Heat exchange system

Abstract

To provide a heat exchange system in which carbon dioxide is liquefied using cold heat of a low-temperature liquefied gas, and carbon dioxide is suppressed from being solidified by the cold heat of the low-temperature liquefied gas.SOLUTION: A heat exchange system 1 includes: a low-temperature flow path 2 in which a low-temperature liquefied gas is introduced and flows; a liquefaction target flow path 3 in which gaseous carbon dioxide as a liquefaction target is introduced and flows; a heat source medium flow path 4 in which water as a heat source medium is introduced and flows; a first heat exchanger 10 which gasifies the low-temperature liquefaction gas by the heat exchange between the low-temperature gas flowing in the low-temperature flow path 2 and the water flowing in the heat source medium flow path 4; and a second heat exchange 20 which is provided on the further downstream side than the first heat exchanger 10 in a flow direction of the low-temperature liquefied gas, and in which the carbon dioxide is liquefied by the heat exchange between a gasified gas gasified from the low-temperature liquefied gas in the first heat exchanger 10 and the gaseous carbon dioxide flowing in the liquefaction target flow path 3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a heat exchange system. [Background technology]

[0002] Conventionally, a heat exchange system has been known that liquefies a gas to be liquefied by utilizing the cold energy of a low-temperature liquefied gas such as liquefied natural gas (hereinafter referred to as LNG) or liquefied hydrogen (see, for example, Patent Document 1). This heat exchange system includes a low-temperature flow path through which LNG (an example of a low-temperature liquefied gas) flows, a gas to be liquefied flow path through which nitrogen, the gas to be liquefied, flows, and a heat exchanger disposed across both flow paths to exchange heat between the LNG and the nitrogen. The LNG is heated and vaporized by heat exchange with the nitrogen in the heat exchanger. Meanwhile, the nitrogen, the gas to be liquefied, is cooled and liquefied by heat exchange with the LNG in the heat exchanger. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-168207 Summary of the Invention [Problem to be solved by the invention]

[0004] It is conceivable to use carbon dioxide instead of nitrogen as the gas to be liquefied in the heat exchange system shown in Patent Document 1. Carbon dioxide liquefaction technology has become increasingly important in recent years as the demand for carbon neutrality has increased and it is also used for liquefying and capturing carbon dioxide.

[0005] However, the temperature of the low-temperature liquefied gas used to cool carbon dioxide in the heat exchanger is, for example, -150°C to -160°C, which is significantly lower than the solidification temperature (approximately -56°C) of carbon dioxide at typical pressures (0.52 MPa to 3 MPa) during storage and transportation. Therefore, there is a risk that the carbon dioxide flowing through the gas flow path to be liquefied in the heat exchanger will solidify due to the cold heat of the low-temperature liquefied gas, causing blockage of the flow path.

[0006] The present invention has been made to solve the above-mentioned problems, and aims to suppress solidification of carbon dioxide due to the cold energy of low-temperature liquefied gas in a heat exchange system that liquefies carbon dioxide using the cold energy of low-temperature liquefied gas. [Means for solving the problem]

[0007] The first invention is a heat exchange system that liquefies gaseous carbon dioxide by utilizing the cold energy of a low-temperature liquefied gas, and includes a low-temperature flow path through which the low-temperature liquefied gas is introduced and flows, a liquefaction target flow path through which the gaseous carbon dioxide to be liquefied is introduced and flows, a heat source medium flow path through which water is introduced and flows as a heat source medium, a first heat exchanger that vaporizes the low-temperature liquefied gas by performing heat exchange between the low-temperature liquefied gas flowing through the low-temperature flow path and the water flowing through the heat source medium flow path, and a second heat exchanger that is provided downstream of the first heat exchanger in the flow direction of the low-temperature liquefied gas and liquefies the carbon dioxide by performing heat exchange between the vaporized gas vaporized from the low-temperature liquefied gas in the first heat exchanger and the gaseous carbon dioxide flowing through the liquefaction target flow path.

[0008] This configuration can prevent carbon dioxide liquefied in the second heat exchanger from solidifying in the liquefaction-target flow path of the second heat exchanger. Specifically, the first heat exchanger heats and vaporizes the low-temperature liquefied gas through heat exchange with water, which serves as a heat source medium, to generate a vaporized gas warmer than the low-temperature liquefied gas. In the second heat exchanger, heat exchange occurs between the heated vaporized gas as a low-temperature fluid and carbon dioxide as a high-temperature fluid, thereby liquefying the carbon dioxide. Therefore, the temperature of the low-temperature fluid (fluid that cools the carbon dioxide) flowing into the second heat exchanger can be increased compared to conventional heat exchange systems that do not have a first heat exchanger. This prevents carbon dioxide from solidifying in the liquefaction-target flow path of the second heat exchanger due to the cold energy of the low-temperature fluid. Consequently, the liquefaction-target flow path can be prevented from being blocked by solidified carbon dioxide.

[0009] Furthermore, with the above configuration, there is no need to prepare a high-temperature refrigerant that exchanges heat directly with the carbon dioxide, separate from the low-temperature liquefied gas, in order to prevent excessive cooling of the carbon dioxide. This prevents the configuration of the heat exchange system from becoming complicated, and the entire system can be constructed inexpensively.

[0010] Furthermore, according to the above configuration, by using water instead of carbon dioxide as the heat source medium for the first heat exchanger, it is possible to ensure sufficient heating capacity (vaporization capacity) of the low-temperature liquefied gas in the first heat exchanger even if the flow rate of carbon dioxide, which is the target of liquefaction, is changed.

[0011] In a second invention, in the first invention, it is preferable that the second heat exchanger has a gas inlet portion which serves as an inlet for the vaporized gas, and is equipped with a bypass flow path which branches off from the low-temperature flow path and bypasses the first heat exchanger, and is capable of supplying the low-temperature liquefied gas to the gas inlet portion of the second heat exchanger or a portion in the low-temperature flow path upstream of the second heat exchanger, and a bypass valve which is capable of adjusting the flow rate of the low-temperature liquefied gas flowing from the low-temperature flow path into the bypass flow path.

[0012] According to this configuration, the low-temperature liquefied gas that has passed through the bypass flow path and the vaporized gas discharged from the first heat exchanger join together and are supplied to the second heat exchanger at a lower temperature than the vaporized gas discharged from the first heat exchanger. Here, since the bypass flow path is provided with a bypass valve, by controlling the opening degree of the bypass valve, the flow rate ratio between the low-temperature liquefied gas that has passed through the bypass flow path and the vaporized gas discharged from the first heat exchanger can be adjusted, making it possible to easily control the temperature of the mixed fluid (low-temperature fluid that cools the carbon dioxide) supplied to the second heat exchanger. As a result, it is possible to easily control the temperature of the liquefied carbon dioxide discharged from the second heat exchanger.

[0013] In a third invention, in the second invention, it is preferable that the system further includes a control unit that controls the bypass valve and a temperature detection unit that detects the temperature of the mixed fluid of the vaporized gas vaporized in the first heat exchanger and the low-temperature liquefied gas that has passed through the bypass flow path, or a temperature correlated to that temperature, and the control unit is configured to perform bypass valve control that controls the bypass valve so that the temperature detected by the temperature detection unit becomes a predetermined target temperature.

[0014] According to this configuration, the bypass valve is automatically controlled by the control unit, so that the temperature of the mixed fluid can be more easily controlled than when the bypass valve is opened and closed manually.

[0015] In the fourth invention, in the third invention, it is preferable that the predetermined target temperature is a temperature that is set in advance so that the temperature of the liquefied carbon dioxide discharged from the second heat exchanger becomes a predetermined required temperature and so that the carbon dioxide does not solidify on the inner surface of the liquefaction target flow path of the second heat exchanger.

[0016] According to this configuration, it is possible to reliably prevent the carbon dioxide from solidifying in the liquefaction target flow path of the second heat exchanger while controlling the temperature of the liquefied carbon dioxide discharged from the second heat exchanger to a predetermined required temperature.

[0017] In a fifth aspect of the present invention, in the first aspect of the present invention, it is preferable that the first heat exchanger is an intermediate refrigerant type heat exchanger that performs heat exchange between the low-temperature liquefied gas and the water via an intermediate refrigerant.

[0018] According to this configuration, by configuring the first heat exchanger as an intermediate refrigerant-type heat exchanger, the condensation heat of the intermediate refrigerant can be used to vaporize the low-temperature liquefied gas. That is, according to the configuration, heat is not exchanged directly between the low-temperature liquefied gas and the heat source medium, water, which have a large temperature difference, but is exchanged indirectly via the intermediate refrigerant. This reduces thermal stress occurring in the heat exchange portion of the first heat exchanger. Therefore, even when the first heat exchanger vaporizes a low-temperature liquefied gas (e.g., LNG or liquid hydrogen) that is significantly lower than the solidification temperature of carbon dioxide (e.g., −56°C), the thermal stress resistance performance of the first heat exchanger can be sufficiently ensured. Furthermore, because there is no direct heat exchange between the low-temperature liquefied gas and the heat source medium, water, which have a large temperature difference, can be prevented from freezing due to the cold heat of the low-temperature liquefied gas.

[0019] In a sixth invention, in the first invention, it is preferable that the second heat exchanger is a stacked type heat exchanger having a first flow path forming layer having a flow path portion into which the vaporized gas is introduced, and a second flow path forming layer stacked on the first flow path forming layer and having a flow path portion into which the gaseous carbon dioxide is introduced.

[0020] According to this configuration, the second heat exchanger can be configured to be compact and strong.

[0021] In the seventh invention, it is preferable that, in the fourth invention, the system further includes a third heat exchanger that is located downstream of the second heat exchanger in the flow direction of the vaporized gas and heats the vaporized gas to a predetermined required temperature by performing heat exchange between the vaporized gas discharged from the second heat exchanger and water flowing through the heat source medium flow path.

[0022] According to this configuration, the vaporized gas discharged from the second heat exchanger can be heated to a predetermined required temperature by the third heat exchanger. This improves the usability of the heat exchange system as a vaporized gas generation system. Furthermore, because the heat source of the third heat exchanger is water, the heating capacity of the third heat exchanger can be sufficiently ensured even if the flow rate of carbon dioxide to be liquefied is changed.

[0023] In the eighth invention, in the seventh invention, it is preferable that the system further includes a liquefaction target flow rate adjustment valve capable of adjusting the flow rate of the gaseous carbon dioxide introduced into the liquefaction target flow path, and the control unit is configured to be able to control the liquefaction target flow rate adjustment valve to switch the operating mode of the heat exchange system between a normal operating mode in which the flow rate of carbon dioxide introduced into the liquefaction target flow path is greater than 0, and a vaporization operating mode in which the flow rate of the carbon dioxide is 0 and the system only performs the function of vaporizing the low-temperature liquefied gas.

[0024] According to this configuration, the control unit can switch the operating state of the heat exchange system between a normal operation mode and a vaporization operation mode. In the normal operation mode, both the carbon dioxide liquefaction function and the low-temperature liquefied gas vaporization function are performed, and in the vaporization operation mode, the carbon dioxide flow rate is zero, so the carbon dioxide liquefaction function is disabled and only the low-temperature liquefied gas vaporization function is performed. Therefore, the operating mode of the heat exchange system can be flexibly switched between when there is a demand for carbon dioxide liquefaction and when there is no demand for carbon dioxide liquefaction. This improves the usability of the heat exchange system depending on whether there is a demand for carbon dioxide liquefaction.

[0025] In a ninth invention, in the eighth invention, it is preferable that the control unit is configured to set the predetermined target temperature when executing the bypass valve control in the normal operation mode and the evaporation operation mode to the same temperature.

[0026] With this configuration, even if the operation mode is frequently switched between the vaporization operation mode and the normal operation mode, the target temperature when controlling the bypass valve is the same in each operation mode, making control easier and reducing the calculation load on the control unit.

[0027] In a tenth aspect of the present invention, in the eighth aspect of the present invention, it is preferable that the control unit is configured to execute the bypass valve control by adjusting the opening degree of the bypass valve in the normal operation mode, while in the evaporation operation mode, the control unit is configured to fully close the bypass valve and not execute the bypass valve control.

[0028] According to this configuration, in the normal operation mode, the bypass valve control is performed by adjusting the opening degree of the bypass valve, whereas in the vaporization operation mode, the bypass valve is controlled to a fully closed state. Therefore, in the vaporization operation mode, the heat of the vaporized gas heated in the first heat exchanger is not wasted by mixing with the low-temperature liquefied gas via the bypass flow path. Therefore, in the vaporization operation mode, the temperature of the vaporized gas flowing into the third heat exchanger can be made higher than in the ninth invention, and the risk of water freezing due to the cold energy of the vaporized gas can be reduced.

[0029] According to an eleventh aspect of the present invention, in any one of the first to tenth aspects of the present invention, the second heat exchanger is preferably a counterflow heat exchanger.

[0030] According to this configuration, by configuring the second heat exchanger as a counterflow heat exchanger, it is possible to lower the temperature of the liquefied carbon dioxide discharged from the second heat exchanger compared to when it is configured as a parallel flow heat exchanger. Here, if the temperature of the liquefied carbon dioxide is set low, the problem of solidification of the carbon dioxide described above is more likely to occur, so the configuration of this invention (first invention) is particularly useful. [Effects of the Invention]

[0031] According to the present invention, in a heat exchange system that liquefies carbon dioxide by utilizing the cold energy of a low-temperature liquefied gas, it is possible to suppress the carbon dioxide from solidifying due to the cold energy of the low-temperature liquefied gas. [Brief explanation of the drawings]

[0032] [Figure 1] 1 is a system diagram showing a schematic configuration of a heat exchange system according to a first embodiment of the present invention. [Figure 2A] FIG. 1 is a diagram showing a schematic configuration of a CO2 liquefier. [Figure 2B] FIG. 2B is a view taken in the direction of the arrow IIB in FIG. 2A. [Figure 3] FIG. 3 is a partial cross-sectional view showing a part of the cross section taken along line III-III in FIG. 2A. [Figure 4] This is a TQ diagram showing the temperature change of the working fluid in a heat exchange system, with heat quantity (enthalpy) on the horizontal axis and temperature on the vertical axis. [Figure 5A] FIG. 2B is a view corresponding to FIG. 2A and showing a first modified example of the first embodiment. [Figure 5B] FIG. 5B is a view taken in the direction of the arrow VB in FIG. 5A. [Figure 6] FIG. 5B is a view showing a second modified example. [Figure 7] FIG. 10 is a view showing a second embodiment, corresponding to FIG. [Figure 8] FIG. 5 is a view corresponding to FIG. 4 and showing the second embodiment. [Figure 9] FIG. 10 is a view showing another embodiment, corresponding to FIG. 1, illustrating an example in which an LNG vaporizer is configured with a shell-and-tube type heat exchanger. [Figure 10] FIG. 10 is a view equivalent to FIG. 1 showing another embodiment, in which an LNG vaporizer is configured with an open rack type heat exchanger. DETAILED DESCRIPTION OF THE INVENTION

[0033] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0034] (Embodiment 1) 1 is a system diagram showing a schematic configuration of a heat exchange system 1 in embodiment 1. This heat exchange system 1 has the function of liquefying gaseous carbon dioxide by utilizing the cold energy of liquefied natural gas (hereinafter referred to as LNG), which is an example of a low-temperature liquefied gas, and vaporizing the LNG to produce natural gas (hereinafter referred to as NG).

[0035] [Overall configuration of heat exchange system] Specifically, this heat exchange system 1 includes a low-temperature flow path 2 through which LNG is introduced and flows, a liquefaction target flow path 3 through which gaseous carbon dioxide to be liquefied is introduced and flows, a heat source medium flow path 4 through which seawater (an example of water) is introduced and flows as a heat source medium, a bypass flow path 5 described below, an LNG vaporizer 10 that vaporizes LNG to NG (vaporized gas) by performing heat exchange between the LNG flowing through the low-temperature flow path 2 and the seawater, a CO2 liquefaction device 20 that liquefies the carbon dioxide by performing heat exchange between the NG vaporized from LNG in the LNG vaporizer 10 and the gaseous carbon dioxide flowing through the liquefaction target flow path 3, an NG heater 30 that heats the NG to a predetermined required temperature by performing heat exchange between the NG discharged from the CO2 liquefier 20 and the seawater flowing through the heat source medium flow path 4, and a controller 100 as a control unit. The LNG vaporizer 10 corresponds to a first heat exchanger, the CO2 liquefier 20 corresponds to a second heat exchanger, and the NG warmer 30 corresponds to a third heat exchanger.

[0036] In the direction of flow of LNG in the low-temperature flow path 2, the LNG vaporizer 10, the CO2 liquefier 20, and the NG warmer 30 are arranged in this order from the upstream side to the downstream side.

[0037] In the direction of seawater flow in the heat source medium flow path 4, the NG heater 30 and the LNG vaporizer 10 are arranged in this order from the upstream side to the downstream side.

[0038] The bypass flow path 5 branches off from the low-temperature flow path 2 and is arranged to bypass the LNG vaporizer 10. That is, the bypass flow path 5 has an upstream end connected to a portion of the low-temperature flow path 2 upstream of the LNG vaporizer 10, and a downstream end connected to a portion of the low-temperature flow path 2 between the LNG vaporizer 10 and the CO liquefier 20.

[0039] The heat exchange system 1 further includes a bypass valve 8 disposed in the bypass flow path 5, a first main valve 6 disposed at the upstream end of the low-temperature flow path 2, and a second main valve 7 (corresponding to a liquefaction target flow rate control valve) disposed at the upstream end of the liquefaction target flow path 3. The first main valve 6, the second main valve 7, and the bypass valve 8 are controlled by a controller 100.

[0040] The controller 100 is configured to be able to switch the operation mode of the heat exchange system 1 between a normal operation mode and a vaporization operation mode by controlling the first main valve 6 and the second main valve 7. In the normal operation mode, both the carbon dioxide liquefaction function and the LNG vaporization function of the heat exchange system 1 are enabled, while in the vaporization operation mode, the carbon dioxide liquefaction function of the heat exchange system 1 is disabled and only the LNG vaporization function is enabled. In the following explanation, unless otherwise specified, it is assumed that the heat exchange system 1 is in the normal operation mode.

[0041] The controller 100 is also configured to be able to execute bypass control for controlling the opening degree of the bypass valve 8. The operation mode switching control and bypass control executed by the controller 100 will be described in detail later.

[0042] [LNG vaporizer] In this example, the LNG vaporizer 10 is configured as an intermediate refrigerant type heat exchanger. Specifically, the LNG vaporizer 10 has a shell 11, a number of heat source pipes 12 provided in the shell 11 and into which seawater is introduced, and a number of heat transfer pipes 13 provided in the shell 11 and into which LNG is introduced. A liquid intermediate refrigerant M (in this example, propane, as an example) having a lower boiling point than the seawater introduced into the heat source pipes 12 is stored in the lower part of the shell 11. Each of the heat source pipes 12 is disposed so as to be immersed in this liquid intermediate refrigerant M.

[0043] The heat source pipe 12 constitutes a part of the heat source medium flow path 4, and the heat transfer pipe 13 constitutes a part of the low-temperature flow path 2. For simplification, Fig. 1 shows only one heat source pipe 12 and one heat transfer pipe 13. Furthermore, the number of heat source pipes 12 and heat transfer pipes 13 does not necessarily have to be plural, and they may be one, and their shape may be any shape, such as linear or U-shaped.

[0044] When seawater is introduced into each heat source tube 12 immersed in the intermediate refrigerant M, heat exchange occurs between the liquid intermediate refrigerant M and the seawater. As a result, at least a portion of the liquid intermediate refrigerant M evaporates and becomes intermediate refrigerant gas, heating the heat transfer tubes 13. As a result, the LNG in the heat transfer tubes 13 vaporizes into NG (an example of a vaporized gas), which is supplied to the CO liquefier 20, which will be described later. Meanwhile, the intermediate refrigerant gas that heated the LNG is condensed by recovering the cold energy of the LNG, becoming liquid intermediate refrigerant M, which falls to the bottom of the shell 11. The intermediate refrigerant M circulates within the shell 11, alternately liquefying and gasifying. Thus, the heat source tubes 12 and the shell 11 function as an intermediate refrigerant evaporation section E1 that evaporates the intermediate refrigerant M. Furthermore, the heat transfer tubes 13 and the shell 11 function as a liquefied gas evaporation section E2 that vaporizes the LNG.

[0045] The intermediate refrigerant M used in the LNG vaporizer 10 is not limited to propane, but may be, for example, an HFC-based mixed refrigerant or the like.

[0046] [CO2 liquefier configuration] Next, the CO2 liquefier 20 will be described in detail with reference to Figures 2A, 2B, and 3. Figure 2A is a diagram showing a schematic configuration of the CO2 liquefier 20, and Figure 2B is a view taken in the direction of arrow IIB in Figure 2A. Figure 3 is a partial cross-sectional view showing part of the cross section taken along line III-III in Figure 2A.

[0047] As shown in FIG. 2A, the CO2 liquefier 20 is a counterflow heat exchanger that exchanges heat by circulating carbon dioxide, a high-temperature fluid, and NG, a low-temperature fluid, in opposite directions. As shown in FIG. 3, the CO2 liquefier 20 is, for example, a microchannel stacked heat exchanger 20L. Specifically, the CO2 liquefier 20 is configured by alternately stacking a number of first metal layers 21 (corresponding to first flow path forming layers) and a number of second metal layers 22 (corresponding to second flow path forming layers). The first metal layer 21 has a number of first flow path portions 21a recessed therein through which NG flows. These first flow path portions 21a constitute a part of the low-temperature flow path 2. The second metal layer 22 has a number of second flow path portions 22a recessed therein through which carbon dioxide flows. These second flow path portions 22a constitute a part of the flow path 3 to be liquefied. The first metal layer 21 and the second metal layer 22 are each made of a metal plate with excellent heat transfer properties. Heat exchange occurs between the NG flowing through each first flow path portion 21a formed in the first metal layer 21 and the carbon dioxide flowing through each second flow path portion 22a formed in the second metal layer 22, heating the NG and cooling the carbon dioxide. The carbon dioxide is liquefied by cooling and discharged to the outside of the heat exchange system 1. While FIG. 3 shows an example of a stacking pattern in which one first metal layer 21 and one second metal layer 22 are alternately stacked, this is not limiting. For example, two first metal layers 21 and one second metal layer 22 may be alternately stacked, three first metal layers 21 and one second metal layer 22 may be alternately stacked, or three first metal layers 21 and two second metal layers 22 may be alternately stacked. In other words, the stacking pattern of the first metal layers 21 and the second metal layers 22 may be any pattern. In addition, in the example of Figure 3, the CO2 liquefier 20 is shown as a microchannel type stacked heat exchanger 20L, but this is not limited to this and may be configured as, for example, a plate-fin type stacked heat exchanger or the like.

[0048] As shown in FIGS. 2A and 2B, a first inflow header 23 having an inlet 23a and a first outlet header 24 having an outlet 24a are connected to a first flow path section 21a formed in the first metal layer 21. A second inflow header 25 having an inlet 25a and a second outlet header 26 having an outlet 26a are connected to a second flow path section 22a formed in the second metal layer 22. NG (strictly speaking, a mixed fluid of NG and LNG passing through a bypass flow path 5, described later) supplied from the LNG vaporizer 10 is distributed to each first flow path section 21a through the first inflow header 23 (corresponding to a gas inlet section), flows through each first flow path section 21a, and then gathers in the first outlet header 24 and is discharged from the CO liquefier 20. Meanwhile, gaseous carbon dioxide is supplied to the second inflow header 25 from a CO supply source (not shown) external to the heat exchange system 1. The supplied gaseous carbon dioxide is then distributed to each second flow path section 22a through the second inlet header 25, and after flowing through each second flow path section 22a, it is collected in the second outlet header 26 and discharged from the CO2 liquefier 20.

[0049] [NG heater configuration] Returning to FIG. 1, the configuration of the NG heater 30 will be described. The NG heater 30 is disposed to the side of the intermediate refrigerant-type LNG vaporizer 10 (the intermediate refrigerant evaporation section E1 and the liquefied gas vaporization section E2). In the example of FIG. 1, the NG heater 30 is disposed to the side of the LNG vaporizer 10, but the location is not limited thereto. For example, the NG heater 30 may be disposed above the LNG vaporizer 10 or away from the LNG vaporizer 10. In this example, the NG heater 30 is a shell-and-tube heat exchanger having a shell 31 and a plurality of heat transfer tubes 32 through which seawater, which is a heat source fluid, flows. The NG discharged from the CO2 liquefier 20 flows into the shell 31 and passes between the plurality of heat transfer tubes 32, whereby the NG exchanges heat with the seawater flowing in each heat transfer tube 32, whereby the NG is heated to a predetermined required temperature (for example, 5°C in this example) and is then discharged out of the shell 31. The plurality of heat transfer tubes 32 (only one is shown in FIG. 1) constitute a part of the heat source medium flow path 4, and the space inside the shell 31 constitutes a part of the low-temperature flow path 2.

[0050] [Explanation of basic operation] Next, the basic operation of the heat exchange system 1 will be described with reference to Figures 1 and 4. Figure 4 is a TQ diagram showing the temperature changes of the working fluids (LNG, carbon dioxide, propane as an intermediate refrigerant, and seawater as a heat source medium) of the heat exchange system 1, with the heat quantity (enthalpy) on the horizontal axis and the temperature on the vertical axis. In this TQ diagram, the heat quantity (enthalpy) increases toward the left on the horizontal axis, and the temperature increases toward the top of the vertical axis. In Figure 4, for simplicity of explanation, the bypass flow rate of LNG flowing through the bypass flow path 5 is assumed to be 0.

[0051] LNG is supplied to an LNG vaporizer 10 (see FIG. 1) at a temperature of −150°C from an LNG supply source provided outside the heat exchange system 1. The LNG supplied to the LNG vaporizer 10 exchanges heat with seawater via propane, an intermediate refrigerant, thereby increasing its heat content (see FIG. 4). As a result, the LNG is heated from −150°C to −56°C and vaporizes to become NG.

[0052] The NG vaporized in the LNG vaporizer 10 flows into the first flow path section 21a of the CO2 liquefier 20 (see FIG. 1) at −56°C, and exchanges heat with the carbon dioxide flowing through the second flow path section 22a, increasing its calorific value. At the outlet of the CO2 liquefier 20, the NG is heated to −15°C (see FIG. 4). The heated NG flows into the shell 31 of the NG warmer 30, and exchanges heat with seawater flowing through the heat transfer tube 32, further increasing its calorific value. The NG is then finally heated to 5°C (an example of a predetermined required temperature) before being discharged from the NG warmer 30.

[0053] Meanwhile, carbon dioxide to be liquefied is supplied to the CO2 liquefier 20 as a gas at 30°C from a CO2 supply source provided outside the heat exchange system 1. The gaseous carbon dioxide supplied to the CO2 liquefier 20 exchanges heat with NG flowing through the first flow path 21a of the CO2 liquefier 20, gradually reducing its heat content (see FIG. 4). Accordingly, the temperature of the carbon dioxide gradually decreases from 30°C until it reaches the liquefaction temperature of −17.2°C (under a pressure of 2.1 MPa in this example). Upon reaching the liquefaction temperature, the carbon dioxide begins to transition from a gas to a liquid. During this transition period, the carbon dioxide coexists as a gas and a liquid, and its temperature remains constant regardless of the reduction in heat content. When the heat content of the carbon dioxide further decreases and the transition period ends, all of the carbon dioxide is liquefied. Thereafter, the temperature decreases as the heat content decreases, and the carbon dioxide is finally discharged from the CO2 liquefier 20 as liquid carbon dioxide at −42°C (an example of a predetermined required temperature).

[0054] Seawater, which is a heat source medium, is supplied to the NG heater 30 (see FIG. 1) as seawater at 10°C from a seawater supply source provided outside the heat exchange system 1. The seawater supplied to the NG heater 30 is cooled by the NG flowing inside the shell 31 of the NG heater 30 while flowing inside the heat transfer tube 32, and its temperature is reduced. Thereafter, the seawater flows into the heat source tube 12 of the LNG vaporizer 10, where it receives the cold energy of the intermediate refrigerant M, and its temperature is further reduced, and it is discharged as seawater at 5°C (see FIG. 4).

[0055] Propane is stored at 0°C in the shell 11 of the LNG vaporizer 10 and repeatedly evaporates due to heat exchange with seawater and condenses due to the cold heat of the NG.

[0056] [About bypass control] However, if the heating capacity of the LNG in the LNG vaporizer 10 is too high, the temperature of the NG discharged from the LNG vaporizer 10 becomes excessively high. As a result, the NG, which is the low-temperature fluid in the CO2 liquefier 20, cannot sufficiently cool the carbon dioxide, which is the high-temperature fluid, and the cooling capacity (i.e., liquefaction capacity) of the carbon dioxide in the CO2 liquefier 20 decreases.

[0057] To avoid this problem, in this embodiment, bypass control is executed by the controller 100. In this bypass control, the opening of the bypass valve 8 is controlled to mix the excessively heated NG in the LNG vaporizer 10 with the extremely low temperature LNG that has passed through the bypass flow path 5, and the mixed fluid is controlled to a predetermined target temperature.

[0058] Specifically, the heat exchange system 1 includes a temperature sensor 9 (corresponding to a temperature detection unit) that detects the temperature of this mixed fluid (hereinafter referred to as mixed NG). In this example, the temperature sensor 9 is connected between the CO2 liquefier 20 and a connection position P1 of the low-temperature flow path 2 with the downstream end of the bypass flow path 5, and detects the temperature of the mixed fluid. Note that the temperature sensor 9 may also be a sensor that detects a temperature correlated with the temperature of the mixed fluid (for example, the temperature of a pipe wall through which the mixed fluid flows).

[0059] The controller 100 is configured by a computer having a CPU, ROM, and RAM, and is connected to the temperature sensor 9 and the bypass valve 8 via signal lines.

[0060] The controller 100 then acquires the temperature detected by the temperature sensor 9 and controls the opening of the bypass valve 8 so that the acquired detected temperature becomes a predetermined target temperature.

[0061] This predetermined target temperature is set in advance so that the temperature of the liquefied carbon dioxide discharged from the second flow path section 22a of the CO2 liquefier 20 will be the predetermined required temperature and so that the carbon dioxide will not solidify on the inner surface of the outlet section (downstream end) of the second flow path section 22a. In other words, if this target temperature is too high, the temperature of the mixed NG, which is the low-temperature fluid flowing into the CO2 liquefier 20, will also be high, reducing the cooling capacity of the mixed NG for the carbon dioxide, resulting in problems such as not liquefying some or all of the carbon dioxide or the temperature of the liquefied carbon dioxide being higher than the predetermined required temperature. On the other hand, if this target temperature is too low, the temperature of the mixed NG, which is the low-temperature fluid flowing into the CO2 liquefier 20, will also be low, increasing the cooling capacity of the mixed NG for the carbon dioxide, resulting in problems such as the temperature of the liquefied carbon dioxide being lower than the predetermined required temperature. Furthermore, if the target temperature is too low, the cold energy of the mixed gas will cause the inner surface temperature of the outlet of the second flow path section 22a of the CO2 liquefier 20 to fall below the solidification temperature of carbon dioxide (−56°C in this example), resulting in the problem of carbon dioxide solidifying at the outlet of the second flow path section 22a. Therefore, in this example, the predetermined target temperature is set in advance to a temperature at which all of the gaseous carbon dioxide is liquefied, the temperature of the liquefied carbon dioxide reaches a predetermined required temperature, and the carbon dioxide does not solidify at the outlet (downstream end) of the second flow path section 22a. The reason why the target temperature is set to a temperature at which carbon dioxide does not solidify at the outlet of the second flow path section 22a is because the temperature of carbon dioxide is lowest at this outlet in the heat exchange system 1 of this example, making it more likely to solidify. In this example, the target temperature is set to −56°C as an example, but is not limited thereto and may be, for example, −50°C to −60°C. That is, the target temperature may be any temperature at which all of the carbon dioxide can be liquefied, the temperature of the liquefied carbon dioxide reaches a predetermined required temperature, and the carbon dioxide does not solidify on the inner surface of the second flow path section 22a. Also, in this example, as an example, the predetermined target temperature is set to a temperature at which all of the gaseous carbon dioxide is liquefied, but the present invention is not limited to this and may be set to a temperature at which only a portion of the gaseous carbon dioxide is liquefied.

[0062] [Operation mode switching control] Next, the control of switching between the normal operation mode and the vaporization operation mode by the controller 100 will be described.

[0063] The controller 100 is connected to an operation panel (not shown) via a signal line. An operator can set the operation mode of the heat exchange system 1 to either a normal operation mode or an evaporation operation mode via this operation panel.

[0064] When the controller 100 receives a setting signal for the normal operation mode from the operation panel, it opens both the first main valve 6 and the second main valve 7 at a predetermined opening (an opening corresponding to the required flow rate). This allows the heat exchange system 1 to function both as a carbon dioxide liquefaction system and as an LNG vaporization system. On the other hand, when the controller 100 receives a setting signal for the vaporization operation mode from the operation panel, it opens the first main valve 6 at a predetermined opening and fully closes the second main valve 7. This prevents gaseous carbon dioxide from being supplied to the CO2 liquefier 20, disabling the carbon dioxide liquefaction function of the CO2 liquefier, and the heat exchange system 1 functions only as an LNG vaporization system.

[0065] In the normal operation mode, when the bypass control is executed, the controller 100 controls the aperture of the bypass valve 8 while keeping the bypass valve 8 open. The controller 100 is configured to increase the aperture of the bypass valve 8 as the aperture of the second main valve 7 increases (in other words, as the flow rate of gaseous carbon dioxide supplied to the CO2 liquefier 20 as the target for liquefaction increases) under predetermined conditions (for example, conditions under which the flow rate of LNG supplied is constant).

[0066] Controller 100 executes bypass control in the vaporization operation mode as well as in the normal operation mode. In the vaporization operation mode, CO2 liquefier 20 is disabled, so the problem arises of how to set the target temperature detected by temperature sensor 9, which is the control target value. In this example, this target temperature is set to the same temperature as the target temperature in the normal operation mode.

[0067] Here, in the vaporization operation mode, the NG heating capacity of the CO2 liquefier 20 is disabled. For this reason, if the target temperature is set to the same temperature as in the normal operation mode, and assuming, for example, that the flow rate of LNG is the same, it is necessary to increase the amount of heating by the NG heater 30 in order to raise the temperature of the NG discharged from the NG heater 30 to a predetermined required temperature (5°C in the example of FIG. 4). Therefore, in this embodiment, the maximum heat exchange capacity (maximum heating capacity) of the NG heater 30 is set in advance to be higher than the heat exchange capacity (heating capacity) required in the normal operation mode.

[0068] [Action and effect] As described above, the heat exchange system 1 of this embodiment includes a low-temperature flow path 2 through which LNG is introduced and flows, a liquefaction target flow path 3 through which the gaseous carbon dioxide to be liquefied is introduced and flows, a heat source medium flow path 4 through which seawater is introduced and flows as a heat source medium, an LNG vaporizer 10 that vaporizes the LNG by performing heat exchange between the LNG flowing through the low-temperature flow path 2 and the seawater flowing through the heat source medium flow path 4, and a CO2 liquefaction device 20 that is provided downstream of the LNG vaporizer 10 in the flow direction of the LNG and liquefies the carbon dioxide by performing heat exchange between the NG vaporized from the LNG in the LNG vaporizer 10 and the gaseous carbon dioxide flowing through the liquefaction target flow path 3.

[0069] This configuration can prevent carbon dioxide liquefied by the CO2 liquefier 20 from solidifying in the liquefaction-target flow path 3 of the CO2 liquefier 20. That is, in the LNG vaporizer 10, LNG is heated and vaporized through heat exchange with seawater, which serves as a heat source medium, to generate NG, and in the CO2 liquefier 20, heat exchange occurs between the heated NG and carbon dioxide to liquefy the carbon dioxide. Therefore, the temperature of the low-temperature fluid (fluid that cools the carbon dioxide) flowing into the CO2 liquefier 20 can be increased compared to a conventional heat exchange system that does not include the LNG vaporizer 10. This can prevent carbon dioxide in the liquefaction-target flow path 3 of the CO2 liquefier 20 from solidifying due to the cold energy of the low-temperature fluid. Consequently, it is possible to prevent the liquefaction-target flow path 3 from being blocked by solidified carbon dioxide.

[0070] Furthermore, with the above configuration, there is no need to prepare a high-temperature refrigerant separate from LNG to directly exchange heat with the carbon dioxide so as not to cool the carbon dioxide excessively, which prevents the configuration of the heat exchange system 1 from becoming complicated and allows the entire system to be constructed inexpensively.

[0071] Furthermore, according to the above configuration, by using seawater instead of carbon dioxide as the heat source medium for the LNG vaporizer 10, it is possible to ensure sufficient LNG heating capacity (vaporization capacity) in the LNG vaporizer 10 even if the flow rate of carbon dioxide, which is the target for liquefaction, is changed.

[0072] In addition, in this embodiment, the heat exchange system 1 is equipped with a bypass flow path 5 that branches off from the low-temperature flow path 2, bypasses the LNG vaporizer 10, and is capable of supplying the LNG to a location in the low-temperature flow path 2 upstream of the CO2 liquefier 20, and a bypass valve 8 that is capable of adjusting the flow rate of the LNG flowing from the low-temperature flow path 2 into the bypass flow path 5.

[0073] According to this configuration, mixed NG, which is a mixture of low-temperature LNG that has passed through the bypass flow path 5 and NG discharged from the LNG vaporizer 10, is supplied to the CO2 liquefier 20. Because the bypass flow path 5 is provided with a bypass valve 8, by controlling the opening degree of the bypass valve 8, the mixing ratio of the LNG that has passed through the bypass flow path 5 and the LNG that has passed through the LNG vaporizer 10 can be adjusted, and the temperature of the mixed NG (low-temperature fluid that cools the carbon dioxide) supplied to the CO2 liquefier 20 can be easily controlled. Consequently, the temperature of the liquefied carbon dioxide discharged from the CO2 liquefier 20 can be easily controlled.

[0074] In this embodiment, the heat exchange system 1 further includes a controller 100 that controls the bypass valve 8 and a temperature sensor 9 that detects the temperature of the mixed NG, and the controller 100 is configured to perform bypass valve 8 control that controls the bypass valve 8 so that the temperature detected by the temperature sensor 9 becomes a predetermined target temperature.

[0075] According to this configuration, the bypass valve 8 is automatically controlled by the controller 100, so that the temperature of the mixed NG can be controlled to the target temperature with higher precision and the burden on the operator can be reduced compared to when the bypass valve 8 is opened and closed manually.

[0076] In this embodiment, the predetermined target temperature is a temperature that is set in advance so that the temperature of the liquefied carbon dioxide discharged from the CO2 liquefier 20 becomes a predetermined required temperature and so that the carbon dioxide does not solidify in the second flow path section 22a of the CO2 liquefier 20.

[0077] According to this configuration, it is possible to reliably prevent carbon dioxide from solidifying in the second flow path section 22a of the CO2 liquefier 20 while controlling the temperature of the liquefied carbon dioxide discharged from the CO2 liquefier 20 to a predetermined required temperature.

[0078] In this embodiment, the LNG vaporizer 10 is an intermediate refrigerant type heat exchanger that exchanges heat between the LNG and the seawater via an intermediate refrigerant.

[0079] According to this configuration, by configuring the LNG vaporizer 10 as an intermediate refrigerant-type heat exchanger, it is possible to vaporize LNG by utilizing the condensation heat of the intermediate refrigerant. Furthermore, instead of directly exchanging heat between LNG and seawater, which serves as a heat source medium, which have a large temperature difference, indirectly exchanging heat via the intermediate refrigerant can reduce thermal stress occurring in the heat exchange parts of the LNG vaporizer 10. Therefore, even if the LNG vaporizer 10 vaporizes LNG that is significantly below the solidification temperature of carbon dioxide (e.g., −56°C), the thermal stress resistance performance of the LNG vaporizer 10 can be sufficiently ensured. Furthermore, because there is no direct heat exchange between LNG, which has a large temperature difference, and seawater (an example of water), which serves as a heat source medium, it is possible to prevent freezing of seawater due to the cold energy of the LNG.

[0080] In addition, in this embodiment, the CO2 liquefier 20 is a stacked heat exchanger 20L that includes a first metal layer 21 having a first flow path portion 21a into which the NG is introduced, and a second metal layer 22 that is stacked on the first metal layer 21 and has a second flow path portion 22a into which the gaseous carbon dioxide is introduced.

[0081] According to this configuration, the CO2 liquefier 20 can be configured to be compact and strong.

[0082] In addition, this embodiment further includes an NG heater 30 that is located downstream of the CO2 liquefier 20 in the flow direction of the NG and heats the NG discharged from the CO2 liquefier 20 to a predetermined required temperature by performing heat exchange between the NG and seawater flowing through the heat source medium flow path 4.

[0083] According to this configuration, the NG discharged from the CO2 liquefier 20 can be heated to a predetermined required temperature by the NG heater 30. This improves the usability of the heat exchange system 1 as an LNG vaporization system. Furthermore, because the heat source of the NG heater 30 is seawater, the heating capacity of the NG heater 30 can be sufficiently ensured even if the flow rate of the carbon dioxide to be liquefied changes.

[0084] In addition, this embodiment further includes a second main valve 7 that can adjust the flow rate of the gaseous carbon dioxide introduced into the liquefaction target flow path 3, and the controller 100 is configured to be able to control the second main valve 7 to switch the operating mode of the heat exchange system 1 between a normal operating mode in which the flow rate of carbon dioxide introduced into the liquefaction target flow path 3 is greater than 0, and a vaporization operating mode in which the flow rate of the carbon dioxide is 0 and the system only performs the function of vaporizing the LNG.

[0085] According to this configuration, under the control of the controller 100, the operating state of the heat exchange system 1 can be switched between a normal operation mode and a vaporization operation mode. In the normal operation mode, both the carbon dioxide liquefaction function and the LNG vaporization function are performed, and in the vaporization operation mode, the carbon dioxide flow rate is zero, so the carbon dioxide liquefaction function is disabled and only the LNG vaporization function is performed. Therefore, the operating mode of the heat exchange system 1 can be flexibly switched between when there is a request for carbon dioxide liquefaction and when there is no request for carbon dioxide liquefaction. This improves the usability of the heat exchange system 1 depending on whether there is a request for carbon dioxide liquefaction.

[0086] The predetermined target temperature when the bypass valve 8 control is executed in the normal operation mode and the vaporization operation mode is set to the same temperature.

[0087] With this configuration, even if the operation mode is frequently switched between the vaporization operation mode and the normal operation mode, the target temperature during bypass valve control is the same in each operation mode, making control easier and reducing the computational load on the controller 100.

[0088] In this embodiment, the CO2 liquefier 20 is configured as a counterflow heat exchanger.

[0089] According to this configuration, by configuring the CO2 liquefier 20 as a counterflow heat exchanger, it is possible to set as low as possible the discharge temperature of the liquefied carbon dioxide cooled in the CO2 liquefier 20. Here, if the discharge temperature of the liquefied carbon dioxide is set low, the problem of solidification of the carbon dioxide described above is likely to occur, so the configuration of the present disclosure, in which LNG is heated and vaporized in advance by the LNG vaporizer 10 as described above and then supplied to the CO2 liquefier 20, is particularly useful.

[0090] (Variation 1) Fig. 5A is a view corresponding to Fig. 2A showing Modification 1, and Fig. 5B is a view taken in the direction of the arrow VB in Fig. 5A. This modification differs from Embodiment 1 in that the CO2 liquefier 20 is composed of a plurality of stacked heat exchangers 20L. In Figs. 5A and 5B, the same components as those in Figs. 2A and 2B are designated by the same reference numerals, and detailed description thereof will be omitted.

[0091] Specifically, in this modification, the CO2 liquefier 20 is configured by arranging four stacked heat exchangers 20L in parallel. Note that the number of stacked heat exchangers 20L is not limited to four.

[0092] The gaseous carbon dioxide inlet ports 25a of the four stacked heat exchangers 20L are each connected to one inlet-side collecting pipe 203 via four inlet-side branch pipes (not shown). The liquefied carbon dioxide outlet ports 26a of the four stacked heat exchangers 20L are each connected to one outlet-side collecting pipe 204 via four outlet-side branch pipes (not shown). The inlet-side collecting pipe 203 and the outlet-side collecting pipe 204 form part of the liquefaction target flow path 3. The gaseous carbon dioxide that flows into the inlet-side collecting pipe 203 is distributed to the four stacked heat exchangers 20L via each inlet-side branch pipe. After being liquefied in each stacked heat exchanger 20L, the distributed gaseous carbon dioxide passes through four outlet-side branch pipes (not shown) to join in the outlet-side collecting pipe 204 and further flows downstream.

[0093] 5B, the mixed NG inlets 23a of the four stacked heat exchangers 20L are each connected to one inlet-side collecting pipe section 201 via four inlet-side branch pipes (not shown). The NG outlets 24a of the four stacked heat exchangers 20L are each connected to one outlet-side collecting pipe section 202 via four outlet-side branch pipes (not shown). The inlet-side collecting pipe section 201 and the outlet-side collecting pipe section 202 form part of the low-temperature flow path 2.

[0094] The downstream end of the bypass flow path 5 is connected to the upstream side of the inlet collecting pipe 201 in the low-temperature flow path 2. Therefore, the LNG flowing through the bypass flow path 5 merges with and is mixed with NG upstream of the inlet collecting pipe 201, and the mixed NG after the merger flows into the inlet collecting pipe 201.

[0095] 5B, the mixed NG that has flowed into the inlet-side collecting pipe section 201 is distributed to the four stacked heat exchangers 20L via the inlet-side branch pipes. The distributed mixed NG is heated in each stacked heat exchanger 20L and becomes NG, and then passes through four outlet-side connecting pipes (not shown) to join in the outlet-side collecting pipe section 202 and flow further downstream.

[0096] As described above, in this variant example 1, the CO2 liquefier 20 is constructed from multiple stacked heat exchangers 20L, which makes it possible to increase the carbon dioxide cooling capacity of the CO2 liquefier 20 as much as possible compared to embodiment 1.

[0097] The heat exchange system 1 of Modification 1 has the same configuration as that of Embodiment 1, except that the CO2 liquefier 20 is configured with four stacked heat exchangers 20L. Therefore, the same effects as those of Embodiment 1 can be obtained.

[0098] (Variation 2) Fig. 6 is a view equivalent to Fig. 5B showing Modification 2. Modification 2 differs from Modification 1 in that the bypass flow path 5 is directly connected to each of the stacked heat exchangers 20L. In Fig. 6, the same components as those in Fig. 5 are designated by the same reference numerals, and detailed description thereof will be omitted.

[0099] That is, in the heat exchange system 1 of this modified example, the downstream end of the bypass flow path 5 branches into four paths that are connected to each of the stacked heat exchangers 20L. Specifically, the bypass flow path 5 has one main flow path portion 5a and four branch flow path portions 5b that branch from the main flow path portion 5a and are connected to each of the stacked heat exchangers 20L. The downstream end of each of the branch flow path portions 5b is connected to the first inlet header 23 of each of the stacked heat exchangers 20L and is configured to be able to supply LNG to the space within the first inlet header 23.

[0100] Although not shown, in this example, the temperature sensor 9 is disposed so as to be able to measure the temperature of the first inflow header 23, which is the mixing point of NG and LNG.

[0101] According to the heat exchange system 1 of the present modified example 2 configured as above, the mixed NG can be supplied evenly to each of the stacked heat exchangers 20L, compared to the above-described modified example 1.

[0102] That is, in the heat exchange system 1 of the above-described first modification, the mixing position (confluence position) of the NG flowing through the low-temperature flow path 2 and the LNG that has passed through the bypass flow path 5 is set upstream of the inlet-side collecting pipe section 201. Therefore, if the mixed NG becomes a gas-liquid two-phase flow after mixing at the mixing position, there is a risk that the flow rate of the mixed NG distributed to each stacked-type heat exchanger 20L will be uneven.

[0103] In contrast, in the heat exchange system 1 of this modification, the mixing position (confluence position) of the NG flowing through the low-temperature flow path 2 and the LNG that has passed through the bypass flow path 5 is set at the NG inlet portion of the stacked heat exchanger 20L (the space within the first inlet header 23, as an example in this example). Therefore, NG and LNG can be individually distributed to each stacked heat exchanger 20L and mixed within each stacked heat exchanger 20L. Therefore, unlike the first modification, there is no bias in the flow rate of the mixed NG flowing into each stacked heat exchanger 20L. This makes it possible to equalize the amount of heat exchanged between the carbon dioxide and the mixed NG in each stacked heat exchanger 20L and suppress temperature variations in the liquefied carbon dioxide discharged from each stacked heat exchanger 20L. This in turn prevents the risk of the carbon dioxide dropping below its solidification temperature due to temperature variations. Therefore, it is possible to avoid the problem of flow path blockage caused by the solidification of carbon dioxide.

[0104] (Embodiment 2) Fig. 7 is a diagram corresponding to Fig. 1 showing embodiment 2, and Fig. 8 is a diagram corresponding to Fig. 4 showing embodiment 2. This embodiment differs from embodiment 1 in that the liquefaction temperature of carbon dioxide to be liquefied is high, so NG is heated to a predetermined required temperature without providing an NG heater 30.

[0105] That is, in this embodiment, as shown in Fig. 8, the pressure of the gaseous carbon dioxide supplied to the CO2 liquefier 20 is 4 MPa, which is set higher than 2.1 MPa in the heat exchange system 1 of embodiment 1. As a result, in the heat exchange system 1 of this embodiment, the liquefaction temperature of carbon dioxide is 5.6°C, which is higher than the liquefaction temperature of carbon dioxide of -17.2°C in embodiment 1. When the liquefaction temperature of carbon dioxide is high like this, the temperature of NG can be heated to a predetermined required temperature (5°C, for example) in the CO2 liquefier 20 as shown in Fig. 8, even without providing the NG heater 30.

[0106] As described above, in this embodiment, since there is no need to provide the NG heater 30, the heat exchange system 1 can be configured at low cost.

[0107] Moreover, the heat exchange system 1 of this embodiment has the same configuration as that of the first embodiment except that it does not have the NG heater 30. Therefore, the same effects as those of the first embodiment can be obtained.

[0108] (Other embodiments) Although the heat exchange system 1 according to the embodiment of the present invention has been described above, the present invention is not limited to this.

[0109] (1) In other words, in each of the above-described embodiments and modifications, the bypass control is executed in both the normal operation mode and the vaporization operation mode. However, this is not limited to this. That is, the bypass valve control may be executed with the bypass valve 8 open in the normal operation mode, while the bypass valve 8 may be fully closed in the vaporization operation mode to prevent the bypass valve control from being executed. With this configuration, the bypass valve control is executed by adjusting the opening of the bypass valve 8 in the normal operation mode, whereas the bypass valve 8 is controlled to be fully closed in the vaporization operation mode. Therefore, in the vaporization operation mode, the heat of the NG heated by the LNG vaporizer 10 is not wasted by mixing with the LNG that has passed through the bypass flow path 5. Therefore, in the vaporization operation mode, the temperature of the NG flowing into the NG heater 30 as the third heat exchanger can be increased compared to the above-described embodiments, thereby reducing the risk of seawater freezing due to the cold energy of the NG.

[0110] (2) Furthermore, in each of the above embodiments and variants, a bypass flow path 5 and a bypass valve 8 are provided, but this is not limited to this, and the heat exchange system 1 may be configured without the bypass flow path 5 and the bypass valve 8.

[0111] (3) Furthermore, in each of the above embodiments and variants, the operating mode of the heat exchange system 1 is configured to be switchable between a normal operating mode and an evaporation operating mode, but it is not necessarily required to have two modes, and for example, it may have only a normal operating mode.

[0112] (4) In each of the above embodiments and variant examples, an intermediate refrigerant type heat exchanger has been described as an example of the LNG vaporizer 10 (first heat exchanger), but this is not limited to this. For example, a shell-and-tube type heat exchanger (see, for example, Patent Publication No. 2020-70922) or an open rack type heat exchanger (see, for example, Patent Publication No. 2021-148206) may be adopted.

[0113] FIG. 9 is a diagram equivalent to FIG. 1 , illustrating an example in which a shell-and-tube heat exchanger is used as the LNG vaporizer 10. In this example, the LNG vaporizer 10 includes a shell 14 into which water, a heat source medium, is introduced, and a number of heat transfer tubes 15 disposed within the shell 14 and into which LNG is introduced. The space within the shell 14 constitutes a part of the heat source medium flow path 4, and the heat transfer tubes 15 constitute a part of the low-temperature flow path 2. The LNG introduced into the heat transfer tube 15 is heated and vaporized by heat exchange with the water flowing within the shell. The heat transfer tube 15 may be a single tube, and may have any shape, such as a straight or U-shape. The water may be any type of water, such as glycol water, industrial water, or seawater. However, from the viewpoint of improving corrosion resistance, it is preferable to use glycol water, industrial water, or the like.

[0114] FIG. 10 is a diagram equivalent to FIG. 1 , illustrating an example in which an open rack-type heat exchanger is used as the LNG vaporizer 10. In this example, the LNG vaporizer 10 includes a number of heat transfer tubes 16 extending vertically and adjacent to each other in the horizontal direction, a lower-end manifold 17 connecting the lower ends of the heat transfer tubes 16, and an upper-end manifold 18 connecting the upper ends of the heat transfer tubes 16. After being supplied to the lower-end manifold 17, LNG is distributed among the heat transfer tubes 16 and flows upward, exchanging heat with water to become NG. The LNG then collects in the upper-end manifold 18 and is discharged to the CO liquefier 20. Meanwhile, water, which serves as a heat source medium, is supplied to a box-shaped trough (not shown) that is open upward and located above the heat transfer tubes 16. The water overflows from the trough and flows downward over the surfaces of the heat transfer tubes 16. The LNG receives heat from the water via the heat transfer tubes 16 and is vaporized. Each heat transfer tube 16 functions as a part of the low-temperature flow path 2. If the required outlet temperature of the NG discharged from the LNG vaporizer 10 is low, the NG warmer 30 as the third heat exchanger does not need to be provided. The water may be any water, such as glycol water, industrial water, or seawater.

[0115] (5) In the above embodiments and modifications, LNG (liquefied natural gas) has been described as an example of liquefied gas, but the present invention is not limited to this and may be, for example, LH2 (liquefied hydrogen), etc.

[0116] (6) In the above-described embodiments and modifications, seawater is used as an example of the heat source medium. However, the heat source medium is not limited to seawater and may be, for example, industrial water or glycol water.

[0117] (7) In each of the above embodiments and variants, a stacked heat exchanger 20L has been described as an example of the CO2 liquefier 20 (second heat exchanger), but this is not limited to this and may be, for example, a multi-tube heat exchanger.

[0118] (8) In each of the above embodiments and variants, a shell-and-tube type heat exchanger has been described as an example of the NG heater 30 (third heat exchanger), but this is not limited to this, and any other type, such as a stacked type heat exchanger, may be adopted.

[0119] (9) In the above-described embodiments and modifications, propane has been described as an example of the intermediate refrigerant. However, the intermediate refrigerant is not limited to this and may be, for example, ammonia or a fluorocarbon.

[0120] (10) In each of the above embodiments and modifications, the gaseous carbon dioxide supplied to the liquefaction target flow path 3 does not need to be 100% carbon dioxide, and for example, the gaseous carbon dioxide may contain components with low melting points such as nitrogen as impurities. In other words, the fluid supplied to the liquefaction target flow path 3 may be any fluid containing carbon dioxide.

[0121] (11) In each of the above embodiments and variants, the temperature of the NG at the inlet of the low-temperature flow path 2 (first flow path section 21a) of the CO2 liquefier 20 is set to a temperature at which all of the carbon dioxide in the gas supplied to the liquefaction target flow path 3 can be liquefied, but this is not limited to this and the temperature may be set to a temperature at which only a portion of the carbon dioxide can be liquefied.

[0122] (12) In each of the above-described embodiments and modifications, the temperature of the NG at the inlet of the low-temperature flow path 2 (first flow path section 21a) of the CO2 liquefier 20 is −56° C. However, this is not limited to this and may be, for example, −50° C. to −60° C. In other words, the temperature of the NG flowing into the CO2 liquefier 20 may be any temperature that does not interfere with the liquefaction of carbon dioxide in the CO2 liquefier 20 (a temperature at which some or all of the carbon dioxide can be liquefied), and that allows the temperature of the carbon dioxide discharged from the CO2 liquefier 20 to reach a predetermined required temperature without solidifying the carbon dioxide. [Explanation of symbols]

[0123] M: Intermediate refrigerant 1: Heat exchange system 2: Low temperature flow path 3: Flow path for liquefaction 4: Heat source medium flow path 5: Bypass flow path 7: Second main valve (liquefaction target flow control valve) 8: Bypass valve 9: Temperature sensor (temperature detection part) 10: LNG vaporizer (first heat exchanger) 20:CO2 liquefier (second heat exchanger) 20L: Stacked heat exchanger 21: First metal layer (first flow path forming layer) 21a: First flow path section (flow path section) 22: Second metal layer (second flow path forming layer) 22a: Second flow path section (flow path section) 24: First outflow header (gas inlet) 30:NG warmer (third heat exchanger) 100: Controller (control unit)

Claims

1. A heat exchange system that liquefies gaseous carbon dioxide using the cold heat of a low-temperature liquefied gas, a low-temperature flow path through which the low-temperature liquefied gas flows; a liquefaction target flow path into which the gaseous carbon dioxide to be liquefied is introduced and flows; a heat source medium flow path through which water as a heat source medium is introduced and flows; a first heat exchanger that vaporizes the low-temperature liquefied gas by performing heat exchange between the low-temperature liquefied gas flowing through the low-temperature flow path and the water flowing through the heat source medium flow path; A heat exchange system comprising: a second heat exchanger that is provided downstream of the first heat exchanger in the flow direction of the low-temperature liquefied gas, and that liquefies the carbon dioxide by performing heat exchange between the vaporized gas vaporized from the low-temperature liquefied gas in the first heat exchanger and the gaseous carbon dioxide flowing through the liquefaction target flow path.

2. 2. The heat exchange system of claim 1, the second heat exchanger has a gas inlet portion serving as an inlet for the vaporized gas, a bypass flow path that branches off from the low-temperature flow path, bypasses the first heat exchanger, and is capable of supplying the low-temperature liquefied gas to the gas inlet port of the second heat exchanger or to a portion of the low-temperature flow path upstream of the second heat exchanger; a bypass valve capable of adjusting the flow rate of the low-temperature liquefied gas flowing from the low-temperature flow path into the bypass flow path.

3. 3. The heat exchange system according to claim 2, a control unit that controls the bypass valve; a temperature detection unit that detects a temperature of a mixed fluid of the vaporized gas vaporized in the first heat exchanger and the low-temperature liquefied gas passing through the bypass flow path or a temperature correlated with the temperature, The control unit is configured to execute bypass valve control to control the bypass valve so that the temperature detected by the temperature detection unit becomes a predetermined target temperature.

4. 4. The heat exchange system according to claim 3, A heat exchange system in which the specified target temperature is a temperature that is set in advance so that the temperature of the liquefied carbon dioxide discharged from the second heat exchanger becomes a specified required temperature and so that the carbon dioxide does not solidify on the inner surface of the liquefaction target flow path of the second heat exchanger.

5. 2. The heat exchange system of claim 1, a heat exchange system, wherein the first heat exchanger is an intermediate refrigerant type heat exchanger that performs heat exchange between the low-temperature liquefied gas and the water via an intermediate refrigerant;

6. 2. The heat exchange system of claim 1, The second heat exchanger is a stacked heat exchanger having a first flow path forming layer having a flow path portion into which the vaporized gas is introduced, and a second flow path forming layer stacked on the first flow path forming layer and having a flow path portion into which the gaseous carbon dioxide is introduced.

7. 5. The heat exchange system according to claim 4, The heat exchange system further includes a third heat exchanger that is located downstream of the second heat exchanger in the flow direction of the vaporized gas and heats the vaporized gas to a predetermined required temperature by performing heat exchange between the vaporized gas discharged from the second heat exchanger and water flowing through the heat source medium flow path.

8. 8. The heat exchange system according to claim 7, a liquefaction target flow rate adjustment valve capable of adjusting the flow rate of the gaseous carbon dioxide introduced into the liquefaction target flow path, The control unit is configured to control the liquefaction target flow rate control valve to switch the operating mode of the heat exchange system between a normal operating mode in which the flow rate of carbon dioxide introduced into the liquefaction target flow path is greater than 0, and a vaporization operating mode in which the flow rate of carbon dioxide is 0 and the heat exchange system only performs the function of vaporizing the low-temperature liquefied gas.

9. 9. The heat exchange system of claim 8, The control unit is configured to set the predetermined target temperature when performing the bypass valve control in the normal operation mode and the evaporation operation mode to the same temperature.

10. 9. The heat exchange system of claim 8, The control unit is configured to perform the bypass valve control by adjusting the opening degree of the bypass valve in the normal operation mode, and to fully close the bypass valve and not perform the bypass valve control in the evaporation operation mode.

11. 11. The heat exchange system according to claim 1, The heat exchange system, wherein the second heat exchanger is a counterflow heat exchanger.

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