A BOG reliquefaction system for LNG ships combined with absorption refrigeration and its use method

Abstract

The present invention provides an LNG ship BOG reliquefaction system combined with absorption refrigeration and a method for using the system, wherein a liquefied natural gas boil-off gas booster module is sequentially connected to a first-stage heat exchanger, a second-stage heat exchanger, a third-stage heat exchanger, a natural gas throttle valve, and a liquefied natural gas separator; a liquefied natural gas separator liquid phase outlet is connected to a liquefied natural gas storage module; a nitrogen compression module is sequentially connected to a second-stage heat exchanger, an ammonia heat exchanger, and a three-way pipe fitting inlet; one outlet of the three-way pipe fitting is sequentially connected to a first-stage refrigerant throttling device and a first-stage mixer inlet; the other outlet of the three-way pipe fitting is sequentially connected to a third-stage heat exchanger, a second-stage refrigerant throttling device, and a first-stage mixer inlet; and the first-stage mixer outlet is sequentially connected to a third-stage heat exchanger, a second-stage heat exchanger, and a nitrogen compression module. The liquefaction process of the present invention has low energy consumption and strong adaptability to different gas sources.

Description

Technical Field

[0001] The present invention relates to the field of chemical engineering and cryogenic engineering technology, and in particular to a BOG reliquefaction system for an LNG ship combined with absorption refrigeration and a method for using the system. Background Art

[0002] As a clean energy, natural gas has been widely used around the world. my country's demand for natural gas is also growing year by year. The market pattern of supply exceeding demand has made my country a major importer of natural gas, of which liquefied natural gas imports account for more than 50% of the total imports. Natural gas is generally transported in the form of gas through pipelines or in the form of liquid through railways, roads or ships, but for long-distance cross-sea transportation, liquefied natural gas ships are generally selected.

[0003] For LNG ships, LNG is generally stored in highly insulated cryogenic tanks at -164℃. However, due to a series of objective factors such as hull shaking and tank heat leakage, part of the LNG is heated and evaporated to produce boil-off gas. With the continuous output of boil-off gas, the tank pressure gradually increases. If this part of boil-off gas is not handled in time, it will cause danger. There are generally three ways to handle boil-off gas: one is to burn the boil-off gas and discharge it through the drain valve, which is obviously not conducive to environmental protection and causes economic losses; the second is to send the boil-off gas into the gas turbine to provide it with power, but when the LNG ship is at port, it is necessary to additionally handle the boil-off gas; the third is to re-liquefy the boil-off gas through the re-liquefaction system and return it to the LNG tank. Therefore, it is very necessary for LNG ships to be equipped with re-liquefaction equipment.

[0004] Traditional liquefied natural gas carrier boil-off gas reliquefaction systems mainly include nitrogen reverse Brayton reliquefaction system, Claude reliquefaction system, Kapitza reliquefaction system, mixed refrigerant reliquefaction system and direct reliquefaction system (partial reliquefaction system). However, with the advancement of science and technology, the use of new refrigeration technology and the rational use of solar energy, waste heat, waste heat, etc. to replace traditional methods will become an inevitable trend for future development to reduce energy consumption.

[0005] The patent with the publication number CN1808027A discloses an operating system of a liquefied natural gas ship for subcooling and liquefying boil-off gas, the operating system comprising: a boil-off gas compressor, a low-temperature heat exchanger connected to a refrigeration system, a first check valve and a first pressure control valve (installed on a pipeline between a liquefied natural gas phase separator and a gas combustion unit), and a second check valve and a second pressure control valve (installed in parallel on a parallel pipeline connected to the aforementioned pipeline). The parallel pipeline is connected to the pipeline between the boil-off gas compressor and the low-temperature heat exchanger so as to provide boil-off gas to the upper steam zone of the liquefied natural gas phase separator. In this way, the pressure and liquid level of the liquefied natural gas phase separator are stably controlled. By stably operating the pressure and liquid level of the liquefied natural gas phase separator, the power consumption is effectively reduced and economic benefits are obtained. The above invention has a high energy consumption. Summary of the invention

[0006] In view of the defects in the prior art, an object of the present invention is to provide a BOG reliquefaction system for an LNG ship combined with absorption refrigeration and a method of using the system.

[0007] According to the present invention, a LNG ship BOG reliquefaction system combined with absorption refrigeration includes a liquefied natural gas boil-off gas booster module, a liquefied cold box module, a nitrogen compression module, an absorption refrigeration module and a liquefied natural gas storage module;

[0008] The liquefied cold box module includes a first-stage heat exchanger, a second-stage heat exchanger, a third-stage heat exchanger, a natural gas throttle valve, a liquefied natural gas separator, an ammonia heat exchanger, a three-way pipe fitting, a first-stage refrigerant throttling device, a first-stage mixer and a second-stage refrigerant throttling device;

[0009] The liquefied natural gas boil-off gas boosting module is connected to the first stage heat exchanger, the second stage heat exchanger, the third stage heat exchanger, the natural gas throttle valve, and the liquefied natural gas separator in sequence;

[0010] The liquid phase outlet of the liquefied natural gas separator is connected to the liquefied natural gas storage module;

[0011] The nitrogen compression module is connected to the second-stage heat exchanger, the ammonia heat exchanger, and the three-way pipe inlet in sequence;

[0012] One outlet of the three-way pipe fitting is connected to the first-stage refrigerant throttling device and the first-stage mixer inlet in sequence; the other outlet of the three-way pipe fitting is connected to the third-stage heat exchanger, the second-stage refrigerant throttling device, and the first-stage mixer inlet in sequence;

[0013] The outlet of the first-stage mixer is connected to the third-stage heat exchanger, the second-stage heat exchanger, and the nitrogen compression module in sequence.

[0014] Preferably, the liquefied natural gas boil-off gas boosting module comprises a boil-off gas first-stage compressor, a boil-off gas first-stage cooler, a boil-off gas second-stage compressor, a boil-off gas second-stage water cooler, a boil-off gas third-stage compressor and a boil-off gas third-stage water cooler connected in sequence;

[0015] The third-stage water cooler for evaporation gas is connected to the second-stage heat exchanger, and the first-stage compressor for evaporation gas is connected to the first-stage heat exchanger.

[0016] Preferably, the first-stage refrigerant throttling device and the second-stage refrigerant throttling device are throttling valves or expansion machines.

[0017] Preferably, the nitrogen compression module comprises a first-stage nitrogen compressor, a first-stage nitrogen cooler, a second-stage nitrogen compressor, a second-stage nitrogen cooler, a third-stage nitrogen compressor and a third-stage nitrogen cooler connected in sequence;

[0018] The first-stage nitrogen compressor is connected to the second-stage heat exchanger, and the third-stage nitrogen cooler is connected to the second-stage heat exchanger.

[0019] Preferably, the absorption refrigeration module comprises a first-stage water cooler, an ammonia solution pump, a fourth-stage heat exchanger, a distillation tower, a second-stage water cooler, a concentrated ammonia solution throttle valve, a second-stage mixer, a dilute ammonia solution throttle valve and a third-stage water cooler;

[0020] The first-stage water cooler is connected to the ammonia solution pump, the fourth-stage heat exchanger and the distillation tower in sequence;

[0021] The top outlet of the rectification tower is connected to the second-stage water cooler, the concentrated ammonia solution throttle valve, the ammonia heat exchanger, and the inlet of the second-stage mixer in sequence;

[0022] The bottom outlet of the rectification tower is connected to the fourth-stage heat exchanger, the dilute ammonia solution throttle valve and the inlet of the second-stage mixer;

[0023] The outlet of the second stage mixer is connected to the first stage water cooler; and the third stage water cooler is connected to the fourth stage heat exchanger.

[0024] Preferably, the first-stage heat exchanger, the second-stage heat exchanger and the third-stage heat exchanger are all multi-stream heat exchangers, and the heat exchanger is in the form of a plate-fin heat exchanger or a coiled-tube heat exchanger.

[0025] The present invention also provides a method for using the LNG ship BOG reliquefaction system combined with absorption refrigeration, comprising the following steps:

[0026] Step 1: After the raw boil-off gas comes out of the liquefied natural gas storage tank and releases cold energy through the first-stage heat exchanger, it is compressed and cooled through the liquefied natural gas boil-off gas booster module, and sequentially enters the second-stage heat exchanger, the first-stage heat exchanger, and the third-stage heat exchanger for cooling and liquefaction. After being throttled and reduced in pressure to the liquefied natural gas storage pressure by the natural gas throttle valve, it enters the liquefied natural gas separator, and the liquefied natural gas product obtained from the bottom enters the liquefied natural gas storage module;

[0027] Step 2: The nitrogen raw material is pressurized and cooled by the nitrogen compression module, and then enters the second-stage heat exchanger and the ammonia heat exchanger for heat exchange and cooling. After passing through the three-way pipe fitting, it is divided into two streams: one stream is throttled and cooled by the first-stage refrigerant throttling device, and the other stream passes through the third-stage heat exchanger and the second-stage refrigerant throttling device in sequence and then enters the third-stage heat exchanger to provide cooling capacity. The two streams are mixed by the first-stage mixer to provide cooling capacity for the third-stage heat exchanger and the second-stage heat exchanger. The nitrogen coming out from the second-stage heat exchanger after providing cooling capacity returns to the nitrogen compression module to complete the refrigeration cycle;

[0028] Step 3: After the ammonia solution is pumped out by the ammonia solution pump, it is heated up by the fourth-stage heat exchanger and then enters the distillation tower for distillation; the concentrated ammonia solution coming out of the top of the distillation tower is cooled by the second-stage water cooler and then throttled and reduced in pressure by the concentrated ammonia solution throttle valve; after releasing latent heat through the ammonia heat exchanger, it enters the inlet of the second-stage mixer; the dilute ammonia solution coming out of the bottom of the distillation tower is cooled by the fourth-stage heat exchanger and then throttled and reduced in pressure by the dilute ammonia solution throttle valve; the two fluids are mixed by the second-stage mixer, cooled by the first-stage water cooler, and then returned to the inlet of the ammonia solution pump to complete the cycle.

[0029] Preferably, in the step 1, when the boil-off gas pressure of the raw liquefied natural gas is higher than 3.8 MPa, the liquefied natural gas boil-off gas boosting module is not enabled.

[0030] Preferably, in step 1, the liquefied natural gas storage pressure is 0.14 MPa.

[0031] Preferably, the heat required by the absorption refrigeration module comes from solar energy, waste heat or industrial waste heat.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. The present invention combines nitrogen expansion with absorption refrigeration to liquefy the boil-off gas of LNG ships, explores the feasibility of combining with new technologies, and has low energy consumption;

[0034] 2. The present invention uses simulation calculations of HYSYS software widely used in the oil and gas industry to prove that the liquefaction process has low energy consumption and strong adaptability to different gas sources, and is more suitable for the reliquefaction process of the boil-off gas reliquefaction device of the nitrogen expansion liquefied natural gas ship combined with absorption refrigeration;

[0035] 3. The liquefaction process of the present invention has low energy consumption and strong adaptability to different gas sources. It also provides the possibility of combining the traditional onboard boil-off gas reliquefaction process with new refrigeration technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0037] Figure 1 This is a process flow chart of the BOG reliquefaction system of an LNG ship combined with absorption refrigeration according to the present invention.

[0038] The figure shows:

[0039] 1. Boiler gas first stage compressor; 2. Boiler gas first stage cooler; 3. Boiler gas second stage compressor; 4. Boiler gas second stage water cooler; 5. Boiler gas third stage compressor; 6. Boiler gas third stage water cooler; 7. Second stage heat exchanger; 8. First stage heat exchanger; 9. Third stage heat exchanger; 10. Natural gas throttle valve; 11. Liquefied natural gas separator; 12. First stage nitrogen compressor; 13. First stage nitrogen cooler; 14. Second stage nitrogen compressor; 15. Second stage nitrogen cooler; 16. Third stage nitrogen compressor; 17. Third stage nitrogen cooler; 18. Ammonia exchanger Heater; 19. Three-way pipe fitting; 20. First-stage refrigerant throttling device; 21. First-stage mixer; 22. Second-stage refrigerant throttling device; 23. First-stage water cooler; 24. Ammonia solution pump; 25. Fourth-stage heat exchanger; 26. Distillation tower; 27. Second-stage water cooler; 28. Concentrated ammonia solution throttle valve; 29. ​​Second-stage mixer; 30. Diluted ammonia solution throttle valve; 31. Third-stage water cooler; 32. Liquefied natural gas storage module; 33. Liquefied natural gas boil-off gas booster module; 34. Liquefied cold box module; 35. Nitrogen compression module; 36. Absorption refrigeration module. DETAILED DESCRIPTION

[0040] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0041] According to the present invention, a LNG ship BOG reliquefaction system combined with absorption refrigeration includes a liquefied natural gas boil-off gas boosting module 33, a liquefied cold box module 34, a nitrogen compression module 35, an absorption refrigeration module 36 and a liquefied natural gas storage module 32.

[0042] The liquefied cold box module 34 includes a first-stage heat exchanger 8, a second-stage heat exchanger 7, a third-stage heat exchanger 9, a natural gas throttle valve 10, a liquefied natural gas separator 11, an ammonia heat exchanger 18, a three-way pipe fitting 19, a first-stage refrigerant throttling device 20, a first-stage mixer 21 and a second-stage refrigerant throttling device 22.

[0043] The liquefied natural gas vapor boosting module 33 is connected to the first-stage heat exchanger 8, the second-stage heat exchanger 7, the third-stage heat exchanger 9, the natural gas throttle valve 10, and the liquefied natural gas separator 11 in sequence. The liquid phase outlet of the liquefied natural gas separator 11 is connected to the liquefied natural gas storage module 32. The nitrogen compression module 35 is connected to the second-stage heat exchanger 7, the ammonia heat exchanger 18, and the inlet of the three-way pipe fitting 19 in sequence. One outlet of the three-way pipe fitting 19 is connected to the first-stage refrigerant throttling device 20 and the inlet of the first-stage mixer 21 in sequence. The other outlet of the three-way pipe fitting 19 is connected to the third-stage heat exchanger 9, the second-stage refrigerant throttling device 22, and the inlet of the first-stage mixer 21 in sequence. The outlet of the first-stage mixer 21 is connected to the third-stage heat exchanger 9, the second-stage heat exchanger 7, and the nitrogen compression module 35 in sequence. The first stage heat exchanger 8, the second stage heat exchanger 7 and the third stage heat exchanger 9 are all multi-stream heat exchangers, and the heat exchanger form is a plate-fin heat exchanger or a coil heat exchanger. The first stage refrigerant throttling device 20 and the second stage refrigerant throttling device 22 are throttling valves or expanders.

[0044] The absorption refrigeration module 36 includes a first-stage water cooler 23, an ammonia solution pump 24, a fourth-stage heat exchanger 25, a distillation tower 26, a second-stage water cooler 27, a concentrated ammonia solution throttle valve 28, a second-stage mixer 29, a dilute ammonia solution throttle valve 30 and a third-stage water cooler 31. The first-stage water cooler is connected to the ammonia solution pump 24, the fourth-stage heat exchanger 25 and the distillation tower 26 in sequence, the top outlet of the distillation tower 26 is connected to the second-stage water cooler 27, the concentrated ammonia solution throttle valve 28, the ammonia heat exchanger 18 and the inlet of the second-stage mixer 29 in sequence, the bottom outlet of the distillation tower 26 is connected to the fourth-stage heat exchanger 25, the dilute ammonia solution throttle valve 30 and the inlet of the second-stage mixer 29, the outlet of the second-stage mixer 29 is connected to the first-stage water cooler 23, and the third-stage water cooler 31 is connected to the fourth-stage heat exchanger 25.

[0045] The nitrogen compression module 35 includes a first-stage nitrogen compressor 12, a first-stage nitrogen cooler 13, a second-stage nitrogen compressor 14, a second-stage nitrogen cooler 15, a third-stage nitrogen compressor 16 and a third-stage nitrogen cooler 17 which are connected in sequence. The first-stage nitrogen compressor 12 is connected to the second-stage heat exchanger 7, and the third-stage nitrogen cooler 17 is connected to the second-stage heat exchanger 7.

[0046] The liquefied natural gas boil-off gas boosting module 33 includes a boil-off gas first-stage compressor 1, a boil-off gas first-stage cooler 2, a boil-off gas second-stage compressor 3, a boil-off gas second-stage water cooler 4, a boil-off gas third-stage compressor 5 and a boil-off gas third-stage water cooler 6, which are connected in sequence. The boil-off gas third-stage water cooler 6 is connected to the second-stage heat exchanger 7, and the boil-off gas first-stage compressor 1 is connected to the first-stage heat exchanger 8.

[0047] The system of the present invention comprises a liquefied natural gas boil-off gas reliquefaction loop, a nitrogen refrigeration cycle loop and an absorption refrigeration cycle loop:

[0048] The specific structure of the liquefied natural gas boil-off gas reliquefaction circuit is as follows: boil-off gas first-stage compressor 1, boil-off gas first-stage cooler 2, boil-off gas second-stage compressor 3, boil-off gas second-stage water cooler 4, boil-off gas third-stage compressor 5, boil-off gas third-stage water cooler 6, first-stage heat exchanger 8, second-stage heat exchanger 7, third-stage heat exchanger 9, natural gas throttle valve 10, and liquefied natural gas separator 11 are connected in sequence;

[0049] The specific structure of the nitrogen refrigeration cycle loop is: a first-stage nitrogen compressor 12, a first-stage nitrogen cooler 13, a second-stage nitrogen compressor 14, a second-stage nitrogen cooler 15, a third-stage nitrogen compressor 16, a third-stage nitrogen cooler 17, a second-stage heat exchanger 7, an ammonia heat exchanger 18 and a three-way pipe fitting 19;

[0050] The specific structure of the absorption refrigeration cycle loop is: a first-stage water cooler 23, an ammonia solution pump 24, a fourth-stage heat exchanger 25, a distillation tower 26, a second-stage water cooler 27, a concentrated ammonia solution throttle valve 28, an ammonia heat exchanger 18, a second-stage mixer 29, a dilute ammonia solution throttle valve 30 and a third-stage water cooler 31.

[0051] As a preferred technical solution of the present invention, the system of the present invention may also selectively include a nitrogen storage unit, an instrument control unit, an instrument air and PSA nitrogen production module, and a generator module. The generator module is used to provide electrical energy for the reliquefaction process when there is no power supply system.

[0052] The present invention also provides a method for using the above-mentioned LNG ship BOG reliquefaction system combined with absorption refrigeration, comprising the following steps:

[0053] Step 1: After the raw boil-off gas comes out of the liquefied natural gas storage tank and releases cold energy through the first-stage heat exchanger 8, it is compressed and cooled through the liquefied natural gas boil-off gas booster module 33, and sequentially enters the second-stage heat exchanger 7, the first-stage heat exchanger 8, and the third-stage heat exchanger 9 for cooling and liquefaction. After being throttled and reduced in pressure to the liquefied natural gas storage pressure through the natural gas throttle valve 10, it enters the liquefied natural gas separator 11, and the liquefied natural gas product obtained from the bottom enters the liquefied natural gas storage module 32;

[0054] Step 2: The nitrogen raw material is pressurized and cooled by the nitrogen compression module 35, and then enters the second-stage heat exchanger 7 and the ammonia heat exchanger 18 for heat exchange and cooling. After passing through the three-way pipe fitting 19, it is divided into two streams: one stream passes through the first-stage refrigerant throttling device 20 for throttling and cooling, and the other stream passes through the third-stage heat exchanger 9 and the second-stage refrigerant throttling device 22 in sequence and then enters the third-stage heat exchanger 9 to provide cooling. The two streams are mixed by the first-stage mixer 21 to provide cooling for the third-stage heat exchanger 9 and the second-stage heat exchanger 7. The nitrogen coming out from the second-stage heat exchanger 7 after providing cooling returns to the nitrogen compression module 35 to complete the refrigeration cycle;

[0055] Step 3: The ammonia solution is pumped out by the ammonia solution pump 24, heated by the fourth-stage heat exchanger 25, and then enters the distillation tower 26 for distillation. The concentrated ammonia solution coming out from the top of the distillation tower 26 is cooled by the second-stage water cooler 27, throttled and reduced in pressure by the concentrated ammonia solution throttle valve 28, and enters the inlet of the second-stage mixer 29 after releasing latent heat through the ammonia heat exchanger 18. The dilute ammonia solution coming out from the bottom of the distillation tower 26 is throttled and reduced in pressure by the dilute ammonia solution throttle valve 30 after heat exchange and cooling in the fourth-stage heat exchanger 25. The two fluids are mixed through the second-stage mixer 29, cooled by the first-stage water cooler 23, and then returned to the inlet of the ammonia solution pump 24 to complete the cycle.

[0056] In step 1, when the boil-off gas pressure of the raw liquefied natural gas is higher than 3.8 MPa, the liquefied natural gas boil-off gas pressurizing module 33 is not activated. The storage pressure of liquefied natural gas is 0.14 MPa.

[0057] The heat required by the absorption refrigeration module 36 comes from solar energy, waste heat or industrial waste heat.

[0058] The method for reliquefying boil-off gas by using the boil-off gas reliquefaction system of liquefied natural gas ship with nitrogen expansion combined with absorption refrigeration of the present invention is specifically shown in the following embodiments:

[0059] Example 1

[0060] The molar composition of liquefied natural gas boil-off gas is 95% CH4+2% C2H6+3% N2, pressure 0.14MPa, temperature -125℃, flow rate 350kg / h; nitrogen flow rate 2000kg / h. The specific steps of the liquefied natural gas ship boil-off gas reliquefaction process combined with nitrogen expansion by absorption refrigeration are as follows:

[0061] 1. The liquefied natural gas boil-off gas raw material releases cold energy through the first-stage heat exchanger 8 and is heated to -35°C;

[0062] 2. The evaporation gas heated in step 1 is compressed to 3.8 MPa by a three-stage compressor (evaporation gas first-stage compressor 1, evaporation gas second-stage compressor 3, evaporation gas third-stage compressor 5), and cooled to 30° C. by a cooler (evaporation gas first-stage cooler 2, evaporation gas second-stage water cooler 4, evaporation gas third-stage water cooler 6);

[0063] 3. The evaporated gas from step 2 is cooled to -19°C through the second stage heat exchanger 7, cooled to -81°C through the first stage heat exchanger 8, and cooled to -136°C through the third stage heat exchanger 9;

[0064] 4. The high-pressure and low-temperature liquefied natural gas obtained in step 3 passes through a natural gas throttle valve 10, and after throttling and reducing the pressure to 0.14 MPa, enters a liquefied natural gas separator 11, and obtains a liquefied natural gas product from the bottom, which is input into a liquefied natural gas storage tank;

[0065] 5. The nitrogen raw material is first pressurized to 2.47 MPa by the first-stage nitrogen compressor 12, the second-stage nitrogen compressor 14, and the third-stage nitrogen compressor 16, and then cooled to 30° C. by the first-stage nitrogen cooler 13, the first-stage nitrogen cooler 15, and the first-stage nitrogen cooler 17;

[0066] 6. The nitrogen cooled in step 5 is cooled to -26°C through the second stage heat exchanger 7 and the ammonia heat exchanger 18;

[0067] 7. The nitrogen cooled in step 6 is divided into two streams by the three-way pipe fitting 19. One stream is throttled and cooled to 0.18 MPa by the first-stage refrigerant throttling device 20; the other stream is cooled to -78°C by the third-stage heat exchanger 9, and the second-stage refrigerant throttling device 22 is used to reduce the pressure to 0.18 MPa, and then enters the third-stage heat exchanger 9 to provide cooling capacity and heat it to -117°C.

[0068] 8. The two fluids from step 7 are mixed through the first stage mixer 21;

[0069] 9. The nitrogen mixed in step 8 provides cooling capacity for the third-stage heat exchanger 9 and the second-stage heat exchanger 7. The nitrogen is reheated to 25°C and then returned to the first-stage nitrogen compressor 12 to complete the refrigeration cycle;

[0070] 10. The ammonia solution is first pumped out through the ammonia solution pump 24 to increase the pressure to 1.3 MPa;

[0071] 11. The ammonia solution pressurized in step 10 is heated to 123° C. in the fourth-stage heat exchanger 25;

[0072] 12. The ammonia solution heated in step 11 enters the distillation tower 26 for rectification. The concentrated ammonia solution coming out of the top of the distillation tower 26 is cooled to 30° C. by the second-stage water cooler 27, throttled and depressurized to 0.12 MPa by the concentrated ammonia solution throttle valve 28, and enters the inlet of the second-stage mixer 29 after releasing latent heat by the ammonia heat exchanger 18; the dilute ammonia solution coming out of the bottom of the distillation tower 26 is cooled to 38° C. by the fourth-stage heat exchanger 25, and throttled and depressurized to 0.12 MPa by the dilute ammonia solution throttle valve 30;

[0073] 13. The two fluids from step 12 are mixed by the second stage mixer 29.

[0074] 14. The ammonia solution mixed in step 13 is cooled to 30° C. by the first-stage water cooler 23, and then returns to the inlet end of the ammonia solution pump 24 to complete the cycle.

[0075] Through simulation calculation, it is concluded that the unit energy consumption of the evaporated gas reliquefaction process of the LNG carrier with nitrogen expansion combined with absorption refrigeration is 0.7878 kWh / kgLNG.

[0076] Example 2

[0077] The molar composition of liquefied natural gas boil-off gas is 93% CH4+2% C2H6+5% N2, pressure is 0.16MPa, temperature is -120℃, flow rate is 350kg / h, and nitrogen flow rate is 2030kg / h. The specific steps of the process of reliquefaction of boil-off gas of liquefied natural gas ship with nitrogen expansion combined with absorption refrigeration are as follows:

[0078] 1. The liquefied natural gas boil-off gas raw material releases cold energy through the first-stage heat exchanger 8 and is heated to -35°C;

[0079] 2. The evaporation gas heated in step 1 is compressed to 3.8 MPa by a three-stage compressor (evaporation gas first-stage compressor 1, evaporation gas second-stage compressor 3, evaporation gas third-stage compressor 5), and cooled to 30° C. by a cooler (evaporation gas first-stage cooler 2, evaporation gas second-stage water cooler 4, evaporation gas third-stage water cooler 6);

[0080] 3. The evaporated gas in step 2 is cooled to -19°C through the second stage heat exchanger 7, cooled to -79°C through the first stage heat exchanger 8, and cooled to -139°C through the third stage heat exchanger 9;

[0081] 4. The high-pressure and low-temperature liquefied natural gas obtained in step 3 passes through a natural gas throttle valve 10, and after throttling and reducing the pressure to 0.14 MPa, enters a liquefied natural gas separator 11, and obtains a liquefied natural gas product from the bottom, which is input into a liquefied natural gas storage tank;

[0082] 5. The nitrogen raw material is first pressurized to 2.47 MPa by the first-stage nitrogen compressor 12, the second-stage nitrogen compressor 14, and the third-stage nitrogen compressor 16, and then cooled to 30° C. by the first-stage nitrogen cooler 13, the first-stage nitrogen cooler 15, and the first-stage nitrogen cooler 17;

[0083] 6. The nitrogen cooled in step 5 is cooled to -26°C through the second stage heat exchanger 7 and the ammonia heat exchanger 18;

[0084] 7. The nitrogen cooled in step 6 is divided into two streams by the three-way pipe fitting 19. One stream is throttled and cooled to 0.18 MPa by the first-stage refrigerant throttling device 20; the other stream is cooled to -78°C by the third-stage heat exchanger 9, and the second-stage refrigerant throttling device 22 is used to reduce the pressure to 0.18 MPa, and then enters the third-stage heat exchanger 9 to provide cooling capacity and heat it to -117°C.

[0085] 8. The two fluids from step 7 are mixed through the first stage mixer 21;

[0086] 9. The nitrogen mixed in step 8 provides cooling capacity for the third-stage heat exchanger 9 and the second-stage heat exchanger 7. The nitrogen is reheated to 25°C and then returned to the first-stage nitrogen compressor 12 to complete the refrigeration cycle;

[0087] 10. The ammonia solution is first pumped out through the ammonia solution pump 24 to increase the pressure to 1.3 MPa;

[0088] 11. The ammonia solution pressurized in step 10 is heated to 123° C. in the fourth-stage heat exchanger 25;

[0089] 12. The ammonia solution heated in step 11 enters the distillation tower 26 for rectification. The concentrated ammonia solution coming out of the top of the distillation tower 26 is cooled to 30° C. by the second-stage water cooler 27, throttled and depressurized to 0.12 MPa by the concentrated ammonia solution throttle valve 28, and enters the inlet of the second-stage mixer 29 after releasing latent heat by the ammonia heat exchanger 18; the dilute ammonia solution coming out of the bottom of the distillation tower 26 is cooled to 38° C. by the fourth-stage heat exchanger 25, and throttled and depressurized to 0.12 MPa by the dilute ammonia solution throttle valve 30;

[0090] 13. The two fluids from step 12 are mixed by the second stage mixer 29.

[0091] 14. The ammonia solution mixed in step 13 is cooled to 30° C. by the first-stage water cooler 23, and then returns to the inlet end of the ammonia solution pump 24 to complete the cycle.

[0092] Through simulation calculation, it is concluded that the unit energy consumption of the evaporated gas reliquefaction process of the LNG carrier with nitrogen expansion combined with absorption refrigeration is 0.7844 kWh / kgLNG.

[0093] Example 3

[0094] The molar composition of liquefied natural gas boil-off gas is 80% CH4+5% C2H6+15% N2, pressure is 0.2MPa, temperature is -100℃, flow rate is 350kg / h, and nitrogen flow rate is 2100kg / h. The specific steps of the process of reliquefaction of boil-off gas of liquefied natural gas ship with nitrogen expansion combined with absorption refrigeration are as follows:

[0095] 1. The liquefied natural gas boil-off gas raw material releases cold energy through the first-stage heat exchanger 8 and is heated to -35°C;

[0096] 2. The evaporation gas heated in step 1 is compressed to 3.8 MPa by a three-stage compressor (evaporation gas first-stage compressor 1, evaporation gas second-stage compressor 3, evaporation gas third-stage compressor 5), and cooled to 30° C. by a cooler (evaporation gas first-stage cooler 2, evaporation gas second-stage water cooler 4, evaporation gas third-stage water cooler 6);

[0097] 3. The evaporated gas from step 2 is cooled to -19°C through the second stage heat exchanger 7, cooled to -68°C through the first stage heat exchanger 8, and cooled to -154°C through the third stage heat exchanger 9;

[0098] 4. The high-pressure and low-temperature liquefied natural gas obtained in step 3 passes through a natural gas throttle valve 10, and after throttling and reducing the pressure to 0.14 MPa, enters a liquefied natural gas separator 11, and obtains a liquefied natural gas product from the bottom, which is input into a liquefied natural gas storage tank;

[0099] 5. The nitrogen raw material is first pressurized to 2.47 MPa by the first-stage nitrogen compressor 12, the second-stage nitrogen compressor 14, and the third-stage nitrogen compressor 16, and then cooled to 30° C. by the first-stage nitrogen cooler 13, the first-stage nitrogen cooler 15, and the first-stage nitrogen cooler 17;

[0100] 6. The nitrogen cooled in step 5 is cooled to -26°C through the second stage heat exchanger 7 and the ammonia heat exchanger 18;

[0101] 7. The nitrogen cooled in step 6 is divided into two streams by the three-way pipe fitting 19. One stream is throttled and cooled to 0.18 MPa by the first-stage refrigerant throttling device 20; the other stream is cooled to -78°C by the third-stage heat exchanger 9, and the second-stage refrigerant throttling device 22 is used to reduce the pressure to 0.18 MPa, and then enters the third-stage heat exchanger 9 to provide cooling capacity and heat it to -117°C.

[0102] 8. The two fluids from step 7 are mixed through the first stage mixer 21;

[0103] 9. The nitrogen mixed in step 8 provides cooling capacity for the third-stage heat exchanger 9 and the second-stage heat exchanger 7. The nitrogen is reheated to 25°C and then returned to the first-stage nitrogen compressor 12 to complete the refrigeration cycle;

[0104] 10. The ammonia solution is first pumped out through the ammonia solution pump 24 to increase the pressure to 1.3 MPa;

[0105] 11. The ammonia solution pressurized in step 10 is heated to 123° C. in the fourth-stage heat exchanger 25;

[0106] 12. The ammonia solution heated in step 11 enters the distillation tower 26 for rectification. The concentrated ammonia solution coming out of the top of the distillation tower 26 is cooled to 30° C. by the second-stage water cooler 27, throttled and depressurized to 0.12 MPa by the concentrated ammonia solution throttle valve 28, and enters the inlet of the second-stage mixer 29 after releasing latent heat by the ammonia heat exchanger 18; the dilute ammonia solution coming out of the bottom of the distillation tower 26 is cooled to 38° C. by the fourth-stage heat exchanger 25, and throttled and depressurized to 0.12 MPa by the dilute ammonia solution throttle valve 30;

[0107] 13. The two fluids from step 12 are mixed by the second stage mixer 29.

[0108] 14. The ammonia solution mixed in step 13 is cooled to 30° C. by the first-stage water cooler 23, and then returns to the inlet end of the ammonia solution pump 24 to complete the cycle.

[0109] Through simulation calculation, it is concluded that the unit energy consumption of the evaporated gas reliquefaction process of the LNG carrier with nitrogen expansion combined with absorption refrigeration is 0.7867 kWh / kgLNG.

[0110] According to different liquefied natural gas boil-off gas components, the nitrogen flow rate also needs to be adjusted accordingly when necessary to minimize the process energy consumption. By comparing Example 1, Example 2 and Example 3, it can be found that the liquefied natural gas ship boil-off gas reliquefaction process combined with nitrogen expansion absorption refrigeration has good adaptability to boil-off gas from different gas sources, and most process parameters do not need to be adjusted, only the nitrogen flow rate needs to be adjusted according to the boil-off gas components.

[0111] Those skilled in the art know that, in addition to realizing the system and its various devices, modules, and units provided by the present invention in a purely computer-readable program code, it is entirely possible to realize the same functions in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered as a hardware component, and the devices, modules, and units included therein for realizing various functions can also be regarded as structures within the hardware component; the devices, modules, and units for realizing various functions can also be regarded as both software modules for realizing the method and structures within the hardware component.

[0112] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A BOG reliquefaction system for LNG carriers combined with absorption refrigeration, characterized in that: It comprises a liquefied natural gas boil-off gas pressurizing module (33), a liquefied cold box module (34), a nitrogen compression module (35), an absorption refrigeration module (36) and a liquefied natural gas storage module (32); The liquefied cold box module (34) comprises a first-stage heat exchanger (8), a second-stage heat exchanger (7), a third-stage heat exchanger (9), a natural gas throttle valve (10), a liquefied natural gas separator (11), an ammonia heat exchanger (18), a three-way pipe fitting (19), a first-stage refrigerant throttling device (20), a first-stage mixer (21) and a second-stage refrigerant throttling device (22); The liquefied natural gas boil-off gas boosting module (33) is connected in sequence to the first-stage heat exchanger (8), the second-stage heat exchanger (7), the third-stage heat exchanger (9), the natural gas throttle valve (10), and the liquefied natural gas separator (11); The liquid phase outlet of the liquefied natural gas separator (11) is connected to the liquefied natural gas storage module (32); The nitrogen compression module (35) is connected to the second-stage heat exchanger (7), the ammonia heat exchanger (18), and the inlet of the three-way pipe (19) in sequence; One outlet of the three-way pipe fitting (19) is connected to the first-stage refrigerant throttling device (20) and the inlet of the first-stage mixer (21) in sequence; the other outlet of the three-way pipe fitting (19) is connected to the third-stage heat exchanger (9), the second-stage refrigerant throttling device (22), and the inlet of the first-stage mixer (21) in sequence; The outlet of the first-stage mixer (21) is connected to the third-stage heat exchanger (9), the second-stage heat exchanger (7), and the nitrogen compression module (35) in sequence; The absorption refrigeration module (36) comprises a first-stage water cooler (23), an ammonia solution pump (24), a fourth-stage heat exchanger (25), a distillation tower (26), a second-stage water cooler (27), a concentrated ammonia solution throttle valve (28), a second-stage mixer (29), a dilute ammonia solution throttle valve (30) and a third-stage water cooler (31); The first-stage water cooler (23) is connected to the ammonia solution pump (24), the fourth-stage heat exchanger (25) and the distillation tower (26) in sequence; The top outlet of the distillation tower (26) is connected in sequence to the second-stage water cooler (27), the concentrated ammonia solution throttle valve (28), the ammonia heat exchanger (18), and the inlet of the second-stage mixer (29); The bottom outlet of the distillation tower (26) is connected to the fourth-stage heat exchanger (25), the dilute ammonia solution throttle valve (30) and the inlet of the second-stage mixer (29); The outlet of the second stage mixer (29) is connected to the first stage water cooler (23); and the third stage water cooler (31) is connected to the fourth stage heat exchanger (25).

2. The LNG ship BOG reliquefaction system combined with absorption refrigeration according to claim 1, characterized in that: The liquefied natural gas boil-off gas boosting module (33) comprises a boil-off gas first-stage compressor (1), a boil-off gas first-stage cooler (2), a boil-off gas second-stage compressor (3), a boil-off gas second-stage water cooler (4), a boil-off gas third-stage compressor (5) and a boil-off gas third-stage water cooler (6) which are connected in sequence; The evaporated gas third-stage water cooler (6) is connected to the second-stage heat exchanger (7), and the evaporated gas first-stage compressor (1) is connected to the first-stage heat exchanger (8).

3. The LNG ship BOG reliquefaction system combined with absorption refrigeration according to claim 1, characterized in that: The first-stage refrigerant throttling device (20) and the second-stage refrigerant throttling device (22) are throttling valves or expansion machines.

4. The LNG ship BOG reliquefaction system combined with absorption refrigeration according to claim 1, characterized in that: The nitrogen compression module (35) comprises a first-stage nitrogen compressor (12), a first-stage nitrogen cooler (13), a second-stage nitrogen compressor (14), a second-stage nitrogen cooler (15), a third-stage nitrogen compressor (16) and a third-stage nitrogen cooler (17) which are connected in sequence; The first-stage nitrogen compressor (12) is connected to the second-stage heat exchanger (7), and the third-stage nitrogen cooler (17) is connected to the second-stage heat exchanger (7).

5. The LNG ship BOG reliquefaction system combined with absorption refrigeration according to claim 1, characterized in that: The first-stage heat exchanger (8), the second-stage heat exchanger (7) and the third-stage heat exchanger (9) are all multi-stream heat exchangers, and the heat exchanger form is a plate-fin heat exchanger or a coiled-tube heat exchanger.

6. A method for using the LNG ship BOG reliquefaction system combined with absorption refrigeration according to any one of claims 1 to 5, characterized in that: The steps include: Step 1: After the raw boil-off gas comes out of the liquefied natural gas storage tank and releases cold energy through the first-stage heat exchanger (8), it is compressed and cooled through the liquefied natural gas boil-off gas booster module (33), and sequentially enters the second-stage heat exchanger (7), the first-stage heat exchanger (8), and the third-stage heat exchanger (9) for cooling and liquefaction. After being throttled and reduced in pressure to the liquefied natural gas storage pressure through the natural gas throttle valve (10), it enters the liquefied natural gas separator (11), and the liquefied natural gas product obtained from the bottom enters the liquefied natural gas storage module (32); Step 2: The nitrogen raw material is pressurized and cooled by the nitrogen compression module (35), and then enters the second-stage heat exchanger (7) and the ammonia heat exchanger (18) for heat exchange and cooling. After passing through the three-way pipe (19), it is divided into two streams of fluid: one stream passes through the first-stage refrigerant throttling device (20) for throttling and cooling, and the other stream passes through the third-stage heat exchanger (9) and the second-stage refrigerant throttling device (22) in sequence and then enters the third-stage heat exchanger (9) to provide cooling. The two streams of fluid are mixed by the first-stage mixer (21) to provide cooling for the third-stage heat exchanger (9) and the second-stage heat exchanger (7). The nitrogen that comes out of the second-stage heat exchanger (7) after providing cooling returns to the nitrogen compression module (35) to complete the refrigeration cycle; Step 3: The ammonia solution is pumped out by the ammonia solution pump (24), heated by the fourth-stage heat exchanger (25), and then enters the distillation tower (26) for distillation. The concentrated ammonia solution coming out from the top of the distillation tower (26) is cooled by the second-stage water cooler (27), throttled and reduced in pressure by the concentrated ammonia solution throttle valve (28), and then enters the inlet of the second-stage mixer (29) after releasing latent heat by the ammonia heat exchanger (18). The dilute ammonia solution coming out from the bottom of the distillation tower (26) is cooled by the fourth-stage heat exchanger (25), throttled and reduced in pressure by the dilute ammonia solution throttle valve (30), and the two fluids are mixed by the second-stage mixer (29), cooled by the first-stage water cooler (23), and then returned to the inlet of the ammonia solution pump (24) to complete the cycle.

7. The method for using the LNG ship BOG reliquefaction system combined with absorption refrigeration according to claim 6, characterized in that: In the step 1, when the boil-off gas pressure of the raw liquefied natural gas is higher than 3.8 MPa, the liquefied natural gas boil-off gas boosting module (33) is not activated.

8. The method for using the LNG ship BOG reliquefaction system combined with absorption refrigeration according to claim 6, characterized in that: In the step 1, the liquefied natural gas storage pressure is 0.14 MPa.

9. The method for using the LNG ship BOG reliquefaction system combined with absorption refrigeration according to claim 6, characterized in that: The heat required by the absorption refrigeration module (36) comes from solar energy, waste heat or industrial waste heat.

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