Boil-off gas reliquefaction system of LNG storage tank and liquefaction method thereof

Abstract

This invention discloses a BOG reliquefaction system and liquefaction method for an LNG storage tank, comprising: an LNG storage tank and a heat exchanger arranged parallel to the LNG storage tank, wherein a first delivery pipe for outputting to the heat exchanger is provided at the top of the LNG storage tank; a rewarming heat exchanger, disposed on one side of the heat exchanger to regulate the BOG temperature, and the rewarming heat exchanger is connected to the interior of the heat exchanger; and a BOG compressor, disposed at the lower end of the rewarming heat exchanger to pressurize the BOG. ​​This invention, by incorporating a heat exchanger, a rewarming heat exchanger, a BOG compressor, a refrigerant assembly, and a pressure regulating assembly, facilitates the adjustment of the pressure inside the LNG storage tank, ensuring storage safety. Gas and liquid phase refrigerant are separated in a gas-liquid separator for different applications, thereby improving system energy utilization. The pressure regulating assembly controls the pressure of the gas phase space at the top of the LNG storage tank to reduce LNG evaporation, and the pressure difference is reduced through devices such as throttle valves to ensure pressure balance and safe system operation during the BOG reliquefaction process.

Description

Technical Field

[0001] This invention specifically relates to a BOG reliquefaction system for LNG storage tanks and a liquefaction method thereof. Background Technology

[0002] Natural gas at normal pressure changes from a gaseous state to a liquid state at -162℃, reducing its volume. This type of natural gas stored in liquid form is called liquefied natural gas (LNG). LNG utilization is considered the most effective method for solving problems such as long-distance natural gas transportation and oil and gas field development. However, during the entire production and transportation process of LNG, due to factors such as the inflow of ambient heat, volume replacement during loading and unloading, flash evaporation, and the accumulation and reduction of atmospheric pressure, a considerable portion of the liquefied natural gas will inevitably vaporize. This vaporized BOG (Boiled Gas) will cause an increase in the pressure inside the tank. To avoid accidents, the vaporized gas must be promptly removed and treated.

[0003] Existing patent number CN104792114B, entitled "BOG Reliquefaction Process and Reliquefaction Recovery System," provides a technical solution that involves liquefying BOG through compression and cooling, then throttling and depressurizing it before returning it to the LNG storage tank. However, the pressures of the BOG volatilized from the LNG storage tank, the LNG after throttling, and the gas phase after gas-liquid separation are different, which may cause a blockage in a certain channel, potentially damaging the equipment. Existing patent number CN105444523B, entitled "Reliquefaction System and Process Using BOG Self-Compression and Expansion to Liquefy BOG," uses the BOG's own expansion pressure to liquefy it. However, this solution does not consider the losses that occur during BOG recirculation, and the pressure difference between the BOG from the LNG storage tank and the BOG after the turbine expander is not considered when they are combined in the BOG buffer tank. This may cause the high-pressure BOG to rush into another flow channel, which is detrimental to equipment safety. Therefore, we propose a BOG reliquefaction system and liquefaction method for LNG storage tanks. Summary of the Invention

[0004] The purpose of this invention is to provide a BOG reliquefaction system and liquefaction method for LNG storage tanks, in order to solve the problems mentioned in the background art, such as the difficulty in properly processing BOG, the existing processing methods having different pressures in the gas phase, resulting in a blockage in a certain channel after collection, and the fact that the pressure difference between the two is not considered when BOG is collected in the BOG buffer tank, which may cause high-pressure BOG to rush into another flow channel, which is detrimental to equipment safety.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a BOG reliquefaction system for an LNG storage tank, comprising:

[0006] An LNG storage tank and a heat exchanger arranged side by side with the LNG storage tank, and a first delivery pipe for outputting to the heat exchanger is provided at the top end of the LNG storage tank;

[0007] A rewarming heat exchange component is arranged on one side of the heat exchanger to adjust the BOG temperature, and the rewarming heat exchange component is internally connected to the heat exchanger;

[0008] A BOG compressor is arranged at the lower end of the rewarming heat exchange component to boost the pressure of the BOG, and a second delivery pipe for outputting to the heat exchanger through the rewarming heat exchange component is provided on the BOG compressor;

[0009] A refrigerant assembly is connected to the lower end of one side of the heat exchanger through a pipeline to provide cooling capacity to the heat exchanger to cool and liquefy the BOG;

[0010] A pressure regulating assembly is arranged at the top end of the LNG storage tank to adjust the internal pressure of the LNG storage tank.

[0011] Preferably, the refrigerant assembly includes an MRC compressor and a gas-liquid separator. One end of the input pipe of the MRC compressor is connected to the second delivery pipe, and the output pipe of the MRC compressor is connected to the input pipe of the gas-liquid separator. The gas phase end and the liquid phase end of the gas-liquid separator are respectively internally connected to the heat exchanger through pipelines.

[0012] Preferably, the other other other other side wall of the heat exchanger includes a second return pipe and a third return pipe arranged side by side. The input end of the second return pipe is connected to the gas phase end of the gas-liquid separator through the heat exchanger, the third return pipe is connected to the liquid phase end of the gas-liquid separator through the heat exchanger, a second throttle valve J-2 is provided on the second return pipe, and a third throttle valve J-3 is provided on the third return pipe.

[0013] Preferably, both the second return pipe and the third return pipe are in the shape of "匸".

[0014] Preferably, a first return pipe for connecting to the LNG storage tank is provided at the upper end of the other side wall of the heat exchanger, and a first throttle valve J-1 is installed on the first return pipe.

[0015] Preferably, the pressure regulating assembly includes a pressure detection component and an adjustment component. The pressure detection component is arranged on the LNG storage tank, an adjustment component is installed on the first output pipe, and the adjustment component and the pressure detection component are interlocked through a wire.

[0016] Preferably, the BOG compressor includes an inlet pipe and an outlet pipe. The inlet pipe and the outlet pipe are arranged side by side and are not connected to each other.

[0017] A liquefaction method for a BOG re-liquefaction system of an LNG storage tank includes the following steps:

[0018] Step A: The BOG from the LNG storage tank is first reheated to -40°C through a heat exchanger and then reheated to room temperature through a reheating heat exchanger. After reheating, the BOG enters the BOG compressor and is pressurized to -MPa. The exhaust temperature is 75-85°C and then enters the reheating heat exchanger to be cooled to room temperature.

[0019] Step B: The cooled BOG is divided into two parts. One part enters the heat exchanger and is cooled to -162°C to obtain liquefied LNG. The LNG exiting the heat exchanger is depressurized to 0.2-0.3MPa through the first throttle valve J-1 and then returned to the LNG storage tank. The other part is used as a refrigerant component to supplement the refrigerant circulation loss.

[0020] Step C: The refrigerant compressed by the MRC compressor first enters the gas-liquid separator to separate the gaseous and liquid phases of the refrigerant. The separated gaseous and liquid phases of the refrigerant enter the cold box inside the heat exchanger from different output pipes.

[0021] Step D: After exiting the cold box, the gaseous refrigerant is depressurized to 0.3 MPa and cooled to -165°C by the second throttling valve J-2, then returns to the cold box to provide the cooling capacity required for biogas liquefaction, and then returns to the MRC compressor. The liquid refrigerant is depressurized to 0.3 MPa and cooled to -55°C by the third throttling valve J-3, then returns to the cold box to provide the cooling capacity required for biogas liquefaction, and then returns to the MRC compressor.

[0022] Step E: The pressure detection and regulation components are interlocked to control the pressure in the gas phase space at the top of the LNG storage tank, thereby reducing the volatilization of LNG inside the tank.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] This invention, by incorporating a heat exchanger, a reheating heat exchanger, a BOG compressor, a refrigerant assembly, and a pressure regulating assembly, facilitates the adjustment of liquefaction temperature and pressurization for BOG reliquefaction. This prevents excessive pressure within the LNG storage tank caused by vaporized BOG, ensuring storage safety. A gas-liquid separator separates the gaseous and liquid refrigerant phases for different applications, thereby improving system energy utilization. The pressure regulating assembly controls the pressure in the gas phase space at the top of the LNG storage tank to reduce LNG evaporation, and devices such as throttle valves reduce the pressure difference, ensuring pressure balance and safe system operation during BOG reliquefaction. This invention effectively solves the problems of pressure imbalance, energy loss, and safety hazards during BOG reliquefaction, improving system stability and efficiency. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of the present invention.

[0026] In the diagram: 1. LNG storage tank; 2. Heat exchanger; 3. Reheating heat exchanger; 4. BOG compressor; 5. MRC compressor; 6. Gas-liquid separator; 7. Pressure detection device; 8. Regulator; J-1, First throttle valve; J-2, Second throttle valve; J-3, Third throttle valve. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Please see Figure 1 This invention provides a technical solution: a BOG reliquefaction system for an LNG storage tank, comprising:

[0029] LNG storage tank 1 and heat exchanger 2 arranged side by side with LNG storage tank 1, and a first delivery pipe for output to heat exchanger 2 is provided at the top of LNG storage tank 1;

[0030] The reheating heat exchanger 3 is set on one side of the heat exchanger 2 to regulate the BOG temperature. The reheating heat exchanger 3 is connected to the inside of the heat exchanger 2. The BOG is reheated through the heat exchanger 2 and the BOG reheating heat exchanger 3 to bring it to about room temperature. Then it is pressurized by the BOG compressor 4 and finally cooled to about room temperature by the BOG reheating heat exchanger 3, thereby realizing the reliquefaction of BOG.

[0031] BOG compressor 4 is located at the lower end of the reheat heat exchanger 3 to pressurize BOG. ​​BOG compressor 4 is also equipped with a second delivery pipe that outputs to heat exchanger 2 via the reheat heat exchanger 3, which facilitates the pressurization and liquefaction of the vaporized BOG.

[0032] The refrigerant assembly is connected to the lower end of one side of the heat exchanger 2 via a pipeline to provide cooling capacity to the heat exchanger 2 so that the BOG is cooled and liquefied, which facilitates the provision of cooling capacity and reduces the regasification of BOG returning to the LNG storage tank 1.

[0033] The pressure regulating component is installed at the top of LNG storage tank 1 to adjust the internal pressure of LNG storage tank 1, reduce the pressure in the top space of LNG storage tank 1, and reduce the volatilization of LNG in LNG storage tank 1.

[0034] Preferably, the refrigerant assembly includes an MRC compressor 5 and a gas-liquid separator 6. One end of the input pipe of the MRC compressor 5 is connected to the second delivery pipe, and the output pipe of the MRC compressor 5 is connected to the input pipe of the gas-liquid separator 6. The gas phase end and the liquid phase end of the gas-liquid separator 6 are respectively connected to the interior of the heat exchanger 2 through pipes, facilitating the separation of gaseous and liquid refrigerants in the gas-liquid separator 6 for different purposes, thereby improving the energy utilization rate of the system.

[0035] Preferably, the other side wall of the heat exchanger 2 includes a second return pipe and a third return pipe arranged side by side. The input end of the second return pipe is connected to the gas phase end of the gas-liquid separator 6 through the heat exchanger 2, and the third return pipe is connected to the liquid phase end of the gas-liquid separator 6 through the heat exchanger 2. A second throttle valve J-2 is provided on the second return pipe, and a third throttle valve J-3 is provided on the third return pipe, facilitating the pressure reduction of gaseous and liquid refrigerants through the second throttle valve J-2 and the third throttle valve J-3.

[0036] Preferably, both the second return pipe and the third return pipe are in an "L" shape, facilitating the return to the MRC compressor 5 after pressure reduction.

[0037] Preferably, a first return pipe connected to the LNG storage tank 1 is provided at the upper end of the other side wall of the heat exchanger 2, and a first throttle valve J-1 is installed on the first return pipe, facilitating the pressure reduction of the liquefied BOG and returning it to the inside of the LNG storage tank 1.

[0038] Preferably, the pressure regulating assembly includes a pressure detection component 7 and an adjustment component 8. The pressure detection component 7 is provided on the LNG storage tank 1, and the adjustment component 8 is installed on the first output pipe. The adjustment component 8 and the pressure detection component 7 are interlocked through a wire, facilitating the control of the pressure in the gas phase space at the top of the LNG storage tank 1 to reduce the volatilization of LNG, and reducing the pressure difference through devices such as throttle valves to ensure pressure balance and system safe operation during the BOG re-liquefaction process.

[0039] Preferably, the BOG compressor 4 includes an inlet pipe and an outlet pipe. The inlet pipe and the outlet pipe are arranged side by side and are not connected to each other, facilitating the better extraction and compression of BOG waste gas for subsequent treatment or reuse, and introducing it into the rewarming heat exchange component 3 through the non-connected inlet pipe and outlet pipe.

[0040] A liquefaction method for a BOG re-liquefaction system of an LNG storage tank includes the following steps:

[0041] Step A: The BOG from the LNG storage tank 1 is first rewarmed to -40°C through the heat exchanger 2 and then to room temperature through the rewarming heat exchange component 3; the rewarmed BOG enters the BOG compressor 4 to be pressurized to 4 - 6 MPa, with the discharge temperature reaching 75 - 85°C and then entering the rewarming heat exchange component 3 to be cooled to room temperature;

[0042] Step B: The cooled BOG is divided into two parts. One part enters heat exchanger 2 and is cooled to -162°C to obtain liquefied LNG. The LNG exiting heat exchanger 2 is depressurized to 0.2-0.3MPa through the first throttle valve J-1 and then returned to LNG storage tank 1. The other part is used as a refrigerant component to supplement the refrigerant circulation loss.

[0043] Step C: The refrigerant compressed by the MRC compressor 5 first enters the gas-liquid separator 6 to separate the gas and liquid phases of the refrigerant. The separated gas and liquid phases of the refrigerant enter the cold box inside the heat exchanger 2 from different output pipes.

[0044] Step D: After exiting the cold box, the gaseous refrigerant is depressurized to 0.3 MPa and cooled to -165°C by the second throttling valve J-2, and then returns to the cold box to provide the cooling capacity required for biogas liquefaction. It then returns to the MRC compressor 5. The liquid refrigerant is depressurized to 0.3 MPa and cooled to -55°C by the third throttling valve J-3, and then returns to the cold box to provide the cooling capacity required for biogas liquefaction before returning to the MRC compressor 5.

[0045] Step E: The pressure detection element 7 and the regulating element 8 are interlocked to control the pressure in the gas phase space at the top of the LNG storage tank 1, thereby reducing the volatilization of LNG in the LNG storage tank 1.

[0046] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A BOG reliquefaction system for an LNG storage tank, characterized in that, include: An LNG storage tank (1) and a heat exchanger (2) arranged side by side with the LNG storage tank (1), and a first delivery pipe for output to the heat exchanger (2) is provided at the top of the LNG storage tank (1); A reheating heat exchanger (3) is installed on one side of the heat exchanger (2) to regulate the BOG temperature, and the reheating heat exchanger (3) is connected to the inside of the heat exchanger (2); The BOG compressor (4) is located at the lower end of the reheat heat exchanger (3) to pressurize the BOG, and the BOG compressor (4) is provided with a second delivery pipe that outputs to the heat exchanger (2) via the reheat heat exchanger (3); The refrigerant assembly is connected to the lower end of one side of the heat exchanger (2) via a pipe to provide cooling capacity to the heat exchanger (2) to cool and liquefy the BOG. A pressure regulating component is installed at the top of the LNG storage tank (1) to adjust the internal pressure of the LNG storage tank (1); The refrigerant assembly includes an MRC compressor (5) and a gas-liquid separator (6). One end of the input pipe of the MRC compressor (5) is connected to the second delivery pipe, and the output pipe of the MRC compressor (5) is connected to the input pipe of the gas-liquid separator (6). The gas phase end and the liquid phase end of the gas-liquid separator (6) are respectively connected to the interior of the heat exchanger (2) through pipes. The other side wall of the heat exchanger (2) includes a second reflux pipe and a third reflux pipe arranged side by side. The input end of the second reflux pipe is connected to the gas phase end of the gas-liquid separator (6) through the heat exchanger (2), and the third reflux pipe is connected to the liquid phase end of the gas-liquid separator (6) through the heat exchanger (2). A second throttle valve (J-2) is provided on the second reflux pipe, and a third throttle valve (J-3) is provided on the third reflux pipe. The heat exchanger (2) has a first reflux pipe connected to the LNG storage tank (1) at the upper end of the other side wall, and a first throttle valve (J-1) is installed on the first reflux pipe. The pressure regulating component includes a pressure detection element (7) and an adjusting element (8). The pressure detection element (7) is installed on the LNG storage tank (1), and the adjusting element (8) is installed on the first delivery pipe. The adjusting element (8) and the pressure detection element (7) are interlocked by a wire. The BOG reliquefaction method for LNG storage tanks includes the following steps: Step A: The BOG from the LNG storage tank (1) is first reheated to -40°C through the heat exchanger (2) and then reheated to room temperature through the reheating heat exchanger (3); the reheated BOG enters the BOG compressor (4) and is pressurized to 4-6MPa. The exhaust temperature is 75-85°C and it enters the reheating heat exchanger (3) to be cooled to room temperature. Step B: The cooled BOG is divided into two parts. One part enters the heat exchanger (2) and is cooled to -162°C to obtain liquefied LNG. The LNG after exiting the heat exchanger (2) is depressurized to 0.2-0.3MPa through the first throttle valve (J-1) and then returned to the LNG storage tank (1). The other part is used as a refrigerant component to supplement the refrigerant circulation loss. Step C: The refrigerant compressed by the MRC compressor (5) first enters the gas-liquid separator (6) to separate the gaseous and liquid-phase refrigerants. The separated gaseous and liquid-phase refrigerants enter the cold box inside the heat exchanger (2) from different output pipes; Step D: After the gaseous refrigerant exits the cold box, it is depressurized to 0.3 MPa and cooled to -165 °C through the second throttle valve (J-2), then returns to the cold box to provide the cooling capacity required for the liquefaction of biomethane, and then returns to the MRC compressor (5). The liquid-phase refrigerant is depressurized to 0.3 MPa and cooled to -55 °C after passing through the third throttle valve (J-3), and then returns to the cold box to provide the cooling capacity required for the liquefaction of biomethane and then returns to the MRC compressor (5); Step E: The pressure detection component (7) and the adjustment component (8) are interlocked with each other to control the pressure in the gas phase space at the top of the LNG storage tank (1), reducing the volatilization of LNG in the LNG storage tank (1).

2. The BOG reliquefaction system for an LNG storage tank according to claim 1, characterized in that: Both the second return pipe and the third return pipe are in an "L" shape.

3. The BOG reliquefaction system for an LNG storage tank according to claim 1, characterized in that: The BOG compressor (4) is equipped with an inlet pipe and an outlet pipe. The inlet pipe and the outlet pipe are arranged side by side and are not connected to each other.

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