A method for recovering ethane and producing small-volume multi-temperature liquefied natural gas

Abstract

The present invention relates to the technical field of liquefied natural gas, and specifically to a method for ethane recovery and co-production of small-volume multi-temperature liquefied natural gas. Under normal circumstances, the natural gas ethane recovery process device and the natural gas liquefaction device are separately arranged, with high comprehensive energy consumption and low device cold energy utilization rate. In view of the above problems, the present invention provides a method for ethane recovery and co-production of small-volume multi-temperature liquefied natural gas, including an ethane recovery unit, a natural gas liquefaction unit with multi-temperature external refrigeration, and a refrigerant circulation and reuse unit. Only one set of refrigeration equipment is used to achieve "natural gas deep-cold ethane recovery and ultra-deep-cold natural gas liquefaction", which significantly reduces the investment cost. At the same time, the multi-temperature refrigeration process of the present invention can also achieve the effects of heat exchange temperature matching, medium step refrigeration, and energy step-by-step utilization, effectively reducing refrigeration energy consumption.

Description

Technical Field

[0001] The invention relates to the technical field of liquefied natural gas, and in particular to a method for recovering ethane and co-producing small-volume multi-temperature liquefied natural gas. Background Art

[0002] Ethane recovery refers to the process of condensing and recovering ethane and above heavy components from hydrocarbon mixtures such as natural gas and coalbed methane. By recovering heavy components, we can solve the condensation and blockage of liquid hydrocarbons during pipeline transportation and improve the quality of natural gas products.

[0003] In recent years, the domestic market demand for ethylene has continued to increase, and the ethane-to-ethylene process has the advantages of low operating costs and high ethylene yields. The profits of the domestic ethylene industry are greatly affected by raw material prices. Therefore, improving the resource utilization rate of domestic ethane products is of great significance to promoting the healthy development of ethylene products.

[0004] At present, typical domestic natural gas ethane recovery process units and natural gas liquefaction units are set up separately, which has the problems of high comprehensive energy consumption and low cold energy utilization rate of the units. At the same time, the construction of small-capacity natural gas liquefaction units for the scattered liquefied gas market also has the problems of high investment costs and low economic benefits.

[0005] The comprehensive energy consumption in the natural gas liquefaction process is mainly the compression power consumption of the external refrigeration process. At the same time, the energy consumption of the refrigeration process in the ethane recovery process is as high as about 40%. By utilizing the multi-temperature mixed refrigerant refrigeration process for natural gas ethane recovery and natural gas liquefaction, the utilization rate of equipment and cold energy can be improved, and the investment and operation and maintenance costs can be reduced. It is of great significance to promote oil field technological transformation, open up external markets, and improve quality and efficiency. Summary of the invention

[0006] The problem in the prior art is that, under normal circumstances, the natural gas ethane recovery process device and the natural gas liquefaction device are separately set up, resulting in high comprehensive energy consumption and low cold energy utilization rate of the device. In view of the above problems, the present invention provides a method for ethane recovery and co-production of small-volume multi-temperature liquefied natural gas.

[0007] It includes an ethane recovery unit, a natural gas liquefaction unit with multi-temperature external refrigeration, and a refrigerant recycling unit.

[0008] The method for recovering ethane by the ethane recovery unit comprises the following steps:

[0009] 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 subcooling cold box in turn for heat exchange and temperature reduction, and then throttles and reduces pressure before entering the top of the demethanizer;

[0010] 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 low-temperature separator. The gas phase separated by the low-temperature separator is depressurized by the expansion unit and then enters the middle of the demethanizer;

[0011] Step 3: The liquid phase separated by the low temperature separator is divided into two paths. The first path of liquid phase enters the main cold box for heat exchange and temperature rise after throttling, and then enters the lower part of the stripping tower. The second path of liquid phase enters the upper part of the stripping tower after throttling and temperature reduction;

[0012] Step 4: the gas phase separated from the stripping tower is mixed with part of the liquid phase separated from the stripping tower, and after heat exchange and cooling in the top supercooling cold box, it is throttled and depressurized as the second feed at the top of the demethanizer; the liquid phase separated from the stripping tower is divided into two paths, the gas phase separated from the stripping tower is mixed into the first liquid phase, and the second liquid phase enters the lower part of the demethanizer after throttling and depressurization;

[0013] Step 5: The liquid phase 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 gas phase enters the tower top subcooling cold box, the main cold box, and the raw gas precooling cold box in turn for heat exchange and temperature increase, 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 gas phase at the top of the tower enters the natural gas liquefaction unit with multi-temperature external refrigeration.

[0014] The refrigerant used in the ethane recovery unit is recycled through the refrigerant recycling unit.

[0015] Preferably, the liquefaction process of the natural gas liquefaction unit with multi-temperature external refrigeration comprises the following steps:

[0016] The second gas phase separated from the top of the demethanizer is pressurized by the top gas compressor, and then enters the top subcooling cold box and the natural gas liquefaction cold box in turn for heat exchange and liquefaction. After throttling and pressure reduction, it enters the liquefied natural gas flash tank. The liquid phase separated from the liquefied natural gas flash tank is exported as a liquefied natural gas product, and the gas phase separated from the liquefied natural gas flash tank enters the fuel gas system as a BOG product.

[0017] Preferably, the refrigerant recycling process in the refrigerant recycling unit comprises the following steps:

[0018] The refrigerant enters the mixed refrigerant low-temperature separator after heat exchange and cooling in the raw gas pre-cooling cold box and the main cold box. The gas phase separated by the mixed refrigerant low-temperature separator is completely liquefied by heat exchange and cooling in the tower top supercooling cold box and the natural gas liquefaction cold box in turn. After throttling and supercooling, it enters the natural gas liquefaction cold box and the tower top supercooling cold box in turn for heat exchange and temperature increase. Finally, it is mixed with the liquid phase separated by the mixed refrigerant low-temperature separator and enters the main cold box for heat exchange and temperature increase and gasification. Then, it enters the first mixed refrigerant compressor and the first mixed refrigerant cooler in turn for pressurization and temperature reduction, and then enters the mixed refrigerant separator. The gas phase separated by the mixed refrigerant separator is pressurized by the second mixed refrigerant compressor and cooled by the second mixed refrigerant cooler, and then mixed with the liquid phase pressurized by the mixed refrigerant booster pump 15, and finally enters the raw gas pre-cooling cold box and the main cold box for heat exchange and temperature reduction, forming a mixed refrigerant refrigeration cycle.

[0019] Preferably, the gas phase separated by the mixed refrigerant separator in the refrigerant cycle is pressurized to 2.1 MPa by a second mixed refrigerant compressor, and the liquid phase separated by the mixed refrigerant separator in the refrigerant cycle is pressurized to 2.1 MPa by a mixed refrigerant booster pump.

[0020] Preferably, the refrigerant is composed of methane, ethane, propane, n-pentane and nitrogen, each component is a pure component, and the volume proportion of each component is 20%, 32%, 25%, 22% and 1% respectively.

[0021] Preferably, in step three, the split flow ratio between the first liquid phase separated by the low-temperature separator and the second liquid phase is 1:1.

[0022] Preferably, the split flow ratio of the first feed gas to the second feed gas is 10-18:82-90.

[0023] Preferably, in step 4, the split flow ratio of the first liquid phase separated by the stripping tower to the second liquid phase is 35-55:45-65.

[0024] Preferably, in step five, the split flow ratio of the first gas phase separated from the top of the demethanizer tower to the second gas phase is 5-15:85-95.

[0025] The present invention has the following beneficial effects:

[0026] (1) The present invention adopts the method of adding a gas stripping tower to recover part of the liquid phase cooling capacity, thereby increasing the methane content in the upper feed of the demethanizer and the refrigeration and distillation effect, reducing the reflux ratio of the external dry gas, and improving the ethane recovery rate and the thermal integration of the device;

[0027] (2) The processing capacity of liquefied natural gas in the present invention is 10×10 4 t / a-50×10 4t / a. The present invention simultaneously recovers ethane and liquefies natural gas at a small processing volume, which is beneficial to reducing investment costs, effectively integrating operation and maintenance personnel and equipment, and reducing operation and maintenance costs and safety management costs. The method of the present invention has the advantages of high equipment utilization, low energy consumption and high yield, and is suitable for hydrocarbon mixtures such as natural gas and coalbed methane;

[0028] (3) The present invention adopts a method of combining split gasification and absorption tower distillation to perform split throttling heat exchange on the liquid phase of the low-temperature separator. The lower feed of the stripping tower with a high gasification rate performs gas stripping distillation on the upper feed of the stripping tower with a high liquefaction rate, thereby recovering part of the liquid phase cooling capacity and improving the gasification refrigeration and distillation effects by increasing the methane content in the second feed of the demethanizer, thereby achieving the purpose of low energy consumption and high yield.

[0029] (4) The present invention realizes "deep-cold ethane recovery of natural gas and ultra-deep-cold natural gas liquefaction" with only one set of refrigeration equipment, significantly reducing the investment cost. At the same time, the multi-temperature refrigeration process of the present invention can also achieve the effects of heat exchange temperature matching, medium step refrigeration, and energy step-by-step utilization, effectively reducing refrigeration energy consumption;

[0030] (5) The method of the present invention has the advantages of low investment cost, low operating cost, high economic benefit and low energy consumption, and is of great significance to promoting oil field technological transformation and improving quality and efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 : It is a device system used in a method for ethane recovery and co-production of small-volume multi-temperature liquefied natural gas provided by the present invention.

[0032] 1. Raw gas pre-cooling cold box, 2. Main cold box, 3. Low-temperature separator, 4. Gas stripping tower, 5. Top subcooling cold box, 6. Turbine expander, 7. Demethanizer, 8. Top gas compressor, 9. Liquefaction cold box, 10. Liquefied natural gas flash tank, 11. Mixed refrigerant low-temperature separator, 12. First mixed refrigerant compressor, 13. First mixed refrigerant cooler, 14. Mixed refrigerant separator, 15. Mixed refrigerant booster pump, 16. Second mixed refrigerant compressor, 17. Second mixed refrigerant cooler, 18. External gas compressor, 19. Air cooler; 20. Water cooler, Ⅰ. Raw gas, Ⅱ. External gas, Ⅲ. Condensate, Ⅳ. LNG, Ⅴ. BOG product. DETAILED DESCRIPTION

[0033] 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.

[0034] As the instruction manual Figure 1As shown, it is a device system used in a method for ethane recovery and co-production of small-volume multi-temperature liquefied natural gas provided by the present invention.

[0035] The gas quality components and operating conditions of raw gas I are as follows:

[0036] Raw gas I processing scale: 900×10 4 m³ / d.

[0037] Raw gas I pressure: 6.9MPa.

[0038] Entry temperature: 33℃.

[0039] Dry gas transmission pressure: 6.2MPa.

[0040] LNG scale: 13.5×10 4 t / a.

[0041] The composition of raw gas I is shown in Table 1.

[0042] The refrigerant is composed of methane, ethane, propane, n-pentane and nitrogen, and the mass percentage of each component is 20%, 32%, 25%, 22% and 1% respectively.

[0043] Table 1

[0044] ,

[0045] In Table 1, C1, C2, C3, iC4, nC4, iC5, nC5, C6, C7, and C8 represent methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, hexane, heptane, and octane in natural gas components, respectively.

[0046] A method for recovering ethane and producing small-volume multi-temperature liquefied natural gas, comprising an ethane recovery unit, a multi-temperature external refrigeration natural gas liquefaction unit, and a refrigerant recycling unit.

[0047] The method for recovering ethane by an ethane recovery unit comprises the following steps:

[0048] Step 1: Raw gas I (6.9 MPa, 33 °C, 900 × 10 4 m³ / d) is divided into two routes. The first route of raw gas I (10%) enters the raw gas I pre-cooling cold box 1, the main cooling cold box 2, and the tower top subcooling cold box 5 in turn for heat exchange and temperature reduction to -95℃, and then throttles and reduces the pressure to -111℃ and 2.85MPa before entering the top of the demethanizer 7;

[0049] Step 2: The second feed gas I enters the feed gas I pre-cooling box 1 and the main cooling box 2 in turn for heat exchange and cooling to -57°C before entering the low-temperature separator 3. The gas phase separated by the low-temperature separator 3 is depressurized to 2.85MPa by the expansion unit and enters the middle of the demethanizer 7;

[0050] Step 3: The liquid phase separated by the low-temperature separator 3 is divided into two paths. The first liquid phase (50%) enters the main cold box 2 for heat exchange and temperature rise to 4.6MPa, -36℃ after throttling and cooling, and then enters the lower part of the stripping tower 4. The second liquid phase enters the upper part of the stripping tower 4 after throttling and cooling to 4.6MPa, -58.7℃;

[0051] Step 4: The gas phase separated from the stripping tower 4 is mixed with part of the liquid phase separated from the stripping tower 4. The first liquid phase separated from the stripping tower 4 is cooled by heat exchange in the top supercooling cold box 5, and then throttled and depressurized to -106°C and 2.85MPa, and then used as the second feed at the top of the demethanizer 7; the liquid phase separated from the stripping tower 4 is divided into two paths, the first liquid phase (14.5%) is mixed with the gas phase separated from the stripping tower 4, and the second liquid phase (85.5%) enters the lower part of the demethanizer 7 after throttling and depressurizing to 2.8M5Pa;

[0052] Step 5: The liquid phase at the bottom of the demethanizer 7 enters the post-processing device with a recovery rate of 97%. The gas phase at the top of the demethanizer 7 is divided into two paths. The first gas phase (92%) enters the tower top subcooling cold box 5, the main cold box 2, and the raw gas I precooling cold box 1 in turn for heat exchange and temperature increase, and then enters the expansion unit, the external gas II compressor 18, and is pressurized to 6.2MPa. After cooling to 30°C by the air cooler 19 and the water cooler 20, the external gas II (6.15MPa, 30°C, 750×10 4 m³ / d), the second tower top gas phase enters the natural gas liquefaction unit with multi-temperature external refrigeration,

[0053] The refrigerant used in the ethane recovery unit is recycled through the refrigerant recycling unit.

[0054] The liquefaction process of the natural gas liquefaction unit with multi-temperature external refrigeration is as follows:

[0055] The second gas phase separated from the top of the demethanizer 7 is pressurized to 2.7 MPa by the top gas compressor 8, and then enters the top subcooling cold box 5 and the natural gas liquefaction cold box 9 in turn for heat exchange and cooling to -92°C and -160°C for liquefaction, and then throttles and reduces the pressure to 112 kPa to enter the liquefied natural gas flash tank 10. The liquid phase (13.5×10 4 t / a) is transported out as liquefied natural gas LNG IV product, and the gas phase separated by the liquefied natural gas flash tank 10 enters the fuel gas system as BOG product V.

[0056] The refrigerant recycling process in the refrigerant recycling unit has the following steps:

[0057] The refrigerant is cooled to -12°C by heat exchange in the raw gas I pre-cooling cold box 1 and the main cold box 2, and then enters the mixed refrigerant low-temperature separator 11. The gas phase separated by the mixed refrigerant low-temperature separator 11 is successively heated to -161°C by the tower top subcooling cold box 5 and the natural gas liquefaction cold box 9 to be completely liquefied. After throttling and subcooling to -168°C and 195kPa, it enters the natural gas liquefaction cold box 9 and the tower top subcooling cold box 5 in turn to heat exchange and heat up to 35°C. It is then mixed with the liquid phase separated by the mixed refrigerant low-temperature separator 11 and enters the main cold box 2 for heat exchange and heat up to 145kPa, 3 ℃ for vaporization, and then enters the first mixed refrigerant compressor 12 and the first mixed refrigerant cooler 13 in turn for pressurization and cooling to 1.1MPa and 32℃, and then enters the mixed refrigerant separator 14. The gas phase separated by the mixed refrigerant separator 14 is pressurized by the second mixed refrigerant compressor 16 and cooled to 2.2MPa and 35℃ by the second mixed refrigerant cooler 17, and then mixed with the liquid phase pressurized by the mixed refrigerant booster pump 15 to 2.2MPa and 33℃, and then enters the raw gas I pre-cooling cold box 1 and the main cooling cold box 2 for heat exchange and cooling to -12℃, forming a mixed refrigerant refrigeration cycle.

[0058] In the above method of the present invention, the cold box pinch point is controlled above 3.5°C, the external refrigeration adopts the mixed refrigerant refrigeration process, the recovery rate of condensate III ethane and above components is 97%, and the amount of liquefied natural gas is 13.5×10 4 t / a.

[0059] 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 recovering ethane and producing small-volume multi-temperature liquefied natural gas, characterized in that: It includes ethane recovery unit, natural gas liquefaction unit with multi-temperature external refrigeration and refrigerant recycling unit. The method for recovering ethane by the 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 cold box (1), the main cooling cold box (2), and the tower top subcooling cold 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 low-temperature separator (3). The gas phase separated by the low-temperature separator (3) is depressurized by the expansion unit and then enters the middle part of the demethanizer (7); Step 3: The liquid phase separated by the low-temperature separator (3) is divided into two paths. The first path of liquid phase enters the main cooling box (2) for heat exchange and temperature increase after throttling, and then enters the lower part of the stripping tower (4). The second path of liquid phase enters the upper part of the stripping tower (4) after throttling and temperature reduction; Step 4: the gas phase separated by the stripping tower (4) is mixed with part of the liquid phase separated by the stripping tower (4), and after heat exchange and cooling in the tower top supercooling cold box (5), the second stream of feed to the top of the demethanizer (7) is throttled and depressurized; the liquid phase separated by the stripping tower (4) is divided into two paths, the gas phase separated by the stripping tower (4) is mixed into the first liquid phase, and the second liquid phase enters the lower part of the demethanizer (7) after throttling and depressurization; Step 5: The liquid phase 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 first path of gas phase enters the tower top subcooling cold box (5), the main 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 (18) for pressurization, and then cools down in the air cooler (19) and the water cooler (20) to obtain the external gas (II). The second path of gas phase at the top of the tower enters the natural gas liquefaction unit with multi-temperature external refrigeration. The refrigerant used in the ethane recovery unit is recycled through the refrigerant recycling unit.

2. The method for recovering ethane and producing liquefied natural gas with small processing volume and multiple temperatures according to claim 1, characterized in that: The liquefaction process of the natural gas liquefaction unit with multi-temperature external refrigeration includes the following steps: The second gas phase separated at the top of the demethanizer (7) is pressurized by the top gas compressor (8) and then enters the top subcooling cold box (5) and the natural gas liquefaction cold box (9) in sequence for heat exchange and cooling to be completely liquefied. The gas phase is then throttled and depressurized and enters the liquefied natural gas flash tank (10). The liquid phase separated in the liquefied natural gas flash tank (10) is transported out as a liquefied natural gas product, and the gas phase separated in the liquefied natural gas flash tank (10) enters the fuel gas system as a BOG product (V).

3. The method for recovering ethane and producing liquefied natural gas with small processing volume and multiple temperatures according to claim 1, characterized in that: The refrigerant recycling process in the refrigerant recycling unit includes the following steps: The refrigerant is cooled by heat exchange in the raw gas pre-cooling box (1) and the main cooling box (2) and then enters the mixed refrigerant low-temperature separator (11). The gas phase separated by the mixed refrigerant low-temperature separator (11) is completely liquefied by heat exchange in the tower top sub-cooling box (5) and the natural gas liquefaction box (9) in turn. After throttling and sub-cooling, the gas phase enters the natural gas liquefaction box (9) and the tower top sub-cooling box (5) in turn for heat exchange and temperature increase. The gas phase is then mixed with the liquid phase separated by the mixed refrigerant low-temperature separator (11) and enters the main cooling box ( 2) heat exchange and temperature increase to gasification, and then successively enter the first mixed refrigerant compressor (12), the first mixed refrigerant cooler (13) for pressurization and temperature reduction, and then enter the mixed refrigerant separator (14). The gas phase separated by the mixed refrigerant separator (14) is pressurized by the mixed refrigerant compressor and cooled by the second mixed refrigerant cooler (17), and then mixed with the liquid phase pressurized by the mixed refrigerant booster pump (15), and then enters the raw gas pre-cooling cold box (1) and the main cooling cold box (2) for heat exchange and temperature reduction, thus forming a mixed refrigerant refrigeration cycle.

4. The method for recovering ethane and producing liquefied natural gas with small processing volume and multiple temperatures according to claim 3, characterized in that: The gas phase separated by the mixed refrigerant separator (14) in the refrigerant cycle is pressurized to 2.1 MPa by a second mixed refrigerant compressor (16), and the liquid phase separated by the mixed refrigerant separator (14) in the refrigerant cycle is pressurized to 2.1 MPa by a mixed refrigerant booster pump (15).

5. The method for recovering ethane and producing liquefied natural gas with small processing volume and multiple temperatures according to claim 3, characterized in that: The refrigerant is composed of methane, ethane, propane, n-pentane and nitrogen, each component is a pure component, and the volume proportion of each component is 20%, 32%, 25%, 22% and 1% respectively.

6. The method for recovering ethane and producing liquefied natural gas with small processing volume and multiple temperatures according to claim 1, characterized in that: In step 3, the split flow ratio between the first liquid phase separated by the low-temperature separator (3) and the second liquid phase is 1:

1.

7. The method for recovering ethane and producing liquefied natural gas with small processing volume and multiple temperatures according to claim 1, characterized in that: The split flow ratio of the first feed gas (I) to the second feed gas (I) is 10:90.

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