A method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure and deep cooling

Abstract

This invention relates to a method for recovering light hydrocarbons from benzene-containing natural gas under medium-pressure cryogenic conditions, belonging to the field of natural gas processing technology. The method includes the following steps: pressurizing and drying the raw natural gas, cryogenically separating it, and then sending the liquid phase obtained from the cryogenic separation through a J-T valve for throttling and expansion before being fed into a demethanizer to separate methane; C3-C5 alkanes are mixed into the liquid phase obtained from the cryogenic separation before the J-T valve or into the raw natural gas before cryogenic separation. This method, by mixing C3-C5 alkanes into the liquid phase obtained from the cryogenic separation before the J-T valve or into the raw natural gas upstream of the cryogenic separator, can intervene to lower the temperature at which benzene precipitates from the material without introducing other substances, thereby alleviating / solving the problem of freezing and blockage downstream of the J-T valve in the natural gas medium-pressure cryogenic light hydrocarbon recovery unit caused by the presence of benzene in the raw natural gas.

Description

Technical Field

[0001] This invention relates to a method for recovering light hydrocarbons from medium-pressure cryogenic natural gas containing benzene, belonging to the field of natural gas processing technology. Background Technology

[0002] The natural gas medium-pressure cryogenic light hydrocarbon recovery unit employs a propane and expander mixed refrigeration process. The feed gas is compressed to 4.5 MPa in the compression unit and dehydrated before entering the NGL recovery unit. There, it undergoes propane refrigerant refrigeration and expander refrigeration to cool to -80°C. The cooled feed gas then enters the distillation tower for light hydrocarbon recovery. As crude oil extraction enters its later stages, the amount of crude oil increases towards heavier oil, while the associated gas volume and light hydrocarbon content gradually decrease. These changes in gas volume and quality increase the operational difficulty of the natural gas processing unit. The throttling and cooling effect of the JT valve is enhanced, leading to a decrease in the downstream temperature and a higher risk of freezing and blockage. Severe freezing and blockage can cause the feed gas compressor and expander to shut down, making safe and stable operation of the unit difficult and potentially causing a plant-wide shutdown. Introducing a general-purpose defrosting agent, injected directly into the blockage site, introduces other components besides natural gas into the system. This requires additional equipment for removal, complicating the process, increasing operational difficulty, and raising construction and operating costs. Summary of the Invention

[0003] The purpose of this invention is to provide a method for recovering light hydrocarbons from benzene-containing natural gas under medium-pressure cryogenic conditions, which can alleviate the problem of freezing and blockage after the JT valve of the unit caused by the depletion of the quality of the raw natural gas without introducing impurities.

[0004] To achieve the above objectives, the technical solution adopted by this invention is as follows:

[0005] A method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions includes the following steps: pressurizing and drying the raw natural gas, cryogenically cooling it, and then sending it to a cryogenic separator for cryogenic separation; then, the liquid phase obtained from cryogenic separation is throttled and expanded through a JT valve and sent to a dealkane tower; C3-C5 alkanes are mixed into the liquid phase obtained from cryogenic separation before the JT valve or into the raw natural gas upstream of the cryogenic separator; the alkanes are one or any of propane, butane, and pentane.

[0006] The method for recovering benzene-containing natural gas under medium-pressure cryogenic light hydrocarbons of the present invention involves mixing C3-C5 alkanes into the liquid phase obtained by low-temperature separation before the JT valve or into the raw natural gas upstream of the cryogenic separator. This can achieve the goal of intervening to reduce the temperature at which benzene precipitates from the material (the freezing temperature of benzene) without introducing other substances, thereby alleviating / solving the problem of freezing after the JT valve of the natural gas medium-pressure cryogenic light hydrocarbon recovery device caused by the depletion of the raw natural gas quality.

[0007] To completely resolve the issue of freezing and blockage downstream of the JT valve caused by the depletion of the feedstock natural gas, the amount of alkane incorporated is further optimized so that the benzene precipitation temperature in the material flowing through the JT valve is lower than the temperature of the material after throttling and expansion through the JT valve. For example, the amount of alkane incorporated does not exceed 40% of the mass of the feedstock natural gas, such as 17-20%. Furthermore, alkane is incorporated into the feedstock natural gas before pressurization. Incorporating alkane into the feedstock natural gas before pressurization allows for effective control of freezing and blockage with minimal changes to the original process and increased investment, increasing the reliability of unit operation, improving the yield of subsequent ethane products, and without causing system disturbance.

[0008] It is understood that the C3-C5 alkanes are one or more of propane, butane, and pentane. Furthermore, the propane, butane, and pentane are all products of the medium-pressure cryogenic light hydrocarbon recovery method for benzene-containing natural gas.

[0009] Furthermore, C3-C5 alkanes are mixed into the feedstock natural gas before pressurization. The feedstock gas is then buffered in a feedstock gas buffer tank before pressurization, and the C3-C5 alkanes are introduced into the feedstock buffer tank to mix them into the feedstock natural gas. By introducing the C3-C5 alkanes into the feedstock buffer tank, the disturbance to the system caused by the mixing of C3-C5 alkanes can be reduced, improving system stability. Especially when retrofitting existing medium-pressure cryogenic light hydrocarbon recovery units to implement the recovery method of this invention, the disturbance to the system caused by the mixing of C3-C5 alkanes can be significantly reduced, system stability can be improved, and retrofit investment and production costs can be reduced simultaneously.

[0010] The C3-C5 alkanes from the medium-pressure cryogenic light hydrocarbon recovery product of benzene-containing natural gas can be vaporized by mixing them into the feedstock natural gas before pressurization. Since the temperature after the JT valve is approximately -101℃, while the light hydrocarbon product is at approximately room temperature, the basic parameters of the two differ significantly. To ensure that the operating parameters of the unit remain constant, when the defreezing agent is mixed in from other locations, it is necessary to add equipment such as condensers and compressors in the middle, increasing equipment investment.

[0011] The dealkane tower can be configured according to specific production needs, and can be used to remove methane or both methane and ethane. Further, the dealkane tower is a demethane tower, and the method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure includes the following steps: sending the effluent from the demethane tower bottom to a deethanene tower to separate ethane, and then sending the effluent from the deethanene tower bottom to a propane tower to separate propane. Even further, a portion of the propane separated by the propane tower is mixed into the feed natural gas before pressurization; and / or the propane separated by the propane tower is stored in a propane storage tank, and the propane in the propane storage tank is transported and mixed into the feed natural gas before pressurization.

[0012] Furthermore, the method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions further includes the following steps: sending the effluent from the depropanizer tower to a debutanizer tower to separate butane. Even further, mixing a portion of the butane separated by the debutanizer tower into the feed natural gas before pressurization; and / or storing the butane separated by the debutanizer tower in a butane storage tank, and then transporting the butane from the butane storage tank into the feed natural gas before pressurization.

[0013] Furthermore, the method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions further includes the following steps: sending the effluent from the butane removal tower to a pentane removal tower to separate pentane. Even further, mixing a portion of the pentane separated from the pentane removal tower into the feed natural gas before pressurization; and / or storing the pentane separated from the pentane removal tower in a pentane storage tank, and then conveying the pentane from the pentane storage tank into the feed natural gas before pressurization. Attached Figure Description

[0014] Figure 1 A schematic diagram of the recovery unit used in the existing medium-pressure cryogenic light hydrocarbon recovery method for natural gas;

[0015] Figure 2 This is a schematic diagram of the recovery device used in the medium-pressure cryogenic light hydrocarbon recovery method for benzene-containing natural gas according to Embodiment 1 of the present invention;

[0016] Figure 3 This is a schematic diagram of the recovery device used in the medium-pressure cryogenic light hydrocarbon recovery method for benzene-containing natural gas according to Embodiment 2 of the present invention;

[0017] Figure 4 This is a schematic diagram of the recovery device used in the medium-pressure cryogenic light hydrocarbon recovery method for benzene-containing natural gas according to Embodiment 3 of the present invention;

[0018] Among them, V01 - feed gas buffer tank, V02 - primary compression buffer tank, V03 - secondary compression buffer tank, V04 - cryogenic separator, V05 - ethane condensate buffer tank, V06 - propane condensate buffer tank, V07 - butane condensate buffer tank, V08 - pentane condensate buffer tank, V09A / V09B - drying tower, E01 - cold box, E02 - primary compression air cooler, E03 - secondary compression air cooler, E04 - reboiler for demethanizer, E05 - ethane condenser, E06 - propane condenser, E07 - butane condenser, E08 - pentane condenser, E09 - reboiler for deethanerizer, E10 - depropanizer. Matching reboilers: E11 - reboiler for the butane removal tower; E12 - reboiler for the pentane removal tower; E13 - secondary compression heat exchanger; K1 - primary compressor; 2TK1 - secondary compressor; 2TK2 - expander; P01 - booster pump; P02 - ethane reflux pump; P03 - propane reflux pump; P04 - butane reflux pump; P05 - pentane reflux pump; F01 - filter; T101 - methanation tower; T102 - ethane removal tower; T103 - propane removal tower; T104 - butane removal tower; T105 - pentane removal tower; V21 / V22 - propane storage tank; V31 / V32 - butane storage tank; V41 / V42 - pentane storage tank. Detailed Implementation

[0019] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0020] Existing methods for recovering light hydrocarbons from medium-pressure cryogenic natural gas employ recovery devices such as... Figure 1 As shown, the system includes a raw material gas buffer unit, a raw material gas compression unit, a drying unit, a recovery unit, and a storage unit, which are arranged sequentially in the material flow direction. The raw material gas buffer unit includes a raw material gas buffer tank V01. The raw material gas compression unit includes a primary compressor K1, a primary air cooler E02, a primary air cooler V02, a secondary compressor 2TK1, a secondary air cooler E03, a secondary heat exchanger E13, and a secondary air cooler V03, arranged sequentially in the material flow direction. The raw material gas outlet of the raw material gas buffer tank V01 is connected to the air inlet of the primary compressor K1. The drying unit includes drying towers V09A and V09B connected in parallel. The raw material inlets of drying towers V09A and V09B are connected to the raw material gas outlet of the secondary air cooler V03.

[0021] The recovery unit includes cold box E01, low-temperature separator V04, demethanizer T101, deethanizer T102, depropanizer T103, debutanizer T104 and depentanizer T105; the raw gas inlet of cold box E01 is connected to the raw gas outlets of drying towers V09A and V09B through a pipeline, a filter F01 is provided on the connecting pipeline, a branch pipeline is provided downstream of filter F01 on the connecting pipeline, and the branch pipeline is connected to the heating fluid inlet of reboiler E04 supporting the demethanizer, and the heating fluid outlet of reboiler E04 supporting the demethanizer is connected to the raw natural gas channel in the cold box; the raw gas outlet of cold box E01 is connected to the feed inlet of low-temperature separator V04 through a pipeline, the liquid phase outlet and gas phase outlet of low-temperature separator V04 are both connected to demethanizer T101. To use cold box E01 to cool down the liquid phase and gas phase separated by low-temperature separator V04, liquid phase flow channels and gas phase flow channels for cooling the liquid phase and gas phase separated by low-temperature separator V04 are respectively provided in cold box E01. The liquid phase outlet of low-temperature separator V04 is connected to the inlet of the liquid phase flow channel through a pipeline, and the gas phase outlet is connected to the inlet of the gas phase flow channel through a pipeline; the outlet of the liquid phase flow channel is connected to the liquid phase feed inlet at the top of demethanizer T101 through a pipeline, and a J-T valve is provided on the connecting pipeline; the outlet of the gas phase flow channel is also connected to another feed inlet at the top of T101 through a pipeline, and an expander 2TK2 is provided on the connecting pipeline; a flow channel for the draw liquid from the bottom of T101 to be heated is also provided in cold box 01. The inlet of this flow channel is connected to the draw outlet at the bottom of T101 through a pipeline, a booster pump P01 is provided on the connecting pipeline, and the outlet of this connecting channel is connected to the feed inlet of deethanizer T102 through a pipeline; an ethane condenser E05, an ethane condensate buffer tank V05 and an ethane reflux pump P02 are successively provided at the top of deethanizer T102, a reboiler E09 supporting the deethanizer is provided at the bottom of deethanizer T102, the draw pipeline at the bottom of deethanizer T102 is connected to the feed inlet of depropanizer T103, a propane condenser E06, a propane condensate buffer tank V06 and a propane reflux pump P03 are successively provided at the top of depropanizer T103, a reboiler E10 supporting the depropanizer is provided at the bottom of depropanizer T103, the draw pipeline at the bottom of depropanizer T103 is connected to the feed inlet of debutanizer T104, a butane condenser E07, a butane condensate buffer tank V07 and a butane reflux pump P04 are successively provided at the top of debutanizer T104, a reboiler E11 supporting the debutanizer is provided at the bottom of debutanizer T104, the draw pipeline at the bottom of debutanizer T104 is connected to the feed inlet of depentanizer T105, a pentane condenser E08, a pentane condensate buffer tank V08 and a pentane reflux pump P05 are successively provided at the top of depentanizer T105, and a reboiler E12 supporting the depentanizer is provided at the bottom of depentanizer T105;The storage unit includes propane tanks V21 and V22, butane tanks V31 and V32, and pentane tanks V41 and V42. The inlet of the propane tanks is connected to the reflux pipe at the top of the propane stripping tower T103; the inlet of the butane tanks is connected to the reflux pipe at the top of the butane stripping tower T104; and the inlet of the pentane tanks is connected to the reflux pipe at the top of the pentane stripping tower T105. The ethane product from the ethane stripping tower is directly exported.

[0022] When using this recovery device for the recovery of benzene-containing natural gas under medium-pressure cryogenic light hydrocarbons, freezing blockage is prone to occur after the JT valve. The inventors analyzed and calculated the freezing blockage points of the light hydrocarbon components that easily precipitate solids in the material after the JT valve, finding that benzene had the highest freezing blockage temperature. This proves that benzene is the component most likely to precipitate solids, and the freezing blockage point of benzene is consistent with the temperature of freezing blockage in the pipeline after the JT valve recorded on-site. This further confirms that benzene is the component causing pipeline freezing blockage after the JT valve. This is because as the associated gas becomes leaner, the solubility of benzene in the system weakens due to the vapor-liquid phase equilibrium in the compression unit of the natural gas medium-pressure cryogenic light hydrocarbon recovery device. Some benzene enters the gas phase, increasing the partial pressure of benzene in the gas phase. This benzene then enters the subsequent recovery unit along with the gas phase. In the subsequent recovery unit, when passing through the JT valve, the temperature drops sharply. When the temperature falls below the freezing blockage point of benzene, solids precipitate, causing pipeline freezing blockage. The inventors proposed the technical solution of this invention based on this. The recovery devices used in the following embodiments are all modified versions of the existing natural gas medium-pressure cryogenic light hydrocarbon recovery device.

[0023] Example 1

[0024] The method for recovering benzene-containing natural gas under medium-pressure cryogenic light hydrocarbons in this embodiment uses a recovery device that is an existing natural gas medium-pressure cryogenic light hydrocarbon recovery device (as described above). Figure 1 The feed gas inlet pipe of the feed gas buffer tank V01 (as shown) and the top reflux pipe of the propane depropanizer T103 are connected by a pipeline. The modified recovery unit is as follows: Figure 2 As shown, it will not be elaborated further here.

[0025] The method for recovering light hydrocarbons from benzene-containing natural gas under medium-pressure cryogenic conditions in this embodiment includes the following steps:

[0026] Through the connecting pipe between the top reflux pipe of the depropanizer and the raw gas inlet pipe of the raw gas buffer tank, part of the produced propane product is directly mixed with the raw natural gas (the mass ratio of propane to raw natural gas is 17:100), and then enters the raw gas buffer tank for buffering. The buffered mixed gas enters the compression unit to be pressurized to 4.5 MPa, and then enters the cold box E01 after dehydration by the drying unit (part directly enters the cold box E01, and part enters the cold box E01 after being cooled by the reboiler E04配套 with the demethanizer) for cryogenic cooling. After heat exchange in the cold box E01 step by step, it enters the low-temperature separator V04 for low-temperature separation. The gas phase obtained by low-temperature separation returns to the cold box to be heat-exchanged to -80 °C and then enters the demethanizer T101 through an expander. The liquid phase obtained by low-temperature separation returns to the cold box for heat exchange and then flows through the J-T valve to be throttled and cooled to about -101 °C and enters the demethanizer. The top product of the demethanizer is exported after being cooled and pressurized to 1.2 MPa, and the bottom draw is sent to the deethanizer T102 for separation. The top product of the deethanizer is the ethane product, which is exported, and the bottom draw is sent to the depropanizer T103 to separate propane. The top product of the depropanizer is the propane product. Except for the part transported to be mixed with the raw natural gas in the raw gas inlet pipe of the raw gas buffer tank V01 before the raw gas buffer tank V01, the remaining propane product is transported to the propane storage tank for storage, and the bottom draw is sent to the debutanizer T104 to separate butane. The top product of the debutanizer is the mixed butane product, which is transported to the butane storage tank for storage, and the bottom draw is sent to the depentanizer T105 for separation. The top product is the mixed pentane product, which is transported to the pentane storage tank for storage.

[0027] In this embodiment, by connecting the top reflux pipe of the depropanizer with the raw gas inlet pipe of the raw gas buffer tank, the freezing blockage temperature after the J-T valve can be reduced, enabling the logistics to enter the demethanizer at a lower temperature, thereby reducing the top temperature of the demethanizer, improving the separation ability of the demethanizer for methane and ethane, reducing the ethane content in the dry gas, increasing the ethane product yield, and creating obvious economic and social benefits. The mass percentage content of benzene in the raw natural gas of this embodiment is 0.52%. When the introduced mass of propane is 17% of the mass of the raw natural gas, the freezing blockage temperature after the J-T valve drops from -101 °C to -105.7 °C, meeting the process requirements.

[0028] Example 2

[0029] For the medium-pressure cryogenic light hydrocarbon recovery method of benzene-containing natural gas in this embodiment, the recovery device used is to connect the raw gas inlet pipe of the raw gas buffer tank V01 of the above existing medium-pressure cryogenic light hydrocarbon recovery device for natural gas ( Figure 1 shown) to the propane storage tank and the butane storage tank through pipes at the same time. The modified recovery device is as shown in Figure 3 [[ID=1:3]]shown, and will not be elaborated here.

[0030] The method for medium-pressure cryogenic light hydrocarbon recovery from benzene-containing natural gas in this embodiment includes the following steps:

[0031] Through the raw gas inlet pipeline of the raw gas buffer tank V01 and the connecting pipelines of the propane storage tank and the butane storage tank, the stored propane product and butane product are directly mixed with the raw natural gas (the total mass ratio of propane, butane and raw natural gas mixed in the raw natural gas is 4:16:100), and then enter the raw gas buffer tank for buffering. The buffered mixed gas enters the compression unit to be pressurized to 4.5 MPa, and then enters the cold box E01 after dehydration by the drying unit (part directly enters the cold box E01, and part enters the cold box E01 after being cooled by heat exchange with the reboiler E04 supporting the demethanizer) for cryogenic treatment. After heat exchange step by step in the cold box E01, it enters the low-temperature separator V04 for low-temperature separation. The gas phase obtained from the low-temperature separation returns to the cold box for heat exchange to -80 °C and then enters the demethanizer T101 through expansion by the expander. The liquid phase obtained from the low-temperature separation returns to the cold box for heat exchange and then flows through the J-T valve for throttling and cooling to about -101 °C and enters the demethanizer to separate methane. The methane product at the top of the demethanizer is externally transported after being cooled and pressurized to 1.2 MPa, and the liquid drawn from the bottom of the tower enters the deethanizer T102 to separate ethane. The product at the top of the deethanizer is the ethane product, which is externally transported, and the liquid drawn from the bottom of the tower enters the depropanizer T103 to separate propane. The product at the top of the depropanizer is the propane product. Except for the part transported to the raw gas buffer tank V01 and mixed with the raw natural gas in the raw gas inlet pipeline of the raw gas buffer tank V01, the remaining propane product is transported to the propane storage tank for storage. The liquid drawn from the bottom of the tower enters the debutanizer T104 to separate butane. The product at the top of the debutanizer is the mixed butane product, which is transported to the butane storage tank for storage. The liquid drawn from the bottom of the tower enters the depentanizer T105 to separate pentane. The product at the top of the tower is the mixed pentane product, which is transported to the pentane storage tank for storage.

[0032] In this embodiment, by mixing the propane in the propane storage tank and the butane in the butane storage tank with the raw natural gas in the raw gas inlet pipeline of the raw gas buffer tank through pipelines, the freezing temperature after the J-T valve can be reduced. The mass percentage content of benzene in the raw natural gas in this embodiment is 0.52%. When propane and butane are mixed into the raw natural gas at a mass ratio of 1:4 and the total mass of propane and butane is 20% of the mass of the raw natural gas, the freezing temperature after the J-T valve drops from -101 °C to -108.4 °C, meeting the process requirements.

[0033] Example 3

[0034] The recovery device used in the method for medium-pressure cryogenic light hydrocarbon recovery from benzene-containing natural gas in this embodiment is to use the above-mentioned existing medium-pressure cryogenic light hydrocarbon recovery device for natural gas ( Figure 1The raw gas inlet pipeline of the raw gas buffer tank V01 (as shown) is connected to the top reflux pipeline of the depropanizer T103, the pentane storage tank, and the butane storage tank through pipelines. The modified recovery device is as Figure 4 shown, which will not be elaborated here.

[0035] The medium-pressure cryogenic light hydrocarbon recovery method for benzene-containing natural gas in this embodiment includes the following steps:

[0036] By connecting the top reflux pipeline of the depropanizer, the pentane storage tank, and the butane storage tank to the raw gas inlet pipeline of the raw gas buffer tank through pipelines, part of the propane product produced by the depropanizer, the stored butane product, and the stored pentane product are directly mixed with the raw natural gas (the mass ratio of propane, butane, pentane, and raw natural gas mixed in the raw natural gas is 4:4:12:100), and then enter the raw gas buffer tank for buffering. The buffered mixture enters the compression unit to be pressurized to 4.5 MPa, and then enters the cold box E01 (part directly enters the cold box E01, and part enters the cold box E01 after being cooled by the reboiler E04 supporting the demethanizer) for cryogenic cooling after dehydration by the drying unit. After heat exchange in the cold box E01 step by step, it enters the low-temperature separator V04 for low-temperature separation. The gas phase obtained from the low-temperature separation returns to the cold box for heat exchange to -80 °C and then enters the demethanizer T101 through expansion by the expander. The liquid phase obtained from the low-temperature separation returns to the cold box for heat exchange and then flows through the J-T valve for throttling and cooling to about -101 °C and enters the demethanizer. The methane product at the top of the demethanizer is externally transported after being cooled and pressurized to 1.2 MPa, and the bottom draw is sent to the deethanizer T102 to separate ethane. The product at the top of the deethanizer is the ethane product, which is externally transported, and the bottom draw is sent to the depropanizer T103 to separate propane. The product at the top of the depropanizer is the propane product. Except for the part transported to the raw gas buffer tank V01 and mixed with the raw natural gas in the raw gas inlet pipeline of the raw gas buffer tank V01, the remaining propane product is transported to the propane storage tank for storage. The bottom draw is sent to the debutanizer T104 to separate butane. The product at the top of the debutanizer is the mixed butane product, which is transported to the butane storage tank for storage. The bottom draw is sent to the depentanizer T105 to separate pentane. The product at the top is the mixed pentane product, which is transported to the pentane storage tank for storage.

[0037] In this embodiment, by mixing the propane in the propane storage tank and the butane in the butane storage tank with the raw natural gas in the raw gas inlet pipeline of the raw gas buffer tank through pipelines, the freezing temperature after the J-T valve can be reduced. The mass percentage content of benzene in the raw natural gas in this embodiment is 0.52%. When the mass ratio of propane, butane, and pentane is 1:1:3 and the total mass of propane, butane, and pentane is 20% of the mass of the raw natural gas and introduced into the raw natural gas, the freezing temperature after the J-T valve drops from -101 °C to -113.6 °C, meeting the process requirements.

Claims

1. A method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions, wherein the recovered products comprise propane, butane, and pentane, characterized in that: Includes the following steps: After the raw natural gas is pressurized, dried and dehydrated, and then cryogenically cooled, it is sent to a cryogenic separator for cryogenic separation. The liquid phase obtained from cryogenic separation is then sent to the dealkane tower after being throttled and expanded through a JT valve. C3-C5 alkanes are mixed into the liquid phase obtained by low-temperature separation before the JT valve or into the raw natural gas upstream of the low-temperature separator. The C3-C5 alkanes are one or any of propane, butane, and pentane. The amount of C3-C5 alkanes mixed in is such that the precipitation temperature of benzene in the material flowing through the JT valve is lower than the temperature of the material after throttling and expansion by the JT valve.

2. The method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions according to claim 1, characterized in that: The amount of alkane mixed in shall not exceed 40% of the mass of the feedstock natural gas.

3. The method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions according to claim 2, characterized in that: The amount of alkane mixed in is 17-20% of the mass of the feedstock natural gas.

4. The method for recovering benzene-containing natural gas under medium-pressure cryogenic light hydrocarbons according to any one of claims 1-3, characterized in that: C3-C5 alkanes are mixed into the feedstock natural gas before pressurization.

5. The method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions according to claim 4, characterized in that: The dealkane tower is a demethanizer tower, and the method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic pressure further includes the following steps: sending the effluent from the bottom of the demethanizer tower to the deethaner tower to separate ethane, and then sending the effluent from the bottom of the deethaner tower to the depropanizer tower to separate propane.

6. The method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions according to claim 5, characterized in that: Part of the propane separated from the propane stripper is mixed into the feedstock natural gas before pressurization; and / or the propane separated from the propane stripper is stored in a propane storage tank, and the propane in the propane storage tank is transported and mixed into the feedstock natural gas before pressurization.

7. The method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions according to claim 5, characterized in that: It also includes the following steps: The liquid extracted from the bottom of the propane stripper is sent to the butane stripper to separate butane.

8. The method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions according to claim 7, characterized in that: Part of the butane separated from the butane de-butane tower is mixed into the feedstock natural gas before pressurization; and / or the butane separated from the butane de-butane tower is stored in a butane storage tank, and the butane in the butane storage tank is transported and mixed into the feedstock natural gas before pressurization.

9. The method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions according to claim 7, characterized in that: It also includes the following steps: The liquid extracted from the bottom of the butane removal tower is sent to the pentane removal tower to separate pentane.

10. The method for recovering light hydrocarbons from benzene-containing natural gas under medium pressure cryogenic conditions according to claim 9, characterized in that: Part of the pentane separated from the depentane tower is mixed into the feedstock natural gas before pressurization; and / or the pentane separated from the depentane tower is stored in a pentane storage tank, and the pentane in the pentane storage tank is transported and mixed into the feedstock natural gas before pressurization.

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