Gas replacement method

Abstract

A method for replacing air in a tank for storing a liquefied gas which is obtained by liquefying a simple substance gas that is composed of a single element with the simple substance gas, the method including: supplying an inert gas to the tank; replacing the air in the tank with the inert gas; supplying the simple substance gas to the tank after the oxygen concentration in the tank becomes equal to or lower than a first concentration and the dew point temperature in the tank becomes equal to or lower than a first dew point temperature; replacing the inert gas in the tank with the simple substance gas; and stopping the supply of the simple substance gas to the tank after the concentration of the simple substance gas in the tank becomes equal to or higher than a second concentration and the dew point temperature in the tank becomes equal to or lower than a second dew point temperature that is lower than the first dew point temperature.

Description

Gas replacement method

[0001] The present disclosure relates to a gas replacement method.

[0002] In Patent Document 1, when starting the storage of liquefied gas in a storage tank, a first replacement operation of replacing the air in the storage tank with an inert gas and a second replacement operation of replacing the inert gas with a process gas are disclosed.

[0003] Japanese Patent Application Laid-Open No. 2023-085748

[0004] When storing a liquefied gas composed of only a single element such as liquefied hydrogen in a storage tank, it may be required to maintain the liquefied gas in the storage tank at a high purity. However, Patent Document 1 does not describe the purity of the liquefied gas.

[0005] An object of the present disclosure is to improve the purity of the liquefied gas stored in a tank.

[0006] One aspect of the present disclosure is a method for replacing the air in a tank for storing a liquefied gas obtained by liquefying a single gas composed of a single element with the single gas, comprising supplying an inert gas to the tank, replacing the air in the tank with the inert gas, and after the oxygen concentration in the tank becomes equal to or lower than a predetermined concentration and the dew point temperature in the tank becomes equal to or lower than a first dew point temperature, supplying the single gas to the tank, replacing the inert gas in the tank with the single gas, and after the concentration of the single gas in the tank becomes equal to or higher than a predetermined concentration and the dew point temperature in the tank becomes equal to or lower than a second dew point temperature lower than the first dew point temperature, stopping the supply of the single gas to the tank.

[0007] According to the present disclosure, the purity of the liquefied gas stored in the tank can be improved.

[0008] FIG. 1 is a schematic diagram showing the configuration of a tank according to an embodiment of the present disclosure. FIG. 2 is a flowchart of a gas replacement method according to an embodiment of the present disclosure. FIG. 3 is a diagram for explaining an inerting process according to an embodiment of the present disclosure. FIG. 4 is a diagram for explaining a gassing-up process according to an embodiment of the present disclosure.

[0009] The gas replacement method relating to this disclosure will be described below with reference to the attached drawings.

[0010] Figure 1 is a schematic diagram showing the configuration of a tank 1 according to one embodiment of the present disclosure. Referring to Figure 1, the tank 1 comprises a first tank 10 for storing liquefied hydrogen, which is obtained by liquefying hydrogen gas composed of a single element, and a second tank 11 surrounding the first tank 10. The tank 1 of this embodiment is a double-shell tank having two tanks, the first tank 10 and the second tank 11. The tank 1 of this embodiment is a cargo tank installed on a ship and for storing liquefied hydrogen as cargo. The liquefied hydrogen of this embodiment is an example of a liquefied gas according to the present disclosure. The hydrogen gas of this embodiment is an example of a single-component gas according to the present disclosure.

[0011] A storage space 12 for storing liquefied gas is defined inside the first tank 10. Furthermore, an inter-tank space 13 is defined between the first tank 10 and the second tank 11. The volume of the storage space 12 is greater than the volume of the inter-tank space 13.

[0012] The first tank 10 is equipped with a dew point thermometer 14 for measuring the dew point temperature in the storage space 12, an oxygen concentration meter 15 for measuring the oxygen concentration in the storage space 12, and a hydrogen gas concentration meter 16 for measuring the hydrogen gas concentration in the storage space 12. The second tank 11 is equipped with a dew point thermometer 17 for measuring the dew point temperature in the space between tanks 13, an oxygen concentration meter 18 for measuring the oxygen concentration in the space between tanks 13, and a hydrogen gas concentration meter 19 for measuring the hydrogen gas concentration in the space between tanks 13. In this specification, dew point temperature refers to the dew point temperature under atmospheric pressure.

[0013] Tank 1 includes first to fourth supply pipes 20 to 23 for supplying gas to the storage space 12 and the inter-tank space 13, and first to third discharge pipes 30 to 32 for discharging gas from the storage space 12 and the inter-tank space 13.

[0014] In the following description, when there is no need to distinguish between the first to fourth supply pipes 20 to 23 and the first to third discharge pipes 30 to 32, one of the first to fourth supply pipes 20 to 23 and the first to third discharge pipes 30 to 32 may be simply referred to as "piping." In the following description, the upstream side in the direction of gas flow through the pipe may be simply referred to as "upstream of the pipe," and the downstream side in the direction of gas flow through the pipe may be simply referred to as "downstream of the pipe." The gas flows through the first to fourth supply pipes 20 to 23 toward the storage space 12 and the inter-tank space 13, and flows through the first to third discharge pipes 30 to 32 away from the storage space 12 and the inter-tank space 13.

[0015] The first supply pipe 20 is a pipe for supplying gas to the storage space 12. The first supply pipe 20 is used to supply combustion gas as an inert gas into the storage space 12. One end of the first supply pipe 20 is located in the storage space 12. One end of the first supply pipe 20 is open to the storage space 12. The combustion gas according to this embodiment is an example of an inert gas according to the disclosure.

[0016] The second supply pipe 21 is a pipe for supplying gas to the inter-tank space 13. The second supply pipe 21 is mainly used to supply nitrogen gas as an inert gas to the inter-tank space 13. One end of the second supply pipe 21 is located in the inter-tank space 13. One end of the second supply pipe 21 is open to the inter-tank space 13. The nitrogen gas according to this embodiment is an example of an inert gas according to this disclosure.

[0017] The third supply pipe 22 fluidly connects the first supply pipe 20 and the second supply pipe 21. One end of the third supply pipe 22 is connected to the first supply pipe 20 at a connection part 24, and the other end of the third supply pipe 22 is connected to the second supply pipe 21 at a connection part 25.

[0018] The fourth supply pipe 23 is a pipe for supplying hydrogen gas to the storage space 12 and the inter-tank space 13. One end of the fourth supply pipe 23 is connected to the second supply pipe 21 at a connection section 26. A vaporizer 27 for vaporizing liquefied gas is located on the fourth supply pipe 23.

[0019] The first to third supply pipes 20 to 22 are each equipped with corresponding first to third supply valves 40 to 42. The first to third supply valves 40 to 42 open and close the corresponding first to third supply pipes 20 to 22. The first supply valve 40 is located downstream of the connection section 24 in the first supply pipe 20. The second supply valve 41 is located downstream of the connection section 25 in the second supply pipe 21.

[0020] The first discharge pipe 30 is a pipe for discharging gas from the storage space 12. One end of the first discharge pipe 30 is located in the storage space 12. One end of the first discharge pipe 30 is open to the storage space 12.

[0021] The second discharge pipe 31 is a pipe for discharging gas from the inter-tank space 13. One end of the second discharge pipe 31 is located in the inter-tank space 13. The second discharge pipe 31 is open to the inter-tank space 13.

[0022] The third discharge pipe 32 fluidly connects the first discharge pipe 30 and the second discharge pipe 31. One end of the third discharge pipe 32 is connected to the first discharge pipe 30 by a connection part 33, and the other end of the third discharge pipe 32 is connected to the second discharge pipe 31 by a connection part 34.

[0023] The first to third discharge pipes 30 to 32 are each equipped with corresponding first to third discharge valves 50 to 52. The first to third discharge valves 50 to 52 open and close the corresponding first to third discharge pipes 30 to 32. The first discharge valve 50 is located upstream of the connection 33 in the first discharge pipe 30. The second discharge valve 51 is located upstream of the connection 34 in the second discharge pipe 31. The first discharge pipe 30 is also equipped with a fourth discharge valve 53 that opens and closes the first discharge pipe 30. The fourth discharge valve 53 is located downstream of the connection 33 in the first discharge pipe 30. The second discharge pipe 31 is equipped with a fifth discharge valve 54 that opens and closes the second discharge pipe 31. The fifth discharge valve 54 is located downstream of the connection 34 in the second discharge pipe 31.

[0024] An inert gas generator 2 is located upstream of the first supply pipe 20. In this embodiment, the inert gas generator 2 is installed on a ship. The inert gas generator 2 produces combustion gas as an inert gas by burning fuel. The combustion gas produced by the inert gas generator 2 is supplied to the first supply pipe 20. The combustion gas supplied to the first supply pipe 20 is supplied to the storage space 12 (see Figure 3).

[0025] A nitrogen gas generator 3 is located upstream of the second supply pipe 21. The nitrogen gas generator 3 in this embodiment is installed on a ship. The nitrogen gas generator 3 produces nitrogen gas. The nitrogen gas generator 3 is a membrane separation type nitrogen gas generator. The dew point temperature of the nitrogen gas produced by the nitrogen gas generator 3 in this embodiment is lower than the dew point temperature of the inert gas produced by the inert gas generator 2 in this embodiment. In addition, the nitrogen gas generator 3 in this embodiment can change the purity of the nitrogen gas produced according to the settings. The nitrogen gas produced by the nitrogen gas generator 3 is supplied to the second supply pipe 21. The nitrogen gas supplied to the second supply pipe 21 is supplied to the inter-tank space 13 (see Figure 3). The amount of impurities contained in the nitrogen gas produced by the nitrogen gas generator 3 is less than the amount of impurities contained in the combustion gas produced by the inert gas generator 2. Also, the amount of nitrogen gas produced per unit time by the nitrogen gas generator 3 is less than the amount of combustion gas produced per unit time by the inert gas generator 2. The amount of nitrogen gas supplied per unit time from the nitrogen gas generator 3 to the second supply pipe 21 is less than the amount of combustion gas supplied per unit time from the inert gas generator 2 to the first supply pipe 20.

[0026] Upstream of the fourth supply pipe 23, a hydrogen supply source 4 is located. The hydrogen supply source 4 is, for example, a liquefied hydrogen tank installed at a liquefied hydrogen handling station (not shown). The hydrogen supply source 4 supplies liquefied hydrogen to the fourth supply pipe 23. The liquefied hydrogen supplied to the fourth supply pipe 23 is vaporized by a vaporizer 27 to become hydrogen gas, which is then supplied to the storage space 12 and the inter-tank space 13. Specifically, the hydrogen gas flows sequentially through the fourth supply pipe 23, the second supply pipe 21, the third supply pipe 22, and the first supply pipe 20 to supply the storage space 12. The hydrogen gas also flows sequentially through the fourth supply pipe 23 and the second supply pipe 21 to supply the inter-tank space 13. The dew point temperature of the hydrogen gas supplied to the storage space 12 and the inter-tank space 13 is lower than both the dew point temperature of the combustion gas supplied to the storage space 12 and the dew point temperature of the nitrogen gas supplied to the inter-tank space 13. In other words, both the dew point temperature of the combustion gas supplied to the storage space 12 and the dew point temperature of the nitrogen gas supplied to the inter-tank space 13 are higher than the dew point temperature of the hydrogen gas supplied to the storage space 12 and the inter-tank space 13.

[0027] Downstream of the first discharge pipe 30, a venting device 5 is located for discharging gas to the outside. In this embodiment, the venting device 5 is a vent mast mounted on a ship. The venting device 5 is used when discharging gas from the storage space 12 and the inter-tank space 13. The gas in the storage space 12 flows through the first discharge pipe 30 and is discharged to the outside via the venting device 5. The gas in the inter-tank space 13 flows sequentially through the second discharge pipe 31, the third discharge pipe 32, and the first discharge pipe 30 and is discharged to the outside via the venting device 5.

[0028] Downstream of the second discharge pipe 31, a combustion facility 6 for burning gas is located. In this embodiment, the combustion facility 6 is, for example, a boiler. The combustion facility 6 is used to burn flammable gas (for example, hydrogen gas) discharged from the storage space 12 and the inter-tank space 13. The flammable gas discharged from the storage space 12 and the inter-tank space 13 is treated by combustion in the combustion facility 6 without being released into the atmosphere. The flammable gas in the storage space 12 flows sequentially through the first discharge pipe 30, the third discharge pipe 32, and the second discharge pipe 31 and is supplied to the combustion facility 6. The flammable gas in the inter-tank space 13 flows through the second discharge pipe 31 and is supplied to the combustion facility 6.

[0029] Figure 2 is a flowchart of the gas replacement method according to this embodiment. Figures 3 and 4 are diagrams illustrating the gas replacement method according to this embodiment. The gas replacement method according to this embodiment is performed after the tank 1 has been constructed or after the inside of the tank 1 has been opened for inspection or repair, and before liquefied hydrogen is stored. The gas replacement method according to this embodiment may be performed in a dock or after the ship has left the dock.

[0030] The gas replacement method according to this embodiment includes an inerting step of replacing the air in the tank 1 with an inert gas, and a gassing-up step of replacing the inert gas in the tank 1 with hydrogen gas. The gassing-up step is performed following the inerting step. In the gas replacement method according to this embodiment, the dew point temperature in the tank 1 is controlled in both the inerting step and the gassing-up step so that the dew point temperature in the tank 1 is below the target dew point temperature when the gassing-up step is completed. In the gas replacement method according to this embodiment, the dew point temperature in the tank 1 is efficiently lowered to the target dew point temperature by gradually lowering the dew point temperature in the tank 1 in the inerting step and the gassing-up step.

[0031] Referring to Figure 2, in step S1, the inerting process is initiated. At the start of the inerting process, air is present in the storage space 12 and the inter-tank space 13. During the inerting process, the air in the storage space 12 is replaced with combustion gas, and the air in the inter-tank space 13 is replaced with nitrogen gas. As shown in Figure 3, the air in the storage space 12 is replaced with combustion gas by supplying combustion gas to the storage space 12 and discharging air from the storage space 12. In addition, the air in the inter-tank space 13 is replaced with nitrogen gas by supplying nitrogen gas to the inter-tank space 13 and discharging air from the inter-tank space 13. The combustion gas-mixed air discharged from the storage space 12 and the nitrogen gas-mixed air discharged from the inter-tank space 13 are discharged to the outside via the vent device 5. Once the inerting process is initiated in step S1, the gas replacement method proceeds to step S2.

[0032] In step S2, it is determined whether the following conditions 1 and 2, which are the termination conditions for the inerting process, have been met as a result of replacing the air in tank 1 with an inert gas. If it is determined in step S2 that both conditions 1 and 2 are met, that is, if the answer is Yes in step S2, the gas replacement method proceeds to step S3. If it is determined in step S2 that neither conditions 1 and 2 are met, that is, if the answer is No in step S2, the gas replacement method repeats step S2.

[0033] <Condition 1> The oxygen concentration in tank 1 is below the first concentration. Specifically, both the oxygen concentration in the storage space 12 and the oxygen concentration in the inter-tank space 13 are below the first concentration. Here, the first concentration is set to a range in which no explosion occurs due to a chemical reaction between oxygen and hydrogen in tank 1.

[0034] <Condition 2> The dew point temperature inside tank 1 is below a predetermined first dew point temperature. Specifically, both the dew point temperature inside the storage space 12 and the dew point temperature inside the inter-tank space 13 are below the first dew point temperature. Here, the first dew point temperature is higher than the dew point temperature of the combustion gas supplied to the storage space 12. Preferably, the first dew point temperature is 10°C or more higher than the dew point temperature of the combustion gas supplied to the storage space 12. Also, the first dew point temperature is lower than the dew point temperature of the air inside the storage space 12.

[0035] In step S3, the inerting process is completed. In step S3, the supply of inert gas to tank 1 is stopped. Specifically, in step S3, the supply of combustion gas to storage space 12 is stopped, and the supply of nitrogen gas to inter-tank space 13 is stopped. Once the inerting process is completed in step S3, the gas replacement method proceeds to step S4.

[0036] In step S4, the gassing-up process is initiated. In the gassing-up process, the combustion gas in the storage space 12 is replaced with hydrogen gas, and the nitrogen gas in the inter-tank space 13 is replaced with hydrogen gas. As shown in Figure 4, hydrogen gas is supplied to the storage space 12, and combustion gas is discharged from the storage space 12, thereby replacing the combustion gas in the storage space 12 with hydrogen gas. In addition, hydrogen gas is supplied to the inter-tank space 13, and nitrogen gas is discharged from the inter-tank space 13, thereby replacing the nitrogen gas in the inter-tank space 13 with hydrogen gas. The combustion gas mixed with hydrogen gas discharged from the storage space 12 and the nitrogen gas mixed with hydrogen gas discharged from the inter-tank space 13 are supplied to the combustion equipment 6. Once the gassing-up process is initiated in step S4, the gas replacement method proceeds to step S5.

[0037] In step S5, it is determined whether the following conditions 3 and 4, which are the completion conditions for the gassing-up process, have been met as a result of replacing the inert gas in tank 1 with hydrogen gas. If it is determined in step S5 that both conditions 3 and 4 are met, that is, if the answer is Yes in step S5, the gas replacement method proceeds to step S6. If it is determined in step S5 that neither conditions 3 and 4 are met, that is, if the answer is No in step S5, the gas replacement method repeats step S5.

[0038] <Condition 3> The hydrogen gas concentration in tank 1 is equal to or greater than the second concentration. Specifically, both the hydrogen gas concentration in the storage space 12 and the hydrogen gas concentration in the inter-tank space 13 are equal to or greater than the second concentration. Here, it is preferable that the second concentration be as high as possible in order to improve the purity of the hydrogen gas in the storage space 12 and minimize the effects of liquefaction of the gas in the inter-tank space 13.

[0039] <Condition 4> The dew point temperature inside tank 1 is below a predetermined second dew point temperature, i.e., the target dew point temperature. Specifically, both the dew point temperature inside the storage space 12 and the dew point temperature inside the inter-tank space 13 are below the second dew point temperature. Here, the second dew point temperature is lower than the first dew point temperature. Also, the second dew point temperature is higher than the dew point temperature of the hydrogen gas supplied to tank 1. Preferably, the second dew point temperature is 10°C or more higher than the dew point temperature of the hydrogen gas supplied to tank 1.

[0040] In step S6, the gassing-up process is completed. In step S6, the supply of hydrogen gas to tank 1 is stopped. Specifically, in step S6, the supply of hydrogen gas to the storage space 12 and the inter-tank space 13 is stopped. When the gassing-up process is completed in step S6, the gas replacement method is completed.

[0041] After the gas replacement method according to this embodiment is completed, the tank 1 is cooled, and then liquefied hydrogen is loaded into the tank 1.

[0042] The gas replacement method according to this embodiment provides the following effects.

[0043] In the gas replacement method according to this embodiment, since the dew point temperature in the tank 1 is controlled, by controlling to lower the dew point temperature in the tank 1, the amount of moisture present in the tank 1 can be reduced. As a result, the occurrence of condensation when the tank 1 is cooled can be suppressed, and the purity of the liquefied hydrogen stored in the tank 1 can be improved.

[0044] In particular, in the gas replacement method according to this embodiment, the dew point temperature in the tank 1 is controlled in both the inerting process and the gassing-up process. Therefore, compared with the case where the dew point temperature in the tank 1 is controlled only in either the inerting process or the gassing-up process to lower it to the target dew point temperature, the dew point temperature in the tank 1 can be efficiently lowered to the target dew point temperature. For example, when the dew point temperature in the tank 1 is controlled only in the inerting process to lower it to the target dew point temperature, it may be necessary to continue the inerting process to lower the dew point temperature in the tank 1 even after the oxygen concentration in the tank 1 has sufficiently decreased. As a result, the time of the inerting process may become longer than necessary. In particular, when the dew point temperature of the inert gas used in the inerting process is a temperature slightly lower than the target dew point temperature, it is difficult to lower the dew point temperature in the tank 1 to the target dew point temperature or it takes a very long time. Also, for example, when the dew point temperature in the tank 1 is controlled only in the gassing-up process to lower it to the target dew point temperature, the dew point temperature in the tank 1 at the start of the gassing-up process (at the end of the inerting process) may not be sufficiently lowered. In this case, it may be necessary to continue the gassing-up process to lower the dew point temperature in the tank 1 even after the hydrogen gas concentration in the tank 1 has become sufficiently high, and the time of the gassing-up process may become longer than necessary. In contrast, in the gas replacement method according to this embodiment, the dew point temperature in the tank 1 is controlled in both the inerting process and the gassing-up process. Therefore, compared with the case where the dew point temperature in the tank 1 is controlled only in either the inerting process or the gassing-up process to lower it to the target dew point temperature, the dew point temperature in the tank 1 can be efficiently lowered to the target dew point temperature.

[0045] Generally, since impurities are removed during the stage of cooling hydrogen gas to liquefy it, liquefied hydrogen has a high purity, for example, a purity of 99.999% or more. Therefore, the dew point temperature of the hydrogen gas obtained by vaporizing the liquefied hydrogen is lower than, for example, the dew point temperature of the hydrogen gas immediately after being manufactured in a factory. In the gas replacement method according to this embodiment, supplying hydrogen gas to the tank 1 includes supplying the hydrogen gas obtained by vaporizing the liquefied hydrogen to the tank 1. As a result, the dew point temperature inside the tank 1 can be efficiently lowered.

[0046] When the dew point temperature of the hydrogen gas supplied to the tank 1 is higher than the dew point temperatures of the combustion gas and nitrogen gas supplied to the tank 1, the time of the gassing-up process may become longer than necessary. On the contrary, since the dew point temperature of the hydrogen gas supplied to the tank 1 is lower than the dew point temperatures of the combustion gas and nitrogen gas supplied to the tank 1, the dew point temperature inside the tank 1 can be efficiently lowered step by step in the inerting process and the gassing-up process.

[0047] When the first dew point temperature is slightly higher than the dew point temperature of the inert gas supplied to the tank 1, it is difficult or may take a very long time to lower the dew point temperature inside the tank 1 to the first dew point temperature in the inerting process. On the contrary, in this embodiment, the first dew point temperature is 10°C or more higher than both the dew point temperature of the combustion gas and the dew point temperature of the nitrogen gas supplied to the tank 1. As a result, it is possible to suppress the time required for the inerting process from becoming excessively long.

[0048] [Modification] The present disclosure is not limited to the configurations described in the above embodiments, and various modifications are possible.

[0049] In the above embodiment, an example where the tank 1 is installed on a ship has been described, but the tank 1 may be installed on a floating structure or a land base.

[0050] In the above embodiment, an example was described in which the tank 1 is a double-shell tank. However, the tank 1 may be a single-shell tank without a second tank 11, or it may be a multi-shell tank of three or more shells with further tanks outside the second tank 11. By reinterpreting the relationship between the first tank 10 and the second tank 11 in the above description as the relationship between two tanks arranged opposite each other in a multi-shell tank of three or more shells, this disclosure can be applied to multi-shell tanks of three or more shells.

[0051] In the above embodiment, an example was described in which the elemental gas is hydrogen gas and the liquefied gas is liquefied hydrogen. However, the elemental gas according to this disclosure may be helium gas or nitrogen gas, and the liquefied gas according to this disclosure may be liquefied helium or liquefied nitrogen. In this case, in the gassing-up process, the inert gas in the tank 1 is replaced with helium gas or nitrogen gas.

[0052] In the above embodiment, during the inert process, combustion gas was supplied to the storage space 12 and nitrogen gas was supplied to the inter-tank space 13. However, combustion gas may be supplied to both the storage space 12 and the inter-tank space 13. Nitrogen gas may be supplied to both the storage space 12 and the inter-tank space 13. Nitrogen gas may be supplied to the storage space 12 and combustion gas may be supplied to the inter-tank space 13. Other inert gases may be used instead of combustion gas, and other inert gases may be used instead of nitrogen gas.

[0053] In the above embodiment, the supply of inert gas to the storage space 12 and the inter-tank space 13 was stopped when conditions 1 and 2 were met in both the storage space 12 and the inter-tank space 13, but the disclosure is not limited thereto. When conditions 1 and 2 are met in the storage space 12 earlier than in the inter-tank space 13, the supply of combustion gas to the storage space 12 may be stopped before the supply of nitrogen gas to the inter-tank space 13 is stopped. Also, when conditions 1 and 2 are met in the inter-tank space 13 earlier than in the storage space 12, the supply of nitrogen gas to the inter-tank space 13 may be stopped before the supply of combustion gas to the storage space 12 is stopped.

[0054] In the above embodiment, an example was described in which the nitrogen gas generator 3 is a membrane separation type nitrogen gas generator, but it may also be a pressure swing adsorption type nitrogen gas generator.

[0055] In the above embodiment, the dew point thermometer 14, the oxygen concentration meter 15, and the hydrogen gas concentration meter 16 were located in the first tank 10, but they may be located in a different location from the first tank 10. In this case, the dew point temperature, oxygen concentration, and hydrogen gas concentration of the gas in the first tank 10 may be measured by testing the gas sampled from the first tank 10 at a location away from the first tank 10. For example, the dew point temperature, oxygen concentration, and hydrogen gas concentration of the gas in the first tank 10 may be measured by filling a sampling bottle with the gas from the first tank 10 on a ship and testing the gas in the sampling bottle at a land-based base such as a liquefied hydrogen handling station.

[0056] In the above embodiment, the dew point thermometer 17, the oxygen concentration meter 18, and the hydrogen gas concentration meter 19 were located in the second tank 11, but they may be located in a different location from the second tank 11. In this case, the dew point temperature, oxygen concentration, and hydrogen gas concentration of the gas in the second tank 11 may be measured by testing the gas sampled from the second tank 11 at a location away from the second tank 11. For example, the dew point temperature, oxygen concentration, and hydrogen gas concentration of the gas in the second tank 11 may be measured by filling a sampling bottle with the gas from the second tank 11 on a ship and testing the gas in the sampling bottle at a land-based base such as a liquefied hydrogen handling station.

[0057] In the above embodiment, an example was described in which a hydrogen supply source 4 supplies liquefied hydrogen to the fourth supply pipe 23, and the liquefied hydrogen supplied to the fourth supply pipe 23 is vaporized by a vaporizer 27. However, this disclosure is not limited thereto. The hydrogen supply source 4 may supply hydrogen gas to the fourth supply pipe 23. The hydrogen supply source 4 may be, for example, a hydrogen gas tank. In this case, the vaporizer 27 may not be provided.

[0058] The discussions of this disclosure described above are presented for illustrative and explanatory purposes only and are not intended to limit the disclosure to the forms disclosed herein. For example, in the foregoing description, various features of the disclosure are grouped into one embodiment for the purpose of concisely illustrating the disclosure, but some of the various features may be combined.

[0059] [Note] The gas replacement method relating to this disclosure provides the following embodiments.

[0060] [Aspect 1] A method for replacing air in a tank for storing liquefied gas obtained by liquefying a single element gas, the method comprising: supplying an inert gas to the tank to replace the air in the tank with the inert gas; supplying the single element gas to the tank after the oxygen concentration in the tank has fallen to a first concentration or less and the dew point temperature in the tank has fallen to a first dew point temperature or less; and stopping the supply of the single element gas to the tank after the concentration of the single element gas in the tank has fallen to a second concentration or more and the dew point temperature in the tank has fallen to a second dew point temperature or less which is lower than the first dew point temperature.

[0061] According to the gas replacement method of Embodiment 1, the dew point temperature inside the tank is controlled, and by controlling the dew point temperature inside the tank to lower it, the amount of moisture present inside the tank can be reduced. As a result, condensation when the tank is cooled is suppressed, and the purity of the liquefied hydrogen stored in the tank can be improved.

[0062] In particular, in the gas replacement method according to Embodiment 1, the dew point temperature inside the tank is controlled in both the inerting step, in which the air inside the tank is replaced with an inert gas, and the gassing-up step, in which the inert gas inside the tank is replaced with a gas. Therefore, compared to the case where the dew point temperature inside the tank is controlled and lowered to the target dew point temperature using only either the inerting step or the gassing-up step, the dew point temperature inside the tank can be lowered to the target dew point temperature more efficiently. For example, when the dew point temperature inside the tank is controlled and lowered to the target dew point temperature using only the inerting step, it may be necessary to continue the inerting step even after the oxygen concentration inside the tank has sufficiently decreased in order to lower the dew point temperature inside the tank. As a result, the time required for the inerting step may become unnecessarily long. In particular, if the dew point temperature of the inert gas used in the inerting step is slightly below the target dew point temperature, it may be difficult or very time-consuming to lower the dew point temperature inside the tank to the target dew point temperature. Furthermore, when the dew point temperature inside the tank is controlled and lowered to the target dew point temperature using only the gassing-up process, the dew point temperature inside the tank may not have dropped sufficiently at the start of the gassing-up process (end of the inerting process). In this case, it may be necessary to continue the gassing-up process to lower the dew point temperature inside the tank even after the concentration of the elemental gas inside the tank has become sufficiently high, which can result in the gassing-up process taking longer than necessary. In contrast, the gas replacement method according to Embodiment 1 controls the dew point temperature inside the tank in both the inerting process and the gassing-up process. Therefore, compared to the case where the dew point temperature inside the tank is controlled and lowered to the target dew point temperature using only either the inerting process or the gassing-up process, the dew point temperature inside the tank can be lowered to the target dew point temperature more efficiently.

[0063] [Aspect 2] The gas replacement method according to aspect 1, wherein supplying the elemental gas to the tank includes supplying the elemental gas obtained by vaporizing the liquefied gas to the tank.

[0064] Generally, liquefied gases often have high purity because impurities are removed during the cooling and liquefaction process. Therefore, the dew point temperature of the gas obtained by vaporizing the liquefied gas is lower than, for example, the dew point temperature of the gas immediately after its manufacture in a factory. In the gas replacement method according to Embodiment 2, supplying gas to the tank includes supplying the gas obtained by vaporizing the liquefied gas to the tank. As a result, the dew point temperature inside the tank can be efficiently lowered.

[0065] [Aspect 3] The gas replacement method according to aspect 1 or 2, wherein the dew point temperature of the single gas supplied to the tank is lower than the dew point temperature of the inert gas supplied to the tank.

[0066] If the dew point temperature of the individual gas supplied to the tank is higher than the dew point temperature of the inert gas supplied to the tank, the gassing-up process may take longer than necessary. In contrast, in the gas replacement method according to embodiment 3, since the dew point temperature of the individual gas supplied to the tank is lower than the dew point temperature of the inert gas supplied to the tank, the dew point temperature inside the tank can be efficiently and gradually lowered in the inerting process and the gassing-up process.

[0067] [Aspect 4] The gas replacement method according to any one of aspects 1 to 3, wherein the first dew point temperature is 10°C or higher than the dew point temperature of the inert gas supplied to the tank.

[0068] If the first dew point temperature is slightly higher than the dew point temperature of the inert gas supplied to the tank, it may be difficult or very time-consuming to lower the dew point temperature in the tank to the first dew point temperature during the inert process. In contrast, in the gas replacement method according to embodiment 4, the first dew point temperature is 10°C or more higher than the dew point temperature of the inert gas supplied to the tank. As a result, it is possible to suppress the time required for the inert process from becoming excessively long.

Claims

1. A method for replacing air in a tank for storing liquefied gas, which is a single element gas obtained by liquefying a single element gas, the method comprising: supplying an inert gas to the tank to replace the air in the tank with the inert gas; supplying the single element gas to the tank after the oxygen concentration in the tank has fallen to a first concentration or less and the dew point temperature in the tank has fallen to a first dew point temperature or less; and stopping the supply of the single element gas to the tank after the concentration of the single element gas in the tank has fallen to a second concentration or more and the dew point temperature in the tank has fallen to a second dew point temperature or less which is lower than the first dew point temperature.

2. The gas replacement method according to claim 1, wherein supplying the single gas to the tank includes supplying the single gas obtained by vaporizing the liquefied gas to the tank.

3. The gas replacement method according to claim 1, wherein the dew point temperature of the single gas supplied to the tank is lower than the dew point temperature of the inert gas supplied to the tank.

4. The gas replacement method according to claim 1, wherein the first dew point temperature is 10°C or higher than the dew point temperature of the inert gas supplied to the tank.

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