A super-large-scale nitrogen-containing natural gas liquefaction device and process thereof

Abstract

The invention relates to a super-large-scale nitrogen-containing natural gas liquefaction device and a process thereof, comprising a precooling heat exchanger, the precooling heat exchanger is connected with a low-temperature separator through a pipeline, the gas phase outlet of the low-temperature separator is connected with a liquefied heat exchanger through a pipeline, the liquefied heat exchanger is connected with a denitrification tower, a cold heat exchanger and a throttle valve through a pipeline, the precooling heat exchanger is connected with a precooling circulation system, a precooling unit in the precooling circulation system is pressurized to 2.5MPa in three stages and cooled to 20-40°C, and then diverted to the precooling heat exchanger and the three-stage throttle valve, the precooling mixed refrigerant entering the three-stage throttle valve is throttled and depressurized, and then enters the precooling separator to be separated into a gas phase precooling refrigerant and a liquid phase precooling refrigerant, the liquid phase precooling refrigerant flows into the precooling heat exchanger and is reheated to 35.8°C, and then merges with the gas phase precooling refrigerant and returns to the high-pressure section of the precooling unit for cyclic compression; the invention has the advantages of simple process flow, small number of equipment, wide adaptability, low energy consumption and being suitable for super-large-scale nitrogen-containing natural gas liquefaction.

Description

Technical Field

[0001] The present invention belongs to the technical field of natural gas liquefaction, and in particular relates to an ultra-large-scale nitrogen-containing natural gas liquefaction device and a process thereof. Background Art

[0002] At present, the main liquefaction processes of large-scale LNG plants abroad are C3MR, CPOCP, AP-X TM , DMR, MFC, etc. The most widely used in industrialization are AP-C3MR and AP-XTM developed by APCI. The LNG production line with the largest single-line processing capacity in the world is the Ras Laffan Natural Gas Phase III project invested and constructed by Qatar Petroleum and ExxonMobil. It adopts the AP-XTM three-cycle process developed by APCI, with a single-line capacity of 7.8 million tons / year (processing scale of 35 million cubic meters / day).

[0003] Compared with foreign countries, China's LNG technology developed relatively late, and there is a large gap in large-scale LNG liquefaction technology and equipment. The main industrial applications in China are DMR and MSC (multi-stage single-component cascade) liquefaction processes. Shandong Tai'an LNG project is the earliest demonstration project for the localization of large-scale LNG technology and equipment in China. It adopts the DMR liquefaction technology independently developed by China Huanqiu Company, with a single-line production capacity of 600,000 tons / year (processing scale of 2.6 million cubic meters / day); Hubei Huanggang LNG Plant is the largest LNG production base in Asia and the first million-ton domestically produced plant in China (currently the only one), using the MSC liquefaction technology independently developed by the Southwest Branch of China Petroleum Engineering Construction Company, with a single-line production capacity of 1.2 million tons / year (processing scale of 5 million cubic meters / day).

[0004] LNG is an important and effective way to supply natural gas. It is clean and environmentally friendly. The industry is developing rapidly and the demand is soaring. As the production of large-scale LNG in China faces resource and technical bottlenecks, overseas LNG resource supply has become a consensus in my country's LNG industry. Therefore, the development of large-scale LNG technology and the localization of complete sets of equipment have become an urgent need to ensure my country's energy security.

[0005] In recent years, in view of the ultra-large-scale natural gas liquefaction technology, many domestic engineering companies have carried out technical research to break through the localization of ultra-large-scale LNG plant complete set technology and key equipment. The Chinese invention patent with the publication number CN110186251A discloses a three-cycle natural gas liquefaction device and method suitable for ultra-large-scale, which adopts a three-stage refrigeration cycle process and is suitable for the construction needs of ultra-large-scale liquefaction devices with a single-line capacity of 6 million to 8 million tons / year. However, the liquefaction process of this process is complicated, with more pre-cooling units and heat exchange equipment, low adaptability, and high energy consumption. On the other hand, it does not involve the removal of nitrogen in the raw gas (the nitrogen content in the LNG product index is required to be less than 0.5%). Summary of the invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art and provide an ultra-large-scale nitrogen-containing natural gas liquefaction device and process thereof with a simple process flow, a small number of equipment, wide adaptability and low energy consumption.

[0007] The technical solution of the present invention is as follows:

[0008] A super-large-scale nitrogen-containing natural gas liquefaction device, comprising a precooling heat exchanger, the precooling heat exchanger is connected to a low-temperature separator through a pipeline, the gas phase outlet of the low-temperature separator is connected to a liquefaction heat exchanger through a pipeline, the liquefaction heat exchanger is connected to a denitrification tower through a pipeline, and the denitrification tower is connected to a subcooling heat exchanger and a first throttle valve in series through a pipeline;

[0009] The precooling heat exchanger is connected to a precooling circulation system to achieve precooling heat exchange of the precooling heat exchanger;

[0010] The outlet and inlet of the precooling heat exchanger are connected in series with a cryogenic unit, the outlet of the precooling heat exchanger is connected with a first cryogenic separator, the gas phase outlet of the first cryogenic separator is connected in series with a liquefaction heat exchanger, a subcooling heat exchanger, a second throttle valve, and a denitrification tower, the denitrification tower is connected in series with a subcooling heat exchanger and a second cryogenic separator, the liquid phase outlet of the first cryogenic separator is connected in series with a liquefaction heat exchanger and a second cryogenic separator, the gas phase outlet and liquid phase outlet of the second cryogenic separator are both connected with a liquefaction heat exchanger, and the liquefaction heat exchanger is connected with the precooling heat exchanger.

[0011] Further, the precooling circulation system includes a precooling unit, a third-stage throttle valve, a second-stage throttle valve, a first-stage throttle valve, and a precooling mixed refrigerant, the precooling unit is respectively connected to the precooling heat exchanger and the third-stage throttle valve, the outlet and the inlet of the precooling heat exchanger are respectively connected in series to the first-stage throttle valve and the second-stage throttle valve, so that the precooling mixed refrigerant in the precooling heat exchanger returns to the precooling heat exchanger after throttling and reducing the pressure through the first-stage throttle valve and the second-stage throttle valve, and the outlet of the precooling heat exchanger is connected to the precooling unit;

[0012] The three-stage throttle valve is connected to a precooling separator, a liquid phase outlet of the precooling separator is connected in series to a precooling heat exchanger and a precooling unit, and a gas phase outlet of the precooling separator is connected to the precooling unit.

[0013] Furthermore, the precooling unit includes a low-pressure section connected to the outlet of the precooling heat exchanger, a medium-pressure section connected to the low-pressure section, and a high-pressure section respectively connected to the precooling heat exchanger and the three-stage throttle valve, the high-pressure section is connected to the first air cooler connected to the medium-pressure section, the high-pressure section is connected to the second air cooler connected to the precooling heat exchanger and the three-stage throttle valve, so as to realize cooling of the precooling mixed refrigerant, the first-stage throttle valve, the precooling heat exchanger, and the low-pressure end are connected in series, the second-stage throttle valve, the precooling heat exchanger, and the medium-pressure section are connected in series, the liquid phase outlet of the precooling separator, the precooling heat exchanger, and the high-pressure section are connected in series, and the gas phase outlet of the precooling separator is connected to the high-pressure section.

[0014] The present invention also provides a super-large-scale nitrogen-containing natural gas liquefaction process of a super-large-scale nitrogen-containing natural gas liquefaction device, comprising the following steps:

[0015] S1: The precooling mixed refrigerant is pressurized to 2.5MPa at the low-pressure end, medium-pressure section and high-pressure section of the precooling unit and cooled to 20-40℃ before being diverted to the precooling heat exchanger and the three-stage throttle valve. The precooling mixed refrigerant flowing into the precooling heat exchanger is precooled to -35℃ and then diverted into diverted refrigerant 1 and diverted refrigerant 2. Diverted refrigerant 1 is cooled to -60℃ by the precooling heat exchanger and then throttled and depressurized by the first-stage throttle valve and then flows back to the precooling heat exchanger. Diverted refrigerant 1 is reheated to -24.7℃ by the precooling heat exchanger and then returns to the low-pressure section of the precooling unit for circulation compression. Diverted refrigerant 2 enters the second-stage throttle valve through the precooling heat exchanger and then throttles and depressurizes and then flows back to the precooling heat exchanger for reheating to 0.2℃ and then returns to the medium-pressure section of the precooling unit for circulation compression;

[0016] S2: The precooling mixed refrigerant entering the three-stage throttle valve is throttled and depressurized, and then enters the precooling separator to separate into gaseous precooling refrigerant and liquid precooling refrigerant. The liquid precooling refrigerant flows into the precooling heat exchanger to be reheated to 35.8℃, and then merges with the gaseous precooling refrigerant and returns to the high-pressure section of the precooling unit for cyclic compression;

[0017] S3: The cryogenic mixed refrigerant is compressed to 3.23MPa and cooled to 20-40℃ in two stages by the cryogenic unit to obtain liquid phase high-pressure cryogenic refrigerant. The liquid phase high-pressure cryogenic refrigerant enters the pre-cooling heat exchanger and is cooled to -58℃ before entering the first cryogenic separator for gas-liquid separation. The gas phase of the first cryogenic separator is cooled to -161.3℃ by the liquefaction heat exchanger and the subcooling heat exchanger, and then throttled and cooled. It is then sent to the top of the denitrification tower as a tower top cold source, and after heat exchange to -165.2℃, it returns to the subcooling heat exchanger to be reheated to -124.9℃ before entering the second cryogenic separator.

[0018] S4: The liquid phase of the first cryogenic separator enters the liquefaction heat exchanger and is cooled to -117°C, after which it is throttled and flows into the second cryogenic separator. The gas phase and liquid phase separated by the second cryogenic separator are reheated to -9°C through the liquefaction heat exchanger and the precooling heat exchanger and then returned to the cryogenic unit.

[0019] S5: The purified natural gas is pre-cooled by the pre-cooling heat exchanger and then enters the low-temperature separator for heavy hydrocarbon separation. The natural gas at the gas phase outlet of the low-temperature separator is liquefied by the liquefaction heat exchanger and then extracted and enters the denitrogenation tower. The natural gas is distilled in the denitrogenation tower to separate nitrogen and obtain denitrogenated natural gas. The denitrogenated natural gas enters the sub-cooling heat exchanger for cooling and is transported to the throttle valve for throttling and pressure reduction before entering the LNG storage system.

[0020] Furthermore, the pre-cooling mixed refrigerant comprises 64 mol% of propane, 11 mol% of isobutane and 25 mol% of ethylene.

[0021] Furthermore, the cryogenic mixed refrigerant comprises 13 mol% propane, 39 mol% ethylene, 31.5 mol% methane and 16.5 mol% nitrogen.

[0022] Furthermore, the subcooling heat exchanger is a plate-fin heat exchanger, and the precooling heat exchanger and the liquefaction heat exchanger are both coil heat exchangers.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. The present invention realizes heat exchange by using a precooling heat exchanger, a liquefaction heat exchanger, and a subcooling heat exchanger, thereby achieving the effects of high integration of heat exchange equipment, high efficiency, and small footprint;

[0025] 2. When the nitrogen content in the purified natural gas is higher than 1%, the present invention can flexibly and effectively separate the nitrogen in the raw gas during the liquefaction process; when the nitrogen content is lower than 1%, a bypass can be connected without denitrification, and the raw material adaptability is strong and the process flexibility is high;

[0026] 3. The present invention adopts a mixed refrigerant. When the ambient temperature changes greatly, the natural gas pre-cooling, liquefaction and supercooling components can be flexibly adjusted to reasonably distribute the load of each section, so that the equipment is in a state of efficient operation and has good operational adaptability.

[0027] In short, the present invention has the advantages of simple process flow, small number of equipment, wide adaptability and low energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the pipeline structure of the present invention.

[0029] In the figure, 1, natural gas; 2, precooling heat exchanger; 3, low-temperature separator; 4, heavy hydrocarbons; 5, liquefaction heat exchanger; 6, denitrogenation tower; 7, nitrogen; 8, denitrogenated natural gas; 9, subcooling heat exchanger; 10, first throttle valve, 11, LNG storage system; 12, precooling unit; 13, gas-phase precooling refrigerant; 14, liquid-phase precooling refrigerant; 15, cryogenic unit; 16, cryogenic mixed refrigerant; 17, third-stage throttle valve; 18, precooling separator; 19, second-stage throttle valve; 20, first-stage throttle valve; 21, first cryogenic separator; 22, second cryogenic separator. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] Example 1

[0032] like Figure 1 As shown, a super-large-scale nitrogen-containing natural gas liquefaction device comprises a precooling heat exchanger 2, the precooling heat exchanger 2 is connected to a low-temperature separator 3 through a pipeline, the gas phase outlet of the low-temperature separator 3 is connected to a liquefaction heat exchanger 5 through a pipeline, the liquefaction heat exchanger 5 is connected to a denitrification tower 6 through a pipeline, a condensing coil is arranged in the top of the denitrification tower 6, a heating coil is arranged in the bottom of the denitrification tower 6, and the denitrification tower 6 is connected in series with a subcooling heat exchanger 9 and a first throttle valve 10 through a pipeline;

[0033] The precooling heat exchanger 2 is connected to a precooling circulation system to achieve precooling heat exchange of the precooling heat exchanger 2;

[0034] The outlet and inlet of the precooling heat exchanger 2 are connected in series with the cryogenic unit 15 through pipelines, the outlet of the precooling heat exchanger 2 is connected with the first cryogenic separator 21 through pipelines, the gas phase outlet of the first cryogenic separator 21 is connected in series with the liquefaction heat exchanger 5, the subcooling heat exchanger 9, the second throttle valve, and the denitrification tower 6 through pipelines, the denitrification tower 6 is connected in series with the subcooling heat exchanger 9 and the second cryogenic separator 22 through pipelines, the liquid phase outlet of the first cryogenic separator 21 is connected in series with the liquefaction heat exchanger 5 and the second cryogenic separator 22 through pipelines, the gas phase outlet and liquid phase outlet of the second cryogenic separator 22 are both connected to the liquefaction heat exchanger 5 through pipelines, and the liquefaction heat exchanger 5 is connected to the precooling heat exchanger 2 through pipelines.

[0035] When in use, the natural gas 1 is precooled to -60°C by the precooling heat exchanger 2 and enters the low-temperature separator 3. The low-temperature separator 3 separates the heavy hydrocarbons from the natural gas 1. The liquid phase of the natural gas 1 after the heavy hydrocarbon separation enters the heavy hydrocarbon separation device, and the gas phase of the natural gas 1 is cooled to -117.5°C by the liquefaction heat exchanger 5, extracted and transported to the denitrification tower 6. The denitrification of the liquefied natural gas is achieved by the heating coil and the condensing coil in the denitrification tower 6. The denitrified natural gas 8 is supercooled to 162°C by the subcooling heat exchanger 9, and then throttled and depressurized to obtain the LNG product.

[0036] During use, the precooling circulation system realizes heat exchange with the precooling mixed refrigerant of the precooling heat exchanger 2, ensuring that the precooling heat exchanger 2 precools the natural gas 1;

[0037] A low-pressure and high-pressure cascade refrigeration cycle is implemented through the precooling heat exchanger 2, the cryogenic unit 15, the first cryogenic separator 21, the liquefaction heat exchanger 5, the subcooling heat exchanger 9, the second throttle valve, and the second cryogenic separator 22 to ensure the precooling, liquefaction, and cryogenic cooling of the natural gas 1.

[0038] The precooling circulation system includes a precooling unit 12, a third-stage throttle valve 17, a second-stage throttle valve 19, a first-stage throttle valve 20, and a precooling mixed refrigerant. The precooling unit 12 is connected to the precooling heat exchanger 2 and the third-stage throttle valve 17 through pipelines, respectively. The outlet and the inlet of the precooling heat exchanger 2 are connected in series to the first-stage throttle valve 20 and the second-stage throttle valve 19 through pipelines, respectively, so that the precooling mixed refrigerant in the precooling heat exchanger 2 returns to the precooling heat exchanger 2 after throttling and reducing the pressure through the first-stage throttle valve 20 and the second-stage throttle valve 19. The outlet of the precooling heat exchanger 2 is connected to the precooling unit 12 through a pipeline.

[0039] The third-stage throttle valve 17 is connected to the precooling separator 18 through a pipeline, the liquid phase outlet of the precooling separator 18 is connected in series to the precooling heat exchanger 2 and the precooling unit 12 through a pipeline, and the gas phase outlet of the precooling separator 18 is connected to the precooling unit 12 through a pipeline;

[0040] The precooling unit 12 includes a low-pressure section connected to the outlet of the precooling heat exchanger 2 through a pipeline, a medium-pressure section connected to the low-pressure section, and a high-pressure section connected to the precooling heat exchanger 2 and the three-stage throttle valve 17 through pipelines, the high-pressure section is connected to the first air cooler through a pipeline, the first air cooler is connected to the medium-pressure section through a pipeline, the high-pressure section is connected to the second air cooler through a pipeline, the second air cooler is connected to the precooling heat exchanger 2 and the three-stage throttle valve 17 through pipelines, so as to realize the cooling of the precooling mixed refrigerant, the first throttle valve 20, the precooling heat exchanger 2, and the low-pressure end are connected in series through a pipeline, the second throttle valve 19, the precooling heat exchanger 2, and the medium-pressure section are connected in series through a pipeline, the liquid phase outlet of the precooling separator 18, the precooling heat exchanger 2, and the high-pressure section are connected in series through a pipeline, and the gas phase outlet of the precooling separator 18 is connected to the high-pressure section through a pipeline;

[0041] When in use, the precooling mixed refrigerant is pressurized to 2.5MPa and cooled to 20-40℃ by the low-pressure end, medium-pressure section and high-pressure section of the precooling unit 12, and then diverted to the precooling heat exchanger 2 and the three-stage throttle valve 17. The precooling mixed refrigerant flowing into the precooling heat exchanger 2 is precooled to -35℃ and then diverted into diverted refrigerant 1 and diverted refrigerant 2. The diverted refrigerant 1 is cooled to -60℃ by the precooling heat exchanger 2 and then throttled and reduced in pressure by the first-stage throttle valve 20. It flows back to the precooling heat exchanger 2, the bypass refrigerant 1 is reheated to -24.7℃ in the precooling heat exchanger 2 and then returns to the low-pressure section of the precooling unit 12 for circulation compression, the bypass refrigerant 2 enters the secondary throttle valve 19 through the precooling heat exchanger 2 for throttling and pressure reduction, then flows back to the precooling heat exchanger 2 for reheating to 0.2℃ and then returns to the medium-pressure section of the precooling unit 12 for circulation compression, thereby realizing three-stage precooling of the precooling mixed refrigerant and ensuring the heat exchange of the precooling mixed refrigerant in the precooling heat exchanger 2.

[0042] Example 2

[0043] like Figure 1 As shown, a super-large-scale nitrogen-containing natural gas liquefaction process includes a super-large-scale nitrogen-containing natural gas liquefaction device described in Example 1, and further includes the following steps:

[0044] S1: The precooling mixed refrigerant is pressurized to 2.5MPa and cooled to 20-40℃ by the low-pressure end, medium-pressure section and high-pressure section of the precooling unit 12, and then diverted to the precooling heat exchanger 2 and the three-stage throttle valve 17. The precooling mixed refrigerant (about 78mol%) flowing into the precooling heat exchanger 2 is precooled to -35℃ and then diverted into diverted refrigerant 1 (36mol%) and diverted refrigerant 2 (36mol%). The diverted refrigerant 1 is cooled to -60℃ by the precooling heat exchanger 2. After throttling and reducing the pressure to 110KPa and -59.5℃ through the first-stage throttle valve 20, it flows back to the precooling heat exchanger 2. The split refrigerant 1 passes through the precooling heat exchanger 2 to be reheated to -24.7℃ and then returns to the low-pressure section of the precooling unit 12 for circulation compression. The split refrigerant 2 passes through the precooling heat exchanger 2 and enters the second-stage throttle valve 19 to be throttling and reducing the pressure to 0.4MPa and -34.36℃ and then flows back to the precooling heat exchanger 2 to be reheated to 0.2℃ and then returns to the medium-pressure section of the precooling unit 12 for circulation compression.

[0045] S2: The precooling mixed refrigerant (22 mol%) entering the three-stage throttle valve 17 is throttled and depressurized to 1.1 MPa and 13.96°C by the three-stage throttle valve 17, and then enters the precooling separator 18 to separate into gas-phase precooling refrigerant 13 and liquid-phase precooling refrigerant 14. The liquid-phase precooling refrigerant 14 flows into the precooling heat exchanger 2 to recover the temperature to 35.8°C, and then merges with the gas-phase precooling refrigerant 13 and returns to the high-pressure section of the precooling unit 12 for cyclic compression;

[0046] S3: The cryogenic mixed refrigerant 16 is compressed to 3.23MPa by the low pressure (0.31MPa) and high pressure (1.3MPa) of the cryogenic unit 15 and cooled to 20-40°C by an air cooler to obtain a liquid phase high pressure cryogenic refrigerant. The liquid phase high pressure cryogenic refrigerant enters the precooling heat exchanger 2 and is cooled to -58°C and enters the first cryogenic separator 21 for gas-liquid separation. The gas phase of the first cryogenic separator 21 is cooled to -117°C by the liquefied heat exchanger 5, and then enters the subcooling heat exchanger 9 and is cooled to -161.3°C, and then throttled and cooled to -350KPa and -171°C, and then sent to the top of the denitrification tower 6 as a tower top cold source, and after heat exchange to -165.2°C, it returns to the subcooling heat exchanger 9 and is reheated to -124.9°C and enters the second cryogenic separator 22;

[0047] S4: The liquid phase of the first cryogenic separator 21 enters the liquefaction heat exchanger 5 and is cooled to -117°C, after which it is throttled to 330 KPa and -122.2°C, and then flows into the second cryogenic separator 22. The gas phase and liquid phase separated by the second cryogenic separator 22 are reheated to -64°C through the liquefaction heat exchanger 5, then enter the pre-cooling heat exchanger 2 and are reheated to -9°C, and then return to the cryogenic unit 15.

[0048] S5: The purified natural gas 1 is precooled to -60°C by the precooling heat exchanger 2 and then enters the low-temperature separator 3 for separation of heavy hydrocarbons 4. The natural gas 1 at the gas phase outlet of the low-temperature separator 3 is liquefied to -117.5°C by the liquefaction heat exchanger 5 and then extracted and enters the denitrogenation tower 6. A condensing coil is arranged at the top of the denitrogenation tower 6. The gas at the top of the denitrogenation tower 6 is condensed to -167.6°C and then separated into nitrogen 7. The nitrogen 7 escaping from the top of the denitrogenation tower 6 is vented at a high point. A heating coil is arranged at the bottom of the denitrogenation tower 6. The natural gas 1 is heated to -129.7°C by the heating coil to obtain denitrogenated natural gas 8. The denitrogenated natural gas 8 enters the subcooling heat exchanger 9 and is subcooled to -162°C, and is transported to the first throttle valve 10 for throttling and pressure reduction to obtain a normal pressure, -161.8°C LNG product. The LNG product enters the LNG storage system 11.

[0049] In the above steps, by increasing the number of pre-cooling units (from 2 to 3), the single-line capacity is increased from 600,000 tons / year to 1-2 million tons / year, maximizing the economies of scale. When the nitrogen content in the raw gas is higher than 1%, the nitrogen in the raw gas can be flexibly and effectively separated during the liquefaction process. When the nitrogen content is lower than 1%, a bypass can be made without denitrification, which makes the process highly adaptable and flexible.

[0050] In this embodiment, the pre-cooling mixed refrigerant is composed of 64 mol% propane, 11 mol% isobutane and 25 mol% ethylene, and the deep-cold mixed refrigerant is composed of 13 mol% propane, 39 mol% ethylene, 31.5 mol% methane and 16.5 mol% nitrogen, respectively. The subcooling heat exchanger 9 is a plate-fin heat exchanger, and the pre-cooling heat exchanger 2 and the liquefaction heat exchanger 5 are both coil heat exchangers; two coil heat exchangers and one plate-fin heat exchanger are used, and the heat exchange equipment has high integration, high efficiency, and small footprint. They are all domestically produced equipment with a high localization rate. The formula of the pre-cooling mixed refrigerant is improved (adjusted from two components of ethylene and propane to three components of propane, isobutane and ethylene) to reduce the outlet pressure, thereby reducing the load of the pre-cooling unit and saving energy.

[0051] Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An ultra-large-scale nitrogen-containing natural gas liquefaction device, characterized in that: It includes a precooling heat exchanger, the precooling heat exchanger is connected to a low-temperature separator through a pipeline, the gas phase outlet of the low-temperature separator is connected to a liquefaction heat exchanger through a pipeline, the liquefaction heat exchanger is connected to a denitrification tower through a pipeline, and the denitrification tower is connected in series with a subcooling heat exchanger and a first throttle valve through a pipeline; The precooling heat exchanger is connected to a precooling circulation system to achieve precooling heat exchange of the precooling heat exchanger; The outlet and inlet of the precooling heat exchanger are connected in series with a cryogenic unit, the outlet of the precooling heat exchanger is connected with a first cryogenic separator, the gas phase outlet of the first cryogenic separator is connected in series with a liquefaction heat exchanger, a subcooling heat exchanger, a second throttle valve, and a denitrification tower, the denitrification tower is connected in series with a subcooling heat exchanger and a second cryogenic separator, the liquid phase outlet of the first cryogenic separator is connected in series with a liquefaction heat exchanger and a second cryogenic separator, the gas phase outlet and liquid phase outlet of the second cryogenic separator are both connected with a liquefaction heat exchanger, and the liquefaction heat exchanger is connected with the precooling heat exchanger; The cryogenic mixed refrigerant is compressed and cooled by the cryogenic unit, then enters the precooling heat exchanger for cooling, and then enters the first cryogenic separator for gas-liquid separation. The gas phase of the first cryogenic separator is throttled and cooled after being cooled by the liquefied heat exchanger and the subcooled heat exchanger, and is sent to the top of the denitrification tower as the tower top cold source. After heat exchange, it returns to the subcooled heat exchanger for reheating and enters the second cryogenic separator; the liquid phase of the first cryogenic separator enters the liquefied heat exchanger for cooling, then throttled and flows into the second cryogenic separator. The gas phase and liquid phase separated by the second cryogenic separator are reheated by the liquefied heat exchanger and the precooled heat exchanger and then returned to the cryogenic unit; the purified natural gas is precooled by the precooling heat exchanger and then enters the low-temperature separator. The natural gas at the gas phase outlet of the low-temperature separator is liquefied by the liquefied heat exchanger and then extracted and enters the denitrification tower. The natural gas is distilled in the denitrification tower to separate nitrogen to obtain denitrified natural gas. The denitrified natural gas enters the subcooled heat exchanger for cooling and is transported to the first throttle valve for throttling and pressure reduction; The precooling circulation system includes a precooling unit, a three-stage throttle valve, a two-stage throttle valve, a first-stage throttle valve, and a precooling mixed refrigerant. The precooling unit is connected to the precooling heat exchanger and the three-stage throttle valve respectively. The outlet and the inlet of the precooling heat exchanger are connected in series to the first-stage throttle valve and the second-stage throttle valve respectively, so that the precooling mixed refrigerant in the precooling heat exchanger returns to the precooling heat exchanger after throttling and reducing the pressure through the first-stage throttle valve and the second-stage throttle valve. The outlet of the precooling heat exchanger is connected to the precooling unit; The three-stage throttle valve is connected to a precooling separator, the liquid phase outlet of the precooling separator is connected in series to a precooling heat exchanger and a precooling unit, and the gas phase outlet of the precooling separator is connected to the precooling unit; The precooling unit includes a low-pressure section connected to the outlet of the precooling heat exchanger, a medium-pressure section connected to the low-pressure section, and a high-pressure section connected to the precooling heat exchanger and the three-stage throttle valve respectively, the high-pressure section is connected to a first air cooler connected to the medium-pressure section, and the high-pressure section is connected to a second air cooler connected to the precooling heat exchanger and the three-stage throttle valve, so as to realize cooling of the precooling mixed refrigerant, the first-stage throttle valve, the precooling heat exchanger, and the low-pressure end are connected in series, the second-stage throttle valve, the precooling heat exchanger, and the medium-pressure section are connected in series, the liquid phase outlet of the precooling separator, the precooling heat exchanger, and the high-pressure section are connected in series, and the gas phase outlet of the precooling separator is connected to the high-pressure section; The precooling mixed refrigerant is pressurized at the low-pressure end, medium-pressure section and high-pressure section of the precooling unit and cooled by the second air cooler before being diverted to the precooling heat exchanger and the three-stage throttle valve. The precooling mixed refrigerant flowing into the precooling heat exchanger is diverted into divert refrigerant 1 and divert refrigerant 2 after precooling. Divert refrigerant 1 is throttled and depressurized by the first-stage throttle valve after being cooled by the precooling heat exchanger, and then flows back to the precooling heat exchanger. Divert refrigerant 1 is reheated by the precooling heat exchanger and then returns to the precooling unit. The low-pressure section is circulated and compressed, and the split refrigerant enters the secondary throttle valve through the precooling heat exchanger for throttling and pressure reduction, then flows back to the precooling heat exchanger for reheating and returns to the medium-pressure section of the precooling unit for circulated compression; the precooling mixed refrigerant entering the third-stage throttle valve is throttled and pressure reduced, then enters the precooling separator for separation into gaseous precooling refrigerant and liquid precooling refrigerant, the liquid precooling refrigerant flows into the precooling heat exchanger for reheating, then merges with the gaseous precooling refrigerant and returns to the high-pressure section of the precooling unit for circulated compression.

2. A super-large-scale nitrogen-containing natural gas liquefaction process, comprising a super-large-scale nitrogen-containing natural gas liquefaction device, characterized in that: The following steps are also included: S1: The precooling mixed refrigerant is pressurized to 2.5MPa at the low-pressure end, medium-pressure section and high-pressure section of the precooling unit and cooled to 20-40℃ before being diverted to the precooling heat exchanger and the three-stage throttle valve. The precooling mixed refrigerant flowing into the precooling heat exchanger is precooled to -35℃ and then diverted into diverted refrigerant 1 and diverted refrigerant 2. Diverted refrigerant 1 is cooled to -60℃ by the precooling heat exchanger and then throttled and depressurized by the first-stage throttle valve and then flows back to the precooling heat exchanger. Diverted refrigerant 1 is reheated to -24.7℃ by the precooling heat exchanger and then returns to the low-pressure section of the precooling unit for circulation compression. Diverted refrigerant 2 enters the second-stage throttle valve through the precooling heat exchanger and then throttles and depressurizes and then flows back to the precooling heat exchanger for reheating to 0.2℃ and then returns to the medium-pressure section of the precooling unit for circulation compression; S2: The precooling mixed refrigerant entering the three-stage throttle valve is throttled and depressurized, and then enters the precooling separator to separate into gaseous precooling refrigerant and liquid precooling refrigerant. The liquid precooling refrigerant flows into the precooling heat exchanger to be reheated to 35.8℃, and then merges with the gaseous precooling refrigerant and returns to the high-pressure section of the precooling unit for cyclic compression; S3: The cryogenic mixed refrigerant is compressed to 3.23MPa and cooled to 20-40℃ in two stages by the cryogenic unit to obtain liquid phase high-pressure cryogenic refrigerant. The liquid phase high-pressure cryogenic refrigerant enters the pre-cooling heat exchanger and is cooled to -58℃ before entering the first cryogenic separator for gas-liquid separation. The gas phase of the first cryogenic separator is cooled to -161.3℃ by the liquefaction heat exchanger and the subcooling heat exchanger, and then throttled and cooled. It is then sent to the top of the denitrification tower as a tower top cold source, and after heat exchange to -165.2℃, it returns to the subcooling heat exchanger to be reheated to -124.9℃ before entering the second cryogenic separator. S4: The liquid phase of the first cryogenic separator enters the liquefaction heat exchanger and is cooled to -117°C, after which it is throttled and flows into the second cryogenic separator. The gas phase and liquid phase separated by the second cryogenic separator are reheated to -9°C through the liquefaction heat exchanger and the precooling heat exchanger and then returned to the cryogenic unit; S5: The purified natural gas is pre-cooled by the pre-cooling heat exchanger and then enters the low-temperature separator for heavy hydrocarbon separation. The natural gas at the gas phase outlet of the low-temperature separator is liquefied by the liquefaction heat exchanger and then extracted and enters the denitrogenation tower. The natural gas is distilled in the denitrogenation tower to separate nitrogen to obtain denitrogenated natural gas. The denitrogenated natural gas enters the subcooling heat exchanger for cooling and is transported to the first throttle valve for throttling and pressure reduction before entering the LNG storage system.

3. The ultra-large-scale nitrogen-containing natural gas liquefaction process according to claim 2, characterized in that: The precooling mixed refrigerant comprises 64 mol% of propane, 11 mol% of isobutane and 25 mol% of ethylene.

4. The ultra-large-scale nitrogen-containing natural gas liquefaction process according to claim 2, characterized in that: The cryogenic mixed refrigerant comprises 13 mol% of propane, 39 mol% of ethylene, 31.5 mol% of methane and 16.5 mol% of nitrogen.

5. The ultra-large-scale nitrogen-containing natural gas liquefaction process according to claim 2, characterized in that: The subcooling heat exchanger is a plate-fin heat exchanger, and the precooling heat exchanger and the liquefaction heat exchanger are both coil heat exchangers.

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