Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DUAL NITROGEN EXPANSION PROCESS
The present invention relates to a natural gas liquefaction process in which
nitrogen is used as the main refrigeration component. The process is
particularly,
but not exclusively, suited for use offshore.
Natural gas can be obtained from the earth to form a natural gas feed which
must be processed before it can be used commercially. The gas is often
liquefied
before being transported.to its point of use. This enables the volume of the
gas to be
reduced by about 600 fold, which greatly reduces the costs associated with
storing
and transporting the gas. Since natural gas is a mixture of gases, it
liquefies over a
range of temperatures. At atmospheric pressure, the usual temperature range
within
which liquefaction occurs is between -165 C and -155 C. Since the critical
temperature of natural gas is about -80 C to -90 C, the gas cannot be
liquefied
purely by compressing it. It is therefore necessary to use cooling processes.
It is known to cool natural gas by using heat exchangers in which a gaseous
refrigerant is used. One known method comprises a number of cooling circuits,
typically three, in the form of a cascade. In such cascades, refrigeration
maybe
provided by methane, ethane and propane, or other hydrocarbons, with each
cycle of
the cascade operating at a lower temperature than the last.
In each cycle cool, compressed refrigerant is expanded, causing further
cooling, and then fed into a heat exchanger where it is placed in indirect
contact with
the natural gas. Heat from the natural gas warms and often vaporises the
refrigerant,
thus cooling the natural gas. The heated refrigerant exits the heat exchanger
and is
then compressed and cooled, whereupon the cycle is repeated. Often the
compressed refrigerant is cooled within the same heat exchanger as the natural
gas,
i.e. the compressed refrigerant is cooled by the same refrigerant in expanded
form.
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In a cascade system, in addition to cooling the natural gas, each cycle is
also
used to cool the refrigerants of the later, cooler refrigeration cycles. This
cooling
can take place in the same heat exchanger as the cooling of the natural gas or
in a
separate heat exchanger.
A cascade arrangement which uses mixed refrigerant streams is described in
WO 98/48227.
It will be appreciated that the use of hydrocarbons as refrigerants poses a
safety issue and this is particularly significant in the offshore environment,
where it
is highly undesirable to have large liquid hydrocarbon inventories in what is
inevitably a confined space.
Several systems have been proposed in which carbon dioxide acts as a
refrigerant fluid. For example, US 6023942 discloses a natural gas
liquefaction
process in which carbon dioxide maybe used as a refrigerant. This process is
not
suitable however for large scale or offshore applications, since it relies not
on a
cascade arrangement but on an open loop expansion process as the primary means
of
cooling the LNG (liquefied natural gas) stream. Expansion processes such as
this do
not allow sufficiently low temperatures to be attained, and hence the LNG has
to be
kept at very high pressures to maintain it in liquid form. Both from a safety
and an
economic point of view, these high pressures are not suitable for industrial
production of LNG and particularly not for large scale or offshore
applications.
US 2003/0089125 discloses the use of carbon dioxide within a closed loop
cascade system to pre-cool the natural gas. While this pre-cooling circuit
reduces
the amount of hydrocarbon refrigerant required, hydrocarbons are still used in
the
subsequent liquefaction and sub-cooling cycles. This is because carbon dioxide
cannot be cooled to low enough temperatures to fully liquefy natural gas
without
solidifying.
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Another known alternative is to. use a nitrogen refrigerant in a gas expansion
process. Traditionally this has the disadvantage that the thermal
efficiency'of
nitrogen is much lower than in a hydrocarbon based system. In addition,
because a
gaseous refrigerant has a low heat transfer coefficient compared to an
evaporating
refrigerant, a large heat transfer area is required to dissipate the waste
heat from the
process into a cooling medium.
US 6446465 discloses a liquefaction process using nitrogen in which two
separate streams of nitrogen are, used to liquefy the natural gas. The two
refrigeration streams are compressed and cooled whereupon one of these streams
is
fed through a heat exchanger where it is cooled, together with the natural
gas, which
has already been pre-cooled within a separate pre-cooling system. The cooled
nitrogen stream is then expanded to lower its temperature further and is used
within
a second heat exchanger to further cool the gas. The second nitrogen stream
meanwhile is expanded to the same pressure as the first nitrogen stream and
combined with the first stream upo'n'its exit from the second heat exchanger.
The
combined first and second refrigerant streams are then introduced back into
the first
heat exchanger to provide cooling to the natural gas and first, compressed
nitrogen
stream. As the natural gas is pre-cooled prior to reaching the nitrogen
refrigeration
circuit the power requirements of this circuit are significantly reduced. In
addition,
by feeding the second refrigerant stream directly to the expansion means
without
passing through the first heat exchanger the heat transfer area in the first
heat
exchanger is reduced.
US 2005/0056051 discloses a further liquefaction system in which a nitrogen
refrigeration circuit is used to cool at least partially liquefied natural
gas. The
natural gas is pre-cooled and substantially liquefied by a separate circuit in
which
hydrocarbons are used as the refrigerant. This substantially liquefied natural
gas is
then fed to the nitrogen cooling system for further cooling. In this document
a
number of configurations for the nitrogen refrigeration circuit are disclosed,
all of
which involve a first heat exchanger, in which expanded, low pressure nitrogen
cools the LNG, and a second heat exchanger in which the warmed, expanded
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nitrogen from the first heat exchanger is used to cool the compressed high
pressure
nitrogen prior to expansion. In several of these embodiments the first and
second
nitrogen streams are expanded to different pressures.
Therefore no system currently exists for providing complete liquefaction of
LNG without the use of hydrocarbons. Further, there is a need within the
industry to
provide a. nitrogen refrigeration system having reduced complexity, simple
operation
and higher efficiency. Such a system would provide large benefits,
particularly in
relation to offshore LNG production.
In accordance with one aspect of the present invention there is provided a
method of natural gas liquefaction comprising first and second nitrogen
refrigerant
streams, each stream undergoing a cycle of compression, cooling, expansion and
heating, during which the first nitrogen stream is expanded to a first,
intermediate
pressure and the second nitrogen stream is expanded to a second, lower
pressure,
and the heating occurs in one or more heat exchangers in which at least one of
the
expanded nitrogen streams is in heat exchanging relationship with natural gas,
wherein, in at least one of said one or more heat exchangers, the first and
second
expanded nitrogen streams are in a heat exchanging relationship with the
natural gas
and both the first and second compressed nitrogen streams.
Viewed from another aspect the present invention provides a natural gas
liquefaction apparatus comprising one or more heat exchangers for placing the
natural gas in a heat exchanging relationship with first and second nitrogen
refrigerant streams; one or more compressors for compressing the first and
second
nitrogen refrigerant streams; a first expander for expanding the first
nitrogen stream
to a first pressure and a second expander for expanding the second nitrogen
stream
to a second, lower pressure; wherein the apparatus is arranged such that, in
at least
one of said one or more heat exchangers, the first and second expanded
nitrogen
streams are in heat exchanging relationship with the natural gas and both the
first
and second compressed nitrogen streams.
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Therefore, in the present invention the nitrogen is expanded to two different
pressures. In comparison to a single expansion process the use of two
expanders
reduces the volume of nitrogen which must be expanded to and compressed from
the
lowest pressure and therefore reduces the required size and power consumption
of
these components.
Both the first, intermediate pressure, and second, lower pressure, nitrogen
streams are used to cool the natural gas as well as the compressed nitrogen
streams
prior to their expansion. Unlike prior art systems the natural gas is cooled
within
each nitrogen heat exchanger. In addition, in at least one heat exchanger the
expanded nitrogen streams are also used to cool the compressed streams. This
ensures that the maximum. amount. of heat exchange occurs between the
refrigerant
and the natural gas 'and allows this system to operate independently, i.e.
without the
need for additional heat exchanger circuits to cool the natural gas.
Preferably the liquefaction method comprises three cooling stages. These.
consist of a final cooling stage, in which the second nitrogen stream is
expanded to a
low pressure and placed in heat exchanging relationship with the natural gas,
an
intermediate stage, in which the first nitrogen stream is expanded to an
intermediate
pressure and placed in heat exchanging relationship with the natural gas to
cool the
natural gas prior to the final stage, and an initial cooling stage in which
the first and
second nitrogen streams, after undergoing heating in the final and/or
intermediate
stages, are placed in heat exchanging relationship with the natural gas to
cool this
prior to the intermediate stage.
Preferably the second nitrogen stream is also used within the intermediate
stage, after heating in the final stage; to provide further cooling to the
natural gas.
Preferably the intermediate stage also provides cooling for the compressed
second
nitrogen stream, prior to its expansion and use in the final stage.
The use of three nitrogen stages to cool natural gas is considered inventive
in
its own right and therefore, viewedtfrom a;further aspect the present
invention
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provides a natural gas liquefaction method in which natural gas is cooled by
first and
second nitrogen refrigerant streams, each stream undergoing a cycle of
compression,
cooling, expansion and heating, the method comprising three stages in which:
in an initial stage the heated, expanded first nitrogen stream and the heated,
expanded second nitrogen stream are used to cool the natural gas;
in an intermediate stage the compressed first nitrogen stream is expanded to
an intermediate pressure and used to cool the natural gas; and
in a final stage the compressed, second nitrogen stream is expanded to a low
pressure and used to cool the natural gas.
Preferably the heated expanded second nitrogen stream is also used to cool
the natural gas within the intermediate stage. Preferably the compressed
second
nitrogen stream is cooled in the intermediate stage.
By re-using the heated, expanded (low pressure) nitrogen streams to
contribute to earlier cooling stages this system can operate independently,
without
the need for an additional pre-cooling or liquefaction circuit. However, in
some
embodiments, the system can also be used with a pre-cooler which applies
external
pre-cooling (preferably in the range of 0 C to -60 C) of the natural gas, and
preferably also the nitrogen streams. Although the use of a pre-cooler
increases the
complexity of the system, it reduces the energy consumption of the system.
As only one refrigeration circuit is required the liquefaction apparatus is
greatly simplified.
Although described as independent streams, the first and second nitrogen
streams do not always need to be separate. It is possible for example for the
first
and second compressed streams to be combined during the first stage and during
compression. It is only necessary for the,streams to be transported separately
through the system at those stages of the cycle in which the streams are at
different
pressures.
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Preferably, in the initial stage the expanded first nitrogen stream and the
expanded second nitrogen stream are used to cool the compressed first and
second
nitrogen streams as well as the natural gas. This increases the efficiency of
the
process.
Preferably, the expanded first nitrogen stream is compressed from the
intermediate pressure after cooling the natural gas in the initial and
intermediate
stages. This reduces the energy required at the compression stage of the
process,
since only the expanded second nitrogen stream must be compressed from the
lower
pressure to a higher pressure. This is an improvement over existing processes,
in
which the entire refrigerant must be compressed from the lower pressure.
By intermediate pressure it is meant any pressure lower than that of the
compressed nitrogen stream but greater than that of the expanded second
refrigerant
stream. Preferably the intermediate pressure is in the range of 15-25 bar. By
low
pressure it is meant any pressure lower than the intermediate pressure.
Preferably
the low pressure is in the range of 5-20 bar.
The stages described by the present invention can each occur in a single heat
exchanger or multiple heat exchangers. Alternatively, one or more stages can
be
combined in a single heat exchanger and it is possible for all three stages to
occur
within a single heat exchanger. Therefore the cooling stages are not defined
by heat
exchangers but by the nitrogen streams which are used to provide cooling.
Preferably the first nitrogen stream, which is expanded to a first
intermediate
pressure, comprises a larger volume of nitrogen than the second nitrogen
stream.
This significantly reduces the volume of nitrogen being expanded to low
pressure
and thus reduces the power requirements of the low pressure expander. This.
also
reduces the power requirements of the low pressure compressor, or the first
stage of
a multistage compressor, which is used to compress the expanded second
refrigerant
stream.
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In a preferred embodiment the natural gas liquefaction method provides
complete cooling of the natural gas. That is the natural gas does not need to
undergo
any pre-cooling or partial liquefaction prior to cooling by the present
invention.
Instead the nitrogen refrigerant streams provide all necessary cooling to the
natural
gas from ambient temperature to storage temperature. This simplifies the
liquefaction system.
By pre-cooling it is meant cooling of the natural gas flow to a temperature at
which liquefaction of C3 components starts to occur. This allows these heavier
components to be separated out of the natural gas stream prior to further
cooling.
This is advantageous as otherwise these heavier components may "freeze out"
during liquefaction and impede the flow of natural gas. Typically the pre-
cooling
phase of a natural gas liquefaction process cools the gas to a temperature of
approximately -50 C.
By subcooling is meant cooling of the condensed liquefied gas below the
bubble point temperature.
In such "complete cooling" embodiments the outlet temperature of the
natural gas from the initial cooling stage is typically between -10 C and.-30
C.
Following passage through the intermediate stage, in which the first nitrogen
stream
is expanded, the outlet temperature is typically between -70 C and -90 C.
Following the final stage, in which the second nitrogen stream is expanded,
the
outlet temperature of the natural gas is typically in the range of -140 C to -
160 C.
This allows the natural gas to be stored and transported in a liquid state
without the
requirement for this to be pressurised. However, the methods and the apparatus
in
accordance with the present invention can produce liquid natural gas at
elevated
pressure (1 to 20 bar) with a corresponding temperature of -100 C to -165 C.
Preferably the method further includes the step of removing C5+
hydrocarbons from the natural gas. Most preferably C3+ hydrocarbons are
removed.
This prevents these heavier hydrocarbons from "freezing out". during
liquefaction.
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This separation step preferably occurs in between the pre-cooling and
liquefaction
phases. Most usually this point occurs within the intermediate cooling stage
however in some embodiments separation may take place between the initial and
intermediate stages.
The separation step is carried out by a heavy hydrocarbons removal column.
Such columns are well known in the art. As stated above, the exact location of
the
heavy hydrocarbon column (HHC) will depend on the temperature of the natural
gas
at differing points within the process.
Preferably the first and second nitrogen streams are compressed in a three
ti
stage compression process. This can be provided by individual compressors or
by a
multi-stage compressor. In the initial compression stage the expanded second
nitrogen stream is compressed to the intermediate pressure of the expanded
first
nitrogen stream and then cooled, preferably by sea water or air. The second
compressor stage compresses both the partially compressed second nitrogen
stream
and the expanded first nitrogen stream. The final compression stage compresses
both first and second nitrogen streams. Any of these stages can be provided by
two
or more parallel compressors. Such an arrangement enables the first and second
expanders to drive the third compressor stage, which makes the system more
efficient. Alternatively a two stage compression process can be provided.
Viewed from a further aspect the present invention provides.a natural gas
liquefaction apparatus arranged to carry out the method of the present
invention.
Preferred embodiments of the. present invention will now be described, by
way of example only, with reference to the accompanying figures in which :
Figure 1 shows a natural gas liquefaction process comprising two nitrogen
refrigerant circuits in accordance with the present invention;
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Figure 2 shows a further embodiment of the present invention in which a
heavy hydrocarbon column is used.
Figure 1 shows a natural gas liquefaction system 100. Feed gas enters the
system via line 1. This gas could be at ambient temperature or pre-cooled via
heat
exchange with air or water. The feed gas is successively cooled by a number of
heat
exchangers 101, 102, 103 such that sub-cooled liquefied natural gas exits the
last
heat exchanger 103 via line 4. This LNG is expanded to atmospheric pressure by
expansion valve 109 and fed via line 5 into separator column 110. LNG exits
this
column 110 via bottom stream 6 while any remaining gaseous elements are
removed
via line 7 for further cooling.
The natural gas is cooled within heat exchangers 101, 102, 103 by first and
second nitrogen streams 121, 122. These streams 121, 122 are cooled
collectively
by a compression system 108, which will be described later. The combined
streams
exit the compressor system via line 22 and are then split prior to entry into
the first
heat exchanger 101. Both nitrogen streams 121, 122 are cooled within this heat
exchanger 101. The first nitrogen stream 121 exits the heat exchanger via line
24
and is expanded to an intermediate pressure by expander 111. = The expanded
first
nitrogen stream 121 is then fed via line 25 into second heat exchanger 102 as
a
cooling fluid. The expanded first nitrogen stream 121 cools both the natural
gas and
the second nitrogen stream 122 within this heat exchanger 102. Upon exit of
the
second heat exchanger 102 the warmed expanded first nitrogen stream 121 is re-
introduced into the first heat exchanger 101 via line 26. Here it again acts
to cool
the natural gas as well as the compressed first and second nitrogen streams
121,.122.
The warmed, expanded nitrogen stream 121 is then directed through line 27 to
the
compressor system 108.
After cooling in the first heat exchanger 101 compressed second nitrogen
stream 122 is fed via line 29 into, the second heat exchanger 102 for further
cooling.
After exiting the second heat exchanger 102 the compressed second nitrogen
stream
122 is fed via line 30 into second. expander 112 for expansion to a pressure
lower
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than the intermediate pressure provided by expander 111. The expanded, cooled
second nitrogen stream 122 is then fed via line 31 into third heat exchanger
103 for
heat exchange with the natural gas. Upon exit from the third heat exchanger
103 the
warmed, expanded second nitrogen stream is fed into the second heat exchanger
102
via line 32 to assist in cooling the natural gas and compressed second
nitrogen
stream 122 before finally being fed into the first heat exchanger 101 by line
33 to
assist in the initial stage cooling of the natural gas and first and second
compressed
nitrogen streams 121, 122. Following exit from the first heat exchanger 101
the
warmed, expanded second nitrogen stream 122 is returned to the compressor
system
108 byline 10.
In the present embodiment the compressor system 108 comprises three
compressor stages. The first compressor stage comprises a single compressor
113
which compresses the low pressure warmed second nitrogen stream 122, delivered
by line 10 from the first heat exchanger 101. The partially compressed second
nitrogen stream is then combined with the warmed intermediate pressure first
nitrogen stream 121 provided byline 27. The combined first and second nitrogen
streams 121, 122 are then further compressed in second stage compressor 114.
The
last compressor stage comprises two compressors 115a, 115b which operate in
parallel and are driven by expanders 111, 112. The combined first and second
nitrogen streams 121, 122 are split into lines .16, 19 and compressed within
the third
stage compressors 11 5a, 11 5b. The splitting of combined nitrogen streams
121, 122
at this point does not necessarily result in all of first nitrogen stream
passing through
one third stage compressor while the second nitrogen stream passes through the
other. Instead, the compressed streams can be split in any ratio at this
point. In
between each compression stage the nitrogen is cooled by heat exchangers 116,
117,
118a and 118b. After final stage compression the combined nitrogen streams are
returned to line 22 for separation and re-introduction into the first heat
exchanger
101.
In the above described embodiment of the present invention nitrogen streams
121 and 122 provide all necessary cooling to the natural gas. The first heat
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exchanger 101 cools the natural gas to between -10 C and -30 C, the second
heat
exchanger 102 to between -70 C and -90 C while the final heat exchanger 103
cools the natural gas to between -140 C to -160 C. Expander 111 typically
expands
the first nitrogen stream 121 to a pressure of 15-20 bar while expander 112
typically
expands the second nitrogen stream 122 to a pressure of 5-20 bar. The first
and
second nitrogen streams 121, 122 do not contain the same volume of nitrogen.
Instead the largest flow is found in the first nitrogen stream 121. This
reduces the
power requirements of the low pressure expander 112 and first stage compressor
113.
The use of the warmed expanded nitrogen streams 121, 122 to continue to
provide cooling within the earlier heat exchangers ensures that the system is
efficient and allows the nitrogen to provide complete cooling without relying
on any
other refrigeration means to provide liquefaction, partial liquefaction or pre-
cooling.
Figure 2 shows a very similar refrigeration system 200 to Figure 1. Identical
components have been indicated by use of the same reference numerals. Again
first
and second nitrogen streams 121, 122 are cooled collectively by compression
system
108 and further cooled within heat exchanger 101. The first refrigerant stream
121
is. then sent via line. 24 to expander 111 and.expanded to an intermediate
pressure.
The expanded nitrogen is then fed via line 25 into the intermediate, or second
cooling stage. Unlike the system of Figure 1, in this system 200 the
intermediate
cooling stage occurs in two separate heat exchangers 202a, 202b. In both of
these
heat exchangers 202a, 202b the expanded first refrigerant stream 121 and the
warmed, expanded second refrigerant stream 122 exiting heat exchanger 103 are
used to provide cooling to the natural gas and the compressed second nitrogen
stream 122..
In between heat exchangers 202a, 202b the natural gas is fed via line 2a into
a Heavy Hydrocarbon Column (HHC) 219. This separates the heavier components,
such as C3+, from the natural gas stream. These heavier components are removed
via line 8 while the natural gas stream is .fed via line 2b to heat exchanger
202b to
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continue the cooling process. The natural gas is diverted and fed through the
HHC
219 at a stage in the cooling process at which pre-cooling has occurred but
prior to
liquefaction. Removing heavier hydrocarbons at this stage prevents these from
"freezing out" during later parts of the cooling process.
The second nitrogen stream 122, which is cooled in the intermediate stage, is
not fed through the HHC 219 but is transferred straight from the heat
exchanger
202a to heat exchanger 202b via line 29a. Similarly, the expanded first
refrigerant
stream and warmed, expanded second nitrogen stream are transported via lines
25a
and 32a respectively directly between the heat exchangers 202a, 202b.
The remainder of the process 200 is identical to that of process 100.
