U.S. patent application number 11/938599 was filed with the patent office on 2008-06-12 for method and apparatus for cooling a hydrocarbon stream.
Invention is credited to Willem Dam, Casper Krijno Groothuis.
Application Number | 20080134717 11/938599 |
Document ID | / |
Family ID | 37905342 |
Filed Date | 2008-06-12 |
United States Patent
Application |
20080134717 |
Kind Code |
A1 |
Dam; Willem ; et
al. |
June 12, 2008 |
METHOD AND APPARATUS FOR COOLING A HYDROCARBON STREAM
Abstract
A method of cooling a hydrocarbon stream such as natural gas,
the method at least comprising the steps of: (a) providing a feed
stream; (b) passing the feed stream through a gas treatment stage
comprising one or more (number: X) parallel gas treatment units,
the feed stream being divided into two or more part-feed streams if
there is more than one gas treatment unit, to provide one or more
first treated streams; (c) passing the first treated stream or
streams of step (b) through an NGL extraction stage comprising one
or more (number: Y) parallel NGL extraction units, the first
treated stream or streams being shared to match the number of NGL
extraction units, to provide one or more second treated streams;
and (d) passing the second treated stream or streams of step (c)
through a cooling stage comprising one or more (number: Z) parallel
cooling systems, the second treated stream or streams being shared
to match the number of cooling systems, to provide a cooled
hydrocarbon stream or streams.
Inventors: |
Dam; Willem; (The Hague,
NL) ; Groothuis; Casper Krijno; (The Hague,
NL) |
Correspondence
Address: |
Shell Oil Company
910 Louisiana
Houston
TX
77002
US
|
Family ID: |
37905342 |
Appl. No.: |
11/938599 |
Filed: |
November 12, 2007 |
Current U.S.
Class: |
62/618 ; 62/611;
62/620 |
Current CPC
Class: |
F25J 1/0271 20130101;
F25J 1/027 20130101; F25J 1/0022 20130101; F25J 2220/66 20130101;
F25J 2290/10 20130101; F25J 2220/64 20130101 |
Class at
Publication: |
62/618 ; 62/611;
62/620 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 14, 2006 |
EP |
06124012.3 |
Claims
1. Method of cooling a hydrocarbon stream such as natural gas, the
method at least comprising the steps of: (a) providing a feed
stream; (b) passing the feed stream through a gas treatment stage
comprising one or more (number: X) parallel gas treatment units,
the feed stream being divided into two or more part-feed streams if
there is more than one gas treatment unit, to provide one or more
first treated streams; (c) passing the first treated stream or
streams of step (b) through an NGL extraction stage comprising one
or more (number: Y) parallel NGL extraction units, the first
treated stream or streams being shared to match the number of NGL
extraction units, to provide one or more second treated streams;
and (d) passing the second treated stream or streams of step (c)
through a cooling stage comprising one or more (number: Z) parallel
cooling systems, the second treated stream or streams being shared
to match the number of cooling systems, to provide a cooled
hydrocarbon stream or streams; wherein Y>Z.
2. Method as claimed in claim 1, wherein the feed stream is divided
into two part-feed streams, and X is 2 and Z is 1.
3. Method as claimed in claim 1, wherein Y is 2 and Z is 1.
4. Method as claimed in claim 1, wherein the feed stream is divided
into three part-feed streams, and X is 3 and Z is 1 or 2.
5. Method as claimed in claim 1, wherein Y is 3 and Z is 1 or
2.
6. Method according to claim 1, wherein the or each NGL extraction
unit provides a second treated stream and one or more separate
streams.
7. Method as claimed in claim 6, wherein the one or more separate
streams comprise one or more of a propane stream, a butane stream
and a C.sub.5+ stream.
8. Method as claimed in claim 6, wherein the one or more separate
streams are fractionated to provide one or more fractionated
streams.
9. Method as claimed in claim 7, wherein the one or more
fractionated streams comprise one or more of a propane stream, a
butane stream and a C.sub.5+ stream.
10. Method according to claim 1, wherein X is different from Z.
11. Method according to claim 1, wherein the cooling step (d)
liquefies the second treated stream or streams to provide a
liquefied hydrocarbon stream or streams.
12. Liquefied hydrocarbons comprising the liquefied hydrocarbon
stream obtained from the method of claim 11.
13. Apparatus for cooling a hydrocarbon stream such as natural gas,
the apparatus at least comprising: (i) a gas treatment stage to
receive a feed stream comprising one or more (number: X) parallel
gas treatment units, the feed stream being divided if there is more
than one gas treatment unit; (ii) an NGL extraction stage to
receive the treated feed stream or streams from the gas treatment
stage, comprising one or more (number: Y) parallel NGL extraction
units; and (iii) a cooling stage to receive the NGL extracted
stream or streams from the NGL extraction stage, comprising one or
more (number: Z) parallel cooling systems to provide a treated
hydrocarbon stream or streams; wherein Y>Z.
14. Apparatus as claimed in claim 13, wherein X is 2 and Z is
1.
15. Apparatus as claimed in claim 13, wherein Y is 2 and Z is
1.
16. Apparatus as claimed in claim 13, wherein X is 3 and Z is 1 or
2.
17. Apparatus as claimed in claim 13, wherein Y is 3 and Z is 1 or
2.
18. Apparatus as claimed in claim 13, further comprising a
fractionator to fractionate all of one or more separate streams
provided by the NGL extraction units.
19. Apparatus according to claim 13, wherein X is different from
Z.
20. Apparatus as claimed in claim 13, wherein the or each cooling
system is a liquefying system.
21. Method as claimed in claim 2, wherein Y is 2 and Z is 1.
22. Method as claimed in claim 4, wherein Y is 3 and Z is 1 or
2.
23. Method according to claim 1, wherein the cooling step (d)
liquefies the second treated stream or streams to provide liquefied
natural gas.
24. Apparatus as claimed in claim 14, wherein Y is 2 and Z is
1.
25. Apparatus as claimed in claim 16, wherein Y is 3 and Z is 1 or
2.
Description
[0001] This application claims priority from European Patent
Application No. 06124012.3, filed Nov. 14, 2006, which is
incorporated herein by reference.
[0002] The present invention relates to a method and apparatus for
cooling, optionally also liquefying, a hydrocarbon stream such as
natural gas.
[0003] 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.
[0004] 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 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.
[0005] In addition to methane, natural gas usually includes some
heavier hydrocarbons and impurities, including but not limited to
carbon dioxide, sulphur, hydrogen sulphide and other sulphur
compounds, nitrogen, helium, water and other non-hydrocarbon acid
gases, ethane, propane, butanes, C.sub.5+ hydrocarbons and aromatic
hydrocarbons. These and any other common or known heavier
hydrocarbons and impurities either prevent or hinder the usual
known methods of liquefying the methane, especially the most
efficient methods of liquefying methane. Most if not all known or
proposed methods of liquefying hydrocarbons, especially liquefying
natural gas, are based on reducing as far as possible the levels of
at least most of the heavier hydrocarbons and impurities prior to
the liquefying process.
[0006] Meanwhile, the capacity of a liquefaction plant is usually
determined by the availability of the gas feed stream, and the
market or expected markets for products provided by the plant.
Thus, it can be desirable to be flexible as to the capacity of the
liquefaction plant. Hitherto, extra capacity at a liquefaction
plant has simply involved the complete replication of all of the
units dealing with the gas feed stream, from its source to its
storage. All such units are often termed a "train". Some
commonality of some units of a train or its support units such as
refrigeration units have been suggested, but no flexibility in the
pre-treatment of the gas has been suggested.
[0007] The costs in creating and running a liquefying natural gas
(LNG) plant or system are naturally high. Thus any flexibility and
so reduction in the energy requirements of the plant or system has
significant cost benefit.
[0008] In a paper entitled "Large Capacity LNG Plant Development",
Paper PS5-3, J. J. B. Pek et al, presented at 14.sup.th
International Conference & Exhibition on Liquefied Natural Gas
(Doha, Qatar), on 21 Mar. 2004, there is shown a general process
for liquefying natural gas comprising the general steps of a gas
receiving stage, a gas treating stage, a pre-cooling stage which
feeds two separate but parallel liquefaction stages, and an end
flash stage to provide LNG to an LNG tank.
[0009] WO 2006/009646 A2 relates to a hydrocarbon fluid processing
plant including a plurality of process unit module types, including
first and second process unit module types having modules intended
to be sized at their respective substantially maximum processing
efficiency. WO 2006/009646 A2 shows a number of plant arrangements
but a problem with these arrangements is that there are always
complete `trains` for treating and liquefying the hydrocarbon
fluids, without flexibility therebetween.
[0010] It is an object of the present invention to improve the
efficiency of a treatment plant or method.
[0011] It is a further object of the present invention to better
streamline the hydrocarbon stream or streams through a treatment
plant or method.
[0012] The present invention provides a method of cooling a
hydrocarbon stream such as natural gas, the method at least
comprising the steps of:
(a) providing a feed stream; (b) passing the feed stream through a
gas treatment stage comprising one or more (number: X) parallel gas
treatment units, the feed stream being divided into two or more
part-feed streams if there is more than one gas treatment unit, to
provide one or more first treated streams; (c) passing the first
treated stream or streams of step (b) through an NGL extraction
stage comprising one or more (number: Y) parallel NGL extraction
units, the first treated stream or streams being shared to match
the number of NGL extraction units, to provide one or more second
treated streams; and (d) passing the second treated stream or
streams of step (c) through a cooling stage comprising one or more
(number: Z) parallel cooling systems, the second treated stream or
streams being shared to match the number of cooling systems, to
provide a cooled hydrocarbon stream or streams; wherein the number
of parallel extraction units is higher than the number of parallel
cooling systems.
[0013] In a further aspect, the present invention provides
apparatus for cooling a hydrocarbon stream such as natural gas, the
apparatus at least comprising:
(i) a gas treatment stage to receive a feed stream comprising one
or more (number: X) parallel gas treatment units, the feed stream
being divided if there is more than one gas treatment unit; (ii) an
NGL extraction stage to receive the treated feed stream or streams
from the gas treatment stage, comprising one or more (number: Y)
parallel NGL extraction units; and (iii) a cooling stage to receive
the NGL extracted stream or streams from the NGL extraction stage,
comprising one or more (number: Z) parallel cooling systems to
provide a treated hydrocarbon stream or streams; wherein the number
of parallel NGL extraction units is higher than the number of
parallel cooling systems.
[0014] The invention will now be further illustrated by way of
example only, and with reference to embodiments, examples and the
accompanying diagrammatic and non-limiting drawings in which:
[0015] FIG. 1 is a block scheme of part of an LNG plant according
to one embodiment of the present invention;
[0016] FIG. 2 is a block scheme of part of an LNG plant according
to a second embodiment of the present invention; and
[0017] FIG. 3 is a block scheme of part of an LNG plant according
to a third embodiment of the present invention.
[0018] 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.
[0019] The method and apparatus described herein comprises a gas
treatment stage, an NGL extraction stage and a cooling stage. The
gas treatment stage comprises one or more parallel gas treatment
units; the NGL extraction stage comprises one or more parallel NGL
extraction units; and the cooling stage comprises one or more
parallel cooling systems. The number of gas treatment units will
hereinafter be represented by the letter X; the number of NGL
extraction units as Y, and the number of cooling systems as Z.
[0020] For the purpose of this specification, "parallel" means
arranged or operated in parallel with respect to other units or
systems of same type when more than one of that type of unit or
system is provided or alone when one unit or system of a type is
provided.
[0021] As described herein, an asymmetric arrangement between the
number of gas treatment units, the number of NGL extraction units,
and the number of cooling systems is provided. An asymmetric
configuration offers benefits in terms of overall plant
availability and matching of individual unit equipment sized design
capacity constraints.
[0022] In an advantageous group of embodiments, the number of NLG
extraction units is higher than the number of cooling systems (i.e.
Y>Z). This advantageous choice is based on an insight that
limits in vapour expander size are typically more restrictive than
the size of a cooling system in terms of production capacity. NGL
extraction units are typically based on an expander. Thus,
production capacity can be increased by operating in parallel more
expander-based extraction units than there are cooling systems.
[0023] Moreover, providing Y>Z offers the possibility of
providing an excess of NGL extraction capacity relative to cooling
capacity. Generating excess NGL-extracted gas may be useful to
provide domestic gas from a hydrocarbon stream cooling site.
[0024] In another group of embodiments, X or Y, preferably X and Y
both, are unequal to Z, whereby at least one of X and Y is greater
than one.
[0025] The asymmetry provided herein also allows the energy
requirements of the liquefaction facility or plant to be more
balanced.
[0026] An asymmetric configuration also offers the user of a plant
options and flexibility should a gas treatment unit or an NGL
extraction unit be off-line, i.e. non-operational, either
accidentally or deliberately for maintenance, etc. Whilst there may
be some reduction in plant output or in one or more stream flows,
overall, the plant can remain operational through the use of other
gas treatment units and NGL extraction units.
[0027] In the gas treatment stage, the level or levels of certain
impurities generally not being hydrocarbons can be reduced. Two
common impurities are carbon dioxide and sulphur (and sulphur-based
compounds), usually present with water in the form of `acid gas`.
Many processes for the removal of acid gas from a feed stream are
known to those skilled in the art. One common method is the use of
an aqueous amine solution, often used in an extraction column
termed a `scrubber`. The aqueous amine may be one or more of known
materials including for example DGA, DEA, MDEA, MEA and
SULFINOL.TM. (Shell), and combinations thereof. Typically acid gas
removal can result in the reduction of carbon dioxide to levels of
less than about 60 ppm, whilst sulphur can be reduced to levels of
less than about 4 ppm. The gas treatment stage comprises one or
more parallel gas treatment units. By "gas treatment unit" is meant
a unit comprising one or more solvent-fed extraction columns which
remove acid gas from the hydrocarbon stream under elevated pressure
(relative to solvent regeneration pressure) to produce enriched
solvent and a treated hydrocarbon stream. The enriched solvent is
then passed within the gas treatment unit to one or more pressure
let-down devices and then to one or more heaters or regeneration
columns which separate the solvent from the acid gas and recycle at
least a portion of the solvent to the one or more extraction
columns. The extraction columns and heaters or regeneration columns
of the gas treatment unit represent equipment which is dedicated to
a single gas treatment unit such that they are not shared with any
other gas treatment units.
[0028] The NGL extraction stage comprises one or more parallel NGL
extraction units. By "NGL extraction unit" is meant a unit
comprising one or more knock-out drums from which the hydrocarbon
stream is passed, via an expansion step, to one or more NGL
separators which produce an NGL stream or streams, which each
comprise, for instance, less than 5 mole % methane. The knock-out
drums and NGL separators of the NGL extraction unit represent
equipment which is dedicated to a single NGL extraction unit such
that they are not shared with any other NGL extraction units.
However, subsequent separation of the NGL stream into individual
hydrocarbon streams, often referred to as fractionation, can be
carried out in a facility shared between NGL extraction units.
[0029] NGL extraction units as such are known to those skilled in
the art. Generally, they are designed to reduce the level or levels
of hydrocarbon compounds other than methane in a feed stream. One
common NGL extraction unit includes a separator or separation
vessel, able to provide a gaseous stream that is methane-enriched,
and one or more other streams. Such other stream or streams usually
but not always include separate or combined streams of heavier
hydrocarbons. The methane-enriched stream may be passed through the
cooling stage.
[0030] In one example, an NGL extraction unit provides a single
heavier hydrocarbon rich stream, which is subsequently used either
per se, or is further divided into particular heavier hydrocarbon
rich streams in a separate location or unit. The division of a
heavier hydrocarbon rich stream can be carried out by one or more
separators known in the art, such as a fractionator. A fractionator
using one or more columns could provide individual streams of
certain heavier hydrocarbons. For example, with multiple columns,
each column could be designed to provide an individual hydrocarbon
stream, such as an ethane-rich stream, a propane-rich stream, a
butane-rich stream, and a C.sub.5+-rich stream, the latter
sometimes also termed a `light condensate stream`. Propane, butane
and C.sub.5+ hydrocarbons are sometimes collectively termed
"natural gas liquids" (NGL), and have known uses.
[0031] An example of a fractionation tower as a conventional
distillation column for NGL extraction is shown in US 2004/0079107
A1.
[0032] In another example, an NGL extraction unit can include a
fractionator which integrally provides individual streams of
certain heavier hydrocarbons such as those listed hereinbefore.
[0033] Cooling systems for the cooling stage are known in the art.
The cooling system may be a liquefying system. Cooling systems and
liquefying systems may be embodied in various ways, and generally
involve one or more heat exchangers and refrigerant circuits.
[0034] The cooling stage comprises one or more parallel cooling
systems. By "cooling system" is meant a cooling system comprising
one or more closed, independent refrigerant cycles. A closed
refrigerant cycle does not exchange refrigerant with another
cooling cycle under normal operation. The closed, independent
refrigerant cycle or cycles are dedicated to a single cooling
system such that they are not shared with any other cooling
systems.
[0035] A liquefying system useable with the present invention may
involve a number of separate serial cooling steps, and the or each
cooling step may involve one or more heat exchangers, levels or
sections. One arrangement involves the cooling stage having a first
cooling step for pre-cooling, followed by a second cooling step for
main cryogenic cooling and liquefying.
[0036] A first or pre-cooling step may involve reducing the
temperature of a feed stream to below -0.degree. C., for example in
the range -10.degree. C. to -30.degree. C.
[0037] A second or main cryogenic cooling step may involve cooling
a feed stream to below -90.degree. C. or below -100.degree. C., for
example between -100.degree. C. to -130.degree. C., which usually
creates a hydrocarbon stream which is now liquefied, such as
liquefied natural gas.
[0038] Each unit or system of the stages of the present invention
may use the same or different parameters, such as flowrate,
temperature, pressure, etc.
[0039] By providing units and systems which may operate
independently, a unit or system may be taken off-line for
maintenance without having to shut down the entire hydrocarbon
cooling plant.
[0040] The present invention includes a combination of any and all
of the methods herein described.
[0041] As described herein, the number of streams for each of the
stages is intended to be shared to match the number of units or
systems in each stage. This may therefore require the division,
sharing or combination of the feed stream or the stream or streams
provided by the previous stage. Such division, sharing and/or
combination may or may not involve the complete mixing of previous
streams, or the complete separation of previous streams.
[0042] Any division, sharing and/or combination of the feed stream
or stream or streams provided by a previous stage may be unequally
distributed. Preferably the distribution is equal, that is, there
is equidistribution of the stream or streams amongst the lines to
the units or systems to which the stream or streams are intended to
be passed to.
[0043] The division or sharing of any of the feed streams or any of
the subsequent treated or cooled hydrocarbon stream or streams,
could be provided by any suitable divisor, for example a stream
splitter. Preferably any division or sharing creates two or more
streams having the same composition and phases.
[0044] All possible arrangements for changes in the number of
streams before or after each stage are envisaged. For example,
where X is 1, Y is 2 and Z is 1, the first treated stream provided
by the gas treatment stage is divided into two first treated
streams for each of the NGL extraction units in the NGL extraction
stage. The two second treated streams provided thereby are then
combined into a single second treated stream for the single cooling
system in the cooling stage.
[0045] In another example, X can be 2, Y can be 3, and Z can be 2.
In this example, the feed stream is shared or divided into two
part-feed streams for the two gas treatment units in the gas
treatment stage. The two first treated streams provided thereby are
then shared to provide three first treated streams, one for each of
the NGL extraction units in the NGL extraction stage. The change
from two to three first treated streams may be through a common
manifold, or may be by any other sharing arrangement not having a
common union or junction of all streams. Similarly, the three
second treated streams provided by the NGL extraction stage are
then shared to create two second treated streams for the two
cooling systems. This sharing of the second treated streams may be
through a common manifold, or through any other arrangement not
involving commonality or union of all the second treated
streams.
[0046] The present invention may also be used to create a facility
or plant where X is 2, Y is 2 and Z is 1. Should there be a wish or
need to enlarge the capacity of the facility or plant, one or more
further gas treatment units, NGL extraction units, and/or cooling
systems, can be added, taking into account the design capacity of
existing units, thereby avoiding the hitherto expedient of having
to supply a complete and separated liquefaction `train`.
[0047] The feed stream may be any suitable hydrocarbon-containing
gas stream to be cooled, but is usually 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.
[0048] 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 % methane.
[0049] Although the method described herein 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 here.
[0050] FIG. 1 shows a block scheme of part of a liquefied natural
gas plant 1. It shows an initial feed stream 10 containing natural
gas. The feed stream 10 is divided by a stream splitter 12 into two
part-feed streams 20a, 20b. 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, systems or units of the plant, or due to other
considerations. One example of the ratio is an equal division of
the cooled stream mass.
[0051] The two part-feed streams 20a, 20b pass to a gas treatment
stage 2 comprising two parallel gas treatment units 14a, 14b. Such
gas treatment units 14a, 14b are adapted to reduce impurities,
including but not limited to acid gas, in the part feed-streams
20a, 20b, and so provide two first treated streams 30a, 30b
respectively. Operation of gas treatment units 14a, 14b such as
scrubbers are well known in the art. FIG. 1 shows an exit stream
15a, 15b for carbon dioxide, sulphur and any sulphur-based
compounds from each gas treatment unit 14a, 14b.
[0052] The first treated streams 30a, 30b are passed to an NGL
extraction stage 4 which comprises two parallel NGL extraction
units 16a, 16b. The NGL extraction units 16a, 16b provide two
second treated streams 40a, 40b, which streams are
methane-enriched, and which are combined to provide a combined
treated stream 50. The second treated feed streams 40a, 40b can be
combined using a combiner 18 known in the art. The combiner may be
any suitable arrangement, generally involving a union or junction
or piping or conduits, optionally involving one or more valves.
[0053] The NGL extraction units 16a, 16b also provide two heavier
hydrocarbon enriched streams 17a, 17b which pass to a common
fractionator 24. The enriched streams 17a, 17b may be combined
prior to entry into the common fractionator 24 as shown in FIG. 1,
or may be passed separately into the common fractionator 24.
[0054] The common fractionator 24 is designed to provide separate
enriched streams of one or more hydrocarbons such as propane,
butane and C.sub.5+ hydrocarbons, and optionally also ethane. Such
enriched streams are useful products for use in the liquefying
plant 1 or outside the plant. Such enriched streams are shown
collectively in FIG. 1 as streams 26.
[0055] The fractionator 24 may be a single fractionation unit, or
have one or more columns, wherein each column is usually dedicated
to separating and providing a particular heavier hydrocarbon.
Fractionation is well known in the art and the benefit and use of
individual streams of propane, butane and C.sub.5+ are also well
known in the art.
[0056] Although not shown in FIG. 1, any methane stream or
methane-enriched stream provided by the fractionator 24 may be
returned or recycled back into the path of the second treated
streams 40a, 40b or the combined stream 50.
[0057] The combined treated stream 50 is then passed to a cooling
stage 6. In FIG. 1, the cooling stage 6 provides a cooled
hydrocarbon stream 60.
[0058] The cooling of the combined treated stream 50 in the cooling
stage 6 may involve any degree of cooling using any number of
units, devices or systems or combinations thereof known in the art.
One example is the use of one or more heat exchangers. Usually,
cooling is effected by passing the combined treated stream 50
against one or more cooling or refrigerant streams and/or through
one or more valves and/or separators, as known in the art.
[0059] In one embodiment of the present invention, the cooling
stage 6 is adapted to liquefy the combined treated stream 50 so as
to provide a liquefied hydrocarbon stream such as liquefied natural
gas. Liquefaction of the combined treated stream 50 can be carried
out by passing it through a cooling system being a liquefying
system 22 using one or more heat exchangers and cooling it against
one or more refrigerants, either being dedicated refrigerants or
other cooled streams. The liquefying can involve one or more
cooling and/or liquefying steps.
[0060] Generally, it is intended to provide a liquefied natural gas
stream having a temperature below -150.degree. C., more usually
between -160.degree. C. and 165.degree. C.
[0061] Between the gas treatment stage 2, NGL extraction stage 4
and the cooling stage 6, one or more other units or features may be
involved such as valves or further treatments, or to control of the
path or flow of the streams thereinbetween.
[0062] FIG. 1 shows a method of treating a hydrocarbon stream such
as natural gas wherein X is 2, Y is 2, and Z is 1. There is
therefore asymmetry between the number of gas treatment units, the
number of NGL extraction units, and the number of cooling systems.
An asymmetric configuration, particularly where Y>Z as in the
embodiment of FIG. 1, offers benefits as explained hereinabove.
[0063] FIG. 2 shows a block scheme of part of a liquefied natural
gas plant 1a according to a second embodiment of the present
invention. In FIG. 2, a feed stream 10 is divided by a divider 12
into three part-feed streams 20a, 20b and 20c, either equally or
unequally as described above. The three part-feed streams 20a, 20b,
20c pass into three respective and parallel gas treatment units
14a, 14b and 14c which comprise the gas treatment stage 2. The
action and effect of the gas treatment units 14a, 14b, 14c is
similar to those described hereinbefore. They provide three first
treated streams 30a, 30b, 30c, and three other streams 15a, 15b,
15c, being for example, sulphur, carbon dioxide, etc.
[0064] The three first treatment streams 30a,b,c pass into
respective and parallel NGL extraction units 16a, 16b, 16c, whose
action and effect is similar to those described hereinbefore. The
units 16a, 16b, 16c provide three second treated streams 40a, 40b,
40c which are then shared for the cooling stage 6, and three
heavier hydrocarbon streams 17a, 17b, 17c, whose use and/or
separation can be the same or similar to that described above in
FIG. 1.
[0065] The sharing of the three second treated streams 40a, 40b,
40c may not involve the complete combination and mixing of the
second treated streams 40a, 40b, 40c within one position, location,
line or stream. Thus, references herein to the second treated
streams being "shared", include the second treated streams being
able to provide one or more streams to a cooling stage, which
cooling stage may comprise one or more cooling systems. The one or
more streams supplying the cooling system or systems may therefore
comprise flow from two or more second treated streams, without
involving flow from all of the second treated streams.
[0066] It is intended by the present invention that all the second
treated streams collectively supply their flow to the or all of the
cooling systems of the cooling stage.
[0067] In FIG. 2, it is diagrammatically represented that the three
second treated streams 40a, 40b, 40c provide, via linked piping or
conduits at junctions 19a, 19b, combined treated streams 50a, 50b,
which pass respectively into two cooling systems 22a, 22b which
comprise the cooling stage 6. The cooling systems 22a, 22b
preferably liquefy the combined treated streams 50a, 50b to provide
two liquefied hydrocarbon streams 52a, 52b, which can be combined
to provide a combined liquefied hydrocarbon stream 60, such as
liquefied natural gas.
[0068] Thus, FIG. 2 shows a method of treating a hydrocarbon stream
such as natural gas wherein X is 3, Y is 3, and Z is 2.
[0069] FIG. 3 shows a simplified block scheme of part of a
liquefied natural gas plant 1b according to a third embodiment of
the present invention. In FIG. 3, a feed stream 10 passes into a
single gas treatment unit 100 for the gas treatment stage 2. The
action and effect of the gas treatment unit 100 is similar to those
described hereinbefore. This provides a first treated stream 30d
which is optionally passed through a separate or integral drying
unit 101 to reduce its water content, and provide a drier treated
stream 30e.
[0070] The drier treated stream 30e is then divided by a divider
32, preferably equally, into 3 part-streams 30f, 30g, 30h, which
pass into three respective and parallel NGL extraction units 102a,
102b and 102c, whose action and effect is similar to those
described hereinbefore, and which comprise the NGL extraction stage
4. The units 102a, b, c provide three second treated streams 40a,
40b, 40c, which are then combined using a combiner 34 to provide a
single combined stream 50.
[0071] The combined stream 50 is then divided by a divider 36 into
two part-streams 50a, 50b, optionally unequally or equally, to pass
into two cooling systems 103a, 103b, similar to those described
above, and which comprise the cooling stage 6. The cooling systems
103a,b are preferably adapted to liquefy the part-streams 50a, 50b,
to provide two liquefied hydrocarbon streams 52a, 52b, which can
then be combined by a combiner 38 to provide a combined liquefied
hydrocarbon stream 60, such as liquefied natural gas. The liquefied
natural gas 60 can pass through an endflash unit 104 in a manner
commonly known in the art, to provide a final liquefied product
stream 70.
[0072] Thus, FIG. 3 shows another embodiment of the present
invention having a gas treatment stage with one gas treatment unit,
an NGL extraction stage having three NGL extraction units, and a
cooling stage comprising two parallel cooling systems, i.e. a
method of treating a hydrocarbon stream such as natural gas wherein
X is 1, Y is 3, and Z is 2.
[0073] 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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