LNG plant station BOG reliquefaction recovery management system using liquid nitrogen for refrigeration

Abstract

The invention discloses an LNG plant station BOG reliquefaction recovery management system using liquid nitrogen for refrigeration, and belongs to the technical field of BOG reliquefaction recovery. The system comprises: a BOG collection module for receiving boil-off gas BOG generated by a storage tank and stably outputting pressurized BOG; the multi-stage supercharging module is used for performing three-stage compression treatment on the supercharged BOG, cooling after each stage of compression, and outputting the compressed BOG with set pressure; the pre-cooling regulation and control module is used for receiving the compressed BOG, carrying out countercurrent flow heat exchange on the compressed BOG and liquid nitrogen return gas and outputting pre-cooled BOG; the liquid nitrogen injection refrigeration module is used for atomizing liquid nitrogen, then carrying out rotational flow mixing on the atomized liquid nitrogen and the pre-cooled BOG and outputting a low-temperature two-phase flow; the phase separation and recovery module is used for carrying out gas-liquid separation on the low-temperature two-phase flow and outputting liquefied natural gas and residual nitrogen; the BOG is re-liquefied and recycled through liquid nitrogen refrigeration, the compression efficiency is improved, and the BOG recycling device has the advantages of being low in energy consumption, high in heat exchange efficiency, high in recycling efficiency and high in safety.

Description

Technical Field

[0001] The present invention relates to the technical field of BOG reliquefaction and recovery, and in particular to a BOG reliquefaction and recovery management system for an LNG plant station utilizing liquid nitrogen refrigeration. Background Art

[0002] During the storage and transportation of liquefied natural gas (LNG), boil-off gas (BOG) is continuously generated due to environmental heat transfer and tank pressure fluctuations. If these BOG are not handled in time, the pressure inside the tank will increase, affecting the safety of LNG storage and possibly causing waste of resources. Therefore, how to efficiently recover and re-liquefy BOG, reduce energy loss, and improve LNG recovery rate is an important technical challenge facing LNG plants.

[0003] Existing BOG reliquefaction technologies mainly include mechanical compression condensation method, expander cooling method and mixed refrigeration cycle method, but these methods have the following problems: High energy consumption: The mechanical compression condensation method relies on a high-pressure compressor to pressurize and cool BOG, which consumes a lot of energy and has high system maintenance costs; Low heat exchange efficiency: The traditional heat exchange system uses ordinary fin heat exchangers, which have the risk of frost and blockage, resulting in reduced heat exchange performance; Unstable liquefaction rate: Some cryogenic expander systems have control response lag problems, making it difficult to adapt to BOG flow fluctuations, affecting the stable recovery of liquefied natural gas. Summary of the invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide an LNG plant BOG reliquefaction and recovery management system using liquid nitrogen refrigeration, which reliquefies and recovers BOG by using liquid nitrogen refrigeration, and has the characteristics of low energy consumption, high heat exchange efficiency, high recovery efficiency and high safety.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is: A BOG reliquefaction recovery management system for an LNG plant using liquid nitrogen refrigeration, comprising: BOG collection module: including an insulated buffer tank and a gas pressure-stabilizing output device; used to receive BOG generated by the storage tank and output pressurized BOG stably by adjusting the internal pressure of the insulated buffer tank; Multi-stage boost module: performs three-stage compression on the boosted BOG, cools down the BOG after each stage of compression, and outputs compressed BOG at the set pressure; Precooling control module: including an asymmetric fin heat exchanger and an anti-icing device, used to receive compressed BOG and perform countercurrent heat exchange with liquid nitrogen return gas to output precooled BOG; Liquid nitrogen jet refrigeration module: including micro-hole injector and pipeline vortex mixer, which atomizes liquid nitrogen and mixes it with pre-cooled BOG in a vortex flow to output low-temperature two-phase flow; Phase separation recovery module: includes a cryogenic separation tank and a liquid level control device, which performs gas-liquid separation on the cryogenic two-phase flow and outputs liquefied natural gas and residual nitrogen.

[0006] The insulated buffer tank adopts a multi-layer vacuum insulation cavity structure; the gas pressure stabilization output device includes a first pressure sensor array and an electric control valve, the first pressure sensor array is distributed at the air inlet, the inside of the tank body and the air outlet of the insulated buffer tank, and the electric control valve is arranged on the air outlet pipeline of the insulated 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.

[0007] The three-stage compression process specifically includes: The first stage compression unit uses a centrifugal compressor to preliminarily boost the pressure of the boosted BOG and output a first stage compressed BOG with a pressure of 0.4 MPa to 0.6 MPa; The first interstage cooler uses a finned tube heat exchanger to cool the first-stage compressed BOG using a low-temperature coolant to reduce its temperature to -50℃~-30℃; The second stage compression unit uses a piston compressor 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 interstage cooler uses a spray heat exchanger to cool the secondary compressed BOG using low-temperature circulating coolant to reduce its temperature to -80℃~-50℃; The third-stage compression unit uses a screw compressor to perform 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.

[0008] 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 process, the following conditions are met: in, For the Stage compression inlet pressure, For the The first stage compression outlet pressure is also used as the Stage compression inlet pressure; For the Stage compression inlet temperature, For the The compression outlet temperature of the first stage is also Stage compression inlet temperature; is the specific heat ratio of natural gas, is the compressor efficiency, is the compressor power, is the BOG mass flow rate, is the specific heat capacity of BOG at constant pressure.

[0009] The asymmetric fin heat exchanger comprises 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-pitch fin structure.

[0010] The anti-icing 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 pipe 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.

[0011] The micro-hole injector adopts a multi-hole distribution structure, and the diameter of the injection hole ranges from 50 μm to 200 μm, which atomizes the liquid nitrogen into fine droplets to form atomized liquid nitrogen; The pipeline vortex mixer is composed of a mixing chamber, a flow guide and a nozzle. The mixing chamber is provided with a plurality of tangential air inlets, so that the atomized liquid nitrogen and the pre-cooled BOG form a rotating flow, mix and exchange heat, and output a low-temperature two-phase flow.

[0012] The low-temperature separation tank adopts a vertical double-chamber structure, including an inner chamber and an outer chamber, wherein 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.

[0013] 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 the PID liquid level controller.

[0014] A second pressure sensor is provided in the top gas phase zone in the low-temperature separation tank to monitor the pressure of the gas phase zone in the low-temperature separation tank in real time. An electronic regulating valve is provided 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.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention reduces environmental heat conduction through a multi-layer vacuum insulation 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 fluctuation of the evaporated gas in the storage tank and avoids the influence of unstable pressure on the subsequent multi-stage boosting module, but also ensures the optimal input conditions of the multi-stage boosting module, improves compression efficiency and reduces energy consumption.

[0016] 2. The present invention adopts an asymmetric fin heat exchanger, in which the high-pressure side channel adopts a dense fin structure to improve the heat exchange efficiency, and the low-pressure side channel adopts a variable-pitch fin structure to optimize the airflow distribution and reduce the pressure loss, thereby improving the countercurrent heat exchange efficiency between BOG and liquid nitrogen return gas. In addition, the anti-icing device can automatically adjust the pipeline airflow when the risk of low-temperature frosting is detected, 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 pre-cooled BOG in the mixing chamber of the pipeline vortex mixer, outputting 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 improving the heat exchange rate, reducing energy consumption, and avoiding low-temperature blockage through an intelligent anti-icing device, thereby improving the long-term stability and reliability of the system.

[0017] 3. The present invention adopts a vertical double-chamber low-temperature separation tank to perform cyclone separation and gravity sedimentation, thereby improving the gas-liquid separation efficiency of the low-temperature two-phase flow and ensuring the purity of the liquefied natural gas. At the same time, a liquid level control device is adopted to monitor the liquefied natural gas level in real time and adjust the opening of the adjustable liquid outlet valve in real time to ensure the stable output of the liquefied natural gas and avoid the influence of liquid level fluctuation on the system operation.

[0018] In summary, the present invention uses liquid nitrogen refrigeration to reliquefy and recover BOG, thereby improving compression efficiency and having the characteristics of low energy consumption, high heat exchange efficiency, high recovery efficiency and high safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a module schematic diagram of the BOG reliquefaction recovery management system of an LNG plant using liquid nitrogen refrigeration according to the present invention.

[0020] Figure 2 It is a structural schematic diagram of the BOG reliquefaction recovery management system of an LNG plant station using liquid nitrogen refrigeration according to the present invention.

[0021] In the figure: 1-storage tank; 2-insulated buffer tank; 3-first pressure sensor; 4-electric control valve; 5-centrifugal compressor; 6-fin 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-microporous injector; 14-pipeline vortex mixer; 15-low-temperature separation tank; 16-capacitive liquid level sensor; 17-adjustable liquid outlet valve; 18-second pressure sensor; 19-electronic regulating valve. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is described in detail below in conjunction with the accompanying drawings.

[0023] like Figure 1 and Figure 2 As shown, a BOG reliquefaction recovery management system for an LNG plant station using liquid nitrogen refrigeration includes: BOG collection module: including an insulated 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 insulated buffer tank 2, and stably output the pressurized BOG; the insulated buffer tank 2 adopts a multi-layer vacuum insulation cavity structure to reduce environmental heat conduction, improve BOG storage stability, and buffer the flow fluctuation of the evaporated gas in the storage tank 1; 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, the inside of the tank body and the air outlet of the adiabatic buffer tank 2, and is used to monitor the BOG pressure change in real time, and the electric control valve 4 is arranged on the air 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 result of the first pressure sensor 3 array, the internal pressure of the adiabatic buffer tank 2 is adjusted to stabilize the output boosted BOG pressure within a preset range to match the input requirements of the subsequent multi-stage boosting module; 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 stabilize the output boost BOG pressure within the preset range of 0.2 MPa to 0.4 MPa. The specific calculation is as follows: in, The output pressure of the adiabatic buffer tank 2 needs to be stabilized at 0.2 MPa to 0.4 MPa; is the current pressure inside the adiabatic buffer tank 2, is the instantaneous BOG outflow mass flow rate, is the gas constant of natural gas, which is 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, calculated by the first PID pressure controller; is the BOG density, about 0.72 kg / m 3 ; is the BOG density under standard conditions, which is 0.717 kg / m 3 ; For the preset stable pressure, take 0.3MPa; is the pressure error, are the control parameters of the first PID pressure controller, which are proportional, integral and differential 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.

[0024] Multi-stage boost module: used to receive the boosted BOG and perform three-stage compression on it, cool down the BOG after each stage of compression, and output compressed BOG of set pressure; The three-stage compression process is to gradually increase the pressure of BOG and reduce its temperature through multi-stage pressurization and cooling cross-processing, and finally achieve effective reliquefaction and recovery of BOG. ​​Setting up a three-stage compression unit and using different types of compressors can improve compression efficiency and reduce temperature rise at different compression ratios, ensuring that the final BOG can be liquefied and recovered within the specified temperature and pressure range. Specifically including: The first stage compression unit uses a centrifugal compressor 5 to preliminarily pressurize the pressurized BOG, and outputs a first-stage compressed BOG with a pressure of 0.4 MPa to 0.6 MPa. This stage mainly plays a role of preliminary compression. BOG usually has a large gas volume. The centrifugal compressor 5 is suitable for large-flow gas processing, especially when the gas flow is large but the pressure requirement is relatively low. It can efficiently process a large amount of gas in a wide flow range, provide a stable compression effect, and is suitable for preliminary pressurization. The first interstage cooler uses a fin tube heat exchanger 6 to cool the first-stage compressed BOG using a low-temperature coolant to reduce its temperature to -50°C to -30°C, thereby reducing the temperature rise in the subsequent compression process; the low-temperature coolant uses liquid nitrogen (LN2); The second stage compression unit uses a piston compressor 7 to perform secondary supercharging on the cooled first stage compressed BOG, and outputs a second stage compressed BOG with a pressure of 0.8 MPa to 1.0 MPa. This stage mainly compresses the BOG to a medium to high pressure range. The piston compressor 7 used can generate a high output pressure in a relatively small volume, is suitable for medium to high pressure gas compression, and can accurately control the supercharging process of BOG. The second interstage cooler uses a spray heat exchanger 8 to cool the secondary compressed BOG to -80°C to -50°C through a low-temperature circulating coolant, thereby reducing the effect of gas expansion on compression efficiency; the low-temperature circulating coolant uses liquid chlorine (CCl4); The third-stage compression unit adopts a screw compressor 9 to perform final pressurization on the cooled second-stage compressed BOG, and the output pressure of the third-stage compressed BOG is stabilized at 1.5 MPa to 2.0 MPa, meeting the pressure requirement of liquefaction recovery; the adopted screw compressor 9 can reduce excessive gas temperature rise during the compression process, which is more important for the subsequent liquefaction process, reduces heat accumulation, maintains a low gas temperature, and provides a stable high-pressure output under high pressure and small gas flow.

[0025] 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 process, the following conditions are met: in, For the Stage compression inlet pressure, For the The first stage compression outlet pressure is also used as the Stage compression inlet pressure; For the Stage compression inlet temperature, For the The compression outlet temperature of the first stage is also Stage compression inlet temperature; is the specific heat ratio of natural gas, which is taken as 1.3; is the compressor efficiency, which is taken as 0.85; is the compressor power, is the BOG mass flow rate, is the specific heat capacity of BOG at constant pressure, which is 2.3 kJ / (kg·K).

[0026] Precooling control module: including an asymmetric fin heat exchanger 10 and an anti-icing device, used to receive compressed BOG and perform countercurrent heat exchange with liquid nitrogen return gas to output 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 to increase the heat exchange area; the low-pressure side channel adopts a variable-pitch fin structure to optimize the airflow distribution; the three-stage compressed BOG enters the high-pressure side channel of the asymmetric fin heat exchanger 10, and the liquid nitrogen return gas enters the low-pressure side channel, and the two perform countercurrent heat exchange; due to the design of the asymmetric fins, the heat exchange area on the high-pressure side is larger, which improves the heat transfer efficiency, while the low-pressure side optimizes the airflow distribution and reduces the pressure loss. Specifically: 1. The high-pressure side channel adopts a dense fin structure: The air flow velocity on the high-pressure side is higher and the heat exchange demand is greater. 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, depending on the flow velocity and heat exchange demand of the high-pressure air flow. By increasing the fin density, the heat exchange efficiency is improved, which helps the BOG after the three-stage compression at a higher compression ratio to more effectively exchange heat with the cryogenic liquid nitrogen return gas.

[0027] 2. The low-pressure side channel adopts a variable-pitch fin structure: The airflow velocity on the low-pressure side is lower, and the heat exchange efficiency is more sensitive to the airflow distribution. Therefore, a variable-pitch fin structure is adopted to optimize the airflow distribution, reduce dead zones and flow resistance, and ensure uniform heat exchange. The fin spacing can be adjusted according to the airflow conditions to optimize the airflow distribution and heat exchange efficiency. The spacing can be changed by adjusting the mechanical structure or the fluid pressure difference of the 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 moderately increased to avoid excessive pressure drop and improve the smoothness of the airflow. The variable spacing design generally adopts an adjustment range of 1 mm to 4 mm, depending on the airflow conditions. By adjusting the spacing, the airflow on the low-pressure side is ensured to be evenly distributed, and excessive airflow is avoided from being concentrated in a certain position, resulting in uneven heat exchange.

[0028] The anti-icing and blocking 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 outlet pipe 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, so as 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 is not frozen and blocked. The calculation of the opening of the electric bypass valve 12 is based on the PID temperature controller, and the calculation formula is: in, is the heat transfer 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 tertiary compressed BOG entering the asymmetric fin heat exchanger 10, is the return gas temperature of liquid nitrogen discharged from the asymmetric fin heat exchanger 10.

[0029] Finally, the pre-cooling BOG temperature is stabilized in the range of -120°C to -100°C.

[0030] Liquid nitrogen injection refrigeration module: includes a microporous injector 13 and a pipeline vortex mixer 14; the outlet of the microporous injector 13 is connected to the inlet of the pipeline vortex mixer 14 through a pipeline, and the liquid nitrogen is atomized and mixed with the pre-cooled BOG by swirl flow to output a low-temperature two-phase flow; The micro-hole injector 13 adopts a multi-hole distribution structure, and the diameter of the injection hole ranges from 50 μm to 200 μm, which atomizes the liquid nitrogen (LN2) into fine droplets to form atomized liquid nitrogen; The pipeline vortex mixer 14 is composed of a mixing chamber, a guide 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 airflow, so that the atomized liquid nitrogen and pre-cooled BOG airflow rotate along the wall of the chamber; the guide is installed inside the mixing chamber, which can effectively control the flow rate, improve the flow stability and mixing efficiency; the atomized liquid nitrogen and pre-cooled BOG enter the mixing chamber through the nozzle, and the atomized liquid nitrogen and pre-cooled BOG form a rotating flow in the mixing chamber under the action of the tangential air inlet and the guide. The rotating flow increases the contact area between the atomized liquid nitrogen and the pre-cooled BOG, and promotes the full mixing of the two. The liquid nitrogen atomized droplets evaporate quickly, thereby absorbing the heat of the BOG, reducing the temperature of the BOG, and the fluid output temperature after mixing and heat exchange is a low-temperature two-phase flow of less than -150°C. In this process, the mixing and temperature change of the atomized liquid nitrogen and the pre-cooled BOG realizes rapid and uniform heat exchange.

[0031] Phase separation recovery module: including a low-temperature separation tank 15 and a liquid level control device, used to receive the low-temperature two-phase flow, and perform gas-liquid separation through cyclone separation and gravity sedimentation, and output liquefied natural gas (LNG) and residual nitrogen (N2); The low-temperature separation tank 15 adopts a vertical double-chamber structure, including an inner chamber and an outer chamber. The two chambers are connected by a partition or an airflow guiding pipe. The inner chamber is used to receive the low-temperature two-phase flow, and the outer chamber is used 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.

[0032] The upper or middle part of the inner cavity of the low-temperature separation tank 15 is provided with an inclined and fixed gas-liquid separation baffle, and the surface of the gas-liquid separation baffle is processed with a plurality of holes, which slow down the gas-liquid flow rate through physical barrier effect, and prevent the gas from flowing out with liquid droplets during the rising process, and the gas-liquid separation baffle effectively improves the separation effect. A cyclone separator with a cylindrical cone structure is provided at the lower part of the inner cavity of the low-temperature separation tank 15, and the low-temperature two-phase flow enters the low-temperature separation tank 15 through the inlet pipe. When the low-temperature two-phase flow passes through the cyclone separator, it is accelerated to separate under the action of centrifugal force to form two-phase stratification, and part of the liquid sinks along the tank wall due to inertia and gravity, and is concentrated at the bottom of the tank body, and is discharged through the liquid outlet pipe at the bottom of the tank body, and the gas flows upward along the center of the cyclone separator, and when passing through the gas-liquid separation baffle, the gas is blocked with liquid droplets during the rising process, and the gas is concentrated at the top of the tank body through the gas-liquid separation baffle, and is discharged through the gas outlet pipe at the top of the tank body.

[0033] 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 cryogenic separation tank 15 to monitor the change of the liquefied natural gas liquid level in the cryogenic separation tank 15 in real time; the adjustable liquid outlet valve 17 is arranged on the liquid outlet pipeline of the cryogenic 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 liquefied natural gas liquid level, 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 of the cryogenic separation tank 15 to ensure the stable output of liquefied natural gas; in, is the liquid level error, To set the liquid level (60% of the volume of the cryogenic separation tank 15), For real-time measurement of liquid level, The real-time opening of the liquid outlet valve 17 can be adjusted. is the control parameter of the PID liquid level controller.

[0034] A high-precision second pressure sensor 18 is provided in the top gas phase region of the low-temperature separation tank 15 (the measurement accuracy is selected to be 0.1% FS, i.e., 0.1% of the full scale, and the range should cover the range of 1.0 MPa to 2.0 MPa) to monitor the pressure of the gas phase region in the low-temperature separation tank 15 in real time, and the pressure range is set to 1.2 MPa to 1.5 MPa; an electronic regulating valve 19 is provided on the gas outlet pipeline of the low-temperature separation tank 15, and 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 residual nitrogen. Under normal operating conditions, the valve remains slightly open to maintain stable low-speed emissions to prevent system instability caused by sudden pressure changes.

[0035] Specifically, if the pressure rises to 1.5 MPa, the valve will automatically increase its opening to speed up the residual nitrogen emission rate; if the pressure drops to 1.2 MPa, the valve will reduce its opening to reduce emissions.

[0036] Residual nitrogen recovery guide module: A gas flow distribution valve is provided on the outlet pipe of the cryogenic separation tank 15. The gas flow distribution valve is used to control the direction of the residual nitrogen. The following two recovery paths can be selected: 1. Nitrogen recovery system: The gas outlet of the cryogenic separation tank 15 is connected to the gas inlet of the nitrogen recovery device through the first valve port of the gas flow distribution valve; if the nitrogen concentration detection probe detects that the residual nitrogen purity is high (nitrogen content>95%), it is introduced into the nitrogen recovery device for low-temperature cooling or reuse; 2. Cryogenic expander: The gas outlet of the cryogenic separation tank 15 is connected to the gas inlet of the cryogenic expander through the second valve port of the gas flow distribution valve; if the gas detector detects that the nitrogen still contains some natural gas, it can be first introduced into the cryogenic expander for energy recovery, and then discharged after lowering the temperature to reduce cold energy loss.

Claims

1. A BOG reliquefaction recovery management system for LNG plants using liquid nitrogen refrigeration, characterized in that: include: BOG collection module: comprising 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 to output the pressurized BOG stably by adjusting the internal pressure of the adiabatic buffer tank (2); Multi-stage boost module: performs three-stage compression on the boosted BOG, cools the BOG after each stage of compression, and outputs compressed BOG at a set pressure; Precooling control module: comprising an asymmetric fin heat exchanger (10) and an anti-icing device, used for receiving compressed BOG and performing countercurrent heat exchange with liquid nitrogen return gas to output precooled BOG; Liquid nitrogen jet refrigeration module: comprising a microporous injector (13) and a pipeline vortex mixer (14), atomizing liquid nitrogen and performing vortex mixing with pre-cooled BOG to output a low-temperature two-phase flow; Phase separation recovery module: comprising a low temperature separation tank (15) and a liquid level control device, for performing gas-liquid separation on the low temperature two-phase flow, and outputting liquefied natural gas and residual nitrogen.

2. The LNG plant BOG reliquefaction recovery management system using liquid nitrogen refrigeration according to claim 1, characterized in that: The thermal insulation buffer tank (2) adopts a multi-layer vacuum insulation cavity structure; the gas pressure stabilization output device comprises a first pressure sensor (3) array and an electric control valve (4); the first pressure sensor (3) array is distributed at the air inlet, the inside of the tank body and the air outlet of the thermal insulation buffer tank (2); the electric control valve (4) is arranged on the air outlet pipeline of the thermal insulation 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. The LNG plant BOG reliquefaction recovery management system using liquid nitrogen refrigeration according to claim 1 is characterized in that: The three-stage compression process specifically includes: The first-stage compression unit uses a centrifugal compressor (5) to preliminarily pressurize the pressurized BOG and output a first-stage compressed BOG with a pressure of 0.4 MPa to 0.6 MPa; The first interstage cooler adopts a fin tube heat exchanger (6) to cool the first stage compressed BOG using a low temperature coolant to reduce its temperature to -50°C to -30°C; The second-stage compression unit adopts a piston compressor (7) to perform secondary pressurization on the cooled first-stage compressed BOG, and outputs a second-stage compressed BOG with a pressure of 0.8 MPa to 1.0 MPa; The second interstage cooler adopts a spray heat exchanger (8) to cool the secondary compressed BOG using low-temperature circulating coolant to reduce 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 stable pressure of 1.5 MPa to 2.0 MPa.

4. The LNG plant BOG reliquefaction recovery management system using liquid nitrogen refrigeration according to claim 3 is 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 process, the following conditions are met: in, For the Stage compression inlet pressure, For the The first stage compression outlet pressure is also used as the Stage compression inlet pressure; For the Stage compression inlet temperature, For the The compression outlet temperature of the first stage is also Stage compression inlet temperature; is the specific heat ratio of natural gas, is the compressor efficiency, is the compressor power, is the BOG mass flow rate, is the constant pressure specific heat capacity of BOG.

5. The LNG plant BOG reliquefaction recovery management system using liquid nitrogen refrigeration according to claim 1, characterized in that: The asymmetric fin heat exchanger (10) comprises 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-pitch fin structure.

6. The LNG plant BOG reliquefaction recovery management system using liquid nitrogen refrigeration according to claim 1, characterized in that: The anti-icing and blocking device comprises a temperature sensor (11) array and an electric bypass valve (12); the temperature sensor (11) array is arranged inside the asymmetric fin heat exchanger (10); and the electric bypass valve (12) is arranged on an air outlet pipeline of the asymmetric fin heat exchanger (10); a signal output end of the temperature sensor (11) is connected to a signal input end of the electric bypass valve (12) via a PID temperature controller.

7. The LNG plant BOG reliquefaction recovery management system using liquid nitrogen refrigeration according to claim 1, characterized in that: The micro-pore injector (13) adopts a multi-pore distribution structure, and the diameter of the injection hole ranges from 50 μm to 200 μm, and atomizes the liquid nitrogen into fine droplets to form atomized liquid nitrogen; The pipeline vortex mixer (14) is composed of a mixing chamber, a flow guide and a nozzle. The mixing chamber is provided with a plurality of tangential air inlets, so that the atomized liquid nitrogen and the pre-cooled BOG form a rotating flow, mix and exchange heat, and output a low-temperature two-phase flow.

8. The LNG plant BOG reliquefaction 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, comprising an inner chamber and an outer chamber, wherein 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.

9. The LNG plant BOG reliquefaction recovery management system using liquid nitrogen refrigeration according to claim 1, characterized in that: The liquid level control device comprises 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); and 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) via the PID liquid level controller.

10. The LNG plant BOG reliquefaction recovery management system using liquid nitrogen refrigeration according to claim 1, characterized in that: A second pressure sensor (18) is provided in the top gas phase region of the low-temperature separation tank (15) to monitor the pressure of the gas phase region in the low-temperature separation tank (15) in real time. An electronic regulating valve (19) is provided on the gas outlet pipeline of the low-temperature separation tank (15); a signal output end of the second pressure sensor (18) is connected to a signal input end of the electronic regulating valve (19) via a second PID pressure controller.

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