gas streams of the one or more parallel treatment
and liquefaction trains may be referred to as Ci-enriched stream or C2 -
depleted
streams. The respective light natural gas streams may for instance be obtained
from
the vapour phase of the reflux vessel of a scrub column, in which case the
light
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natural gas streams may be at a temperature in the range of minus 40 C ¨ minus
50 C, e.g. minus 45 C, and at a pressure in the range of 40 ¨ 55 bara. Side
streams
taken from these light natural gas stream to be comprised in the additional
feed
stream can be passed to the additional liquefaction train directly, i.e.
without
5 recompression.
The light natural gas streams may also be heat integrated with a first or pre-
cooling unit to recover at least some of the cold present in these streams
before
obtaining a side stream. In such an embodiment, the side streams taken from
the light
natural gas streams after cold recovery may be at a temperature in the range
of +10 C
¨ +20 C, e.g. +15 C, and at a pressure in the range of 40 ¨ 55 bara.
Preferably, the respective light natural gas streams are heat-integrated with
pre-
cool step, especially when a collecting and compression unit is provided
(collecting
and compression unit will be explained in more detail below).
The side streams taken from the respective light natural gas streams have a
relatively high pressure related to the pressure of the contaminated natural
gas feed
stream and/or the cleaned natural gas stream (30-100 bar) which can be fed
into the
additional liquefaction train, without recompression or with moderate
recompression
only. In certain embodiments, the pressure of the additional feed stream is
selected
higher (e.g. 10 or 20 bar higher) than the pressure of the contaminated
natural gas
feed stream to facilitate efficient cooling and liquefaction, which higher
pressure can
be obtained at relatively low costs given the relatively high pressure of at
least some
of the streams comprised by the additional feed stream.
In another aspect there is provided a method of retrofitting an existing
liquid
natural gas plant to increase the liquefied natural gas production capacity
thereof,
wherein the existing liquid natural gas plant comprises two or more parallel
treatment
and liquefaction trains for producing liquefied natural gas from a
contaminated
natural gas feed stream, wherein the two or more parallel treatment and
liquefaction
trains are arranged to process portions of the contaminated natural gas feed
stream in
parallel and are each arranged to:
- receive a portion of a contaminated natural gas feed stream,
- remove contaminants from the respective portion of the contaminated natural
gas feed stream thereby generating a cleaned natural gas stream,
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- cool at least part of the cleaned natural gas stream and extract natural gas
liquids, thereby generating a light natural gas stream to be at least
partially further
cooled by the cooling stage,
wherein the method of retrofitting comprises
- providing an additional liquefaction train, the additional liquefaction
train
comprising an additional cooling stage arranged to receive and liquefy an
additional
feed stream to generate additional liquefied natural gas,
- fluidly connecting the additional liquefaction train to two or more of the
light
natural gas streams of two or more of the treatment and liquefaction trains to
obtain
side streams of the two or more light natural gas streams to be comprised in
the
additional feed stream, thereby creating a retrofitted liquid natural gas
plant.
The method may further comprise fluidly connecting the additional
liquefaction train to one or more further streams (as described above),
including side
streams taken downstream from the inlet of the treatment and liquefaction
trains,
such as from an end flash stream, a boil-off gas stream (taken from one or
more LNG
storage tanks, the cleaned natural gas stream, an intermediate partially
cleaned
natural gas streams in between respective gas treatment units and side streams
obtained from a fractionation unit provided to receive and fractionate the
natural gas
liquids obtained from the NGL-extraction unit (e.g. side streams from one or
more
methane enriched streams and/or one or more ethane enriched streams).
The additional liquefaction train may be as described above. The additional
liquefaction train may preferably not comprise a NGL-extraction unit, as the
additional feed stream already passed through a NGL-extraction unit. The
additional
liquefaction train may also not comprise a gas treatment stage as the
additional feed
stream is already cleaned.
The method may further comprise debottlenecking the existing liquid natural
gas plant, for instance by replacing or maintaining parts of the existing LNG
plant, in
particular upstream of the position where natural gas liquids are extracted.
Debottlenecking means improving the design throughput of the existing liquid
natural gas plant by increasing the design throughput of the most constraining
part of
the liquid natural gas plant.
The method of retrofitting is in particular advantageous in situations in
which
the capacity of the warm ends, i.e. the pre-treatment unit (described in more
detail
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below), the gas treatment stage, the NGL extraction unit were designed for a
richer
and more contaminated gas composition than actually experienced, thereby
resulting
in spare ullage in the warm ends of the treatment and liquefaction trains,
while the cold
end, i.e. the equipment downstream of the NGL extraction unit, are already
working at
or close to their design capacity.
Furthermore, this method is in particular advantageous when the existing
treatment and liquefaction trains are constrained by the available gas turbine
power
output, limiting the available refrigeration capacity.
According to a further aspect there is provided a method of operating a
(retrofitted) liquid natural gas plant as described above, wherein the method
comprises
- operating the two or more parallel treatment and liquefaction trains
whereby
operating the two or more parallel treatment and liquefaction trains comprises
receiving respective portions of the contaminated natural gas feed streams by
the two
or more parallel treatment and liquefaction trains at a feed pressure and
- operating the additional liquefaction train, whereby operating the
additional
liquefaction train comprises providing an additional feed stream comprising
two or
more light natural gas streams taken from the two or more parallel treatment
and
liquefaction trains (A, B), wherein the additional feed stream is provided at
an
additional feed pressure, being at least 10 bar above the feed pressure.
Similar to above, the additional feed stream may comprise one or more further
streams, including side streams taken downstream from the inlet of the
treatment and
liquefaction trains, such as from an end flash stream, a boil-off gas stream
(taken
from one or more LNG storage tanks), the cleaned natural gas stream, an
intermediate partially cleaned natural gas streams in between respective gas
treatment units and side streams obtained from a fractionation unit provided
to
receive and fractionate the natural gas liquids obtained from the NGL-
extraction unit
(e.g. side streams from one or more methane enriched streams and/or one or
more
ethane enriched streams).
Because for the additional liquefaction train, which doesn't comprise NGL-
extraction, there are no pressure limitations to take into account associated
with the
NGL-extraction. The additional feed pressure may therefore be selected higher
than
the feed pressure to contribute to efficient cooling and liquefaction. The
additional
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feed pressure may even be at least 20 bar above the feed pressure. The
additional
feed pressure may be above 50 bara, or even may be above 60 bara (bara = bar
absolute).
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings depict one or more implementations in according with the
present teachings, by way of example only, not by way of limitation. In the
figures,
like reference numerals refer to the same or similar elements. Furthermore, a
single
reference number will be used to identify a conduit or line as well as the
stream
conveyed by that line.
Figure 1 is a schematic illustration of an embodiment.
DETAILED DESCRIPTION OF THE INVENTION
The following examples of certain aspects of some embodiments are given to
facilitate a better understanding of the present invention. In no way should
these
examples be read to limit, or define, the scope of the invention.
There is provided a liquid natural gas plant comprising a plurality of
liquefaction trains, of which at least one train, referred to as an additional
liquefaction train, liquefies gas streams received from the other trains
(referred to as
treating and liquefaction trains). The additional liquefaction train is
preferably added
to an existing plant as retrofit.
In existing plant designs, i.e. plants not comprising an additional
liquefaction
train as described here, typically fuel is obtained from gas streams obtained
from the
treatment and liquefaction trains, such as portions of the end flash gas and
the
overhead of the scrub column used for NGL-extraction, which are relatively
lean and
clean.
Now provided is a liquid natural gas plant in which fuel is mainly obtained
from the natural gas feed stream upstream of the cooling and gas treatment
units. The
relatively lean and clean gas streams mentioned above are now fed to the
additional
liquefaction train as additional feed stream.
It is recognized that the treatment required for fuel gas, i.e. for fuel gas
fed to
gas turbines, boilers, LNG plant furnaces) are less stringent than the
requirements for
LNG. For example, for fuel gas, CO2 removal is less stringent, heavy
hydrocarbon
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removal is less stringent, H2S removal is less stringent, water removal is
less
stringent and mercury removal is less stringent. The treatment requirements
for
making the contaminated natural gas feed stream suitable for fuel, may
comprise
some sort of hydrate formation control (i.e. pre-heating, dew-pointing),
pressure
control and possibly superheating, but over-all requires less hardware than
gas
treatment to meet LNG specs.
It is therefore recognized that the relatively clean and lean gas streams can
more efficiently be liquefied to produce additional LNG, than used as fuel
stream.
The additional liquefaction train can advantageously be added to existing
liquid
natural gas plants comprising two or more treatment and liquefaction trains,
in
particular in situations wherein there is overcapacity available upstream in
the NGL
extraction unit and upstream thereof.
According to W0201576975 the temperature of the main cryogenic heat
exchanger is increased to ensure sufficient production of end flash gas and
boil off
gas to feed the small scale LNG plant. This approach is disadvantageous, as it
depends on the additional capacity available in the main cryogenic heat
exchanger.
Also, changing the temperature could take the operating parameters outside the
original design window of the main cryogenic heat exchanger, thereby resulting
in a
less efficient operation. According to the current embodiments, there is no
need to
change the temperature of the main cryogenic heat exchanger.
The embodiments provided here allow to operate the LNG plant including the
additional liquefaction train, without such a need of increase of the
temperature.
Instead of increasing the temperature of the main cryogenic heat exchanger, or
more generally said: of the cold end of the train, different measures are
suggested to
ensure a sufficiently large additional feed stream to benefit from the economy
of
scale. One of these measures is to take fuel gas from another location in the
plant, i.e.
a position upstream of the second cooling unit (see detailed description
below, also
known as the main cryogenic heat exchanger), thereby allowing to use more
clean
and lean gas from the treatment and liquefaction trains to provide to the
additional
liquefaction train.
With reference to Fig. 1, there is provided a liquid natural gas plant (1) for
producing liquefied natural gas from a contaminated natural gas feed stream
(10).
The contaminated natural gas feed stream 10 may originate from a pre-treatment
unit
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2, which is arranged to receive one or more natural gas streams 30 from a
source or
well and discharge a contaminated natural gas feed stream 10 having a well-
defined
constant pressure, flow rate and composition. Therefore, the pre-treatment
unit 2 may
be equipped with a slug catcher, pressure control devices, stabilizers and
metering
5 equipment. The slug catcher and the stabilizer are provided to remove
heaver liquids
(condensates).
The liquid natural gas plant comprises two or more parallel treatment and
liquefaction trains A, B. According to an embodiment, the liquid natural gas
plant 1
comprises three or more parallel treatment and liquefaction trains, for
instance four
10 or six parallel treatment and liquefaction trains. The term parallel is
used to indicate
that the trains are arranged to process portions of the contaminated natural
gas feed
stream in parallel. There may however be integration between the parallel
treatment
and liquefaction trains A, B, for instance by having shared refrigerant loops,
shared
utility functionality, shared refrigerant make-up facilities.
The respective treatment and liquefaction trains A, B comprise an inlet 11 for
receiving a portion of the contaminated natural gas feed stream 10', 10". The
inlet
11 may be in fluid communication with the pre-treatment unit 2 described
above.
The respective treatment and liquefaction trains A, B may further comprise a
gas treatment stage 12. The gas treatment stage is arranged to receive the
respective
portion 10', 10" of the contaminated natural gas feed stream 10 and remove
certain
contaminants therefrom. The gas treatment stage 12 may comprise one or more of
the
following gas treatment units:
- acid gas removal unit (AGRU) to remove acid components, CO2
and/or
H2S, the acid gas removal unit comprising equipment for separation
purposes, such as an absorption column, a regenerator column including
reboilers, condensers and other heat exchangers, wherein the absorption
column and regenerator column comprise internals, e.g. trays or packing
(structured/random),
- dehydration unit to remove H20, and
- mercury removal unit to remove mercury, and
The acid gas removal unit (AGRU) may also remove aromatic components
(co-absorption), to help achieving benzene/ aromatics specifications for
liquefaction.
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The gas treatment stages 12 comprise an outlet for discharging a cleaned
natural gas stream 13. The gas treatment stages 12 may also comprise one or
more
outlets for respective contaminant streams 36. The respective treatment and
liquefaction trains A, B comprise a cooling stage 14 in fluid connection with
the
outlet of the gas treatment stage 12 to receive the cleaned natural gas stream
13.
The cooling stage 14 is equipped to cool and optionally liquefy at least part
of
the cleaned natural gas stream 13 received. The cooling stage 14 may comprise
any
suitable cooling process, such as Single Mixed Refrigerant (SMR) process,
Double
Mixed Refrigerant (DMR) process, Cascade process, C3MR process, LiquefinTm
process as well as expansion based cooling processes. It will be understood
that any
suitable cooling process may be applied. The parallel treatment and
liquefaction train
A, B do not necessarily comprise the same cooling process.
By way of example, Figure 1 schematically shows a cooling process
comprising a first cooling unit 15, e.g. using a first refrigerant, and a
second cooling
unit 17, using a second refrigerant. The first refrigerant may be a first
mixed
refrigerant and the second refrigerant may be a second mixed refrigerant (DMR
process). Alternatively, the first refrigerant may be a single component
refrigerant
(typically propane) and the second refrigerant may be a second mixed
refrigerant
(SMR process).
Depending on the cooling process and the operating parameters, the cooling
stage 14 cools and liquefies at least part of the cleaned natural gas stream
13, or only
cools at least part of the cleaned natural gas stream 13, while the phase
transition to
liquid takes place downstream thereof, e.g. in an end flash unit (described
below).
The first cooling unit 15 is arranged to generate a pre-cooled cleaned natural
gas stream 151. The second cooling unit 17 is arranged to generate a further
cooled
stream 171 (being at an operating temperature). The further cooled stream may
be
substantially liquid (i.e. more than 99 mol% liquid) at the outlet pressure of
the
second cooling unit 17 (main cryogenic heat exchanger (MCHE)). In treatment
and
liquefaction train(s) A, including an end-flash unit 18 (as will be described
in more
detail below), the conditions of the further cooled stream 171' are selected
such to
produce a liquefied natural gas stream 184 and end flash stream 182 by means
of
reducing pressure and separation of liquid and vapor phases (described in more
detail
below). In treatment and liquefaction trains(s) B (a train not comprising an
end-flash
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unit), the conditions of the further cooled stream 171" are selected such to
produce a
liquefied natural gas stream 186. The liquefied natural gas stream 186 is
substantially
liquid. Herein, a vapor phase may be separated from the liquefied natural gas
stream
186 in the LNG storage tank 100. Such vapor is commonly referred to as boil-
off gas
(BOG). This process may be referred to as flash-in-tank. Further cooled stream
171"
may be a sub-cooled liquid before pressure let down and may typically contain,
for
instance, about 1 ¨ 3 mol% vapour after pressure let down,. This vapour will
be
separated, for instance, in the LNG storage tank 100.
It will be understood that many variations exist, which variations typically
are
the amount of vapor generated and the location where vapor is generated (in
LNG
storage tank 100 or in end flash unit 18).
Although schematically shown as separate blocks, it will be understood that
the
first and second cooling units 15, 17 may be integrated. For instance, the
second
(mixed) refrigerant may be passed through the first cooling unit 15 to be pre-
cooled
by the first (mixed) refrigerant.
According to the embodiments, the cooling stage 14 comprises a NGL-
extraction unit 16 (also known as a NGL removal unit) for extracting natural
gas
liquids 161 from the cleaned natural gas stream 13. Natural Gas Liquids (NGLs)
are
hydrocarbon molecules having two or more carbon atoms (C2+-molecules), such as
ethane, propane etc..
According to the embodiment schematically depicted in Fig. 1, the NGL-
extraction unit 16 is positioned in between the first and second cooling units
15, 17.
The NGL extraction unit 16 is therefore arranged to receive the pre-cooled
cleaned
natural gas stream 151 from the first cooling unit 15. However, other line-ups
may be
conceivable, for instance depending on the cooling process and gas
composition.
The NGL-extraction unit 16 may be a scrub column or any other suitable
separator, including a flash vessel. The appropriate NGL-extraction unit 16
depends
on the liquid content of the feed gas. For instance, pipeline gas typically
contains
little NGLs, while associated gas typically contains high amounts of NGLs.
The NGL-extraction unit 16 discharges a natural gas liquid stream 161, which
may also be referred to as a C2+-enriched stream 161. The term C2+-enriched is
used
to indicate that the stream is enriched in C2+ molecules compared to the
stream
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received by the NGL extraction unit 16, in this embodiment stream pre-cooled
cleaned natural gas stream 151.
The natural gas liquid stream 161 may be passed on to a fractionation unit,
comprising a fractionation column or series of fractionation columns (de-
methanizer,
de-ethanizer, de-propanizer, etc.), to further separate the components of the
natural
gas liquid stream 161. The fractionation unit, in use, may generate a methane
enriched stream and an ethane enriched stream. The methane enriched stream is
enriched in methane compared to the natural gas liquid stream 161. The ethane
enriched stream is enriched in ethane compared to the natural gas liquid
stream 161.
Some components may be stored separately for separate sale or refrigerant
make-up; other components may be fed back to the cooling stage 14, e.g.
methane
that came with the natural gas liquid stream 161.
According to an embodiment, the additional feed stream may comprise a
further stream comprising at least a portion of the one or more methane
enriched
streams generated by the respective fractionation units of the respective
treatment
and liquefaction trains (A, B).
According to an embodiment, the additional feed stream may comprise a
further stream comprising at least a portion of the one or more ethane
enriched
streams generated by the respective fractionation units of the respective
treatment
and liquefaction trains (A, B).
Advantageously, the methane enriched streams and the ethane enriched streams
are at a substantial pressure, typically in the range of 20 ¨ 30 bara, e.g. 25
bara, so
need relatively little compression for being fed to the additional
liquefaction train C.
The NGL-extraction unit 16 further discharges a light natural gas stream 162,
in fact being the stream received by the NGL-extraction unit 16 without the
natural
gas liquid stream 161, which light natural gas stream 162 is at least
partially to be
further cooled by the second cooling unit 17 and to be at least partially
liquefied to
generate liquefied natural gas 181. The light natural gas stream 162 may also
be
referred to as CI-enriched stream 162.
The term CI-enriched is used to indicate that the stream is enriched in Ci
molecules (methane) compared to the stream received by the NGL extraction unit
16,
in this embodiment stream pre-cooled cleaned natural gas stream 151.
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The respective treatment and liquefaction trains A, B comprise an outlet 181
for discharging liquefied natural gas. According to the embodiment
schematically
depicted in Fig. 1, the liquefied natural gas streams 181 discharged by the
outlets 181
are collected in a LNG storage tank 100. However, it will be understood that
more
than one LNG storage tank 100 may be present, and not all trains are
necessarily
connected to all LNG storage tanks 100.
The liquid natural gas plant 1 comprises an additional liquefaction train C.
The additional liquefaction train C can be added as retrofit to an existing
LNG
plant 1, or can be part of the original design of a LNG plant. Retrofitting
may in
particular be advantageous in situations in which there is overcapacity in the
warm
end, i.e. in the NGL-extraction unit 16 and upstream thereof, which may for
instance
be the case when there is a change in the composition of the feed gas, e.g.
less CO2
content, or as a result of (partial) replacement of the equipment for
separation
purposes of the acid gas removal unit (as described above) and/or
retrofitting/expanding other equipment of the acid gas removal unit, such as
heat
exchangers, allowing a higher throughput through the gas treatment stage 12.
It will
be understood that replacement of the equipment for separation purposes of the
acid
gas removal unit, in particular a replacement of internals of the absorption
column,
and/or replacement/expansion of smaller equipment is a relatively cost-
efficient way
to increase the throughput of the gas treatment stage 12, in case the acid gas
removal
unit is the constraining part of the gas treatment stage 12.
Also, retrofitting may be advantageous in case the existing LNG plant was
originally overdesigned.
It is noted that the additional liquefaction train C preferably does not
comprise
gas treatment, i.e. does not comprise an acid gas removal unit, dehydration
unit,
mercury removal unit and co-absorption unit, and the additional liquefaction
train
further doesn't comprise NGL-extraction.
According to an embodiment, the additional liquefaction train C comprises less
gas treatment units than the treatment and liquefaction trains A, B. For
instance, in
case the treatment and liquefaction trains A, B comprise four gas treatment
units, e.g.
an acid gas removal unit (AGRU), a dehydration unit, a mercury removal unit,
and a
co-absorption unit, while the additional liquefaction train comprises no more
than a
subset thereof (three or less gas treatment units).
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The additional liquefaction train comprises an additional cooling stage 214
arranged to receive and liquefy an additional feed stream 210 thereby
generating
additional liquefied natural gas. The additional cooling stage C may again use
any
suitable cooling process, such as the examples provided above, and may be
similar or
5 different to the cooling processes comprised by the treatment and
liquefaction trains
A, B.
According to the embodiment depicted in Fig. 1, the additional liquefaction
train C comprises a similar cooling stage as the treatment and liquefaction
trains A, B
described above, i.e. comprises a first cooling unit 215 is arranged to
generate a pre-
10 cooled natural gas stream 2151 and a second cooling unit 217 is arranged
to generate
a further cooled stream 2171. The second cooling unit 217 is arranged to
receive the
pre-cooled cleaned natural gas stream 2151 from the first cooling unit 215
directly.
The additional liquefaction train C optionally comprises an end-flash unit
218,
which is arranged to receive the further cooled stream 2171 and discharge a
flash
15 stream 282 and a liquid natural gas stream 284 via outlet 281.
The additional liquefaction train C receives an additional feed stream 210
which comprises at least two side streams 163 taken from the respective light
natural
gas streams 162 discharged by the NGL-extraction units 16 of the one or more
parallel treatment and liquefaction trains (A, B). The at least two side
streams 163 are
substantially gaseous. Depending on the pressure of the at least two side
streams 163,
the side streams 163 may be passed through a collecting and compression unit
202,
described in more detail below. The second cooling unit 217 can directly
receive the
pre-cooled cleaned natural gas stream 2151 from the first cooling unit 215,
because
NGL 161 has already been removed from the additional feed stream 210 by the
NGL extraction units 16 of the respective treatment and liquefaction trains A,
B.
The additional liquefaction train C may be a stand-alone train, i.e. being
separate
from the treatment and liquefaction trains A, B. However, the additional
liquefaction
train C may also be integrated with the one or more treatment and liquefaction
trains A,
B, for instance, by sharing refrigerant make-up facilities, utilities, etc.
According to an embodiment, one or more of the parallel treatment and
liquefaction trains (A) comprise an end flash unit 18 arranged to receive and
flash at
least part of the further cooled stream 171 to generate an end flash stream
182 and a
liquefied natural gas stream 184. The end flash stream 182 is substantially
gaseous.
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The liquefied natural gas stream 184 is substantially liquid. Herein, the
additional
feed stream 210 may further comprise at least a portion of the one or more
flash
streams 182 generated by the respective end flash units 18 of the one or more
of the
parallel treatment and liquefaction trains (A).
The end-flash unit 18 may be positioned downstream of the cooling stage 14
and arranged to receive (part of) the further cooled stream 171.
Fig. 1 shows one train with end flash unit 18 and one train without end flash
unit 18, but it will be understood that any number of the parallel treatment
and
liquefaction trains present may comprise an end flash unit 18, including none
and all.
In addition to only using side streams 163 taken from the respective light
natural gas streams 162 from the NGL extraction unit 16, end-flash gas 182 may
be
used in addition. In use, any suitable portion of the end flash stream 182 may
be
used, ranging from 0% to 100%. The portion of the end flash stream 182 not fed
to
the additional liquefaction train C, may be used for fuel or may be flared.
According to an embodiment, the entire flash streams 182 of one or more
treatment and liquefaction trains A comprising an end flash unit 18 are
comprised by
the additional feed stream. According to an embodiment, the entire flash
streams 182
of all the treatment and liquefaction trains A comprising an end flash unit 18
are
comprised by the additional feed stream.
As the flash streams 182 are already relatively clean and lean, the additional
liquefaction train C doesn't require (all of the) a gas treatment stage or NGL-
extraction unit.
According to an embodiment, the flash streams 182 are first passed through the
respective cooling stages of the treatment and liquefaction trains A from
which they
are obtained for cold-recovery purposes before being (partially) passed on to
the
additional liquefaction train C.
The different streams to be comprised in the additional feed stream 210 may
not all be at the same pressure and may not (all) be at a suitable pressure.
This is in
particular the case when the additional feed stream 210 also comprises (a
portion of)
flash streams 182, as flash streams are typically at a relatively low
pressure, such as
close to ambient.
According to an embodiment the liquid natural gas plant 1 comprises a
collecting and compression unit 202, comprising a plurality of inlets to
receive the
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respective streams to be comprised by the additional feed stream 210. The
collecting
and compression unit 202 is arranged to pressurize and combine the different
streams
to form and discharge the additional feed stream.
The collecting and compression unit 202 may comprise a single or multistage
compressor 203, optionally with inter and/or after-coolers (not shown), to
compress
the streams to be comprised by the additional feed stream to a predetermined
additional feed pressure. The compressor 203 may have one or more inlets
allowing
inflow of streams with different pressures.
In a further embodiment, the driver of the compressor 203 is mechanically or
electrically connected to the driver of one or more refrigerant compressors in
train C.
The additional liquefaction train C may comprise one or more refrigerant
compressors arranged to compress refrigerant, being part of the refrigeration
loop, as
will be understood by a skilled person. Combination of compressors to a driver
(less
drivers than compressors) enables a better matching of driver(s) power to
compressor(s) power, especially if the drivers are gas turbines, thereby
benefitting of
the economy of scale of less but larger drivers, for the same capacity of
additional
liquefaction train C.
In a further embodiment, the additional feed pressure (stream 210) is selected
such that the compressor power for compressor 203 and the refrigerant
compressor
power for additional cooling stage 214 are selected to further match driver
and
compressor power.
The additional feed pressure may be selected substantially higher than the
feed
pressure of the contaminated natural gas feed stream 10 for the treatment and
liquefaction trains A, B. Since preferably no NGL-extraction is required in
the
additional train C, there are no pressure limitations/considerations to take
into
account associated with the NGL-extraction. NGL-extraction typically takes
place at
a predetermined pressure, typically in the range of 30 ¨ 60 bara, e.g. 50
bara,
depending on the optimal conditions for performing NGL extraction for the
particular composition of the stream. NGL-extraction is preferably done at a
relatively low pressure, while liquefaction can typically be done more
efficiently at a
relatively higher pressure. These two effects need to be balanced. The absence
of
NGL-extraction in the additional liquefaction train C eliminates this
balancing and
allows for a more optimal pressure for liquefaction.
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A relatively high pressure in the NGL-extraction units of the treatment and
liquefaction trains A, B minimizes the need for (re-)compression of the two or
more
side streams (163) taken from the respective light natural gas streams (162)
to be
comprised in the additional feed stream.
According to an embodiment, the additional feed pressure of the additional
feed stream 210 may be more than 10 bar higher than the feed pressure of the
contaminated natural gas feed stream 10, more preferably even more than 20 bar
higher. The higher additional feed pressure contributes to more efficient
cooling and
liquefaction of the additional feed stream.
The one or more compressors may be suitable to compress the streams to be
comprised by the additional feed stream to a pressure that is more than 10 bar
of even
more than 20 bar above a feed pressure at which the contaminated natural gas
feed
stream (10, 10', 10") is received by the parallel treatment and liquefaction
trains (A,
B).
In embodiments in which the additional feed stream 210 comprises at least a
portion of one or more flash streams 182, the additional feed 210 stream may
be
relatively rich in nitrogen.
According to an embodiment the liquid natural gas plant comprises a nitrogen
removal stage (not shown), the nitrogen removal stage being arranged to
receive one
or more streams to be comprised by the additional feed stream and discharge
one or
more nitrogen depleted streams.
The nitrogen removal stage (also referred to as nitrogen removal unit) may be
incorporated in the additional liquefaction train C, maybe incorporated in the
collecting and compression unit 202 or may be incorporated in between the end
flash
unit 18 and upstream of the collecting and compression unit 202/additional
liquefaction train C. In the latter embodiment, there is preferably a single
nitrogen
removal stage provided to process all the end flash streams to be comprised by
the
additional feed stream 210. The nitrogen removal stage comprises an outlet
arranged
to discharge the one or more nitrogen depleted streams, the outlet being in
fluid
communication with the collecting and compression unit 202 or with the
collecting
and compression unit 202.
However, the use of an additional liquefaction train as described may in
particular be suitable in situations wherein the contaminated natural gas feed
stream
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has a low nitrogen content, which would eliminate the requirement for a
nitrogen
removal stage as additional treatment step, i.e. would eliminate the
requirement for a
nitrogen removal stage to be comprised by the additional liquefaction train C
or
positioned upstream thereof.
5 According to an embodiment, the natural gas feed stream 10 preferably
has a
nitrogen content of less than 1,0 mol% or less than 0.5 mol%.
More specifically, the nitrogen content of the portion of the contaminated
natural gas feed stream 10' provided to treatment and liquefaction train A
comprising
an end flash unit 18 is preferably less than 0.5 mol%, while the nitrogen
content of
10 the portion of the contaminated natural gas feed stream 10" provided to
treatment
and liquefaction train B, not comprising an end flash unit 18, is preferably
less than
1,0 mol%.
Fig. 1 further shows a fuel unit 300. The fuel unit 300 is depicted
schematically
and comprises a fuel inlet 301 to receive a fuel stream, comprising a side
stream 32
of the contaminated natural gas feed stream 10. The fuel unit 300 is arranged
to burn
the fuel stream to generate power (electricity) to provide power or energy
(heat) to
LNG plant 1, including in particular gas turbine drivers for refrigerant
compressors,
gas turbine generators for electrical power generation, fuel for the heat
transfer
furnaces/steam boilers, flare pilots and to supply fuel for heat demand, in
particular
gas treatment stage 12 may require heat. In Fig. 1 the parts of the liquid
natural gas
plant requiring power from the fuel unit 300 are schematically indicated by
power
consumers 302.
The fuel stream comprises at least a portion 32 of the contaminated natural
gas
feed stream 10. Preferably, at least 50% of the fuel stream is formed by the
fuel side-
stream 32 of the contaminated natural gas feed stream 10, a portion 34 of the
end
flash stream 182, and/or a portion of the cleaned natural gas stream 13.
According to an embodiment, there is provided a method of retrofitting a
liquid
natural gas plant comprising at least two or more first liquefaction trains A,
B
arranged in parallel with an additional liquefaction train C. After completing
the
retrofit, the liquid natural gas plant 1 can be operated with a flow rate for
the
contaminated natural gas feed stream 10 as outputted by the pretreatment unit
2,
which can be increased with respect to a flow rate for the contaminated
natural gas
feed stream 10 prior to the retrofit (i.e. without additional train C).
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Preferably the fuel side stream is taken from the contaminated natural gas
feed
stream 10. However, in situations where the contaminated natural gas feed
comprises
relatively high amounts of H2S, the fuel side-stream is preferably taken from
the
cleaned natural gas stream 13.
5 According to an embodiment, the liquid natural gas plant comprises at
least
four or more parallel treatment and liquefaction trains (A, B).
The treatment and liquefaction trains preferably each have a capacity of at
least
2 mmtpa, preferably at least 3 mmtpa (mmtpa: million metric tonnes of LNG per
year). This way the additional liquefaction train (C) may have a capacity of
at least 2
10 mmtpa to benefit from economy of scale.
Also, in case the additional liquefaction train is added to an existing liquid
natural gas plant, having at least four or more, e.g. six, parallel treatment
and
liquefaction trains with the above mentioned capacity, a sufficiently large
additional
feed stream can be generated without the need to change the operating
parameters of
15 the existing parallel treatment and liquefaction trains.
According to an embodiment there is provided a method of retrofitting an
existing liquid natural gas plant to increase the liquefied natural gas
production
capacity thereof. The resulting liquid natural gas plant may be referred to as
a
retrofitted liquid natural gas plant.
20 The existing liquid natural gas plant (1) may comprise two or more
parallel
treatment and liquefaction trains (A, B) as described above, which are
arranged to
- receive a portion of a contaminated natural gas feed stream (10', 10"),
- remove contaminants from the respective portion of the contaminated
natural gas
feed stream (10', 10") thereby generating a cleaned natural gas stream (13),
- cool at least part of the cleaned natural gas stream (13) and extract
natural gas
liquids (161) from the cleaned natural gas stream (13), thereby generating a
light
natural gas stream (162) to be at least partially further cooled by the
cooling stage
(14).
The method of retrofitting comprises
- providing an additional liquefaction train (C), the additional liquefaction
train
comprising an additional cooling stage arranged to receive and liquefy an
additional
feed stream (210) thereby generating additional liquefied natural gas. The
additional
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liquefaction train may be as described above, i.e. may not comprise NGL-
extraction
and/or gas treatment.
The method of retrofitting comprises
- fluidly connecting the additional liquefaction train (C) to two or more
of the
light natural gas streams (162) of two or more of the treatment and
liquefaction trains
(A, B) to obtain side streams (163) of the two or more light natural gas
streams to be
comprised in the additional feed stream (210).
In case the existing liquid natural gas plant comprises one or more parallel
treatment and liquefaction trains (A) comprising an end flash unit (18) as
described
above, the method of retrofitting may comprise
- fluidly connecting the additional liquefaction train (C) to one or more
of the
end flash streams (182) to receive at least a portion thereof to be comprised
in the
additional feed stream (210).
The method of retrofitting may comprise
- providing a collecting and compression unit (202) comprising a plurality of
inlets to receive the respective streams to be comprised by the additional
feed stream
from the two or more parallel treatment and liquefaction trains (A, B), the
collecting
and compression unit comprising one or more compressors to compress the
streams
to be comprised by the additional feed stream to a predetermined additional
feed
pressure and combine the streams to be comprised by the additional feed stream
to
form the additional feed stream, the collecting and compression unit (202)
further
comprising an outlet to discharge the additional feed stream (210),
- fluidly connecting the respective inlets of the collecting and
compression unit
(202) with the two or more parallel treatment and liquefaction trains (A, B)
and
fluidly connecting the outlet of the collecting and compression unit (202)
with the
additional liquefaction train (C).
The respective inlets of the collecting and compression unit are fluidly
connected with those streams of the parallel treatment and liquefaction trains
from
which gas is taken to be comprised in the additional feed stream 210.
According to an embodiment, the existing liquid natural gas plant comprises a
fuel unit (300), the fuel unit (300) being arranged to receive and burn a fuel
stream,
thereby generating power and/or heat to provide the liquid natural gas plant
(1) with
energy and/or heat,
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wherein the method of retrofitting comprises
- fluidly connecting the fuel unit (300) to receive a side-stream
of the
contaminated natural gas feed stream (10) or the cleaned natural gas stream
(13).
The existing liquid natural gas plant may comprise a fluid fuel connection
between the fuel unit and the respective parallel treatment and liquefaction
trains,
e.g. between the fuel unit and end flash streams (182), to provide the fuel
unit with
fuel. The method of retrofitting may comprise disconnecting this fuel
connection.
The method of retrofitting may also comprise leaving the existing fuel
connection in
place, but in use, the flow rate through the existing fuel connection will be
significantly reduced compared to prior to the retrofit, typically reduced
with more
than 50%.
As described above, the treatment and liquefaction trains may comprise a first
cooling unit 15 and a second cooling unit 17, the second cooling unit
generating a
further cooled stream 171 being at an operating temperature, wherein the
operating
temperature in the treatment and liquefaction trains A, B is substantially
equal prior
and after retrofitting. The term substantial equal is used here to indicate
that the
operating temperatures differ less than 4 C, preferably less than 2 C.
Once a retrofitted liquid natural gas plant has been provided, the retrofitted
liquid natural gas plant may be operated, wherein the method of operating
comprises
- receiving the respective portions of the contaminated natural gas feed
streams
(10', 10") at a feed pressure and
- pressurizing the additional feed stream to an additional feed pressure,
being at
least 10 bar above the feed pressure,
- feeding the additional feed stream to the additional liquefaction train
(C).
According to an embodiment, the additional feed pressure of the additional
feed stream 210 may be more than 10 bar higher than the feed pressure of the
contaminated natural gas feed stream 10, more preferably even more than 20 bar
higher. The higher additional feed pressure contributes to more efficient
cooling and
liquefaction of the additional feed stream.
The present disclosure is not limited to the embodiments as described above
and the appended claims. Many modifications are conceivable and features of
respective embodiments may be combined.
Additional embodiments
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According to a further embodiment, there is provided a liquid natural gas
plant
(1) for producing liquefied natural gas from a contaminated natural gas feed
stream
(10), the liquid natural gas plant (1) comprising one or more parallel
treatment and
liquefaction trains (A, B), wherein the respective treatment and liquefaction
trains
(A, B) comprise:
- an inlet (11) for receiving a portion of the contaminated natural gas
feed
stream (10', 10"),
- a gas treatment stage (12) for removing contaminants from the respective
portion of the contaminated natural gas feed stream (10', 10") thereby
generating a
cleaned natural gas stream (13),
- a cooling stage (14) arranged to receive the cleaned natural gas stream
(13)
from the gas treatment stage (12) for cooling at least part of the cleaned
natural gas
stream (13), wherein the cooling stage (14) comprises a NGL-extraction unit
(16) for
extracting natural gas liquids (161), thereby generating a light natural gas
stream
(162) to be at least partially further cooled by the cooling stage (14) to be
at least
partially liquefied and
- an outlet (181) for discharging liquefied natural gas,
wherein the liquid natural gas plant (1) comprises an additional liquefaction
train (C), the additional liquefaction train comprising
- an inlet (211) for receiving an additional feed stream (210),
- an additional cooling stage (214) arranged to receive the additional feed
stream for cooling the additional feed stream (210) thereby generating
additional
liquefied natural gas, and
- an outlet (281) for discharging the additional liquefied natural gas
(284),
wherein the additional feed stream (210) comprises one or more side streams
(163) taken from the respective light natural gas streams (162) of the one or
more
parallel treatment and liquefaction trains (A, B).
According to a further embodiment there is provided a method of retrofitting
an existing liquid natural gas plant to increase the liquefied natural gas
production
capacity thereof, wherein the existing liquid natural gas plant (1) comprises
one or
more parallel treatment and liquefaction trains (A, B) for producing liquefied
natural
gas from a contaminated natural gas feed stream (10), wherein the one or more
parallel treatment and liquefaction trains (A, B) are arranged to
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- receive a portion of a contaminated natural gas feed stream (10', 10"),
- remove contaminants from the respective portion of the contaminated
natural
gas feed stream (10', 10") thereby generating a cleaned natural gas stream
(13),
- cool at least part of the cleaned natural gas stream (13) and extract
natural gas
liquids (161), thereby generating a light natural gas stream (162) to be at
least
partially further cooled by the cooling stage (14),
wherein the method of retrofitting comprises
- providing an additional liquefaction train (C), the additional
liquefaction train
comprising an additional cooling stage arranged to receive and liquefy an
additional
feed stream (210) to generate additional liquefied natural gas,
- fluidly connecting the additional liquefaction train (C) to one or more
of the
light natural gas streams (162) of one or more of the treatment and
liquefaction trains
(A, B) to obtain side streams (163) of the one or more light natural gas
streams to be
comprised in the additional feed stream (210), thereby creating a retrofitted
liquid
natural gas plant.
According to a further embodiment there is provided a method of operating a
retrofitted liquid natural gas plant (1) according to the above or being
provided
according to method or retrofitting provided above, wherein the method
comprises
- operating the one or more parallel treatment and liquefaction trains (A,
B)
whereby operating the one or more parallel treatment and liquefaction trains
(A, B)
comprises receiving respective portions of the contaminated natural gas feed
streams
(10', 10") by the one or more parallel treatment and liquefaction trains (A,
B) at a
feed pressure and
- operating the additional liquefaction train (C), whereby operating the
additional liquefaction train (C) comprises providing an additional feed
stream
comprising one or more light natural gas streams (163) taken from the one or
more
parallel treatment and liquefaction trains (A, B), wherein the additional feed
stream is
provided at an additional feed pressure, being at least 10 bar above the feed
pressure.
It is recognized that an additional liquefaction train may also be added to a
single treatment and liquefaction train. The additional feed stream may
comprise any
combination of the above disclosed streams taken from the (single) treatment
and
liquefaction trains, such as end flash.
