U.S. patent application number 16/485883 was filed with the patent office on 2020-02-20 for lng production system equipped with recondenser.
The applicant listed for this patent is L'Air Liquide, Societe Anonyme pour l'Etude et l'Exploitation des Procedes Georges Claude. Invention is credited to Kenji HIROSE, Loic JOLY, Daisuke NAGATA, Shinji TOMITA.
Application Number | 20200056837 16/485883 |
Document ID | / |
Family ID | 63249524 |
Filed Date | 2020-02-20 |
![](/patent/app/20200056837/US20200056837A1-20200220-D00000.png)
![](/patent/app/20200056837/US20200056837A1-20200220-D00001.png)
![](/patent/app/20200056837/US20200056837A1-20200220-D00002.png)
![](/patent/app/20200056837/US20200056837A1-20200220-D00003.png)
![](/patent/app/20200056837/US20200056837A1-20200220-D00004.png)
![](/patent/app/20200056837/US20200056837A1-20200220-D00005.png)
![](/patent/app/20200056837/US20200056837A1-20200220-D00006.png)
![](/patent/app/20200056837/US20200056837A1-20200220-D00007.png)
United States Patent
Application |
20200056837 |
Kind Code |
A1 |
HIROSE; Kenji ; et
al. |
February 20, 2020 |
LNG PRODUCTION SYSTEM EQUIPPED WITH RECONDENSER
Abstract
An LNG production system including a boil off gas recondenser
that can recondense boil off gas without using a BOG compressor and
without depending on an LNG liquefaction process is provided.
Inventors: |
HIROSE; Kenji; (Kobe,
JP) ; JOLY; Loic; (Paris, FR) ; NAGATA;
Daisuke; (Kobe, JP) ; TOMITA; Shinji; (Kobe,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
L'Air Liquide, Societe Anonyme pour l'Etude et l'Exploitation des
Procedes Georges Claude |
Paris |
|
FR |
|
|
Family ID: |
63249524 |
Appl. No.: |
16/485883 |
Filed: |
February 13, 2018 |
PCT Filed: |
February 13, 2018 |
PCT NO: |
PCT/EP2018/053562 |
371 Date: |
August 14, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2290/34 20130101;
F25J 1/0245 20130101; F25J 1/0249 20130101; F25J 1/0272 20130101;
F25J 2235/60 20130101; F25J 1/0025 20130101; F25J 1/0244 20130101;
F25J 1/0262 20130101; F25J 1/0204 20130101; F25J 2205/90 20130101;
F25J 1/0022 20130101; F25J 1/0258 20130101; F25J 1/0251 20130101;
F25J 1/0261 20130101; F25J 2240/40 20130101; F25J 1/0254 20130101;
F25J 1/0072 20130101; F25J 1/0212 20130101; F25J 1/0052 20130101;
F25J 2210/04 20130101; F25J 1/0045 20130101; F25J 2210/42 20130101;
F25J 1/0264 20130101 |
International
Class: |
F25J 1/00 20060101
F25J001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 14, 2017 |
JP |
2017-024938 |
Feb 28, 2017 |
JP |
2017-036432 |
Claims
1-12. (canceled)
13. An LNG production system, comprising: a liquefier that cools
and liquefies natural gas by indirect heat exchange with a
refrigerant that is fed from a first refrigerator; an LNG tank that
stores liquid natural gas liquefied in the liquefier; a transfer
line for transferring the liquid natural gas from the LNG tank to
an LNG carrier; a recondenser that recondenses boil off gas that is
generated by the liquid natural gas, the recondensing performed by
indirect heat exchange with the refrigerant fed from a second
refrigerator; and a return line that feeds the recondensed boil off
gas to the LNG tank from the recondenser.
14. The LNG production system of claim 13, wherein the first
refrigerator and the second refrigerator are the same.
15. The LNG production system of claim 13, wherein the first
refrigerator and the second refrigerator are different.
16. The LNG production system of claim 13, wherein the recondenser
switches to perform, alternately, a first recondensation in a
recondenser that recondenses boil off gas that is generated by the
liquid natural gas, the recondensing performed by indirect heat
exchange with the refrigerant fed from a second refrigerator, and a
second recondensation wherein the boil off gas is recondensed by
indirect heat exchange with the refrigerant from the second
refrigerator and refrigerant from a refrigerant buffer, thereby
processing more boil off gas than the boil off gas at a time of the
first recondensation processing.
17. The LNG production system of claim 13, wherein the recondenser
is designed to recondense the boil off gas by a refrigerant under a
pressure lower than an operating pressure of the LNG tank.
18. The LNG production system of claim 13, wherein the recondenser
is internally provided with a heat exchanger in which the
refrigerant is introduced, and the boil off gas is introduced into
the heat exchanger, and is cooled by the refrigerant.
19. The LNG production system of claim 18, wherein an external
capacity of the heat exchanger is smaller than an internal space
capacity of the recondenser, and the heat exchanger is disposed in
the internal space of the recondenser.
20. A recondenser within an LNG production system that liquefies
natural gas to produce liquid natural gas, and recondenses boil off
gas in an LNG tank in which the liquid natural gas that is
liquefied is stored, by a refrigerant, the condenser comprising: a
return line that directly feeds liquid natural gas that is
recondensed to the LNG tank.
21. The recondenser of claim 20, wherein the recondenser is
designed to recondense the boil off gas by a refrigerant under a
pressure lower than an operating pressure of the LNG tank.
22. The recondenser of claim 20, wherein the recondenser is
internally provided with a heat exchanger in which the refrigerant
is introduced, and the boil off gas is introduced into the heat
exchanger, and is cooled by the refrigerant.
23. The recondenser of claim 22, wherein an external capacity of
the heat exchanger is smaller than an internal space capacity of
the recondenser, and the heat exchanger is disposed in the internal
space of the recondenser.
Description
TECHNICAL FIELD
[0001] The present invention relates to an LNG production system
equipped with a recondenser (Recondenser) that recondenses
(reliquefies) boil off gas (BOG, Boil off Gas).
BACKGROUND ART
[0002] FIG. 6 illustrates a general LNG production system. Natural
gas (NG, Natural Gas) is fed to a CO.sub.2 removal process 62 in a
subsequent stage by a compressor 61. In the CO.sub.2 removal
process 62, CO.sub.2 is removed from the natural gas by using a
predetermined solvent. The NG after removal from which CO.sub.2 is
removed is fed to a dry process 63. In the dry process 63,
predetermined drying treatment is applied to the NG after removal.
The dried NG is fed to a liquefaction process 64. In the
liquefaction process 64, the dried NG is liquefied by using a
liquid refrigerant that is fed from a refrigeration system 65. The
liquid natural gas (LNG) obtained by liquefaction is fed to an LNG
storage tank 66. Subsequently, in an LNG loading process 68, LNG is
fed from the LNG storage tank 66 at a predetermined timing (for
example, at the timing at which the LNG is transferred to a
transport ship tank, or the like). Here, LNG in the LNG storage
tank 66 sometimes evaporates by natural heat input and generates
BOG. Further, when LNG is transferred to the transport ship tank
from the LNG storage tank 66 in the LNG loading process 68, a large
amount of BOG is sometimes generated. Further, piping is cooled
when LNG is transferred to the transport ship tank, so that BOG is
sometimes generated.
[0003] Many LNG storage tanks 66 which are installed in the LNG
production bases have large capacities, and design pressure thereof
is generally set at a vicinity of the atmospheric pressure from the
viewpoints of technology and cost. Consequently, it is necessary to
discharge BOG from the LNG storage tank 66 even at the time of a
slight pressure increase. Further, the pressure increase also
occurs by a piston effect (also referred to as "pushup effect")
accompanying supply of LNG to the LNG storage tank 66 from the
liquefaction process 64, and therefore supply of LNG to the LNG
storage tank 66 causes regular discharge of BOG. Further, a flash
loss of decompression accompanying charge to the LNG storage tank
66 from a liquefier of the liquefaction process 64 also occurs.
Note that LNG loss by a flash loss sometimes occupies approximately
50% of the BOG generation amount.
[0004] However, it is undesirable to discharge BOG into the
atmosphere in terms of environmental and economical aspects, so
that BOG is conventionally returned to the dry process 63 by a
compressor 67, and is supplied to the liquefaction process 64 with
the dried NG. Thereby, it is possible to reliquefy BOG.
Alternatively, BOG is sometimes used as the heat source for
regeneration of a drying material or the like in the dry process
63.
[0005] However, in the case of reliquefaction of the BOG described
above, a total amount of NG that is fed to the liquefaction process
64 from the dry process 63 includes the recycled BOG. Further, use
as the fuel gas means that the total amount of LNG which is
produced cannot be transferred to the transport ship tank.
[0006] Further, it is required to bring BOG into a high pressure
state in order to reliquefy the BOG, so that the compressor 67 is
required at the time of recycle that returns the BOG to the dry
process 63. Consequently, consumption of a large amount of energy
is required to compress the BOG.
[0007] Further, when BOG is returned to the dry process 63 as
described above, BOG can be reliquefied only at the lime of LNG
production, so that when BOG has to be discharged from the LNG
storage tank at the time of producing no LNG, BOG has to be
discharged into the air. That is, the timing for reliquefaction is
limited, and there is no flexibility in reliquefaction process.
[0008] The LNG production system described in Patent Literature 1
presents no method of solution to the above described problem.
CITATION LIST
Patent Literature
[0009] [Patent Literature 1] U. S. Patent No. 2011/0094261
SUMMARY OF INVENTION
Technical Problem
[0010] An object of the present invention is to provide an LNG
production system including a recondenser that can recondense BOG
(boil off gas) without using a BOG compressor and without depending
on an LNG liquefaction process.
Solution to Problem
[0011] A first LNG production system of the present invention
includes
[0012] a liquefier that cools and liquefies natural gas by a
refrigerant that is fed from a refrigerator,
[0013] an LNG tank that stores liquid natural gas (LNG) liquefied
in the liquefier,
[0014] a transfer line for transferring the liquid natural gas from
the LNG tank,
[0015] an LNG carrier that is disposed in a subsequent stage of the
transfer line and is for transferring the liquid natural gas,
[0016] a recondenser that recondenses (reliquefies) boil off gas
that is generated by heat being given to the liquid natural gas, by
the refrigerant fed from the refrigerator, and
[0017] a return line that feeds liquid natural gas that is
liquefied to the LNG tank from the recondenser.
[0018] A second LNG production system of the present invention
includes
[0019] a liquefier that cools and liquefies natural gas by a
refrigerant that is fed from a first refrigerator,
[0020] an LNG tank that stores liquid natural gas liquefied in the
liquefier,
[0021] a transfer line for transferring the liquid natural gas from
the LNG tank,
[0022] an LNG carrier that is disposed in a subsequent stage of the
transfer line and is for transferring the liquid natural gas,
[0023] a recondenser that recondenses boil off gas (BOG) that is
generated by heat being given to the liquid natural gas, by a
refrigerant fed from the second refrigerator, and
[0024] a return line that feeds liquid natural gas that is
liquefied to the LNG tank from the recondenser.
[0025] In the above described invention, the refrigerant that is
fed from the first refrigerator, and the refrigerant that is fed
from the second refrigerator may be the same refrigerants, or may
be different refrigerants. For example, as the refrigerant from the
first refrigerator, a mixture such as hydrocarbon is cited, and as
the refrigerant from the second refrigerator, nitrogen or the like
is cited.
[0026] A third LNG production system of the present invention
includes
[0027] a liquefier that cools and liquefies natural gas by a
refrigerant that is fed from a refrigerator,
[0028] an LNG tank that stores liquid natural gas liquefied in the
liquefier,
[0029] a transfer line for transferring the liquid natural gas from
the LNG tank,
[0030] an LNG carrier that is disposed in a subsequent stage of the
transfer line and is for transferring the liquid natural gas,
[0031] a recondenser that switches to perform, alternately, a first
recondensation processing of liquefying boil off gas that is
generated by heat being given to the liquid natural gas, by the
refrigerant fed from the refrigerator, and a second recondensation
processing of liquefying boil off gas by the refrigerant fed from
the refrigerator and a refrigerant fed from a refrigerant buffer to
process more boil off gas than the boil off gas at a time of the
first recondensation processing, and
[0032] a return line that feeds liquid natural gas that is
liquefied to the LNG tank from the recondenser.
[0033] In the above described invention, the refrigerant that is
fed from the refrigerator, and the refrigerant that is fed from the
refrigerant buffer may be the same refrigerants, or may be
different refrigerants. For example, as the refrigerant from the
refrigerator, a mixture such as hydrocarbon is cited, and as the
refrigerant from the refrigerant buffer, nitrogen or the like is
cited.
[0034] In the aforementioned first recondensation processing, the
refrigerant may be fed to the recondenser from the refrigerator,
and in the second recondensation processing, the refrigerant from
the refrigerant buffer may be fed to the recondenser in addition to
that the refrigerant is fed to the recondenser from the
refrigerator. In the case of switching to the second recondensation
processing from the first recondensation processing, the operation
of the refrigerator may be stopped, or the refrigerator may be
continuously operated without stopping.
[0035] The recondenser may have a switch control section that
switches the first recondensation processing and the second
recondensation processing to each other.
[0036] The switch control section may switch from the first
recondensation processing to the second recondensation processing,
in the case of transferring BOG to the LNG carrier.
[0037] The switch control section may switch from the first
recondensation processing to the second recondensation processing
when a pressure value measured by a pressure gauge disposed in the
LNG tank or the feed line that feeds the BOG to the recondenser
becomes a predetermined value or more.
[0038] According to the above described respective configurations,
in the case of processing BOG in an amount in a predetermined range
(a flow rate per unit time) or of a pressure value in a
predetermined range that is set in advance, the first
recondensation processing (processing of liquefying BOG with the
refrigerant from the refrigerator) is executed, and in the case of
processing BOG that exceeds the above described amount in the
predetermined range or pressure value in the predetermined range,
the second recondensation processing (processing of liquefaction
also by the refrigerant fed from the refrigerant buffer while
keeping liquefaction by the refrigerant fed from the refrigerator)
can be executed, so that BOG can be recondensed without using a BOG
compressor and without depending on the LNG liquefaction
process.
[0039] In each of the above described first to third LNG production
system, predetermined treatment may be applied to the
aforementioned natural gas that is fed to the liquefier in advance.
For example, each of the LNG production systems may include a
removing device that removes predetermined impurities from natural
gas, and a dryer that dries the natural gas that is treated by the
removing device.
[0040] The transfer line may be provided with piping and a sluice
valve.
[0041] The return line may be provided with piping, a pump for
feeding LNG and an automatic on-off valve.
[0042] A feed line that feeds the BOG to the recondenser from the
LNG tank may be included. The feed line may be provided with any
one or more of piping, an automatic on-off valve, a flow rate
control valve and a pressure regulating valve.
[0043] A pressure gauge that measures the pressure of the LNG tank
may be provided. When a pressure value of the pressure gauge
reaches a predetermined value or more, valves of the feed line and
the return line may open, and BOG may be fed to the recondenser
through the feed line.
[0044] The recondenser may be controlled so as to increase a
cooling capability of the recondenser when the pressure value of
the pressure gauge installed in the feed line reaches the
predetermined value or more. For example, control may be performed
so as to increase the feeding amount of the refrigerant that is fed
from the refrigerator (the first or the second refrigerator), for
example.
[0045] The LNG carrier may be, for example, a loading station
container, a loading pier, a loading station truck, and the
like.
[0046] A recovery line for returning BOG that is present in the LNG
carrier to the LNG tank may be provided.
[0047] In the third LNG production system, the refrigerant stored
in the refrigerant buffer may be supplied from the refrigerator or
an external refrigerator.
[0048] The recondenser may have a piping through which the
refrigerant fed from the refrigerator passes, and a piping through
which the refrigerant fed from the refrigerant buffer passes as
separate components, and the returning refrigerants may be returned
together to the refrigerator.
[0049] The recondenser may have a first heat exchanger to which the
refrigerant fed from the refrigerator is introduced, and a second
heat exchanger to which the refrigerant fed from the refrigerant
buffer is introduced.
[0050] In each of the above described first to third LNG production
systems, the recondenser preferably has the following
configuration.
[0051] The recondenser is designed to recondense (liquefy) the boil
off gas by a refrigerant under a pressure lower than an operating
pressure of the LNG tank.
[0052] According to the configuration, BOG can be recondensed under
the pressure lower than the operating pressure of the LNG tank
without using the conventional BOG compressor.
[0053] The recondenser may be internally provided with a heat
exchanger into which the refrigerant is introduced, and the BOG may
be introduced into the heat exchanger, and is cooled by the
refrigerant. Thereby, BOG can be effectively liquefied in a mode of
the heat exchanger.
[0054] A volume (an external capacity) of the heat exchanger may be
smaller than an internal volume (an internal space capacity) of the
recondenser, and the heat exchanger may be disposed in the internal
space of the recondenser.
[0055] Thereby, BOG can be effectively liquefied in the mode of the
heat exchanger. The liquefied LNG accumulates on the bottom of the
recondenser. The accumulating LNG can be fed to the LNG tank by a
liquid feed pump.
[0056] The pressure in the recondenser or in the heat exchanger may
be regulated as follows.
(1) Before sending BOG, feed the refrigerant, and pre-cool the
inside of the recondenser or the inside of the heat exchanger.
After a lapse of a predetermined time period, or when the inside of
the recondenser or the inside of the heat exchanger reaches a
predetermined temperature, start introduction of BOG. (2) The
introduced BOG is liquefied, and accumulates on the bottom of the
recondenser or the heat exchanger. The liquefied LNG accumulating
on the bottom can be fed to the LNG tank by the pump, a
pressurizing device or the like.
[0057] A fourth LNG production system of the present invention
includes
[0058] a liquefier that cools and liquefies natural gas by a
refrigerant that is fed from a refrigerator,
[0059] an LNG tank that stores liquid natural gas liquefied in the
liquefier,
[0060] a transfer line for transferring the liquid natural gas from
the LNG tank,
[0061] an LNG carrier that is disposed in a subsequent stage of the
transfer line and is for transferring the liquid natural gas,
[0062] an LNG lead-out line that leads out the liquid natural gas
from the LNG tank,
[0063] a sub-cooler that is provided in the LNG lead-out line and
cools the liquid natural gas with a refrigerant (for example,
liquid nitrogen or the like),
[0064] a recondenser that recondenses boil off gas that is
generated by heat being given to the liquid natural gas, by the
liquid natural gas cooled in the sub-cooler, and
[0065] a return line that feeds liquid natural gas that is
liquefied to the LNG tank from the recondenser.
[0066] In the present invention, the recondenser may recondense
(liquefy) boil off gas with LNG that is cooled by the sub-cooler
under a pressure lower than an operating pressure of the LNG
tank.
[0067] According to the configuration, the liquid natural gas is
firstly cooled by using the refrigerant such as LN.sub.2, and boil
off gas is liquefied with the cooled liquid natural gas. Thereby,
recondensation of boil off gas can be effectively performed under
the pressure lower than the operating pressure of the LNG tank.
[0068] In the fourth invention, the sub-cooler may be controlled so
that liquid natural gas has a higher temperature than a solidifying
point of the liquid natural gas by the pressure regulating valve or
the flow rate regulating valve that is installed in the refrigerant
line in which the refrigerant flows.
[0069] In the fourth invention, two or more of the sub-coolers may
be adopted. In the case of the two sub-coolers, the first
recondensation processing of liquefying BOG by the refrigerant fed
from the first sub-cooler, and the second recondensation processing
of liquefying BOG by the refrigerant fed from the first sub-cooler
and the refrigerant fed from the second sub-cooler in order to
process more boil off gas than the boil off gas at a time of the
first recondensation processing may be performed by switching the
first recondensation processing and the second recondensation
processing to each other. In the case of switching from the first
recondensation processing to the second recondensation processing,
the operation of the refrigerator may be stopped, or the
refrigerator may be continuously operated without stopping.
[0070] The recondenser may have a switch control section that
switches the first recondensation processing and the second
recondensation processing to each other.
[0071] The switch control section may switch from the first
recondensation processing to the second recondensation processing
in the case of transferring boil off gas to the LNG carrier.
[0072] The switch control section may switch from the first
recondensation processing to the second recondensation processing
when a pressure value measured by a pressure gauge disposed in the
LNG tank or the feed line that feeds the BOG to the recondenser
becomes a predetermined value or more.
[0073] The refrigerant in the second sub-cooler may be supplied
from the refrigerant buffer in which the refrigerant is stored in
advance.
[0074] In the above described LNG production system, the pump for
feeding liquid natural gas (LNG) to the transfer line from the LNG
tank may be an in-tank type pump that is installed inside the LNG
tank, or may be a pump that is disposed on the transfer line.
BRIEF DESCRIPTION OF DRAWINGS
[0075] FIG. 1 is a diagram illustrating a configuration example of
an LNG production system of embodiment 1.
[0076] FIG. 2 is a diagram illustrating a configuration example of
an LNG production system of embodiment 2.
[0077] FIG. 3 is a diagram illustrating a configuration example of
an LNG production system of embodiment 3.
[0078] FIG. 4A is a diagram illustrating a configuration example of
a recondenser.
[0079] FIG. 4B is a diagram illustrating a configuration example of
the recondenser.
[0080] FIG. 4C is a diagram illustrating a configuration example of
the recondenser.
[0081] FIG. 5A is a diagram illustrating a configuration example of
an LNG production system in embodiment 4.
[0082] FIG. 5B is a diagram illustrating a configuration example of
the recondenser.
[0083] FIG. 5C is a diagram illustrating a configuration example of
the recondenser.
[0084] FIG. 6 is a diagram illustrating a configuration example of
a conventional LNG production system.
DESCRIPTION OF EMBODIMENTS
[0085] Hereunder, several embodiments of the present invention will
be described. The embodiments described as follows explain only
examples of the present invention. The present invention is not
limited by the following embodiments in any way, and also includes
various modified modes that are carried out within the range
without changing the gist of the present invention. Note that all
of components described as follows are not always indispensable
components of the present invention.
Embodiment 1
[0086] An LNG production system 1 of embodiment 1 will be described
with reference to FIG. 1. The LNG production system 1 has a first
line L1 for transferring natural gas to a process in a subsequent
stage, a compressor 11 and a second line L2 (a pipe, for example).
As the process in the subsequent stage, a removing unit 12 is
disposed, and a predetermined substance (CO2, for example) is
removed from NG here. Next, the NG after removal is fed to a dryer
13 through a third line L3, and is subjected to drying treatment.
Next, the dried NG is fed to a liquefier 14 through a fourth line
L4 and is liquefied. A refrigerant (a liquid refrigerant) is fed to
the liquefier 14 from a refrigerator 15 to cool NG, and LNG is
obtained. Further, the refrigerant which is subjected to heat
exchange returns to the refrigerator 15 in an evaporated state. LNG
is fed to the LNG tank 16 through a fifth line L5 and stored. The
first line L1 to the fifth line L5 are configured by pipes and
on-off valves, for example. A predetermined control device
(controller) controls operation of the respective devices, opening
and closing of the valves, a production amount of LNG, and the like
of the LNG production system 1.
[0087] In the LNG tank 16, an in-tank type first pump P1 is
disposed, and LNG in the tank is fed into an LNG carrier 18 through
a transfer line L6 by the first pump P1. As the LNG carrier 18, for
example, a loading station container, a loading pier, a loading
station track, and the like are cited. BOG that is present in the
LNG carrier 18 is fed to the LNG tank 16 through a recovery line
A2. Instead of or in addition to the recovery line A2, a second
feed line for feeding BOG present in the LNG carrier 18 to a
recondenser 17 may be provided.
[0088] In the LNG tank 16, BOG is generated by heat input. Further,
when LNG is fed from the liquefier 14, BOG is also generated.
Further, when LNG is fed to the LNG carrier 18, BOG is also
generated. In this way, BOG in the LNG tank 16 is fed to the
recondenser 17 through a first feed line A1. Further, BOG in the
transfer line L6 is fed to the recondenser 17 through a third feed
line A3.
[0089] A refrigerant (liquid refrigerant) is introduced into the
recondenser 17 through a refrigerant line B1 from the refrigerator
15. By the refrigerant, BOG fed by each of the feed lines is
recondensed (liquefied). A configuration of the recondenser 17 is
described later. The LNG that is obtained by being recondensed
(liquefied) is returned to the LNG tank 16 through a return line
A4. In the return line A4, a second pump P2 is disposed, and LNG is
fed to the LNG tank 16 by operating the second pump P2.
[0090] According to the present embodiment, a series of processes
is not required, which feeds BOG to the dryer and feeds BOG to the
liquefier with NG to liquefy BOG as in the conventional art.
Consequently, it is not necessary to operate the entire LNG
production system, and only the refrigerator 15 can be operated.
The recondenser 17 can recondense BOG to LNG, so that all of the
liquefaction capability of the liquefier 14 can be used in
liquefaction of NG that is fed from the dryer.
(Recondenser)
[0091] FIGS. 4A and 4B illustrate an embodiment of the recondenser
17. In FIG. 4A, the recondenser 17 has an outer wall 171, and a
heat exchanger 172 covered with the outer wall 171. A refrigerant
(a liquid refrigerant) is introduced into the heat exchanger 172
from the refrigerator 15 through a refrigerant line B1, and BOG is
cooled by cold energy of the refrigerant. The refrigerant
evaporates and returns to the refrigerator 15 through a refrigerant
return line B2. LNG is fed to the LNG tank 16 from the recondenser
17 by the second pump P2.
[0092] The recondenser 17 is designed to recondense (liquefy) BOG
with the refrigerant under a pressure lower than an operating
pressure of the LNG tank 16.
[0093] The first feed line A1 may be provided with a safety valve
for a time when the pressure in the LNG tank 16 becomes abnormally
high. Further, in the first feed line A1, an automatic on-off valve
42 for performing feeding control of BOG to the recondenser 17 is
provided. Further, in the first feed line A1, a pressure gauge, and
a pressure regulating valve 41 controlled in accordance with a
value of the pressure gauge are provided.
[0094] The operating pressure in the LNG tank 16 is an average of
1.2 barA (120 KPaA) in absolute pressure, and is controlled within
.+-.15% as upper and lower limit values. When a large amount of BOG
is generated, a tank internal pressure becomes high. The tank
internal pressure is measured with a pressure gauge, and based on a
measurement result (a conversion result), a valve control section
(not illustrated) controls opening and closing of the automatic
on-off valve 42. For example, when the tank internal pressure
becomes 1.3 times as high as 1.2 barA (120 KPaA), BOG is fed to the
recondenser 17. As for the pressure regulating valve 41, in-pipe
pressure of the first feed line A1 is measured, and a valve opening
degree is controlled based on the measurement result.
[0095] The refrigerant which is supplied from the refrigerator 15
may be any medium having a lower temperature than a boiling point
of LNG, and LN2 may be used, for example.
[0096] Internal pressure of the heat exchanger 172 is controlled to
be a pressure lower than the operating pressure (average of 1.2
barA (120 KPaA) in absolute pressure) of the LNG tank 16, in a BOG
recondensation processing. The internal pressure of the heat
exchanger 172 is measured with a pressure gauge, and is regulated
to be lower than the operating pressure of the LNG tank 16. In the
present embodiment, the refrigerant contacts BOG in the heat
exchanger 172, whereby a volume of BOG decreases by liquefaction
and the pressure in the heat exchanger 172 is reduced. During
operation, the low pressure state is kept by the refrigerant being
continuously supplied. The internal pressure of the heat exchanger
172 is regulated by controlling a flow rate of the refrigerant. A
flow rate regulating valve (not illustrated) is provided in the
refrigerant feed line B1, and the flow rate of the refrigerant may
be controlled with the flow rate regulating valve in accordance
with a measurement result of the pressure gauge that measures the
internal pressure of the above described heat exchanger 172.
[0097] Note that the recondenser 17 is not limited to a mode of the
heat exchanger 172, but may be a mode in which BOG and the
refrigerant are brought into direct contact with each other. As a
method for contacting both BOG and the refrigerant to each other,
means of spraying the refrigerant by a shower, means of contacting
both of them by using a filler and the like are cited. A lower
portion of the heat exchanger 172 and the return line A4 are
connected. An automatic on-off valve (not illustrated) provided in
the return line A4 is controlled to open and close, and the second
pump P2 is controlled, whereby LNG can be fed back to the LNG tank
16 from the recondenser 17. A processing procedure of
recondensation processing of boil off gas (BOG) will be described
hereunder.
(1) Feed the refrigerant to the heat exchanger 172 from the
refrigerator 15, and pre-cool the heat exchanger 172, when the tank
internal pressure of the LNG tank 16 exceeds a first threshold
value. A temperature of the refrigerant is preferably set at a
temperature higher than an LNG solidifying point, and lower than a
temperature of LNG in the LNG tank 16, for example. The temperature
of LNG which is cooled may be set based on an amount of BOG and an
amount of LNG which is cooled. (2) When the heat exchanger 172
reaches a predetermined temperature or less, regulate the supply
amount of the refrigerant by the flow rate regulating valve (not
illustrated) provided in the refrigerant line B1 to keep the
temperature. (3) When the tank internal pressure of the LNG tank 16
exceeds a second threshold value (the second threshold value>the
first threshold value), open the automatic on-off valve 42 and the
pressure regulating valve 41, and introduce BOG directly into the
heat exchanger 172 of the recondenser 17 from the LNG tank 16. At
the time of introducing the BOG, regulate the supply amount of the
refrigerant to the heat exchanger 172, and keep the inside of the
heat exchanger 172 under a negative pressure (or under a pressure
lower than the operating pressure of the LNG tank 16). (4) The heat
exchanger 172 is pre-cooled, and BOG is cooled immediately and
changes state to LNG, and the LNG drops onto a bottom of the heat
exchanger 172. (5) LNG is fed back to the LNG tank 16 through the
return line A4. (6) Close the respective valves after the
recondensation processing ends.
[0098] It is assumed that a condition of (3) (the tank internal
pressure>the second threshold value) is established during
processing of (1) and/or (2), so that BOG may be configured to be
discharged from the LNG tank 16 with a safety valve (not
illustrated), or BOG may be discharged to external air by a vent
not illustrated.
[0099] The above described "first threshold value" is a pressure
that is 1.26 times as high as 1.2 barA (120 KPaA), for example.
[0100] The above described "second threshold value" is a pressure
that is 1.3 times as high as 1.2 barA (120 KPaA), for example.
[0101] A pressure regulating valve (not illustrated) or a flow rate
regulating valve (not illustrated) may be installed in the
refrigerant line B1, and a refrigerant feed amount (VN) and a BOG
feed amount (VB) may be controlled to VN>VB.
Other Embodiments
[0102] A recondenser in FIG. 4B will be described. In FIG. 4B, in
the recondenser 17, a volume (an external capacity) of the heat
exchanger 172 is smaller than an internal volume (an internal space
capacity) of the recondenser 17, and the heat exchanger 172 is
disposed in an internal space 173 of the recondenser. In the mode
of the heat exchanger, BOG can be effectively liquefied. The
liquefied LNG accumulates on a bottom of the internal space 173 of
the recondenser 17. The accumulating LNG can be fed to the LNG tank
16 by the second pump P2.
[0103] An upper portion (preferably an upper side of the heat
exchanger 172) of the internal space 173 of the recondenser 17 and
the first feed line A1 are directly connected. Further, a lower
portion of the internal space 173 of the recondenser 17 and the
return line A4 are directly connected.
[0104] An internal pressure of the recondenser 17 is controlled to
be a pressure lower than the operating pressure (an average of 1.2
barA (120 KPaA) in absolute pressure) of the LNG tank 16 in the BOG
recondensation processing. The internal pressure of the recondenser
17 is measured by a pressure gauge, and is regulated to be lower
than the operating pressure of the LNG tank 16.
[0105] In the present embodiment, the refrigerant is fed to the
heat exchanger 172, and thereby the inside of the recondenser 17 is
cooled. When BOG is introduced into the cooled recondenser 17, the
volume of BOG decreases by liquefaction and the pressure inside the
recondenser 17 is reduced. During operation, the refrigerant is
continuously supplied to the heat exchanger 172 and thereby
continues to cool the inside of the recondenser 17 to liquefy BOG
to keep the inside of the recondenser 17 in a low-pressure state.
The internal pressure of the recondenser 17 is regulated by
controlling the flow rate of the refrigerant. A flow rate
regulating valve is provided in the refrigerant line B1, and the
flow rate of the refrigerant may be controlled by the flow rate
regulating valve in accordance with the measurement result of the
pressure gauge that measures the internal pressure of the above
described recondenser 17, the flow rate of the refrigerant may be
controlled by controlling the opening degree of the automatic
on-off valve provided in the refrigerant line B1, or both of them
may be controlled.
[0106] A processing procedure of the recondensation processing of
boil off gas (BOG) will be described hereunder.
(1) Feed the refrigerant to the heat exchanger 172, and pre-cool
the heat recondenser 17, when the tank internal pressure of the LNG
tank 16 exceeds a first threshold value. A temperature of the
refrigerant is preferably set at a temperature higher than the LNG
solidifying point, and lower than a temperature of LNG in the LNG
tank 16, for example. The temperature of LNG which is cooled may be
set based on an amount of BOG and an amount of LNG which is cooled.
(2) When the recondenser 17 reaches a predetermined temperature or
less, regulate the supply amount of the refrigerant by the flow
rate regulating valve (not illustrated) provided in the refrigerant
line B1 to keep the temperature. (3) When the tank internal
pressure of the LNG tank 16 exceeds a second threshold value (the
second threshold value>the first threshold value), open the
automatic on-off valve 42 and the pressure regulating valve 41, and
introduce BOG into the recondenser 17 from the LNG tank 16. At the
time of introducing the BOG, regulate the supply amount of the
refrigerant to the heat exchanger 172, and keep the inside of the
heat exchanger 172 under a negative pressure (or under a pressure
lower than the operating pressure of the LNG tank 16). (4) The
recondenser 17 is pre-cooled, and BOG is cooled immediately and
changes state to LNG, and the LNG accumulates on the bottom of the
recondenser 17. (5) LNG that accumulates on the bottom of the
recondenser 17 is fed back to the LNG tank 16 through the return
line A4. (6) Close the respective valves after the recondensation
processing ends.
[0107] It is assumed that a condition of (3) (the tank internal
pressure>the second threshold value) is established during
processing of (1) and/or (2), so that BOG may be configured to be
discharged from the LNG tank 16 with a safety valve (not
illustrated), or BOG may be discharged to external air by a vent
not illustrated.
[0108] The above described "first threshold value" is a pressure
that is 1.26 times as high as 1.2 barA (120 KPaA), for example.
[0109] The above described "second threshold value" is a pressure
that is 1.3 times as high as 1.2 barA (120 KPaA), for example.
[0110] A pressure regulating valve (not illustrated) or a flow rate
regulating valve (not illustrated) may be installed in the
refrigerant line B1, and a refrigerant feed amount (VN) and a BOG
feed amount (VB) may be controlled to VN>VB.
Embodiment 2
[0111] An LNG production system 2 of embodiment 2 will be described
with use of FIG. 2. Components with the same reference signs as
those in the LNG production system 1 of embodiment 1 have the same
functions, and therefore, explanation of the components will be
omitted, or will be made briefly.
[0112] The LNG production system 2 of embodiment 2 has the first
refrigerator 15 and a second refrigerator 20. The first
refrigerator feeds a refrigerant to a cooling device 14. The second
refrigerator 20 feeds the refrigerant (liquid refrigerant) to the
recondenser 17 through a refrigerant line C1 (corresponding to B1
in FIG. 1), and returns the refrigerant used as a cold source in
the recondenser 17 through a return line C2 (corresponding to B2 in
FIG. 1).
[0113] Consequently, since the second refrigerator 20 is provided
separately from the first refrigerator 15, it is not necessary to
supply the refrigerant to the operating cooling device 14 from the
large refrigerator and supply the refrigerant to the recondenser
17, it is not necessary to install the refrigerator that is larger
than necessary, and only a medium or small refrigerator can be
installed, so that the installation space can be small, and the
initial cost and the running cost can be reduced.
Embodiment 3
[0114] An LNG production system 3 of embodiment 3 will be described
with reference to FIG. 3. Components with the same reference signs
as those in the LNG production system 1 of embodiment 1 have the
same functions, and therefore, explanation of the components will
be omitted, or will be made briefly.
[0115] The recondenser 17 of the LNG production system 3 of
embodiment 3 can perform a first recondensation processing of
liquefying BOG by the refrigerant fed from the refrigerator 15, and
a second recondensation processing that liquefies BOG by the
refrigerant fed from the refrigerator 15 and a refrigerant fed from
a refrigerant buffer 30 in order to process more BOG than BOG at
the time of the first recondensation processing by switching the
first and the second recondensation processings to each other.
[0116] In the refrigerant buffer 30, the refrigerant is supplied
from the refrigerator 15 through a first supply line E1 and/or the
refrigerant is supplied from an external refrigerant source through
a second supply line E2 and is stored in advance. When the
recondenser 17 is operated, the refrigerant is introduced into the
recondenser 17 through a buffer line D1 from the refrigerant buffer
30.
[0117] The recondenser 17 has a switch control section (not
illustrated) that switches the first recondensation processing and
the second recondensation processing to each other.
[0118] When BOG is transferred to the LNG carrier 18, the switch
control section can switch from the first recondensation processing
to the second recondensation processing in response to a timing of
start of transfer that is scheduled, or a timing at which a
detecting section detects that LNG is transferred from the LNG tank
16, for example. As the detecting section, there are cited a
detecting section that detects that the transportation ship enters
a harbor, a detecting section that detects that the automatic
on-off valve of the transfer line L6 opens, a detecting section
that uses a control signal for controlling the automatic on-off
valve as a detection signal, a detecting section that detects that
a measurement result of a flow meter disposed in the transfer line
L6 reaches a threshold value or more and the like.
[0119] Further, the switch control section can switch from the
first recondensation processing to the second recondensation
processing when a pressure value of the inside of the LNG tank 16
measured by the pressure gauge, or a pressure value measured by a
pressure gauge disposed in at least any one of the feed line A1,
the recovery line A2 and the feed line A3 reaches a predetermined
value or more.
[0120] An example of the recondenser 17 of embodiment 3 will be
described with FIG. 4C. The recondenser 17 has a first heat
exchanger 172 and a second heat exchanger 174 in an internal space
173 thereof. A refrigerant is introduced into the first heat
exchanger 172 from the refrigerator 15 at a time of the first
recondensation processing, and cools BOG. When a generation amount
of BOG is large, the switch control section switches from the first
recondensation processing to the second recondensation processing.
While the first heat exchanger 172 is operated, the second heat
exchanger 174 is further operated. The refrigerant is introduced
into the second heat exchanger 174 through the buffer line D1 from
the refrigerant buffer 30. Thereby, cooling by the two heat
exchangers is executed, so that at a peak time (for example, in a
case of processing a large amount of BOG that is generated when LNG
is fed into the LNG transport ship, for example) at which the
amount of BOG that is fed is larger than the amount of BOG at a
normal time, BOG is also effectively cooled to be converted into
LNG and the LNG can be returned to the LNG tank 16. Note that the
refrigerant that is used in the second heat exchanger 174 is
configured to join the refrigerant return line B2 of the first heat
exchanger 172 through a refrigerant return line D2, but the present
invention is not limited to this, and the refrigerant return line
D2 may be connected to the refrigerator 15.
[0121] Further, when BOG is fed to the LNG carrier 18, the switch
control section can switch from the second recondensation
processing to the first recondensation processing in response to a
timing of end of transfer that is scheduled, or timing at which a
detecting section detects that LNG transfer from the LNG tank 16 is
completed, for example. As the detecting section, there are cited a
detecting section that detects that the automatic on-off valve in
the transfer line L6 is closed, a detecting section that detects
the control signal that controls the automatic on-off valve as a
detection signal, a detecting section that detects that the
measurement result of the flow meter disposed in the transfer line
L6 becomes a threshold value or less and the like.
[0122] Further, the switch control section can switch from the
second recondensation processing to the first recondensation
processing when a pressure value of the inside of the LNG tank 16
measured by the pressure gauge, or a pressure value measured by a
pressure gauge disposed in at least one of the feed line A1, the
recovery line A2 and the feed line A3 becomes less than a
predetermined value.
Other Embodiments
[0123] In the above described embodiments, two heat exchangers are
disposed in the recondenser 17, and cooling capabilities of the
heat exchangers may be the same or different.
[0124] In the present embodiment, a combination of the refrigerant
buffer and the heat exchanger is provided to be one, but the
present invention is not limited to this, and two or more
combinations may be provided.
Embodiment 4
[0125] An LNG production system 5 in embodiment 4 will be described
with reference to FIGS. 5A to 5B. Components with the same
reference signs as those in the LNG production system 1 of
embodiment 1 have the same functions, and therefore, explanation of
the components will be omitted, or will be made briefly.
[0126] In embodiment 4, the refrigerator in the LNG production
system is not used, but LNG in the LNG tank is used. That is, in
embodiment 4, LNG is sub-cooled to a predetermined temperature by a
refrigerant, and LNG is fed to the recondenser to be brought into
contact with BOG to liquefy BOG.
[0127] Embodiment 4 has a first feed line A1 that feeds BOG from
the LNG tank 16, an LNG lead-out line E1 that leads out LNG from
the LNG tank 16, a sub-cooler 52 that cools LNG with the
refrigerant, a recondenser 57 that liquefies BOG that is fed
through the first feed line A1 for BOG by LNG that is cooled in the
sub-cooler 52, under a pressure lower than the operating pressure
of the LNG tank 16, and a return line A4 that returns LNG that is
BOG liquefied in the recondenser 57 to the LNG tank 16. The
respective components will be described in detail hereunder.
[0128] The first feed line A1 may be provided with a safety valve
(not illustrated) for a time when the pressure in the LNG tank 16
becomes abnormally high. Further, the first feed line A1 is
provided with an automatic on-off valve 42 and a pressure
regulating valve 41 for performing feeding control of BOG to the
condenser 10.
[0129] The operating pressure in the LNG tank 16 is an average of
1.2 barA (120 KPaA) in absolute pressure, and is controlled within
.+-.15% as an upper and lower limit values. When a large amount of
BOG is generated, the tank internal pressure becomes high. The tank
internal pressure is measured by a pressure gauge, and a valve
control section (not illustrated) controls opening and closing of
the automatic on-off valve 42 based on a measurement result
(conversion result) thereof. For example, when the tank internal
pressure becomes 1.3 times as high as 1.2 barA (120 KPaA), BOG is
fed to the recondenser 57. The pressure regulating valve 41
measures a pipe internal pressure of the first feed line A1, and
controls a valve opening degree based on a measurement result.
[0130] LNG is introduced into the sub-cooler 52 through the LNG
lead-out line E1 from the LNG tank 16. The valve control section
(not illustrated) performs opening and closing control of the
automatic on-off valve 51 provided in the LNG lead-out line E1, and
controls a liquid feed pump P5, whereby LNG is fed to the
sub-cooler 52 from the LNG tank 16, and is fed to the recondenser
57 in a subsequent stage. The in-tank liquid feed pump P1 may be
configured to feed LNG, instead of the liquid feed pump P5.
[0131] The refrigerant in the sub-cooler 52 can be a medium having
a temperature lower than the boiling point of LNG, and LN2 is used
in the present embodiment. LN2 is introduced into the sub-cooler 52
through a refrigerant line F1 from an LN2 source (LN2 tank, for
example), and is used as a cold source for cooling LNG passing
through an inside of the sub-cooler 52. LN2 may be gasified, or may
be discharged as a fluid in which a liquid and gas are mixed, when
LN2 is discharged from the sub-cooler 52 through a discharge line
F2. The fluid (LN2 and/or GN2) which is discharged may be subjected
to discharge treatment to the atmosphere or may be subjected to
recycle processing.
[0132] In the sub-cooler 52, LNG may be controlled to have a
temperature higher than the LNG solidifying point by a pressure
regulating valve (not illustrated), or a flow rate regulating valve
(not illustrated) that is installed in the refrigerant line F1 in
which the refrigerant (LN2) flows.
[0133] An internal pressure of the recondenser 57 is controlled to
be a pressure lower than the operating pressure (an average of 1.2
barA (120 KPaA) in the absolute pressure) of the LNG tank 16 in the
BOG recondensation processing. The internal pressure of the
recondenser 57 is measured by the pressure gauge, and is regulated
to be lower than the operating pressure of the LNG tank 16.
[0134] In the present embodiment, LNG that is cooled in the
sub-cooler 52 contacts BOG in the recondenser 57, whereby the
volume of BOG decreases by liquefaction, and the pressure in the
recondenser 57 is reduced. During operation, the low pressure state
is kept by the cooled LNG being continuously supplied. The internal
pressure of the recondenser 57 is regulated by controlling the flow
rate of the cooled LNG. A flow rate regulating valve is provided in
an LNG feed line E2 between the sub-cooler 52 and the recondenser
57, and the flow rate of LNG may be controlled by the flow rate
regulating valve in accordance with a measurement result of the
pressure gauge that measures the internal pressure of the above
described recondenser 57, the flow rate of LNG may be controlled by
controlling the opening degree of an automatic on-off valve 51, or
both of them may be controlled.
[0135] The BOG that is introduced into the recondenser 57 is
brought into contact with cooled LNG, whereby BOG is liquefied to
be LNG, and LNG accumulates on a bottom of the recondenser 57. As a
method for bringing both BOG and LNG into contact with each other,
there are means of spraying LNG cooled in the sub-cooler 52 by
shower, means of bringing both of them into contact with each other
by using a filler, and the like.
[0136] A lower part of the recondenser 57 and the return line A4
are connected. The valve control section (not illustrated) performs
on-off control of an automatic on-off valve 54 provided in the
return line A4, and controls the liquid feed pump P2, whereby the
valve control section can feed LNG back to the LNG tank 16 from the
recondenser 57.
[0137] A processing procedure of the recondensation processing of
BOG will be described hereunder. The respective valves 41 to 42, 51
and 54 are in closed states except for the recondensation
processing.
(1) Feed the refrigerant (LN2, for example) to the sub-cooler 52,
when the tank internal pressure of the LNG tank 16 exceeds a first
threshold value. (2) When the sub-cooler 52 reaches a predetermined
temperature or less, feed LNG to the sub-cooler 52 from the LNG
tank 16, and is cooled. For example, a temperature of the LNG which
is cooled is preferably set at a temperature higher than the LNG
solidifying point, and lower than the temperature of LNG in the LNG
tank 16. The temperature of LNG which is cooled may be set based on
the amount of BOG and the amount of LNG which is cooled. (3) Feed
the cooled LNG to the recondenser 57, and pre-cool the recondenser
57. The automatic on-off valve 54 of the return line A4 is closed.
(4) When the tank internal pressure of the LNG tank 16 exceeds a
second threshold value (the second threshold value>the first
threshold value), open the automatic on-off valve 42 and the
pressure regulating valve 41, and introduce BOG to the recondenser
57 from the LNG tank 16. (5) The recondenser 57 is pre-cooled, and
the cooled LNG is introduced into the recondenser 57 with BOG,
whereby BOG is cooled and changes state to LNG, and the LNG
accumulates on the bottom of the recondenser 57. (6) When LNG that
accumulates on the bottom of the recondenser 57 reaches a
predetermined amount (or at a predetermined timing), open the
automatic on-off valve 54, control the liquid feed pump P2, and
feed LNG to the LNG tank 16 from the recondenser 57. (7) Close the
respective valves after the recondensation processing ends.
[0138] It is also assumed that a condition of (4) (the tank
internal pressure>the second threshold value) is established
during processing of (1) to (3), so that BOG may be configured to
be discharged from the LNG tank 16 with a safety valve (not
illustrated), or BOG may be discharged to external air by a vent
not illustrated.
[0139] The above described "first threshold value" is the pressure
that is 1.26 times as high as 1.2 barA (120 KPaA), for example.
[0140] The above described "second threshold value" is the pressure
that is 1.3 times as high as 1.2 barA (120 KPaA), for example.
Embodiment 5
[0141] An LNG production system of embodiment 5 will be described
with use of FIG. 5C. Components with the same reference signs as
those in the LNG production systems 1 and 5 of embodiments 1 and 4
have the same functions, and therefore, explanation of the
components will be omitted, or will be made briefly.
[0142] In embodiment 5, a first and second sub-coolers are
disposed, and a first recondensation processing of liquefying BOG
by a refrigerant fed from the first sub-cooler 52, and a second
recondensation processing of liquefying BOG by the refrigerant fed
from the first sub-cooler 52 and a refrigerant fed from a second
sub-cooler 521 in order to process more BOG than BOG at the time of
the first recondensation processing are switched. According to the
embodiment, in the case of processing BOG in an amount in a
predetermined range (a flow rate per unit time) or of a pressure
value in a predetermined range set in advance, the first
recondensation processing (processing of liquefying BOG with LNG
cooled in the first sub-cooler) is executed, and in the case of
processing BOG exceeding the amount in the predetermined range or
of the pressure value in the predetermined range described above,
the second recondensation processing (simultaneously performing the
processing of liquefying BOG by LNG cooled in the second sub-cooler
while keeping the processing of liquefying BOG by LNG cooled in the
first sub-cooler) can be executed.
[0143] The switch control section (not illustrated) may switch from
the first recondensation processing to the second recondensation
processing in the case of transferring BOG to the LNG carrier, or
may switch from the first recondensation processing to the second
recondensation processing when the pressure value that is measured
by the pressure gauge disposed in the LNG tank or in the feed line
A1 that feeds BOG to the recondenser 57 reaches a predetermined
value or more.
[0144] In the present embodiment, the refrigerant which is fed to
the first sub-cooler 52, and the refrigerant which is fed to the
second sub-cooler 521 may be the same refrigerants, or may be
different refrigerants. For example, as the refrigerant to the
first sub-cooler 52, a mixture such as hydrocarbon can be cited,
and as the refrigerant to the second sub-cooler 521, nitrogen and
the like are cited.
[0145] The switch control section can perform switching from the
first recondensation processing to the second recondensation
processing in the timing of embodiment 3 described above. When the
processing is switched to the second recondensation processing, the
valve control section (not illustrated) performs opening and
closing control of a sluice valve 53, feeds LNG to the second
sub-cooler 521, and feeds LNG to the recondenser 57 in the
subsequent stage. That is, in the first recondensation processing,
cooled LNG is fed to the recondenser 57 through the LNG feed line
E2 from the first sub-cooler 52, but the processing is switched to
the second recondensation processing, and cooled LNG is fed to the
recondenser 57 through an LNG feed line E21 from the second
sub-cooler 521, in addition to the cooled LNG being fed to the
recondenser 57 from the first sub-cooler 52.
[0146] The refrigerant of the second sub-cooler 521 can be any
medium with a temperature lower than the boiling point of LNG, and
LN2 is used in the present embodiment. LN2 is introduced into the
second sub-cooler 521 through a refrigerant line F11 from an LN2
source (an LN2 tank, for example), and is used as a cold source for
cooling LNG that passes through an inside of the second sub-cooler
521. LN2 may be gasified, or may be discharged as a fluid in which
a liquid and gas are mixed, when the LN2 is discharged through a
discharge line F21 from the second sub-cooler 521. The fluid (LN2
and/or GN2) which is discharged may be subjected to discharge
treatment into the atmosphere, or may be subjected to recycling
processing. Further, in the second sub-cooler 521, LNG may be
controlled to have a temperature higher than the LNG solidifying
point, by a pressure regulating valve (not illustrated) or a flow
rate regulating valve (not illustrated) that is installed in a
refrigerant line F11 in which the refrigerant (LN2) flows.
Other Embodiments
[0147] In embodiments 4 and 5 described above, the respective
automatic on-off valves, pressure regulating valves and liquid feed
pumps are provided in the respective lines, but some or all of them
may be omitted in accordance with the use purpose without being
limited to the above described dispositions.
[0148] Further, in embodiments 4 and 5 described above, in the BOG
recondensation processing, LNG that is cooled is fed to pre-cool
the recondenser 57 before BOG is fed into the recondenser 57, but
the present invention is not limited to this, and cooled LNG and
BOG may be fed together. In that case, a feeding amount (VL) of the
cooled LNG and a feeding amount (VB) of BOG may be controlled to
VL>VB. The flow rate regulating valves may be provided in the
respective LNG introduction line E1, line E2 and first feed line A1
for BOG to perform flow rate control of the respective feeding
amounts.
[0149] Further, in embodiments 4 and 5 described above, the liquid
feed pump P2 is provided in the return line A4, but a configuration
in which the liquid feed pump is not provided in the return line A4
may be adopted. LNG changed in state from BOG in the recondenser 57
may be fed into the LNG tank 16 by the gravity.
[0150] Further, the refrigerant in the aforementioned second
sub-cooler 521 may be supplied from a refrigerant buffer in which
the refrigerant is stored in advance.
[0151] In each of all the embodiments described above, the pump P1
is of an in-tank type, but the pump P1 is not limited to this, and
the pump P1 may be a pump disposed on the transfer line L6.
REFERENCE SIGNS LIST
[0152] 1 LNG production system
[0153] 14 Cooling device
[0154] 15 Refrigerator
[0155] 16 LNG tank
[0156] 17 Recondenser
[0157] 18 LNG carrier
[0158] L6 Transfer line
* * * * *