Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND APPARATUS
FOR COOLING DOWN A CRYOGENIC HEAT EXCHANGER
FIELD OF THE INVENTION
The present invention relates to a method for cooling
down a liquefaction system for liquefying hydrocarbon-
containing gas.
Such liquefaction systems typically comprise a (main)
cryogenic heat exchanger arranged to cool a hydrocarbon-
containing gas stream, such as a natural gas stream, which is
typically treated and pre-cooled before being received by the
cryogenic heat exchanger.
In another aspect, the present invention relates to a
system and liquefaction system arranged to perform such a
method.
BACKGROUND
Methods and systems for liquefying hydrocarbon-containing
gas streams are well known in the art. It is desirable to
liquefy a hydrocarbon-containing gas stream such as a natural
gas stream. For instance, natural gas can be stored and
transported over long distances more readily as a liquid than
in gaseous form, because it occupies a smaller volume and
does not need to be stored at high pressures.
Typically, before being liquefied, the hydrocarbon-
containing gas stream is treated to remove one or more
contaminants (such as H20, CO2, H25 and the like) which may
freeze out during the liquefaction process.
Processes of liquefaction are known from the prior art in
which one or more refrigerant cycles are used to cool and
liquefy the hydrocarbon-containing gas stream.
Typically, the liquefaction system comprises
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- a pre-cooling stage comprising one or more pre-cooling
heat exchangers and a separator, such as a scrub column,
and
-a main cooling stage comprising one or more (main)
cryogenic heat exchangers and
-at least one LNG storage tank.
During normal operation a hydrocarbon-containing gas
stream is subsequently passed through the pre-cooling stage,
the main cooling stage and into the LNG storage tank.
For reliability reasons, the pre-cooled hydrocarbon-
containing gas stream passed from the pre-cooling stage to
main cooling stage, in particular to the cryogenic heat
exchanger, is a relatively clean hydrocarbon-containing gas
stream with low amounts of heavy ends. Typically the pre-
cooled hydrocarbon-containing gas stream should comply with a
predetermined Cn+-specification, the Cn+-specification
specifying a maximum amount of molecules having n or more
carbon molecules. The Cn+-specification may for instance be a
C5-'-specification specifying that the maximum amount of
molecules having 5 or more carbon molecules is less than a
predetermined value, such as less than 0.15 mol%. The
separator, such as a separator or a scrub column, is provided
to remove the heavy molecules from the hydrocarbon-containing
stream to meet this specification.
Typically, the separator is positioned in between two
pre-cooling heat exchangers positioned in series.
The C5-'-specification is associated with the one or more
cryogenic heat exchangers to prevent solids being formed in
the cryogenic heat exchangers.
Examples of liquefaction processes are a C3-MR process
and a DMR process.
In a C3-MR process the pre-cooling stage uses mainly
propane (i.e. >99 mol% propane) as refrigerant and the main
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cooling stage uses a mixed refrigerant, i.e. a mixture of two
or more refrigerants, such as a mixture of propane, ethane,
methane and nitrogen.
In a DMR process, both the pre-cooling stage and the main
cooling-stage use (different) mixed refrigerants.
Other liquefaction processes are known to the skilled
person.
Before a liquefaction system is ready for normal
operation, the liquefaction system including the cryogenic
heat exchanger of the main cooling stage needs to be cooled
down to or close to operating temperatures. This process is
referred to as a cool-down procedure. Cool-down procedures
are needed at first start up and after maintenance.
The cool-down procedure comprises two sub-procedures: the
pre-cool-down procedure in which the pre-cooling stage
including the separator are cooled down and prepared for
operation and the cryogenic cool-down procedure in which the
cryogenic heat exchanger is cooled down and prepared for
operation.
Only when the separator of the pre-cooling stage is fully
functioning, a pre-cooled hydrocarbon-containing gas stream
can be obtained from the pre-cooling stage that meets the
predetermined Cn+-specification. Only then the cryogenic
cool-down procedure can commence. It typically takes several
hours after starting the pre-cool-down procedure before the
pre-cooling stage is able to produce a pre-cooled
hydrocarbon-containing gas stream that meets the
predetermined Cn+-specification.
So, in a typical cool-down procedure, the pre-cooling
stage is started up and is used to generate clean/dew pointed
natural gas that meets the Cn+-specification of the cryogenic
heat exchanger, which can then be used to cool down the
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cryogenic heat exchanger and start up the cryogenic cool-down
procedure.
Cool-down procedures take a substantial amount of time,
for instance more than 48 hours. Stabilizing the separator,
e.g. the scrub column, such that a stream becomes available
in the pre-cooling stage that meets the predetermined Cn+-
specification may take many hours.
Once the cool-down procedure is finished, normal
operation can start.
Automated manners to perform a cool-down procedure are
known in the art, such as from U54809154 and W009098278.
SHORT DESCRIPTION
It is an object of the invention to provide a method and
system for cooling down a cryogenic heat exchanger in a
liquid natural gas generating liquefaction system in a time
efficient manner.
There is provided a method for cooling down a
liquefaction system for liquefying a hydrocarbon-containing
gas stream, the liquefaction system comprising a pre-cooling
stage comprising one or more pre-cooling heat exchangers and
a separator, a main cooling stage comprising one or more
cryogenic heat exchangers and the liquefaction system further
comprising at least one LNG storage tank, the pre-cooling
stage being arranged to generate a pre-cooled hydrocarbon
containing gas stream by pre-cooling the hydrocarbon-
containing gas stream and passing the pre-cooled hydrocarbon
containing gas stream to the main cooling stage, the method
for cooling down the liquefaction system comprising:
a) performing a pre-cool-down procedure to cool down the
pre-cooling stage, wherein the pre-cool-down procedure
comprises feeding the hydrocarbon-containing gas stream to
the pre-cooling stage,
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b) performing a cryogenic cool-down procedure to cool
down the main cooling stage, wherein the cryogenic cool-down
procedure comprises feeding a main cool-down stream (201) to
the main cooling stage,
wherein b) comprises forming the main cool-down stream
(201) and passing the main cool-down stream (201) to the main
cooling stage, the main cool-down stream (201) meeting a
predetermined Cn+-specification, the Cn+-specification
specifying a maximum amount of molecules having n or more
carbon molecules,
wherein the main cool-down stream (201) is formed out of
at least one auxiliary stream not being the pre-cooled
hydrocarbon containing gas stream, and
wherein the cryogenic cool down procedure and the pre-
cool down procedure are at least partially performed
simultaneously.
The Cn+-specification is associated with the one or more
cryogenic heat exchangers to prevent solids being formed in
the cryogenic heat exchangers and specifies a maximum amount
of molecules having n or more carbon molecules. Typically, n
2, e.g. n = 3, n = 4 or n = 5. Typically, n = 5, as C5+-
molecules will solidify in the cryogenic heat exchanger 210.
During most of the pre-cool down procedure (a), the
(partially) pre-cooled hydrocarbon containing gas stream
doesn't meet the Cn+-specification. As long as the pre-
cooling stage is not sufficiently cold yet, the pre-cooling
stage is not able to remove the heavy molecules and the pre-
cooled hydrocarbon containing gas stream doesn't meet the
Cn+-specification.
By forming the main cool-down stream from at least one
auxiliary stream not being the pre-cooled hydrocarbon
containing gas stream, it is possible to provide a main cool-
down stream that meets the Cn+-specification before the pre-
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cool down procedure is able to generate such a stream and the
cryogenic cool down procedure can start at an earlier time.
For all embodiments, the main cool-down stream 201 is
formed out of at least one auxiliary stream not being the
pre-cooled hydrocarbon containing gas stream and not being a
split-off stream thereof.
Accordingly there is provided a liquefaction system for
liquefying a hydrocarbon-containing gas stream, the
liquefaction system comprising a pre-cooling stage comprising
one or more pre-cooling heat exchangers and a separator, a
main cooling stage comprising one or more cryogenic heat
exchangers and the liquefaction system further comprising at
least one LNG storage tank, the pre-cooling stage being
arranged to generate a pre-cooled hydrocarbon containing gas
stream by pre-cooling the hydrocarbon-containing gas stream
and passing the pre-cooled hydrocarbon containing gas stream
to the main cooling stage, the system comprising a control
unit being arranged to cool down the liquefaction system by:
a) performing a pre-cool-down procedure to cool down the
pre-cooling stage, wherein the pre-cool-down procedure
comprises feeding the hydrocarbon-containing gas stream to
the pre-cooling stage,
b) performing a cryogenic cool-down procedure to cool
down the main cooling stage, wherein the cryogenic cool-down
procedure comprises feeding a main cool-down stream via a
main cool-down stream feed to the main cooling stage,
wherein the main cool-down stream feed is arranged to
receive at least one auxiliary stream not being the pre-
cooled hydrocarbon containing gas stream, and
wherein the control unit is arranged to perform the
cryogenic cool down procedure and the pre-cool down procedure
at least partially performed simultaneously.
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According to an embodiment, the main cool-down stream
feed is arranged to receive at least one auxiliary stream not
being the pre-cooled hydrocarbon containing gas stream and at
least part of the pre-cooled hydrocarbon containing gas
stream.
SHORT DESCRIPTION OF THE DRAWINGS
The present invention will now be illustrated by way of
example only, and with reference to embodiments and the
accompanying non-limiting schematic drawings in which:
Fig. 1 schematically shows a liquefaction system
encompassing a number of embodiments.
DETAILED DESCRIPTION
For the purpose of this description, a single reference
number will be assigned to a line (conduit, pipe) as well as
a stream carried by that line (conduit, pipe).
Were the term step is used in this text this term should
not be understood as being limited to the embodiments
provided here. Also, the steps may be performed in any
technically possible order, including (partially) overlapping
in time, as will be apparent to the skilled person.
Fig. 1 schematically depicts a liquefaction system 1
arranged to receive a hydrocarbon-containing gas stream 10,
such as a natural gas stream. The hydrocarbon-containing gas
stream 10 is preferably received from a gas treating plant
(not shown). The gas treating plant removes contaminants,
such as
-acid components (e.g. carbon dioxide, hydrogen sulphide)
using an acid gas removal unit,
-mercury using a mercury removal unit and/or
-water using a dehydration unit.
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The hydrocarbon-containing gas stream 10 may therefore
also be referred to as a (pre-)treated hydrocarbon-containing
gas stream 10.
The hydrocarbon-containing gas stream 10 mainly comprises
methane, i.e. comprises >50 mol% methane, typically >80 mol%.
First, a high-level explanation of the normal steady-
state operation of the liquefaction system 1 will be
provided.
The liquefaction system 1 comprises a pre-cooling stage
100 comprising one or more pre-cooling heat exchangers 110
and a separator 120. The separator 120 is preferably a
refluxed separator or a, optionally reboiled, scrub column.
Fig. 1 shows two, serial, pre-cooling heat exchangers 110,
but in practice more serial and/or parallel pre-cooling heat
exchangers 110 may be present. Also, pre-cooling heat
exchangers 110 may be present that are arranged to provide
cooling to the mixed refrigerant of the main refrigerant
cycle described in more detail below.
The separator 120 may be positioned in between pre-
cooling heat exchangers 110, arranged to receive the
hydrocarbon-containing gas stream 10 from a first pre-cooling
heat exchanger and forward a light top stream 125 from the
separator 120 to the second pre-cooling heat exchanger. The
separator 120 may receive a reflux stream, fed to the stop of
the separator, obtained as bottom stream from a reflux vessel
121.
The liquefaction system 1 further comprises a main
cooling stage 200 comprising one or more (main) cryogenic
heat exchangers 210. Again, Fig. 1 shows a single cryogenic
heat exchanger 210, but in practice two or more serial and/or
parallel cryogenic heat exchangers 210 may be present.
Finally, the liquefaction system 1 comprises at least one
LNG storage tank 300 (one shown). Where reference is made to
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a/the LNG storage tank in this text, it will be understood
that this is done for convenience only and that in fact more
than one LNG storage tank 300 may be present.
A boil-off gas stream 301 is obtained from the LNG
storage tank 300, which will be at least partially passed to
a BOG-compressor 303 to obtain pressurized boil-off gas
stream 301' and a BOG heater or cooler, for instance using
air as heating or cooling medium, to receive the pressurized
boil-off gas stream 301' and generate a heated or cooled and
pressurized boil-off gas stream 301", which may for instance
be used as fuel stream.
The choice between a BOG heater and BOG cooler depends on
the temperature of the pressurized boil-off gas stream 301'.
For a low pressure (e.g. 4-7 barg) BOG compressor 303, the
compressor discharge temperature is cryogenic (-40 to -60 C)
and therefore a discharge BOG heater is provided. For high
pressure (e.g. 20-27 barg) BOG compressor 303, the compressor
discharge temperature is >100 C and therefore a discharge
BOG cooler is provided.
In this text the unit bar and bara are used to refer to
bar absolute. The unit barg is used to refer to bar gauge,
wherein bara - baratmõ phermc = barg.
The boil-off gas stream 301 typically has a pressure
close to ambient pressure and a cryogenic temperature
(typically below minus 150 C). The heated or cooled and
pressurized boil-off gas stream 301" typically has a
pressure above 5 or 20 bar, e.g. 6 bar or 25 bar, and a
temperature above 10 C or 30 C, e.g. 40 C.
It will be understood that Fig. 1 shows a schematic and
simplified liquefaction system 1. For instance, the
refrigerant cycles associated with the pre-cooling stage 100
and the main cooling stage 200 are only shown partially and
schematically and in practice may comprise a compressor, a
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condenser and a pressure reduction device. The cooling stages
using a mixed refrigerant may be arranged to split the mixed
refrigerant in a light and heavy portion which are passed
through the heat exchanger(s) separately. The refrigerant of
the main cooling stage 200 may receive cooling duty from the
refrigerant of the pre-cooling stage 100.
Also, it will be understood that further stages,
including further cooling stages and devices may be present,
such as a flashing stage in between the main cooling stage
200 and the LNG storage tank 300.
During normal operation, the hydrocarbon-containing gas
stream 10 is passed to the separator 120, from which a heavy
bottom stream 126 and a light top stream 125 are obtained.
The heavy bottom stream 126 may be further processed by a
NGL stage (not shown), typically comprising a de-methanizer,
a de-ethanizer etc. as will be known by the skilled person.
The light top stream 125 is passed to the at least one
pre-cooling heat exchangers 110 via conduit(s) 125 to obtain
a pre-cooled intermediate stream 111 which may be passed to
reflux vessel 121 directly.
From the reflux vessel 121 a liquid bottom stream 122 is
obtained which is passed to the top of the separator 120 as
reflux stream 122, 124 optionally using a reflux pump 123.
The reflux stream 124 is passed to (the top of) the separator
120 to increase the separation effect of the separator 120
(e.g. scrubbing effect in case of a scrub column) and thereby
lower the amount of Cr,-' molecules in the light top stream 125
obtained from the separator, n 2.
Typically, n = 5, as C5+-
molecules will solidify in the cryogenic heat exchanger 210.
A pre-cooled hydrocarbon containing gas stream 112 is
obtained as top stream from the from the reflux vessel 121
which is passed to the main cryogenic cooling stage 200 to be
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cooled further by the one or more cryogenic heat exchanger
210.
According to an alternative embodiment (not shown), the
pre-cooled intermediate stream 111 is not passed to reflux
vessel 121 directly, but the pre-cooled intermediate stream
111 is first passed to a warm bundle of the main cooling
stage 200 to be further cooled by the main cooling stage 200,
i.e. in the warm bundle, to generate a further cooled stream
at a lower temperature to enable meet the C5-'-specification.
The further cooled stream is fed to reflux vessel 121. The
reflux vessel 121 generates an overhead vapour stream which
is rooted back the main cooling stage 200 for further
cooling, i.e. in the mid and cold bundle thereof, and a
bottom liquid stream which is passed to the separator 120 as
reflux stream.
Where in this text reference is made to a or the
cryogenic heat exchanger 210, it will be understood that this
encompasses one or more (serial and/or parallel) cryogenic
heat exchangers.
The cryogenic heat exchanger 210 may be a coil-wound heat
exchanger.
The cryogenic heat exchanger 210 may have an associated
main refrigerant cycle 200 arranged to separate the mixed
refrigerant into a light and heavy mixed refrigerant and may
be a heat exchanger arranged to receive the light mixed
refrigerant and the heavy mixed refrigerant separately. The
cryogenic heat exchanger 210 may be divided in different
sections with different bundles to carry the hydrocarbon
containing gas stream to be liquefied, which may be referred
to as the warm bundle and the cold bundle, with optionally a
mid-bundle in between.
The pre-cooled hydrocarbon containing gas stream 112 may
be cooled by the cryogenic heat exchanger 210 to obtain a
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further cooled stream 211 which may be passed to the LNG
storage tank 300. It will be understood that further
equipment may be present between the cryogenic cooling stage
200 and the LNG storage tank 300, such as a final cooling
stage and/or a flash stage.
During normal operation, pre-cooled hydrocarbon
containing gas stream 112 should meet a predetermined C+-
specification of the cryogenic heat exchanger 210. The Cn+-
specification specifies a maximum fraction of molecules
having n or more carbon molecules allowed to be comprised by
the main cool down stream. According to an embodiment, n = 2
or n = 3 or n = 4 or n = 5. Typically, n 2
and preferably n
= 5.
As explained above, the liquefaction system 1 needs to go
through a cool-down procedure after maintenance before it is
ready for normal operation. There is provided a method for
cooling down the liquefaction system, wherein the method
comprises:
a) performing a pre-cool-down procedure to cool down the
pre-cooling stage, wherein the pre-cool-down procedure
comprises feeding the hydrocarbon-containing gas stream to
the pre-cooling stage,
b) performing a cryogenic cool-down procedure to cool
down the main cooling stage, wherein the cryogenic cool-down
procedure comprises feeding a main cool-down stream to the
main cooling stage,
wherein b) comprises forming a main cool-down stream
(201) and passing the main cool-down stream to the main
cooling stage, the main cool-down stream meeting a
predetermined Cn+-specification, the Cn+-specification
specifying a maximum amount of molecules having n or more
carbon molecules,
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wherein the main cool-down stream is formed out of at
least one auxiliary stream not being the pre-cooled
hydrocarbon containing gas stream, and
wherein the cryogenic cool down procedure and the pre-
cool down procedure are at least partially performed
simultaneously.
The main cool-down stream is formed out of at least one
auxiliary stream not being the pre-cooled hydrocarbon
containing gas stream from the pre-cooling stage including
pre-cooled hydrocarbon containing gas streams from parallel
pre-cooling stages. It will be understood and become clear
from below described embodiments that part of the main cool-
down stream may be formed from the pre-cooled hydrocarbon
containing gas stream or part thereof.
The embodiments can advantageously be used in single train
liquefaction systems, such as floating LNG facilities.
The pre-cool down procedure may start with step al) which
is pressurizing the separator to operating pressure,
typically in the range of 55 - 60 bars.
Next, step a2), the hydrocarbon-containing gas stream 10
is started to flow to the separator 120 and the pre-cooling
refrigerant cycle 130 is started by starting pre-cooling
refrigerant compressor(s) (not shown) to cool down the one or
more pre-cooling heat exchangers 110.
A pre-cooled intermediate stream 111 starts to be
produced and is sent to the reflux vessel 121 (directly or
via the (warm bundle of the) cooling stage 200). As this flow
is not sufficiently cold, no liquid bottom stream 122 is
obtained from the reflux vessel 121. The pre-cooled gaseous
top stream 112 obtained from the reflux vessel 121, which
during this phase could be referred to as partially pre-
cooled gaseous top stream 112, does not meet the Cn+-
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specification and may (partially) be flared via flare stack
40.
In step a3) cooling of the reflux pump 123 is commenced,
for instance using a partially pre-cooled gaseous stream or
(later) using liquid bottom stream 122.
Next, in step a4), a liquid bottom stream 122 is obtained
from the reflux vessel 121 and the Cn-'-content of the pre-
cooled gaseous top stream 112 obtained from the reflux vessel
121 starts to decrease towards the Cn-'-specification. At the
end of step a4) the Cn-'-content of the pre-cooled gaseous top
stream 112 obtained from the reflux vessel 121 meets the
predetermined Cn+-specification.
During most of the pre-cool down procedure, no or only a
small liquid bottom stream 122 is obtained from the reflux
vessel 121 and there is no stream available in the pre-
cooling stage 100 that meets the Cn+-specification, in
particular: the (partially) pre-cooled gaseous top stream 112
obtained from the reflux vessel 121 doesn't meet the Cn+-
specification and can't be passed to the main cooling stage
200.
The currently proposed embodiments overcome this by using
at least one auxiliary stream not being the pre-cooled
hydrocarbon containing gas stream and not being a split-
stream of the pre-cooled hydrocarbon containing gas stream,
to be part of the main cool-down stream thereby allowing the
cryogenic cool down procedure and the pre-cool down procedure
to be performed at least partially simultaneously.
The cryogenic cool down procedure and the pre-cool down
procedure have an overlap in time. Typically, the cryogenic
cool down procedure and the pre-cool down procedure are
performed simultaneously for at least one hour, preferably at
least three hours, more preferably at least eight hours and
most preferably at least 10 hours.
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So, in the context of the pre-cool-down procedure and the
cryogenic cool-down procedure, the pre-cooled hydrocarbon
containing gas stream 112 may also be referred to as a
partially pre-cooled hydrocarbon containing gas stream as the
pre-cool-down procedure to cool down the pre-cooling stage
has not been completed at the time the main cryogenic cool-
down procedure starts.
The temperature of the partially pre-cooled hydrocarbon
containing gas stream is above a predetermined pre-cool
temperature, the predetermined pre-cool temperature being
below minus 20 C, e.g. minus 30 C.
The partially pre-cooled hydrocarbon containing gas
stream 112 obtained from the pre-cooling stage may also be
passed through a pressure reduction device 113 to reduce the
pressure to a predetermined main cool down pressure. The
partially pre-cooled hydrocarbon containing gas stream 112
may be passed through pressure reduction device 113 to reduce
the temperature of the partially pre-cooled hydrocarbon
containing gas stream to less than minus 70 C to provide cold
to the main cool-down stream.
According to an embodiment, at least one auxiliary stream
meets a second Cn+-specification, the second Cn+-specification
specifying a second maximum amount of molecules having n or
more carbon molecules, the second maximum amount being lower
than the maximum amount. The second Cn+-specification
mentioned above may be Cs+ < 0.01 mol%.
This is in particular advantageous when the auxiliary
stream is mixed with the pre-cooled hydrocarbon containing
gas stream to form the main cool-down stream. According to
such an embodiment, the mass flow rate of the one or more
auxiliary streams with respect to the mass flow rate of the
pre-cooled hydrocarbon containing gas stream from the pre-
cooling stage are preferably controlled in relation to each
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other such that a main cool-down stream 201 is obtained that
meets the predetermined Cn+-specification.
The mass flow rate of the one or more auxiliary streams
with respect to the mass flow rate of the pre-cooled
hydrocarbon containing gas stream from the pre-cooling stage
are preferably controlled in relation to each other such that
a main cool-down stream 201 is obtained having a temperature
below minus 20 C, preferably below minus 25 C, for instance
at a temperature of minus 35 C. This may be achieved by
passing the partially pre-cooled hydrocarbon containing gas
stream 112 through pressure reduction device 113, thereby
reducing the temperature of the partially pre-cooled
hydrocarbon containing gas stream. This may be done to ensure
the main cool-down stream 201 is sufficiently cold.
Furthermore, the mass flow rate of the one or more
auxiliary streams with respect to the mass flow rate of the
pre-cooled hydrocarbon containing gas stream from the pre-
cooling stage are preferably controlled in relation to each
other such that a main cool-down stream 201 is obtained that
meets a predetermined mass flow needed for performing the
cryogenic cool down procedure.
According to an embodiment, the Cn+-specification is Cs+ <
0.15 mol%. According to such a specification, the main cool-
down stream 201 comprises less than 0.15 mol% hydrocarbon
molecules with five or more carbon atoms. According to a
further embodiment, the Cn+-specification is Cs+ < 0.25 mol%.
The second Cn+-specification mentioned above may be Cs+ < 0.01
mol%.
According to a further embodiment, the Cn+-specification
is a function of the temperature of the main cooling stage,
wherein the Cn+-specification decreases with decreasing
temperature of the main cooling stage. According to such an
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embodiment, forming the main cool-down stream comprises
obtaining a temperature indication of the main cooling stage.
The mass flow rate of the one or more auxiliary streams
with respect to the mass flow rate of the pre-cooled
hydrocarbon containing gas stream from the pre-cooling stage
are preferably controlled in relation to each based on the
temperature indication of the main cooling stage or a Cn+-
specification derived therefrom.
According to an embodiment, b) comprises
-obtaining a pre-cooled stream from the pre-cooling stage
(112), the pre-cooled stream being derived from the
hydrocarbon-containing gas stream (10),
-obtaining one or more auxiliary streams (501; 502; 505;
506; 507) and
-mixing the pre-cooled stream (112) and the auxiliary
stream to obtain the main cool-down stream (201).
The one or more auxiliary streams are not the pre-cooled
hydrocarbon containing gas stream. The auxiliary streams meet
a second Cn+-specification, the second C-specification
specifying a second maximum amount of molecules having n or
more carbon molecules, the second maximum amount being lower
than the maximum amount associated with the predetermined
C-specification.
According to an embodiment, the at least one auxiliary
stream comprises one or more of the following:
- boil-off gas (501, 502) obtained from the at least one
LNG storage tank,
- liquid natural gas (505) obtained from the at least one
LNG storage tank and
- nitrogen (506) obtained from a N2-source.
The embodiments using a boil-off stream and/or using
liquid natural gas as auxiliary stream(s) are in particular
relevant to situations wherein the cool down procedure is
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applied after maintenance, as in such situations an at least
partially filled LNG storage tank 300 is usually available.
Different embodiments will be described in more detail
below.
According to an embodiment, the hydrocarbon-containing gas
stream 10 is a lean hydrocarbon-containing gas stream.
The embodiments are in particular advantageous for lean
hydrocarbon-containing gas streams 10, as lean hydrocarbon-
containing gas streams 10 require more cooling and thus more
cool-down time in order to generate a liquid bottom stream
122 to be used in the separator 120 to stabilise the
hydrocarbon-containing gas stream. The liquid bottom stream
122 comprises mostly heavy ends and these are easier to
generate if their composition is higher, as is the case for a
rich or non-lean stream.
The term lean is used to indicate a hydrocarbon-
containing gas stream 10 comprising relatively low amount of
hydrocarbon molecules heavier than methane, i.e. C2+-content
< 0.10 mol% or < 0.07 mol%. Table 1 below provides a typical
composition of a lean hydrocarbon-containing gas stream 10 as
may be used in the embodiments:
Molar Component Mol-Fraction
N2 0.037
C1 0.899
C2 0.036
C3 0.016
C4+ 0.012
Table 1
It is noted that during the pre-cool-down procedure to
cool down the pre-cooling stage the C5+-composition of the
light top stream 125 from the separator 120 and of the pre-
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cooled hydrocarbon containing gas stream 112 obtained as top
stream from the from the reflux vessel 121 may be higher than
that of the hydrocarbon-containing gas stream 10 due to the
not optimally functioning separator 120 lacking a reflux
stream 124.
According to an embodiment the one or more cryogenic heat
exchangers (210) comprises refrigerant tubes, hydrocarbon
tubes and a shell side and b) comprises feeding the main cool
down stream (201) to the refrigerant tubes, hydrocarbon tubes
and the shell side.
The main cool-down stream 112 is typically fed to all
parts of the one or more cryogenic heat exchangers 210,
including the flow paths/tubes of the natural gas stream
(during normal operation), i.e. pre-cooled gaseous top stream
as well as the flow paths of the main refrigerant. In case
the cryogenic heat exchanger is a coil-wound heat exchanger,
during the cryogenic cool-down procedure, the pre-cooled
gaseous top stream may be passed through all tubes and the
shell side to cool down the cryogenic heat exchanger.
As schematically shown in Fig. 1, the main cool-down
stream line 112 is split into a first main cool-down stream
line 1221 being in fluid communication with the flow
paths/tubes of the natural gas stream, a second main cool-
down stream line 1222 being in fluid communication with the
flow paths/tubes of the shell side a third main cool-down
stream line 1223 being in fluid communication with the flow
paths/tubes of the main refrigerant.
According to an embodiment, schematically depicted in
Fig. 1, b) comprises
-obtaining a pre-cooled stream from the pre-cooling stage
(112), the (partially) pre-cooled stream being derived
from the hydrocarbon-containing gas stream (10),
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-obtaining an auxiliary stream (501; 502) obtained from a
boil-off stream (301) obtained from the at least one LNG
storage tank (300) and
-mixing the pre-cooled stream (112) and the auxiliary
stream to obtain the main cool-down stream (201).
The (partially) pre-cooled stream 112 from the pre-
cooling stage 100 doesn't meet the predetermined C+-
specification. The boil-off stream 301 from the at least one
LNG storage tank 300 has a Cn+-content that is below the
predetermined Cn+-specification. The C5-'-content of the boil-
off stream 301 is typically zero.
By mixing the at least partially pre-cooled stream 112
and the at least part of the boil-off stream 301, a main
cool-down stream 201 is obtained that meets the predetermined
Cn-'-specification. In other words, by mixing or blending the
at least partially pre-cooled stream 112 which doesn't meet
the Cn+-specification with the at least part of the boil-off
stream which more than meets the Cn+-specification (i.e.
meets the second Cn+-specification), a combined stream 201 is
obtained that meets the predetermined Cn+-specification
before the pre-cool down procedure has been finished.
This has the advantage that the cryogenic cool down
procedure can start earlier, i.e. before the pre-cool down
procedure has finished and before the pre-cool down procedure
is able to generate a stream that meets the predetermined
Cn-'-specification. As a result, the total time needed for the
cool-down procedure can be reduced significantly, for
instance with more than 10 hours.
The pre-cooled hydrocarbon containing gas stream 112 is
formed from the entire top stream from the reflux vessel 121
or a portion thereof.
It is noted that according to the alternative embodiment,
wherein the pre-cooled intermediate stream 111 is not passed
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to reflux vessel 121 (the warm bundle of) the main cooling
stage 200 to be further cooled by the main cooling stage 200,
stream 112 is still referred to as pre-cooled hydrocarbon
containing gas stream 112 which is passed to the main cooling
stage from the pre-cooling stage. Reflux vessel 121 is
considered to form part of the pre-cooling stage.
As described above, the boil-off gas stream 301 is
obtained from the LNG storage tank 300, which will be at
least partially passed to a BOG-compressor 303 to obtain
pressurized boil-off gas stream 301' and a BOG heater or
cooler (depending on the temperature of the pressurized boil-
off gas stream 301'), for instance using air as heating or
cooling medium, to receive the pressurized boil-off gas
stream 301' and generate a heated or cooled and pressurized
boil-off gas stream 301", which may for instance be used as
fuel stream.
The auxiliary streams may be formed as a side stream 501
from the pressurized boil-off gas stream 301' and/or as a
side stream 502 from the cooled or heated pressurized boil-
off gas stream 301".
According to a preferred embodiment, the pre-cooled
stream from the pre-cooling stage 112 is mixed with an
auxiliary stream 502 obtained as a side stream from the
heated or cooled and pressurized boil-off gas stream 301" to
obtain the main cool-down stream 201.
Pressure reduction devices 503, 504 may be present to let
down the pressure of the auxiliary streams 501, 502 to obtain
an auxiliary stream having a predetermined main cool down
pressure.
The pre-cooled stream 112 may also be passed through a
pressure reduction device 113 to reduce the pressure to the
predetermined main cool down pressure.
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The pressure reduction devices 113, 503, 504 may be
embodied as a valve, a JT-valve and/or an expander. The
pressure reduction may be performed by pressure reduction
valves, such as pressure reduction valve 113 present in
conduit 112 carrying the at least partially pre-cooled stream
112 and pressure reduction valve 503, 504 present in lines
501, 502 respectively carrying the boil-off stream 302.
So, according to an embodiment, the at least partially
pre-cooled stream 112 and the auxiliary stream 501; 502
obtained from the boil-off stream 301 are reduced in pressure
to the predetermined main cool down pressure (P201) before
being mixed. Alternatively, the pressure reduction step may
be applied to the main cool-down stream 201, thus after
mixing.
The main cool-down stream 201 provided to the cryogenic
heat exchanger 210 has a pressure equal to the predetermined
main cool down pressure. The main cool down pressure is
typically in the range of 2 - 4 barg, for instance 2.5 barg.
According to an alternative embodiment, also
schematically depicted in Fig. 1, b) comprises
-obtaining a pre-cooled stream from the pre-cooling stage
112, the (partially) pre-cooled stream being derived
from the hydrocarbon-containing gas stream 10,
-obtaining an auxiliary stream 506 from a nitrogen-source
(N2), the auxiliary stream 506 mainly consisting of
nitrogen and
-mixing the pre-cooled stream 112 and the auxiliary stream
506 from the nitrogen-source (N2) to obtain the main
cool-down stream 201.
The auxiliary stream 506 mainly consisting of nitrogen,
may have a pressure of 7 barg and a temperature of 30 C.
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The auxiliary stream 506 mainly consisting of nitrogen
may be obtained from a dedicated nitrogen-source (N2) or
process plant nitrogen supply system. The auxiliary stream
mainly consisting of nitrogen, typically comprises more than
90 mol% nitrogen or even more than 99 mol% nitrogen.
Although not shown, conduit 506 carrying the auxiliary
stream 506 mainly consisting of nitrogen may comprise a
pressure reduction device, i.e. a (JT-)valve or expander to
let down the pressure of the auxiliary stream 506 to the
appropriate pressure, i.e. the predetermined main cool down
pressure (F201) =
Embodiments described above have in common that the
auxiliary stream is mixed with the pre-cooled stream 112
obtained from the pre-cooling stage.
According to these embodiments the mass flow rate of the
partially pre-cooled stream (from the pre-cooling stage 112)
is MF112 and the mass flow rate of the auxiliary streams
combined is MFaux and MF112 > MFaux and MF112 > 1.5* MFaux. The
mass flow rate of the auxiliary streams combined is the sum
of the mass flow rates of the one or more auxiliary streams
(which may also be referred to as the mass flow rate of the
combined auxiliary streams). It will be understood that this
also encompasses the situation of a single auxiliary stream.
According to a further embodiment the ratio (R) of the
mass flow rate of the partially pre-cooled stream (from the
pre-cooling stage 112) MF112 and the mass flow rate of the
auxiliary streams MFaux combined is determined based on a
temperature indication (TmcHE) of the main cooling stage or
based on a Cn+-specification derived from the temperature
indication. The ratio R (= MF112/MFaux) may thus be a function
of the temperature indication (TmcHE): R = f(TmcHE). The ratio
may further depend on an obtained, i.e. measured or computed,
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C,+-value (Cõ,112) of the partially pre-cooled stream 112: R =
f (TmCHEr Cri,1121 =
The method may further comprise controlling the mass flow
rates of the partially pre-cooled stream and the mass flow
rate of the auxiliary stream(s) to meet the determine ratio
(R). The liquefaction according to this embodiment may
comprise a controller arranged to obtain the temperature
indication (TmcHE) from a suitable temperature sensor,
determine the ratio R and control the mass flow rates of the
partially pre-cooled stream and the mass flow rate of the
auxiliary stream(s) to meet the determined ratio (R).
According to an embodiment, b) comprises
-obtaining a first auxiliary stream by taking a side-
stream 501 from the pressurized boil-off gas stream 301'
- obtaining a second auxiliary stream 502 by taking a
side-stream from the heated or cooled and pressurized
boil-off gas stream 301"and
-mixing the first and second auxiliary streams 501, 502 to
obtain the main cool-down stream 201.
As shown in Fig. 1, pressure reduction devices 503, 504
may be present to let down and equalize the pressure of the
first and second auxiliary stream 501, 502.
As explained above, the boil-off gas stream 301 obtained
from the LNG storage tank 300 is at least partially passed
through a BOG-compressor 303 to obtain pressurized boil-off
gas stream 301' and a BOG heater or cooler, for instance
using air as heating medium, to receive the pressurized boil-
off gas stream 301' and generate a heated or cooled and
pressurized boil-off gas stream 301", which may for instance
be used as fuel stream.
The first auxiliary stream 501 is obtained as side stream
from the pressurized boil-off gas stream 301' (i.e.
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downstream the BOG-compressor 303 and upstream the BOG heater
or cooler 310) and the second auxiliary stream 502 is
obtained as side stream from the heated pressurized boil-off
gas stream 301".
According to an embodiment, b) comprises
-obtaining a first auxiliary stream 501; 502 obtained from
a boil-off stream 301 obtained from the at least one LNG
storage tank 300 and
- obtaining a second auxiliary stream 507 by taking a
side-stream from the hydrocarbon-containing gas stream
10 upstream of the pre-cooling stage 100,
-mixing the first and second auxiliary streams 501, 502;
507 to obtain the main cool-down stream 201.
According to this embodiment, the first auxiliary stream
301 is preferably obtained as side stream from the
pressurized boil-off gas stream 301' (i.e. downstream the
BOG-compressor 303 and upstream the BOG heater or cooler
310).
The second auxiliary stream 507 is preferably taken
downstream of any pre-treatment stages but upstream of the
pre-cooling stage. The second auxiliary stream 507 may be
cooled using an ambient stream by a suitable ambient cooler
508, such as an air or water cooler.
According to an embodiment, b) comprises
-obtaining a first auxiliary stream 501; 502 being a
liquid natural gas stream 505 from the at least one LNG
storage tank and
-obtaining a second auxiliary stream 502 by taking a side-
stream from the hydrocarbon-containing gas stream (10)
upstream of the pre-cooling stage 100,
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-mixing the first and second auxiliary streams 501, 502 to
obtain the main cool-down stream 201.
The second auxiliary stream is preferably taken
downstream of any pre-treatment stages but upstream of the
pre-cooling stage.
With reference to all embodiments described above, the
main cool-down stream is fed to the main cooling stage
typically at a pressure 1-4 or 2 - 4 barg (i.e. below 40
barg).
This is done until one cryogenic heat exchanger (210) has
reached a predetermined first temperature or first
temperature profile.
The predetermined first temperature may be in the range
of minus 15 C - minus 35 C and may be applied to one or more
predetermined positions in the at least one cryogenic heat
exchangers 210. Also a first temperature profile may be
applied in which different predetermined temperatures at
different predetermined positions in the at least one
cryogenic heat exchangers 210 are used, such as minus 20 C
for a position at or near the top and minus 30 C for a
position at or close to the bottom of the respective
cryogenic heat exchanger 210.
According to an embodiment, b) is performed until the one
or more cryogenic heat exchangers have reached a
predetermined first temperature or first temperature profile,
subsequently the method continuing with
-feeding a pressurized main cool-down stream 201 to the
main cooling stage to further cool-down the main cooling
stage, the main cool-down stream 201 having a pressure
above 40 barg.
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The main cool-down stream 201 may be provided at a
pressure above 40 barg by using suitable compressors and/or
by-passing pressure reduction devices for the different
streams together forming the main cool-down stream 201.
During this phase the main cooling stage and in
particular the cryogenic heat exchanger(s) can be further
cooled down to reach a predetermined second temperature or
predetermined second temperature profile. The predetermined
second temperature is lower than the predetermined first
temperature and the predetermined second temperature profile
is lower than the predetermined first temperature profile.
Liquefaction in the main cooling stage 200 is more
efficiently at higher pressures. Therefore, it is
advantageous to provide a main cool-down stream 201 at a
pressure above 40 barg. Also, this allows to bring the at
least one cryogenic heat exchangers 210 to the pressure used
during actual production and thereby a smooth transition from
cool-down to actual production is ensured.
The predetermined second temperature may be in the range
of minus 130 C - minus 150 C and may be applied to one or
more predetermined positions in the at least one cryogenic
heat exchangers 210. Also a second temperature profile may be
applied in which different predetermined temperatures at
different predetermined positions in the at least one
cryogenic heat exchangers 210 are used, such as minus 135 C
for a position at or near the top and minus 150 C for a
position at or close to the bottom of the respective
cryogenic heat exchanger 210.
This embodiment further reduces the time required for
performing the pre-cool down procedure and the cryogenic cool
down procedure as it allows cooling down the main cooling
stage to temperatures below minus 130 C and pressurize the
main cooling stage before the pre-cooling stage 100 is able
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to generate a stream that meets the predetermined C+-
specification.
By further reducing the start-up time for the liquefaction
system, the production of liquefied natural gas can commence
early thereby increasing the up-time. Also, flaring is further
reduced.
The person skilled in the art will understand that the
present invention can be carried out in many various ways
without departing from the scope of the appended claims.
