U.S. patent application number 12/446543 was filed with the patent office on 2010-12-23 for method and apparatus for liquefying hydrocarbon streams.
Invention is credited to Willem Dam, Ming Teck Kong, David Bertil Runbalk.
Application Number | 20100319396 12/446543 |
Document ID | / |
Family ID | 37898417 |
Filed Date | 2010-12-23 |
United States Patent
Application |
20100319396 |
Kind Code |
A1 |
Dam; Willem ; et
al. |
December 23, 2010 |
METHOD AND APPARATUS FOR LIQUEFYING HYDROCARBON STREAMS
Abstract
A method of cooling at least two hydrocarbon streams such as
natural gas, the method at least comprising the steps of: (a)
providing at least first and second hydrocarbon streams (20, 20a);
(b) passing the first hydrocarbon stream (20) through one or more
first heat exchangers (12, 14) to provide a first cooled
hydrocarbon stream (30); and (c) passing the second hydrocarbon
stream (20a) through one or more second heat exchangers (12a, 14a)
to provide a second cooled hydrocarbon stream (30a); wherein a
refrigerant circuit (100) provides cooling to the first heat
exchanger (s) (12, 14) and the second heat exchanger (s) (12a,
14a).
Inventors: |
Dam; Willem; (NL-2596 HR The
Hague, NL) ; Kong; Ming Teck; (NL-2596 HR The Hague,
NL) ; Runbalk; David Bertil; (NL-2596 HR The Hague,
NL) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
37898417 |
Appl. No.: |
12/446543 |
Filed: |
October 23, 2007 |
PCT Filed: |
October 23, 2007 |
PCT NO: |
PCT/EP07/61316 |
371 Date: |
April 21, 2009 |
Current U.S.
Class: |
62/613 |
Current CPC
Class: |
F25J 1/0042 20130101;
F25J 1/0055 20130101; F25J 1/0057 20130101; F25J 1/0292 20130101;
F25J 1/0271 20130101; F25J 2220/62 20130101; F25J 1/0052 20130101;
F25J 1/0022 20130101; F25J 1/0294 20130101; F25J 1/0214
20130101 |
Class at
Publication: |
62/613 |
International
Class: |
F25J 1/00 20060101
F25J001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 23, 2006 |
EP |
06122725.2 |
Claims
1. A method of liquefying at least two hydrocarbon streams, the
method at least comprising the steps of: (a) providing at least
first and second hydrocarbon streams; (b) passing the first
hydrocarbon stream through one or more first heat exchangers to
provide a first cooled hydrocarbon stream; (c) passing the second
hydrocarbon stream through one or more second heat exchangers to
provide a second cooled hydrocarbon stream; and (d) subsequently
liquefying said first and second cooled hydrocarbon streams;
wherein a refrigerant circuit provides cooling to the one or more
first heat exchangers and the one or more second heat exchangers,
by passing separate streams of refrigerant through the one or more
first heat exchangers of step (b) and through the one or more
second heat exchangers of step (c), and subsequently compressing
the refrigerant streams and commonly cooling the compressed
refrigerant streams in one or more common coolers.
2. A method as claimed in claim 1, wherein step (b) comprises
passing the first hydrocarbon stream through 2, 3, 4 or 5 first
heat exchangers; and wherein step (c) comprises passing the second
hydrocarbon stream through 2, 3, 4 or 5 second heat exchangers.
3. A method as claimed in claim 1, wherein said subsequently
compressing the refrigerant streams comprises separately
compressing each separate stream of refrigerant.
4. A method as claimed in claim 3, further comprising combining the
separately compressed streams of refrigerant prior to said commonly
cooling in said one or more common coolers.
5. A method as claimed in claim 1, wherein the passing of steps (b)
and (c) forms part of a first cooling stage, and wherein the
subsequently liquefying of step (d) comprises further cooling said
first and second cooled hydrocarbon streams in a second cooling
stage.
6. A method as claimed in claim 1, wherein the first and second
cooled hydrocarbon streams in step (d) are liquefied as separate
streams.
7. A method as claimed in claim 1, wherein the first and second
hydrocarbon streams are feed streams.
8. Apparatus for liquefying at least two hydrocarbon streams, the
apparatus at least comprising: one or more first heat exchangers to
cool a first hydrocarbon stream and to provide a first cooled
hydrocarbon stream; one or more second heat exchangers to cool a
second hydrocarbon stream and to provide a second cooled
hydrocarbon stream; at least one liquefaction system arranged to
liquefy said first and second cooled hydrocarbon streams; and a
refrigerant circuit comprising at least two separate streams of
refrigerant one of which to provide cooling to the one or more
first heat exchangers and the other one of which to provide cooling
to the one or more second heat exchangers; at least one compressor
for compressing the refrigerant streams; and one or more common
coolers for commonly cooling the compressed refrigerant
streams.
9. Apparatus as claimed in claim 8, further comprising a stream
splitter to divide a feed stream into at least the first and second
hydrocarbon streams.
10. Apparatus as claimed in claim 8, further comprising at least
one separate compressor in each of the separate streams of
refrigerant.
11. Apparatus as claimed in claim 10, further comprising a combiner
arranged downstream of the separate compressors and upstream of the
one or more common coolers for combining the separate compressed
refrigerant streams.
12. A method of liquefying at least two hydrocarbon streams, the
method at least comprising the steps of: (a) providing at least
first and second hydrocarbon streams; (b) passing the first
hydrocarbon stream through one or more first heat exchangers to
provide a first cooled hydrocarbon stream; (c) passing the second
hydrocarbon stream through one or more second heat exchangers to
provide a second cooled hydrocarbon stream; and (d) subsequently
liquefying said first and second cooled hydrocarbon streams;
wherein a refrigerant circuit provides cooling to the one or more
first heat exchangers and the one or more second heat exchangers,
by passing separate streams of refrigerant through the one or more
first heat exchangers of step (b) and through the one or more
second heat exchangers of step (c), separately compressing each
separate stream of refrigerant after passing through the at least
one or more first heat exchangers and respectively the at least one
or more second heat exchangers, and combining the compressed
separate streams of refrigerant.
13. Apparatus for liquefying at least two hydrocarbon streams, the
apparatus at least comprising: one or more first heat exchangers to
cool a first hydrocarbon stream and to provide a first cooled
hydrocarbon stream; one or more second heat exchangers to cool a
second hydrocarbon stream and to provide a second cooled
hydrocarbon stream; at least one liquefaction system arranged to
liquefy said first and second cooled hydrocarbon streams; and a
refrigerant circuit comprising at least two separate streams of
refrigerant one of which to provide cooling to the one or more
first heat exchangers and the other one of which to provide cooling
to the one or more second heat exchangers and at least one separate
compressor in each of the separate streams of refrigerant and a
combiner arranged downstream of the separate compressors for
combining the compressed separate streams of refrigerant.
14. A method as claimed in claim 2, wherein step (b) comprises
passing the first hydrocarbon stream through two first heat
exchangers; and wherein step (c) comprises passing the second
hydrocarbon stream through two second heat exchangers.
15. A method as claimed in claim 2, wherein said subsequently
compressing the refrigerant streams comprises separately
compressing each separate stream of refrigerant.
16. A method as claimed in claim 14, wherein said subsequently
compressing the refrigerant streams comprises separately
compressing each separate stream of refrigerant.
17. A method as claimed in claim 15, further comprising combining
the separately compressed streams of refrigerant prior to said
commonly cooling in said one or more common coolers.
18. A method as claimed in claim 16, further comprising combining
the separately compressed streams of refrigerant prior to said
commonly cooling in said one or more common coolers.
19. A method as claimed in claim 2, wherein the passing of steps
(b) and (c) forms part of a first cooling stage, and wherein the
subsequently liquefying of step (d) comprises further cooling said
first and second cooled hydrocarbon streams in a second cooling
stage.
20. A method as claimed in claim 3, wherein the passing of steps
(b) and (c) forms part of a first cooling stage, and wherein the
subsequently liquefying of step (d) comprises further cooling said
first and second cooled hydrocarbon streams in a second cooling
stage.
Description
[0001] The present invention relates to a method and apparatus for
liquefying at least two hydrocarbon streams, such as at least two
natural gas streams.
[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 a high
pressure.
[0003] Usually natural gas, comprising predominantly methane,
enters an LNG plant at elevated pressures and is pre-treated to
produce a purified feed stock suitable for liquefaction at
cryogenic temperatures. The purified gas is processed through a
plurality of cooling stages using heat exchangers to progressively
reduce its temperature until liquefaction is achieved. The liquid
natural gas is then further cooled and expanded through one or more
expansion stages to final atmospheric pressure suitable for storage
and transportation. The flashed vapour from each expansion stage
can be used as a source of plant fuel gas.
[0004] The costs in creating and running a liquefying natural gas
(LNG) plant or system are naturally high, and a significant part is
for the cooling configurations. Any reduction in the energy
requirements of the plant or system has significant cost benefit.
Reducing any cost of any cooling configuration is particularly
advantageous.
[0005] U.S. Pat. No. 6,272,882 B1 relates to a process of
liquefying a gaseous, methane-enriched feed to obtain a liquefied
product. The liquefaction process comprises a number of steps, one
of which is to separate the partly-condensed refrigerant for the
main heat exchanger into a liquid heavy refrigerant fraction and a
gaseous light refrigerant fraction. At least part of the liquid
refrigerant fraction is cooled, liquefied and sub-cooled against
off-gas removed from a flash vessel used after the main heat
exchanger. The process of U.S. Pat. No. 6,272,882 B1 shows a single
`train` for liquefaction.
[0006] U.S. Pat. No. 6,389,844 B1 relates to a plant for liquefying
natural gas. More specifically, a pre-cooled dual heat exchanger,
dual refrigerant system. The plant in U.S. Pat. No. 6,389,844 B1
has a liquefaction capacity which is 40 to 60% higher than that of
a single liquefaction train, and comprises one pre-cooling heat
exchanger, and at least two main heat exchangers. Each main heat
exchanger uses a main refrigerant which is separated into a heavy
liquid fraction and a light gaseous fraction which are only seen to
be cooled in the main heat exchanger, prior to expansion.
[0007] It is an object of the present invention to improve the
efficiency of a treatment plant or method, in particular of a
liquefaction plant or method.
[0008] It is a further object of the present invention to reduce
the energy requirements of a treatment plant or method, in
particular of a liquefaction plant or method.
[0009] It is another object of the present invention to provide an
alternative method and apparatus for treating a hydrocarbon stream,
in particular for liquefying a hydrocarbon stream.
[0010] The present invention provides a method of liquefying at
least two hydrocarbon streams, such as at least two natural gas
streams, the method at least comprising the steps of:
(a) providing at least first and second hydrocarbon streams; (b)
passing the first hydrocarbon stream through one or more first heat
exchangers to provide a first cooled hydrocarbon stream; (c)
passing the second hydrocarbon stream through one or more second
heat exchangers to provide a second cooled hydrocarbon stream; and
(d) subsequently liquefying said first and second cooled
hydrocarbon streams.
[0011] A refrigerant circuit provides cooling to the one or more
first heat exchangers and the one or more second heat exchangers,
by passing separate streams of refrigerant through the one or more
first heat exchangers of step (b) and through the one or more
second heat exchangers of step (c).
[0012] The separate refrigerant streams, after respectively passing
through the first heat exchanger(s) the second heat exchanger(s)
and/or having exchanged heat with the respective hydrocarbon
streams, may be compressed, either commonly or separately.
[0013] The compressed refrigerant streams may subsequently be
commonly cooled in one or more common coolers. In this way, there
can be a reduction in the number of coolers required, reducing
capital and running costs.
[0014] In a further aspect, the present invention provides an
apparatus for liquefying at least two hydrocarbon streams, such as
at least two natural gas streams, the apparatus at least
comprising:
[0015] one or more first heat exchangers to cool a first
hydrocarbon stream and to provide a first cooled hydrocarbon
stream;
[0016] one or more second heat exchangers to cool a second
hydrocarbon stream and to provide a second cooled hydrocarbon
stream;
[0017] at least one liquefaction system arranged to liquefy said
first and second cooled hydrocarbon streams; and
[0018] a refrigerant circuit comprising at least two separate
streams of refrigerant one of which to provide cooling to the one
or more first heat exchangers and the other one of which to provide
cooling to the one or more second heat exchangers.
[0019] The refrigerant circuit may further be provided with at
least one compressor for compressing the refrigerant streams,
either commonly or separately, after having provided their cooling
to the first and second heat exchanger(s).
[0020] The refrigerant circuit may further be provided with one or
more common coolers for commonly cooling the compressed refrigerant
streams.
[0021] Embodiments of the present invention will now be described
by way of example only, and with reference to the accompanying
non-limiting schematic drawings in which:
[0022] FIG. 1 is a generalised scheme of part of a liquefaction
plant according to one embodiment of the present invention; and
[0023] FIG. 2 is a more detailed scheme of the liquefaction plant
in FIG. 1.
[0024] 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,
streams or lines.
[0025] Described herein are methods and apparatuses wherein two
hydrocarbon streams are cooled against a refrigerant in a
refrigerant circuit, by passing a stream of the refrigerant through
one or more first heat exchangers and passing a separate stream of
the refrigerant through one or more second heat exchangers and
subsequently compressing the separate streams of the refrigerant
using at least one compressor.
[0026] By using a, preferably one, refrigerant circuit to provide
cooling to the first heat exchanger(s) and the second heat
exchanger(s), a reduction in the capital costs and running costs
can be provided by commonality of elements and features in a single
refrigerant circuit serving the different heat exchangers for the
two hydrocarbon streams.
[0027] The refrigerant circuit may involve any number of separate
lines or streams of refrigerant to cool different hydrocarbon
streams, and any number of common elements or features, including
compressors, coolers, etc. Some refrigerant streams may be common
and some may be separate.
[0028] The first and second heat exchangers may be separate, and
any alteration in existing first and second heat exchanger
arrangements can be avoided to effect the invention.
[0029] Embodiments of the invention comprise common cooling of two
or more compressed refrigerant streams.
[0030] The compressed refrigerant streams may be combined for
commonly cooling through the one or more common coolers. Usually,
the common or combined cooling will involve the refrigerant of the
refrigeration circuit being condensed. The common cooling equates
to common heat rejection from the separate refrigerant streams
after their compression. One or more other coolers, being separate
or integrated, may also be involved or associated with the
compressors as is known in the art.
[0031] By using some, preferably the majority, of common cooling in
the refrigerant circuit, the present invention can reduce the
overall energy requirements of a method or plant or apparatus for
treating, in particular liquefying, a hydrocarbon stream, and/or
make the method, plant or apparatus more efficient and so more
economical.
[0032] The refrigerant of the refrigerant circuit may be a single
component such as propane. Preferably it is a mixed refrigerant
based on two or more components, said components preferably
selected from the group comprising nitrogen, methane, ethane,
ethylene, propane, propylene, butane and pentane.
[0033] In a step (b) of the methods described herein, a first
hydrocarbon stream is passed through one or more first heat
exchangers to provide a first cooled hydrocarbon stream, while in a
step (c) a second hydrocarbon stream is passed through one or more
second heat exchangers to provide a second cooled hydrocarbon
stream.
[0034] In embodiments of the present invention, each of the steps
(b) and (c) comprises passing the hydrocarbon stream through 2, 3,
4 or 5 first and second heat exchangers, preferably two first heat
exchangers and two second heat exchangers.
[0035] The first and second cooled hydrocarbon streams could be
further treated, for example liquefied.
[0036] Preferably, the first and second hydrocarbon streams are
feed streams, preferably provided from a single feed stream. Where
the first and second hydrocarbon streams are provided in this way,
they may be equally or unequally divided; preferably they are the
same. The feed stream can be divided by any suitable divider,
stream splitter, or similar known in the art.
[0037] In general, a feed stream or streams can be liquefied by
passing it through at least two cooling stages. Any number of
cooling stages can be used, and each cooling stage can involve one
or more heat exchangers, as well as optionally one or more steps,
levels or sections. Each cooling stage may involve two or more heat
exchangers either in series, or in parallel, or a combination of
same.
[0038] Arrangements of suitable heat exchangers able to cool and
liquefy a feed stream such as a hydrocarbon stream such as natural
gas are known in the art.
[0039] In one arrangement, this involves the two cooling stages
comprising a first cooling stage and a second cooling stage, the
first stage being preferably a pre-cooling stage to cool below
0.degree. C., and the second stage preferably being a main
cryogenic stage to liquefy below -100.degree. C.
[0040] In a particular embodiment of the present invention, the
method of treating hydrocarbon streams is part of a method of
liquefying a hydrocarbon stream such as natural gas from a feed
stream, wherein the method of treating comprises a first cooling
stage, and there is a subsequent second cooling stage for
liquefying the first and second cooled hydrocarbon streams.
[0041] The hydrocarbon streams may be any suitable
hydrocarbon-containing streams to be liquefied, but they are
usually from a natural gas stream obtained from natural gas or
petroleum reservoirs. As an alternative the natural gas stream may
also be obtained from another source, also including a synthetic
source such as a Fischer-Tropsch process.
[0042] Usually natural gas is comprised substantially of methane.
Preferably the feed stream comprises at least 60 mol % methane,
more preferably at least 80 mol % methane.
[0043] 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 sulfur compounds, and the like.
[0044] If desired, the hydrocarbon streams may be pre-treated
before using them in the present invention. This pre-treatment may
comprise removal of any undesired components present 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.
[0045] Although the method according to the present invention is
applicable to various hydrocarbon feed streams, it is particularly
suitable for natural gas streams to be liquefied. As the person
skilled readily understands how to liquefy a hydrocarbon stream,
this is not further discussed in detail herein.
[0046] 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.
[0047] The present invention may involve one or more other or
further refrigerant circuits, for example in or passing through a
first cooling stage. Any other or further refrigerant circuits
could optionally be connected with and/or concurrent with the
refrigerant circuit for cooling the first and second hydrocarbon
streams.
[0048] FIG. 1 shows a general arrangement of part of a liquefied
natural gas (LNG) plant. It shows an initial feed stream 10
containing natural gas. In addition to methane, natural gas usually
includes some heavier hydrocarbons and impurities, e.g. carbon
dioxide, nitrogen, helium, water and non-hydrocarbon acid gases.
The feed stream 10 has usually been pre-treated to separate out
these impurities as far as possible, and to provide a purified feed
stock suitable for liquefying at cryogenic temperatures.
[0049] The feed stream 10 is divided by a stream splitter 15 to
provide first and second hydrocarbon streams 20, 20a prior to a
first cooling stage 2. The feed stream 10 may be divided into any
number of hydrocarbon streams, and FIG. 1 shows the division into
two hydrocarbon streams by way of preferred example only. The
division of the feed stream 10 could be based on any ratio of mass
and/or volume and/or flow rate. The ratio may be based on the size
or capacity of the subsequent parts of the liquefaction stages or
systems or units, or due to other considerations. One example of
the ratio is an equal division of the feed stream mass.
[0050] In the first cooling stage 2, the first hydrocarbon stream
20 passes through a first set of two first heat exchangers 12, 14
to provide a first cooled hydrocarbon stream 30. The second
hydrocarbon stream 20a passes through a second set of second heat
exchangers 12a, 14a, which may be identical or different to the
first set of first heat exchangers 12, 14, to provide a second
cooled hydrocarbon stream 30a.
[0051] The first heat exchangers 12, 14, and second heat exchangers
12a, 14a, are provided with cooling by a first refrigerant circuit
100. The first refrigerant circuit 100 has two refrigerant streams
101 and 101a which separately cool the first heat exchangers 12, 14
and second heat exchangers 12a, 14a respectively. After providing
their cooling, the refrigerant streams 101, 101a are passed into
one or more separate compressors 32, 32a, before the compressed
refrigerant streams 101d, 101e are combined to provide a single
stream 101f for common heat rejection. The single stream 101f
passes through one or more common water and/or air coolers, two of
which coolers 34, 34a are shown in FIG. 1. The (usually) condensed
refrigerant stream 101g is then divided to provide the separate
refrigerant streams 101, 101a for cooling.
[0052] The first cooling stage 2 may comprise any number of heat
exchangers for each hydrocarbon stream, and the feed stream 10 may
be divided into more than two hydrocarbon streams.
[0053] The first cooling stage 2 will generally cool the first and
second hydrocarbon streams 20,20a to a temperature below 0.degree.
C., and preferably between -20.degree. C. to -60.degree. C.
[0054] In FIG. 1, the first and second cooled hydrocarbon streams
30, 30a pass through a second cooling stage 4, wherein they are
liquefied by two separate liquefaction systems, each generally
including at least one heat exchanger respectively, to provide
separate liquefied streams 40, 40a respectively. Liquefaction
systems and process conditions for liquefaction are well known in
the art, and are not described further herein. In FIG. 1, the two
liquefaction systems are symbolically represented by liquefaction
heat exchangers 16 and 16a. These are also heat exchangers, but
they are referred to as liquefaction heat exchangers merely in
order to label them differently (by their function) from the first
and second heat exchangers discussed hereinabove.
[0055] Each of the liquefaction heat exchangers 16, 16a in the
second cooling stage 4 of the example shown in FIG. 1 uses a
refrigerant circuit: the first liquefaction heat exchanger 16 uses
a first refrigerant circuit 102, and the second liquefaction heat
exchanger 16a uses a second refrigerant circuit 103. Each of these
refrigerant circuits 102, 103 may use the same or different
refrigerants. Preferably, each uses the same refrigerant, and more
preferably the refrigerant for each of the refrigerant circuits
102, 103 is a mixed refrigerant. The mixed refrigerant may be based
on two or more components, preferably selected from the group
comprising nitrogen, methane, ethane, ethylene, propane, propylene,
butane and pentane.
[0056] Generally, the first and second cooled hydrocarbon streams
30, 30a, are cooled by the second cooling stage 4 to a temperature
of at least below -100.degree. C.
[0057] The liquefied streams 40 and 40a are then combined. They may
be combined in any known manner, and in any known combination of
steps. Such combination of streams may be prior to or after any
expansion of any of the liquefied streams 40, 40a. The combining of
the liquefied streams may not require full integration or mixing
for their subsequent passage through a gas/liquid separator.
Preferably the streams are combined before passing through a
gas/liquid separator.
[0058] Arrangements for combining streams are known to the person
skilled in the art. The example arrangement shown in FIG. 1 is for
the combination of the liquefied streams 40, 40a using a combiner
18 known in the art, to provide a combined liquefied hydrocarbon
stream 50. The combiner may be any suitable arrangement, generally
involving a union or junction or piping or conduits, optionally
involving one or more valves.
[0059] The combined liquefied hydrocarbon stream 50 provided by the
second cooling stage 4 can pass through a flash valve (not shown)
and then on to a gas/liquid separator such as an end flash vessel
22, wherein the liquid stream is generally recovered as a liquefied
hydrocarbon product stream 60, and the vapour is provided as a
gaseous stream 70. The liquefied hydrocarbon stream 60 is then sent
by one or more pumps (not shown) to storage and/or transportation
facilities.
[0060] FIG. 2 shows a more detailed scheme of the embodiment of the
present invention shown in FIG. 1, wherein the feed stream 10 is
divided into the first and second hydrocarbon streams 20a, 20b,
which pass through the two separate but parallel and identical sets
of first heat exchangers, 12, 14, and second heat exchangers 12a,
14a, as the first cooling stage 2.
[0061] Both the sets of first and second heat exchangers 12, 14,
12a, 14a are provided with cooling by the one refrigerant circuit
100. The first refrigerant circuit 100 has the two refrigerant
lines 101 and 101a which separately cool the first set of first
heat exchangers 12, 14 at two different pressure levels in a manner
known in the art, and cool the second set of second heat exchangers
12a, 14a at two different pressure levels in a manner known in the
art, respectively.
[0062] After providing their cooling, the refrigerant streams 101,
101a are passed into two sets of compressors 36 and 36a
respectively. Each stream of compressed refrigerant is passed
through separate water and/or air coolers 38, 38a respectively, and
then combined to form a single refrigerant stream 101f. The
separate coolers 38, 38a also provide cooling of the compressors
36, 36a in their recycle operation.
[0063] The single refrigerant stream 101f then passes through a
large water and/or air cooler 34, where the majority of the heat in
the refrigerant is exchanged by being rejected to ambient, as
condensation of the refrigerant takes place. The refrigerant then
passes into an accumulator 42 known in the art. From the
accumulator 42, a stream of refrigerant passes through a final and
usually smaller water and/or air cooler 34a before being divided
into the two refrigerant lines 101 and 101a.
[0064] Preferably, the large cooler 34 provides the same level of
cooling as prior art coolers of separate refrigerant circuits used
hitherto fro cooling two hydrocarbon streams.
[0065] It is noted that not all the cooling of the refrigerant in
the first refrigerant circuit 100 is or need be carried out by a
common cooling unit or units, such as the large cooler 34. The
separate coolers 38 and 38a will provide some initial cooling,
although they are dedicated to their compressors 36 and 36a to
enable their recycling of gas in a manner known in the art. By way
of example only, the ratio of cooling power of the large (and
common) cooler 34 compared to the cooling power of the compressor
coolers 38 and 38a can be from 5:1 up to 20:1 or more; but
preferably approximately a 10:1 ratio. For the present invention,
at least the majority of the cooling of the refrigerant in the
refrigerant circuit 100 is provided by a common cooler or coolers
after recombination of all the separate refrigerant streams (after
their provision of cooling to the hydrocarbon streams in the
relevant heat exchangers).
[0066] The arrangement of the first refrigeration circuit 100 in
FIGS. 1 and 2 simplifies the cooling provided to one heat
exchanger, or some of the heat exchangers or all of the heat
exchangers, of the first cooling stage 2, or any cooling stage,
configuration or arrangement. In particular, the arrangements shown
in FIGS. 1 and 2 reduce the number of water and/or air units and
accumulators required in a first refrigerant circuit 100, which can
nevertheless still provide two refrigerant streams for separate
sets of heat exchangers. It may be possible to further reduce the
number of features regarding the first refrigerant circuit 100 by
further combination of coolers, valves and/or compressors, in order
to further reduce the capital and running costs of the first
refrigerant circuit 100 and/or the first cooling stage 2.
[0067] Downstream of the second one 14 of the first heat exchangers
12,14, there is a cooled hydrocarbon stream 30. This stream 30, and
the equivalent cooled hydrocarbon stream 30a from the second set of
second heat exchangers 12a, 14a of the first cooling stage 2, then
pass into two parallel and preferably identical liquefaction heat
exchangers 16, 16a, which form the second cooling stage 4.
[0068] The liquefaction heat exchangers 16, 16a of the second
cooling stage 4 are preferably spool-wound or spiral-wound
cryogenic heat exchangers, whose operation is known in the art and
whose cooling is provided by the second and third refrigerant
circuits 102, 103 respectively.
[0069] Each of the liquefaction heat exchangers 16, 16a provides a
liquefied hydrocarbon stream 40, 40a, which streams 40, 40a are
then combined into a combined liquefied hydrocarbon stream 50.
After passage through a third heat exchanger 24, the cooled
combined liquefied hydrocarbon stream 50a passes through an
expander, and into a gas/liquid separator, being an end flash
vessel 22 known in the art. From the end flash vessel 22 there is
provided a liquefied hydrocarbon product stream 60, which can then
be passed along by a pump 26 to storage and/or transportation, and
a gaseous stream 70, which after any heat exchange, may be used as
a fuel gas, and/or used in other parts of the LNG plant.
[0070] In the example shown in FIG. 2, the first, second and third
refrigerant circuits 100, 102, 103 preferably use a mixed
refrigerant. The second and third refrigerant circuits 102 and 103
preferably use the same mixed refrigerant.
[0071] The mixed refrigerant of each refrigerant circuit may be
based on two or more components, more preferably selected from the
group comprising nitrogen, methane, ethane, ethylene, propane,
propylene, butane and pentane. The average molar weight of the
refrigerant in the first refrigerant circuit 100 is preferably
higher than the average molar weight of refrigerant in the second
and third refrigerant circuits 102 and 103.
[0072] For clarity, the second refrigerant circuit 102 will now be
described in more detail. From the liquefaction heat exchanger 16,
a stream 102e of vapourised refrigerant is provided, and compressed
and cooled by two compressors and two water or air coolers, to
provide a cooled refrigerant stream 102a. This cooled refrigerant
stream 102a then passes through the set of two heat exchangers 12,
14 of one part of the first cooling stage 2, which provides some
cooling to the second refrigerant. This further cooled refrigerant
stream 102b is then passed into a gas/liquid separator 46. The
separator 46 provides a light refrigerant fraction 102c, and a
heavy refrigerant fraction 102d, which both pass into the
liquefaction heat exchanger 16 to be cooled and expanded to use
their cold energy in the liquefaction heat exchanger 16 in a manner
known in the art.
[0073] Table 1 gives a representative working example of
temperatures, pressures and flows of streams at various parts an
example process of the present invention referring to FIG. 2.
TABLE-US-00001 TABLE 1 Stream Temperature Pressure Mass flow number
(.degree. C.) (bar) (kg/s) Phase 10 51.0 92.6 277.7 Vapor 20 51.0
92.6 139.5 Vapor 30 -41.5 89.0 140.0 Vapor 40 -151.4 83.5 140.0
Liquid 50a -156.8 81.0 280.0 Liquid 60 -162.5 1.1 251.6 Liquid 70
-165.1 1.0 28.4 Vapor 102a 46.0 53.3 205.0 Vapor 102b -41.5 49.0
205.0 Mixed 102c -41.6 48.9 36.0 Vapor 102d -41.6 48.9 169.0 Liquid
102e 62.3 19.9 205.0 Vapor 101 41.0 37.8 442.9 Liquid 101f 68.7
38.6 885.9 Vapor
[0074] The person skilled in the art will understand that the
present invention can be carried out in many various ways without
departing from the scope of the appended claims.
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