BOG reliquefaction recovery processing method and system

Abstract

The invention discloses a BOG reliquefaction recovery processing method and system, and the BOG reliquefaction recovery processing method comprises the following steps: when the water dew point of BOG raw material gas processed by a raw material gas drying device is smaller than a first preset value, the BOG raw material gas enters a liquefaction cold box device; adjusting a liquefaction cold box device, a refrigerant matching device and a refrigerant compression device, and circularly providing different cold quantities for the multi-level cold sections of the liquefaction cold box through multi-level compression of the refrigerant compression device and multi-level expansion cooling coupling of a corresponding expansion mechanism until the temperature of a backflow refrigerant reaches a first preset value; the treated BOG raw material gas is subjected to multi-level cooling by a liquefaction cold box device to form LNG (Liquefied Natural Gas); and the output quantity of the refrigerant compression device is adjusted to the maximum value, the refrigerant adding quantity is increased, the opening degree of an HV-1 throttling valve is increased, liquid in the denitrification flash evaporation device is controlled to be within a fifth threshold value range, and LNG is subjected to flash evaporation separation through the denitrification flash evaporation device and recycled into the LNG storage tank. The system and the method have the beneficial effects that BOG feed gas is liquefied to form LNG, and rapid recovery of BOG is realized.

Description

Technical Field

[0001] This invention relates to the field of liquefied natural gas technology, and more particularly to a method and system for BOG reliquefaction and recovery. Background Technology

[0002] Currently, BOG (Bottle-Off Gas) is inevitably generated during the operation of liquefied natural gas (LNG) receiving terminals. BOG refers to the gas produced by LNG during storage and transportation due to heat intrusion or pressure changes, and there are some problems in its handling.

[0003] Existing methods typically involve direct emissions, flaring, or recovery via reliquefaction systems. While these methods address instantaneous pressure issues, they directly lead to energy waste and greenhouse gas emissions. On one hand, the systems suffer from low energy efficiency. Traditional reliquefaction processes rely on complex and energy-intensive multi-stage compression and cryogenic refrigeration cycles. Under dynamically fluctuating BOG loads, especially at low loads, the system consumes a large amount of energy and is inefficient, unable to achieve stepless adjustment of cooling capacity, resulting in a precipitous drop in operating efficiency.

[0004] On the other hand, the complexity of the equipment and operation brings high costs and potential risks. The system needs to integrate a series of large equipment such as compressors, refrigeration units, and heat exchangers, resulting in huge initial investment and land area requirements, as well as considerable daily maintenance costs. More importantly, the system operates unstablely and is difficult to precisely match in the face of BOG gas volume pulsations. This not only reduces efficiency but also introduces operational safety hazards such as liquid slugging, affecting the long-term stability and reliability of the unit.

[0005] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0006] The technical problem to be solved by this invention is the low efficiency of BOG evaporation gas reliquefaction.

[0007] The present invention solves the above-mentioned technical problems through the following technical means:

[0008] This invention claims a method for BOG reliquefaction and recycling, comprising the following steps: When the dew point of the BOG feed gas after processing by the feed gas drying device is less than the first predetermined value, the BOG feed gas enters the liquefaction cold box device. Adjust the liquefied cold box device, refrigerant matching device and refrigerant compression device, and through the multi-stage compression of the refrigerant compression device and the multi-stage expansion and cooling coupling of the corresponding expansion mechanism, cyclically provide different cooling capacities to the multi-stage cold sections of the liquefied cold box until the temperature of the reflux refrigerant reaches the first predetermined value; The processed BOG feedstock gas is cooled in multiple stages by a liquefaction cold box unit to form LNG; When the temperature of the reflux refrigerant reaches the first predetermined value, set the pressure of the PV-1 pressure regulating valve to the second predetermined value, adjust the HV-1 throttle valve, and vent the denitrification flash evaporator. When the inlet temperature of the HV-1 pressure regulating valve reaches the third predetermined value, the ES-1 emergency shut-off valve is opened, and the opening of the HV-1 throttle valve is adjusted so that the precooling rate of the pipeline between the liquefied cold box unit and the LNG storage tank is within the fourth threshold range. When the temperature after the HV-1 pressure regulating valve is less than the third predetermined value, adjust the HV-1 throttle valve to the target opening. Adjust the output of the refrigerant compression unit to the maximum value, increase the amount of refrigerant added, increase the opening of the HV-1 throttle valve, control the liquid in the denitrification flash evaporation unit to be within the fifth threshold range, and the denitrification flash evaporation unit flashes and separates the LNG, which is then returned to the LNG storage tank.

[0009] BOG feedstock gas is dried by a feedstock gas drying unit, and after confirming that the water dew point meets the standard, it enters the liquefaction cold box unit. Through multi-stage compression and corresponding multi-stage expansion and cooling coupling of the expansion mechanism, the BOG feedstock gas is liquefied into LNG. LNG is then separated by a denitrification flash evaporation unit, realizing rapid recovery of BOG, avoiding direct emission of methane, and contributing to air safety.

[0010] Furthermore, the processed raw gas is cooled in stages, so that the cooling temperature provided by the refrigerant can closely follow the actual needs of the BOG raw gas to gradually cool down; this reduces the ineffective energy dissipation caused by excessive temperature difference, thereby improving the energy utilization efficiency of the liquefied gas cooling box unit.

[0011] Preferably, the liquefied cold box device and the refrigerant compression device are interconnected to form a loop, including the following steps: The reflux refrigerant is compressed in multiple stages by a refrigerant compression device to separate the gas phase component and the liquid phase component; The liquid component flows through the liquid flow branch into the liquefaction cold box device, where it expands and cools down through the first expansion mechanism to form a gas-liquid mixture, providing cooling capacity for the shallow cooling section. The gaseous components flow through the gaseous flow branch into the liquefied cold box device, where they expand and cool down through the second expansion mechanism to form an ultra-low temperature refrigerant, which provides cooling capacity for the cryogenic section. The ultra-low temperature fluid and gas-liquid mixture that have undergone heat exchange merge through the mixing branch to provide cooling capacity to the shallow cooling section; After mixing, the mixture undergoes heat exchange to form a reflux refrigerant, which is then fed into a refrigerant compression device to form a cycle.

[0012] The refrigerant circulation process, in conjunction with the refrigerant compression unit, sequentially compresses the return refrigerant from the liquefied gas tank through a low-pressure stage mixed refrigerant compressor unit and a high-pressure stage mixed refrigerant compressor unit. This separates the gaseous and liquid phase components with different boiling points. After throttling, the liquid phase component is used for cooling in the shallow cooling section, while the gaseous component is used for cooling in the deep cooling section. At each stage of the cooling curve, the temperature of the fluid providing the cold source is very close to the temperature of the natural gas, greatly reducing energy loss caused by excessive temperature differences and improving energy utilization efficiency. After heat exchange, the liquid and gaseous components are further mixed and returned to the refrigerant compression unit, forming a cycle to ensure sustainable operation. Ultimately, through the above multi-stage compression and each stage of expansion and cooling coupling process, the reliquefaction energy is reduced on the basis of efficient liquefaction of feed gas into LNG.

[0013] Preferably, multi-level compression specifically includes the following steps: The refrigerant returning from the liquefied cold box unit via pipes 5-6 passes through the suction filter and enters the low-pressure stage mixed refrigerant compressor unit for pressurization and cooling. It then enters the high-pressure stage mixed refrigerant compressor unit for further pressurization and cooling to form a mixed refrigerant. The mixed refrigerant is then separated into gas phase and liquid phase components by the outlet gas-liquid separator.

[0014] The refrigerant compression unit is a screw-type mixed refrigerant compressor.

[0015] Preferably, providing cooling capacity to the shallow cooling section includes the following steps: The liquid phase component enters the liquefaction cold box device from the top through pipes 4-4 and 5-5 in sequence. After being pre-cooled in the shallow cooling section, it is throttled and cooled by the second throttle valve to form a gas-liquid mixture, which then enters the medium-temperature refrigerant separator.

[0016] Preferably, providing cooling capacity to the cryogenic section specifically includes the following steps: The gaseous components enter the liquefaction cold box from the top through pipes 4-3 and 5-4 for pre-cooling. After being pre-cooled in the shallow cooling section, the gaseous components enter the deep cooling section for gradual cooling. After being throttled and cooled by the first throttling valve, they enter the cryogenic refrigerant separator to form an ultra-low temperature fluid.

[0017] The gaseous and liquid phases with different boiling points are separated and throttled separately. The liquid phase is cooled in the shallow cooling section, while the gaseous phase is cooled in the deep cooling section. At each stage of the cooling curve, the temperature of the fluid providing the cold source is very close to the temperature of the natural gas, which greatly reduces energy loss caused by excessive temperature difference and improves energy utilization efficiency.

[0018] This invention claims protection for a BOG reliquefaction and recovery system applied to the BOG reliquefaction and recovery treatment method, including a BOG reliquefaction and recovery system comprising a feed gas drying unit, a liquefaction cold box unit, a denitrification flash evaporation unit, and an LNG storage tank connected in sequence by pipelines; an HV-1 throttling valve is installed on the pipeline between the liquefaction cold box unit and the denitrification flash evaporation unit; a PV-1 pressure regulating valve is installed on the pipeline between the top of the denitrification flash evaporation unit and the liquefaction cold box unit; and an ES-1 emergency shut-off valve is installed on the pipeline between the denitrification flash evaporation unit and the LNG storage tank. It also includes a refrigerant compression device and a refrigerant matching device. The liquefaction cold box device and the refrigerant compression device are connected to each other to form a cycle. The refrigerant matching device is connected between the liquefaction cold box device and the refrigerant compression device. Multiple cold sections are formed sequentially in the liquefaction cold box device. Each cold section is equipped with a corresponding expansion mechanism. Through the multi-level compression of the refrigerant compression device and the multi-level expansion and cooling coupling of the expansion mechanism, different cooling capacities are provided to the multi-level cold sections in a cycle.

[0019] Preferably, the liquefied cold box device is a liquefied cold box, which uses a multi-flow plate heat exchanger to divide the interior of the liquefied cold box into an upper shallow cooling section and a lower deep cooling section; the shallow cooling section corresponds to the first expansion mechanism, and the deep cooling section corresponds to the second expansion mechanism. The refrigerant compression device is connected to the first expansion mechanism through a liquid phase flow branch; the deep cooling section is connected to the second expansion mechanism through a gas phase flow branch, and the second expansion mechanism and the first expansion mechanism are connected and merged through a mixing branch. The mixing branch is connected to the refrigerant compression device to form a cycle.

[0020] Preferably, the multi-stage compression specifically includes a low-pressure stage mixed refrigerant compressor unit, a high-pressure stage mixed refrigerant compressor unit, and a gas-liquid separator connected in sequence.

[0021] Preferably, the first expansion mechanism includes a second throttle valve and a medium-temperature refrigerant separator. The gas-liquid separator enters the liquefied cold box device through pipes 4-4 and 5-5, passes through the shallow cooling section, and is connected to the second throttle valve and the medium-temperature refrigerant separator in sequence.

[0022] Preferably, the second expansion mechanism includes a first throttle valve and a cryogenic refrigerant separator. The gas-liquid separator enters the liquefied cold box from the top through pipes 4-3 and 5-4, passing through the shallow cooling section to the deep cooling section, and is connected to the first throttle valve and the cryogenic refrigerant separator in sequence.

[0023] The advantages of this invention are: by using multi-stage compression and each stage of expansion and cooling coupled processes, the energy consumption for reliquefaction is reduced on the basis of efficient liquefaction of raw gas to form LNG. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the BOG reliquefaction and recycling system in Embodiment 1 of the present invention; Figure 2This is a schematic diagram of the hot blowing process of the raw material gas drying device in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the cold blowing process of the raw material gas drying device in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the liquefaction cold box device in Embodiment 1 of the present invention; 1. Raw material gas drying unit; 10. Main pipeline; 11. Regeneration gas pipeline; 12. Adsorption dehydration branch pipeline; 120. First drying tower; 121. Dust filter; 13. Hot blowing regeneration branch pipeline; 130. Regeneration gas heater; 131. Second drying tower; 132. Regeneration gas separator; 133. Regeneration gas cooler; 14. Online analyzer; 2. Liquefaction cold box unit; a. Shallow cooling section; b. Deep cooling section; 20. Natural gas liquefaction branch; 21. Second throttle valve; 22. Medium-temperature refrigerant separator; 23. First throttle valve; 24. Low-temperature refrigerant separator; 25. Mixing branch; 26. Recirculation branch; 3. Refrigerant matching device; 4. Refrigerant compression unit; 40. Low-pressure stage mixed refrigerant compressor unit; 41. High-pressure stage mixed refrigerant compressor unit; 5. Denitrification flash evaporation unit; 6. LNG storage tank. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0026] Example 1 See Figure 1 This embodiment requires protection of a BOG reliquefaction and recovery system for efficiently reliquefying and recovering BOG evaporated during storage and transportation due to heat intrusion or pressure changes through a multi-stage compression and cryogenic expansion coupling process. The system includes a feed gas drying unit 1, a liquefaction cold box unit 2, a refrigerant matching unit 3, a refrigerant compression unit 4, a denitrification flash evaporation unit 5, an LNG storage tank 6, and an air-cooling unit (not shown in the figure). The main bodies of all the above units are skid-mounted, and the modular design reduces the system's volume, making it suitable for space-constrained LNG carriers or bunkering stations.

[0027] The raw material gas drying unit 1 is used to filter and dehydrate the high-pressure BOG raw material gas from outside the boundary. Specifically, 4A molecular sieve is used as the adsorbent, and two drying towers are set up. The high-pressure BOG raw material gas is dehydrated through an isobaric temperature variable process. The two drying towers alternately perform adsorption and regeneration. The regeneration process includes two stages: heating and cooling. The cycle is preferably 8 hours for adsorption and 8 hours for regeneration, including 4.5 hours of hot blowing and 3.5 hours of cold blowing.

[0028] See Figure 2 and Figure 3 The high-pressure BOG feed gas is divided into two paths by the flow regulating valve FV-1: the main path 10 and the regeneration gas path 11. Main channel 10, accounting for approximately 85%–90% of the total flow, after depressurization, merges with the regenerated gas and enters the drying tower in an adsorption state for dehydration.

[0029] Regeneration gas path 11, accounting for approximately 10%–15% of the total flow, is used for hot and cold blowing of the tower in the regeneration state.

[0030] Two drying towers alternately perform adsorption and regeneration for dehydration treatment, representing an existing technology in the application of pressure swing / temperature swing adsorption (PSA) in industrial gas purification and drying. Therefore, this explanation will focus on the first drying tower 120 for adsorption and the second drying tower 131 for regeneration, without further elaboration.

[0031] The adsorption-dehydration branch 12 is used for filtering and pre-treating the high-pressure BOG feed gas. Specifically, after the feed gas and regenerated gas from the main line 10 are combined, they enter the first drying tower 120 from the top through the XV-1 remote control valve. Moisture is adsorbed by the molecular sieve, and the dried gas flows out from the bottom of the first drying tower 120. It then passes through the dust filter 121 to remove dust. The dust filter 121 is preferably TS-X-10343 with a filtration accuracy of 1μm, and is then sent to the liquefaction cold box device 2.

[0032] The hot-blowing regeneration branch 13 is used for dehydration of the first drying tower 120. Specifically, the gas from the regeneration gas path 11 first passes through the operating regeneration gas heater 130 and is electrically heated to above 220°C. The high-temperature regeneration gas then enters the second drying tower 131 from the bottom, heating the adsorbent bed in a counter-current manner to decompose and remove the adsorbed water. The regeneration gas carrying a large amount of water vapor flows out from the top of the second drying tower 131 and enters the regeneration gas cooler 133 to be cooled to approximately 40°C. Subsequently, it enters the regeneration gas separator 132, where the condensate is separated and discharged. The separated gas, still containing trace amounts of moisture, is incorporated into the main path 10 and enters the first drying tower 120, which is still in the adsorption state, for final dehydration.

[0033] The cold-blowing regeneration branch is used to cool the second drying tower 131 to near room temperature; specifically, the electric heater is turned off. The gas in the regeneration gas path 11 is redirected to enter from the top of the second drying tower 131. The room-temperature regeneration gas flows down through the heated bed of the second drying tower 131, cooling it to near room temperature; the gas heated by the bed flows out from the bottom of the tower, still passing through the pipes of the heater that is no longer heated, and then enters the regeneration gas cooler 133 and the regeneration gas separator 132 for processing, and finally returns to the main path 10 to enter the first drying tower 120.

[0034] The raw gas outlet of the raw gas drying unit 1 is connected to the liquefied gas cooling box unit 2 through pipe 5-1. An online analyzer 14 with water dew point analysis function is installed on pipe 5-1, and a dynamic sampling port is set at the skid inlet of the raw gas drying unit 1 for detecting the water dew point of the BOG raw gas using a portable water dew point meter.

[0035] The preferred liquefied gas cold box unit 2 has a processing capacity of one 20×10⁴ Nm³ / d liquefied gas cold box. A multi-flow plate heat exchanger divides the liquefied gas cold box into an upper shallow cooling section a and a lower deep cooling section b. The liquefied gas cold box's operating process consists of a natural gas liquefaction process and a refrigerant circulation process.

[0036] The natural gas liquefaction process, via natural gas liquefaction branch 20, liquefies the processed feed gas into LNG at -162°C. Specifically, after processing by feed gas drying unit 1, the processed feed gas at 6.08 MPaG and 40°C enters the liquefaction cold box via pipeline 5-1. In the shallow cooling section a, the processed feed gas is pre-cooled to -50°C by a low-temperature reflux mixed refrigerant. The pre-cooled natural gas then enters the deep cooling section b, where it is further cooled to -162°C by an ultra-low temperature reflux refrigerant, at which point the natural gas liquefies to form LNG at -162°C.

[0037] See Figure 4 The refrigerant circulation process, in conjunction with the refrigerant matching device 3 and the refrigerant compression device 4, provides different cooling capacities for the shallow cooling section a and the deep cooling section b. Specifically, the refrigerant compression device 4 is a screw-type mixed refrigerant compressor, which includes a low-pressure stage mixed refrigerant compressor unit 40 and a high-pressure stage mixed refrigerant compressor unit 41. The return refrigerant, at 20.315 MPaG and 36°C, from the liquefied cold box device via pipes 5-6, enters the low-pressure stage mixed refrigerant compressor unit 40 through the suction filter (not shown in the figure), is pressurized to 1.6 MPaG and cooled, and then enters the high-pressure stage mixed refrigerant compressor unit 41 for further pressurization to 3.1 MPaG and cooling to approximately 4000°C, forming a mixed refrigerant. The mixed refrigerant is then separated into gas phase components and liquid phase components by the outlet gas-liquid separator (not shown in the figure).

[0038] In the liquid phase flow branch, the liquid phase component sequentially passes through pipes 4-4 and 5-5 and enters the liquefaction cold box from the top for pre-cooling, with the cooling capacity coming from the throttled return refrigerant. The liquid phase component is pre-cooled to -50°C in the shallow cooling section a, then throttled to 0.335 MPaG by the second throttle valve 21 and cooled to -54.40°C, forming a gas-liquid mixture. This gas-liquid mixture enters the medium-temperature refrigerant separator 22, providing the main cooling source for the shallow cooling section a.

[0039] In the gas phase flow branch, the gas phase components sequentially pass through pipes 4-3 and 5-4 from the top into the liquefied cold box for pre-cooling, with the cooling capacity coming from the throttled refrigerant. The gas phase components are pre-cooled to -50°C in the shallow cooling section a, and then enter the deep cooling section b, where they are gradually cooled to -162°C. The -162°C gas phase refrigerant passes through the first throttling valve 23, generating a cryogenic fluid of 0.355 MPaG and -166.2°C, which enters the cryogenic refrigerant separator 24. The cryogenic refrigerant flows back in the deep cooling section b, serving as the main cold source for the deep cooling section b.

[0040] In the mixing branch 25, after the ultra-low temperature refrigerant completes the heat exchange in the cryogenic section b, its temperature rises to about -54°C. It then flows out of the cryogenic section b and merges with the gas-liquid mixture in the medium-temperature refrigerant separator 22, and together they enter the shallow cryogenic section a as a cold source.

[0041] The reflux branch 26, after merging, forms a reflux refrigerant at 0.315 MPaG and 36°C. It enters the low-pressure stage mixed refrigerant compressor unit 40 through the suction filter via pipe 5-6, is pressurized to 1.6 MPaG and cooled, and then enters the high-pressure stage mixed refrigerant compressor unit 41 for further pressurization, forming the entire refrigerant cycle.

[0042] The refrigerant matching device 3 is used to store, replenish, and adjust the components of the mixed refrigerant in the liquefied cold box device 2. Specifically, the refrigerant matching device 3 includes a refrigerant collection tank (not shown in the figure), a refrigerant heater (not shown in the figure), and a refrigerant dryer (not shown in the figure). The outlet of the refrigerant collection tank is connected to the inlet pipeline of the refrigerant heater, and the inlet pipeline of the refrigerant heater is equipped with a methane filling port, an ethylene filling port, an isobutane filling port, a nitrogen filling port, and a spare filling port. The outlet of the refrigerant dryer is connected to the 4-4 pipeline. A refrigerant dryer is installed at the outlet of the refrigerant dryer to ensure that the water content in the refrigerant entering the refrigerant compression device 4 meets the requirements for refrigeration liquefaction, ensuring long-term stable operation of the equipment. The main function of the refrigerant collection tank is to recover the system refrigerant when the system is shut down or the load is reduced, so that it can be reused when restarting or increasing the load, avoiding waste caused by discharge and eliminating safety hazards; restarting the recovery and treatment system can achieve rapid start-up, reduce investment costs, and reduce operation and maintenance costs.

[0043] The denitrification flash evaporation unit 5 separates nitrogen from the LNG formed after liquefaction in the liquefaction cold box unit 2. Specifically, the denitrification flash evaporation unit 5 includes a denitrification flash tank. LNG liquefied at -162°C via the natural gas liquefaction branch line 20 enters the denitrification flash tank through pipeline 6-1, where some nitrogen is flashed out. The LNG is then sent to the receiving terminal's LNG storage tank 6 via pipeline 6-3 for storage. Furthermore, an HV-1 throttle valve is installed on the 6-1 pipeline to reduce pressure; the HV-1 throttle valve is interlocked with the liquid level of the denitrification flash tank to control the flow rate and ensure safety.

[0044] An ES-1 emergency shut-off valve is installed on the 6-3 pipeline. When the ES-1 emergency shut-off valve malfunctions and becomes blocked, causing the liquid level in the flash tank to be high, the HV-throttle valve can be interlocked and closed in time.

[0045] The BOG at the top of the denitrification flash tank is led out through pipe 6-2, and the tank pressure is maintained at about 0.3 MPaG by the PV-1 pressure regulating valve. Then it enters the liquefaction cold box for reheating, and finally is transported to the venting main to prevent it from circulating and accumulating in the system.

[0046] The air-cooling unit is used to cool the raw gas drying unit 1 and the refrigerant compression unit 4. Specifically, the air-cooling unit includes five air coolers and two cooling water circulating pumps, one in use and one on standby. The air-cooling unit uses demineralized water as the medium. After being cooled by the air coolers, the water is pressurized by the circulating pumps and delivered to the regenerated gas cooler 133 and the cooler of the refrigerant compression unit 4 to cool the process medium. The circulating water, after absorbing heat and increasing its temperature, returns to the air-cooling unit, releases heat in the air coolers, and is cooled to the design temperature. It then re-enters the circulating pump inlet to form a closed-loop process. This is existing technology and will not be described in detail further.

[0047] In this embodiment, the wasteful evaporated BOG can be collected and liquefied again through the recycling system. The processed raw gas is filtered and dehydrated by the raw gas drying device 1, and then passed through the liquefaction cold box device 2 to achieve multi-stage cooling and liquefaction to form LNG. After being separated by the denitrification flash evaporation device 5 and nitrogen removed, it is stored in the LNG storage tank 6, realizing rapid BOG recovery, avoiding direct methane emission, and meeting environmental protection requirements.

[0048] Based on this, the traditional method of using compressors and circulating pumps for refrigeration has been abandoned. First, the liquefied gas cold box is divided into a shallow cooling section a and a deep cooling section b according to the cooling capacity. In the natural gas liquefaction process, the processed feed gas is cooled step by step, so that the cooling temperature provided by the refrigerant can closely follow the actual needs of the BOG feed gas to gradually cool down. This reduces the ineffective energy dissipation caused by excessive temperature difference, thereby improving the energy utilization efficiency of the liquefied gas cold box device 2.

[0049] Secondly, the refrigerant circulation process, in conjunction with the refrigerant compression unit 4, sequentially compresses the return refrigerant from the liquefied cold box through the low-pressure stage mixed refrigerant compressor unit 40 and the high-pressure stage mixed refrigerant compressor unit 41, separating the gas phase component and liquid phase component with different boiling points. After throttling, the liquid phase component is cooled in the shallow cooling section a, ranging from -50°C to ambient temperature. The gas phase component is cooled in the deep cooling section b, ranging from -166.2°C to -50°C. At each stage of the cooling curve, the temperature of the fluid providing the cold source is very close to the temperature of the natural gas, greatly reducing energy loss caused by excessive temperature difference and improving energy utilization efficiency. After heat exchange, the liquid and gas phase components are further mixed and returned to the refrigerant compression unit 4, forming a cycle to ensure sustainable operation. Finally, through the above multi-stage compression and each stage expansion and cooling coupling process, the energy consumption for reliquefaction is reduced on the basis of efficient liquefaction of raw gas into LNG.

[0050] Example 2 This embodiment, based on Embodiment 1, claims protection for a BOG reliquefaction and recycling method, including the following steps: S1. Start the air-cooling device and put it in standby mode; specifically, turn on the circulating pump, maintain the circulating pump outlet pressure at 0.25MPa, and control the flow rate at 570m³ / h.

[0051] S2. Observe the online analyzer 14 to monitor the water dew point of the BOG feed gas after it has been processed by the drying device. When the water dew point is less than -65℃, open the outlet valve of the dust filter 121 and the inlet valve of the liquefaction cold box to prevent BOG feed gas that has not met the drying index from entering the liquefaction cold box; at the same time, confirm that the HV-1 throttle valve is in the closed state.

[0052] S3. Adjusting the refrigerant compression device 4, the refrigerant matching device 3, and the refrigerant compression device 4 to provide different cooling capacities for the shallow cooling section a and the deep cooling section b of the liquefied cold box. Specifically, this includes: S30. Open the first throttle valve 23 to 1% opening, and open the second throttle valve 21 to 1% opening; S31. Refrigerant is gradually added through the refrigerant filling port of the refrigerant matching device 3, and the refrigerant enters the refrigerant compression device 4.

[0053] S32. The reflux refrigerant is pressurized step by step through the low-pressure stage mixed refrigerant compressor unit 40 and the high-pressure stage mixed refrigerant compressor unit 41 to form a mixed refrigerant. The mixed refrigerant is separated into gas phase components and liquid phase components by the outlet gas-liquid separator.

[0054] The liquid phase component is throttled by the second throttle valve 21 to form a gas-liquid mixture, which enters the medium-temperature refrigerant separator 22. The gas-liquid mixture formed becomes the main cold source of the shallow cooling section a.

[0055] The gaseous components are throttled by the first throttle valve 23 to form an ultra-low temperature fluid, which enters the cryogenic refrigerant separator 24 and becomes the main cold source for the cryogenic section b.

[0056] The ultra-low temperature fluid and gas-liquid mixture that have undergone heat exchange merge to provide cooling for the shallow cooling section a.

[0057] After mixing, the refrigerant is exchanged to form a reflux refrigerant, which is then introduced into the low-pressure stage mixed refrigerant compressor unit 40 and the high-pressure stage mixed refrigerant compressor unit 41 to form a cycle.

[0058] S4. Adjust the opening of the first throttle valve 23, the opening of the second throttle valve 21, and the refrigerant replenishment to gradually lower the temperature of the liquefied cold box. When the temperature of the liquefied cold box drops to -120°C, ensure that the cooling rate of the cold box does not exceed 20°C / h.

[0059] S5. When the liquefied cold box cools down to -120℃, the pressure of the PV-1 pressure regulating valve is set to 0.35MPa, and the HV-1 throttle valve gradually introduces the low-temperature natural gas into the denitrification flash tank, and vents through the PV-1 pressure regulating valve.

[0060] S6. When the temperature before the HV-1 throttle valve reaches -130℃, open the ES-1 emergency shut-off valve. By controlling the opening of the HV-1 throttle valve, ensure that the precooling rate of the LNG product pipeline does not exceed 20℃ / h. The LNG product pipeline refers to the pipeline between the liquefied cold box unit 2 and the LNG storage tank 6.

[0061] S7. When the temperature after the HV-1 throttle valve reaches below -130℃, adjust the HV-1 throttle valve to the target opening. S8. Set the slide valve 40 of the low-pressure stage mixed refrigerant compressor unit and the slide valve 41 of the high-pressure stage mixed refrigerant compressor unit to 100%. 100% means that the output of the two compressor units reaches the maximum value under the current operating conditions. Observe that the temperature points before and after the HV-1 throttle valve show a downward trend. Simultaneously increase the amount of refrigerant added, open the HV-1 throttle valve to control the liquid level of the denitrification flash tank to 30% to 50%, and recover the finished LNG to the LNG storage tank 6.

[0062] S9. Parking.

[0063] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for BOG reliquefaction and recycling, characterized in that, Includes the following steps: When the dew point of the BOG feed gas after processing by the feed gas drying device is less than the first predetermined value, the BOG feed gas enters the liquefaction cold box device. Adjust the liquefied cold box device, refrigerant matching device and refrigerant compression device, and through the multi-stage compression of the refrigerant compression device and the multi-stage expansion and cooling coupling of the corresponding expansion mechanism, cyclically provide different cooling capacities to the multi-stage cold sections of the liquefied cold box until the temperature of the reflux refrigerant reaches the first predetermined value; The processed BOG feedstock gas is cooled in multiple stages by a liquefaction cold box unit to form LNG; When the temperature of the reflux refrigerant reaches the first predetermined value, set the pressure of the PV-1 pressure regulating valve to the second predetermined value, adjust the HV-1 throttle valve, and vent the denitrification flash evaporator. When the inlet temperature of the HV-1 pressure regulating valve reaches the third predetermined value, the ES-1 emergency shut-off valve is opened, and the opening of the HV-1 throttle valve is adjusted so that the precooling rate of the pipeline between the liquefied cold box unit and the LNG storage tank is within the fourth threshold range. When the temperature after the HV-1 pressure regulating valve is less than the third predetermined value, adjust the HV-1 throttle valve to the target opening. Adjust the output of the refrigerant compression unit to the maximum value, increase the amount of refrigerant added, increase the opening of the HV-1 throttle valve, control the liquid in the denitrification flash evaporation unit to be within the fifth threshold range, and the denitrification flash evaporation unit flashes and separates the LNG, which is then returned to the LNG storage tank.

2. The BOG reliquefaction and recycling method according to claim 1, characterized in that, The liquefied cold box unit and the refrigerant compression unit are interconnected to form a loop, including the following steps: The reflux refrigerant is compressed in multiple stages by a refrigerant compression device to separate the gas phase component and the liquid phase component; The liquid component flows through the liquid flow branch into the liquefaction cold box device, where it expands and cools down through the first expansion mechanism to form a gas-liquid mixture, providing cooling capacity for the shallow cooling section. The gaseous components flow through the gaseous flow branch into the liquefied cold box device, where they expand and cool down through the second expansion mechanism to form an ultra-low temperature refrigerant, which provides cooling capacity for the cryogenic section. The ultra-low temperature fluid and gas-liquid mixture that have undergone heat exchange merge through the mixing branch to provide cooling capacity to the shallow cooling section; After mixing, the mixture undergoes heat exchange to form a reflux refrigerant, which is then fed into a refrigerant compression device to form a cycle.

3. The BOG reliquefaction and recycling method according to claim 2, characterized in that, Multi-level compression specifically includes the following steps: The refrigerant returning from the liquefied cold box unit via pipes 5-6 passes through the suction filter and enters the low-pressure stage mixed refrigerant compressor unit for pressurization and cooling. It then enters the high-pressure stage mixed refrigerant compressor unit for further pressurization and cooling to form a mixed refrigerant. The mixed refrigerant is then separated into gas phase and liquid phase components by the outlet gas-liquid separator.

4. The BOG reliquefaction and recycling method according to claim 2, characterized in that, Providing cooling to the shallow cooling section includes the following steps: The liquid phase component enters the liquefaction cold box device from the top through pipes 4-4 and 5-5 in sequence. After being pre-cooled in the shallow cooling section, it is throttled and cooled by the second throttle valve to form a gas-liquid mixture, which then enters the medium-temperature refrigerant separator.

5. The BOG reliquefaction and recycling method according to claim 2, characterized in that, Providing cooling capacity to the cryogenic section specifically includes the following steps: The gaseous components enter the liquefaction cold box from the top through pipes 4-3 and 5-4 for pre-cooling. After being pre-cooled in the shallow cooling section, the gaseous components enter the deep cooling section for gradual cooling. After being throttled and cooled by the first throttling valve, they enter the cryogenic refrigerant separator to form an ultra-low temperature fluid.

6. A BOG reliquefaction and recycling system applied to the BOG reliquefaction and recycling method according to any one of claims 1 to 5, characterized in that, This includes a BOG reliquefaction and recovery system, which consists of a feed gas drying unit, a liquefaction cold box unit, a denitrification flash evaporation unit, and an LNG storage tank connected in sequence by pipelines; an HV-1 throttle valve is installed on the pipeline between the liquefaction cold box unit and the denitrification flash evaporation unit, a PV-1 pressure regulating valve is installed on the pipeline between the top of the denitrification flash evaporation unit and the liquefaction cold box unit, and an ES-1 emergency shut-off valve is installed on the pipeline between the denitrification flash evaporation unit and the LNG storage tank; It also includes a refrigerant compression device and a refrigerant matching device. The liquefaction cold box device and the refrigerant compression device are connected to each other to form a cycle. The refrigerant matching device is connected between the liquefaction cold box device and the refrigerant compression device. Multiple cold sections are formed sequentially in the liquefaction cold box device. Each cold section is equipped with a corresponding expansion mechanism. Through the multi-level compression of the refrigerant compression device and the multi-level expansion and cooling coupling of the expansion mechanism, different cooling capacities are provided to the multi-level cold sections in a cycle.

7. The BOG reliquefaction and recycling system according to claim 6, characterized in that, The liquefied gas cold box device is a liquefied gas cold box that uses a multi-flow plate heat exchanger to divide the interior of the liquefied gas cold box into an upper shallow cooling section and a lower deep cooling section. The shallow cooling section corresponds to the first expansion mechanism, and the deep cooling section corresponds to the second expansion mechanism. The refrigerant compression device is connected to the first expansion mechanism through a liquid phase flow branch. The deep cooling section is connected to the second expansion mechanism through a gas phase flow branch. The second expansion mechanism and the first expansion mechanism are connected and merged through a mixing branch. The mixing branch is connected to the refrigerant compression device to form a circulation.

8. The BOG reliquefaction and recycling system according to claim 6, characterized in that, The multi-stage compression specifically includes a low-pressure stage mixed refrigerant compressor unit, a high-pressure stage mixed refrigerant compressor unit, and a gas-liquid separator connected in sequence.

9. The BOG reliquefaction and recycling system according to claim 8, characterized in that, The first expansion mechanism includes a second throttle valve and a medium-temperature refrigerant separator. The gas-liquid separator enters the liquefied cold box device through pipes 4-4 and 5-5, passes through the shallow cooling section, and is connected to the second throttle valve and the medium-temperature refrigerant separator in sequence.

10. The BOG reliquefaction and recycling system according to claim 8, characterized in that, The second expansion mechanism includes a first throttle valve and a cryogenic refrigerant separator. The gas-liquid separator enters the liquefied cold box from the top through pipes 4-3 and 5-4, passing through the shallow cooling section to the deep cooling section, and is connected to the first throttle valve and the cryogenic refrigerant separator in sequence.

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