An LNG plant BOG re-liquefaction recovery management system using liquid nitrogen refrigeration

Abstract

The present invention discloses an LNG plant BOG re-liquefaction recovery management system using liquid nitrogen refrigeration, belonging to the technical field of BOG re-liquefaction recovery; the system includes: a BOG collection module for receiving the evaporated gas BOG generated by the storage tank and stably outputting pressurized BOG; a multi-stage pressurization module for performing three-stage compression treatment on the pressurized BOG, and cooling and temperature reduction are carried out after each stage of compression to output compressed BOG with a set pressure; a precooling regulation module for receiving the compressed BOG and performing countercurrent heat exchange with the liquid nitrogen return gas to output precooled BOG; a liquid nitrogen injection refrigeration module for atomizing the liquid nitrogen and performing swirling mixing with the precooled BOG to output a low-temperature two-phase flow; a phase separation and recovery module for performing gas-liquid separation on the low-temperature two-phase flow to output liquefied natural gas and residual nitrogen; the present invention improves the compression efficiency by using liquid nitrogen refrigeration for BOG re-liquefaction recovery, and has the characteristics of low energy consumption, high heat exchange efficiency, high recovery efficiency and high safety.

Description

Technical Field

[0001] The present invention relates to the technical field of BOG re-liquefaction recovery, and particularly to a BOG re-liquefaction recovery management system for an LNG plant using liquid nitrogen refrigeration. Background Art

[0002] During the storage and transportation of liquefied natural gas (LNG), due to environmental heat transfer and storage tank pressure fluctuations, evaporation gas (BOG, Boil-Off Gas) is continuously generated. If this BOG is not processed in a timely manner, it will cause the internal pressure of the storage tank to rise, affecting the safety of LNG storage and potentially resulting in resource waste. Therefore, how to efficiently recover and re-liquefy BOG, reduce energy losses, and improve the LNG recovery rate is an important technical challenge faced by LNG plants.

[0003] Existing BOG re-liquefaction technologies mainly include mechanical compression condensation method, expander cooling method, and mixed refrigeration cycle method. However, these methods have the following problems:

[0004] High energy consumption: The mechanical compression condensation method relies on a high-pressure compressor to pressurize and cool BOG, resulting in high operating energy consumption and high system maintenance costs;

[0005] Low heat exchange efficiency: The traditional heat exchange system uses ordinary finned heat exchangers, which have a risk of frosting and blockage, leading to a decline in heat exchange performance;

[0006] Unstable liquefaction rate: Some cryogenic expander systems have problems with lagging control response, making it difficult to adapt to BOG flow fluctuations and affecting the stable recovery of liquefied natural gas. Summary of the Invention

[0007] In order to overcome the above-mentioned shortcomings in the prior art, the purpose of the present invention is to provide a BOG re-liquefaction recovery management system for an LNG plant using liquid nitrogen refrigeration. By using liquid nitrogen refrigeration to re-liquefy and recover BOG, it has the characteristics of low energy consumption, high heat exchange efficiency, high recovery efficiency, and high safety.

[0008] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] A BOG re-liquefaction recovery management system for an LNG plant using liquid nitrogen refrigeration, comprising:

[0010] BOG collection module: including an adiabatic buffer tank and a gas pressure stabilizing output device; used to receive BOG generated by the storage tank and stably output pressurized BOG by adjusting the internal pressure of the adiabatic buffer tank;

[0011] Multi-stage pressurization module: performing three-stage compression processing on the pressurized BOG, and cooling and reducing the temperature of the BOG after each stage of compression, and outputting compressed BOG with a set pressure;

[0012] Precooling control module: It includes an asymmetric fin heat exchanger and an anti-ice blockage device, which is used to receive the compressed BOG and conduct countercurrent heat exchange with the return gas of liquid nitrogen, and output precooled BOG;

[0013] Liquid nitrogen injection refrigeration module: It includes a microporous injector and a pipe-type vortex mixer, atomizes the liquid nitrogen and conducts swirling mixing with the precooled BOG, and outputs a low-temperature two-phase flow;

[0014] Phase separation and recovery module: It includes a cryogenic separation tank and a liquid level control device, separates the gas-liquid of the low-temperature two-phase flow, and outputs liquefied natural gas and residual nitrogen.

[0015] The adiabatic buffer tank adopts a multi-layer vacuum adiabatic cavity structure; the gas pressure stabilizing output device includes a first pressure sensor array and an electric control valve. The first pressure sensor array is distributed at the inlet, inside the tank body and the outlet of the adiabatic buffer tank, and the electric control valve is arranged on the outlet pipeline of the adiabatic buffer tank; the signal output end of the first pressure sensor is connected to the signal input end of the electric control valve through a first PID pressure controller.

[0016] The three-stage compression treatment specifically includes:

[0017] The first-stage compression unit, using a centrifugal compressor, preliminarily pressurizes the boosted BOG, and outputs the first-stage compressed BOG with a pressure of 0.4 MPa to 0.6 MPa;

[0018] The first-stage intercooler, using a finned tube heat exchanger, cools the first-stage compressed BOG with a low-temperature coolant, and reduces its temperature to -50°C to -30°C;

[0019] The second-stage compression unit, using a piston compressor, performs secondary pressurization on the cooled first-stage compressed BOG, and outputs the second-stage compressed BOG with a pressure of 0.8 MPa to 1.0 MPa;

[0020] The second-stage intercooler, using a spray heat exchanger, cools the second-stage compressed BOG with a low-temperature circulating coolant, and reduces its temperature to -80°C to -50°C;

[0021] The third-stage compression unit, using a screw compressor, performs final pressurization on the cooled second-stage compressed BOG, and outputs the third-stage compressed BOG with a stable pressure of 1.5 MPa to 2.0 MPa.

[0022] The first-stage compression unit, the second-stage compression unit and the third-stage compression unit all adopt a multi-stage adiabatic compression model, and satisfy the following in each stage of compression:

[0023]

[0024] Among them, is the inlet pressure of the -stage compression, is the outlet pressure of the -stage compression, and at the same time serves as the inlet pressure of the -stage compression; is the inlet temperature of the -stage compression, is the outlet temperature of the -stage compression, and at the same time serves as the inlet temperature of the -stage compression; is the specific heat ratio of natural gas, is the compressor efficiency, is the compressor power, is the mass flow rate of BOG, is the specific heat capacity of BOG at constant pressure.

[0025] The asymmetric fin heat exchanger includes a high-pressure side channel and a low-pressure side channel; the high-pressure side channel adopts a dense fin structure, and the low-pressure side channel adopts a variable-spacing fin structure.

[0026] The anti-icing plugging device includes a temperature sensor array and an electric bypass valve. The temperature sensor array is arranged inside the asymmetric fin heat exchanger, and the electric bypass valve is arranged on the outlet gas pipeline of the asymmetric fin heat exchanger; the signal output end of the temperature sensor is connected to the signal input end of the electric bypass valve through a PID temperature controller.

[0027] The micro-hole injector adopts a porous distribution structure, and the diameter range of the injection holes is 50 μm to 200 μm, atomizing liquid nitrogen into fine droplets to form atomized liquid nitrogen;

[0028] The pipe-type vortex mixer is composed of a mixing chamber, a deflector and a nozzle. The mixing chamber is provided with a plurality of tangential air inlets, so that the atomized liquid nitrogen and the precooled BOG form a swirling flow, and mix and exchange heat to output a low-temperature two-phase flow.

[0029] The low-temperature separation tank adopts a vertical double-chamber structure, including an inner chamber and an outer chamber. The inner chamber is provided with a gas-liquid separation baffle and a cyclone separator, receives the low-temperature two-phase flow, and performs gas-liquid separation through cyclone separation and gravity sedimentation.

[0030] The liquid level control device includes a capacitive liquid level controller and an adjustable liquid outlet valve. The capacitive liquid level controller is arranged inside the low-temperature separation tank, and the adjustable liquid outlet valve is arranged on the liquid outlet pipeline of the low-temperature separation tank; the signal output end of the capacitive liquid level sensor is connected to the signal input end of the adjustable liquid outlet valve through a PID liquid level controller.

[0031] A second pressure sensor is arranged in the gas phase area at the top inside the low-temperature separation tank to monitor the pressure in the gas phase area of the low-temperature separation tank in real time. An electronic regulating valve is arranged on the gas outlet pipeline of the low-temperature separation tank. The signal output end of the second pressure sensor is connected to the signal input end of the electronic regulating valve through a second PID pressure controller.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] 1. The present invention reduces the environmental heat conduction through a multi-layer vacuum adiabatic buffer tank and adopts a gas pressure stabilizing output device to achieve precise control of BOG pressurization, so that the output pressure is within a stable range. It not only buffers the flow rate fluctuation of the evaporation gas in the storage tank, avoids the influence of unstable pressure on the subsequent multi-stage pressurization module, but also ensures the optimal input conditions of the multi-stage pressurization module, improves the compression efficiency, and reduces the energy consumption.

[0034] 2. The present invention adopts an asymmetric fin heat exchanger. The high-pressure side channel adopts a dense fin structure to improve the heat exchange efficiency, while the low-pressure side channel adopts a variable-spacing fin structure to optimize the air flow distribution and reduce the pressure loss, thereby improving the countercurrent heat exchange efficiency between BOG and the return gas of liquid nitrogen. In addition, the anti-ice blockage device can automatically adjust the pipeline air flow when detecting the risk of low-temperature frosting, ensuring the long-term stable operation of the heat exchanger and avoiding the ice blockage problem. At the same time, the liquid nitrogen injection refrigeration module atomizes the liquid nitrogen through a micro-hole injector and performs efficient rotational mixing heat exchange with the precooled BOG in the mixing chamber of the pipeline vortex mixer to output a low-temperature two-phase flow, so that the temperature is stably controlled at -160°C to -140°C, improving the subsequent liquefaction efficiency, significantly increasing the heat exchange rate, reducing the energy consumption, and avoiding low-temperature blockage through the intelligent anti-ice blockage device, improving the long-term stability and reliability of the system.

[0035] 3. The present invention adopts a low-temperature separation tank with a vertical double-chamber structure for swirl separation and gravity sedimentation to improve the gas-liquid separation efficiency of the low-temperature two-phase flow and ensure the purity of liquefied natural gas. At the same time, a liquid level control device is adopted to adjust the opening degree of the adjustable liquid outlet valve in real time by monitoring the liquid level of liquefied natural gas in real time, ensuring the stable output of liquefied natural gas and avoiding the influence of liquid level fluctuation on the system operation.

[0036] In summary, the present invention uses liquid nitrogen refrigeration to re-liquefy and recover BOG, improves the compression efficiency, and has the characteristics of low energy consumption, high heat exchange efficiency, high recovery efficiency, and high safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a module schematic diagram of the LNG plant BOG re-liquefaction recovery management system using liquid nitrogen refrigeration according to the present invention.

[0038] Figure 2It is a schematic structural diagram of the BOG re-liquefaction recovery management system of the LNG plant and station using liquid nitrogen refrigeration according to the present invention.

[0039] In the figure: 1 - storage tank; 2 - adiabatic buffer tank; 3 - first pressure sensor; 4 - electric control valve; 5 - centrifugal compressor; 6 - finned tube heat exchanger; 7 - piston compressor; 8 - spray heat exchanger; 9 - screw compressor; 10 - asymmetric fin heat exchanger; 11 - temperature sensor; 12 - electric bypass valve; 13 - micro-hole injector; 14 - pipeline vortex mixer; 15 - cryogenic separation tank; 16 - capacitive liquid level sensor; 17 - adjustable liquid outlet valve; 18 - second pressure sensor; 19 - electronic control valve. Specific embodiments

[0040] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0041] As Figure 1 and Figure 2 shown, a BOG re-liquefaction recovery management system of the LNG plant and station using liquid nitrogen refrigeration includes:

[0042] BOG collection module: It includes an adiabatic buffer tank 2 and a gas pressure stabilizing output device, which is used to receive the BOG generated by the storage tank 1, adjust the internal pressure of the adiabatic buffer tank 2, and stably output pressurized BOG; the adiabatic buffer tank 2 adopts a multi-layer vacuum adiabatic cavity structure to reduce environmental heat conduction, improve the storage stability of BOG, and buffer the flow rate fluctuation of the evaporation gas of the storage tank 1;

[0043] The gas pressure stabilizing output device includes a first pressure sensor 3 array and an electric control valve 4; the first pressure sensor 3 array is arranged at the air inlet, inside the tank body and the air outlet of the adiabatic buffer tank 2 for real-time monitoring of the BOG pressure change, and the electric control valve 4 is arranged on the outlet pipeline of the adiabatic buffer tank 2; the signal output end of the first pressure sensor 3 is connected to the signal input end of the electric control valve 4 through a first PID pressure controller; based on the pressure detection results of the first pressure sensor 3 array, the internal pressure of the adiabatic buffer tank 2 is adjusted to make the pressure of the output pressurized BOG stable within a preset range to match the input requirements of the subsequent multi-stage pressurization module;

[0044] The first PID pressure controller calculates the optimal valve opening based on the detection data of the first pressure sensor 3 array, adjusts the opening of the electric control valve 4 in real time, and adjusts the internal pressure of the adiabatic buffer tank 2 to make the pressure of the output pressurized BOG stable within the preset range of 0.2 MPa to 0.4 MPa. The specific calculation is as follows:

[0045]

[0046] Among them, is the output pressure of the adiabatic buffer tank 2, which needs to be stabilized at 0.2 MPa to 0.4 MPa; is the current pressure inside the adiabatic buffer tank 2, is the mass flow rate of the instantaneous BOG outflow, is the natural gas gas constant, taking 518.3 J / (kg·K); is the BOG gas temperature, the effective volume of the adiabatic buffer tank 2, is the flow coefficient of the electric control valve 4, which is determined by the valve structure; is the real-time opening of the electric control valve 4, which is calculated by the first PID pressure controller; is the BOG density, about 0.72 kg / m 3 ; is the BOG density under standard conditions, taking 0.717 kg / m 3 ; is the preset stable pressure, taking 0.3 MPa; is the pressure error, are the control parameters of the first PID pressure controller, which are the proportional, integral and derivative gains respectively, is the proportional control term of the first PID pressure controller, indicating the direct response of the first PID pressure controller to the current control error signal, represents the control error signal.

[0047] Multi-stage supercharging module: used to receive the supercharged BOG and perform three-stage compression treatment on it, and cool down the BOG after each stage of compression, and output the compressed BOG with a set pressure;

[0048] The three-stage compression treatment gradually increases the pressure of the BOG and reduces its temperature through the cross-treatment of multi-stage supercharging and cooling, and finally realizes the effective reliquefaction and recovery of the BOG. Setting up a three-stage compression unit and using different types of compressors can improve the compression efficiency at different compression ratios, reduce the temperature rise, and ensure that the final BOG can be liquefied and recovered within the specified temperature and pressure range. Specifically, it includes:

[0049] The first-stage compression unit uses a centrifugal compressor 5 to perform preliminary supercharging on the supercharged BOG and outputs the first-stage compressed BOG with a pressure of 0.4 MPa to 0.6 MPa. This stage mainly plays a role in preliminary compression; the BOG usually has a large gas volume, and the centrifugal compressor 5 is suitable for processing large-flow gases. Especially when the gas flow is large but the pressure requirement is relatively low, it can efficiently process a large amount of gas within a wide flow range and provide a stable compression effect, which is suitable for preliminary supercharging;

[0050] The first-stage intercooler uses a finned-tube heat exchanger 6 to cool the first-stage compressed BOG with a low-temperature coolant, reducing its temperature to -50°C to -30°C and minimizing the temperature rise during subsequent compression. The low-temperature coolant is liquid nitrogen (LN2).

[0051] The second-stage compression unit uses a piston compressor 7 to further boost the pressure of the cooled first-stage compressed BOG, outputting second-stage compressed BOG with a pressure of 0.8 MPa to 1.0 MPa. This stage mainly compresses the BOG to the medium-high pressure range. The piston compressor 7 used can generate a relatively high output pressure within a relatively small volume, is suitable for compressing medium to high-pressure gases, and can precisely control the pressurization process of the BOG.

[0052] The second-stage intercooler uses a spray heat exchanger 8 to cool the second-stage compressed BOG with a low-temperature circulating coolant, reducing its temperature to -80°C to -50°C and minimizing the impact of gas expansion on compression efficiency. The low-temperature circulating coolant is carbon tetrachloride (CCl4).

[0053] The third-stage compression unit uses a screw compressor 9 to finally boost the pressure of the cooled second-stage compressed BOG, outputting third-stage compressed BOG with a stable pressure of 1.5 MPa to 2.0 MPa, meeting the pressure requirements for liquefaction recovery. The screw compressor 9 used can minimize excessive gas temperature rise during compression, which is important for subsequent liquefaction, reduce heat accumulation, maintain a relatively low gas temperature, and provide a stable high-pressure output under high pressure and relatively low gas flow conditions.

[0054] The first-stage compression unit, the second-stage compression unit, and the third-stage compression unit all adopt a multi-stage adiabatic compression model, satisfying the following conditions during each stage of compression:

[0055]

[0056] Among them, is the inlet pressure of the -th stage of compression, is the outlet pressure of the -th stage of compression, which also serves as the inlet pressure of the -th stage of compression; is the inlet temperature of the -th stage of compression, is the outlet temperature of the -th stage of compression, which also serves as the inlet temperature of the -th stage of compression; is the specific heat ratio of natural gas, taken as 1.3; is the compressor efficiency, taken as 0.85; is the compressor power, is the mass flow rate of BOG, The specific heat capacity at constant pressure of BOG is taken as 2.3 kJ / (kg·K).

[0057] Precooling control module: It includes an asymmetric fin heat exchanger 10 and an anti-ice blockage device, and is used to receive the compressed BOG and conduct countercurrent heat exchange with the return gas of liquid nitrogen, and output the precooled BOG.

[0058] The asymmetric fin heat exchanger 10 includes a high-pressure side channel and a low-pressure side channel; the high-pressure side channel adopts a dense fin structure to increase the heat exchange area; the low-pressure side channel adopts a variable-spacing fin structure to optimize the air flow distribution; the three-stage compressed BOG enters the high-pressure side channel of the asymmetric fin heat exchanger 10, and the return gas of liquid nitrogen enters the low-pressure side channel, and the two conduct countercurrent heat exchange; due to the design of the asymmetric fins, the heat exchange area on the high-pressure side is larger, improving the heat transfer efficiency, and at the same time, the air flow distribution on the low-pressure side is optimized, reducing the pressure loss. Specifically:

[0059] 1. The high-pressure side channel adopts a dense fin structure: The air flow velocity on the high-pressure side is relatively high and the heat exchange demand is large. Therefore, a dense fin structure is adopted to increase the heat exchange area, thereby improving the heat exchange efficiency. The spacing of the dense fins is between 0.5 mm and 2 mm, specifically depending on the flow velocity of the high-pressure air flow and the heat exchange demand. By increasing the fin density, the heat exchange efficiency is improved, which helps the three-stage compressed BOG under a higher compression ratio to conduct heat exchange with the return gas of low-temperature liquid nitrogen more effectively.

[0060] 2. The low-pressure side channel adopts a variable-spacing fin structure: The air flow velocity on the low-pressure side is relatively low, and the heat exchange efficiency is more sensitive to the air flow distribution. Therefore, a variable-spacing fin structure is adopted to optimize the air flow distribution, reduce the dead zone and flow resistance, and at the same time ensure uniform heat exchange. The spacing of the fins can be adjusted according to the air flow conditions to optimize the air flow distribution and heat exchange efficiency. The spacing can be changed by a mechanical structure or the fluid pressure difference of a spring structure, so that when the flow rate is low, the spacing between the fins can be reduced to increase the contact area; when the flow rate is large, the fin spacing is appropriately increased to avoid excessive pressure drop and improve the smoothness of the air flow. The adjustment range of the variable spacing is generally 1 mm to 4 mm, depending on the air flow conditions. By adjusting the spacing, it is ensured that the air flow on the low-pressure side is evenly distributed and excessive air flow concentration at a certain position is avoided, resulting in uneven heat exchange.

[0061] The anti-icing blockage device includes a temperature sensor array 11 and an electric bypass valve 12. The temperature sensor array 11 is arranged inside the asymmetric fin heat exchanger 10, and the electric bypass valve 12 is arranged on the gas outlet pipeline of the asymmetric fin heat exchanger 10. The signal output end of the temperature sensor 11 is connected to the signal input end of the electric bypass valve 12 through a PID temperature controller, which is used to detect the real-time temperature inside the asymmetric fin heat exchanger 10 and adjust the opening of the electric bypass valve 12 based on the real-time temperature feedback to ensure that the flow channel of the asymmetric fin heat exchanger 10 does not freeze and block.

[0062] The calculation of the opening of the electric bypass valve 12 is based on the PID temperature controller, and the calculation formula is:

[0063]

[0064] Where, is the heat exchange temperature error, is the real-time opening of the electric bypass valve 12, is the control parameter of the PID temperature controller, is the temperature of the three-stage compressed BOG entering the asymmetric fin heat exchanger 10, is the temperature of the returned liquid nitrogen gas discharged from the asymmetric fin heat exchanger 10.

[0065] Finally, the pre-cooled BOG temperature is stabilized within the range of -120°C to -100°C.

[0066] Liquid nitrogen injection refrigeration module: It includes a micro-hole injector 13 and a pipe-type vortex mixer 14. The outlet of the micro-hole injector 13 is connected to the inlet of the pipe-type vortex mixer 14 through a pipeline. After the liquid nitrogen is atomized, it is mixed with the pre-cooled BOG in a swirling manner to output a low-temperature two-phase flow.

[0067] The micro-hole injector 13 adopts a porous distribution structure, and the diameter range of the injection holes is 50 μm to 200 μm, which atomizes the liquid nitrogen (LN2) into fine droplets to form atomized liquid nitrogen.

[0068] The pipeline vortex mixer 14 is composed of a mixing chamber, a deflector and a nozzle. The nozzle is located at the inlet of the mixing chamber. The mixing chamber is provided with a plurality of tangential air inlets, which can change the inlet angle of the air flow, so that the atomized liquid nitrogen and the precooled BOG air flow rotate along the cavity wall; the deflector is installed inside the mixing chamber, which can effectively control the flow rate and improve the stability and mixing efficiency of the flow; the atomized liquid nitrogen and the precooled BOG enter the mixing chamber through the nozzle. Under the action of the tangential air inlets and the deflector, a rotating flow is formed in the mixing chamber. The rotating flow increases the contact area between the atomized liquid nitrogen and the precooled BOG, and promotes the full mixing of the two. The liquid nitrogen atomized droplets evaporate rapidly, thereby absorbing the heat of the BOG, reducing the temperature of the BOG, and the output temperature of the fluid after mixing and heat exchange is a cryogenic two-phase flow below -150°C. In this process, the mixing of the atomized liquid nitrogen and the precooled BOG and the temperature change achieve rapid and uniform heat exchange.

[0069] Phase separation and recovery module: It includes a cryogenic separation tank 15 and a liquid level control device, which are used to receive the cryogenic two-phase flow and perform gas-liquid separation through cyclone separation and gravity sedimentation, and output liquefied natural gas (LNG) and residual nitrogen (N2);

[0070] The cryogenic separation tank 15 adopts a vertical double-chamber structure, including an inner chamber and an outer chamber. These two chambers are connected by a partition or an air flow guiding pipe. The inner chamber is used to receive the cryogenic two-phase flow, and the outer chamber serves as a storage space for the separated gas or liquid. The vertical double-chamber structure can ensure that the gas and liquid fluids in the gas-liquid separation process have sufficient residence time and separation space in the tank.

[0071] An inclined fixed gas-liquid separation baffle is arranged at the upper or middle part of the inner chamber of the cryogenic separation tank 15. A number of holes are processed on the surface of the gas-liquid separation baffle. Through the physical blocking effect, the gas-liquid flow rate is slowed down, and the liquid droplets are prevented from flowing out together with the gas during the rising process. The gas-liquid separation baffle effectively improves the separation effect. A cyclone separator with a cylindrical conical structure is arranged at the lower part of the inner chamber of the cryogenic separation tank 15. The cryogenic two-phase flow enters the cryogenic separation tank 15 through the inlet pipe. When the cryogenic two-phase flow passes through the cyclone separator, it is accelerated and separated under the action of centrifugal force to form two-phase stratification. Part of the liquid sinks along the tank wall due to inertia and gravity, concentrates at the bottom of the tank, and is discharged through the liquid outlet pipe at the bottom of the tank. The gas flows upward along the center of the cyclone separator. When passing through the gas-liquid separation baffle, the liquid droplets carried by the gas during the rising process are blocked. The gas is concentrated at the top of the tank through the gas-liquid separation baffle and is discharged through the gas outlet pipe at the top of the tank.

[0072] The liquid level control device includes a capacitive liquid level sensor 16 and an adjustable liquid outlet valve 17. The capacitive liquid level sensor 16 is arranged inside the low-temperature separation tank 15 for real-time monitoring of the change in the liquid level of liquefied natural gas in the low-temperature separation tank 15. The adjustable liquid outlet valve 17 is arranged on the liquid outlet pipeline of the low-temperature separation tank 15. The signal output end of the capacitive liquid level sensor 16 is connected to the signal input end of the adjustable liquid outlet valve 17 through a PID liquid level controller. Based on the real-time monitored liquid level of liquefied natural gas, the opening of the adjustable liquid outlet valve 17 is adjusted in real time through the PID liquid level controller to adjust the liquid outlet flow rate of the low-temperature separation tank 15, ensuring the stable output of liquefied natural gas.

[0073]

[0074] Among them, is the liquid level error, is the set liquid level (60% of the volume of the low-temperature separation tank 15), is the real-time measured liquid level, is the real-time opening of the adjustable liquid outlet valve 17, is the control parameter of the PID liquid level controller.

[0075] A high-precision second pressure sensor 18 (measurement accuracy is selected as 0.1% FS, that is, 0.1% of the full scale, and the range should cover the range of 1.0 MPa to 2.0 MPa) is arranged in the top gas phase area inside the low-temperature separation tank 15 to real-time monitor the pressure in the gas phase area of the low-temperature separation tank 15, and the set pressure range is 1.2 MPa to 1.5 MPa. An electronic regulating valve 19 is arranged on the gas outlet pipeline of the low-temperature separation tank 15. The signal output end of the second pressure sensor 18 is connected to the signal input end of the electronic regulating valve 19 through a second PID pressure controller. When the pressure approaches the upper limit, the emission control program is started, and the electronic regulating valve 19 is used to control the flow rate of the residual nitrogen. In the normal operation state, the valve remains slightly open to maintain a stable low-speed emission to prevent system instability caused by sudden pressure changes.

[0076] Specifically, if the pressure rises to 1.5 MPa, the valve automatically increases the opening to accelerate the emission rate of the residual nitrogen; if the pressure drops to 1.2 MPa, the valve reduces the opening to reduce the emission.

[0077] Residual nitrogen recovery guiding module: A gas flow direction distribution valve is arranged on the gas outlet pipeline of the low-temperature separation tank 15. Through the gas flow direction distribution valve, the destination of the residual nitrogen is controlled, and the following two recovery paths can be selected:

[0078] 1. Nitrogen recovery system: The gas outlet of the low-temperature separation tank 15 is connected to the inlet of the nitrogen recovery device through the first valve port of the gas flow distribution valve; if the residual nitrogen purity detected by the nitrogen concentration detection probe is relatively high (nitrogen content > 95%), it is introduced into the nitrogen recovery device for low-temperature cooling or reuse;

[0079] 2. Low-temperature expander: The gas outlet of the low-temperature separation tank 15 is connected to the inlet of the low-temperature expander through the second valve port of the gas flow distribution valve; if the gas detector detects that there is still some natural gas in the nitrogen, it can be first introduced into the low-temperature expander for energy recovery, and then discharged after the temperature is reduced to reduce cold energy loss.

Claims

1. An LNG plant BOG re-liquefaction recovery management system using liquid nitrogen refrigeration, characterized in that, Including: BOG collection module: including an adiabatic buffer tank (2) and a gas pressure stabilizing output device; It is used to receive the boil-off gas BOG generated by the storage tank (1), and stably output the pressurized BOG by adjusting the internal pressure of the adiabatic buffer tank (2); Multi-stage pressurization module: performs three-stage compression treatment on the pressurized BOG, and cools down the BOG after each stage of compression, and outputs the compressed BOG with a set pressure; The specific three-stage compression treatment includes: The first-stage compression unit uses a centrifugal compressor (5) to perform preliminary pressurization on the pressurized BOG, and outputs the first-stage compressed BOG with a pressure of 0.4 MPa to 0.6 MPa; The first-stage intercooler uses a finned tube heat exchanger (6) to cool the first-stage compressed BOG with a low-temperature coolant, and reduces its temperature to -50°C to -30°C; The second-stage compression unit uses a piston compressor (7) to perform secondary pressurization on the cooled first-stage compressed BOG, and outputs the second-stage compressed BOG with a pressure of 0.8 MPa to 1.0 MPa; The second-stage intercooler uses a spray heat exchanger (8) to cool down the second-stage compressed BOG with a low-temperature circulating coolant, and reduces its temperature to -80°C to -50°C; The third-stage compression unit uses a screw compressor (9) to perform final pressurization on the cooled second-stage compressed BOG, and outputs the third-stage compressed BOG with a pressure stably at 1.5 MPa to 2.0 MPa; Precooling control module: including an asymmetric fin heat exchanger (10) and an anti-ice blockage device, which is used to receive the compressed BOG and perform countercurrent heat exchange with the return gas of liquid nitrogen, and output the precooled BOG; The asymmetric fin heat exchanger (10) includes a high-pressure side channel and a low-pressure side channel; the high-pressure side channel adopts a dense fin structure, and the spacing of the dense fins is 0.5 mm to 2 mm, and the low-pressure side channel adopts a variable-spacing fin structure, and the variable spacing is 1 mm to 4 mm; The anti-ice blockage device includes an array of temperature sensors (11) and an electric bypass valve (12). The array of temperature sensors (11) is arranged inside the asymmetric fin heat exchanger (10), and the electric bypass valve (12) is arranged on the gas outlet pipeline of the asymmetric fin heat exchanger (10); the signal output end of the temperature sensor (11) is connected to the signal input end of the electric bypass valve (12) through a PID temperature controller; Liquid nitrogen injection refrigeration module: including a microporous injector (13) and a pipe-type vortex mixer (14), atomizes the liquid nitrogen and performs swirling mixing with the precooled BOG, and outputs a low-temperature two-phase flow; Phase separation and recovery module: including a low-temperature separation tank (15) and a liquid level control device, performs gas-liquid separation on the low-temperature two-phase flow, and outputs liquefied natural gas and residual nitrogen.

2. The LNG plant BOG re-liquefaction recovery management system using liquid nitrogen refrigeration according to claim 1, wherein: The adiabatic buffer tank (2) adopts a multi-layer vacuum adiabatic cavity structure; the gas pressure stabilizing output device includes a first pressure sensor (3) array and an electric control valve (4). The first pressure sensor (3) array is distributed at the inlet, inside the tank body, and the outlet of the adiabatic buffer tank (2), and the electric control valve (4) is arranged on the outlet pipeline of the adiabatic buffer tank (2); the signal output end of the first pressure sensor (3) is connected to the signal input end of the electric control valve (4) through a first PID pressure controller.

3. A BOG re-liquefaction recovery management system for an LNG plant using liquid nitrogen refrigeration according to claim 1, characterized in that: The first-stage compression unit, the second-stage compression unit, and the third-stage compression unit all adopt a multi-stage adiabatic compression model, and in each stage of compression, it satisfies: Among them, P n is the inlet pressure of the nth-stage compression, and P n ' is the outlet pressure of the nth-stage compression and also serves as the inlet pressure of the (n + 1)th-stage compression; T n is the inlet temperature of the nth-stage compression, and T n ' is the outlet temperature of the nth-stage compression and also serves as the inlet temperature of the (n + 1)th-stage compression; γ is the specific heat ratio of natural gas, and η c is the compressor efficiency, W is the compressor power, m is the mass flow rate of BOG, and C p is the specific heat capacity at constant pressure of BOG.

4. A BOG re-liquefaction recovery management system for an LNG plant and station using liquid nitrogen refrigeration according to claim 1, characterized in that: The micro-hole injector (13) adopts a porous distribution structure, and the diameter range of the injection holes is 50μm to 200μm, atomizing liquid nitrogen into fine liquid droplets to form atomized liquid nitrogen; The pipeline type vortex mixer (14) is composed of a mixing chamber, a flow guide device, and a nozzle. The mixing chamber is provided with a plurality of tangential air inlets, so that the atomized liquid nitrogen and the precooled BOG form a rotating flow, and mix and exchange heat to output a low-temperature two-phase flow.

5. The LNG plant BOG re-liquefaction recovery management system using liquid nitrogen refrigeration according to claim 1, characterized in that: The low-temperature separation tank (15) adopts a vertical double-chamber structure, including an inner chamber and an outer chamber. The inner chamber is provided with a gas-liquid separation baffle and a cyclone separator, receives the low-temperature two-phase flow, and performs gas-liquid separation through cyclone separation and gravity sedimentation.

6. The LNG plant BOG re-liquefaction recovery management system using liquid nitrogen refrigeration according to claim 1, wherein: The liquid level control device includes a capacitive liquid level controller (16) and an adjustable liquid outlet valve (17). The capacitive liquid level controller (16) is arranged inside the low-temperature separation tank (15), and the adjustable liquid outlet valve (17) is arranged on the liquid outlet pipeline of the low-temperature separation tank (15); the signal output end of the capacitive liquid level sensor (16) is connected to the signal input end of the adjustable liquid outlet valve (17) through a PID liquid level controller.

7. A BOG re-liquefaction recovery management system for an LNG plant and station using liquid nitrogen refrigeration according to claim 1, characterized in that: A second pressure sensor (18) is arranged in the gas phase area at the top inside the low-temperature separation tank (15) to monitor the pressure in the gas phase area of the low-temperature separation tank (15) in real time. An electronic control valve (19) is arranged on the outlet pipeline of the low-temperature separation tank (15); the signal output end of the second pressure sensor (18) is connected to the signal input end of the electronic control valve (19) through a second PID pressure controller.

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