Boil-off gas re-liquefaction system

Abstract

To provide a versatile boil-off gas re-liquefaction system capable of minimizing a discharge flow rate of LNG necessary for re-liquefaction of boil-off gas.SOLUTION: A boil-off gas re-liquefaction system 1 includes: an intermediate pressure LNG pump 70 and a high pressure LNG pump 80 for boosting pressure of low-temperature liquefied gas stored in a low-temperature liquefied gas storage tank and discharging the gas; a re-liquefaction BOG drum 10 for storing re-liquefied boil-off gas obtained through re-liquefaction of the boil-off gas; a re-liquefaction BOG circulation line 170 for forming a circulation path of the re-liquefied boil-off gas; a BOG ejector 40 that is disposed in the re-liquefaction BOG circulation line 170 and uses the re-liquefied boil-off gas as drive fluid and the boil-off gas as suction fluid; and a re-liquefaction BOG cooler 30 that is disposed in the re-liquefaction BOG circulation line 170 to cool the re-liquefied boil-off gas flowing in the re-liquefaction BOG circulation line 170 by using the discharged low-temperature liquefied gas.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a boil-off gas reliquefaction system that reliquefies boil-off gas in a cryogenic liquefied gas storage tank that stores cryogenic liquefied gas. [Background technology]

[0002] Generally, boil-off gas (hereinafter referred to as BOG) which is evaporated gas from LNG storage tanks at LNG terminals is compressed to high pressure by a BOG compressor and injected into gas transmission piping. However, the compression power required by a BOG compressor is enormous, which goes against the recent trend toward energy conservation.

[0003] In this regard, Patent Document 1 (Patent No. 6345965) discloses a technology in which BOG in an LNG storage tank is sucked in by an ejector installed on an LNG discharge line from the LNG storage tank, and then separated into liquid and gas in a gas-liquid separation drum installed downstream, with the liquid being merged into the LNG discharge line and the gas being returned to the LNG storage tank. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6345965 Summary of the Invention [Problem to be solved by the invention]

[0005] The amount of driving fluid for the ejector is far greater than the amount of suction fluid, and can be, for example, 100 to 200 times greater, so the technology described in Patent Document 1, in which an ejector is installed on an LNG delivery line, presupposes the existence of a large-volume LNG user. If the normal BOG generation rate from an LNG storage tank is 4 ton / h, then 400 to 800 ton / h of LNG users are required. However, finding such users is extremely difficult, and the number of applicable LNG terminals is extremely limited.

[0006] The present invention has been made to solve such problems, and aims to provide a versatile boil-off gas reliquefaction system that can minimize the LNG discharge flow rate required for boil-off gas reliquefaction. [Means for solving the problem]

[0007] (1) The boil-off gas reliquefaction system according to the present invention comprises: a cryogenic liquefied gas boost pump that boosts and discharges the cryogenic liquefied gas stored in a cryogenic liquefied gas storage tank; a re-liquefaction boil-off gas drum for storing re-liquefied boil-off gas obtained by re-liquefying boil-off gas, which is an evaporated gas of the low-temperature liquefied gas; a reliquefied boil-off gas circulation line including the reliquefied boil-off gas drum and forming a circulation path for the reliquefied boil-off gas; an ejector disposed in the reliquefied boil-off gas circulation line, the ejector using the reliquefied boil-off gas flowing through the reliquefied boil-off gas circulation line as a driving fluid and the boil-off gas as a suction fluid; and a re-liquefied boil-off gas cooler that is disposed in the re-liquefied boil-off gas circulation line and that cools the re-liquefied boil-off gas flowing through the re-liquefied boil-off gas circulation line with the low-temperature liquefied gas discharged by the low-temperature liquefied gas boost pump.

[0008] (2) Furthermore, the device described in (1) above is characterized by having a cold storage tank having a cold storage material, storing the cold energy of the low-temperature liquefied gas discharged by the low-temperature liquefied gas boost pump, and imparting the stored cold energy to the reliquefied boil-off gas flowing through the reliquefied boil-off gas circulation line.

[0009] (3) In addition, in the device described in (2) above, the cold storage material contains a metal compound.

[0010] (4) In addition, in the device described in (2) above, the regenerator material contains hydrocarbon. [Effects of the Invention]

[0011] According to the boil-off gas reliquefaction system of the present invention, the LNG flow rate required for reliquefaction can be minimized, thereby providing a highly versatile boil-off gas reliquefaction system. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram showing a boil-off gas reliquefaction system according to a first embodiment of the present invention. [Figure 2] FIG. 4 is a schematic diagram showing a boil-off gas reliquefaction system according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] [Embodiment 1] First, the configuration and functions of the first embodiment of the present invention will be described with reference to FIG. The boil-off gas reliquefaction system 1 according to the first embodiment of the present invention includes an LNG boost pump (medium-pressure LNG pump 70, high-pressure LNG pump 80) that pressurizes and discharges LNG stored in an LNG (liquefied natural gas) storage tank 60, a reliquefied BOG drum 10, a reliquefied BOG circulation line 170, a BOG ejector 40, a reliquefied BOG cooler 30, and a reliquefied BOG discharge pump 50.

[0014] The LNG storage tank 60 is an aboveground PC (Pre-stressed Concrete) storage tank, and has inner and outer steel tanks surrounded by a circular PC wall. The space between the inner and outer tanks is filled with particles called perlite, creating a nitrogen atmosphere that provides excellent thermal insulation. This allows the LNG storage tank 60 to store large quantities of extremely low-temperature LNG (approximately -160°C) while suppressing heat input from the outside.

[0015] In this embodiment, the LNG booster pump of the present invention is composed of a medium-pressure LNG pump 70 and a high-pressure LNG pump 80.

[0016] The medium-pressure LNG pump 70 is, for example, a vertical centrifugal submerged pump that is disposed inside the LNG storage tank 60 and is submerged in the LNG. The medium-pressure LNG pump 70 pressurizes the LNG stored in the LNG storage tank 60 and discharges it.

[0017] The high-pressure LNG pump 80 is, for example, a vertical centrifugal submerged pump disposed downstream of the LNG storage tank 60, and the pump body is submerged inside a pot. The high-pressure LNG pump 80 pressurizes and discharges LNG supplied from the medium-pressure LNG pump discharge line 181.

[0018] The reliquefied BOG drum 10 is a vertical cylindrical drum that temporarily stores the reliquefied BOG received from the BOG ejector outlet line 174.

[0019] The reliquefied BOG circulation line 170 forms a circulation path for the reliquefied boil-off gas, including the reliquefied boil-off gas drum 10, and is composed of a reliquefied BOG circulation pump inlet line 171, a reliquefied BOG circulation pump outlet line 172, a BOG ejector inlet line 173, and a BOG ejector outlet line 174. The reliquefied BOG circulation line 170 is provided with a reliquefied BOG circulation pump 20, a reliquefied BOG cooler 30, and a BOG ejector 40.

[0020] The reliquefied BOG circulation pump 20 is a centrifugal pump that receives reliquefied BOG temporarily stored in the reliquefied BOG drum 10 through a reliquefied BOG circulation pump inlet line 171, pressurizes the reliquefied BOG, and discharges it to a reliquefied BOG circulation pump outlet line 172. The amount of pressure increase by the reliquefied BOG circulation pump 20 corresponds to the pressure loss in the system including the reliquefied BOG cooler 30 and the BOG ejector 40.

[0021] The reliquefaction BOG cooler 30 is a typical shell-and-tube type heat exchanger, and cools the reliquefaction BOG received from the reliquefaction BOG circulation pump outlet line 172 by the discharged LNG.

[0022] The BOG ejector 40 is a general ejector that creates a reduced pressure state using reliquefied BOG supplied from a BOG ejector inlet line 173 as a driving fluid, and receives BOG from a BOG receiving line 175. After the BOG ejector 40, reliquefied BOG flows through a BOG ejector outlet line 173 toward the reliquefied BOG drum 10.

[0023] The reliquefied BOG discharge pump 50 is a centrifugal pump that receives reliquefied BOG temporarily stored in the reliquefied BOG drum 10 from a reliquefied BOG discharge pump inlet line 176, pressurizes the gas, and discharges it to a reliquefied BOG discharge pump outlet line 177.

[0024] Next, the operation of the boil-off gas reliquefaction system 1 of this embodiment will be described. The boil-off gas reliquefaction system 1 circulates reliquefied BOG within the system by operating the reliquefied BOG circulation pump 20. Specifically, the reliquefied BOG in the reliquefied BOG drum 10 is sent to a reliquefied BOG circulation pump inlet line 171, passes through the reliquefied BOG circulation pump 20, the reliquefied BOG circulation pump outlet line 172, the reliquefied BOG cooler 30, the BOG ejector inlet line 173, the BOG ejector 40, and the BOG ejector outlet line 174, and is returned to the reliquefied BOG drum 10.

[0025] In the re-liquefied BOG drum 10, the re-liquefied BOG is stored at, for example, 0.7 MPaG and −133°C, and is pressurized to, for example, 1.4 MPaG by the re-liquefied BOG pump 20. Here, the flow rate of the re-liquefied BOG that causes 4 ton / h of BOG to be sucked into the BOG ejector 40 is assumed to be 550 ton / h.

[0026] The reliquefied BOG pressurized by the reliquefied BOG circulation pump 20 is heated by the work heat input from the reliquefied BOG circulation pump 20, but in the reliquefied BOG cooler 30, the reliquefied BOG is cooled by heat exchange with the high-pressure LNG supplied from the high-pressure LNG pump 80, thereby canceling out the heat input. Meanwhile, the high-pressure LNG is heated, and its temperature rises from −155°C to −135°C, for example.

[0027] The re-liquefied BOG cooled by the re-liquefied BOG cooler 30 is supplied to the BOG ejector 40, and flows as a high-speed flow through the small diameter portion inside the BOG ejector 40, creating a reduced pressure state using Bernoulli's theorem. On the other hand, in the LNG storage tank 60, BOG generated by natural heat input from the outside or work heat input from the medium-pressure LNG pump 70 is supplied to the BOG ejector 40 via a BOG receiving line 175. BOG at atmospheric pressure or low pressure is supplied from a BOG receiving line 175 to the inside of the BOG ejector 40 in a depressurized state.

[0028] Inside the BOG ejector 40, the BOG supplied from the BOG receiving line 175 mixes with the re-liquefied BOG, which is flowing at a high speed. As the diameter of the BOG ejector 40 expands and the flow rate decreases, the pressure of the mixed fluid that has passed through the small-diameter section of the BOG ejector 40 recovers to 0.7 MPaG, and the BOG in the mixed fluid is compressed, cooled, and condensed. During this process, the BOG is compressed while being cooled by contact with the re-liquefied BOG, so the BOG compression process is close to isothermal compression, resulting in high compression efficiency.

[0029] In the downstream of the BOG ejector 40, the re-liquefied BOG flows through the BOG ejector outlet line 170 toward the re-liquefied BOG drum 10, but if BOG that has not been completely condensed remains inside the BOG ejector 40, it becomes a multiphase flow.

[0030] The liquefied carbon dioxide that flows into the reliquefaction BOG drum 10 becomes saturated at 0.7 MPaG and separates into a gas layer and a liquid layer. As BOG is supplied from the BOG receiving line 175, the amount of BOG held in the system increases. Therefore, an amount of BOG equal to the amount of BOG supplied is discharged from the reliquefaction BOG drum 10 to the first high-pressure LNG discharge line 185 by the reliquefaction BOG discharge pump 50.

[0031] Of the medium-pressure LNG discharged by the medium-pressure LNG pump 70 to the medium-pressure LNG pump discharge line 181, a portion is supplied to the high-pressure LNG pump 80, and the remainder is supplied to other medium-pressure LNG users (e.g., medium-pressure LNG vaporizers or LNG tanker shipping facilities) via the medium-pressure LNG discharge line 182.

[0032] The intermediate-pressure LNG supplied to the high-pressure LNG pump 80 is further pressurized by the high-pressure LNG pump 80 and is discharged to the high-pressure LNG pump discharge line 183. A portion of the high-pressure LNG discharged to the high-pressure LNG pump discharge line 183 is supplied to the re-liquefaction BOG cooler 30, and the remainder is supplied to a high-pressure LNG user (e.g., another high-pressure LNG vaporizer) via a second high-pressure LNG discharge line 184.

[0033] According to the boil-off gas reliquefaction system 1 configured as described above, the large flow rate required for suction of BOG in the BOG ejector 40 can be ensured in the reliquefaction BOG circulation line (a circulation line including the reliquefaction BOG drum 10, the reliquefaction BOG circulation pump 20, the reliquefaction BOG cooler 30, and the BOG ejector 40), so that BOG can be sucked by generating negative pressure in the BOG ejector 40 regardless of the flow rate of the discharged LNG. Therefore, the system can be applied to a variety of LNG terminals, improving versatility.

[0034] In FIG. 1 , which explains this embodiment, all of the BOG generated in the LNG storage tank 60 is received by the BOG ejector 40. However, it is not necessary to receive all of the BOG in the BOG ejector 40, and the BOG may be processed by another BOG processing device such as a BOG compressor.

[0035] In addition, in this embodiment, LNG and its evaporated gas are used as an example of low-temperature liquefied gas, but the present invention is also applicable to other low-temperature liquefied gases such as ammonia and its evaporated gas, liquefied hydrogen and its evaporated gas, ethane and its evaporated gas, and propane and its evaporated gas.

[0036] Furthermore, the types of components such as the pump and cooler described in this embodiment are merely examples, and may be selected appropriately within the scope of design.

[0037] [Embodiment 2] Next, the configuration and functions of the second embodiment will be described with reference to FIG. The same numbers are used for components having the same configurations and functions as those in the first embodiment. The boil-off gas reliquefaction system 200 according to the second embodiment includes a cold storage tank 210, a reliquefaction BOG cooler flow control valve 271, a reliquefaction BOG cold storage tank via line 272, a reliquefaction BOG cold storage tank via flow control valve 273, a medium-pressure LNG cold storage tank bypass flow control valve 274, a high-pressure LNG cold storage tank bypass flow control valve 275, a medium-pressure LNG cold storage tank via line 276, a medium-pressure LNG cold storage tank via flow control valve 277, a high-pressure LNG cold storage tank via line 281, and a high-pressure LNG cold storage tank via flow control valve 282.

[0038] The cold storage tank 210 is a vertical cylindrical container filled with a metal compound containing bismuth as a cold storage material. The cold storage material stores the cold energy of the medium-pressure LNG supplied via the medium-pressure LNG cold storage tank via line 276 and the high-pressure LNG supplied via the high-pressure LNG cold storage tank via line 281, and cools the re-liquefied BOG supplied via the re-liquefied BOG cold storage tank via line 272 using the cold energy of the cold storage material.

[0039] The reliquefied BOG cooler flow rate control valve 271 is a remotely controlled globe valve, and adjusts the flow rate of the reliquefied BOG supplied to the reliquefied BOG cooler 30.

[0040] The re-liquefied BOG cold storage tank via line 272 is a pipe whose starting end is connected to the upstream part of the re-liquefied BOG cooler flow control valve 271 in the re-liquefied BOG circulation pump outlet line 172 and whose terminal end is connected to the BOG ejector inlet line 173, and supplies part or all of the re-liquefied BOG discharged by the re-liquefied BOG circulation pump 20 to the cold storage tank 210.

[0041] The re-liquefied BOG via cold storage tank flow rate adjustment valve 273 is a remote-controlled globe valve, and adjusts the flow rate of the re-liquefied BOG supplied to the cold storage tank 210.

[0042] The medium-pressure LNG cold storage tank bypass flow rate control valve 274 is a remotely controlled globe valve, and adjusts the flow rate of the medium-pressure LNG that bypasses the cold storage tank 210 and flows through the medium-pressure LNG delivery line 182.

[0043] The high-pressure LNG regenerator bypass flow rate control valve 275 is a remotely controlled globe valve, and adjusts the flow rate of the high-pressure LNG that bypasses the regenerator 210 and flows through the second high-pressure LNG delivery line 184 .

[0044] The medium-pressure LNG cold storage tank via line 276 is a pipe whose starting end is connected to the upstream part of the medium-pressure LNG cold storage tank bypass flow control valve 274 in the medium-pressure LNG discharge line 182 and whose ending end is connected to the downstream part of the medium-pressure LNG cold storage tank bypass flow control valve 274, and which has a flow path that passes through the cold storage tank 210, and which directs some or all of the medium-pressure LNG flowing through the medium-pressure LNG discharge line 182 to the cold storage tank 210.

[0045] The medium-pressure LNG via cold storage tank flow rate control valve 277 is a remote-controlled globe valve, and adjusts the flow rate of the medium-pressure LNG flowing into the cold storage tank 210.

[0046] The high-pressure LNG cold storage tank via line 281 is a pipe whose starting end is connected to the upstream part of the high-pressure LNG cold storage tank bypass flow control valve 275 in the high-pressure LNG discharge line 184 and whose end is connected to the downstream part of the high-pressure LNG cold storage tank bypass flow control valve 275, and which has a flow path that passes through the cold storage tank 210, and which directs some or all of the high-pressure LNG flowing through the high-pressure LNG discharge line 184 to the cold storage tank 210.

[0047] The high-pressure LNG via cold storage tank flow rate adjustment valve 282 is a remote-controlled globe valve, and adjusts the flow rate of high-pressure LNG flowing into the cold storage tank 210.

[0048] Next, the operation of the boil-off gas reliquefaction system 200 according to this embodiment will be described. The explanation of the same functions as those in the first embodiment will be omitted.

[0049] During a time period when there is sufficient electricity demand and the discharge flow rate of high-pressure LNG is sufficiently high, such as during the daytime, high-pressure LNG is passed through the re-liquefaction BOG cooler 30 at a sufficient flow rate to re-liquefy the BOG received in the BOG ejector 40. In this case, the re-liquefaction BOG via cold storage tank flow rate control valve 273 is closed, and the flow rate of re-liquefied BOG supplied to the re-liquefaction BOG cooler 30 is adjusted by the re-liquefaction BOG cooler flow rate control valve 271.

[0050] High-pressure LNG and / or medium-pressure LNG is also passed through the cold storage tank 210 to store cold energy in the cold storage material. In this case, the flow rates of the medium-pressure LNG and high-pressure LNG are adjusted according to the discharge demand by the medium-pressure LNG cold storage tank bypass flow rate adjustment valve 274 and the high-pressure LNG cold storage tank bypass flow rate adjustment valve 275, while the flow rates of the medium-pressure LNG and / or high-pressure LNG supplied to the cold storage tank 210 are adjusted by the medium-pressure LNG cold storage tank-via flow rate adjustment valve 277 and the high-pressure LNG cold storage tank-via flow rate adjustment valve 282.

[0051] In this way, during times when the discharge flow rate of high-pressure LNG is sufficiently high, the cold energy required to re-liquefy the BOG received by the BOG ejector 40 is supplied in the re-liquefaction BOG cooler 30, while the cold energy is stored in the cold storage tank 210.

[0052] On the other hand, during times such as nighttime when the demand for electricity decreases and the discharge flow rate of high-pressure LNG becomes zero, the re-liquefaction BOG cooler flow control valve 271 is closed and the re-liquefaction BOG via re-liquefaction BOG cold storage tank flow control valve 273 supplies re-liquefied BOG to the cold storage tank 210. As a result, the cold energy stored in the cold storage tank 210 during times when the discharge flow rate of high-pressure LNG is sufficiently high is used to cover the cold energy required to re-liquefy the BOG received by the BOG ejector 40.

[0053] According to the boil-off gas reliquefaction system 200 of this embodiment, the cold energy of LNG is stored in the cold storage tank 210 during a time period when the demand for LNG discharge is sufficient to cover the cold energy required by the reliquefaction BOG cooler 30, and the cold energy of the cold storage tank 210 can be used to reliquefy BOG during a time period when the demand for LNG discharge is insufficient for the cold energy required by the reliquefaction BOG cooler 30.Therefore, BOG can be reliquefied stably regardless of fluctuations in the demand for LNG.

[0054] In this embodiment, the cold storage material contained in the cold storage tank 210 is described as a metal compound containing bismuth, but a liquid refrigerant such as methane, ethane, propane, or a fluorocarbon-based liquid may also be used as the cold storage material. [Industrial Applicability]

[0055] INDUSTRIAL APPLICABILITY The present invention can be used as a highly versatile boil-off gas reliquefaction system that can minimize the discharge flow rate of LNG required for reliquefying boil-off gas. [Explanation of symbols]

[0056] 1. Boil-off gas reliquefaction system (embodiment 1) 10 Reliquefied BOG drum 20 Re-liquefied BOG circulation pump 30 Reliquefaction BOG Cooler 40 BOG ejector 50 Re-liquefied BOG discharge pump 60 LNG storage tank 70 Medium pressure LNG pump 80 High-pressure LNG pump 170 Re-liquefied BOG circulation line 171 Circulation pump inlet line 172 Reliquefaction BOG Circulation Pump Outlet Line 173 BOG ejector inlet line 174 BOG ejector outlet line 175 BOG receiving line 176 Reliquefied BOG discharge pump inlet line 177 Reliquefied BOG discharge pump outlet line 181 Medium pressure LNG pump discharge line 182 Medium-pressure LNG delivery line 183 High-pressure LNG pump discharge line 184 Second high-pressure LNG delivery line 185 First high-pressure LNG delivery line 200 Boil-off gas reliquefaction system (embodiment 2) 210 Cold storage tank 271 Re-liquefaction BOG cooler flow control valve 272 Re-liquefied BOG cool storage tank route 273 Re-liquefied BOG cool storage tank flow control valve 274 Medium-pressure LNG cool storage tank bypass flow control valve 275 High-pressure LNG cool storage tank bypass flow control valve 276 Medium pressure LNG cool storage tank line 277 Medium pressure LNG cool storage tank flow control valve 281 High-pressure LNG cool storage tank line 282 High-pressure LNG cool storage tank flow control valve

Claims

1. a low-temperature liquefied gas booster pump that boosts and dispenses the low-temperature liquefied gas stored in the low-temperature liquefied gas storage tank; a re-liquefaction boil-off gas drum for storing re-liquefied boil-off gas obtained by re-liquefying boil-off gas, which is an evaporated gas of the low-temperature liquefied gas; a reliquefied boil-off gas circulation line including the reliquefied boil-off gas drum and forming a circulation path for the reliquefied boil-off gas; an ejector disposed in the reliquefied boil-off gas circulation line, the ejector using the reliquefied boil-off gas flowing through the reliquefied boil-off gas circulation line as a driving fluid and the boil-off gas as a suction fluid; a re-liquefaction boil-off gas cooler disposed in the re-liquefaction boil-off gas circulation line for cooling the re-liquefaction boil-off gas flowing through the re-liquefaction boil-off gas circulation line with the low-temperature liquefied gas discharged by the low-temperature liquefied gas boost pump.

2. 2. The boil-off gas reliquefaction system according to claim 1, further comprising a cold storage tank having a cold storage material, storing the cold energy of the low-temperature liquefied gas discharged by the low-temperature liquefied gas boost pump, and imparting the stored cold energy to the reliquefied boil-off gas flowing through the reliquefied boil-off gas circulation line.

3. The boil-off gas reliquefaction system according to claim 2, wherein the regenerator material contains a metal compound.

4. 3. The boil-off gas reliquefaction system according to claim 2, wherein the regenerator material comprises a hydrocarbon.

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