U.S. patent application number 13/444061 was filed with the patent office on 2013-10-17 for natural gas liquefaction with feed water removal.
This patent application is currently assigned to AIR PRODUCTS AND CHEMICALS, INC.. The applicant listed for this patent is Adam Adrian Brostow, Glenn Eugene Kinard. Invention is credited to Adam Adrian Brostow, Glenn Eugene Kinard.
Application Number | 20130269386 13/444061 |
Document ID | / |
Family ID | 48050574 |
Filed Date | 2013-10-17 |
United States Patent
Application |
20130269386 |
Kind Code |
A1 |
Brostow; Adam Adrian ; et
al. |
October 17, 2013 |
Natural Gas Liquefaction With Feed Water Removal
Abstract
A method and apparatus for drying and liquefying a natural gas
stream is described, in which: (a) the water containing natural gas
feed stream is cooled; (b) the cooled natural gas feed stream is
dried and further cooled; (c) the dried cooled natural gas stream
is heated; (d) the dried rewarmed natural gas stream is cooled and
liquefied and at least one compressed refrigerant feed stream is
cooled by counter-current indirect heat exchange with an expanded
cold refrigerant; and (e) the compressed cold refrigerant stream or
streams are expanded, and thereby further cooled, to provide said
expanded cold refrigerant; wherein the cooling of the natural gas
feed stream in step (a) and heating of the dried cooled natural gas
stream in step (c) is by indirect heat exchange between said two
streams.
Inventors: |
Brostow; Adam Adrian;
(Emmaus, PA) ; Kinard; Glenn Eugene; (Allentown,
PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Brostow; Adam Adrian
Kinard; Glenn Eugene |
Emmaus
Allentown |
PA
PA |
US
US |
|
|
Assignee: |
AIR PRODUCTS AND CHEMICALS,
INC.
Allentown
PA
|
Family ID: |
48050574 |
Appl. No.: |
13/444061 |
Filed: |
April 11, 2012 |
Current U.S.
Class: |
62/613 |
Current CPC
Class: |
F25J 1/0055 20130101;
F25J 1/0022 20130101; F25J 2220/68 20130101; F25J 1/0215 20130101;
F25J 2270/90 20130101; F25J 1/005 20130101; F25J 2205/60 20130101;
F25J 1/0077 20130101; F25J 1/0072 20130101; F25J 1/0212 20130101;
F25J 1/0204 20130101; F25J 1/0205 20130101; F25J 2220/64 20130101;
F25J 1/0262 20130101; F25J 2205/02 20130101; F25J 1/0057
20130101 |
Class at
Publication: |
62/613 |
International
Class: |
F25J 1/02 20060101
F25J001/02 |
Claims
1. A method for drying and liquefying a natural gas stream, the
method comprising: (a) cooling a natural gas feed stream, that
contains water, to produce a cooled natural gas stream; (b)
removing water from and further cooling the cooled natural gas feed
stream to produce a dried cooled natural gas stream; (c) heating
the dried cooled natural gas stream to produce a dried rewarmed
natural gas stream; (d) cooling and liquefying the dried rewarmed
natural gas stream and cooling at least one compressed refrigerant
feed stream by counter-current indirect heat exchange with an
expanded cold refrigerant, to produce a liquefied natural gas
product stream, at least one compressed cold refrigerant stream,
and an expanded warmed refrigerant stream; and (e) expanding and
thereby further cooling the compressed cold refrigerant stream or
streams to provide said expanded cold refrigerant; wherein the
cooling of the natural gas feed stream in step (a) and heating of
the dried cooled natural gas stream in step (c) is by indirect heat
exchange between said two streams.
2. The method of claim 1, wherein in step (c) the dried cooled
natural gas stream is heated to a temperature that is the same as
or within 20.degree. C. of the temperature of the at least one
compressed refrigerant feed stream, such that there is no or less
than 20.degree. C. temperature difference between the dried
rewarmed natural gas stream and the at least one compressed
refrigerant feed stream at the start of step (d).
3. The method of claim 2, wherein the temperature of the natural
gas feed stream at the start of step (a) is also the same as or
within 20.degree. C. of the temperatures of the dried rewarmed
natural gas stream and the at least one compressed refrigerant feed
stream at the start of step (d).
4. The method of claim 1, wherein step (d) is carried out in a
wound coil cryogenic heat exchanger.
5. The method of claim 1, wherein in step (b) the cooled natural
gas feed stream is first dried, to remove water therefrom, and is
then further cooled to produce the dried cooled natural gas
stream.
6. The method of claim 1, wherein the refrigerant in steps (d) and
(e) is either a mixed refrigerant, the compressed cold refrigerant
stream or streams in step (d) being liquid or mixed phase streams
and the expanded warmed refrigerant stream in step (d) being a
mixed phase or vapor stream, or is a gaseous refrigerant that
remains in substantially gaseous form throughout steps (d) and
(e).
7. The method of claim 1, wherein the method further comprises: (f)
compressing the expanded warmed refrigerant stream to provide said
at least one compressed refrigerant feed stream that is cooled in
step (d).
8. The method of claim 7, wherein step (f) comprises compressing
and cooling the expanded warmed refrigerant stream to provide both
said at least one compressed refrigerant feed stream that is cooled
in step (d) and an additional compressed refrigerant stream, the
method further comprising expanding said additional compressed
refrigerant stream to further cool said stream and using said
further cooled additional refrigerant stream in step (b) to further
cool the cooled natural gas feed stream by indirect heat
exchange.
9. The method of claim 8, wherein step (f) comprises compressing,
cooling and phase separating the expanded warmed refrigerant stream
to provide a vapor stream of compressed refrigerant and a liquid
stream of compressed refrigerant, said vapor stream forming at
least one compressed refrigerant feed stream that is cooled and at
least partially liquefied in step (d), and at least a portion of
said liquid stream forming the additional refrigerant stream that
is expanded and then used in step (b) to further cool the cooled
natural gas feed stream by indirect heat exchange.
10. The method of claim 1, wherein in step (d) the dried rewarmed
natural gas stream is cooled and liquefied to produce the liquefied
natural gas product stream and an additional liquefied natural gas
stream, said additional liquefied natural gas stream being used in
step (b) to further cool the cooled natural gas feed stream.
11. The method of claim 10, wherein in step (b) the cooled natural
gas feed stream is further cooled by countercurrent direct heat
exchange with said additional liquefied natural gas stream.
12. An apparatus for drying and liquefying a natural gas stream,
the apparatus comprising: an economizer heat exchanger for
receiving a water-containing natural gas feed stream and a dried
cooled natural gas stream and for cooling the water-containing
natural gas feed stream and warming the dried cooled natural gas
stream by indirect heat exchange with each other, so as to produce
a cooled water-containing natural gas feed stream and a dried
rewarmed natural gas stream; natural gas feed water removal and
natural gas feed cooling systems, in fluid flow communication with
the economizer heat exchanger and each other, for receiving the
cooled water-containing natural gas feed stream from the economizer
heat exchanger, drying and further cooling said stream, and
returning the resulting dried cooled natural gas stream to the
economizer heat exchanger; a main cryogenic heat exchanger for
cooling and liquefying the dried rewarmed natural gas stream and
for cooling at least one compressed refrigerant feed stream by
counter-current indirect heat exchange with an expanded cold
refrigerant, so as to produce a liquefied natural gas product
stream, at least one compressed cold refrigerant stream, and an
expanded warmed refrigerant stream; a conduit arrangement for
transferring the dried rewarmed natural gas stream from the
economizer heat exchanger to the warm end of the main cryogenic
heat exchanger, and for withdrawing the liquefied natural gas
product stream from the cold end of the main cryogenic heat
exchanger; and a refrigerant expansion system, in fluid flow
communication with the main cryogenic heat exchanger, for receiving
at least one compressed cold refrigerant stream from the cold end
of the cryogenic heat exchanger, expanding and thereby further
cooling said cold refrigerant, and returning expanded cold
refrigerant to the cold end of the cryogenic heat exchanger.
13. An apparatus according to claim 12, wherein the main cryogenic
heat exchanger is a wound coil heat exchanger.
14. An apparatus according to claim 12, wherein the natural gas
feed water removal system is upstream of the natural gas feed
cooling system, such that cooled water-containing natural gas from
the economizer heat exchanger is first dried in said water removal
system, and dried natural gas from said water removal system is
then further cooled in said cooling system to produce dried cooled
natural gas that is then returned to the economizer heat
exchanger.
15. An apparatus according to claim 12, wherein the apparatus
further comprises: a refrigerant compression system, in fluid flow
communication with the main cryogenic heat exchanger, for receiving
the expanded warmed refrigerant stream from the warm end of the
cryogenic heat exchanger, compressing said refrigerant, and
returning at least one compressed refrigerant feed stream to the
warm end of the cryogenic heat exchanger.
16. An apparatus according to claim 15, wherein the main cryogenic
heat exchanger, refrigerant expansion system, and refrigerant
compression system form or form part of a closed loop refrigerant
system, the refrigerant contained and circulating within said
closed loop system comprising said compressed and expanded
refrigerant streams, said refrigerant being a mixed refrigerant or
pure nitrogen or argon.
17. An apparatus according to claim 15, wherein the refrigerant
compression system compresses and cools the expanded warmed
refrigerant, and the natural gas feed cooling system is an indirect
heat exchanger, and wherein the apparatus further comprises an
additional expansion system, in fluid flow communication with the
refrigerant compression system and the natural gas feed cooling
system, for receiving a stream of compressed and cooled refrigerant
from the refrigerant compression system and expanding said stream
to further cool said stream, the natural gas feed cooling system
using said further cooled stream to further cool the cooled natural
gas feed stream by indirect heat exchange.
18. An apparatus according to claim 17, wherein the refrigerant
compression system further comprises at least one phase separator,
for separating the compressed and cooled refrigerant into liquid
and vapor phases, said phase separator or separators being in fluid
flow communication with the main cryogenic heat exchanger and the
additional expansion system such that a vapor stream of compressed
refrigerant is fed to the warm end of the cryogenic heat exchanger
and a liquid stream of compressed refrigerant is fed to the
additional expansion system.
19. An apparatus according to claim 12, wherein the apparatus
further comprises a conduit arrangement for transferring an
additional liquefied natural gas stream from the main cryogenic
heat exchanger to the natural gas feed cooling system, the feed
cooling system using said additional liquefied natural gas stream
to further cool the cooled natural gas feed stream.
20. An apparatus according to claim 19, wherein the natural gas
feed cooling system is a scrub column, in which the cooled natural
gas feed stream is further cooled by countercurrent direct heat
exchange with said additional liquefied natural gas stream.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
drying and liquefying a natural gas stream.
[0002] Where a natural gas stream contains water it is necessary
first to dry the stream, to remove all, or substantially all, of
the water therefrom, before liquefaction of the natural gas can
take place. In order to remove water (and other impurities, such as
mercury) from the natural gas prior to liquefaction, it is moreover
common practice to first cool the feed to below ambient
temperature, especially when the ambient temperature is high.
[0003] An example of a liquefaction cycle that includes such a
pre-cooling step is the propane pre-cooled mixed refrigerant
("C3MR") cycle. In such methods, propane (or a different liquid
refrigerant in a vapor compression cycle) may be used to cool the
natural gas feed to a desired temperature prior to the drying step
taking place. Then the same liquid refrigerant can be used at a
lower pressure to further cool the now dried feed prior to said
feed being introduced into the main cryogenic heat exchanger
("MCHE"). The refrigerant used for liquefaction, for example mixed
refrigerant ("MR"), is also pre-cooled to about the same
temperature. Therefore, all streams entering the warm end of the
MCHE are at about the same temperature, thereby minimizing thermal
stresses in MCHE.
[0004] The pre-cooling of a water-containing natural gas may pose
an additional problem. That is, if the temperature of the natural
gas during this cooling step is not tightly controlled, there can
be a risk of hydrate formation. Using a single-component liquid
refrigerant, such as propane, that is vaporized to pre-cool the
natural gas feed allows for good temperature control, because at
any given pressure the temperature at which such a refrigerant
vaporizes will not vary (whereas, for example, with a mixed
refrigerant stream the vaporization temperature will vary with any
changes in the ratio of the different refrigerants present in the
stream as well as with any changes in the pressure of the
stream).
[0005] An example of prior art that addresses hydrate formation in
a C3MR cycle is that described in U.S. Pat. No. 4,755,200, the
disclosure of which is incorporated herein by reference, where a
natural gas feed is chilled using a single component vapor
compression cycle prior to water removal. The resulting dry natural
gas is further cooled by indirect heat exchange with mixed
refrigerant vapor from the MCHE prior to being fed to the MCHE.
[0006] However, one drawback with the C3MR process discussed in
U.S. Pat. No. 4,755,200, and other such processes where a
single-component liquid refrigerant in a vapor compression cycle is
used to pre-cool the natural gas feed prior to liquefaction of the
natural gas in a MCHE that uses a mixed refrigerant, is that they
require the use of an additional refrigeration loop (namely the
propane or other single-component loop). This increases both the
footprint and the capital investment cost of the liquefaction
plant.
[0007] There are also known natural gas liquefaction cycles that do
not involve the use of propane for pre-cooling the natural gas feed
stream. These include the single mixed refrigerant ("SMR") cycle,
such as for example that described in U.S. Pat. No. 6,347,531, the
dual mixed refrigerant ("DMR") cycle, such as for example that
described in U.S. Pat. No. 6,119,479, and the nitrogen recycle ("N2
recycle") cycle, such as for example that described in US
2010/0122551, the disclosures of each of which are incorporated
herein by reference.
[0008] In such methods a portion of refrigerant (MR or gaseous
nitrogen) may be withdrawn from one of the main refrigeration loops
and used to cool the feed prior to water removal. Since MR, or a
pure gaseous refrigerant such as nitrogen, provide refrigeration
over a range of temperatures, it is difficult to control
temperature to prevent hydrate formation. In addition, the MR
composition is optimized to provide refrigeration at colder
temperatures, and the nitrogen recycle cycle (reverse-Brayton
cycle) is inherently inefficient at warmer temperatures. From the
standpoint of efficiency there is a need to minimize the
pre-cooling duty.
[0009] U.S. Pat. No. 6,793,712, the disclosure of which is
incorporated herein by reference, discloses a cascade process where
a water-containing natural gas feed is first dried and then
expanded in an isentropic expander. The resulting cold natural gas
is heated in a first heat exchanger and then in a second heat
exchanger, prior to acid gas removal. It is then cooled back down
by indirect heat exchange with the cold natural gas in the second
heat exchanger prior to further water removal, and is then further
cooled in the first heat exchanger, again by indirect heat exchange
with the cold natural gas. The natural gas is then further cooled
and liquefied. The disadvantage of such a process is that it
requires at least one piece of rotating machinery (i.e. the
isentropic expander) and two heat exchangers (or one heat exchanger
with side-headers), and the reduced feed pressure resulting from
expansion in the isentropic expander lowers liquefaction
efficiency.
[0010] Thus, there is a need in the art for alternative and/or
improved natural gas liquefaction cycles (such as, but not limited
to, the SMR, DMR, and N2 recycle cycles) in situations where the
natural gas feed is required to be pre-cooled for water
removal.
[0011] It is an object of embodiments of the present invention to
provide a liquefaction cycle in which the temperature mismatch at
the bottom of the MCHE is minimized, and the overall efficiency of
the liquefaction cycle is enhanced.
[0012] It is further an object of preferred embodiments of the
present invention to provide for good temperature control during
pre-cooling of the natural gas feed so as to prevent or minimize
hydrate formation.
BRIEF SUMMARY OF THE INVENTION
[0013] According to a first aspect of the present invention a
method for drying and liquefying a natural gas stream is provided,
the method comprising:
(a) cooling a natural gas feed stream, that contains water, to
produce a cooled natural gas stream; (b) removing water from and
further cooling the cooled natural gas feed stream to produce a
dried cooled natural gas stream; (c) heating the dried cooled
natural gas stream to produce a dried rewarmed natural gas stream;
(d) cooling and liquefying the dried rewarmed natural gas stream
and cooling at least one compressed refrigerant feed stream by
counter-current indirect heat exchange with an expanded cold
refrigerant, to produce a liquefied natural gas product stream, at
least one compressed cold refrigerant stream, and an expanded
warmed refrigerant stream; and (e) expanding and thereby further
cooling the compressed cold refrigerant stream or streams to
provide said expanded cold refrigerant; wherein the cooling of the
natural gas feed stream in step (a) and heating of the dried cooled
natural gas stream in step (c) is by indirect heat exchange between
said two streams.
[0014] According to a second aspect of the present invention an
apparatus for drying and liquefying a natural gas stream is
provided, the apparatus comprising:
[0015] an economizer heat exchanger for receiving a
water-containing natural gas feed stream and a dried cooled natural
gas stream and for cooling the water-containing natural gas feed
stream and warming the dried cooled natural gas stream by indirect
heat exchange with each other, so as to produce a cooled
water-containing natural gas feed stream and a dried rewarmed
natural gas stream;
[0016] natural gas feed water removal and natural gas feed cooling
systems, in fluid flow communication with the economizer heat
exchanger and each other, for receiving the cooled water-containing
natural gas feed stream from the economizer heat exchanger, drying
and further cooling said stream, and returning the resulting dried
cooled natural gas stream to the economizer heat exchanger;
[0017] a main cryogenic heat exchanger for cooling and liquefying
the dried rewarmed natural gas stream and for cooling at least one
compressed refrigerant feed stream by counter-current indirect heat
exchange with an expanded cold refrigerant, so as to produce a
liquefied natural gas product stream, at least one compressed cold
refrigerant stream, and an expanded warmed refrigerant stream;
[0018] a conduit arrangement for transferring the dried rewarmed
natural gas stream from the economizer heat exchanger to the warm
end of the main cryogenic heat exchanger, and for withdrawing the
liquefied natural gas product stream from the cold end of the main
cryogenic heat exchanger; and
[0019] a refrigerant expansion system, in fluid flow communication
with the main cryogenic heat exchanger, for receiving at least one
compressed cold refrigerant stream from the cold end of the
cryogenic heat exchanger, expanding and thereby further cooling
said cold refrigerant, and returning expanded cold refrigerant to
the cold end of the cryogenic heat exchanger.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0020] FIG. 1 is a schematic flow diagram of an apparatus and
method for drying and liquefying a natural gas stream in accordance
with one embodiment of the present invention.
[0021] FIG. 2 is a schematic flow diagram of an exemplary closed
loop mixed refrigerant system and process for use in the apparatus
and method for drying and liquefying a natural gas stream depicted
in FIG. 1.
[0022] FIG. 3 is a schematic flow diagram of an apparatus and
method for drying and liquefying a natural gas stream in accordance
with another embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] As described above, in the first aspect of the present
invention a method for drying and liquefying a natural gas stream
is provided comprising the above mentioned steps of (a) cooling a
natural gas feed stream, (b) removing water from and further
cooling the cooled natural gas feed stream, (c) heating the dried
cooled natural gas stream, (d) cooling and liquefying the dried
rewarmed natural gas stream and cooling at least one compressed
refrigerant feed stream by counter-current indirect heat exchange
with an expanded cold refrigerant, and (e) expanding and thereby
further cooling the compressed cold refrigerant stream or streams
to provide said expanded cold refrigerant, wherein the cooling of
the natural gas feed stream in step (a) and heating of the dried
cooled natural gas stream in step (c) is by indirect heat exchange
between said two streams.
[0024] As used herein, the term "expanding" refers to the reduction
in pressure of the fluid in question by any suitable means, and in
the case of a liquid may, unless otherwise indicated, involve at
least partial vaporization or simply a reduction in pressure.
[0025] As used herein, the term "dried" refers to a fluid from
which all or substantially all the water has been removed. More
specifically, it means that either all water has been removed, or
that the residual amount of water that remains is sufficiently low
as to have a negligible effect on the subsequent processing of the
fluid. In particular, in the case of a "dried natural gas stream"
either all water has been removed, or any residual water remaining
in said stream is present in a sufficiently low amount as to not
cause any operational problems in the downstream cooling and
liquefaction process due to water freezeout.
[0026] As used herein, the phrase "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 (for
example, in a shell and tube heat exchanger indirect heat exchange
occurs, as the tube-side fluid is kept separate from the shell-side
fluid by the walls of the tubes). This is in contrast to "direct
heat exchange", where the fluids come into contact and can mix (as,
for example, in a scrubbing column, where mass transfer in addition
to heat transfer can take place between the counter-current streams
flowing through the column).
[0027] In a preferred embodiment, in step (c) the dried cooled
natural gas stream is heated to about the same temperature as the
temperature of the at least one compressed refrigerant feed stream,
such that the temperatures of the dried rewarmed natural gas stream
and the at least one compressed refrigerant feed stream at the
start of step (d) are about the same. Preferably, the natural gas
feed stream at the start of step (a) is also at about the same
temperature as the temperatures of the dried rewarmed natural gas
stream and the at least one compressed refrigerant feed stream at
the start of step (d).
[0028] Preferably, in step (c) the dried cooled natural gas stream
is heated to a temperature that is the same as or within 20.degree.
C. of, more preferably within 10.degree. C. of, the temperature of
the at least one compressed refrigerant feed stream, such that
there is no or less than 20.degree. C., more preferably less than
10.degree. C., temperature difference between the dried rewarmed
natural gas stream and the at least one compressed refrigerant feed
stream at the start of step (d). Preferably, temperature of the
natural gas feed stream at the start of step (a) is also the same
as or within 20.degree. C., more preferably within 10.degree. C.,
of the temperatures of the dried rewarmed natural gas stream and
the at least one compressed refrigerant feed stream at the start of
step (d).
[0029] In a preferred embodiment, step (d) is carried out in a
cryogenic heat exchanger of the shell and tube type, most
preferably a wound coil heat exchanger.
[0030] In a preferred embodiment, in step (b) of the method the
cooled natural gas feed stream is first dried, to remove water
therefrom, and is then further cooled to produce the dried cooled
natural gas stream. Alternatively, said steps can be carried out in
the reverse order, wherein the feed stream is first further cooled
and then dried to produce the dried cooled natural gas stream.
However, the latter option is generally less preferred.
[0031] In one embodiment, the refrigerant in steps (d) and (e) is
either a mixed refrigerant (comprising for example a mixture of
hydrocarbons and/or perfluorohydrocarbons), in which case the
compressed cold refrigerant stream or streams in step (d) are
liquid or mixed phase streams and the expanded warmed refrigerant
stream in step (d) is a mixed phase or vapor stream, or is a
gaseous refrigerant (such as for example a pure, e.g. 99 mol. % or
higher, nitrogen or argon) that remains in substantially gaseous
form throughout steps (d) and (e) (i.e. wherein at most 12 mol. %,
more preferably at most 5 mol. %, and most preferably none of the
refrigerant is in liquid form at any point throughout steps (d) and
(e)).
[0032] In preferred embodiments, the method further comprises the
step:
(f) compressing, and preferably cooling (for example by way of one
or more interstage and/or after-coolers), the expanded warmed
refrigerant stream to provide said at least one compressed
refrigerant feed stream that is cooled in step (d).
[0033] In one embodiment, step (f) comprises compressing and
cooling the expanded warmed refrigerant stream to provide both said
at least one compressed refrigerant feed stream that is cooled in
step (d) and an additional compressed refrigerant stream, the
method further comprising expanding said additional compressed
refrigerant stream to further cool said stream and using said
further cooled additional refrigerant stream in step (b) to further
cool the cooled natural gas feed stream by indirect heat exchange.
Preferably, step (f) comprises compressing, cooling and phase
separating the expanded warmed refrigerant stream to provide a
vapor stream of compressed refrigerant and a liquid stream of
compressed refrigerant, said vapor stream forming at least one
compressed refrigerant feed stream that is cooled and at least
partially liquefied in step (d), and at least a portion of said
liquid stream forming the additional refrigerant stream that is
expanded and then used in step (b) to further cool the cooled
natural gas feed stream by indirect heat exchange.
[0034] In another embodiment, in step (d) the dried rewarmed
natural gas stream is cooled and liquefied to produce the liquefied
natural gas product stream and an additional liquefied natural gas
stream, said additional liquefied natural gas stream being used in
step (b) to further cool the cooled natural gas feed stream.
Preferably, in step (b) the cooled natural gas feed stream is
further cooled by countercurrent direct heat exchange with said
additional liquefied natural gas stream.
[0035] As also described above, in the second aspect of the present
invention an apparatus for drying and liquefying a natural gas
stream is provided which comprises the above mentioned economizer
heat exchanger, natural gas feed water removal and natural gas feed
cooling systems, main cryogenic heat exchanger, conduit
arrangement, and refrigerant expansion system. Said apparatus is
suitable for carrying out the method according to the first aspect.
Accordingly, in a further preferred embodiment of the first aspect,
the method according to the first aspect of the invention is
carried out in the apparatus according to the second aspect.
[0036] In a preferred embodiment of the apparatus according to the
second aspect, the main cryogenic heat exchanger is of the shell
and tube type, and most preferably is a wound coil heat
exchanger.
[0037] The natural gas feed water removal system is, preferably,
upstream of the natural gas feed cooling system, such that cooled
water-containing natural gas from the economizer heat exchanger is
first dried in said water removal system, and dried natural gas
from said water removal system is then further cooled in said
cooling system to produce dried cooled natural gas that is then
returned to the economizer heat exchanger. Alternatively, the order
of said systems can be reversed, such that cooled water-containing
natural gas from the economizer heat exchanger is first further
cooled in said cooling system and then dried in said water removal
system. Again, however, said latter option is generally less
preferred.
[0038] Optionally, where the natural gas feed water removal system
is upstream of the natural gas feed cooling system, the dried
natural gas from the water removal system may be returned to the
economizer heat exchanger and further cooled therein prior to being
sent to and further cooled in the cooling system to produce dried
cooled natural gas that is then returned to the economizer heat
exchanger.
[0039] In a preferred embodiment, the apparatus further comprises a
refrigerant compression system (which is preferably a refrigerant
compression and cooling system, said cooling being for example
provided by one or more interstage and/or after-coolers), in fluid
flow communication with the main cryogenic heat exchanger, for
receiving the expanded warmed refrigerant stream from the warm end
of the cryogenic heat exchanger, compressing (and preferably
cooling) said refrigerant, and returning at least one compressed
refrigerant feed stream to the warm end of the cryogenic heat
exchanger. The main cryogenic heat exchanger, refrigerant expansion
system, and refrigerant compression system may form, or form part
of a closed loop refrigerant system, the refrigerant contained and
circulating within said closed loop system comprising said
compressed and expanded refrigerant streams. As in the method
according to the first aspect, said refrigerant may, for example,
be a mixed refrigerant (comprising for example a mixture of
hydrocarbons and/or perfluorohydrocarbons), or or a pure gaseous
refrigerant such as, for example, pure (e.g. 99 mol. % or higher)
nitrogen or argon.
[0040] In one embodiment, the natural gas feed cooling system is an
indirect heat exchanger, and the apparatus further comprises an
additional expansion system, in fluid flow communication with the
refrigerant compression and cooling system and the natural gas feed
cooling system, for receiving a stream of compressed and cooled
refrigerant from the refrigerant compression and cooling system and
expanding said stream to further cool said stream, the natural gas
feed cooling system using said further cooled stream to further
cool the cooled natural gas feed stream by indirect heat exchange.
The refrigerant compression and cooling system may further comprise
at least one phase separator, for separating the compressed and
cooled refrigerant into liquid and vapor phases, said phase
separator or separators being in fluid flow communication with the
main cryogenic heat exchanger and the additional expansion system
such that a vapor stream of compressed refrigerant is fed to the
warm end of the cryogenic heat exchanger and a liquid stream of
compressed refrigerant is fed to the additional expansion
system.
[0041] In an alternative embodiment, the apparatus further
comprises a conduit arrangement for transferring an additional
liquefied natural gas stream from the main cryogenic heat exchanger
to the natural gas feed cooling system, said feed cooling system
using said additional liquefied natural gas stream to further cool
the cooled natural gas feed stream. For example, the natural gas
feed cooling system may in this case be a system (such as for
example a scrub column) in which the cooled natural gas feed stream
is further cooled by countercurrent direct heat exchange with said
additional liquefied natural gas stream.
[0042] Accordingly, the present invention includes the following
aspects, numbered #1 to #20:
#1. A method for drying and liquefying a natural gas stream, the
method comprising: (a) cooling a natural gas feed stream, that
contains water, to produce a cooled natural gas stream; (b)
removing water from and further cooling the cooled natural gas feed
stream to produce a dried cooled natural gas stream; (c) heating
the dried cooled natural gas stream to produce a dried rewarmed
natural gas stream; (d) cooling and liquefying the dried rewarmed
natural gas stream and cooling at least one compressed refrigerant
feed stream by counter-current indirect heat exchange with an
expanded cold refrigerant, to produce a liquefied natural gas
product stream, at least one compressed cold refrigerant stream,
and an expanded warmed refrigerant stream; and (e) expanding and
thereby further cooling the compressed cold refrigerant stream or
streams to provide said expanded cold refrigerant;
[0043] wherein the cooling of the natural gas feed stream in step
(a) and heating of the dried cooled natural gas stream in step (c)
is by indirect heat exchange between said two streams.
#2. A method according to #1, wherein in step (c) the dried cooled
natural gas stream is heated to a temperature that is the same as
or within 20.degree. C. of the temperature of the at least one
compressed refrigerant feed stream, such that there is no or less
than 20.degree. C. temperature difference between the dried
rewarmed natural gas stream and the at least one compressed
refrigerant feed stream at the start of step (d). #3. A method
according to #2, wherein the temperature of the natural gas feed
stream at the start of step (a) is also the same as or within
20.degree. C. of the temperatures of the dried rewarmed natural gas
stream and the at least one compressed refrigerant feed stream at
the start of step (d). #4. A method according to any one of #1 to
#3, wherein step (d) is carried out in a wound coil cryogenic heat
exchanger. #5. A method according to any one of #1 to #4, wherein
in step (b) the cooled natural gas feed stream is first dried, to
remove water therefrom, and is then further cooled to produce the
dried cooled natural gas stream. #6. A method according to any one
of #1 to #5, wherein the refrigerant in steps (d) and (e) is either
a mixed refrigerant, the compressed cold refrigerant stream or
streams in step (d) being liquid or mixed phase streams and the
expanded warmed refrigerant stream in step (d) being a mixed phase
or vapor stream, or is a gaseous refrigerant that remains in
substantially gaseous form throughout steps (d) and (e). #7. A
method according to any one of #1 to #6, wherein the method further
comprises: (f) compressing the expanded warmed refrigerant stream
to provide said at least one compressed refrigerant feed stream
that is cooled in step (d). #8. A method according to #7, wherein
step (f) comprises compressing and cooling the expanded warmed
refrigerant stream to provide both said at least one compressed
refrigerant feed stream that is cooled in step (d) and an
additional compressed refrigerant stream, the method further
comprising expanding said additional compressed refrigerant stream
to further cool said stream and using said further cooled
additional refrigerant stream in step (b) to further cool the
cooled natural gas feed stream by indirect heat exchange. #9. A
method according to #8, wherein step (f) comprises compressing,
cooling and phase separating the expanded warmed refrigerant stream
to provide a vapor stream of compressed refrigerant and a liquid
stream of compressed refrigerant, said vapor stream forming at
least one compressed refrigerant feed stream that is cooled and at
least partially liquefied in step (d), and at least a portion of
said liquid stream forming the additional refrigerant stream that
is expanded and then used in step (b) to further cool the cooled
natural gas feed stream by indirect heat exchange. #10. A method
according to any one of #1 to #7, wherein in step (d) the dried
rewarmed natural gas stream is cooled and liquefied to produce the
liquefied natural gas product stream and an additional liquefied
natural gas stream, said additional liquefied natural gas stream
being used in step (b) to further cool the cooled natural gas feed
stream. #11. A method according to #10, wherein in step (b) the
cooled natural gas feed stream is further cooled by countercurrent
direct heat exchange with said additional liquefied natural gas
stream. #12. An apparatus for drying and liquefying a natural gas
stream, the apparatus comprising:
[0044] an economizer heat exchanger for receiving a
water-containing natural gas feed stream and a dried cooled natural
gas stream and for cooling the water-containing natural gas feed
stream and warming the dried cooled natural gas stream by indirect
heat exchange with each other, so as to produce a cooled
water-containing natural gas feed stream and a dried rewarmed
natural gas stream;
[0045] natural gas feed water removal and natural gas feed cooling
systems, in fluid flow communication with the economizer heat
exchanger and each other, for receiving the cooled water-containing
natural gas feed stream from the economizer heat exchanger, drying
and further cooling said stream, and returning the resulting dried
cooled natural gas stream to the economizer heat exchanger;
[0046] a main cryogenic heat exchanger for cooling and liquefying
the dried rewarmed natural gas stream and for cooling at least one
compressed refrigerant feed stream by counter-current indirect heat
exchange with an expanded cold refrigerant, so as to produce a
liquefied natural gas product stream, at least one compressed cold
refrigerant stream, and an expanded warmed refrigerant stream;
[0047] a conduit arrangement for transferring the dried rewarmed
natural gas stream from the economizer heat exchanger to the warm
end of the main cryogenic heat exchanger, and for withdrawing the
liquefied natural gas product stream from the cold end of the main
cryogenic heat exchanger; and
[0048] a refrigerant expansion system, in fluid flow communication
with the main cryogenic heat exchanger, for receiving at least one
compressed cold refrigerant stream from the cold end of the
cryogenic heat exchanger, expanding and thereby further cooling
said cold refrigerant, and returning expanded cold refrigerant to
the cold end of the cryogenic heat exchanger.
#13. An apparatus according to #12, wherein the main cryogenic heat
exchanger is a wound coil heat exchanger. #14. An apparatus
according to #12 or #13, wherein the natural gas feed water removal
system is upstream of the natural gas feed cooling system, such
that cooled water-containing natural gas from the economizer heat
exchanger is first dried in said water removal system, and dried
natural gas from said water removal system is then further cooled
in said cooling system to produce dried cooled natural gas that is
then returned to the economizer heat exchanger. #15. An apparatus
according to any one of #12 to #14, wherein the apparatus further
comprises:
[0049] a refrigerant compression system, in fluid flow
communication with the main cryogenic heat exchanger, for receiving
the expanded warmed refrigerant stream from the warm end of the
cryogenic heat exchanger, compressing said refrigerant, and
returning at least one compressed refrigerant feed stream to the
warm end of the cryogenic heat exchanger.
#16. An apparatus according to #15, wherein the main cryogenic heat
exchanger, refrigerant expansion system, and refrigerant
compression system form or form part of a closed loop refrigerant
system, the refrigerant contained and circulating within said
closed loop system comprising said compressed and expanded
refrigerant streams, said refrigerant being a mixed refrigerant or
pure nitrogen or argon. #17. An apparatus according to #15 or #16,
wherein the refrigerant compression system compresses and cools the
expanded warmed refrigerant, and the natural gas feed cooling
system is an indirect heat exchanger, and wherein the apparatus
further comprises an additional expansion system, in fluid flow
communication with the refrigerant compression system and the
natural gas feed cooling system, for receiving a stream of
compressed and cooled refrigerant from the refrigerant compression
system and expanding said stream to further cool said stream, the
natural gas feed cooling system using said further cooled stream to
further cool the cooled natural gas feed stream by indirect heat
exchange. #18. An apparatus according to #17, wherein the
refrigerant compression system further comprises at least one phase
separator, for separating the compressed and cooled refrigerant
into liquid and vapor phases, said phase separator or separators
being in fluid flow communication with the main cryogenic heat
exchanger and the additional expansion system such that a vapor
stream of compressed refrigerant is fed to the warm end of the
cryogenic heat exchanger and a liquid stream of compressed
refrigerant is fed to the additional expansion system. #19. An
apparatus according to any one of #12 to #16, wherein the apparatus
further comprises a conduit arrangement for transferring an
additional liquefied natural gas stream from the main cryogenic
heat exchanger to the natural gas feed cooling system, the feed
cooling system using said additional liquefied natural gas stream
to further cool the cooled natural gas feed stream. #20. An
apparatus according to #19, wherein the natural gas feed cooling
system is a scrub column, in which the cooled natural gas feed
stream is further cooled by countercurrent direct heat exchange
with said additional liquefied natural gas stream.
[0050] Solely by way of example, certain specific embodiments of
the invention will now be described, with reference to the
accompanying drawings.
[0051] Referring to FIG. 1, an exemplary apparatus and method for
drying and liquefying a natural gas stream in accordance with one
embodiment of the present invention is depicted. Water-containing
natural gas feed stream 10 is first cooled in an economizer heat
exchanger 11. The resulting cooled water-containing natural gas
feed stream 12 is fed to natural gas feed water removal system 13
to cold dry the stream, thereby producing dried natural gas stream
14. There may a phase separator on stream 12 (not shown for
simplicity) to remove any condensed water from said stream prior to
its introduction into water removal system 13. Dried natural gas
stream 14 is further cooled in natural gas feed cooling system
(feed cooler) 15 to produce dried cooled natural gas stream 16.
Dried cooled natural gas stream 16 is then warmed back up in
economizer heat exchanger 11, by countercurrent indirect heat
exchange with water-containing natural gas feed stream 10, to
produce rewarmed dried natural gas stream 17. Thus, in the
economizer heat exchanger 11 the water-containing natural gas feed
stream 10 is cooled, and dried cooled natural gas stream 16 is
warmed, by indirect heat exchange between the two streams. Rewarmed
dried natural gas stream 17 is then sent to the main cryogenic heat
exchanger (MCHE) 1 for further cooling and liquefaction.
[0052] Economizer heat exchanger 11 can be any type of heat
exchange suitable for for effecting countercurrent indirect heat
exchange between the water-containing natural gas feed stream 10
and dried cooled natural gas stream 16, such as, for example, a
shell-and-tube, plate-and-fin, or printed circuit heat
exchanger.
[0053] Water removal system 13 may be any type of system suitable
for drying/dehydrating a water-containing natural gas stream.
Various types of water removal system are known in the art,
including both absorption systems, such as, for example, a glycol
dehydrator or adsorption systems, such as, for example, molecular
sieves and activated alumina.
[0054] Feed cooler 15 uses a colder-than-ambient stream to further
cool the natural gas stream. Feed cooler 15 may, for example, be an
indirect heat exchanger that uses as said colder-than-ambient
stream a stream of refrigerant from the same closed loop of
refrigerant that is also used to provide the cooling duty for the
MCHE 1, an example of such an arrangement being depicted in FIG. 2,
which will be described in further detail below. Alternatively (and
albeit less preferably), said colder-than-ambient stream may, for
example, form part of a separate refrigerant loop, such as where
feed cooler 15 is a packaged chiller that uses its own, separate
refrigerant loop. In either case, feed cooler 15 can be of any flow
arrangement, such as countercurrent or kettle, and type, such as
shell-and-tube, plate-and-fin, or diffusion-bonded, for effecting
indirect heat exchange between the natural gas and
colder-than-ambient streams.
[0055] In FIG. 1, water removal system 13 is located upstream of
feed cooler 15, such that the cooled water-containing natural gas
feed stream from economizer heat exchanger 11 is dried by the water
removal system before being further cooled in the feed cooler. If
feed cooler 15 is downstream of the water removal system 13 (as
shown in FIG. 1) then the colder-than-ambient stream used by feed
cooler 15 cools an already dried natural gas stream, with the
resulting dried and cooled natural gas stream 16 then being used in
economizer heat exchanger 11 to cool the water-containing natural
gas feed stream 10 as required prior to water removal. This is
particularly advantageous where the colder-than-ambient stream used
by feed cooler 15 is a mixed refrigerant or a pure gaseous
refrigerant (such as nitrogen), such as may be the case where the
colder-than-ambient stream is a stream of refrigerant from the same
closed loop of refrigerant that is also used to provide the cooling
duty for the MCHE 1, as cooling the water-containing natural gas
feed stream 10 in the economizer heat exchanger 11 in this manner
provides better control over the temperature of water-containing
natural gas feed stream 10 during cooling than if a mixed
refrigerant or a pure gaseous refrigerant (such as nitrogen) were
to be used to cool said water-containing natural gas feed stream 10
directly. Hence, cooling the water-containing natural gas feed
stream 10 in this manner can reduce the risk of hydrate formation
in the water-containing natural gas feed stream during cooling
prior to water removal.
[0056] In an alternative arrangement (not depicted), feed cooler 15
may instead be placed upstream of the water removal system 13.
However, where the feed cooler 15 is using a mixed refrigerant or a
pure gaseous refrigerant there will then also be a greater risk of
hydrate formation in the water-containing natural gas feed stream
during cooling (in said feed cooler 15) prior to water removal. If
feed cooler 15 is instead a packaged chiller that uses a separate
refrigeration loop comprising, for example, a pure liquid
refrigerant (or an azeotrope) in a vapor compression cycle then
there may not be any increased risk of hydrate formation, but the
requirement for an additional refrigeration loop (i.e. that of the
packaged chiller) will increase the capital investment cost and
footprint of the plant.
[0057] Thus, positioning the water removal system 13 upstream of
the feed cooler 15 is, in general, preferred.
[0058] The rewarmed dried natural gas stream 17 exiting economizer
heat exchanger 11 is, as noted above, introduced into the warm end
of main cryogenic heat exchanger (MCHE) 1 and is cooled and
liquefied to produce liquefied natural gas product stream 18, which
is withdrawn from the cold end of heat exchanger 1. MCHE 1 forms
part of a refrigeration system 2, such as a closed-loop refrigerant
system (using, for example, a mixed refrigerant or pure gaseous
refrigerant), for cooling and liquefying the rewarmed dried natural
gas stream 17. In said system, one or more feed streams of
compressed refrigerant 3 are also cooled in the MCHE 1 to produced
one or more compressed cold refrigerant streams, which are then
withdrawn from the MCHE 1 and expanded to further cool the
refrigerant, the expanded cold refrigerant then being returned to
the MCHE 1 to provide the cooling duty for cooling and liquefying
the rewarmed dried natural gas stream 17 and cooling the compressed
cold refrigerant streams 3. The expanded warmed refrigerant,
resulting from countercurrent heat exchange with said rewarmed
dried natural gas stream 17 and compressed refrigerant streams 3,
is then withdrawn from the MCHE 1, compressed, and returned to the
MCHE 1 as the one or more feed streams 3 of compressed
refrigerant.
[0059] In the example illustrated in FIG. 1, two feed streams of
compressed refrigerant 3 are introduced into the warm end of MCHE
1, with one stream being cooled and withdrawn as a compressed cold
refrigerant stream from the cold end of the MCHE 1, and the other
being cooled and withdrawn as a compressed cold refrigerant stream
from an intermediate location of the MCHE 1. The compressed cold
refrigerant stream withdrawn from the cold end is then expanded
across throttle valve 38, which adiabatic (isenthalpic) expansion
further cools the refrigerant, thereby providing an expanded cold
refrigerant that is returned to the cold end of the MCHE 1 to
provide cooling duty. Similarly, the compressed cold refrigerant
stream withdrawn from the intermediate location is expanded across
throttle valve 39, which adiabatic (isenthalpic) expansion further
cools the refrigerant, thereby providing an expanded cold
refrigerant that is returned to the intermediate location of the
MCHE 1 to provides cooling duty. The expanded cold refrigerant
flows through the MCHE in the opposite direction to the rewarmed
dried natural gas stream 17 and compressed refrigerant feed streams
3, cooling said streams by countercurrent indirect heat exchange.
Further aspects of an exemplary refrigeration system 2, of which
the MCHE 1 forms a part, will be described in further detail below
with reference to FIG. 2.
[0060] Although in FIG. 1, throttle valves 38 and 39 are used to
expand the cooled compressed refrigerant streams, in the present
invention any type of device or system for expanding (i.e. reducing
the pressure of) said streams in order to reduce the temperature of
said streams may be used. Thus, any device or system for
adiabatically expanding said streams may be used, including
centrifugal or reciprocating expanders that expand the refrigerant
stream while producing external work (i.e. wherein isentropic,
rather than isenthalpic, expansion of the refrigerant is
occurring). For example, where the cooled compressed refrigerant
streams are liquid streams, hydraulic turbines (dense fluid
expanders) that isentropically expand the refrigerant could be
used.
[0061] As depicted in FIG. 1, the MCHE 1 is a wound-coil heat
exchanger (or other heat exchanger of the shell-and-tube type)
containing two bundles (the intermediate location, from which one
of the two compressed cold refrigerant streams is withdrawn, being
between the two bundles), rewarmed dried natural gas stream 17
being for example cooled and optionally partially or fully
liquefied in the first bundle, and fully liquefied (if not already
so) and/or sub-cooled in the second bundle. Equally, however, other
types and arrangements of heat exchanger may be used. For example,
where the MCHE is a would-coil (or other type of shell-and-tube)
heat exchanger, it may contain more or less bundles, and the
bundles may be located in the same or different shells
(interconnected by suitable conduits, in the case of the latter).
The MCHE may also be any other type of cryogenic heat exchange
suitable for effecting counter-current indirect heat exchange. For
example, the MCHE could be of the plate-and-fin type. Nevertheless,
the use of a MCHE of the wound-coil type is generally
preferred.
[0062] The rewarmed dried natural gas stream 17 and compressed
refrigerant feed stream or steams 3 preferably enter the MCHE 1 at
the same or a similar temperature, so as to minimize any
temperature mismatch between the streams entering the warm end of
the MCHE. Preferably, rewarmed dried natural gas stream 17 and
stream or streams 3 are within 10.degree. C. of each other.
Typically, the water-containing natural gas feed stream 10 is also
at a similar temperature to streams 17 and 3, and thus is typically
likewise within 10.degree. C. of the temperatures of streams 17 and
3.
[0063] The use of the economizer heat exchanger 11, arranged and
operating as described above, provides a number of benefits. The
cooling of water-containing natural gas stream 10 in economizer
heat exchanger 11 results in a colder natural gas stream (stream
12) being fed to water removal system 13 than would otherwise be
the case if water-containing natural gas stream 10 were fed
directly to water removal system 13, which in turn allows for more
optimal water removal from said natural gas stream. More
specifically, cooling the water-containing natural gas stream prior
to its introduction into water removal system 13 can reduce the
load on said system (as where the cooling results in some of the
water in the stream being condensed out and removed prior to
introduction of the stream into the water removal system) and/or
increase the efficiency with which said system removes water (as,
for example, where the water removal system is an adsorptive
system, where the adsorbent can adsorb more water at lower
temperatures). Cooling the water-containing natural gas stream
prior to its introduction into water removal system 13 also allows
the temperature of the natural gas being fed into the water removal
system 13 to be controlled, and so avoids operational difficulties
as could otherwise result from the temperature of the natural gas
straying above the temperature at which the water removal system 13
is designed to operate (which could result in inadequate removal of
water from the natural gas in system 13, and accordingly
unacceptable levels of water in the natural gas downstream of said
system).
[0064] Moreover (and as is demonstrated in the Examples that
follow), the inventors have found that by cooling the
water-containing natural gas stream 10 in economizer heat exchanger
11 against the dried cooled natural gas stream 16 the overall
efficiency of the drying and liquefaction process can be improved.
The use of econmizer heat exchanger 11 significantly reduces the
cooling duty required of natural gas feed cooling system 15, as a
significant proportion of the cooling duty for cooling the
water-containing natural gas stream 10 prior to water removal in
water removal system 13 is in this case supplied by recovering the
cold from the dried cooled natural gas (i.e. from stream 16) after
water removal. Although this also means that the natural gas stream
(i.e. rewarmed natural gas stream 17) that is fed into the MCHE 1
is warmer than would otherwise be the case if the dried cooled
natural gas stream 16 were instead to be fed directly into the MCHE
1, the inventors have nevertheless found that the overall power
consumption of the process is still reduced (in particular where
the refrigerant used by the feed cooling system 15 is a stream of
refrigerant from the same closed loop of refrigerant that is used
to provide cooling duty in the MCHE 1). Where the feed cooling
system 15 is a packaged chiller that uses its own, separate,
refrigeration loop, the reduction in the cooling duty required of
feed cooling system 15 may also allow a smaller packaged chiller to
be used, thereby allow for capital cost savings.
[0065] Furthermore, the heating of dried cooled natural gas stream
16 in economizer heat exchanger 11 to provide a dried rewarmed
natural gas (i.e. stream 17) at a similar temperature to that of
the compressed refrigerant (i.e. streams 3) also entering the warm
end of the MCHE 1 allows any temperature mismatch between the
streams entereing the warm end of the MCHE 1 to be minimized. This
in turn minimizes mechanical stresses that would otherwise occur
(in particular in a wound coil heat exchanger) due to differential
thermal expansion of components at the warm end of the MCHE 1, and
thus minimizes the potential for damage to the MCHE 1 as a result
thereof. In certain types of MCHE, such as a MCHE of the brazed
aluminum coil type, a possible alternative arrangement (not in
accordance with the present invention) for avoiding any such
temperature mismatch would be to introduce the dried natural gas
stream into the MCHE at a colder temperature than that of the
compressed refrigerant streams and at a different location, more
towards the cold end of the exchanger, through a so-called
side-header instead of using an economizer heat exchanger to rewarm
the natural gas stream after it has been dried. However, the use of
side-headers makes the manufacturing of such an MCHE more
complicated, and thus this is also undesirable.
[0066] Referring now to FIG. 2, an exemplary closed loop
refrigerant system and process (indicated as system 2 in FIG. 1) is
depicted that may be employed in the system depicted in FIG. 1. The
closed loop system and process in this case contains and uses a
mixed-refrigerant, and comprises, in addition to the MCHE 1,
throttle valves 38 and 39 and feed cooler 15 (all as previously
described), a refrigerant compression and cooling system
(comprising refrigerant compressor 21, refrigerant cooler 22, and
phase separator 23) and further throttle valves 28 and 29.
[0067] In this case, the two feed streams of compressed refrigerant
3 that are introduced into the warm end of MCHE 1 are a liquid
stream of mixed-refrigerant 27 and a vapor stream of mixed
refrigerant 24. The vapor stream 24 is cooled and partially or
fully liquefied and withdrawn as a compressed cold (liquid or
mixed-phase) refrigerant stream from the cold end of the MCHE 1,
and the liquid stream is cooled and withdrawn as a compressed cold
(liquid) refrigerant stream from the intermediate location of the
MCHE 1. These streams are then adiabatically expanded across
throttle valves 38 and 39, respectively, to provide expanded cold
refrigerant streams (that may be at least partially vaporized as a
result of said expansion) that are returned to the cold end and
intermediate location, respectively, of the MCHE 1.
[0068] The expanded cold mixed refrigerant flows through MCHE (i.e.
through the shell side in the case of a wound-core or other type of
shell and tube heat exchanger) and is warmed (and, if still liquid
or mixed phase, vaporized) by indirect heat exchange with the
natural gas in stream 17 and compressed mixed refrigerant in
streams 3. The expanded warmed refrigerant (i.e. the mixed
refrigerant obtained after it has undergone heat exchange with the
natural gas in stream 17 and compressed mixed refrigerant in
streams 3) is collected and withdrawn from the warm end of the heat
exchanger as stream 20.
[0069] Stream 20 is compressed in refrigerant compressor 21, cooled
in refrigerant cooler 22, and separated into a liquid stream 25
(MRL) and vapor stream 24 (MRV) in phase separator 23. Although in
the illustrated embodiment refrigerant cooler 22 is depicted as a
after-cooler separate from refrigerant compressor 21, refrigerant
compressor 21 could be a multi-stage compressor and in this case
refrigerant cooler 22 could comprise one or a series of
intercoolers, in addition to or instead of an after-cooler.
Refrigerant cooler 22 may, for example, also comprise a pre-cooler
for cooling stream 20 prior to compression in refrigerant
compressor 21. The vapor stream 24 from the phase separator 23 is
then introduced into the cold end of the MCHE 1 as the
aforementioned vapor stream of compressed mixed refrigerant (that
is cooled and partially or fully liquefied in the MCHE 1 and
withdrawn from the cold end thereof). The liquid stream 25 from the
phase separator 23 is split into liquid stream 27 (a major portion)
and liquid stream 26. Liquid stream 27 is introduced into the cold
end of the MCHE 1 as the aforementioned liquid stream of compressed
mixed refrigerant (that is cooled in the MCHE 1 and withdrawn from
the intermediate location thereof).
[0070] Liquid steam 26 of mixed refrigerant is expanded across
throttle valve 28 to further cool (and in this case at least
partially vaporize) the stream, and this further cooled stream is
used in feed cooler 15 to provide refrigeration (via indirect heat
exchange) for further cooling dried natural gas stream 14 to
produce dried cooled natural gas stream 16. The resulting warmed
(and now fully vaporized) mixed refrigerant stream exiting feed
cooler 15 may optionally be back-pressured by valve 29, and is then
recombined at the suction of refrigerant compressor 21 with the
expanded warmed refrigerant stream 20 withdrawn from the warm end
of MCHE 1.
[0071] Referring now to FIG. 3, an alternative exemplary apparatus
and method of drying and liquefying a natural gas stream is
depicted, which is modified from that depicted in FIG. 1 in two
main respects (either of which modifications to the method of FIG.
1 could be made independently, as well as in combination as is
shown in FIG. 3).
[0072] The first modification is that, in the apparatus and method
depicted in FIG. 3, the dried natural gas stream exiting natural
gas feed water removal system 13 is first returned to economizer
heat exchanger 11 for further cooling therein prior to being sent
to the natural gas feed cooling system. Thus, as shown in FIG. 3,
the water-containing natural gas feed stream 10 is first cooled in
economizer heat exchanger 11 to produce cooled water-containing
natural gas feed stream 12 that is, in this case, withdrawn from an
intermediate location of the economizer heat exchanger 11. The
cooled water-containing natural gas feed stream 12 is fed to water
removal system 13 to cold dry said stream, thereby producing dried
natural gas stream 14 which is then returned to the intermediate
location of the economizer heat exchanger 11 and further cooled
therein prior to being withdrawn from the cold end of the
economizer heat exchanger 11 as stream 30 and sent to the natural
gas feed cooling system.
[0073] The second modification is that in, in the apparatus and
method depicted in FIG. 3, the natural gas feed cooling system uses
a stream of liquefied natural gas 34 obtained from the MCHE 1 as
the colder-than-ambient stream for cooling the dried natural gas
stream 14/30. Natural gas feed cooling system could again be an
indirect heat exchange system but, in this arrangement, it is
preferred that said natural gas feed cooling system comprises a
scrub column 31 (or other system in which the natural gas stream is
further cooled by direct countercurrent heat exchange with the
liquefied natural gas, thereby also allowing mass transfer to take
place between the countercurrent streams). This allows the method
to be applied to a situation where there is a need to remove heavy
components from natural gas feed prior to liquefaction, as the
scrub column 31 can remove these components in addition to cooling
the natural gas.
[0074] More specifically, dried natural gas stream 30, which has
already been cooled in the economizer heat exchanger 11, enters the
scrub column 31 (which, in this example, is a simple rectifier) and
brought into direct countercurrent contact with a reflux stream of
liquefied natural gas, which both further cools and strips heavy
components from stream 30. Heavy bottoms product is removed as
stream 32. Lighter overhead product, constituting the dried cooled
natural gas stream 33, is then (as before) rewarmed in economizer
heat exchanger 11 and enters the warm end of MCHE 1 as rewarmed
dried natural gas stream 17. The rewarmed dried natural gas stream
17 is partially liquefied (for example in the first wound-coil
bundle of the MCHE) to produce a mixed-phase natural gas stream 34
that is withdrawn from an intermediate location of the MCHE. This
mixed-phase stream 34 is then separated in a reflux drum 35 or
other phase separator into a liquid stream of natural gas 36 and a
vapor stream of natural gas 37. The liquid stream 36 is then
returned to scrub column 31, to provide reflux as described above.
The vapor stream 37 is returned to the intermediate location of the
MCHE and is cooled and liquefied (for example in a second
wound-coil bundle of the MCHE) to produce the liquefied natural gas
(LNG) product stream 18.
[0075] In a possible modification of the system/apparatus and
method depicted in FIG. 3, the MCHE 1 could for example be a
wound-coil heat exchanger that has three bundles (instead of the
two depicted), one for pre-cooling the feed to generate reflux for
the scrub column, one to liquefy it, and one to sub-cool it.
[0076] It will be apparent to those skilled in the art that the
methods and apparatus illustrated in FIGS. 1 to 3 represent also
only some of the possible arrangements. Different MR arrangements
in accordance with the present invention could involve multiple
phase separators, multiple stages of compression, liquid pumps, and
so forth. Any MR liquid stream could be utilized by feed cooler 15
and returned fully or partially vaporized to different locations
within the closed loop MR system. In a closed loop nitrogen recycle
cycle, a portion of gaseous refrigerant could likewise be used for
the same purpose.
EXAMPLE
[0077] Referring to FIG. 1, water-containing natural gas feed
stream 10, comprising 0.8% nitrogen, 88.2% methane, 6.9% ethane,
2.5% propane, and balance heavier hydrocarbons, saturated with
water, and at a pressure of 1024 psia (7060 kPa) and temperature of
118.6.degree. F. (48.1.degree. C.) is to be liquefied. Natural gas
stream 12 leaves the economizer heat exchanger 11 at 71.6.degree.
F. (22.degree. C.). Natural gas stream 14 leaves the water removal
system 13 dry at 78.8.degree. F. (26.degree. C.) (a little warmer
due to heat of absorption). It is then cooled in the feed cooler 15
to 66.1.degree. F. (18.9.degree. C.). The cooling utility fluid
used in feed cooler 15 is a portion of MR withdrawn from the main
MR loop. The MR enters feed cooler 15 as a two-phase stream,
comprising 52.5% vapor, at -76.2.degree. F. (-60.1.degree. C.). It
leaves as a fully vaporized stream at 57.0.degree. F. (13.9.degree.
C.). It contains 1.7% nitrogen, 24.5% methane, 43.7% ethane, 13.7%
propane, and 17.1% isopentane. Dried cooled natural gas stream 16
is warmed back up in the economizer heat exchanger 11 to
115.0.degree. F. (46.1.degree. C.). Dried rewarmed natural gas
stream 17 enters the MCHE 1 and leaves as liquefied stream 18 at
-247.9.degree. F. (-155.5.degree. C.).
[0078] The MR streams (in this case a vapor MR stream and a liquid
MR stream) 3, comprising nitrogen, methane, ethane, propane, and
isopentane, enter the warm end of the MCHE at 116.6.degree. F.
(47.degree. C.), a temperature close to the temperature of the
rewarmed dried natural gas stream 17.
TABLE-US-00001 TABLE 1 Case 1 2 Power % 100.0% 97.6% Cooler Duty %
100.0% 26.6%
[0079] Table 1 compares the current invention to a conventional
prior art arrangement. Case 1 is a conventional SMR cycle producing
about 2 million tons per annum of LNG, with no feed heat exchanger,
and the feed cooler exchanger (necessarily) upstream of the water
removal system. Case 2 is the configuration (according to FIG. 1 of
the present application) described in the above example. As can be
seen, in the present invention the feed cooler duty (i.e. the
cooling duty required of feed cooler 15) is reduced by about 73%,
and the liquefaction power requirement (i.e. the total power
required by the operation of both MCHE 1 and feed cooler 15) is
reduced by 2.4%.
[0080] The quantitative result shown in Table 1 is almost exactly
the same if in the method according to the present invention the
feed cooler 15 is placed upstream instead of downstream of the
water removal system 13. In this case, natural gas stream 12 leaves
the economizer heat exchanger 11 at 83.9.degree. F. (28.8.degree.
C.) and is cooled in feed cooler 15 to 71.6.degree. F. (22.degree.
C.) prior to being fed to water removal system 13, and dried cooled
natural gas stream 16 re-enters economizer heat exchanger 11 at
78.8.degree. F. (26.degree. C.). The same feed cooler duty and
liquefaction power savings (as compared to the conventional prior
art arrangement) are achieved. However, the configuration shown in
FIG. 1 is better suited to avoiding hydrate formation in the feed
as feed cooler 15 (and, thus, mixed refrigerant from the
refrigerant loop providing also cooling duty to the MCHE) is in the
FIG. 1 configuration not being used to cool the natural gas stream
before the drying step has taken place.
[0081] It will be appreciated that the invention is not restricted
to the details described above with reference to the preferred
embodiments, and that numerous modifications and variations can be
made without departing form the spirit or scope of the invention as
defined in the following claims.
* * * * *