Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR REMOVING NITROGEN FROM A
CRYOGENIC HYDROCARBON COMPOSITION
The present invention relates to a method and
apparatus for removing nitrogen from a cryogenic
hydrocarbon composition.
Liquefied natural gas (LNG) forms an economically
important example of such a cryogenic hydrocarbon
composition. 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.
WO 2011/009832 describes a method for treating a
multi-phase hydrocarbon stream produced from natural gas,
wherein lower boiling point components, such as nitrogen,
are separated from the multi-phase hydrocarbon stream, to
produce a liquefied natural gas stream with a lower
content of such lower boiling point components. It
employs two subsequent gas/liquid separators operating at
different pressures. The multi-phase hydrocarbon stream
is fed into the first gas/liquid separator at a first
pressure. The bottom stream of the first gas/liquid
separator is passed to the second gas/liquid separator,
which provides vapour at a second pressure that is lower
than the first pressure. The vapour is compressed in an
overhead stream compressor, and returned to the first
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gas/liquid separator as a stripping vapour stream.
Compressed boil-off gas from a cryogenic storage tank may
be added to the stripping vapour stream. The first
gas/liquid separator comprises a contacting zone, with
contact enhancing means such as trays or packing,
arranged gravitationally between the inlet for the
multiphase hydrocarbon stream into the first gas/liquid
separator and the inlet for the stripping vapour stream.
A low pressure fuel gas stream is prepared from the
overhead vapour stream discharged from the first
gas/liquid separator, which low pressure fuel gas stream
is passed to a combustion device.
A drawback of the method and apparatus as described
in WO 2011/009832 is that the equilibrium in the first
gas/liquid separator can be disturbed if the amount of
stripping vapour changes substantially, which could be
the case when the plant transits between holding mode and
loading mode operation.
The present invention provides a method of removing
nitrogen from a cryogenic hydrocarbon composition
comprising a nitrogen- and methane-containing liquid
phase, the method comprising:
- providing a cryogenic hydrocarbon composition
comprising a nitrogen- and methane-containing liquid
phase;
feeding a first nitrogen stripper feed stream, at a
stripping pressure, into a nitrogen stripper column
comprising at least one internal stripping section
positioned within the nitrogen stripper column, said
first nitrogen stripper feed stream comprising a first
portion of the cryogenic hydrocarbon composition;
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drawing a nitrogen-stripped liquid from a sump space
of the nitrogen stripper column below the stripping
section;
- producing at least a liquid hydrocarbon product
stream and a process vapour from the nitrogen-stripped
liquid, comprising at least a step of depressurizing the
nitrogen-stripped liquid to a flash pressure;
compressing said process vapour to at least the
stripping pressure, thereby obtaining a compressed
vapour;
selectively splitting the compressed vapour into a
stripping portion and a non-stripping portion that does
not comprise the stripping portion, which non-stripping
portion comprises a bypass portion of said compressed
vapour;
- passing a stripping vapour stream into the nitrogen
stripper column at a level gravitationally below said
stripping section, said stripping vapour stream
comprising at least the stripping portion of said
compressed vapour;
- passing an intermediate vapour through a condenser
whereby indirectly heat exchanging of the intermediate
vapour against an auxiliary refrigerant stream and
partially condensing the intermediate vapour, wherein
said intermediate vapour comprises at least the non-
stripping portion of said compressed vapour, and wherein
said heat exchanging comprises passing heat from the
intermediate vapour to the auxiliary refrigerant stream
at a cooling duty, whereby an excess liquid is formed
from the intermediate vapour and whereby at least said
bypass portion from the compressed vapour remains in
vapour phase;
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discharging a vapour fraction as off gas, comprising
a discharge fraction of an overhead vapour obtained from
an overhead space of the nitrogen stripper column and
comprising at least the bypass portion; and
returning at least part of a liquid recycle portion
to the liquid hydrocarbon product stream, wherein the
liquid recycle portion comprises at least part of the
excess liquid;
wherein from said selectively splitting to the
discharging of the bypass portion in the vapour fraction
of the off gas the bypass portion bypasses the at least
one internal stripping section.
In another aspect, the present invention provides an
apparatus for removing nitrogen from a cryogenic
hydrocarbon composition comprising a nitrogen- and
methane-containing liquid phase, the apparatus
comprising: for removing nitrogen from a cryogenic
hydrocarbon composition comprising a nitrogen- and
methane-containing liquid phase, the apparatus
comprising:
a cryogenic feed line connected to a source of a
cryogenic hydrocarbon composition comprising nitrogen and
a methane-containing liquid phase;
a nitrogen stripper column in fluid communication
with the cryogenic feed line, said nitrogen stripper
column comprising at least one internal stripping section
positioned within the nitrogen stripper column and a sump
space defined gravitationally below the stripping
section;
a nitrogen-stripped liquid discharge line comprising
an intermediate depressurizer, in fluid communication
with the sump space of the nitrogen stripper column
arranged to receive a nitrogen-stripped liquid from the
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sump space and to depressurize the nitrogen-stripped
liquid, said intermediate depressurizer located on an
interface between a stripping pressure side comprising
the nitrogen stripper column and a flash pressure side;
a liquid hydrocarbon product line arranged on the
flash pressure side in communication with the
intermediate depressurizer, to discharge a liquid
hydrocarbon product stream produced from the nitrogen-
stripped liquid;
a process vapour line arranged on the flash pressure
side in communication with the intermediate
depressurizer, to receive a process vapour produced from
the nitrogen-stripped liquid;
a process compressor arranged in the process vapour
line arranged to receive the process vapour and compress
the process vapour to provide a compressed vapour at a
process compressor discharge outlet of the process
compressor, said process compressor being on said
interface between the stripping pressure side and the
flash pressure side;
a bypass splitter, whereby an upstream side thereof
is in fluid communication with the discharge outlet of
the process compressor to receive the compressed vapour,
and of which bypass splitter a first discharge side is in
fluid communication with the nitrogen stripper column via
a stripping vapour line and a second inlet system
arranged at a level gravitationally below the stripping
section and arranged to receive at least a stripping
portion of said compressed vapour from the process
compressor, and of which bypass splitter a second
discharge side is in fluid communication with a vapour
bypass line containing a non-stripping portion of the
compressed vapour;
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a condenser arranged in fluid communication with the
vapour bypass line to bring an intermediate vapour that
comprises at least the non-stripping portion from the
vapour bypass line, which condenser comprises a heat
exchanging surface providing indirect heat exchange
contact between the intermediate vapour and an auxiliary
refrigerant stream;
a discharge line in communication with both the
condenser and with an overhead space of the nitrogen
stripper column, and arranged to discharge a vapour
fraction as off gas comprising an overhead vapour
obtained from the overhead space of the nitrogen stripper
column and a bypass portion comprising a non-condensed
vapour from the intermediate vapour that has passed
though the condenser; and
a liquid recycle line on its upstream side in fluid
communication with the condenser and on its downstream
side in liquid communication with the liquid hydrocarbon
product line;
wherein a bypass path extends between the bypass splitter
and the discharge line, wherein the bypass path bypasses
the at least one internal stripping section.
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 incorporating an
embodiment of the invention; and
Fig. 2 schematically represents a process flow scheme
representing a method and apparatus incorporating another
embodiment of the invention.
In these figures, same reference numbers will be used
to refer to same or similar parts. Furthermore, a single
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reference number will be used to identify a conduit or
line as well as the stream conveyed by that line.
The present description concerns removal of nitrogen
from a cryogenic hydrocarbon composition comprising a
nitrogen- and methane-containing liquid phase. A least a
first portion of the cryogenic hydrocarbon composition is
fed to a nitrogen stripper column as a first nitrogen
stripper feed stream. A nitrogen-stripped liquid is
drawn from the nitrogen stripper column. A liquid
hydrocarbon product stream and a process vapour are
produced comprising at least a step of depressurizing the
nitrogen-stripped liquid to a flash pressure. The
process vapour is compressed, and selectively split into
a stripping portion and a non-stripping portion. A
stripping vapour stream comprising at least the stripping
portion is passed into the nitrogen stripper column
gravitationally below a stripping section positioned
therein. An intermediate vapour, comprising at least the
non-stripping portion of the compressed process vapour,
is passed through a condenser whereby an excess liquid is
formed from the intermediate vapour and whereby at least
a bypass portion from the compressed vapour remains in
vapour phase. A vapour fraction is discharged as off
gas, comprising a discharge fraction of overhead vapour
from the nitrogen stripper column and comprising at least
the bypass portion from the compressed vapour which
bypasses the stripping section positioned in the nitrogen
stripper column. A liquid recycle portion comprises at
least part of the excess liquid. At least part of the
liquid recycle portion is returned to the liquid
hydrocarbon product stream.
An advantage of splitting off the non-stripping
portion, which contains the bypass portion from the
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compressed process vapour, and passing at least the
bypass portion to the off gas whereby bypassing at least
the stripping section positioned within the nitrogen
stripper column, is that the nitrogen stripper column can
be protected against excess flow of stripping vapour
flowing through the stripping section. Such excess flow
may cause disturbance of the equilibrium conditions.
Thanks to passing the non-stripping portion through the
condenser, it is avoided that valuable parts of the
process vapour that are split off in the non-stripping
portion, such as typically vaporous methane, are lost
through the off gas but instead can be re-condensed and
added to excess liquid, which is led back ultimately to
the liquid hydrocarbon product stream.
The vapour fraction in the off gas generally has a
heating value. Preferably, the cooling duty in the
condenser is adjusted to regulate the heating value of
the vapour fraction being discharged. The ability to
regulate the heating value is advantagous allows to
stabilize the heating value of the vapour fraction in the
off gas against variation or fluctuations in the flow
rate and/or the composition of the bypass portion from
the compressed process vapour compared to the flow rate
and/or composition of the overhead vapour from the
nitrogen stripper column. Variations in both flow rate
and compositions can be expected in an LNG plant when
transiting from holding mode operation to loading mode
operation. Not only is the vapour flow rate higher
during loading mode, the composition is leaner as well
(particularly containing more nitrogen). The ability to
adjust the bypass portion as well as the cooling duty in
the condenser both contribute to the ability to handle
the additional vapour load during the loading mode.
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The process vapour may comprise vaporous methane that
has previously formed part of the raw liquefied product.
Vaporous methane that has previously formed part of the
raw liquefied product can be formed in an LNG
liquefaction plant due to various reasons. During normal
operation of a natural gas liquefaction facility, methane
containing vapour is formed from the (raw) liquefied
product in the form of:
- flash vapour resulting from flashing of the raw
liquefied product during depressurizing; and
- boil-off gas resulting 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. During this mode of
operation, known as holding mode operation, the storage
tanks are being filled with the liquefied hydrocarbon
product as it comes out of the plant without any
transporter loading operations taking place at the same
time. When in holding mode, the methane-containing
vapours are generated on the plant side of the storage
tanks.
The operation mode of an LNG plant while there are
ongoing transporter loading operations (typically ship
loading operations) is known as loading mode operation.
During loading mode operation, boil-off gas is
additionally produced on the ship side of the storage
tanks, for instance due to initial chilling of the ship
tanks; vapour displacement from the ship tanks; heat
leakage through piping and vessels connecting the storage
tanks and the ships, and heat input from LNG loading
pumps.
The proposed solution may facilitate the handling of
these vapours both during holding mode and loading mode
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operations. It combines the removal of nitrogen from the
cryogenic hydrocarbon composition with re-condensation of
excess vaporous methane. This forms an elegant solution
in situations where little plant fuel is demanded, such
as could he the case in an electrically driven plant
using electric power from an external power grid.
While the process vapour may comprise one or both of
flash vapour and boil-off gas, it is particularly suited
for boil-off gas. The flow rate of boil-off gas is the
most subject to variation in a typical LNG plant. Since
the proposed solution allows for selectively stripping of
the compressed vapour into stripping and non-stripping
portions, it allows to selectively bypass the stripping
section in the nitrogen stripper column with any process
vapour in excess of what is needed as stripping vapour.
This makes the proposed solution particularly suited to
accommodate boil-off gas into the process vapour.
Figure 1 illustrates an apparatus comprising an
embodiment of the invention. A cryogenic feed line 8 is
in fluid communication with a nitrogen stripper column
20, via a first inlet system 21. A first feed line 10
connects the cryogenic feed line 8 with the first inlet
system 21 of the nitrogen stripper column 20, optionally
via an initial stream splitter 9 arranged between the
cryogenic feed line 8 and the first feed line 10.
Upstream of the cryogenic feed line 8, a liquefaction
system 100 may be provided. The liquefaction system 100
functions as a source of a cryogenic hydrocarbon
composition. The liquefaction system 100 is in fluid
communication with the cryogenic feed line 8 via a main
depressurizing system 5, which communicates with the
liquefaction system 100 via a raw liquefied product line
1. In the embodiment as shown, the main depressurizing
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system 5 consists of a dynamic unit, such as an expander
turbine 6, and a static unit, such as a Joule Thomson
valve 6, but other variants are possible. Preferably,
but not necessarily, any compressor forming part of the
hydrocarbon liquefaction process in the liquefaction
system, particularly any refrigerant compressor, is
driven by one or more electric motors, without being
mechanically driven by any steam- and/or gas turbine.
Such compressor may be driven exclusively by one or more
electric motors.
The nitrogen stripper column 20 comprises an internal
stripping section 24 positioned within the nitrogen
stripper column 20. An overhead vapour discharge line 30
communicates with the nitrogen stripper column 20 via an
overhead space 26 within the nitrogen stripper column 20.
A nitrogen-stripped liquid discharge line 40 communicates
with the nitrogen stripper column 20 via a sump space 28
within the nitrogen stripper column 20 gravitationally
below the stripping section 24.
The nitrogen stripper column 20 may comprise
vapour/liquid contact-enhancing means to enhance
component separation and nitrogen rejection. Depending
on the tolerable amount of nitrogen in the nitrogen
stripped liquid and the amount of nitrogen in the
cryogenic feed line 8, between 2 and 8 theoretical stages
may typically be needed in total. In one particular
embodiment, 4 theoretical stages were required. Such
contact-enhancing means may be provided in the form of
trays and/or packing, in the form of either structured or
non-structured packing. At least part of the
vapour/liquid contact-enhancing means suitably forms part
of the internal stripping section 24.
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An intermediate depressurizer 45 is arranged in the
nitrogen-stripped liquid discharge line 40, and thereby
fluidly connected to the nitrogen stripper column 20. The
intermediate depressurizer 45 is functionally coupled to
a level controller LC, which cooperates with the sump
space 28 of the nitrogen stripper column 20.
The intermediate depressurizer 45 is located on an
interface between a stripping pressure side comprising
the nitrogen stripper column 20, and a flash pressure
side. The flash pressure side comprises a liquid
hydrocarbon product line 90, arranged to discharge a
liquid hydrocarbon product stream produced from the
nitrogen-stripped liquid 40, and a process vapour line
60, arranged to receive a process vapour produced from
the nitrogen-stripped liquid 40. In the embodiment as
shown, the flash pressure side furthermore comprises a
cryogenic storage tank 210 connected to the liquid
hydrocarbon product line 90 for storing the liquid
hydrocarbon product stream, an optional boil-off gas
supply line 230, and an optional end flash separator 50.
If such end flash separator 50 is provided, such as
is the case in the embodiment of Figure 1, it may be
configured in fluid communication with the nitrogen
stripper column 20 via the intermediate depressurizer 45
and the nitrogen-stripped liquid discharge line 40. The
end flash separator 50 may then be connected to the
cryogenic storage tank 210 via the liquid hydrocarbon
product line 90. A cryogenic pump 95 may be present in
the liquid hydrocarbon product line 90 to assist the
transport of the liquid hydrocarbon product to the
cryogenic storage tank 210.
If the initial stream splitter 9 is provided, the
cryogenic feed line 8 is also connected to at least one
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of the group consisting of: the nitrogen-stripped liquid
discharge line 40, the liquid hydrocarbon product line 90
and the process vapour line 60. To this end, a second
feed line 11 is connected at an upstream side thereof to
the optional initial splitter 9. This second feed line
11 bypasses the nitrogen stripper column 20. A bypass
stream flow control valve 15 is arranged in the second
feed line 11. The bypass stream flow control valve is
functionally connected to a flow controller PC provided
in the first feed line 10. Suitably, the second feed
line 11 feeds into the optional end flash separator 50.
A benefit of the optional second feed line 11 and the
optional initial splitter 9 is that the nitrogen stripper
column 20 can be sized smaller than in the case that the
cryogenic feed line 8 and the first feed line 10 are
directly connected without a splitter such that all of
the cryogenic hydrocarbon composition is let into the
nitrogen stripper column 20 via the first inlet system
21.
The process vapour line 60, as shown in the
embodiment of Fig. 1, may be connected to the optional
end flash separator 50 via a flash vapour line 64 and
flash vapour flow control valve 65, as well as to the
cryogenic storage tank 210 via the optional boil-off gas
supply line 230. An advantage of the latter connection
is that it allows for re-condensing of at least part of
the boil-off gas from the cryogenic storage tank 210 by
means of a condenser, which will be further discussed
herein below.
Also configured on the interface between the
stripping pressure side and the flash pressure side, is a
process compressor 260. Preferably, the process
compressor 260 is driven by an electric motor. The
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process compressor 260 is arranged in the process vapour
line 60 to receive the process vapour and to compress the
process vapour. A compressed vapour discharge line 70 is
fluidly connected with a process compressor discharge
outlet 261 of the process compressor 260. Suitably, the
process compressor 260 is provided with anti-surge
control and a recycle cooler which is used when the
process compressor is on recycle and during start-up (not
shown in the drawing).
A stripping vapour line 71 is in fluid communication
with the nitrogen stripper column 20 via a second inlet
system 23 configured at a level gravitationally below the
stripping section 24 and preferably above the sump space
28. The stripping vapour line 71 is connected to the
compressed vapour discharge line 70 via a bypass splitter
79. A stripping vapour valve 75 is provided in the
stripping vapour line 71.
Optionally, an external stripping vapour supply line
74 is provided in fluid communication with the second
inlet system 23 of the nitrogen stripper column 20. In
one embodiment, as shown in Fig. 1, the optional external
stripping vapour supply line 74 connects to the
compressed vapour discharge line 70. An external
stripping vapour flow control valve 73 is provided in the
optional external stripping vapour supply line 74. In
one embodiment, the optional external stripping vapour
supply line 74 is suitably connected to a hydrocarbon
vapour line in, or upstream of, the liquefaction system
100.
The bypass splitter 79 is also in fluid communication
with a condenser via at least a vapour bypass line 76. A
vapour bypass control valve 77 is preferably provided in
the vapour bypass line 76. The vapour bypass line 76
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contains a non-stripping portion of the compressed vapour
from the compressed vapour discharge line 70. The
condenser can be any type of indirect heat exchanger in
fluid communication with the bypass splitter 79 via the
vapour bypass line 76. Such condenser is advantageously
utilized to re-condense at least part of compressed
process vapour from the compressed vapour discharge line
70.
Figure 1 shows a convenient embodiment wherein the
condenser is provided in the form of an overhead
condenser 35 external to the nitrogen stripper column 20.
The overhead condenser 35 is arranged in fluid
communication with both the overhead vapour discharge
line 30 and the vapour bypass line 76, to partially
condense an intermediate vapour stream that contains the
non-stripping portion from the vapour bypass line 76 in
addition to any overhead vapour being discharged from the
nitrogen stripper column 20. The condenser comprises a
heat exchanging surface that provides Indirect heat
exchange contact between the intermediate vapour and the
auxiliary refrigerant stream 132, whereby heat can pass
from the intermediate vapour to the auxiliary refrigerant
stream 132 at a cooling duty. An auxiliary refrigerant
stream flow control valve 135 is provided in the
auxiliary refrigerant line 132.
In the embodiment of Figure 1, the vapour bypass line
76 suitably extends along a bypass path extending between
the bypass splitter 79 and the overhead vapour discharge
line 30 on an upstream side of the overhead condenser 35.
The bypass path extends between the bypass splitter 79
and the overhead vapour discharge line 30 and/or the
vapour fraction discharge line 80. The bypass path does
not pass through the internal stripping section 24 in the
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nitrogen stripper column 20. This way it can be avoided
that the non-stripping portion passes through the
internal stripping section 24, which helps to avoid
disturbing the equilibrium in the nitrogen stripper
column 20.
Still referring to Figure 1, an overhead separator 33
is arranged on a downstream side of the overhead vapour
discharge line 30. The overhead vapour discharge line 30
discharges into the overhead separator 33. The overhead
separator 33 is arranged to separate any, non-condensed,
vapour fraction from any condensed fraction of the
overhead vapour.
A vapour fraction discharge line 80 is arranged to
discharge the vapour fraction mentioned above. The
vapour fraction discharge line 80 is in fluid
communication with both the condenser and with the
overhead space 26 of the nitrogen stripper column 20. In
embodiments such as the one of Figure l, wherein the
intermediate vapour contains both the overhead vapour and
the non-stripping vapour the vapour fraction discharge
line 80 is inherently in communication with both the
condenser and with the overhead space 26 of the nitrogen
stripper column 20. The bypass path in this embodiment
extends to the vapour fraction discharge line 80.
A benefit of the vapour bypass line 76 is that at
times when there is an excess of process vapour, this can
be processed together with the off gas in the vapour
fraction discharge line 80 without upsetting the material
balance in the nitrogen stripper column 20.
The condenser is also in fluid communication with a
liquid recycle line 13. The liquid recycle line 13 is in
liquid communication with the liquid hydrocarbon product
line 90. Liquid communication means that the liquid
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recycle line 13 is connected to any suitable location
from where at least a part of a liquid recycle portion
can flow into the liquid hydrocarbon product line 90
while staying in the liquid phase. Thus, the liquid
recycle line 13 may for instance he connected directly to
one or more selected from the group consisting of: the
nitrogen stripper column 20, the cryogenic feed line 8,
the first feed line 10, the optional second feed line 11,
the nitrogen-stripped liquid discharge line 40, the
optional end flash separator 50 and the liquid
hydrocarbon product line 90. A recycle valve 14 is
configured in the liquid recycle line 13.
Optionally, the nitrogen stripper column 20 comprises
an internal rectifying section 22 in addition to the
internal stripping section 24. The internal rectifying
section 22 is positioned within the nitrogen stripper
column 20, gravitationally higher than the stripping
section 24. The overhead space 26 is preferably defined
gravitationally above the rectifying section 22. The
first inlet system 21 is provided gravitationally between
the internal rectifying section 22 and the internal
stripping section 24. The overhead space 26 is
gravitationally above the rectifying section 22.
The optional internal rectifying section 22 may
comprise vapour/liquid contact-enhancing means similar to
the internal stripping section 24, to further enhance
component separation and nitrogen rejection.
A reflux system may be arranged to allow at least a
reflux portion 36 of the condensed fraction into the
nitrogen stripper column 20 at a level above the
rectifying section 22. In the embodiment of Figure 1,
the reflux system comprises a condensed fraction
discharge line 37 fluidly connected to a lower part of
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the overhead separator 33, an optional reflux pump 38
provided in the condensed fraction discharge line 37, and
a condensed fraction splitter 39. The condensed fraction
splitter 39 fluidly connects the condensed fraction
discharge line 37 with the nitrogen stripper column 20,
via a reflux portion line 36 and a reflux inlet system
25, and with the liquid recycle line 13. An optional
reflux flow valve 32 functionally controlled by a reflux
flow controller (not shown) may preferably be provided in
the reflux portion line 36.
In embodiments wherein the nitrogen stripper column
comprises the optional internal rectifying section 22,
the liquid recycle line 13 is preferably in liquid
communication with the liquid hydrocarbon product line 90
15 via a recycle path that does not pass through the
rectifying section 22 if it is provided. This way the
liquid recycle line 13 helps to avoid feeding too much
liquid onto the rectifying section 22 and to avoid
passing the recycle liquid through the rectifying section
20 22. This is beneficial to avoid disturbing the
equilibrium in the nitrogen stripper column 20.
A cooling duty controller 34 may be provided to
control the cooling duty, being the rate at which heat
passes from the intermediate vapour to the auxiliary
refrigerant stream. Suitably, the cooling duty
controller 34 is configured to control the cooling duty
in response to an indicator of heating value of the off
gas relative to a demand for heating power. In the
embodiment as shown, the cooling duty controller 34 is
embodied in the form of a pressure controller PC and the
auxiliary refrigerant stream flow control valve 135,
which are functionally coupled to each other.
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A combustion device 220 is suitably arranged on a
downstream end of the vapour fraction discharge line 80,
to receive at least a fuel portion of the vapour fraction
in the vapour fraction discharge line 80. The combustion
device may comprise multiple combustion units, and/or it
may include for example one or more of a furnace, a
boiler, an incinerator, a dual fuel diesel engine, or
combinations thereof. A boiler and a duel fuel diesel
engine may be coupled to an electric power generator.
The amount of methane in the off gas can be
controlled to meet a specific demand for methane. This
renders the off gas suitable for use as fuel gas stream,
preferably at a fuel gas pressure not higher than the
stripping pressure, even in circumstances where the
demand for heating value is variable.
A vapour recycle line 87 is optionally configured to
receive at least a vaporous recycle portion of the vapour
from the overhead discharge line 30. The vapour recycle
line 87 bypasses the nitrogen stripper column 20, and
feeds back into at least one of the group consisting of:
the liquid hydrocarbon product line 90 and the process
vapour line 60. A vapour recycle flow control valve 88
is preferably provided in the vapour recycle line 87. A
benefit of the proposed vapour recycle line 87 is that it
allows for selectively increasing of the nitrogen content
in the liquid hydrocarbon product stream 90. If the
optional end flash separator 50 is provided, the vapour
recycle line 87 suitably feeds into the end flash
separator 50.
Suitably, the configuration of the optional vapour
recycle line 87 comprises an optional vapour fraction
splitter 89, which may be provided in the vapour fraction
line 80, allowing controlled fluid communication between
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the vapour fraction line 80 and the vapour recycle line
87.
A cold recovery heat exchanger 85 may be provided in
the vapour fraction discharge line 80, to preserve the
cold vested in the vapour fraction 80 by heat exchanging
against a cold recovery stream 86 prior to feeding the
vapour fraction 80 to any combustion device.
In one embodiment, the cold recovery stream 86 may
comprise or consist of a side stream sourced from the
hydrocarbon feed stream in the hydrocarbon feed line 110
of the liquefaction system 100. The resulting cooled
side stream may for instance be combined with the
cryogenic hydrocarbon composition in the cryogenic feed
line 8. Thus, the cold recovery heat exchanging in the
cold recovery heat exchanger 85 supplements the
production rate of the cryogenic hydrocarbon composition.
In another embodiment, the cold recovery stream 86 may
comprise or consist of the overhead vapour in the
overhead vapour discharge line 30, preferably in the part
of the overhead vapour discharge line 30 where through
the overhead vapour is passed from the nitrogen stripper
column 20 to the overhead condenser 35. Herewith the
duty required from the auxiliary refrigerant stream 132
in the overhead condenser 35 would be reduced.
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 -
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processes, such as PRICO described in the paper "LNG
Production on floating platforms" 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 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 a/, presented at the 22nd
World Gas Conference in Tokyo, Japan (2003). Other
suitable three-cycle processes include for example US
Pat. 6,962,060; WO 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-
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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.
Preferably, but not necessarily, any compressor
forming part of the hydrocarbon liquefaction process in
the liquefaction system, particularly any refrigerant
compressor, is driven by one or more electric motors,
without being mechanically driven by any steam- and/or
gas turbine. Such compressor may be driven exclusively
by one or more electric motors. Not all examples listed
above employ electric motors as refrigerant compressor
drivers. It will be clear that any drivers other than
electric motors can be replaced for an electric motor to
enjoy the most benefit 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 Figure 2. It employs a cryogenic heat
exchanger 180, in this case in the form of a coil wound
heat exchanger comprising 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 lower and upper hydrocarbon product tube bundles
181 and 182 fluidly connect the raw liquefied product
line 1 with a hydrocarbon feed line 110. At least one
refrigerated hydrocarbon pre-cooling heat exchanger 115
may be provided in the hydrocarbon feed line 110 upstream
of the cryogenic heat exchanger 180.
A main refrigerant, in the form of a mixed
refrigerant, is provided in a main refrigerant circuit
101. The main refrigerant circuit 101 comprises a spent
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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
a main refrigerant compressor 160, and a compressed
refrigerant line 120 connecting a main refrigerant
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 ambient heat exchanger 124 and at
least one refrigerated main refrigerant pre-cooling heat
exchanger 125. 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 a 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.
The optional external stripping vapour supply line 74
(if provided) may suitably be connected to the
hydrocarbon feed line 110, either at a point upstream of
the at least one refrigerated hydrocarbon pre-cooling
heat exchanger 115, downstream of the at least one
refrigerated hydrocarbon pre-cooling heat exchanger 115,
or (for instance possible if two or more refrigerated
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hydrocarbon pre-cooling heat exchangers are provided)
between two consecutive refrigerated hydrocarbon pre-
cooling heat exchangers, to be sourced with a part of the
hydrocarbon feed stream from the hydrocarbon feed line
110.
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 in which
an HMR control valve 144 is configured. The HMR line 141
is in fluid communication with the shell side 186 of the
cryogenic heat exchanger 180 and, via 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,
with the spent refrigerant line 150.
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 an LMR line 131.
A first LMR return line 133 establishes fluid
communication between the LMR line 131 and the shell side
186 of the cryogenic heat exchanger 180. An LMR control
valve 134 is configured in the first LMR return line 133.
The first LMR 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, and each one of the LMR tube bundles 183 and
184, and the HMR tube bundle 185.
Figure 2 reveals one possible source of the auxiliary
refrigerant. The LMR line 131 is split into the
auxiliary refrigerant line 132 and the first LMR return
line 133. A second LMR return line 138 on an upstream
end thereof fluidly connects with the auxiliary
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refrigerant line 132 via the overhead condenser (for
example the overhead condenser 35 of Figure 1, or an
integrated internal overhead condenser 235 as depicted in
Figure 2), and on a downstream end the second LMR return
line 138 ultimately connects with the spent refrigerant
line 150, suitably via the first HMR line 141.
The line up around the nitrogen stripper column 20 in
Figure 2 is similar to the one shown in Figure 1 and will
not be set forth in detail again. Optional lines
including the optional second feed line 11, the optional
external stripping vapour supply line 74, and the
optional vapour recycle line 87 may be provided but have
not been reproduced in Figure 2 for purpose of clarity.
One difference to be noted, however, between the
embodiment of Figure 2 with that of Figure 1 is that the
overhead condenser 35, the overhead separator 33 and the
reflux system have been embodied in the form of the
integrated internal overhead condenser 235, which is
internally configured within the overhead space 26 in the
nitrogen stripper column 20. Such internal overhead
condenser 235, as such, is known in the art. The liquid
recycle line 13 is provided in liquid communication with
a partial liquid draw off tray 27 provided inside the
nitrogen stripper column 20 gravitationally above the
rectifying section 22 and below the internal overhead
condenser 235. The partial liquid draw off tray 27
functions equivalently to the condensed fraction splitter
39 of Figure 1.
Regardless of whether in the form of the (external)
overhead condenser 35 or the internal overhead consenser
235, the condenser is preferably arranged in fluid
communication with both the vapour bypass line 76 and the
overhead space 25 of the nitrogen stripper column 20,
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whereby the intermediate vapour passing through the
condenser preferably comprises both the non-stripping
portion from the vapour bypass line 76 and the overhead
vapour obtained from the overhead space 26 of the
nitrogen stripper column 20.
The apparatus and method for removing nitrogen from a
cryogenic hydrocarbon composition comprising a nitrogen-
and methane-containing liquid phase may be operated as
follows.
A cryogenic hydrocarbon composition 8 comprising a
nitrogen- and methane-containing liquid phase is
provided, preferably at an initial pressure of between 2
and 15 bar absolute (bara), and preferably at a
temperature lower than -130 C.
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.
In typical embodiments, the temperature of lower than
-130 C can be achieved by passing a hydrocarbon feed
stream 110 through the liquefaction system 100. In such a
liquefaction system 100, the hydrocarbon feed 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
a raw liquefied stream within the raw liquefied product
line 1. The desired cryogenic hydrocarbon composition 8
may then be obtained from the raw liquefied stream 1.
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The main refrigerant stream may be generated by
cycling the main refrigerant in the main refrigerant
circuit 101, whereby spent refrigerant 150 is compressed
in the main 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 main refrigerant compressor 160 is
via the one or more heat exchangers that are provided in
the compressed refrigerant line 120. 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
which completes the cycle. Simultaneously, the
hydrocarbon feed 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 and sub-cooled against the same
evaporating light and heavy refrigerant fractions.
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Depending on the source, the hydrocarbon feed 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 CO2, 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 C2+ hydrocarbons.
If desired, the hydrocarbon feed 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 feed stream 110
thus varies depending upon the type and location of the
gas and the applied pre-treatment(s).
The raw liquefied stream 1 may comprise between from
1 molt to 5 mol% nitrogen, be at a raw temperature of
between from -165 C to -120 C and typically at a
liquefaction pressure of between from 15 bara to 120
bara. In many cases, the raw temperature may be between
from -155 C to -140 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 depressurizing to between 1 and 2 bara.
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The cryogenic hydrocarbon composition 8 may be
obtained from the raw liquefied stream 1 by main
depressurizing the raw liquefied stream 1 from the
liquefaction pressure to the initial pressure. A first
nitrogen stripper feed stream 10 is derived from the
cryogenic hydrocarbon composition 8, and fed into the
nitrogen stripper column 20 at a stripping pressure via
the first inlet system 21.
The stripping pressure is usually equal to or lower
than the initial pressure. The stripping pressure in
preferred embodiments is selected in a range of between 2
and 15 bar absolute. Preferably, the stripping pressure
is at least 4 bara, because with a somewhat higher
stripping pressure the stripping vapour in stripping
vapour line 71 can benefit from some additional enthalpy
(in the form of heat of compression) that is added to the
process stream 60 in the process compressor 260.
Preferably, the stripping pressure is at most 8 bara in
order to facilitate the separation efficiency in the
nitrogen stripper column 20. Moreover, if the stripping
pressure is within a range of between from 4 to 8 bara,
the off gas in the vapour fraction line 80 can readily be
used as so-called low pressure fuel stream without a need
to further compress.
In one example, the raw temperature of the raw
liquefied stream 1 was -161 C while the liquefaction
pressure was 55 bara. The main depressurization may be
effected in two stages: first a dynamic stage using the
expansion turbine 6 to reduce the pressure from 55 bara
to about 10 bara, followed by a further depressurization
in a static stage using the Joule Thomson valve 7 to a
pressure of 7 bara. The stripping pressure in this case
was assumed to be 6 bara.
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An overhead vapour stream 30 is obtained from the
overhead space 26 of the nitrogen stripping column 20. A
vapour fraction 80 obtained from the overhead vapour
stream 30, and comprising a discharge fraction of the
overhead vapour 30, is discharged as off gas. Suitably,
at least a fuel portion of the vapour fraction 80 is
passed to the combustion device 220 at a fuel gas
pressure that is not higher than the stripping pressure.
A nitrogen-stripped liquid 40 is drawn from the sump
space 26 of the nitrogen stripper column 20. The
temperature of the nitrogen-stripped liquid 40 is
typically higher than that of the first nitrogen stripper
feed stream 10. Typically, it is envisaged that the
temperature of the nitrogen-stripped liquid 40 is higher
than that of the first nitrogen stripper feed stream 10
and between -140 C and -80 C, preferably between
-140 C and -120 C.
The nitrogen-stripped liquid 40 is then
depressurized, preferably employing the intermediate
depressurizer 45, to a flash pressure that is lower than
the stripping pressure, suitably in a range of between
from 1 and 2 bar absolute. Preferably, the flash
pressure lies in a range of between from 1.0 and 1.4
bara. With a somewhat higher differential between the
flash pressure and the stripping pressure, the stripping
vapour in stripping vapour line 71 can benefit from some
additional heat of compression that is added to the
process stream 60 in the process compressor 260.
The intermediate depressurizer 45 may be controlled
by the level controller LC, set to increase the flow rate
through the intermediate depressurizer if the level of
liquid accumulated in the sump space 26 of the nitrogen
stripper column 20 increases above a target level. As a
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result of the depressurization, the temperature is
generally lowered to below -160 C. The liquid
hydrocarbon product stream 90 that is produced hereby can
typically be kept at an atmospheric pressure in an open
insulated cryogenic storage tank.
Process vapour 60 is produced as well. The process
vapour 60 may comprise flash vapour 64 that is often
generated upon the depressurization of the nitrogen-
stripped liquid 40 and/or depressurization of a bypass
feed stream 11 (further discussed later herein below).
The first nitrogen stripper feed stream 10 comprises
a first portion of the cryogenic hydrocarbon composition
8. It may contain all of the cryogenic hydrocarbon
composition 8, but in practice it is preferred to split
the cryogenic hydrocarbon composition 8 into the first
portion 10 and a second portion 11 having the same
composition and phase as the first portion 10. The
second portion is preferably diverted, in form of the
bypass feed stream, from the stripping pressure side to a
suitable location on the flash pressure side.
The split ratio, defined as the flow rate of the
second portion relative to the flow rate of the cryogenic
hydrocarbon composition in the cryogenic hydrocarbon
composition line 8, may be controlled using the bypass
stream flow control valve 15. This bypass stream flow
control valve 15 may be controlled by the flow controller
FC to maintain a predetermined target flow rate of the
first nitrogen stripper feed stream 10 into the nitrogen
stripper column 20. The flow controller FC will increase
the open fraction of the bypass stream flow control valve
15 if there is a surplus flow rate that exceeds the
target flow rate, and decrease the open fraction if there
is a flow rate deficit compared to the target flow rate.
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As a general guideline, the split ratio may
advantageously be selected between 50 % and 95 %. The
lower values are typically recommended for higher content
of nitrogen in the cryogenic hydrocarbon composition,
while higher values are preferred for lower content of
nitrogen. In one example, the content of nitrogen in the
cryogenic hydrocarbon composition 8 was 3.0 mol% whereby
the selected split ratio was 75%.
The second portion originating from the initial
stream splitter 9 is also be depressurized to said flash
pressure, before subsequently feeding it into at least
one of the group consisting of: the nitrogen-stripped
liquid discharge line 40, the liquid hydrocarbon product
line 90 and the process vapour line 60; while bypassing
the nitrogen stripper column 20. Suitably the optional
second portion is passed into the optional end flash
separator 50.
The process vapour 60 may comprise boil-off gas.
Boil-off gas 230 typically results from adding of heat to
the liquid hydrocarbon product stream 90 whereby a part
of the liquid hydrocarbon product stream 90 evaporates to
form the boil-off gas. In a typical LNG plant the
generation of boil-gas can exceed the flow rate of flash
vapour by multiple times, particularly during operating
the plant in so-called loading mode, and hence it is an
important benefit to not only re-condense the flash
vapour but to re-condense the boil-off gas as well, if
there is not enough on-site demand for heating power to
use all of the methane contained in the boil-off gas.
In order to facilitate transferring of the boil-off
gas to the process vapour stream 60, preferably the
optional boil-off gas supply line 230 connects a vapour
space in the cryogenic storage tank 210 with the process
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vapour line 60. In order to facilitate transferring the
flash vapour 64 to the process vapour stream 60, and to
further denitrogenate the liquid hydrocarbon product
stream 90, preferably, the nitrogen-stripped liquid after
its depressurization is fed into the optional end flash
separator where it is phase separated at a flash
separation pressure into the liquid hydrocarbon product
stream 90 and the flash vapour 64. The flash separation
pressure is equal to or lower than the flash pressure,
and suitably lies in the range of between from 1 to 2 bar
absolute into the liquid hydrocarbon product stream 90
and the flash vapour 64. In one embodiment the flash
separation pressure is envisaged to be 1.05 bara.
The process vapour 60 is compressed to at least the
stripping pressure, thereby obtaining a compressed vapour
stream 70. A stripping vapour stream 71 is obtained from
the compressed vapour stream 70, and passed into the
nitrogen stripper column 20 via the second inlet system
23. This stripping vapour can percolate upward through
the stripping section 23 in contacting counter current
with liquids percolating downward through the stripping
section 23.
If the external stripping vapour supply line 74 is
provided in fluid communication with the second inlet
system 23, an external stripping vapour may selectively
be fed into the nitrogen stripper column 20 via the
second inlet system 23. Herewith major disruption of the
nitrogen stripper column 20 may be avoided, for instance,
in case the process compressor 260 is not functioning to
provide the compressed vapour stream 70 in sufficient
amounts.
Obtaining of the stripping vapour stream 71 from the
compressed vapour stream 70 involves selectively
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splitting the compressed vapour stream 70 into a
stripping portion and a non-stripping portion. The non-
stripping portion comprises a bypass portion of the
compressed vapour, which bypass portion may herein below
also be referred to as vapour bypass portion. It does
not contain the stripping portion. The stripping vapour
stream 71 contains at least the stripping portion.
The selective injection may be controlled using the
vapour bypass control valve 77. Suitably, the vapour
bypass control valve 77 is controlled by a pressure
controller on the compressed vapour line 70, which is set
to increase the open fraction of the vapour bypass
control valve 77 in response to an increasing pressure in
the compressed vapour line 70. It is envisaged that the
flow rate of the vapour bypass portion that is allowed to
flow through the vapour bypass line 76 into the overhead
vapour stream 30 is particularly high during so-called
loading mode at which time usually the amount of boil-off
gas is much higher than in is usually the case during so-
called holding mode. Preferably, the vapour bypass
control valve 77 is fully closed during normal operation
in holding mode.
A partially condensed intermediate stream is formed
from an intermediate vapour by passing the intermediate
vapour through the condenser. The intermediate vapour
that comprises at least the non-stripping portion of the
compressed vapour. In preferred embodiments, such as
illustrated in Figure 1, the intermediate vapour also
contains the overhead vapour 30. This may be achieved by
selectively injecting the non-stripping portion of the
compressed vapour into the overhead vapour stream 30,
thereby forming the intermediate vapour. The forming of
the partially condensed intermediate stream suitably
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involves indirectly heat exchanging the intermediate
vapour against the auxiliary refrigerant stream 132 and
partially condensing the intermediate vapour, whereby
heat is passed from the intermediate vapour to the
auxiliary refrigerant stream 132 at a selected cooling
duty. The resulting partially condensed intermediate
stream comprises a condensed fraction containing an
excess liquid, and a vapour fraction. The vapour
fraction contains the bypass portion from the compressed
vapour, which remains in the vapour phase throughout the
partially condensing.
The condensed fraction is separated from the vapour
fraction in the overhead separator 33, at a separation
pressure that may be lower than the stripping pressure,
and preferably lies in a range of between 2 and 15 bar
absolute. The vapour fraction is discharged via the
vapour fraction discharge line 80 as off gas. It
contains a discharge fraction of the overhead vapour
obtained from the overhead space 26 of the nitrogen
stripper column 20 as well as at least the vapour bypass
portion. The condensed fraction is discharged from the
overhead separator 33 into a reflux system, for instance
via the condensed fraction discharge line 37.
This way, from the selective splitting of the
compressed vapour in the stripping and non-stripping
portions all the way to the discharging of the bypass
portion in the vapour fraction of the off gas, the bypass
portion bypasses the at least one internal stripping
section 24. In other words, on the route from the bypass
splitter 79 to the overhead vapour discharge line 30
and/or the vapour fraction discharge line 80 the bypass
portion does not pass through the at least one internal
stripping section 24. Herewith it is achieved that any
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compressed vapour in the compressed vapour line 70 in
excess of the amount of stripping vapour consumed during
normal operation of the nitrogen stripper column 20 in
equilibrium, is diverted around the stripping section 24
so that the equilibrium in the stripping within the
nitrogen stripper column 20 is not disturbed. In
preferred embodiments, the bypass portion bypasses not
only the stripping section 24 but the entire nitrogen
stripper column 20, such as is shown in the embodiment of
Figure 1.
At least part of the condensed fraction discharged
from the overhead separator 33 is led into the liquid
recycle line 13 to form a liquid recycle portion. The
recycle valve 14 may suitably be controlled using a flow
controller provided in the condensed fraction discharge
line 37 and/or a level controller provided on the
overhead separator 33. The liquid recycle portion
contains at least part of the excess liquid. At least
part of liquid recycle portion is returned to the liquid
hydrocarbon product stream, while keeping this at least
part in liquid phase. This may be done by feeding the
liquid recycle portion into at least one of the group
consisting of: the nitrogen stripper column 20, the
cryogenic hydrocarbon composition 8, the first nitrogen
stripper feed stream 10, the optional bypass feed stream
11, the nitrogen-stripped liquid 40, the optional end
flash separator 50 and the liquid hydrocarbon product
stream 90.
The condenser, which in the embodiment of Figure 1 is
embodied in the form of the overhead condenser 35, thus
allows for re-condensation of vaporous methane that has
previously formed part of the raw liquefied product 1 (or
the cryogenic hydrocarbon composition 8), by adding any
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such vaporous methane containing stream to the
(compressed) process vapour stream. Preferably, the
methane is condensed to the extent that it is in excess
of a target amount of methane in the discharged vapour
fraction 80. Once forming part of the process vapour 60
or compressed process vapour 70, the vaporous methane can
find its way to the heat exchanging with the auxiliary
refrigerant 132 by which it is selectively condensed out
of the overhead vapour 30 from the nitrogen stripper
column 20, while allowing the majority of the nitrogen to
be discharged with the off gas. Herewith it becomes
possible to remove sufficient nitrogen from the cryogenic
hydrocarbon composition 8 to produce a liquid hydrocarbon
product stream 90 within a desired maximum specification
of nitrogen content, while as the same time not producing
more heating capacity in the off gas than needed.
The vapour fraction 80 in the off gas generally has a
heating value. The heating value of the vapour fraction
80 being discharged is suitably regulated by adjusting
the cooling duty in the overhead condenser 35. This may
be done by the cooling duty controller 34. By adjusting
the cooling duty at which heat is passed from the
overhead vapour to the auxiliary refrigerant stream, the
relative amount of methane in the off gas can be
regulated. As a result, the heating value of the
discharged vapour fraction can be regulated to match with
a specific demand of heating power. This renders the off
gas suitable for use as fuel gas stream, even in
circumstances where the demand for heating value is
variable.
When the vapour fraction 80 is passed to and consumed
by a combustion device 220 as fuel, the heating value may
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be regulated to match with an actual demand of heating
power by the combustion device 220.
The heating value being regulated may be selected in
accordance with the appropriate circumstances of the
intended use of the off gas as fuel gas. The heating
value may be determined in accordance with DIN 51857
standards. For many applications, the heating value
being regulated may be proportional to the lower heating
value (LHV; sometimes referred to as net calorific
value), which may be defined as the amount of heat
released by combusting a specified quantity (initially at
25 C) and returning the temperature of the combustion
products to 150 C. This assumes the latent heat of
vaporization of water in the reaction products is not
recovered.
However, for the purpose of regulating the heating
value in the context of the present disclosure, the
actual heating value of the vapour fraction being
discharged does not need to be determined on an absolute
basis. Generally it is sufficient to regulate the
heating value relative to an actual demand for heating
power, with the aim to minimize any shortage and excess
of heating power being delivered.
In the context of the present description, cooling
duty reflects the rate at which heat is exchanged in the
condenser, which can be expressed in units of power (e.g.
Watt or MWatt). The cooling duty is related to the flow
rate of the auxiliary refrigerant being subjected to the
heat exchanging against the overhead vapour.
Preferably, the cooling duty is automatically
adjusted in response to a signal that is causally related
to the heating value being regulated. In embodiments
wherein the vapour fraction is passed to one or more
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selective consumers of methane, such as for instance the
combustion device 220 shown in Figure 1, the controlling
can be done in response to the demanded heating power,
whereby the partial flow rate of methane is controlled to
achieve a heating value that matches the demand.
Suitably, the auxiliary refrigerant stream flow control
valve 135 may be controlled by the pressure controller PC
to maintain a predetermined target flow rate of auxiliary
refrigerant stream 132 through the overhead condenser 35.
The actual pressure in the vapour fraction discharge line
80 is causally related to the heating value that is being
regulated. The pressure controller PC will be set to
decrease the open fraction of the auxiliary refrigerant
stream flow control valve 135 when the pressure drops
below a pre-determined target level, which is indicative
of a higher consumption rate of methane than supply rate
in the vapour fraction 80. Conversely, the pressure
controller PC will be set to increase the open fraction
of the auxiliary refrigerant stream flow control valve
135 when the pressure exceeds the pre-determined target
level.
The vapour fraction 80 is envisaged to contain
between from 50 mol% to 95 mol% of nitrogen, preferably
between from 70 mol% to 95 mol% of nitrogen or between
from 50 mol% to 90 mol% of nitrogen, more preferably
between from 70 mol% to 90 mol% of nitrogen, still more
preferably from 75 mol% to 95 mol% of nitrogen, most
preferably from 75 mol% to 90 mol% of nitrogen. The
condensed fraction 37 is contemplated to contain less
than 35 mol% of nitrogen.
The auxiliary refrigerant 132 stream preferably has a
bubble point under standard conditions at a lower
temperature than the bubble point of the overhead vapour
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stream 30 under standard conditions (ISO 13443 standard:
15 C under 1.0 atmosphere). This facilitates
recondensing a relatively high amount of the methane that
is present in the overhead vapour stream 30, which in
turn facilitates the controllability of the methane
content in the vapour fraction 80. For instance, the
auxiliary refrigerant may contain between from 5 mol% to
75 mol% of nitrogen. In a preferred embodiment, the
auxiliary refrigerant stream is formed by a slip stream
of the main refrigerant stream, more preferably by a slip
stream of the light refrigerant fraction. This latter
case is illustrated in Figure 2 but may also be applied
in the embodiment of Figure 1. Such a slip stream may
conveniently be passed back into the main refrigerant
circuit via the shell side 186 of the cryogenic heat
exchanger 180, where it may still assist in withdrawing
heat from the stream in the upper and/or lower tube
bundles.
In one example, a contemplated composition of the
auxiliary refrigerant contains between 25 mol% and
40 mol% of nitrogen; between 30 mol% and 60 mol% of
methane and up to 30 mol% of C2 (ethane and/or ethylene),
whereby the auxiliary refrigerant contains at least 95%
of these constituents and/or the total of nitrogen and
methane is at least 65 mol%. A composition within these
ranges is may be readily available from the main
refrigerant circuit if a mixed refrigerant is employed
for sub-cooling of the liquefied hydrocarbon stream.
It is also possible to employ a separate
refrigeration cycle for the purpose of partially
condensing the overhead vapour stream 30. However,
employing a slip stream from the main refrigerant stream
has as advantage that the amount of additional equipment
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to be installed is minimal. For instance, no additional
auxiliary refrigerant compressor and auxiliary
refrigerant condenser would be needed.
If the nitrogen stripper column 20 is equipped with
the optional internal rectifying section 22 as described
above, the overhead vapour stream 30 is preferably
obtained from an overhead space of the nitrogen stripping
column 20 above the rectifying section 22.
At least a reflux portion 36 of the condensed
fraction is allowed onto the rectifying section 22 in the
nitrogen stripper column 20, starting at a level above
the rectifying section 22. From here the reflux portion
can percolate downward through the rectifying section 22,
in contact with vapours rising upward through the
rectifying section 22. In the case of the embodiment of
Figure 1, the condensed fraction may pass through into
the nitrogen stripper column 20 via the reflux inlet
system 25. The reflux portion is suitably obtained from
the condensed fraction and charged into the nitrogen
stripper column 20 via the optional reflux pump 38
(and/or it may flow under the influence of gravity) and
the reflux portion line 36. In the case of the
embodiment of Figure 2, the condensed fraction is
separated inside the overhead space of the nitrogen
stripper column 20 and therefore already available above
the rectifying section 22.
The reflux portion may contain all of the condensed
fraction, but optionally, the condensed fraction is split
in the optionally provided condensed fraction splitter 39
into a liquid recycle portion which is charged via liquid
recycle line 13 into, for instance, the first feed stream
10, and the reflux portion which is charged into the
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nitrogen stripper column 20 via reflux inlet system 25
and reflux portion line 36.
The capability of splitting the condensed fraction
into the reflux portion 36 and the liquid recycle portion
13 is beneficial to divert any excess liquid of the
condensed fraction around the rectifying section 22 as a
liquid recycle, such as not to upset the operation of the
rectifying section 22. In embodiments wherein the liquid
recycle portion is recycled into the nitrogen stripper
column 20, bypassing of the internal rectifying section
22 can be accomplished by feeding the liquid recycle
portion into the nitrogen stripper column 22 at a point
gravitationally below the rectifying section 22.
The partially condensing may also involve direct
and/or indirect heat exchanging with other streams in
other consecutively arranged overhead heat exchangers.
For instance, the cold recovery heat exchanger 85 may be
such an overhead heat exchanger whereby the partially
condensing of the overhead stream further comprises
indirect heat exchanging against the vapour fraction 80.
The optional vapour recycle line 87 may be
selectively employed, suitably by selectively opening the
vapour recycle control valve 88, to increase the amount
of nitrogen that remains in the liquid hydrocarbon
product stream 90. This may be done by drawing a
vaporous recycle portion from the vapour fraction,
depressurising the vaporous recycle portion to the flash
pressure and subsequently injecting the vaporous recycle
portion into the nitrogen-stripped liquid 40. The
remaining part of the vapour fraction 80 that is not
passed into the vapour recycle line 87 may form the fuel
portion that may be conveyed to the combustion device
220.
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In some embodiments, the target amount of nitrogen
dissolved in the liquid hydrocarbon product stream 90 is
between 0.5 and 1 mol%, preferably as close to 1.0 mol%
as possible yet not exceeding 1.1 mol%. The vapour
recycle flow control valve 88 regulates the amount of the
vapour fraction stream 80 that is fed back into, for
instance, the end flash separator 50 while bypassing the
nitrogen stripper column 20. Herewith the amount of
nitrogen in the liquid hydrocarbon product stream 90 can
19 be influenced. To further assist in meeting the target
nitrogen content, the vapour recycle flow control valve
88 may be controlled in response to a signal from a
quality measurement instrument QMI that is optionally
provided in the liquid hydrocarbon product line 90.
The proposed method and apparatus are specifically
suitable for application in combination with a
hydrocarbon liquefaction system, such as a natural gas
liquefaction system, in order to remove nitrogen from the
raw liquefied product. It has been found that even when
the raw liquefied product - or the cryogenic hydrocarbon
composition - contains a fairly high amount of from 1
mol% (or from about 1 mol%) up to 5 mol% (or up to about
5 mol%) of nitrogen, the resulting liquid hydrocarbon
product can meet a nitrogen content within a
specification of between from 0.5 to 1 mol% nitrogen.
The remainder of the nitrogen is discharged as part of
the vapour fraction in the off gas, together with a
controlled amount of methane.
It is suggested that the presently proposed method
and apparatus are most beneficial when the raw liquefied
product, or the cryogenic hydrocarbon composition,
contains from 1.5 mol%, preferably from 1.8 mol%, up to
5 mol% of nitrogen. Existing alternative approaches may
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also work adequately when the nitrogen content is below
about 1.8 mol% and/or below about 1.5 mol%.
Static simulations have been performed on the
embodiment shown in Figure 1, for both holding mode
(Table 1) and loading mode (Table 2). The cryogenic
hydrocarbon composition 8 was assumed to consist for more
than 90 mol% of a mixture of nitrogen and methane
(98.204 mol%). In the example, the amount of nitrogen
(1.654 mol%) and methane (98.204 mol%) is more than
99.8 mol%, the balance of 0.142 mol% consisting of carbon
dioxide (0.005 mol%). The carbon dioxide leaves the
process via the nitrogen stripped liquid 40 and the
liquid hydrocarbon product stream 90. The split ratio in
the initial stream splitter 9 was about 75 % in both
cases.
It can be seen that in both holding mode and loading
mode, despite the large difference in amount of process
vapour, the amount of methane in the discharged vapour
fraction 80 could be kept at about 80 mol% and well
within the range of between 10 mol% and 25 mol% while at
the same time the nitrogen content in the liquid
hydrocarbon product stream 90 was kept within the target
of close to 1.0 mol% and not exceeding 1.1 molt.
In holding mode, about 2.0 kg/s of boil-off gas
consisting of about 17 mol% nitrogen and 83 mol% methane
was added to the process via the boil-off gas supply line
230, while in loading mode this was about 4.4 kg/s.
In holding mode no vapour was guided through the
vapour bypass line 76, while in the loading mode 30% of
the compressed vapour 70 was guided through the vapour
bypass line 76 in order to accommodate the additional
vapour brought about by the additional inflow of boil-off
gas. The liquid recycle 13 in the loading mode also went
0
ts.)
=
Table 1: Holding mode; Reference numbers correspond to Figure 1.
LI
=
=
-1
Ref. 1 8 10 11 13 30 36 40 60 64 70 71
76 80 87 90 ul
-z.
v;
number
Phase L L L L L V L L V V V V
V V L
(V/L)
Flow 134 134 36.1 99 0.55 11.3 6.60 45.8 14.4 12.4 14.4 14.4 0.00 4.1 1.44
134 p
2
rate
.
u,
0
(kg/s)
13;
N,
Temp. -162 -163 -163 -163 -159 -143 -159 -137 -162 -164 -72 -72 -
-159 -159 -164 0
1-
,_i=
.
1
01
0
( C)
m
,
I
0
Pressure 55 6.4 6.4 6.4 6.4 6.2 6.2 6.3 1.00 1.05 6.8 6.3 -
5.8 5.8 1.05
(bara)
Nitrogen 1.66 1.66 1.91 1.66 20.1 37.7 20.1 1.77 18.0 18.3 18.0 18.0 -
80.0 80.0 0.86
"d
(mol%)
n
m
-:. . . .
. .
nJ
Methane 98.3 98.3 98.1 98.3 79.9 62.3 79.9 98.2 82.0 81.7 82.0 82.0 -
20.0 20.0 99.1
nJ
(mol%)
-I-
-.1
.r.,
v;
ul
-.1
0
ts.)
=
Table 2: loading mode; Reference numbers correspond to Figure 1.
LI
=
=
-1
Ref. 1 8 10 11 13 30 36 40 60 64 70 71
76 80 87 90 ul
c"
v;
number
Phase L L L L L V L L V V V V
V V V L
(V/L)
Flow 134 134 36.8 102 4.80 17.8 6.91 45.0 19.1 14.6 19.1 13.5 5.53 6.1 3.3
136 p
2
rate
,..
m
m
(kg/s)
13;
m
Temp. -162 -163 -162 -162 -160 -115 -160 -138 -154 -164 -56 -57 -57 -160 -
160 -164 .
,_i=
.
,
m
m
( C)
m
,
I
m
Pressure 55 6.4 6.4 6.4 6.4 6.2 6.2 6.3 1.00 1.05 6.8 6.3 6.2 5.8 5.8 1.05
(bara)
Nitrogen 1.66 1.66 3.90 1.66 20.9 37.3 20.9 2.15 21.3 22.5 21.3 21.3 21.3 81.0
81.0 1.09
"d
(mol%)
n
m
t.,
Methane 98.3 98.3 96.1 98.3 79.1 62.7 79.1 97.9 78.7 77.5 78.7 78.7 78.7 19.0
19.0 98.9
(mol%)
-I-
-..1
.6,
v;
ul
-.1
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up, from about 8% to about 41% of the condensed fraction
in the condensed fraction discharge line 37. The
additional flow of condensed fraction is a result of
additional re-condensed methane.
The liquefaction system 100 in the calculation used a
line up as shown in Figure 2 with a mixed refrigerant in
the compressed refrigerant line 120 with a composition as
listed in Table 3 in the column labelled "120".
Table 3: mixed refrigerant composition
(in mol%)
121; 131; 132
120
Holding Loading
Nitrogen 21.5 33.1 33.5
Methane 33.3 40.9 40.8
Ethane 0.13 0.07 0.07
Ethylene 32.6 23.1 22.8
Propane 12.2 2.79 2.81
Butanes 0.25 0.02 0.02
In holding mode the pressure in the compressed
refrigerant line 120 was 58 bara, in loading mode higher,
61 bara. The aggregated pressure drop in the lower and
upper LMR tube bundles (183 and 184, respectively) of the
cryogenic heat exchanger is 13 bar in both cases. The
pressure drop Imposed by the auxiliary refrigerant stream
flow control valve 135 was 39 bar in the holding mode
case and 42 bar in the loading mode operation so that the
shell pressure in shell side 186 of the cryogenic heat
exchanger 180 was the same for both the holding mode as
the loading mode.
The relative flow rate of the auxiliary refrigerant
stream 132 consisted of 11 % of the total LMR flow rate
- 48 -
in LMR line 131. In loading mode this was 18 %. Also the
actual flow rate was 1.6x higher than in the holding mode
case, but the separation between HMR and LMR in MR
separator 128 was made to favour HMR a little bit more in
the loading mode operation than in the holding mode
operation.
In the above example, the cryogenic hydrocarbon
composition was assumed to contain no hydrocarbons heavier
than methane (C2+ hydrocarbons), such as could be the case
if the cryogenic hydrocarbon composition is derived from
non-conventional gas sources, such as coal bed methane,
shale gas, or perhaps certain synthetic sources. However,
the proposed methods and apparatus may also be applied
where the cryogenic hydrocarbon composition would contain
up to about 15 mol% of C2+ hydrocarbons, including one or
more selected from the group consisting of ethane, propane,
i-butane, n-butane, and pentane. In essence these
additional C2+ hydrocarbons are not expected to change the
functioning of the proposed methods and apparatus, as it is
anticipated that none of such C2+ hydrocarbons would be
found in the overhead vapour 30 or the off gas in vapour
fraction discharge line 80, like the carbon dioxide of the
example.
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 invention.
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