A device and method for extracting high-purity methane from LNG flash gas

Abstract

The present invention discloses a device and method for extracting high-purity methane from LNG flash gas. The device comprises a heat exchanger (1), a heavy hydrocarbon removal tower (2) and a high-purity methane tower (3). The heat exchanger (1) is provided with a circulating methane positive flow channel (21) and a circulating methane reflux channel (26). The circulating methane positive flow channel (21) and the circulating methane reflux channel (26) are connected to the high-purity methane tower (3) through a circulating pipeline. The heat exchanger (1) is also provided with a high-purity methane channel (22), a methane-rich gas channel (23) and an LNG flash gas channel (25). The present invention has a simple structure and low cost. An independent methane cycle is used as a cold source for a tower top condenser, which is not affected by a raw material end, can better ensure the purity of a high-purity methane product, and is easy to operate.

Description

Technical Field

[0001] The present invention relates to the field of deep cryogenic technology, and in particular to a device and method for extracting high-purity methane from LNG flash gas. Background Art

[0002] High-purity methane is widely used as calibration gas, standard gas, catalyst evaluation gas, online instrument standard gas, certain special mixed gas configuration, and research in chemical, electronic, metallurgical, petroleum and other industrial sectors and scientific research, atomic energy, aviation and other fields. It is used as auxiliary additive gas for large-scale integrated circuit dry etching or plasma etching gas in the production of semiconductors, integrated circuits, microelectronics, solar cells, optical fibers and other electronic industries. With the continuous growth of my country's semiconductor, integrated circuit, round crystal, laser technology, liquid crystal display, gas insulation, printed circuit and other industries, the demand for high-purity methane and other electronic special gases will continue to rise. The state encourages domestic enterprises to seek technological innovation, find new sources and production methods of electronic special gases, reduce production costs, improve product quality, break monopolies, and realize capacity transfer.

[0003] Due to the stringent requirements on impurities in high-purity methane, there are two main sources of suitable raw materials: one is liquefied natural gas (LNG). The high-purity methane extraction device can be completely independent and does not rely on the LNG liquefaction plant; the other is LNG flash steam (i.e. flash steam that has been pressurized and cooled by a BOG compressor). The LNG liquefaction device can provide the utilities and cooling capacity (LNG / LIN) required for the high-purity methane extraction device.

[0004] Chinese patent CN1145003C describes a cryogenic distillation system for producing fuel and high-purity methane. After the natural gas is decarbonated and dehydrated, it goes to the heat exchanger 214 for cooling: one path is used as the heat source of the reboiler of the distillation tower 220, and is condensed as its reflux liquid; the other path goes to the distillation tower 220 to separate the heavy components and enters the heat exchanger 234 for cooling: one path is used as the heat source of the reboiler of the cryogenic distillation tower 240, and is condensed as its reflux liquid; the other path goes to the cryogenic distillation tower 240 to separate the light components, and high-purity liquid methane is obtained at the bottom of the tower: one path is throttled as the cold source of the condenser of the cryogenic distillation tower, and then goes to the heat exchanger 234 and the heat exchanger 214 in turn for reheating, and then is compressed by the compressor 260 as part of the product gas; the other path is pressurized by the pump and goes to the heat exchanger 234 and the heat exchanger 214 in turn for reheating as another part of the product gas. The light components at the top of the deep cold distillation tower 240 are used as the cooling source of the condenser of the distillation tower 220 together with the separated heavy components at the bottom of the distillation tower 220 after passing through the heat exchanger 234, and then go to the heat exchanger 214 for reheating and drying at the front end for use as fuel gas. The selection of raw materials leads to complex equipment and high investment; the removed heavy components and light components provide cooling for the condenser of the distillation tower 220, and the composition of the raw materials directly affects the distillation effect of the distillation tower 220, and the equipment has poor applicability and is not easy to operate. Summary of the invention

[0005] The object of the present invention is to overcome the shortcomings of the prior art and to provide a device and method for extracting high-purity methane from LNG flash vapor which has a simple structure, wide applicability, simple operation and low cost.

[0006] The objective of the present invention is achieved by adopting the following technical scheme: a device for extracting high-purity methane from LNG flash gas, comprising a heat exchanger, a heavy hydrocarbon removal tower and a high-purity methane tower, wherein a circulating methane positive flow channel and a circulating methane reflux channel are arranged in the heat exchanger, and the circulating methane positive flow channel and the circulating methane reflux channel are connected to the high-purity methane tower through a circulating pipeline; a high-purity methane channel, a methane-rich gas channel and an LNG flash gas channel are also arranged in the heat exchanger, the high-purity methane channel is connected to the high-purity methane tower through a high-purity methane pipeline, the methane-rich gas channel is respectively connected to the heavy hydrocarbon removal tower and the high-purity methane tower through a methane-rich gas pipeline, the LNG flash gas channel is connected to the heavy hydrocarbon removal tower through the LNG flash gas pipeline, and the heavy hydrocarbon removal tower is connected to the high-purity methane tower through a tower connecting pipeline.

[0007] Furthermore, an external liquid supply channel is provided in the heat exchanger, and the external liquid supply channel is connected to an external liquid supply pipeline, and the external liquid supply pipeline is also connected to a methane-rich gas pipeline.

[0008] Furthermore, the circulation pipeline includes a circulating methane positive flow pipeline and a circulating methane reflux pipeline, the circulating methane positive flow channel is respectively connected to the second inlet I and the fourth outlet L of the high-purity methane tower through the circulating methane positive flow pipeline, wherein the circulating methane positive flow pipeline is also connected to the second inlet D of the heavy hydrocarbon removal tower; the circulating methane reflux channel is respectively connected to the second outlet J and the third inlet K of the high-purity methane tower through the circulating methane reflux pipeline, wherein the circulating methane reflux pipeline is also connected to the second outlet E of the heavy hydrocarbon removal tower.

[0009] Furthermore, the high-purity methane channel is connected to the third outlet H of the high-purity methane tower through a high-purity methane pipeline.

[0010] Furthermore, the methane-rich gas channel is connected to the third outlet C of the heavy hydrocarbon removal tower and the first outlet G of the high-purity methane tower respectively through a methane-rich gas pipeline.

[0011] Furthermore, the LNG flash steam channel is connected to the first inlet A of the heavy hydrocarbon removal tower through an LNG flash steam pipeline.

[0012] Furthermore, the first outlet B of the heavy hydrocarbon removal tower is connected to the first inlet F of the high-purity methane tower through a tower connecting pipeline.

[0013] Furthermore, a methane compressor is provided on the circulating methane reflux pipeline, and the methane compressor is connected to the third inlet K of the high-purity methane tower. The third inlet K is the inlet of the bottom reboiler, and the bottom reboiler is provided at the bottom of the high-purity methane tower.

[0014] A method for extracting high-purity methane from LNG flash gas comprises the following steps:

[0015] S1: LNG flash gas enters the LNG flash gas channel of the heat exchanger through the LNG flash gas pipeline, and enters the first inlet A of the heavy hydrocarbon removal tower after being cooled to a saturated gas phase to remove heavy components. The heavy components enter the methane-rich gas channel of the heat exchanger through the third outlet C of the heavy hydrocarbon removal tower;

[0016] S2: The gas from the top of the heavy hydrocarbon removal tower enters the first inlet F of the high-purity methane tower through the first outlet B to remove light components. The high-purity liquid methane at the bottom of the high-purity methane tower enters the high-purity methane channel of the heat exchanger through the third outlet H, and the light components at the top of the high-purity methane tower enter the methane-rich gas channel of the heat exchanger through the first outlet G.

[0017] S3: high-purity liquid methane is supercooled or reheated in the high-purity methane channel of the heat exchanger and then discharged to the outside through the high-purity methane pipeline;

[0018] S4: the externally supplied liquid enters the externally supplied liquid channel of the heat exchanger, is vaporized, and is reheated to room temperature. Then, together with the methane-rich gas reheated in the methane-rich gas channel, it is sent to the LNG liquefaction device outside the boundary through the methane-rich gas pipeline as fuel gas;

[0019] S5: After the circulating methane is pressurized by the methane compressor, it goes to the third inlet K of the high-purity methane tower to provide heat for the tower bottom reboiler. After being cooled, it goes to the circulating methane positive flow channel of the heat exchanger to be liquefied and supercooled. The outlet end is divided into two paths: providing cold capacity for the heavy hydrocarbon removal tower top condenser of the heavy hydrocarbon removal tower and the high-purity methane tower top condenser of the high-purity methane tower, respectively. After being heated, they go to the circulating methane reflux channel of the heat exchanger for reheating and return to the inlet of the methane compressor.

[0020] S6: The flow rate of the heat source is controlled according to the temperature of the circulating methane exiting the tower bottom reboiler, and the bottom liquid level of the heavy hydrocarbon removal tower and the high-purity methane tower, the tower pressure of the high-purity methane tower, and the flow rate of the externally supplied liquid are controlled respectively.

[0021] Furthermore, the inlet pressure of the LNG flash gas is 0.4-2.0 MPa; the externally supplied liquid is LNG or liquid nitrogen; the distillation pressure of the heavy hydrocarbon removal tower and the high-purity methane tower is 0.4-2.0 MPa; the inlet and outlet pressures of the methane compressor are 0.05-0.5 MPa and 1.0-3.0 MPa respectively; the medium of the methane compressor is pure methane or a mixed refrigerant of methane + nitrogen; the high-purity liquid methane product is pumped into a Dewar tank / storage tank or pressurized into a bottle.

[0022] The beneficial effects of the present invention are: simple structure and low investment cost; independent methane circulation is used as the cold source of the tower top condenser, which is not affected by the raw material end, can better ensure the purity of the high-purity methane product, and is simple to operate; it is suitable for various LNG liquefaction plants: LNG flash gas prepared from pipeline natural gas, coalbed methane and coke oven gas; the heavy component and light component methane-rich gas of the present invention are recycled and utilized, which avoids the waste of raw materials, saves production costs, improves the economic benefits of the enterprise, and protects the ecological environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0024] Figure 1 This is a structural schematic diagram of Embodiment 1 of the present invention;

[0025] Figure 2 This is a structural diagram of Embodiment 2 of the present invention;

[0026] Figure 3 It is a schematic diagram of the structure of the condenser of the present invention;

[0027] In the figure, 1-heat exchanger, 2-heavy hydrocarbon removal tower, 3-high purity methane tower, 4-methane compressor, 5-heavy hydrocarbon removal tower top condenser, 6-high purity methane tower top condenser, 7-tower bottom reboiler, 8-LNG flash gas pipeline, 9-external liquid pipeline, 10-high purity methane pipeline, 11-fuel gas pipeline, 12-regulating valve a, 13-regulating valve b, 14-regulating valve c, 15-regulating valve d, 16-regulating valve e, 17-regulating valve f, 18-regulating valve g, 19-regulating valve h, 20-regulating valve i, 21-circulating methane positive flow channel, 22-high purity methane channel, 23-methane-rich gas channel, 24-external liquid channel, 25-LNG flash gas channel, 26-circulating methane reflux channel, 27-circulating methane inlet, 28-circulating methane outlet, 29-methane-rich gas outlet, 30-high purity liquid methane outlet. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0029] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0030] Some embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments can be combined with each other.

[0031] Embodiment 1:

[0032] See also Figure 1 , a device for extracting high-purity methane from LNG flash gas, comprising a heat exchanger 1, a heavy hydrocarbon removal tower 2 and a high-purity methane tower 3, wherein the heat exchanger 1 is provided with a circulating methane positive flow channel 21 and a circulating methane reflux channel 26, wherein the circulating methane positive flow channel 21 and the circulating methane reflux channel 26 are connected to the high-purity methane tower 3 through a circulating pipeline; the heat exchanger 1 is also provided with a high-purity methane channel 22, a methane-rich gas channel 23 and an LNG flash gas channel 25, wherein the high-purity methane channel 22 is connected to the high-purity methane tower 3 through a high-purity methane pipeline 10, wherein the methane-rich gas channel 23 is respectively connected to the heavy hydrocarbon removal tower 2 and the high-purity methane tower 3 through a methane-rich gas pipeline 11, wherein the LNG flash gas channel 25 is connected to the heavy hydrocarbon removal tower 2 through an LNG flash gas pipeline 8, and the heavy hydrocarbon removal tower 2 is connected to the high-purity methane tower 3 through a tower connecting pipeline.

[0033] In this embodiment, an external liquid supply channel 24 is further provided in the heat exchanger 1, and the external liquid supply channel 24 is connected to the external liquid supply pipeline 9, and the external liquid supply pipeline 9 is also connected to the methane-rich gas pipeline 11. Furthermore, a regulating valve i20 is provided on the external liquid supply pipeline 9.

[0034] In the present embodiment, the circulation pipeline includes a circulation methane positive flow pipeline and a circulation methane reflux pipeline, and the circulation methane positive flow channel 21 is respectively connected to the second inlet I and the fourth outlet L of the high-purity methane tower 3 through the circulation methane positive flow pipeline, wherein the circulation methane positive flow pipeline is also connected to the second inlet D of the heavy hydrocarbon removal tower 2, and further, a regulating valve g18 is provided at the second inlet D of the heavy hydrocarbon removal tower 2, and a regulating valve h19 is provided at the second inlet I of the high-purity methane tower 3; the circulation methane reflux channel 26 is respectively connected to the second outlet J and the third inlet K of the high-purity methane tower 3 through the circulation methane reflux pipeline, wherein the circulation methane reflux pipeline is also connected to the second outlet E of the heavy hydrocarbon removal tower 2, and further, a regulating valve f17 is provided at the third inlet K.

[0035] In this embodiment, the high-purity methane channel 22 is connected to the third outlet H of the high-purity methane tower 3 through the high-purity methane pipeline 10 , and a regulating valve e16 is also provided on the high-purity methane pipeline 10 .

[0036] In this embodiment, the methane-rich gas channel 23 is connected to the third outlet C of the heavy hydrocarbon removal tower 2 and the first outlet G of the high-purity methane tower 3 through the methane-rich gas pipeline 11. Furthermore, a regulating valve c14 is provided at the third outlet C, and a regulating valve d15 is provided at the first outlet G.

[0037] In this embodiment, the LNG flash gas channel 25 is connected to the first inlet A of the heavy hydrocarbon removal tower 2 through the LNG flash gas pipeline 8. Furthermore, a regulating valve a12 is provided at the first inlet A.

[0038] In this embodiment, the first outlet B of the heavy hydrocarbon removal tower 2 is connected to the first inlet F of the high-purity methane tower 3 through a tower connecting pipeline. Furthermore, a regulating valve b13 is provided at the first inlet F.

[0039] In this embodiment, a methane compressor 4 is provided on the circulating methane reflux pipeline, and the methane compressor 4 is a screw type or a piston type. The methane compressor 4 is connected to the third inlet K of the high-purity methane tower 3, and the third inlet K is the inlet of the bottom reboiler 7, wherein the fourth outlet L is the outlet of the bottom reboiler 7, and the bottom reboiler 7 is arranged at the bottom of the high-purity methane tower 3, and the bottom reboiler 7 is a coiled tube type or a plate-fin type.

[0040] In this embodiment, a heavy hydrocarbon removal tower top condenser 5 is provided at the top of the heavy hydrocarbon removal tower 2, and a high-purity methane tower top condenser 6 is provided at the top of the high-purity methane tower 3. The heavy hydrocarbon removal tower top condenser 5 and the high-purity methane tower top condenser 6 have the same structure, which can be referred to in Figure 3 Taking the high-purity methane tower top condenser 6 as an example, the circulating methane inlet 27 is the second inlet I of the high-purity methane tower 3, the circulating methane outlet 28 is the second outlet J of the high-purity methane tower 3, the methane-rich gas outlet 29 is the first outlet G of the high-purity methane tower 3, and the high-purity liquid methane outlet 30 is the third outlet H of the high-purity methane tower 3.

[0041] The extraction method of this embodiment comprises the following steps:

[0042] S1: LNG flash gas (1.2MPa, 40℃) enters the LNG flash gas channel 25 of the heat exchanger through the LNG flash gas pipeline 8 and is cooled to -126℃. It is throttled to 1.15MPa by the regulating valve a12 and enters the heavy hydrocarbon removal tower 2 to remove the heavy components. The temperature at the bottom of the tower is -126.4℃, and the temperature at the top of the tower is -127.2℃. The heavy components at the bottom of the heavy hydrocarbon removal tower 2 are throttled to 0.3MPa by the regulating valve c14 from its third outlet D and then go to the methane-rich gas channel 23 of the heat exchanger 1.

[0043] S2: The gas from the top of the heavy hydrocarbon removal tower 2 is throttled to 1.12MPa through the regulating valve b13 from its first outlet B and then goes to the first inlet F of the high-purity methane tower 3 to remove light components. The tower top temperature is -139°C and the tower bottom temperature is -121.7°C. The bottom of the high-purity methane tower 3 is high-purity liquid methane. The gas from the top of the heavy hydrocarbon removal tower 2 is throttled to 0.45MPa through the regulating valve e16 from its third outlet H and then goes to the high-purity methane channel 22 of the heat exchanger 1. The light components from the top of the high-purity methane tower 3 are throttled to 0.3MPa through the regulating valve d15 from the first outlet G and then go to the methane-rich gas channel 23 of the heat exchanger 1.

[0044] S3: The high-purity liquid methane is heated to 5°C in the high-purity methane channel 22 of the heat exchanger 1 and then discharged to the outside through the high-purity methane pipeline 10;

[0045] S4: LNG is vaporized in the liquid supply channel 24 outside the heat exchanger 1, and then reheated to 5°C. It is sent to the LNG liquefaction device outside the boundary as fuel gas through the methane-rich gas pipeline 11 together with the 5°C methane-rich gas coming out of the methane-rich gas channel 23 of the heat exchanger 1;

[0046] S5: After the circulating methane is pressurized to 1.6 MPa by the methane compressor 4, it goes to the third inlet K of the high-purity methane tower 3 to provide heat for the tower bottom reboiler 7. After being cooled, it goes to the circulating methane positive flow channel 21 of the heat exchanger 1 to be liquefied and supercooled to -140°C. The outlet end is divided into two paths: the circulating methane is throttled to -144°C through the regulating valve g18 and the regulating valve h19 respectively to provide cold capacity for the heavy hydrocarbon removal tower top condenser 5 and the high-purity methane tower top condenser 6. After being heated, it goes to the circulating methane reflux channel 26 of the heat exchanger 1 to be reheated to 10°C and returns to the inlet of the methane compressor 4;

[0047] S6: regulating valve f17 controls the flow of heat source according to the temperature of -20℃ of circulating methane out of the tower bottom reboiler 7. At the same time, regulating valve c14 and regulating valve e16 are used to control the bottom liquid levels of heavy hydrocarbon removal tower 2 and high-purity methane tower 3 respectively to ensure the normal operation of the distillation tower; regulating valve i20 controls the flow of external liquid to ensure the normal operation of heat exchanger 1.

[0048] Embodiment 2:

[0049] See also Figure 2The difference between this embodiment and the first embodiment is that the positions of the internal circulating methane positive flow channel 21 and the high-purity methane channel 22 of the heat exchanger 1 are swapped, and the other connection structures are the same as those of the first embodiment.

[0050] The extraction method of this embodiment comprises the following steps:

[0051] S1: LNG flash gas (1.2MPa, 40℃) enters the LNG flash gas channel 25 of the heat exchanger through the LNG flash gas pipeline 8 and is cooled to -126℃. It is throttled to 1.15MPa by the regulating valve a12 and enters the heavy hydrocarbon removal tower 2 to remove the heavy components. The tower bottom temperature is -126.5℃ and the tower top temperature is -137.2℃. The heavy components at the bottom of the heavy hydrocarbon removal tower 2 are throttled to 0.3MPa by the regulating valve c14 through its third outlet D and then go to the methane-rich gas channel 23 of the heat exchanger 1.

[0052] S2: The gas from the top of the heavy hydrocarbon removal tower 2 is throttled to 1.12MPa through the regulating valve b13 from its first outlet B and then goes to the first inlet F of the high-purity methane tower 3 to remove light components. The tower top temperature is -139°C and the tower bottom temperature is -121.7°C. The bottom of the high-purity methane tower 3 is high-purity liquid methane. The gas from the top of the heavy hydrocarbon removal tower 2 is throttled to 0.45MPa through the regulating valve e16 from its third outlet H and then goes to the high-purity methane channel 22 of the heat exchanger 1. The light components from the top of the high-purity methane tower 3 are throttled to 0.3MPa through the regulating valve d15 from the first outlet G and then go to the methane-rich gas channel 23 of the heat exchanger 1.

[0053] S3: high-purity liquid methane is supercooled to -143°C in the high-purity methane channel 22 of the heat exchanger 1 and then discharged to the outside through the high-purity methane pipeline 10;

[0054] S4: LNG is vaporized in the liquid supply channel 24 outside the heat exchanger 1, and then re-warmed to 0°C. It is sent to an off-site LNG liquefaction device as fuel gas together with the 0°C methane-rich gas coming out of the methane-rich gas channel 23 of the heat exchanger 1 through the methane-rich gas pipeline 11;

[0055] S5: After the circulating methane is pressurized to 1.6 MPa by the methane compressor 4, it goes to the third inlet K of the high-purity methane tower 3 to provide heat for the tower bottom reboiler 7. After being cooled, it goes to the circulating methane positive flow channel 21 of the heat exchanger 1 to be liquefied and supercooled to -140°C. The outlet end is divided into two paths: the circulating methane is throttled to -144°C through the regulating valve g18 and the regulating valve h19 respectively, and then provides cold capacity for the heavy hydrocarbon removal tower top condenser 5 and the high-purity methane tower top condenser 6. After being heated, it goes to the circulating methane reflux channel 26 of the heat exchanger 1 to be reheated to -5°C, and returns to the inlet of the methane compressor 4;

[0056] S6: regulating valve f17 controls the flow of heat source according to the temperature of -20℃ of circulating methane out of the tower bottom reboiler 7. At the same time, regulating valve c14 and regulating valve e16 are used to control the bottom liquid levels of heavy hydrocarbon removal tower 2 and high-purity methane tower 3 respectively to ensure the normal operation of the distillation tower; regulating valve i20 controls the flow of external liquid to ensure the normal operation of heat exchanger 1.

[0057] In order to more intuitively demonstrate the advantages of the present invention, the designed production capacity, energy consumption, product yield, product purity, etc. are shown in Table 1:

[0058] Table 1

[0059] Embodiment 1 Embodiment 2 <![CDATA[Raw material gas volume Nm 3 / h]]> 100 100 Raw gas methane content (V / V) % 80 80 <![CDATA[Design capacity Nm 3 / h]]> 54.8 54.8 Compressor power kW 5.0 5.5 <![CDATA[Unit energy consumption kW.h / Nm 3 > 0.091 0.1 Product yield% 68.5 68.5 Product purity % 99.9998 99.9998 <![CDATA[External supply liquid (LNG) Nm 3 / h]]> 80 20

[0060] The present invention has at least the following technical effects:

[0061] The device of the present invention is relatively simple and has low investment cost; an independent methane cycle is used as a cold source for the tower top condenser, which is not affected by the raw material end, can better ensure the purity of the high-purity methane product, and is simple to operate; it is suitable for various LNG liquefaction plants: LNG flash gas prepared from pipeline natural gas, coalbed methane and coke oven gas; the heavy component and light component methane-rich gas of the present invention are recycled and utilized, thereby avoiding the waste of raw materials, saving production costs, improving the economic benefits of the enterprise, and protecting the ecological environment.

[0062] It should be noted that, for the above embodiments, for the sake of simplicity, they are all described as a series of action combinations, but those skilled in the art should be aware that the present application is not limited by the order of the actions described, because according to the present application, some steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in the specification belong to preferred embodiments, and the actions involved are not necessarily required by the present application.

[0063] The above embodiments describe the basic principles and main features of the present invention and the advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the changes and modifications made by those skilled in the art shall be within the scope of protection of the appended claims of the present invention without departing from the spirit and scope of the present invention.

Claims

1. A device for extracting high-purity methane from LNG flash gas, characterized in that: The invention comprises a heat exchanger (1), a heavy hydrocarbon removal tower (2) and a high-purity methane tower (3), wherein the heat exchanger (1) is provided with a circulating methane positive flow channel (21) and a circulating methane reflux channel (26), and the circulating methane positive flow channel (21) and the circulating methane reflux channel (26) are connected to the high-purity methane tower (3) through a circulating pipeline; the heat exchanger (1) is also provided with a high-purity methane channel (22), a methane-rich gas channel (23) and an LNG flash vapor channel (24). 5), the high-purity methane channel (22) is connected to the high-purity methane tower (3) through the high-purity methane pipeline (10), the methane-rich gas channel (23) is connected to the heavy hydrocarbon removal tower (2) and the high-purity methane tower (3) through the methane-rich gas pipeline (11), the LNG flash vapor channel (25) is connected to the heavy hydrocarbon removal tower (2) through the LNG flash vapor pipeline (8), and the heavy hydrocarbon removal tower (2) is connected to the high-purity methane tower (3) through the tower connecting pipeline; The heat exchanger (1) is also provided with an external liquid supply channel (24), the external liquid supply channel (24) is connected to the external liquid supply pipeline (9), and the external liquid supply pipeline (9) is also connected to the methane-rich gas pipeline (11); The circulation pipeline comprises a circulation methane positive flow pipeline and a circulation methane reflux pipeline. The circulation methane positive flow channel (21) is respectively connected to the second inlet I and the fourth outlet L of the high-purity methane tower (3) through the circulation methane positive flow pipeline, wherein the circulation methane positive flow pipeline is also connected to the second inlet D of the heavy hydrocarbon removal tower (2); the circulation methane reflux channel (26) is respectively connected to the second outlet J and the third inlet K of the high-purity methane tower (3) through the circulation methane reflux pipeline, wherein the circulation methane reflux pipeline is also connected to the second outlet E of the heavy hydrocarbon removal tower (2).

2. The device for extracting high-purity methane from LNG flash gas according to claim 1, characterized in that: The high-purity methane channel (22) is connected to the third outlet H of the high-purity methane tower (3) via a high-purity methane pipeline (10).

3. The device for extracting high-purity methane from LNG flash gas according to claim 1, characterized in that: The methane-rich gas channel (23) is connected to the third outlet C of the heavy hydrocarbon removal tower (2) and the first outlet G of the high-purity methane tower (3) respectively through the methane-rich gas pipeline (11).

4. The device for extracting high-purity methane from LNG flash gas according to claim 1, characterized in that: The LNG flash steam channel (25) is connected to the first inlet A of the heavy hydrocarbon removal tower (2) via an LNG flash steam pipeline (8).

5. The device for extracting high-purity methane from LNG flash gas as claimed in claim 1, characterized in that: The first outlet B of the heavy hydrocarbon removal tower (2) is connected to the first inlet F of the high-purity methane tower (3) via a tower connecting pipeline.

6. The device for extracting high-purity methane from LNG flash gas according to claim 1, characterized in that: A methane compressor (4) is provided on the circulating methane reflux pipeline. The methane compressor (4) is connected to a third inlet K of the high-purity methane tower (3). The third inlet K is an inlet of a tower bottom reboiler (7). The tower bottom reboiler (7) is provided at the bottom of the high-purity methane tower (3).

7. A method for extracting high-purity methane from LNG flash gas, using the device for extracting high-purity methane from LNG flash gas according to any one of claims 1 to 6, characterized in that: The steps include: S1: LNG flash gas enters the LNG flash gas channel (25) of the heat exchanger (1) through the LNG flash gas pipeline (8), and after being cooled to a saturated gas phase, enters the first inlet A of the heavy hydrocarbon removal tower (2) to remove heavy components. The heavy components enter the methane-rich gas channel (23) of the heat exchanger (1) through the third outlet C of the heavy hydrocarbon removal tower (2); S2: The gas at the top of the heavy hydrocarbon removal tower (2) enters the first inlet F of the high-purity methane tower (3) through the first outlet B to remove light components. The high-purity liquid methane at the bottom of the high-purity methane tower (3) enters the high-purity methane channel (22) of the heat exchanger (1) through the third outlet H. The light components at the top of the high-purity methane tower (3) enter the methane-rich gas channel (23) of the heat exchanger (1) through the first outlet G. S3: The high-purity liquid methane is supercooled or reheated in the high-purity methane channel (22) of the heat exchanger (1) and then discharged to the outside through the high-purity methane pipeline (10); S4: the externally supplied liquid enters the externally supplied liquid passage (24) of the heat exchanger (1), is vaporized, and is reheated to room temperature. After that, the externally supplied liquid is sent to an off-site LNG liquefaction device through a methane-rich gas pipeline (11) together with the methane-rich gas reheated in the methane-rich gas passage (23) as fuel gas. S5: After being pressurized by the methane compressor (4), the circulating methane goes to the third inlet K of the high-purity methane tower (3) to provide heat for the tower bottom reboiler (7). After being cooled, it goes to the circulating methane positive flow channel (21) of the heat exchanger (1) to be liquefied and supercooled. The outlet end is divided into two paths: providing cold for the top condenser (5) of the heavy hydrocarbon removal tower (2) and the top condenser (6) of the high-purity methane tower (3). After being heated, they go to the circulating methane reflux channel (26) of the heat exchanger (1) to recover temperature and return to the inlet of the methane compressor (4); S6: The flow rate of the heat source is controlled according to the temperature of the circulating methane exiting the tower bottom reboiler (7), and the bottom liquid levels of the heavy hydrocarbon removal tower (2) and the high-purity methane tower (3), the tower pressure of the high-purity methane tower (3), and the flow rate of the externally supplied liquid are controlled respectively.

8. A method for extracting high-purity methane from LNG flash gas as claimed in claim 7, characterized in that: The externally supplied liquid is LNG or liquid nitrogen.

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