Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR COOLING A GASEOUS HYDROCARBON
STREAM
The present invention relates to a method of cooling
a gaseous hydrocarbon stream to produce a liquefied
hydrocarbon stream.
A common hydrocarbon stream to be liquefied is
natural gas. There are many types of processes that can
be used to liquefy natural gas. Many of these processes
involve two or more successive refrigerant cycles, often
in a cascaded arrangement, for progressively lowering the
temperature of the natural gas. Such refrigeration cycles
typically comprise refrigerant compressors to recompress
the refrigerants in the respective cycles after they have
absorbed heat from the natural gas.
The refrigerant compressors may be driven by gas
turbines. Such gas turbines comprise an air compressor to
compress a stream of inlet air. It is a known
characteristic of gas turbines that the power that they
can generate decreases with increasing ambient
temperature. The decrease in generated power may be
mitigated at least in part by chilling the inlet air to
the gas turbine.
US patent 6,324,867 to Exxon Mobil discloses a
natural gas liquefaction system and process wherein
excess refrigeration available in a typical natural gas
liquefaction system is used to cool the inlet air to gas
turbines in the system to thereby improve the overall
efficiency of the system. A coolant (e.g. water) is
flowed through coolers positioned in front of the air
inlet of each gas turbine. The coolant, in turn, is
cooled with propane taken from a refrigerant circuit in
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the system which is used to initially cool the natural
gas which is to be liquefied. The coolant flows through
the coolers in a parallel fashion, because the cooled
coolant is split to flow to each cooler and recombined
downstream of the coolers. A control valve is provided in
each line after the split, and controlled independently
by an unspecified property of the inlet air in the
corresponding gas turbine.
A drawback of this method is that it does not take
into account which of the gas turbines is presenting the
most severe constraint on the LNG production.
Another drawback of the method described in US patent
6,324,867 to Exxon Mobil is that it uses cooling duty
from a refrigerant circuit which cooling duty is
therefore not available to cool the natural gas that is
to be liquefied.
The present invention provides a method of cooling a
gaseous hydrocarbon stream to produce a liquefied
hydrocarbon stream, comprising:
- cooling the gaseous hydrocarbon stream in one or more
heat exchangers using a first refrigerant from a first
refrigerant circuit in which said first refrigerant is
compressed in a first compressor driven by a first gas
turbine having a first inlet air stream, said cooling
providing a cooled hydrocarbon stream;
- liquefying the cooled hydrocarbon stream using a second
refrigerant, which second refrigerant is compressed in a
second compressor driven by a second gas turbine having a
second inlet air stream, and cooled at least by heat
exchanging with said first refrigerant from the first
refrigerant circuit, said liquefying providing a
liquefied hydrocarbon stream;
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- providing a stream of a chilled coolant comprising a
chilling fluid;
- dividing the cooling duty available in the chilled
coolant over at least first and second parts;
- cooling one or both of said first and second inlet air
streams with the chilled coolant, whereby the cooling
duty available in the first part is used to cool the
first inlet air stream, and the cooling duty available in
the second part is used to cool the second inlet air
stream.
The cooled hydrocarbon stream is provided by
producing a partially condensed hydrocarbon stream from
the gaseous hydrocarbon stream and passing the partially
condensed hydrocarbon stream through a gas/liquid phase
separator and drawing a liquid bottom stream and a
vaporous overhead stream from the gas/liquid phase
separator, wherein the cooled hydrocarbon stream is
provided from the vaporous overhead stream from the
gas/liquid phase separator. The fluid is actively
chilled using refrigeration duty taken from the liquid
bottom stream.
Moreover, an apparatus is provided, arranged to carry
out these process steps.
The invention further provides an apparatus for
cooling a gaseous hydrocarbon stream to produce a
liquefied hydrocarbon stream, comprising:
- a first refrigerant circuit comprising a first
refrigerant, a first compressor, a first gas turbine
coupled to the first compressor to drive the first
compressor, and a first inlet air stream to the first gas
turbine; the first compressor arranged to compress said
first refrigerant;
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- a second refrigerant circuit comprising a second
refrigerant, a second compressor, a second gas turbine
coupled to the second compressor to drive the second
compressor, and a second inlet air stream to the second
gas turbine; the second compressor arranged to compress
said second refrigerant;
- one or more first heat exchangers arranged to receive
and cool the gaseous hydrocarbon stream and the second
refrigerant, using said first refrigerant from said
cooling providing a cooled hydrocarbon stream and a
cooled second refrigerant stream;
- one or more second heat exchangers arranged to receive
and liquefy the cooled hydrocarbon stream using the
cooled second refrigerant stream, so as to provide a
liquefied hydrocarbon stream;
- a stream of a chilled coolant, comprising a chilling
fluid;
- a divider to divide the chilled coolant over at least
first and second parts;
- a first inlet air cooling heat exchanger arranged in
the first inlet air stream to cool the first inlet air
stream with the first part of the chilled coolant;
- a second inlet air cooling heat exchanger arranged in
the second inlet air stream to cool the second inlet air
stream with the second part of the chilled coolant.
The apparatus further comprises:
- a gas/liquid phase separator arranged between at least
one of the one or more first heat exchangers and the one
or more second heat exchangers, arranged to receive the
hydrocarbon stream after it has passed through the at
least one of the one or more first heat exchangers,
wherein said gas/liquid separator is connected to a
vaporous overhead stream line and connected to a liquid
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bottom stream line, which vaporous overhead stream line
is connected to a line that connects to the one or more
second heat exchangers;
- a bottom stream heat exchanger arranged in the liquid
bottom stream line; and- a chiller arranged to actively
chill a fluid to provide the chilled coolant.
The bottom stream heat exchanger is connected to a
heat source and arranged to add heat to at least part of
the liquid bottom stream in the liquid bottom stream line
The heat souce is formed by said fluid.
The invention will now be further illustrated by way
of example and with reference to one or more figures in
the accompanying drawing, wherein
Fig. 1 schematically shows an apparatus and method
for cooling and liquefying a hydrocarbon stream according
to an embodiment of the invention;
Fig. 2 schematically shows an example of a chilling
refrigerant circuit for actively chilling of the coolant
fluid;
Fig. 3 schematically shows an alternative drive
scheme that can be used in the invention;
Fig. 4 schematically shows another alternative drive
scheme that can be used in the invention; and
Fig. 5 schematically shows still another alternative
drive scheme that can be used in the invention.
In the description of these figures hereinbelow, a
single reference number has been assigned to a line as
well as a stream carried in that line. The same reference
numbers refer to similar components, streams or lines.
It is presently proposed to chill the coolant using
refrigeration duty provided by another cold stream
available in the process in addition to and/or instead of
one or more of the refrigerant streams indicated above.
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Moreover, the cooling duty available in the chilled
coolant may be divided over at least first and second
parts in accordance with a common input parameter, and to
cool at least first and second gas turbine inlet air
streams with the chilled coolant, whereby the cooling
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duty available in first part is used to cool the first
inlet air stream and the cooling duty available in the
second part is used to cool the second inlet air stream.
By dividing the cooling duty available in accordance
with a common input parameter, a better optimum in the
division of available cooling duty over the at least two
inlet air streams can be achieved.
For instance, if the common input parameter is a
parameter representative of ambient temperature, the
division of cooling duty can be made in accordance with
ambient temperature. Depending on ambient temperature,
the compression power needed in the first and second
compressors in a hydrocarbon cooling process, as well as
the available power in the first and second gas turbines,
shifts. At low ambient temperature the condensing
pressure of the first refrigerant is relatively low and
hence relatively less compression power is needed in the
first compressor compared to the second compressor,
making the compression power in the main refrigerant
circuit the limiting factor in the amount of LNG that is
produced. In that case, the division of cooling duty can
be made leaning towards favoring the cooling duty in the
second part to increase the compression power available
in the second compressor.
However, as the ambient temperature increases, the
production limitation starts shifting towards the first
compressor, because of increasing discharge pressure of
the first compressor. Then the division can be made
differently, favoring less the second part and thereby
freeing up cooling duty for the first part. Hereby the
LNG production can be maximised and/or the power
consumption for a fixed production rate of LNG minimized.
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The cooling duty available in the chilled coolant may
be divided in any ratio between the first and second
parts, ranging from 0:1 to 1:0. For instance, during cold
ambient conditions the duty attributed to the first part
may be zero such that the full available cooling duty in
the chilled coolant can be used to cool the second air
inlet stream.
Suitably, said dividing of the cooling duty in
accordance with the common input parameter comprises
deriving an optimum ratio of division based on the common
input parameter, and controlling the ratio at which the
cooling duty available in the chilled coolant is actually
divided between the first and second parts whereby
causing this ratio to be changed to or be maintained at
the derived optimum ratio.
For the purpose of the present specification,
"chilled coolant" is understood to be a fluid that has a
temperature lower than that of the ambient air
temperature. The chilled coolant can be prepared by
actively chilling the fluid, using refrigeration duty
from any refrigerant or cold stream, including
refrigeration duty taken from the first refrigerant
circuit and/or refrigeration duty taken from the second
refrigerant circuit, and/or refrigeration duty from any
type of refrigerant circuit.
There are also other cold streams available in a
hydrocarbon liquefaction process, which are not cycled in
a refrigerant circuit. Examples include the liquid bottom
stream of an extraction column and/or a fractionation
column and/or an overhead stream from a fractionation
column, a stream of end-flash gas that may be generated
when letting down the pressure of the liquefied
hydrocarbon stream, a stream of boil-off gas that may be
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evaporated off of the liquefied hydrocarbon while in
storage. Typical examples of extraction columns used in a
hydrocarbon liquefaction line-up include a simple
gas/liquid phase separator vessel, or a more advanced
distillation column such as a scrub column and a natural
gas liquids extraction column, which typically operates
at a lower pressure than a scrub column. Typical
fractionation columns in use in a natural gas liquids
fractionation train are a demethanizer, a deethanizer, a
depropanizer and a debutanizer.
One or more other common input parameters may be used
instead of or in addition to the parameter indicative of
ambient temperature. Suitable examples include parameters
representative of: first compressor discharge pressure;
cut point temperature between first and second
refrigerant cycle; first compressor adsorbed power;
second compressor absorbed power; difference between
first or second gas turbine power output; flow rate of
liquefied hydrocarbon.
Referring now to Figure 1, there is shown an
apparatus for cooling a gaseous hydrocarbon stream 10 to
produce a liquefied hydrocarbon stream 20. The apparatus
comprises a first refrigerant circuit 100 and a second
refrigerant circuit 200.
The first refrigerant circuit 100 comprises a system
of lines containing a first refrigerant that can be
cycled through the circuit. The second refrigerant
circuit comprises a separate system of lines, containing
a second refrigerant that can be cycled through the
second refrigerant circuit 200.
The first refrigerant circuit 100 comprises a first
compressor 110. A first gas turbine 120 is coupled to the
first compressor 110 via a first drive shaft 115, to
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directly drive the first compressor 110. The first gas
turbine 120 is associated with a first inlet air stream
125 to the first gas turbine 120. The first compressor
110 is arranged to compress the first refrigerant in line
130. As a precaution, the refrigerant in line 130 may
have passed though an optional suction drum 132 to ensure
that no liquid constituents are fed into the first
compressor 110.
The second refrigerant circuit 200 comprises a second
compressor 210 and a second gas turbine 220. The second
gas turbine 220 is coupled to the second compressor 210
via a second drive shaft 215, to drive the second
compressor 210. The second gas turbine 220 is associated
with a second inlet air stream 225 to the second gas
turbine. The second compressor 210 is arranged to
compress the second refrigerant in line 230. As a
precaution, the refrigerant in line 230 may have passed
though an optional suction drum 232 to ensure that no
liquid constituents are fed into the second compressor
210.
The respective first and second gas turbines 120, 220
are each associated with an inlet air cooling heat
exchanger, in the form of a first inlet air cooling heat
exchanger 127 and a second inlet air cooling heat
exchanger 227, respectively. These inlet air cooling heat
exchangers are arranged in the first respectively second
inlet air stream 125, 225 to cool the first and second
inlet air streams. Optionally, filters may be provided in
the first and second inlet air streams 125, 225 (not
shown) to filter the air before it is compressed in the
respective gas turbine 120, 220. Separators (not shown),
such as vertical vane type separators, and associated
drain facilities may be provided downstream of the inlet
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air cooling heat exchangers 127, 227 to remove moisture
that may develop during the cooling of the inlet air
stream(s). Drain facilities may also be provided in the
air cooling heat exchangers 127, 227 to drain of moisture
from these heat exchangers.
The suction inlet of the second compressor 210 is
connected to a second refrigerant outlet 262 of a second
heat exchanger 260 via the line 230 and the optional
suction drum 232. The second heat exchanger 260 is one of
one or more second heat exchangers, arranged to receive
and liquefy a cooled hydrocarbon stream in line 80, so as
to provide a liquefied hydrocarbon stream 20.
The outlet of the second compressor 210 is connected
to line 119 that is provided with one or more ambient
coolers 217.
The outlet of the first compressor 110 is connected
to one or more first heat exchangers 140a, 140b via a
refrigerant line 119. Upstream of the one or more first
heat exchangers 140a, 140b, one or more ambient coolers
117 are provided in the refrigerant line 119. Pressure
reduction devices 142a, 142b are provided upstream of the
one or more first heat exchangers 140a, 140b to regulate
the pressure in these heat exchangers. The one or more
heat exchangers 140a, 140b have refrigerant outlets that
are connected to the first refrigerant compressor 110 via
lines 134a and 134b. In the embodiment shown in Figure 1,
the lines 134a and 134b connect to the first refrigerant
compressor 110 via the optional suction drum 132.
In the embodiment as shown, two of the one or more
heat exchangers 140a, 140b are arranged in a parallel
configuration, and each have a single warm tube or warm
tube bundle 141a, 141b. Alternatively, it is possible to
arrange two parallel warm tubes or warm tube bundles in
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one heat exchanger. This may be in various types of heat
exchangers, such as the kettle type as presently shown in
Figure 1 and spool-wound type as for instance shown in US
Patent 6,370,910.
One of the one or more first heat exchangers is
arranged to receive and cool the gaseous hydrocarbon
stream 10. This one will be referred to as first
hydrocarbon feed heat exchanger 140a. Optionally, there
are one or more other first heat exchangers arranged in
the hydrocarbon feed line 10 upstream of the first
hydrocarbon feed heat exchanger 140a, to be operated at
higher pressures than the first hydrocarbon feed heat
exchanger 140a.
Line 40 downstream of the first hydrocarbon feed heat
exchanger may be connected directly to line 80 that
connects to the second heat exchanger 260 in order to
provide the cooled hydrocarbon stream to line 80.
However, as shown in the embodiment of Figure 1, the line
40 is connected to withdrawing means in the form of an
optional gas/liquid separator 50 that is provided to
receive the hydrocarbon stream 40 at approximately the
hydrocarbon feed gas pressure, after it has passed
through the first hydrocarbon feed heat exchanger 140a.
The optional gas/liquid separator may suitably be a
natural gas liquids extraction column and/or employed for
the purpose of extraction of natural gas liquids. Typical
examples of extraction columns used in a hydrocarbon
liquefaction line-up for extraction of natural gas
liquids include a simple gas/liquid phase separator
vessel, or a more advanced distillation column such as a
scrub column and a natural gas liquids extraction column,
which typically operates at a lower pressure than a scrub
column. In the embodiment shown in Figure 1, the
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optional gas/liquid separator is provided in the form of
a scrub column.
The optional gas/liquid separator 50 has an overhead
outlet for discharging a vaporous overhead stream 60 and
a bottom outlet for discharging a liquid bottom stream
70. Line 60 for the vaporous overhead stream 60 may be
connected to line 80 to provide the cooled hydrocarbon
stream in line 80. A splitter 63 may be provided in line
60 or line 80, to draw off a fuel gas stream 62 from the
vaporous overhead stream 60.
The liquid bottom stream 70, which may typically
comprise C2 to C4 constituents as well as C5+, may be
connected to an optional fractionation train 75 to
fractionate at least a part of the bottom stream 70 into
fractionation product streams 76. A bottom stream heat
exchanger 73 may optionally be provided to add heat to at
least a part of the bottom stream 70. Part of the bottom
stream 70 may be fed back to the optional gas/liquid
separator 50 as a reboiled stream 74, preferably
comprising, more preferably consisting of, vapour to
function as stripping vapour in the optional gas/liquid
separator 50. The heat source may be formed by stream
320, for instance by employing the bottom stream 70 as
cold fluid CF. An advantage of this arrangement is that
the part of the bottom stream 70 that needs to be fed
back to the optional gas/liquid separator 50 is cold and
needs to receive heat to generate the reboiled stream 74,
while the coolant fluid is available and needs to be
chilled.
Another one of the one or more first heat exchangers,
which will hereinafter be referenced as first second
refrigerant heat exchanger 140b, is arranged to receive
the second refrigerant from line 219. To this end, line
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219 is connected to the warm tube (or warm tube bundle)
141b. Optionally, there are one or more other first heat
exchangers arranged in the second refrigerant line 219
upstream of the first second refrigerant heat exchanger
140b, to be operated at higher pressures than the first
second refrigerant heat exchanger 140b. Downstream of the
first second refrigerant heat exchanger an optional
refrigerant gas/liquid separator 250 is provided to
receive the cooled second refrigerant stream 240 after it
has passed through the first second refrigerant heat
exchanger 140b and separate it into cooled at least by
heat exchanging with said first refrigerant from the
first refrigerant circuit.
The cooled hydrocarbon stream 80 and the second
refrigerant stream 240 (or vapour and liquid second
refrigerant streams 252 respectively 254) are connected
to the one or more second heat exchangers 260, to further
cool and liquefy the cooled hydrocarbon stream 80 to
obtain at least an intermediate liquefied hydrocarbon
stream 90 and an at least partially or fully evaporated
refrigerant stream 265 at the outlet 262.
Line 90 may be connected to depressurizing equipment
comprising optional phase separation equipment to
separate flash vapour from the remaining liquid. This
may be employed as withdrawal means in order to withdraw
a fraction from the hydrocarbon stream that may be used
as stream CF in chiller 325 to provide the cold coolant
fluid 320. There are various systems known in the art.
As example, the depressurizing equipment is here embodied
as one or more expansion devices 97 to produce a
depressurized stream 98 followed by a phase separator 99.
The expansion devices may be embodied in the form of an
isentropic expander such as work-expander 95 which may be
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provided in the form of a turbine, and/or an isenthalpic
expander such as a Joule-Thomson valve 96. In the
embodiment of Figure 1, the isenthalpic expander 96 is
suitably be provided downstream of the isentropic
expander 95.
Still referring to Figure 1, the apparatus further
comprises a coolant circuit 300, wherein a coolant fluid
can be circulated for chilling the first and/or second
inlet air streams 125, 225. In the embodiment as shown,
there is provided a storage tank 310 wherein the coolant
fluid can be stored. The coolant fluid is preferably a
liquid and/or inflammable for safety reasons. Suitable
coolants include water and brine, possibly admixed with
an anti-freezant such as a glycol and/or a corrosion
inhibitor.
The coolant circuit 300 further comprises means for
actively chilling the fluid to provide a chilled coolant
320. In the embodiment of Figure 1, a chiller 325 is
provided for that purpose. The chiller 325 is arranged to
receive a cold fluid CF capable of withdrawing heat from
the coolant fluid and thereby to provide the chilled
coolant in line 320. The cold fluid CF can be obtained
from numerous sources, as will be further illustrated
hereinbelow.
The cold fluid CF may be obtained from a single
source or it may comprise a mixture of fluids from two or
more sources. Alternatively, instead of one cold fluid
CF, there may be two or more cold fluids, each arranged
to remove heat from the coolant fluid in line 320. In
this case, it may be a suitable choice of design to use a
plurality of chillers, either arranged in parallel or in
series in line 320. Suitably, a separate chiller is
provided for each source of cold fluid.
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To assist the flow of the fluid in the coolant
circuit, a pump 305 is provided. The pump may be provided
anywhere in the circuit. Suitably, as proposed in the
embodiment of Figure 1, the pump 305 has its low pressure
inlet connected to the storage tank 310 via line 315 and
its high-pressure outlet connected to the chiller 325.
Downstream of the chiller 325 there is provided a
divider 335, to divide the chilled coolant 320 over at
least a first part 340 and a second part 350. The divider
will be discussed in more detail below.
The first inlet air cooling heat exchanger 127 is
arranged in line 340 to cool the first inlet air stream
125 with the first part of the chilled coolant. The
second inlet air cooling heat exchanger 227 is arranged
in the second inlet air stream to cool the second inlet
air stream with the second part of the chilled coolant.
The divider as shown in Figure 1 comprises a junction
337, such as a T-piece, a first flow control valve 338 in
line 340, and a second flow control valve 339 in line
350. Both control valves have been depicted as
controllable valves, to provide freedom to add other
streams. However, the skilled person will understand that
in the apparatus as depicted in Figure 1 only one of both
flow control valves needs to be controllable because
there are only two lines downstream of the junction 337.
The apparatus in the embodiment as shown in Figure 1
further comprises a controller C. In a preferred
embodiment, the controller is arranged to receive a
signal representative of a common input parameter. The
controller is further arranged to determine an optimum
division of the cooling duty available in the chilled
refrigerant over the first and second parts 340, 350
based on the common input parameter.
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As represented in Figure 1, the common input
parameter is indicative of ambient temperature. The
signal may be provided from a temperature sensor Ta,
which is for instance located in one or more of the inlet
air streams 125, 225. Means, for example controller C,
are provided for sending a control signal to one or more
of the flow control valves 338, 339. The control signal
may be provided in the form of a valve setting set point.
Alternatively, the signal may be provided to
represent a common input parameter that is indicative of
another relevant entity. For instance, the common input
parameter may be indicative of first compressor discharge
pressure.
The apparatus works as follows. The gas turbines 120
and 220 each take in an inlet air stream and a fuel
stream and provide mechanical power on the respective
drive shafts 115, 215. The drive shafts are mechanically
coupled to respective first and second compressors 110,
210 and thus the compressors are driven.
The first refrigerant in the first refrigerant
circuit 100 is compressed in compressor 110, cooled
against ambient in one or more coolers 117 and
distributed over one or more first heat exchangers 140a,
140b. Typically, cooling of the first refrigerant in the
cooler(s) 117 causes it to partially, preferably fully,
condense. Upstream of each of the first heat exchangers
the first refrigerant the pressure is let down in the
reduction devices 142a, 142b. The first refrigerant is
then allowed to evaporate in the first heat exchangers
140a, 140b by drawing heat from the warm tubes or tube
bundles 141a, 141b. The evaporated first refrigerant is
led back to the first compressor 110.
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A gaseous hydrocarbon stream 10 cooled in one or more
of the first heat exchangers, as shown in Figure 1 by
passing the gaseous hydrocarbon stream through the warm
tube 141a in first hydrocarbon feed heat exchanger 140a,
to produce a partially condensed hydrocarbon stream 40.
The second refrigerant in the second refrigerant
circuit 200 is compressed in compressor 210, cooled
against ambient in one or more coolers 217 and then
further cooled in one or more of the first heat
exchangers. As depicted in Figure 1, the further cooling
of the second refrigerant is achieved by passing it
through warm tube 141b in first second refrigerant heat
exchanger 140b where it is cooled at least by heat
exchanging with said first refrigerant, to produce a
partially condensed second refrigerant stream 240.
The partially condensed second refrigerant stream 240
is separated into vapour and liquid second refrigerant
phases 252 respectively 254. These streams are then
condensed and sub-cooled, respectively subcooled, in the
one or more second heat exchangers 260 in a manner well
known in the art.
The partially condensed hydrocarbon stream 40 is
separated into vaporous overhead stream 60 and liquid
bottom stream 70. Optionally, at least a part of the
bottom stream 70 is warmed in bottom stream heat
exchanger 73 and at least part of the warmed bottom
stream 74 may be fed back to the optional gas/liquid
separator 50 as a reboiled stream. The remaining part is
typically led to the fractionation train 75 where it is
fractionated into one or more fractionation product
streams. Typical fractionation columns in use in a
natural gas liquids fractionation train are a
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demethanizer, a deethanizer, a depropanizer and a
debutanizer.
The vaporous overhead stream 60 is fed into line 80
as a cooled hydrocarbon stream 80. The cooled hydrocarbon
stream 80 is then fed to one or more of the second heat
exchangers 260 in a manner known in the art where it is
liquefied using the second refrigerant. Herewith is
produced an intermediate liquefied hydrocarbon stream 90.
This intermediate liquefied hydrocarbon stream 90 may
be depressurized in the one or more expansion devices 97
and the depressurized stream 98 led to phase separator
99, where any vaporous constituents, mainly flash vapour,
are separated from the liquid hydrocarbons in stream 98.
The liquid hydrocarbons are removed from the phase
separator 99 as liquefied hydrocarbon product stream 20,
the vaporous constituents are drawn from the phase
separator 99 as end flash stream 92.
The coolant fluid in the storage tank 310 is pumped
or otherwise led to chiller 325, wherein it is actively
chilled to provide chilled coolant 320 by heat exchanging
against cold fluid CF. The cooling duty available in the
chilled coolant 320 is used to chill the inlet air stream
of at least one of the gas turbines.
The available cooling duty can suitably be divided
over at least first and second parts. The cooling duty
may for instance be divided by physically splitting the
chilled coolant 320 over two or more part streams, such
as two part streams 340, 350 in the embodiment of Figure
1. The first part stream 340 is used to cool the first
inlet air stream 125 and the second part stream 350 is
used to cool the second inlet air stream 225.
The dividing of the cooling duty may be done in
accordance with a common input parameter. This allows to
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control the division of available cooling duty over the
at least two inlet air streams in the best possible way.
Of course, it is possible that at times all of the
chilling duty is sent to only one of the part streams,
depending on the common input parameter. Preferably, the
control of the division of the cooling duty in accordance
with the common input parameters allows control over the
power balance between the various refrigerant circuits.
Suitably, the common input parameter allows the
controller C to determine which refrigeration circuit is
the constraining refrigeration circuit in terms of not
being able to deliver enough cooling duty to allow one or
more of the other refrigeration circuits to operate at a
higher (or full) capacity. By then providing relatively
more chilling duty to the inlet air stream(s) of the
turbine(s) driving the constraining refrigerant circuit,
it is possible to selectively increase the gas turbine
efficiency (resulting in increased shaft power output) of
the constraining gas turbine relative to the other gas
turbines driving other refrigerant circuits. This then
allows to increase the production rate of the liquefied
hydrocarbon product (or to produce the liquefied
hydrocarbon product stream at lower specific energy
consumption).
Suitably the common input parameter is indicative of
the ambient temperature, such as the temperature Ta of
one or both of the inlet air streams 125, 225. A
consequence of the cascaded refrigeration arrangement of
Figure 1 is that, as the ambient temperature increases,
relatively more cooling duty is needed from the first
refrigerant circuit 100 relative to the cooling duty
needed from the second refrigerant circuit 200. The
controller can then cause relatively more cooling duty
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from the chilled coolant to be made available for cooling
of the first inlet air stream 125. Depending on the
design of the process, it may be possible that all
cooling duty from the chilled coolant is made available
to cooling of the first inlet air stream 125,
particularly in situations that the first gas turbine
power output is constraining the process. However,
particularly when no or insufficient additional (helper)
driver power is provided to supplement the gas turbine
drive power for the second compressor 210, it is likely
preferable that at least some of the cooling duty is
always used for cooling the second inlet air stream 225,
even when the first gas turbine power output is
constraining, in order to ensure that the second
compressor is not driven out of its operating window into
surge as a result of too low a drive power.
At relatively lower ambient temperature, the second
refrigerant circuit 200 may become the constraining
circuit and the controller may cause relatively more
cooling duty from the chilled coolant to be made
available for cooling of the second inlet air stream 225.
At very cold ambient temperatures the controller may
cause to send all cooling duty from the chilled coolant
to be made available for cooling of the second inlet air
stream 225.
Similar effects may be achieved by using another
common input parameter, such as for example a common
input parameter that is indicative of one of the group
of: first compressor discharge pressure; first gas
turbine load/poweroutput; second gas turbine
load/poweroutput; first gas turbine fuel gas valve
opening; second gas turbine fuel gas valve opening; cut
point temperature Tc between first and second refrigerant
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cycle; first compressor adsorbed power; second compressor
absorbed power; difference between first or second gas
turbine power output; flow rate of liquefied hydrocarbon.
The latter is symbolically indicated in Figure 1 by the
flow sensor F, which may feed its signal to controller C
(not shown) similar to sensor Ta.
After having cooled the first and/or second inlet air
streams 125, 225, the coolant may be recombined and led
to the storage tank 310 for re-use.
The inlet air streams 125, 225 are preferably not
cooled to lower than about 5 C to ensure that formation
of ice is avoided.
The cooling duty for actively chilling of the coolant
fluid in the one or more chillers 325 may be obtained
from a wide variety of sources. For instance, it may use
chilling duty provided by a thermally driven chilling
process. Particularly, the one or more chillers 325 may
comprise one or more thermally driven chillers. The
thermally driven chilling process and/or the thermally
driven chillers may be operated using waste heat from the
liquefaction process, e.g. the waste heat from one or
more of the first and second gas turbines 120,220.
Thermally driven chillers are known in the art. A
relatively common example is formed by the group
consisting of absorption chillers. One example of an
absorption chilling is based on evaporating liquid
ammonia in the presence of hydrogen gas, providing the
cooling. More common in large commercial plants are so-
called lithium/bromide absorption chillers. A
lithium/bromide absorption chiller uses a solution of
lithium/bromide salt and water. Another example of
thermally driven chillers known in the art is formed by
the group consisting of adsorption chillers. Still
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another example is formed by the group consisting of
absorption heat pumps. Their principle of operation is
similar to absorption chillers.
Alternatively or in addition to thermally driven
chilling, the actively chilling of the coolant fluid may
use a chilling refrigerant from a dedicated mechanically
driven chilling refrigerant circuit. As illustrated in
Figure 2, the dedicated chilling refrigerant circuit 380
is provided with its own compressor 381 and means for
rejecting heat from the compressed chilling refrigerant
to the ambient such as cooler 382. The compressor 381 may
be driven by any suitable driver 383, suitably an
electric motor but not necessarily so. The chiller 325 is
depicted in the form of a kettle. A Joule Thomson valve
386 is provided between the kettle 325 and the cooler
382, downstream of an optional accumulator 385. A
knockout drum 384 may be provided between the kettle 325
and the suction inlet of the compressor 381 as a
precaution. The chilling refrigerant may consist of any
component or mixture suitable for removing heat at
approximately the temperature level of the ambient
temperature. Examples include butane, iso-butane,
propane, ammonia.
Alternatively or in addition thereto, it may use
refrigeration duty from a stream that is already
available in the liquefaction line-up. For instance, it
may use refrigeration duty taken from the first
refrigerant circuit and/or the second refrigerant
circuit.
Of these two, it is preferred to use refrigeration
duty from the first refrigerant circuit 100 because the
refrigerant in the first refrigeration circuit 100 is
generally more efficient at removing heat at the desired
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temperature level of the chilling coolant. This can or
instance be done by providing chiller 325 in the form of
a kettle wherein the first refrigerant from line 119 is
evaporated at a desired suitable pressure level.
Downstream of the chiller 325 the first refrigerant may
be recompressed, e.g. via a dedicated compressor and then
recombined with the first refrigerant in the first
refrigerant circuit downstream of first refrigerant
compressor 110, or via first refrigerant compressor 110
itself for instance by feeding the refrigerant downstream
of chiller 325 to the knock out drum 132.
Refrigeration duty from the second refrigerant
circuit may be used by allowing a slip stream from for
instance line 240 to evaporate in and/or pass through
chiller 325 at a desired pressure level as cold stream
CF. The slip stream may also be drawn from other suitable
places in the second refrigerant circuit 200, such as
from the liquid second refrigerant stream in line 254 if
optional refrigerant gas/liquid separator 250 is present.
Irrespective of the origin of the slipstream, downstream
of the chiller the slip stream may be fed back to the
second compressor 210 and/or recompressed using a
dedicated compressor.
Optionally, the controller C is arranged to control
the selection of the source of refrigeration duty between
first and second refrigeration circuit based on the
refrigeration circuit that is the least constraining of
the two.
In addition to and/or instead of one or more of the
refrigerant streams indicated above, the coolant may be
chilled using refrigeration duty provided by any other
cold stream available in the process. For instance, if
gas/liquid phase separator 50 is present, the cold fluid
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CF may be derived from or contain the liquid bottom
stream 70. In this case, optional heat exchanger 73 may
be in communication with stream 320 or the optional heat
exchanger 73 may be one of the one or more chillers 325.
Other examples of cold streams that may be used to
provide part of or all of the refrigeration duty for the
active cooling of the coolant include fuel gas stream 62,
end flash stream 92, and any cold stream from (optional)
fractionation train 75. Figure 1 shows symbolically
optional chillers 61 and 91 that could be used as
chiller(s) 325 or be otherwise positioned in
communication with line 320. Boil off gas, for instance
from a storage tank wherein the liquefied hydrocarbon
stream 20 may be stored, may also be used to provide part
of or all of the refrigeration duty for the active
cooling of the coolant.
In alternative embodiments, the second refrigerant
may be fully condensed after its cooling against the
first refrigerant. In such embodiments, obviously the
optional refrigerant gas/liquid separator 250 need not be
provided. There are also alternative embodiments wherein
the second refrigerant is not fully condensed but wherein
nevertheless no gas/liquid phase separation is needed,
for instance because full condensation is achieved in a
subsequent heat exchanging against a further refrigerant
or by auto-cooling.
The apparatus may have various modifications compared
to what is specifically depicted in Figure 1. Some
modifications and alternatives have already been
mentioned hereinabove. In another optional modification,
for instance, first compressor 110 may have a multiple of
inlets at different pressure levels in a manner known in
the art. The first compressor 110 and/or the second
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compressor 210 may each be embodied in the form of two or
more successive or parallel arranged frames, in a manner
known in the art.
First and/or second gas turbines 120, 220 may be of
an aeroderivative type, such as for example a Rolls Royce
Trent 60 or RB211, and General Electric LMS100TM, LM6000,
LM5000 and LM2500. The presently proposed inlet air
chilling is particularly advantageous when using
aeroderivative type turbines, as this can replace the
need for helper drivers (typically a steam turbine or
electric motor) to compensate for power loss.
Alternatively, the first and/or second gas turbines may
be of a heavy industrial frame type, such as for example
a General Electric Frame 6, Frame 7 or Frame 9 to enhance
the efficiency, although in this case an additional
driver may still need to be provided for starting up the
turbine. Clearly, equivalent gas turbines from other
manufactures may be employed as well.
Optionally, (not shown), an overhead heat exchanger
may be provided in line 60 in a way known in the art.
Such an overhead heat exchanger may form part of the one
or more first heat exchangers, and it may for instance be
connected to line 119 to obtain a fraction of the first
refrigerant. Where such an overhead heat exchanger is
provided in line 60, an optional overhead gas/liquid
separator is provided downstream of the overhead heat
exchanger in order to remove any condensed fraction from
stream downstream of the overhead heat exchanger. The
vapour outlet of the overhead gas/liquid separator may
then be connected to line 80 to provide the cooled
hydrocarbon stream. The bottom liquid outlet of the
overhead gas/liquid separator may be connected to the
gas/liquid separator 50 to feed back at least a portion
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of the condensed fraction as a reflux stream. The fuel
gas stream 62 may be drawn from the vapour stream.
In an alternative embodiment, the optional gas/liquid
separator 50 is located upstream of the first hydrocarbon
feed heat exchanger 140a. The overhead outlet of the
gas/liquid separator could in such an alternative
embodiment be connected to line 10 in Figure 1, and line
40 could the be connected directly to line 80 to provide
the cooled hydrocarbon stream to the second heat
exchanger 260. Such embodiments may have an expander
upstream of the optional gas/liquid separator 50, and
typically one or more recompressors and/or booster
compressors upstream of the first hydrocarbon feed heat
exchanger 140a, and/or other heat exchangers to pre-cool
the feed to the optional gas/liquid separator 50. Such
embodiments are known in the art and need not be further
detailed here.
In the embodiment as shown in Figure 1, the first
refrigerant is a single component refrigerant consisting
essentially of propane, while the second refrigerant is a
mixed refrigerant. A mixed refrigerant or a mixed
refrigerant stream as referred to herein comprises at
least 5 mol% of two different components. The mixed
refrigerant may contain two or more components selected
from the group consisting of: nitrogen, methane, ethane,
ethylene, propane, propylene, butanes. A common
composition for a mixed refrigerant can be:
Nitrogen 0-10 mol%
Methane (Cl) 30-70 mol%
Ethane (C2) 30-70 mol%
Propane (C3) 0-30 mol%
Butanes (C4) 0-15 mol%
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The total composition comprises 100 mol%.
However, the methods and apparatus disclosed herein
may further involve the use of one or more other
refrigerants, in separate or overlapping refrigerant
circuits or other cooling circuits. Moreover, the first
refrigerant may be a mixed refrigerant (such as described
for instance in US Pat. 6,370,910) and/or the second
refrigerant may be a single component refrigerant (such
as consisting essentially of ethane, ethylene, methane or
nitrogen). The invention may also be applied in 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).
The gaseous hydrocarbon stream 10 to be cooled and
liquefied may be derived from any suitable gas stream to
be cooled and liquefied, such as a natural gas stream
obtained from natural gas or petroleum reservoirs or coal
beds. As an alternative the gaseous hydrocarbon stream 10
may also be obtained from another source, including as an
example a synthetic source such as a Fischer-Tropsch
process.
When the gaseous hydrocarbon stream 10 is a natural
gas stream, it is usually comprised substantially of
methane. Preferably the gaseous hydrocarbon stream 10
comprises at least 50 mol% methane, more preferably at
least 80 mol% methane.
Depending on the source, natural gas may contain
varying amounts of hydrocarbons heavier than methane such
as in particular ethane, propane and the butanes, and
possibly lesser amounts of pentanes and aromatic
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hydrocarbons. The composition varies depending upon the
type and location of the gas.
Conventionally, the hydrocarbons heavier than methane
are removed as far as needed to produce a liquefied
hydrocarbon product stream in accordance with a desired
specification. Hydrocarbons heavier than butanes (C4) are
removed as far as efficiently possible from the natural
gas prior to any significant cooling for several reasons,
such as having different freezing or liquefaction
temperatures that may cause them to block parts of a
methane liquefaction plant.
The natural gas may also contain non-hydrocarbons
such as H20r N2r 002, Hg, H2S and other sulphur compounds,
and the like. Thus, if desired, the gaseous hydrocarbon
stream 10 comprising the natural gas may be pre-treated
before cooling and at least partial liquefaction. This
pre-treatment may comprise reduction and/or removal of
undesired components such as 002 and H2S or other steps
such as early cooling, pre-pressurizing or the like. As
these steps are well known to the person skilled in the
art, their mechanisms are not further discussed here.
It will be understood that the present invention is
applicable not only to the drive scheme as specifically
illustrated in Figure 1, but to other drives schemes as
well. Figures 3 to 5, which are not intended to form an
exclusive list, illustrate some possible alternative
options. Similar and/or various other options are also
briefly depicted in for instance in LNG-14 paper entitled
"REDUCING LNG CAPITAL COST IN TODAY'S COMPETITIVE
ENVIRONMENT" by Mark J. Roberts et al, paper 2.6 (2004).
For instance, Figure 3 shows the first refrigerant in
line 130 being offered to a plurality of inlets in the
first compressor 110 each at a different pressure. The
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compressor 210 for compressing the second refrigerant is
embodied in successively arranged low pressure second
refrigerant compressor 210a and high pressure second
refrigerant compressor 210b, both being driven on a
single axis 215 by the second gas turbine 220. The second
refrigerant in line 230 is fed to the low pressure second
refrigerant compressor 210a and the high pressure second
refrigerant compressor 210b discharges into line 219.
As illustrated in Figure 4, the invention can be
applied to the so-called Split-MRTM introduced by Air
Products and Chemicals Inc and briefly described in for
instance LNG-13 paper entitled "REDUCING LNG COSTS BY
BETTER CAPITAL UTILIZATION" by Dr. Yu Nan Liu et al,
paper PS5-4 (2001). In essence, the second compressor 210
driven by the second gas turbine 220 functions as the low
pressure second refrigerant compressor 210a of Figure 3
but the first gas turbine 120 drives both the first
compressor 110 as well as a second second compressor 211
which functions as the high pressure second refrigerant
compressor 210b of Figure 3. Thus, the second refrigerant
in line 230 is fed to second compressor 210, and the
second second compressor 211 discharges into line 219.
Figure 5 illustrates an embodiment using an auxiliary
second compressor 410 which is driven by a third gas
turbine 420. Like Figure 4, the second compressor 210 is
driven by the second gas turbine 220 and functions as the
low pressure second refrigerant compressor 210a of Figure
3, but in this case the third gas turbine 420, via shaft
415, drives the auxiliary second compressor 410 which
functions as the high pressure second refrigerant
compressor 210b of Figure 3. Thus, the second refrigerant
in line 230 is fed to second compressor 210, and the
auxiliary second compressor 410 discharges into line 219.
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As depicted in Figure 5 only the first and second gas
turbines inlet air streams 125 and 225 are cooled,
whereas the third gas turbine inlet air stream 425 is
provided to the third gas turbine at ambient temperature
and not cooled. Alternative embodiments also cool the
third inlet air stream 425 to the third gas turbine 420
(either sharing cooling duty from the chilled coolant 320
or by a separate cooling source), or cool the third inlet
air stream 425 instead of the second inlet air stream
225.
An intercooler, such as an air-cooled or water-cooled
intercooler, may be provided in the line between
consecutive pressure stages of the second refrigerant
circuit, such as the low pressure and high pressure
refrigerant compressors in any of the embodiments of
Figures 3 to 5.
The invention can be applied on still other drive
schemes as well. One typical modification, for instance,
of the Split-MR drive scheme as shown in Figure 4 is that
two pressure stages (e.g. LP and MP) are driven by the
second gas turbine 220 on a single shaft 215, in which
case of course the medium pressure compressor discharges
to the second second compressor 211 (which functions as
high pressure compressor). Likewise, multiple compressor
stages can be driven on shaft 215 in Figure 5.
The invention described hereinabove is not limited to
two refrigeration circuits: it can also be applied for
dividing cooling duty of the chilled coolant over three
or more parts for cooling third or more inlet air streams
of other refrigerant circuits.
The embodiments described above contain another
invention, which can be applied both in combination with
the features associated with the dividing of cooling duty
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available in the chilled coolant over at least first and
second parts and the cooling of at least first and second
gas turbine inlet air streams with the chilled coolant,
and separately therefrom. The other invention can even
be applied in cooling and/or liquefaction processes based
on a single refrigerant cycle. This other invention,
which relates to a method of producing a liquefied
hydrocarbon stream and an apparatus therefor, will be
described in the remainder of the present specification.
Another drawback of the method described in US patent
6,324,867 to Exxon Mobil is that it uses cooling duty
from a refrigerant circuit which cooling duty is
therefore not available to cool the natural gas that is
to be liquefied.
In one aspect, the other invention described herein
may be defined as providing a method of producing a
liquefied hydrocarbon stream, comprising:
- indirect heat exchanging a hydrocarbon stream in one or
more heat exchangers against one or more refrigerants
from one or more refrigerant circuits, at least one of
which refrigerant circuits comprising a compressor driven
by a gas turbine by which compressor the refrigerant of
that refrigerant circuit is compressed;
- withdrawing a fraction from the hydrocarbon stream
after it has been heat exchanged in at least one of the
one or more heat exchangers;
- providing a stream of a chilled coolant by indirect
heat exchanging the chilled coolant against at least a
part of the withdrawn fraction of the hydrocarbon stream;
- chilling an inlet air stream comprising heat exchanging
with the chilled coolant to produce a chilled inlet air
stream, and feeding the chilled inlet air stream to the
gas turbine;
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wherein the produced liquefied hydrocarbon stream
comprises at least part of the hydrocarbon stream that
has not been withdrawn.
Thus, in embodiments of the other invention, a
fraction is withdrawn from the hydrocarbon stream after
it has been heat exchanged in at least one of one or more
heat exchangers, thus suitably downstream of at least one
of one or more heat exchangers, to provide a stream of a
chilled coolant that, in turn, is used to produce a
chilled inlet air stream at least by heat exchanging the
chilled coolant with the inlet air stream. The chilled
inlet air stream to the gas turbine that drives a
refrigerant circuit employed for cooling a hydrocarbon
stream in the one or more heat exchangers, and producing
a liquefied hydrocarbon stream therefrom.
Such fractions of hydrocarbon stream are often
removed from the hydrocarbon stream to be liquefied
anyway, for various uses or reasons. Since the fraction
is removed downstream of at least one of the one or more
heat exchangers, it has the ability to chill the inlet
air stream before its other use or before being
discarded.
Any cooling duty that can be provided from the
removed fraction for the purpose of gas turbine inlet air
chilling, does not need to be removed from a refrigerant
cycle that is intended to cool the hydrocarbon stream to
be liquefied. This way, the invention helps to further
increase the production rate of the liquefied hydrocarbon
without a need to install additional refrigeration power.
Examples of removed fractions that can be employed
for chilling the inlet air stream of a gas turbine
include:
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- a natural gas liquids stream that has been
extracted from the hydrocarbon stream in order to meet a
composition specification for the liquefied hydrocarbon
stream;
- a fuel gas stream removed from the hydrocarbon
stream for the purpose of being combusted, for instance
in one or more of the gas turbines;
- an end flash stream created upon depressurizing a
pressurized liquefied hydrocarbon stream;
- a boil-off gas stream originating from the
liquefied hydrocarbon stream during its storage in a
storage tank.
Again, in the context of the other invention, the
term "chilled coolant" is understood to be a fluid that
has a temperature lower than that of the ambient air
temperature. But in this case, the chilled coolant can
be prepared by actively chilling the fluid, using
refrigeration duty from any cold stream available in the
hydrocarbon liquefaction process that is not cycled in a
refrigerant circuit.
Favorable examples include the liquid bottom stream
of an extraction column and/or a fractionation column
and/or an overhead stream from a fractionation column, a
stream of end-flash gas that may be generated when
letting down the pressure of the liquefied hydrocarbon
stream, a stream of boil-off gas that may be evaporated
off of the liquefied hydrocarbon while in storage.
The cooling duty available in one or more of these
removed fractions may be supplemented by cooling duty
obtained from a refrigerant cycled in a refrigerant
circuit. Examples include mechanical chilling or
absorption chilling. The cooling duty may for instance
be supplemented using an external chilling package.
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The indirect heat exchanging of the hydrocarbon
stream in one or more heat exchangers against one or more
refrigerants from one or more refrigerant circuits may
comprise:
- cooling the hydrocarbon stream by heat exchanging
against a first refrigerant from a first refrigerant
circuit in which said first refrigerant is compressed in
a first compressor driven by a first gas turbine having a
first inlet air stream, said cooling providing a cooled
hydrocarbon stream;
- liquefying at least part of the cooled hydrocarbon
stream using a second refrigerant, which second
refrigerant is compressed in a second compressor driven
by a second gas turbine having a second inlet air stream,
and cooled at least by heat exchanging with said first
refrigerant from the first refrigerant circuit, said
liquefying providing a liquefied hydrocarbon stream;
wherein said chilling of said inlet air stream comprises
cooling one or both of said first and second inlet air
streams with at least a part of the chilled coolant.
These features have been amply described in the
preceding parts of the specification. It also follows
from the preceding parts of the specification that
advantages embodiments may further comprise:
- dividing the cooling duty available in the chilled
coolant over at least first and second parts, whereby the
cooling duty available in the first part is used to cool
the first inlet air stream, and the cooling duty
available in the second part is used to cool the second
inlet air stream. Said cooling duty may be divided in
accordance with the common input parameter as set forth
in the preceding parts of the specification, preferably
to divide the cooling duty available in the chilled
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coolant such as to provide relatively more chilling duty
to the inlet air stream of the gas turbine that drives
the most constraining refrigerant circuit of the first
and second refrigerant circuits.
However, it should be stressed that the other
invention now being described is not limited to two
refrigeration circuits. It can for instance also be
applied for dividing cooling duty of the chilled coolant
over three or more parts for cooling third or more inlet
air streams of other refrigerant circuits. And the other
invention is also useful in liquefaction processes that
use only one refrigeration circuit, typically consisting
of so-called single mixed refrigerant processes. Amongst
others, an example is formed by the Shell single mixed
refrigerant process described in U.S. Patent 5,832,745.
Said withdrawing of the fraction from the hydrocarbon
stream after it has been heat exchanged in at least one
of the one or more heat exchangers may comprise:
- producing a partially condensed hydrocarbon stream from
the gaseous hydrocarbon stream;
- passing the partially condensed hydrocarbon stream
through a gas/liquid phase separator; and
- drawing a liquid bottom stream and a vaporous overhead
stream from the gas/liquid phase separator. In such
embodiments, said fraction from the hydrocarbon stream
may advantageously comprises the liquid bottom stream and
said liquefied hydrocarbon stream is produced from the
vaporous overhead stream. Alternatively or in addition
thereto, such embodiments may comprise drawing off a fuel
gas stream from the vaporous overhead stream and wherein
said fraction from the hydrocarbon stream comprises the
fuel gas stream.
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Said withdrawing of the fraction from the hydrocarbon
stream after it has been heat exchanged in at least one
of the one or more heat exchangers may also, or instead,
comprise:
- obtaining at least an intermediate liquefied
hydrocarbon stream out of the hydrocarbon stream;
- depressurizing the intermediate liquefied hydrocarbon
stream;
- passing the depressurized stream into a phase
separator;
- separating any vaporous constituents from any liquid
hydrocarbons in the depressurized stream;
- removing the liquid hydrocarbons from the phase
separator as the produced liquefied hydrocarbon product
stream;
- removing the vaporous constituents from the phase
separator,
wherein said fraction from the hydrocarbon stream
comprises the vaporous constituents withdrawn from the
phase separator.
Said withdrawing of the fraction from the hydrocarbon
stream after it has been heat exchanged in at least one
of the one or more heat exchangers may also, or instead,
comprise:
- storing the produced liquefied hydrocarbon stream in a
storage tank; and
- withdrawing boil off gas, originating from the
liquefied hydrocarbon stream being stored, from the
storage tank,
wherein said fraction from the hydrocarbon stream
comprises the boil off gas.
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In another aspect, the other invention may be defined
as providing an apparatus for producing a liquefied
hydrocarbon stream, comprising:
- one or more refrigerant circuits each comprising a
refrigerant, at least one of which refrigerant circuits
comprising a compressor driven by a gas turbine, for
compressing the refrigerant of that refrigerant circuit;
- an inlet air stream to the gas turbine;
- one or more heat exchangers for indirectly heat
exchanging a hydrocarbon stream against one or more
refrigerants from the one or more refrigerant circuits,
including said at least one;
- withdrawing means for withdrawing a fraction of the
hydrocarbon stream downstream of at least one of the one
or more heat exchangers and providing a remaining
hydrocarbon stream from which the fraction has been
withdrawn;
- a chiller connected to the withdrawing means and
arranged to receive at least part of the withdrawn
fraction from the withdrawing means, and further arranged
to indirectly heat exchange a coolant fluid against the
at least part of the withdrawn fraction to produce a
stream of a chilled coolant from the coolant fluid
- an inlet air cooling heat exchanger arranged in the
inlet air stream to cool the inlet air stream with the
the chilled coolant;
- a feed duct to feed the cooled inlet air stream from
the inlet air cooling heat exchanger into the gas
turbine;
- conduit means for conveying a liquefied hydrocarbon
stream that comprises at least part of the remaining
hydrocarbon stream.
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As in embodiments described in the preceding parts of
the specification, the one or more refrigerant circuits
may comprise:
- a first refrigerant circuit comprising a first
refrigerant, a first compressor, a first gas turbine
coupled to the first compressor to drive the first
compressor, and a first inlet air stream to the first gas
turbine; the first compressor arranged to compress said
first refrigerant;
- a second refrigerant circuit comprising a second
refrigerant, a second compressor, a second gas turbine
coupled to the second compressor to drive the second
compressor, and a second inlet air stream to the second
gas turbine; the second compressor arranged to compress
said second refrigerant;
and wherein the one or more heat exchangers comprise:
- one or more first heat exchangers arranged to receive
and cool the gaseous hydrocarbon stream and the second
refrigerant, using said first refrigerant from said
cooling providing a cooled hydrocarbon stream and a
cooled second refrigerant stream;
- one or more second heat exchangers arranged to receive
and liquefy the cooled hydrocarbon stream using the
cooled second refrigerant stream, so as to provide the
liquefied hydrocarbon stream;
and wherein the inlet air cooling heat exchanger is
arranged in at least one of the first and the second
inlet air streams.
Such embodiments may further comprise:
- a divider to divide the chilled coolant over at least
first and second parts in accordance with a common input
parameter;
wherein the inlet air cooling heat exchanger comprises:
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- a first inlet air cooling heat exchanger arranged in
the first inlet air stream to cool the first inlet air
stream with the first part of the chilled coolant;
- a second inlet air cooling heat exchanger arranged in
the second inlet air stream to cool the second inlet air
stream with the second part of the chilled coolant.
In preferred embodiments, the withdrawing means may
comprise a gas/liquid separator having an overhead outlet
for discharging a vaporous overhead stream and a bottom
outlet for discharging a liquid bottom stream. In such
embodiments, said fraction of the hydrocarbon stream may
advantageously comprise the liquid bottom stream and said
remaining stream comprises the vaporous overhead stream.
Alternatively or in addition thereto, such embodiments
may further comprise a splitter in the vaporous overhead
stream for drawing off a fuel gas stream from the
vaporous overhead stream and wherein said fraction of the
hydrocarbon stream comprises the fuel gas stream.
Alternatively or in addition thereto, the withdrawing
means may comprise:
- depressurizing equipment arranged to receive an
intermediate liquefied hydrocarbon stream formed out of
the hydrocarbon stream and to form a depressurized stream
therefrom;
- phase separation equipment arranged downstream of the
depressurizing equipment to receive the depressurized
stream and to separate any vaporous constituents from any
liquid hydrocarbons in the depressurized stream;
- a liquid discharge line connected to the phase
separation equipment for removing liquid hydrocarbons
from the phase separator as the produced liquefied
hydrocarbon product stream;
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- a vapour discharge line connected to the phase
separation equipment withdrawing the vaporous
constituents from the phase separator,
wherein said fraction from the hydrocarbon stream
comprises the vaporous constituents removed from the
phase separator.
The apparatus may comprise a storage tank for storing
the produced liquefied hydrocarbon stream. In such a
case, the withdrawing means may comprise:
- a boil-off gas conduit connected to the storage tank
for withdrawing boil-off gas, originating from the
liquefied hydrocarbon stream being stored, from the
storage tank. In such embodiments, said fraction from the
hydrocarbon stream may comprise the boil off gas.
The other invention will now be further illustrated
in more detail by way of example and with reference to
figures in the accompanying drawing.
Referring to Figure 1, a liquefied hydrocarbon stream
is produced by indirect heat exchanging a hydrocarbon
20 stream 10 in one or more heat exchangers 140 (and/or 260)
against one or more refrigerants from one or more
refrigerant circuits 100 (and/or 200). At least one of
these refrigerant circuits comprises a compressor 110
(and/or 210) driven by a gas turbine 120 (and/or 220) by
which compressor the refrigerant of that refrigerant
circuit is compressed. A fraction 70 (and/or 62 and/or
92) is withdrawn from the hydrocarbon stream after it has
been heat exchanged in at least one of the one or more
heat exchangers and a stream of a chilled coolant 320 is
provided by indirect heat exchanging the coolant 315
against at least a part CF of the withdrawn fraction of
the hydrocarbon stream. An inlet air stream 125 (and/or
225) is chilled with the chilled coolant 320 to produce a
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chilled inlet air stream, which is fed to the gas
turbine. The produced liquefied hydrocarbon stream 20
comprises at least part of the hydrocarbon stream that
has not been withdrawn.
It is herewith proposed to use refrigeration duty
provided by any cold stream available in the process that
is not circulated in a refrigerant circuit. More
specifically, the refrigeration duty may be provided by a
fraction withdrawn from the hydrocarbon stream after it
has been heat exchanged in at least one of the one or
more heat exchangers, thus suitably downstream of at
least one of the one or more heat exchangers. Suitably,
the fraction is subsequently discarded from the process
or subsequently used in the process in a way that it
needed to be warmer. In both these cases, the cold in
the fraction is favourably used to chill inlet air and
thereby increase the LNG production.
For instance, referring to Figure 1, if gas/liquid
phase separator 50 is present it may be comprised in the
withdrawing means, in which case the cold fluid CF may
for instance be derived from or contain the liquid bottom
stream 70. In this case, optional heat exchanger 73 may
be in communication with stream 320 or the optional heat
exchanger 73 may be one of the one or more chillers 325.
Still referring to Figure 1, other examples of cold
streams that may be used to provide part of or all of the
refrigeration duty for the active cooling of the coolant
include fuel gas stream 62, end flash stream 92, and any
cold stream from (optional) fractionation train 75.
Figure 1 shows symbolically optional chillers 61 and 91
that could be used as chiller(s) 325 or be otherwise
positioned in communication with line 320. Boil off gas,
for instance from a storage tank wherein the liquefied
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hydrocarbon stream 20 may be stored, may also be used to
provide part of or all of the refrigeration duty for the
active cooling of the coolant.
In addition to any one of these streams mentioned
above, other sources chilling duty provided may be
employed for the inlet air chilling, including any
refrigerant cycled in a refrigerant circuit and
undergoing compression and expansion in the circuit (as
known in the art) or a refrigerant cycled in a thermally
driven chilling process. Reference is made to the
earlier description of Figure 1 for further details.
The alternative drive schemes as illustrated in
Figures 3 to 5 can also be applied with the other
invention now being described.
The person skilled in the art will understand that
each of the present inventions can be carried out in many
various ways without departing from the scope of the
appended claims.
