A system and method for producing LNG and co-producing pure methane from natural gas

Abstract

The present invention discloses a system and method for co-producing pure methane in the production of LNG from natural gas. The system includes: a natural gas feed pipeline, a pure methane output pipeline, an LNG output pipeline, a heavy hydrocarbon output pipeline, a fuel gas output pipeline, a cold box including four heat exchangers from top to bottom, a heavy hydrocarbon separator, a denitrification tower, and a methane tower. In order to make the product LNG and pure methane meet the specification requirements, the present invention sets up a denitrification tower and a methane tower. By adjusting the bottom load of the denitrification tower and the reflux ratio of the denitrification tower, the nitrogen content in the LNG and pure methane products meets the specification requirements; by adjusting the bottom load of the methane tower, the product methane meets the index requirement of 99.9%. The present invention can co-produce 99.9% pure methane while producing LNG. The theoretical calculation shows that the unit energy consumption of liquefaction of this co-production device is about 1.07 to 1.20 times that of the device only producing LNG, but the economic benefit is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of liquefied natural gas, and particularly to a system and method for producing LNG and co-producing pure methane from natural gas. Background Art

[0002] The main component of natural gas is methane, and the main components of liquefied natural gas (abbreviated as LNG) and pure methane are both methane. Under normal pressure, natural gas is cryogenically cooled to -162°C to produce liquefied natural gas (LNG). The production of pure methane also requires cryogenically cooling natural gas to -162°C. The production process of pure methane is similar to the production process of LNG, and it is necessary to carry out acid removal, dehydration, mercury removal, nitrogen removal, and liquefaction treatment on natural gas.

[0003] Pure methane can be used for the preparation of standard mixed gas, catalyst evaluation, carburization of metals and alloys, formation of carbides, cultivation of microorganisms, refrigerant, and as a chemical raw material (such as for synthesizing ammonia, urea, and can also be used for producing methanol, acetylene, and ethylene), etc.

[0004] The economic value of pure methane is about more than 10 times that of LNG. Taking pure methane as another product of the LNG production device can not only improve the economic benefits of the device, but also make the production more flexible and maneuverable. Summary of the Invention

[0005] The purpose of the present invention is to provide a system and method for producing LNG and co-producing pure methane from natural gas, so as to produce methane with a purity of 99.9% while liquefying natural gas to produce LNG.

[0006] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0007] On the one hand, the present invention provides a system for producing LNG and co-producing pure methane from natural gas, and the system includes:

[0008] A natural gas feed pipeline, a pure methane output pipeline, an LNG output pipeline, a heavy hydrocarbon output pipeline, a fuel gas output pipeline, a cold box including four heat exchangers from top to bottom, a heavy hydrocarbon separator, a denitrification tower, and a methane tower;

[0009] The natural gas feed pipeline enters from the top of the first heat exchanger of the cold box and exits from the bottom, and then is connected to the inlet of the heavy hydrocarbon separator; the bottom liquid phase outlet of the heavy hydrocarbon separator is connected to the heavy hydrocarbon output pipeline, and the heavy hydrocarbon output pipeline enters from the bottom of the first heat exchanger of the cold box and exits from the top; the pipeline connected to the top gas phase outlet of the heavy hydrocarbon separator enters from the top of the second heat exchanger of the cold box and exits from the bottom, and then is connected to the top of the denitrification tower;

[0010] The gas-phase outlet at the top of the denitrification tower is connected to the reflux drum at the top of the denitrification tower. The gas-phase outlet at the top of the reflux drum at the top of the denitrification tower is connected to the fuel gas output pipeline. The fuel gas output pipeline enters from the bottom and exits from the top of the second heat exchanger and the first heat exchanger of the cold box in sequence. The pipeline connected to the liquid-phase outlet at the bottom of the denitrification tower enters from the top and exits from the bottom of the third heat exchanger of the cold box, and then is divided into two paths. One path is connected to the top of the methane tower, and the other path merges with the pipeline connected to the liquid-phase outlet at the bottom of the methane tower to form the LNG output pipeline. The LNG output pipeline enters from the top and exits from the bottom of the fourth heat exchanger of the cold box. The gas-phase outlet at the top of the methane tower is connected to the pure methane output pipeline. The pure methane output pipeline enters from the top and exits from the bottom of the fourth heat exchanger of the cold box.

[0011] In the system according to the present invention, preferably, the denitrification tower is provided with a reflux drum at the top of the denitrification tower.

[0012] More preferably, the pipeline connected to the gas-phase outlet at the top of the denitrification tower enters from the top and exits from the bottom of the third heat exchanger of the cold box, and then is connected to the reflux drum at the top of the denitrification tower. The gas-phase outlet at the top of the reflux drum at the top of the denitrification tower is connected to the fuel gas output pipeline. The fuel gas output pipeline enters from the bottom and exits from the top of the third heat exchanger, the second heat exchanger and the first heat exchanger of the cold box in sequence. The pipeline connected to the liquid-phase outlet at the bottom of the reflux drum at the top of the denitrification tower is connected to the top of the denitrification tower.

[0013] Further preferably, a reflux pump is provided on the pipeline connected to the liquid-phase outlet at the bottom of the reflux drum at the top of the denitrification tower.

[0014] In the system according to the present invention, preferably, the denitrification tower is provided with a reboiler at the bottom of the denitrification tower.

[0015] More preferably, after the pipeline connected to the gas-phase outlet at the top of the heavy hydrocarbon separator is led out from the bottom of the second heat exchanger, it provides heat source for the reboiler at the bottom of the denitrification tower, and then is connected to the top of the denitrification tower.

[0016] Further preferably, a pressure regulating valve is also provided on the pipeline connected to the gas-phase outlet at the top of the heavy hydrocarbon separator after passing through the reboiler at the bottom of the denitrification tower.

[0017] In the system according to the present invention, preferably, the methane tower is provided with a reboiler at the bottom of the methane tower.

[0018] More preferably, the heat source of the reboiler at the bottom of the methane tower is provided by the mixed refrigerant in the cold box during cyclic operation.

[0019] In the system according to the present invention, preferably, the mixed refrigerant circulation mechanism of the cold box includes: a low-pressure unit of a refrigerant compressor, a low-pressure stage cooler of the refrigerant compressor, a low-pressure stage separator of the refrigerant compressor, a high-pressure unit of the refrigerant compressor, a high-pressure stage cooler of the refrigerant compressor, a high-pressure stage separator of the refrigerant compressor, a low-pressure liquid refrigerant throttle valve, a low-pressure liquid-phase refrigerant separator, a high-pressure liquid-phase refrigerant throttle valve, a high-pressure liquid-phase refrigerant separator, a high-pressure gas-phase refrigerant throttle valve, and a high-pressure gas-phase refrigerant separator;

[0020] After the outlet of the low-pressure unit of the refrigerant compressor is connected to the low-pressure stage cooler of the refrigerant compressor, it is connected to the inlet of the low-pressure stage separator of the refrigerant compressor;

[0021] The top gas-phase outlet of the low-pressure stage separator of the refrigerant compressor sequentially passes through the high-pressure unit of the refrigerant compressor and the high-pressure stage cooler of the refrigerant compressor, and then is connected to the inlet of the high-pressure stage separator of the refrigerant compressor; the pipeline connected to the bottom liquid-phase outlet of the low-pressure stage separator of the refrigerant compressor enters from the top of the first-layer heat exchanger, exits from the bottom, and then passes through the low-pressure liquid refrigerant throttle valve and is connected to the inlet of the low-pressure liquid-phase refrigerant separator;

[0022] The pipeline connected to the top gas-phase outlet of the high-pressure stage separator of the refrigerant compressor sequentially enters from the top of the first-layer heat exchanger and the second-layer heat exchanger, exits from the bottom, passes through the reboiler at the bottom of the methane tower, and provides heat source for it; then it sequentially enters from the top of the third-layer heat exchanger and the fourth-layer heat exchanger, exits from the bottom, and passes through the high-pressure gas-phase refrigerant throttle valve and is connected to the inlet of the high-pressure gas-phase refrigerant separator; the pipeline connected to the bottom liquid-phase outlet of the high-pressure stage separator of the refrigerant compressor sequentially enters from the top of the first-layer heat exchanger and the second-layer heat exchanger, exits from the bottom, and passes through the high-pressure liquid-phase refrigerant throttle valve and is connected to the inlet of the high-pressure liquid-phase refrigerant separator;

[0023] The connecting pipelines of the top gas-phase outlet and the bottom liquid-phase outlet of the high-pressure gas-phase refrigerant separator are separately led into the cold box and then converge, and then sequentially enter from the bottom of the fourth-layer heat exchanger, the third-layer heat exchanger, and the second-layer heat exchanger, exit from the top, and are connected to the inlet of the low-pressure liquid-phase refrigerant separator; the connecting pipelines of the top gas-phase outlet and the bottom liquid-phase outlet of the high-pressure liquid-phase refrigerant separator are separately led into the cold box and then converge, and then enter from the bottom of the second-layer heat exchanger, exit from the top, and are connected to the inlet of the low-pressure liquid-phase refrigerant separator; the connecting pipelines of the top gas-phase outlet and the bottom liquid-phase outlet of the low-pressure liquid-phase refrigerant separator are separately led into the cold box and then converge, and then enter from the bottom of the first-layer heat exchanger, exit from the top, and return to the low-pressure unit of the refrigerant compressor.

[0024] For the system according to the present invention, preferably, the cold box is selected from an aluminum plate-fin heat exchanger or a wound-tube heat exchanger, and the refrigerant compressor is preferably a single-stage centrifugal compressor. To save energy consumption, the inter-stage cooler of the refrigerant compressor is preferably in a combined mode of air cooling and water cooling.

[0025] For the system according to the present invention, preferably, the system further includes a pure methane storage tank and an LNG storage tank; the pure methane output pipeline and the LNG output pipeline are respectively connected to the pure methane storage tank and the LNG storage tank after leading out from the bottom of the fourth-layer heat exchanger.

[0026] For the system according to the present invention, preferably, throttle valves are provided on both the pure methane output pipeline and the LNG output pipeline after they lead out from the bottom of the fourth-layer heat exchanger.

[0027] On the other hand, the present invention provides a method for producing LNG and co-producing pure methane from natural gas, which is realized by the above system.

[0028] Preferably, the method includes the following processes:

[0029] Natural gas is input through the natural gas feed pipeline, cooled by the first-layer heat exchanger (-30°C) and then enters the heavy hydrocarbon separator; the heavy hydrocarbons separated from the bottom of the heavy hydrocarbon separator are output after recovering the cold energy and rewarming through the first-layer heat exchanger; the gaseous raw material gas separated from the top of the heavy hydrocarbon separator is continuously cooled by the second-layer heat exchanger and then enters the top of the denitrification tower.

[0030] The denitrification tower performs the denitrification process of natural gas. The gas at the top of the tower is sequentially recovered for cold energy and rewarmed through the second-layer heat exchanger and the first-layer heat exchanger and then output as fuel gas. The low-temperature natural gas liquid at the bottom of the tower is cooled by the third-layer heat exchanger (-about 140°C) and then divided into two paths. One path enters the top of the methane tower; the pure methane produced at the top of the methane tower continues to be cooled by the fourth-layer heat exchanger (-162°C) and then pure methane (with a purity of 99.9%) is output; the bottom liquid of the methane tower is combined with the other path of low-temperature natural gas liquid and then cooled by the fourth-layer heat exchanger (-162°C) to output LNG.

[0031] In order to make the product LNG and pure methane meet the specification requirements, a denitrification tower and a methane tower are provided. By adjusting the bottom load of the denitrification tower and the reflux ratio of the denitrification tower, the nitrogen content in the LNG and pure methane products meets the specification requirements; by adjusting the bottom load of the methane tower, the product methane meets the index requirement of 99.9%.

[0032] According to the method of the present invention, preferably, the denitrification tower is provided with a reflux drum at the top of the denitrification tower. The gas at the top of the denitrification tower enters the reflux drum at the top of the denitrification tower for gas-liquid separation after being cooled by the third-layer heat exchanger. The separated gas is sequentially passed through the third-layer heat exchanger, the second-layer heat exchanger, and the first-layer heat exchanger to recover the cold energy and be reheated, and then output as fuel gas. The separated liquid is returned to the top reflux of the denitrification tower after being pressurized.

[0033] According to the method of the present invention, preferably, the denitrification tower is provided with a reboiler at the bottom of the denitrification tower;

[0034] The gaseous raw material gas separated from the top of the heavy hydrocarbon separator is cooled by the second-layer heat exchanger (about -80 °C), provides heat source for the reboiler at the bottom of the denitrification tower and is continuously cooled, and then enters the top of the denitrification tower after being regulated by a pressure regulating valve.

[0035] According to the method of the present invention, preferably, the methane tower is provided with a reboiler at the bottom of the methane tower; the heat source of the reboiler at the bottom of the methane tower is provided by the mixed refrigerant in the cold box during the cyclic operation. Specifically, it is the high-pressure gaseous refrigerant during the circulation process of the mixed refrigerant.

[0036] According to the method of the present invention, the operating pressures of the methane tower and the denitrification tower should meet the pressure drops of the subsequent pipelines and product storage tanks of the device. Taking a 10,000 m³ LNG storage tank as an example, in order to overcome the frictional resistance along the pipeline (considering the frictional resistance along the pipeline as 100 kPa) and the loss of liquid entering the tank, the operating pressure of the methane tower needs to reach above 240 kPaG; the operating pressure of the upstream denitrification tower needs to reach above 440 kPaG.

[0037] The mixed refrigerant in the present invention consists of N2 and CH4~iC5H 12The composition of the mixture (for example, composed of a ratio of N2, methane, propane, ethylene, and isopentane), and the mixed refrigerant circulates within the system. The mixed refrigerant at ~0.3 MPa.A is compressed by the low-pressure unit of the refrigerant compressor and then cooled to 40°C by the low-pressure stage cooler of the refrigerant compressor. Subsequently, it enters the low-pressure stage separator of the refrigerant compressor. The low-pressure liquid phase of V-101 goes to the first heat exchanger, and the gas phase enters the high-pressure unit of the refrigerant compressor to be further pressurized to around 4.0 MPa.A. After being cooled to 40°C by the high-pressure stage cooler of the refrigerant compressor, it enters the high-pressure stage separator of the refrigerant compressor for gas-liquid separation; the high-pressure gas phase and liquid refrigerant separated by the high-pressure stage separator of the refrigerant compressor respectively enter the cold box; the low-pressure liquid refrigerant is subcooled in the first heat exchanger and then throttled by the low-pressure liquid refrigerant throttle valve to lower the temperature, resulting in a gas-liquid two-phase state. To achieve better heat exchange effect and reduce the vibration of the two-phase flow pipeline, the low-temperature fluid is separated into gas and liquid phases in the low-pressure liquid refrigerant separator and separately led to the cold box for confluence. After mixing with the reflux refrigerant, it returns to the first heat exchanger to jointly provide cooling capacity for this section of the heat flow, cooling the raw natural gas, gaseous refrigerant, and the liquid refrigerant that needs to be subcooled; the liquid refrigerant coming out of the high-pressure stage separator of the refrigerant compressor enters the cold box (passing through LNG-101 and LNG-102 in sequence), is gradually cooled, and then throttled by the high-pressure liquid refrigerant throttle valve to lower the temperature, resulting in a gas-liquid two-phase state. The low-temperature fluid is separated into gas and liquid phases in the high-pressure liquid refrigerant separator and separately led to the cold box for confluence. The throttled refrigerant and the reflux refrigerant converge and heat up to vaporize; the gaseous refrigerant separated by the high-pressure stage separator of the refrigerant compressor enters the cold box (passing through LNG-101 and LNG-102 in sequence), is gradually cooled to provide heat source for the bottom of the methane tower, and at the same time, after being cooled itself, it continues to be cooled in the subsequent section going to the cold box (passing through LNG-103 and LNG-104 in sequence). Then, it is throttled by the high-pressure gas refrigerant throttle valve to lower the temperature, resulting in a gas-liquid two-phase state. The low-temperature fluid is separated into gas and liquid phases in the high-pressure gas refrigerant separator and separately led to the cold box for confluence. The throttled refrigerant (reflux refrigerant) returns to the cold box (passing through LNG-104, LNG-103, LNG-102, and LNG-101 in sequence) to provide cooling capacity and at the same time heat up and vaporize itself to return to the inlet of the refrigerant compressor.

[0038] The mixed refrigerant of the present invention adopts two-stage compression, and the low-pressure refrigerant generated between stages is introduced into the cold box, and the entire refrigerant system is in a closed cycle.

[0039] The normal-temperature low-pressure mixed refrigerant after heat exchange in the cold box returns to the buffer tank at the inlet of the refrigerant compressor, is buffered, and then enters the inlet of the mixed refrigerant compressor (the inlet of the low-pressure unit of the refrigerant compressor), and is recycled in this way to provide cooling capacity for natural gas liquefaction. The gaseous refrigerant in the buffer tank at the inlet of the refrigerant compressor is recycled, and no liquid phase is generated under normal operating conditions. If a small amount of liquid phase may precipitate when the refrigerant is subcooled in the cold box, a small stream of high-temperature gas (the inter-stage gas of the refrigerant compressor) is introduced at the bottom of the refrigerant buffer tank to vaporize this part of the liquid and recycle it back to the system.

[0040] In the system of the present invention, natural gas and normal-temperature high-pressure refrigerant flow through each layer of the cold box from top to bottom, and the reflux refrigerant returns to the inlet of the refrigerant compressor unit from bottom to top.

[0041] The treatment processes before natural gas liquefaction, such as natural gas pressurization, purification (acid removal, dehydration, mercury removal, etc.), and the buffer tank at the inlet of the refrigerant compressor, all belong to conventional technologies and are not within the technical scope of the present invention, so they will not be elaborated here.

[0042] The beneficial effects of the present invention include:

[0043] 1) While producing LNG, the present invention can co-produce 99.9% pure methane. The theoretical calculation shows that the unit energy consumption of liquefaction of this co-production device is about 1.07 - 1.20 times that of the device only producing LNG, and it is more applicable to large-scale natural gas liquefaction plants.

[0044] 2) The present invention can adjust the bottom load of the denitrification tower and the reflux ratio of the denitrification tower according to the nitrogen content in the raw material gas, so that the nitrogen content in the LNG and pure methane products meets the specification requirements.

[0045] 3) The bottom liquid of the denitrification tower is divided into two paths. One path produces LNG, and the other path enters the methane tower to produce pure methane; the present invention can adjust the liquid amount going to the methane tower to adjust the production ratio of pure methane and LNG products within a certain range, and at the same time, the matching of the refrigerant compressor within a certain range needs to be considered.

[0046] 4) In the present invention, the heat at the bottom of the denitrification tower and the methane tower is taken from within the system, and no external heat source is required. Description of the Drawings

[0047] Figure 1 It is a schematic diagram of the system and method flow for producing LNG and co-producing pure methane from natural gas in a preferred embodiment.

[0048] Description of the Reference Numerals in the Drawings

[0049] C-101A, low-pressure unit of the refrigerant compressor; E-101, low-pressure stage cooler of the refrigerant compressor; V-101, low-pressure stage separator of the refrigerant compressor;

[0050] C-101B, high-pressure unit of the refrigerant compressor; E-102, high-pressure stage cooler of the refrigerant compressor; V-102, high-pressure stage separator of the refrigerant compressor;

[0051] LNG-101, first-layer heat exchanger; LNG-102, second-layer heat exchanger; LNG-103, third-layer heat exchanger; LNG-104, fourth-layer heat exchanger;

[0052] V-103, heavy hydrocarbon separator; V-104, low-pressure liquid refrigerant separator; V-105, high-pressure liquid refrigerant separator; V-106, high-pressure gas refrigerant separator; V-107, reflux drum at the top of the denitrification tower

[0053] P-101, reflux pump at the top of the denitrification tower; J-T-101, throttle valve for low-pressure liquid refrigerant; J-T-102, throttle valve for high-pressure liquid refrigerant; J-T-103, throttle valve for high-pressure gas refrigerant; J-T-104, throttle valve for liquefied natural gas; J-T-105, throttle valve for pure methane; VLV-104, pressure regulating valve

[0054] T-101, denitrification tower; T-102, methane tower

[0055] E-103, reboiler at the bottom of the denitrification tower; E-104, reboiler at the bottom of the methane tower

[0056] 100, natural gas feed pipeline; 200, pure methane output pipeline; 300, LNG output pipeline; 400, heavy hydrocarbon output pipeline; 500, fuel gas output pipeline Detailed implementation mode

[0057] To illustrate the present invention more clearly, the present invention will be further described below in conjunction with preferred embodiments. Those skilled in the art should understand that the content specifically described below is illustrative rather than restrictive, and should not be used to limit the protection scope of the present invention

[0058] The system for producing LNG and co-producing pure methane from natural gas provided by the present invention includes: natural gas feed pipeline 100, pure methane output pipeline 200, LNG output pipeline 300, heavy hydrocarbon output pipeline 400, fuel gas output pipeline 500, cold box including four heat exchangers from top to bottom, heavy hydrocarbon separator V-103, denitrification tower T-101, methane tower T-102

[0059] The natural gas feed pipeline 100 enters from the top and exits from the bottom of the first heat exchanger LNG-101 of the cold box, and then is connected to the inlet of the heavy hydrocarbon separator V-103; the bottom liquid phase outlet of the heavy hydrocarbon separator V-103 is connected to the heavy hydrocarbon output pipeline 400, and the heavy hydrocarbon output pipeline 400 enters from the bottom and exits from the top of the first heat exchanger LNG-101 of the cold box; the pipeline connected to the top gas phase outlet of the heavy hydrocarbon separator V-103 enters from the top and exits from the bottom of the second heat exchanger LNG-102 of the cold box, and then is connected to the top of the denitrification tower T-101

[0060] The gas-phase outlet at the top of the denitrification tower T-101 is connected to the reflux drum V-107 at the top of the denitrification tower. The top gas-phase outlet of the reflux drum V-107 at the top of the denitrification tower is connected to the fuel gas output pipeline 500. The fuel gas output pipeline 500 enters from the bottom and exits from the top of the second heat exchanger LNG-102 and the first heat exchanger LNG-101 in the cold box in sequence. The pipeline connected to the liquid-phase outlet at the bottom of the denitrification tower T-101 enters from the top and exits from the bottom of the third heat exchanger LNG-103 in the cold box, and then is divided into two paths. One path is connected to the top of the methane tower T-102, and the other path merges with the pipeline connected to the liquid-phase outlet at the bottom of the methane tower T-102 to form the LNG output pipeline 300. The LNG output pipeline 300 enters from the top and exits from the bottom of the fourth heat exchanger LNG-104 in the cold box. The gas-phase outlet at the top of the methane tower T-102 is connected to the pure methane output pipeline 200. The pure methane output pipeline 200 enters from the top and exits from the bottom of the fourth heat exchanger LNG-104 in the cold box.

[0061] Preferably, the pipeline connected to the gas-phase outlet at the top of the denitrification tower T-101 enters from the top and exits from the bottom of the third heat exchanger LNG-103 in the cold box, and then is connected to the reflux drum V-107 at the top of the denitrification tower. The top gas-phase outlet of the reflux drum V-107 at the top of the denitrification tower is connected to the fuel gas output pipeline. The fuel gas output pipeline enters from the bottom and exits from the top of the third heat exchanger LNG-103, the second heat exchanger LNG-102, and the first heat exchanger LNG-101 in sequence. The pipeline connected to the liquid-phase outlet at the bottom of the reflux drum V-107 at the top of the denitrification tower is connected to the top of the denitrification tower T-101. Further preferably, a reflux pump P-101 is provided on the pipeline connected to the liquid-phase outlet at the bottom of the reflux drum V-107 at the top of the denitrification tower.

[0062] Preferably, the denitrification tower T-101 is provided with a reboiler E-103 at the bottom of the denitrification tower. More preferably, after the pipeline connected to the top gas-phase outlet of the heavy hydrocarbon separator V-103 is led out from the bottom of the second heat exchanger LNG-102, it provides heat source for the reboiler E-103 at the bottom of the denitrification tower, and then is connected to the top of the denitrification tower T-101. Further preferably, a pressure regulating valve VLV-104 is also provided on the pipeline connected to the top gas-phase outlet of the heavy hydrocarbon separator V-103 after passing through the reboiler E-103 at the bottom of the denitrification tower.

[0063] Preferably, the methane tower T-102 is also provided with a reboiler E-104 at the bottom of the methane tower. More preferably, the heat source of the reboiler E-104 at the bottom of the methane tower is provided by the mixed refrigerant in the cold box during the cyclic operation.

[0064] Specifically, the refrigerant circulation mechanism of the cold box includes: a low-pressure refrigerant compressor unit C-101A, a low-pressure stage cooler of the refrigerant compressor E-101, a low-pressure stage separator of the refrigerant compressor V-101, a high-pressure refrigerant compressor unit C-101B, a high-pressure stage cooler of the refrigerant compressor E-102, a high-pressure stage separator of the refrigerant compressor V-102, a low-pressure liquid refrigerant throttle valve J-T-101, a low-pressure liquid-phase refrigerant separator V-104, a high-pressure liquid-phase refrigerant throttle valve J-T-102, a high-pressure liquid-phase refrigerant separator V-105, a high-pressure gas-phase refrigerant throttle valve J-T-103, and a high-pressure gas-phase refrigerant separator V-106;

[0065] After the outlet of the low-pressure refrigerant compressor unit C-101A is connected to the low-pressure stage cooler of the refrigerant compressor E-101, it is connected to the inlet of the low-pressure stage separator of the refrigerant compressor V-101;

[0066] The top gas-phase outlet of the low-pressure stage separator of the refrigerant compressor V-101 sequentially passes through the high-pressure refrigerant compressor unit C-101B and the high-pressure stage cooler of the refrigerant compressor E-102, and then is connected to the inlet of the high-pressure stage separator of the refrigerant compressor V-102; the pipeline connected to the bottom liquid-phase outlet of the low-pressure stage separator of the refrigerant compressor V-101 enters from the top of the first-layer heat exchanger LNG-101, exits from the bottom, and then passes through the low-pressure liquid refrigerant throttle valve J-T-101 and is connected to the inlet of the low-pressure liquid-phase refrigerant separator V-104;

[0067] The pipeline connected to the top gas-phase outlet of the high-pressure stage separator of the refrigerant compressor V-102 sequentially enters from the top of the first-layer heat exchanger LNG-101 and the second-layer heat exchanger LNG-102, exits from the bottom, passes through the bottom reboiler of the methane tower E-104 to provide heat source for it; then it sequentially enters from the top of the third-layer heat exchanger LNG-103 and the fourth-layer heat exchanger LNG-104, exits from the bottom, and passes through the high-pressure gas-phase refrigerant throttle valve J-T-103 and is connected to the inlet of the high-pressure gas-phase refrigerant separator V-106; the pipeline connected to the bottom liquid-phase outlet of the high-pressure stage separator of the refrigerant compressor V-102 sequentially enters from the top of the first-layer heat exchanger LNG-101 and the second-layer heat exchanger LNG-102, exits from the bottom, and passes through the high-pressure liquid-phase refrigerant throttle valve J-T-102 and is connected to the inlet of the high-pressure liquid-phase refrigerant separator V-105;

[0068] The connecting pipelines of the top gas phase outlet and the bottom liquid phase outlet of the high-pressure gas refrigerant separator V-106 are separately led into the cold box and then converge. After that, they enter from the bottom and exit from the top of the fourth-layer heat exchanger LNG-104, the third-layer heat exchanger LNG-103, and the second-layer heat exchanger LNG-102 in sequence, and are connected to the inlet of the low-pressure liquid refrigerant separator V-104; the connecting pipelines of the top gas phase outlet and the bottom liquid phase outlet of the high-pressure liquid refrigerant separator V-105 are separately led into the cold box and then converge. After that, they enter from the bottom and exit from the top of the second-layer heat exchanger LNG-102, and are connected to the inlet of the low-pressure liquid refrigerant separator V-104; the connecting pipelines of the top gas phase outlet and the bottom liquid phase outlet of the low-pressure liquid refrigerant separator V-104 are separately led into the cold box and then converge. After that, they enter from the bottom and exit from the top of the first-layer heat exchanger LNG-101, and return to the low-pressure unit C-101A of the refrigerant compressor.

[0069] The cold box of the present invention can be an aluminum plate-fin heat exchanger or a wound tube heat exchanger, and the refrigerant compressor is preferably a single centrifugal compressor. To save energy consumption, the inter-stage cooler of the refrigerant compressor is preferably in a combined mode of air cooling and water cooling.

[0070] Preferably, the system further includes a pure methane storage tank and an LNG storage tank; the pure methane output pipeline and the LNG output pipeline are respectively connected to the pure methane storage tank and the LNG storage tank after being led out from the bottom of the fourth-layer heat exchanger LNG-104. More preferably, throttle valves (liquefied natural gas throttle valve J-T-104 and pure methane throttle valve J-T-105) are provided on the pure methane output pipeline and the LNG output pipeline after being led out from the bottom of the fourth-layer heat exchanger LNG-104.

[0071] The method for producing LNG and co-producing pure methane from natural gas by the system of the above preferred scheme includes the following process:

[0072] The dry natural gas from the purification device is input through the natural gas feed pipeline 100, and enters the heavy hydrocarbon separator V-103 after being cooled by the first-layer heat exchanger LNG-101 (-30°C); the heavy hydrocarbons separated from the bottom of the heavy hydrocarbon separator V-103 are output after recovering the cold energy and rewarming through the first-layer heat exchanger LNG-101; the gaseous raw material gas separated from the top of the heavy hydrocarbon separator V-103 is continuously cooled (-80°C or so) by the second-layer heat exchanger LNG-102, provides heat source for the bottom reboiler E-103 of the denitrification tower and is continuously cooled, and then enters the top of the denitrification tower T-101 after being pressure-regulated by the pressure regulating valve VLV-104;

[0073] The denitrification tower T-101 conducts the denitrification process of natural gas. The gas at the top of the tower is cooled by the third-layer heat exchanger LNG-103 and then enters the top reflux drum V-107 of the denitrification tower for gas-liquid separation. The separated gas passes through the third-layer heat exchanger LNG-103, the second-layer heat exchanger LNG-102, and the first-layer heat exchanger LNG-101 in sequence to recover the cold energy and reheat, and then is output as fuel gas. The separated liquid is pressurized and returned to the top reflux of the denitrification tower T-101. The low-temperature natural gas liquid at the bottom of the tower is cooled by the third-layer heat exchanger LNG-103 (about -140°C) and then divided into two paths. One path enters the top of the methane tower T-102. The pure methane produced at the top of the methane tower T-102 continues to be cooled by the fourth-layer heat exchanger LNG-104 (-162°C) and then is discharged into the pure methane storage tank outside the cold box through the pure methane throttle valve J-T-105. The bottom liquid of the methane tower T-102 converges with the other path of low-temperature natural gas liquid, is cooled by the fourth-layer heat exchanger LNG-104 (-162°C), and then is discharged into the LNG storage tank outside the cold box through the liquefied natural gas throttle valve J-T-104.

[0074] In order to make the product LNG and pure methane meet the specification requirements, a denitrification tower and a methane tower are set up. By adjusting the bottom load of the denitrification tower and the reflux ratio of the denitrification tower, the nitrogen content in the LNG and pure methane products meets the specification requirements; by adjusting the bottom load of the methane tower, the product methane meets the index requirement of 99.9%.

[0075] The operating pressures of the methane tower T-102 and the denitrification tower T-101 should meet the pressure drops of the subsequent pipelines and product storage tanks of the device. Taking a 10,000-cubic-meter LNG storage tank as an example, in order to overcome the pipeline frictional resistance (considering the pipeline frictional resistance as 100 kPa) and the liquid inlet loss into the tank, the operating pressure of the methane tower T-102 needs to reach above 240 kPaG; the operating pressure of the upstream denitrification tower T-101 needs to reach above 440 kPaG.

[0076] The mixed refrigerant in the present invention consists of N2 and CH4~iC5H 12The composition of the mixture (for example, composed of a ratio of N2, methane, propane, ethylene, and isopentane), and the mixed refrigerant circulates within the system. The mixed refrigerant at ~0.3 MPa.A is compressed by the low-pressure unit C-101A of the refrigerant compressor and then cooled to 40°C by the low-pressure stage cooler E-101 of the refrigerant compressor. Subsequently, it enters the low-pressure stage separator V-101 of the refrigerant compressor. The low-pressure liquid phase of V-101 goes to the first heat exchanger LNG-101, and the gas phase enters the high-pressure unit C-101B of the refrigerant compressor to be further pressurized to about 4.0 MPa.A. After being cooled to 40°C by the high-pressure stage cooler E-102 of the refrigerant compressor, it enters the high-pressure stage separator V-102 of the refrigerant compressor for gas-liquid separation; the high-pressure gas phase and liquid phase refrigerants separated by the high-pressure stage separator V-102 of the refrigerant compressor enter the cold box respectively; the low-pressure liquid phase refrigerant is subcooled in the first heat exchanger LNG-101 and then throttled and cooled by the low-pressure liquid refrigerant throttle valve J-T-101 to form a gas-liquid two-phase state. To achieve better heat exchange effect and reduce the vibration of the two-phase flow pipeline simultaneously, the low-temperature fluid goes to the low-pressure liquid phase refrigerant separator V-104 to separate the gas-liquid phase and is separately led to the cold box for confluence. After mixing with the return refrigerant, it returns to the first heat exchanger LNG-101 to jointly provide cooling capacity for this section of the heat flow, cooling the raw natural gas, gaseous refrigerant, and the liquid refrigerant to be subcooled; the liquid refrigerant coming out of the high-pressure stage separator V-102 of the refrigerant compressor enters the cold box (passing through LNG-101 and LNG-102 in sequence), is cooled step by step, and then throttled and cooled by the high-pressure liquid phase refrigerant throttle valve J-T-102 to form a gas-liquid two-phase state. The low-temperature fluid goes to the high-pressure liquid phase refrigerant separator V-105 to separate the gas-liquid phase and is separately led to the cold box for confluence. The throttled refrigerant mixes with the return refrigerant and heats up and vaporizes; the gaseous refrigerant separated by the high-pressure stage separator V-102 of the refrigerant compressor enters the cold box (passing through LNG-101 and LNG-102 in sequence), is cooled step by step to provide heat source for the bottom of the methane tower T-102. At the same time, after being cooled itself, it continues to be cooled in the subsequent section going to the cold box (passing through LNG-103 and LNG-104 in sequence), and then throttled and cooled by the high-pressure gas phase refrigerant throttle valve J-T-103 to form a gas-liquid two-phase state. The low-temperature fluid goes to the high-pressure gas phase refrigerant separator V-106 to separate the gas-liquid phase and is separately led to the cold box for confluence. The throttled refrigerant (return refrigerant) returns to the cold box (passing through LNG-104, LNG-103, LNG-102, and LNG-101 in sequence) to provide cooling capacity, and at the same time, it heats up and vaporizes and returns to the inlet of the refrigerant compressor.

[0077] The mixed refrigerant of the present invention adopts two-stage compression. The low-pressure refrigerant generated between stages is introduced into the cold box, and the entire refrigerant system operates in a closed cycle.

[0078] The normal-temperature and low-pressure mixed refrigerant after heat exchange in the cold box returns to the inlet buffer tank of the refrigerant compressor, and after buffering, it enters the inlet of the mixed refrigerant compressor (the inlet of the low-pressure unit C-101A of the refrigerant compressor), and is recycled in this way to provide cold energy for natural gas liquefaction. The gaseous refrigerant in the inlet buffer tank of the refrigerant compressor is recycled, and no liquid phase is generated under normal operating conditions. If a small amount of liquid phase may precipitate when the refrigerant is subcooled in the cold box, a small stream of high-temperature gas (the inter-stage gas of the refrigerant compressor) is introduced at the bottom of the refrigerant buffer tank to vaporize this part of the liquid and recycle it back to the system.

[0079] In the system of the present invention, natural gas and normal-temperature and high-pressure refrigerant flow through each layer of the cold box from top to bottom, and the reflux refrigerant returns to the inlet of the refrigerant compressor unit from bottom to top. The treatment processes before natural gas liquefaction, such as natural gas pressurization, purification (acid removal, dehydration, mercury removal, etc.) and the inlet buffer tank of the refrigerant compressor, are all conventional technologies and do not belong to the technical scope of the present invention, so they will not be elaborated.

[0080] Using the above Figure 1 The process of co-producing LNG and pure methane using the above system includes the low-temperature liquefaction of natural gas (i.e., from gaseous state to liquid state). During this period, in order to meet the nitrogen index requirements in the product, a nitrogen removal tower needs to be set up. In order to obtain pure methane, a methane tower is set up, high-purity methane is obtained at the top of the tower, and LNG products are obtained at the bottom of the tower. The cold energy required for refrigeration is provided by the circulating mixed refrigerant.

[0081] The main operating parameters in a specific application process are shown in Table 1 below:

[0082] Table 1

[0083] Serial number Main operating parameter Unit Value Remarks 1 Intake air volume <![CDATA[Nm 3 / d]]> <![CDATA[50.0×10 4 > At 101.3 kPa and 0 °C 2 Operating pressure of purified gas entering cold box MPa.G 4.6 2 LNG outlet temperature from cold box ℃ -162 3 Operating pressure of denitrification tower MPa.G 0.44 4 Operating pressure of methane tower MPa.G 0.24

[0084] The reference components of the feed natural gas are shown in Table 2:

[0085] Table 2

[0086] Serial number Component mol(%) Serial number Component mol(%) 1 <![CDATA[CH4]]> 90.0 6 <![CDATA[i-C5H 12 > 0.0024 2 <![CDATA[C2H6]]> 5.89 7 <![CDATA[n-C5H 12 > 0.0016 3 <![CDATA[C3H8]]> 1.09 8 <![CDATA[n-C6H 14 > 0.0009 4 <![CDATA[i-C4H 10 > 0.22 9 <![CDATA[N2]]> 2.5 5 <![CDATA[n-C4H 10 > 0.29

[0087] In Table 3 below, based on 1,000,000 cubic meters of raw gas per day, because the treatment processes in the front-end purification part are the same and the consumption is the same, this table only considers the power consumption of the liquefaction part (co-production ratio: 77% of LNG and about 23% of methane). The specific data are shown in Table 3 below:

[0088] Table 3

[0089]

[0090] As can be seen from the comparison in Table 3 above, compared with only producing LNG, when co-producing 77% of LNG and about 23% of methane, the incremental power consumption per unit is smaller while the revenue is larger; for a natural gas processing volume of 1,000,000 m³ / day, the annual co-production revenue increases by approximately 313,000,000 yuan (annual production time: 8000 h).

[0091] In Table 4 below, based on a raw gas volume of 500,000 m³ / day, since the front-end purification part has the same processing technology and consumption, this table only considers the power consumption of the liquefaction part (co-production ratio: 94% of LNG and about 6% of methane), and the specific data is shown in Table 4 below:

[0092] Table 4

[0093]

[0094] As can be seen from the comparison in Table 4 above, compared with only producing LNG, when co-producing 94% of LNG and about 6% of methane, the incremental power consumption per unit is also smaller and the revenue increases; for a natural gas processing volume of 500,000 m³ / day, the annual co-production revenue increases by approximately 28,333,000 yuan (annual production time: 8000 h).

[0095] The larger the production scale of the device and the larger the proportion of methane products, the better the annual revenue.

[0096] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, based on the above description, other different forms of changes or modifications can be made. It is impossible to enumerate all implementation manners here. Any obvious changes or modifications derived from the technical solutions of the present invention still fall within the protection scope of the present invention.

Claims

1. A system for producing LNG and co-producing pure methane from natural gas, characterized in that, The system includes: A natural gas feed pipeline, a pure methane output pipeline, an LNG output pipeline, a heavy hydrocarbon output pipeline, a fuel gas output pipeline, a cold box including four heat exchangers from top to bottom, a heavy hydrocarbon separator (V-103), a denitrification tower (T-101), and a methane tower (T-102); The natural gas feed pipeline enters from the top and exits from the bottom of the first heat exchanger (LNG-101) of the cold box, and then is connected to the inlet of the heavy hydrocarbon separator (V-103); the bottom liquid phase outlet of the heavy hydrocarbon separator (V-103) is connected to the heavy hydrocarbon output pipeline, and the heavy hydrocarbon output pipeline enters from the bottom and exits from the top of the first heat exchanger (LNG-101) of the cold box; the pipeline connected to the top gas phase outlet of the heavy hydrocarbon separator (V-103) enters from the top and exits from the bottom of the second heat exchanger (LNG-102) of the cold box, and then is connected to the top of the denitrification tower (T-101); The top gas phase outlet of the denitrification tower (T-101) is connected to a denitrification tower top reflux tank (V-107), the top gas phase outlet of the denitrification tower top reflux tank (V-107) is connected to the fuel gas output pipeline, and the fuel gas output pipeline enters from the bottom and exits from the top of the second heat exchanger (LNG-102) and the first heat exchanger (LNG-101) of the cold box in sequence; the pipeline connected to the bottom liquid phase outlet of the denitrification tower (T-101) enters from the top and exits from the bottom of the third heat exchanger (LNG-103) of the cold box, and then is divided into two paths, one path is connected to the top of the methane tower (T-102), and the other path is merged with the pipeline connected to the bottom liquid phase outlet of the methane tower (T-102) to form the LNG output pipeline, and the LNG output pipeline enters from the top and exits from the bottom of the fourth heat exchanger (LNG-104) of the cold box; the top gas phase outlet of the methane tower (T-102) is connected to the pure methane output pipeline, and the pure methane output pipeline enters from the top and exits from the bottom of the fourth heat exchanger (LNG-104) of the cold box; The mixed refrigerant circulation mechanism of the cold box includes: a low-pressure refrigerant compressor unit (C-101A), a low-pressure stage cooler of the refrigerant compressor (E-101), a low-pressure stage separator of the refrigerant compressor (V-101), a high-pressure refrigerant compressor unit (C-101B), a high-pressure stage cooler of the refrigerant compressor (E-102), a high-pressure stage separator of the refrigerant compressor (V-102), a low-pressure liquid refrigerant throttle valve (J-T-101), a low-pressure liquid phase refrigerant separator (V-104), a high-pressure liquid phase refrigerant throttle valve (J-T-102), a high-pressure liquid phase refrigerant separator (V-105), a high-pressure gas phase refrigerant throttle valve (J-T-103), and a high-pressure gas phase refrigerant separator (V-106); After the outlet of the low-pressure refrigerant compressor unit (C-101A) is connected to the low-pressure stage cooler of the refrigerant compressor (E-101), it is connected to the inlet of the low-pressure stage separator of the refrigerant compressor (V-101); The gas-phase outlet at the top of the low-pressure stage separator (V-101) of the refrigerant compressor is sequentially connected to the inlet of the high-pressure stage separator (V-102) of the refrigerant compressor after passing through the high-pressure unit (C-101B) of the refrigerant compressor and the high-pressure stage cooler (E-102) of the refrigerant compressor; the pipeline connected to the liquid-phase outlet at the bottom of the low-pressure stage separator (V-101) of the refrigerant compressor enters from the top of the first-layer heat exchanger (LNG-101), exits from the bottom, and then passes through the low-pressure liquid refrigerant throttle valve (J-T-101) and is connected to the inlet of the low-pressure liquid-phase refrigerant separator (V-104); The pipeline connected to the gas-phase outlet at the top of the high-pressure stage separator (V-102) of the refrigerant compressor enters from the top of the first-layer heat exchanger (LNG-101) and the second-layer heat exchanger (LNG-102) sequentially, exits from the bottom, and then passes through the reboiler (E-104) at the bottom of the methane tower to provide heat source for it; then it enters from the top of the third-layer heat exchanger (LNG-103) and the fourth-layer heat exchanger (LNG-104) sequentially, exits from the bottom, and passes through the high-pressure gas-phase refrigerant throttle valve (J-T-103) and is connected to the inlet of the high-pressure gas-phase refrigerant separator (V-106); the pipeline connected to the liquid-phase outlet at the bottom of the high-pressure stage separator (V-102) of the refrigerant compressor enters from the top of the first-layer heat exchanger (LNG-101) and the second-layer heat exchanger (LNG-102) sequentially, exits from the bottom, and passes through the high-pressure liquid-phase refrigerant throttle valve (J-T-102) and is connected to the inlet of the high-pressure liquid-phase refrigerant separator (V-105); The connecting pipelines of the gas-phase outlet at the top and the liquid-phase outlet at the bottom of the high-pressure gas-phase refrigerant separator (V-106) are separately led into the cold box and then merged. Then they enter from the bottom of the fourth-layer heat exchanger (LNG-104), the third-layer heat exchanger (LNG-103) and the second-layer heat exchanger (LNG-102) sequentially, exit from the top, and are connected to the inlet of the low-pressure liquid-phase refrigerant separator (V-104); the connecting pipelines of the gas-phase outlet at the top and the liquid-phase outlet at the bottom of the high-pressure liquid-phase refrigerant separator (V-105) are separately led into the cold box and then merged. Then it enters from the bottom of the second-layer heat exchanger (LNG-102), exits from the top, and is connected to the inlet of the low-pressure liquid-phase refrigerant separator (V-104); the connecting pipelines of the gas-phase outlet at the top and the liquid-phase outlet at the bottom of the low-pressure liquid-phase refrigerant separator (V-104) are separately led into the cold box and then merged. Then it enters from the bottom of the first-layer heat exchanger (LNG-101), exits from the top, and returns to the low-pressure unit (C-101A) of the refrigerant compressor.

2. The system according to claim 1, wherein The pipeline connected to the top gas phase outlet of the denitrification tower (T-101) enters from the top of the third-layer heat exchanger (LNG-103) of the cold box and exits from the bottom, and then is connected to the top reflux drum (V-107) of the denitrification tower; the top gas phase outlet of the top reflux drum (V-107) of the denitrification tower is connected to the fuel gas output pipeline, and the fuel gas output pipeline enters from the bottom and exits from the top of the third-layer heat exchanger (LNG-103), the second-layer heat exchanger (LNG-102), and the first-layer heat exchanger (LNG-101) of the cold box in sequence; the pipeline connected to the bottom liquid phase outlet of the top reflux drum (V-107) of the denitrification tower is connected to the top of the denitrification tower (T-101).

3. The system according to claim 2, wherein A reflux pump (P-101) is provided on the pipeline connected to the bottom liquid phase outlet of the top reflux drum (V-107) of the denitrification tower.

4. The system according to claim 1, characterized in that, The denitrification tower (T-101) is provided with a denitrification tower bottom reboiler (E-103).

5. The system according to claim 4, wherein The pipeline connected to the top gas phase outlet of the heavy hydrocarbon separator (V-103) is led out from the bottom of the second-layer heat exchanger (LNG-102), then provides heat source for the denitrification tower bottom reboiler (E-103), and then is connected to the top of the denitrification tower (T-101).

6. The system according to claim 5, wherein A pressure regulating valve (VLV-104) is further provided on the pipeline connected to the top gas phase outlet of the heavy hydrocarbon separator (V-103) after passing through the denitrification tower bottom reboiler (E-103).

7. The system according to claim 1, wherein The methane tower (T-102) is provided with a methane tower bottom reboiler (E-104).

8. The system according to claim 7, wherein The heat source of the methane tower bottom reboiler (E-104) is provided by the mixed refrigerant in the cold box during cyclic operation.

9. The system according to claim 1, wherein The system further includes a pure methane storage tank and an LNG storage tank; the pure methane output pipeline and the LNG output pipeline are respectively connected to the pure methane storage tank and the LNG storage tank after being led out from the bottom of the fourth-layer heat exchanger (LNG-104).

10. The system according to claim 1, wherein Throttle valves (J-T-104 and J-T-105) are provided on the pure methane output pipeline and the LNG output pipeline after being led out from the bottom of the fourth-layer heat exchanger (LNG-104).

11. A method for producing LNG and co-producing pure methane from natural gas, which is realized by the system according to any one of claims 1-10.

12. The method according to claim 11, wherein, The method includes the following processes: Natural gas is input from the natural gas feed pipeline, cooled by the first-layer heat exchanger (LNG-101), and then enters the heavy hydrocarbon separator (V-103); the heavy hydrocarbons separated at the bottom of the heavy hydrocarbon separator (V-103) are output after recovering cold energy and rewarming through the first-layer heat exchanger (LNG-101); the gaseous raw material gas separated at the top of the heavy hydrocarbon separator (V-103) is continuously cooled by the second-layer heat exchanger (LNG-102), and then enters the top of the denitrification tower (T-101). The denitrification tower (T-101) conducts the denitrification process of natural gas. The gas at the top of the tower passes through the second-layer heat exchanger (LNG-102) and the first-layer heat exchanger (LNG-101) in sequence to recover cold energy and is reheated before being output as fuel gas. The low-temperature natural gas liquid at the bottom of the tower is cooled by the third-layer heat exchanger (LNG-103) and then divided into two paths. One path enters the top of the methane tower (T-102). The pure methane produced at the top of the methane tower (T-102) continues to be cooled by the fourth-layer heat exchanger (LNG-104) and then pure methane is output. The liquid at the bottom of the methane tower (T-102) is combined with the other path of low-temperature natural gas liquid, cooled by the fourth-layer heat exchanger (LNG-104), and then LNG is output.

13. The method according to claim 12, wherein The denitrification tower (T-101) is provided with a denitrification tower top reflux drum (V-107). The gas at the top of the denitrification tower (T-101) is cooled by the third-layer heat exchanger (LNG-103) and then enters the denitrification tower top reflux drum (V-107) for gas-liquid separation. The separated gas passes through the third-layer heat exchanger (LNG-103), the second-layer heat exchanger (LNG-102), and the first-layer heat exchanger (LNG-101) in sequence to recover cold energy and is reheated before being output as fuel gas. The separated liquid is pressurized and then returned to the top of the denitrification tower (T-101) for reflux.

14. The method according to claim 13, wherein The denitrification tower (T-101) is provided with a denitrification tower bottom reboiler (E-103); The gaseous raw material gas separated from the top of the heavy hydrocarbon separator (V-103) is cooled by the second-layer heat exchanger (LNG-102), provides heat source for the denitrification tower bottom reboiler (E-103) and is continuously cooled, and then enters the top of the denitrification tower (T-101) after being regulated by a pressure regulating valve (VLV-104).

15. The method according to claim 13, characterized in that, The methane tower (T-102) is provided with a methane tower bottom reboiler (E-104); the heat source of the methane tower bottom reboiler (E-104) is provided by the mixed refrigerant in the cold box during cyclic operation.

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