High-purity liquefied methane production equipment

Abstract

To provide a device for producing high-purity liquefied methane from liquefied natural gas and capable of relatively easily producing liquefied methane with high purity from liquefied natural gas while effectively using boil-off gas generated in a liquefied natural gas storage tank that stores liquefied natural gas.SOLUTION: A production device for high-purity liquefied methane is a device for producing high-purity liquefied methane from liquefied natural gas, and includes: a liquefied natural gas storage tank for storing liquefied natural gas; a rectification column for separating methane gas from the liquefied natural gas; a methane gas condenser for liquefying methane gas; a liquefied methane storage tank for storing liquefied methane; and a first boil-off gas supply passage for supplying boil-off gas generated in the liquefied natural gas storage tank to the rectification column.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an apparatus for producing high-purity liquefied methane from liquefied natural gas. [Background technology]

[0002] Methane is used as a raw material in chemical processes, and typically, ultra-high-purity liquefied methane with a purity of 99.9999% or higher is used. The raw material for methane is liquefied natural gas (LNG), and methane is the main component of LNG. The methane concentration in LNG varies depending on the origin of the LNG; for example, Australian LNG contains about 87% methane, Indonesian and Malaysian LNG contains about 89%, and Alaskan LNG contains about 99% methane. Therefore, in order to use methane as a raw material in chemical processes, it is necessary to purify the LNG to produce ultra-high-purity liquefied methane with a methane purity of 99.9999% or higher.

[0003] Patent documents 1 and 2 describe apparatus capable of producing ultra-high-purity methane with a methane purity of 99.9999% or higher by refining LNG.

[0004] Patent Document 1 describes an apparatus for producing ultra-high-purity methane, comprising: a first distillation apparatus for separating high-boiling-point components, equipped with a condenser and a reboiler, into which liquefied natural gas is introduced; a second distillation apparatus for separating low-boiling-point components, equipped with a condenser and a reboiler, from which the bottom liquid is extracted as ultra-high-purity methane; a transfer passage for extracting a mixed fluid of nitrogen and methane from the top of the first distillation apparatus and sending it to the second distillation apparatus; and a condenser cooling passage for passing the liquid at the bottom of the first distillation apparatus through the condenser of at least one of the distillation apparatuses for evaporation in the condenser.

[0005] Patent Document 2 describes a high-purity methane purification apparatus comprising a crude methane rectification column for crudely purifying natural gas and a methane rectification column for further purifying the methane crudely purified in the crude methane rectification column.

[0006] Methane is the main component of city gas and is also used as fuel for industrial and household purposes. Since the required methane concentration for city gas is around 90%, LNG can be used as is. However, if the methane concentration in LNG falls below the standard value, it needs to be purified to raise the methane concentration to around 90%.

[0007] Methane is used as a raw material in chemical processes and as an industrial and household fuel. In recent years, it has also been used as fuel for mobile vehicles such as rockets, as well as for LNG trucks by mixing liquefied biomethane (LBM) with a methane purity of 99.9% or higher with LNG, and as fuel for combustion equipment such as boilers. These fuels use high-purity liquefied methane with a purity of 99% or higher. Alaskan LNG contains about 99% methane, so Alaskan LNG can be used as is without refining to increase the methane purity. However, Alaskan LNG is not always available, and because LNG is a natural product, its methane concentration fluctuates and can fall below 99%. In such cases, it is necessary to refine LNG with a methane concentration of less than 99% to increase the methane concentration to about 99%. The apparatus described in the aforementioned Patent Documents 1 and 2 can be used to increase the methane concentration. However, the methane obtained by the apparatus described in Patent Documents 1 and 2 has an ultra-high purity of over 99.9999%, making it over-specced for use as industrial or household fuel or as fuel for mobile vehicles such as rockets. Furthermore, the apparatus described in Patent Documents 1 and 2 requires a large rectification column to increase the methane purity to over 99.9999%. The large size of the rectification column increases the time required for pre-cooling, etc., thus delaying the production of high-purity liquefied methane. In addition, constructing large-scale equipment requires a vast site, making it difficult to construct a rectification facility at each location where liquefied methane is used. Therefore, it is necessary to load liquefied methane purified to the required purity at a refining facility onto trucks or other vehicles and transport it to the location where it will be used. However, liquefied methane vaporizes during transport, resulting in resource loss. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 4-225778 [Patent Document 2] Japanese Patent Publication No. 2010-275215 [Overview of the project] [Problems that the invention aims to solve]

[0009] In the apparatus described in Patent Documents 1 and 2 above, the liquefied natural gas (LNG) before purification is temporarily stored in a LNG storage tank. The LNG stored in the LNG storage tank is in liquid form, but if the temperature inside the tank changes, some of the components contained in the LNG may vaporize. The gas produced by vaporization is generally called boil-off gas. The boil-off gas generated in the LNG storage tank is usually discharged from the LNG storage tank to the outside of the system. For example, in the case of a large LNG terminal, it is burned and exhausted using a flare stack or vented into the atmosphere from a high place using a vent stack. In addition, the boil-off gas generated in the LNG storage tank may be compressed and supplied to users via pipeline, or the boil-off gas may be reliquefied.

[0010] The present invention has been made in view of the circumstances described above, and its purpose is to provide an apparatus for producing high-purity liquefied methane from liquefied natural gas, which can produce high-purity liquefied methane from liquefied natural gas relatively easily while effectively utilizing the boil-off gas generated in a liquefied natural gas storage tank for storing liquefied natural gas. [Means for solving the problem]

[0011] The present invention is as follows: [1] An apparatus for producing high-purity liquefied methane from liquefied natural gas, comprising: a liquefied natural gas storage tank for storing the liquefied natural gas; a rectification column for separating methane gas from the liquefied natural gas; a methane gas condenser for liquefying the methane gas; a liquefied methane storage tank for storing the liquefied methane; and a first boil-off gas supply path for supplying boil-off gas generated in the liquefied natural gas storage tank to the rectification column. [2] An apparatus for producing high-purity liquefied methane from liquefied natural gas, comprising: a liquefied natural gas storage tank for storing the liquefied natural gas; a rectification column for separating methane gas from the liquefied natural gas; a methane gas condenser for liquefying the methane gas; a liquefied methane storage tank for storing the liquefied methane; and a second boil-off gas supply path for supplying boil-off gas generated in the liquefied natural gas storage tank to the liquefied methane storage tank. [3] The manufacturing apparatus according to [2], wherein the second boil-off gas supply path is equipped with a device capable of determining the methane concentration in the second boil-off gas supply path. [4] The manufacturing apparatus according to [3], further comprising a first boil-off gas supply path for supplying boil-off gas generated in the liquefied natural gas storage tank to the rectification column. [5] A manufacturing apparatus according to any one of [1] to [4], wherein the liquefied methane storage tank is equipped with a reliquefaction device, the reliquefaction device is connected to a heat transfer medium supply path for supplying a heat transfer medium, and the reliquefaction device produces liquefied methane by heat exchange between the heat transfer medium and the methane gas in the liquefied methane storage tank. [6] The manufacturing apparatus according to [5], wherein the rectification column is equipped with a heat exchanger and has a path for supplying the heat transfer medium that has been heat-exchanged in the re-liquefaction device to the heat exchanger of the rectification column. [7] A manufacturing apparatus according to any one of [1] to [6], comprising a liquefied natural gas supply route for supplying the liquefied natural gas in the liquefied natural gas storage tank to the rectification column. [8] A manufacturing apparatus according to any one of [1] to [7], comprising a discharge route for discharging boil-off gas generated in the liquefied natural gas storage tank. [9] The production apparatus according to [8], wherein the boil-off gas contains nitrogen.

[10] The production apparatus according to any one of [1] to [9], comprising a discharge path for discharging boil-off gas generated in the liquefied methane storage tank.

[11] The production apparatus according to

[10] , wherein the boil-off gas contains nitrogen.

[12] The production apparatus according to any one of [1] to

[11] , wherein the purity of the high-purity liquefied methane is 99 mol% or more. Effects of the Invention

[0012] The apparatus for producing high-purity liquefied methane according to the present invention comprises: a liquefied natural gas storage tank that stores liquefied natural gas; a rectification column that separates methane gas from the liquefied natural gas; a methane gas condenser that liquefies the methane gas; and a liquefied methane storage tank that stores liquefied methane. Since the apparatus has a simple configuration, high-purity liquefied methane can be easily produced from liquefied natural gas. Furthermore, the apparatus is relatively compact and can be constructed at each location where liquefied methane is used. As a result, long-distance transportation is not required, so loss of liquefied methane during transportation can be reduced. Furthermore, the apparatus for producing high-purity liquefied methane according to the present invention comprises a path for supplying boil-off gas generated in the liquefied natural gas storage tank to the rectification column or the liquefied methane storage tank, so that the boil-off gas can be effectively utilized. Brief Description of the Drawings

[0013] [Figure 1] FIG. 1 is a schematic diagram showing Embodiment 1 of the apparatus for producing high-purity liquefied methane according to the present invention. [Figure 2] FIG. 2 is a schematic diagram showing Embodiment 2 of the apparatus for producing high-purity liquefied methane according to the present invention. Mode for Carrying Out the Invention

[0014] The apparatus for producing high-purity liquefied methane according to the present invention is an apparatus for producing high-purity liquefied methane from liquefied natural gas, and comprises: a liquefied natural gas storage tank that stores liquefied natural gas; a rectification column that separates methane gas from liquefied natural gas; a methane gas condenser that liquefies methane gas; and a liquefied methane storage tank that stores liquefied methane. In Embodiment 1 of the apparatus for producing high-purity liquefied methane, a first boil-off gas supply path for supplying boil-off gas generated in the liquefied natural gas storage tank to the rectification column is provided, and in Embodiment 2, a second boil-off gas supply path for supplying boil-off gas generated in the liquefied natural gas storage tank to the liquefied methane storage tank is provided. Embodiment 1 and Embodiment 2 are common to each other except for the difference in the supply destination to which boil-off gas generated in the liquefied natural gas storage tank is supplied.

[0015] The apparatus for producing high-purity liquefied methane according to the present invention has a simple apparatus configuration, so high-purity liquefied methane can be produced relatively easily from liquefied natural gas. In addition, since the apparatus is relatively compact, it can be constructed at each site where liquefied methane is used. As a result, long-distance transportation is eliminated, and loss of liquefied methane during transportation can be reduced. Furthermore, in the apparatus for producing high-purity liquefied methane of the present invention, boil-off gas generated in the liquefied natural gas storage tank can be effectively utilized.

[0016] Hereinafter, the apparatus for producing high-purity liquefied methane according to the present invention will be specifically described with reference to the drawings showing embodiments, but the present invention is not limited to the illustrated examples, and modifications may be made within a range compatible with the spirit described above and hereinafter, all of which are encompassed within the technical scope of the present invention.

[0017] (Embodiment 1) Embodiment 1 of the apparatus for producing high-purity liquefied methane according to the present invention will be described with reference to Fig. 1. The apparatus for producing high-purity liquefied methane in Embodiment 1 comprises: a liquefied natural gas storage tank 1 that stores liquefied natural gas; a rectification column 2 that separates methane gas from liquefied natural gas; a methane gas condenser 3 that liquefies methane gas; and a liquefied methane storage tank 4 that stores liquefied methane.

[0018] The liquefied natural gas storage tank 1 is a container for storing liquefied natural gas. The methane concentration in the liquefied natural gas stored in the liquefied natural gas storage tank 1 is not particularly limited and may be, for example, less than 85 mol%, but 85 mol% or higher is preferred. The methane concentration is more preferably 89 mol% or higher, and even more preferably 90 mol% or higher. There is no particular upper limit to the methane concentration, but it may be, for example, 99.9 mol%.

[0019] It is preferable to maintain conditions (i.e., temperature and pressure) inside the liquefied natural gas storage tank 1 such that the liquefied methane contained in the liquefied natural gas does not vaporize. Specifically, it is preferable to maintain the temperature below the boiling point of methane (below -161°C at normal pressure). By maintaining the temperature below the boiling point of methane, the vaporization of methane contained in the liquefied natural gas can be suppressed.

[0020] The rectification column 2 is a device for separating methane gas from liquefied natural gas. Preferably, the rectification column 2 is equipped with a heat exchanger 13 at its lower part.

[0021] The methane gas condenser 3 is a device that cools and liquefies the methane gas separated in the rectification column 2 to produce liquefied methane. The methane gas condenser 3 is housed in container 3a.

[0022] The liquefied methane storage tank 4 is a container for storing the liquefied methane obtained from the methane gas condenser 3. It is preferable to maintain conditions (i.e., temperature and pressure) inside the liquefied methane storage tank 4 that prevent the liquefied methane from vaporizing. Specifically, it is preferable to maintain the temperature below the boiling point of methane (below -161°C at normal pressure). By maintaining the temperature below the boiling point of methane, the vaporization of methane can be suppressed.

[0023] The apparatus for producing high-purity liquefied methane according to the present invention preferably further comprises a heat transfer medium storage tank 5. The heat transfer medium storage tank 5 is a container for storing a heat transfer medium at a temperature lower than the boiling point of methane. The heat transfer medium at a temperature lower than the boiling point of methane is preferably in the form of a liquid, and specifically, examples include liquids such as oxygen, argon, nitrogen, and helium. Among these, liquid nitrogen is preferred in terms of safety and cost.

[0024] The liquefied natural gas (LNG) storage tank 1 and the rectification column 2 are connected by a LNG supply path 103 that supplies LNG from the LNG storage tank 1 to the rectification column 2. The rectification column 2 and the methane gas condenser 3 are connected by a path 114, and the methane gas condenser 3 and the LNG storage tank 4 are connected by a path 116. The heat transfer medium storage tank 5 and the heat exchanger 13 installed in the rectification column 2 are connected by a path 111. This path 111 branches off midway, and the branch path 115 is connected to a container 3a equipped with the methane gas condenser 3.

[0025] The liquefied natural gas stored in the liquefied natural gas storage tank 1 is supplied to the rectification column 2 via the liquefied natural gas supply path 103. The liquefied natural gas supplied to the rectification column 2 undergoes heat exchange with a heat transfer medium supplied from a path 111 connected to the heat exchanger 13 in the rectification column 2.

[0026] The temperature of the heat transfer medium supplied to the heat exchanger 13 via path 111 is preferably above the boiling point of methane. By setting the temperature of the heat transfer medium above the boiling point of methane, the methane contained in the liquefied natural gas supplied to the rectification column 2 can be vaporized. Specifically, the temperature of the heat transfer medium should be controlled to -161°C or higher. On the other hand, there is no particular upper limit to the temperature of the heat transfer medium, but it is preferable to control the temperature of the heat transfer medium to, for example, 35°C or lower. The temperature of the heat transfer medium may also be controlled to be below the boiling point of ethane (specifically, below -88°C). By setting the temperature of the heat transfer medium below the boiling point of ethane, the vaporization of hydrocarbons with more carbon atoms than methane contained in the liquefied natural gas supplied to the rectification column 2 can be suppressed, thereby increasing the purity of the methane discharged from the rectification column 2.

[0027] In order to control the temperature of the heat transfer medium supplied to the heat exchanger 13 within the above range, it is preferable to place a vaporizer 7 in the middle of the path 111 connecting the heat transfer medium storage tank 5 and the heat exchanger 13 provided in the rectification column 2, which vaporizes the liquid supplied from the heat transfer medium storage tank 5. In the vaporizer 7, the temperature of the heat transfer medium can be controlled within the above range by exchanging heat between the heat transfer medium supplied from the heat transfer medium storage tank 5 and the atmosphere.

[0028] The heat transfer medium is supplied to the heat exchanger 13 of the rectification column 2 via path 111, and after heat exchange in the heat exchanger 13, it can be discharged out of the system via path 113. The heat transfer medium discharged from path 113 can be used, for example, as an instrumentation gas or a process purge gas.

[0029] The gas obtained by heat exchange and vaporization in the heat exchanger 13 rises within the rectification column 2 and accumulates at the top of the rectification column 2. An outlet for removing the gas produced within the rectification column 2 is provided at the top of the rectification column 2. A passage 114 is connected to this outlet, and this passage 114 is connected to the methane gas condenser 3.

[0030] A heat transfer medium is supplied to a container 3a equipped with a methane gas condenser 3 from a heat transfer medium storage tank 5 via a branch line 115. In the methane gas condenser 3, heat exchange occurs between the heat transfer medium supplied via the branch line 115 and the gas supplied via path 114. The methane gas contained in the heat-exchanged gas is cooled and liquefied, and the resulting liquefied methane is supplied to a liquefied methane storage tank 4 via path 116 and stored there.

[0031] Preferably, the container 3a is provided with an outlet for removing the gas accumulated at the top of the container 3a. A path 118 is connected to this outlet, and preferably, the path 118 is connected downstream of the vaporizer 7 located in the path 111 that connects the heat transfer medium storage tank 5 and the heat exchanger 13 of the rectification column 2, and upstream of the heat exchanger 13. By supplying the gas accumulated at the top of the container 3a to the heat exchanger 13 of the rectification column 2 via paths 118 and 111, the gas accumulated at the top of the container 3a can be reused as a heat transfer medium.

[0032] As described above, the liquefied natural gas (LNG) storage tank 1 is controlled to maintain conditions (i.e., temperature and pressure) that prevent the liquefied methane contained in the LNG from vaporizing. However, boil-off gas is generated within the LNG storage tank 1. Boil-off gas is the gas that vaporizes when a portion of the LNG stored in the LNG storage tank 1 evaporates. Conventionally, the boil-off gas generated within the LNG storage tank 1 was exhausted into the atmosphere.

[0033] On the other hand, Embodiment 1 includes a first boil-off gas supply path 101 that supplies boil-off gas generated in the liquefied natural gas storage tank 1 to the rectification column 2. The first boil-off gas supply path 101 is connected to a boil-off gas outlet located at the top of the liquefied natural gas storage tank 1. The boil-off gas generated in the liquefied natural gas storage tank 1 is supplied to the rectification column 2 via the first boil-off gas supply path 101, and is purified in the rectification column 2, separating the methane gas contained in the boil-off gas from a liquid containing hydrocarbons heavier than methane. As a result, the methane contained in the boil-off gas can be effectively utilized.

[0034] The liquid containing hydrocarbons heavier than methane separated in the rectification column 2 descends within the rectification column 2 and accumulates at the bottom of the rectification column 2. Preferably, an outlet is provided at the bottom of the rectification column 2 for removing the liquid accumulated at the bottom of the rectification column 2. A passage 112 is connected to this outlet, and the liquid accumulated at the bottom of the rectification column 2 is discharged out of the system through the passage 112. The liquid discharged through the passage 112 can be used, for example, as fuel for other equipment.

[0035] In the high-purity liquefied methane production apparatus of the present invention, the number of rectification columns 2 may be multiple, but one is sufficient.

[0036] It is preferable that the rectification column 2 is equipped with a rectification means inside. Examples of rectification means include rectification shelves and packing materials. Examples of packing materials include regular packing materials and irregular packing materials.

[0037] Furthermore, the liquefied natural gas storage tank 1 may also be equipped with a discharge route 104 for discharging boil-off gas generated within the liquefied natural gas storage tank 1 to the outside of the system, separate from the first boil-off gas supply route 101. Preferably, the discharge route 104 is connected to an outlet provided at the top of the liquefied natural gas storage tank 1. By discharging boil-off gas to the outside of the system through the discharge route 104, the methane purity in the liquefied natural gas stored in the liquefied natural gas storage tank 1 can be increased.

[0038] The boil-off gas discharged from the discharge path 104 preferably contains a substance with a boiling point lower than that of methane, and preferably contains nitrogen, for example.

[0039] The high-purity liquefied methane production apparatus of the present invention may include a gas-liquid separator 6 in the middle of the path 116 connecting the methane gas condenser 3 and the liquefied methane storage tank 4. In the gas-liquid separator 6, the liquid supplied from the methane gas condenser 3 via the path 116 is separated into gas and liquid, and by separating the gas from the liquid, the purity of the methane contained in the liquid can be further increased. On the other hand, the gas separated from the liquid in the gas-liquid separator 6 can be discharged out of the system through a path 121 connected to an outlet provided at the top of the gas-liquid separator 6.

[0040] It is preferable to control the temperature inside the gas-liquid separator 6 so that it is higher than the boiling point of the heat transfer medium supplied to the container 3a, and below the boiling point of methane. This allows substances with lower boiling points than methane (such as nitrogen) contained in the liquefied methane to be vaporized, thereby increasing the purity of the methane.

[0041] The path 116 connecting the gas-liquid separator 6 and the liquefied methane storage tank 4 may be branched along the way, and if it is branched, it is preferable that the branch path 117 is connected to the rectification column 2. The liquid separated in the gas-liquid separator 6 is returned to the rectification column 2 via the branch path 117, and the purity of the liquefied methane can be further increased by purifying it again in the rectification column 2.

[0042] In the high-purity liquefied methane production apparatus according to the present invention, it is preferable that the liquefied methane storage tank 4 is equipped with a reliquefaction device 12, and that a heat medium supply path 119 for supplying a heat medium is connected to the reliquefaction device 12.

[0043] The heat transfer medium supply path 119 is a path connecting the heat transfer medium storage tank 5 and the re-liquefaction device 12 provided in the liquefied methane storage tank 4. For example, the heat transfer medium supply path 119 can be a branch path that branches off from the path 111 connecting the heat transfer medium storage tank 5 and the heat exchanger 13 of the rectification column 2.

[0044] The reliquefaction device 12 is a device that generates liquefied methane by exchanging heat between a heat transfer medium supplied to the reliquefaction device 12 and methane gas generated in the liquefied methane storage tank 4. By reliquefying the methane gas generated in the liquefied methane storage tank 4, the loss of liquefied methane stored in the liquefied methane storage tank 4 can be reduced.

[0045] The heat transfer medium that has undergone heat exchange in the reliquefaction unit 12 can be discharged out of the system via a path 120 connected to the reliquefaction unit 12. The path 120 may be connected to, for example, a heat exchanger 13 in the rectification column 2, or, as shown in Figure 1, the path 120 may be connected to a path 111 that connects the vaporizer 7 and the heat exchanger 13 of the rectification column 2. The heat transfer medium can be reused by supplying the heat transfer medium that has undergone heat exchange in the reliquefaction unit 12 to the heat exchanger 13 in the rectification column 2 via the path 120.

[0046] The high-purity liquefied methane production apparatus according to the present invention preferably includes a discharge path 105 for discharging boil-off gas generated in the liquefied methane storage tank 4. The discharge path 105 is preferably connected to an outlet provided at the top of the liquefied methane storage tank 4. By discharging the boil-off gas from the discharge path 105 to the outside of the system, the methane purity in the liquefied methane stored in the liquefied methane storage tank 4 can be increased.

[0047] The boil-off gas discharged from the discharge path 105 preferably contains a substance with a boiling point lower than that of methane, and preferably contains nitrogen, for example.

[0048] (Embodiment 2) Next, Embodiment 2 of the high-purity liquefied methane production apparatus according to the present invention will be described with reference to Figure 2. In Figure 2, the same reference numerals are used for parts that overlap with Figure 1 to avoid redundant explanations.

[0049] The apparatus for producing high-purity liquefied methane in Embodiment 2 is consistent with Embodiment 1 in that it comprises a liquefied natural gas storage tank 1 for storing liquefied natural gas, a distillation column 2 for separating methane gas from the liquefied natural gas, a methane gas condenser 3 for liquefying the methane gas, and a liquefied methane storage tank 4 for storing the liquefied methane.

[0050] On the other hand, Embodiment 1 is equipped with the first boil-off gas supply path 101, whereas Embodiment 2 is equipped with a second boil-off gas supply path 102 that supplies the boil-off gas generated in the liquefied natural gas storage tank 1 to the liquefied methane storage tank 4. If the methane concentration in the boil-off gas generated in the liquefied natural gas storage tank 1 is high, the boil-off gas can be effectively utilized as liquefied methane by supplying it to the liquefied methane storage tank 4 instead of supplying it to the rectification column 2 as in Embodiment 1. The boil-off gas generated in the liquefied natural gas storage tank 1 may be supplied to the liquefied methane storage tank 4 in a gaseous state and liquefied in the reliquefaction device 12 provided in the liquefied methane storage tank 4 to produce liquefied methane, or a methane gas condenser (not shown) may be provided in the middle of the second boil-off gas supply path 102 to convert it into a liquid state and supply it to the liquefied methane storage tank 4. Furthermore, the second boil-off gas supply route 102 only needs to be connected to a boil-off gas outlet located at the top of the liquefied natural gas storage tank 1.

[0051] In Embodiment 2, the liquefied methane storage tank 4 is equipped with a reliquefaction device 12, and a heat medium supply path 119 for supplying a heat medium is connected to the reliquefaction device 12. In the reliquefaction device 12, heat exchange occurs between the heat medium supplied to the reliquefaction device 12 and the methane gas generated in the liquefied methane storage tank 4 and the boil-off gas supplied from the second boil-off gas supply path 102 to produce liquefied methane. By reliquefying the methane gas generated in the liquefied methane storage tank 4 and the boil-off gas supplied from the second boil-off gas supply path 102, the loss of liquefied methane stored in the liquefied methane storage tank 4 can be reduced, and the boil-off gas supplied from the liquefied natural gas storage tank 1 can be effectively utilized as a resource.

[0052] Preferably, the second boil-off gas supply path 102 is equipped with a device that can identify the methane concentration in the boil-off gas supplied to the liquefied methane storage tank 4 via the second boil-off gas supply path 102. If the device that can identify the methane concentration is detected to have fallen below a predetermined value, the supply of boil-off gas from the liquefied natural gas storage tank 1 to the liquefied methane storage tank 4 can be stopped. This prevents boil-off gas with a methane concentration below a predetermined value from flowing into the liquefied methane storage tank 4, thereby maintaining the purity of liquefied methane in the liquefied methane storage tank 4. When the methane concentration in the boil-off gas falls below a predetermined value, the boil-off gas can be discharged outside the system.

[0053] The configuration of the device capable of determining the methane concentration is not particularly limited; it may be a concentration meter capable of measuring the methane concentration in boil-off gas, or a concentration meter capable of measuring the concentration of other gases (for example, the concentration of impurity gases such as nitrogen) in boil-off gas. If a concentration meter capable of measuring the concentration of other gases in boil-off gas is used, the methane concentration in the boil-off gas can be calculated by subtracting the concentration of other gases measured by the concentration meter from 100 mol%.

[0054] The threshold methane concentration for deciding whether or not to stop the supply of boil-off gas from the liquefied natural gas storage tank 1 to the liquefied methane storage tank 4 is preferably, for example, 99 mol%. If the methane concentration is 99 mol% or higher, it is preferable to continue supplying boil-off gas from the liquefied natural gas storage tank 1 to the liquefied methane storage tank 4, and if the methane concentration is less than 99 mol%, it is preferable to stop supplying boil-off gas from the liquefied natural gas storage tank 1 to the liquefied methane storage tank 4. If the methane concentration is less than 99 mol%, it is preferable to, for example, discharge the boil-off gas generated in the liquefied natural gas storage tank 1 to the outside of the system via the discharge path 104 to increase the methane gas concentration in the boil-off gas within the liquefied natural gas storage tank 1. The threshold methane concentration may also be, for example, 98 mol%, 97 mol%, or 99.5 mol%.

[0055] The location where the device capable of determining the methane concentration is installed is not particularly limited; it may be at the connection point between the liquefied natural gas storage tank 1 and the second boil-off gas supply path 102, or at the connection point between the second boil-off gas supply path 102 and the liquefied methane storage tank 4. Of course, it may also be located somewhere along the second boil-off gas supply path 102.

[0056] If the second boil-off gas supply path 102 is equipped with a device capable of determining the methane concentration in the boil-off gas supplied to the liquefied methane storage tank 4, it is preferable that the high-purity liquefied methane production apparatus further includes a first boil-off gas supply path 101a.

[0057] The first boil-off gas supply route 101a may be provided, for example, as a route connecting the liquefied natural gas storage tank 1 and the rectification column 2, separate from the second boil-off gas supply route 102, but it is preferable to provide it as a branch of the second boil-off gas supply route 102 that connects the liquefied natural gas storage tank 1 and the liquefied methane storage tank 4. When the first boil-off gas supply route 101a is provided as a branch of the second boil-off gas supply route 102, it is preferable to place a valve 122 at the connection point between the second boil-off gas supply route 102 and the first boil-off gas supply route 101a. When the methane concentration detected by a device capable of determining the methane concentration in the boil-off gas falls below a threshold, the valve 122 is switched, and the boil-off gas in the liquefied natural gas storage tank 1 is supplied to the rectification column 2, where it is purified to increase the methane concentration.

[0058] As described above, while Embodiment 1 and Embodiment 2 differ in the destination of the boil-off gas generated in the liquefied natural gas storage tank 1, they are similar in that they effectively utilize the boil-off gas generated in the liquefied natural gas storage tank 1. Therefore, in either Embodiment 1 or Embodiment 2, high-purity liquefied methane can be produced relatively easily from liquefied natural gas while effectively utilizing the boil-off gas generated in the liquefied natural gas storage tank 1 that stores the liquefied natural gas.

[0059] According to the apparatus for producing high-purity liquefied methane according to the present invention, high-purity liquefied methane with a methane purity of 99 mol% or higher can be produced. The methane purity is more preferably 99.9 mol% or higher, even more preferably 99.99 mol% or higher, and particularly preferably 99.999 mol% or higher. For example, a methane purity of less than 99.9999 mol% is preferred.

[0060] The high-purity liquefied methane production apparatus according to the present invention can reduce the amount of boil-off gas (BOG) released into the atmosphere and produce high-purity liquefied methane from liquefied natural gas (LNG) in high yield. Therefore, it can reduce greenhouse gas emissions and contribute to some of the Sustainable Development Goals (SDGs).

[0061] The present invention will be described in more detail below with reference to examples, but the present invention is not limited by the following examples, and it is of course possible to implement it with modifications within the scope that is consistent with the spirit described above and below, and all such modifications are included within the technical scope of the present invention. [Examples]

[0062] (Example 1) Using the high-purity liquefied methane production apparatus of Embodiment 1, the purity of liquefied methane obtained from liquefied natural gas was calculated using software for calculating steady-state conditions. The calculation was performed on a high-purity liquefied methane production apparatus comprising a liquefied natural gas storage tank 1 for storing liquefied natural gas, a rectification column 2 for separating methane gas from liquefied natural gas, a methane gas condenser 3 for liquefying methane gas, a gas-liquid separator 6, a liquefied methane storage tank 4 for storing liquefied methane, and a first boil-off gas supply path 101 for supplying boil-off gas generated in the liquefied natural gas storage tank 1 to the rectification column 2. The calculation conditions were as follows.

[0063] The temperature inside the liquefied natural gas storage tank 1 was set to -140°C and the pressure to 300 kPaG. The temperature inside the rectification column 2 was set to -139°C and the pressure to 270 kPaG. The temperature inside the liquefied methane storage tank 4 was set to -145°C and the pressure to 270 kPaG. Liquid nitrogen at 20°C was supplied as a heat transfer medium to the heat exchanger 13 installed in the rectification column 2 via the path 111. The flow rate of liquefied natural gas supplied from the liquefied natural gas storage tank 1 to the rectification column 2 via the liquefied natural gas supply path 103 was 76.6 Nm³. 3 The flow rate of boil-off gas supplied from liquefied natural gas storage tank 1 to distillation column 2 via the first boil-off gas supply path 101 is 2.3 Nm³ / hour. 3 The flow rate of the liquid discharged out of the system via the path 112 connected to the bottom of the rectification column 2 is 7.1 Nm³ / hour. 3 The flow rate of liquefied methane supplied to the liquefied methane storage tank 4 via the path 116 connected to the lower part of the gas-liquid separator 6 is 69.8 Nm³ / hour. 3 The flow rate of gas discharged outside the system via the path 121 connected to the top of the gas-liquid separator 6 is 2.0 Nm³ / hour. 3 The unit was given as " / hour". 3 " refers to the amount of substance converted to the volume of a gas at 0°C and 101.3 kPa. In the case of a liquid, the amount of the liquid is converted to the volume of a gas at 0°C and 101.3 kPa. The same applies hereafter.

[0064] The composition of the liquefied natural gas stored in liquefied natural gas storage tank 1 was assumed to be 0.03 mol% nitrogen, 91.49 mol% methane, and 8.48 mol% hydrocarbons with more carbon atoms than methane, such as ethane. The composition of the boil-off gas generated in liquefied natural gas storage tank 1 was assumed to be 5 mol% nitrogen and 95 mol% methane. As a result, 8.8 mol% of methane was recovered as liquid in rectification column 2, and 91.2 mol% of hydrocarbons with more carbon atoms than methane, such as ethane, were recovered as liquid.

[0065] The liquefied methane recovered in the liquefied methane storage tank 4 after passing through the methane gas condenser 3 and the gas-liquid separator 6 had a purity of 99.02 mol%, and contained 0.98 mol% nitrogen as an impurity.

[0066] As described above, according to the apparatus for producing high-purity liquefied methane in Embodiment 1, liquefied methane having a methane purity of 99.02 mol% can be produced from liquefied natural gas having a methane purity of 91.49 mol%.

[0067] (Example 2) Using the apparatus for producing high-purity liquefied methane in Embodiment 2, the purity of liquefied methane obtained from liquefied natural gas was calculated using software for steady-state calculation. The calculation was performed for an apparatus for producing high-purity liquefied methane comprising: a liquefied natural gas storage tank 1 for storing liquefied natural gas; a rectification column 2 for separating methane gas from liquefied natural gas; a methane gas condenser 3 for liquefying methane gas; a gas-liquid separator 6; a liquefied methane storage tank 4 for storing liquefied methane; and a second boil-off gas supply path 102 for supplying boil-off gas generated in the liquefied natural gas storage tank 1 to the liquefied methane storage tank 4. The calculation conditions are as follows.

[0068] In the liquefied natural gas storage tank 1, the temperature was set to -140°C and the pressure was set to 300 kPaG; in the rectification column 2, the temperature was set to -86°C and the pressure was set to 270 kPaG; and in the liquefied methane storage tank 4, the temperature was set to -143°C and the pressure was set to 270 kPaG. To the heat exchanger 13 provided in the rectification column 2, liquefied nitrogen at 20°C as a heat medium was supplied via a path 111. The flow rate of the liquefied natural gas supplied from the liquefied natural gas storage tank 1 to the rectification column 2 via the liquefied natural gas supply path 103 was 79 Nm 3 / hour, and the flow rate of boil-off gas supplied from the liquefied natural gas storage tank 1 to the liquefied methane storage tank 4 via the second boil-off gas supply path 102 was 2.3 Nm 3 / hour, and the flow rate of liquid discharged out of the system via a path 112 connected to the lower part of the rectification column 2 was 7.2 Nm 3 / hour, and the flow rate of liquefied methane supplied to the liquefied methane storage tank 4 via a path 116 connected to the lower part of the gas-liquid separator 6 was 69.8 Nm 3 / hour, and the flow rate of gas discharged out of the system via a path 121 connected to the upper part of the gas-liquid separator 6 was 2.0 Nm 3 / hour.

[0069] The composition of the liquefied natural gas stored in liquefied natural gas storage tank 1 was assumed to be 0.03 mol% nitrogen, 91.49 mol% methane, and 8.48 mol% hydrocarbons with more carbon atoms than methane, such as ethane. The composition of the boil-off gas generated in liquefied natural gas storage tank 1 was assumed to be 99 mol% methane. As a result, the recovered methane in liquid form in rectification column 2 was 7.6 mol%, and the recovered hydrocarbons with more carbon atoms than methane, such as ethane, was 92.4 mol%.

[0070] The liquefied methane recovered in the liquefied methane storage tank 4 after passing through the methane gas condenser 3 and the gas-liquid separator 6 had a purity of 99.97 mol%, and contained 0.03 mol% nitrogen as an impurity.

[0071] As described above, the high-purity liquefied methane production apparatus in Embodiment 2 was able to produce liquefied methane with a methane purity of 99.97 mol% from liquefied natural gas with a methane purity of 91.49 mol%. [Explanation of Symbols]

[0072] 1. Liquefied natural gas storage tank 2 Rectification tower 3. Methane gas condenser 3a container 4. Liquefied methane storage tank 5. Heat transfer medium storage tank 6 Gas-liquid separator 7. Vaporizer 12 Reliquefaction equipment 13 Heat exchanger 101 First boil-off gas supply path 102 Second boil-off gas supply route 103 Liquefied Natural Gas Supply Routes 104, 105 Emission routes Routes 111-114, 116, 118, 120, 121 115, 117 Junction 119 Heat transfer medium supply path 122 valves

Claims

1. A apparatus for producing high-purity liquefied methane from liquefied natural gas, A liquefied natural gas storage tank for storing the aforementioned liquefied natural gas, A rectification column for separating methane gas from a liquid containing hydrocarbons heavier than methane gas, A methane gas condenser that liquefies methane gas, A liquefied methane storage tank for storing liquefied methane, A first boil-off gas supply path supplies boil-off gas generated in the liquefied natural gas storage tank to the rectification column, A production apparatus for high-purity liquefied methane equipped with [specific features / equipment].

2. A apparatus for producing high-purity liquefied methane from liquefied natural gas, A liquefied natural gas storage tank for storing the aforementioned liquefied natural gas, A rectification column for separating methane gas from the liquefied natural gas, A methane gas condenser that liquefies methane gas, A liquefied methane storage tank for storing liquefied methane, A second boil-off gas supply path supplies boil-off gas generated in the liquefied natural gas storage tank to the liquefied methane storage tank, A discharge route for discharging boil-off gas generated in the liquefied natural gas storage tank, A production apparatus for high-purity liquefied methane equipped with [specific features / equipment].

3. The manufacturing apparatus according to claim 2, wherein the second boil-off gas supply path is equipped with a device capable of determining the methane concentration within the second boil-off gas supply path.

4. The manufacturing apparatus according to claim 3, further comprising a first boil-off gas supply path for supplying boil-off gas generated in the liquefied natural gas storage tank to the rectification column.

5. The aforementioned liquefied methane storage tank is equipped with a reliquefaction device, The aforementioned reliquefaction device is connected to a heat transfer medium supply path for supplying a heat transfer medium. The manufacturing apparatus according to claim 1 or 2, wherein the reliquefaction apparatus generates liquefied methane by heat exchange between the heat transfer medium and the methane gas in the liquefied methane storage tank.

6. The aforementioned rectification column is equipped with a heat exchanger, The manufacturing apparatus according to claim 5, further comprising a path for supplying the heat transfer medium, which has been heat-exchanged in the re-liquefaction apparatus, to the heat exchanger of the rectification column.

7. The manufacturing apparatus according to claim 1 or 2, further comprising a liquefied natural gas supply path for supplying the liquefied natural gas in the liquefied natural gas storage tank to the rectification column.

8. The manufacturing apparatus according to claim 1, further comprising a discharge path for discharging boil-off gas generated in the liquefied natural gas storage tank.

9. The manufacturing apparatus according to claim 8, wherein the boil-off gas contains nitrogen.

10. The manufacturing apparatus according to claim 1 or 2, further comprising a discharge path for discharging boil-off gas generated in the liquefied methane storage tank.

11. The manufacturing apparatus according to claim 10, wherein the boil-off gas contains nitrogen.

12. The manufacturing apparatus according to claim 1 or 2, wherein the purity of the high-purity liquefied methane is 99 mol% or more.

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