A Shallow Cooling Process LNG Recovery System and Recovery Method

Abstract

The present invention relates to a shallow cooling process LNG recovery system and a recovery method. The shallow cooling process LNG recovery system includes a raw gas pressurization and cooling unit, an ammonia-cooled raw gas unit, a raw gas separation LPG and NGL unit, a nitrogen refrigeration cycle unit, and a raw gas conversion LNG unit. The present invention uses ammonia as a refrigerant, circulates the natural gas into a four-stream plate-fin heat exchanger after being pressurized by a nitrogen booster, continuously cools the raw natural gas in the cold energy generated by the expansion in the expander, has a low cost and a high LNG recovery rate. At the same time, the circulating nitrogen continuously absorbs the cold energy released by the natural gas to generate LNG, improving the energy utilization efficiency of the system, and can be widely applied to the recovery of LNG.

Description

Technical Field

[0001] The present invention belongs to the technical field of LNG recovery, and particularly relates to a shallow cooling process LNG recovery system and a recovery method. Background Art

[0002] The volume of LNG (liquefied natural gas) is 1 / 625 of its gaseous volume (20 °C, 101.325 kPa), which is convenient for storage and transportation. Moreover, it is the cleanest fossil energy at present, and its consumption has been growing rapidly. In actual production, LNG can not only be used as a clean alternative fuel for petroleum products, but also be used to produce methanol, ammonia and other chemical products. In addition, LNG can also be used for civil gas peak shaving, and its vaporization latent heat can also be used in industries such as refrigeration and cold storage, with a wide range of uses.

[0003] With the development of China's natural gas industry, the demand for liquefied natural gas is increasing continuously, and the design calculation of liquefied natural gas restricts its development. At present, the main LNG recovery used is mixed refrigerant. However, for the composition of the mixed refrigerant, generally a confidentiality method is adopted, and its composition and ratio are the core technical secrets of the enterprise. The price of purchasing such mixed refrigerant is expensive, which causes certain difficulties for the further development of natural gas liquefaction. Summary of the Invention

[0004] Aiming at the above problems, the purpose of the present invention is to provide a shallow cooling process LNG recovery system and a recovery method, which use ammonia as a refrigerant, circulate the natural gas into a multi-stream heat exchanger after being pressurized by a nitrogen booster, and continuously cool the raw natural gas in the cold energy generated by the expansion of the expander, with low cost and high LNG recovery rate.

[0005] The technical solution of the present invention lies in: a shallow-cooling process LNG recovery system, which includes a raw gas pressurization and cooling unit, an ammonia-cooled raw gas unit, a raw gas separation LPG and NGL unit, a nitrogen refrigeration cycle unit, and a raw gas conversion to LNG unit; wherein: the raw gas pressurization and cooling unit includes a raw gas compressor, a first cooler, and a second cooler connected in sequence through pipelines; the ammonia-cooled raw gas unit includes a first mixer, an ammonia compressor, a third cooler, a first throttle valve, a diverter, and a first circulator connected in sequence through pipelines; the inlet of the first mixer is connected to the outlet of the second cooler of the raw gas pressurization and cooling unit, and the outlet of the first circulator is connected to the inlet of the second cooler of the raw gas pressurization and cooling unit on one hand and the inlet of the first mixer on the other hand; the raw gas separation LPG and NGL unit includes a separator, a second mixer, a deethanizer, a pump, and a liquefied gas tower connected in sequence through pipelines; the nitrogen refrigeration cycle unit includes an expander, a nitrogen booster three, a sixth cooler, a nitrogen booster one, a fifth cooler, a nitrogen booster two, a fourth cooler, and a second circulator connected in sequence through pipelines; the raw gas conversion to LNG unit includes a multi-stream heat exchanger and a second throttle valve connected in sequence through pipelines; the inlets of the multi-stream heat exchanger are respectively connected to the outlet of the second cooler of the raw gas pressurization and cooling unit, the outlet of the second mixer of the raw gas separation LPG and NGL unit, the outlet of the expander of the nitrogen refrigeration cycle unit, and the outlet of the second circulator; the outlets of the multi-stream heat exchanger are respectively connected to the second throttle valve, the inlet of the expander of the nitrogen refrigeration cycle unit, the inlet of the nitrogen booster three, and the outlet of the separator of the raw gas separation LPG and NGL unit.

[0006] The first cooler is connected to the outlet of the raw gas compressor, and the inlet of the second cooler is connected to the outlet of the first cooler.

[0007] The inlet of the third cooler is connected to the outlet of the ammonia compressor; the inlet of the first throttle valve is connected to the outlet of the third cooler; the inlet of the diverter is connected to the outlet of the first throttle valve; the outlet of the diverter is connected to the inlet of the first circulator.

[0008] The liquid outlet of the separator is connected to the deethanizer, the gas outlet of the separator is connected to the second mixer, the inlet of the pump is connected to the outlet of the deethanizer, the outlet of the pump is connected to the inlet of the liquefied gas tower, and the gas outlet of the deethanizer is connected to the second mixer.

[0009] The third outlet of the nitrogen booster is connected to the inlet of the sixth cooler; the outlet of the sixth cooler is connected to the first inlet of the nitrogen booster; the first outlet of the nitrogen booster is connected to the inlet of the fifth cooler; the outlet of the fifth cooler is connected to the second inlet of the nitrogen booster; the second outlet of the nitrogen booster is connected to the inlet of the fourth cooler, and the outlet of the fourth cooler is connected to the inlet of the second circulator.

[0010] The multi-stream heat exchanger is a four-stream plate-fin heat exchanger.

[0011] The liquefied gas tower is provided with 24 trays.

[0012] The deethanizer is provided with 25 trays.

[0013] A method for recovering LNG by a shallow cooling process uses any one of the shallow cooling process LNG recovery systems described above, and the specific process is as follows:

[0014] S1: The raw gas is pressurized, cooled, heat-exchanged and then throttled to produce LNG;

[0015] The raw gas, with a temperature of 30°C and a pressure of 900 kPa, enters the raw gas compressor for compression. After compression, the outlet gas stream has a temperature of 56.11°C and a pressure of 1200 kPa. It enters the first cooler for cooling. After cooling, the gas stream has a temperature of 32°C and a pressure of 1170 kPa. It enters the second cooler for further cooling. After cooling, the gas stream has a temperature of -30°C and a pressure of 1155 kPa. It enters the multi-stream heat exchanger for sufficient heat exchange. After heat exchange, the gas stream has a temperature of -100°C and a pressure of 1140 kPa. It enters the separator for gas-liquid separation. After separation, the gas stream has a temperature of -100°C and a pressure of 1140 kPa. It enters the mixer. The gas stream coming out of the mixer has a temperature of -98.13°C and a pressure of 950 kPa. It enters the multi-stream heat exchanger again for heat exchange. After heat exchange, the gas stream has a temperature of -153°C and a pressure of 935 kPa. Finally, it enters the first throttle valve for throttling. The LNG coming out has a temperature of -152.8°C and a pressure of 40 kPa;

[0016] S2: The process of recycling the raw gas;

[0017] The gas stream coming out of the bottom of the separator has a temperature of -100°C and a pressure of 1140 kPa. It enters the deethanizer for sufficient rectification. The deethanizer is provided with 25 trays. The top pressure is set to 950 kPa, and the bottom pressure is set to 958 kPa. The molar content of ethane in the reboiler of the deethanizer is 8%. After sufficient rectification, the gas stream coming out of the top has a temperature of -49.01°C and a pressure of 950 kPa and enters the mixer together with the gas stream coming out of the separator, repeating the steps in S1, entering the multi-stream heat exchanger for heat exchange and then throttling to produce LNG;

[0018] S3: The process of separating LPG and NGL from the raw gas;

[0019] The temperature of the stream coming out of the bottom of the deethanizer is 33.61°C and the pressure is 958 kPa. It enters a pump for pressurization. After pressurization, the temperature of the stream is 34.56°C and the pressure is 2000 kPa. Then it enters the LPG tower for full rectification. The LPG tower is equipped with trays. The top pressure is designed to be 1500 kPa and the bottom pressure is designed to be 1508 kPa. The reflux ratio is set to 1, and the nC5 flow rate at the bottom is set to 1 kgmole / h. The temperature of the LPG coming out of the top of the tower is 47.51°C and the pressure is 1500 kPa. The temperature of the NGL coming out of the bottom of the tower is 183.5°C and the pressure is 1508 kPa.

[0020] In the step S1, the ammonia stream used to provide cooling capacity for the second cooler has a temperature of -35°C and a pressure of 91.52 kPa. It enters the first mixer 5. The ammonia stream coming out of the first mixer has a temperature of -35°C and a pressure of 91.52 kPa. It enters an ammonia compressor for compression. After compression, the temperature of the stream is 114.4°C and the pressure is 1700 kPa. It enters the third cooler for cooling. After cooling, the temperature of the stream is 35°C and the pressure is 1680 kPa. It enters the second throttle valve for throttling. After throttling, the temperature of the stream is -35°C and the pressure is 91.52 kPa. It enters a diverter to split into two parts of the stream. One part of the stream enters the first mixer, and the other part of the stream enters the second cooler through a recycle loop.

[0021] In the step S1, to make full use of the cold energy released by the generation of LNG from natural gas, a nitrogen refrigeration cycle unit is used for cyclic absorption. The added nitrogen has a temperature of 28.45°C and a pressure of 425 kPa and enters the third nitrogen booster for pressurization. After pressurization, the temperature is 158.9°C and the pressure is 1150 kPa. It enters the sixth cooler for cooling. After cooling, the temperature of the stream is 32°C and the pressure is 1130 kPa. It enters the second nitrogen booster for compression. After compression, the temperature of the stream is 155.03°C and the pressure is 2880 kPa. Then it enters the fourth cooler. After cooling, the temperature of the stream is 35°C and the pressure is 5300 kPa. It enters the second circulator and is recycled to the multi-stream heat exchanger. The temperature of the stream coming out is -70°C and the pressure is 5285 kPa. The stream coming out of the multi-stream heat exchanger enters an expander for expansion refrigeration. In the expander, nitrogen undergoes adiabatic expansion and does external work, enabling the gas to obtain a low temperature. The temperature of the stream coming out of the expander is -168.6°C and the pressure is 440 kPa. It enters the multi-stream heat exchanger again for heat exchange. The temperature of the stream coming out is 28.45°C and the pressure is 425 kPa and enters the first nitrogen booster.

[0022] The technical effects of the present invention are as follows: 1. The present invention uses ammonia as a refrigerant, and after the natural gas is pressurized by a nitrogen booster, it circulates into a multi-stream heat exchanger, continuously cooling the raw natural gas in the cold energy generated by the expansion in the expander, improving the LNG recovery rate. At the same time, the circulating nitrogen continuously absorbs the cold energy released by the natural gas to generate LNG, improving the energy utilization efficiency of the system; 2. While recovering LNG, the deethanizer and the liquefied gas tower in the system can simultaneously convert a part of the natural gas into liquefied petroleum gas products LPG and NGL, improving the utilization rate of natural gas.

[0023] The following will be further described with reference to the accompanying drawings. Brief Description of the Drawings

[0024] Figure 1 It is a schematic flow diagram of a shallow-cooling process LNG recovery system of the present invention.

[0025] Reference numerals in the drawings: 1 - raw gas; 2 - raw gas compressor; 3 - first cooler; 4 - second cooler; 5 - first mixer; 6 - third cooler; 7 - first throttle valve; 8 - splitter; 9 - ammonia compressor; 10 - first circulator; 11 - separator; 12 - second mixer; 13 - deethanizer; 14 - pump; 15 - liquefied gas tower; 16 - multi-stream heat exchanger; 17 - expander; 18 - second circulator; 19 - fourth cooler; 20 - fifth cooler; 21 - first nitrogen booster; 22 - second nitrogen booster; 23 - sixth cooler; 24 - third nitrogen booster; 25 - second throttle valve; 26 - LNG; 27 - LPG, 28 - NGL. Detailed Embodiments

[0026] Example 1

[0027] As Figure 1As shown in the figure, a shallow-cooling process LNG recovery system includes a raw gas pressurization and cooling unit, an ammonia-cooled raw gas unit, a raw gas separation LPG and NGL unit, a nitrogen refrigeration cycle unit, and a raw gas conversion to LNG unit; among which: the raw gas pressurization and cooling unit includes a raw gas compressor 2, a first cooler 3, and a second cooler 4 connected in sequence through pipelines; the ammonia-cooled raw gas unit includes a first mixer 5, an ammonia compressor 9, a third cooler 6, a first throttle valve 7, a diverter 8, and a first circulator 10 connected in sequence through pipelines; the inlet of the first mixer 5 is connected to the outlet of the second cooler 4 of the raw gas pressurization and cooling unit, and the outlet of the first circulator 10 is connected to the inlet of the second cooler 4 of the raw gas pressurization and cooling unit on one hand and the inlet of the first mixer 5 on the other hand; the raw gas separation LPG and NGL unit includes a separator 11, a second mixer 12, a deethanizer 13, a pump 14, and a liquefied gas tower 15 connected in sequence through pipelines; the nitrogen refrigeration cycle unit includes an expander 17, a nitrogen booster three 24, a sixth cooler 23, a nitrogen booster one 21, a fifth cooler 20, a nitrogen booster two 22, a fourth cooler 19, and a second circulator 18 connected in sequence through pipelines; the raw gas conversion to LNG unit includes a multi-stream heat exchanger 16 and a second throttle valve 25 connected in sequence through pipelines; the inlets of the multi-stream heat exchanger 16 are respectively connected to the outlet of the second cooler 4 of the raw gas pressurization and cooling unit, the outlet of the second mixer 12 of the raw gas separation LPG and NGL unit, the outlet of the expander 17 of the nitrogen refrigeration cycle unit, and the outlet of the second circulator 18; the outlets of the multi-stream heat exchanger 16 are respectively connected to the second throttle valve 25, the inlet of the expander 17 of the nitrogen refrigeration cycle unit, the inlet of the nitrogen booster three 24, and the outlet of the separator 11 of the raw gas separation LPG and NGL unit.

[0028] In the present invention, through the ammonia-cooled raw gas unit, ammonia is used as a refrigerant. After the initial pressurization of the raw gas pressurization and cooling unit, it enters the nitrogen booster 24 for further pressurization and circulation into the multi-stream heat exchanger, and continuously cools the raw natural gas in the cold generated by the expansion of the expander 17. The cost is low and the LNG recovery rate is high. At the same time, the nitrogen refrigeration cycle unit continuously absorbs the cold released by the generation of LNG from natural gas, improving the energy utilization efficiency of the system; while recovering LNG26, the deethanizer 13 and the liquefied gas tower 15 in the system can simultaneously convert a part of the natural gas into liquefied petroleum gas products LPG27 and NGL28, improving the utilization rate of natural gas.

[0029] Example 2

[0030] Preferably, on the basis of Embodiment 1, in this embodiment, the first cooler 3 is connected to the outlet of the raw gas compressor 2, and the inlet of the second cooler 4 is connected to the outlet of the first cooler 3. The inlet of the third cooler 6 is connected to the outlet of the ammonia compressor 9; the inlet of the first throttle valve 7 is connected to the outlet of the third cooler 6; the inlet of the diverter 8 is connected to the outlet of the first throttle valve 7; the outlet of the diverter 8 is connected to the inlet of the first circulator 10. The liquid outlet of the separator 11 is connected to the deethanizer 13, the gas outlet of the separator 11 is connected to the second mixer 12, the inlet of the pump 14 is connected to the outlet of the deethanizer 13, the outlet of the pump 14 is connected to the inlet of the liquefied gas tower 15, and the gas outlet of the deethanizer 13 is connected to the second mixer 12. The outlet of the nitrogen booster three 24 is connected to the inlet of the sixth cooler 23; the outlet of the sixth cooler 23 is connected to the inlet of the nitrogen booster one 21; the outlet of the nitrogen booster one 21 is connected to the inlet of the fifth cooler 20; the outlet of the fifth cooler 20 is connected to the inlet of the nitrogen booster two 22; the outlet of the nitrogen booster two 22 is connected to the inlet of the fourth cooler 19, and the outlet of the fourth cooler 19 is connected to the inlet of the second circulator 18. The multi-stream heat exchanger 16 is a four-stream plate-fin heat exchanger. The liquefied gas tower 15 is provided with 24 trays. The deethanizer 13 is provided with 25 trays.

[0031] In actual use, the multi-stream heat exchanger 16 is a four-stream plate-fin heat exchanger. Using a four-stream heat exchanger for heat exchange saves energy consumption and improves the conversion rate of energy. The liquefied gas tower 15 is provided with 24 trays, which is conducive to the full rectification of the material flow entering the liquefied gas tower 15. The deethanizer 13 is provided with 25 trays, which facilitates the full reaction of the material flow and is beneficial to improving the conversion rates of LPG27 and NGL28.

[0032] Embodiment 3

[0033] On the basis of Embodiment 2, a method for recovering LNG by a shallow cooling process uses any one of the above-mentioned shallow cooling process LNG recovery systems, and the specific process is as follows:

[0034] S1: The raw gas is pressurized, cooled, heat-exchanged and then throttled to produce LNG;

[0035] The raw material gas 1, with a temperature of 30°C and a pressure of 900 kPa, enters the raw material gas compressor 2 for compression. After compression, the outlet stream has a temperature of 56.11°C and a pressure of 1200 kPa. It then enters the first cooler 3 for cooling. After cooling, the stream has a temperature of 32°C and a pressure of 1170 kPa. It enters the second cooler 4 for further cooling. After cooling, the stream has a temperature of -30°C and a pressure of 1155 kPa. It enters the multi-stream heat exchanger 16 for sufficient heat exchange. After heat exchange, the stream has a temperature of -100°C and a pressure of 1140 kPa. It enters the separator 11 for gas-liquid separation. After separation, the stream has a temperature of -100°C and a pressure of 1140 kPa. It enters the mixer 12. The stream coming out of the mixer 12 has a temperature of -98.13°C and a pressure of 950 kPa. It enters the multi-stream heat exchanger 16 again for heat exchange. After heat exchange, the stream has a temperature of -153°C and a pressure of 935 kPa. Finally, it enters the second throttle valve 25 for throttling. The LNG coming out has a temperature of -152.8°C and a pressure of 40 kPa;

[0036] S2: The process of raw material gas recycling and utilization;

[0037] The stream coming out of the bottom of the separator 11 has a temperature of -100°C and a pressure of 1140 kPa. It enters the deethanizer 13 for sufficient rectification. The deethanizer 13 is provided with 25 trays. The top pressure is set to 950 kPa, and the bottom pressure is set to 958 kPa. The molar content of ethane in the reboiler in the deethanizer 13 is 8%. After sufficient rectification, the stream coming out of the top has a temperature of -49.01°C and a pressure of 950 kPa and enters the mixer 12 together with the stream coming out of the separator 11, repeating the steps in S1, entering the multi-stream heat exchanger 16 for heat exchange and then throttling to produce LNG;

[0038] S3: The process of separating LPG and NGL from the raw material gas;

[0039] The stream coming out of the bottom of the deethanizer 13 has a temperature of 33.61°C and a pressure of 958 kPa. It enters the pump 14 for pressurization. After pressurization, the stream has a temperature of 34.56°C and a pressure of 2000 kPa. It enters the liquefied gas tower 15 for sufficient rectification. The liquefied gas tower 15 is provided with 24 trays. The top pressure is designed to be 1500 kPa, the bottom pressure is designed to be 1508 kPa, the reflux ratio is set to 1, and the bottom nC5 flow rate is set to 1 kgmole / h. The LPG 27 coming out of the top has a temperature of 47.51°C and a pressure of 1500 kPa, and the NGL28 coming out of the bottom has a temperature of 183.5°C and a pressure of 1508 kPa.

[0040] In the step S1, the ammonia logistics for providing cooling capacity to the second cooler 4 has a temperature of -35°C and a pressure of 91.52 kPa, enters the first mixer 5, and the ammonia logistics exiting the first mixer 5 has a temperature of -35°C and a pressure of 91.52 kPa, enters the ammonia compressor 9 for compression. After compression, the logistics has a temperature of 114.4°C and a pressure of 1700 kPa, enters the third cooler 6 for cooling. After cooling, the logistics has a temperature of 35°C and a pressure of 1680 kPa, enters the first throttle valve 7 for throttling. After throttling, the logistics has a temperature of -35°C and a pressure of 91.52 kPa, enters the diverter 8 to split into two parts of logistics. One part of the logistics enters the first mixer 5, and the other part of the logistics enters the second cooler 4 through circulation.

[0041] In the step S1, to make full use of the cold energy released by the generation of LNG from natural gas, a nitrogen refrigeration cycle unit is used for cyclic absorption. The added nitrogen has a temperature of 28.45°C and a pressure of 425 kPa, enters the nitrogen booster three 24 for boosting. After boosting, the temperature is 158.9°C and the pressure is 1150 kPa, enters the sixth cooler 23 for cooling. After cooling, the logistics has a temperature of 32°C and a pressure of 1130 kPa, enters the nitrogen booster two 22 for compression. After compression, the logistics has a temperature of 155.03°C and a pressure of 2880 kPa, then enters the fourth cooler 19. After cooling, the logistics has a temperature of 35°C and a pressure of 5300 kPa, enters the second circulator 18 to circulate to the multi-stream heat exchanger 16. The exiting logistics has a temperature of -70°C and a pressure of 5285 kPa. The logistics exiting the multi-stream heat exchanger 16 enters the expander 17 for expansion refrigeration. In the expander 17, nitrogen undergoes adiabatic expansion and does external work, enabling the gas to obtain a low temperature. The logistics exiting the expander 17 has a temperature of -168.6°C and a pressure of 440 kPa, and enters the multi-stream heat exchanger 16 again for heat exchange. The exiting logistics has a temperature of 28.45°C and a pressure of 425 kPa, and enters the nitrogen booster one 21.

[0042] In actual use, the multi-stream heat exchanger 16 is a four-stream plate-fin heat exchanger. Using a four-stream heat exchanger for heat exchange saves energy consumption and improves the energy conversion rate. The liquefied gas tower 15 is provided with 24 trays, which is conducive to the full rectification of the logistics entering the liquefied gas tower 15. The deethanizer 13 is provided with 25 trays, which is convenient for the full reaction of the logistics and is conducive to improving the conversion rates of LPG27 and NGL28.

[0043] In actual production, the raw material gas and the produced LNG, LPG, and NGL components are shown in Tables 1, 2, 3, and 4. LNG (Liquefied Natural Gas) is liquefied natural gas. Liquefied natural gas is a liquid hydrocarbon mixture mainly composed of methane. LPG (Liquefied Petroleum Gas), whose main components are composed of C3 and C4 hydrocarbons, is in a liquid state at normal temperature. NGL (Natural Gas Liquids) is also called natural gas liquid or natural gas liquids, and is commonly called light hydrocarbons.

[0044] Table - 1 Composition Table of Raw Material Gas

[0045]

[0046] Table - 2 Composition Table of LNG

[0047]

[0048] Table - 3 Composition Table of LPG

[0049]

[0050] Table - 4 Composition Table of NGL

[0051]

[0052] It can be seen from Tables 1 to 4 that: Since the liquefaction temperature of LNG is -152.8°C, after the LNG recovery system of the shallow refrigeration process, most of the C1 and a small amount of C2 and C3 have been liquefied. Most of LPG at normal temperature is C3, iC4, and nC4, and C5 accounts for a small part. After the above process, C6 in NGL accounts for most, while C5 only accounts for a small part. The energy consumption of each main equipment in the LNG recovery system of the shallow refrigeration process is shown in Table 5.

[0053] Table 5 Energy Consumption Table of Each Main Equipment in the System

[0054]

[0055] As described above, it is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A shallow cooling process LNG recovery system, characterized in that: It includes a raw gas pressurization and cooling unit, an ammonia-cooled raw gas unit, a raw gas separation LPG and NGL unit, a nitrogen refrigeration cycle unit, and a raw gas conversion to LNG unit; where: The raw gas pressurization and cooling unit includes a raw gas compressor (2), a first cooler (3), and a second cooler (4) connected in sequence through pipelines; The ammonia-cooled raw gas unit includes a first mixer (5), an ammonia compressor (9), a third cooler (6), a first throttle valve (7), a diverter (8), and a first circulator (10) connected in sequence through pipelines; the inlet of the first mixer (5) is connected to the outlet of the second cooler (4) of the raw gas pressurization and cooling unit, and the outlet of the first circulator (10) is connected to the inlet of the second cooler (4) of the raw gas pressurization and cooling unit and the inlet of the first mixer (5); The raw gas separation LPG and NGL unit includes a separator (11), a second mixer (12), a deethanizer (13), a pump (14), and a liquefied gas tower (15) connected in sequence through pipelines; The nitrogen refrigeration cycle unit includes an expander (17), a third nitrogen booster (24), a sixth cooler (23), a first nitrogen booster (21), a fifth cooler (20), a second nitrogen booster (22), a fourth cooler (19), and a second circulator (18) connected in sequence through pipelines; The raw gas conversion to LNG unit includes a multi-stream heat exchanger (16) and a second throttle valve (25) connected in sequence through pipelines; the inlets of the multi-stream heat exchanger (16) are respectively connected to the outlet of the second cooler (4) of the raw gas pressurization and cooling unit, the outlet of the second mixer (12) of the raw gas separation LPG and NGL unit, the outlet of the expander (17) of the nitrogen refrigeration cycle unit, and the outlet of the second circulator (18); the outlets of the multi-stream heat exchanger (16) are respectively connected to the second throttle valve (25), the inlet of the expander (17) of the nitrogen refrigeration cycle unit, the inlet of the third nitrogen booster (24), and the outlet of the separator (11) of the raw gas separation LPG and NGL unit; The first cooler (3) is connected to the outlet of the raw gas compressor (2), the inlet of the second cooler (4) is connected to the outlet of the first cooler (3), and the inlet of the third cooler (6) is connected to the outlet of the ammonia compressor (9); the inlet of the first throttle valve (7) is connected to the outlet of the third cooler (6); the inlet of the diverter (8) is connected to the outlet of the first throttle valve (7); the outlet of the diverter (8) is connected to the inlet of the first circulator (10), the liquid outlet of the separator (11) is connected to the deethanizer (13), the gas outlet of the separator (11) is connected to the second mixer (12), the inlet of the pump (14) is connected to the outlet of the deethanizer (13), the outlet of the pump (14) is connected to the inlet of the liquefied gas column (15), the gas outlet of the deethanizer (13) is connected to the second mixer (12), the outlet of the nitrogen booster three (24) is connected to the inlet of the sixth cooler (23); the outlet of the sixth cooler (23) is connected to the inlet of the nitrogen booster one (21); the outlet of the nitrogen booster one (21) is connected to the inlet of the fifth cooler (20); the outlet of the fifth cooler (20) is connected to the inlet of the nitrogen booster two (22); the outlet of the nitrogen booster two (22) is connected to the inlet of the fourth cooler (19), and the outlet of the fourth cooler (19) is connected to the inlet of the second circulator (18).

2. The LNG recovery system using the shallow refrigeration process according to claim 1, characterized in that: The multi-stream heat exchanger (16) is a four-stream plate-fin heat exchanger.

3. The LNG recovery system for the shallow cooling process according to claim 1, wherein: The liquefied gas column (15) is provided with 24 trays, and the deethanizer (13) is provided with 25 trays.

4. The recovery method of a shallow cooling process LNG recovery system according to claims 1-3, characterized in that: It includes the following processes: S1: The raw gas is pressurized, cooled, heat-exchanged and then throttled to produce LNG; The raw gas (1), with a temperature of 30°C and a pressure of 900 kPa, enters the raw gas compressor (2) for compression. The outlet stream has a temperature of 56.11°C and a pressure of 1200 kPa, enters the first cooler (3) for cooling. After cooling, the stream has a temperature of 32°C and a pressure of 1170 kPa, enters the second cooler (4) for further cooling. After cooling, the stream has a temperature of -30°C and a pressure of 1155 kPa, enters the multi-stream heat exchanger (16) for sufficient heat exchange. After heat exchange, the stream has a temperature of -100°C and a pressure of 1140 kPa, enters the separator (11) for gas-liquid separation. The separated stream has a temperature of -100°C and a pressure of 1140 kPa, enters the mixer (12). The stream coming out of the mixer (12) has a temperature of -98.13°C and a pressure of 950 kPa, enters the multi-stream heat exchanger (16) again for heat exchange. After heat exchange, the stream has a temperature of -153°C and a pressure of 935 kPa, and finally enters the second throttle valve (25) for throttling. The LNG coming out has a temperature of -152.8°C and 40 kPa; S2: The process of recycling the raw gas; The temperature of the stream exiting from the bottom of the separator (11) is -100°C and the pressure is 1140 kPa. It enters the deethanizer (13) for sufficient rectification. The deethanizer (13) is equipped with 25 trays. The top pressure is set to 950 kPa and the bottom pressure is set to 958 kPa. The molar content of ethane in the reboiler of the deethanizer (13) is 8%. After sufficient rectification, the stream exiting from the top has a temperature of -49.01°C and a pressure of 950 kPa, which enters the mixer (12) together with the stream exiting from the separator (11). The steps in S1 are repeated, and after heat exchange in the multi-stream heat exchanger (16), it is throttled to produce LNG. S3: The process of separating LPG and NGL from the raw gas; The stream exiting from the bottom of the deethanizer (13) has a temperature of 33.61°C and a pressure of 958 kPa. It enters the pump (14) for pressurization. After pressurization, the stream has a temperature of 34.56°C and a pressure of 2000 kPa. It enters the liquefied petroleum gas tower (15) for sufficient rectification. The liquefied petroleum gas tower (15) is equipped with 24 trays. The top pressure is designed to be 1500 kPa and the bottom pressure is designed to be 1508 kPa. The reflux ratio is set to 1, and the nC5 flow rate at the bottom is set to 1 kgmole / h. The LPG (27) exiting from the top of the tower has a temperature of 47.51°C and a pressure of 1500 kPa, and the NGL (28) exiting from the bottom of the tower has a temperature of 183.5°C and a pressure of 1508 kPa.

5. The recovery method of a shallow cooling process LNG recovery system according to claim 4, characterized in that: In the step S1, the ammonia stream used to provide cooling capacity to the second cooler (4) has a temperature of -35°C and a pressure of 91.52 kPa. It enters the first mixer (5). The ammonia stream exiting from the first mixer (5) has a temperature of -35°C and a pressure of 91.52 kPa. It enters the ammonia compressor (9) for compression. After compression, the stream has a temperature of 114.4°C and a pressure of 1700 kPa. It enters the third cooler (6) for cooling. After cooling, the stream has a temperature of 35°C and a pressure of 1680 kPa. It enters the first throttle valve (7) for throttling. After throttling, the stream has a temperature of -35°C and a pressure of 91.52 kPa. It enters the diverter (8) to split into two parts of the stream. One part of the stream enters the first mixer (5), and the other part of the stream enters the second cooler (4) through circulation.

6. The recovery method of a shallow cooling process LNG recovery system according to claim 4, characterized in that: In the step S1, to make full use of the cold energy released by the generation of LNG from natural gas, a nitrogen refrigeration cycle unit is used for cyclic absorption. The added nitrogen with a temperature of 28.45 °C and a pressure of 425 kPa enters the third nitrogen booster (24) for boosting. After boosting, the temperature is 158.9 °C and the pressure is 1150 kPa. Then it enters the sixth cooler (23) for cooling. After cooling, the temperature of the logistics is 32 °C and the pressure is 1130 kPa. Then it enters the second nitrogen booster (22) for compression. After compression, the temperature of the logistics is 155.03 °C and the pressure is 2880 kPa. Then it enters the fourth cooler (19). After cooling, the temperature of the logistics is 35 °C and the pressure is 5300 kPa. It enters the second circulator (18) and circulates to the multi-stream heat exchanger (16). The temperature of the outgoing logistics is -70 °C and the pressure is 5285 kPa. The logistics coming out of the multi-stream heat exchanger (16) enters the expander (17) for expansion refrigeration. In the expander (17), nitrogen undergoes adiabatic expansion and does external work, enabling the gas to obtain a low temperature. The temperature of the logistics coming out of the expander (17) is -168.6 °C and the pressure is 440 kPa. It enters the multi-stream heat exchanger (16) again for heat exchange. The temperature of the outgoing logistics is 28.45 °C and the pressure is 425 kPa, and then it enters the first nitrogen booster (21).

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