CN111295559A - BOG recondenser and LNG storage system that is provided with it - Google Patents

BOG recondenser and LNG storage system that is provided with it Download PDF

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Publication number
CN111295559A
CN111295559A CN201880070490.9A CN201880070490A CN111295559A CN 111295559 A CN111295559 A CN 111295559A CN 201880070490 A CN201880070490 A CN 201880070490A CN 111295559 A CN111295559 A CN 111295559A
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China
Prior art keywords
bog
coolant
condenser
lng
heat exchanger
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CN201880070490.9A
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Chinese (zh)
Inventor
富田伸二
广濑献儿
永田大祐
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LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
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LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
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Publication of CN111295559A publication Critical patent/CN111295559A/en
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F25J1/0025—Boil-off gases "BOG" from storages
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C13/00—Details of vessels or of the filling or discharging of vessels
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C7/00—Methods or apparatus for discharging liquefied, solidified, or compressed gases from pressure vessels, not covered by another subclass
    • F17C7/02—Discharging liquefied gases
    • F17C7/04—Discharging liquefied gases with change of state, e.g. vaporisation
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C9/00—Methods or apparatus for discharging liquefied or solidified gases from vessels not under pressure
    • F17C9/02—Methods or apparatus for discharging liquefied or solidified gases from vessels not under pressure with change of state, e.g. vaporisation
    • F17C9/04—Recovery of thermal energy
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0221—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using the cold stored in an external cryogenic component in an open refrigeration loop
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F25J1/0256—Safety aspects of operation
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2221/00—Handled fluid, in particular type of fluid
    • F17C2221/03—Mixtures
    • F17C2221/032—Hydrocarbons
    • F17C2221/033—Methane, e.g. natural gas, CNG, LNG, GNL, GNC, PLNG
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
    • F17C2223/01—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the phase
    • F17C2223/0146—Two-phase
    • F17C2223/0153—Liquefied gas, e.g. LPG, GPL
    • F17C2223/0161—Liquefied gas, e.g. LPG, GPL cryogenic, e.g. LNG, GNL, PLNG
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
    • F17C2223/03—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the pressure level
    • F17C2223/033—Small pressure, e.g. for liquefied gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2265/00—Effects achieved by gas storage or gas handling
    • F17C2265/03—Treating the boil-off
    • F17C2265/032—Treating the boil-off by recovery
    • F17C2265/033—Treating the boil-off by recovery with cooling
    • F17C2265/034—Treating the boil-off by recovery with cooling with condensing the gas phase
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/40—Air or oxygen enriched air, i.e. generally less than 30mol% of O2
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/42—Nitrogen
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2220/00—Processes or apparatus involving steps for the removal of impurities
    • F25J2220/60—Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
    • F25J2220/62—Separating low boiling components, e.g. He, H2, N2, Air
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2245/00—Processes or apparatus involving steps for recycling of process streams
    • F25J2245/42—Processes or apparatus involving steps for recycling of process streams the recycled stream being nitrogen
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2250/00—Details related to the use of reboiler-condensers
    • F25J2250/02—Bath type boiler-condenser using thermo-siphon effect, e.g. with natural or forced circulation or pool boiling, i.e. core-in-kettle heat exchanger
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2250/00—Details related to the use of reboiler-condensers
    • F25J2250/10—Boiler-condenser with superposed stages
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2290/00—Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/62—Details of storing a fluid in a tank

Abstract

The problems to be solved are as follows: provided is a BOG recondenser for recondensing BOG of LNG with high heat exchange efficiency while removing nitrogen in BOG of NG and without using a compressor. The solution is as follows: the BOG recondenser 1 has an LNG buffer tank (12), a first condenser (111), and a second condenser (211). The first (condenser 111) has a first heat exchanger (112). The second condenser (211) has a second heat exchanger (212). The BOG produced in the LNG buffer tank (12) is recondensed by heat exchange with the coolant in the first heat exchanger (112). The BOG that has not been recondensed is introduced into a second condenser (211). Some of the BOG introduced into the second condenser (211) is recondensed by the second heat exchanger (212) and returned to the LNG buffer tank (12). At least some of the coolant that exchanges heat in the second heat exchanger (212) with the BOG in the second condenser (211) is also exchanged heat in the first heat exchanger (112) with the BOG in the first condenser (111).

Description

BOG recondenser and LNG storage system that is provided with it
The present invention relates to a BOG recondenser for recondensing BOG of LNG and an LNG storage system provided therewith.
When storing cryogenic liquids such as Liquefied Natural Gas (LNG) or Liquefied Petroleum Gas (LPG), a recondenser is typically used to liquefy and condense Boil Off Gas (BOG), for example, which is vaporized by natural external heat input.
A method is known for compressing BOG generated from a storage tank for storing LNG by a compressor and recondensing the BOG by heat exchange with LNG in a supercooled state supplied from the LNG storage tank (for example, patent document 1). According to the method, the recondensed LNG is returned to the LNG storage tank.
A method has been proposed for using liquid nitrogen as a coolant for a heat exchanger in place of LNG in a recondenser used when storing LNG (for example, patent document 2).
It is well known that LNG stored in storage tanks contains nitrogen. This is because nitrogen used for purging the natural gas storage facility and nitrogen used for instrumentation are found in LNG in addition to nitrogen contained in nitrogen produced from the gas field. The nitrogen mixed with the LNG reduces the liquid density of the LNG. As a result, LNG having different liquid densities exists in the same LNG tank and liquid layers having different liquid densities are formed in the LNG tank, which is a cause of rapid vaporization (referred to as rollover) of LNG in the LNG tank. When the pressure in the tank increases rapidly due to vaporization, there is a risk of damage to the tank. Therefore, a technique for removing nitrogen in LNG has been developed (for example, patent document 3).
Documents of the prior art
Patent document
Patent document 1: japanese unexamined Utility model patent publication No. H5-6299
Patent document 2: japanese unexamined patent publication No. 2002-
Patent document 3: international patent publication No. 2011/064605
Disclosure of Invention
Problems to be solved by the invention
A problem with a recondenser using a compressor (for example, patent document 1) is that: the compressor is expensive and has a complicated structure with a rotating member, which complicates maintenance.
On the premise of continuous consumption of LNG, a system for recondensing compressed BOG by heat exchange with LNG in a supercooled state supplied from an LNG tank has been designed (for example, patent document 1). However, if LNG is not consumed continuously or the LNG consumption fluctuates greatly, BOG is not suitable for recondensation because the fluctuation of heat exchange is too large.
In the case where liquid nitrogen is used as a coolant in the BOG recondenser, the latent heat of the liquid nitrogen is generally used (for example, patent document 2). However, using only latent heat increases the temperature difference between liquid nitrogen and BOG, and makes the thermal efficiency poor. In addition, sensible heat of the low-temperature nitrogen gas vaporized by heat exchange with BOG is not used, which also makes thermal efficiency poor. Therefore, this method has problems in that: the consumption of liquid nitrogen for use in the heat exchange is increased.
Using both the latent heat of the liquid nitrogen in a single heat exchanger and the sensible heat of the vaporized nitrogen gas means: the coolant is processed into different states and applied to systems with large load fluctuations (such as BOG recondensation) may have difficulty in controlling the temperature of the coolant and eventually cause a sudden pressure rise or pressure drop on the BOG side. Although how to cope with such pressure rise or pressure drop may affect the design of the LNG buffer tank and the heat exchanger, such design is not easy in terms of material selection and structural complexity.
Since all of these methods for recondensing the entire amount of BOG also recondensing the nitrogen contained in the BOG, the LNG in the storage tank continues to contain nitrogen. Therefore, there is a risk that rapid vaporization (referred to as rollover) of LNG occurs in the LNG storage tank.
Patent document 3 proposes a method for removing nitrogen in LNG by rectification or the like, but this method has problems in that: a large rectification apparatus must be installed and a large amount of electricity is required to operate the rectification apparatus. Specifically, in order to rectify the raw LNG after temporary depressurization by the expansion turbine, the gas or liquid recovered from the bottom of the column must be repressurized, and this process requires electric power. In the case of condensing nitrogen in the top of the column before removing it, more power is required to compress and recondense the gas in the top to return to the rectification column as a fixed reflux liquid.
In view of this situation, it is an object of the present invention to provide a BOG recondenser for recondensing BOG of LNG with high heat exchange efficiency while removing nitrogen in the BOG of LNG and without using a compressor, and an LNG storage system using the same.
Means for solving the problems
Invention 1
The BOG recondenser according to one aspect of the present invention is
-a BOG recondenser for recondensing Boil Off Gas (BOG) vaporized from LNG in an LNG buffer tank, characterized by: is provided with
-a BOG draw off pipe for drawing BOG from the LNG buffer tank,
-a first condenser for condensing at least some of the BOG conveyed by the BOG draw off pipe,
a first gas supply section for supplying at least some of the gas in the first condenser from the first condenser to a second condenser,
-a first return line for returning at least some of the recondensed BOG in the first condenser from the first condenser to the LNG buffer tank,
-a second return line for returning recondensed BOG in the second condenser from the second condenser to the LNG buffer tank, and
-a second gas exhaust for exhausting at least some of the gas in the second condenser from the second condenser;
-the first condenser has a first heat exchanger;
-the second condenser has a second heat exchanger; and is
-at least some of the coolant that exchanges heat in the second heat exchanger with the BOG in the second condenser is also exchanged heat in the first heat exchanger with the BOG in the first condenser.
The BOG recondenser according to the present invention does not require an expensive compressor rotator and does not require cumbersome compressor maintenance. Since the coolant used in the second heat exchanger is also used in the first heat exchanger, the BOG recondenser according to the present invention can effectively utilize the cold energy of the coolant and obtain high heat exchange efficiency.
According to the present invention, in the first heat exchanger, the relatively high temperature BOG generated from the LNG buffer tank is cooled by heat exchange with the coolant in a state in which: the temperature of the coolant itself has been raised by heat exchange in the second heat exchanger. In the second heat exchanger, the BOG cooled in the first heat exchanger is further cooled by a coolant in the following state: the temperature is lower than the coolant in the first heat exchanger. Therefore, the temperature difference between the heat exchange fluids in the first heat exchanger or the second heat exchanger is relatively small compared to when BOG generated from the LNG buffer tank is heat exchanged with the coolant in a liquid state in a single heat exchanger.
The BOG recondenser according to the present invention may also remove nitrogen from the BOG by venting at least some of the nitrogen-rich gas in the second condenser through the second vent line.
The first condenser and the second condenser in the present invention may be installed in parallel at the upper portion of the LNG buffer tank. In this case, the first gas supply section may be, for example, a gas supply pipe for introducing the gas extracted from the first condenser into the second condenser.
The first condenser and the second condenser in the present invention may be installed in series at the upper portion of the LNG buffer tank. In this case, the first gas supply section is located in an intermediate region between the first condenser and the second condenser.
The LNG buffer tank in the present invention is not particularly limited as long as it is a storage tank for supplying and storing LNG, and may be a main storage tank for storing LNG or a buffer tank for temporarily storing LNG until BOG condensed in the first condenser and/or the second condenser is returned.
The coolant used in the present invention is not particularly limited as long as it is a coolant at a temperature at or below the condensation point of BOG, and may be, for example, liquid nitrogen or liquid air.
Invention 2
In the BOG recondenser according to an aspect of the present invention, the second heat exchanger is a latent heat exchanger for exchanging heat between latent heat of the coolant and heat of the BOG in the second condenser, and the first heat exchanger is a sensible heat exchanger for exchanging heat between sensible heat of the coolant and heat of the BOG in the first condenser.
According to the invention, the coolant in the liquid state is first introduced into the second heat exchanger for heat exchange. The temperature is raised by heat exchange, which vaporizes the coolant into a gaseous state. The coolant in a gaseous state is introduced into the first heat exchanger to perform heat exchange. Since the cold of the latent heat part of the coolant is used for the heat exchange in the second heat exchanger and the sensible heat of the coolant is used for the heat exchange in the first heat exchanger, the heat of the coolant can be effectively used for the efficient heat exchange. Therefore, the consumption amount of the coolant for cooling the BOG can be reduced.
According to the present invention, in the first heat exchanger, the relatively high temperature BOG generated from the LNG buffer tank is cooled by heat exchange with the coolant in the gaseous state, which is higher in temperature than the coolant in the liquid state. In the second heat exchanger, the BOG cooled by heat exchange with the coolant in the gaseous state is further cooled by the coolant in the liquid state, which is lower in temperature than the coolant in the gaseous state. Thus, a single-phase coolant is used in both the first heat exchanger and the second heat exchanger. As a result, the structural design of the heat exchanger is simplified.
The present invention uses latent and sensible heat, respectively, and is particularly advantageous in cases where the temperature of the BOG fluctuates widely.
BOG is known to contain nitrogen because nitrogen is mixed with LNG during production or used to purge LNG facilities. Nitrogen content in BOG fluctuates widely due to factors such as the structure of the facility to be purged and the length of time the LNG is stored in the facility. The condensation point of BOG also fluctuates with fluctuations in the nitrogen content in BOG.
The temperature of the BOG varies depending on the characteristics of the LNG plant and the temperature of the transfer line for transferring LNG to the LNG buffer tank. In the case of transferring LNG from an LNG ship or the like to an LNG buffer tank (including receiving, filling, or transporting LNG from the LNG ship to the LNG buffer tank), the temperature of the BOG tends to change to a higher temperature.
In this case, the invention is particularly advantageous, wherein the BOG pre-cooled in the first heat exchanger is further cooled and condensed in the second heat exchanger. Pre-cooling by coolant in the gaseous state in the first heat exchanger improves the heat load and minimizes the consumption of liquid coolant in the second heat exchanger in case the condensation point of the BOG has changed to a higher temperature or the temperature of the BOG has changed to a high temperature. This object is to improve the thermal efficiency of the entire heat exchanger system comprising the first heat exchanger and the second heat exchanger.
According to the conventional method using a single heat exchanger, for example, in the case of heat exchange using liquid nitrogen at-170 ℃, the amount of liquid nitrogen required increases by about 5% when the BOG having a temperature of-150 ℃ is introduced into the heat exchanger, as compared with the BOG having a temperature of-162 ℃. In this case, the heat exchanger produces gaseous nitrogen at-170 ℃.
According to the invention, wherein gaseous nitrogen is used for pre-cooling in the first heat exchanger, the heat required for re-condensing the BOG in the second heat exchanger is reduced in comparison, since BOG at-150 ℃ can be cooled to about-162 ℃ in the first heat exchanger and the temperature of the BOG can be reduced when introduced into the second heat exchanger. As a result, the consumption of liquid nitrogen can be minimized.
Invention 3
In the BOG recondenser according to an aspect of the present invention, an end of the BOG draw tube opposite to the first condenser may be disposed lower than the first heat exchanger;
the end of the first return line opposite the first condenser may be set lower than the end of the BOG draw line opposite the first condenser;
the end of the first gas supply section opposite the first condenser may be arranged higher than the first heat exchanger;
the end of the first gas supply section opposite the second condenser may be disposed lower than the second heat exchanger; and is
The end of the second return line opposite the second condenser may be arranged lower than the end of the first gas supply section opposite the second condenser.
According to the present invention, BOG is supplied from the lower portions of the first condenser and the second condenser, and the recondensed BOG is discharged from the bottom portion to the outside of the condensers. At the same time, the uncondensed components are discharged from the upper portions of the condensers to the outside of the condensers. This produces a rectification effect by contact between the BOG and the recondensed BOG in the first condenser and the second condenser. This rectification effect concentrates components having a condensation point lower than that of BOG, such as nitrogen, in the upper portion of the condenser, and can reduce the content of low condensation point components (e.g., nitrogen) in the BOG after recondensation.
Invention 4
In the BOG recondenser according to one aspect of the present invention, the second condenser may be provided with a second exhaust pipe for drawing a gas in the second condenser, and an exhaust pressure control valve for performing control so that a pressure in the second exhaust pipe is a predetermined value or lower; and is
The second exhaust pipe may be disposed higher than the second heat exchanger.
The second exhaust pipe is a pipe for removing waste nitrogen gas from the gas phase portion of the second condenser.
The gas phase portion in the second condenser comprises BOG containing a large amount of nitrogen. The concentration of this nitrogen is determined by the temperature and pressure in the second condenser. Therefore, nitrogen gas having a predetermined concentration can be discharged from the second exhaust pipe by using an exhaust pressure control valve to keep the pressure in the second condenser at or below a predetermined value (for example, a value below the range of 1.013 bar to 1.5 bar). As a result, nitrogen contained in BOG can be removed, and recondensed BOG from which nitrogen has been removed can be returned to the LNG buffer tank, which improves the quality of heat of LNG in the LNG buffer tank.
Invention 5
In the BOG recondenser according to an aspect of the present invention, the second heat exchanger may be provided with: a second coolant delivery passage for drawing the coolant from the second heat exchanger; a coolant buffer tank for collecting the coolant delivered via the second coolant delivery passage; a second coolant return passage for returning at least some of the liquid phase coolant in the coolant buffer tank to the second heat exchanger; and a second coolant flow control valve for controlling the circulation amount of the coolant.
Invention 6
In the BOG recondenser according to an aspect of the present invention, the coolant buffer tank may be further provided with a first coolant return passage for drawing at least some of the gas-phase coolant in the coolant buffer tank to the first heat exchanger.
Invention 7
In the BOG recondenser according to an aspect of the present invention, the coolant may be liquid nitrogen and/or liquid air.
The coolant in the second heat exchanger is recirculated to the second heat exchanger via the second coolant delivery passage, the coolant buffer tank, and the second coolant return passage. This makes it possible to circulate the coolant using coolant density fluctuation caused by a coolant temperature difference generated by heat exchange between the BOG and the coolant (thermosiphon). In the coolant buffer tank, the heat exchange function using latent heat and the heat exchange function using sensible heat may be separated by sending the gas-phase portion of the coolant to the first heat exchanger and the liquid-phase portion of the coolant to the second heat exchanger.
After the coolant has been separated into gas and liquid in the coolant surge tank, the coolant in the heat exchangers is a single phase, rather than a mixed gas-liquid phase (the coolant used in the first heat exchanger is only a gas phase, while the coolant used in the second heat exchanger is only a liquid phase). This may facilitate temperature control in the first heat exchanger and the second heat exchanger.
Specifically, the temperature of the first heat exchanger may be controlled by adjusting the flow rate of the coolant in the first coolant return passage for introducing the gas-phase coolant from the coolant surge tank to the first heat exchanger.
The temperature of the second heat exchanger is achieved by controlling the level of coolant in the second heat exchanger to control the heating surface area between the coolant and the BOG. If the temperature in the second heat exchanger has decreased below a desired temperature, the second coolant flow control valve is closed or the opening degree is decreased to collect the gas-phase coolant in front of the second coolant flow control valve. This causes the temperature of the second heat exchanger to rise by reducing the amount of the gas-phase coolant flowing from the coolant buffer tank into the second heat exchanger via the second coolant return passage. Conversely, if the temperature of the second heat exchanger has to be lowered, the second coolant flow control valve is closed to reduce the pressure of the gas-phase coolant in front of the second coolant flow control valve. This increases the amount of gas-phase coolant flowing from the second coolant return passage to the second heat exchanger, and decreases the temperature of the second heat exchanger.
It can be said that the use of a single-phase coolant instead of a gas-liquid mixed phase facilitates temperature control in both heat exchangers.
Although the coolant supplied to the second heat exchanger may be supplied at a temperature lower than the freezing point of BOG, such as liquid nitrogen in a supercooled state (e.g., -196 ℃), the buffering effect of the coolant buffer tank is set to prevent the operating temperature of the heat exchanger from reaching the freezing point of BOG. This means that the coolant can be used in a state of having more cooling capacity than the coolant whose temperature is controlled to avoid condensation of BOG, and the consumption amount of the coolant can be reduced.
The coolant may be a coolant capable of cooling and condensing the BOG to or below the condensation point of the BOG, and may be, for example, liquid nitrogen or liquid air. The coolant may also be a mixture of liquid nitrogen and liquid air. The coolant may be in a liquid or gaseous state.
Liquid nitrogen is inert and flammable, and is particularly advantageous in terms of safety and for use in facilities handling flammable LNG. In the case where liquid nitrogen is required to separate nitrogen from air, the liquid air does not require a separation operation, and is therefore useful in terms of energy. Thus, liquid air may be used instead of liquid nitrogen as a coolant for recondensing the BOG, or nitrogen may be used as an intermediate medium for heat exchange with the liquid air, and liquefied liquid nitrogen may be used for heat exchange with the BOG.
Invention 8
An LNG storage system according to an aspect of the present invention is provided with a BOG recondenser according to any one of inventions 1 to 7, an LNG tank for storing LNG, an LNG tank BOG vent pipe for introducing BOG in the LNG tank to the LNG buffer tank, and an LNG buffer tank LNG vent pipe for delivering at least some of liquid-phase LNG in the LNG buffer tank to the LNG tank.
A condenser is attached for directly recondensing LNG in the LNG tank receiving LNG from an LNG ship or the like, and the condenser is capable of directly returning recondensed BOG to the LNG tank. Alternatively, the recondensed BOG may be temporarily received in the LNG buffer tank and then returned from the LNG buffer tank to the LNG tank using a pump or other means. The LNG buffer tank has the function of ensuring a Net Positive Suction Head (NPSH). The LNG buffer tank has a function of receiving a gas phase portion in the LNG tank when the recondensed BOG is returned from the LNG buffer tank to the LNG tank, to alleviate a pressure rise in the LNG tank.
Drawings
Fig. 1 is a diagram showing a configuration example of the BOG recondenser of embodiment 1;
fig. 2 is a diagram showing a configuration example of the LNG storage system of embodiment 2; and
fig. 3 is a diagram showing a configuration example of the BOG recondenser of embodiment 1;
Detailed Description
Several embodiments of the invention will be described below. The embodiments described below describe examples of the invention. The present invention is by no means limited to the following embodiments, but includes various modifications performed within a scope not changing the essence of the present invention. The configuration described below does not necessarily include all necessary configurations of the present invention.
Example 1
The BOG recondenser of example 1 will be described with reference to fig. 1.
The BOG recondenser 1 has an LNG buffer tank 12, a first condenser 111, and a second condenser 211. The first condenser 111 has a first heat exchanger 112. The second condenser 211 has a second heat exchanger 212.
The LNG buffer tank 12 may be any tank having a structure capable of storing LNG and may directly receive LNG from an LNG ship or the like, but may also be a buffer tank for temporarily holding recondensed BOG recondensed from BOG generated from an LNG tank (not shown) receiving LNG from the LNG ship.
The BOG generated in the LNG buffer tank 12 is introduced into the first condenser 111 through the BOG draw-out pipe 11. At least some of the BOG introduced to the first condenser 111 is recondensed by heat exchange with the coolant in the first heat exchanger 112. The recondensed BOG is returned to the LNG buffer tank 12 via the first return pipe 113. The portion of the BOG introduced into the first condenser 111 that is not recondensed in the first condenser 111 is introduced into the second condenser 211 through the first gas supply section 114. At least some of the BOG introduced into the second condenser 211 is recondensed by heat exchange with the coolant in the second heat exchanger 212. The recondensed BOG is returned to the LNG buffer tank 12 via the second return pipe 213.
The first gas supply section 114 is a pipe for circulating BOG.
The LNG buffer tank 12 in the present invention is not particularly limited as long as it is a storage tank for supplying and storing LNG, and may be a main storage tank for storing LNG or a buffer tank for temporarily storing LNG until BOG condensed in the first and second condensers 111 and 211 is returned to the main storage tank for storing LNG.
The coolant used in the second heat exchanger 212 is introduced into the second heat exchanger 212, and after heat exchange with the BOG in the second condenser 211, is introduced into the first heat exchanger 112 via the second coolant transfer passage 216. The coolant introduced into the first heat exchanger 112 exchanges more heat with the BOG in the first condenser 111.
The coolant in the present embodiment may be a coolant capable of cooling and condensing BOG to or below the condensation point of BOG, and may be, for example, liquid nitrogen or liquid air. The coolant (e.g., nitrogen) is introduced into the second heat exchanger 212 in a liquid state. The temperature of the coolant (liquid nitrogen) at this time may be any temperature at or below the liquefaction temperature of BOG; for example-170 deg.C. After heat exchanging with the BOG in the second heat exchanger 212, the liquid nitrogen is introduced into the first heat exchanger 112 through the first coolant return passage 115. Although the coolant may be introduced into the first heat exchanger 112 in a liquid state, some or all of the coolant may be introduced into the first heat exchanger 112 in a vaporized state. In the first heat exchanger 112, heat exchange is performed at a higher temperature (e.g., -162 ℃) than the second heat exchanger 212, and some of the BOG in the first condenser 111 is condensed. After heat exchange in the first heat exchanger 111, some or all of the coolant is in a vaporized state. Although this coolant may be discarded, it may be cooled again to be liquefied and reused.
Positional relationship between condenser and tube
The end of the BOG draw tube 11 opposite the first condenser 111 is disposed lower than the lower end of the first heat exchanger 112. This allows the heat exchange to be performed while circulating the BOG upward from the lower end of the first heat exchanger 112 to bring the BOG circulating upward from below into contact with the recondensed BOG circulating downward from above to obtain the rectification effect. As a result of the rectification, the gas containing many low boiling point compounds (e.g., nitrogen) is collected in the upper portion of the first condenser 111, and this gas is sent from the upper portion of the first condenser 111 to the second condenser 211 via the first gas supply section 114.
For the same reason, the end of the first gas supply section 114 opposite to the second condenser 211 is disposed lower than the lower end of the second heat exchanger 212. In the second condenser 211, the BOG circulates upward from below the second heat exchanger 212 and contacts the recondensed BOG circulating downward from above. As a result of the rectification, the gas containing more low boiling point compounds (e.g., nitrogen) is collected in the upper portion of the second condenser 211 and discharged as waste nitrogen through the second exhaust pipe 214.
The recondensed BOG collected in the lower portion of the first condenser 111 is returned to the LNG buffer tank 12 through the first return pipe 113. The recondensed BOG collected in the lower portion of the second condenser 211 is returned to the LNG buffer tank 12 through the second return pipe 213. Since a certain amount of recondensed BOG collects at the bottom of the first and second condensers 111, 211, the end of the BOG draw tube 11 opposite the first condenser 111 is preferably positioned above the level of the collected recondensed BOG.
Coolant buffer tank
The coolant may be introduced directly from the second heat exchanger 212 to the first heat exchanger 112, or may be introduced by means of the coolant buffer tank 13. The coolant extracted from the second heat exchanger 212 is introduced into the coolant surge tank 13 through the second coolant transfer passage 216. The liquid phase portion of the coolant introduced into the coolant buffer tank 13 is collected in the lower portion of the coolant buffer tank 13 and is again delivered to the second heat exchanger 212 through the second coolant return passage 215. The gas phase portion of the coolant introduced into the coolant buffer tank 13 is collected in the upper portion of the coolant buffer tank 13 and is delivered to the first heat exchanger 112 through the first coolant return passage 115.
The coolant may be cooled in the coolant buffer tank 13 to be partially liquefied. For example, liquid air or liquid nitrogen may be used to cool the coolant. Although liquid nitrogen may be used as the coolant and may be cooled, liquid air may also be used.
The coolant is temporarily introduced into the coolant buffer tank 13 and mixed with the circulating coolant to be supplied to the second heat exchanger 212. The amount of coolant in the system is indicated by a level indicator 301 and if the amount of coolant decreases, the second coolant flow control valve 22 is opened to add more coolant.
If some of the coolant is vaporized by heat exchange with the BOG in the heat exchanger 212, the pressure of the gas phase portion of the coolant buffer tank 13 is raised through the second coolant conveying passage 216, and the gas phase portion of the coolant is pushed upward from the lower portion of the coolant buffer tank 13 by the liquid phase portion of the coolant. The coolant pushed upward is introduced into the second heat exchanger 212 through the second coolant return passage 215. Thus, the coolant can be transferred between the coolant buffer tank 13 and the second heat exchanger 212 without using a motive force such as a pump.
The first coolant flow control valve 21 is disposed in the second coolant delivery passage 216. During normal operation, the first coolant flow control valve 21 is in a fully open state.
If the pressure of the BOG in the second heat exchanger 212 drops due to too much BOG being condensed by the second heat exchanger 212 or the like, the pressure in the second heat exchanger 212 becomes negative pressure with respect to atmospheric pressure. As a result, contamination or damage may occur to second heat exchanger 212 due to air mixing with BOG in second heat exchanger 212.
To correct this problem, the pressure of the BOG in the second heat exchanger 212 is detected by the first pressure indicator controller 304, and if it is judged that the pressure on the BOG side detected by the arithmetic logic unit 303 is lower than the threshold value, the first coolant flow control valve 21 is closed to control the pressure.
Although the first pressure indicator controller 304 is disposed on the second exhaust pipe 214, the first pressure indicator controller 304 may detect the pressure in the second heat exchanger 212 because the pressure in the second exhaust pipe 214 is equal to the pressure in the second heat exchanger 212.
By controlling the first coolant flow rate control valve 21 to be closed, the evaporation gas generated by the heat exchange in the second heat exchanger 212 is accumulated in the upper portion of the second heat exchanger 212, and the pressure thereof returns the liquid coolant to the coolant buffer tank 13. This may terminate the heat exchange in second heat exchanger 212, stop any further condensation of the BOG, and may bring the pressure of the BOG in second heat exchanger 212 to a negative pressure. When the liquid-phase portion of the coolant in the second heat exchanger 212 is returned to the coolant buffer tank 13 through the second coolant return passage, the level of the coolant in the second heat exchanger 212 falls. As a result, the heating surface area between the BOG and the liquid-phase coolant in the second heat exchanger 212 is reduced, which can minimize the supercooling phenomenon of the BOG. In the case where the temperature in the second heat exchanger 212 has risen, the opening degree of the first coolant flow rate control valve 21 may be increased to increase the level of the coolant and decrease the BOG temperature in the second heat exchanger 212.
The temperature of the second heat exchanger 212 may be measured by detecting the wall temperature of the second heat exchanger 212 or the temperature of the internal coolant, or may be known by detecting the temperature of the exhaust nitrogen gas discharged from the second heat exchanger 212.
The coolant must be operated at a temperature at which the BOG in the second heat exchanger 212 is not solidified, and pressure control in consideration of the liquid-vapor equilibrium of the coolant is advantageous for controlling the temperature of the coolant. To this end, the coolant pressure control valve 25 is opened and closed by the first pressure indicator controller 302 to measure and regulate the pressure of the first cooling supply passage 115 to control the operating pressure of the second heat exchanger 212.
The coolant pressure control valve 23 is opened and closed by the third pressure indicator controller 305 to control the pressure of the BOG in the second heat exchanger 212.
Other embodiments
Although the first condenser 111 and the second condenser 211 may be arranged in parallel as shown in fig. 1, as another embodiment, the second condenser 211 may be arranged lower than the first condenser 111. In this case, the first gas supply section 114 is a gas circulation section positioned between the first condenser 111 and the second condenser 211.
As another example, the first coolant flow rate control valve 21 may be disposed on the second coolant return passage 215. In this case, the second coolant flow control valve 21 is controlled to be closed if the temperature in the second heat exchanger 212 falls below the desired temperature, and is controlled to be opened if the temperature rises above the desired temperature. As described above, in the case where the heat of the BOG greatly fluctuates, the first coolant flow control valve 21 may be controlled to quickly adjust the temperature and efficiently recondense the BOG.
Example 2
The LNG storage system 2 of embodiment 2 will be described with reference to fig. 2. Elements denoted by the same reference numerals as those of the BOG recondenser 1 of embodiment 1 have the same functions and will not be described again.
The LNG storage system 2 of embodiment 2 has an LNG tank 33 for receiving the transferred LNG and an LNG buffer tank 12 for receiving BOG in the LNG tank. The BOG in the LNG tank 33 is temporarily collected in the LNG buffer tank 12 and then recondensed by the LNG recondenser 1 of embodiment 1. The recondensed BOG recondensed and collected in the LNG buffer tank 12 is returned to the LNG tank 33 by using a pump. When recondensed BOG is received from the LNG buffer tank 12, the volume of the liquid phase (LNG) in the LNG tank 33 increases, and the pressure of the gas phase (BOG) portion increases. In the case where the pressure in the LNG tank 33 is greater than a predetermined threshold value (e.g., 1.1 bar), control may be performed to receive the BOG in the LNG tank 33 in the LNG buffer tank 12.
Example 1
The pressure (barA), temperature (c), flow (kg/h), methane concentration (wt%) and nitrogen concentration (wt%) in each section were simulated to verify when the LNG storage system according to example 1 was used to store LNG with 80 wt% methane and 20 wt% nitrogen as feedstock. Liquid nitrogen was used as the coolant.
Results
When BOG (-150 ℃ and 1.2barA) of LNG is supplied from an LNG tank to the LNG surge tank 12 at a flow rate of 11,740kg/h, the results shown in table 1 are obtained for the pressure (barA), temperature (° c), flow rate (kg/h), methane concentration (wt%), and nitrogen concentration (wt%) in sections a to F and sections a to e in fig. 3.
The sections a-F in fig. 3 are positions for measuring the temperature of BOG and the like, and the sections a-e in fig. 3 are positions for measuring the temperature of nitrogen and the like. The locations of sections a-F and sections a-e in fig. 3 are as follows.
A is located directly in front of the location where BOG is introduced from the LNG tank (not shown) to the LNG buffer tank 12. The measurement at position a is equal to the measurement at position in the BOG draw tube 11 (as shown in fig. 3 (a)).
B is located on the first gas supply section 114 between the first condenser 111 and the second condenser 211.
C is located on the first return line 113 between the first condenser 111 and the LNG buffer tank 12.
D is located on the second exhaust pipe 214 at the outlet of the upper portion of the second condenser 211.
E is located on the second return line 213 between the second condenser 211 and the LNG buffer tank 12.
F is located at the bottom outlet of the LNG buffer tank 12 between the LNG buffer tank 12 and the LNG tank (not shown).
a is located directly in front of the position where the coolant liquid nitrogen is introduced into the coolant surge tank 13, between the coolant surge tank 13 and the coolant flow control valve 22 disposed in front of the coolant surge tank 13.
b are located on the second coolant return passage 215 between the coolant buffer tank 13 and the second heat exchanger 212.
c are located on the second coolant delivery passage 216 between the second heat exchanger 212 and the first coolant flow control valve 21.
d are located on the first coolant return passage 115 between the coolant buffer tank 13 and the first heat exchanger 112.
eAt the outlet of the first heat exchanger 112.
TABLE 1
Pressure of Temperature of Flow rate Methane Nitrogen is present in
MPaA ℃ kg/h wt％ wt％
A 1.20 -150.0 11,740 80.00 20.00
B 1.13 -162.1 10,547 77.85 22.15
C 1.20 -162.1 1,202 98.88 1.12
D 1.06 -182.0 220 6.49 93.51
E 1.20 -182.0 10,317 79.37 20.63
F 1.20 -179.9 11,520 81.40 18.60
a 3.80 -196.0 25,524 0.00 100.00
b 3.80 -186.0 104,483 0.00 100.00
C 3.80 -182.8 104,483 0.00 100.00
d 3.80 -182.8 25,524 0.00 100.00
e 3.70 -152.0 25,524 0.00 100.00
Based on the results of example 1, the BOG of LNG can be recondensed by using both latent heat and sensible heat of liquid nitrogen including a coolant and at high thermal efficiency without using a compressor. When the BOG is introduced from the LNG tank to the LNG buffer tank 12, the nitrogen concentration in the LNG is 20.0 wt%, but when the BOG is returned from the first condenser 111 to the LNG buffer tank 12 (C in fig. 3), it has been reduced to 1.1 wt%. When the BOG is returned from the second condenser 211 to the LNG buffer tank 12 (E in fig. 3), the nitrogen concentration slightly rises to 20.6 wt%, but when the BOG is returned from the LNG buffer tank 12 to the LNG tank (F in fig. 3), it has fallen to 18.6 wt%. Thus, in this example, nitrogen in the BOG of the LNG may be reduced.
Explanation of reference numerals
1 BOG recondensor
11 BOG extraction pipe
12 LNG buffer tank
13 coolant buffer tank
21 first coolant flow control valve
22 second coolant flow control valve
23 exhaust pressure control valve
25 refrigerant pressure control valve
33 LNG tank
111 first condenser
112 first heat exchanger
113 first return pipe
114 first gas supply section
115 first coolant return passage
116 first coolant delivery passage
211 second condenser
212 second heat exchanger
213 second return pipe
214 second exhaust pipe
215 second coolant return passage
216 second coolant delivery passage
301 liquid level indicator
302 first pressure indicator controller
303 arithmetic logic unit
304 second pressure indicator controller
305 third pressure indicator control

Claims (8)

1. A boil-off gas (BOG) recondenser for recondensing BOG vaporized from LNG in an LNG buffer tank, characterized by: is provided with:
-a BOG draw off pipe for drawing BOG from the LNG buffer tank,
-a first condenser for condensing at least some of the BOG conveyed by the BOG draw off pipe,
a first gas supply section for supplying at least some of the gas in the first condenser from the first condenser to a second condenser,
-a first return line for returning at least some of the recondensed BOG in the first condenser from the first condenser to the LNG buffer tank,
-a second return line for returning recondensed BOG in the second condenser from the second condenser to the LNG buffer tank, and
-a second gas exhaust for exhausting at least some of the gas in the second condenser from the second condenser;
the first condenser having a first heat exchanger;
the second condenser having a second heat exchanger; and is
At least some of the coolant that exchanges heat in the second heat exchanger with the BOG in the second condenser is also exchanged heat in the first heat exchanger with the BOG in the first condenser.
2. The BOG recondenser of claim 1, wherein the second heat exchanger is a latent heat exchanger for exchanging heat between latent heat of the coolant and heat of the BOG in the second condenser, and
the first heat exchanger is a sensible heat exchanger for exchanging heat between sensible heat of the coolant and heat of the BOG in the first condenser.
3. The BOG recondenser of claim 1 or claim 2, wherein an end of the BOG draw tube opposite the first condenser is disposed lower than the first heat exchanger;
the end of the first return line opposite the first condenser is disposed lower than the end of the BOG draw line opposite the first condenser;
an end of the first gas supply section opposite the first condenser is disposed higher than the first heat exchanger;
an end of the first gas supply section opposite the second condenser is disposed lower than the second heat exchanger; and is
The end of the second return pipe opposite the second condenser is disposed lower than the end of the first gas supply section opposite the second condenser.
4. The BOG recondenser of any one of claims 1 to 3, wherein the second condenser is provided with a second vent pipe for withdrawing a gas in the second condenser and a vent pressure control valve for controlling so that a pressure in the second vent pipe is a predetermined value or lower; and the second exhaust pipe is disposed higher than the second heat exchanger.
5. The BOG recondenser of claim 1 or claim 2, wherein the second heat exchanger is provided with:
-a second coolant conveying channel for extracting the coolant from the second heat exchanger,
a coolant buffer tank for collecting the coolant conveyed via the second coolant conveying channel,
-a second coolant return channel for returning at least some of the liquid phase coolant in the coolant buffer tank to the second heat exchanger,
and a second coolant flow control valve for controlling the circulation amount of the coolant.
6. The BOG recondenser of any one of claims 1 to 3, further provided with a first coolant return passage for drawing at least some of the gas phase coolant in the coolant buffer tank to the first heat exchanger.
7. The BOG recondenser of any one of claims 1 to 4, wherein the coolant is liquid nitrogen and/or liquid air.
8. An LNG storage system provided with a BOG recondenser according to any one of claims 1 to 7, an LNG tank for storing LNG, an LNG tank BOG vent pipe for introducing BOG in the LNG tank to the LNG buffer tank, and an LNG buffer tank LNG vent pipe for delivering at least some of liquid-phase LNG in the LNG buffer tank to the LNG tank.
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