System and method for realizing liquefaction recovery of low-temperature BOG gas through liquid nitrogen cooling

Abstract

The invention discloses a system and a method for realizing liquefaction recovery of low-temperature BOG gas through liquid nitrogen cooling, and belongs to the technical field of liquefied natural gas recovery. The system comprises an LNG storage tank, a main heat exchanger, a liquid nitrogen storage tank, a bypass valve and a pressure indication control alarm system. BOG enters the hot side inlet of the main heat exchanger from the top outlet of the LNG storage tank to be liquefied, and liquefied LNG flows back to the LNG storage tank through the hot side outlet. A liquid nitrogen outlet of the liquid nitrogen storage tank is divided into two branches; a second pipeline is connected with a cold side inlet of the main heat exchanger, and the reheated liquid nitrogen serves as nitrogen to be emptied from a cold side outlet; and a bypass valve is arranged on the third pipeline and is connected with a liquid nitrogen storage tank. The pressure indication control alarm system monitors the pressure of the LNG storage tank and controls opening and closing of a bypass valve, and safe operation of the system is ensured. The system is simple in process, high in adaptability, low in energy consumption, remarkable in economic benefit and good in operation safety.

Description

Technical Field

[0001] The present invention belongs to the technical field of liquefied natural gas recovery, and in particular relates to a system and method for realizing liquefaction recovery of low-temperature BOG gas by cooling low-temperature BOG gas with liquid nitrogen. Background Art

[0002] Natural gas is a clean and high-quality energy source that contains almost no sulfur, dust and other harmful substances. Compared with traditional fossil fuels, natural gas produces less carbon dioxide when burned and has a lower impact on the greenhouse effect. From an economic point of view, the price of natural gas is comparable to that of manufactured gas at the same calorific value, and due to its clean characteristics, it does not leave any residue after combustion. As a clean energy source with a stable supply, natural gas will not only not pollute the air quality, but also inject new vitality into the economic development of the region, promote economic prosperity and improve the environment. In addition, natural gas is non-toxic, easy to disperse, lighter than air, not easy to accumulate to form explosive gas, and has a high degree of safety.

[0003] Liquefied natural gas (LNG) is a cryogenic liquid obtained by compressing and cooling natural gas (mainly methane) at extremely low temperatures (about -162°C). The main advantage of LNG is that it is easy to store and transport, especially when transported over long distances, it is more economical and efficient than pipeline transportation. However, during the storage process, heat leakage, changes in ambient temperature or pressure will cause part of the LNG to evaporate and produce gas, which is called BOG gas. The generation of BOG gas will increase the pressure in the storage tank. Excessive pressure may damage the structure of the storage tank and pose a danger to its maintenance system. When the internal pressure in the existing storage tank is too high, the safety valve will open and discharge the BOG gas directly into the atmosphere. This practice is not only dangerous, but also causes direct economic losses. The conventional BOG gas recovery process includes recovering the BOG gas generated from the LNG storage tank, recovering the cold capacity of the BOG gas, and then pressurizing it through a compressor and sending it to a cold box for liquefaction recovery. Conventional technologies have defects such as high energy consumption, complex equipment and high cost, limited liquefaction efficiency and high safety risks. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects in the prior art and specifically provide a system and method for liquefying and recovering low-temperature BOG gas by cooling it with liquid nitrogen.

[0005] The specific technical solutions adopted by the present invention are as follows:

[0006] In a first aspect, the present invention provides a system for liquefying and recovering low-temperature BOG gas by cooling it with liquid nitrogen, comprising an LNG storage tank, a main heat exchanger, a liquid nitrogen storage tank, a bypass valve, and a pressure indication control alarm system;

[0007] The outlet at the top of the LNG storage tank for discharging BOG gas is connected to the hot side inlet of the main heat exchanger through a pipeline, so that the BOG gas enters the main heat exchanger for liquefaction; the hot side outlet of the main heat exchanger is connected to the LNG storage tank through a pipeline, and the liquefied liquefied natural gas is returned to the LNG storage tank; the LNG storage tank is also provided with a pressure indicating control alarm system for monitoring the internal pressure of the storage tank;

[0008] The liquid nitrogen outlet of the liquid nitrogen storage tank is divided into two branches, namely, a second pipeline and a third pipeline, through a first pipeline; the second pipeline is connected to the cold side inlet of the main heat exchanger, so that the liquid nitrogen enters the main heat exchanger for heat exchange; the cold side outlet of the main heat exchanger discharges the nitrogen obtained after reheating to a safe high point through a pipeline; the third pipeline is connected to the liquid nitrogen storage tank, and a bypass valve is provided on the third pipeline; the pressure indication control alarm system controls the opening and closing of the bypass valve.

[0009] Preferably, a liquid nitrogen pump for providing conveying power is provided on the first pipeline.

[0010] Preferably, a regulating valve is provided on the second pipeline.

[0011] Preferably, a U-shaped liquid tank is provided on the pipeline connecting the hot side outlet of the main heat exchanger and the LNG storage tank to prevent BOG gas from flowing out along the pipeline.

[0012] Preferably, when the pressure indicating control alarm system detects that the pressure in the LNG storage tank is greater than a set threshold, the bypass valve is closed; when the pressure in the LNG storage tank is about to fall below the threshold, the bypass valve is opened.

[0013] Preferably, the main heat exchanger is arranged on the top of the LNG storage tank.

[0014] Preferably, the main heat exchanger is a plate-fin heat exchanger or a coiled-tube heat exchanger.

[0015] In a second aspect, the present invention provides a method for liquefying and recovering low-temperature BOG gas by cooling with liquid nitrogen using the system described in the first aspect, which is as follows:

[0016] The BOG gas generated in the LNG storage tank enters the main heat exchanger through a pipeline; the liquid nitrogen in the liquid nitrogen storage tank enters the main heat exchanger through a pipeline equipped with a liquid nitrogen pump; the BOG gas and liquid nitrogen exchange heat in the main heat exchanger, and the liquid nitrogen is converted into nitrogen after reheating. The nitrogen is sent to a safe high point through a pipeline for discharge, and the BOG gas is cooled and liquefied into liquefied natural gas and flows back to the LNG storage tank;

[0017] The pressure indicating control alarm system monitors the pressure inside the LNG storage tank; when too much BOG gas accumulates and the pressure inside the LNG storage tank is higher than a threshold value, the pressure indicating control alarm system controls the bypass valve to close, so that the flow of liquid nitrogen delivered to the main heat exchanger increases, and the condensed and liquefied BOG gas increases. After the pressure inside the LNG storage tank drops to a threshold value, the pressure indicating control alarm system controls the bypass valve to open.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) The system provided by the present invention can continuously process low-temperature BOG gas, has good adaptability to changes in working conditions, and can simultaneously meet different conditions such as normal production, start-up and shutdown, or emergency shutdown.

[0020] (2) The overall process of the system provided by the present invention is simple, and no complicated expander or other auxiliary equipment is required, which reduces the equipment investment cost. In addition, the system adopts a high-efficiency plate-fin heat exchanger, which further reduces the equipment volume and floor space.

[0021] (3) The system provided by the present invention has high operational safety. By efficiently recovering BOG and liquefying it and returning it to the LNG storage tank, the direct emission of BOG is reduced, the greenhouse gas emission is reduced, and it is more environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The overall flow chart of the system provided for this embodiment;

[0023] In the figure: LNG storage tank 1; main heat exchanger 2; liquid nitrogen storage tank 3; liquid nitrogen pump 4; bypass valve 5; pressure indication control alarm system 6; U-shaped liquid tank 7; first pipeline H1; second pipeline H2, third pipeline H3, BOG gas A; liquid nitrogen B; nitrogen C; liquefied natural gas D. DETAILED DESCRIPTION

[0024] In order to make the above-mentioned purpose, features and advantages of the present invention more obvious and easy to understand, the specific implementation mode of the present invention is described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0025] In the description of the present invention, it should be understood that the terms "first" and "second" are only used for the purpose of distinguishing descriptions, and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features.

[0026] like Figure 1 As shown, as a preferred embodiment of the present invention, this embodiment provides a system for liquefying and recovering low-temperature BOG gas by cooling it with liquid nitrogen. The system includes an LNG storage tank 1, a main heat exchanger 2, a liquid nitrogen storage tank 3, a bypass valve 5, a pressure indicating control alarm system 6 and a U-shaped liquid tank 7.

[0027] In the system provided in this embodiment, an outlet for discharging BOG gas A is provided at the top of the LNG storage tank 1, and the outlet is connected to the hot side inlet of the main heat exchanger 2 through a pipeline, so that the BOG gas A enters the main heat exchanger 2 for liquefaction. In order to ensure the compactness of the overall system, the main heat exchanger 2 is arranged on the top of the LNG storage tank 1, shortening the distance of the pipeline arranged between the main heat exchanger 2 and the LNG storage tank 1, reducing the area occupied by the system and also reducing energy consumption, which is convenient for later inspection and maintenance.

[0028] The hot side outlet of the main heat exchanger 2 is connected to the LNG storage tank 1 through a pipeline, and the liquefied liquefied natural gas D is returned to the LNG storage tank 1. A U-shaped liquid tank 7 is provided on the pipeline connecting the hot side outlet of the main heat exchanger 2 and the LNG storage tank 1. A liquid seal is formed in the U-shaped liquid tank 7, which prevents the BOG gas in the LNG storage tank 1 from flowing out of this pipeline, affecting the overall process. In addition, the U-shaped liquid tank 7 can also serve as a pressure buffer device to stabilize the pressure; when the pipeline expands and contracts due to heat and cold and generates stress, the U-shaped liquid tank can absorb the stress, prevent the pipeline from being damaged, and protect the overall pipeline system.

[0029] In the system provided in this embodiment, the liquid nitrogen outlet of the liquid nitrogen storage tank 3 is divided into two branches, the second pipeline H2 and the third pipeline H3, after passing through the first pipeline H1 provided with a liquid nitrogen pump 4. Liquid nitrogen has an extremely low temperature under normal pressure and is easily vaporized due to heat absorption during transportation. The liquid nitrogen pump 4 is provided to provide transportation power and ensure sufficient pressure, so that the liquid nitrogen remains in a liquid state during transportation, avoiding unstable flow or pipeline pressure fluctuations caused by vaporization, and has high stability and safety. A regulating valve is provided on the second pipeline H2, and the second pipeline H2 is connected to the cold side inlet of the main heat exchanger 2, so that liquid nitrogen B enters the main heat exchanger 2 for heat exchange; the cold side outlet of the main heat exchanger 2 discharges the nitrogen C obtained after reheating to a safe high point through a pipeline; the third pipeline H3 is connected to the liquid nitrogen storage tank 3, and a bypass valve 5 is provided on the third pipeline H3.

[0030] In the system provided in this embodiment, the LNG storage tank 1 is also provided with a pressure indicating control alarm system 6 for monitoring the internal pressure of the storage tank. The pressure indicating control alarm system 6 controls the opening and closing of the bypass valve 5. When the pressure indicating control alarm system 6 detects that the pressure in the LNG storage tank 1 is greater than the set threshold, the bypass valve 5 is closed; when the pressure in the LNG storage tank 1 is about to fall below the threshold, the bypass valve 5 is opened. It should be noted that the present invention does not specifically limit the threshold range, and those skilled in the art can select it according to the design pressure and actual working pressure of the LNG storage tank.

[0031] It should be noted that the high safety point for venting nitrogen C is set at more than 3 meters above the highest equipment or building around it, and at least 15 meters away from the nearest operating platform and personnel activity area to avoid affecting personnel safety. The venting point is far away from the vent, equipment air inlet or confined space to avoid gas backflow. The present invention does not limit the specific setting of the high safety point, and those skilled in the art can select it according to specific working conditions.

[0032] In this embodiment, the main heat exchanger 2 adopts a plate-fin heat exchanger. The plate-fin heat exchanger has a compact structure, small size, and light weight, and can be installed on the top of the LNG storage tank 1 to ensure the compactness of the overall system. It should be noted that those skilled in the art may also choose a coiled heat exchanger.

[0033] This embodiment also provides a method for utilizing the above system to perform liquid nitrogen cooling and liquefaction recovery of low-temperature BOG gas, which is specifically as follows:

[0034] The low-temperature BOG gas A generated in the LNG storage tank 1 enters the main heat exchanger 2 through a pipeline; the liquid nitrogen B in the liquid nitrogen storage tank 3 enters the main heat exchanger 2 through a pipeline provided with a liquid nitrogen pump 4; the low-temperature BOG gas A and the liquid nitrogen B exchange heat in the main heat exchanger 2, and the liquid nitrogen B is reheated to become nitrogen C, which is sent to a safe high point through a pipeline for discharge, and the BOG gas A is cooled and liquefied into liquefied natural gas D and flows back to the LNG storage tank 2. A part of the liquefied natural gas liquid is left in the U-shaped liquid tank 7 to form a liquid seal to prevent the BOG gas from escaping from this pipeline and affecting the overall process.

[0035] The pressure indicating control alarm system 6 monitors the pressure in the LNG storage tank 1; when the BOG gas accumulates too much and the pressure in the LNG storage tank 1 is higher than the threshold, the pressure indicating control alarm system 6 controls the bypass valve 5 to close, so that the flow of liquid nitrogen delivered to the main heat exchanger 2 increases, a large amount of low-temperature BOG gas A is liquefied and recovered, and the pressure of the LNG storage tank 1 decreases. When the pressure in the LNG storage tank 1 drops to the threshold, the pressure indicating control alarm system 6 controls the bypass valve 5 to open. A part of the delivered liquid nitrogen B returns to the liquid nitrogen storage tank 3 through the bypass, the flow of liquid nitrogen delivered to the main heat exchanger 2 decreases, the liquefied and recovered BOG gas decreases, and the pressure of the LNG storage tank 1 increases.

[0036] The system process provided by the present invention is simpler and has outstanding high efficiency, energy saving and environmental protection characteristics, and is particularly suitable for the diversified and key configuration devices for energy saving and emission reduction in the entire plant of large factories.

[0037] The above-described embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A system for liquefying and recovering low-temperature BOG gas by cooling it with liquid nitrogen, characterized in that: It comprises an LNG storage tank (1), a main heat exchanger (2), a liquid nitrogen storage tank (3), a bypass valve (5) and a pressure indicating control alarm system (6); The outlet at the top of the LNG storage tank (1) for discharging the BOG gas (A) is connected to the hot side inlet of the main heat exchanger (2) through a pipeline, so that the BOG gas (A) enters the main heat exchanger (2) for liquefaction; the hot side outlet of the main heat exchanger (2) is connected to the LNG storage tank (1) through a pipeline, and the liquefied liquefied natural gas (D) is refluxed to the LNG storage tank (1); the LNG storage tank (1) is also provided with a pressure indicating control alarm system (6) for monitoring the internal pressure of the storage tank; The liquid nitrogen outlet of the liquid nitrogen storage tank (3) is divided into two branches, namely a second pipeline (H2) and a third pipeline (H3), through a first pipeline (H1); the second pipeline (H2) is connected to the cold side inlet of the main heat exchanger (2), so that the liquid nitrogen (B) enters the main heat exchanger (2) for heat exchange; the cold side outlet of the main heat exchanger (2) discharges the nitrogen (C) obtained after reheating to a safe high point through a pipeline; the third pipeline (H3) is connected to the liquid nitrogen storage tank (3), and a bypass valve (5) is provided on the third pipeline (H3); the pressure indication control alarm system (6) controls the opening and closing of the bypass valve (5).

2. The system for liquefying and recovering low-temperature BOG gas by cooling with liquid nitrogen according to claim 1 is characterized in that: The first pipeline (H1) is provided with a liquid nitrogen pump (4) for providing conveying power.

3. The system for liquefying and recovering low-temperature BOG gas by cooling with liquid nitrogen according to claim 1, characterized in that: The second pipeline (H2) is provided with a regulating valve.

4. The system for liquefying and recovering low-temperature BOG gas by cooling with liquid nitrogen according to claim 1 is characterized in that: A U-shaped liquid tank (7) is provided on the pipeline connecting the hot side outlet of the main heat exchanger (2) and the LNG storage tank (1) to prevent BOG gas from flowing out along the pipeline.

5. The system for liquefying and recovering low-temperature BOG gas by cooling with liquid nitrogen according to claim 1, characterized in that: When the pressure indicating control alarm system (6) detects that the pressure in the LNG storage tank (1) is greater than a set threshold, the bypass valve (5) is closed; when the pressure in the LNG storage tank (1) is about to fall below the threshold, the bypass valve (5) is opened.

6. The system for liquefying and recovering low-temperature BOG gas by cooling with liquid nitrogen according to claim 1, characterized in that: The main heat exchanger (2) is arranged on the top of the LNG storage tank (1).

7. The system for liquefying and recovering low-temperature BOG gas by cooling with liquid nitrogen according to claim 1, characterized in that: The main heat exchanger (2) is a plate-fin heat exchanger or a coiled-tube heat exchanger.

8. A method for liquefying and recovering low-temperature BOG gas by cooling with liquid nitrogen using the system described in any one of claims 1 to 7, characterized in that: The details are as follows: The BOG gas (A) generated in the LNG storage tank (1) enters the main heat exchanger (2) through a pipeline; the liquid nitrogen (B) in the liquid nitrogen storage tank (3) enters the main heat exchanger (2) through a pipeline provided with a liquid nitrogen pump (4); the BOG gas (A) and the liquid nitrogen (B) perform heat exchange in the main heat exchanger (2), the liquid nitrogen (B) is reheated and converted into nitrogen (C), the nitrogen (C) is sent to a safe high point through a pipeline for discharge, and the BOG gas (A) is cooled and liquefied into liquefied natural gas (D) and flows back to the LNG storage tank (2); The pressure indicating control alarm system (6) monitors the pressure in the LNG storage tank (1); when the BOG gas accumulates too much and the pressure in the LNG storage tank (1) is higher than a threshold value, the pressure indicating control alarm system (6) controls the bypass valve (5) to close, thereby increasing the flow rate of liquid nitrogen delivered to the main heat exchanger (2), increasing the amount of condensed and liquefied BOG gas, and after the pressure in the LNG storage tank (1) drops to a threshold value, the pressure indicating control alarm system (6) controls the bypass valve (5) to open.

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