Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR PRODUCING A LIQUEFIED
HYDROCARBON STREAM
The present invention relates to a method and
apparatus for producing a liquefied hydrocarbon stream.
Liquefied natural gas (LNG) forms an economically
important example of such a cryogenic hydrocarbon stream.
Natural gas is a useful fuel source, as well as a source
of various hydrocarbon compounds. It is often desirable
to liquefy natural gas in a liquefied natural gas plant
at or near the source of a natural gas stream for a
number of reasons. As an example, 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
pressure.
NO 2006/120127 describes an LNG separation process
and installation. Liquefied natural gas in liquid form
is sent to a separation unit, wherein a stream of LNG
purified of nitrogen, and a nitrogen-enriched vapour are
produced. The separation unit employs two columns. An
LNG stream which has been liquefied in a liquefier is
first separated in a first column operating at about 1.25
bar producing a nitrogen-depleted liquid and an overhead
gas stream. The overhead gas stream is recompressed to
about 4 bar and passed to a second column, where any
remaining methane is recondensed. The recondensed
methane is withdrawn as liquid from the second column and
mixed with the nitrogen-depleted liquid from the first
column to form the stream of LNG purified of nitrogen.
Gaseous nitrogen is withdrawn from the top of the second
column, allowing for the nitrogen contained in the
natural gas to be utilized at commercial purity.
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The refrigeration or said recondensing of the methane
in the second column is provided by a nitrogen cycle
independent of the liquefier, which employs a refrigerant
fluid is of which the nitrogen content is greater than
80 mol9o.
A drawback of this LNG separation process is that an
independent refrigeration cycle is required which
involves both capital expenditure as well as operational
expenditure. Moreover, as the recondensed methane is
added to the purified LNG stream, it becomes increasingly
demanding to maintain the nitrogen level in the purified
LNG stream below the specification required for
commercial LNG.
The present invention provides a method of producing
a liquefied hydrocarbon stream, comprising:
- providing a cryogenic hydrocarbon composition
comprising a nitrogen- and methane-containing liquid
phase at an initial pressure of between 1 and 2 bar
absolute;
- phase separating the cryogenic hydrocarbon
composition, in an end flash separator at a first
separation pressure of between 1 and 2 bar absolute, into
a vaporous reject stream and a liquid stream;
- discharging the liquid stream from the end flash
separator in the form of the liquefied hydrocarbon
stream;
- compressing the vaporous reject stream in an end-flash
compressor to a pressure of above 2 bar absolute, thereby
obtaining a compressed vapour stream;
- forming a partially condensed intermediate stream from
the compressed vapour, said partially condensed
intermediate stream comprising a condensed fraction and a
vapour fraction, by partially condensing the compressed
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vapour stream, wherein said partially condensing
comprises indirectly heat exchanging the compressed
vapour stream against an auxiliary refrigerant stream
formed by a slip stream of the liquefied hydrocarbon
stream whereby passing heat from at least part of the
compressed vapour stream to the auxiliary refrigerant
stream;
separating the condensed fraction from the vapour
fraction in a gas/liquid separator, at a second
separation pressure;
- discharging the vapour fraction from the gas/liquid
separator, said vapour fraction having a first heating
value;
combusting the vapour fraction in a combustion device
other than a gas turbine;
- discharging the condensed fraction from the
gas/liquid separator;
revaporizing the condensed fraction whereby
transforming the condensed fraction into a fully
vaporized stream having a second heating value that is
higher than the first heating value;
- combusting the fully vaporized stream in a gas
turbine.
In another aspect, the present invention provides an
apparatus for producing a liquefied hydrocarbon stream,
comprising:
- a cryogenic feed line connected to a source of a
cryogenic hydrocarbon composition comprising nitrogen and
a methane-containing liquid phase
- an end-flash separator arranged to receive the
cryogenic hydrocarbon composition and to separate the
cryogenic hydrocarbon composition into a liquid stream
and vaporous reject stream;
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- a liquid hydrocarbon product line fluidly connected
to a bottom part of the end-flash separator to discharge
said liquid stream in the form of the liquefied
hydrocarbon stream from the end-flash separator;
- a vapour reject line fluidly connected to an overhead
part of the end-flash separator to discharge said
vaporous reject stream from the end-flash separator;
- an end-flash compressor arranged in the vapour reject
line to compress the vaporous reject stream, thereby
obtaining a compressed vapour stream;
- a condenser arranged in the vapour reject line
downstream of the end-flash compressor, arranged to
receive the compressed vapour stream and to form a
partially condensed intermediate stream from the
compressed vapour stream, said partially condensed
intermediate stream comprising a condensed fraction and a
vapour fraction, which condenser is arranged to establish
indirect heat exchanging contact between at least part of
the compressed vapour stream and an auxiliary refrigerant
stream;
- an auxiliary refrigerant feed line extending between
the liquid hydrocarbon product line and the condenser, to
supply a slip stream of the liquid hydrocarbon product
stream to the condenser;
- a gas/liquid separator arranged downstream of the
condenser and arranged to receive the condensed fraction
and vapour fraction;
- a vapour fraction discharge line fluidly connected
with an overhead part of the gas/liquid separator
arranged to receive the vapour fraction from the
gas/liquid separator;
- a combustion device other than a gas turbine fluidly
connected with the gas/liquid separator by means of the
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vapour fraction discharge line to receive and combust the
discharged vapour fraction;
- a condensed fraction discharge line fluidly connected
with a bottom part of the gas/liquid separator arranged
to receive the condensed fraction from the gas/liquid
separator;
- a gas turbine fluidly connected with the gas/liquid
separator by means of the condensed fraction discharge
line to receive and combust the discharged condensed
fraction;
- a revaporizer arranged in the condensed fraction
discharge line between the gas/liquid separator and the
gas turbine and arranged to transform the condensed
fraction into a fully vaporized stream prior to
combustion in the gas turbine.
The invention will be further illustrated
hereinafter, using examples and with reference to the
drawing in which;
Fig. 1 schematically represents a process flow scheme
representing a method and apparatus according to an
embodiment of the invention;
Fig. 2 schematically represents an embodiment of a
pressure reduction system for use in the invention;
Fig. 3 schematically represents a process flow scheme
representing a method and apparatus according to another
embodiment of the invention;
Fig. 4 schematically represents a process flow scheme
representing a method and apparatus according to still
another embodiment of the invention;
Fig. 5 schematically represents a process flow scheme
wherein the embodiment of Fig. 3 is applied in
combination with a selected liquefier;
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Fig. 6 schematically represents a process flow scheme
wherein the embodiment of Fig. 4 is applied in
combination with the selected liquefier; and
Fig. 7 schematically represents a process flow scheme
wherein the embodiment of Fig. 4 is applied with a
specific type of end-flash separator.
In these figures, same reference numbers will be used
to refer to same or similar parts. Furthermore, a single
reference number will be used to Identify a conduit or
line as well as the stream conveyed by that line.
The present description concerns producing of a
liquefied hydrocarbon stream, such as for instance a
liquefied natural gas stream. A cryogenic hydrocarbon
composition is first separated into a vaporous reject
stream and a liquid stream. The liquid stream is
discharged in the form of the liquefied hydrocarbon
stream. The vaporous reject stream is recompressed,
partially condensed by indirectly heat exchanging the
compressed vapour stream against an auxiliary refrigerant
stream, and separated. The condensed fraction is
revaporized and combusted in a gas turbine. This fuel
gas vapour stream is identified as high quality fuel gas
stream.
The vapour fraction, which generally has a higher
nitrogen content and a lower heating value than the
condensed fraction, is combusted in a combustion device
other than a gas turbine. In the context of the present
description and compared to the condensed fraction, this
fuel is referred to as low quality fuel gas. Low quality
in this context means having a heating value that is
lower compared to the heating value of the high quality
fuel gas vapour stream, which is combusted in the gas
turbine.
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The auxiliary refrigerant stream is advantageously
formed by a slip stream of the liquefied hydrocarbon
stream. No high degree of separation between the methane
and the nitrogen in the intermediate condensed stream
formed from the vaporous reject stream is required, as
both the vapour fraction and the condensed fraction are
combusted. Therefore the vapour fraction does not have
to be free from methane while the condensed fraction is
bound to less stringent requirements for its nitrogen
content than if it would be added to the liquefied
hydrocarbon stream.
The proposed method and apparatus thus do not require
a full nitrogen rejection unit, since a combustible fuel
gas stream is produced instead of a ventable nitrogen
stream.
The proposed method and apparatus can be
advantageously applied for instance if the raw liquefied
stream comprises in the range of from 1 mol% to 7 mol%
nitrogen. However, most benefit is enjoyed in cases
wherein the raw liquefied stream comprises more than
3 mol% of nitrogen, as in such cases a relatively high
flow rate of vaporous reject gas is generated in order to
maintain the liquid stream from which the liquefied
hydrocarbon stream is derived within specification with
regard to maximum content of lower boiling constituents
such as nitrogen in commercially tradable liquefied
natural gas. The high flow rate of vaporous reject gas
generally contains too much nitrogen for use as fuel in
gas turbines, and it usually exceeds plant fuel
requirements if gas turbines are used to drive the
refrigeration cycles in the liquefier.
More than 30 mol% of the vaporous reject stream
and/or more than 30 mol% of the partially condensed
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i nt e rmedi at e stream may consist of nitrogen. Such
nitrogen content would be too high to meet the fuel gas
requirements of most gas turbines. The proposed method
and apparatus may then be advantageously employed to
recondense a fraction of the vaporous reject stream, to
obtain a condensed fraction of which less than 30 mol%
consists of nitrogen so that, after revaporization, it
can be used to fuel a gas turbine.
If the nitrogen content is still too high for the
selected gas turbine, the condensed fraction (preferably
after revaporization) may be blended with other fuel gas
to bring the fuel on specification. In such cases the
invention provides the benefit that the blending
requirements are less demanding than if the fuel gas had
more than 30 mol% of nitrogen.
The revaporized condensed fraction may have to be
subjected to compression in order to meet a pre-
determined gas turbine fuel gas pressure specification.
Alternatively, the condensed fraction may be pressurized,
e.g. by means of a liquid pump, before revaporizing so
that the condensed fraction can be revaporized at a
pressure that is already sufficiently high to meet the
fuel pressure specification of the gas turbine in which
the revaporized condensed fraction will be combusted.
The cryogenic hydrocarbon composition may be obtained
by subjecting a raw liquefied hydrocarbon stream to a
pressure reduction step.
The cryogenic hydrocarbon composition may be sourced
from a liquefier. Such liquefier may comprise a
refrigerant circuit for cycling a refrigerant stream.
The refrigerant circuit may comprise a refrigerant
compressor coupled to a refrigerant compressor driver,
and arranged to compress the refrigerant stream; and a
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cryogenic heat exchanger arranged to establish an
indirect heat exchanging contact between a hydrocarbon
stream and the refrigerant stream of the refrigerant
circuit, whereby a raw liquefied stream is formed out of
the hydrocarbon stream comprising a subcooled hydrocarbon
stream. The liquefier may further comprise a pressure
reduction system arranged downstream of the cryogenic
heat exchanger and in fluid communication therewith, to
receive the raw liquefied stream and to reduce pressure
of the raw liquefied stream. A rundown line may fluidly
connect the pressure reduction system with the cryogenic
heat exchanger to establish fluid communication for the
raw liquefied stream to pass from the cryogenic heat
exchanger to the pressure reduction system, wherein the
end-flash separator is arranged downstream of the
pressure reduction system and in fluid communication
therewith to receive the cryogenic hydrocarbon
composition from the pressure reduction system.
Suitably, the gas turbine in which the condensed fraction
is revaporized and combusted is the refrigerant
compressor driver of the refrigerant circuit in the
liquefier. The gas turbine is preferably selected from
the group consisting of aeroderivative gas turbines.
Accordingly, the method may suitably comprise cycling
a refrigerant stream in the liquefier, comprising driving
a refrigerant compressor and compressing said refrigerant
stream in the refrigerant compressor. A hydrocarbon
stream may be condensed and subcooled, comprising
indirectly heat exchanging said hydrocarbon stream
against the refrigerant stream in the liquefier, thereby
forming a raw liquefied stream at a liquefaction pressure
of higher than 2 bara. The raw liquefied stream may be
passed through a pressure reduction step, thereby
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obtaining the cryogenic hydrocarbon composition
comprising nitrogen and a methane-containing liquid
phase. Suitably, the refrigerant compressor is driven by
the mentioned gas turbine in which the fully vaporized
condensed fraction is combusted.
The slip stream for the auxiliary refrigerant stream
is preferably formed of a part of the liquefied
hydrocarbon stream. An advantage of employing a slip
stream of the liquefied hydrocarbon stream for this
purpose is that it can relatively easily be implemented
on an already existing plant without the need to
interrupt or modify any part relating to the source of
the cryogenic hydrocarbon composition. Moreover, it is
the coldest stream readily available in the plant,
without the need for providing a dedicated refrigeration
cycle, and there is generally plenty of it.
An auxiliary refrigerant return line suitably extends
between the condenser and the end-flash separator.
Herewith, the auxiliary refrigerant stream containing
heat from the at least part of the compressed vapour
stream can be passed to and into the end-flash separator,
such that a half-open refrigeration cycle is formed.
Preferably, a pump is configured in the auxiliary
refrigerant line wherein the slip stream of the liquefied
hydrocarbon stream can be pumped to the condenser.
Figure 1 illustrates an embodiment of the invention.
A cryogenic hydrocarbon composition comprising a
nitrogen- and methane-containing liquid phase is conveyed
in a cryogenic feed line 8. The source of the cryogenic
hydrocarbon composition is not a limitation of the
invention in its broadest definition, but for the sake of
completeness one embodiment is illustrated wherein the
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cryogenic hydrocarbon composition is sourced from a
liquefier 100.
Such a liquefier 100 would typically be provided
upstream of the cryogenic feed line 8. The liquefier 100
may be in fluid communication with the cryogenic feed
line 8 via a pressure reduction system 5, which
communicates with the liquefier 100 via a rundown line 1.
The pressure reduction system 5 is arranged downstream of
the cryogenic heat exchanger 180 and arranged to receive
and reduce the pressure of a raw liquefied stream from
the main cryogenic heat exchanger 5.
The pressure reduction system 5 may comprise a
dynamic unit, such as an expander turbine, a static unit,
such as a Joule Thomson valve, or a combination thereof.
An example of a pressure reduction system 5 with a Joule
Thomson valve 7 in series with an expander turbine 6 is
shown in Fig. 2. If an expander turbine is used, it may
optionally be drivingly connected to a power generator.
Many arrangements are possible and known to the person
skilled in the art.
In the example embodiment shown in Fig. 1, liquefier
100 comprises a refrigerant circuit 101 for cycling a
refrigerant. The refrigerant circuit 101 comprises a
refrigerant compressor 160 coupled to a refrigerant
compressor driver 190 in a mechanical driving engagement.
The refrigerant compressor 160 is arranged to compress a
spent refrigerant stream 150 and to discharge the
refrigerant, in a pressurized condition, into a
compressed refrigerant line 120. At least one reject
heat exchanger 124 is normally provided in the compressed
refrigerant line 120 of the refrigerant circuit 101. The
reject heat exchanger 124 is arranged to reject heat from
the pressurized refrigerant stream carried in the
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compressed refrigerant line 120 to the ambient, either to
the air or to a body of water such as a lake, a river, or
the sea.
The liquefier 100 typically comprises a refrigerant
refrigerator arranged to refrigerate the pressurized
refrigerant from the compressed refrigerant line 120 from
which heat has been rejected in the reject heat exchanger
124. Herewith a refrigerated refrigerant stream is
obtained in a refrigerated refrigerant line 131.
The liquefier 100 further comprises a cryogenic heat
exchanger 180 connected to the refrigerant compressor 160
discharge outlet via the compressed refrigerant line 120.
In the embodiment of Figure 1, the cryogenic heat
exchanger 180 also fulfils the function of the
refrigerant refrigerator discussed in the previous
paragraph, but this is not a requirement of the
invention. The cryogenic heat exchanger is generally
arranged to establish an indirect heat exchanging contact
between a hydrocarbon stream 110 and the refrigerant of
the refrigerant circuit 101.
A spent refrigerant line 150 connects the cryogenic
heat exchanger 180 with a main suction end of the
refrigerant compressor 160. The refrigerated refrigerant
line 131 is in fluid communication with the spent
refrigerant line 150, via a cold side of the cryogenic
heat exchanger 180. The hydrocarbon stream 110 flows
through a warm side of the cryogenic heat exchanger 180.
The cold side and the warm side are in heat exchanging
contact with each other.
A main refrigerant return line 133 establishes fluid
communication between the refrigerated refrigerant line
131 and the cold side of the cryogenic heat exchanger
180. The main refrigerant return line 133 is in fluid
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communication with the spent refrigerant line 150, via
said cold side and in heat exchanging arrangement with
the hot side. A main refrigerant control valve 134 is
configured in the main refrigerant return line 133.
The cryogenic heat exchanger 180 receives the
refrigerant stream in a depressurized condition from the
main refrigerant return line 133 via the main refrigerant
control valve 134, and discharges into the refrigerant
compressor 160. Thus, the cryogenic heat exchanger 180
forms part of the refrigerant circuit 101.
The cryogenic heat exchanger 180 may be provided in
any suitable form, including a printed circuit type, a
plate fin type, optionally in a cold box configuration,
or a tube-in-shell type heat exchanger such as a coil
wound heat exchanger or a spool wound heat exchanger.
A specific non-limiting example of the liquefier and
its refrigerant circuit based on a tube-in-shell type
heat exchanger and including the refrigerant compressor
and the cryogenic heat exchanger is shown in Figures 5
and 6. These figures will be described in detail later
below.
Back to the invention, an end-flash separator 50 is
arranged to receive the cryogenic hydrocarbon composition
8, optionally downstream of the pressure reduction system
5 and in fluid communication therewith, if such system is
provided. Depending on the separation requirements, the
end flash separator 50 may be provided in the form of a
simple drum which separates vapour from liquid phases in
a single equilibrium stage (such as depicted in Fig. 1),
or a more sophisticated distillation column. Non-
limiting examples of possibilities are disclosed in US
Patents 5,421,165; 5,893,274; 6,014,869; 6,105,391; and
pre-grant publication US 2008/0066492.
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A liquid hydrocarbon product line 90 is fluidly
connected to a bottom part of the end-flash separator 50.
The liquid hydrocarbon product line 90 connects the end-
flash separator 50 to a cryogenic storage tank 210. An
optional cryogenic pump (not shown) may be present in the
liquid hydrocarbon product line 90, to assist the
transport of any liquid hydrocarbon product that is being
discharged from the end-flash separator 50 to the
cryogenic storage tank 210.
A vapour reject line 64 is fluidly connected to an
overhead part of the end-flash separator 50. An end-
flash compressor 260 is arranged in the vapour reject
line 64, to compress the vaporous reject stream from the
end-flash separator 50. A condenser 35 is arranged in
the vapour reject line 64, downstream of the end-flash
compressor 260. This part of the vapour reject line will
be referred to as compressed vapour stream line 70.
The condenser 35 is arranged to receive the
compressed vapour stream, and to form a partially
condensed intermediate stream from the compressed vapour
stream. The condenser is arranged to establish indirect
heat exchanging contact between at least part of the
compressed vapour stream, and an auxiliary refrigerant
stream.
An aftercooler 69 may be provided in the compressed
vapour stream line 70 between the end-flash compressor
260 and the condenser 35. The aftercooler is arranged to
reject heat from the compressed vapour to the ambient
(for instance by heat exchanging against an ambient air
stream or an ambient water stream). Such an aftercooler
is recommended in embodiments where the temperature of
the compressed vapour stream as it is discharged from the
end-flash compressor exceeds the temperature of the
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ambient air and/or ambient water so that at least part of
the heat added to the vapour in the end-flash compressor
can be rejected to the ambient.
A cold recovery heat exchanger 65 may optionally be
provided in the vapour reject line 64 resulting in that
the reject vapour is fed into the end flash compressor
260 at an end-flash compressor suction temperature that
is higher than the temperature at which the reject vapour
is discharged from the end-flash separator 50 into the
vapour reject line 64. Herewith the cold vested in the
reject vapour in vapour reject 64 is preserved in a cold
recovery stream 66, by heat exchanging against the cold
recovery stream 66 prior to compressing the reject vapour
to the end-flash compressor 260.
In one embodiment, the cold recovery stream 66 may
comprise or consist of a side stream sourced from the
hydrocarbon stream 110 in the liquefier 100. The
resulting cooled side stream may for instance be combined
with the cryogenic hydrocarbon composition in the
cryogenic feed line B. Thus, the cold recovery heat
exchanging in the cold recovery heat exchanger 65
supplements the production rate of the cryogenic
hydrocarbon composition.
In another embodiment, the cold recovery stream 66
may comprise or consist of a refrigerant stream being
cycled in the liquefier 100 whereby the refrigerant
stream (or a slipstream thereof) is condensed or sub-
cooled. For instance, a slip stream of the compressed
refrigerant can be drawn from the compressed refrigerant
line 120 and be refrigerated by vapour reject line 64.
In still another embodiment, the cold recovery stream
66 may comprise or consist of the aftercooled reject
vapour in the compressed vapour stream line 70,
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preferably in the part of the compressed vapour stream
line 70 that extends between the aftercooler 69 and the
condenser 35 through which the compressed vapour is
passed from the end-flash compressor 260 to the condenser
35. Herewith the duty required from the auxiliary
refrigerant stream 132 in the condenser 35 would be
reduced.
The auxiliary refrigerant stream is supplied from an
auxiliary refrigerant feed line 132 that extends between
the liquid hydrocarbon product line 90 and the condenser
35. In the example as shown, the liquefied hydrocarbon
product line 90 is split into the auxiliary refrigerant
feed line 132 and a main product line 91. An auxiliary
refrigerant return line 138 extends between the condenser
35 and the end-flash separator 50, and is arranged to
return the auxiliary refrigerant containing heat from the
compressed vapour stream back to the end-flash separator
50. An auxiliary refrigerant control valve 135 is
arranged in the auxiliary refrigerant feed line 132. An
optional auxiliary refrigerant pump (not shown) may
optionally be provided in the auxiliary refrigerant feed
line 132.
A gas/liquid separator 33 is arranged downstream of
the condenser 35. A vapour fraction discharge line 80 is
fluidly connected with an overhead part of the gas/liquid
separator 33 and a condensed fraction discharge line 40
is fluidly connected with a bottom part of the gas/liquid
separator 33. A gas turbine 320 is fluidly connected
with the gas/liquid separator, by means of the condensed
fraction discharge line 40. A combustion device 220
other than a gas turbine is fluidly connected with the
gas/liquid separator by means of the vapour fraction
discharge line 80.
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The combustion device 220 may comprise multiple
combustion units. It may include, for example, one or
more of a furnace, a boiler, an incinerator, a dual fuel
diesel engine, or cross-combinations thereof. A boiler
and a duel fuel diesel engine may advantageously be
coupled to an electric power generator.
A revaporizer 285 is arranged in the condensed
fraction discharge line 40, between the gas/liquid
separator 33 and the gas turbine 320. The revaporizer is
arranged to bring the condensed fraction in the condensed
fraction discharge line 40 in indirect heat exchanging
contact with a heating fluid 286 whereby during operation
heat is transferred from the heating fluid 286 to the
condensed fraction in the condensed fraction discharge
line 40. Optionally, a fuel gas compressor 360 is
arranged in the condensed fraction discharge line 40
between the revaporizer 285 and the gas turbine 320.
A cold recovery heat exchanger 85 may optionally be
provided in the vapour fraction discharge line 80 to
recover cold vested in the vapour fraction prior to
combusting it in the combustion device 220. The cold
recovery heat exchanger 85 is arranged to bring the
vapour fraction in the vapour fraction discharge line 80
in indirect heat exchanging contact with a cold recovery
stream 86. During operation heat is transferred from the
cold recovery stream 86 to the vapour fraction in the
vapour fraction discharge line 80. This cold recovery
heat exchanger 85 may be referred to as second cold
recovery heat exchanger in embodiments wherein the cold
recovery heat exchanger 65 is provided in the vapour
reject line 64. In such embodiments, the cold recovery
heat exchanger 65 in the vapour reject line 64 may be
referred to as first cold recovery heat exchanger.
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The apparatus described above may be used in a method
described as follows.
A cryogenic hydrocarbon composition 8 comprising a
nitrogen- and methane-containing liquid phase is provided
at an initial pressure of between 1 and 2 bar absolute,
and an initial temperature. Providing of the cryogenic
hydrocarbon composition 8 may comprise passing a
hydrocarbon stream 110 through the liquefier 100. The
hydrocarbon stream 110 may be condensed and subcooled in
the liquefier 100. The condensing and subcooling of the
hydrocarbon stream 110 preferably involves indirectly
heat exchanging the hydrocarbon stream 110 against the
refrigerant in the liquefier 100. The thus formed
subcooled liquefied hydrocarbons stream is referred to as
the raw liquefied stream. Thus the raw liquefied stream
is formed out of the hydrocarbon stream by condensing and
subsequently subcooling the hydrocarbon stream.
For example, in such a liquefier 100, the hydrocarbon
stream 110 comprising a hydrocarbon-containing feed
vapour may be heat exchanged, for example in the
cryogenic heat exchanger 180, against a main refrigerant
stream, thereby liquefying the feed vapour of the feed
stream to provide the raw liquefied stream within the
rundown line 1. The desired cryogenic hydrocarbon
composition 8 may then be obtained from the raw liquefied
stream 1. The raw liquefied stream may be discharged in
the rundown line 1 from the liquefier 100. The cryogenic
hydrocarbon composition 8 may be obtained from the raw
liquefied stream, for instance by passing the raw
liquefied stream through a pressure reduction step in
pressure reduction system 5. In this pressure reduction
step, the pressure may be reduced from the liquefaction
pressure to the initial pressure.
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The cryogenic hydrocarbon composition 8 is
subsequently phase separated, at a first separation
pressure of between 1 and 2 bar absolute, into a vaporous
reject stream 64 and a liquid stream 90. Suitably, this
phase separating is performed in the end-flash separator
50. The vaporous reject stream 64 comprises a majority
of, preferably all of, any flash vapour that has been
generated during the pressure reduction step. The liquid
stream 90 is discharged in the form of the liquefied
hydrocarbon stream, which may be a liquefied natural gas
stream provided that the methane content is at least
81 mo195. The liquid stream 90 is typically conveyed to
the cryogenic storage tank 210.
The vaporous reject stream 64 is discharged from the
end-flash separator 50, and subsequently compressed in
the end-flash compressor 260 to a pressure of above 2 bar
absolute, thereby obtaining a compressed vapour stream
70.
The compressed vapour stream 70 is passed to the
condenser 35. If an aftercooler 69 is provided in the
compressed vapour stream line 70, the compressed vapour
stream 70 is passed through the aftercooler 69 as it is
being passed to the condenser 35. In the aftercooler 69,
heat is rejected from the compressed vapour to the
ambient (for instance by heat exchanging against an
ambient air stream or an ambient water stream). The
compressed vapour 70 is discharged from the optional
aftercooler 69 at an aftercooled temperature that is
close to ambient temperature, for instance 2 C above
ambient temperature. Ambient temperature is considered
to be the temperature of the ambient medium (air or
water) to which the heat is rejected.
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In the condenser 35, a partially condensed
intermediate stream is formed from the compressed vapour
70, by partially condensing the compressed vapour stream.
The partially condensed intermediate stream comprises a
condensed fraction 40 and a vapour fraction 80. The
partially condensing comprises indirect heat exchanging
of the compressed vapour stream 70 against an auxiliary
refrigerant stream 132, formed by a slip stream of the
liquid stream 90. During the indirect heat exchanging,
heat passes from, at least part of, the compressed vapour
stream 70 to the auxiliary refrigerant stream 132. It is
conceived that only about 0.2 % of the liquid stream 90
is needed as the auxiliary refrigerant stream 132.
Generally, between 0.05 and 0.40 % of the liquid stream
90 may be needed as the auxiliary refrigerant stream 132.
The condensed fraction 40 is then separated from the
vapour fraction 80 in the gas/liquid separator 33, at a
second separation pressure. Preferably, the vapour
fraction and the condensed fraction co-exist in the
gas/liquid separator 33, and are separated in a single
thermodynamic equilibrium state between said vapour
fraction and the condensed fraction residing inside the
gas/liquid separator 33. This can generally be achieved
if the gas/liquid separator 33 is embodied in the form of
simple drum with no gas/liquid contacting internals such
as trays or packing, thus essentially representing one
single theoretical stage.
The vapour fraction 80 is discharged from the
gas/liquid separator 33, typically as a vapour phase in
its dew point. The vapour fraction 80, as it is being
discharged from the gas/liquid separator 33, has a first
heating value. The vapour fraction 80 is combusted in
the combustion device 220.
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The condensed fraction 40 is also discharged from the
gas/liquid separator 33, but as liquid phase at its
bubble point. The condensed fraction 40 is subsequently
revaporized in revaporizer 285. The revaporizing
comprises bringing the condensed fraction 40 in indirect
heat exchanging contact with the heating fluid 286,
whereby heat is transferred from the heating fluid 286 to
the condensed fraction 40. During revaporizing the
condensed fraction 40 is transformed into a fully
vaporized stream having a second heating value. After
revaporizing, the condensed fraction 40 is fully in
vapour phase. The fully vaporized stream resulting from
the condensed fraction 40 is combusted in a gas turbine
320.
The auxiliary cooling duty transferred by the
auxiliary refrigerant stream 132 in the condenser 35 can
be altered by manipulating the auxiliary refrigerant
control valve 135. Various control strategies are
possible. For example, the auxiliary refrigerant control
valve 135 is functionally coupled to an optional level
controller 37 arranged in the gas/liquid separator 33 to
establish a constant liquid level in gas/liquid separator
33 by regulating the amount of partial condensation of
the compressed vapour 70 that occurs in the condenser 35.
Another example involves the auxiliary refrigerant
control valve 135 is functionally coupled to an optional
temperature sensor (not shown) arranged between the
condenser 35 and the gas/liquid separator 33 to establish
a constant temperature of the partially condensed
intermediate stream.
A suitable set point for the temperature sensor can
be derived from a desired nitrogen content of the
condensed fraction 40 that is consisted with the fuel gas
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composition specification of the gas turbine 320. The
remaining nitrogen stays in the vapour fraction. The
rundown temperature of raw liquefied stream in the
rundown line 1 can be regulated such as to ensure that
the total available heating power in the reject vapour 64
and/or the partially condensed intermediate stream meets
the combined fuel gas requirement of the combustion
device(s) 220 and the gas turbine(s) 320. For instance,
if there is too much heating power in the reject vapour
64, the rundown temperature can be lowered to reduce the
amount of methane that is flashed in the pressure
reduction step in pressure reduction system 5. The
partitioning of the nitrogen over the vapour fraction 80
and the condensed fraction 40 is regulated by the
auxiliary cooling duty.
The nitrogen content of the liquid stream 90 may be
kept within specification over the range of rundown
temperatures anticipated during operation by the correct
choice and dimensioning of the end-flash separator 50 at
the design stage.
The first and second heating values define the amount
of heat that can be released by combustion of a mole of
the fuel gas. This can be either the so-called "high"
heating value as the "low" heating value as long as the
same conditions are used for comparing the two heating
values. Preferably the "low" heating value is used to
compare the two heating values, as this is the closest to
the combustion conditions used in the invention. The
heating value may be determined using ASTM D3588-98
applied regardless of the composition of the vapour
fraction 80 and/or the condensed fraction 40. As a
result of the separation in the cooled gas/liquid
separator 33, the second heating value (belonging to the
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condensed fraction 40) is higher than the first heating
value (belonging to the vapour fraction 80). However, as
the partially condensed intermediate stream essentially
consists of two components, methane and nitrogen, the
first and second heating values uniquely map onto
nitrogen content of the vapour fraction 80 and the
condensed fraction 40, respectively.
The vapour fraction 80 is combusted the combustion
device 220 preferably at a first fuel gas pressure that
is not higher than the second separation pressure. This
way a compressor can be avoided as the pressure in the
vapour fraction 80 does not have to be increased.
Preferably, the vapour fraction 80 is combusted in the
combustion device at a pressure of between 2 and 15 bara,
more preferably at a pressure of between 2 and 6 bara.
The condensed fraction 40 may have to be pressurized
to a second fuel gas pressure that is higher than the
second separation pressure. If a fuel gas compressor 360
is arranged in the condensed fraction discharge line 40
between the revaporizer 285 and the gas turbine 320, the
fully vaporized stream may optionally be compressed in
such fuel gas compressor 360 to the second fuel gas
pressure before combusting the fully vaporized stream in
the gas turbine 320. The second fuel gas pressure is
generally higher than the second separation pressure, and
preferably adapted to meet fuel gas pressure requirements
imposed by the selected gas turbine 320.
A pressure reduction valve 245 may optionally be
arranged in the condensed fraction discharge line 40
between the gas/liquid separator 33 and the revaporizer
285. This allows for flashing some of the condensed
fraction 40 prior to passing the condensed fraction 40
through the revaporizer 285, by first passing the
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condensed fraction 40 through the pressure reduction
valve 245 and subsequently performing the indirectly heat
exchanging of the condensed fraction 40 against the
heating fluid 286. Herewith a lower temperature of the
heating fluid 286 can be achieved when it is discharged
from the revaporizer 285 and/or more of the cold vested
in the condensed fraction 40 can be recovered into the
heating fluid 286. The pressure reduction valve 245
controls the discharge temperature of the vaporized
stream being discharged from the revaporizer 285. The
pressure reduction valve 245 may suitably be functionally
coupled to a first temperature sensor 247, arranged in
the condensed fraction discharge line 40 downstream of
the revaporizer 285, whereby the valve setting is
controlled in response to a first temperature signal
generated in the first temperature sensor 247. If the
optional fuel gas compressor 390 is provided, the first
temperature sensor is suitably arranged in the condensed
fraction discharge line 40 between the revaporizer 285
and the optional fuel gas compressor 390. A first target
temperature setting for this control loop may be set at a
few degrees below, e.g. 2 C below, the temperature of
the heating fluid 286 at the inlet of the revaporizer
285. Preferably, the temperature of the condensed
fraction 40 at the outlet of the revaporizer is between
ambient temperature and 10 C below ambient temperature
to effectively obtain the most benefit from the cold
vested in the condensed fraction 40.
The second separation pressure is preferably higher
than the first separation pressure. The second
separation pressure may suitably be between 2 and 22
bara, preferably between 5 and 22 bara, more preferably
between 5 and 15 bara. A second separation pressure in
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the higher end of the range of 2 to 22 bara helps the
partial condensation of the compressed stream 70 and to
provide clearance for a higher pressure drop in the
optional pressure reduction valve 245 and/or to maintain
a higher pressure even after the pressure reduction valve
245, which saves on fuel gas compression duty in the
optional fuel gas compressor 360. The lower end of the
range helps the separation efficiency in the gas/liquid
separator 33 and causes less over compressing of the
vapor fraction 80 which is to be combusted in the
combustion device 220 at a relatively low pressure of
typically less than 15 bara. The proposed range of
between 5 and 15 bara for the second separation pressure
strikes a proper balance between the beneficial and the
adverse effects summarized earlier in this paragraph.
A typical pressure drop of between 1.0 and 4.0 bar
over the optional pressure reduction valve 245 has been
found adequate in typical cases.
In some embodiments, the second separation pressure
is in the range of from 5 to 8 bara, which pressure most
often meets the requirements of a low-pressure fuel gas
stream suitable for conveying the vapour fraction 80 to
the combustion device 220 without need for further
compression. A higher pressure may be selected if the
combustion device 220 is at a relatively large distance
from the first gas/liquid phase separator and/or when the
vapour fraction 80 is intended to pass through one or
more cold recovery heat exchangers 85. In such
circumstances an additional pressure drop may be expected
in the course of conveying the off gas to the combustion
device 220. In one embodiment, the second separation
pressure is about 6.5 bara.
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The cryogenic hydrocarbon composition 8 may be
obtained from natural gas or petroleum reservoirs or coal
beds. As an alternative the cryogenic hydrocarbon
composition 8 may also be obtained from another source,
including as an example a synthetic source such as a
Fischer-Tropsch process. Preferably the cryogenic
hydrocarbon composition 8 comprises at least 50 mol%
methane, more preferably at least 80 mol% methane. A
preferred initial temperature of lower than -130 C may
be achieved by passing a hydrocarbon stream 110 through a
liquefaction system 100. An embodiment of passing the
hydrocarbon stream 110 through the liquefaction system
100 will be described in more detail below.
A refrigerant is cycled in the refrigerant circuit
101 of the liquefier 100. Cycling comprises driving the
refrigerant compressor 160, and compressing the
refrigerant stream in the refrigerant compressor 160. A
hydrocarbon stream 110 is condensed and subcooled. The
condensing and subcooling involves indirectly heat
exchanging the hydrocarbon stream 110 against the
refrigerant in the liquefier 100. The thus formed
subcooled liquefied hydrocarbons stream is referred to as
raw liquefied stream. Thus the raw liquefied stream is
formed out of the hydrocarbon stream by condensing and
subsequently subcooling the hydrocarbon stream.
The hydrocarbon stream 110 in any of the examples
disclosed herein may be obtained from natural gas or
petroleum reservoirs or coal beds. As an alternative the
cryogenic hydrocarbon composition 8 may also be obtained
from another source, including as an example a synthetic
source such as a Fischer-Tropsch process. Preferably the
cryogenic hydrocarbon stream 110 comprises at least
50 mol% methane, more preferably at least 80 mol%
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methane. The resulting liquid hydrocarbon product
conveyed in the liquid hydrocarbon product line 90 and/or
stored in the cryogenic storage tank 210 is preferably
liquefied natural gas (LNG).
Depending on the source, the hydrocarbon stream 110
may contain varying amounts of components other than
methane and nitrogen, including one or more non-
hydrocarbon components other than water, such as 002, Hg,
H2S and other sulphur compounds; and one or more
hydrocarbons heavier than methane such as in particular
ethane, propane and butanes, and, possibly lesser amounts
of pentanes and aromatic hydrocarbons. Hydrocarbons with
a molecular mass of at least that of propane may herein
be referred to as C3+ hydrocarbons, and hydrocarbons with
a molecular mass of at least that of ethane may herein be
referred to as 02+ hydrocarbons.
If desired, the hydrocarbon stream 110 may have been
pre-treated to reduce and/or remove one or more of
undesired components such as CO2 and H2S, or have
undergone other steps such as pre-pressurizing or the
like. Such steps are well known to the person skilled in
the art, and their mechanisms are not further discussed
here. The composition of the hydrocarbon stream 110 thus
varies depending upon the type and location of the gas
and the applied pre-treatment(s).
The raw liquefied stream is discharged in the rundown
line 1 from the liquefier 100. The raw liquefied stream
may comprise in the range of from 1 mol% to 7 mol%
nitrogen and more than 81 mol% of methane. The
temperature of the raw liquefied stream in the rundown
line 1 may be anywhere between -165 C and -120 C. A
cryogenic hydrocarbon composition 8 is obtained from the
raw liquefied stream by passing the raw liquefied stream
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through a pressure reduction step in pressure reduction
system 5, whereby reducing the pressure from the
liquefaction pressure to an initial pressure of between 1
and 2 bar absolute. Flash vapour is usually generated
during such pressure reduction step.
The cryogenic hydrocarbon composition 8 comprises a
nitrogen- and methane-containing liquid phase, and is
usually at a temperature lower than -130 C.
In many cases, the temperature of the raw liquefied
stream in the rundown line 1 may be in the range of from
-160 C to -145 C. Within this more narrow range the
cooling duty needed in the liquefaction system 100 is
lower than when lower temperatures are desired, while the
amount of sub-cooling at the pressure of above 15 bara is
sufficiently high to avoid excessive production of flash
vapours upon reducing the pressure to the initial
pressure of between 1 and 2 bara.
The liquefaction system 100 in the present
specification has so far been depicted very
schematically. It can represent any suitable hydrocarbon
liquefaction system and/or process, in particular any
natural gas liquefaction process producing liquefied
natural gas, and the invention is not limited by the
specific choice of liquefaction system. Examples of
suitable liquefaction systems employ single refrigerant
cycle processes (usually single mixed refrigerant - SMR -
processes, such as PRICO described in a paper by K R
Johnsen and P Christiansen, presented at Gastech 1998
(Dubai), but also possible is a single component
refrigerant such as for instance the BHP-cLNG process
also described in the afore-mentioned paper by Johnsen
and Christiansen); double refrigerant cycle processes
(for instance the much applied Propane-Mixed-Refrigerant
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process, often abbreviated C3MR, such as described in for
instance US Patent 4,404,008, or for instance double
mixed refrigerant - DMR - processes of which an example
is described in US Patent 6,658,891, or for instance two-
cycle processes wherein each refrigerant cycle contains a
single component refrigerant); and processes based on
three or more compressor trains for three or more
refrigeration cycles of which an example is described in
US Patent 7,114,351.
Other examples of suitable liquefaction systems are
described in: US Patent 5,832,745 (Shell SMR); US Patent
6,295,833; US Patent 5,657,643 (both are variants of
Black and Veatch SMR); US Pat. 6,370,910 (Shell DMR).
Another suitable example of DMR is the so-called Axens
LIQUEFIN process, such as described in for instance the
paper entitled "LIQUEFIN: AN INNOVATIVE PROCESS TO REDUCE
LNG COSTS" by P-Y Martin et al, presented at the 22nd
World Gas Conference in Tokyo, Japan (2003). Other
suitable three-cycle processes include for example US
Pat. 6,962,060; NO 2008/020044; US Pat. 7,127,914;
DE3521060A1; US Pat. 5,669,234 (commercially known as
optimized cascade process); US Pat. 6,253,574
(commercially known as mixed fluid cascade process); US
Pat. 6,308,531; US application publication 2008/0141711;
Mark J. Roberts et al "Large capacity single train AP-
X(TM) Hybrid LNG Process", Gastech 2002, Doha, Qatar (13-
16 October 2002). These suggestions are provided to
demonstrate wide applicability of the invention, and are
not intended to be an exclusive and/or exhaustive list of
possibilities. Not all examples listed above employ
(aeroderivative) gas turbines as primary refrigerant
compressor drivers. It will be clear that any drivers
other than gas turbines can be replaced for a gas turbine
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to enjoy the certain preferred benefits of the present
invention.
An example, wherein in the liquefaction system 100 is
based on, for instance C3MR or Shell DMR, is briefly
illustrated in Figures 5 and 6. In both cases the
cryogenic heat exchanger 180 in the liquefaction system
100 is selected to be a coil wound heat exchanger,
comprising a warm side comprising all the tubes,
including lower and upper hydrocarbon product tube
bundles (181 and 182, respectively), lower and upper LMR
tube bundles (183 and 184, respectively) and an HMR tube
bundle 185. The cold side is formed by the shell side of
the cryogenic heat exchanger 180.
The lower and upper hydrocarbon product tube bundles
181 and 182 fluidly connect the hydrocarbon stream line
110 with the rundown line 1. At least one refrigerated
hydrocarbon pre-cooling heat exchanger 115 may be
provided in the hydrocarbon stream line 110, upstream of
the cryogenic heat exchanger 180.
The refrigerant provided in the refrigerant circuit
101 will be referred to as "main refrigerant" to
distinguish it from other refrigerants that may used in
the liquefaction system 100 such as a pre-cooling
refrigerant 127 which may provide cooling duty to the
refrigerated hydrocarbon pre-cooling heat exchanger 115.
The main refrigerant in the present embodiment is a mixed
refrigerant.
The refrigerant circuit 101 comprises a spent
refrigerant line 150, connecting the cryogenic heat
exchanger 180 (in this case a shell side 186 of the
cryogenic heat exchanger 180) with a main suction end of
the refrigerant compressor 160, and a compressed
refrigerant line 120 connecting the refrigerant
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compressor 160 discharge outlet with an MR separator 128.
One or more heat exchangers are provided in the
compressed refrigerant line 120, including in the present
example at least one reject heat exchanger 124. The MR
separator 128 is in fluid connection with the lower LMR
tube bundle 183 via a light refrigerant fraction line
121, and with the HMR tube bundle via a heavy refrigerant
fraction line 122.
The at least one refrigerated hydrocarbon pre-cooling
heat exchanger 115 and the at least one refrigerated main
refrigerant pre-cooling heat exchanger 125 are
refrigerated by the pre-cooling refrigerant (via lines
127 and 126, respectively). The same pre-cooling
refrigerant may be shared from the same pre-cooling
refrigerant cycle. Moreover, the at least one
refrigerated hydrocarbon pre-cooling heat exchanger 115
and the at least one refrigerated main refrigerant pre-
cooling heat exchanger 125 may be combined into one pre-
cooling heat exchanger unit (not shown). Reference is
made to US Pat. 6,370,910 as a non-limiting example.
At a transition point between the upper (182, 184)
and lower (181, 183) tube bundles, the HMR tube bundle
185 is in fluid connection with an HMR line 141. The HMR
line 141 is in fluid communication with the shell side
186 of the cryogenic heat exchanger 180 via a first HMR
return line 143, in which an HMR control valve 144 is
configured. Via the said shell side 186, and in heat
exchanging arrangement with each of one of the lower
hydrocarbon product tube bundle 181 and the lower LMR
tube bundle 183 and the HMR tube bundle 185, first HRM
return line 143 is fluidly connected to the spent
refrigerant line 150.
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Above the upper tube bundles 182 and 184, near the
top of the cryogenic heat exchanger 180, the LMR tube
bundle 184 is in fluid connection with the refrigerated
refrigerant line 131. A main refrigerant return line 133
establishes fluid communication between the refrigerated
refrigerant line 131 and the shell side 186 of the
cryogenic heat exchanger 180. A main refrigerant control
valve 134 is configured in the main refrigerant return
line 133. The main refrigerant return line 133 is in
fluid communication with the spent refrigerant line 150,
via said shell side 186 and in heat exchanging
arrangement with each of one of the upper and lower
hydrocarbon product tube bundles 182 and 181,
respectively, and each one of the LMR tube bundles 183
and 184, and the HMR tube bundle 185.
The line-up around the end flash separator 50 and the
gas/liquid separator 33 as shown in Figure 5 corresponds
to the line-up shown in Fig. 3. The line-up around the
end flash separator 50 and the gas/liquid separator 33 as
shown in Figure 6 corresponds to the line-up shown in
Fig. 4. In both cases, the auxiliary refrigerant line,
on an upstream end thereof, fluidly connects with the
liquid hydrocarbon product line 90, and on a downstream
end thereof with the condenser 35. The optional
auxiliary refrigerant pump 96 is configured in the
auxiliary refrigerant feed line 132. The auxiliary
refrigerant return line 133, on an upstream end thereof,
fluidly connects with the auxiliary refrigerant feed line
132 via the condenser 35. In the embodiments of Figures
5 and 6, the auxiliary refrigerant return line 138, on a
downstream end thereof, ultimately connects with the end-
flash separator 50.
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Employing a slip stream from the liquid hydrocarbon
product stream has as advantage that the amount of
additional equipment to be installed is minimal. For
instance, no additional auxiliary refrigerant compressor
and auxiliary refrigerant condenser would be needed,
which would be the case if a separate independent
auxiliary refrigerant cycle would be proposed.
The refrigerant in the liquefier 100 is cycled in the
refrigerant circuit 101, whereby spent refrigerant 150 is
compressed in the refrigerant compressor 160 to form a
compressed refrigerant 120 out of the spent refrigerant
150. Heat is removed from the compressed refrigerant
discharged from the refrigerant compressor 160, via the
one or more heat exchangers that are provided in the
compressed refrigerant line 120 including the least one
reject heat exchanger 124. This results in a partially
condensed compressed refrigerant, which is phase
separated in the MR separator 128 into a light
refrigerant fraction 121 consisting of the vaporous
constituents of the partially condensed compressed
refrigerant, and a heavy refrigerant fraction 122
consisting of the liquid constituents of the partially
condensed compressed refrigerant.
The light refrigerant fraction 121 is passed via
successively the lower LMR bundle 183 and the upper LMR
bundle 184 through the cryogenic heat exchanger 180,
while the heavy refrigerant fraction 122 is passed via
the HMR bundle 185 through the cryogenic heat exchanger
180 to the transition point. While passing through these
respective tube bundles, the respective light- and heavy
refrigerant fractions are cooled against the light and
heavy refrigerant fractions that are evaporating in the
shell side 186 again producing spent refrigerant 150
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which completes the cycle. Simultaneously, the
hydrocarbon stream 110 passes through the cryogenic heat
exchanger 180 via successively the lower hydrocarbon
bundle 181 and the upper hydrocarbon bundle 182 and is
being liquefied evaporating heavy refrigerant fraction
and sub-cooled against the evaporating light refrigerant
fraction.
Preferably, the vaporizer 285 is arranged in the
compressed vapour line 70 between the end-flash
compressor 260 and upstream of the gas/liquid separator
33, such that the heating fluid 286 can be provided in
the form of the compressed vapour stream 70. Examples of
such embodiments, wherein the condensed fraction 40 is
indirectly heat exchanged against at least a part of the
compressed vapour 70 stream whereby fully vaporizing the
condensed fraction 40, are shown in Figures 3 and 4. As
a result, the cooling duty required from the auxiliary
refrigerant 132 in condenser 35 to generate the same
degree of condensation is smaller.
Also illustrated in Figs. 3 and 4 is that the end
flash compressor 260 and the optional fuel gas compressor
360 may share a single compressor driver 290. These
compressors may be embodied as two separate compressor
casings on a common drive shaft, or they may actually be
two compressor stages within a single casing.
Figure 4 shows a special group of embodiments wherein
a stream splitter 75 is provided in the compressed vapour
stream line 70, whereby the compressed vapour stream line
70 is divided over a first branch 71 and a second branch
72. The first branch 71 is arranged to convey a first
compressed vapour part stream to the gas/liquid separator
33, and the second branch 72 is arranged to convey a
second compressed vapour part stream to the same
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gas/liquid separator 33. The stream splitter 70 merely
divides the incoming compressed vapour stream 70 into two
part streams of equal composition and phase. The stream
splitter 70 may be a pipe junction in the form a simple
T-junction, preferably in conjunction with a split ratio
control valve 76 in one of the first and second branches.
The condenser 35 is arranged in the first branch 71.
Advantageously, the optional cold recovery heat exchanger
85 is also arranged in the first branch 71 such that the
first compressed vapour part stream is employed as the
cold recovery fluid 86 generally referenced in Figure 1.
The revaporizer 285 may be arranged in the second branch
72, such that the second compressed vapour part stream is
employed as the heating fluid 286 generally referenced in
Figure 1.
In the embodiment illustrated in Fig. 4, the
compressed vapour stream 70 is split into a first
compressed vapour part stream and a second compressed
vapour part stream. The first compressed vapour part
stream is discharged from the stream splitter 75 into and
conveyed to the gas/liquid separator 33 in the first
branch 71, while the second compressed vapour part stream
is discharged from the stream splitter 75 into and
conveyed to the gas/liquid separator 33 in the second
branch 72. The first compressed vapour part stream and
second compressed vapour part stream both have the same
composition and phase as the compressed vapour 70. The
part of the compressed vapour stream 70 from which heat
is passed to the auxiliary refrigerant stream 132 is
formed by the first compressed vapour part stream.
However, prior to passing the heat from the first
compressed vapour part stream to the auxiliary
refrigerant stream 132, the first compressed vapour part
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stream is indirectly heat exchanged against the vapour
fraction 80 from the gas/liquid separator 33 in the cold
recovery heat exchanger 85. Downstream of this heat
exchanging, the vapour fraction 80 is warmer than between
the gas/liquid separator 33 and the cold recovery heat
exchanger 85. It may then be combusted in the combustion
device 220 as explained hereinbefore.
Still referring to Figure 4, the part of the
compressed vapour stream that is indirectly heat
exchanged in the revaporizer 285 against the condensed
fraction 40 is formed by the second compressed vapour
part stream. This way, the partially condensed
intermediate stream that is fed into the gas/liquid
separator 33 is formed by the combination of the first
and second compressed part streams.
The splitting of the compressed vapour is preferably
performed with an adjustable split ratio. The split
ratio corresponds to the quotient of mass flow rates in
the second branch 71 and the compressed vapour line 70.
The split ratio may be adjusted in response to a
temperature signal representative of the temperature of
the vapour fraction 80 from the gas/liquid separator 33
being discharged from the cold recovery heat exchanger 85
before being combusted. This temperature is preferably
maintained at a pre-determined target value by adjusting
the split ratio, and this way it will be possible to
achieve a certain degree of cold recovery from the vapour
fraction 80 regardless of variations in the flow rate of
the vapour fraction 80. The flow rate of first
compressed vapour part stream, which functions as the
cold recovery fluid, is effectively assimilated to the
available flow rate of the vapour fraction 80. To this
end, a second temperature sensor 77 may be provided in
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the vapour fraction line 80 between the cold recovery
heat exchanger 85 and the combustion device 220, which is
electronically coupled to the split ratio control valve
76 such that the valve setting of the split ratio control
valve 76 is controlled using the signal representative of
the temperature generated in the second temperature
sensor 77. A second target temperature setting for this
control loop may be set at a few degrees below, e.g. 2 C
below, the temperature of the cold recovery fluid 86 at
the inlet of the cold recovery heat exchanger 85. If
the temperature of the vapour fraction 80 at the outlet
of the cold recovery heat exchanger 85 is still lower
than the second target temperature, the split ratio may
be adjusted to be increased (for instance by reducing the
flow opening in the split ratio control valve 76).
Additional control strategies known to the person skilled
in the art may be implemented to avoid pinching of the
cold recovery heat exchanger 85 when it is regulated on
the outlet temperature.
Preferably, the temperature of the vapour fraction 80
at the outlet of the cold recovery heat exchanger 85 is
between ambient temperature and at most 10 C below
ambient temperature, to obtain the most cold recovery out
of the vapour fraction 80.
Material and heat balance calculations have been
performed using Pro2 simulation software, to demonstrate
the feasibility of the proposed methods and apparatuses.
Tables 1 to 4 show results for embodiments based on
Figure 7. The embodiment of Figure 7 implements an
extended end-flash separator system as previously been
described in US Pat. 6,014,869. The end-flash separator
50 in this case comprises a gas/liquid
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contacting device (for instance in the form of packing or
a collection of contacting trays), a lower inlet device
52 connected to a reboiler 55 and an upper inlet device
53. The cold recovery stream 66 consists of a side
stream of natural gas with the same composition as the
raw liquefied stream 1. It should be noted that the
majority of the auxiliary refrigerant feed line 132 and
the auxiliary refrigerant return line 138 have not been
drawn for clarity reasons. Only the end parts near the
condenser 35 and near the end-flash separator 50 have
been indicated, and it should be understood that the ends
parts of the auxiliary refrigerant feed line 132 are
mutually connected, as well as the end parts of the
auxiliary refrigerant feed line 138.
The cryogenic hydrocarbon composition 8 is assumed to
consist of the cold recovery stream 66 and the raw
liquefied stream 1 coming from the pressure reduction
system 5. The remainder of Figure 7 corresponds to
Figure 4.
For the calculations it is further assumed that the
reject vapour stream 64 between the cold recovery heat
exchanger 65 and the end flash compressor 260 contains
the vapour from the end-flash separator 50 together with
a boil-off gas stream from the cryogenic storage tank
210. It is further assumed that vapour bypass control
valve 77, vapour recycle control valve 88, recycle valve
14, and external stripping vapour flow control valve 73
are closed and in no-flow condition.
Tables 1 and 2 correspond to one calculation wherein
the second separation pressure falls in the range of from
4 to 8 bara. This is referred to as the low pressure
case. Tables 3 and 4 correspond to another calculation,
which is referred to as the high pressure case. In the
r.)
Table 1 (low pressure case)
Stream Mr. 1 la lb a 60 66 90 138
Pressure
74.8 74.2 9.88 1.05 1.05 9.88 1.12 1.50
(bara)
Temperature
-155 -163 -164 -167 -167 -157 -163 -134 0
( C)
Flow rate
211 211 211 218 19.4 6.65 198 0.30
(kg/s) 5
kr)
Nitrogen 5
3.93 3.93 3.93 3.93 41.4 3.93 1.09 1.09
(mol.%)
Methane
95.7 95.7 95.7 95.7 58.6 95.7 98.5 98.5
(mol.%)
c2+
0.39 0.39 0.39 0.39 0.00 0.39 0.42 0.42
(mol.%)
JI
JI
r.)
Table 2 (low pressure case, continued)
Stream Nr. 8 40 64 70 71 71a 71b 72 80 90
132 138 230 240
Pressure
1.05 6.50 0.85 7.50 7.50 7.00 6.50 7.00 6.50 1.12
2.00 1.50 1.00 3.00
(bara)
Temperature
-167 -162 -27 +31 +31 -131 -142 -169 -162 -163
-163 -134 -159 -172 0
( C)
Flow rate
218 19.6 23.7 23.7 3.48 3.48 3.48 20.3 4.12 198
0.30 0.30 3.20 19.6
(kg/s)
0
5
Nitrogen
(1,1
3.93 29.0 36.8 36.8 36.8 36.8 36.8 36.8 86.4 1.09
1.09 1.09 17.3 29.0
(mol.%)
Methane
95.7 71.0 63.2 63.2 63.2 63.2 63.2 63.2 13.6 98.5
98.5 98.5 82.7 71.0
(mol.%)
c2+
0.39 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.42
0.42 0.42 0.00 0.00
(mol.%)
JI
JI
r.)
Table 3 (high pressure case)
Stream Mr. 1 la lb a 60 66 90 138
Pressure
74.8 74.2 9.88 1.05 1.05 9.73 1.12 1.50
(bara)
Temperature
-155 -163 -164 -167 -167 -157 -163 -128 0
( C)
Flow rate
210 210 210 217 19.4 6.58 198 0.49
(kg/s)
Nitrogen 5
3.93 3.93 3.93 3.93 41.6 3.93 1.11 1.11
(mol.%)
Methane
95.7 95.7 95.7 95.7 58.4 95.7 98.5 98.5
(mol.%)
c2+
0.39 0.39 0.39 0.39 0.00 0.39 0.42 0.42
(mol.%)
JI
JI
r.)
Table 4 (high pressure case, continued)
Stream Nr. 8 40 64 70 71 71a 71b 72 80 90
132 138 230 240
Pressure
1.05 16.5 0.85 17.5 17.5 17.0 16.5 17.0 16.5 1.12
2.00 1.50 1.00 9.50
(bara)
Temperature
-167 -135 -27 +31 +31 -125 -127 -140 -135 -163
-163 -128 -160 -144 0
( C)
Flow rate
217 12.5 23.5 23.5 8.18 8.18 8.18 15.3 10.9 198
0.49 0.49 4.32 12.5
(kg/s)
0
5
Nitrogen
(1,1
3.93 19.3 36.9 36.9 36.9 36.9 36.9 36.9 62.9 1.09
1.11 1.11 18.9 19.3
(mol.%)
Methane
95.7 80.7 63.1 63.1 63.1 63.1 63.1 63.1 37.1 98.5
98.5 98.5 81.1 80.7
(mol.%)
c2+
0.39 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.42
0.42 0.42 0.00 0.00
(mol.%)
JI
JI
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other calculation the second separation pressure falls in
the range of from 10 to 20 bara. This affects the
pressure drop that is available over the pressure
reduction valve 245, which in turn affects the cooling
duty that is available in revaporizer 285. This, of
course, at the cost of extra compression power. It can
be seen that the temperature under which the phase
separation in the gas/liquid separator 33 is performed
can be higher in this pressure range. Notwithstanding, a
larger amount of the liquid hydrocarbon stream has to be
used as the auxiliary refrigerant stream.
The low pressure case as calculated in the present
example yields a low quality fuel gas that is discharged
from the cold recovery heat exchanger 85 at a pressure of
5.00 bara and a temperature of 22 C; and a revaporized
condensed fraction that is discharged from the
revaporizer 285 at a pressure of 3.00 bara and a
temperature of 25 C. The latter may be utilized as high
quality fuel gas.
The high pressure case as calculated in the present
example yields a low quality fuel gas that is discharged
from the cold recovery heat exchanger 85 at a pressure of
5.00 bara and a temperature of 28 C; and a revaporized
condensed fraction that is discharged from the
revaporizer 285 at a pressure of 9.00 bara and a
temperature of 19 C. The latter may be utilized as high
quality fuel gas.
In both the low pressure case and the high pressure
case, the ultimate composition of the liquefied
hydrocarbon inventory as stored in the cryogenic storage
tank 210 is 0.83 mol.% nitrogen; 98.74 mol.% methane and
0.43 mol.% 02+, whereby C2+ indicates all hydrocarbons
having a mass corresponding to that of ethane, and
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upward. The liquefied hydrocarbon stream being passed
through the main product line 91 to the cryogenic storage
tank 210 has slightly more nitrogen than the liquefied
hydrocarbon inventory as stored in the cryogenic storage
tank 210.
In any of the examples above, a preferred range of
liquefaction pressure, at which raw liquefied stream is
discharged in the rundown line i from the liquefier 100,
is from 15 bara to 120 bara, more preferably from 15 bara
to 90 bara or from 45 bara to 120 bara. The most
preferably range for the liquefaction pressure is from 45
bara to 90 bara. In case that the raw liquefied stream
consists for at least 80 mol% of methane and nitrogen, a
preferred temperature range for the raw liquefied stream
in the rundown line 1 may be from -165 C to -120 C.
In any of the examples above, the vapour fraction 80
is envisaged to contain in the range of from 30 mol% to
90 mol% of nitrogen, preferably in the range of from 30
mol% to 80 mol% of nitrogen or in the range of from 45
mol% to 90 mol% of nitrogen, preferably in the range of
from 45 mol% to 80 mol% of nitrogen, most preferably from
50 mol% to 80 mol% of nitrogen. To achieve a content of
nitrogen of between 50 mol% and 80 mol%, such as about
60 mol%, sufficient methane must be recondensed from the
compressed vapour stream 70. This may for instance be
done using a pressure of the compressed vapour stream 70
of between 4 and 8 bara, and achieving a temperature of
the partially condensed intermediate stream of in the
range of from -150 C to -135 C. The temperature range
may have higher end points if the pressure is higher than
8 bara.
Furthermore, the condensed fraction 40 generally
contains up to 30 mol% of nitrogen, and not less than
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mol%, preferably not less than 10 mol%. Striving for
lower values would cost more auxiliary cooling duty
whereas that is not needed for typical gas turbines and
particularly not for aero derivative gas turbines.
5 Compressors forming part of the hydrocarbon
liquefaction process in the liquefaction system 100,
particularly any refrigerant compressor including
refrigerant compressor 160, may be driven by any type of
suitable compressor driver 190, including any selected
from the group consisting of gas turbine; steam turbine;
and electric motor; and inter combinations thereof. This
generally applies also to refrigerant compressor driver
190.
The gas turbine may be selected from the group of so-
called industrial gas turbines, or the group of so-called
aeroderivative gas turbines. The group of aeroderivative
gas turbines includes: Rolls Royce Trent 60, RB211, or
6761, and General Electric LMS100TM, LM6000, LM5000 and
LM2500, and variants of any of these (e.g. 1M2500+).
Suitably, the gas turbine 320 in which the condensed
fraction 40 is ultimately combusted is the refrigerant
compressor driver 190 that is in driving engagement with
the refrigerant compressor 160. The gas turbine 320 may
drive the refrigerant compressor 160.
Typically, the second fuel gas pressure is selected
in a range between 15 and 75 bara, more preferably in a
range of between 45 and 75 bara. The usual prescribed
fuel gas pressure for most conventional types of
industrial gas turbines is between around 15 and around
25 bara, on average. However, the latest generation of
industrial gas turbine requires relatively high pressure
fuel gas, such as in the range of from 35 to 45 bara.
The range of between 45 and 75 bara is recommended to
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meet fuel gas pressure requirements of typical
aeroderivative gas turbines.
The nitrogen content of the liquid stream 90
typically does not exceed a desired maximum of about
1.1 mol%. In some embodiments, the amount of nitrogen in
the liquid hydrocarbon stream 90 is between 0.5 and
1 mol%, preferably as close to 1.0 mol% as possible yet
not exceeding said maximum of about 1.1 mol%.
It is a known phenomenon that boil-off gas results
from thermal evaporation caused by heat added to the
liquefied product, for instance in the form of heat
leakage into storage tanks, LNG piping, and heat input
from plant LNG pumps. In any of the examples and
embodiments illustrated herein, boil-off gas may
optionally be injected into the vapour reject line 64,
either upstream or downstream of the end-flash compressor
260 to be subject to the phase separation in the
gas/liquid separator 33. This may suitably comprise
collecting boil-off gas from the cryogenic storage tank
210, possibly via a boil-off gas supply line 230 as is
illustrated for example in Figure 5. Boil-off gas
results from adding heat to at least part of the
liquefied hydrocarbons, whereby a part of the methane-
containing liquid phase in the liquefied hydrocarbons
evaporates to form said boil-off gas.
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.
