Multi-stage BOG expansion offshore LNG flash gas reliquefaction device and process with precooling

Abstract

The present invention provides a LNG flash vapor reliquefaction device and process with multi-stage BOG expansion with precooling, wherein the reliquefaction device comprises the following parts: a BOG pretreatment system, a precoolant circulation system, a multi-stage BOG circulation system, and a BOG liquefaction storage system. The process comprises the following steps: (i) a BOG pretreatment step, (ii) a precoolant circulation step, (iii) a multi-stage BOG circulation step, and (iv) a BOG liquefaction storage step. The BOG circulation refrigeration of the present invention is realized by simply compressing first and then throttling, and the precooling refrigeration cycle is realized by throttling after compression, which minimizes the use of separators, does not have equipment such as expanders and pumps, has few auxiliary equipment, and is quick to start and stop, and can be applied to environments with harsh sea conditions. The precooling refrigeration is combined with the multi-stage BOG expansion refrigeration, and the high-grade cold source is partially replaced by the low-grade cold source, thereby reducing the power consumption of the BOG refrigeration cycle and improving the liquefaction efficiency. The present invention uses local materials and utilizes the BOG generated on board as an expansion refrigerant, thereby reducing the cost of preparing and storing the refrigerant. All systems adopt a skid design, which improves the economy, safety and compactness of the device.

Description

Technical Field

[0001] The present invention relates to an LNG flash vapor reliquefaction device and process, and in particular to an LNG flash vapor reliquefaction process with multi-stage BOG expansion with precooling applicable to offshore. Background Art

[0002] LNG (Liquefied Natural Gas) is a high-quality energy widely used in the fields of industry, power generation, city gas, automobile fuel, and ships. It has the advantages of being clean, low-carbon, and highly efficient. LNG can be transported by ship, pipeline, rail, and road. Ship transportation is widely used due to its large capacity, safety, and reliability. LNG has a low boiling point under normal pressure, and will produce a large amount of flash gas (BOG) during storage and transportation. If it is connected to a flare combustion or venting system, it will not only cause a huge waste of energy, but also pollute the environment.

[0003] Usually, LNG storage tanks will produce 0.2% to 0.3% BOG. ​​For high-pressure LNG gas supply systems, if low-pressure auxiliary machines (generators, boilers) cannot completely digest BOG, the direct discharge of excess BOG into the atmosphere will cause waste and pollution. The commonly adopted measures are to use high-pressure compressors to boost the pressure and supply it to the main engine as fuel or to set up a BOG reliquefaction system for recovery. Selecting a suitable reliquefaction process will reduce the cost of ship construction and operation, improve system operability, and achieve lower energy consumption and higher liquefaction yield. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a multi-stage BOG expansion offshore LNG flash vapor reliquefaction device and process with precooling. The device and process adopt multi-stage BOG expansion technology with precooling, use local materials, do not need to prepare and store refrigerants, reduce operating costs, adopt precooling technology, reduce energy consumption, and improve reliquefaction yield.

[0005] Firstly, the present invention provides a multi-stage BOG expansion offshore LNG flash vapor reliquefaction device with precooling, comprising the following parts: a BOG pretreatment system, a precoolant circulation system, a multi-stage BOG circulation system, and a BOG liquefaction storage system.

[0006] The BOG pre-treatment system includes a heat exchanger E-101, a compressor K-100 and a water cooler E-102.

[0007] The precoolant circulation system includes compressor K-101, throttle valve JT1, heat exchanger E-103, separation tank V-102, throttle valve JT2, heat exchanger E-104, and compressor K-102;

[0008] The multi-stage BOG circulation system includes a compressor module, a refrigerant heat exchanger LNG-101, and a throttle valve JT4; wherein the compressor module includes two or more BOG compression cycles, specifically a two-stage BOG cycle;

[0009] The BOG liquefaction storage system includes heat exchangers LNG-102, LNG-103, LNG-104, throttle valves JT5, JT3 and storage tank V-101; among which LNG-102, LNG-103, LNG-104 are multi-stream plate heat exchangers.

[0010] The tube side outlet of the heat exchanger E-101 of the BOG pretreatment system is connected to the inlet of the compressor K-100 through a pipeline; the heat exchanger E-101 heats the BOG to the working temperature range of the compressor, and the heat source used comes from the precoolant circulation system. The heat exchanger E-101 also plays a role in cooling the precoolant; the heat exchange form adopts a shell and tube heat exchanger, BOG goes through the tube side, and the precoolant goes through the shell side;

[0011] The compressor K-100 uses a screw BOG compressor, and the water cooler E-102 uses a shell and tube heat exchanger. BOG flows through the tube side, and cooling water flows through the shell side. The cooling water temperature is required to be 20-30℃; the outlet pipeline of the compressor K-100 is connected to the inlet of the E-102 tube side; the outlet BOG temperature of the BOG pretreatment system is 30-40℃, and the pressure is 800-900KPa;

[0012] The precoolant circulation system includes a compressor K-101, a throttle valve JT1, a heat exchanger E-103, a separation tank V-102, a throttle valve JT2, a heat exchanger E-104, and a compressor K-102;

[0013] The precooling agent used is one of carbon dioxide, propane, freon, lithium bromide and other substances; K-101 uses a centrifugal compressor, and the outlet pipeline of K-101 is connected to the shell side inlet pipeline of the heat exchanger E-101 of the BOG pretreatment system, and the precooling agent exchanges heat with BOG to reduce the temperature; the precooling agent of E-101 returns to the refrigerant circulation system from the BOG pretreatment system, and a throttle valve JT1 is set on the pipeline, which is connected to the tube side inlet pipeline of the heat exchanger E-103; E-103 is a shell and tube heat exchanger, BOG goes through the tube side, and the refrigerant goes through the shell side; the shell side outlet pipeline is connected to the separator V-102, and the working pressure of the separation tank The pressure is 1500KPa-2000KPa; a throttle valve JT2 is arranged on the liquid phase pipeline of the separation tank and connected to the shell side inlet of the heat exchanger E-104; E-104 is a shell and tube heat exchanger, BOG goes through the tube side, and the precoolant goes through the shell side; the upstream of the tube side is connected to the water cooler E-102, and the downstream is connected to the LNG-102 of the BOG liquefaction storage system; the shell side outlet pipeline is connected to the inlet pipeline of the compressor K-102, and the outlet pressure of the K-102 compressor is equal to the pressure of the separation tank V-102, and the compressor outlet pipeline is merged with the gas phase pipeline of the separation tank V-102 and connected to the inlet pipeline of the compressor K-101.

[0014] The multi-stage BOG circulation system comprises a compressor module, a refrigerant heat exchanger LNG-101, and a throttle valve JT4; wherein the compressor module comprises two or more BOG compression cycles, specifically a two-stage BOG cycle; the compressor of this module adopts a multi-stage screw compressor with interstage cooling; the compressor of the one-stage BOG circulation system is a two-stage compression, K-106 and K-107 are compressors, E-108 and E-106 are interstage coolers, the interstage cooler is cooled by cooling water, the cooling water flows through the shell side, and the cooling water temperature is required to be 20-30°C; the connection method is that the inlet of the first-stage compressor K-106 is connected to the cold stream outlet of LNG-101, the outlet is connected to the water cooler E-108, the pipe side outlet of E-108 is connected to the second-stage compressor K-107, the outlet pipeline of K-107 is connected to the water cooler E-106, the pipe side outlet pipeline of E-106 is connected to the refrigerant heat exchanger LNG-101 for cooling, and the exchange A throttle valve JT4 is arranged on the corresponding stream pipeline coming out of the heat exchanger, and its downstream is connected to the heat exchanger LNG-102 of the BOG liquefaction storage system; the compressor of the secondary BOG circulation system is a three-stage compression, K-103, K-104, K-105 are compressors, and E-100, E-107 and E-105 are interstage coolers; the interstage cooler is cooled by cooling water, and the cooling water flows through the shell side, and the cooling water temperature is required to be 20-30°C; the connection method is that the inlet of the first-stage compressor K-103 is connected to the cold stream outlet of LNG-101, and the outlet is connected to the water cooler E-100, the pipe outlet of E-100 is connected to the secondary compressor K-104, the outlet pipeline of K-104 is connected to the water cooler E-107, the pipe outlet of E-107 is connected to the third-stage compressor, the outlet pipeline of K-104 is connected to the water cooler E-105, and the pipe outlet pipeline of E-105 is connected to the refrigerant heat exchanger LNG-101 for cooling;

[0015] LNG-101 is a multi-stream plate heat exchanger, with two hot streams and two cold streams for heat exchange, and both the hot and cold media are BOG used as refrigerant; its hot stream inlet is connected to the E-106 pipe outlet pipeline and the E-105 pipe outlet pipeline respectively, the hot stream from E-106 is connected to LNG-101 through pipeline NC1-3 for cooling, and then connected to throttle valve JT-4 through pipeline NC-1-4 for further cooling, and the hot stream from E-105 is connected to LNG-102 through pipeline NC2-4 after cooling through LNG-101; Its cold stream comes from the two cold streams at the outlet of LNG-102; the cold stream from JT-4 passes through LNG-102 and is connected to the cold stream inlet pipe of LNG-101 through pipeline NC1-8, and the outlet of LNG-101 of this stream is connected to the inlet pipeline of K-106; the cold stream from JT-5 passes through LNG-104, LNG-103, and LNG-102 in turn and is connected to another inlet pipe of LNG-101 cold stream through pipeline NC2-10, and the outlet of LNG-101 of this stream is connected to the inlet pipeline of K-103.

[0016] The BOG liquefaction storage system comprises heat exchangers LNG-102, LNG-103, LNG-104, throttle valves JT5, JT3 and storage tank V-101; LNG-102 is a multi-stream plate heat exchanger, the heat exchange streams are two hot streams and two cold streams, the two hot streams are respectively BOG to be liquefied and refrigerant BOG, and the two cold streams are respectively from two-stage BOG cycles; the outlet pipeline of the E-104 pipe of the precoolant circulation system is connected to the hot stream inlet pipeline B2 of LNG-102, the hot stream medium is BOG to be liquefied, and the outlet is connected to LNG-103 through the pipeline B3; LNG-10 The hot stream from JT4 is connected to the inlet of another hot stream of LNG-1 through the pipeline NC2-4. The hot stream is the refrigerant BOG, and the outlet is connected to LNG-103 through the pipeline NC2-5; the outlet pipeline NC1-6 of JT4 is connected to the inlet of one cold stream of LNG-102, and the outlet pipeline NC1-8 of the stream is connected to the inlet of the cold stream of LNG-101; the outlet pipeline NC2-9 of the cold stream of LNG-103 is connected to the inlet of another cold stream of LNG-102, and the outlet pipeline NC2-10 of the stream is connected to the inlet of the cold stream of LNG-101; LNG-103 is a multi-stream plate heat exchanger, and the heat exchange streams are two hot streams and one The two hot stream media are BOG to be liquefied and refrigerant BOG, and the cold stream comes from the secondary BOG cycle; the outlet pipeline of the hot stream of BOG to be liquefied of LNG-102 is connected to one of the hot stream pipelines of LNG-103, and the outlet pipeline B4 of the stream LNG-103 is connected to the hot stream inlet of LNG-104; the outlet pipeline of the refrigerant BOG hot stream of LNG-102 is connected to another hot stream pipeline of LNG-103, and the outlet pipeline NC2-6 of the stream LNG-103 is connected to the throttle valve JT5, and the valve is connected to the cold stream inlet pipeline of LNG-104; the cold stream inlet of LNG-103 is connected to the throttle valve JT5 through the throttle valve JT5. NC2-8 is connected to the cold stream outlet of LNG-104, and the cold stream outlet of LNG-103 is connected to the cold stream inlet of LNG-102 through the pipeline NC2-9; LNG-104 is a two-stream plate heat exchanger, and the cold stream is connected to the cold stream inlet pipeline of LNG-104 by the JT5 outlet pipeline, and the cold stream outlet pipeline is connected to the cold stream inlet pipeline of LNG-103; the BOG hot stream outlet pipeline of LNG-103 is connected to the hot stream inlet of LNG-104, and the hot stream outlet pipeline of LNG-104 B5 is connected to the throttle valve JT3 and then connected to the storage tank V-101, and the working pressure of the storage tank is 110-300KPa.

[0017] Utilizing the above-mentioned flash gas reliquefaction device, the present invention also provides a multi-stage BOG expansion offshore LNG flash gas reliquefaction process with precooling, characterized in that it comprises the following steps:

[0018] (i) BOG pretreatment step, (ii) precoolant circulation step, (iii) multi-stage BOG circulation step, and (iv) BOG liquefaction storage step.

[0019] In the step (i) BOG pre-treatment, BOG from the storage tank is first heated by heat exchange with the refrigerant of the pre-cooling system in the heat exchanger E101, then enters the compressor for pressurization, and enters the circulating water cooler E102 for cooling.

[0020] (ii) The pre-coolant circulation step is a process in which a refrigerant is circulated to cool BOG. ​​The refrigerant used may be one of carbon dioxide, propane, freon, lithium bromide and the like.

[0021] The specific steps are as follows: the refrigerant (C1) is pressurized by the compressor K-101 and enters E101 to exchange heat with the BOG of the pre-treatment system and cool it down. After passing through the throttle valve JT1, it enters the heat exchanger E103 to cool the BOG from the pre-treatment system. The refrigerant coming out of E103 enters the gas-liquid separation tank V-102, and the working pressure of the separation tank is 1500KPa-2000KPa. The liquid refrigerant in the separation tank first passes through the throttle valve JT2 to further cool down and pressurize it, and then enters the heat exchanger E104 to further cool the BOG from E103. After the heat exchange, the refrigerant changes from liquid to gas (C8), enters the compressor K-102 to increase the pressure, and then mixes with the gas refrigerant (C5) in the separation tank V-102 and enters K-101, completing the pre-cooling refrigerant cycle.

[0022] In this step, the BOG from the pre-treatment system enters the heat exchangers E-103 and E-104 in turn to exchange heat with the pre-coolant for cooling.

[0023] (III) The multi-stage BOG cycle includes multiple BOG expansion and refrigeration cycles. The difference between the multiple cycles is that the temperature before each stage of expansion into the throttle valve is different. The secondary cycle adds further cooling than the primary cycle, so the temperature of the secondary cycle is lower than that of the primary cycle. The BOG consumption of the primary cycle is 1-3 times that of the secondary cycle.

[0024] First-stage BOG cycle process: BOG (NC1-2) comes out of compressor K-106, passes through water cooler E-108, and enters compressor K-107. This gas compression process can be divided into multiple stages of compression. Then it enters E-106 for cooling, enters heat exchanger LNG-101 for deep cooling, and then enters throttle valve JT4. The low-temperature BOG obtained by decompression enters heat exchangers LNG-102 and LNG-101 in turn to participate in refrigeration and heat exchange. The heated BOG enters the inlet of compressor K-106 for compression, completing the first-stage BOG cycle.

[0025] Secondary BOG circulation process: BOG (NC2-2) comes out of compressor K-103, passes through water cooler E-100, enters compressor K-105, and then enters compressor K-104 after cooling through water cooler E-107. This gas compression process can be divided into multiple stages of compression. Then it enters E-105 for cooling, enters heat exchangers LNG-101, LNG-102 and LNG-103 in turn for deep cooling, and then enters throttle valve JT5. After decompression, the low-temperature BOG is obtained and enters heat exchangers LNG-104, LNG-103, LNG-102 and LNG-101 in turn to participate in refrigeration and heat exchange. The heated BOG enters the inlet of compressor K-103 for compression, completing the secondary BOG circulation.

[0026] In this step, the BOG from the precooling system first enters the precooling heat exchanger LNG-102 for cooling, then enters the liquefaction heat exchanger LNG-103 for liquefaction, and finally enters the subcooling heat exchanger LNG-104 to increase the degree of subcooling.

[0027] (IV) BOG liquefaction storage steps: BOG supercooled by LNG-104 enters the storage tank V-101 after being depressurized by throttle valve JT3. The working pressure of the storage tank is 110-300KPa.

[0028] Beneficial effects of the present invention:

[0029] The present invention has good adaptability at sea. The main refrigerant used is BOG gas produced by the storage tank, and there is no need to produce refrigerant separately. The precooling cycle and the multi-stage BOG expansion cycle are both realized by throttling after compression, without expansion machines, pumps and other equipment, saving equipment investment and maintenance. A skid structure is adopted, in which the pre-treatment system, the precooling refrigerant circulation system, and the precooling system are integrated in a skid, the multi-stage BOG compression and expansion system is integrated in a skid, and the heat exchanger of the BOG liquefaction system is integrated in a skid. After integration, it has the characteristics of simple process flow, small number of equipment, compact equipment, and small deck area. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Attached Figure 1 This is the BOG pre-treatment system diagram.

[0031] Attached Figure 2 This is a diagram of the precoolant circulation system.

[0032] Attached Figure 3 It is a system diagram of multi-stage BOG cycle steps.

[0033] Attached Figure 4 This is a diagram of the BOG liquefaction storage system.

[0034] In the figure: heat exchanger E-101, compressor K-100, water cooler E-102, compressor K-101, throttle valve JT1, heat exchanger E-103, separation tank V-102, throttle valve JT2, heat exchanger E-104, compressor K-102, heat exchanger LNG-101, throttle valve JT4, compressor K-106, compressor K-107, interstage cooler E-108, interstage cooler E-106, compressor K-103, compressor K-104, compressor K-105, interstage cooler E-100, interstage cooler E-107, interstage cooler E-105, heat exchanger LNG-102, LNG-103, LNG-104, throttle valves JT5, JT3, storage tank V-101. DETAILED DESCRIPTION

[0035] Example 1

[0036] As attached Figure 1-4 As shown, this embodiment provides a multi-stage BOG expansion offshore LNG flash vapor reliquefaction device with precooling, including the following parts: a BOG pretreatment system, a precoolant circulation system, a multi-stage BOG circulation system, and a BOG liquefaction storage system.

[0037] The BOG pre-treatment system includes a heat exchanger E-101, a compressor K-100 and a water cooler E-102.

[0038] The precoolant circulation system includes compressor K-101, throttle valve JT1, heat exchanger E-103, separation tank V-102, throttle valve JT2, heat exchanger E-104, and compressor K-102;

[0039] The multi-stage BOG circulation system includes a compressor module, a refrigerant heat exchanger LNG-101, and a throttle valve JT4; wherein the compressor module includes two or more BOG compression cycles, specifically a two-stage BOG cycle;

[0040] The BOG liquefaction storage system includes heat exchangers LNG-102, LNG-103, LNG-104, throttle valves JT5, JT3 and storage tank V-101; LNG-102 is a multi-stream plate heat exchanger.

[0041] The tube side outlet of the heat exchanger E-101 of the BOG pretreatment system is connected to the inlet of the compressor K-100 through a pipeline; the heat exchanger E-101 heats the BOG to the working temperature range of the compressor, and the heat source used comes from the precoolant circulation system. The heat exchanger E-101 also plays a role in cooling the precoolant; the heat exchange form adopts a shell and tube heat exchanger, BOG goes through the tube side, and the precoolant goes through the shell side;

[0042] The compressor K-100 uses a screw BOG compressor, and the water cooler E-102 uses a shell and tube heat exchanger. BOG flows through the tube side, and cooling water flows through the shell side. The cooling water temperature is required to be 20-30℃; the outlet pipeline of the compressor K-100 is connected to the inlet of the E-102 tube side; the outlet BOG temperature of the BOG pretreatment system is 30-40℃, and the pressure is 800-900KPa;

[0043] The precoolant circulation system includes a compressor K-101, a throttle valve JT1, a heat exchanger E-103, a separation tank V-102, a throttle valve JT2, a heat exchanger E-104, and a compressor K-102;

[0044] The precooling agent used is one of carbon dioxide, propane, freon, lithium bromide and other substances; K-101 uses a centrifugal compressor, and the outlet pipeline of K-101 is connected to the shell side inlet pipeline of the heat exchanger E-101 of the BOG pretreatment system, and the precooling agent exchanges heat with BOG to reduce the temperature; the precooling agent of E-101 returns to the refrigerant circulation system from the BOG pretreatment system, and a throttle valve JT1 is set on the pipeline, which is connected to the tube side inlet pipeline of the heat exchanger E-103; E-103 is a shell and tube heat exchanger, BOG goes through the tube side, and the refrigerant goes through the shell side; the shell side outlet pipeline is connected to the separator V-102, and the working pressure of the separation tank The pressure is 1500KPa-2000KPa; a throttle valve JT2 is arranged on the liquid phase pipeline of the separation tank and connected to the shell side inlet of the heat exchanger E-104; E-104 is a shell and tube heat exchanger, BOG goes through the tube side, and the precoolant goes through the shell side; the upstream of the tube side is connected to the water cooler E-102, and the downstream is connected to the LNG-102 of the BOG liquefaction storage system; the shell side outlet pipeline is connected to the inlet pipeline of the compressor K-102, and the outlet pressure of the K-102 compressor is equal to the pressure of the separation tank V-102, and the compressor outlet pipeline is merged with the gas phase pipeline of the separation tank V-102 and connected to the inlet pipeline of the compressor K-101.

[0045] The multi-stage BOG circulation system comprises a compressor module, a refrigerant heat exchanger LNG-101, and a throttle valve JT4; wherein the compressor module comprises two or more BOG compression cycles, specifically a two-stage BOG cycle; the compressor of this module adopts a multi-stage screw compressor with interstage cooling; the compressor of the one-stage BOG circulation system is a two-stage compression, K-106 and K-107 are compressors, E-108 and E-106 are interstage coolers, the interstage cooler is cooled by cooling water, the cooling water flows through the shell side, and the cooling water temperature is required to be 20-30°C; the connection method is that the inlet of the first-stage compressor K-106 is connected to the cold stream outlet of LNG-101, the outlet is connected to the water cooler E-108, the pipe side outlet of E-108 is connected to the second-stage compressor K-107, the outlet pipeline of K-107 is connected to the water cooler E-106, the pipe side outlet pipeline of E-106 is connected to the refrigerant heat exchanger LNG-101 for cooling, and the exchange A throttle valve JT4 is arranged on the corresponding stream pipeline coming out of the heat exchanger, and its downstream is connected to the heat exchanger LNG-102 of the BOG liquefaction storage system; the compressor of the secondary BOG circulation system is a three-stage compression, K-103, K-104, K-105 are compressors, and E-100, E-107 and E-105 are interstage coolers; the interstage cooler is cooled by cooling water, and the cooling water flows through the shell side, and the cooling water temperature is required to be 20-30°C; the connection method is that the inlet of the first-stage compressor K-103 is connected to the cold stream outlet of LNG-101, and the outlet is connected to the water cooler E-100, the pipe outlet of E-100 is connected to the secondary compressor K-104, the outlet pipeline of K-104 is connected to the water cooler E-107, the pipe outlet of E-107 is connected to the third-stage compressor, the outlet pipeline of K-104 is connected to the water cooler E-105, and the pipe outlet pipeline of E-105 is connected to the refrigerant heat exchanger LNG-101 for cooling;

[0046] LNG-101 is a multi-stream plate heat exchanger, with two hot streams and two cold streams for heat exchange, and both the hot and cold media are BOG used as refrigerant; its hot stream inlet is connected to the E-106 pipe outlet pipeline and the E-105 pipe outlet pipeline respectively, the hot stream from E-106 is connected to LNG-101 through pipeline NC1-3 for cooling, and then connected to throttle valve JT-4 through pipeline NC-1-4 for further cooling, and the hot stream from E-105 is connected to LNG-102 through pipeline NC2-4 after cooling through LNG-101; Its cold stream comes from the two cold streams at the outlet of LNG-102; the cold stream from JT-4 passes through LNG-102 and is connected to the cold stream inlet pipe of LNG-101 through pipeline NC1-8, and the outlet of LNG-101 of this stream is connected to the inlet pipeline of K-106; the cold stream from JT-5 passes through LNG-104, LNG-103, and LNG-102 in turn and is connected to another inlet pipe of LNG-101 cold stream through pipeline NC2-10, and the outlet of LNG-101 of this stream is connected to the inlet pipeline of K-103.

[0047] The BOG liquefaction storage system comprises heat exchangers LNG-102, LNG-103, LNG-104, throttle valves JT5, JT3 and storage tank V-101; LNG-102 is a multi-stream plate heat exchanger, the heat exchange streams are two hot streams and two cold streams, the two hot streams are respectively BOG to be liquefied and refrigerant BOG, and the two cold streams are respectively from two-stage BOG cycles; the outlet pipeline of the E-104 pipe of the precoolant circulation system is connected to the hot stream inlet pipeline B2 of LNG-102, the hot stream medium is BOG to be liquefied, and the outlet is connected to LNG-103 through the pipeline B3; LNG-10 The hot stream from JT4 is connected to the inlet of another hot stream of LNG-1 through the pipeline NC2-4. The hot stream is the refrigerant BOG, and the outlet is connected to LNG-103 through the pipeline NC2-5; the outlet pipeline NC1-6 of JT4 is connected to the inlet of one cold stream of LNG-102, and the outlet pipeline NC1-8 of the stream is connected to the inlet of the cold stream of LNG-101; the outlet pipeline NC2-9 of the cold stream of LNG-103 is connected to the inlet of another cold stream of LNG-102, and the outlet pipeline NC2-10 of the stream is connected to the inlet of the cold stream of LNG-101; LNG-103 is a multi-stream plate heat exchanger, and the heat exchange streams are two hot streams and one The two hot stream media are BOG to be liquefied and refrigerant BOG, and the cold stream comes from the secondary BOG cycle; the outlet pipeline of the hot stream of BOG to be liquefied of LNG-102 is connected to one of the hot stream pipelines of LNG-103, and the outlet pipeline B4 of the stream LNG-103 is connected to the hot stream inlet of LNG-104; the outlet pipeline of the refrigerant BOG hot stream of LNG-102 is connected to another hot stream pipeline of LNG-103, and the outlet pipeline NC2-6 of the stream LNG-103 is connected to the throttle valve JT5, and the valve is connected to the cold stream inlet pipeline of LNG-104; the cold stream inlet of LNG-103 is connected to the throttle valve JT5 through the throttle valve JT5. NC2-8 is connected to the cold stream outlet of LNG-104, and the cold stream outlet of LNG-103 is connected to the cold stream inlet of LNG-102 through the pipeline NC2-9; LNG-104 is a two-stream plate heat exchanger, and the cold stream is connected to the cold stream inlet pipeline of LNG-104 by the JT5 outlet pipeline, and the cold stream outlet pipeline is connected to the cold stream inlet pipeline of LNG-103; the BOG hot stream outlet pipeline of LNG-103 is connected to the hot stream inlet of LNG-104, and the hot stream outlet pipeline of LNG-104 B5 is connected to the throttle valve JT3 and then connected to the storage tank V-101, and the working pressure of the storage tank is 110-300KPa.

[0048] Example 2

[0049] The present embodiment provides a multi-stage BOG expansion offshore LNG flash vapor reliquefaction process with precooling, comprising (i) a BOG pretreatment step, (ii) a precoolant circulation step, (iii) a multi-stage BOG circulation step, and (iv) a BOG liquefaction storage step.

[0050] In the step (i) BOG pre-treatment, BOG from the storage tank is first heated by heat exchange with the refrigerant of the pre-cooling system in the heat exchanger E101, then enters the compressor for pressurization, and enters the circulating water cooler E102 for cooling.

[0051] (ii) The pre-coolant circulation step is a process in which a refrigerant is circulated to cool BOG. ​​The refrigerant used can be one of carbon dioxide, propane, Freon, lithium bromide and other substances.

[0052] The specific steps are as follows: the refrigerant (C1) is pressurized by the compressor K-101 and enters E101 to exchange heat with the BOG of the pre-treatment system and cool it down. After passing through the throttle valve JT1, it enters the heat exchanger E103 to cool the BOG from the pre-treatment system. The refrigerant coming out of E103 enters the gas-liquid separation tank V-102, and the working pressure of the separation tank is 1500KPa-2000KPa. The liquid refrigerant in the separation tank first passes through the throttle valve JT2 to further cool down and pressurize it, and then enters the heat exchanger E104 to further cool the BOG from E103. After the heat exchange, the refrigerant changes from liquid to gas (C8), enters the compressor K-102 to increase the pressure, and then mixes with the gas refrigerant (C5) in the separation tank V-102 and enters K-101, completing the pre-cooling refrigerant cycle.

[0053] In this step, the BOG from the pre-treatment system enters the heat exchangers E-103 and E-104 in turn to exchange heat with the pre-coolant for cooling.

[0054] (III) The multi-stage BOG cycle includes multiple BOG expansion and refrigeration cycles. The difference between the multiple cycles is that the temperature before each stage of expansion into the throttle valve is different. The secondary cycle adds further cooling than the primary cycle, so the temperature of the secondary cycle is lower than that of the primary cycle. The BOG consumption of the primary cycle is 1-3 times that of the secondary cycle.

[0055] First-stage BOG cycle process: BOG (NC1-2) comes out of compressor K-106, passes through water cooler E-108, and enters compressor K-107. This gas compression process can be divided into multiple stages of compression. Then it enters E-106 for cooling, enters heat exchanger LNG-101 for deep cooling, and then enters throttle valve JT4. The low-temperature BOG obtained by decompression enters heat exchangers LNG-102 and LNG-101 in turn to participate in refrigeration and heat exchange. The heated BOG enters the inlet of compressor K-106 for compression, completing the first-stage BOG cycle.

[0056] Secondary BOG circulation process: BOG (NC2-2) comes out of compressor K-103, passes through water cooler E-100, enters compressor K-105, and then enters compressor K-104 after cooling through water cooler E-107. This gas compression process can be divided into multiple stages of compression. Then it enters E-105 for cooling, enters heat exchangers LNG-101, LNG-102 and LNG-103 in turn for deep cooling, and then enters throttle valve JT5. After decompression, the low-temperature BOG is obtained and enters heat exchangers LNG-104, LNG-103, LNG-102 and LNG-101 in turn to participate in refrigeration and heat exchange. The heated BOG enters the inlet of compressor K-103 for compression, completing the secondary BOG circulation.

[0057] In this step, the BOG from the precooling system first enters the precooling heat exchanger LNG-102 for cooling, then enters the liquefaction heat exchanger LNG-103 for liquefaction, and finally enters the subcooling heat exchanger LNG-104 to increase the degree of subcooling.

[0058] (IV) BOG liquefaction storage steps: BOG supercooled by LNG-104 enters the storage tank V-101 after being depressurized by throttle valve JT3. The working pressure of the storage tank is 110-300KPa.

Claims

1. Multi-stage BOG expansion offshore LNG flash gas reliquefaction unit with precooling, characterized in that: Includes the following parts: BOG pre-treatment system, pre-coolant circulation system, multi-stage BOG circulation system, BOG liquefaction storage system; The BOG pretreatment system includes a heat exchanger E-101, a compressor K-100 and a water cooler E-102; the outlet of the heat exchanger E-101 is connected to the inlet of the compressor K-100 through a pipeline; the heat exchanger E-101 heats the BOG to the working temperature range of the compressor, and the heat source used comes from the precoolant circulation system. The heat exchange form adopts a shell and tube heat exchanger, with BOG passing through the tube side and the precoolant passing through the shell side; the water cooler E-102 adopts a shell and tube heat exchanger, with BOG passing through the tube side and cooling water passing through the shell side, and the outlet pipeline of the compressor K-100 is connected to the inlet of the water cooler E-102 tube side; The precoolant circulation system includes compressor K-101, throttle valve JT1, heat exchanger E-103, separation tank V-102, throttle valve JT2, heat exchanger E-104, and compressor K-102; The outlet pipeline of compressor K-101 is connected to the shell side inlet pipeline of heat exchanger E-101 of BOG pre-treatment system, and the precoolant exchanges heat with BOG for cooling; the precoolant of heat exchanger E-101 returns to the refrigerant circulation system from the BOG pre-treatment system, and a throttle valve JT1 is set on the pipeline, which is connected to the tube side inlet pipeline of heat exchanger E-103; heat exchanger E-103 is a shell and tube heat exchanger, BOG goes through the tube side, and refrigerant goes through the shell side; the shell side outlet pipeline is connected to separator V-102, and a throttle valve JT2 is set on the liquid phase pipeline of the separation tank to The shell side inlet of heat exchanger E-104 is connected; heat exchanger E-104 is a shell and tube heat exchanger, BOG goes through the tube side, and precoolant goes through the shell side; the upstream of the tube side is connected to the water cooler E-102, and the downstream is connected to the heat exchanger LNG-102 of the BOG liquefaction storage system; the shell side outlet pipeline is connected to the inlet pipeline of compressor K-102, the compressor outlet pressure of compressor K-102 is equal to the pressure of separation tank V-102, the compressor outlet pipeline is combined with the gas phase pipeline of separation tank V-102, and connected to the inlet pipeline of compressor K-101; The multi-stage BOG circulation system includes a compressor module, a refrigerant heat exchanger LNG-101, and a throttle valve JT4; The compressor module includes a multi-stage BOG cycle, which is typically two stages; The compressor of the primary BOG circulation system is a two-stage compression system, specifically: K-106 and K-107 are compressors, E-108 and E-106 are interstage coolers, the interstage cooler is cooled by cooling water, and the cooling water flows through the shell side. The connection method is that the inlet of the primary compressor K-106 is connected to the cold stream outlet of the heat exchanger LNG-101, and the outlet is connected to the interstage cooler E-108. The pipe outlet of the interstage cooler E-108 is connected to the secondary compressor K-107. The outlet pipeline of the secondary compressor K-107 is connected to the interstage cooler E-106. The pipe outlet pipeline of the interstage cooler E-106 is connected to the refrigerant heat exchanger LNG-101 for cooling. A throttle valve JT4 is set on the corresponding stream pipeline coming out of the heat exchanger, and its downstream is connected to the heat exchanger LNG-102 of the BOG liquefaction storage system; the secondary BOG circulation system The compressor of the system is a three-stage compression, K-103, K-104, K-105 are compressors, E-100, E-107 and E-105 are interstage coolers; the interstage cooler is cooled by cooling water, and the cooling water flows through the shell side. The connection method is that the inlet of the first-stage compressor K-103 is connected to the cold stream outlet of the heat exchanger LNG-101, and the outlet is connected to the interstage cooler E-100, and the pipe outlet of the interstage cooler E-100 is connected to the second-stage compressor K-105, and the outlet pipeline of the second-stage compressor K-105 is connected to the interstage cooler E-107, and the pipe outlet of the interstage cooler E-107 is connected to the third-stage compressor K-104, and the outlet pipeline of the third-stage compressor K-104 is connected to the interstage cooler E-105, and the pipe outlet pipeline of the interstage cooler E-105 is connected to the refrigerant heat exchanger LNG-101 for cooling; Heat exchanger LNG-101 is a multi-stream plate heat exchanger, with two hot streams and two cold streams. Both the hot and cold media are BOG used as refrigerant. Its hot stream inlet is connected to the interstage cooler E-106 pipe outlet pipeline and the interstage cooler E-105 pipe outlet pipeline respectively. The hot stream from interstage cooler E-106 is connected to heat exchanger LNG-101 through pipeline NC1-3 for cooling, and then connected to throttle valve JT4 through pipeline NC-1-4 for further cooling. The hot stream from interstage cooler E-105 is connected to heat exchanger LNG-102 through pipeline NC2-4 after cooling in heat exchanger LNG-101. Its cold stream comes from Two cold streams from the outlet of heat exchanger LNG-102; the cold stream from throttle valve JT4 passes through heat exchanger LNG-102 and is connected to the cold stream inlet pipe of heat exchanger LNG-101 through pipeline NC1-8, and the outlet of heat exchanger LNG-101 of this stream is connected to the inlet pipeline of compressor K-106; the cold stream from throttle valve JT5 passes through heat exchanger LNG-104, heat exchanger LNG-103, and heat exchanger LNG-102 in sequence and is connected to another inlet pipe of cold stream of heat exchanger LNG-101 through pipeline NC2-10, and the outlet of heat exchanger LNG-101 of this stream is connected to the inlet pipeline of primary compressor K-103; The BOG liquefaction storage system includes heat exchanger LNG-102, heat exchanger LNG-103, heat exchanger LNG-104, throttle valve JT5, throttle valve JT3 and storage tank V-101; heat exchanger LNG-102 is a multi-stream plate heat exchanger, and the heat exchange streams are two hot streams and two cold streams. The two hot stream media are BOG to be liquefied and refrigerant BOG, respectively, and the two cold streams come from two-stage BOG cycles; the outlet pipeline of the heat exchanger E-104 of the precoolant circulation system is connected to the hot stream inlet pipeline B2 of the heat exchanger LNG-102, and the hot stream medium is BOG to be liquefied, and the outlet is connected to the heat exchanger LNG-103 through the pipeline B3; the heat exchanger LNG-101 The hot stream from LNG-2 is connected to the other hot stream inlet of LNG-2 through pipeline NC2-4. The hot stream is the refrigerant BOG, and its outlet is connected to the heat exchanger LNG-103 through pipeline NC2-5; the outlet pipeline NC1-6 of the throttle valve JT4 is connected to one cold stream inlet of the heat exchanger LNG-102, and the outlet pipeline NC1-8 of the stream is connected to the cold stream inlet of the heat exchanger LNG-101; the cold stream outlet pipeline NC2-9 of the heat exchanger LNG-103 is connected to the other cold stream inlet of the heat exchanger LNG-102, and the outlet pipeline NC2-10 of the stream is connected to the cold stream inlet of the heat exchanger LNG-101; the heat exchanger LNG-103 is a multi-stream plate heat exchanger, and the heat exchange streams are two hot streams and one The two hot stream media are BOG to be liquefied and refrigerant BOG, and the cold stream comes from the secondary BOG cycle; the outlet pipeline of the hot stream of BOG to be liquefied of the heat exchanger LNG-102 is connected to one of the hot stream pipelines of the heat exchanger LNG-103, and the outlet pipeline B4 of the heat exchanger LNG-103 is connected to the hot stream inlet of the heat exchanger LNG-104; the outlet pipeline of the refrigerant BOG hot stream of the heat exchanger LNG-102 is connected to another hot stream pipeline of the heat exchanger LNG-103, and the outlet pipeline NC2-6 of the heat exchanger LNG-103 is connected to the throttle valve JT5, and the throttle valve JT5 is connected to the cold stream inlet pipeline of the heat exchanger LNG-104; the heat exchanger LNG -103's cold stream inlet is connected to the cold stream outlet of the heat exchanger LNG-104 through NC2-8, and the cold stream outlet of the heat exchanger LNG-103 is connected to the cold stream inlet of the heat exchanger LNG-102 through the pipeline NC2-9; the heat exchanger LNG-104 is a two-stream plate heat exchanger, and the cold stream is connected to the cold stream inlet pipeline of the heat exchanger LNG-104 by the outlet pipeline JT5, and the cold stream outlet pipeline is connected to the cold stream inlet pipeline of the heat exchanger LNG-103; the BOG hot stream outlet pipeline of the heat exchanger LNG-103 is connected to the hot stream inlet of the heat exchanger LNG-104, and the hot stream outlet pipeline B5 of the heat exchanger LNG-104 is connected to the throttle valve JT3 and then connected to the storage tank V-101.

2. The LNG flash vapor reliquefaction process using the multi-stage BOG expansion offshore LNG flash vapor reliquefaction device with precooling according to claim 1 is characterized in that: The following steps are involved:

1. BOG pre-treatment steps (II) Precoolant circulation steps (III) Multi-stage BOG cycle steps, (iv) BOG liquefaction and storage steps; Step (ii) The precoolant circulation steps are specifically as follows: The refrigerant (C1) is pressurized by the compressor K-101 and enters the heat exchanger E-101 to exchange heat with the BOG in the pre-treatment system and cool it down. After passing through the throttle valve JT1, it enters the heat exchanger E-103 to cool the BOG from the BOG pre-treatment step. The refrigerant coming out of the heat exchanger E-103 enters the gas-liquid separation tank V-102. The working pressure of the separation tank is 1500KPa-2000KPa. The liquid refrigerant in the separation tank first passes through the throttle valve JT2 to further cool down and pressurize it, and then enters the heat exchanger E-104 to further cool the BOG from the heat exchanger E-103. After the heat exchange, the refrigerant changes from liquid to gas (C8). After entering the compressor K-102 to increase the pressure, it is mixed with the gas refrigerant (C5) in the separation tank V-102 and enters the compressor K-101 to complete the pre-cooling refrigerant cycle. Step (iii) Multi-stage BOG cycle refrigeration liquefaction treatment includes more than two BOG expansion refrigeration cycles, typically in the form of two-stage cycles, namely, a primary BOG cycle process and a secondary BOG cycle process; The temperature before each stage of expansion into the throttle valve is different. The secondary cycle has a further cooling than the primary cycle. The temperature of the secondary cycle is lower than that of the primary cycle. The BOG consumption of the primary cycle is 1-3 times that of the secondary cycle. First-stage BOG cycle process: BOG (NC1-2) comes out of compressor K-106, passes through interstage cooler E-108, and enters compressor K-107. This gas compression process can be divided into multiple stages of compression; then it enters interstage cooler E-106 for cooling, enters heat exchanger LNG-101 for deep cooling, and then enters throttle valve JT4. The low-temperature BOG obtained by decompression enters heat exchanger LNG-102 and heat exchanger LNG-101 in turn to participate in refrigeration and heat exchange. The heated BOG enters the inlet of compressor K-106 for compression, completing the first-stage BOG cycle; Secondary BOG cycle process: BOG (NC2-2) comes out of compressor K-103, passes through interstage cooler E-100, enters compressor K-105, and then enters compressor K-104 after being cooled by interstage cooler E-107. This gas compression process can be divided into multi-stage compression; then enters interstage cooler E-105 for cooling, and enters heat exchanger LNG-101, heat exchanger LNG-102 and heat exchanger LNG-103 in turn for deep cooling, and then enters throttle valve JT5. After decompression, the low-temperature BOG obtained enters heat exchanger LNG-104, heat exchanger LNG-103, heat exchanger LNG-102 and heat exchanger LNG-101 in turn to participate in refrigeration and heat exchange. The heated BOG enters the inlet of compressor K-103 for compression, completing the secondary BOG cycle; Step (IV) BOG liquefaction storage step: BOG supercooled by heat exchanger LNG-104 enters storage tank V-101 after being depressurized by throttle valve JT3. The working pressure of the storage tank is 110-300Kpa.

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