A concealed NAPL phase extraction device and method

Abstract

The application discloses a concealed NAPL phase extraction device and an extraction method. The device comprises a relay tank, a vacuum pump, an oil-water separation head, an oil receiving tank and a remote control system. The vacuum pump is used to adjust the vacuum degree in the relay tank. The oil-water separation head is connected to the middle part of the relay tank through an oil extraction connecting pipe to extract the NAPL phase into the relay tank. The oil receiving tank is connected to the bottom of the relay tank through an oil discharge pipe and an oil discharge pump to store the NAPL phase removed from the relay tank. The remote control system is connected to the vacuum pump, the oil discharge pump and a liquid level switch through electric signals. The remote control system can receive the liquid level information of the relay tank detected by the liquid level switch and send control instructions to start or stop the vacuum pump and the oil discharge pump. The extraction device and the extraction method can overcome the problem that the general device causes the local groundwater level to drop when removing the NAPL phase, thereby avoiding more soil pollution and further avoiding the increase of the repair engineering quantity. In addition, the device has fewer faults and is more suitable for the extraction of the NAPL phase in the difficult migration stratum in cooperation with the oil film thickness measuring device.

Description

Technical Field

[0001] This invention relates to the field of groundwater pollution prevention and control technology, specifically to a concealed NAPL phase extraction device and extraction method. Background Technology

[0002] Petroleum and other water-insoluble organic liquids are collectively referred to as non-aqueous liquids (NAPLs). Accidents during production, transportation, sale, or use can cause NAPL leaks. Severe leaks or accidents can lead to large amounts of NAPL entering the soil environment. These NAPL phases migrate downwards along soil fissures into groundwater, forming NAPL phases on or below the groundwater surface. For example, the NAPL phases formed from less dense petroleum hydrocarbons float on the groundwater surface and are called low-density NAPL phases; while the NAPL phases formed from halogenated hydrocarbons, which are denser than water, accumulate above the impermeable floor below the groundwater level, forming low-density NAPL phases. Although the migration rate of NAPL phases within a site is relatively slow due to groundwater flow, they continuously diffuse into the groundwater in dissolved form, causing long-term groundwater pollution, or invade the human living environment in gaseous form. This not only disrupts the microbial community structure of the area but also poses a health hazard to humans living in the area. With the increasing perfection of domestic environmental protection laws and regulations, the aforementioned contaminated sites urgently need effective remediation using environmentally friendly and relatively concealed methods.

[0003] Currently, the main method for treating LNAPL facies in land parcels is through groundwater extraction via wells, as described in patent reference 1.

[0004] Reference 1: Chinese patent document with patent publication number CN211027493U.

[0005] Reference 1 discloses a device for the precise extraction of NAPL contaminants from groundwater, comprising a locator capable of moving up and down along a well pipe; a densitometer or viscometer probe fixed to the locator; and an extraction tube, the lower end of which is fixed to the locator and located on the same horizontal plane as the densitometer or viscometer probe, with the upper end of the extraction tube connected to the inlet of a water pump. Using data returned from the densitometer or viscometer probe, the boundary between the NAPL phase and the aqueous phase can be accurately located, thereby precisely extracting the NAPL contaminants from the groundwater. A temporary storage tank and a separation sedimentation tank can be connected downstream of the water pump. Separating the sedimentation phase allows for the removal of a small amount of water from the extracted liquid, and the NAPL contaminants can be recovered as a resource.

[0006] However, while this method can indeed rapidly extract NAPL phases from groundwater wells, NAPL phases often exist in regional areas. The migration rate of NAPL phases around the groundwater well is relatively slow, and it takes time for them to migrate to the well. Therefore, this method often indiscriminately extracts groundwater along with the NAPL phases, causing a significant drop in the groundwater level. This leads to NAPL phases infiltrating the underlying soil, thus contaminating more soil. Alternatively, if the groundwater level drops too low and the impermeable floor is uneven, it can cause discontinuity in the NAPL phases, affecting their migration to the NAPL extraction well. This necessitates increasing the number of NAPL extraction wells, and in more serious cases, it can affect the NAPL extraction assessment results. All of these situations will increase the amount of remediation work.

[0007] How to effectively remove the regional NAPL phase without causing significant changes in the groundwater level, thereby avoiding artificially increasing the amount of engineering work, has become an urgent technical problem to be solved. To overcome the above technical difficulties, the applicant previously applied for a utility model patent (Reference 2).

[0008] Reference 2: Chinese patent document with patent publication number CN216911507U.

[0009] Reference 2 discloses a NAPL phase removal device and a contaminated site remediation system, relating to the field of organic contaminated site remediation devices. The device includes a conveying mechanism and a regeneration mechanism. The conveying mechanism includes a drive component and a conveying component connected to it. The conveying component is equipped with absorbent material that can absorb NAPL phases and release the absorbed NAPL phases under external force. The conveying component drives the absorbent material from the wellhead of a groundwater well into the area below the NAPL phase to absorb the NAPL phase, and then drives the absorbent material with absorbed NAPL phases out of the wellhead. The regeneration mechanism applies external force to the absorbent material extending from the wellhead to release the absorbed NAPL phases. This effectively removes NAPL phases from contaminated sites while preventing the downward diffusion of NAPL phases caused by local groundwater level drops, thus avoiding further soil contamination and minimizing the need for additional remediation work.

[0010] However, the aforementioned devices have a relatively high failure rate due to their numerous moving parts, and their large size prevents concealed operation. Therefore, based on this research, the applicant has redeveloped a NAPL extraction device and its usage method. Summary of the Invention

[0011] The purpose of this invention is to solve the technical problems of existing NAPL extraction devices having a slightly high failure rate and large size, which prevents concealed operation, and to provide a concealed NAPL phase extraction device and extraction method.

[0012] To address the shortcomings of the aforementioned technical problems, the present invention provides a concealed NAPL phase extraction device, which comprises:

[0013] The relay tank is used to temporarily store the NAPL phase and is equipped with a level switch.

[0014] A vacuum pump, connected to the top of the relay tank via a pressure-reducing connecting pipe, is used to regulate the vacuum level within the relay tank; and

[0015] An oil-water separator head, connected to the middle of the relay tank via an oil extraction connection pipe, draws the NAPL phase into the relay tank; and

[0016] An oil receiving tank, connected to the bottom of the relay tank via a drain pipe and a drain pump, is used to store the NAPL phase removed from the relay tank; and

[0017] The remote control system is connected to the vacuum pump, the oil discharge pump, and the level switch via electrical signals. It can receive the level information in the relay tank detected by the level switch and issue control commands to start or stop the vacuum pump and the oil discharge pump.

[0018] As a further optimization of the concealed NAPL phase extraction device of the present invention: the relay tank can withstand a vacuum of at least one atmosphere.

[0019] As a further optimization of the concealed NAPL phase extraction device of the present invention: the vacuum pump has a pumping capacity of 1-10 times the volume of the relay tank per minute, and can evacuate the relay tank to an absolute pressure of 0.05-90 kPa. The outlet of the vacuum pump is connected to an exhaust gas purification device.

[0020] As a further optimization of the concealed NAPL phase extraction device of the present invention: the liquid level switch is a float switch that can sense the liquid level in the relay tank at 5% and 95%.

[0021] As a further optimization of the concealed NAPL phase extraction device of the present invention: the oil-water separation head is made of an oleophilic and hydrophobic material.

[0022] This invention also provides a concealed NAPL phase extraction method, which utilizes the above-mentioned extraction device to complete the extraction of the NAPL phase, specifically as follows:

[0023] Detect the NAPL phase thickness, place the relay tank above the NAPL phase in the extraction well, select an appropriate oil-water separator head length based on the NAPL phase thickness, adjust the length of the oil-water separator head and the oil extraction connection pipe connecting the oil-water separator head and the relay tank, and keep part of the oil-water separator head in the air.

[0024] Start the vacuum pump and adjust the absolute pressure inside the relay tank to 0-90 kPa, so that the NAPL phase enters the relay tank through the oil-water separator.

[0025] When there is a large amount of NAPL phase in the relay tank, the upper limit of the liquid level set by the level switch is reached. The remote control system receives the signal from the level switch and sends a shutdown command to the vacuum pump and a start command to the oil discharge pump. After the oil discharge pump starts, it discharges the NAPL phase in the relay tank to the oil receiving tank.

[0026] When the NAPL phase in the relay tank is low and reaches the lower limit set by the level switch, the remote control system receives the signal from the level switch and sends a shutdown command to the oil discharge pump. The vacuum pump continues to maintain the vacuum in the relay tank, and the NAPL phase continues to enter the relay tank through the oil-water separator, thus achieving the extraction of the NAPL phase.

[0027] As a further optimization of the concealed NAPL phase extraction method of the present invention: when the NAPL phase thickness is >10cm, the length of the oil-water separation head is <30cm; when the NAPL phase thickness is ≤10cm, the length of the oil-water separation head is less than twice the thickness of the NAPL layer.

[0028] As a further optimization of the concealed NAPL phase extraction method of the present invention: when the NAPL phase thickness is ≤2cm, keep at least 30% of the oil-water separation head in the air.

[0029] As a further optimization of the concealed NAPL phase extraction method of the present invention: the NAPL phase thickness is detected by a water surface NAPL phase thickness detection device, the detection device having:

[0030] A light source, placed within the aqueous phase, emits light outward at a non-perpendicular angle; and

[0031] A light receiver, positioned above the water surface, receives light emitted by a light source; and

[0032] A laser rangefinder, positioned above the water surface, is used to measure the distance between a light receiver and the upper surface of the oil layer; and

[0033] A support bracket is used to hold the light source, light receiver, and laser rangefinder; and

[0034] The data processing equipment is used to receive data detected by the optical receiver and the laser rangefinder, perform calculations, and output the LNAPL layer thickness value.

[0035] The specific method for detecting NAPL phase thickness includes the following steps:

[0036] S1. Roughly detect the thickness of the oil layer in the well, and take the refractive index no of the oil layer above the groundwater to test the light intensity.

[0037] S2. Adjust the length of the distance rod to be greater than the rough measured oil layer thickness, and then use the rope fixed to the device traction hole to lower the device into the groundwater monitoring well. Place the light source in the water phase, and keep the laser rangefinder and light receiver in the gas phase above the oil layer to ensure that the distance rod of the detection device is perpendicular to the underground horizontal plane. Turn on the light source, light receiver and laser rangefinder.

[0038] S3. The data processing equipment receives data from the optical receiver and laser rangefinder and calculates the real-time thickness H of the LNAPL layer using the following formula. o :

[0039] L 测 -(H L tanα+H A tany)=H o (tanβ-tanα)

[0040] Among them, L 测 The lateral propagation distance of light as measured by the light receiver;

[0041] H A The distance from the light receiver to the upper surface of the oil layer, as measured by the laser rangefinder;

[0042] α is the incident angle set by the light source;

[0043] β is the exit angle of light entering the oil layer;

[0044] γ is the exit angle of light as it exits the water surface and enters the air.

[0045] β and γ are calculated using the following formula:

[0046]

[0047]

[0048] Where, n o The refractive index of oil, n W Let n be the refractive index of water. A is the refractive index of air.

[0049] As a further optimization of the concealed NAPL phase extraction method of the present invention: the thickness of the oil layer in the groundwater monitoring well is roughly detected by sampling the liquid in the monitoring well using the vertical interception method.

[0050] The present invention has the following beneficial effects: The extraction device and method of the present invention overcome the problem of local groundwater level drop caused by conventional devices when removing NAPL phases, thereby avoiding further soil pollution and thus avoiding artificially increased remediation work. Furthermore, the device has fewer moving parts and a low failure rate, and when used with an oil film thickness measuring instrument, it is more suitable for extracting NAPL phases from difficult-to-migrate strata. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of the extraction device of the present invention;

[0052] Figure 2 This is a schematic diagram of the structure of the online monitoring device of the present invention;

[0053] Figure 3 This is a schematic diagram illustrating the principle of the monitoring method of the present invention;

[0054] Marked in the image: 102. Relay tank; 103. Pressure-reducing connecting pipe; 104. Oil extraction connection pipe; 105. Oil drain pipe; 106. Liquid level switch; 107. Oil-water separator head; 108. Oil discharge pump; 109. Vacuum pump; 110. Exhaust gas purification device; 111. Oil receiving tank; 112. Remote control system; 202. Rope; 203. Laser rangefinder; 204. Optical receiver; 205. Install the support frame; 206. Lower support; 207. Lower support; 208. Light source; 209. Wire; 210. Data processing unit; 211. Output transmission unit. Detailed Implementation

[0055] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0056] <Example 1>

[0057] like Figure 1 As shown, a concealed NAPL phase extraction device includes a relay tank 102, a vacuum pump 109, an oil-water separator 107, an oil receiving tank 111, and a remote control system 112.

[0058] The relay tank 102 is used to temporarily store the NAPL phase. It is equipped with a level switch 106 and can withstand a vacuum of at least one atmosphere. The level switch 106 is a float switch that can sense the liquid level in the relay tank 102 at 5% and 95% or at 0% and 100%.

[0059] The vacuum pump 109 is connected to the top of the relay tank 102 via a pressure-reducing connecting pipe 103 to adjust the vacuum level inside the relay tank 102. The vacuum pump 109 has a pumping capacity of 1-10 times the volume of the relay tank per minute, and can evacuate the relay tank to an absolute pressure of 0.05-90 kPa. The outlet of the vacuum pump 109 is connected to an exhaust gas purification device 110. The exhaust gas purifier 110 is a device with an outer diameter slightly smaller than the inner diameter of the NAPL phase extraction well, filled with adsorbent material that can adsorb the NAPL phase. Its length is determined by a breakthrough curve test and it uses a quick-connect connection for easy replacement.

[0060] The oil-water separator 107 is connected to the middle of the relay tank 102 via an oil extraction connection pipe 104 to extract the NAPL phase into the relay tank 102. The oil-water separator 107 is made of an oleophilic and hydrophobic material, which is existing technology and will not be described in detail here.

[0061] The oil receiving tank 111 is connected to the bottom of the relay tank 102 via the oil drain pipe 105 and the oil drain pump 108, and is used to store the NAPL phase removed from the relay tank 102.

[0062] The remote control system 112 is connected to the vacuum pump 109, the oil discharge pump 108 and the liquid level switch 106 via electrical signals. It can receive the liquid level information in the relay tank 102 detected by the liquid level switch 106 and issue control commands to start or stop the vacuum pump 109 and the oil discharge pump 108.

[0063] The relay tank 102 has three connecting pipes and one level switch: a pressure-reducing connecting pipe 103, an oil-suction connecting pipe 104, an oil-draining pipe 105, and a float switch 106. The pressure-reducing connecting pipe 103 is connected to a vacuum pump 109 via a pipeline, and the vacuum pump outlet is connected to a tail gas purification device 110. The oil-suction connecting pipe 104 is connected to an oil-water separator 107 via a pipeline. This oil-water separator allows air and oil to pass through while preventing water from passing. When the pressure in the relay tank decreases, the oil-water separator 107, located at the NAPL phase-water phase interface, begins to pump NAPL phase liquid into the relay tank. The oil-draining pipe 105 is connected to an oil-draining pump 108 via a pipeline. When the oil-draining pump 108 starts, it discharges the NAPL phase in the relay tank 102 into the oil receiving tank 111. The float switch 106 monitors the liquid level in the relay tank 102 and, through the device's remote control system 112, sends start and stop signals to the oil-draining pump 108.

[0064] The remote control system 112 can set the start-stop frequency of the device and transmit device power information, instrument status, and other information in real time, thereby realizing real-time management of the device status. The remote control system 112 enables remote control and monitoring of the device's operation. All pumps and exhaust gas treatment devices used in the entire device are small-volume, high-power equipment. Therefore, the NAPL phase extraction device can be placed in the NAPL phase extraction well. The only parts extending out of the NAPL phase extraction well are the electrical wires and the outlet hose of the oil pump, thus enabling concealed extraction of the NAPL phase without surface equipment.

[0065] <Example 2>

[0066] A concealed NAPL phase extraction method utilizes the extraction device described in Example 1 to extract the NAPL phase. The specific method is as follows:

[0067] Select a relay tank with an outer diameter slightly smaller than the inner diameter of the NAPL phase extraction well. Determine the suspension depth of the device based on the groundwater depth and place the device above the NAPL phase in the NAPL phase extraction well. Select an appropriate oil-water separator head length based on the NAPL phase thickness. When the NAPL phase is thick, the oil-water separator head length should not be too long, generally within 30cm. When the NAPL phase thickness is thin (within 10cm), the oil-water separator head length should not exceed twice the NAPL layer thickness. Adjust the hose length connecting the oil-water separator head and the relay tank to keep a portion of the oil-water separator head in the air. When the NAPL phase thickness is thin (0-2cm), ensure that at least 30% of the oil-water separator head is in the air.

[0068] Start the vacuum pump and adjust the absolute pressure in the relay tank to 0-90 kPa. The NAPL phase will then enter the relay tank through the oil-water separator. When there is a significant amount of NAPL phase in the relay tank, the float switch opens, controlling the vacuum pump to shut down and the oil discharge pump to start. After the oil discharge pump starts, it discharges the NAPL phase from the relay tank to the oil receiving tank. When the NAPL phase level in the relay tank is lower than the lower float of the float switch, the float switch closes, controlling the oil discharge pump to shut down. The vacuum pump continues to maintain the vacuum in the relay tank, and the NAPL phase continues to enter the relay tank through the oil-water separator, thus achieving NAPL phase extraction. The remote control system of the device can set the start-up and stop frequency of the device and can also transmit real-time information such as device power and instrument status, thereby achieving real-time management of the device status.

[0069] The NAPL phase thickness can be detected by a water surface NAPL layer thickness detection device based on the principle of light refraction. The detection device includes a light source 208, a light receiver 204, a laser rangefinder 203, a support, and data processing equipment.

[0070] The bracket supports the light source 208, the light receiver 204, and the laser rangefinder 203. The bracket includes a spacer rod 206, an upper bracket 205, and a lower bracket 207. The upper bracket 205 and lower bracket 207 are parallel to each other and perpendicular to the spacer rod 206 at its upper and lower ends, respectively. The laser rangefinder 203 and the light receiver 204 are mounted on the upper bracket 205, and the light source 208 is mounted on the lower bracket 207. This structural design ensures that the longitudinal distance between the light source 208 and the spacer rod 206 does not change with the lateral distance, allowing the light source 208 to rotate at any angle or be mounted at a fixed angle on the lower bracket 207. It also ensures that the longitudinal distance between the laser rangefinder 203 and the spacer rod 206 does not change with the lateral distance.

[0071] The spacer bar 206 is a telescopic straight bar. After telescopic extension, the spacer bar can bear a load of 10kg without bending and the length remains unchanged.

[0072] The upper support 205 is provided with a traction hole, and the line connecting the center of the traction hole and the center of gravity of the device is parallel to the spacer rod 206. A hoisting rope 202 is installed on the traction hole. The device is lowered into the groundwater monitoring well or other places where it is needed by means of the rope 202 fixed to the traction hole.

[0073] The light source 208 is placed in the water phase and emits light outward at a non-vertical angle. The light source can emit direct light, and the thickness of the light is no greater than 0.01mm.

[0074] The light receiver 204 is placed above the water surface and receives the light emitted by the light source 208. The resolution of the light receiver is higher than 0.01mm.

[0075] The laser rangefinder 203 is positioned above the water surface to measure the distance between the optical receiver 204 and the upper surface of the oil layer. The rangefinder has a testing accuracy of 0.01 mm. The laser rangefinder 203 is a small-volume laser rangefinder that can be placed in a groundwater monitoring well.

[0076] The data processing device is used to receive data detected by the optical receiver 204 and the laser rangefinder 203, perform calculations, and output the NAPL layer thickness value.

[0077] The data processing device includes a data processing unit 210 and a data transmission unit 211. The data processing unit 210 has data display, input, and editing / calculation functions. The data transmission unit 211 has components for transmitting data from the data processor to a remote memory, components for the application to retrieve data from the remote memory, and components for running the application. The data transmitted by the data transmission unit 211 includes input data, measured data, test time, calculated oil layer thickness data, and instrument status.

[0078] The method for detecting NAPL layer thickness using the aforementioned detection device is as follows: The distance rod length is adjusted to be greater than the estimated oil layer thickness. The device is lowered into the groundwater monitoring well via a rope fixed to the device's traction hole. The light source is positioned in the aqueous phase, and the laser rangefinder is kept in the gas phase above the oil layer. The distance rod of the detection device is ensured to be perpendicular to the groundwater level. All components of the oil layer thickness measuring instrument are activated, and the light source is directed towards the oil-water interface at an incident angle α. The oil layer thickness above the groundwater surface is then calculated by the data processor.

[0079] The light source is incident at an angle α towards the oil-water interface. Assuming there is no water on the surface, the exit angle of the light from the water surface into the air is γ, and the relationship between α and β is shown in Equation 1. If there is an oil layer on the water surface, the exit angle β of the light entering the oil layer is related to the refractive index of the oil layer, and the relationship between β and α is shown in Equation 2. The light is incident at the oil-air interface in the direction of the incident angle β. From Equations 1, 2, and 3, it can be seen that the exit angle is still γ. The length (L) of the distance measuring rod is a set value, and the distance from the upper support to the upper surface of the oil layer measured by the laser rangefinder is H. A H L The depth to which the fixed-distance rod is submerged in the NAPL and aqueous phases is L, and the lateral distance from the light source to the photosensitive point on the light source receiving surface, as measured by the photosensitive material receiver, is L. 测 The thickness H of the oil layer above the groundwater surface can be calculated based on Equation 4. o The value of . L 测 -(H L tanα+H A tanγ)=H o (tanβ-tanα) Equation 4

[0080] <Example 3>

[0081] The experiment used a sandbox to simulate a real-world pollution scenario. The sandbox measured 150×150×100cm (length×width×depth), and included a 10cm inner diameter screen pipe as a NAPL phase extraction well, filled with native soil. 30cm of water was added to the sandbox, followed by a 10cm thick layer of NAPL phase composed of diesel fuel. The patented device was used, with the following dimensions and model selected: a relay tank with an outer diameter of 9.5cm and a length of 25cm; a 23cm long double-ball float switch; a 15cm long oil-water separator head; a 10cm suspended depth for the relay tank; the oil-water separator head was positioned vertically within the NAPL phase, with the top 2cm exposed to air; and commercially available miniature pumps were used for both the vacuum pump and the oil discharge pump. Start the main power supply of the device and adjust the pressure in the relay tank to an absolute pressure of 80 kPa. After 10 minutes, the float switch automatically opens, controlling the vacuum pump to stop and the oil discharge pump to start. Within 1 minute, the oil receiving bottle collects 1.4 L of oil, after which the float switch automatically closes, controlling the oil discharge pump to stop and the vacuum pump to start. After 35 hours, the NAPL layer thickness in the sand box is 1 cm. Adjust the depth of the oil-water separator head submerged in water to 7 cm. Check the device's power information and instrument status via the remote control system; it is normal. Adjust the device's start-up frequency to 2 hours / time, 30 minutes each time via the remote control system. After 24 hours, the NAPL phase thickness in the sand box is 0.2 cm.

[0082] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A concealed NAPL phase extraction method, comprising using an extraction device to extract the NAPL phase, characterized in that, The extraction device has: The intermediate tank (102) is used to temporarily store the NAPL phase, and it is equipped with a level switch (106). A vacuum pump (109), connected to the top of a relay tank (102) via a pressure-reducing connecting pipe (103), is used to regulate the vacuum level within the relay tank (102); and An oil-water separator (107) is connected to the middle of a relay tank (102) via an oil extraction connection pipe (104) to draw the NAPL phase into the relay tank (102); and An oil receiving tank (111), connected to the bottom of a relay tank (102) via a drain pipe (105) and a drain pump (108), is used to store the NAPL phase removed from the relay tank (102); and The remote control system (112) is connected to the vacuum pump (109), the oil pump (108) and the level switch (106) via electrical signals. It can receive the liquid level information in the relay tank (102) detected by the level switch (106) and issue control commands to start or stop the vacuum pump (109) and the oil pump (108). The extraction method is as follows: Detect the NAPL phase thickness, place the relay tank above the NAPL phase in the extraction well, select the oil-water separator head length according to the NAPL phase thickness, adjust the length of the oil-water separator head and the oil extraction connection pipe connecting the oil-water separator head and the relay tank, and keep part of the oil-water separator head in the air. Start the vacuum pump and adjust the absolute pressure inside the relay tank to 0-90 kPa, so that the NAPL phase enters the relay tank through the oil-water separator. When the NAPL phase in the relay tank reaches the upper limit of the liquid level set by the liquid level switch, the remote control system receives the signal from the liquid level switch and sends a shutdown command to the vacuum pump and a start command to the oil discharge pump. After the oil discharge pump starts, it discharges the NAPL phase in the relay tank to the oil receiving tank. When the NAPL phase in the relay tank reaches the lower limit of the liquid level set by the level switch, the remote control system receives the signal from the level switch and sends a shutdown command to the oil discharge pump. The vacuum pump continues to maintain the vacuum in the relay tank, and the NAPL phase continues to enter the relay tank through the oil-water separator, thus realizing the extraction of the NAPL phase.

2. The concealed NAPL phase extraction method as described in claim 1, characterized in that, The relay tank (102) can withstand a vacuum of at least one atmosphere, and its outer diameter is selected to be smaller than the inner diameter of the monitoring well to be treated.

3. The concealed NAPL phase extraction method as described in claim 2, characterized in that, The vacuum pump (109) has a pumping capacity of 1-10 times the volume of the relay tank (102) per minute, and can evacuate the relay tank (102) to an absolute pressure of 0.05-90 kPa. The outlet of the vacuum pump (109) is connected to a tail gas purification device (110).

4. The concealed NAPL phase extraction method as described in claim 1, characterized in that, The liquid level switch (106) is a float switch that senses the liquid level in the relay tank (102) at 5% and 95%.

5. The concealed NAPL phase extraction method as described in claim 4, characterized in that, The oil-water separator head (107) is made of an oleophilic and hydrophobic material.

6. The concealed NAPL phase extraction method as described in claim 1, characterized in that, When the NAPL phase thickness is greater than 10 cm, the length of the oil-water separation head is less than 30 cm. When the NAPL phase thickness is less than or equal to 10 cm, the length of the oil-water separation head is less than twice the thickness of the NAPL layer.

7. The concealed NAPL phase extraction method as described in claim 1, characterized in that, When the NAPL phase thickness is ≤2cm, keep at least 30% of the oil-water separator head exposed to air.

8. The concealed NAPL phase extraction method as described in claim 1, characterized in that, The thickness of the NAPL phase is detected using a surface NAPL phase thickness detection device. The detection device has the following features: The light source (208) is placed in the aqueous phase and emits light outward at a non-perpendicular angle; as well as A light receiver (204) is placed above the water surface and receives light emitted by a light source (208); as well as A laser rangefinder (203), positioned above the water surface, is used to measure the distance between the light receiver (204) and the upper surface of the oil layer; and A support bracket for supporting the light source (208), the light receiver (204), and the laser rangefinder (203); and A data processing device is used to receive and calculate the data detected by the optical receiver (204) and the laser rangefinder (203) and output the NAPL layer thickness value. The specific method for detecting NAPL phase thickness includes the following steps: S1. Roughly detect the oil layer thickness in the well, and test the refractive index n of the oil layer above the groundwater. o ; S2. Adjust the length of the distance rod to be greater than the rough measured oil layer thickness, and then use the rope fixed to the device traction hole to lower the device into the groundwater monitoring well. Place the light source in the water phase, and keep the laser rangefinder and light receiver in the gas phase above the oil layer to ensure that the distance rod of the detection device is perpendicular to the underground horizontal plane. Turn on the light source, light receiver and laser rangefinder. S3. The data processing equipment receives data from the optical receiver and laser rangefinder and calculates the real-time thickness H of the NAPL layer using the following formula. o : Among them, L 测 The lateral propagation distance of light as measured by the light receiver; H A The distance from the light receiver to the upper surface of the oil layer, as measured by the laser rangefinder; H L The depth to which the spacer rod is submerged in the oil and water phases; α is the incident angle set by the light source; β is the exit angle of light entering the oil layer; γ is the exit angle of light as it exits the water surface and enters the air; β and γ are calculated using the following formula: in, n o For the refractive index of oil, n W The refractive index of water, n A is the refractive index of air.

9. The concealed NAPL phase extraction method as described in claim 1, characterized in that, The thickness of the oil layer in the groundwater monitoring well was roughly determined by sampling the liquid in the monitoring well using the vertical interception method.

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