System and method for vaporizing cryogenic liquid and gas mixture from a barrier space of a cryogenic storage tank

Abstract

A system for vaporizing a cryogenic gas-liquid mixture includes a cryogenic gas-liquid mixture supply line, a vaporizer, an injection device for injecting cryogenic liquid into the vaporized cryogenic gas-liquid mixture at an outlet of the vaporizer, a mist separator for separating liquid droplets of cryogenic liquid from the vaporized cryogenic gas-liquid mixture, and a compressor. The temperature at the inlet of the compressor is controlled by a control valve located on the cryogenic liquid supply line.

Description

[0001] This invention relates to a system for vaporizing cryogenic gas-liquid mixtures.

[0002] More specifically, the present invention relates to a system for vaporizing a cryogenic gas-liquid mixture extracted from the barrier space of a cryogenic storage tank.

[0003] The present invention also relates to a method for operating a system for vaporizing cryogenic gas-liquid mixtures according to the invention, and to liquefied gas carriers using such a system. Background Technology

[0004] Gases, such as natural gas, can be stored and transported by sea in liquid form (such as liquefied gas or liquefied natural gas (LNG)). If the gas being transported is natural gas, it is stored in an insulated cryogenic tank at a temperature below -150°C (typically -161°C) and at a pressure a few millibars higher than atmospheric pressure.

[0005] Typically, cryogenic insulated tanks used for liquefied natural gas are fully enclosed.

[0006] The first thin metal membrane barrier provides liquid tightness between the liquefied natural gas and the first porous insulation material layer.

[0007] The second thin metal membrane barrier is disposed between the first porous insulation material layer and the second porous insulation material layer.

[0008] Then, the second porous insulation layer is supported by the ship's inner hull.

[0009] In the event of a rupture or leak in the first thin-film metal barrier, LNG will accumulate in the space between the first and second metal barrier. This space is called the interbarrier space (IBS).

[0010] Then, the second metal membrane barrier will provide liquid tightness and prevent cryogenic liquids from contacting the ship's inner hull, which could lead to catastrophic failure of the ship.

[0011] In the event of LNG buildup in the IBS, during unloading of the tank, the liquid level inside the tank may drop faster than the liquid level in the IBS, resulting in pressure exerted from the liquid load onto the first barrier from the IBS side toward the inside of the tank. This pressure could then compromise the overall integrity of the first barrier and the closed system.

[0012] To avoid this situation, suction pipes are installed inside the IBS to drain LNG from it. These suction pipes are arranged so that all liquids can be removed from the IBS.

[0013] LNG is removed from the IBS via a gas lift through a discharge pipe.

[0014] Decompression, or overall pressure reduction, is generated by the suction of the compressor connected to the suction pipe. Due to the simultaneous decompression of the space between the compressor suction and the IBS, a portion of the LNG accumulated in the IBS will vaporize and rise in the pipe, thereby lifting any remaining liquid along with it in the form of a cryogenic gas-liquid mixture, in this case, a mixture of LNG and vaporized LNG.

[0015] Then, the liquid portion of the cryogenic gas-liquid mixture removed from the IBS must be vaporized before it can be incinerated using a marine gas combustion device.

[0016] Vaporization is achieved using an onboard forced vaporizer and a shell-and-tube heat exchanger. The liquid portion of the cryogenic gas-liquid mixture (i.e., LNG from IBS) vaporizes through indirect heat exchange with steam, while the gaseous portion of the mixture is heated to a superheated state, resulting in a vaporized cryogenic gas-liquid mixture. However, because this forced vaporizer is sized to accommodate flow rates much higher than those occurring during IBS venting, the temperature of the vaporized cryogenic gas-liquid mixture at the vaporizer outlet is excessively high and can potentially damage the centrifugal compressor impeller, which is typically made of aluminum.

[0017] Therefore, the vaporized cryogenic gas-liquid mixture must be cooled. This is typically done using cold natural gas (NG) received from an onshore terminal. This means that ships must be moored and connected to an onshore LNG terminal to empty the interbarrier space (IBS), making it difficult (if not impossible) to empty the IBS while the ship is at sea.

[0018] Therefore, one object of the present invention is to provide an improved system for vaporizing cryogenic gas-liquid mixtures that avoids the aforementioned disadvantages. Summary of the Invention

[0019] This objective is achieved by the system for vaporizing cryogenic gas-liquid mixtures according to the present invention, the method for vaporizing cryogenic gas-liquid mixtures according to the present invention, and the liquefied gas carrier including the system for vaporizing cryogenic gas-liquid mixtures according to the present invention.

[0020] A further technical solution of the present invention refers to a preferred embodiment of the present invention.

[0021] Therefore, the present invention provides a system for vaporizing cryogenic gas-liquid mixtures, comprising:

[0022] - A cryogenic gas-liquid mixture supply line for extracting a cryogenic gas-liquid mixture from the barrier space of a cryogenic storage tank;

[0023] - A vaporizer for vaporizing the cryogenic gas-liquid mixture, located downstream of the cryogenic gas-liquid mixture supply line;

[0024] Instead of cooling the vaporized cryogenic gas-liquid mixture with cold gas from a land-based terminal as described above, the present invention advantageously uses cryogenic liquid from a ship to mix the cryogenic liquid with the vaporized cryogenic gas-liquid mixture. The cryogenic liquid is supplied from the ship itself via a cryogenic liquid supply line, and the cryogenic liquid supplied via the cryogenic supply line is stored onboard in a cryogenic storage tank.

[0025] The mixing of the vaporized cryogenic gas-liquid mixture with the cryogenic liquid is performed by a first injection device that injects the cryogenic liquid from the cryogenic liquid supply line into the vaporized cryogenic gas-liquid mixture. Therefore, direct heat exchange occurs between the two fluids, and the cryogenic liquid vaporizes within the vaporized cryogenic gas-liquid mixture, reducing the overall temperature of the resulting cold mixture to a temperature level suitable for the compressor, i.e., below -50°C.

[0026] - A mist separator is configured to separate droplets of the cryogenic liquid from the cold mixture in the event that the cryogenic liquid is not completely vaporized within the cold mixture. The mist separator is located downstream of the injection device and upstream of the compressor to capture any remaining droplets so that they cannot reach the compressor.

[0027] - A compressor located downstream of the gas outlet of the mist separator, the compressor's suction creates a pressure reduction within the system to vaporize the cryogenic gas-liquid mixture.

[0028] Therefore, there is no need to connect to a land-based terminal.

[0029] To improve the system's accuracy, a temperature control valve can be positioned on the cryogenic liquid supply line to control the temperature at the compressor inlet by adjusting the flow rate of the cryogenic liquid injected into the vaporized cryogenic gas-liquid mixture.

[0030] If the amount of cryogenic gas-liquid mixture removed from the inter-barrier space (IBS) is sufficient (i.e., exceeds a certain value), the temperature of the vaporized cryogenic gas-liquid mixture can be reduced by having a portion of the cryogenic gas-liquid mixture removed from the IBS bypass the vaporizer by passing that portion through a bypass line (which bypasses the vaporizer) and by using a second injection device located on the line exiting the vaporizer (i.e., the line downstream of the vaporizer) to inject that portion of the cryogenic gas-liquid mixture into the vaporized cryogenic gas-liquid mixture.

[0031] Preferably, the compressor is a centrifugal compressor.

[0032] To prevent excessive pressure reduction in the system, a controlled vaporized liquefied gas stream is fed into the system via an LPG vapor supply line, downstream of the vaporizer and upstream of the first injection device. The LPG vapor supply line then connects the line exiting the vaporizer to the upstream of the first injection device. A pressure control valve located on the LPG vapor supply line can be used to regulate the flow rate of the LPG vapor stream, thereby regulating the pressure within the system.

[0033] To accelerate the vaporization of LNG in the barrier space and remove the cryogenic gas-liquid mixture via the air lift effect, the system can operate below atmospheric pressure.

[0034] In a preferred embodiment, the pressure in the vaporization system is higher than 50 kPa absolute pressure.

[0035] According to a second aspect, the present invention relates to a method for vaporizing a cryogenic gas-liquid mixture, the method comprising the following steps:

[0036] - Remove the cryogenic gas-liquid mixture from the barrier space of the cryogenic storage tank;

[0037] - The low-temperature gas-liquid mixture is vaporized in a vaporizer;

[0038] - A cryogenic liquid is injected into a vaporized cryogenic gas-liquid mixture to obtain the resulting cold mixture.

[0039] - Separate the droplets of the cryogenic liquid from the cold mixture to obtain the resulting droplet-free mixture.

[0040] - The above steps are performed under the reduced pressure generated by the suction of the compressor.

[0041] Preferably, the temperature at the compressor inlet is controlled by a temperature control valve located on the cryogenic liquid supply line supplying the cryogenic liquid.

[0042] In a preferred embodiment, the pressure inside the system is controlled by a pressure control valve located on a liquefied gas vapor supply line that connects the line leaving the vaporizer to the upstream of the first injection device.

[0043] In another implementation, the pressure inside the system is controlled below atmospheric pressure.

[0044] In another preferred embodiment, the pressure within the system is controlled within the range of 50 kPa absolute pressure to atmospheric pressure.

[0045] The third aspect seeking protection (but which also represents an embodiment of the invention according to the first and second aspects) relates to a liquefied gas carrier that includes a system according to the invention for vaporizing a cryogenic gas-liquid mixture. Attached Figure Description

[0046] Figure 1 A first embodiment of the invention having a first injection device is schematically shown.

[0047] Figure 2 A second embodiment of the invention having a first spraying device and a second spraying device is schematically shown.

[0048] Detailed description of the attached figures

[0049] The following section will discuss in detail the basis... Figure 1 and Figure 2 Different embodiments of the drawings are shown, with the same reference numerals indicating the same or substantially the same units. It should be understood that those skilled in the art can combine certain components of the embodiments shown in the drawings with features of the present invention, without necessarily including components other than this particular component of the embodiment shown in the drawings, or even all other components.

[0050] Figure 1 A system 1 for vaporizing cryogenic gas-liquid mixtures according to the present invention is illustrated schematically.

[0051] The cryogenic storage tank 4 for LNG, which has a first thin metal membrane barrier and a second thin metal membrane barrier, is used onboard in LNG carriers for storing and transporting LNG. The two barriers are separated by a porous cryogenic insulation material layer called the inter-barrier space 3.

[0052] Since the first thin metal membrane barrier is in direct contact with the LNG inside the cryogenic tank 4, the LNG will accumulate in the space between the barriers 3 if the first thin metal membrane barrier ruptures.

[0053] During unloading, the liquid level in tank 4 may drop faster than the liquid level in the interbar space 3. To avoid damaging the first thin metal membrane barrier, a suction pipe is installed in the IBS to drain the accumulated LNG.

[0054] During IBS venting, the accumulated LNG is lifted in the draw-out pipe in the form of a cryogenic gas-liquid mixture through the gas lift effect. This cryogenic gas-liquid mixture is a mixture of gaseous LNG and liquid LNG.

[0055] The cryogenic gas-liquid mixture is supplied to the vaporization system 1 according to the invention via a cryogenic gas-liquid mixture supply line 2, which is fluidly connected to the intake pipe and the vaporizer 5, wherein the cryogenic gas-liquid mixture is vaporized by indirect heat exchange with steam.

[0056] Vaporizer 5 is designed to deliver a higher yield than required for IBS venting, so the vaporized cryogenic gas-liquid mixture may exit vaporizer 5 at excessively high temperatures.

[0057] To avoid this situation, the vaporized cryogenic gas-liquid mixture leaving the vaporizer 5 and flowing through line 16 is cooled with a cryogenic liquid (LNG in this embodiment), which is supplied by a cryogenic liquid supply line 6. The cryogenic supply line 6 is fluidly connected to the interior space of the cryogenic tank 4 in which the LNG is stored.

[0058] The vaporized cryogenic gas-liquid mixture leaving the vaporizer 5 and cryogenic liquid forming line 6 is mixed with a first injection device 7 located downstream of the vaporizer 5, wherein cryogenic liquid is injected into the vaporized cryogenic gas-liquid mixture.

[0059] During injection, the cryogenic liquid vaporizes within the vaporized cryogenic gas-liquid mixture, thereby reducing the temperature of the vaporized cryogenic gas-liquid mixture to that of the resulting mixture.

[0060] The mist separator 8 is located downstream of the first injection device 7 so as to separate the remaining droplets of cryogenic liquid from the resulting cold mixture if the cryogenic liquid is not completely vaporized in the resulting cold mixture.

[0061] A compressor 9 is located downstream of the gas outlet of the mist separator 8. The suction of the compressor 9 creates a pressure reduction in the system 1, thereby lowering the boiling point of the LNG and triggering the vaporization of the LNG in the interbar space IBS 3. The resulting gas rises along the suction pipe and carries some liquid with it.

[0062] Due to the overall low temperature within the system, it is advantageous to use a centrifugal compressor 9 because this type of compressor has a lower risk of the mechanical lubricating oil coming into contact with the compressed cryogenic gas, which could lead to contamination of the compressed cryogenic gas or freezing of the lubricating oil.

[0063] However, since centrifugal compressors 9 are typically equipped with impellers made of aluminum alloy, the temperature at the compressor inlet must be controlled below -50°C.

[0064] Temperature control valve 11 is installed on the cryogenic liquid supply line 6 upstream of the first injection device 7 to adjust the flow rate of cryogenic liquid injected into the vaporized cryogenic gas-liquid mixture based on the temperature measured at the inlet of the compressor 9.

[0065] The pressure in the vaporization system 1 of the present invention is controlled below atmospheric pressure by allowing a flow of liquefied gas vapor (in this case, natural gas (NG)) into the vaporization system 1. A pressure control valve 14 is provided on the liquefied gas vapor supply line 15, which connects the line 16 exiting the vaporizer to the upstream of the first injection device 7. The flow rate of liquefied gas vapor into the system 1 can be controlled using the pressure control valve 14. The pressure can be controlled within the range of 50 kPa to atmospheric pressure, as pressures below 50 kPa may damage the mechanical components of the compressor 9.

[0066] Figure 2 A system 1 for vaporizing a cryogenic gas-liquid mixture according to a second embodiment of the present invention is illustrated schematically.

[0067] Apart from Figure 1 In addition to the first embodiment shown, the second embodiment also includes a second injection device 12 located on the line 16 leaving the vaporizer, upstream of the first injection device 7, and upstream of the connection between the line 16 leaving the vaporizer and the liquefied gas vapor supply line 15.

[0068] If the flow rate of the cryogenic gas-liquid mixture supplied by the cryogenic gas-liquid mixture supply line 2 exceeds a certain value, the second injection device 12 can be used. A portion of the cryogenic gas-liquid mixture will bypass the vaporizer 5 through the vaporizer bypass line 13 and will be mixed into the vaporized cryogenic gas-liquid mixture through the second injection device 12 located downstream of the vaporizer 5 to reduce the temperature of the vaporized cryogenic gas-liquid mixture.

[0069] List of reference numerals

[0070] 1: Systems for vaporizing cryogenic gas-liquid mixtures

[0071] 2: Low-temperature gas-liquid mixture supply pipeline

[0072] 3: Space between barriers

[0073] 4: Storage tank

[0074] 5: Carburetor

[0075] 6: Cryogenic liquid supply pipeline

[0076] 7: First injection device

[0077] 8: Fog Separator

[0078] 9: Compressor

[0079] 10: Gas outlet of the mist separator

[0080] 11: Temperature control valve

[0081] 12: Second injection device

[0082] 13: Carburetor bypass line

[0083] 14: Pressure control valve

[0084] 15: Liquefied petroleum gas (LPG) steam supply pipeline

[0085] 16: Lines leaving the carburetor

Claims

1. A system (1) for vaporizing a cryogenic gas-liquid mixture, comprising: - Cryogenic gas-liquid mixture supply line (2), the cryogenic gas-liquid mixture supply line is used to extract cryogenic gas-liquid mixture from the barrier space (3) of the cryogenic storage tank (4); - Vaporizer (5), the vaporizer being used to vaporize the cryogenic gas-liquid mixture, the vaporizer (5) being arranged downstream of the cryogenic gas-liquid mixture supply line (2); - Cryogenic liquid supply line (6); - First injection device (7), the first injection device is used to inject cryogenic liquid from the cryogenic liquid supply line (6) into the vaporized cryogenic gas-liquid mixture downstream of the vaporizer (5) to obtain the resulting cold mixture; - Fog separator (8), the fog separator is used to separate droplets of the cryogenic liquid from the cold mixture, the fog separator is located downstream of the first spray device (7); - Compressor (9), which is located downstream of the gas outlet (10) of the mist separator (8), the compressor (9) draws in the system (1) to create a pressure reduction to vaporize the low-temperature gas-liquid mixture.

2. The system (1) for vaporizing a cryogenic gas-liquid mixture according to claim 1, comprising a temperature control valve (11) located on the cryogenic liquid supply line (6) for controlling the temperature at the inlet of the compressor (9).

3. The system (1) for vaporizing a cryogenic gas-liquid mixture according to claim 1 or 2, comprising a second injection device (12) for injecting a portion of the cryogenic gas-liquid mixture passing through a bypass line (13) of the vaporizer (5) into the vaporized cryogenic gas-liquid mixture exiting the vaporizer (5), the second injection device (12) being located on the line (16) exiting the vaporizer.

4. The system (1) for vaporizing a cryogenic gas-liquid mixture according to claim 1 or 2, wherein the compressor (9) is a centrifugal compressor.

5. The system (1) for vaporizing a cryogenic gas-liquid mixture according to claim 1 or 2, comprising a pressure control valve (14) located on a liquefied gas vapor supply line (15) for controlling the pressure within the system (1) to vaporize the cryogenic gas-liquid mixture, the liquefied gas vapor supply line (15) connecting the line (16) exiting the vaporizer to the upstream of the first injection device (7).

6. The system (1) for vaporizing a cryogenic gas-liquid mixture according to claim 5, characterized in that... The pressure inside the system (1) is lower than atmospheric pressure.

7. The system (1) for vaporizing a cryogenic gas-liquid mixture according to claim 6, characterized in that... The pressure inside the system (1) is in the range of 50 kPa absolute pressure to atmospheric pressure.

8. A method for vaporizing a cryogenic gas-liquid mixture, comprising the following steps: - The cryogenic gas-liquid mixture is supplied from the barrier space (3) of the cryogenic storage tank (4); - The low-temperature gas-liquid mixture is vaporized in the vaporizer (5); - Injecting a cryogenic liquid into a vaporized cryogenic gas-liquid mixture to obtain a cold mixture; - Separate droplets of the cryogenic liquid from the cold mixture; - The above steps are performed under the reduced pressure generated by the suction of the compressor (9).

9. The method for vaporizing a cryogenic gas-liquid mixture according to claim 8, wherein the temperature at the inlet of the compressor (9) is controlled by a temperature control valve (11) located on the cryogenic liquid supply line (6).

10. The method for vaporizing a cryogenic gas-liquid mixture according to any one of claims 8 to 9, wherein the pressure inside the system (1) is controlled by a pressure control valve (14) located on a liquefied gas vapor supply line (15), the liquefied gas vapor supply line (15) connecting a line (16) exiting the vaporizer to the upstream of a first injection device (7).

11. The method for vaporizing a cryogenic gas-liquid mixture according to claim 10, wherein the pressure inside the system (1) is controlled below atmospheric pressure.

12. The method for vaporizing a cryogenic gas-liquid mixture according to claim 11, wherein the pressure inside the system (1) is controlled within the range of 50 kPa absolute pressure to atmospheric pressure.

13. A liquefied gas carrier, the liquefied gas carrier comprising a system (1) for vaporizing a cryogenic gas-liquid mixture according to any one of claims 1 to 7.

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