A CO2 purification system coupling low-temperature separation and phase change absorption

Abstract

The present invention provides a CO2 purification system coupling low-temperature separation and phase change absorption, which includes an associated oilfield gas pretreatment system, a low-temperature separation system and a phase change absorption system; the associated oilfield gas pretreatment system includes a filter, a light hydrocarbon recovery device and a dehydrator; the low-temperature separation system includes a compressor, a first heat exchanger, a second heat exchanger, a first cooler, a first gas-liquid separator, a second cooler, a second gas-liquid separator, a purification tower and a compression-expansion integrated machine; the phase change absorption system includes a phase change absorption membrane tower, a third heat exchanger and a phase change desorption tower. The device is ingeniously designed, has a simple process and low energy consumption, and is applicable to the treatment and purification of associated gas for CO2 flooding.

Description

Technical Field

[0001] The present invention relates to the technical field of gas separation and purification, and in particular to a CO2 purification system coupling low-temperature separation and phase change absorption. Background Art

[0002] With the development of oil and gas fields, most domestic oil fields have entered the middle and late stages of exploitation, and the difficulty of exploitation has also increased. In recent years, major oil fields have begun to generally adopt CO2 flooding technology in order to improve recovery. However, the problem that comes with it is that part of the CO2 injected into the reservoir to improve recovery will return to the ground with the oil and gas, and its content can sometimes reach 80% of the total amount of associated gas in the oil field. For the associated gas of this oil field, the traditional oil and gas field produced gas decarbonization process cannot meet the separation requirements. At the same time, under the goals of carbon peak and carbon neutrality, higher requirements are placed on the separation and purification purity of CO2 and associated gas. The above background has brought new challenges to the treatment and purification of CO2 flooding associated gas.

[0003] At present, the main technologies for decarbonizing produced gas from oil and gas fields include chemical absorption, pressure swing adsorption, membrane separation and cryogenic separation. The traditional chemical absorption method is suitable for the treatment of low-carbon natural gas, and has the advantages of good separation effect and high absorption rate, but it has disadvantages such as high solvent regeneration energy consumption and complex equipment. If it is used for the treatment of high-carbon associated gas, a large amount of absorption solvent is required, and the separation cost will be further increased. The pressure swing adsorption method has poor selectivity, and the separation effect is easily affected by gas impurities. It also has high automation requirements, frequent equipment adsorption and desorption, and the adsorbent is easy to fail. Membrane separation technology has the advantages of simple process, low separation cost, and small footprint, but the separation purity cannot meet the requirements. It is only suitable for the rough removal of high-carbon gas, and the membrane components are easily damaged and the cost is high. The cryogenic separation method is more suitable for the treatment of high-carbon gas, but it has the disadvantages of large equipment investment, high operating energy consumption, and low separation and purification purity.

[0004] There are deficiencies in the use of single separation technology to process high-carbon natural gas. It is necessary to further improve the design of high-carbon natural gas processing in view of the deficiencies of single separation technology. Summary of the invention

[0005] The technical problem to be solved by the present invention is: in order to overcome the shortcomings of the prior art, the present invention provides a CO2 purification system that couples low-temperature separation and phase change absorption, which couples the low-temperature separation technology and the phase change absorption technology, effectively avoiding the shortcomings of high energy consumption and low separation and purification purity when using a single separation technology. The purification system is cleverly designed, simple in process, and low in energy consumption, and is suitable for the treatment and purification of associated gas in CO2-driven oil fields.

[0006] The technical solution adopted by the present invention to solve its technical problems is: a CO2 purification system coupling low-temperature separation and phase change absorption, including an associated oilfield gas pretreatment system, a low-temperature separation system, and a phase change absorption system.

[0007] The described associated oilfield gas pretreatment system includes a filter, a light hydrocarbon recovery device, and a dehydrator connected in sequence by pipelines. The associated oilfield gas enters the associated oilfield gas pretreatment system through the filter. The bottom of the light hydrocarbon recovery device has a light hydrocarbon discharge port.

[0008] The described low-temperature separation system includes a first compressor, a first heat exchanger, a second heat exchanger, a first cooler, a first gas-liquid separator, a second cooler, a second gas-liquid separator, a purification tower, and a compression-expansion integrated machine. The inlet pipeline of the first compressor is connected to the outlet pipeline of the dehydrator. The first compressor, the first heat exchanger, the second heat exchanger, the first cooler, the first gas-liquid separator, the second cooler, and the second gas-liquid separator are connected in sequence by pipelines. The gas-phase outlet of the first gas-liquid separator is connected to the inlet pipeline of the second cooler. The bottom of the second cooler has a cooling discharge pipeline. The liquid-phase outlet of the first gas-liquid separator is connected to the pipeline of the purification tower by a pipeline. The gas-phase outlet of the second gas-liquid separator is connected to the inlet pipeline of the compression-expansion integrated machine by a pipeline. The liquid-phase outlet of the second gas-liquid separator is connected to the pipeline of the purification tower by a pipeline. The bottom of the purification tower has a liquid CO2 discharge port, and the top pipeline is connected to a condenser. The low-temperature separation system also has a refrigerant pipeline. The first cooler, the second cooler, and the condenser are respectively connected to the refrigerant pipeline.

[0009] The described phase change absorption system includes a phase change absorption membrane tower, a third heat exchanger, and a phase change desorption tower. The top of the phase change absorption membrane tower has an air outlet, which is connected to the pipeline of the compression-expansion integrated machine. The upper part of the phase change absorption membrane tower has a liquid inlet, and the bottom has a first liquid outlet. The first liquid outlet is connected back to the upper liquid inlet through a pipeline. The first heat exchanger is connected to the third heat exchanger by a pipeline. The third heat exchanger is connected to the liquid inlet of the upper part of the phase change desorption tower by a pipeline. The bottom liquid outlet of the phase change desorption tower is connected to the third heat exchange pipeline and this pipeline passes through the third heat exchanger and then enters the liquid inlet of the upper part of the phase change absorption membrane tower. The top of the phase change desorption tower has a CO2 discharge port.

[0010] Preferably, the bottom of the phase change absorption membrane tower has a second liquid outlet, and the second liquid outlet is connected to a phase separation heating pipeline that transports back to the bottom of the phase change absorption membrane tower. A first heater for providing phase separation heat for the phase change absorbent is provided on the phase separation heating pipeline.

[0011] Preferably, the bottom of the phase change desorption tower has a liquid outlet, and this liquid outlet is connected to a phase change recovery pipeline that transports back to the bottom of the phase change desorption tower. A second heater for providing desorption heat for the phase change desorption tower is provided on the phase change recovery pipeline.

[0012] Further, there is a phase change absorbent in the phase change absorption membrane tower. A tubular hollow membrane separator is vertically installed in the phase change absorption membrane tower. After the phase change absorbent absorbs the associated oilfield gas introduced into the second heat exchanger, under the absorption action of the phase change absorbent, the associated oilfield gas forms a CO₂-rich phase change absorbent and a CO₂-lean phase change absorbent. The CO₂-lean phase change absorbent can pass through the tubular hollow membrane separator. A third liquid outlet is opened on the side of the tubular hollow membrane separator, and the third liquid outlet is connected to the first heat exchanger through a pipeline. The CO₂-rich phase change absorbent enters the first heat exchanger through the third liquid outlet, is heated and temperature-raised, and then enters the phase change desorption tower through the third heat exchanger.

[0013] Furthermore, the phase change absorbent is a phase change absorbent prepared by compounding dimethylbutylamine (DMBA) and triethylamine (TEA). The concentration of DMBA is 5 mol / L, and the concentration of TEA is 3 mol / L. The preparation steps of the phase change absorbent are to heat DMBA and TEA to about 50 °C, and then mix and stir well to obtain the phase change absorbent.

[0014] Still further, a number of air inlets are installed in the middle and lower part of the phase change absorption membrane tower. The air inlets are fixed in a ring on the tower wall of the phase change absorption membrane tower. A porous hollow disc is installed inside the top of the phase change absorption membrane tower, and a number of atomizing nozzles are installed on the porous hollow disc.

[0015] The compression-expansion integrated machine includes an expander and a second compressor. Among them, the gas phase outlet of the second gas-liquid separator is connected to the inlet pipeline of the expander, the outlet pipeline of the expander is introduced into the second heat exchanger, the inlet pipeline of the second compressor is connected to the outlet pipeline of the top gas outlet of the phase change absorption membrane tower, and the outlet pipeline of the second compressor is the oil and gas discharge pipeline.

[0016] Preferably, a recovery pipeline for transporting back into the purification tower is connected to the pipeline of the liquid CO₂ discharge outlet at the bottom of the purification tower. A reboiler is provided on this recovery pipeline. The outlet pipeline of the compressor is connected to the reboiler to provide heat source for the reboiler. After heat exchange in the reboiler, the heat source is transported into the second heat exchanger through a pipeline.

[0017] The beneficial effects of the present invention are:

[0018] 1. Coupling the low-temperature separation technology and the phase change absorption technology effectively avoids the disadvantages of high energy consumption and low separation and purification purity of using a single separation technology.

[0019] 2. By using a heat exchanger, the heat of the pressurized associated oilfield gas is transferred to the phase change absorbent and the reboiler corresponding to the purification column. At the same time, the temperature of the associated oilfield gas is also reduced, saving energy consumption for the next phase change desorption and CO2 liquefaction. A compression-expansion integrated machine is used to convert the pressure energy of the medium-high pressure gas coming out of the top of the second gas-liquid separator into kinetic energy and cold energy for the system to reuse.

[0020] 3. A phase change absorbent prepared by compounding dimethylbutylamine (DMBA) and triethylamine (TEA) is used. The phase change absorption membrane column separates the lean CO2 phase change absorbent from the rich CO2 phase change absorbent, reducing the desorption amount of the phase change absorbent. Compared with the traditional absorption process, the absorption energy consumption can be reduced by more than 40%.

[0021] This device is ingeniously designed, has a simple process, and low energy consumption, and is suitable for the treatment and purification of associated oilfield gas in CO2 flooding. Brief Description of the Drawings

[0022] The present invention will be further described below in conjunction with the drawings and embodiments.

[0023] Figure 1 It is a schematic structural diagram of the optimal embodiment of the present invention.

[0024] Figure 2 It is a distribution diagram of the air inlet nozzles of the phase change absorption membrane column in the optimal embodiment of the present invention.

[0025] Figure 3 It is a schematic structural diagram of the porous hollow disk in the phase change absorption membrane column in the optimal embodiment of the present invention.

[0026] In the figure: 1. Pretreatment system; 1-1. Filter; 1-2. Light hydrocarbon recovery device; 1-3. Dehydrator; 2. Low-temperature separation system; 2-1. First compressor; 2-2. First heat exchanger; 2-3. Second heat exchanger; 2-4. First cooler; 2-5. First gas-liquid separator; 2-6. Second cooler; 2-7. Second gas-liquid separator; 2-8. Purification column; 2-8-1. Reboiler; 2-8-2. Condenser; 2-9. Compression-expansion integrated machine; 2-9-1. Expander; 2-9-2. Second compressor; 3. Phase change absorption system; 3-1. Phase change absorption membrane column; 3-1-1. Tubular hollow membrane separator; 3-1-2. Porous hollow disk; 3-1-3. Atomizing nozzle; 3-1-4. Air inlet nozzle; 3-1-5. First heater; 3-2. Third heat exchanger; 3-3. Phase change desorption tower; 3-4. Second heater. Detailed Embodiments

[0027] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are all simplified schematic diagrams, only illustrating the basic structure of the present invention in a schematic manner. Therefore, they only show the components related to the present invention, and directions and references (such as up, down, left, right, etc.) can only be used to assist in the description of the features in the drawings. Therefore, the following specific embodiments are not adopted in a restrictive sense, and the scope of the claimed subject matter is defined only by the appended claims and their equivalents.

[0028] As Figure 1 shown, a CO2 purification system coupling low-temperature separation and phase change absorption is the optimal embodiment of the present invention.

[0029] The purification system includes an associated gas pretreatment system 1 for oil fields, a low-temperature separation system 2, and a phase change absorption system 3.

[0030] The associated gas pretreatment system 1 for oil fields includes a filter 1-1, a light hydrocarbon recovery device 1-2, and a dehydrator 1-3. The filter 1-1, the light hydrocarbon recovery device 1-2, and the dehydrator 1-3 are connected in sequence through pipe fittings. The associated gas from the oil field enters the pretreatment system, is filtered by the filter, and after the light hydrocarbon is recovered by the light hydrocarbon recovery device, it enters the dehydrator for dehydration. The pretreated associated gas from the oil field enters the low-temperature separation system.

[0031] The low-temperature separation system 2 includes at least a first compressor 2-1, a first heat exchanger 2-2, a second heat exchanger 2-3, a first cooler 2-4, a first gas-liquid separator 2-5, a second cooler 2-6, a second gas-liquid separator 2-7, a purification tower 2-8, and a compression-expansion integrated machine 2-9. The phase change absorption system includes at least a phase change absorption membrane tower 3-1, a third heat exchanger 3-3, and a phase change desorption tower 3-4.

[0032] The first compressor 2-1 of the low-temperature separation system is connected to the first heat exchanger 2-2 through pipe fittings, the first heat exchanger 2-2 is connected to the second heat exchanger 2-3 through pipe fittings, the second heat exchanger 2-3 is connected to the first cooler 2-4 through pipe fittings, the first cooler 2-4 is connected to the first gas-liquid separator 2-5 through pipe fittings, the gas-phase outlet of the first gas-liquid separator 2-5 is connected to the inlet of the second cooler 2-6, the liquid-phase outlet of the first gas-liquid separator 2-5 is connected to the purification tower 2-8, the second cooler 2-6 is connected to the second gas-liquid separator 2-7 through pipe fittings, the gas-phase outlet of the second gas-liquid separator 2-7 is connected to the inlet of the expander 2-9-1 of the compression-expansion integrated machine 2-9 through pipe fittings, the outlet of the expander 2-9-1 is connected to the upper inlet of the second heat exchanger 2-3 through pipe fittings, and the lower outlet of the second heat exchanger 2-3 is connected to the phase change absorption membrane tower 3-1 through pipe fittings. The liquid-phase outlet of the second gas-liquid separator 2-7 is connected to the liquid-phase pipe fitting of the first gas-liquid separator 2-5 through pipe fittings.

[0033] The reboiler 2-8-1 at the bottom of the purification tower 2-8 is connected to the associated gas pipeline output by the first compressor 2-1 through pipe fittings, and provides heat source for the reboiler 2-8-1 by introducing the associated gas partially pressurized and heated by the first compressor 2-1. The condenser 2-8-2 at the top of the purification tower 2-8, the first cooler 2-4 and the second cooler 2-6 are all connected to the refrigerant pipe through pipe fittings, and the refrigerant can be any one of LNG / liquid ammonia / propane. The compression-expansion integrated machine 2-9 includes an expander 2-9-1 and a second compressor 2-9-2. The expander 2-9-1 and the second compressor 2-9-2 are coaxial. The medium-high pressure gas enters the expander 2-9-1 to drive the expander 2-9-1 to rotate and reduce the gas temperature. The second compressor 2-9-2 is used to recover the kinetic energy of the expander 2-9-1.

[0034] The first liquid outlet at the bottom of the phase change absorption membrane tower 3-1 of the phase change absorption system 3 is connected to the liquid flowing into the third heat exchanger 3-2 at the bottom outlet of the phase change desorption tower 3-3 through pipe fittings after passing through the third heat exchanger 3-2, and is transported back to the phase change absorption membrane tower 3-1 through a pipeline. The liquid outlet on the lower right side of the phase change absorption membrane tower 3-1 is connected to the first heat exchanger 2-2 through pipe fittings. The first heat exchanger 2-2 is connected to the third heat exchanger 3-2 through pipe fittings. The third heat exchanger 3-2 is connected to the upper liquid inlet of the phase change desorption tower 3-3. The bottom liquid outlet of the phase change desorption tower 3-3 is connected to the third heat exchanger 3-2 through pipe fittings. The third heat exchanger 3-2 is connected to the upper liquid inlet of the phase change absorption membrane tower 3-1 through pipe fittings.

[0035] The phase change absorbent used in this system is a phase change absorbent prepared by compounding dimethylbutylamine (DMBA) and triethylamine (TEA). The concentration of DMBA is 5 mol / L, and the concentration of TEA is 3 mol / L. The preparation steps are to heat DMBA and TEA to about 50 °C, and then mix and stir well. The absorption principle of the phase change absorbent is to contact countercurrently with the crude purified gas from the low-temperature separation system in the phase change absorption membrane tower. After the phase change absorbent absorbs the CO2 gas in the crude purified gas, it is enriched at the bottom of the phase change absorption membrane tower. The heater provides phase separation heat for the phase change absorbent that has absorbed CO2 at the bottom of the phase change absorption membrane tower. After the temperature rises, the absorbent undergoes phase separation to produce CO2-lean and CO2-rich phase change absorbents. It is measured by experiments that the CO2-lean absorbent has a smaller viscosity than the CO2-rich absorbent.

[0036] At the same time, a vertical tubular hollow membrane separator 3-1-1 is fixed in the middle of the phase change absorption membrane tower 3-1. This membrane module can permeate the CO2-lean phase change absorbent with a smaller viscosity.

[0037] According to research, the pipeline transportation pressure of the oilfield gathering and transportation system is generally between 0.5 and 1 MPa. Therefore, the pressure of the associated gas in the oilfield before purification should also be between 0.5 and 1 MPa. According to the technological process of this system, before cryogenic separation, the pressure of the associated gas in the oilfield needs to be increased to about 4.5 MPa. According to the relevant knowledge of engineering thermodynamics, the temperature of the adiabatically compressed gas can be obtained from T2 = T1(P2 / P1) (K-1) / K where the temperature T1 of the associated gas in the oilfield is about 300 K, the initial pressure P1 is 0.5 - 1 MPa, P2 is 4.5 MPa, and the adiabatic index K of the gas is calculated as 1.3. From the above data, the temperature T2 of the associated gas in the oilfield after pressurization is calculated to be between 424 and 498 K, that is, between 151 and 225 °C. This part of the heat generated by compressing the associated gas in the oilfield can be used for the reboiler 2-8-1 of the purification column 2-8 and the desorption of the phase change absorbent, saving energy consumption for the system.

[0038] Purification steps:

[0039] The associated gas in the oilfield enters the pretreatment system 1 and passes through the filter 1-1, the light hydrocarbon recovery device 3-2 and the dehydrator in sequence to remove impurities, recover light hydrocarbons and dehydrate the associated gas in the oilfield, preventing it from damaging the separation equipment or affecting the separation effect.

[0040] The associated gas in the oilfield passing through the pretreatment system enters the first compressor 2-1 for pressurization, raising the pressure to about 4.5 MPa, and the temperature of the pressurized gas rises to between 151 and 225 °C. The heated gas exchanges heat with the rich CO2 phase change absorbent in the phase change absorption system 3 in the first heat exchanger 2-2, raising the temperature of the phase change absorbent to prepare for the desorption of the phase change absorbent, and at the same time reducing the temperature of the associated gas in the oilfield to prepare for liquefying the CO2 in the associated gas in the oilfield. This heat exchange process can not only utilize the heat of the compressed and heated gas to raise the temperature of the phase change absorbent, reducing the heating energy consumption in the phase change absorption part, but also reduce the temperature of the associated gas in the oilfield, reducing the refrigeration energy consumption for liquefying CO2.

[0041] The associated gas in the oilfield after temperature reduction enters the second heat exchanger 2-3 and the second gas-liquid separator 2-7, and exchanges heat with the crude oil and gas undergoing expansion cooling in the compression-expansion integrated machine 2-9 from the top of the second gas-liquid separator 2-7. The associated gas in the oilfield after temperature reduction enters the first cooler 2-4, where most of the CO2 in the associated gas in the oilfield is liquefied. The resulting gas-liquid mixture enters the first gas-liquid separator 2-5 for gas-liquid separation. The gas coming out of the first gas-liquid separator 2-5 enters the second cooler 2-6 to continue liquefying the CO2 gas in it. The resulting gas-liquid mixture enters the second gas-liquid separator 2-7, and the gas after gas-liquid separation enters the phase change absorption system 3.

[0042] The liquid CO2 separated by the first gas-liquid separator 2-5 and the second gas-liquid separator 2-7 enters the purification tower 2-8 to remove impurity gases such as CH4 dissolved in the liquid CO2. The heat of the reboiler 2-8-1 at the bottom of the purification tower 2-8 is provided by the associated oilfield gas coming out of the compressor 2-1, and the cooling capacity of the condenser 2-8-2 at the top of the purification tower 2-8 is provided by the system refrigerant.

[0043] The oil and gas heated by heat exchange through the second heat exchanger 2-3 enters the phase change absorption membrane tower 3-1 through the gas inlet nozzle 3-1-4 on the phase change absorption membrane tower, and countercurrently contacts the atomized phase change absorbent sprayed by the atomizing nozzle 3-1-3 at the top of the phase change absorption membrane tower 3-1. After the phase change absorbent absorbs the CO2 gas in the oil and gas, it is enriched at the bottom of the phase change absorption membrane tower 3-1. The first heater 3-1-5 provides phase separation heat for the phase change absorbent that has absorbed CO2, and the phase change absorbent after the temperature rises undergoes phase separation to produce lean CO2 and rich CO2 phase change absorbents. The lean CO2 absorbent with low viscosity is enriched in the tubular hollow membrane separator 3-1-1, and then converges with the phase change absorbent of the third heat exchanger 3-2 (sent by the phase change desorption tower 3-3) through the pipeline at the bottom of the phase change absorption membrane tower 3-1 and returns to the top of the phase change absorption membrane tower 3-1 for reuse.

[0044] The rich CO2 phase change absorbent generated at the bottom of the phase change absorption membrane tower 3-1 enters the first heat exchanger 2-2 through the third liquid outlet, is heated by heat exchange through the third heat exchanger 3-2, and then enters the phase change desorption tower 3-3. The second heater 3-4 provides heat for the phase change desorption tower. After the rich CO2 phase change absorbent releases CO2, it comes out from the bottom of the tower, exchanges heat with the third heat exchanger 3-2, and finally returns to the top of the phase change absorption membrane tower 3-1 for continuous use. The oil and gas coming out of the top of the phase change absorption membrane tower 3-1 enters the second compressor 2-9-2 of the compression-expansion integrated machine 2-9, and after pressurization, it is transported to the outside.

[0045] As Figure 2 shown, the gas inlet nozzle 3-1-4 at the lower part of the phase change absorption membrane tower 3-1 is fixed on the tower wall of the phase change absorption membrane tower in a circular manner for dispersing gas and improving the separation and purification effect.

[0046] As Figure 3 shown, the multi-hole hollow disk is used for the oil and gas passing through purification. The atomizing nozzle 3-1-3 at the top of the phase change absorption membrane tower 3-1 is fixed on the multi-hole hollow disk 3-1-2, and the atomizing nozzle 3-1-3 is connected to the pipeline of the tubular hollow membrane separator 3-1-1 for spraying the phase change absorbent downward.

[0047] In this purification system, the use of absorbent phase separation technology reduces the desorption amount of the phase change absorbent. Compared with the traditional absorption process, it can save more than 40% of the energy consumption. The liquid CO2 discharged from the bottom of the purification tower 2-8 and the CO2 flowing out from the top of the phase change desorption tower 3-4 can be used for CO2 flooding in oil fields, food processing, etc.

[0048] Inspired by the above ideal embodiments based on the present invention, through the above description, relevant staff can completely make various changes and modifications without departing from the technical idea of the present invention. The technical scope of the present invention is not limited to the content in the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A CO2 purification system coupling low-temperature separation and phase change absorption, characterized in that: It includes an associated gas pretreatment system (1) for oil fields, a cryogenic separation system (2), and a phase change absorption system (3); The said associated gas pretreatment system (1) for oil fields includes a filter (1-1), a light hydrocarbon recovery unit (1-2), and a dehydrator (1-3) connected in sequence by pipelines. The associated gas from oil fields enters the associated gas pretreatment system (1) through the filter (1-1). The bottom of the said light hydrocarbon recovery unit (1-2) has a light hydrocarbon discharge port; The said cryogenic separation system (2) includes a first compressor (2-1), a first heat exchanger (2-2), a second heat exchanger (2-3), a first cooler (2-4), a first gas-liquid separator (2-5), a second cooler (2-6), a second gas-liquid separator (2-7), a purification tower (2-8), and a compression-expansion integrated machine (2-9). Among them, the inlet pipeline of the first compressor (2-1) is connected to the outlet pipeline of the dehydrator (1-3). The first compressor (2-1), the first heat exchanger (2-2), the second heat exchanger (2-3), the first cooler (2-4), the first gas-liquid separator (2-5), the second cooler (2-6), and the second gas-liquid separator (2-7) are connected in sequence by pipelines. The gas-phase outlet of the first gas-liquid separator (2-5) is connected to the inlet pipeline of the second cooler (2-6). The bottom of the second cooler (2-6) has a cooling discharge pipeline. The liquid-phase outlet of the first gas-liquid separator (2-5) is connected to the pipeline of the purification tower (2-8) by a pipeline. The gas-phase outlet of the second gas-liquid separator (2-7) is connected to the inlet pipeline of the compression-expansion integrated machine (2-9) by a pipeline. The liquid-phase outlet of the second gas-liquid separator (2-7) is connected to the pipeline of the purification tower (2-8) by a pipeline. The bottom of the said purification tower (2-8) has a liquid CO₂ discharge port, and the top pipeline is connected to a condenser (2-8-2). The said cryogenic separation system (2) also has a refrigerant pipeline. The first cooler (2-4), the second cooler (2-6), and the condenser (2-8-2) are respectively connected to the refrigerant pipeline; The said phase change absorption system (3) includes a phase change absorption membrane tower (3-1), a third heat exchanger (3-2), and a phase change desorption tower (3-3). The top of the said phase change absorption membrane tower (3-1) has an air outlet, which is connected to the pipeline of the compression-expansion integrated machine (2-9). The upper part of the phase change absorption membrane tower (3-1) has a liquid inlet, and the bottom has a first liquid outlet. The first liquid outlet is connected back to the upper liquid inlet by a pipeline. The first heat exchanger (2-2) is connected to the third heat exchanger (3-2) by a pipeline. The third heat exchanger (3-2) is connected to the liquid inlet of the upper part of the phase change desorption tower (3-3) by a pipeline. The bottom liquid outlet of the phase change desorption tower (3-3) is connected to the third heat exchange pipeline, and this pipeline passes through the third heat exchanger (3-2) and then enters the upper liquid inlet of the phase change absorption membrane tower (3-1). The top of the said phase change desorption tower has a CO₂ discharge port.

2. The CO2 purification system coupling low-temperature separation and phase change absorption according to claim 1, characterized in that: The bottom of the phase change absorption membrane tower (3-1) has a second liquid outlet, and the second liquid outlet is connected to a phase separation heating pipeline that transports back to the bottom of the phase change absorption membrane tower (3-1). A first heater (3-1-5) for providing phase separation heat to the phase change absorbent is provided on the phase separation heating pipeline.

3. The CO2 purification system coupling low-temperature separation and phase change absorption according to claim 1, characterized in that: The bottom of the phase change desorption tower (3-3) has a liquid outlet, and the liquid outlet is connected to a phase change recovery pipeline that transports back to the bottom of the phase change desorption tower (3-3). A second heater (3-4) for providing desorption heat to the phase change desorption tower (3-3) is provided on the phase change recovery pipeline.

4. The CO2 purification system coupling low-temperature separation with phase change absorption according to claim 2, characterized in that: The phase change absorption membrane tower (3-1) contains a phase change absorbent. A tubular hollow membrane separator (3-1-1) is vertically installed in the phase change absorption membrane tower (3-1). After the phase change absorbent absorbs the associated oilfield gas introduced by the second heat exchanger (2-3), under the absorption action of the phase change absorbent, the associated oilfield gas forms a rich CO2 phase change absorbent and a lean CO2 phase change absorbent. The lean CO2 phase change absorbent can pass through the tubular hollow membrane separator (3-1-1). A third liquid outlet is opened on the side of the tubular hollow membrane separator (3-1-1), and the third liquid outlet is connected to the pipeline of the first heat exchanger (2-2). The rich CO2 phase change absorbent enters the first heat exchanger (2-2) through the third liquid outlet, is heated and raised in temperature, and then enters the phase change desorption tower (3-3) through the third heat exchanger (3-2).

5. The CO2 purification system coupling low-temperature separation and phase change absorption according to claim 4, characterized in that: A number of air inlets (3-1-4) are installed in the middle and lower part of the phase change absorption membrane tower (3-1). The air inlets (3-1-4) are fixed in a ring on the tower wall of the phase change absorption membrane tower (3-1). A porous hollow disc (3-1-2) is installed inside the top of the phase change absorption membrane tower (3-1), and a number of atomizing nozzles (3-1-3) are installed on the porous hollow disc (3-1-2).

6. The CO2 purification system coupling low-temperature separation and phase change absorption according to claim 1, characterized in that: The compression-expansion integrated machine (2-9) includes an expander (2-9-1) and a second compressor (2-9-2). Among them, the gas phase outlet of the second gas-liquid separator (2-7) is connected to the inlet pipeline of the expander (2-9-1). The outlet pipeline of the expander (2-9-1) leads into the second heat exchanger (2-3). The inlet pipeline of the second compressor (2-9-2) is connected to the outlet pipeline of the top gas outlet of the phase change absorption membrane tower (3-1). The outlet pipeline of the second compressor (2-9-2) is an oil and gas discharge pipeline.

7. The CO2 purification system coupling low-temperature separation and phase change absorption as claimed in claim 1, wherein: A recovery pipeline that transports back into the purification tower (2-8) is connected to the pipeline of the liquid CO2 discharge outlet at the bottom of the purification tower (2-8). A reboiler (2-8-1) is provided on the recovery pipeline. The outlet pipeline of the first compressor (2-1) is connected to the reboiler (2-8-1) to provide heat source for the reboiler (2-8-1). After heat exchange in the reboiler (2-8-1), the heat source is transported into the second heat exchanger (2-3) through a pipeline.

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