U.S. patent application number 10/573213 was filed with the patent office on 2008-01-10 for natural gas liquefaction process.
This patent application is currently assigned to Linde AG. Invention is credited to Heinz Bauer, Manfred Bolt, Hubert Franke, Arne Olav Fredheim, Pentti Paurola, Jostein Pettersen, Rainer Sapper, Marc Schier.
Application Number | 20080006053 10/573213 |
Document ID | / |
Family ID | 34379077 |
Filed Date | 2008-01-10 |
United States Patent
Application |
20080006053 |
Kind Code |
A1 |
Bauer; Heinz ; et
al. |
January 10, 2008 |
Natural Gas Liquefaction Process
Abstract
A method of liquefying a hydrocarbon-rich gas, wherein the gas
flows through a cascade of three refrigeration stages, each stage
comprising a refrigerant circuit and a compressor, wherein at least
part of the flow of refrigerant from the second circuit is used for
the pre-cooling of the hydrocarbon rich gas in the first
refrigeration stage. This balances the load on each of the
compressors. By standardizing the drive units and compressors of
the three coolant circuits, it is possible to maximize the
attainable liquefaction capacity of the liquefaction process using
tried-and-trusted drive units and compressors respectively. This
method can be applied to mixed refrigerant cascades and circuits
with a carbon dioxide pre-cooling circuit. This latter option has
benefits for offshore use where large amounts of hydrocarbons are
undesirable.
Inventors: |
Bauer; Heinz; (Ebenhausen,
DE) ; Franke; Hubert; (Pulach, DE) ; Sapper;
Rainer; (Aufkirchen, DE) ; Schier; Marc;
(Grunwald, DE) ; Bolt; Manfred; (Grobenzell,
DE) ; Pettersen; Jostein; (Ranheim, NO) ;
Fredheim; Arne Olav; (Trondheim, NO) ; Paurola;
Pentti; (Hafrsfjord, NO) |
Correspondence
Address: |
NEEDLE & ROSENBERG, P.C.
SUITE 1000, 999 PEACHTREE STREET
ATLANTA
GA
30309-3915
US
|
Assignee: |
Linde AG
Wiesbaden
DE
Statoil ASA
Stavanger
NO
|
Family ID: |
34379077 |
Appl. No.: |
10/573213 |
Filed: |
September 23, 2004 |
PCT Filed: |
September 23, 2004 |
PCT NO: |
PCT/GB04/04047 |
371 Date: |
January 16, 2007 |
Current U.S.
Class: |
62/612 |
Current CPC
Class: |
F25J 1/0292 20130101;
F25B 1/10 20130101; F25J 1/0217 20130101; F25B 5/02 20130101; F25J
1/0297 20130101; F25B 2400/13 20130101; F25B 9/008 20130101; F25J
1/0022 20130101; F25J 1/0296 20130101; F25B 2309/06 20130101; F25J
1/0095 20130101; F25J 1/0279 20130101; F25J 1/0278 20130101; F25J
1/0295 20130101; F25J 2220/64 20130101; F25J 1/0052 20130101; F25J
1/0283 20130101; F25J 1/0218 20130101 |
Class at
Publication: |
62/612 |
International
Class: |
F25J 1/02 20060101
F25J001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 23, 2003 |
DE |
10344030.5 |
Apr 23, 2004 |
GB |
0409103.9 |
Claims
1. A method for the liquefaction of a hydrocarbon-rich flow,
whereby the liquefaction of the hydrocarbon-rich flow is effected
against a refrigerant circuit cascade consisting of three
refrigeration circuits, whereby the first of the three
refrigeration circuits serves to provide preliminary cooling, the
second refrigeration circuit serves to provide the actual
liquefaction, and the third refrigeration circuit serves the
sub-cooling of the liquefied hydrocarbon-rich flow, and whereby
each refrigeration circuit comprises at least one single-stage or
multi-stage compressor, characterised in that at least one part
flow of the refrigerant of the second refrigeration circuit is used
for the preliminary cooling of the hydrocarbon-rich flow.
2. A method as claimed in claim 1 wherein the part flow of the
refrigerant of the second refrigeration circuit used for the
pre-cooling of the hydrocarbon-rich flow is evaporated at a
pressure which is higher than the evaporation pressure of the
remaining part flow of the refrigerant of the second cooling
circuit, and is conducted to the compressor of the second cooling
circuit at an intermediate pressure level.
3. A method as claimed in claim 1 wherein the separation of
unwanted components and/or components of the hydrocarbon-rich flow
which freeze out during the liquefaction of the hydrocarbon-right
flow takes place before the actual liquefaction of the
hydrocarbon-rich flow.
4. A method as claimed in claim 3, wherein at least one part flow
of one of the two part flows of the second cooling circuit is used
for the provision of cooling in the separation unit.
5. A method as claimed in claim 1 wherein the volumes and/or
evaporation pressures of the two part flows of the second
refrigeration circuit are changeable.
6. A method as claimed in claim 1 wherein the hydrocarbon rich
stream is a natural gas flow.
7. A method as claimed in claim 1 wherein each compressor has a
substantially equal share of the load.
8. A method as claimed in claim 1 wherein the first refrigeration
circuit comprises carbon dioxide.
9. A method as claimed in claim 1 wherein all the refrigeration
circuits comprise mixed refrigerants.
10. A method of liquefying a hydrocarbon-rich gas, wherein the gas
flows through a cascade of three refrigeration stages, each stage
comprising a refrigerant circuit and a compressor, wherein at least
part of the flow of refrigerant from the second circuit is used for
the preliminary cooling of the hydrocarbon rich gas in the first
refrigeration stage.
11. A method of liquefaction comprising a plurality of cooling
circuits arranged in a cascade formation, each circuit comprising a
compressor, wherein each compressor has a substantially equal share
of the total load.
12. A method as claimed in claim 11 wherein the cascade comprises
at least first and second cooling circuits, the second cooling
circuit being used at least partially for pre-cooling the substance
to be liquefied.
13. A method as claimed in claim 11 wherein the method is a method
of liquefaction of a hydrocarbon rich flow.
14. A method as claimed in claim 13 wherein the first cooling
circuit comprises carbon dioxide.
15. A substantially load balanced mixed refrigerant cascade process
comprising a carbon dioxide pre-cooling circuit.
16. A substantially load balanced mixed refrigerant cascade process
as claimed in claim 15 wherein the carbon dioxide is cooled after
condensation to a temperature of 2020 C. or less.
17. A substantially load balanced process as claimed in claim 16
wherein the carbon dioxide is cooled to a temperature of 15.degree.
C. or less.
18. A substantially load balanced process as claimed in claim 16
wherein cold cooling water is used to cool the carbon dioxide.
19. A substantially load balanced process as claimed in claim 18
wherein the cold cooling water is sea water.
20. A substantially load balanced process as claimed in claim 15,
wherein the carbon dioxide pre-cooling circuit includes a
sub-cooling heat exchanger installed after the condenser.
21. A substantially load balanced process as claimed in claim 15,
wherein the carbon dioxide cooling circuit comprises three pressure
levels.
22. A substantially load balanced process as claimed in claim 15,
wherein the carbon dioxide is not subcooled in the pre-cooling
circuit.
23. A substantially load balanced process as claimed in claim 15,
wherein a high pressure casing is used with the carbon dioxide
compressor.
24. A substantially load balanced process as claimed in claim 23
wherein the compressor is split into two casings and a barrel type
casing used for the high pressure stage.
25. An LNG liquefaction process comprising three cascade cycles
each driven by a compressor, wherein the compressors are
substantially equally loaded and one of the cascade cycles is a
carbon dioxide cycle.
26. A method for the liquefaction of a hydrocarbon-rich flow,
whereby the liquefaction of the hydrocarbon-rich flow is effected
against a refrigerant circuit cascade consisting of three mixed
refrigeration circuits, whereby the first of the three
refrigeration circuits serves to provide preliminary cooling, the
second refrigeration circuit serves to provide the actual
liquefaction, and the third refrigeration circuit serves the
sub-cooling of the liquefied hydrocarbon-rich flow, and whereby
each refrigeration circuit comprises at least one single-stage or
multi-stage compressor, characterized in that at least one part
flow of the refrigerant of the second refrigeration circuit is used
for the preliminary cooling of the hydrocarbon-rich flow.
Description
[0001] The present invention relates to a method for the
liquefaction of a hydrocarbon-rich flow.
[0002] Natural gas can be obtained from the earth to form a natural
gas feed which must be processed before it can be used
commercially. Normally the gas is first pre-treated to remove or
reduce the content of impurities such as carbon dioxide, water,
hydrogen sulphide, mercury, etc.
[0003] The gas is often liquefied before being transported to its
point of use to provide liquefied natural gas (LNG). This enables
the volume of gas to be reduced by about 600 fold, which greatly
reduces the transportation costs. Since natural gas is a mixture of
gases, it liquefies over a range of temperatures. At atmospheric
pressure, the usual temperature range within which complete
liquefaction occurs is -165.degree. C. to -155.degree. C. However,
since the critical temperature of natural gas is about -80.degree.
C. to -90.degree. C., the gas cannot be liquefied purely by
compressing it. It is therefore necessary to use refrigeration
processes.
[0004] Natural gas liquefaction plants are either designed as what
are known as LNG baseload plants, i.e. plants for the liquefaction
of natural gas for the supply of natural gas as primary energy, or
as what are known as peak-shaving plants, i.e. plants for the
liquefaction of natural gas to cover peak demand.
[0005] It is known to cool natural gas by using heat exchangers in
which a refrigerant or coolant is used. One well-known method
comprises a number of coolant or refrigeration cycles in the form
of a cascade.
[0006] LNG baseload plants are operated as a rule with coolant
circuits consisting of a mixture of hydrocarbons. These mixed
refrigerant circuits are more efficient in terms of energy than
expander circuits and make it possible, with the high liquefaction
capacities of the baseload plants, for correspondingly relatively
low energy consumptions to be achieved.
[0007] Conventional liquefaction processes using only two
refrigerant cycles are limited to about 5 million tons per annum
(mtpa) LNG, unless parallel strings within a single train are
considered.
[0008] The Mixed Fluid Cascade process is known, e.g. from the U.S.
Pat. No. 6,253,574, and uses three independent refrigeration
cycles, which shifts the limit of a real single train concept with
proven compressor drivers to above 8 mtpa LNG.
[0009] This method is also known from the German published
application 197 16 415.
[0010] With liquefaction methods of this type, in principle the
first coolant circuit serves to provide pre-cooling, the second
coolant circuit serves to provide the liquefaction, and the third
coolant circuit serves to provide the sub-cooling for the
hydrocarbon-rich flow or natural gas respectively.
[0011] Between the pre-cooling and the liquefaction, if necessary,
the separation of hydrocarbons with higher boiling points takes
place. These are at least those components of the hydrocarbon-rich
flow or natural gas which would freeze out during the following
cooling stage, i.e. C.sub.5+ hydrocarbons and aromates. Often, in
addition, those hydrocarbons, meaning in this situation in
particular propane and butane, which would undesirably increase the
calorific value of the liquefied natural gas are also separated out
before the liquefaction stage.
[0012] This separation of hydrocarbons with higher boiling points
usually takes place by provision being made for what is known as an
HHC (Heavy Hydrocarbon) column, which serves to separate the heavy
hydrocarbons as well as benzene out of the hydrocarbon-rich flow
which is to be liquefied. A process stage of this kind is likewise
described in the German published application 197 16 415 already
mentioned.
[0013] Due to the provision of this separation, designated
hereinafter as C.sub.3+ separation, at a given pressure of the raw
gas the temperature level of the separation of these components is
set within comparatively narrow limits.
[0014] If the first coolant circuit is now used exclusively for the
pre-cooling of the hydrocarbon-rich flow which is to be liquefied
before this C.sub.3+ separation, then a part of the overall
compression effect of some 40 to 50% will necessarily be spent on
this, while the remaining compression effect of 60 to 50% will be
divided over the second and third coolant circuits.
[0015] In the sense of an economical exploitation of the available
compressor and drive units, however, the inventors have realised
that it is desirable for the (circuit) compressors of the three
circuits to retain approximately the same drive capacity, i.e. in
each case about a third of the overall drive capacity. This applies
in particular to large liquefaction plants with a liquefaction
capacity greater than 5 mtpa, because the number of available
compressors and drive units for such orders of magnitude is
severely restricted. By standardizing the drive units and
compressors of the three coolant circuits, it is possible to
maximize the attainable liquefaction capacity of the liquefaction
process using tried-and-trusted drive units and compressors
respectively.
[0016] Thus according to one aspect of the invention there is
provided an LNG liquefaction process having first and second
refrigeration circuits wherein the second refrigeration circuit is
used at least partially for pre-cooling the hydrocarbon-rich stream
to be liquefied. Thus the installed power of the gas turbines and
starters--at least during normal liquefying operation--can be
exploited to the full.
[0017] Part of the refrigerant of the liquefaction cycle (LC) may
be vaporized under elevated pressure in the precooling section of
the process and fed to the LC compressor as a side stream. In this
way a substantial load balancing between all the refrigeration
cycles can be achieved.
[0018] Therefore, according to one aspect of the present invention
there is provided a method for the liquefaction of a
hydrocarbon-rich flow, in particular of a natural gas flow, whereby
the liquefaction of the hydrocarbon-rich flow is effected against a
refrigerant circuit cascade consisting of three refrigeration
circuits, whereby the first of the three refrigeration circuits
serves to provide preliminary cooling, the second refrigeration
circuit serves to provide the actual liquefaction, and the third
refrigeration circuit serves the sub-cooling of the liquefied
hydrocarbon-rich flow, and whereby each refrigeration circuit
comprises at least one single-stage or multi-stage compressor,
characterised in that at least one part flow of the refrigerant of
the second refrigeration circuit is used for the preliminary
cooling of the hydrocarbon-rich flow.
[0019] From another aspect, the invention provides a method of
liquefying a hydrocarbon-rich gas, wherein the gas flows through a
cascade of three refrigeration stages, each stage comprising a
refrigerant circuit and a compressor, wherein at least part of the
flow of refrigerant from the second circuit is used for the
preliminary cooling of the hydrocarbon rich gas in the first
refrigeration stage.
[0020] Preferably the part flow of the refrigerant of the second
refrigeration (or cooling) circuit, used for the pre-cooling of the
hydrocarbon-rich flow is evaporated at a pressure which is higher
than the evaporation pressure of the remaining part flow of the
refrigerant of the second cooling circuit, and is conducted to
the-compressor of the second cooling circuit at an intermediate
pressure level.
[0021] Preferably the separation of heavier components and/or
components of the hydrocarbon-rich flow which freeze out during the
liquefaction of the hydrocarbon-rich flow takes place before the
actual liquefaction of the hydrocarbon-rich flow.
[0022] Preferably the volumes and/or evaporation pressures of the
two part flows of the second cooling circuit are changeable.
[0023] Preferably at least one part flow of one of the two part
flows of the second cooling circuit is used for the provision of
cooling in the heavy hydrocarbon separation unit.
[0024] The invention therefore provides a load balanced
liquefaction process for LNG in which each compressor may have a
substantially equal share of the total load, and preferably an
equal share. This concept can be applied more widely and hence from
another aspect the present invention provides a method of
liquefaction comprising a plurality of cooling circuits arranged in
a cascade formation, each circuit comprising a compressor, wherein
each compressor has a substantially equal share of the total
load.
[0025] The benefits of load balancing the refrigeration circuits
are not limited to any particular type of refrigerant used.
However, as mentioned above mixed refrigerant cascades provide an
efficient system and therefore in one preferred embodiment the
refrigeration circuits are mixed refrigerant circuits.
[0026] Therefore, according to another aspect of the present
invention there is provided a method for the liquefaction of a
hydrocarbon-rich flow, in particular of a natural gas flow, whereby
the liquefaction of the hydrocarbon-rich flow is effected against a
mixed refrigerant circuit cascade consisting of three refrigeration
circuits, whereby the first of the three refrigeration circuits
serves to provide preliminary cooling, the second refrigeration
circuit serves to provide the actual liquefaction, and the third
refrigeration circuit serves the sub-cooling of the liquefied
hydrocarbon-rich flow, and whereby each refrigeration circuit
comprises at least one single-stage or multi-stage compressor,
characterised in that at least one part flow of the refrigerant of
the second refrigeration circuit is used for the preliminary
cooling of the hydrocarbon-rich flow.
[0027] It will be appreciated that the use of hydrocarbons as
refrigerants poses a safety issue and this is particularly
significant in the offshore environment, where it is highly
undesirable to have large liquid hydrocarbon inventories in what is
inevitably a confined space.
[0028] Floating LNG production, storage and offloading facilities
(LNG FPSOs) are now considered a realistic option for remote
offshore gas fields that cannot be economically exploited with
conventional onshore technology. A floating concept may soon become
the preferred solution for draining deep-water gas reserves.
[0029] Therefore the need to increase the safety of such a system
is of great importance.
[0030] One possibility is to use a nitrogen based process, but this
has the significant disadvantage that the thermal efficiency is
much lower than a hydrocarbon based system. In addition, because
nitrogen has a low heat transfer coefficient, a large heat transfer
area is required to dissipate the waste heat from the process into
a cooling medium. Consequently, despite the safety hazards
involved, hydrocarbon-based refrigeration cycles continue to be
used.
[0031] Another non-flammable and inert refrigerant option is carbon
dioxide, which may operate in a vapour compression cycle giving
reasonable efficiency. Carbon dioxide has a freezing point of
-56.6.degree. C., which restricts the minimum possible evaporating
temperature due to the risk of dry ice formation. Therefore carbon
dioxide is an option for the precooling process only. Since most of
the hydrocarbon refrigerant inventory is in the precooling cycle, a
change over to CO.sub.2may still improve the safety of the
liquefaction process significantly.
[0032] Apart from being non-flammable and its high triple point,
carbon dioxide is also distinguished from the common hydrocarbon
refrigerants for natural gas precooling by its rather low critical
temperature (31.1.degree. C.), which is comparable to that of
ethane (32.3.degree. C.).
[0033] WO 01/69149 discloses the possibility of providing a carbon
dioxide precooling circuit in a cascade arrangement with a main
cooling circuit.
[0034] The low critical temperature of CO.sub.2 is a disadvantage
since the throttling loss and heat rejection loss in the
refrigerating cycle will be larger than for C.sub.3 and
C.sub.3/C.sub.2 mixtures. In addition, the heat transfer loss will
be larger than with mixed refrigerant due to constant-temperature
evaporation.
[0035] It has been found that replacing a traditional
C.sub.3/C.sub.2 precooling process, for example that disclosed in
U.S. Pat. No. 6,253,574, with an equivalent CO.sub.2 process
increases the total power consumption for liquefaction by about
10%, which is considered unacceptable. This consumption increase is
due to the reduction in efficiency of the cycle due to the low
critical temperature of carbon dioxide. In addition, the
evaporating temperature in the first stage of the CO.sub.2
precooling cycle is only a few degrees higher than the CO.sub.2
triple point. This leads to operational problems and a danger of
dry ice formation.
[0036] There therefore exists a need for an efficient liquefaction
process containing a CO.sub.2 precooling circuit.
[0037] The applicants of the present invention have realised that a
carbon dioxide pre-cooling circuit can be combined with the load
balanced liquefaction process described above in order to overcome
the above discussed problems with using carbon dioxide.
[0038] Therefore, in a preferred embodiment of the present
invention the first refrigeration circuit comprises carbon
dioxide.
[0039] This concept is considered inventive in its own right and
therefore, according to another aspect of the present invention
there is provided a substantially load balanced mixed refrigerant
cascade process comprising a carbon dioxide pre-cooling
circuit.
[0040] As the liquefaction compressor takes over some of the
pre-cooling cycle load, the carbon dioxide circuit can be operated
to provide a higher minimum evaporation temperature and thus the
risk of dry ice formation is reduced. In addition, as the load of
the carbon dioxide cycle is reduced the impact of the lower
thermodynamic efficiency of CO.sub.2 compared with C.sub.2/C.sub.3
is alleviated. In a load balanced process where each compressor
contributes a third of the total power consumption, the increase in
power consumption caused by using CO.sub.2 can be reduced to only a
few percent greater than when using hydrocarbons.
[0041] In order to achieve maximum efficiency from the carbon
dioxide circuit it is preferable that the carbon dioxide is cooled
after condensation to a temperature of 20.degree. C. or less, more
preferably to 15.degree. C. or less. This can be achieved using air
cooling although preferably cold cooling water is used. As the
invention is particularly suited for offshore application the water
is preferably sea water, preferably extracted from a depth suitable
to give the required low temperature.
[0042] Preferably therefore the carbon dioxide pre-cooling cycle
includes a sub-cooling heat exchanger installed after the
condenser.
[0043] Using this method the reduction in total power consumption
is great enough to make using a CO.sub.2 pre-cooling circuit a
viable option in both on and offshore LNG facilities.
[0044] Preferably the carbon dioxide cooling circuit comprises
three pressure levels in order to improve the thermodynamic
efficiency of the process.
[0045] In order to reduce the internal heat load of the precooling
circuit it is preferable that only a substream of carbon dioxide is
subcooled in the pre-cooling circuit. This is unlike the second and
third cooling cycle refrigerants, the full sub-cooling of which
increases the efficiency of the process.
[0046] The higher operating pressure required when using CO.sub.2
means that it my be preferable to use a high pressure casing with
the carbon dioxide compressor. More preferably the compressor can
be split into two casings and a barrel type casing used for the
high pressure stage.
[0047] According to another aspect of the present invention there
is provided a LNG liquefaction process comprising three cascade
cycles each driven by a compressor, wherein the compressors are
substantially equally loaded and one of the cascade cycles is a
carbon dioxide cycle.
[0048] According to a further aspect of the present invention there
is provided a carbon dioxide pre-cooling circuit for LNG
liquefaction wherein the carbon dioxide has a minimum evaporation
temperature of no less then -50.degree. C., preferably no less than
-40.degree. C. and most preferably no less than -35.degree. C.
[0049] Preferred embodiments of the present invention shall now be
described, by way of example only, with reference to the following
drawings, in which:
[0050] FIG. 1 shows a load balanced liquefaction process in
accordance with a preferred embodiment of the invention;
[0051] FIG. 2 show an alternative embodiment of a load balanced
process;
[0052] FIG. 3 shows a graph of overall power demand as a function
of a reference temperature;
[0053] FIG. 4 shows a load balanced liquefaction process containing
a carbon dioxide pre-cooling circuit;
[0054] FIG. 5 shows hot/cold composite curves for the processes
shown in FIGS. 2 and 4; and
[0055] FIG. 6 shows a comparison of refrigerant inventories of the
processes shown in FIGS. 2 and 4.
[0056] In FIG. 1 the cooling and liquefaction of the
hydrocarbon-rich flow, which is conducted via line 1, are effected
against a mixed refrigerant circuit cascade, consisting of three
mixed refrigerant circuits. These as a rule have different
compositions, such as are described, for example, in the
aforementioned German published application 197 16 415.
[0057] The hydrocarbon-rich flow which is to be liquefied is cooled
in the heat exchanger E1 against the two evaporating mixed
refrigerant flows 4b and 4d of the first mixture circuit 4a to 4e,
then cooled by the evaporating mixed refrigerant flow 3d, and then
conducted via line 1a to a heavy hydrocarbon separation unit S,
represented simply as a box.
[0058] In this separation unit S the C.sub.3+ separation described
heretofore takes place, whereby the components separated out of the
hydrocarbon-rich flow are drawn off from the heavy hydrocarbon
separation unit S via line 1b.
[0059] According to one advantageous embodiment of the method
according to the invention, not shown in the drawing, at least one
part flow of one of the two part flows 3b and 3d of the second
cooling agent mixture circuit 3a to 3e, which will be discussed in
greater detail hereinafter, is used for the provision of cooling in
the separation unit S. In this situation, the choice of which of
the two part flows 3b and/or 3d is drawn from for this provision of
cooling is determined by the temperature level(s) required in the
heavy hydrocarbon separation unit S.
[0060] The hydrocarbon-rich flow to be liquefied is then conducted
via line 1c to a second heat exchanger E2, and is liquefied in this
against the evaporating mixed refrigerant flow 3b of the second
cooling circuit 3a to 3e.
[0061] Once liquefaction has taken place, the hydrocarbon-rich flow
is conducted via line 1d to a third heat exchanger E3, and is
subcooled here against the mixed refrigerant flow 2b of the third
cooling circuit 2a to 2c. The subcooled liquid product is then
conducted via line le to its further use.
[0062] As can be seen from the drawing, each of the three cooling
circuits 2a to 2c, 3a to 3e, and 4a to 4e, has a compressor, V2,
V3, and V4 respectively. Not shown in the drawing are the
corresponding drives for these compressors V2, V3, and V4. In
addition, the coolers or heat exchangers which are located
downstream of the compressors V2, V3, and V4 respectively are not
shown in the drawing, in which the refrigerant mixture is cooled
against a cooling medium, such as water.
[0063] The refrigerant mixture of the first refrigerant circuit,
compressed in the compressor V4, is conducted via line 4a to the
heat exchanger E1, and is divided here into two part flows 4b and
4d after cooling has taken place. The refrigerant mixture in these
part flows 4b and 4d, after throttling has been effected in the
valves d and e or expansion devices, is evaporated to different
pressure levels in the heat exchanger E1 and then conducted via
line 4c or 4e to the compressor V4 before the first stage (part
flow 4c) or to an intermediate pressure level (part flow 4e).
[0064] The refrigerant mixture of the second cooling circuit 3a to
3e, compressed in the compressor V3, is conducted via line 3a
through heat exchangers E1 and E2, and is cooled in these. That
part flow 3b of this refrigerant mixture flow, which is conducted
through heat exchanger E2, after expansion in valve b, is
evaporated in heat exchanger E2 against cooling process flows, and
is then conducted via line 3c to the intake stage of compressor
V3.
[0065] According to the invention, a part flow 3d of the
refrigerant mixture of the second refrigerant mixture circuit 3a to
3e is drawn off after the heat exchanger E1, expanded in valve c,
and then evaporated in heat exchanger E1 against cooling process
flows, before being conducted via line 3e, at an intermediate
pressure level, to the circuit compressor V3. Accordingly, the
refrigerant mixture part flow 3d, according to the invention, makes
a contribution to the pre-cooling of the hydrocarbon-rich flow in
heat exchanger E1.
[0066] In order for this to be achieved, the part flow 3d of the
refrigerant mixture of the second mixed refrigerant circuit 3a to
3e, used for the pre-cooling of the hydrocarbon-rich flow, must be
evaporated at a pressure which is higher than the evaporation
pressure of the mixed refrigerant part flow 3b of the second mixed
refrigerant circuit 3a to 3e.
[0067] By selecting the intermediate pressure at which the mixed
refrigerant part flow 3e is evaporated and conducted to the
compressor V3, and by regulating the volume distribution of the two
mixed refrigerant part flows 3b and 3d, the distribution of the
cooling capacity of the second refrigerant circuit onto the heat
exchangers E1 and E2, and therefore to the pre-cooling and
liquefaction of the hydrocarbon-rich flow which is to be liquefied,
can be adjusted almost at will.
[0068] If, for example, 40% of the total drive capacity is required
for the pre-cooling and 60% for the liquefaction and subcooling of
the hydrocarbon-rich flow, then, with the concept and method
according to the invention, one compressor is used in each case
with a third of the total drive capacity in the first and third
refrigerant mixture circuit, i.e. for the pre-cooling as well as
for the subcooling of the hydrocarbon-rich flow which is to be
liquefied. The compressor of the second refrigerant mixture circuit
is operated according to the invention in such a way that it uses
20% of its capacity, and consequently 6.66% of the total capacity,
for pre-cooling, while the remaining 80%, i.e. 26.66% of the total
capacity, is used for liquefaction.
[0069] The method according to the invention accordingly makes
possible the economical exploitation of the available compressors
and drive units, because the (circuit) compressors of the three
refrigerant circuits obtain approximately the same drive capacity,
i.e. a third of total capacity in each case.
[0070] Accordingly, large liquefaction plants in particular, with a
liquefaction capacity greater than 5 million tonnes LNG per year,
can be operated substantially more economically, since, by
standardizing the drives and compressors of the three cooling
circuits, the achievable liquefaction capacity of the liquefaction
process can be maximised with the use of tried-and-trusted drive
units and compressors.
[0071] FIG. 2 shows an alternative version of the load balanced
process. As with FIG. 1 the pre-cooling cycle C10 comprises a first
circuit driven by a first compressor V10 and one part 22 of the
refrigerant stream 21 from the second cycle C20. Three General
Electric MS 7121 EA (Frame 7) gas turbines are used to drive the
compressors V10, V20, V30. If highest availability is of the
essence, the three refrigeration cycles can be designed with two
times 50% gas turbine/compressor trains. In this case six GE MS
6581 B (Frame 6) gas turbines would replace the three Frame 7s.
[0072] All LNG plants require the extraction of at least of those
hydrocarbons, which would freeze in the LNG under storage
conditions (e.g. aromatics and C.sub.5+). In an LNG plant
precooling is usually considered as first cooling step between
ambient temperature and extraction of the mentioned
hydrocarbons.
[0073] It should be emphasised that the method according to the
invention can be combined with all known separation methods
considered to be prior art for relatively high-boiling
hydrocarbons.
[0074] The precooling portion of the overall power demand of all
refrigeration compressors for the two gases defined in Table 1 is
shown in FIG. 3 as a function of a reference temperature. This is
the temperature, under which all main process streams (natural gas,
refrigerant fluids) enter into the cryogenic heat exchangers.
TABLE-US-00001 TABLE 1 Lean Rich mol % Gas Gas N.sub.2 5.00 5.00
CH.sub.4 88.93 84.07 C.sub.2H.sub.6 3.96 5.58 C.sub.3H.sub.8 1.37
2.73 C.sub.4H.sub.10 0.48 1.34 C.sub.5H.sub.12 0.17 0.65
C.sub.6H.sub.14 0.06 0.32 C.sub.7H.sub.16 0.02 0.16 C.sub.8H.sub.18
0.01 0.08 Benzene 0.01 0.08 100.00 100.00
[0075] The lower the reference temperature and the richer the gas
the smaller the required compressor power for precooling becomes.
This situation can be addressed reasonably well by designers of
dual flow liquefaction processes, if the power mismatch between
precooling and liquefaction plus subcooling is compensated by
helpers for the gas turbines.
[0076] A process with three refrigeration cycles offers a much
wider field for even load distribution between the cycles. If part
of the refrigerant of the liquefaction cycle C20 is vaporized under
elevated pressure in the precooling section C10 and is fed to the
LC compressor V20 as side stream 22, a perfect load balancing
between all three refrigeration cycles can be achieved. This
feature is a major aspect of a cost effective design for large
production capacities. As all three (3) cycles are symmetrically
driven this arrangement is referred to as MFC*s3.
[0077] Unlike the embodiment of FIG. 1, the final compressor V30 of
FIG. 2 is split into two casings V31, V32. The second casing V32 is
designed to deal with high pressures at which the multistage
compressor operates.
[0078] In order to provide actual figures for a realistic process
design a large LNG train has been studied. On the basis of the lean
gas composition with a pressure of 62 bar and a temperature of 35
deg C. at the inlet to precooling a conceptual process design was
made. The refrigeration compressors are driven by Frame 7's with
additional 20 MW on each shaft, which have been recruited from the
starter/helpers. The resulting LNG rundown amounts to 8.5 mtpa at
333 stream days, which is accompanied by an additional quantity of
0.4 mtpa NGL (C.sub.3+ hydrocarbons). The specific energy
consumption of the refrigeration compressors is 259
kWh/t.sub.LNG.
[0079] In FIG. 4 the precooling circuit C10 of FIG. 2 has been
replaced with a pre-cooling circuit C100 which comprises a carbon
dioxide stream 101. After compression and condensation/subcooling
the stream 101 is split into three separate streams, 102, 103, 104
which are then expanded to different pressures. This compensates
for the constant temperature evaporation of CO.sub.2. Unlike
hydrocarbon streams 201, 301 only part of the carbon dioxide stream
101 is sub-cooled by the pre-cooling heat exchanger E100 prior to
expansion, in order to reduce the internal heat load of this
exchanger.
[0080] Owing to the higher operating pressure, the CO.sub.2
precooling compressor V100 is split into two casings, V110, V120
with a barrel type casing V120 for the high-pressure stage. After
compression the carbon dioxide is cooled by a water cooled
condenser C20 and an additional subcooling heat exchanger C22,
using seawater to subcool the liquid refrigerant after the
condenser C20, in order to improve process efficiency. In addition
a desuperheater can also be provided after the compressor, as in
many conventional systems.
[0081] As with the previous embodiments, "load balancing" is
achieved by allowing the liquefaction compressor V200 to take over
some of the precooling cycle load, leading to a "symmetrical"
process.
[0082] Process simulations of the above embodiment as shown in FIG.
4 and FIG. 2 gave power requirement data as shown in Table 2, using
the design data as shown in Table 3. As a result of the
load-balanced process, the power input to the CO.sub.2-precooled
case was only 4.4% higher than the baseline. For a given maximum
available power as defined by the hydrocarbon process case, this
would correspond to a LNG capacity of 95.6% with CO.sub.2
precooling unless more driver capacity is installed.
TABLE-US-00002 TABLE 2 C.sub.2/C.sub.3 CO.sub.2 precooling
precooling Total shaft power 162.7 155.8 MW (104%) (100%)
Precooling 49.6 47.6 MW compressor Liquefaction 50.5 47.7 MW
compressor Subcooling 50.5 48.5 MW compressor Other power 12.1 12.0
MW consumers
TABLE-US-00003 TABLE 3 LNG production capacity 5.8 mtpa Gross
calorific value 40 MJ/Sm.sup.3 of LNG Feed gas pressure 69 bar
(liquefaction inlet) Sea cooling water 5 .degree. C.
temperature
[0083] Temperature profiles in the form of hot/cold composite
curves for the two cases are shown in FIG. 5. The three CO.sub.2
precooling temperature levels are easily observed in the left
diagram. The highest pressure level to the liquefaction compressor
is also considered part of precooling. Changes in the subcooling
process are minimal between the two cases.
[0084] Layout, size and weight of an offshore LNG liquefaction
module with CO.sub.2 precooling were compared to the baseline
hydrocarbon case (that shown in FIGS. 1 and 2). Among the factors
that contributed to reduce the equipment footprints and give
smaller dimensions with CO.sub.2 were reduced precooling compressor
suction drum sizes and smaller precooling piping dimensions.
Additional equipment caused by the third precooling pressure
level/drum and the installation of a refrigerant subcooler made the
net reduction in footprint area marginal, however. The plate-fin
heat exchangers were reduced in size due to larger LMTD
(Logarithmic Mean Temperature Difference) and less internal duty.
While plate fin heat exchangers were used in this instance it is of
course also possible to use other types of heat exchangers, which
could also be reduced in size. Some of the major pipe sizes in the
liquefaction and subcooling circuit did not change much, and it is
these pipes that to a large degree set out the deck heights, so no
changes were envisaged relative to deck elevations. In total, it
was concluded that the liquefaction module size would be no greater
when using a CO.sub.2 precooling circuit, and indeed a reduction of
a few square meters is possible. In addition, the weight of the
module dropped by 100 tons.
[0085] A major safety concern of the LNG process with hydrocarbon
precooling, especially when applied offshore, is the possible
formation of a flammable and explosive hydrocarbon/air mixture in
case of a major leakage in one of the refrigerant circuits. Thus,
the minimization of hydrocarbon refrigerant inventory is very
important in terms of safety.
[0086] As may be observed from the FIG. 6, the HC refrigerant
inventory is reduced by about 70% in the CO.sub.2-precooled
process. The reduced hydrocarbon charge is positive in relation to
loss prevention and to the availability of the three main safety
functions of the LNG barge, which are (i) main structural strength,
(ii) main escape routes, and (iii) means of evacuation.
[0087] If the molecular weight of the hydrocarbon refrigerant is
higher than that of air, a flammable cloud can accumulate inside or
between the modules, and on the deck surfaces. Thus, in addition to
minimizing the total hydrocarbon inventory it is of special
importance to eliminate the heavier components, especially propane
(52% heavier than air), but also ethane (4% heavier than air). By
replacing the hydrocarbon precooling with CO.sub.2, all propane is
eliminated from the liquefaction module, and even though ethane is
present in the liquefaction and subcooling refrigerants, both these
mixtures have a molar mass that is lower than air.
[0088] From the above results it has been found that the
introduction of CO.sub.2 precooling in a load-balanced MFC*s3
process does not give a significant increase in specific power
requirement, or equipment size/weight/cost, while the safety of the
process can be improved.
* * * * *