System and method for enriching carbon dioxide in oilfield associated gas

Abstract

The invention discloses a system and a method for enriching carbon dioxide in oilfield associated gas. The method comprises the following steps: step 1, sequentially pressurizing and cooling raw material gas obtained after impurity removal of the associated gas to obtain cooled raw material gas; step 2, carrying out membrane separation on the cooled raw material gas to obtain permeated gas and retentate gas, and sequentially compressing and liquefying the permeated gas to obtain a gas-liquid mixture; and step 3, rectifying and purifying the gas-liquid mixture to obtain the product gas. Membrane separation, low-temperature liquefaction and rectification are combined, CO2 is primarily concentrated through membrane separation, then low-temperature liquefaction and rectification are carried out, the concentration of carbon dioxide in product gas is further improved, and finally the purity of CO2 in the product gas reaches 95% or above. In addition, the method can be used for enriching CO2 in associated gas with the CO2 volume fraction of 10%-70%, multi-stage membrane treatment is not needed, and the investment of membrane separation equipment is reduced; meanwhile, the process is simple, and equipment investment is low.

Description

Technical Field

[0001] The invention relates to the technical field of petrochemical engineering, and in particular to a system and method for enriching carbon dioxide in associated gas of oil fields. Background Art

[0002] During the oil field exploitation process, a large amount of associated gas will be produced, which contains a large amount of carbon dioxide, and there is still a large amount of high-carbon dioxide natural gas to be exploited in the formation gas reservoir. The extraction of a large amount of carbon dioxide from offshore oil and gas fields not only keeps the carbon emissions of offshore gas fields high, but also affects the purity of natural gas. At present, carbon dioxide capture, utilization and storage (CCUS) technology is a necessary means to achieve carbon emission reduction and natural gas purification. If relevant technologies are used to separate and concentrate the carbon dioxide in the associated gas of the oil field and inject it back into the underground for oil recovery, it can reduce environmental pollution and bring certain economic benefits. However, offshore oil and gas fields are far away from the land. If the produced gas is carbon captured on land and then the carbon dioxide is transported to the sea for gas recovery and storage, the investment cost of the carbon dioxide transmission pipeline is too high. Therefore, it is necessary to adopt relevant technologies to decarbonize natural gas at sea and directly inject the captured carbon dioxide back, which will greatly reduce the transportation cost.

[0003] At present, the known technical methods for carbon dioxide capture include chemical absorption, adsorption separation, cryogenic separation and membrane separation. In the chemical absorption method, the absorption liquid is in direct contact with the carbon dioxide gas, so it has a high selectivity. Its disadvantage is that it consumes a lot of energy and is generally only suitable for the separation of gases with a low carbon dioxide volume fraction. Although the adsorption separation method has low energy consumption, the adsorption capacity of the adsorbent is limited, frequent adsorption-desorption operations are required, and the recovery rate is low, so its application is limited. Although the cryogenic separation method has a good separation effect, the process is complicated, the equipment investment is large, and the cost is high, so it needs to be used in combination with other processes. The membrane separation method has many advantages such as low energy consumption, simple equipment, no pollution, easy operation and reliable operation, but it is impossible to obtain high purity carbon dioxide and high recovery rate at the same time.

[0004] However, the available space and weight of offshore oil and gas field production platforms are limited; based on this, there is an urgent need for a recovery equipment that occupies a small space and has a high CO2 recovery rate. Summary of the invention

[0005] In order to solve the above problems, one of the objects of the present invention is to provide a method for enriching carbon dioxide in associated gas of oil fields.

[0006] The technical solution of the present invention to solve the above technical problems is as follows:

[0007] A method for enriching carbon dioxide in oilfield associated gas comprises the following steps:

[0008] Step 1, pressurizing and cooling the raw gas obtained after removing impurities from the associated gas in sequence to obtain cooled raw gas;

[0009] Step 2: performing membrane separation on the cooled raw gas to obtain permeate gas and retentate gas, and sequentially compressing and liquefying the permeate gas to obtain a gas-liquid mixture;

[0010] Step 3: Distill and purify the gas-liquid mixture to obtain product gas.

[0011] On the basis of the above technical solution, the present invention can also be improved as follows:

[0012] Furthermore, the volume fraction of CO2 in the associated gas in step 1 is 10%-70%; the particle size in the raw gas in step 1 is less than 0.01um, and the content is less than 10mg / m 3 ; Sulfur dioxide content <10mg / m 3 ; Water content <0.17g / m 3 ; Volume fraction of heavy hydrocarbons <0.1%.

[0013] Furthermore, the volume fraction of CO2 in the associated gas in step 1 is 30%-60%.

[0014] Furthermore, in step 1, the pressure of the cooled raw gas is 5-7 MPa, and the temperature of the cooled raw gas is 30-50°C; preferably, the temperature of the cooled raw gas is 30°C.

[0015] Furthermore, the membrane separation conditions in step 2 are: the cooled raw gas is subjected to membrane separation at a pressure of 5 to 7 MPa and a flow rate of 20 to 50 L / min; the pressure of the retentate gas is 0.2 MPa lower than the pressure of the raw gas, and the pressure of the permeate gas is normal pressure.

[0016] Furthermore, the pressure of the permeate gas after compression in step 2 is 3.0-7.0 MPa; the liquefaction conditions in step 2 are: cooling temperature of -30-10°C and pressure of 3.0-7.0 MPa; preferably, the cooling temperature is -25-5°C.

[0017] Furthermore, the distillation conditions are: condensation temperature is -35 to -10°C; heating temperature is 0 to 20°C, preferably, heating temperature is 5 to 10°C; pressure in the tower is 3.0 to 7.0 MPa; tower top pressure and tower bottom pressure are both 50 Kpa lower than the tower pressure.

[0018] The second object of the present invention is to provide a system for enriching carbon dioxide in associated gas of oil fields, comprising: a pretreatment unit, a membrane separation unit and a liquefaction distillation unit;

[0019] The pretreatment unit includes a first compressor, a condenser and a first gas storage tank connected in sequence;

[0020] The membrane separation unit comprises a membrane separator, a second compressor and a second gas storage tank which are connected in sequence, the membrane separator separates the feed gas into permeate gas and retentate gas, the membrane separator is connected to the first gas storage tank; a permeate gas pipe located at one side of the membrane separator and connected to the membrane separator, and a retentate gas pipe located at the other side of the membrane separator and connected to the membrane separator, and both the retentate gas pipe and the permeate gas pipe are provided with a back pressure valve;

[0021] The liquefaction and distillation unit includes a liquefier connected to the second gas storage tank, a cooling circulation device connected to the liquefied gas, a distillation tower connected to the liquefier, a bottom reboiler located at the bottom of the distillation tower, and a top condenser located at the top of the distillation tower.

[0022] On the basis of the above technical solution, the present invention can also be improved as follows:

[0023] Furthermore, the membrane of the membrane separator is a hollow fiber membrane; preferably, the membrane of the membrane separator is a polyimide hollow fiber membrane.

[0024] Furthermore, the second compressor is a multi-stage compressor; preferably, the compression ratio of the second compressor is 10:1.

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

[0026] 1. The method of enriching CO2 from associated gas of the present invention can achieve separation and purification of carbon dioxide, and has the advantages of stable process operation and efficient capture of carbon dioxide. In addition, the capture method of the present invention has low operating cost, simple process, small footprint, and is very suitable for use on offshore platforms with limited platforms.

[0027] In addition, when the present invention is applicable to high-pressure associated gas generated during offshore oil field exploitation, when membrane separation is performed, the raw gas obtained after the associated gas is impurity-removed does not need to be compressed, which can further reduce equipment costs and equipment space.

[0028] In addition, later experiments have shown that the pressure of the product gas obtained by the present invention is greater than 2.5 MPa, which has a relatively high pressure. Therefore, when the product gas is injected back into the ground, there is no need to pressurize it, which can reduce costs and energy consumption.

[0029] 2. The present invention combines membrane separation, cryogenic liquefaction and distillation. In the present invention, polyimide hollow fiber membrane is first used for membrane separation to achieve preliminary separation and purification of carbon dioxide, and then the high-concentration carbon dioxide gas obtained after separation is further subjected to cryogenic liquefaction and distillation to further increase the carbon dioxide concentration in the product gas. Later experiments have proved that the CO2 purity in the product gas obtained by this method is as high as 95% or more. In addition, the method has a wide range of application, and associated gas with a CO2 volume fraction of 10% to 70% can be applied to the method; and from the later test analysis, it can be seen that the method in the present invention has more advantages in investment and operating costs when the carbon dioxide volume fraction in the associated gas is low and the methane volume fraction is high. The reason is that the CO2 concentration is low, and after membrane separation, a high-concentration CO2 permeate gas is obtained on the permeate side, and a higher concentration of CH4 permeate gas is obtained on the retentate side, so that the separation and purification cost of CH4 in the retentate gas can be reduced. In addition, compared with a simple membrane separation process, the present invention does not require a multi-stage membrane treatment process, reducing the investment in membrane separation equipment; at the same time, compared with simple cryogenic separation, the technical process is simple and the equipment investment is low.

[0030] 3. The present invention installs back pressure valves on the retentate gas pipe and the permeate gas pipe to control the composition and flow of the gas on both sides of the membrane separator through the back pressure valve, thereby controlling the composition of the gas entering the liquefier and the distillation tower and the gas flow, and having a certain regulatory effect on the recovery rate of carbon dioxide in the entire system.

[0031] In addition, the present invention also regulates the state of the raw materials entering the distillation tower by regulating the temperature and pressure of the liquefier, thereby regulating the distillation effect. Further, the reflux rate is regulated by regulating the temperature and pressure of the top condenser of the distillation tower, thereby regulating the recovery rate of carbon dioxide in the entire system; and the evaporation amount is regulated by regulating the temperature of the bottom reboiler of the distillation tower, thereby regulating the purity of carbon dioxide in the product gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a system diagram of the present invention;

[0033] Description of reference numerals:

[0034] 1-first compressor, 2-first storage tank, 3-membrane separator, 4-back pressure valve, 5-second compressor, 6-second storage tank, 7-liquefier, 8-tower top condenser, 9-distillation tower, 10-tower bottom reboiler, 11-condensation circulation device, 12-retentate gas pipe, 13-permeate gas pipe, 14-condenser. DETAILED DESCRIPTION

[0035] The following will describe a system and method for enriching carbon dioxide in oilfield associated gas in the present application in conjunction with embodiments. However, the present application can be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein. Rather, the purpose of providing these embodiments is to make the present application thorough and complete and to fully convey the scope of the present application to those skilled in the art.

[0036] The inventor found in the process of investigating the recovery technology of carbon dioxide (CO2) in associated gas that the existing recovery methods of CO2 in associated gas (such as chemical absorption, adsorption separation, cryogenic separation and membrane separation) cannot achieve simple process, low energy consumption and high efficiency of enriching carbon dioxide in associated gas. Based on this, the present invention proposes to combine membrane separation with cryogenic liquefaction and distillation technology to improve the low energy consumption and high efficiency capture of carbon dioxide in associated gas, and at the same time enable the process to be used on a large scale. The technical scheme of the present invention is clearly and completely described by examples below.

[0037] An embodiment of the first aspect of the present invention provides a method for enriching carbon dioxide in oilfield associated gas, comprising the following steps:

[0038] Step 1, pressurizing and cooling the raw gas obtained after removing impurities from the associated gas in sequence to obtain cooled raw gas;

[0039] Step 2: performing membrane separation on the cooled raw gas to obtain permeate gas and retentate gas, and sequentially compressing and liquefying the permeate gas to obtain a gas-liquid mixture;

[0040] Step 3: Distill and purify the gas-liquid mixture to obtain product gas.

[0041] In this embodiment, associated gas mainly refers to gas containing different concentrations of carbon dioxide produced during the natural gas extraction process; among them, in this embodiment, the volume fraction of CO2 in the associated gas is preferably 10%-70%, and further preferably, the volume fraction of CO2 in the associated gas is 30%-60%.

[0042] In addition, in this embodiment, the removal of impurities in the associated gas is mainly carried out by existing technologies, and reference can be made to "Optimization of integrated deoiling, dehydration and dust removal device and its application in natural gas purification"; preferably, the particle size of the raw gas obtained after impurity removal in this embodiment is less than 0.01um, and the content is less than 10mg / m 3 ; Sulfur dioxide content <10mg / m 3 ; Water content <0.17g / m 3 ; The volume fraction of heavy hydrocarbons is less than 0.1%; the raw gas obtained after impurity removal improves the effect of subsequent raw gas membrane separation, while also avoiding irreversible damage to the membrane of the membrane separator during subsequent membrane separation.

[0043] In addition, in some embodiments, the pressure of the cooled raw gas in step 1 is 5-7 MPa, and the temperature of the cooled raw gas is 30-50°C. Preferably, the temperature of the cooled raw gas is 30°C. Under this condition, membrane separation of the raw gas can reduce the cost of enriching CO2 in the associated gas.

[0044] In addition, in some embodiments, the membrane separation conditions in step 2 are: the cooled raw gas is subjected to membrane separation at a pressure of 5 to 7 MPa and a flow rate of 20 to 50 L / min; the pressure of the retentate gas is 0.2 MPa lower than the pressure of the raw gas, and the pressure of the permeate gas is normal pressure. In addition, after membrane separation under this condition, the volume fraction of CO2 in the permeate gas is 75% to 90%, which has the characteristic of a high volume fraction of CO2. In this embodiment, the cooled raw gas is subjected to membrane separation in a membrane separator, and a retentate gas pipe and a permeate gas pipe are connected to both sides of the membrane separator, and a back pressure valve is provided on the retentate gas pipe and the permeate gas pipe. In this embodiment, the pressure of the retentate gas on the retentate side of the membrane separator is mainly adjusted to 0.2 MPa lower than the pressure of the raw gas through the back pressure valve.

[0045] In addition, in some embodiments, the pressure of the permeate gas after compression in step 2 is 3.0-7.0 MPa; the conditions for liquefaction in step 2 are: cooling temperature of -30-10°C and pressure of 3.0-7.0 MPa; preferably, the cooling temperature during liquefaction is -25-5°C. In this embodiment, in order to compress the permeate gas at normal pressure to 3.0-7.0 MPa, a multi-stage compressor is selected in this embodiment, and its compression ratio is 10:1. In addition, in this embodiment, industrial alcohol is preferably used as the cooling medium during liquefaction, and the flow rate of the cooling medium during liquefaction is 10-30 L / min, and the temperature of the cooling medium is -30-10°C, preferably, the temperature of the cooling medium is -25-5°C; in addition, the permeate gas after liquefaction under the liquefaction conditions of this embodiment is a gas-liquid mixture, and most of the CO2 is stored in the liquid of the gas-liquid mixture.

[0046] In addition, in some embodiments, the conditions of distillation are: condensation temperature is -35 to -10 ° C; heating temperature is 0 to 20 ° C, preferably, heating temperature is 5 to 10 ° C; pressure in the tower is 3.0 to 7.0 MPa; tower top pressure and tower bottom pressure are both 50 Kpa lower than the tower pressure. In addition, this embodiment mainly performs distillation and purification in a distillation tower, and a tower top condenser is provided at the top of the distillation tower, and a tower bottom reboiler is provided at the bottom of the distillation tower. In this embodiment, during distillation separation, the bottom liquid of the distillation tower contains a part of methane and nitrogen, and the top gas of the distillation tower contains a part of carbon dioxide. By configuring a tower top condenser at the top of the distillation tower, a part of the carbon dioxide is refluxed to improve the overall recovery rate of carbon dioxide; by setting a tower bottom reboiler at the bottom of the distillation tower, the liquid is appropriately heated, so that the methane and nitrogen in the liquid are converted into gas and discharged from the top of the tower, and the volume fraction of carbon dioxide in the bottom product is increased. In order to maintain the pressure in the distillation tower, a back pressure valve is set after the top condenser and the bottom reboiler of the distillation tower to control the pressure. At the same time, in order to reduce the loss of cold during the distillation process, the pipeline of the liquefied distillation system is wrapped with insulation material. In addition, in this embodiment, the top pressure refers to the pressure in the top condenser of the distillation tower, and the bottom pressure refers to the pressure in the bottom reboiler of the distillation tower.

[0047] In addition, in this embodiment, industrial alcohol is preferably used as the cooling medium for cooling the top of the tower; and seawater is preferably used as the heating medium for heating the bottom of the tower, thereby reducing the cost of enriching CO2. In addition, during distillation, the pressure in the top condenser and the pressure in the bottom reboiler are 50Kpa lower than the pressure inside the distillation tower.

[0048] The embodiment of the second aspect of the present invention provides a system for enriching carbon dioxide in associated gas of an oil field, such as Figure 1 As shown, in this embodiment, a system for enriching carbon dioxide in associated gas of an oil field comprises: a pretreatment unit, a membrane separation unit and a liquefaction distillation unit;

[0049] Specifically, the pretreatment unit includes a first compressor 1, a condenser 14 and a first gas storage tank 2 which are connected in sequence.

[0050] The membrane separation unit includes a membrane separator 3, a second compressor 5 and a second gas storage tank 6 which are connected in sequence. The membrane separator separates the feed gas into permeate gas and retentate gas, and the membrane separator is connected to the first gas storage tank. The permeate gas pipe 13 is located on one side of the membrane separator and connected to the membrane separator, and the retentate gas pipe 12 is located on the other side of the membrane separator and connected to the membrane separator. Back pressure valves 4 are provided on the retentate gas pipe and the permeate gas pipe.

[0051] The liquefaction and distillation unit includes a liquefier 7 connected to the second gas storage tank 6, a cooling circulation device 11 connected to the liquefied gas, a distillation tower 9 connected to the liquefier, a bottom reboiler 10 located at the bottom of the distillation tower, and a top condenser 8 located at the top of the distillation tower. Figure 1 Schematic diagram, the liquefied gas-liquid mixture is distilled and separated to obtain product gas and purified gas.

[0052] In this embodiment, the back pressure valve 4 located on the retentate gas pipe 12 and the permeate gas pipe 13 controls the flow and composition of the gas on the permeate side and the retentate side of the membrane separator, thereby controlling the gas composition and gas flow entering the liquefaction distillation unit, and finally achieving the regulation of the CO2 recovery rate.

[0053] In addition, in this embodiment, the permeate gas entering the liquefier 6 is liquefied by the cooling circulation device 7. Preferably, in this embodiment, industrial alcohol is used as the cooling medium, and the flow rate of the cooling medium during cooling is 10 to 30 L / min.

[0054] In addition, in some embodiments, the membrane separator is preferably a membrane separator having a hollow fiber membrane structure, and is further preferably a polyimide hollow fiber membrane separator; in this embodiment, the hollow fiber membrane has the characteristics of low production cost and compact packing. In addition, in actual use, the membrane separator can select multiple membrane separators for series use according to the amount of raw gas processing. Similarly, the membrane area in the membrane separator can also be adjusted according to the size of the membrane separator shell, and the number of membranes in the membrane separator can be adjusted in parallel according to the size of the raw gas processing.

[0055] In addition, in some embodiments, in order to pressurize the permeate gas at normal pressure to 3.0-7.0 MPa, the second compressor is preferably a multi-stage compressor in this embodiment, and the compression ratio of the multi-stage compressor is 10:1.

[0056] Example

[0057] The following embodiment provides a system for enriching carbon dioxide in associated gas of oil fields to recover CO2 based on the embodiment of the second aspect described above, and illustrates the method of the present invention using the configured associated gas.

[0058] Example 1

[0059] A method for enriching carbon dioxide in oilfield associated gas comprises the following steps:

[0060] Step 1: Raw gas pretreatment

[0061] The main components of the raw gas in this embodiment (the raw gas is a simulated associated gas and can be directly pretreated without impurity removal) are: 30% CO2, 56% CH4, 14% N2, and the temperature of the raw gas used in this embodiment is 30.0°C and the pressure is 7MPa. The pretreatment of the raw gas is as follows:

[0062] 1) The raw gas is passed into the first compressor for pressurization, and the pressure of the pressurized raw gas is 7 MPa.

[0063] 2) The pressurized raw gas is cooled in a condenser and stored in a first storage tank to obtain a cooled raw gas with a temperature of 30°C.

[0064] Step 2: Membrane separation

[0065] First, the cooled raw gas in step 1 is passed into a membrane separator (wherein the membrane of the membrane separator is a polyimide hollow fiber membrane) at a pressure of 7 MPa and a flow rate of 50 L / min for separation, and the back pressure valves on the retentate gas pipe and the permeate gas pipe are adjusted to make the pressure of the retentate gas 6.8 MPa and the pressure on the permeate side normal pressure. After separation by the membrane separator, permeate gas and retentate gas are obtained, and the flow rates and compositions of the permeate gas and the retentate gas are detected.

[0066] Secondly, the permeate gas is compressed by a second compressor. The compressed permeate gas has a pressure of 7.0 MPa and is stored in a second gas storage tank.

[0067] Step 3: Distillation

[0068] First, the permeate gas in the second gas storage tank is introduced into the liquefier, and cooled at a pressure of 7.0 MPa and a cooling temperature of 5.0° C. to obtain a gas-liquid mixture.

[0069] Secondly, the gas-liquid mixture is passed into a distillation tower (the pressure inside the tower is 7 MPa), and the pressure of the top condenser is adjusted to 6.95 MPa, and the temperature of the coolant is -10°C; the pressure of the bottom reboiler is 6.95 MPa, and the temperature is 10°C. After distillation, purified gas with a lower carbon dioxide volume fraction and product gas with a higher carbon dioxide volume fraction are obtained, wherein the product gas pressure is 6.9 MPa.

[0070] Step 4: Detect the purified gas and product gas obtained in the above step 3 to detect the flow rate and composition of the gas. The results are shown in Table 1.

[0071] Table 1. Flow rate and concentration of purified gas and product gas after distillation

[0072] Air flow <![CDATA[CO2]]> <![CDATA[CH4]]> <![CDATA[N2]]> Purified gas 3.2L / min 48.8％ 32.2％ 19.0％ Product gas 11.8L / min 95.2％ 3.5％ 1.3％

[0073] Example 2

[0074] A method for enriching carbon dioxide in oilfield associated gas comprises the following steps:

[0075] Step 1: Raw gas pretreatment

[0076] The main components of the raw gas in this embodiment (the raw gas is a simulated associated gas and can be directly pretreated) are: 45% CO2, 44% CH4, 11% N2, and the temperature of the raw gas used in this embodiment is 30.0°C and the pressure is 6.0MPa. The pretreatment of the raw gas is as follows:

[0077] 1) The raw gas is passed into the first compressor for pressurization, and the pressure of the pressurized raw gas is 6.0 MPa.

[0078] 2) The pressurized raw gas is cooled in a condenser and stored in a first storage tank to obtain a cooled raw gas with a temperature of 30°C.

[0079] Step 2: Membrane separation

[0080] First, the cooled raw gas in step 1 is passed into a membrane separator (wherein the membrane of the membrane separator is a polyimide hollow fiber membrane) at a pressure of 6.0 MPa and a flow rate of 40 L / min for separation, and the back pressure valves on the retentate gas pipe and the permeate gas pipe are adjusted to make the pressure of the retentate gas 5.8 MPa and the pressure on the permeate side normal pressure. After separation by the membrane separator, permeate gas and retentate gas are obtained, and the flow rates and compositions of the permeate gas and the retentate gas are detected.

[0081] Secondly, the permeate gas is compressed by a second compressor. The compressed permeate gas has a pressure of 5.5 MPa and is stored in a second gas storage tank.

[0082] Step 3: Distillation

[0083] First, the permeate gas in the second gas storage tank is introduced into the liquefier, and cooled at a pressure of 5.5 MPa and a cooling temperature of -10°C to obtain a gas-liquid mixture.

[0084] Secondly, the gas-liquid mixture is passed into a distillation tower (the pressure in the tower is 5 MPa), and the pressure of the top condenser is adjusted to 4.95 MPa, and the temperature of the coolant is -20°C; the pressure of the bottom reboiler is 4.95 MPa, and the temperature is 10°C. After distillation, purified gas with a lower carbon dioxide volume fraction and product gas with a higher carbon dioxide volume fraction are obtained, wherein the product gas pressure is 4.9 MPa.

[0085] Step 4: Detect the purified gas and product gas obtained in the above step 3 to detect the flow rate and composition of the gas. The results are shown in Table 2.

[0086] Table 2. Flow rate and concentration of purified gas and product gas after distillation

[0087] Air flow <![CDATA[CO2]]> <![CDATA[CH4]]> <![CDATA[N2]]> Purified gas 2.4L / min 52.0％ 29.2％ 18.8％ Product gas 13.6L / min 95.8％ 2.4％ 0.8％

[0088] Example 3

[0089] A method for enriching carbon dioxide in oilfield associated gas comprises the following steps:

[0090] Step 1: Raw gas pretreatment

[0091] The main components of the raw gas in this embodiment (the raw gas is a simulated associated gas and can be directly pretreated) are: 60% CO2, 32% CH4, 8% N2, and the temperature of the raw gas used in this embodiment is 30.0°C and the pressure is 5.0MPa. The pretreatment of the raw gas is as follows:

[0092] 1) The raw gas is passed into the first compressor for pressurization, and the pressure of the pressurized raw gas is 5.0 MPa.

[0093] 2) The pressurized raw gas is cooled in a condenser and stored in a first storage tank to obtain a cooled raw gas with a temperature of 30°C.

[0094] Step 2: Membrane separation

[0095] First, the cooled raw gas in step 1 is passed into a membrane separator (wherein the membrane of the membrane separator is a polyimide hollow fiber membrane) at a pressure of 5.0 MPa and a flow rate of 20 L / min for separation, and the back pressure valves on the retentate gas pipe and the permeate gas pipe are adjusted to make the pressure of the retentate gas 4.8 MPa and the pressure on the permeate side normal pressure. After separation by the membrane separator, permeate gas and retentate gas are obtained, and the flow rates and compositions of the permeate gas and the retentate gas are detected.

[0096] Secondly, the permeate gas is compressed by a second compressor. The compressed permeate gas has a pressure of 3.5 MPa and is stored in a second gas storage tank.

[0097] Step 3: Distillation

[0098] First, the permeate gas in the second gas storage tank is introduced into the liquefier, and cooled at a pressure of 3.0 MPa and a cooling temperature of -25°C to obtain a gas-liquid mixture.

[0099] Secondly, the gas-liquid mixture is passed into a distillation tower (the pressure in the tower is 3Mpa), and the pressure of the top condenser is adjusted to 2.95Mpa, and the temperature of the coolant is -35°C; the pressure of the bottom reboiler is 2.95Mpa, and the temperature is 5°C. After distillation, purified gas with a lower carbon dioxide volume fraction and product gas with a higher carbon dioxide volume fraction are obtained, wherein the product gas pressure is 2.9MPa.

[0100] Step 4: Detect the purified gas and product gas obtained in the above step 3 to detect the flow rate and composition of the gas. The results are shown in Table 3.

[0101] Table 3. Flow rate and concentration of purified gas and product gas after distillation

[0102] Air flow <![CDATA[CO2]]> <![CDATA[CH4]]> <![CDATA[N2]]> Purified gas 2.6L / min 71.8％ 16.4％ 11.8％ Product gas 9.4L / min 95.4％ 3.4％ 1.2％

[0103] According to the above test analysis results, the CO2 concentration in the product gas obtained after the treatment of Example 1 is greater than 95.0%, and the CO2 capture rate is 74.9%. The CO2 concentration in the product gas obtained after the treatment of Example 2 is greater than 95.0%, and the CO2 capture rate is 72.4%. The CO2 concentration in the product gas obtained after the treatment of Example 3 is greater than 95.0%, and the CO2 capture rate is 74.7%.

[0104] The above experiments show that the method of the present invention has a large capture rate for capturing CO2 from associated gas, and the volume fraction concentration of CO2 in the final product gas obtained is as high as over 95.0%.

[0105] It should be noted that the above CO2 capture rate R is calculated based on the gas flow rate before and after enrichment and the volume fraction of carbon dioxide in the gas. The specific calculation formula is:

[0106]

[0107] Wherein, f0 is the flow rate of raw gas, in L / min;

[0108] w0 is the volume fraction of CO2 in the raw gas, in %;

[0109] f1 is the flow rate of product gas after distillation, in L / min;

[0110] w1 is the volume fraction of CO2 in the product gas after distillation, in %.

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for enriching carbon dioxide in oilfield associated gas, characterized in that: The following steps are involved: Step 1, pressurizing and cooling the raw gas obtained after removing impurities from the associated gas in sequence to obtain cooled raw gas; Step 2: performing membrane separation on the cooled raw gas to obtain permeate gas and retentate gas, and sequentially compressing and liquefying the permeate gas to obtain a gas-liquid mixture; Step 3: distill and purify the gas-liquid mixture to obtain product gas.

2. The method according to claim 1, characterized in that The volume fraction of CO2 in the associated gas in step 1 is 10%-70%; In step 1, the particle size of the raw gas is less than 0.01um, and the content is less than 10mg / m 3 ; Sulfur dioxide content <10mg / m 3 ; Water content <0.17g / m 3 ; Volume fraction of heavy hydrocarbons <0.1%.

3. The method according to claim 2, characterized in that The volume fraction of CO2 in the associated gas in step 1 is 30%-60%.

4. The method according to claim 3, characterized in that The pressure of the cooled raw gas in step 1 is 5-7 MPa, and the temperature of the cooled raw gas is 30-50°C.

5. The method according to claim 4, characterized in that The membrane separation conditions in step 2 are: The cooled raw gas is subjected to membrane separation at a pressure of 5 to 7 MPa and a flow rate of 20 to 50 L / min; The pressure of the retentate gas is 0.2 MPa lower than the pressure of the feed gas, and the pressure of the permeate gas is normal pressure.

6. The method according to claim 5, characterized in that In step 2, the pressure of the permeate gas after compression is 3.0-7.0 MPa; The liquefaction conditions in step 2 are: cooling temperature of -30 to 10°C and pressure of 3.0 to 7.0 MPa.

7. The method according to claim 6, characterized in that The conditions of the distillation are: Condensation temperature is -35~-10℃; The heating temperature is 0-20°C; The pressure inside the tower is 3.0~7.0MPa; The pressure at the top and bottom of the tower are both 50Kpa lower than the pressure inside the tower.

8. A system for enriching carbon dioxide in associated gas of oil fields for use in the method according to any one of claims 1 to 7, characterized in that: include: Pretreatment unit, membrane separation unit and liquefaction distillation unit; The pretreatment unit comprises a first compressor, a condenser and a first gas storage tank which are connected in sequence; The membrane separation unit comprises a membrane separator, a second compressor and a second gas storage tank which are connected in sequence, wherein the membrane separator separates the feed gas into a permeate gas and a retentate gas, and the membrane separator is connected to the first gas storage tank; a permeate gas pipe located at one side of the membrane separator and connected to the membrane separator, and a retentate gas pipe located at the other side of the membrane separator and connected to the membrane separator, wherein both the retentate gas pipe and the permeate gas pipe are provided with a back pressure valve; The liquefaction and distillation unit includes a liquefier connected to the second gas storage tank, a cooling circulation device connected to the liquefied gas, a distillation tower connected to the liquefier, a bottom reboiler located at the bottom of the distillation tower, and a top condenser located at the top of the distillation tower.

9. The system according to claim 8, characterized in that The membrane of the membrane separator is a hollow fiber membrane.

10. The system according to claim 8, characterized in that The second compressor is a multi-stage compressor.

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