CN116592575B - A natural gas BOG direct reliquefaction system and method based on nitrogen expansion and throttling refrigeration - Google Patents
A natural gas BOG direct reliquefaction system and method based on nitrogen expansion and throttling refrigeration Download PDFInfo
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- CN116592575B CN116592575B CN202310235974.7A CN202310235974A CN116592575B CN 116592575 B CN116592575 B CN 116592575B CN 202310235974 A CN202310235974 A CN 202310235974A CN 116592575 B CN116592575 B CN 116592575B
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- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 title claims abstract description 188
- 229910052757 nitrogen Inorganic materials 0.000 title claims abstract description 93
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 title claims abstract description 56
- 238000000034 method Methods 0.000 title claims abstract description 26
- 239000003345 natural gas Substances 0.000 title claims abstract description 25
- 238000005057 refrigeration Methods 0.000 title claims abstract description 14
- 239000007788 liquid Substances 0.000 claims description 21
- 238000001816 cooling Methods 0.000 claims description 20
- 239000000203 mixture Substances 0.000 claims description 12
- 239000002994 raw material Substances 0.000 claims description 10
- 230000006835 compression Effects 0.000 claims description 9
- 238000007906 compression Methods 0.000 claims description 9
- 239000007789 gas Substances 0.000 claims description 5
- 229910001873 dinitrogen Inorganic materials 0.000 claims 2
- 238000010992 reflux Methods 0.000 claims 1
- 238000005265 energy consumption Methods 0.000 abstract description 3
- 239000003949 liquefied natural gas Substances 0.000 description 19
- 239000003507 refrigerant Substances 0.000 description 6
- 238000005516 engineering process Methods 0.000 description 5
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 3
- 238000002156 mixing Methods 0.000 description 3
- ATUOYWHBWRKTHZ-UHFFFAOYSA-N Propane Chemical compound CCC ATUOYWHBWRKTHZ-UHFFFAOYSA-N 0.000 description 2
- 238000009835 boiling Methods 0.000 description 2
- 239000012530 fluid Substances 0.000 description 2
- 238000011084 recovery Methods 0.000 description 2
- 238000007792 addition Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 239000000969 carrier Substances 0.000 description 1
- 238000002485 combustion reaction Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000001704 evaporation Methods 0.000 description 1
- 230000008020 evaporation Effects 0.000 description 1
- 238000002309 gasification Methods 0.000 description 1
- 239000007791 liquid phase Substances 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000012354 overpressurization Methods 0.000 description 1
- 239000012071 phase Substances 0.000 description 1
- 239000001294 propane Substances 0.000 description 1
- 230000000630 rising effect Effects 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
- F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
- F25J1/0228—Coupling of the liquefaction unit to other units or processes, so-called integrated processes
- F25J1/0235—Heat exchange integration
- F25J1/0237—Heat exchange integration integrating refrigeration provided for liquefaction and purification/treatment of the gas to be liquefied, e.g. heavy hydrocarbon removal from natural gas
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
- F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
- F25J1/0022—Hydrocarbons, e.g. natural gas
- F25J1/0025—Boil-off gases "BOG" from storages
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
- F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
- F25J1/0022—Hydrocarbons, e.g. natural gas
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
- F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
- F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
- F25J1/005—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by expansion of a gaseous refrigerant stream with extraction of work
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
- F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
- F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
- F25J1/0052—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
- F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
- F25J1/007—Primary atmospheric gases, mixtures thereof
- F25J1/0072—Nitrogen
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
- F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
- F25J1/0203—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle
- F25J1/0204—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a single-component refrigerant [SCR] fluid in a closed vapor compression cycle as a single flow SCR cycle
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
- F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
- F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
- F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
- F25J1/0262—Details of the cold heat exchange system
- F25J1/0264—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams
- F25J1/0265—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams comprising cores associated exclusively with the cooling of a refrigerant stream, e.g. for auto-refrigeration or economizer
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
- F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
- F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
- F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
- F25J1/0262—Details of the cold heat exchange system
- F25J1/0264—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams
- F25J1/0265—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams comprising cores associated exclusively with the cooling of a refrigerant stream, e.g. for auto-refrigeration or economizer
- F25J1/0268—Arrangement of heat exchanger cores in parallel with different functions, e.g. different cooling streams comprising cores associated exclusively with the cooling of a refrigerant stream, e.g. for auto-refrigeration or economizer using a dedicated refrigeration means
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- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Mechanical Engineering (AREA)
- Thermal Sciences (AREA)
- General Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Chemical & Material Sciences (AREA)
- Filling Or Discharging Of Gas Storage Vessels (AREA)
Abstract
The invention discloses a natural gas BOG direct reliquefaction system and method based on nitrogen expansion and throttling refrigeration, wherein an outlet of a secondary pressurizing unit is communicated with a first channel of a primary heat regenerator and then divided into two paths, one path is sequentially connected with the first channel of the secondary heat regenerator, an expansion valve, a first channel of a heat exchanger, a second channel of the secondary heat regenerator and an inlet end of a second channel of the primary heat regenerator, the other path is sequentially connected with an expander unit, a third channel of the secondary heat regenerator, a third channel of the primary heat regenerator and an inlet end of a primary pressurizing unit, an outlet of the primary pressurizing unit and an outlet of the second channel of the primary heat regenerator are respectively connected with an inlet end of the secondary pressurizing unit, and a BOG buffer tank is sequentially connected with the second channel of the heat exchanger and an inlet end of an LNG storage tank. By utilizing the method, the liquefaction yield of the BOG can be improved, and the liquefaction energy consumption can be reduced.
Description
Technical Field
The invention relates to the technical field of natural gas liquefaction, in particular to a natural gas BOG direct reliquefaction system and method based on nitrogen expansion and throttling refrigeration.
Background
In the natural gas industry, natural gas is often produced as liquefied natural gas (Liquefied Natural Gas, LNG) (which has a volume of about 1/600 of the original gaseous volume) for ease of storage and transportation. In LNG production, it is necessary to reduce the liquefaction pressure to tank pressure through a final throttling valve. The boiling points of the components in natural Gas are different (nitrogen: 77.36K, methane 111.7K, propane: 231.03K) and the throttling process is equivalent to a simple evaporation of LNG at the storage tank pressure, and Gas molecules with lower boiling points are firstly escaped from the LNG and are called flash gases (BOG). In addition, the gas-liquid mixture formed after LNG flows through the throttle valve is sent to an LNG tank for storage through a low-temperature pipeline, BOG is also generated in the storage process due to heat leakage from the environment to the storage tank, and the components of the BOG are mainly nitrogen and methane. In order to avoid over pressurization of the LNG storage tanks, BOG needs to be removed, and the common treatment is combustion and re-liquefaction, wherein the latter is more economical and social due to methane recovery.
The existing BOG re-liquefaction process mostly adopts a mode of re-heating the BOG to normal temperature, pressurizing (increasing the liquefaction temperature), then secondarily cooling and liquefying, and throttling to normal pressure. The chinese patent document with publication number CN108870866a discloses a BOG re-liquefaction recovery process using BOG as refrigerant for LNG carriers. The process comprises the following steps of enabling raw material BOG to enter a compressor after being reheated by a heat exchanger, enabling the raw material BOG to enter an expansion machine for compression cooling, enabling the raw material BOG to enter the heat exchanger for further cooling, enabling the raw material BOG to enter a cold box for cooling, enabling the raw material BOG to enter a gas-liquid separation tank after being cooled and throttled, enabling liquid-phase LNG to enter a storage tank for storage, enabling the refrigerant BOG and the reheated raw material BOG to enter the compressor, enabling the raw material BOG to enter the expansion machine for compression cooling, enabling the cooled refrigerant BOG to enter the expansion machine for expansion and throttle cooling, enabling the cooled refrigerant BOG to return to the cold box for providing cold energy for the cold box, and enabling the reheated refrigerant BOG and the reheated raw material BOG to repeat the operations.
However, the above process increases heat exchange loss, equipment investment (such as BOG compressor group, multi-stream heat exchanger, throttle valve) and system complexity (such as control of flow rate of each stream in multi-stream heat exchanger), and new BOG generation is unavoidable after throttling, reducing liquefaction yield.
In addition, for the BOG direct reliquefaction process adopting the single-stage nitrogen reverse brayton cycle, the temperature rising curve of low-temperature low-pressure nitrogen (cold fluid) after expansion is a straight line with the slope (inversely proportional to the product of the mass flow and the specific heat capacity of the constant pressure) being larger than zero, the temperature change of the BOG (hot fluid) phase change process is not large, the change of the specific heat capacity of the constant pressure of the nitrogen with the temperature is small (except for a critical point), so that the sensible heat of the nitrogen is used for providing the latent heat required for the BOG reliquefaction, the larger nitrogen flow and the higher low-temperature Duan Chuanre temperature difference are caused, and the comprehensive energy efficiency of the system is lower. On the other hand, the low-temperature nitrogen after gasification is insufficient to pre-cool the nitrogen at high-pressure normal temperature to the pre-throttling temperature.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a natural gas BOG direct reliquefaction system based on nitrogen expansion and throttling refrigeration, which adopts a nitrogen expansion refrigeration technology and a liquid nitrogen throttling refrigeration technology to directly liquefy the low-temperature low-pressure BOG from an LNG storage tank, and has the characteristics of safety, high thermodynamic efficiency, good economy and large product quantity.
A natural gas BOG direct reliquefaction system based on nitrogen expansion and throttling refrigeration comprises a primary supercharging unit, a secondary supercharging unit, a primary heat regenerator, a secondary heat regenerator, an expander unit, an expansion valve, a heat exchanger, a BOG buffer tank and an LNG storage tank;
The outlet of the secondary pressurizing unit is communicated with the first channel of the primary heat regenerator and then is divided into two paths, wherein one path is sequentially connected with the first channel of the secondary heat regenerator, the expansion valve, the first channel of the heat exchanger, the second channel of the secondary heat regenerator and the inlet end of the second channel of the primary heat regenerator;
the outlet of the primary supercharging unit and the outlet of the second channel of the primary regenerator are respectively connected with the inlet end of the secondary supercharging unit;
The BOG buffer tank is sequentially connected with the second channel of the heat exchanger and the inlet end of the LNG storage tank.
Further, the primary supercharging unit and the secondary supercharging unit adopt a single-stage or multi-stage compression and inter-stage cooling mode. Preferably, the primary supercharging unit adopts a two-stage compression and inter-stage water cooling mode, and the secondary supercharging unit adopts a one-stage compression and aftercooler water cooling mode.
Further, the expander unit adopts a multi-stage expansion refrigeration technology. Preferably, a two-stage gas bearing turbine expander is selected, and no liquid is carried after expansion.
Further, the low-temperature nitrogen passing through the outlet of the first channel of the first-stage heat regenerator is divided into two paths, one path enters the expander unit to expand and cool, and the other path enters the first channel of the second-stage heat regenerator to be cooled to be completely liquefied, wherein the temperature is not higher than the saturation temperature of the nitrogen, and then the pressure is reduced through the expansion valve.
Further, the temperature of the nitrogen-liquid mixture throttled by the expansion valve is lower than the saturation temperature of the BOG after liquefaction, and the temperature difference is 1.5-3 ℃. The arrangement is that the two-flow heat exchanger generally has a pinch temperature difference at the cold end of the heat exchanger, a small pinch temperature difference means a larger heat exchanger (influencing investment cost, occupied area and the like), but the heat transfer efficiency is higher (according to the heat law, the low temperature Duan Chuanre temperature difference is more obvious for improving the energy efficiency of the system), and the heat exchange area of the heat exchanger and the heat transfer temperature difference of the low temperature section are comprehensively considered, so that the cold end temperature difference of 1.5-3 ℃ is preferable.
Further, the temperature of the nitrogen heated by the heat exchanger is lower than the temperature of the nitrogen-liquid mixture before expansion of the expansion valve, and the temperature difference is 3-5 ℃. The arrangement is that the temperature of nitrogen heated by the heat exchanger affects the temperature difference of the hot end of the heat exchanger and the temperature difference of the cold end of the secondary heat regenerator simultaneously, so that the heat exchange area of the secondary heat regenerator is prevented from being too large, the heat transfer efficiency of the heat exchanger and the heat transfer efficiency of the secondary heat regenerator are considered, and the temperature difference is preferably 3-5 ℃.
Further, the temperature of the nitrogen at the outlet of the second channel and the outlet of the third channel of the first-stage heat regenerator are equal and lower than that of the nitrogen at the outlet of the second-stage pressurizing unit, and the temperature difference is 3-5 ℃. The temperature difference setting can recover cold nitrogen as much as possible, can reduce the heat transfer temperature difference of the hot end of the primary heat regenerator, improves the energy efficiency of the system, and can avoid overlarge heat exchange area. Further, the temperature of the nitrogen at the outlet of the second channel and the third channel of the second-stage heat regenerator is equal.
Further, the nitrogen pressure from the first stage pressurizing unit is not lower than the nitrogen pressure from the outlet of the second channel of the first stage regenerator. Preferably, the two pressures are chosen to be equal.
Further, the primary heat regenerator, the secondary heat regenerator and the heat exchanger adopt plate-fin structures.
Further, the BOG temperature in the BOG buffer tank is-120 ℃ to-140 ℃, and the air pressure is 120kPa to 160kPa.
The invention also provides a method for reliquefaction of the natural gas BOG, which uses the system and comprises the following steps:
Step 1, pressurizing and cooling low-pressure circulating nitrogen in a first-stage pressurizing unit, mixing the low-pressure circulating nitrogen with low-pressure normal-temperature nitrogen from a second channel of a first-stage heat regenerator, and then entering a second-stage pressurizing unit for further pressurizing;
Step 2, the high-pressure normal-temperature nitrogen from the secondary pressurizing unit enters a first channel of the primary heat regenerator to cool, and the cooled high-pressure nitrogen is divided into two parts, wherein one part enters the first channel of the secondary heat regenerator to exchange heat with the low-pressure cold nitrogen flowing back and from the expander unit and is liquefied by itself;
Step 3, the liquid nitrogen from the first channel of the second-stage heat regenerator is depressurized through an expansion valve to become a gas-liquid mixture;
step 4, the nitrogen-liquid mixture obtained by throttling the expansion throttle valve enters a first channel of a heat exchanger, exchanges heat with raw material BOG from a BOG buffer tank, and the liquefied BOG is sent to an LNG storage tank for storage;
Step 5, the low-pressure nitrogen heated by the heat exchanger and the low-pressure expanded by the expander unit respectively enter a second channel and a third channel of the second-stage heat regenerator to provide the cold energy required by liquefying the high-pressure nitrogen, and the low-pressure nitrogen is further heated;
Step 6, the heated low-pressure nitrogen from the second and third channels of the second-stage regenerator respectively enter the second and third channels of the first-stage regenerator, exchange heat with the high-pressure normal-temperature nitrogen from the second-stage pressurizing unit, and are simultaneously reheated;
And 7, enabling the nitrogen re-warmed in the third channel from the first-stage heat regenerator to enter the first-stage pressurizing unit for pressurizing, mixing with the re-warmed medium-pressure nitrogen from the second channel of the first-stage heat regenerator, and then entering the second-stage pressurizing unit to complete a cycle.
Compared with the prior art, the invention has the following beneficial effects:
1. The invention adopts the nitrogen expansion refrigeration technology and the liquid nitrogen throttling refrigeration technology simultaneously, utilizes the latent heat of the liquid nitrogen generated by the throttling of the expansion valve to provide the cold energy required by the reliquefaction of the BOG, utilizes the cold energy generated by the expansion machine unit to liquefy the nitrogen, and reduces the flow and the heat transfer of the nitrogen Loss (especially low temperature section) improves the energy efficiency and economy of the system. Compared with a natural gas BOG reliquefaction system adopting a single-stage nitrogen reverse Brayton cycle, the BOG reliquefaction system can further improve the BOG liquefaction yield and reduce the liquefaction energy consumption.
2. The invention directly liquefies the low-temperature low-pressure BOG from the LNG storage tank, omits equipment required by BOG rewarming, compression and throttling, simplifies the process structure and improves the economy. Secondly, avoiding the BOG re-heating and secondary cooling processAnd the loss improves the comprehensive energy utilization efficiency of the system. And thirdly, the BOG is not throttled after liquefaction, so that the generation of new BOG is avoided, and the liquefaction yield of the BOG is improved.
Drawings
FIG. 1 is a process flow diagram of a natural gas BOG direct reliquefaction system based on nitrogen expansion and throttling refrigeration.
In the figure, a primary supercharging unit 1, a secondary supercharging unit 2, a primary regenerator 3, a secondary regenerator 4, an expander unit 5, an expansion valve 6, a heat exchanger 7, a BOG buffer tank 8 and an LNG storage tank 9 are arranged.
Detailed Description
The invention will be described in further detail with reference to the drawings and examples, it being noted that the examples described below are intended to facilitate the understanding of the invention and are not intended to limit the invention in any way.
As shown in fig. 1, the natural gas BOG direct reliquefaction system based on nitrogen expansion and throttling refrigeration comprises a primary booster unit 1, a secondary booster unit 2, a primary regenerator 3, a secondary regenerator 4, an expander unit 5, an expansion valve 6, a heat exchanger 7, a BOG buffer tank 8 and an LNG storage tank 9.
The outlet of the secondary supercharging unit 2 is sequentially connected with the first channel of the primary regenerator 3, the first channel of the secondary regenerator 4, the expansion valve 6, the first channel of the heat exchanger 7, the second channel of the secondary regenerator 4 and the inlet end of the second channel of the primary regenerator 3.
The outlet of the first channel of the first-stage heat regenerator 3 is respectively connected with the inlet end of the expander unit 5 and the inlet end of the first channel of the second-stage heat regenerator 4, and the outlet of the expander unit 5 is sequentially connected with the third channel of the second-stage heat regenerator 4, the third channel of the first-stage heat regenerator 3 and the inlet end of the first-stage supercharging unit 1.
The outlet of the primary supercharging unit 1 and the outlet of the second channel of the primary regenerator 3 are respectively connected with the inlet end of the secondary supercharging unit 2. The BOG buffer tank 8 is connected with the second channel of the heat exchanger 7 and the inlet end of the LNG storage tank 9 in sequence.
In the embodiment, the primary booster unit 1 adopts a two-stage compression and interstage water cooling mode, the secondary booster unit 2 adopts a one-stage compression and aftercooler water cooling mode, and the expander unit 5 adopts two common gas bearing turbine expanders, and no liquid is carried after expansion.
The low-temperature nitrogen at the outlet of the first channel of the first-stage heat regenerator 3 is divided into two streams, one stream enters the expander unit 5 to be expanded and cooled, the other stream enters the first channel of the second-stage heat regenerator 4 to be cooled to be completely liquefied, the temperature is not lower than the saturation temperature of the nitrogen, then the pressure is reduced through the expansion valve 6, the temperature of the throttled liquid nitrogen is lower than the saturation temperature of the BOG, and the temperature difference is 3 ℃.
The temperature of the nitrogen at the outlet of the second channel and the outlet of the third channel of the primary heat regenerator 3 are equal and lower than the temperature of the nitrogen at the outlet of the secondary pressurizing unit 2, and the temperature difference is 3.04 ℃. The temperature of the nitrogen at the outlet of the second and third channels of the secondary regenerator 4 is equal.
The nitrogen pressure of the primary pressurizing unit 1 is equal to the nitrogen pressure from the outlet of the second channel of the primary heat regenerator 3, and the nitrogen pressure are mixed and enter the secondary pressurizing unit 2 to complete one-time circulation. The primary heat regenerator 3, the secondary heat regenerator 4 and the heat exchanger 7 adopt plate-fin structures. The temperature of the BOG in BOG buffer tank 8 was-140 ℃ and the pressure was 160kPa.
The method for carrying out the natural gas BOG reliquefaction by utilizing the system comprises the following steps:
In the step 1, circulating nitrogen with the temperature, the pressure and the flow rate of 28.81 ℃ and 423kPa and 2129kmol/h respectively is pressurized and cooled to 1286kPa and 31.85 ℃ in a first-stage pressurizing unit 1, then mixed with nitrogen with the flow rate of 1251kmol/h (1286 kPa and 28.81 ℃) from a second channel of the first-stage regenerator 3, and then enters a second-stage pressurizing unit 2 for further pressurizing.
And 2, introducing pressurized normal-temperature nitrogen (4510 kPa,31.85 ℃) with the flow of 3380kmol/h from the secondary pressurizing unit 2 into the primary heat regenerator 3 for cooling, wherein the cooled high-pressure nitrogen is at-92 ℃, and then dividing into two parts, wherein one part (1251 kmol/h) enters a first channel of the secondary heat regenerator 4 for exchanging heat with the low-pressure cold nitrogen flowing back and from the expander unit 5, liquefying the nitrogen, and the other part (2129 kmol/h) enters the expander unit 5 for expansion and cooling.
Step3, the liquid nitrogen (-147.5 ℃) from the secondary regenerator 4 is depressurized through expansion valve 6 to 1316kPa as a gas-liquid mixture.
And 4, the nitrogen-liquid mixture obtained by throttling the expansion valve 6 enters a first channel of the heat exchanger 7, exchanges heat with raw BOG (446.1 kmol/h) from the BOG buffer tank 8, and the liquefied BOG (-162 ℃) is sent to the LNG storage tank for storage.
And 5, respectively feeding the nitrogen (-150.5 ℃) heated by the heat exchanger 7 and the nitrogen (-170.4 ℃) expanded by the expander unit 5 into the second and third channels of the secondary heat regenerator 4, providing the cold energy required for liquefying the high-pressure nitrogen, and simultaneously further heating the nitrogen.
And 6, the heated nitrogen (-111.1 ℃) from the second and third channels of the second-stage heat regenerator 4 respectively enter the second and third channels of the first-stage heat regenerator 3, exchange heat with high-pressure normal-temperature nitrogen (4500 kPa,31.85 ℃) from the second-stage pressurizing unit 2, and are subjected to self-tempering.
And 7, introducing the re-warmed nitrogen (28.81 ℃) in the third channel from the first-stage heat regenerator 3 into the first-stage pressurizing unit 1 for pressurizing, and then introducing the mixture into the second-stage pressurizing unit 2 after mixing with the re-warmed nitrogen (28.81 ℃) in the second channel from the first-stage heat regenerator 3, so as to complete one cycle.
In this example, the pinch temperature difference of each heat exchanger is 3 ℃, the required refrigerant flow is smaller than that of the single-stage reverse brayton cycle, and the energy consumption of unit liquefied product is as low as 0.976 kW.h/kg (BOG).
The foregoing embodiments have described in detail the technical solution and the advantages of the present invention, it should be understood that the foregoing embodiments are merely illustrative of the present invention and are not intended to limit the invention, and any modifications, additions and equivalents made within the scope of the principles of the present invention should be included in the scope of the invention.
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| CN114659338A (en) * | 2022-03-24 | 2022-06-24 | 浙江大学 | Refrigeration system and method for separating heavy hydrocarbon and methane in natural gas BOG |
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| CN114659338A (en) * | 2022-03-24 | 2022-06-24 | 浙江大学 | Refrigeration system and method for separating heavy hydrocarbon and methane in natural gas BOG |
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