A method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas

Abstract

The present invention relates to the technical field of ethane recovery and natural gas liquefaction, and specifically to a method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas. The natural gas liquefaction process and the ethane recovery process are independent of each other, and two independent refrigeration systems are used, first ethane recovery is performed, and then natural gas liquefaction is performed, and energy consumption accounts for a large proportion. In view of the above problems, the present invention provides a method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas, which includes a multi-stage separation ethane recovery unit, a liquefied natural gas unit, a mixed refrigerant refrigeration cycle unit and a propane refrigeration cycle unit. The natural gas liquefaction process and the ethane recovery process, the mixed refrigerant refrigeration cycle unit and the propane refrigeration cycle unit share a set of device systems, which can realize ethane recovery and natural gas liquefaction. The mixed refrigerant refrigeration cycle unit and the propane refrigeration cycle unit are used in combination, which can reduce investment and operation and maintenance costs, and can also improve the adaptability of the device to the processing volume and gas quality, so as to realize the resource integration and benefit maximization of the upstream and downstream of the oil and gas field natural gas processing.

Description

Technical Field

[0001] The invention relates to the technical field of ethane recovery and natural gas liquefaction, and in particular to a method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas. Background Art

[0002] Ethane recovery refers to the process of recovering ethane and heavier components from hydrocarbon mixtures such as natural gas and coalbed methane. The recovery of heavy components can not only improve the quality of natural gas products, but also improve the economic benefits of oil and gas fields.

[0003] In the next few years, the gap in the domestic ethylene market will continue to increase, and ethane to ethylene has the significant advantages of low operating costs and high ethylene yields. Major domestic oil and gas fields have successively put into production natural gas ethane recovery units. Therefore, improving ethane yields through process optimization can stabilize the price of raw materials for the domestic ethylene industry and increase the profits of the ethane and ethylene industries, which is of great significance to promoting the high-quality and healthy development of the domestic ethylene industry.

[0004] At present, typical domestic natural gas liquefaction processes and ethane recovery processes have the problem of high energy consumption at high recovery rates. In addition, the natural gas liquefaction process and ethane recovery process are independent of each other and use two independent refrigeration systems to first recover ethane and then liquefy natural gas, which accounts for a large proportion of energy consumption. Summary of the invention

[0005] The problem in the prior art is that the natural gas liquefaction process and the ethane recovery process are independent of each other, and two independent refrigeration systems are used to first recover ethane and then liquefy natural gas, which consumes a lot of energy. In view of the above problems, the present invention provides a method for multi-stage separation ethane recovery and co-production of large-volume liquefied natural gas, which includes a multi-stage separation ethane recovery unit, a liquefied natural gas unit, a mixed refrigerant refrigeration cycle unit and a propane refrigeration cycle unit.

[0006] The method for recovering ethane by a multi-stage separation ethane recovery unit comprises the following steps:

[0007] Step 1: The raw gas is divided into two paths. The first path of raw gas enters the raw gas pre-cooling cold box, the main cooling cold box, and the tower top feed subcooling cold box in turn for heat exchange and temperature reduction, and then throttles and reduces pressure before entering the top of the demethanizer;

[0008] Step 2: The second feed gas enters the feed gas pre-cooling box and the main cooling box in turn for heat exchange and cooling, and then enters the first low-temperature separator. The gas phase separated by the first low-temperature separator enters the middle and upper part of the demethanizer after being depressurized and cooled by the expansion unit;

[0009] Step 3: The liquid phase separated by the first low-temperature separator enters the main cold box for heat exchange and temperature increase after throttling, and then enters the second low-temperature separator. The gas phase separated by the second low-temperature separator is mixed with part of the liquid phase separated by the second low-temperature separator. After heat exchange and temperature reduction in the top feed supercooling cold box, the liquid phase enters the upper part of the demethanizer after throttling and pressure reduction;

[0010] Step 4: The liquid phase separated by the second low-temperature separator is divided into two paths, the first liquid phase is mixed with the gas phase separated by the second low-temperature separator, and the second liquid phase enters the lower part of the demethanizer after throttling and pressure reduction;

[0011] Step 5: The condensate at the bottom of the demethanizer tower enters the post-processing device, and the gas phase at the top of the demethanizer tower is divided into two paths. The first path of the gas phase at the top of the tower enters the top feed subcooling cold box, the main cold box, and the raw gas precooling cold box in turn for heat exchange and heating, and then enters the expansion unit and the external gas compressor for pressurization, and then cools down through the air cooler and water cooler to obtain the external gas. The second path of the gas phase at the top of the tower enters the liquefied natural gas unit.

[0012] Preferably, the method for liquefying natural gas by the liquefied natural gas unit comprises the following steps:

[0013] The second gas phase separated from the top of the demethanizer is pressurized by the top gas compressor, and then enters the top feed subcooling cold box and the liquefaction cold box in sequence for heat exchange and liquefaction. After throttling and pressure reduction, it enters the liquefied natural gas flash tank. The gas phase (BOG) and liquefied natural gas (LNG) separated in the liquefied natural gas flash tank are both exported.

[0014] Preferably, the method for circulating propane in the propane refrigeration cycle unit is as follows:

[0015] The gaseous propane separated from the second propane flash tank enters the first propane compressor for compression and pressure boosting, and is mixed with the gaseous phase separated from the first propane flash tank after pressure boosting. The gaseous phase separated from the first propane flash tank is compressed and pressure boosted by the second propane compressor again, and is cooled and liquefied by the propane cooler after pressure boosting, and then throttled and flashed to enter the first propane flash tank; the liquid phase separated from the first propane flash tank is divided into two paths, the first liquid phase enters the first propane flash tank after heat exchange and temperature rise in the raw gas pre-cooling cold box, the second liquid phase enters the second propane flash tank after throttling and flashing, and the liquid phase separated from the second propane flash tank enters the second propane flash tank after heat exchange and temperature rise in the main cold box.

[0016] Preferably, the split ratio of the first liquid phase and the second liquid phase separated by the first propane flash tank is 52.6:47.4.

[0017] Preferably, the method for circulating the refrigerant in the mixed refrigerant refrigeration cycle unit is as follows:

[0018] The mixed refrigerant flash tank contains a mixed refrigerant. The gas phase separated from the mixed refrigerant flash tank is successively liquefied by heat exchange in the tower top feed supercooling cold box and the liquefaction cold box, and then enters the liquefaction cold box and the tower top feed supercooling cold box in turn for heat exchange and temperature increase after throttling supercooling, and then is mixed with the liquid phase separated from the mixed refrigerant flash tank after being cooled by the tower top feed supercooling cold box. The two liquid phases are combined and enter the tower top feed supercooling cold box, and after cooling out of the tower top feed supercooling cold box, enter the first mixed refrigerant compressor for pressurization, and then enter the first mixed refrigerant cooler for cooling, and then enter the second mixed refrigerant compressor for pressurization, and then pressurize and cool through the second air cooler and the second mixed refrigerant cooler, and then enter the raw gas pre-cooling cold box and the main cold box for cooling and flash evaporation, and then enter the mixed refrigerant flash tank.

[0019] Preferably, the mixed refrigerant consists of methane, ethane, ethylene, propane, butane, pentane and nitrogen.

[0020] Preferably, the split ratio of the first feed gas to the second feed gas is 1:4.

[0021] Preferably, the split ratio of the first liquid phase and the second liquid phase separated by the second low-temperature separator is 85:15.

[0022] Preferably, the split ratio of the first top gas phase separated from the top of the demethanizer tower to the second top gas phase is 57.5:42.5.

[0023] The present invention has the following beneficial effects:

[0024] (1) The present invention uses a two-stage separation method to recover part of the cold energy and increase the methane content in the second stream of feed from top to bottom of the demethanizer, thereby improving the distillation effect of the demethanizer and achieving the goal of high ethane yield and low energy consumption;

[0025] (2) The processing capacity of liquefied natural gas in the present invention is 80×10 4 t / a-150×10 4 t / a, using the two-stage combined refrigeration process of propane precooling and mixed refrigerant liquefaction to recover ethane and liquefy natural gas. In this process, the propane refrigeration cycle provides a precooling temperature of about -30°C for the raw gas and the mixed refrigerant, and the mixed refrigerant refrigeration cycle provides an ultra-deep cold temperature of about -162°C for the natural gas. At the same time, the propane refrigeration cycle is selected from the cascade refrigeration process composed of three independent refrigeration cycles of methane, propane and ethylene, and then combined with the mixed refrigerant refrigeration process, integrating the advantages of the cascade refrigeration process and the mixed refrigerant refrigeration process. Therefore, the refrigeration process has the characteristics of less refrigeration equipment, reasonable matching of cold and heat loads, low refrigeration power consumption, large operational flexibility and strong adaptability to raw gas components, which can not only reduce investment and operation and maintenance costs, but also improve the adaptability of the device to processing volume and quality, and realize the resource integration and benefit maximization of upstream and downstream natural gas processing in oil and gas fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 : The present invention is a device system for multi-stage separation of ethane recovery and co-production of large-scale liquefied natural gas.

[0027] In the figure: 1. raw gas pre-cooling cold box, 2. main cold box, 3. first low-temperature separator, 4. second low-temperature separator, 5. top feed subcooling cold box, 6. turbine expander, 7. demethanizer, 8. top gas compressor, 9. liquefaction cold box, 10. liquefied natural gas flash tank, 11. external gas compressor, 12. first air cooler, 13. water cooler, 14. first propane flash tank, 15. second propane flash tank, 16. first propane compressor, 17. second propane compressor, 18. propane cooler, 19. mixed refrigerant flash tank, 20. first mixed refrigerant compressor, 21. first mixed refrigerant cooler, 22. second mixed refrigerant compressor, 23. second air cooler, 24. second mixed refrigerant cooler, I. raw gas, II. external gas, III. condensate, IV. LNG, V. BOG. DETAILED DESCRIPTION

[0028] The present invention is described in detail below with reference to the examples. However, it should be understood that the following examples are only illustrative of the embodiments of the present invention, and are not intended to limit the scope of the present invention.

[0029] Example 1

[0030] The gas quality components and operating conditions of raw gas I are as follows:

[0031] Raw gas I processing scale: 1000×10 4 m3 / d

[0032] Raw gas I pressure: 6.5MPa

[0033] Station temperature: 30℃

[0034] Dry gas transmission pressure: 6.3MPa

[0035] LNG scale: 105×10 4 t / a

[0036] The composition of raw gas I is shown in Table 1.

[0037] Table 1

[0038] composition <![CDATA[N2]]> <![CDATA[CO2]]> <![CDATA[C1]]> <![CDATA[C2]]> <![CDATA[C3]]> <![CDATA[iC4]]> <![CDATA[nC4]]> <![CDATA[iC5]]> <![CDATA[nC5]]> <![CDATA[C6]]> <![CDATA[C7]]> <![CDATA[C8]]> <![CDATA[C9]]> <![CDATA[C 10 ]]> mol% 0.52 0.14 89.2 7.65 1.32 0.321 0.295 0.151 0.092 0.41 1.110 0.06 0 0

[0039] like Figure 1 As shown, a device system for high-efficiency rich gas ethane recovery and co-production of liquefied natural gas provided by the present invention includes a multi-stage separation ethane recovery unit, a liquefied natural gas unit, a mixed refrigerant refrigeration cycle unit and a propane refrigeration cycle unit.

[0040] The method for recovering ethane by a multi-stage separation ethane recovery unit comprises the following steps:

[0041] Step 1: Raw gas I (6.5MPa, 25℃, 1000×10 4 m 3 / d) is divided into two routes, the first route raw gas I (20%) enters the raw gas pre-cooling cold box 1, the main cooling cold box 2, and the tower top feed subcooling cold box 5 in sequence, after heat exchange and cooling to -92°C, throttling and reducing the pressure to 2.9MPa, and enters the top of the demethanizer 7;

[0042] Step 2: The second feed gas I (80%) enters the feed gas pre-cooling box 1 and the main cooling box 2 in sequence for heat exchange and cooling to -57.5°C, and then enters the first low-temperature separator 3. The gas separated by the first low-temperature separator 3 is depressurized to 2.9MPa and cooled by the expansion unit, and then enters the middle and upper part of the demethanizer 7;

[0043] Step 3: The liquid phase separated by the first low-temperature separator 3 is throttled to -59°C and then enters the main cold box 2 for heat exchange and temperature rise to -46°C, and then enters the second low-temperature separator 4. The gas phase separated by the second low-temperature separator 4 is mixed with 85% of the liquid phase separated by the second low-temperature separator 4, and after heat exchange and temperature reduction by the top feed supercooling cold box 5, it is throttled and depressurized to 2.9MPa and enters the upper part of the demethanizer 7;

[0044] Step 4: The liquid phase separated by the second low-temperature separator 4 is divided into two paths, the first liquid phase (85%) is mixed with the gas phase separated by the second low-temperature separator 4, and the second liquid phase (15%) enters the lower part of the demethanizer 7 after throttling and pressure reduction;

[0045] Step 5: The condensate III at the bottom of the demethanizer 7 enters the post-processing unit. The ethane recovery rate is 96%, 76×10 4 m 3 / d, the top gas phase of the demethanizer 7 is divided into two paths. The first top gas phase (57.5%) enters the top feed subcooling cold box 5, the main cooling cold box 2, and the raw gas precooling cold box 1 in turn for heat exchange and temperature increase, and then enters the expansion unit and the external gas compressor 11 to increase the pressure to 6.5MPa, and then passes through the first air cooler 12 and the water cooler 13 to cool down to 30℃ to obtain external gas II (6.35MPa, 30℃, 480×10 4 m 3 / d), the second tower top gas phase enters the liquefied natural gas unit.

[0046] The method for liquefying natural gas by the liquefied natural gas unit comprises the following steps:

[0047] The second gas phase 42.5% separated from the top of the demethanizer 7 is pressurized to 4.5 MPa by the top gas compressor 8, and then enters the top feed subcooling cold box 5 to cool to -93 ° C, and the liquefaction cold box 9 to cool to -156 ° C for heat exchange and liquefaction, and then enters the liquefied natural gas flash tank 10 after throttling and depressurization to 115 kPa. The BOGⅤ and LNGⅣ (115 kPa, -157 ° C, 105×10 4 t / a) are all exported.

[0048] The circulation method of propane in the propane refrigeration cycle unit is as follows:

[0049] The gaseous propane separated from the second propane flash tank 15 enters the first propane compressor 16 for compression and pressure increase to 295 kPa, and after pressure increase, is mixed with the gaseous phase separated from the first propane flash tank 14. The gaseous phase separated from the first propane flash tank 14 is compressed and pressure increased to 1.9 MPa by the second propane compressor 17, and after pressure increase, is cooled to 38° C. by the propane cooler 18 for liquefaction, and then throttled and flashed into the first propane flash tank 14; the liquid phase (1.9 MPa) separated from the first propane flash tank 14 , 13298kmol / h) is divided into two paths, the first path liquid phase accounts for (52.6%, -15℃, 4810kmol / h) and enters the first propane flash tank 14 after heat exchange and temperature rise in the raw gas pre-cooling cold box 1, the second path liquid phase accounts for (47.4%, -15℃, 4325kmol / h) and enters the second propane flash tank 15 after throttling flash, the liquid phase separated from the second propane flash tank 15 enters the second propane flash tank 15 after heat exchange and temperature rise in the main cooling box 2.

[0050] The circulation method of the refrigerant in the mixed refrigerant refrigeration cycle unit is as follows:

[0051] The mixed refrigerant flash tank 19 contains a mixed refrigerant (2.2MPa, 11397kmol / h). The gas phase separated by the mixed refrigerant flash tank 19 is successively liquefied by heat exchange to -156°C in the tower top feed subcooling cold box 5 and the liquefaction cold box 9, and then enters the liquefaction cold box 9 and the tower top feed subcooling cold box 5 for heat exchange and temperature increase, and then is mixed with the liquid phase separated by the mixed refrigerant flash tank 19 after being cooled by the tower top feed subcooling cold box 5. The two liquid phases merge and enter The feed enters the tower top supercooling cold box 5, and the temperature of the feed out of the tower top supercooling cold box 5 is reduced to 145kPa. After entering the first mixed refrigerant compressor 20 at -38℃ and pressurized to 1.1MPa, it enters the first mixed refrigerant cooler 21 and cools down to 38℃, and then enters the second mixed refrigerant compressor 22 and pressurized to 2.2MPa, and then passes through the second air cooler 23 and the second mixed refrigerant cooler 24 and pressurized and cooled to 38℃, and then enters the raw gas precooling cold box 1 and the main cooling cold box 2 in sequence for cooling and flash evaporation, and then enters the mixed refrigerant flash tank 19.

[0052] The mixed refrigerant consists of methane, ethane, ethylene, propane, butane, pentane and nitrogen.

[0053] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas, characterized in that: It includes multi-stage separation ethane recovery unit, liquefied natural gas unit, mixed refrigerant refrigeration cycle unit and propane refrigeration cycle unit. The method for recovering ethane by a multi-stage separation ethane recovery unit comprises the following steps: Step 1: The raw gas (I) is divided into two paths. The first path of raw gas (I) enters the raw gas pre-cooling box (1), the main cooling box (2), and the tower top feed subcooling box (5) in sequence, and then enters the top of the demethanizer (7) after throttling and reducing the temperature through heat exchange; Step 2: The second feed gas (I) enters the feed gas pre-cooling box (1) and the main cooling box (2) in sequence for heat exchange and cooling, and then enters the first low-temperature separator (3). The gas phase separated by the first low-temperature separator (3) enters the middle and upper part of the demethanizer (7) after being reduced in pressure and temperature by the expansion unit; Step 3: The liquid phase separated by the first low-temperature separator (3) enters the main cooling box (2) for heat exchange and temperature increase after throttling, and then enters the second low-temperature separator (4). The gas phase separated by the second low-temperature separator (4) is mixed with part of the liquid phase separated by the second low-temperature separator (4). After heat exchange and temperature reduction in the top feed subcooling box (5), the liquid phase enters the upper part of the demethanizer (7) after throttling and pressure reduction; Step 4: the liquid phase separated by the second low-temperature separator (4) is divided into two paths, the first path of liquid phase is mixed with the gas phase separated by the second low-temperature separator (4), and the second path of liquid phase enters the lower part of the demethanizer (7) after throttling and pressure reduction; Step 5: The condensate (III) at the bottom of the demethanizer (7) enters the post-processing device, and the gas phase at the top of the demethanizer (7) is divided into two paths. The gas phase at the top of the first path enters the top feed subcooling cold box (5), the main cooling cold box (2), and the raw gas precooling cold box (1) in sequence for heat exchange and temperature increase, and then enters the expansion unit and the external gas compressor (11) for pressurization, and then cools down in the first air cooler (12) and the water cooler (13) to obtain the external gas (II). The gas phase at the top of the second path enters the liquefied natural gas unit.

2. The method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas according to claim 1, characterized in that: The method for liquefying natural gas by the liquefied natural gas unit comprises the following steps: The second gas phase separated from the top of the demethanizer (7) is pressurized by the top gas compressor (8) and then enters the top feed subcooling cold box (5) and the liquefaction cold box (9) in sequence for heat exchange and liquefaction. After throttling and pressure reduction, the gas phase enters the liquefied natural gas flash tank (10). The BOG (V) separated from the top of the liquefied natural gas flash tank (10) enters the fuel gas system, and the LNG (IV) separated from the bottom of the liquefied natural gas flash tank (10) is transported out.

3. The method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas according to claim 1, characterized in that: The circulation method of propane in the propane refrigeration cycle unit is as follows: The gaseous propane separated from the second propane flash tank (15) enters the first propane compressor (16) for compression and pressure increase, and after pressure increase, is mixed with the gaseous phase separated from the first propane flash tank (14). The gaseous phase separated from the first propane flash tank (14) is compressed and pressure increased by the second propane compressor (17), and after pressure increase, it is cooled and liquefied by the propane cooler (18), and then throttled and flashed to enter the first propane flash tank (14). The liquid phase separated from the first propane flash tank (14) is divided into two paths. The first path of liquid phase enters the first propane flash tank (14) after heat exchange and temperature increase in the raw gas pre-cooling cold box (1), and the second path of liquid phase enters the second propane flash tank (15) after throttling and flashing. The liquid phase separated from the second propane flash tank (15) enters the second propane flash tank (15) after heat exchange and temperature increase in the main cooling cold box (2).

4. The method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas according to claim 3, characterized in that: The split ratio of the first liquid phase and the second liquid phase separated by the first propane flash tank (14) is 52.6:47.

4.

5. The method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas according to claim 1, characterized in that: The circulation method of the refrigerant in the mixed refrigerant refrigeration cycle unit is as follows: The mixed refrigerant flash tank (19) contains a mixed refrigerant. The gas phase separated by the mixed refrigerant flash tank (19) is sequentially liquefied by heat exchange in the tower top feed subcooling box (5) and the liquefaction box (9), and then enters the liquefaction box (9) and the tower top feed subcooling box (5) for heat exchange and temperature increase. The gas phase is then mixed with the liquid phase separated by the mixed refrigerant flash tank (19) after being cooled by the tower top feed subcooling box (5). The two liquid phases merge and enter the tower top feed subcooling box. The feed from the tower top is cooled in the supercooling box (5) and then enters the first mixed refrigerant compressor (20) for pressurization, and then enters the first mixed refrigerant cooler (21) for cooling, and then enters the second mixed refrigerant compressor (22) for pressurization, and then passes through the second air cooler (23) and the second mixed refrigerant cooler (24) for pressurization and cooling, and then enters the raw gas pre-cooling box (1) and the main cooling box (2) in sequence for cooling and flash evaporation, and then enters the mixed refrigerant flash tank (19).

6. The method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas according to claim 5, characterized in that: The mixed refrigerant consists of methane, ethane, ethylene, propane, butane, pentane and nitrogen.

7. The method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas according to claim 1, characterized in that: The split ratio of the first raw material gas (I) to the second raw material gas (I) is 1:

4.

8. The method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas according to claim 1, characterized in that: The split ratio between the first liquid phase and the second liquid phase separated by the second low-temperature separator (4) is 85:

15.

9. The method for multi-stage separation of ethane recovery and co-production of large-volume liquefied natural gas according to claim 1, characterized in that: The split ratio of the first top gas phase separated at the top of the demethanizer (7) to the second top gas phase is 57.5:42.5.

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