U.S. patent application number 11/300768 was filed with the patent office on 2006-06-22 for method for recovery of natural gas liquids for liquefied natural gas.
This patent application is currently assigned to ABB LUMMUS GLOBAL INC.. Invention is credited to Sanjiv N. Patel.
Application Number | 20060130521 11/300768 |
Document ID | / |
Family ID | 36594004 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060130521 |
Kind Code |
A1 |
Patel; Sanjiv N. |
June 22, 2006 |
Method for recovery of natural gas liquids for liquefied natural
gas
Abstract
The invention includes a process and apparatus for separating a
liquefied natural gas (LNG) stream containing methane and lighter
components and heavy hydrocarbon components into a more volatile
gas fraction containing a substantial amount of the methane and
lighter components and a less volatile fraction containing a large
portion of the heavy hydrocarbon components. The process includes
splitting the LNG stream, preheating and providing to a
fractionation tower at two locations. A tower reflux stream is
utilized.
Inventors: |
Patel; Sanjiv N.; (Sugar
Land, TX) |
Correspondence
Address: |
BRACEWELL & GIULIANI LLP
P.O. BOX 61389
HOUSTON
TX
77208-1389
US
|
Assignee: |
ABB LUMMUS GLOBAL INC.
|
Family ID: |
36594004 |
Appl. No.: |
11/300768 |
Filed: |
December 15, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60637353 |
Dec 17, 2004 |
|
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|
Current U.S.
Class: |
62/620 |
Current CPC
Class: |
F25J 3/0238 20130101;
F25J 2205/02 20130101; F25J 3/0214 20130101; F25J 2200/02 20130101;
F25J 2235/60 20130101; F25J 2230/60 20130101; F25J 2245/02
20130101; F25J 2200/70 20130101; F25J 2200/50 20130101; F25J 3/0233
20130101; F25J 2200/74 20130101; F25J 2200/76 20130101; F25J
2270/04 20130101; F25J 2230/08 20130101; F25J 2270/88 20130101 |
Class at
Publication: |
062/620 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Claims
1. A process for separating a liquefied natural gas (LNG) stream
containing methane and lighter components and heavy hydrocarbon
components into a more volatile gas fraction containing a
substantial amount of the methane and lighter components and a less
volatile fraction containing a large portion of the heavy
hydrocarbon components, the process comprising the steps of: (a)
splitting the LNG stream into a first feed stream and a second feed
stream; (b) preheating at least a portion of the first feed stream
to provide first tower feed stream and supplying the first tower
feed stream to a fractionation tower to produce a tower overhead
stream including the more volatile fraction containing the
substantial amount of the methane and lighter components and a
tower bottoms stream including the less volatile fraction
containing the heavy hydrocarbon components; (c) preheating at
least a portion of the second feed stream to provide second tower
feed stream and supplying second tower feed stream to the
fractionation tower; (d) cooling and partially condensing at least
a portion of the tower overhead stream to produce a partially
condensed tower overhead stream; (e) separating the partially
condensed tower overhead stream into a lean vapor stream and a
tower reflux stream, and supplying the tower reflux stream to the
fractionation tower; (f) cooling the lean vapor stream such that
the lean vapor stream is substantially condensed thereby producing
a lean LNG stream; and (g) pumping the lean LNG stream to a high
pressure lean LNG stream.
2. The process according to claim 1, wherein the heavy hydrocarbon
components include C2 components, C3 components and heavier
components.
3. The process according to claim 1, wherein the heavy hydrocarbon
components include C3 components and heavier components.
4. The process according to claim 1, further comprising the step of
compressing the lean vapor stream prior to the step of cooling the
lean vapor stream.
5. The process according to claim 1, wherein the step of cooling at
least a portion of the tower overhead stream includes cooling at
least a portion of the tower overhead stream by heat exchange
contact with the at least a portion of the second feed stream
thereby providing at least a portion of duty for the step of
preheating the at least a portion of the second feed stream.
6. The process according to claim 1, wherein the step of preheating
at least a portion of the first feed stream and supplying the first
tower feed stream to the fractionation tower includes supplying the
first tower feed stream to the fractionation tower as a tower
bottom feed stream.
7. The process according to claim 1, wherein the step of preheating
at least a portion of the first feed stream and supplying the first
tower feed stream to the fractionation tower includes supplying the
first tower feed stream to the fractionation tower at a position in
the fractionation tower lower than the position where the second
tower feed stream enters the fractionation tower.
8. The process according to claim 1, wherein the step of separating
the partially condensed tower overhead stream into the lean vapor
stream and the tower reflux stream includes supplying the tower
reflux stream as a top tower feed stream to the fractionation
tower.
9. The process according to claim 1, wherein the step of cooling
the lean vapor stream includes cooling the lean vapor stream by
heat exchange contact with at least a portion of the first feed
stream thereby providing at least a portion of the duty for the
step of preheating at least a portion of the first feed stream.
10. The process according to claim 1, wherein the process further
includes the step of cooling the tower reflux stream to create a
cooled tower reflux stream and supplying the cooled tower reflux
stream to the fractionation tower.
11. The process according to claim 10, wherein the step of sending
the cooled tower reflux stream to the fractionation tower includes
sending the cooled tower reflux stream as a top feed stream to the
fractionation tower.
12. The process according to claim 10, wherein the step of cooling
the tower reflux stream includes cooling the tower reflux stream by
heat exchange contact with at least a portion of the first feed
stream.
13. A process for separating a liquefied natural gas (LNG) stream
containing methane and lighter components and heavy hydrocarbon
components into a more volatile gas fraction containing a
substantial amount of the methane and lighter components and a less
volatile fraction containing a large portion of the heavy
hydrocarbon components, the process comprising the steps of:
preheating at least a portion of the LNG stream and supplying the
at least a portion of the LNG stream to a fractionation tower as a
first tower feed stream to produce a tower overhead stream
including the more volatile fraction containing the substantial
amount of the methane and lighter components and a tower bottoms
stream including the less volatile fraction containing the heavy
hydrocarbon components; expanding at least a portion of the tower
overhead stream to a lower pressure such that the at least a
portion of the tower overhead stream is partially condensed to
produce a partially condensed low pressure vapor stream; separating
the partially condensed low pressure vapor stream into a lean vapor
stream and a tower reflux stream; compressing the lean vapor
stream; cooling the lean vapor stream to create a lean LNG stream;
cooling the tower reflux stream thereby producing a cooled lean
tower reflux stream; and pumping the lean LNG stream to a high
pressure lean LNG stream.
14. The process according to claim 13, where the heavy hydrocarbon
components include C2 components, C3 components and heavier
components.
15. The process according to claim 13, where the heavy hydrocarbon
components include C3 components and heavier components.
16. The process according to claim 13, whereby the tower reflux
stream is cooled by heat exchange contact with at least a portion
of the LNG stream prior to the step of supplying the tower reflux
stream 18 to the fractionation tower.
17. The process according to claim 13, the step of supplying the
tower reflux stream to the fractionation tower includes supplying
the tower reflux stream to the fractionation tower as a top feed
stream.
18. The process according to claim 13, wherein the step of cooling
the lean vapor stream includes cooling the lean vapor stream by
heat exchange contact with at least a tower side reboiler stream
thereby providing reboiling duty to the fractionation tower.
19. The process according to claim 13, wherein the step of cooling
the lean vapor stream includes cooling the lean vapor stream by
heat exchange with at least a portion of the LNG stream thereby
providing at least a portion of duty for the step of preheating the
at least a portion of the LNG stream 3.
20. The process according to claim 13, wherein the step of
supplying the at least a portion of the LNG stream to the
fractionation tower includes supplying the at least a portion of
the LNG stream to the fractionation tower as a tower bottom feed
stream.
21. An apparatus for separating a liquefied natural gas (LNG)
stream containing methane and lighter components and heavy
hydrocarbon components into a more volatile gas fraction containing
a substantial amount of the methane and lighter components and a
less volatile fraction containing a large portion of the heavy
hydrocarbon components, the apparatus comprising: means for
splitting the LNG stream into a first feed stream and a second feed
stream; a first exchanger operable to preheat the first feed stream
to provide a first tower feed stream; a fractionation tower for
receiving the first tower feed stream to produce a tower overhead
stream including the more volatile fraction containing the
substantial amount of the methane and lighter components and a
tower bottoms stream including the less volatile fraction
containing the heavy hydrocarbon components; a first cooler
operable to cool and partially condense at least a portion of the
tower overhead stream to produce a partially condensed tower
overhead stream, the first cooler operable to allow for heat
exchange between the portion of the tower overhead stream and the
second feed stream; a separator operable to separate the partially
condensed overhead tower stream into a lean vapor stream and a
tower reflux stream, the tower reflux stream operable to return to
the fractionation tower, the first exchanger being operable to
allow for heat exchange between the lean vapor stream and the first
feed stream, the lean vapor stream being cooled in the first
exchanger to provide a lean LNG stream; and a pump operable for
pumping the lean LNG stream to a high pressure LNG stream.
22. The apparatus of claim 21, further including a first compressor
for compressing the lean vapor stream.
23. The apparatus of claim 21, further including a side reboiler
for supplying at least a portion of reboiling requirements for the
fractionation tower.
Description
Related Applications
[0001] This patent application claims priority to U.S. Provisional
Patent Application Ser. No. 60/637,353, filed on Dec. 17, 2004,
which is incorporated by reference in its entirety. Also claiming
priority to U.S. applications of the same title filed on the same
date as the current application.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field of the Invention
[0003] The present invention relates to the recovery of natural gas
liquids (NGL) from liquefied natural gas (LNG) streams.
[0004] 2. Description of the Related Art
[0005] Many attempts have been made to recover NGL from LNG
streams. One such example can be found in U.S. Pat. No. 6,564,579
issued to McCartney titled Method for Vaporizing and Recovery of
Natural Gas Liquids from Liquefied Natural Gas Streams (hereinafter
"McCartney"). McCartney describes a process, as shown in FIG. 4 of
the present application, that uses of an overhead compressor to
increase the bubble point of a lean gas stream exiting a
fractionation tower 10. In this prior art process, a rich low
pressure LNG stream 1 is pumped in pump 2 to 450 psia. LNG stream 3
exiting pump 2 is heated in exchanger 5 to -132.5.degree. F. to
produce a heated LNG stream 6. Heated LNG stream 6 is introduced to
fractionation tower 10 as a top tower feed stream.
[0006] Fractionation tower 10 is a distillation tower that produces
a tower bottoms stream 12 and a tower overhead stream 14. Tower
bottoms stream 12 is sent to a tower reboiler 13 that is used to
maintain an amount of methane in an NGL product stream 12. A vapor
return stream from reboiler 13 is sent to fractionation tower 10,
while NGL product is drawn off as NGL product stream 12. Tower
overhead stream 14 is compressed in lean gas compressor 27 to 500
psia to produce a partially boosted stream 28. Partially boosted
stream 28 is cooled to -137.degree. F. and completely condensed in
first exchanger 5 to produce a low pressure lean LNG stream 21. Low
pressure lean LNG stream 21 is then pumped by pump 22 to elevate
its pressure to 1422 psia to produce a high pressure lean LNG
product stream 23, which is the high pressure lean LNG product that
is sent for vaporization and/or energy recovery.
[0007] As can be seen in Table II, recovery of LNG products is
limited in the prior art. Simulation results of the embodiments of
the current invention compared to alternate schemes show that for
lower recoveries and NGL production, there is an increase in
capital cost due to the addition of a compressor and increased heat
exchanger area for prior art. Utility consumption is also
affected.
[0008] Co-pending U.S. patent application Ser. No. 10/651,178
titled Optimized Heating Value in Natural Gas Liquids Recovery
Scheme illustrates another attempt at recovering NGL products from
LNG streams. FIG. 2 illustrates one scheme that uses subcooled rich
LNG to increase NGL recovery. In this related process, rich low
pressure LNG stream 1 is pumped in pump 2 to about 525 psia to
produce LNG stream 3. LNG stream 3 is then split into two streams,
a first feed stream 4 and a second feed stream 24. First feed
stream 4, which is the larger of the two streams and contains about
93% of LNG stream 3, is heated in first exchanger 5 to
-132.5.degree. F. to produce a heated first feed stream 6. Heated
first feed stream 6, which is still all liquid, is introduced to
fractionation tower 10 as a tower bottom feed stream. Second feed
stream 24 is sent as a top feed stream to fractionation tower
10.
[0009] As in the prior art process shown in FIG. 4, fractionation
tower 10 is a distillation tower that produces a tower overhead
stream 14 and a tower bottom stream 12. Tower bottom stream 12 is
sent to a tower reboiler 13 that is used to maintain an amount of
methane in NGL product stream 12. Reboiler vapor stream returns to
fractionation tower 10, while NGL product is drawn off of reboiler
13 as NGL product stream 12.
[0010] Tower overhead stream 14 is cooled to -133.7.degree. F. and
completely condensed in first exchanger 5 to produce low pressure
lean LNG stream 21. Low pressure lean LNG stream 21 is pumped in
pump 22 to elevate its pressure to 1422 psia to produce high
pressure lean LNG product stream 23, which is the high pressure
lean LNG product that is sent for vaporization and/or energy
recovery.
SUMMARY OF THE INVENTION
[0011] The present invention includes a process and apparatus to
increase the recovery of NGL. As an embodiment of the present
invention, a process is provided for separating a liquefied natural
gas (LNG) stream into a more volatile gas fraction containing a
substantial amount of methane and lighter components and a less
volatile fraction containing a large portion of heavy hydrocarbon
components is advantageously provided. In this embodiment, the LNG
stream is split into a first feed stream and a second feed stream.
At least a portion of the first feed stream is preheated and
supplied to a fractionation tower as a first tower feed stream.
[0012] In preferred embodiments of the present invention,
fractionation tower produces a tower overhead stream and a tower
bottoms stream. Tower overhead stream preferably includes the more
volatile fraction of the LNG stream that contains a substantial
amount of methane and lighter components. Tower bottoms stream
preferably includes the less volatile fraction of the LNG stream
that contains the heavy hydrocarbon components.
[0013] At least a portion of the second feed stream is heated and
supplied to the fractionation tower as a second tower feed stream.
At least a portion of the tower overhead stream is cooled and
partially condensed to produce a partially condensed tower overhead
stream. Partially condensed tower overhead stream is separated into
a lean vapor stream and a tower reflux stream being sent to the
fractionation tower. Lean vapor stream is subcooled so that the
lean vapor stream is substantially condensed thereby producing a
lean LNG stream. Lean LNG stream is then pumped to a high
pressure.
[0014] In another embodiment of the invention, the liquefied
natural gas (LNG) stream is split into a first feed stream and a
second feed stream. The feed streams define, respectively, a first
feed stream enthalpy and a second feed stream enthalpy. At least a
portion of the first feed stream is preheated and provided to the
fractionation tower as the first tower feed stream. From the
fractionation tower, the tower overhead stream is produced
containing the more volatile fraction. The tower bottoms stream
containing the less volatile fraction is also produced from the
fractionation tower. The second feed stream is supplied to the
fractionation tower such that the first feed stream and the second
tower feed stream have a substantially common composition but the
first tower feed stream enthalpy differs from the second feed
stream enthalpy. The tower overhead stream is compressed to produce
a compressed overhead stream. The compressed overhead stream is
cooled until it is at least substantially condensed thereby
producing a lean LNG stream. In one preferred embodiment, the
compressed overhead stream is cooled until it is subcooled. The
lean LNG stream is pumped to a higher pressure thereby creating a
high pressure lean LNG stream.
[0015] In yet another embodiment of the invention, the process for
separating the liquefied natural (LNG) stream includes preheating
at least a portion of the LNG stream to produce first tower feed
stream. The first tower feed stream is supplied to the
fractionation tower that produces the tower overhead stream and
tower bottoms stream. At least a portion of the tower overhead
stream is cooled so that the portion of the tower overhead stream
is at least substantially condensed thereby producing the lean LNG
stream. The tower overhead stream can be subcooled. The lean LNG
stream is pumped to a higher pressure to produce a high pressure
lean LNG stream. The high pressure lean LNG stream is split into a
lean tower reflux stream and a lean LNG product stream. The lean
tower reflux stream is cooled to produce a cooled lean tower reflux
stream and the cooled lean tower reflux stream is supplied to the
fractionation tower.
[0016] Yet another preferred embodiment includes a process for
separating the liquefied natural gas (LNG) stream including
splitting the LNG stream into first feed stream and second feed
stream. The second feed stream is supplied to the fractionation
tower as second tower feed stream to produce the tower overhead
stream and the tower bottoms stream 12 including the less volatile
fraction containing the heavy hydrocarbon components. At least a
portion of the first feed stream is preheated to produce the first
tower feed stream. The first tower feed stream is supplied to the
fractionation tower. The tower overhead stream is compressed to
produce compressed overhead stream. The compressed overhead stream
is cooled such that the compressed overhead stream is substantially
condensed thereby producing lean LNG stream. The lean LNG stream is
pumped to a higher pressure to produce high pressure lean LNG
stream. The high pressure lean LNG stream is split into lean tower
reflux stream and lean LNG product stream. The lean tower reflux
stream is cooled to produce cooled lean tower reflux stream that is
supplied to the fractionation tower.
[0017] Yet another embodiment of the invention includes a process
for separating a liquefied natural gas (LNG) stream including
preheating at least a portion of the LNG stream and supplying the
at least a portion of the LNG stream to fractionation tower as
first tower feed stream. The fractionation tower produces tower
overhead stream and tower bottoms stream. At least a portion of the
tower overhead stream is expanded to a lower pressure such that the
at least a portion of the tower overhead stream is partially
condensed to produce a partially condensed low pressure vapor
stream. The partially condensed low pressure vapor stream is
separated into a lean vapor stream and tower reflux stream 18. The
lean vapor stream is compressed and cooled to create lean LNG
stream. The tower reflux stream is cooled thereby producing a
cooled lean tower reflux stream and the lean LNG stream is pumped
to a higher pressure creating high pressure lean LNG stream.
[0018] In yet another embodiment of the present invention, the
process for separating a liquefied natural gas (LNG) stream
includes preheating at least a portion of the LNG stream to produce
first tower feed and supplying the first tower feed to
fractionation tower. From the fractionation tower is produced tower
overhead stream and tower bottoms stream. At least a portion of the
tower overhead stream is cooled and thereby partially condensing to
produce partially condensed tower overhead stream. The partially
condensed tower overhead stream is separated into lean vapor stream
and lean LNG stream. The lean LNG stream is pumped to a high
pressure to produce high pressure lean LNG stream. The high
pressure lean LNG stream is split into a lean tower reflux stream
25 and lean LNG product stream. The lean tower reflux stream is
cooled to produce cooled lean tower reflux stream. The cooled lean
tower reflux stream is supplied to the fractionation tower. The
lean vapor stream is compressed to high pressure to produce
compressed second lean vapor stream. The compressed second lean
vapor stream is combined with the lean LNG product stream.
[0019] In another preferred embodiment, the invention includes an
apparatus for separating a liquefied natural gas (LNG) stream, the
apparatus including means for splitting the LNG stream into a first
feed stream and a second feed stream. Also included is a first
exchanger operable to preheat the first feed stream to provide a
first tower feed stream. A fractionation tower is included for
receiving the first tower feed stream and to produce tower overhead
stream and tower bottoms stream. A first cooler is provided that is
operable to cool and partially condense at least a portion of the
tower overhead stream to produce a partially condensed tower
overhead stream. The first cooler is operable to allow for heat
exchange between the portion of the tower overhead stream and the
second feed stream. A separator is operable to separate the
partially condensed overhead tower stream into lean vapor stream
and tower reflux stream, the tower reflux stream returning to the
fractionation tower. The first exchanger is operable to allow for
heat exchange between the lean vapor stream and the first feed
stream, the lean vapor stream being cooled in the first exchanger
to provide lean LNG stream. A pump operable for pumping the lean
LNG stream to high pressure LNG stream is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] So that the manner in which the features, advantages and
objects of the invention, as well as others which will become
apparent, may be understood in more detail, more particular
description of the invention briefly summarized above may be had by
reference to the embodiment thereof that is illustrated in the
appended drawings, which form a part of this specification. It is
to be noted, however, that the drawings illustrate only a preferred
embodiment of the invention and are therefore not to be considered
limiting of the invention's scope as it may admit to other equally
effective embodiments.
[0021] FIG. 1 is a simplified flow diagram of a hydrocarbon
recovery process that is configured for increased recovery of heavy
components from an inlet LNG stream through the use of a split feed
stream and lean tower reflux stream in accordance with an
embodiment of the present invention;
[0022] FIG. 2 is a simplified flow diagram of a hydrocarbon
recovery process that is configured for increased recovery of heavy
components from an inlet LNG stream through the use of a split feed
stream in accordance with an embodiment of a process described in
co-pending U.S. patent application Ser. No. 10/651,178 titled
Optimized Heating Value in Natural Gas Liquids Recovery Scheme;
[0023] FIG. 3 is a simplified flow diagram of a hydrocarbon
recovery process that is configured for increased recovery of heavy
components from an inlet LNG stream through the use of a lean LNG
reflux stream in accordance with an embodiment of the present
invention;
[0024] FIG. 4 is a simplified flow diagram of a prior art process
as shown in U.S. Pat. No. 6,564,579 issued to McCartney, which
illustrates a typical system and process for vaporizing LNG and
separating natural gas liquids from the LNG stream in accordance
with prior art;
[0025] FIG. 5 is a simplified flow diagram of a hydrocarbon
recovery process that is configured for increased recovery of heavy
components from an inlet LNG stream through the use of an overhead
compressor in accordance with an embodiment of the present
invention.
[0026] FIG. 6 is a simplified flow diagram of a hydrocarbon
recovery process that is configured for increased recovery of heavy
components from an inlet LNG stream through the use of an overhead
compressor and a lean LNG reflux stream in accordance with an
embodiment of the present invention.
[0027] FIG. 7 is a simplified flow diagram of a hydrocarbon
recovery process that is configured for increased recovery of heavy
components from an inlet LNG stream through the use of an overhead
condenser and an overhead compressor in accordance with an
embodiment of the present invention.
[0028] FIG. 8 is a simplified flow diagram of a hydrocarbon
recovery process that is configured for increased recovery of heavy
components from an inlet LNG stream through the use of an overhead
expander reflux stream in accordance with an embodiment of the
present invention.
[0029] FIG. 9 is a simplified flow diagram of a hydrocarbon
recovery process that is configured for increased recovery of heavy
components from an inlet LNG stream through the use of a partial
flow compressor in accordance with an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0030] In the description of the figures, the same numbers are used
to refer to similar or same components in all figures. For clarity
and simplicity, not all equipment is shown as they are considered
known to those skilled in the art. For consistency, the same
equipment parameters such as number of trays, efficiencies,
pressure drops, product specifications etc have been used to
compare processes.
[0031] Embodiments of the present invention have been compared with
prior art processes to illustrate improvements in either yield or
power consumptions. The processes described herein were compared
using the inlet LNG stream composition and conditions shown in
Table I. TABLE-US-00001 TABLE I Component Units Nitrogen Mol % 0.46
Methane Mol % 89.79 Ethane Mol % 6.47 Propane Mol % 2.23 i-Butane
Mol % .42 n-Butane Mol % .63 Temperature F. -241.6 Pressure Psia
71.6 Flow MMTPA 5
[0032] In all embodiments of the process, liquefied natural gas
(LNG) stream 3 contains methane and lighter components and heavy
hydrocarbon components. In order to recover NGL products, LNG
stream 3 is separated into a more volatile gas fraction that
contains a substantial amount of the methane and lighter components
and a less volatile gas fraction that contains a large potion of
the heavy hydrocarbon components.
[0033] Referring to the drawings, FIG. 1 shows a process scheme
embodiment of the present invention that is used to recover NGL
products. LNG inlet stream 1 is pumped by LNG pump 2 to about 525
psia. LNG stream 3 exiting LNG pump 2 is split into two streams,
first feed stream 4 and second feed stream 7. First feed stream 4,
which is typically the larger of the two streams and contains about
95% of LNG stream 3, is preheated in first heater or first
exchanger 5 to about -133.5.degree. F. to provide first tower feed
stream 6. First tower feed stream 6, which is still substantially
all liquid, is introduced into fractionation or distillation tower
10. Second feed stream 7 is heated in first cooler 8 to about
-132.9.degree. F. to produce second tower feed stream 9, which is
introduced into the fractionation tower 10. In preferred
embodiments of the present invention, fractionation tower 10
produces a tower overhead stream 14 and a tower bottoms stream 12.
Tower overhead stream 14 contains the more volatile fraction
containing a substantial amount of methane contained within LNG
stream 3. Tower bottoms stream 12 contains the less volatile
fraction containing the heavy hydrocarbon component. In a preferred
embodiment, bottom liquid stream 11 is sent to reboiler 13 to
produce tower bottoms stream 12. Vapor stream from tower reboiler
13 returns to fractionation tower 10. In this embodiment, the
reboiler can be used maintain methane content in a NGL product
stream 12. Alternate embodiments include substituting other heat
sources known in the art to the bottom of the fractionation column
10.
[0034] Tower overhead stream 14 is cooled to about -128.5.degree.
F. in first cooler 8. This partially condenses at least a portion
of the tower overhead stream 14 to produce a partially condensed
tower overhead stream 15. Partially condensed tower overhead stream
15 is sent to reflux accumulator or separator 16. Partially
condensed tower overhead stream 15 is separated into a lean vapor
stream 17 and a tower reflux stream 18. Tower reflux stream 18 is
sent to fractionation tower 10 preferably at a top tower feed
location. Tower reflux stream 18 can be cooled prior to sending
tower reflux stream 18 to fractionation tower 10. Tower reflux
stream 18 is sent to fractionation tower 10 preferably by pumping
with reflux pump 19.
[0035] Lean vapor or residue stream 17 is cooled to about
-134.5.degree. F. and substantially completely condensed in first
exchanger 5 thereby producing lean LNG stream 21. Lean LNG stream
21 is then pumped preferably by pump 22 to elevate its pressure to
about 1422 psia. High pressure lean LNG stream 23 leaving the pump
is a product stream that can be sent for vaporization and/or energy
recovery.
[0036] The process shown in FIG. 1 can be used when the heavy
hydrocarbon components include C2 components, C3 components, and
heavier components, i.e., ethane recovery. The process shown in
FIG. 1 can also be used when the heavy hydrocarbon components
include C3 components, and heavier components, i.e., propane
recovery.
[0037] In embodiments of the present invention, the step of cooling
at least a portion of tower overhead stream 14 includes cooling at
least a portion of tower overhead stream 14 by heat exchange
contact with at least a portion of the second feed stream 7 thereby
providing at least a portion of duty for the step of preheating the
at least a portion of the second feed stream. Heat exchange contact
between second feed stream 7 and tower overhead stream 14 can be
performed in first cooler 8. In alternate embodiments, such as
demonstrated in FIG. 5, In embodiments of the present invention,
the step of cooling lean vapor stream 17 includes cooling lean
vapor stream 17 by heat exchange contact with at least a portion of
first feed stream 4 in first exchanger 5 thereby providing at least
a portion of the duty for the step of preheating at least a portion
of first feed stream 4.
[0038] In another embodiment of the present invention, such as is
demonstrated in FIG. 7, the embodiments described above in
reference to FIG. 1 further include the step of compressing the
lean vapor stream 17 prior to the step of cooling the lean vapor
stream.
[0039] In embodiments of the present invention, the tower reflux
stream 18 is cooled to create cooled tower reflux stream 26, which
is then supplied to the fractionation tower. The cooled tower
reflux stream 26 is preferably supplied to the fractionation tower
at a top feed position. The tower reflux stream 18 is cooled by
heat exchange contact with at least a portion of second feed stream
7 thereby providing at least a portion of the duty for the step of
preheating at least a portion of second feed stream 7. In an
alternate embodiment, the tower reflux stream 18 is cooled by heat
exchange contact with at least a portion of the first feed stream 4
thereby providing at least a portion of the duty for the step of
preheating at least a portion of the first feed stream 4.
[0040] In certain embodiments, the step of preheating at least a
portion of the first feed stream 4 to create first tower feed
stream 6 advantageously includes supplying the first tower feed
stream 6 to the fractionation tower 10 as a tower bottom feed
stream. As noted previously, various means of providing heat or
energy to the bottom of the fractionation tower are encompassed. An
alternate embodiment includes providing the first tower feed stream
at a position in the column lower than the position where the
second tower feed stream enters the fractionation tower 10. This
advantageously provides a driving force to the separation in the
fractionation tower 10.
[0041] As another embodiment of the present invention, FIG. 3 shows
a scheme that uses cooled lean tower reflux stream 26 to increase
NGL recovery in NGL product stream 12. In this embodiment, it is
advantageous that cooled lean tower reflux stream 26 be subcooled.
Rich low pressure LNG stream 1 is pumped in LNG pump 2 to about 525
psia. LNG stream 3 exiting LNG pump 2 is heated in first exchanger
5 to about -133.2.degree. F. to produce first tower feed stream 6.
First tower feed stream 6, which is still substantially all liquid,
is introduced into fractionation tower 10 preferably as a bottom
feed stream.
[0042] In this embodiment, tower overhead stream 14 is cooled to
about -133.7.degree. F. and is substantially completely condensed
in first exchanger 5 to produce a low pressure lean LNG stream 21.
Lean LNG stream 21 exiting exchanger 5 is elevated in pressure
preferably by pump 22 to about 1422 psia to produce high pressure
lean LNG stream 23. High pressure lean LNG stream 23 is then split
into lean tower reflux stream 25 and lean LNG product stream 48.
Lean LNG stream 48 can be sent for vaporization and/or energy
recovery. Lean tower stream 25 is cooled in first exchanger 5 to
about -233.degree. F. and is then sent, preferably as a top feed
stream, to fractionation tower 10. The process described above and
its various alternate embodiments can be used when the heavy
hydrocarbon components include C2 components, C3 components, and
heavier components, i.e., ethane recovery. Similarly, the process
described above and its various alternate embodiments can also be
used when the heavy hydrocarbon components include C3 components,
and heavier components, i.e., propane recovery.
[0043] In one embodiment shown in FIG. 3, the step of cooling at
least a portion of the tower overhead stream 14 includes cooling
the portion of the tower overhead stream by heat exchange contact
with at least a portion of LNG stream 3 thereby providing at least
a portion of the duty for the step of preheating the LNG stream 3.
Alternately or in addition, lean tower reflux stream 25 is cooled
by heat exchange contact with LNG stream 3 thereby providing at
least a portion of the duty for the step of preheating the LNG
stream 3. In one embodiment, the first tower feed stream 6 is
introduced to the fractionation tower below cooled lean tower
reflux stream 26 to provide additional driving force due to
enthalpy differences. The cooled lean tower reflux stream 26 is
preferably supplied to the fractionation tower as a tower top feed
stream.
[0044] In certain instances, first exchanger 5 can act as a
critical path element possibly limiting recoveries due to the need
to produce a completely liquid lean LNG stream from the exchanger
that can be pumped. Attempts to increase recoveries can result in
exchanger pinch on first exchanger 5 and/or possibly result in a
two phase lean LNG stream at the outlet of first exchanger 5, which
would then require further processing. Regarding FIG. 5, lean gas
compressor 27 is introduced to avoid such a pinch and to avoid a
two phase stream at the first exchanger 5 outlet. The bubble point
of the lean residue gas stream leaving the tower is increased in
this manner. Tower overhead stream 14 can be compressed, and then
condensed at a higher temperature.
[0045] As another embodiment of the present invention, FIG. 5 shows
another scheme that can be used to separate LNG stream 3 into the
more volatile gas fraction containing a substantial amount of the
methane and lighter components and a less volatile fraction
containing a large portion of the heavy hydrocarbon components. In
this embodiment, LNG inlet stream 1 is pumped by LNG pump 2 to
about 475 psia. LNG stream 3 exiting LNG pump 2 is split into two
streams, first feed stream 4 and second feed stream 7 defining a
second feed stream enthalpy. First feed stream 4, which is the
larger of the two streams and contains about 82% of LNG stream 3,
is heated in first exchanger 5 to about -129.degree. F. to produce
first tower feed stream 6 defining a first tower feed stream
enthalpy. First tower feeds stream, which is still substantially
all-liquid, and second feed stream 7 are sent to fractionation
tower 10. In a preferred embodiment, second feed stream 7 is
introduced to fractionation tower 10 at a position above the first
tower feed stream. In one embodiment, second feed stream 7 is
introduced as a top feed to the fractionation tower 10. In one
embodiment, first tower feed stream 6 is introduced as a bottom
feed to the fractionation tower.
[0046] In FIG. 5, tower overhead stream 14 is compressed in lean
gas compressor 27 to about 534 psia. The partially boosted tower
overhead stream 28 is then cooled to about -131.1.degree. F. and is
completely condensed in first exchanger 5 to produce low-pressure
lean LNG stream 21. Low pressure lean LNG stream 21 is then pumped
preferably by pump 22 to elevate its pressure to about 1422 psia.
Stream 23 leaving pump 22 is the high pressure lean LNG product
stream that can be sent for vaporization and/or energy
recovery.
[0047] As another embodiment of the present invention, FIG. 6 shows
a dual lean reflux scheme that utilizes a side reboiler 31 to
increase NGL recovery from LNG stream 1. In addition to the
elements shown in FIG. 5, side reboiler 31 introduced in FIG. 6
advantageously maximizes utilization of cold streams and also
minimizes compressor power requirements. As in other embodiments
described herein, rich low pressure LNG stream 1 is pumped by LNG
pump 2 to about 485 psia. LNG stream 3 is then split into two
streams, first feed stream 4 and second feed stream 7. First feed
stream 4, which is the larger of the two streams and contains about
92% of LNG stream 3, is heated in first exchanger or heater 5 to
about -128.4.degree. F. to produce first tower feed stream 6. First
tower feed stream, which is still substantially all liquid, is sent
to fractionation tower 10. IN one embodiment first tower feed
stream is fed to the fractionation tower as a bottom feed stream.
Second feed stream 7 is introduced preferably as a middle feed
stream to fractionation tower 10. Tower overhead stream 14 is
compressed in lean gas compressor 27 to about 543 psia. The
partially boosted tower overhead stream 28 is then cooled in side
reboiler 31 to about -124.1.degree. F.
[0048] To provide a portion of the reboiling energy for
fractionation tower 10, side reboiler 31 is utilized in this
embodiment. Cold tower liquid stream or tower side reboiler stream
29 is heated in side reboiler 31 and returned to fractionation
tower 10 as stream 30. Partially cooled stream 32 is then further
cooled to about -130.7.degree. F. and completely condensed in first
exchanger 5 to produce low pressure lean LNG stream 21. Although
two separate exchangers are shown to provide cooling for tower
overhead stream 28, a single exchanger can be used. Low pressure
lean LNG stream 21 is then pumped, preferably by pump 22, to
elevate its pressure to about 1422 psia to produce high pressure
lean LNG stream 23. Lean LNG stream 23 is then split into a lean
tower reflux stream 25 and lean LNG product stream 48. Lean LNG
product stream 48 is sent for vaporization and/or energy recovery.
Lean tower reflux stream 25 is then cooled in first exchanger 5 to
about -232.degree. F. and is then sent to fractionation tower 10,
preferably as top feed stream 26.
[0049] In this embodiment, the compressed tower overhead stream 28
can cooled by heat exchange with at least a portion of the first
feed stream 4 thereby providing at least a portion of the duty for
the step of preheating at least a portion of the first feed stream
4.
[0050] FIG. 6 demonstrates an embodiment including splitting the
high pressure lean LNG stream 23 to create a lean tower reflux
stream 25 and a lean LNG product stream 48. The lean tower reflux
stream 25 is then cooled to produce a cooled lean tower reflux
stream 26, which is fed to the fractionation tower 10.
[0051] J The embodiments of the invention described herein are
applicable when the heavy hydrocarbon components include C2
components, C3 components, and heavier components, i.e., ethane
recovery. The embodiments described herein are applicable also when
the heavy hydrocarbon components include C3 components, and heavier
components, i.e., propane recovery. 10050] Higher recoveries are
made possible by increasing amount of flow in first tower feed
stream 6, which is cold and rich in the embodiments shown in FIG. 5
and FIG. 6. Recoveries are also enhanced by increase in amount of
cooled lean tower reflux stream 26 that is returned to
fractionation tower 10. Increasing the discharge pressure of lean
gas compressor 27 eliminates exchanger pinch in first exchanger 5
and avoids two phase LNG stream from reaching pump 22. The tower
pressure can also be lowered to increase recovery, at the cost of
higher compression power. This scheme is able to give high ethane
recovery with very high propane recovery. Comparing results through
process simulation with prior art, it can be seen that for a modest
increase in power, the process in FIG. 6 recovers more ethane. In
addition, the recovery of propane is also increased. Although the
simulations have been carried out for C2+ (ethane, ethylene,
propane, propylene and heavier hydrocarbons) component recovery,
the same process can be used for C3+ (propane, propylene and
heavier hydrocarbons) component recovery.
[0052] As yet another embodiment of the present invention, FIG. 7
shows a scheme that uses first cooler 8 as an overhead condenser
along with a lean gas compressor 27 to recover NGL from rich low
pressure LNG 1. In this embodiment, rich low pressure LNG 1 is
pumped by LNG pump 2 to about 525 psia. LNG stream 3 is then split
into two streams, first feed stream 4 and second feed stream 7.
First feed stream 4, which is the larger of the two streams and
contains about 61% of LNG stream 3, is heated in first exchanger 5
to about -126.8.degree. F. to produce first tower feed stream 6.
First tower feed stream 6, which is substantially all liquid, is
introduced to fractionation tower 10, preferably as a bottom feed
stream. Second feed stream 7 is heated in first cooler 8 to about
-10.degree. F. to produce second tower feed stream 9. Second tower
feed stream 9 is sent, preferably at a position lower than first
tower feed stream 7, to fractionation tower 10. In one embodiment,
second tower feed stream 9 is a bottom feed.
[0053] Tower overhead stream 14 is cooled to about -133.3.degree.
F. in first cooler 8 thereby partially condensing tower overhead
stream. Partially condensed tower overhead stream 15 is sent to
reflux accumulator or separator 16. Partially condensed tower
overhead stream 15 is then separated into lean vapor stream 17 and
tower reflux stream 18. Tower reflux stream 18 is pumped by pump 19
and is sent to fractionation tower 10, preferably as a top feed
stream. Lean vapor or residue stream 17 is compressed in lean gas
compressor 27 to about 596 psia to produce partially boosted
compressed overhead stream 28. Partially boosted compressed
overhead stream 28 is cooled, preferably in side reboiler 31 (see
FIG. 6), to about -121.5.degree. F. Alternately, as shown in FIG.
7, partially boosted compressed overhead stream 28 can be cooled in
first cooler 8 to produce partially cooled stream 32. Cold tower
side reboiler stream 29 exchanges heat in first cooler 8 and
returned to fractionation tower 10 as return stream 30. Partially
cooled stream 32 is then further cooled to about -125.3.degree. F.
and completely condensed in first exchanger 5 to produce low
pressure lean LNG stream 21. Lean LNG stream 21 is then pumped
preferably by pump 22 to elevate its pressure to about 1422 psia to
produce lean LNG product stream 23. Stream 23 is the high pressure
lean LNG product that is sent for vaporization and/or energy
recovery.
[0054] Tower reflux stream 18 can be subcooled to enhance recovery.
This subcooling can take place in first exchanger 5.
[0055] FIG. 8 illustrates another embodiment for recovery of NGL
products from a rich lower pressure LNG stream. In this embodiment,
overhead expander 33 is used to generate reflux for fractionation
tower 10 to recover NGL. Rich low pressure LNG stream 1 is pumped
in LNG pump 2 to about 550 psia. LNG stream 3 exiting pump 2 is
heated in first exchanger 5 to about -125.5.degree. F. LNG stream
3, which is still all liquid, is introduced to fractionation tower
10. In one preferred embodiment, LNG stream 3 is introduced to
fractionation tower 10 as a bottom feed stream.
[0056] At least a portion of tower overhead stream 14 is expanded
in expander 33 to about 365 psia so that tower overhead stream 14
is at least partially condensed thereby producing partially
condensed low pressure vapor stream 35. Partially condensed low
pressure vapor stream 35 is then sent to reflux accumulator or
separator 16 where the stream is separated into lean vapor stream
17 and tower reflux stream 18. Tower reflux stream 18 is pumped by
pump 19. Tower reflux stream 18 is then subcooled in first
exchanger 5 to about -232.degree. F. to produce lean tower reflux
stream 26. Lean tower reflux stream 26 is then sent to
fractionation tower 10, preferably as a top feed stream. Lean vapor
stream 17 is boosted in pressure booster compressor 34, which is
driven off or powered by the power of expander 33, and then further
compressed in lean compressor 27 to about 526 psia. Compressed and
warm residue gas stream 28 is cooled in side reboiler 31 to about
-115.8.degree. F. to produce partially cooled stream 32. Cold tower
side reboiler stream 29 is heated in side reboiler 31 and returned
to fractionation tower 10 as return stream 30. Partially cooled
stream 32 is then further cooled to about -132.6.degree. F. and
completely condensed in first exchanger 5 to produce lean LNG
stream 21. Lean LNG stream 21 is pumped by pump 22 to elevate its
pressure to about 1422 psia. Lean LNG product stream 23 is the high
pressure lean LNG product that is sent for vaporization and/or
energy recovery.
[0057] FIG. 9 illustrates yet another embodiment that is used to
separate rich, low pressure LNG stream 1 by utilizing a subcooled
lean LNG stream to increase NGL recovery and lean gas compressor 27
for a portion of a second lean vapor stream 17. In this embodiment,
rich low pressure LNG stream 1 is pumped, preferably by pump 2, to
about 535 psia. LNG stream 3 exiting pump 2 is then heated in first
exchanger 5 to about -133.4.degree. F. to produce first tower feed
stream 6. First tower feed stream 6, which is still all liquid, is
introduced to fractionation tower 10, preferably as a bottom tower
feed stream.
[0058] Tower overhead stream 14 is cooled to about -133.degree. F.
and partially condensed in first exchanger 5 to produce two phase
stream or partially condensed tower overhead stream 15. Two phase
stream 15 is sent to suction scrubber or second separator 38 that
separates two phase stream 15 into second lean vapor stream 17 and
lean LNG stream 21, which is liquid.
[0059] Lean LNG stream 21 is elevated in pressure, preferably by
pump 22, to 1422 psia to produce a high pressure lean LNG stream
23. High pressure lean LNG stream 23 is then split into lean tower
reflux stream 25 and lean LNG product stream 48. Lean tower reflux
stream 25 is cooled in first exchanger 5 to about -232.degree. F.
and is then sent, preferably as lean tower reflux stream 26 as a
top feed to fractionation tower 10.
[0060] Second lean vapor stream 17 is compressed in lean gas
compressor 27 to produce a compressed second lean vapor stream 37.
Compressed second lean vapor stream 37 is then combined with lean
LNG product stream 48 to produce the lean LNG product that is sent
for vaporization and/or energy recovery. If high pressure lean LNG
stream 23 is sent for energy recovery, then second lean vapor
stream 17 can be cooled in side reboilers that can be added to
fractionation tower 10, similar to that shown in FIG. 6. With the
additional cooling for second lean vapor stream 17, energy recovery
will be more efficient due to the stream being colder.
COMPARISON OF PRIOR ART PROCESSES AND EMBODIMENTS OF THE PRESENT
INVENTION
[0061] TABLE-US-00002 TABLE II POWER Tower C2 C3 Pump Compressor
Total UA Power LP LNG FIG. RECOVERY % RECOVERY % NGL BPD hp hp hp
Btu/hr-F Psia .degree. F. Hp/gpm 1 87.63 98.48 41271 8047 8047
4.44E+06 510 -134.5 6.69 2 88.87 97.73 41520 8055 8055 4.25E+06 510
-133.7 6.65 3 88.57 99.32 41635 8304 8304 4.54E+06 510 -133.7 6.84
4 88.05 97.68 41282 7528 2195 9723 5.43E+06 435 -137 8.08 5 92.48
98.41 42658 7789 2195 9984 5.48E+06 460 -131.1 8.02 6 94.74 99.62
43464 8279 2195 10474 5.93E+06 470 -130.7 8.26 7 98 99.99 44457
8257 2195 10452 5.30E+06 510 -125.3 8.06 8 96.83 100 44116 8161
2195 10356 6.98E+06 535 -132.6 8.05 9 96.03 99.99 43882 8097 2186
10283 4.73E+06 520 -133 8.03
[0062] Table II provides a side-by-side comparison of prior art
processes and described embodiment of the present invention. As can
be seen in Table II, the prior art process shown in FIG. 4 has the
lowest recovery rates for both C2 and C3 recoveries when compared
to the process embodiments described herein.
[0063] The invention also encompasses the apparatus necessary for
each process embodiment. A preferred embodiment of the apparatus
for separating a liquefied natural gas (LNG) stream into a more
volatile gas fraction containing a substantial amount of the
methane and lighter components and a less volatile fraction
containing a large portion of the heavy hydrocarbon components
includes means for splitting LNG stream 3 into a first feed stream
4 and a second feed stream 7. Various means are known in the art
from a simple T in a line to more complex vessels. First exchanger
5 is operable to preheat first feed stream 4 thereby creating first
tower feed stream 6. First tower feed stream is fed into
fractionation tower 10. Fractionation tower 10 produces tower
overhead stream 14 including the more volatile fraction containing
the substantial amount of the methane and lighter components and a
tower bottoms stream 12 including the less volatile fraction
containing the heavy hydrocarbon components. At least a portion of
tower overhead stream 14 is cooled by first cooler 8, which is
operable to cool and partially condense at least a portion of the
tower overhead stream 14 to produce a partially condensed tower
overhead stream 15. In a preferred embodiment, first cooler 8 is
operable to allow for heat exchange between the portion of the
tower overhead stream 14 and the second feed stream 7. Separator 16
is provided which is operable to separate the partially condensed
overhead tower stream 15 into a lean vapor stream 17 and a tower
reflux stream 18. Tower reflux stream 18 returns to and is fed to
the fractionation tower 10. In a preferred embodiment, first
exchanger 5 is operable to allow for heat exchange between the lean
vapor stream 17 and the first feed stream 4, the lean vapor stream
17 being cooled in the first exchanger 5 to provide a lean LNG
stream 21. Pump 22 is operable for pumping the lean LNG stream 21
to a higher pressure to produce high pressure LNG stream 23.
[0064] In another embodiment, first compressor 27 is operable to
receive lean vapor stream 17 and boost the pressure to produce
compressed overhead stream 28. Another embodiment includes side
reboiler for supplying at least a portion of reboiling requirements
for the fractionation tower.
[0065] While the invention has been shown or described in only some
of its forms, it should be apparent to those skilled in the art
that it is not so limited, but is susceptible to various changes
without departing from the scope of the invention.
* * * * *