U.S. patent application number 14/604030 was filed with the patent office on 2016-07-28 for separation of heavy hydrocarbons and ngls from natural gas in integration with liquefaction of natural gas.
This patent application is currently assigned to AIR PRODUCTS AND CHEMICALS, INC.. The applicant listed for this patent is AIR PRODUCTS AND CHEMICALS, INC.. Invention is credited to Gowri Krishnamurthy, Mark Julian Roberts, Timothy Truong.
Application Number | 20160216030 14/604030 |
Document ID | / |
Family ID | 55229497 |
Filed Date | 2016-07-28 |
United States Patent
Application |
20160216030 |
Kind Code |
A1 |
Truong; Timothy ; et
al. |
July 28, 2016 |
Separation of Heavy Hydrocarbons and NGLs from Natural Gas in
Integration with Liquefaction of Natural Gas
Abstract
Described herein is a method of and system for fractionating and
liquefying a natural gas feed stream. The natural gas is first
fractionated in a scrub column. The overhead vapor from the scrub
column is cooled, condensed and divided to form a first, a second
and at least one further stream of liquefied first overhead. The
first stream of liquefied first overhead is returned to the scrub
column as a reflux stream. The second stream of liquefied first
overhead forms an LNG product. The further stream of liquefied
first overhead is used to provide or generate reflux for a
de-methanizer column used to fractionate the bottoms liquid from
the scrub column.
Inventors: |
Truong; Timothy; (Macungie,
PA) ; Krishnamurthy; Gowri; (Sellersville, PA)
; Roberts; Mark Julian; (Kempton, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AIR PRODUCTS AND CHEMICALS, INC. |
Allentown |
PA |
US |
|
|
Assignee: |
AIR PRODUCTS AND CHEMICALS,
INC.
Allentown
PA
|
Family ID: |
55229497 |
Appl. No.: |
14/604030 |
Filed: |
January 23, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2270/18 20130101;
F25J 1/025 20130101; F25J 2270/02 20130101; F25J 1/0055 20130101;
F25J 3/0209 20130101; F25J 1/0237 20130101; F25J 2245/02 20130101;
F25J 3/0242 20130101; F25J 1/0216 20130101; F25J 2200/02 20130101;
F25J 1/0241 20130101; F25J 2200/74 20130101; F25J 1/0045 20130101;
F25J 3/0238 20130101; F25J 1/0022 20130101; F25J 3/0233 20130101;
F25J 2210/04 20130101; F25J 1/0052 20130101; F25J 2200/72 20130101;
F25J 2200/78 20130101; F25J 1/0292 20130101; F25J 2215/62 20130101;
F25J 2270/66 20130101; F25J 2260/02 20130101; F25J 3/0214 20130101;
F25J 2200/04 20130101 |
International
Class: |
F25J 3/02 20060101
F25J003/02 |
Claims
1. A method of fractionating and liquefying a natural gas feed
stream, the method comprising: (a) introducing the natural gas feed
stream into a scrub column in which the natural gas feed stream is
separated into a methane-rich vapor fraction collected as a first
overhead vapor at the top of the scrub column, and a liquid
fraction, enriched in hydrocarbons heavier than methane, collected
as a first bottoms liquid at the bottom of the scrub column; (b)
withdrawing a stream of first overhead vapor from the top of the
scrub column, and cooling, condensing and dividing said stream to
form a first stream of liquefied first overhead, second stream of
liquefied first overhead, and at least one further stream of
liquefied first overhead; (c) returning the first stream of
liquefied first overhead to the scrub column as a reflux stream
introduced into the top the scrub column, thereby providing reflux
for the scrub column; (d) forming a liquefied natural gas (LNG)
product stream from the second stream of liquefied first overhead;
(e) withdrawing a stream of first bottoms liquid from the bottom of
the scrub column, and introducing said stream into a de-methanizer
column in which the stream of first bottoms liquid is separated
into a methane-rich vapor fraction collected as a second overhead
vapor at the top of the de-methanizer column, and a liquid
fraction, enriched in hydrocarbons heavier than methane, collected
as a second bottoms liquid at the bottom of the de-methanizer
column; and (f) providing reflux to the de-methanizer column by:
(1) introducing one of said further streams of liquefied first
overhead as a reflux stream into the top of the de-methanizer
column; and/or (2) condensing, by indirect heat exchange with one
of said further streams of liquefied first overhead, a portion of
the second overhead vapor to form a stream of liquefied second
overhead that is reintroduced as a reflux stream into the top the
de-methanizer column.
2. The method of claim 1, wherein step (b) comprises: withdrawing
the stream of first overhead vapor from the top of the scrub
column, cooling and partially condensing said stream, separating
the resulting liquid and vapor phases, dividing the separated
liquid phase to form the first stream of liquefied first overhead
and at least one further stream of liquefied first overhead, and
further cooling and condensing at least a portion of the separated
vapor phase to form the second stream of liquefied first overhead;
and/or withdrawing the stream of first overhead vapor from the top
of the scrub column, cooling and partially condensing said stream,
separating the resulting liquid and vapor phases, forming the first
stream of liquefied first overhead from at least a portion of the
separated liquid phase, further cooling and condensing at least a
portion of the separated vapor phase, and dividing the resulting
further cooled and condensed stream to form the second stream of
liquefied first overhead and at least one further stream of
liquefied first overhead.
3. The method of claim 2, wherein: the second stream of liquefied
first overhead is sub-cooled prior to forming the LNG product
stream; or the cooled and condensed stream is sub-cooled prior to
being divided to form the second stream of liquefied first overhead
and at least one further stream of liquefied first overhead.
4. The method of claim 2, wherein the stream of first overhead
vapor from the top of the scrub column is cooled and partially
condensed in a warm bundle of a coil-wound main heat exchanger, and
the at least a portion of the separated vapor phase is further
cooled and condensed in a middle and/or cold bundle of the
coil-wound main heat exchanger.
5. The method of claim 2, wherein the stream of first overhead
vapor from the top of the scrub column is cooled and partially
condensed in an overhead condenser heat exchanger, and the at least
a portion of the separated vapor phase is further cooled and
condensed in a coil-wound main heat exchanger.
6. The method of claim 1, wherein step (b) comprises: withdrawing
the stream of first overhead vapor from the top of the scrub
column, cooling and condensing said stream, and dividing the cooled
and condensed stream to form the first stream of liquefied first
overhead, the second stream of liquefied first overhead, and at
least one further stream of liquefied first overhead.
7. The method of claim 6, wherein: the cooled and condensed stream
is sub-cooled prior to being divided to form the first stream of
liquefied first overhead, the second stream of liquefied first
overhead, and at least one further stream of liquefied first
overhead; or the cooled and condensed stream is divided to form the
first stream of liquefied first overhead and another liquid stream
that is then sub-cooled prior to being divided to form the second
stream of liquefied first overhead and at least one further stream
of liquefied first overhead; or the second stream of liquefied
first overhead is sub-cooled prior to forming the LNG product
stream.
8. The method of claim 1, wherein the first stream of liquefied
first overhead, used as a reflux stream in step (c), and the
further stream(s) of liquefied first overhead, used in step (f),
have a flow ratio of at least about 9:1.
9. The method of claim 1, wherein the first stream of liquefied
first overhead, used as a reflux stream in step (c), and the
further stream(s) of liquefied first overhead, used in step (f),
each have a temperature of or below about -40.degree. C.
10. The method of claim 1, wherein the first overhead vapor and
second overhead vapor each comprise at least about 95 mole %
methane, and the second bottoms liquid comprises less than about 5
mole % methane.
11. The method of claim 1, wherein the method further comprises:
withdrawing a stream of second overhead vapor from the top of the
de-methanizer column; cooling and condensing all or a portion of
said stream to form additional LNG product; and/or using all or a
portion of said stream as a fuel stream; and/or exporting all or a
portion of said stream as a gaseous natural gas product stream.
12. The method of claim 1, wherein the method further comprises
withdrawing a stream of second bottoms liquid from the bottom of
the de-methanizer column and fractionating the stream of second
bottoms liquid to provide one or more natural gas liquids (NGL)
streams.
13. The method of claim 12, wherein the step of fractionating the
stream of second bottoms liquid comprises: introducing said stream
into a de-ethanizer column in which the stream of second bottoms
liquid is separated into ethane-enriched fraction, collected as a
third overhead vapor at the top of the de-ethanizer column, and a
fraction enriched in hydrocarbons heavier than ethane, collected as
a third bottoms liquid at the bottom of the de-ethanizer
column.
14. The method of claim 13, wherein the method further comprises:
withdrawing a stream of third overhead vapor from the top of the
de-ethanizer column, and cooling and condensing said stream to form
an NGL stream; and/or withdrawing a stream of third bottoms liquid
from the bottom of the de-ethanizer column, and forming one or more
NGL streams therefrom.
15. The method of claim 13, wherein the method further comprises
providing reflux to the de-ethanizer column by condensing, by
indirect heat exchange with a further stream of liquefied first
overhead, a portion of the third overhead vapor to form a stream of
liquefied third overhead that is reintroduced as a reflux stream
into the top the de-ethanizer column.
16. The method of claim 13, wherein the third overhead vapor
comprises at least about 95% ethane, and the third bottoms liquid
comprises less than about 5 mole ethane.
17. A system for fractionating and liquefying a natural gas feed
stream, the system comprising: a scrub column arranged and operable
to receive the natural gas feed stream and separate said stream
into a methane-rich vapor fraction collected as a first overhead
vapor at the top of the scrub column, and a liquid fraction,
enriched in hydrocarbons heavier than methane, collected as a first
bottoms liquid at the bottom of the scrub column; a set of
conduits, one or more heat exchangers, and optionally one or more
separators, arranged and operable to withdraw a stream of first
overhead vapor from the top of the scrub column and to cool,
condense and divide said stream to form a first stream of liquefied
first overhead, second stream of liquefied first overhead, and at
least one further stream of liquefied first overhead; a conduit
arranged and operable to return the first stream of liquefied first
overhead to the scrub column as a reflux stream introduced into the
top the scrub column, thereby providing reflux for the scrub
column; a conduit arranged and operable to withdraw from the system
a liquefied natural gas (LNG) product stream formed from the second
stream of liquefied first overhead; a de-methanizer column arranged
and operable to receive a stream of first bottoms liquid from the
bottom of the scrub column and to separate said stream into a
methane-rich vapor fraction collected as a second overhead vapor at
the top of the de-methanizer column, and a liquid fraction,
enriched in hydrocarbons heavier than methane, collected as a
second bottoms liquid at the bottom of the de-methanizer column; a
conduit arranged and operable to withdraw the stream of first
bottoms liquid from the bottom of the scrub column and introduce
said stream into the de-methanizer column; and one or both of: (1)
a conduit arranged and operable to introduce one of said further
streams of liquefied first overhead as a reflux stream into the top
of the de-methanizer column, thereby providing reflux for the
de-methanizer column; (2) a heat exchanger arranged and operable to
condense, by indirect heat exchange with one of said further
streams of liquefied first overhead, a portion of the second
overhead vapor to form a stream of liquefied second overhead that
is reintroduced as a reflux stream into the top the de-methanizer
column, thereby providing reflux for the de-methanizer column, and
a conduit arranged and operable to introduce said further stream of
liquefied first overhead into said heat exchanger.
18. A system according to claim 17, wherein the set of conduits,
one or more heat exchangers, and one or more separators arranged
and operable to withdraw, cool, condense and divide the stream of
first overhead vapor comprise: a conduit arranged and operable to
withdraw the stream of first overhead vapor from the top of the
scrub column; a heat exchanger or section of a heat exchanger
arranged and operable to cool and partially condense said stream; a
separator arranged and operable to separate the resulting liquid
and vapor phases; a set of conduits arranged and operable to divide
the separated liquid phase to form the first stream of liquefied
first overhead and a further stream of liquefied first overhead;
and a heat exchanger or section of a heat exchanger arranged and
operable to receive, further cool and condense at least a portion
of the separated vapor phase to form the second stream of liquefied
first overhead.
19. A system according to claim 18, wherein the system further
comprises a heat exchanger or section of a heat exchanger arranged
and operable to receive and sub-cool the second stream of liquefied
first overhead.
20. A system according to claim 18, wherein the section of a heat
exchanger arranged and operable to cool and partially condense the
stream of first overhead vapor comprises a warm bundle of a
coil-wound main heat exchanger, and the section of a heat exchanger
arranged and operable to further cool and condense at least a
portion of the separated vapor phase comprises a middle and/or cold
bundle of the coil-wound main heat exchanger.
21. A system according to claim 18, wherein the heat exchanger
arranged and operable to cool and partially condense the stream of
first overhead vapor comprises an overhead condenser heat
exchanger, and the heat exchanger arranged and operable to further
cool and condense at least a portion of the separated vapor phase
comprises a coil-wound main heat exchanger.
22. A system according to claim 17, wherein the set of conduits and
one or more heat exchangers arranged and operable to withdraw,
cool, condense and divide the stream of first overhead vapor
comprise: a conduit arranged and operable to withdraw the stream of
first overhead vapor from the top of the scrub column; a heat
exchanger or section of a heat exchanger arranged and operable to
cool and condense said stream; and a set of conduits arranged and
operable to divide the cooled and condensed stream to form the
first stream of liquefied first overhead, the second stream of
liquefied first overhead, and at least one further stream of
liquefied first overhead.
23. A system according to claim 22, wherein the system further
comprises a heat exchanger or section of a heat exchanger arranged
and operable to sub-cool the cooled and condensed stream prior to
said stream being divided to form the first stream of liquefied
first overhead, the second stream of liquefied first overhead, and
at least one further stream of liquefied first overhead.
24. A system according to claim 22, wherein the system further
comprises a heat exchanger or section of a heat exchanger arranged
and operable to sub-cool the second stream of liquefied first
overhead.
25. A system according to claim 17, wherein the system further
comprises: a de-ethanizer column arranged and operable to receive a
stream of second bottoms liquid from the bottom of the
de-methanizer column and to separate said stream into an
ethane-enriched fraction, collected as a third overhead vapor at
the top of the de-ethanizer column, and a fraction enriched in
hydrocarbons heavier than ethane, collected as a third bottoms
liquid at the bottom of the de-ethanizer column; and a conduit
arranged and operable to withdraw the stream of second bottoms
liquid from the bottom of the de-methanizer column and introduce
said stream into the de-ethanizer column.
26. A system according to claim 25, wherein the system further
comprises: a conduit arranged and operable to withdraw a stream of
third overhead vapor from the top of the de-ethanizer column, and
one or more heat exchangers arranged and operable to receive, cool
and condense said stream to form an NGL stream; and/or a conduit
arranged and operable to withdraw a stream of third bottoms liquid
from the bottom of the de-ethanizer column, for forming one or more
NGL streams therefrom.
27. A system according to claim 25, wherein the system further
comprises a heat exchanger arranged and operable to condense, by
indirect heat exchange with a further stream of liquefied first
overhead, a portion of the third overhead vapor to form a stream of
liquefied third overhead that is reintroduced as a reflux stream
into the top the de-ethanizer column, thereby providing reflux for
the de-ethanizer column, and a conduit arranged and operable to
introduce said further stream of liquefied first overhead into said
heat exchanger.
Description
BACKGROUND
[0001] The present invention relates to a method of and system for
fractionating and liquefying a natural gas feed stream.
[0002] Removal of the heavy hydrocarbons (HHCs), such as C6+
hydrocarbons (hydrocarbons having 6 or more carbon atoms), from
natural gas prior to liquefaction of the natural gas is, typically,
essential in order to lower the concentration of these components
in the natural gas down to a level where freeze-out of these
components in the main heat exchanger will not occur. C2-C5+
hydrocarbons (hydrocarbons having 2 to 5 or more carbon atoms),
also referred to in the art as Natural Gas Liquids (NGLs), are
typically also separated from natural gas since they have a high
market value, and can therefore be sold separately. Additionally,
as NGLs have a higher heating value than methane it may be
necessary to reduce the levels of NGL components in the natural gas
in order that the LNG product meets stipulated product
specifications. Where the refrigerant used in the natural gas
liquefaction process comprises one or more hydrocarbon
refrigerants, such as, in particular, where a cascade or mixed
refrigerant cycle using ethane and/or propane is employed for the
liquefaction process, it may also be desirable to use separated NGL
components (such as ethane or propane) as refrigerant make-up.
Traditionally, distillation columns have been used for this
purpose.
[0003] FIG. 1 depicts, schematically, a conventional arrangement
for fractionating and liquefying a natural gas feed stream. Natural
gas feed stream, 101, which has been pre-treated to remove acid
gases, water and mercury, and which may optionally also have been
pre-cooled in one or more heat exchangers, is introduced into scrub
column, 10, in which it is separated into a methane-rich overhead
vapor and a bottoms liquid enriched in hydrocarbons heavier than
methane. A stream of the overhead vapor, 202, is withdrawn from the
top of the scrub column, and a stream of the bottoms liquid, 103,
is withdrawn from the bottom of the scrub column.
[0004] The stream of overhead vapor, 202, is sent to the warm
bundle, 22, of a coil-wound main heat exchanger, 20, in which the
stream is partially condensed. The partially condensed stream, 203,
is then withdrawn from the warm bundle and separated in a phase
separator, 28, into its liquid and vapor phases to produce a liquid
stream, 120, and vapor stream, 207. The vapor stream, 207, is sent
to the middle bundle, 24, of the main heat exchanger in which the
stream is further cooled and liquefied, and the liquefied stream,
204, is then sub-cooled in the cold bundle, 26, of the main heat
exchanger, producing an LNG product stream, 205. The LNG product
stream, 205, may be flashed and sent to an LNG storage tank, 30,
with the boil-off gas (BOG) or flash gas, 401, from the tank being
sent to a fuel header, flared, or recycled (not shown) to the
overhead vapor stream, 202, fed to the main heat exchanger. The
liquid stream, 120, from the phase separator, 28, is returned to
the top of the scrub column, 10, as a reflux stream in order to
provide the necessary reflux for operation of the scrub column. If
the quantity of reflux generated is larger than required for the
scrub column, a portion of liquid stream, 120, may be mixed with
stream 207 and sent to the middle bundle, 24, of the main heat
exchanger.
[0005] The stream of bottoms liquid, 103, from the scrub column,
10, which is rich in NGLs and HHCs, is expanded to partially
vaporize the stream, and is then sent to the de-methanizer column
of a fractionation unit in which the stream is subjected to further
fractionation/separation. In the arrangement illustrated in FIG. 1,
the fractionation unit comprises a de-methanizer column (also
referred to herein as a DeC1 column), a de-ethanizer column (also
referred to herein as a DeC2 column), a de-propanizer column (also
referred to herein as a DeC3 column), and a de-butanizer column
(also referred to herein as a DeC4 column). Typically, these
columns contain multiple stages to enhance the separation of the
HHCs and NGLs from methane and lighter components of the natural
gas.
[0006] The de-methanizer column, 12, separates the bottoms liquid,
103, from the scrub column into an overhead vapor rich in methane,
and a bottoms liquid enriched in hydrocarbons heavier than methane.
The overhead vapor is partially condensed in an overhead condenser
to produce a liquid reflux stream that is returned to the
de-methanizer column, 12, with the remaining vapor portion being
withdrawn as from the de-methanizer column as an overhead vapor
stream, 104. A portion of the bottoms liquid is heated in a
re-boiler to provide boil-up for the de-methanizer column, 12, and
the remainder of the bottoms liquid is withdrawn as stream 105,
expanded to partially vaporizer the stream, and sent to the
de-ethanizer column 14.
[0007] De-ethanizer column, 14, in turn separates the bottoms
liquid, 105, from the de-methanizer column into an overhead vapor
enriched in ethane, and a bottoms liquid enriched in hydrocarbons
heavier than ethane. The overhead vapor is condensed in an overhead
condenser, with a portion of the condensed overhead being returned
to the de-ethanizer column, 14, as a reflux stream, and the
remainder being withdrawn as a liquefied overhead stream, 106,
enriched in ethane. A portion of the bottoms liquid is heated in a
re-boiler to provide boil-up for the de-ethanizer column, 14, and
the remainder of the bottoms liquid is withdrawn as stream 107,
expanded, and sent to the de-propanizer column 16.
[0008] The de-propanizer column, 16, operating in a similar manner
to the de-ethanizer column, then separates the bottoms liquid, 107,
from the de-ethanizer column to provide a liquefied overhead
stream, 108, enriched in propane and a bottoms liquid stream, 109,
enriched in hydrocarbons heavier than propane. Likewise, the
de-butanizer column, 18, operating in a similar manner to the
de-ethanizer column, then separates the bottoms liquid, 109, from
the de-propanizer column to provide a liquefied overhead stream,
110, enriched in butane and a bottoms liquid stream, 111, enriched
in hydrocarbons heavier than butane.
[0009] As the liquefied overhead streams, 106, 108 and 110, from
columns DeC2, DeC3 and DeC4 mainly comprise ethane, propane and
butane, respectively, and the bottoms liquid 111 from column DeC4
is a C5+(pentane and heavier hydrocarbon) rich stream, these NGL
streams may, as discussed above, be sold or, as and where
appropriate, used as refrigerant make-up. In some cases, a fraction
of one or more of these streams may also be re-injected into the
LNG product stream to adjust the heating value of the LNG product
stream to an optimal value.
[0010] As noted above, each of columns DeC1, DeC2, DeC3 and DeC4 is
equipped with an overhead condenser that generates reflux for the
column by condensing at least a portion of the overhead vapor,
thereby providing a liquid reflux stream that is returned to the
top of the column. Having these overhead condensers adds to the
capital cost of setting up and maintaining the system. However,
eliminating the overhead condensers would lead to carryover of
unwanted components into the overhead vapor streams. The condensers
improve separation efficiency, and it is especially beneficial to
have such condensers on the DeC1 and DeC2 columns. The temperature
of the cold reflux stream needed depends on the composition of the
natural gas stream. The lower the NGL content, the colder the
reflux stream needs to be to efficiently knock out the desired NGL
and HHC components into the liquid condensate stream. Eliminating
the overhead condensers from the DeC1 and DeC2 columns would result
in carryover of ethane and propane into, respectively, the overhead
vapor streams from DeC1 and DeC2. This will result in ethane being
lost with the DeC1 overhead and likewise in additional propane
being present in the DeC2 overhead, thus negatively affecting both
the purity of ethane in the DeC2 overhead (which likewise adversely
affects its use as make-up refrigerant and/or value as a commercial
product).
[0011] Typically, propane is used as a refrigerant for providing
the cooling duty for the overhead condenser used with the DeC1
column. Such an arrangement has, in particular, been considered in
fractionation and liquefaction systems where a propane pre-cooled
mixed refrigerant (C3MR) cycle is being used, and in which propane
refrigeration is therefore easily available.
[0012] US 2012/0090350 A1 describes a system and method of
controlling the heating value of an LNG product in a natural gas
liquefaction plant. The natural gas feed stream, after initial
pre-treatment, is introduced into a scrub column to separate the
natural gas feed into a C5+ depleted overhead vapor and a C3+
enriched bottoms liquid. Reflux for the scrub column is provided,
in part, by an overhead condenser that uses low pressure propane
refrigeration. A stream of the overhead vapor from the scrub column
is sent to a liquefaction plant for liquefaction, and a stream of
bottoms liquid is fractionated in an NGL fractionation unit to
provide a C3 rich liquid stream and a C4+ rich liquid stream.
[0013] U.S. Pat. No. 6,662,589 and US 2006/0260355 A1 describe
processes in which a natural gas feed stream, after initial
pre-treatment, is introduced into a scrub column to separate the
natural gas feed into an overhead vapor and bottoms liquid. The
overhead vapor stream withdrawn from the scrub column is cooled and
partially condensed in the warm section of a main heat exchanger,
and then separated in a phase separator into a liquid phase, which
is returned to the top of the scrub column as a liquid reflux
stream, and a remaining vapor phase which is further cooled and
condensed in the cold section of the main heat exchanger to provide
LNG product. The bottoms liquid stream withdrawn from the scrub
column is fractionated in an NGL fractionation unit, comprising
de-ethanizer, de-propanizer and de-butanizer columns, to provide
C3, C4 and C5+ rich liquid streams, and a C1 and C2 rich vapor
stream. The C1 and C2 rich vapor stream is condensed in the main
heat exchanger to provide additional LNG product.
[0014] US 2008/0115532 A1 describes the operation of a scrub column
used to remove heavier hydrocarbon components from a natural gas
stream prior to liquefaction of the natural gas. Reflux for the
scrub column can be provided by condensate from an overhead
condenser and/or by a stream of LNG.
[0015] US 2008/0016910 A1 and US 2013/0061632 A1 describe processes
in which a pre-treated natural gas feed stream is introduced into a
scrub column to separate the natural gas feed into an overhead
vapor and bottoms liquid. The overhead vapor stream withdrawn from
the scrub column is cooled and condensed in the warm section of a
main heat exchanger. The condensed stream is then divided into two
streams, with one stream being returned to the top of the scrub
column as a liquid reflux stream, and the other being sub-cooled in
the cold section of the main heat exchanger to provide an LNG
product stream. The bottoms liquid stream withdrawn from the scrub
column is fractionated in an NGL fractionation system to provide
C2, C3 and C4 product streams.
[0016] U.S. Pat. No. 4,065,278, CA 1059425 and U.S. Pat. No.
5,659,109 describe processes for producing LNG in which, similar to
the processes described in U.S. Pat. No. 6,662,589 and US
2006/0260355 A1, a pre-treated natural gas feed stream is
fractionated in a scrub column, and reflux to the scrub column is
provided by partially condensing the scrub column overhead in the
main heat exchanger and returning the separated liquid phase to the
scrub column as a liquid reflux stream, with at least a portion of
the remaining vapor phase being further cooled and condensed to
provide the LNG product.
[0017] U.S. Pat. No. 4,445,917 describes a process for producing
LNG in which, similar to the processes described in 2008/0016910 A1
and US 2013/0061632 A1, a pre-treated natural gas feed stream is
fractionated in a scrub column, with the scrub column overhead
being fully condensed and then divided to provide an LNG stream and
a liquid reflux stream that is returned to the top of the scrub
column.
[0018] U.S. Pat. No. 5,956,971 teaches a process form producing LNG
in which the natural gas feed is first processed in a fractionation
column having a controlled freezing zone.
[0019] US2007012071 describes a process and system for producing
LNG in which a pre-treated natural gas feed is separated in a
fractionation column into a methane rich overhead and a C2+ rich
bottoms liquid. The overhead stream is condensed, further cooled,
and then flashed to generate a LNG stream and a flash gas. The
flash gas is used as refrigerant in the main heat exchanger. A
portion of the warmed flash gas is then re-compressed, cooled,
condensed and introduced into the top of the fractionation column
as a reflux stream.
[0020] U.S. Pat. No. 5,588,308 describes a process of removing NGLs
from a natural gas stream, in which the natural gas feed is
partially condensed and separated into liquid and vapor phases, the
vapor phase providing a natural gas product, and the liquid phase
being introduced into and fractionated in a stripping column to
provide an NGL stream.
[0021] US 2004/0200353 A1 describes a process for removing NGLs
from a natural gas feed using a scrub column, reflux for which is
provided by an overhead condenser.
[0022] It would be desirable to have improved methods and systems
for fractionating and liquefying a natural gas feed stream.
BRIEF SUMMARY
[0023] According to a first aspect of the present invention, there
is provided a method of fractionating and liquefying a natural gas
feed stream, the method comprising:
[0024] (a) introducing the natural gas feed stream into a scrub
column in which the natural gas feed stream is separated into a
methane-rich vapor fraction collected as a first overhead vapor at
the top of the scrub column, and a liquid fraction, enriched in
hydrocarbons heavier than methane, collected as a first bottoms
liquid at the bottom of the scrub column;
[0025] (b) withdrawing a stream of first overhead vapor from the
top of the scrub column, and cooling, condensing and dividing said
stream to form a first stream of liquefied first overhead, second
stream of liquefied first overhead, and at least one further stream
of liquefied first overhead;
[0026] (c) returning the first stream of liquefied first overhead
to the scrub column as a reflux stream introduced into the top the
scrub column, thereby providing reflux for the scrub column;
[0027] (d) forming a liquefied natural gas (LNG) product stream
from the second stream of liquefied first overhead;
[0028] (e) withdrawing a stream of first bottoms liquid from the
bottom of the scrub column, and introducing said stream into a
de-methanizer column in which the stream of first bottoms liquid is
separated into a methane-rich vapor fraction collected as a second
overhead vapor at the top of the de-methanizer column, and a liquid
fraction, enriched in hydrocarbons heavier than methane, collected
as a second bottoms liquid at the bottom of the de-methanizer
column; and
[0029] (f) providing reflux to the de-methanizer column by: [0030]
(1) introducing one of said further streams of liquefied first
overhead as a reflux stream into the top of the de-methanizer
column; and/or [0031] (2) condensing, by indirect heat exchange
with one of said further streams of liquefied first overhead, a
portion of the second overhead vapor to form a stream of liquefied
second overhead that is reintroduced as a reflux stream into the
top the de-methanizer column.
[0032] According to a second aspect of the present invention, there
is provided a system for fractionating and liquefying a natural gas
feed stream, the system comprising:
[0033] a scrub column arranged and operable to receive the natural
gas feed stream and separate said stream into a methane-rich vapor
fraction collected as a first overhead vapor at the top of the
scrub column, and a liquid fraction, enriched in hydrocarbons
heavier than methane, collected as a first bottoms liquid at the
bottom of the scrub column;
[0034] a set of conduits, one or more heat exchangers, and
optionally one or more separators, arranged and operable to
withdraw a stream of first overhead vapor from the top of the scrub
column and to cool, condense and divide said stream to form a first
stream of liquefied first overhead, second stream of liquefied
first overhead, and at least one further stream of liquefied first
overhead;
[0035] a conduit arranged and operable to return the first stream
of liquefied first overhead to the scrub column as a reflux stream
introduced into the top the scrub column, thereby providing reflux
for the scrub column;
[0036] a conduit arranged and operable to withdraw from the system
a liquefied natural gas (LNG) product stream formed from the second
stream of liquefied first overhead;
[0037] a de-methanizer column arranged and operable to receive a
stream of first bottoms liquid from the bottom of the scrub column
and to separate said stream into a methane-rich vapor fraction
collected as a second overhead vapor at the top of the
de-methanizer column, and a liquid fraction, enriched in
hydrocarbons heavier than methane, collected as a second bottoms
liquid at the bottom of the de-methanizer column;
[0038] a conduit arranged and operable to withdraw the stream of
first bottoms liquid from the bottom of the scrub column and
introduce said stream into the de-methanizer column; and
[0039] one or both of: [0040] (1) a conduit arranged and operable
to introduce one of said further streams of liquefied first
overhead as a reflux stream into the top of the de-methanizer
column, thereby providing reflux for the de-methanizer column;
[0041] (2) a heat exchanger arranged and operable to condense, by
indirect heat exchange with one of said further streams of
liquefied first overhead, a portion of the second overhead vapor to
form a stream of liquefied second overhead that is reintroduced as
a reflux stream into the top the de-methanizer column, thereby
providing reflux for the de-methanizer column, and a conduit
arranged and operable to introduce said further stream of liquefied
first overhead into said heat exchanger.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1 is a schematic flow diagram depicting a comparative
natural gas fractionation and liquefaction system and method, not
in accordance with the present invention.
[0043] FIG. 2 is a schematic flow diagram depicting a natural gas
fractionation and liquefaction system and method in accordance with
an embodiment of the present invention.
[0044] FIG. 3 is a schematic flow diagram depicting a natural gas
fractionation and liquefaction system and method in accordance with
another embodiment of the present invention.
[0045] FIG. 4 is a schematic flow diagram depicting a natural gas
fractionation and liquefaction system and method in accordance with
another embodiment of the present invention.
[0046] FIG. 5 is a schematic flow diagram depicting a natural gas
fractionation and liquefaction system and method in accordance with
another embodiment of the present invention.
[0047] FIG. 6 is a schematic flow diagram depicting a natural gas
fractionation and liquefaction system and method in accordance with
another embodiment of the present invention.
[0048] FIG. 7 is a schematic flow diagram depicting a natural gas
fractionation and liquefaction system and method in accordance with
another embodiment of the present invention.
DETAILED DESCRIPTION
[0049] This present invention provides novel options for providing
reflux/condensing duty to the de-methanizer column in integration
with the liquefaction of natural gas.
[0050] More specifically, and as noted above, according to the
first aspect of the present invention a method of fractionating and
liquefying a natural gas feed stream is provided, in which the
overhead vapor from the top of the scrub column is cooled,
condensed and divided to form a first stream of liquefied first
overhead, second stream of liquefied first overhead, and at least
one further stream of liquefied first overhead. The first stream of
liquefied first overhead is reintroduced into the scrub column to
provide reflux for the scrub column, the second stream of liquefied
first overhead provides the desired LNG product, and the further
stream of liquefied first overhead is used to provide or generate
reflux for the de-methanizer column.
[0051] In particular, in a preferred embodiment the further stream
of liquefied first overhead (i.e. the further stream of liquefied
scrub column overhead) is introduced as a reflux stream into the
top of the de-methanizer column, thereby providing reflux for said
column.
[0052] This has the benefit, as compared to the conventional
arrangement depicted in FIG. 1 and described above, of allowing the
overhead condenser for the de-methanizer column to be eliminated,
thereby significantly reducing the capital cost of the method and
system.
[0053] Furthermore, the liquefied first overhead (i.e. the
liquefied scrub column overhead) will typically be colder than the
condensed overhead that can be generated by condensing
de-methanizer column overhead in a propane cooled overhead
condenser. For example, the lowest temperature reflux stream
attainable by using a propane kettle to condense overhead from the
de-methanizer column is typically around -31.degree. C., whereas
the reflux streams of -40.degree. C. or below are typically
obtained when condensing, even if only partially, the scrub column
overhead in the main heat exchanger or using the refrigerant used
in the main heat exchanger. As such, using a stream of liquefied
first overhead as a reflux stream in the de-methanizer column,
rather than refluxing the de-methanizer column using a propane
cooled overhead condenser, typically improves the separation of
methane from ethane and heavier components in the de-methanizer
column, which in turn improves the recovery of ethane in the
bottoms liquid of the de-methanizer column.
[0054] In an another embodiment, reflux for the de-methanizer
column is provided by condensing, by indirect heat exchange with
the further stream of liquefied first overhead (i.e. the further
stream of liquefied scrub column overhead), a portion of the second
overhead vapor (i.e. the overhead vapor from the de-methanizer
column) to form a stream of liquefied second overhead that is
reintroduced as a reflux stream into the top the de-methanizer
column.
[0055] In this embodiment, a dedicated overhead condenser for the
de-methanizer column is still required, and so the capital cost
savings provided by the previous embodiment are not obtained.
However, this embodiment retains the benefit of using a, typically,
colder stream to generate reflux for the de-methanizer, since the
de-methanizer column is in this embodiment still using a stream of
liquefied scrub column overhead to provide the cooling duty for
refluxing the de-methanizer column, rather than using a propane
refrigerant to provide said cooling duty. This embodiment is also
beneficial when propane is not the pre-cooling refrigerant and may
not be available. As such, this embodiment still provides the
benefit of improving the separation of methane from ethane and
heavier components in the de-methanizer column.
[0056] The articles "a" and "an", as used herein and unless
otherwise indicated, mean one or more when applied to any feature
in embodiments of the present invention described in the
specification and claims. The use of "a" and "an" does not limit
the meaning to a single feature unless such a limit is specifically
stated. The article "the" preceding singular or plural nouns or
noun phrases denotes a particular specified feature or particular
specified features and may have a singular or plural connotation
depending upon the context in which it is used.
[0057] As used herein, the term "natural gas feed stream"
encompasses also streams comprising synthetic and/or substitute
natural gases. The major component of natural gas is methane (which
typically comprises at least about 85 mole %, more often at least
about 90 mole %, and most typically about 95 mole % of the feed
stream). The natural gas feed stream also contains smaller amounts
of other, heavier hydrocarbons, such as ethane, propane, butanes,
pentanes, etc. Other typical components of raw natural gas include
one or more components such as nitrogen, helium, hydrogen, carbon
dioxide and/or other acid gases, and mercury. However, the natural
gas feed stream processed in accordance with the present invention
will have been pre-treated if and as necessary to reduce the levels
of any (relatively) high freezing point components, such as
moisture, acid gases and mercury, down to such levels as are
necessary to avoid freezing or other operational problems in the
scrub column into which the natural gas feed stream is to be
introduced.
[0058] As used herein, the term "methane-rich" refers to a stream,
fraction or portion that comprises methane as its major component.
The stream, fraction or portion may, in particular, have a
concentration of methane that is similar to, or higher than, that
of the natural gas feed stream. Thus, typically, the stream,
fraction or portion will comprise at least about 85 mole %, more
often at least about 90 mole %, and most typically about 95 mole %
or more than about 95 mole % methane.
[0059] As used herein, the term "hydrocarbons heavier than methane"
refers to hydrocarbons that have a lower volatility (i.e. higher
boiling point) than methane. Similarly, the term "hydrocarbons
heavier than ethane" refers to hydrocarbons that have a lower
volatility (i.e. higher boiling point) than ethane, the term
"hydrocarbons heavier than propane" refers to hydrocarbons that
have a lower volatility (i.e. higher boiling point) than propane,
and so on.
[0060] As used herein, the term "enriched in hydrocarbons heavier
than methane" refers to a stream, fraction or portion that is
enriched, relative to the natural gas feed stream, in hydrocarbons
heavier than methane, and thus that has a higher mole % of
hydrocarbons heavier than methane than that of the natural gas feed
stream. Similarly, the term "enriched in ethane" refers to a
stream, fraction or portion that is enriched, relative to the
natural gas feed stream, in ethane, and so on.
[0061] As used herein, the term "indirect heat exchange" refers to
heat exchange between two fluids where the two fluids are kept
separate from each other by some form of physical barrier.
[0062] In one embodiment, step (b) of the method according to the
first aspect comprises withdrawing the stream of first overhead
vapor from the top of the scrub column, cooling and partially
condensing said stream, separating the resulting liquid and vapor
phases, dividing the separated liquid phase to form the first
stream of liquefied first overhead and at least one further stream
of liquefied first overhead, and further cooling and condensing at
least a portion of the separated vapor phase to form the second
stream of liquefied first overhead. Preferably, the second stream
of liquefied first overhead is then sub-cooled prior to forming the
LNG product stream.
[0063] In another embodiment, step (b) of the method according to
the first aspect comprises withdrawing the stream of first overhead
vapor from the top of the scrub column, cooling and partially
condensing said stream, separating the resulting liquid and vapor
phases, forming the first stream of liquefied first overhead from
at least a portion of the separated liquid phase, further cooling
and condensing at least a portion of the separated vapor phase, and
dividing the resulting further cooled and condensed stream to form
the second stream of liquefied first overhead and at least one
further stream of liquefied first overhead. Preferably, the second
stream of liquefied first overhead is then sub-cooled prior to
forming the LNG product stream, or the cooled and condensed stream
is sub-cooled prior to being divided to form the second stream of
liquefied first overhead and at least one further stream of
liquefied first overhead
[0064] In another embodiment, step (b) of the method according to
the first aspect comprises a combination of the previous two
embodiments.
[0065] In the above-mentioned embodiments, it is preferred that a
coil-wound heat exchanger is used for further cooling and
condensing the separated vapor phase. Thus, in one embodiment the
stream of first overhead vapor from the top of the scrub column may
be cooled and partially condensed in a warm bundle of a coil-wound
main heat exchanger, and the at least a portion of the separated
vapor phase is further cooled and condensed in a middle and/or cold
bundle of the coil-wound main heat exchanger. In another embodiment
the stream of first overhead vapor from the top of the scrub column
may be cooled and partially condensed in an overhead condenser heat
exchanger, and the at least a portion of the separated vapor phase
is further cooled and condensed in a coil-wound main heat
exchanger.
[0066] In another embodiment, step (b) of the method according to
the first aspect comprises withdrawing the stream of first overhead
vapor from the top of the scrub column, cooling and condensing said
stream, and dividing the cooled and condensed stream to form the
first stream of liquefied first overhead, the second stream of
liquefied first overhead, and at least one further stream of
liquefied first overhead.
[0067] The cooled and condensed stream may, in this embodiment, be
sub-cooled prior to being divided to form the first stream of
liquefied first overhead, the second stream of liquefied first
overhead, and at least one further stream of liquefied first
overhead. Alternatively, the cooled and condensed stream may be
divided to form the first stream of liquefied first overhead and
another liquid stream that is then sub-cooled prior to being
divided to form the second stream of liquefied first overhead and
at least one further stream of liquefied first overhead.
Alternatively, the second stream of liquefied first overhead may be
sub-cooled prior to forming the LNG product stream.
[0068] In this embodiment, it is preferred that a coil-wound heat
exchanger is used for cooling and condensing the stream of first
overhead vapor from the top of the scrub column.
[0069] In the method according to the first aspect, it is preferred
that the first stream of liquefied first overhead, used as a reflux
stream in step (c), and the further stream(s) of liquefied first
overhead, used in step (f), have a flow ratio of at least about
9:1.
[0070] Thus, it is preferred that the flow rate of the first stream
of liquefied first overhead, used as a reflux stream in step (c),
is about nine times or more than nine times greater than the flow
rate of the further stream(s) of liquefied first overhead, used in
step (f), compared on a like-for-like basis (e.g. mass flow rate to
mass flow rate, or molar flow rate to molar flow rate, etc.). In
those embodiments where the method uses in step (f) both a further
stream of liquefied first overhead as a reflux stream and a further
stream of liquefied first overhead to condense, by indirect heat, a
portion of the second overhead vapor, it is preferred that flow
rate of the first stream of liquefied first overhead, used as a
reflux stream in step (c), is about nine times or more than nine
times greater than the combined flow rate of both of said further
streams of liquefied first overhead used in step (f).
[0071] In the method according to the first aspect, it is preferred
that the first stream of liquefied first overhead, used as a reflux
stream in step (c), and the further stream(s) of liquefied first
overhead, used in step (f), each have a temperature of or below
about -40.degree. C.
[0072] In a preferred embodiment, the first overhead vapor and
second overhead vapor each comprise at least about 95 mole %
methane.
[0073] In a preferred embodiment, the second bottoms liquid
comprises less than about 5 mole % methane.
[0074] In one embodiment, the method further comprises: withdrawing
a stream of second overhead vapor from the top of the de-methanizer
column; cooling and condensing all or a portion of said stream to
form additional LNG product; and/or using all or a portion of said
stream as a fuel stream; and/or exporting all or a portion of said
stream as a gaseous natural gas product stream.
[0075] In one embodiment, the method further comprises withdrawing
a stream of second bottoms liquid from the bottom of the
de-methanizer column and fractionating the stream of second bottoms
liquid to provide one or more natural gas liquids (NGL)
streams.
[0076] In one embodiment, the step of fractionating the stream of
second bottoms liquid comprises: introducing said stream into a
de-ethanizer column in which the stream of second bottoms liquid is
separated into ethane-enriched fraction, collected as a third
overhead vapor at the top of the de-ethanizer column, and a
fraction enriched in hydrocarbons heavier than ethane, collected as
a third bottoms liquid at the bottom of the de-ethanizer column.
The method may further comprise: withdrawing a stream of third
overhead vapor from the top of the de-ethanizer column, and cooling
and condensing said stream to form an NGL stream; and/or
withdrawing a stream of third bottoms liquid from the bottom of the
de-ethanizer column, and forming one or more NGL streams therefrom.
The method may further comprise providing reflux to the
de-ethanizer column by condensing, by indirect heat exchange with a
further stream of liquefied first overhead, a portion of the third
overhead vapor to form a stream of liquefied third overhead that is
reintroduced as a reflux stream into the top the de-ethanizer
column. Preferably, the third overhead vapor comprises at least
about 95% ethane. Preferably, the third bottoms liquid comprises
less than about 5 mole % ethane. The third bottoms liquid may, if
desired, be further fractionated, such as in a de-propanizer column
and/or in a de-butanizer column, to further separate the third
bottoms liquid into a propane-enriched fraction, a butane-enriched
fraction, and/or a fraction enriched in pentanes and heavier
hydrocarbons.
[0077] As is well known in the art, the terms "scrub column",
"de-methanizer column" and "de-ethanizer column", "de-propanizer
column" and "de-butanizer column" refer to types of distillation
column. The term "distillation column" refers to a column
containing one or more separation stages, composed of devices such
as packing or trays, that increase contact and thus enhance mass
transfer between upward rising vapor and downward flowing liquid
flowing inside the column. In this way, the concentration of
lighter (i.e. higher volatility and lower boiling point) components
increases in the rising vapor that collects as overhead vapor at
the top of the column, and the concentration of heavier (i.e. lower
volatility and higher boiling point) components increases in the
descending liquid that collects as bottoms liquid at the bottom of
the column. The "top" of the distillation column refers to the part
of the column at or above the top-most separation stage. The
"bottom" of the column refers to the part of the column at or below
the bottom-most separation stage. An "intermediate location" of the
column refers to a location between the top and bottom of the
column, between two separation stages.
[0078] In the case of the scrub column, the natural gas feed stream
is introduced (as a gaseous stream or as a partially condensed,
two-phase stream) into the scrub column at an intermediate location
of the column or, more typically, at the bottom of the column.
[0079] The upward rising vapor from the feed stream is then brought
into contact, as it passes through one or more separation stages
inside the scrub column, with a downward flowing liquid reflux
stream, thereby "scrubbing" components heavier than methane from
said vapor (i.e. removing at least some of said less volatile
components from the vapor). This results, as noted above, in the
natural gas feed stream being separated into a methane-rich vapor
fraction collected as an overhead vapor (referred to herein as a
"first overhead vapor") at the top of the scrub column, and a
liquid fraction, enriched in hydrocarbons heavier than methane,
collected as a bottoms liquid (referred to herein as a "first
bottoms liquid") at the bottom of the scrub column.
[0080] In the case of the de-methanizer column, the stream of first
bottoms liquid from the scrub column is, as noted above, further
separated into a methane-rich vapor fraction, collected as an
overhead vapor ("second overhead vapor") at the top of the
de-methanizer column, and a liquid fraction, enriched in
hydrocarbons heavier than methane, collected as a bottoms liquid
("second bottoms liquid") at the bottom of the de-methanizer
column. The stream of first bottoms liquid is typically partially
vaporized (via heating and/or expansion of the stream) prior to
being introduced into the de-methanizer column as a two-phase
stream. Typically the stream is introduced into the de-methanizer
column at an intermediate location of the column, so that upward
rising vapor from the stream is brought into contact, as it passes
through one or more separation stages, with a downward flowing
liquid reflux stream thereby scrubbing components heavier than
methane from said vapor, and so that downward flowing liquid from
the stream is brought into contact, as it passes through one or
more separation stages, with upward rising vapor (typically
provided by boiling a portion of the bottoms liquid collected at
the bottom of the column) thereby "stripping" methane and
components lighter than methane from the said liquid (i.e. removing
at least some of said more volatile components from the
liquid).
[0081] In the case of the de-ethanizer column, the stream of second
bottoms liquid from the de-methanizer column is, as noted above,
further separated into an ethane-enriched fraction, collected as an
overhead vapor ("third overhead vapor") at the top of the
de-ethanizer column, and a fraction enriched in hydrocarbons
heavier than ethane, collected as a bottoms liquid ("third bottoms
liquid") at the bottom of the de-ethanizer column. Operation of the
de-ethanizer column is typically similar to that of the
de-methanizer column, insofar as the stream of second bottoms
liquid is typically partially vaporized prior to being introduced
into the de-ethanizer column at an intermediate location of the
column, so that upward rising vapor from the stream is scrubbed by
a downward flowing reflux liquid of components heavier than ethane,
and so that downward flowing liquid from the stream is stripped by
upward rising vapor of ethane and components lighter than
ethane.
[0082] As used herein, the term "separator" or "phase separator"
refers to a device, such as drum or other form of vessel, in which
a two phase stream can be introduced in order to separate the
stream into its constituent vapor and liquid phases.
[0083] The system according to the second aspect of the present
invention is suitable for carrying out the methods of the first
aspect, and therefore the above-mentioned benefits of the method
according to the first aspect of the invention apply equally to the
systems according to the second aspect of the invention.
[0084] According to one embodiment of the second aspect, the set of
conduits, one or more heat exchangers, and one or more separators
arranged and operable to withdraw, cool, condense and divide the
stream of first overhead vapor comprise: a conduit arranged and
operable to withdraw the stream of first overhead vapor from the
top of the scrub column; a heat exchanger or section of a heat
exchanger arranged and operable to cool and partially condense said
stream; a separator arranged and operable to separate the resulting
liquid and vapor phases; a set of conduits arranged and operable to
divide the separated liquid phase to form the first stream of
liquefied first overhead and a further stream of liquefied first
overhead; and a heat exchanger or section of a heat exchanger
arranged and operable to receive, further cool and condense at
least a portion of the separated vapor phase to form the second
stream of liquefied first overhead.
[0085] The system may further comprise a heat exchanger or section
of a heat exchanger arranged and operable to receive and sub-cool
the second stream of liquefied first overhead. The section of a
heat exchanger arranged and operable to cool and partially condense
the stream of first overhead vapor may comprise a warm bundle of a
coil-wound main heat exchanger, and the section of a heat exchanger
arranged and operable to further cool and condense at least a
portion of the separated vapor phase may comprise a middle and/or
cold bundle of the coil-wound main heat exchanger. Alternatively,
the heat exchanger arranged and operable to cool and partially
condense the stream of first overhead vapor may comprise an
overhead condenser heat exchanger, and the heat exchanger arranged
and operable to further cool and condense at least a portion of the
separated vapor phase may comprise a coil-wound main heat
exchanger.
[0086] According to another embodiment of the second aspect, the
set of conduits and one or more heat exchangers arranged and
operable to withdraw, cool, condense and divide the stream of first
overhead vapor comprise: a conduit arranged and operable to
withdraw the stream of first overhead vapor from the top of the
scrub column; a heat exchanger or section of a heat exchanger
arranged and operable to cool and condense said stream; and a set
of conduits arranged and operable to divide the cooled and
condensed stream to form the first stream of liquefied first
overhead, the second stream of liquefied first overhead, and at
least one further stream of liquefied first overhead.
[0087] The system may further comprise a heat exchanger or section
of a heat exchanger arranged and operable to sub-cool the cooled
and condensed stream prior to said stream being divided to form the
first stream of liquefied first overhead, the second stream of
liquefied first overhead, and at least one further stream of
liquefied first overhead. The system may further comprise a heat
exchanger or section of a heat exchanger arranged and operable to
sub-cool the second stream of liquefied first overhead.
[0088] In an embodiment of the second aspect, the system further
comprises: a de-ethanizer column arranged and operable to receive a
stream of second bottoms liquid from the bottom of the
de-methanizer column and to separate said stream into an
ethane-enriched fraction, collected as a third overhead vapor at
the top of the de-ethanizer column, and a fraction enriched in
hydrocarbons heavier than ethane, collected as a third bottoms
liquid at the bottom of the de-ethanizer column; and a conduit
arranged and operable to withdraw the stream of second bottoms
liquid from the bottom of the de-methanizer column and introduce
said stream into the de-ethanizer column. Said system may further
comprise: a conduit arranged and operable to withdraw a stream of
third overhead vapor from the top of the de-ethanizer column, and
one or more heat exchangers arranged and operable to receive, cool
and condense said stream to form an NGL stream; and/or a conduit
arranged and operable to withdraw a stream of third bottoms liquid
from the bottom of the de-ethanizer column, for forming one or more
NGL streams therefrom. Said system may further comprise a heat
exchanger arranged and operable to condense, by indirect heat
exchange with a further stream of liquefied first overhead, a
portion of the third overhead vapor to form a stream of liquefied
third overhead that is reintroduced as a reflux stream into the top
the de-ethanizer column, thereby providing reflux for the
de-ethanizer column, and a conduit arranged and operable to
introduce said further stream of liquefied first overhead into said
heat exchanger. The heat exchanger operable to cool and condense
the stream of third overhead vapor to form an NGL stream, and the
heat exchanger operable to condense a portion of the third overhead
vapor to form a stream of liquefied third overhead that is
reintroduced as a reflux stream into the top the de-ethanizer
column may be one and the same heat exchanger.
[0089] Further embodiments of the system according to the second
aspect will be apparent from the foregoing discussion of
embodiments of the method according to the first aspect.
[0090] Preferred aspects of the present invention include the
following aspects, numbered #1 to #27:
#1. A method of fractionating and liquefying a natural gas feed
stream, the method comprising:
[0091] (a) introducing the natural gas feed stream into a scrub
column in which the natural gas feed stream is separated into a
methane-rich vapor fraction collected as a first overhead vapor at
the top of the scrub column, and a liquid fraction, enriched in
hydrocarbons heavier than methane, collected as a first bottoms
liquid at the bottom of the scrub column;
[0092] (b) withdrawing a stream of first overhead vapor from the
top of the scrub column, and cooling, condensing and dividing said
stream to form a first stream of liquefied first overhead, second
stream of liquefied first overhead, and at least one further stream
of liquefied first overhead;
[0093] (c) returning the first stream of liquefied first overhead
to the scrub column as a reflux stream introduced into the top the
scrub column, thereby providing reflux for the scrub column;
[0094] (d) forming a liquefied natural gas (LNG) product stream
from the second stream of liquefied first overhead;
[0095] (e) withdrawing a stream of first bottoms liquid from the
bottom of the scrub column, and introducing said stream into a
de-methanizer column in which the stream of first bottoms liquid is
separated into a methane-rich vapor fraction collected as a second
overhead vapor at the top of the de-methanizer column, and a liquid
fraction, enriched in hydrocarbons heavier than methane, collected
as a second bottoms liquid at the bottom of the de-methanizer
column; and
[0096] (f) providing reflux to the de-methanizer column by: [0097]
(1) introducing one of said further streams of liquefied first
overhead as a reflux stream into the top of the de-methanizer
column; and/or [0098] (2) condensing, by indirect heat exchange
with one of said further streams of liquefied first overhead, a
portion of the second overhead vapor to form a stream of liquefied
second overhead that is reintroduced as a reflux stream into the
top the de-methanizer column. #2. The method of Aspect #1, wherein
step (b) comprises:
[0099] withdrawing the stream of first overhead vapor from the top
of the scrub column, cooling and partially condensing said stream,
separating the resulting liquid and vapor phases, dividing the
separated liquid phase to form the first stream of liquefied first
overhead and at least one further stream of liquefied first
overhead, and further cooling and condensing at least a portion of
the separated vapor phase to form the second stream of liquefied
first overhead; and/or
[0100] withdrawing the stream of first overhead vapor from the top
of the scrub column, cooling and partially condensing said stream,
separating the resulting liquid and vapor phases, forming the first
stream of liquefied first overhead from at least a portion of the
separated liquid phase, further cooling and condensing at least a
portion of the separated vapor phase, and dividing the resulting
further cooled and condensed stream to form the second stream of
liquefied first overhead and at least one further stream of
liquefied first overhead.
#3. The method of Aspect #2, wherein:
[0101] the second stream of liquefied first overhead is sub-cooled
prior to forming the LNG product stream; or
[0102] the cooled and condensed stream is sub-cooled prior to being
divided to form the second stream of liquefied first overhead and
at least one further stream of liquefied first overhead.
#4. The method of Aspect #2 or #3, wherein the stream of first
overhead vapor from the top of the scrub column is cooled and
partially condensed in a warm bundle of a coil-wound main heat
exchanger, and the at least a portion of the separated vapor phase
is further cooled and condensed in a middle and/or cold bundle of
the coil-wound main heat exchanger. #5. The method of Aspect #2 or
#3, wherein the stream of first overhead vapor from the top of the
scrub column is cooled and partially condensed in an overhead
condenser heat exchanger, and the at least a portion of the
separated vapor phase is further cooled and condensed in a
coil-wound main heat exchanger. #6. The method of Aspect #1,
wherein step (b) comprises:
[0103] withdrawing the stream of first overhead vapor from the top
of the scrub column, cooling and condensing said stream, and
dividing the cooled and condensed stream to form the first stream
of liquefied first overhead, the second stream of liquefied first
overhead, and at least one further stream of liquefied first
overhead.
#7. The method of Aspect #6, wherein:
[0104] the cooled and condensed stream is sub-cooled prior to being
divided to form the first stream of liquefied first overhead, the
second stream of liquefied first overhead, and at least one further
stream of liquefied first overhead; or
[0105] the cooled and condensed stream is divided to form the first
stream of liquefied first overhead and another liquid stream that
is then sub-cooled prior to being divided to form the second stream
of liquefied first overhead and at least one further stream of
liquefied first overhead; or
[0106] the second stream of liquefied first overhead is sub-cooled
prior to forming the LNG product stream.
#8. The method of any one of Aspects #1 to #7, wherein the first
stream of liquefied first overhead, used as a reflux stream in step
(c), and the further stream(s) of liquefied first overhead, used in
step (f), have a flow ratio of at least about 9:1. #9. The method
of any one of Aspects #1 to #8, wherein the first stream of
liquefied first overhead, used as a reflux stream in step (c), and
the further stream(s) of liquefied first overhead, used in step
(f), each have a temperature of or below about -40.degree. C. #10.
The method of any one of Aspects #1 to #9, wherein the first
overhead vapor and second overhead vapor each comprise at least
about 95 mole % methane, and the second bottoms liquid comprises
less than about 5 mole % methane. #11. The method of any one of
Aspects #1 to #10, wherein the method further comprises:
withdrawing a stream of second overhead vapor from the top of the
de-methanizer column; cooling and condensing all or a portion of
said stream to form additional LNG product; and/or using all or a
portion of said stream as a fuel stream; and/or exporting all or a
portion of said stream as a gaseous natural gas product stream.
#12. The method of any one of Aspects #1 to #11, wherein the method
further comprises withdrawing a stream of second bottoms liquid
from the bottom of the de-methanizer column and fractionating the
stream of second bottoms liquid to provide one or more natural gas
liquids (NGL) streams. #13. The method of Aspect #12, wherein the
step of fractionating the stream of second bottoms liquid
comprises: introducing said stream into a de-ethanizer column in
which the stream of second bottoms liquid is separated into
ethane-enriched fraction, collected as a third overhead vapor at
the top of the de-ethanizer column, and a fraction enriched in
hydrocarbons heavier than ethane, collected as a third bottoms
liquid at the bottom of the de-ethanizer column. #14. The method of
Aspect #13, wherein the method further comprises:
[0107] withdrawing a stream of third overhead vapor from the top of
the de-ethanizer column, and cooling and condensing said stream to
form an NGL stream; and/or
[0108] withdrawing a stream of third bottoms liquid from the bottom
of the de-ethanizer column, and forming one or more NGL streams
therefrom.
#15. The method of Aspect #13 or #14, wherein the method further
comprises providing reflux to the de-ethanizer column by
condensing, by indirect heat exchange with a further stream of
liquefied first overhead, a portion of the third overhead vapor to
form a stream of liquefied third overhead that is reintroduced as a
reflux stream into the top the de-ethanizer column. #16. The method
of any one of Aspects #13 to #15, wherein the third overhead vapor
comprises at least about 95% ethane, and the third bottoms liquid
comprises less than about 5 mole % ethane. #17. A system for
fractionating and liquefying a natural gas feed stream, the system
comprising:
[0109] a scrub column arranged and operable to receive the natural
gas feed stream and separate said stream into a methane-rich vapor
fraction collected as a first overhead vapor at the top of the
scrub column, and a liquid fraction, enriched in hydrocarbons
heavier than methane, collected as a first bottoms liquid at the
bottom of the scrub column;
[0110] a set of conduits, one or more heat exchangers, and
optionally one or more separators, arranged and operable to
withdraw a stream of first overhead vapor from the top of the scrub
column and to cool, condense and divide said stream to form a first
stream of liquefied first overhead, second stream of liquefied
first overhead, and at least one further stream of liquefied first
overhead;
[0111] a conduit arranged and operable to return the first stream
of liquefied first overhead to the scrub column as a reflux stream
introduced into the top the scrub column, thereby providing reflux
for the scrub column;
[0112] a conduit arranged and operable to withdraw from the system
a liquefied natural gas (LNG) product stream formed from the second
stream of liquefied first overhead;
[0113] a de-methanizer column arranged and operable to receive a
stream of first bottoms liquid from the bottom of the scrub column
and to separate said stream into a methane-rich vapor fraction
collected as a second overhead vapor at the top of the
de-methanizer column, and a liquid fraction, enriched in
hydrocarbons heavier than methane, collected as a second bottoms
liquid at the bottom of the de-methanizer column;
[0114] a conduit arranged and operable to withdraw the stream of
first bottoms liquid from the bottom of the scrub column and
introduce said stream into the de-methanizer column; and
[0115] one or both of: [0116] (1) a conduit arranged and operable
to introduce one of said further streams of liquefied first
overhead as a reflux stream into the top of the de-methanizer
column, thereby providing reflux for the de-methanizer column;
[0117] (2) a heat exchanger arranged and operable to condense, by
indirect heat exchange with one of said further streams of
liquefied first overhead, a portion of the second overhead vapor to
form a stream of liquefied second overhead that is reintroduced as
a reflux stream into the top the de-methanizer column, thereby
providing reflux for the de-methanizer column, and a conduit
arranged and operable to introduce said further stream of liquefied
first overhead into said heat exchanger. #18. A system according to
Aspect #17, wherein the set of conduits, one or more heat
exchangers, and one or more separators arranged and operable to
withdraw, cool, condense and divide the stream of first overhead
vapor comprise:
[0118] a conduit arranged and operable to withdraw the stream of
first overhead vapor from the top of the scrub column;
[0119] a heat exchanger or section of a heat exchanger arranged and
operable to cool and partially condense said stream;
[0120] a separator arranged and operable to separate the resulting
liquid and vapor phases;
[0121] a set of conduits arranged and operable to divide the
separated liquid phase to form the first stream of liquefied first
overhead and a further stream of liquefied first overhead; and
[0122] a heat exchanger or section of a heat exchanger arranged and
operable to receive, further cool and condense at least a portion
of the separated vapor phase to form the second stream of liquefied
first overhead.
#19. A system according to Aspect #18, wherein the system further
comprises a heat exchanger or section of a heat exchanger arranged
and operable to receive and sub-cool the second stream of liquefied
first overhead. #20. A system according to Aspect #18 or #19,
wherein the section of a heat exchanger arranged and operable to
cool and partially condense the stream of first overhead vapor
comprises a warm bundle of a coil-wound main heat exchanger, and
the section of a heat exchanger arranged and operable to further
cool and condense at least a portion of the separated vapor phase
comprises a middle and/or cold bundle of the coil-wound main heat
exchanger. #21. A system according to Aspect #18 or #19, wherein
the heat exchanger arranged and operable to cool and partially
condense the stream of first overhead vapor comprises an overhead
condenser heat exchanger, and the heat exchanger arranged and
operable to further cool and condense at least a portion of the
separated vapor phase comprises a coil-wound main heat exchanger.
#22. A system according to Aspect #17, wherein the set of conduits
and one or more heat exchangers arranged and operable to withdraw,
cool, condense and divide the stream of first overhead vapor
comprise:
[0123] a conduit arranged and operable to withdraw the stream of
first overhead vapor from the top of the scrub column;
[0124] a heat exchanger or section of a heat exchanger arranged and
operable to cool and condense said stream; and
[0125] a set of conduits arranged and operable to divide the cooled
and condensed stream to form the first stream of liquefied first
overhead, the second stream of liquefied first overhead, and at
least one further stream of liquefied first overhead.
#23. A system according to Aspect #22, wherein the system further
comprises a heat exchanger or section of a heat exchanger arranged
and operable to sub-cool the cooled and condensed stream prior to
said stream being divided to form the first stream of liquefied
first overhead, the second stream of liquefied first overhead, and
at least one further stream of liquefied first overhead. #24. A
system according to Aspect #22, wherein the system further
comprises a heat exchanger or section of a heat exchanger arranged
and operable to sub-cool the second stream of liquefied first
overhead. #25. A system according to any one of Aspects #17 to #24,
wherein the system further comprises:
[0126] a de-ethanizer column arranged and operable to receive a
stream of second bottoms liquid from the bottom of the
de-methanizer column and to separate said stream into an
ethane-enriched fraction, collected as a third overhead vapor at
the top of the de-ethanizer column, and a fraction enriched in
hydrocarbons heavier than ethane, collected as a third bottoms
liquid at the bottom of the de-ethanizer column; and
[0127] a conduit arranged and operable to withdraw the stream of
second bottoms liquid from the bottom of the de-methanizer column
and introduce said stream into the de-ethanizer column.
#26. A system according to Aspect #25, wherein the system further
comprises:
[0128] a conduit arranged and operable to withdraw a stream of
third overhead vapor from the top of the de-ethanizer column, and
one or more heat exchangers arranged and operable to receive, cool
and condense said stream to form an NGL stream; and/or
[0129] a conduit arranged and operable to withdraw a stream of
third bottoms liquid from the bottom of the de-ethanizer column,
for forming one or more NGL streams therefrom.
#27. A system according to Aspect #25 or #26, wherein the system
further comprises a heat exchanger arranged and operable to
condense, by indirect heat exchange with a further stream of
liquefied first overhead, a portion of the third overhead vapor to
form a stream of liquefied third overhead that is reintroduced as a
reflux stream into the top the de-ethanizer column, thereby
providing reflux for the de-ethanizer column, and a conduit
arranged and operable to introduce said further stream of liquefied
first overhead into said heat exchanger.
[0130] Solely by way of example, certain preferred embodiment of
the invention will now be described with reference to FIGS. 2 to 7.
In FIG. 1, described above, and FIGS. 2 to 7, described below,
where a feature is common to more than one Figure that feature has
been assigned the same reference numeral in each Figure, for
clarity and brevity.
[0131] In the embodiments depicted in FIGS. 2 to 7, the main heat
exchanger, that is used to liquefy the natural gas, is shown as
being a coil-wound heat exchanger. However, while the use of such a
heat exchanger is preferred, the main exchanger may equally be a
plate and fin heat exchanger or any other kind of heat exchanger
known in the art. Likewise, although in the embodiments depicted
the coil bundles of the main heat exchanger are shown as being
housed in a single casing or shell forming a single unit, the main
heat exchanger could equally comprise a series of two or more
units, with each bundle having its own casing/shell, or with one or
more of the bundles being housed in one casing/shell, and with one
or more other bundles being housed in one or more different
casings/shells. The refrigerant cycle used to supply cold
refrigerant to the main heat exchanger may likewise be of any type
suitable for carrying out the liquefaction of natural gas.
Exemplary cycles known and used in the art, and that could be
employed in the present invention, include the propane pre-cooled
mixed refrigeration cycle (C3MR), single mixed refrigerant cycle
(SMR), nitrogen expander cycle, methane expander cycle, dual mixed
refrigerant cycle (DMR), and cascade cycle.
[0132] Referring now to FIG. 2, in one embodiment of the invention
a natural gas feed stream, 101, which typically will have been
pre-treated to remove acid gases, water and mercury, and which may
optionally also have been pre-cooled in one or more heat
exchangers, is introduced into the bottom of a scrub column, 10,
which contains multiple separation stages. The scrub column, 10,
separates the natural gas feed into a methane-rich vapor fraction
collected as overhead vapor (also referred to herein as a the
`first overhead vapor` or `scrub column overhead vapor`) at the top
of the column, and a liquid fraction, enriched in hydrocarbons
heavier than methane, collected as bottoms liquid (also referred to
herein as the `first bottoms liquid` or `scrub column bottoms
liquid`) at the bottom of the column. A stream of the overhead
vapor, 202, is withdrawn from the top of the scrub column, and a
stream of the bottoms liquid, 103, is withdrawn from the bottom of
the scrub column.
[0133] The stream of first overhead vapor, 202, is sent to a main
heat exchanger, 20, for cooling and liquefaction. In the
illustrated embodiment, the main heat exchanger, 20, is a
coil-wound heat exchanger containing warm, 22, middle, 24, and
cold, 26, bundles housed in a single shell casing. The main heat
exchanger may be supplied with any suitable refrigerant (not shown)
by any suitable refrigerant cycle effective for carrying out the
liquefaction of natural gas. The stream of overhead vapor, 202, is
introduced into the warm end of the main heat exchanger, 20, and is
cooled and partially condensed in the warm bundle, 22, to form a
partially condensed (two-phase) stream, 203. The partially
condensed stream, 203, is then withdrawn from the warm bundle and
separated in a phase separator, 28, into its liquid and vapor
phases to produce a liquid stream, 120, and vapor stream, 207.
[0134] The vapor stream, 207, is returned to the main heat
exchanger, 20, in which it is first further cooled and fully
condensed in the middle bundle, 24, to form a stream of liquified
first overhead, 204. This stream of liquefied first overhead is
then sub-cooled in the cold bundle, 26, thereby producing an LNG
product stream, 205. The LNG product stream, 205, may, as shown, be
flashed (for example by passing the stream through a J-T valve) and
sent to an LNG storage tank, 30, for storage. Boil-off gas (BOG) or
flash gas, 401, from the tank may be sent to a fuel header, flared,
or recycled to the overhead vapor stream fed to the main heat
exchanger (not shown).
[0135] The liquid stream, 120, from the phase separator, 28, which
is typically at or between a temperature of about -40.degree. C. to
about -70.degree. C., is divided, thereby forming two streams of
liquefied first overhead, 125 and 121. Stream 125 typically
comprises the majority of the flow of stream 120, the flow ratio of
stream 125 to stream 121 typically being about 9:1 (stream 125
therefore typically comprising at least 90% of stream 120, and
stream 121 typically comprising less than 10% of stream 120).
Stream 125 is returned (typically by pumping or gravity) to the
scrub column, 10, as a reflux stream introduced into the top of the
scrub column, so as to provide the necessary reflux for operation
of the scrub column. Stream 121 is sent (typically of after being
passed through a J-T valve) to the top of a de-methanizer column,
12, as a reflux stream in order to provide the necessary reflux for
operation of the de-methanizer column, as will be described in
further detail below.
[0136] The stream of first bottoms liquid, 103, withdrawn from the
scrub column, 10, is expanded (for example by being passed, as
shown, through a J-T valve) to partially vaporize the stream, and
is then introduced into an intermediate location of the
de-methanizer column, 12, which also contains multiple separation
stages. The de-methanizer column, 12, separates the first bottoms
liquid, 103, into a methane-rich vapor fraction collected as
overhead vapor (also referred to herein as the `second overhead
vapor` or `de-methanizer column overhead vapor`) at the top of the
column, and a liquid fraction, enriched in hydrocarbons heavier
than methane, collected as bottoms liquid (also referred to herein
as the `second bottoms liquid` or `de-methanizer column bottoms
liquid`) at the bottom of the column. Reflux for the de-methanizer
column is, in this embodiment, and as noted above, provided by
introducing a stream of liquefied first overhead, 121, as a reflux
stream into the top of the de-methanizer column. A portion of the
de-methanizer column bottoms liquid is, as shown, heated in a
re-boiler (which may use any suitable heat source) in order to
provide boil-up for the de-methanizer column, 12. The remainder of
the bottoms liquid is withdrawn as a stream of second bottoms
liquid, 105.
[0137] The temperatures and pressures in the scrub column and
de-methanizer column are typically controlled so that the first
overhead vapor and second overhead vapor (i.e. the scrub column
overhead vapor and de-methanizer column overhead vapor) each
comprise at least about 95 mole % methane, and so that the second
bottoms liquid (i.e. the de-methanizer column bottoms liquid)
comprises less than 5 mole % methane. Doing so maximizes the
recovery and concentration of the NLGs and HHCs (i.e. C2.sub.+
hydrocarbons) in the stream of second bottoms liquid withdrawn from
the de-methanizer column. The temperature at the top of the scrub
column and de-methanizer column is controlled by the temperature of
the reflux streams, 125 and 125, formed from the liquefied first
overhead, which reflux streams typically have, as noted above, a
temperature of about -40.degree. C. or below. The temperature at
the bottom of the de-methanizer column is controlled by the
de-methanizer re-boiler. The pressures in the columns are dictated
by the pressure of the natural gas feed stream, 101, and the degree
of pressure let down when the stream of first bottoms liquid (i.e.
the stream of scrub column bottoms liquid), 103, is expanded.
[0138] The de-methanizer column overhead vapor withdrawn from the
top of the de-methanizer column as a stream of second overhead
vapor, 104, may be put to any suitable or desired purpose. It may,
for example, be used as a fuel stream, and/or sent to a pipeline or
otherwise sold and exported as a gaseous natural gas product
stream. Additionally or alternatively it may (not shown) be cooled
and condensed in the main heat exchanger (separately from and/or in
combination with the first overhead vapor) to form and provide
additional LNG product.
[0139] The stream of second bottoms liquid, 105, withdrawn from the
bottom of the de-methanizer column, 12, is further fractionated in
a NGL fractionation system, that in this case comprises a
de-ethanizer column, 14, de-propanizer column 16, and de-butanizer
column, 18, in order to provide the desired NGL streams (that can,
for example and as discussed above, be used as make-up refrigerant,
sold as separate commercial products, and/or selectively
re-combined with the LNG product to tailor the heating value of the
LNG product).
[0140] More specifically, the stream of second bottoms liquid
(de-methanizer column bottoms liquid) is first cooled (for example
by being passed, as shown, through a heat exchanger using air,
water or another ambient temperature cooling medium) and the cooled
stream, 105, is then expanded (for example by being passed, as
shown, through a heat exchanger and a J-T valve) to form a
partially vaporized (two-phase) stream that is introduced into an
intermediate location the de-ethanizer column, 14, which also
contains multiple separation stages. The de-ethanizer column, 14,
separates the second bottoms liquid, 105, into a ethane-enriched
vapor fraction collected as overhead vapor (also referred to herein
as `third overhead vapor` or `de-ethanizer column overhead vapor`)
at the top of the column, and a liquid fraction, enriched in
hydrocarbons heavier than ethane, collected as bottoms liquid (also
referred to herein as the `third bottoms liquid` or `de-ethanizer
column bottoms liquid`) at the bottom of the column. Reflux for the
de-ethanizer column is provided by condensing the de-ethanizer
column overhead vapor in an overhead condenser heat exchanger
(which can be supplied with any suitable refrigerant), a portion of
the condensed overhead being returned to the de-ethanizer column,
14, as a reflux stream, and the remainder being withdrawn as a
stream of liquefied third overhead, 106. A portion of the
de-ethanizer column bottoms liquid is, as shown, heated in a
re-boiler (which may use any suitable heat source) in order to
provide boil-up for the de-ethanizer column, 14. The remainder of
the bottoms liquid is withdrawn as a stream of third bottoms
liquid, 107.
[0141] Due to the fact that (as discussed above) only very small
amounts of methane are, typically, present in the stream of second
bottoms liquid (de-methanizer column bottoms liquid), 105, fed to
the de-ethanizer column, and due to the fact that the
recovery/yield of ethane (and other NGLs) in the second bottoms
liquid is typically very good, the de-ethanizer column in this
embodiment can produce an overhead vapor stream enriched in ethane
that both is of high purity and is produced in relatively high
volumes, thereby maximizing the amount of high purity ethane that
is available as make-up refrigerant and/or as a NGL product stream
for sale. Typically, the third overhead vapor (de-ethanizer column
overhead vapor) comprises at least about 95 mole ethane, and the
third bottoms liquid (de-ethanizer column bottoms liquid) comprises
less than about 5 mole % ethane.
[0142] The stream of third bottoms liquid (de-ethanizer column
bottoms liquid), 107, is expanded (for example by being passed
through a J-T valve) to form a partially vaporized (two-phase)
stream that is introduced into an intermediate location the
de-propanizer column, 16, which also contains multiple separation
stages. The de-propanizer column, 16, separates the third bottoms
liquid, 107, into a propane-enriched vapor fraction collected as
overhead vapor (also referred to herein as `fourth overhead vapor`
or `de-propanizer column overhead vapor`) at the top of the column,
and a liquid fraction, enriched in hydrocarbons heavier than
propane, collected as bottoms liquid (also referred to herein as
the `fourth bottoms liquid` or `de-propanizer column bottoms
liquid`) at the bottom of the column. Reflux for the de-propanizer
column is provided by condensing the de-propanizer column overhead
vapor in an overhead condenser heat exchanger (supplied with any
suitable refrigerant), with a portion of the condensed overhead
being returned to the de-propanizer column, 16, as a reflux stream,
and the remainder being withdrawn as a stream of liquefied fourth
overhead, 108. A portion of the de-propanizer column bottoms liquid
is, as shown, heated in a re-boiler (which may use any suitable
heat source) in order to provide boil-up for the de-propanizer
column, 16. The remainder of the bottoms liquid is withdrawn as a
stream of fourth bottoms liquid, 109.
[0143] The stream of fourth bottoms liquid (de-propanizer column
bottoms liquid), 109, is expanded (for example by being passed
through a J-T valve) to form a partially vaporized (two-phase)
stream that is introduced into an intermediate location the
de-butanizer column, 18, which also contains multiple separation
stages. The de-butanizer column, 18, separates the fourth bottoms
liquid, 109, into a butane-enriched vapor fraction collected as
overhead vapor (also referred to herein as `fifth overhead vapor`
or `de-butanizer column overhead vapor`) at the top of the column,
and a liquid fraction, enriched in hydrocarbons heavier than
butane, collected as bottoms liquid (also referred to herein as the
`fifth bottoms liquid` or `de-butanizer column bottoms liquid`) at
the bottom of the column. Reflux for the de-butanizer column is
provided by condensing the de-butanizer column overhead vapor in an
overhead condenser (supplied with any suitable refrigerant), with a
portion of the condensed overhead being returned to the
de-butanizer column, 18, as a reflux stream, and the remainder
being withdrawn as a stream of liquefied fifth overhead, 110. A
portion of the de-butanizer column bottoms liquid is, as shown,
heated in a re-boiler (which may use any suitable heat source) in
order to provide boil-up for the de-butanizer column, 18. The
remainder of the bottoms liquid is withdrawn as a stream of fifth
bottoms liquid, 111.
[0144] In comparison to the conventional arrangement shown in FIG.
1, the method and system of the embodiment of the present invention
depicted in FIG. 2 therefore differs in the manner in which reflux
is provided to the de-methanizer column. In particular, whereas in
the arrangement in FIG. 1 the stream of overhead vapor withdrawn
from the scrub column is cooled, condensed and divided to provide
just two streams of liquefied overhead, one of which (steam 205) is
taken as the LNG product and the other of which (stream 120) is
used as a reflux stream for the scrub column, in the arrangement in
FIG. 2 the stream of overhead vapor withdrawn from the scrub column
(i.e. the stream of first overhead vapor) is cooled, condensed and
divided to provide at least three streams of liquefied first
overhead. A first stream of liquefied first overhead (stream 125)
is returned to the scrub column as a reflux stream, a second stream
of liquefied first overhead (stream 204) is taken as the LNG
product (stream 205), and at least one further stream of liquefied
first overhead (stream 121) is introduced as a reflux stream into
the top of the de-methanizer column. This provides the benefits,
discussed above, of eliminating the need for a dedicated overhead
condenser for the de-methanizer column, and (typically) providing
colder reflux to the de-methanizer column (thereby enhancing the
recovery of ethane in the de-methanizer column bottoms liquid).
[0145] Instead of refluxing the scrub column and de-methanizer
column using liquid separated in phase separator 28 from a
partially condensed stream of first overhead, 203, the reflux
streams for the scrub column and/or de-methanizer column can, in an
alternative arrangements, be obtained from one or more colder
locations of the main heat exchanger.
[0146] Thus, referring now to FIG. 3, an alternative embodiment of
the invention is depicted in which the main heat exchanger has only
two bundles, namely a warm bundle, 22, and a cold bundle, 26. The
stream of first overhead vapor, 202, withdrawn from the top of the
scrub column, 10, is again introduced into the warm bundle, 22, but
in this case the warm bundle functions to cool and fully condense
the overhead vapor stream to form a stream of liquefied first
overhead that is then divided to form the first stream of liquefied
first overhead, 125, second stream of liquefied first overhead,
204, and further stream of liquefied first overhead, 121. As in the
embodiment shown in FIG. 2, the first stream of liquefied first
overhead, 125, is then returned to the top of the scrub column, 10,
as a reflux stream, the second stream of liquefied first overhead,
204, is sub-cooled in the cold bundle, 26, to provide the LNG
product stream, 205, and the further stream of liquefied first
overhead, 121, is introduced as a reflux stream into the top of the
de-methanizer column.
[0147] Equally, referring now to FIG. 4, in yet another embodiment
the main heat exchanger again has only two bundles, namely a warm
bundle, 22, and a cold bundle, 26, but in this embodiment all of
the stream, 202, of first overhead withdrawn from the scrub column,
10, is cooled and fully condensed in the warm bundle, 22, to form a
stream of liquefied first overhead, 204, and all of this stream is
then sub-cooled in the cold bundle, 26, to form a sub-cooled stream
of liquefied first overhead that is then divided to form the first
stream of liquefied first overhead, 125, second stream of liquefied
first overhead, 205, and further stream of liquefied first
overhead, 121. In this embodiment the second stream of liquefied
first overhead, 205, can then be taken as the LNG product stream
without need for further cooling or processing. As in the
embodiment shown in FIG. 2, the first stream of liquefied first
overhead, 125, is returned as a reflux stream to the top of the
scrub column, 10, and the further stream of liquefied first
overhead, 121, is introduced as a reflux stream into the top of the
de-methanizer column.
[0148] In the embodiments depicted in FIGS. 3 and 4, the first and
further streams of liquefied first overhead, 125 and 121, are
typically much colder than the first and further streams of
liquefied first overhead, 125 and 121, generated in the embodiment
depicted in FIG. 2, due to said streams originating from a colder
location of the main heat exchanger in FIGS. 3 and 4. In the
embodiment shown in FIG. 3, streams 125 and 121 are typically at or
between a temperature of about -100.degree. C. to about
-135.degree. C. In the embodiment shown in FIG. 4, streams 125 and
121 are typically at or between a temperature of about -130.degree.
C. to about -160.degree. C. Since these streams are at a colder
temperature, relatively lower flowrates of these streams are needed
and can be used to provide the necessary reflux to the scrub
column, 10, and de-methanizer column, 12 (the required flow rates
of streams 125 and 121 in FIG. 4 being less than those in FIG. 3;
and likewise the required flow rates of streams 125 and 121 in FIG.
3 being less than those in FIG. 2). The arrangements depicted in
FIGS. 3 and 4 also eliminate the need for phase separator 28 and
reduce the number of bundles required in the main heat exchanger,
thereby further reducing the capital cost of these
arrangements.
[0149] In yet further embodiments (not shown), a combination of two
or more of the arrangements depicted in FIGS. 2, 3 and 4 can be
used to provide the reflux streams for the scrub column and
de-methanizer column. For example, in one arrangement the first
stream of liquefied first overhead, 125, that is used to reflux
scrub column may be generated by partially condensing the stream of
first overhead vapor, 202, withdrawn from the scrub column,
separating the liquid phase in a phase-separator, 28, and using the
liquid phase to form the first stream of liquefied first overhead,
125, in a similar manner to that shown in FIG. 2, whereas the
further stream of liquefied first overhead, 121, that is used to
reflux the de-methanizer column, is instead generated by dividing
the further cooled and liquefied stream or subcooled stream exiting
the middle or cold bundles of the main heat exchanger to form the
second stream of liquefied first overhead. 204/205, and further
stream of liquefied first overhead, 121, in a similar manner to
that depicted in FIG. 3 or 4.
[0150] Equally, in another arrangement the reflux streams for the
scrub column and/or de-methanizer column may be obtained from any
other location downstream of the cold end of the main exchanger,
such as from the LNG tank. They may also be obtained from an LNG
stream which has been produced in another part of the plant such as
an end-flash exchanger (not shown) which liquefies a portion of
natural gas stream, 101, while warming up BOG or flash gas, stream,
401.
[0151] In yet another embodiment, the reflux provided to the scrub
column and/or de-methanizer column by the first and further streams
of liquefied first overhead may be supplemented by, or may be
supplemental to, reflux provided to the column(s) by overhead
condenser(s) supplied with refrigerant and operated to condense at
least a portion of the overhead from said column(s).
[0152] Referring now to FIGS. 5 and 6, yet further embodiments of
the invention are shown which differ from the embodiment shown in
FIG. 2 in that the initial cooling and partial condensation of the
stream of first overhead vapor, 202, does not take place in the
warm bundle of the main heat exchanger, but instead takes place in
a separate overhead condenser heat exchanger that is supplied with
refrigerant by the same refrigeration cycle that supplies the
refrigerant and cooling duty for the main heat exchanger. In FIGS.
5 and 6 a two bundle coil-wound heat exchanger, supplied with
refrigerant by a propane-precooled mixed refrigerant cycle (C3MR),
is depicted. As previously noted, the main heat exchanger may be a
heat exchanger of another type, and different types of
refrigeration cycle can equally be employed, but here just a
coil-wound heat exchanger and a C3MR cycle is shown, for
simplicity.
[0153] In the embodiments depicted in FIGS. 5 and 6, the natural
gas is pre-cooled with propane refrigerant in a pre-cooler heat
exchanger, 34, prior to being introduced as natural gas feed
stream, 101, into the scrub column. The first overhead vapor
stream, 202, withdrawn from the top of the scrub column, 10, is
then partially cooled and condensed in an overhead condenser heat
exchanger, 32, to form a partially condensed (two-phase) stream,
203. The partially condensed stream, 203, is separated in a phase
separator, 28, into its liquid and vapor phases to produce a liquid
stream, 120, and vapor stream, 207. The liquid stream, 120, is then
divided to from the first stream of liquefied first overhead, 125,
and further stream of liquefied first overhead, 121, that are then
introduced as reflux streams into the scrub column, 10, and
de-methanizer column, 12, as previously described in relation to
the embodiment shown in FIG. 2. The vapor stream, 207, is
introduced into the main heat exchanger, 20, where it is further
cooled, condensed and subcooled to form the second stream of
liquefied first overhead that is taken as the LNG product stream,
205. The overhead condenser heat exchanger, 32, may be a plate and
fin heat exchanger, a printed circuit heat exchanger, or any other
suitable exchanger.
[0154] In the embodiment shown in FIG. 5, the warm mixed
refrigerant stream, 309, withdrawn from the bottom of the main heat
exchanger, 20, is sent to a compression system (comprising a motor,
36, and associated compressors and inter- and after-coolers) where
it is compressed to form a high pressure refrigerant stream, 312.
This stream is then phase separated in a separator, 38, to produce
a mixed refrigerant vapor (MRV) stream, 302, and a mixed
refrigerant liquid (MRL) stream, 301. Both streams, 301 and 302,
are sent to the main exchanger to be cooled in separate circuits.
The cooled and at least partially condensed MRV stream is withdrawn
from the cold end of the cold bundle of the main heat exchanger,
expanded (for example by being passed through a J-T valve, as
shown), and reintroduced into the shell-side of the main heat
exchanger as a cold, low pressure, vaporized/vaporizing refrigerant
stream 308 to provide cooling duty to the cold and warm bundles of
the main heat exchanger. The cooled MRL stream is withdrawn from
the cold end of the warm bundle of the main heat exchanger and is
then divided to form two streams. One stream is expanded to a form
cold, vaporized/vaporizing low pressure refrigerant stream, 307,
that is reintroduced into the shell-side of the main heat exchanger
to provide further cooling duty to the warm bundle of the main heat
exchanger. The other stream, 320, is expanded (for example by being
passed through a J-T valve, as shown) to form cold,
vaporized/vaporizing low pressure refrigerant stream, typically at
a temperature of a temperature at or between about -60.degree. C.
to about -120.degree. C., that is passed through overhead condenser
heat exchanger 32 to provide the cooling duty for partially
condensing the first overhead vapor stream, 202, withdrawn from the
top of the scrub column, 10. The warmed refrigerant stream, 350,
exiting the overhead condenser heat exchanger, 32, may then be
re-combined with the warmed refrigerant exiting the warm end of the
main heat exchanger as stream 309. Alternatively, if stream 350 is
still two-phase, it may be further expanded and sent to the
shell-side of the main heat exchanger at an intermediate location
to provide additional cooling duty to the warm bundle of the main
heat exchanger.
[0155] A benefit of partially condensing the first overhead vapor
stream from the scrub column in a separate overhead heat exchanger
via heat exchange with refrigerant from the main heat exchanger,
rather than in the a warm bundle of the main heat exchanger itself,
is that it eliminates multiple circuits and bundles in the main
heat exchanger. It also leads to a lower complexity system that is
easier to operate. The flow split between streams refrigerant
steams 307 and 320 is adjusted based on the overhead temperature of
the columns which is in turn determined based on the required
purity of the overhead streams from the columns.
[0156] In the embodiment shown in FIG. 6, the operation of the
refrigeration cycle is similar to that illustrated in FIG. 5 and
described above. The only difference is that, in the embodiment
depicted in FIG. 6, the refrigerant stream, 320, supplied to the
overhead condenser heat exchanger, 32, is not derived by dividing
cooled MRL stream that is withdrawn from the cold end of the warm
bundle of the main heat exchanger, but rather by dividing cooled
and at least partially condensed MRV stream is withdrawn from the
cold end of the cold bundle of the main heat exchanger. The
temperature of stream 320, after expansion, is in this case
typically at or between about -140.degree. C. to -160.degree. C.,
and is therefore much colder than the equivalent refrigerant
stream, 320, in the embodiment shown in FIG. 5.
[0157] In addition to the MRL and MRV streams shown in FIGS. 5-6,
additional locations exist from which the refrigerant stream sent
to the overhead condenser heat exchanger, 32, may be obtained.
These vary with the refrigeration cycle employed. For instance, in
the SMR cycle which employs up to four mixed refrigerant streams of
different compositions, the refrigerant to generate reflux may be
obtained from any of these.
[0158] Referring now to FIG. 7, yet another embodiment of the
invention is shown, which differs from the embodiment shown in FIG.
2 in that the further stream of liquefied first overhead, 121,
instead of being introduced into the top of the de-methanizer
column, 12, as a reflux stream, is instead used as a refrigerant
stream in an overhead condenser heat exchanger in which a portion
of the second overhead vapor (i.e. a portion of the de-methanizer
column overhead vapor) is condensed, by indirect heat exchange with
the further stream of liquefied first overhead, to form a stream of
liquefied second overhead that is reintroduced as a reflux stream
into the top the de-methanizer column. Thus, although the further
stream of liquefied first overhead is not itself used as a reflux
stream in the de-methanizer column, in this embodiment the further
stream of liquefied first overhead does supply the cooling duty for
generating the reflux in the de-methanizer column, and so is still
indirectly generating reflux for the de-methanizer column.
[0159] More specifically, and as shown in FIG. 7, in this
embodiment the stream of first liquefied overhead, 202, withdrawn
from the top of the scrub column, 10, is again cooled and partially
condensed in the warm bundle 22 of the main heat exchanger, 20, to
form a two phase stream, 203, that is then separated into its
liquid and vapor phases in separator 28. The liquid phase, 120, is
again further divided to form a first stream of liquefied first
overhead, 125, and, in this case, two further streams of liquefied
first overhead, 121 and 122. The first stream of liquefied first
overhead, 125, is, as before, reintroduced into the top of the
scrub column, 10, to provide reflux for this column. However, the
two further streams of liquefied first overhead, 121 and 125, are
in this case used as refrigerant streams to supply cooling duty to
a de-methanizer column overhead condenser heat exchanger, 40, and
to a de-ethanizer column overhead condenser heat exchanger, 42,
respectively.
[0160] In this case, reflux for the de-methanizer column, 12, is,
therefore, generated by: withdrawing a stream of de-methanizer
column overhead vapor (`second overhead vapor`) from the top of the
de-methanizer column; partially condensing said stream in the
de-methanizer column overhead condenser, 40, by indirect heat
exchange with one of the further streams of liquefied first
overhead, 121; separating the liquid and vapor phases; returning
the liquid phase as to the de-methanizer column as a stream of
liquefied second overhead that is reintroduced as a reflux stream
into the top the column; and withdrawing the remaining vapor phase
as a stream of second overhead vapor, 104 (which stream can, for
example and as discussed above, be used as fuel, exported as a
gaseous natural gas product, and/or liquefied to provide additional
LNG product).
[0161] Similarly, in this embodiment reflux for the de-ethanizer
column, 14, is, therefore, generated by: withdrawing a stream of
de-ethanizer column overhead vapor (`third overhead vapor`) from
the top of the de-ethanizer column; condensing said stream in the
de-ethanizer column overhead condenser, 42, by indirect heat
exchange with another of the further streams of liquefied first
overhead, 122; returning a portion of the liquefied overhead to the
de-ethanizer column as a stream of liquefied third overhead that is
reintroduced as a reflux stream into the top the column; and
withdrawing the remaining portion of the liquefied overhead as a
stream of liquefied third overhead, 106 (which stream can, for
example and as discussed above, be sold as an NGL product, used as
make-up refrigerant, and/or recombined with the LNG product to
adjust the heating value of the latter).
[0162] The warmed streams of first overhead, 126 and 127, exiting
the de-methanizer column overhead condenser, 40, and de-ethanizer
column overhead condenser, 42, may be put to any suitable or
desired use. For example, they may be re-injected into the natural
gas feed, and/or used as refrigerant make-up.
[0163] Again, many variations of the arrangement shown in FIG. 7
are also possible. For example, instead of obtaining some or all of
streams 125, 121 and 122 from phase separator 28 as shown in FIG.
7, streams 125, 121 and/or 122 may be obtained from a colder
locations of the main heat exchanger, in a similar manner to the
first and further streams of liquefied first overhead generated in
the embodiments depicted in FIGS. 3 and 4.
Example
[0164] In order to illustrate the operation of the invention, the
methods of removing refrigerant from a natural gas liquefaction
system as described and depicted in FIGS. 2 and 3 were simulated
using ASPEN Plus software.
[0165] Table 1 below lists the conditions and compositions of the
various streams for the simulation of the method depicted in FIG.
2, and Table 2 lists the conditions and compositions of the various
streams for the simulation of the method depicted in FIG. 3. The
data in these tables illustrates that utilizing a portion a further
stream of liquefied first overhead (generated from cooling,
liquefying and dividing the first overhead vapor withdrawn from the
scrub column), in order to reflux the de-methanizer column, leads
to effective separation of heavy hydrocarbons and NGL components
from natural gas. Additionally, these arrangement lead to a
reduction in equipment count, as compared to conventional
arrangements such as depicted in FIG. 1, by eliminating a the
de-methanizer column overhead condenser and reflux drum, thereby
leading to cost savings and simpler operation.
TABLE-US-00001 TABLE 1 Stream No. 101 125 121 103 104 105 202 203
207 Tempera- -34.10 -64.72 -64.72 -35.54 -66.47 30.00 -38.48 -64.72
-64.72 ture .degree. C. Pressure 63.25 55.17 55.17 61.09 26.89
26.95 61.03 55.17 55.17 bara Flowrate 45,410 1,376 16 127 65 78
46,658 46,658 45,266 kgmole/h Flowrate 758,356 31,911 376 6,835
1,082 6,128 783,433 783,433 751,146 kg/h Compo- sition Carbon
0.00500 0.00996 0.00996 0.00132 0.00506 0.00000 0.00516 0.00516
0.00501 Dioxide Mol % Nitrogen 0.494 0.160 0.160 0.096 0.227 0.000
0.485 0.485 0.495 Mol % Methane 96.866 78.358 78.358 40.015 96.771
0.700 96.476 96.476 97.033 Mol % Ethane 1.730 6.403 6.403 4.962
2.400 7.392 1.859 1.859 1.719 Mol % Propane 0.522 5.382 5.382 4.734
0.480 8.410 0.654 0.654 0.508 Mol % Isobutane 0.111 2.827 2.827
3.277 0.063 5.859 0.182 0.182 0.101 Mol % Butane 0.115 3.162 3.162
5.147 0.046 8.982 0.191 0.191 0.100 Mol % Isopentane 0.025 1.269
1.269 2.328 0.005 4.042 0.055 0.055 0.018 Mol % Pentane 0.027 1.535
1.535 3.326 0.004 5.721 0.062 0.062 0.017 Mol % Hexane 0.033 0.852
0.852 10.533 0.000 17.299 0.028 0.028 0.003 Mol % Heptane 0.071
0.027 0.027 25.289 0.000 41.117 0.001 0.001 0.000 Mol % Benzene 9.0
143.8 143.8 2,923.7 0.1 4,782.5 5.0 5.0 0.7 ppm
TABLE-US-00002 TABLE 2 Stream No. 101 125 121 103 104 105 202 204
Temperature -19.94 -125.48 -126.11 121.11 -61.64 30.00 -68.15
-126.11 .degree. C. Pressure 52.13 51.98 43.36 52.24 27.58 27.78
51.98 43.36 bara Flowrate 30,980 13,441 13 42 20 35 44,391 30,925
kgmole/h Flowrate 514,824 222,223 213 3,308 343 3,177 733,952
511,303 kg/h Composition Carbon Dioxide 0.00500 0.00500 0.00500
0.00586 0.01579 0.00000 0.00500 0.00500 Mol % Nitrogen 0.846 0.847
0.847 0.006 0.569 0.000 0.847 0.847 Mol % Methane 96.979 97.093
97.093 12.451 90.363 0.000 97.093 97.093 Mol % Ethane 1.574 1.571
1.571 3.327 8.038 0.050 1.571 1.571 Mol % Propane 0.312 0.308 0.308
3.010 1.012 3.134 0.308 0.308 Mol % Isobutane 0.060 0.058 0.058
1.634 0.001 1.967 0.058 0.058 Mol % Butane 0.071 0.067 0.067 3.078
0.001 3.691 0.067 0.067 Mol % Isopentane 0.036 0.031 0.031 3.608
0.000 4.310 0.031 0.031 Mol % Pentane 0.024 0.019 0.019 3.567 0.000
4.257 0.019 0.019 Mol % Hexane 0.022 0.000 0.000 15.918 0.000
18.967 0.000 0.000 Mol % Heptane 0.022 0.000 0.000 16.241 0.000
19.352 0.000 0.000 Mol % Benzene 113.0 1.0 1.0 83,058.2 0.0
98,966.8 1.0 1.0 ppm
[0166] It will be appreciated that the invention is not restricted
to the details described above with reference to the preferred
embodiments but that numerous modifications and variations can be
made without departing from the spirit or scope of the invention as
defined in the following claims.
* * * * *