U.S. patent application number 16/498514 was filed with the patent office on 2020-04-02 for nitrogen recovery apparatus and method of recovering nitrogen.
This patent application is currently assigned to LINDE AKTIENGESELLSCHAFT. The applicant listed for this patent is LINDE AKTIENGESELLSCHAFT. Invention is credited to Heinz BAUER, Lisa KOPPL.
Application Number | 20200103166 16/498514 |
Document ID | / |
Family ID | 58489175 |
Filed Date | 2020-04-02 |
United States Patent
Application |
20200103166 |
Kind Code |
A1 |
BAUER; Heinz ; et
al. |
April 2, 2020 |
NITROGEN RECOVERY APPARATUS AND METHOD OF RECOVERING NITROGEN
Abstract
A nitrogen recovery apparatus for recovering nitrogen from
natural gas comprises a separator having a liquid fraction port and
a vapour fraction port in fluid communication with a split flow
arrangement, the split flow arrangement having a sub-cooled fluid
path and an expanded fluid path. A fractionating column has a
reflux inlet port in fluid communication with the subcooled fluid
path above a middle feed port thereof, the middle feed port being
in fluid communication with the expanded fluid path. A bottom feed
port of the fractionating column is in fluid communication with the
liquid fraction port of the separator. A side reboiler circuit and
a reboiler circuit are operably coupled to the fractionating column
below the bottom feed port. A bottom hydrocarbon product stream
path is in fluid communication with a bottom hydrocarbon port of
the fractionating column.
Inventors: |
BAUER; Heinz; (Ebenhausen,
DE) ; KOPPL; Lisa; (Neuried, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LINDE AKTIENGESELLSCHAFT |
Munich |
|
DE |
|
|
Assignee: |
LINDE AKTIENGESELLSCHAFT
Munich
DE
|
Family ID: |
58489175 |
Appl. No.: |
16/498514 |
Filed: |
March 23, 2018 |
PCT Filed: |
March 23, 2018 |
PCT NO: |
PCT/EP2018/057464 |
371 Date: |
September 27, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2240/02 20130101;
F25J 2270/02 20130101; F25J 3/0209 20130101; F25J 2230/08 20130101;
F25J 2200/02 20130101; F25J 2230/60 20130101; F25J 2260/42
20130101; F25J 2200/50 20130101; F25J 2205/04 20130101; F25J
2200/76 20130101; F25J 3/0257 20130101; F25J 2200/78 20130101; F25J
3/0233 20130101 |
International
Class: |
F25J 3/02 20060101
F25J003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2017 |
EP |
17164256.4 |
Claims
1. A nitrogen recovery apparatus (100) for recovering nitrogen from
natural gas, the apparatus (100) comprising: a separator (110)
having a liquid fraction port and a vapour fraction port in fluid
communication with a split flow arrangement (112), the split flow
arrangement (112) having a sub-cooled fluid path (118) and an
expanded fluid path (120); a fractionating column (116) having a
reflux inlet port (124) in fluid communication with the subcooled
fluid path (118) above a middle feed port (130) thereof, the middle
feed port (130) being in fluid communication with the expanded
fluid path (120); a bottom feed port (114) of the fractionating
column (116) in fluid communication with the liquid fraction port
of the separator (110); a side reboiler circuit (132) operably
coupled to the fractionating column (116) below the bottom feed
port (114); a reboiler circuit (140) operably coupled to the
fractionating column (116) below the side boiler circuit (132); and
a bottom hydrocarbon product stream path (160) in fluid
communication with a bottom hydrocarbon port (162) of the
fractionating column (116).
2. An apparatus as claimed in claim 1, wherein the separator (110)
comprises a feed inlet port for receiving partially liquefied
natural gas.
3. An apparatus as claimed in claim 2, further comprising: a heat
exchanger (108) having a natural gas feed inlet port, the heat
exchanger (108) also having an output port; wherein the output port
is in fluid communication with the feed inlet port of the separator
(110).
4. An apparatus as claimed in claim 1, wherein the fractionating
column (116) further comprises: an overhead gas stream path (148)
arranged to support sub-cooling in the subcooled fluid path
(118).
5. An apparatus as claimed in claim 4, wherein the overhead gas
stream path (148) is also in fluid communication with the heat
exchanger (108).
6. An apparatus as claimed in claim 1, further comprising: a reflux
condenser stream circuit (184) which is fed by a drawn off from the
fractionating column (116) from below the middle feed (130), the
reflux condenser stream circuit (184) being configured to provide
cooling in the subcooled fluid path (118).
7. An apparatus as claimed in claim 3, further comprising: a
sub-cooler (122); wherein the sub-cooled fluid path (118) is
arranged to pass through the sub-cooler (122).
8. An apparatus as claimed in claim 1, further comprising: an
expander (128); wherein the expanded fluid path (120) is arranged
to pass though the expander (128).
9. An apparatus as claimed in claim 8, wherein the expander (128)
is arranged to drive a first compressor (129).
10. An apparatus as claimed in claim 9, wherein the fractionating
column further comprises: a nitrogen product fraction tapping point
(190) disposed above the reflux inlet port (124) and operably
coupled to a reflux circuit comprising the first compressor (128),
the reflux circuit returning to the fractionating column (116)
above the nitrogen product fraction tapping point (190).
11. An apparatus as claimed in claim 10, further comprising the
sub-cooler (122) in the reflux circuit.
12. An apparatus as claimed in claim 4, further comprising: a
nitrogen return compression and cooling arrangement (151) disposed
in the overhead gas stream path (148).
13. An apparatus as claimed in claim 12, further comprising: an
expander (128), wherein the expanded fluid path (120) is arranged
to pass through the expander (128); and wherein the expander (128)
is arranged to drive at least part of the nitrogen return
compression and cooling arrangement (151).
14. A hydrocarbon capture and recovery system comprising: the
nitrogen recovery apparatus (100) as claimed in claim 1; and a
return fluid path fluidly coupling an output port of a nitrogen
return compression and cooling arrangement (151) to a wellbore for
returning nitrogen recovered from a natural gas feed and the
wellbore under pressure.
15. A method of recovering nitrogen from natural gas, the method
comprising: separating (202) a partially liquefied natural gas feed
into a vapour fraction and a liquid fraction; splitting (206) the
flow of the vapour fraction to form a first portion and a second
portion of vapour fraction; subcooling (208) the first portion of
the vapour fraction using an overhead gas stream (148) of a
fractionating column (116) to form a reflux stream, the reflux
stream being applied to the fractionating column (116); expanding
(210) the second portion of the vapour fraction to form a middle
feed to the fractionating column (116) below the reflux stream;
feeding (204) the liquid fraction as a bottom feed to the
fractionating column (116); a side-reboiler circuit (132) feeding
the fractionating column (116) below the bottom feed; and a
reboiler circuit (140) feeding the fractionating column (116) below
the side reboiler circuit.
Description
[0001] The present invention relates to a nitrogen recovery
apparatus of the type that, for example, comprises a fractionating
column for rectifying natural gas. The present invention also
relates to a method of recovering nitrogen, the method being of the
type that uses a fractionating column.
[0002] In the field of hydrocarbon recovery processing,
particularly methane from natural gas extracted from a wellbore,
known nitrogen/methane separation systems are operated as nitrogen
rejection units with nitrogen treated as a valueless by-product.
However, for some applications, valuable hydrocarbons need to be
recovered from a nitrogen rich gas whilst keeping the pressure of
the nitrogen fraction as high as possible. The hydrocarbon depleted
nitrogen fraction is not vented, but reprocessed instead, for
example for so-called "Enhanced Oil Recovery" (EOR). Processing of
natural gas so as to re-use the nitrogen content of the natural gas
can be considered methane recovery rather than nitrogen
rejection.
[0003] Some wellbores produce natural gas feeds having a
particularly challenging set of content characteristics, for
example the feed from a wellbore can have a nitrogen content
greater than 30 mol % or even greater than 50 mol %, and the
methane content in a recovered nitrogen product fraction can be
less than 10 mol % or even less than 3 mol %. Given the valuable
nature of the hydrocarbons, it is nevertheless desirable to recover
as much of the methane as possible from the recovered nitrogen
product fraction. Additionally, the recovered hydrocarbon fraction
has to be produced under an elevated pressure, for example greater
than 5 bar (500 kPa), such as greater than 10 bar (1000 kPa)
without compression. Also, the recovered nitrogen product fraction
needs to be produced under elevated pressure, typically greater
than 20 bar (2000 kPa) without compression. In order to separate
methane from nitrogen in the face of the above set of
characteristics, a nitrogen/methane fractionating column has to be
operated at a pressure of greater than 20 bar (2000 kPa) and has to
be provided with reflux without vaporising liquid hydrocarbons at a
pressure of less than 5 bar (500 kPa). A competitive and economic
natural gas processing design is required.
[0004] U.S. Pat. No. 4,157,904 discloses a so-called "Gas Subcooled
Process" (GSP) for refraction of hydrocarbons that employs a
separator having a vapour fraction stream separated into two
streams, one of which is used to generate refrigeration by work
expansion, while the other stream is used to provide reflux for a
fractionating column after undergoing at least partial
condensation. The process disclosed also employs a side reboiler
stream and reboiler stream. However, the process is intended for
the separation of methane from heavier hydrocarbons and has
specific process parameters that disregard the recovery and re-use
of nitrogen.
[0005] According to a first aspect of the present invention, there
is provided a nitrogen recovery apparatus for recovering nitrogen
from natural gas, the apparatus comprising: a separator having a
liquid fraction port and a vapour fraction port in fluid
communication with a split flow arrangement, the split flow
arrangement having a sub-cooled fluid path and an expanded fluid
path; a fractionating column having a reflux inlet port in fluid
communication with the subcooled fluid path above a middle feed
port thereof, the middle feed port being in fluid communication
with the expanded fluid path; a bottom feed port of the
fractionating column in fluid communication with the liquid
fraction port of the separator; a side reboiler circuit operably
coupled to the fractionating column below the bottom feed port; a
reboiler circuit operably coupled to the fractionating column below
the side boiler circuit; and a bottom hydrocarbon product stream
path in fluid communication with a bottom hydrocarbon port of the
fractionating column.
[0006] The separator may comprise a feed inlet port for receiving
partially liquefied natural gas.
[0007] The apparatus may further comprise: a heat exchanger having
a natural gas feed inlet port; the heat exchanger may also have an
output port; wherein the output port may be in fluid communication
with the feed inlet port of the separator.
[0008] The fractionating column may further comprise: an overhead
gas stream path arranged to support sub-cooling in the subcooled
fluid path.
[0009] The overhead gas stream path may also be in fluid
communication with the heat exchanger.
[0010] The apparatus may further comprise: a reflux condenser
stream circuit drawn off the fractionating column from below the
middle feed; the reflux condenser stream circuit may support
cooling in the subcooled fluid path. The reflux condenser stream
circuit may be fed by a drawn off from the fractionating column
from below the middle feed, The reflux condenser stream circuit
being configured to provide cooling in the subcooled fluid
path.
[0011] The apparatus may further comprise: a sub-cooler; wherein
the sub-cooled fluid path may be arranged to pass through the
sub-cooler.
[0012] The sub-cooler may be another heat exchanger.
[0013] The apparatus may further comprise: an expander; wherein the
expanded fluid path may be arranged to pass though the
expander.
[0014] The expander may be arranged to drive a first
compressor.
[0015] The fractionating column may further comprise: a nitrogen
product fraction tapping point disposed above the reflux inlet port
and operably coupled to a reflux circuit comprising the first
compressor; the reflux circuit may return to the fractionating
column above the nitrogen product fraction tapping point.
[0016] The apparatus may further comprise the sub-cooler in the
reflux circuit.
[0017] The apparatus may further comprise: a nitrogen return
compression and cooling arrangement disposed in the overhead gas
stream path.
[0018] The nitrogen return compression and cooling arrangement may
have an inlet port in fluid communication with another output port
of the heat exchanger.
[0019] The expander may be arranged to drive at least part of the
nitrogen return compression and cooling arrangement.
[0020] The nitrogen compression and cooling arrangement may
comprise a second compressor in fluid communication with a third
compressor via an inter-cooler. An aftercooler may be in fluid
communication with and downstream of the third compressor.
[0021] The apparatus may further comprise: a reflux path arranged
to extend from a partial compression point in the nitrogen return
compression and cooling arrangement back to the fractionating
column above the reflux inlet port. The reflux path may supply,
when in use, another reflux.
[0022] The reflux path may be arranged to pass through the heat
exchanger and the sub-cooler.
[0023] The apparatus may further comprise: a hydrocarbon fraction
compression and cooling arrangement in fluid communication with the
bottom hydrocarbon product stream path.
[0024] The hydrocarbon fraction compression and cooling arrangement
may be in fluid communication with the heat exchanger.
[0025] The expander may be arranged to drive at least part of the
hydrocarbon fraction compression and cooling arrangement.
[0026] The hydrocarbon fraction compression and cooling arrangement
may comprise a fourth compressor in fluid communication with a
fifth compressor via an inter-cooler. An aftercooler may be in
fluid communication with and downstream of the fifth
compressor.
[0027] The bottom hydrocarbon product stream path may comprise
another split flow arrangement arranged to provide a first
vaporised and superheated fluid path and a second vaporised and
superheated fluid path having a first pressure associated
therewith; the second vaporised and superheated fluid path may be
further vaporised at a second pressure higher than the first
pressure.
[0028] The first vaporised and superheated fluid path may be in
fluid communication with an inlet of the hydrocarbon fraction
compression and cooling arrangement. The second vaporised and
superheated fluid path may be in fluid communication with a partial
compression point in the hydrocarbon fraction compression and
cooling arrangement.
[0029] The expander may be arranged to drive a generator.
[0030] The first, second, third, fourth and/or fifth compressors
may be combined as a train of compressors sharing a common drive
shaft.
[0031] The first, second, third, fourth and/or fifth compressors
may be driven by at least one of an electric motor, a gas turbine,
and/or a steam turbine.
[0032] The fractionating column may comprise a high-pressure
fractionating column operably coupled to a low-pressure
fractionating column.
[0033] According to a second aspect of the present invention, there
is provided a hydrocarbon capture and recovery system comprising:
the nitrogen recovery apparatus as set forth above in relation to
the first aspect of the invention; and a return fluid path fluidly
communicating an output port of the nitrogen return compression and
cooling arrangement to a wellbore for returning nitrogen recovered
from a natural gas feed and the wellbore under pressure.
[0034] According to a third aspect of the present invention, there
is provided a method of recovering nitrogen from natural gas, the
method comprising: separating a partially liquefied natural gas
feed into a vapour fraction and a liquid fraction; splitting the
flow of the vapour fraction to form a first portion and a second
portion of vapour fraction; subcooling the first portion of the
vapour fraction using an overhead gas stream of a fractionating
column to form a reflux stream, the reflux stream being applied to
the fractionating column; expanding the second portion of the
vapour fraction to form a middle feed to the fractionating column
below the reflux stream; feeding the liquid fraction as a bottom
feed to the fractionating column; a side-reboiler circuit feeding
the fractionating column below the bottom feed; and a reboiler
circuit feeding the fractionating column below the side reboiler
circuit.
[0035] It is thus possible to provide an apparatus and method that
recovers nitrogen in an efficient manner, thereby increasing the
methane recovery rate and achieving the pressure level requirements
of all product streams. The amount of product recovered is
therefore improved, whilst reducing operating costs. Additionally,
the provision of a reflux condenser stream improves heat
integration significantly, for example energy requirements for one
or more compressors are reduced. The provision of additional reflux
circuits also improves methane recovery rates. Furthermore, the
provision of the fractionating column as more than one
fractionating component facilitates transportation where size
restrictions exist.
[0036] At least one embodiment of the invention will now be
described, by way of example only, with reference to the
accompanying drawings, in which:
[0037] FIG. 1 is a schematic diagram of a nitrogen recovery
apparatus constituting an embodiment of the invention; and
[0038] FIGS. 2 to 7 are parts of a flow diagram of a method of a
method of recovering nitrogen constituting another embodiment of
the invention.
[0039] Throughout the following description, identical reference
numerals will be used to identify like parts.
[0040] Referring to FIG. 1, a nitrogen recovery apparatus 100
comprises a natural gas feed inlet port 102, a nitrogen feedback
outlet port 104, and a hydrocarbon outlet port 106. The natural gas
feed inlet port 102 is in fluid communication with a first inlet of
a first heat exchanger 108. A first outlet of the first heat
exchanger 108 is in fluid communication with an inlet of a
separator 110, and so a natural gas feed inlet path extends through
the first heat exchanger 108. A vapour fraction port of the
separator 110 being in fluid communication with a split feed
arrangement 112. A liquid fraction port of the separator 110 is in
fluid communication with a bottom feed inlet port 114 of a
fractionating column 116 via a first Joule-Thomson device, for
example a throttling device, such as a first valve 115.
[0041] The split feed arrangement 112 divides a fluid path
originating from the vapour fraction port of the separator 110 into
a sub-cooled fluid path 118 and an expanded fluid path 120.
[0042] The sub-cooled fluid path 118 is in fluid communication with
a first inlet port of a second heat exchanger 122 constituting a
sub-cooler. A first outlet port of the sub-cooler 122 is in fluid
communication with a first reflux inlet port 124 of the
fractionating column 116 via a second Joule-Thomson device, such as
a second valve 126. The sub-cooled fluid path therefore extends
through the sub-cooler 122.
[0043] The expanded fluid path 120 is in fluid communication with
an inlet port of an expander 128, for example a turboexpander, an
outlet port of the expander being in fluid communication with a
middle feed inlet port 130 of the fractionating column 116. The
expander 128 is therefore in the expanded fluid path 120. In
another example, additionally or alternatively, the expander 128 is
operably coupled to a first compressor 129 by a drive shaft 131;
the application of the first condenser 129 will be described in
further detail later herein.
[0044] A side reboiler circuit 132 comprises a side reboiler fluid
path 134 coupled at a first end thereof to a side reboiler outlet
port 136 of the fractionating column 116 and at a second end
thereof to a side reboiler inlet port 138 of the fractionating
column 116. The side reboiler outlet port 136 is disposed below the
bottom feed inlet port 114 of the fractionating column 116, the
side reboiler inlet port 138 being disposed below the side reboiler
outlet port 136. The side reboiler fluid path 134 passes through
the first heat exchanger 108 via a second inlet port and a second
outlet port of the first heat exchanger 108.
[0045] A reboiler circuit 140 comprises a reboiler fluid path 142
coupled at a first end thereof to a reboiler outlet port 144 of the
fractionating column 116 and at a second end thereof to a reboiler
inlet port 146 of the fractionating column 116. The reboiler outlet
port 144 is disposed below the side reboiler inlet port 138 and the
reboiler inlet port 146 is disposed below the reboiler outlet port
144. The reboiler fluid path 142 also passes through the first heat
exchanger 108 via a third inlet port and a fourth outlet port of
the first heat exchanger 108.
[0046] An overhead gas stream path 148 extends from the
fractionating column 116 by way of an overhead gas outlet port 150
of the fractionating column 116 being in fluid communication with a
second inlet port of the sub-cooler 122. A second outlet port of
the sub-cooler 122 is in fluid communication with a fourth inlet
port of the first heat exchanger 108 and the overhead gas stream
path 148 passes through the first heat exchanger 108 to a fourth
outlet port thereof, the fourth outlet port of the first heat
exchanger 108 being in fluid communication with a nitrogen return
compression and cooling arrangement 151. The nitrogen return
compression and cooling arrangement 151 comprises an inlet port of
a second compressor 152 in fluid communication with the fourth
outlet port of the first heat exchanger 108. The nitrogen return
compression and cooling arrangement 151 also comprises an outlet of
the second compressor 152 in fluid communication with an inlet port
of a third compressor 154 via a first intermediary heat exchanger,
for example a first intercooler 156. An outlet port of the third
compressor 154 is in fluid communication with the nitrogen feedback
outlet port 104 via a first post-compression heat exchanger, for
example a post- or after-cooler 158.
[0047] A bottom hydrocarbon product stream path 160 is fluid
communication at one end thereof with a bottom hydrocarbon outlet
port 162 of the fractionating column 116. The bottom hydrocarbon
product stream path 160 comprises another split feed arrangement
164 providing a first product stream path 166 and a second product
stream path 168. The first product stream path 166 is in fluid
communication with a fifth inlet port of the first heat exchanger
108 via a third Joule-Thomson device, such as a third valve 170,
the first product stream path 166 passing through the first heat
exchanger 108 to a fifth outlet port of the first heat exchanger
108. The fifth outlet port of the first heat exchanger 108 is in
fluid communication with a hydrocarbon fraction compression and
cooling arrangement 172. In this example, the hydrocarbon fraction
compression and cooling arrangement 172 comprises a fourth
compressor 174 having an inlet port in fluid communication with the
fifth outlet port of the first heat exchanger 108. The hydrocarbon
fraction compression and cooling arrangement 172 also comprises a
fifth compressor 176, an outlet port of the fourth compressor 174
being in fluid communication with an inlet port of a fifth
compressor 176 via a second intermediary heat exchanger, for
example a second intercooler 178. An outlet port of the fifth
compressor 176 is in fluid communication with the hydrocarbon
outlet port 106 via a second post-compression heat exchanger, for
example a second post- or after-cooler 180.
[0048] The product stream path 168 also passes through the first
heat exchanger 108 by being in fluid communication with a sixth
inlet port of the first heat exchanger 108, a sixth outlet port of
the first heat exchanger 108 being in fluid communication with a
first partial compression point of the hydrocarbon fraction
compression and cooling arrangement 172, for example the inlet of
the fifth compressor 176, via a fourth Joule-Thomson device, such
as a fourth valve 182.
[0049] In another embodiment, a reflux condenser circuit 184 is
provided that is in fluid communication with a reflux condenser
stream outlet port 186 of the fractionating column 116, the reflux
condenser stream outlet port 186 being disposed below the bottom
feed inlet port 114, but above the side reboiler outlet port 136.
In this regard, the reflux condenser stream outlet port 186 is
disposed at a level of the fractionating column 116 so as to draw
partially vaporised product from underneath trays of the
fractionating column 116 fed via the bottom feed inlet port 114.
The reflux condenser circuit 184 passes through the sub-cooler 122
via a third inlet port and a third outlet port of the sub-cooler
122, the reflux condenser circuit 184 returning to the
fractionating column 116 and is in fluid communication, at the
other end thereof, with a reflux condenser stream inlet port 188,
which is below the reflux condenser stream outlet port 186 and
above the side reboiler outlet port 136.
[0050] In yet another embodiment, additionally or alternatively, a
reflux circuit is provided. In this regard, the fractionating
column 116 comprises a nitrogen product fraction tapping port 190
in fluid communication with an inlet port of the first compressor
129, an outlet port of the first compressor 129 being in fluid
communication with a fourth inlet port of the sub-cooler 122 so
that the reflux circuit passes through the sub-cooler 122. A fourth
outlet port of the sub-cooler 122 is in fluid communication with a
second reflux inlet port 192 of the fractionating column 116 via a
fifth Joule-Thomson device, such as a fifth valve 194.
[0051] In another embodiment, additionally or alternatively,
another reflux circuit is formed by feeding back a proportion of
partially condensed nitrogen from the nitrogen return compression
and cooling arrangement 151. In this regard, a reflux feedback path
196 is in fluid communication with a partial compression point in
the nitrogen return compression and cooling arrangement 151, for
example the outlet port of the first intercooler 156. The reflux
feedback path 196 passes through the first heat exchanger 108, via
a seventh inlet port and a seventh outlet port thereof, and the
sub-cooler 122, via a fifth inlet port and a fifth outlet port
thereof, before returning to the fractionating column 116 via a
sixth Joule-Thomson device, such as a sixth valve 197. In this
respect, the reflux feedback path 196 is in fluid communication
with a third reflux inlet port 198.
[0052] In operation (FIGS. 2 to 7), natural gas comprising, inter
alia, a mixture of nitrogen and hydrocarbons, is supplied (Step
200) to the natural gas feed inlet port 102 from, for example, the
wellbore mentioned above. In this example, the gas is at a
temperature of between about 10.degree. C. and about 65.degree. C.,
for example about 50.degree. C., and a pressure of between about 3
MPa (30 bar abs) and about 10 MPa (100 bar abs), for example about
6 MPa (60 bar abs). The first heat exchanger 108 partially
liquefies (Step 202) the natural gas by virtue of the overhead gas
stream path 148 passing through the first heat exchanger 108 to
yield a fluid between -130.degree. C. and about -100.degree. C. in
temperature, for example about -117.degree. C., and between about
3.7 MPa (37 bar abs) and about 9.7 MPa (97 bar abs) in pressure,
for example 5.7 MPa (57 bar abs). The partially liquefied natural
gas then enters (Step 204) the separator 110, the vapour fraction
being between about 0.6 mol/mol and about 0.85 mol/mol, for example
about 0.74 mol/mol. The liquid fractions leave the separator 110
via the liquid fraction port of the separator 110. In this regard,
the liquid fractions exiting the separator 110 pass through the
first valve 115 resulting in cooling of the liquid fractions
further before entering the fractionating column 116 at a
temperature of between about -140.degree. C. and about -110.degree.
C., for example about -129.degree. C. In this example, the vapour
fraction at this point is between about 0.15 mol/mol and about 0.3
mol/mol, for example about 0.25 mol/mol.
[0053] The vapour fractions exiting the vapour fraction port of the
separator 110 are split (Step 206) by the split feed arrangement
112 to form a first vapour fraction of between about 20% and about
50%, for example about 38% of the total vapour exiting the
separator 110, which is cooled (Step 208) by the sub-cooler 122 and
expanded (Step 210) by the second valve 126 to cause liquefaction
of the first vapour fraction before entering (Step 212) the
fractionating column 116 at the first reflux inlet port 124. The
liquefied first vapour fraction travels down the fractionating
column 116 rectifying vapour travelling up the fractionating column
116, thereby removing valuable hydrocarbons from the upwardly
travelling vapour. The upwardly travelling vapour originates, at
least in part, from a second vapour fraction provided by the split
feed arrangement 112 to the expanded vapour path 120. The second
vapour fraction comprises between about 50% and about 80%, for
example about 62%, of the total vapour exiting the separator 110
and passes through the expander 128 undergoing cooling and partial
liquefaction, yielding a distillation reflux that is applied (Step
214) to the fractionating column 116 at the middle feed inlet port
130. At the middle feed inlet port 130, the distillation reflux is
between about -150.degree. C. and about -120.degree. C., for
example about -138.degree. C., in temperature, between about 2 MPa
(20 bar abs) and about 3.2 MPa (32 bar abs), for example about 2.6
MPa (26 bar abs), in pressure, and the vapour fraction is between
about 0.85 mol/mol and about 0.6 mol/mol, for example about 0.73
mol/mol.
[0054] Liquid from the side reboiler outlet port 136 of the
fractionating column 116 is at a temperature of about -130.degree.
C. and about -110.degree. C., for example about -121.degree. C.,
and circulates through the side reboiler circuit 132, which passes
through the first heat exchanger 108 before returning to the side
reboiler inlet port 138 of the fractionating column 116 partially
vaporised. In this respect, the partially vaporised fluid returning
via the side reboiler inlet port 138 is at a temperature of between
about -120.degree. C. and about -100.degree. C., for example about
-110.degree. C., and comprises a vapour fraction of between about
0.15 mol/mol and about 0.3 mol/mol, for example about 0.24 mol/mol.
Similarly, liquid from the reboiler outlet port 144 of the
fractionating column 116 is at a temperature of between about
-120.degree. C. and about -100.degree. C., for example -109.degree.
C. and circulates through the reboiler circuit 140, which passes
through the first heat exchanger 108 before returning to the
reboiler inlet port 146 of the fractionating column 116. At the
reboiler inlet port 146, the partially vaporised fluid is at a
temperature of between about -115.degree. C. and about -95.degree.
C., for example about -104.degree. C., and comprises a vapour
fraction of between about 0.05 mol/mol and about 0.3 mol/mol, for
example about 0.1 mol/mol. The reboiler circuit 140 and the side
reboiler circuit 132 serve to reboil the liquid at the bottom of
the fractionating column 116 causing vapour to travel up the
fractionating column 116. As the function of the reboiler circuit
140 and the side reboiler circuit 132 are known to the skilled
person, the reboiler circuit 140 and the side reboiler circuit 132
will not be described in further detail herein for the sake of
clarity and conciseness of description.
[0055] The vapour travelling up the fractionating column 116 is
rectified by the reflux travelling down the fractionating column
116. The vapour exiting the fractionating column 116 via the
overhead gas outlet port 150 is largely devoid of hydrocarbons to a
required degree of purity and exits at a temperature of between
about -160.degree. C. and about -140.degree. C., for example about
-149.degree. C., and at a pressure of between about 2 MPa (20 bar
abs) and about 3.2 MPa (32 bar abs), for example 3.2 MPa (32 bar
abs). The vapour from the overhead gas outlet port 150 passes
through the sub-cooler 122 and the first heat exchanger 108,
thereby supporting cooling by both heat exchanges, before being
compressed and cooled by the nitrogen return compression and
cooling arrangement 151 and then provided at the nitrogen feedback
outlet port 104. The nitrogen present at the nitrogen feedback
outlet port 104 can be fed back to the wellbore (not shown), for
example under pressure, in order to enhance extraction of natural
gas from the wellbore.
[0056] Hydrocarbon fractions in liquid state exit the fractionating
column 116 via the bottom hydrocarbon outlet port 162 at a
temperature of between about -115.degree. C. and about -95.degree.
C., for example about -104.degree. C., and a pressure of between
about 2 MPa (20 bar abs) and about 3.2 MPa (32 bar abs), for
example 2.6 MPa (26 bar abs), and follow the bottom hydrocarbon
product stream path 160 before being split so as to follow the
first product stream path 166 and the second product stream path
168. In this example, the flow fraction of the first product stream
path 166 is between about 10% and about 30%, for example about 17%,
of the total liquid in the bottom hydrocarbon product stream path
160, and the flow fraction of the second product steam path 168 is
between about 90% and about 70%, for example about 83%, of the
total liquid in the bottom hydrocarbon product stream path 160. The
liquid fraction following the first product stream path 166
undergoes partial vaporisation and superheating, at a first
pressure of between about 500 kPa (5 bar abs) and about 2 MPa (20
bar abs), for example about 1.1 MPa (11 bar abs), in the first heat
exchanger 108 before being supplied to the compression and cooling
arrangement 172. However, the portion of liquid in the second
product stream path 168 is also fully vaporised at an elevated,
second, pressure greater than the first pressure, the second
pressure being between about 2 MPa (20 bar abs) and about 3.2 MPa
(32 bar abs), for example 2.6 MPa (26 bar abs). The fully vaporised
and superheated product is applied to the inlet port of the fifth
compressor 176, thereby reducing the energy consumption of the
fourth compressor 174.
[0057] In another embodiment, employing the reflux condenser
circuit 184 described above, a liquid side stream at a temperature
of between about -145.degree. C. and -125, for example about
-135.degree. C., is drawn off (Step 216) from the underneath the
reflux condenser stream outlet port 186 and passes through the
sub-cooler 122 (Step 218) before returning (220) to the
fractionating column 116 via the reflux condenser stream inlet port
188 at a temperature of between about -140.degree. C. and about
-120.degree. C., for example about -131.degree. C., thereby
supporting refrigeration provided by the sub-cooler 122. This
therefore improves the energy efficiency when generating reflux
through the sub-cooler 122. The fluid returning to the
fractionating column 116 via the reflux condenser stream inlet port
188 comprises a vapour fraction of between about 0.05 mol/mol and
about 0.2 mol/mol, for example about 0.13 mol/mol.
[0058] In yet another embodiment, vapour, at a temperature of
between about -155.degree. C. and about -135.degree. C., for
example about -145.degree. C., is tapped off (Step 222) the
fractionating column 116 at the nitrogen product fraction tapping
port 190 and undergoes compression (Step 224) by the first
compressor 129, cooling by the sub-cooler 122 and then further
cooling by passage through the fifth valve 194, resulting in the
extracted vapour being partially liquefied and providing additional
distillation reflux that is introduced (Step 226) into the
fractionating column 116 at the second reflux inlet port 192. The
additional distillation reflux is at a temperature of between about
-160.degree. C. and about -145.degree. C., for example -153.degree.
C., and comprises a vapour fraction of between about 0 mol/mol and
about 0.15 mol/mol, for example about 0.04 mol/mol.
[0059] In another embodiment, employing the reflux feedback path
196, a portion of the overhead gas stream is tapped off from the
nitrogen return compression and cooling arrangement 151 at the
point of partial compression mentioned above, for example after
passage of the overhead gas stream through the first intercooler
156. The reflux feedback path 196 comprises between about 0% and
about 50%, for example about 30%, of the total vapour in the
overhead gas stream path 148 and the fluid is at a pressure of
between about 3.5 MPa (35 bar abs) and about 10 MPa (100 bar abs),
for example about 6 MPa (60 bar abs). The tapped portion of the
overhead gas stream follows the reflux feedback path 196 so as to
become cooled by passing back (Step 228) through the first heat
exchanger 108 and the sub-cooler 122 before being cooled further
and partially liquefied using the sixth valve 197 and then
reintroduced (Step 230) into the fractionating column 116 at the
third reflux inlet port 198 as further distillation reflux. The
further distillation reflux is at a temperature of between about
-160.degree. C. and about -145.degree. C., for example about
-151.degree. C., and comprises a vapour fraction of between about 0
mol/mol and about 0.2 mol/mol, for example about 0.08 mol/mol.
[0060] The provision of the extra distillation reflux in the above
embodiments serves to provide additional rectification so as to
improve the purity of the nitrogen leaving the fractionating column
116 via the overhead gas outlet port 150 and so increase the
proportion of hydrocarbons that can be recovered. Furthermore, the
nitrogen recovered can be employed in a hydrocarbon capture and
recovery system employing, for example an EOR technique.
[0061] The skilled person should appreciate that the
above-described implementations are merely examples of the various
implementations that are conceivable within the scope of the
appended claims. Indeed, although in these examples, the expander
128 is employed to drive the first compressor 129, the skilled
person should appreciate that the work produced by the expander 128
can be employed to drive some or part of the nitrogen return
compression and cooling arrangement 151 and/or the hydrocarbon
fraction compression and cooling arrangement 172. Furthermore, the
compressors of the nitrogen return compression and cooling
arrangement 151 and/or the hydrocarbon fraction compression and
cooling arrangement 172 can be arranged in combination, optionally
with the first compressor 129, as a train of compressors sharing a
common drive shaft driven by the expander 128. Additionally or
alternatively, the compressors of the nitrogen return compression
and cooling arrangement 151 and/or the hydrocarbon fraction
compression and cooling arrangement 172 can, optionally with the
first compressor 129, be driven by at least one of an electric
motor, a gas turbine and/or a steam turbine. The first compressor
129 does not necessarily have to be driven by the expander 128 and
other modes of drive can be employed. Indeed, the expander 128 can
be used instead to drive an electrical generator.
[0062] In some embodiments, the fractionating column can be formed
from more than one part, for example two parts, such as a first
high-pressure fractionating column operably coupled to a second
lower-pressure fractionating column.
[0063] Although, in the above examples, a specific number and types
of heat exchangers are described, it should be appreciated that
heat exchanges can be implemented using any number and type of heat
exchangers, depending upon implementation requirements. In the
above examples, the number and types of heat exchangers are
employed for reasons of efficiency and sometimes implementation
convenience.
* * * * *