U.S. patent application number 11/918162 was filed with the patent office on 2009-03-12 for method and apparatus for liquefying a natural gas stream.
Invention is credited to Cornelis Buijs, Willem Dam, Emilius Carolus Joanes Nicolaas De Jong.
Application Number | 20090064713 11/918162 |
Document ID | / |
Family ID | 34939248 |
Filed Date | 2009-03-12 |
United States Patent
Application |
20090064713 |
Kind Code |
A1 |
Buijs; Cornelis ; et
al. |
March 12, 2009 |
Method and Apparatus for Liquefying a Natural Gas Stream
Abstract
The present invention relates to a method of liquefying a
natural gas stream, wherein the natural gas stream (10) is provided
at a pressure of 10-80 bar, supplied to a gas/liquid separator
(31), and separated into a vaporous stream (40) and a liquid stream
(30). The vaporous stream (40) is compressed to a pressure of at
least 70, 84 bar heat exchanged against the vaporous stream (40),
and liquefied to obtain a liquefied natural gas stream (100).
Inventors: |
Buijs; Cornelis; (The Hague,
NL) ; Dam; Willem; (The Hague, NL) ; De Jong;
Emilius Carolus Joanes Nicolaas; (The Hague, NL) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
34939248 |
Appl. No.: |
11/918162 |
Filed: |
April 10, 2006 |
PCT Filed: |
April 10, 2006 |
PCT NO: |
PCT/EP2006/061470 |
371 Date: |
October 10, 2007 |
Current U.S.
Class: |
62/613 |
Current CPC
Class: |
F25J 1/0035 20130101;
F25J 1/0042 20130101; F25J 1/0214 20130101; F25J 1/0216 20130101;
F25J 2220/62 20130101; F25J 1/0055 20130101; F25J 1/0022 20130101;
F25J 2205/02 20130101; F25J 1/0238 20130101; F25J 2220/64 20130101;
F25J 2230/60 20130101 |
Class at
Publication: |
62/613 |
International
Class: |
F25J 1/02 20060101
F25J001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 12, 2005 |
EP |
05102884.3 |
Claims
1. A method of liquefying a natural gas stream, the method
comprising the steps of: (a) providing a feed stream containing
natural gas at a pressure of 10-80 bar; (b) supplying the feed
stream provided in step (a) to a gas/liquid separator; (c)
separating the feed stream in the gas/liquid separator into a
vaporous stream and a liquid stream, the vaporous stream being
enriched in methane relative to the feed stream, and the liquid
stream being reduced in methane relative to the feed stream; (d)
compressing the vaporous stream obtained in step (c) thereby
obtaining a compressed stream having a pressure of at least 70 bar;
(e) liquefying the compressed stream obtained in step (d) thereby
obtaining a liquefied natural gas stream; wherein the compressed
stream obtained in step (d), before it is liquefied in step (e), is
heat exchanged against the vaporous stream obtained in step (c),
and wherein the pressure of the feed stream as provided in step (a)
is not increased until the compressing in step (d).
2. The method according to claim 1, wherein in step (d) the
pressure is increased to at least 86 bar.
3. The method according to claim 1, wherein the vapour stream
obtained in step (c) has a C.sub.5.sup.+ content of below 0.5 mol
%.
4. The method according to claim 1, wherein the compressed stream
obtained in step (d) is cooled, before it is heat exchanged against
the vaporous stream obtained in step (c).
5. The method according to claim 1, wherein the feed stream
provided in step (a), before supplying to the gas/liquid separator
in step (b), is expanded.
6. The method according to claim 5, wherein an expander for
expanding the feed stream is functionally coupled to a compressor
for compressing the vaporous stream in step (d).
7. An apparatus for liquefying a natural gas stream, the apparatus
at least comprising: means for providing a feed stream containing
natural gas at a pressure of 10-80 bar; a gas/liquid separator for
separating the feed stream into a vaporous stream and a liquid
stream, the vaporous stream being enriched in methane relative to
the feed stream, and the liquid stream being reduced in methane
relative to the feed stream; a compressor for increasing the
pressure of the vaporous stream obtained in the gas/liquid
separator to a pressure of at least 70 bar, thereby obtaining a
compressed stream; a heat exchanger for heat exchanging the
compressed stream against the vaporous stream obtained from the
gas/liquid separator; and a liquefaction unit for liquefying an
effluent from the heat exchanger having a pressure of at least 70
bar, the liquefaction unit comprising at least one cryogenic heat
exchanger.
8. The apparatus according to claim 7, wherein the apparatus
further comprises an expander for expanding the feed stream before
it is supplied to the gas/liquid separator.
9. The apparatus according to claim 8, wherein the compressor and
expander are functionally coupled.
10. The apparatus according to claim 7, wherein no compressor is
present between the means for providing the feed stream at a
pressure of 10-80 bar and the compressor for increasing the
pressure of the vaporous stream.
11. The method according to claim 1, wherein in step (d) the
pressure is increased to at least 84 bar.
12. The method according to claim 1, wherein in step (d) the
pressure is increased to at least 90 bar.
13. A method of liquefying a natural gas stream, the method
comprising the steps of: (a) providing a feed stream containing
natural gas at a pressure of 10-50 bar; (b) supplying the feed
stream provided in step (a) to a gas/liquid separator; (c)
separating the feed stream in the gas/liquid separator into a
vaporous stream and a liquid stream, the vaporous stream being
enriched in methane relative to the feed stream, and the liquid
stream being reduced in methane relative to the feed stream; (d)
compressing the vaporous stream obtained in step (c) thereby
obtaining a compressed stream having a pressure of at least 70 bar;
(e) liquefying the compressed stream obtained in step (d) thereby
obtaining a liquefied natural gas stream; wherein the compressed
stream obtained in step (d), before it is liquefied in step (e), is
heat exchanged against the vaporous stream obtained in step (c),
and wherein the pressure of the feed stream as provided in step (a)
is not increased until the compressing in step (d).
14. The method according to claim 13, wherein in step (d) the
pressure is increased to at least 84 bar.
15. The method according to claim 13, wherein in step (d) the
pressure is increased to at least 86 bar.
16. The method according to claim 13, where in step (d) the
pressure is increased to at least 90 bar.
17. The method according to claim 2, wherein the vapour stream
obtained in step (c) has a C.sub.5.sup.+ content of below 0.5 mol
%.
18. The method according to claim 11, wherein the vapour stream
obtained in step (c) has a C.sub.5.sup.+ content of below 0.5 mol
%.
19. The method according to claim 12, wherein the vapour stream
obtained in step (c) has a C.sub.5.sup.+ content of below 0.5 mol
%.
20. The method according to claim 1, wherein the vapour stream
obtained in step (c) has a C.sub.5.sup.+ content of below 0.1 mol
%.
Description
[0001] The present invention relates to a method of liquefying a
natural gas stream.
[0002] Several methods of liquefying a natural gas stream thereby
obtaining liquefied natural gas (LNG) are known. It is desirable to
liquefy a natural gas stream for a number of reasons. As an
example, natural gas can be stored and transported over long
distances more readily as a liquid than in gaseous form, because it
occupies a smaller volume and does not need to be stored at high
pressures.
[0003] Examples of known methods of liquefying gas are disclosed in
U.S. Pat. No. 6,272,882 and DE 102 26 597 A1.
[0004] According to FIG. 1 of DE 102 26 597 A1 a natural gas stream
having a pressure of 70-100 bar is expanded (Expander X) to a
pressure range of 40-70 bar, cooled (Heat Exchanger E1) and fed to
a Heavy Hydrocarbon (HHC) column (T1). A C.sub.2-rich fraction
taken from the overhead of the HHC column is further cooled (E2)
and fed to a further column (D). The overhead stream of this
further column (D) is pressurized (V) to a pressure in the range of
50-100 bar and subsequently liquefied.
[0005] A problem of the method according to DE 102 26 597 is that
it is unnecessarily complicated. A further problem of the above
method is that it needs a relatively high cooling duty in the heat
exchanger(s) for liquefying the natural gas.
[0006] It is an object of the present invention to minimize the
above problems.
[0007] It is a further object of the present invention to decrease
the total duty of the heat exchangers used for cooling and
liquefying the natural gas.
[0008] It is an even further object of the present invention to
provide an alternative method for liquefying a natural gas
stream.
[0009] One or more of the above or other objects are achieved
according to the present invention by providing a method of
liquefying a natural gas stream, the method comprising the steps
of:
[0010] (a) providing a feed stream containing natural gas at a
pressure of 10-80 bar, preferably 10-50 bar;
[0011] (b) supplying the feed stream provided in step (a) to a
gas/liquid separator;
[0012] (c) separating the feed stream in the gas/liquid separator
into a vaporous stream and a liquid stream, the vaporous stream
being enriched in methane relative to the feed stream, and the
liquid stream being reduced in methane relative to the feed
stream;
[0013] (d) compressing the vaporous stream obtained in step (c)
thereby obtaining a compressed stream having a pressure of at least
70, preferably at least 84 bar;
[0014] (e) liquefying the compressed stream obtained in step (d)
thereby obtaining a liquefied natural gas stream;
[0015] wherein the compressed stream obtained in step (d), before
it is liquefied in step (e), is heat exchanged against the vaporous
stream obtained in step (c),
[0016] and wherein the pressure of the feed stream as provided in
step (a) is not increased until the compressing in step (d).
[0017] It has surprisingly been found that using the method
according to the present invention, a significantly increased
recovery of compounds heavier than methane can be obtained. An
important advantage of the present invention is that this can be
achieved in a surprisingly simple manner.
[0018] A further advantage of the present invention is that an
increased production of liquefied natural gas can be obtained using
a given refrigeration power. Thus, for a given refrigeration power
(e.g. using a given line-up comprising one or more cryogenic heat
exchangers, compressors, etc.), the method according to the present
invention provides more LNG than a known process. It has been found
that according to the present invention increases in LNG product as
high as 20% may be obtained, while keeping the refrigeration power
constant.
[0019] It is noted that US 2004/0079107 A1 discloses the heat
exchanging of a compressed stream against a vaporous stream
obtained from a distillation column. However, US 2004/0079107 A1
teaches away from the present invention, as paragraphs [0032] and
[0033] (while referring to FIG. 4) of US 2004/0079107 A1 suggest to
perform the liquefaction at lower pressures. Thus according to US
2004/0079107 A1 it is suggested to heat exchange the vaporous
stream obtained from the distillation column against a compressed
stream which is at a relatively low pressure, which is contrary to
the present invention.
[0020] According to the present invention the natural gas stream
may be any suitable gas stream to be liquefied, but is usually
obtained from natural gas or petroleum reservoirs. As an
alternative the natural gas may also be obtained from another
source, also including a synthetic source such as a Fischer-Tropsch
process.
[0021] Usually the natural gas stream is comprised substantially of
methane. Preferably the feed stream comprises at least 60 mol %
methane, more preferably at least 80 mol %, most preferably the
feed stream comprises at least 90 mol % methane.
[0022] Depending on the source, the natural gas may contain varying
amounts of hydrocarbons heavier than methane such as ethane,
propane, butanes and pentanes as well as some aromatic
hydrocarbons. The natural gas stream may also contain
non-hydrocarbons such as H.sub.2O, N.sub.2, CO.sub.2, H.sub.2S and
other sulphur compounds, and the like.
[0023] If desired, the feed stream containing the natural gas may
be pre-treated before feeding it to the gas/liquid separator. This
pre-treatment may comprise removal of undesired components such as
CO.sub.2 and H.sub.2S, or other steps such as pre-cooling,
pre-pressurizing or the like. As these steps are well known to the
person skilled in the art, they are not further discussed here.
[0024] The gas/liquid separator may be any suitable means for
obtaining a vaporous stream and a liquid stream, such as a
scrubber, distillation column, etc. If desired, two or more
gas/liquid separators may be present.
[0025] The person skilled in the art will readily understand that
the increase in pressure of the vaporous stream may be performed in
various ways, provided that a pressure of at least 70, preferably
at least 84 bar is obtained. Preferably, the pressure in step (d)
is increased by compressing the vaporous stream in a compressor,
thereby obtaining a compressed stream. To this end one or more
compressors may be used.
[0026] Also, the person skilled in the art will understand that the
liquefaction of the pressurized vaporous stream may be performed in
various ways, e.g. using one or more cryogenic heat exchangers.
[0027] Further the person skilled in the art will readily
understand that after liquefaction, the liquefied natural gas may
be further processed, if desired. As an example, the obtained LNG
may be depressurized by means of a Joule-Thomson valve or by means
of a cryogenic turbo-expander. Also, further intermediate
processing steps between the gas/liquid separation and the
liquefaction may be performed.
[0028] Preferably in step (d) the pressure is increased to at least
86 bar, preferably at least 90 bar. Herewith the amount of LNG
product obtained may be increased. As a result of the relatively
high pressure used, the vaporous stream may be supercritical,
depending on the prevailing pressure and the composition of the
respective vaporous stream. Preferably the vaporous stream is
supercritical, as this avoids phase changes in the liquefaction
process.
[0029] Further it is preferred that the vaporous stream obtained in
step (c) has a C.sub.5.sup.+ content of below 0.5 mol %, preferably
below 0.1 mol %. This minimizes operating problems in the
downstream liquefaction unit. With "C.sub.5.sup.+ content" is meant
the content of hydrocarbon components having five or more carbon
atoms.
[0030] Further it is preferred that the compressed stream obtained
in step (d) is cooled, e.g. in an ambient heat exchanger. Further
it is preferred that the compressed stream is heat exchanged
against the vaporous stream obtained in step (c).
[0031] Also it is preferred that the feed stream, before supplying
to the gas/liquid separator in step (b), is expanded. Preferably
the feed stream is expanded to a pressure <(below) 35 bar.
[0032] According to a particularly preferred embodiment of the
method according to the present invention, an expander for
expanding the feed stream is functionally coupled to a compressor
for compressing the vaporous stream. As a result, the power
generated by the expander is used at least partially for driving
the compressor to which it is functionally coupled. Hereby, the
expander and compressor form a so-called "compressor-expander
scheme", as a result of which the energy consumption of the whole
process is minimized. As the person skilled in the art will readily
understand what is meant with a "compressor-expander scheme", this
is not further discussed here.
[0033] In a further aspect the present invention relates to LNG
product obtained by the method according to the present invention,
in particular liquefied methane.
[0034] In an even further aspect the present invention relates to
an apparatus suitable for performing the method according to the
present invention, the apparatus at least comprising:
[0035] means for providing a feed stream containing natural gas at
a pressure of 10-80 bar, preferably 10-50 bar;
[0036] a gas/liquid separator for separating the feed stream into a
vaporous stream and a liquid stream, the vaporous stream being
enriched in methane relative to the feed stream, and the liquid
stream being reduced in methane relative to the feed stream;
[0037] a compressor for increasing the pressure of the vaporous
stream obtained in the gas/liquid separator to a pressure of at
least 70, preferably at least 84 bar, thereby obtaining a
compressed stream;
[0038] a heat exchanger for heat exchanging the compressed stream
against the vaporous stream obtained from the gas/liquid separator;
and
[0039] a liquefaction unit for liquefying an effluent from the heat
exchanger having a pressure of at least 70, preferably at least 84
bar, the liquefaction unit comprising at least one cryogenic heat
exchanger.
[0040] Preferably, the apparatus further comprises an expander for
expanding the feed stream.
[0041] According to a particularly preferred embodiment, the
compressor and expander are functionally coupled, thereby forming a
so-called "compressor-expander scheme".
[0042] Hereinafter the invention will be further illustrated by the
following non-limiting drawing. Herein shows:
[0043] FIG. 1 schematically a process scheme in accordance with an
embodiment of the present invention; and
[0044] FIG. 2 schematically a process scheme in accordance with
another embodiment of the present invention.
[0045] For the purpose of this description, a single reference
number will be assigned to a line as well as a stream carried in
that line. Same reference numbers refer to similar components.
[0046] FIG. 1 schematically shows a base load liquefied natural gas
(LNG) export process and an apparatus (generally indicated with
reference number 1) suitable for performing the same. A feed stream
10 containing natural gas is supplied to a gas/liquid separator 31
at a certain inlet pressure and inlet temperature. In the
embodiment of FIG. 1 the feed stream 10 is pre-cooled against a
refrigerant in a heat exchanger 11. Typically, the inlet pressure
to heat exchanger 11 will be between 10 and 80 bar (preferably
<(below) 50 bar), and the temperature will be close to ambient
temperature, usually between 5 and 50.degree. C.
[0047] If desired the feed stream 10 may have been pre-treated
before it is fed to the separator 31. As an example, the feed
stream 10 may be expanded (as also shown in the embodiment of FIG.
2 hereafter; in expander 12).
[0048] As mentioned above, in the embodiment of FIG. 1, the feed
stream 10 is pre-cooled against a refrigerant in a heat exchanger
11, or in a train of heat exchangers, for instance comprising two
or more heat exchangers operating at different refrigerant pressure
levels. The pre-cooled feed stream in line 20 is at a pre-cooling
temperature that is lower than the temperature in line 10. The
pre-cooling temperature is chosen to form a partially condensed
feed stream 20. Further, the pre-cooling temperature is chosen to
optimise a subsequent separation step in separator 31.
[0049] As mentioned above, stream 20 is fed to the gas/liquid
separator 31. There the feed stream in line 20 is separated into a
vaporous overhead stream 40 and a liquid bottom stream 30. The
overhead stream 40 is enriched in methane (and usually also ethane)
relative to the feed stream 20.
[0050] The bottom stream 30 is generally liquid and usually
contains some components that are freezable when they would be
brought to a temperature at which methane is liquefied. Separator
31 can be a separator vessel or a distillation column such as a
scrub column, depending on the separation required to remove
freezable components from the feed stream. Typically the freezable
components are CO.sub.2, H.sub.2S and hydrocarbon components having
the molecular weight of pentane or higher. These freezable
components may also at least partially have been removed from the
feed stream before entering the separator 31.
[0051] The bottom stream 30 may also contain hydrocarbons that can
be separately processed to form liquefied petroleum gas (LPG)
products.
[0052] Usually, the bottom stream 30 is subjected to one or more
fractionation steps to collect various natural gas liquid
products.
[0053] The overhead stream 40 is led through an effluent stream
heat exchanger 41, where it is indirectly heated against a stream
of about ambient temperature (stream 70). Stream 50, which is
discharged from the effluent stream heat exchanger 41 is then
compressed via compressor 51 or a train of two or more compressors.
The compressed stream is discharged at a pressure above 84 bar into
line 60. The pressure-increase in this compression step is chosen
between 30 bar and 150 bar, depending on the choices of
respectively the separation pressure and the liquefaction
pressure.
[0054] Part of the heat added during this compression step is
removed from stream 60 against the ambient, for instance using an
air cooler 61 or a water cooler. The resulting ambient-cooled
stream 70 is then led to the effluent stream heat exchanger 41
where it is cooled in indirect heat exchange with the cold overhead
stream 40.
[0055] The cold stream 80 is then further cooled in one or more
external cooling stages. This may include a pre-cooling stage, here
depicted as heat exchanger 81. A train of subsequent heat
exchangers may be employed instead.
[0056] A pre-cooled stream 90 is then further cooled into
liquefaction in a liquefaction unit (generally indicated by
reference number 5) at least comprising a main cryogenic heat
exchanger 91. Any suitable type of heat exchanger may be employed.
Here depicted is a cryogenic heat exchanger 91 operated by a mixed
refrigerant, of which light and heavy fractions are first
autocooled in tubes running parallel to the pre-cooled stream (not
shown) and then expanded to the shell side via inlet means 95 and
96 respectively. The spent heavy and light fractions are drawn from
the shell side of the main cryogenic heat exchanger 91 via outlet
97. The spent refrigerant in line 97 can be recompressed to form a
liquid, or, in case of a mixed refrigerant, a mixed vaporous light
fraction and liquid heavy fraction.
[0057] Referring again to stream 60, the liquefaction pressure is
chosen to exceed a pressure of at least 70, preferably at least 84
bar, more preferably above 86 bar. As a result, the vapour in
stream 60 may be in a supercritical condition.
[0058] As a next step, the liquefied stream leaving the main
cryogenic heat exchanger 91 via line 100 is further cooled in a
flash step wherein the pressure is let down via a valve or liquid
expander 101. Suitably the pressure after expanding is about
atmospheric. Expansion heat is extracted from the liquefied stream,
so that the temperature is further lowered to a temperature under
which the liquefied product remains liquid at atmospheric pressure.
Flash gas 130, typically containing nitrogen and some methane, is
separated from the stream 110 in flash tank 111. A part of the
flash gas 130 can be employed as fuel gas for providing energy to
the liquefaction process. The liquid part of stream 110 is
discharged from the bottom of flash tank 111 in line 120. This can
be stored and transported as LNG.
[0059] Table I gives an overview of the pressures and temperatures
of a stream at various parts in an example process of FIG. 1. Also
the mol % of methane is indicated. The feed stream in line 10 of
FIG. 1 comprised approximately the following composition: 85%
methane, 6% ethane, 4% propane, 2% butanes, 1% C.sub.5.sup.+ and 2%
N.sub.2. Freezable components such as H.sub.2S, CO.sub.2 and
H.sub.2O were previously removed.
TABLE-US-00001 TABLE I Temperature Mol % Line Pressure (bar)
(.degree. C.) methane 10 37 32 85 20 36.8 -42 85 40 36.8 -42 90 50
36.4 38 90 60 86 125 90 70 85.9 40 90 80 85.5 -38 90 90 85.3 -50 90
100 85.0 -151 90 110 1 -161 90
[0060] FIG. 2 schematically depicts an alternative embodiment of
the process according to the invention. In this embodiment, the
feed stream 10 is expanded in an expander 12 to a pressure below 35
bar before entering the separator 31 as stream 25.
[0061] Preferably, the compressor train 51 uses expansion energy
from at least expander 12. To this end at least one compressor of
the compressor train 51 is functionally coupled to the expander 12
thereby forming a so-called "compressor-expander scheme".
Additional compression power may however be provided to achieve a
pressure above 84 bar. Preferably, the additional compressor motor
power consumed by the compressor 51 is chosen close to or identical
to the power required by the refrigerant compressors so that
identical drivers can be employed for both purposes thereby
providing cost and maintenance benefits.
[0062] Table II gives an indication of decrease in cooling duty in
the heat exchangers for cooling and liquefaction of the natural gas
using the process as described in FIG. 1 according to the present
invention. As a comparison the same line-up as FIG. 1 was used,
but--in contrast to the present invention--no heat exchanging took
place in heat exchanger 41. As shown in Table II the present
invention results in a significantly decreased cooling duty of
about 10%.
TABLE-US-00002 TABLE II FIG. 1 without heat exchanger 41 FIG. 1
(invention) (comparison) Cooling duty in 2.27 3.25 heat exchanger
81 [MW] Cooling duty in 6.38 6.34 heat exchanger 91 [MW] Total [MW]
8.65 9.59
* * * * *