A method of cleaning a finished oil storage tank with supercritical CO2

Abstract

The application discloses a method for cleaning a finished oil storage tank by using supercritical CO2, which comprises the following steps: pretreating the oil storage tank; performing low-pressure flexible water jet pre-cleaning on the inner wall of the oil storage tank by using a nozzle; continuously introducing high-purity nitrogen into the oil storage tank for inertization until the limit oxygen concentration is not higher than a preset value and the combustible gas concentration is not higher than a preset value; then performing vacuumizing treatment on the oil storage tank, and maintaining the negative pressure in the oil storage tank to a preset range, so as to completely discharge the air and light oil gas in the oil storage tank; performing omnibearing spraying on the inner wall of the oil storage tank by using a nozzle; introducing the supercritical CO2 mixed fluid with heavy organic matter into a flash tank for directional precipitation; performing discharge pretreatment on the light component non-condensable gas generated in the flashing process; and replacing the CO2 gas in the oil storage tank by using nitrogen. The method fills the blank of the supercritical cleaning technology for large storage tanks, and provides a coupled tank cleaning method which is safe, compliant, resourceful and can be scaled up.

Description

Technical Field

[0001] This invention relates to the field of oil storage tank cleaning technology, specifically to a method for cleaning finished oil storage tanks using supercritical CO2. Background Technology

[0002] During long-term service, floating roof storage tanks for refined oil products continuously accumulate a composite bottom sediment (BSTF) at the bottom, consisting of C15, heavy paraffin wax, gum, asphalt, rust, silt, and emulsified water. This type of sediment has strong adhesion, mixed components, and is difficult to clean. Long-term accumulation can lead to problems such as reduction in the effective volume of the storage tank, oil quality contamination, and accelerated tank wall corrosion. To ensure the safe and stable operation of the oil depot, regular tank cleaning operations are necessary.

[0003] The current mainstream tank cleaning process for oil depots in China is low-pressure water jet cleaning + three-phase separation + thermal desorption and volume reduction. This process has inherent technical defects: First, water jet cleaning can only remove loose, large pieces of oil sludge, and it has no effective ability to remove dense, heavy residual oil films on the tank wall and the lower surface of the floating roof. The cleanliness of the tank cannot meet the high-standard operation and maintenance requirements. Second, the operation generates a huge amount of oily wastewater and hazardous oil sludge, which has high disposal costs and significant secondary pollution risks. Third, open tank cleaning operations result in large amounts of fugitive VOC emissions, making it difficult to meet the stringent local emission standards of DB11 / 206-2023 "Oil and Gas Emission Control and Limits for Oil Depots". Fourth, high-pressure water flow can easily impact and damage the anti-corrosion coating of the inner floating roof and the tank wall substrate, shortening the service life of the equipment and resulting in high overall operation and maintenance costs.

[0004] Existing supercritical CO2 cleaning technology, with its excellent dissolution and diffusion properties, has been applied in precision manufacturing, semiconductors, food extraction, and small-scale oilfield equipment cleaning. However, all existing supercritical technologies have inherent barriers in terms of scenarios, operating conditions, and process logic, making them completely unsuitable for large storage tank cleaning scenarios. Specifically, they can be divided into two main categories of existing technologies: The first category is supercritical pipeline cleaning technology for oil and gas pipelines (such as CN117102173B): This technology relies on the natural pressure difference of the narrow and closed structure of the pipeline to drive the medium and the pig. Supercritical CO2 serves as both the driving medium and the wax-dissolving medium, making it suitable for pipelines with only wax deposition. Its operating conditions are free from solid sediment, emulsion water interference, large-space flow field inhomogeneity, high-concentration oil and gas explosion risks, and the rigid requirement for end-of-pipe treatment of local VOCs. The process logic and equipment are completely incompatible with the operating conditions of large storage tanks, and those skilled in the art cannot obtain a complete tank cleaning solution by simply modifying this type of technology.

[0005] The second category is small-cavity supercritical cleaning technology: This technology is applied to cubic meter-level sealed cleaning chambers, targeting single, pure mechanical oil stains, photoresist, and other contaminants using a static immersion cleaning mode. Its chamber is small and contains no large amounts of inorganic / water impurities, fundamentally different from the large spaces of 5000–100000 m³ storage tanks, the three-phase mixed sludge, and the Class A explosion-proof conditions, resulting in zero compatibility with these conditions.

[0006] In summary, there is currently no complete process in the existing technology that combines "large storage tanks + two-stage graded impurity removal + native in-situ co-solvent + large-space collaborative flow field cleaning + compliant governance + skid-mounted cluster deployment", indicating a clear technological gap. Summary of the Invention

[0007] To address the aforementioned shortcomings of existing technologies, this invention provides a method for cleaning refined oil storage tanks using supercritical CO2. This method solves the problems of low cleanliness, high pollution, and easy equipment damage associated with existing water jet cleaning. It also overcomes industry bottlenecks such as poor adaptability to various scenarios, inability to handle mixed oils, insufficient safety in high-risk operating conditions, and poor economic viability. This method fills the gap in supercritical cleaning technology for large storage tanks and provides a safe, compliant, resource-efficient, and scalable coupled cleaning method.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for cleaning refined oil storage tanks using supercritical CO2 is provided, comprising the following steps: S1: Empty the oil from the oil storage tank, isolate the process pipelines, seal the blind flanges, cut off all media connection paths, and complete basic safety pretreatment; S2: The inner wall of the oil storage tank is pre-cleaned with low-pressure flexible water jet through a nozzle to remove polar inorganic solid impurities, and the polar inorganic solid impurities are separated by a three-phase separation device; after the pre-cleaning is completed, the amount of residual virgin light oil on the tank wall is controlled, and the virgin light oil is used as an in-situ co-solvent. S3: Continuously introduce high-purity nitrogen into the oil storage tank for inertization until the limiting oxygen concentration and combustible gas concentration are not higher than the preset values; then, vacuum the oil storage tank and maintain the negative pressure inside the oil storage tank to the preset range to completely remove the air and light oil gas inside the oil storage tank. S4: Supercritical CO2 is sprayed all-round on the inner wall of the oil storage tank through the nozzle, and the spraying process adopts gradient pressure increase until the preset pressure is reached. Supercritical CO2, together with in-situ co-solvent, continuously dissolves residual heavy organic oil. S5: Supercritical CO2 mixed fluid containing heavy organic matter is introduced into a flash tank for directional precipitation. The precipitated gaseous CO2 is filtered, compressed and purified for recycling, and the precipitated heavy oil is recovered. S6: Pre-treat the light component non-condensable gases generated during the flash evaporation process until the NMHC concentration is not higher than the preset value before releasing them. S7: After the entire cleaning operation is completed, use nitrogen to replace the CO2 gas in the oil storage tank until the gas composition and safety indicators meet the standards, then the oil storage tank can be opened.

[0009] Furthermore, the nozzle in step S2 is a 180° wide-angle explosion-proof nozzle, which is mounted on an explosion-proof tracked robot.

[0010] Furthermore, in step S2, when the oil storage tank is a gasoline tank, the original light oil is original light gasoline with a content of 1% to 5%; when the oil storage tank is a diesel tank, the original light oil is light naphtha with a content of 0.5% to 1%, and the oxygen content in the tank is stable at <8% before filling.

[0011] Furthermore, in step S3, the preset value for the limiting oxygen concentration is 8%, the preset value for the combustible gas concentration is 10%, and the preset range for the negative pressure inside the oil storage tank is -50 to -100 Pa.

[0012] Furthermore, in step S3, a triple real-time interlock is established for limiting oxygen concentration, combustible gas concentration, and negative pressure. When any one of the limiting oxygen concentration, combustible gas concentration, and negative pressure exceeds a preset value, an alarm is automatically triggered and the operation is stopped.

[0013] Furthermore, the supercritical CO2 in step S4 is obtained by compressing and heating industrial liquid CO2 to 100-120 bar and 50-60°C.

[0014] Furthermore, the method of supercritical CO2 all-round spraying in step S4 is as follows: the inner wall of the oil storage tank is fully covered by a nozzle array fixed inside the oil storage tank, with a working pressure of 80-120 bar; then, the nozzles carried by the explosion-proof tracked robot are used for targeted and enhanced spraying, with a working pressure of 80-100 bar.

[0015] Furthermore, in step S5, the flash tank is equipped with a stratified settling buffer zone inside, an automatic heavy oil discharge valve at its bottom, and a coalescing oil removal filter and a two-stage pressure reducing valve at its gas phase outlet.

[0016] Further, step S6 specifically includes: the light component non-condensable gas generated in the flash evaporation process is sequentially condensed through two main gradients at -30 ℃ and -50 ℃, and equipped with an auxiliary low-temperature cold trap at -70 ℃, and then deeply adsorbed by activated carbon until the NMHC concentration is no higher than 20 g / m³.

[0017] The beneficial effects of this invention are as follows: 1. This solution breaks through industry technical biases and achieves supercritical complete cleaning of large water-containing and solid-containing storage tanks for the first time: It abandons the limitations of existing supercritical technology, which is only suitable for small cavities, clean environments, and single organic matter conditions. It creatively adopts a two-stage forced division of labor: first, mechanically remove all water and inorganic solid impurities, and then supercritically target and dissolve C15+ heavy organic residues. This completely eliminates the interference of polar media on the supercritical system and solves long-standing pain points such as incomplete water jet cleaning, large amounts of hazardous waste, excessive VOCs, and easy damage to the floating roof. It fills the technical gap in supercritical cleaning of large storage tanks of 5,000 to 100,000 m³.

[0018] 2. The in-situ dual co-solvent system of this scheme achieves efficient solubilization and low-cost operation: For gasoline storage tanks, the original light gasoline in the tank is used as a co-solvent, while diesel storage tanks only need to add a small amount of light naphtha to assist in solubilization. There is no need to purchase high-cost organic solvents throughout the process. This system can significantly improve the dissolution efficiency of supercritical CO2 on asphaltenes and heavy paraffins, which is consistent with the conclusions of supercritical extraction research published at home and abroad, achieving a dual breakthrough of efficiency improvement and cost reduction.

[0019] 3. This solution thoroughly solves the problems of cleaning dead corners and uneven flow field by using large-space full-area collaborative spraying. By combining the nozzle array on the tank top with the fixed-point reinforcement of the explosion-proof robot, the CO2 concentration distribution inside the tank is made uniform (after CFD simulation optimization, the concentration difference is controlled within the acceptable range of engineering), ensuring that there are no cleaning dead corners in the 10,000 cubic meter storage tank, and overcoming the engineering obstacles that traditional static soaking or small cavity flow field cannot be directly transplanted.

[0020] 4. This solution establishes a complete multi-level safety protection system, with safety standards that are comprehensively superior to traditional water jet technology, ensuring operational safety in high-risk and confined environments.

[0021] 5. This solution achieves a significant reduction in hazardous waste, wastewater, and VOCs, while meeting environmental protection requirements: Pre-treatment significantly reduces the amount of hazardous waste generated; the supercritical medium recycling recovery rate is ≥90%; after the tail gas undergoes two-stage main condensation (-30℃ / -50℃) plus a -70℃ auxiliary cold trap and activated carbon adsorption, the NMHC is stably ≤20 g / m³, and the -70℃ cold trap design avoids dry ice blockage, increasing the C5+ light hydrocarbon recovery efficiency from approximately 65% ​​in the conventional single-stage -30℃ condensation process to over 95%, thus realizing the resource recovery of heavy waxy colloids. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly described below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. The above and other objects, features, and advantages of the present invention will become clearer through the accompanying drawings. The same reference numerals indicate the same parts in all the drawings. The drawings are not intentionally drawn to scale to actual dimensions; the focus is on illustrating the main points of the invention.

[0023] Figure 1 A flowchart of a method for cleaning refined oil storage tanks with supercritical CO2. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0025] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0026] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0027] Furthermore, the terms "first," "second," etc., are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0028] like Figure 1 As shown, the method for cleaning refined oil storage tanks with supercritical CO2 in this scheme, when implemented, uses 10 4 The m³ storage tank is used as a typical example for explanation; for other storage tanks in the range of 5000~100000 m³, parameters such as the number of nozzles and CO2 usage are scaled proportionally to the tank volume and determined through CFD simulation optimization, specifically including the following steps: S1. Tank Pretreatment: For large internal floating roof finished oil storage tanks of 5,000 to 100,000 m³, complete the emptying of oil, isolation of process pipelines, sealing of blind flanges, cutting off all medium connection paths, implementing safety briefings for confined space operations and on-site closure and isolation, and completing basic safety pretreatment.

[0029] S2. Pre-cleaning mechanical classification and impurity removal: An explosion-proof tracked robot equipped with a 180° wide-angle explosion-proof nozzle performs low-pressure flexible water jet pre-cleaning, specifically removing large loose oil sludge, free water, rust, and sand, as well as other polar inorganic solid impurities, from the bottom of the tank. The oil-water-solid three-phase separation equipment completes the classification and separation, removing 80%-90% of the large-mass mixed contaminants in the tank, leaving only a dense C15+ heavy organic residual oil film on the tank wall, the lower surface of the floating roof, and the dead corners of the tank bottom, completely eliminating the interference of polar media.

[0030] S3. Sealed Inert Low Negative Pressure Safety Pretreatment: High-purity nitrogen is continuously introduced into the storage tank for inerting. The limiting oxygen concentration (LOC) of gasoline vapor inerting with nitrogen is approximately 10% (volume fraction). This scheme sets a 2 percentage point safety margin to stably control the oxygen volume fraction in the tank to <8%; the combustible gas in the tank is stably controlled within 10% of the lower explosive limit for a long period of time; then a low negative pressure replacement mode is adopted, with the vacuum unit controlling the negative pressure in the tank to be stable at -50 to -100 Pa, combined with intermittent nitrogen replacement, and equipped with emergency pressure relief equipment and nitrogen pressure replenishment interlock to avoid instability and deformation of the tank body and internal floating roof, and to completely remove air and light oil and gas; simultaneously, a triple real-time safety interlock is established for oxygen content, combustible gas, and negative pressure in the tank, to stably control the oxygen volume fraction to <8% and keep the combustible gas below 10% of the lower explosive limit for a long period of time, avoiding the dual safety hazards of combustion and explosion and asphyxiation caused by the coexistence of high-pressure CO2 medium and oil and gas.

[0031] S4. Large-space supercritical CO2 in-situ co-solubilization and precise rinsing: 99.9% industrial-grade high-purity liquid CO2 is selected. Through the coordinated control of high-pressure variable frequency compressor unit and constant temperature heating unit, the supercritical operating conditions of 100-120 bar and 50-60 ℃ are stably maintained (preferably 110 bar and 55 ℃ is the optimal steady-state condition, which is in the peak solubility range of C20-C40 long-chain paraffin. According to the isentropic work theory of CO2 multi-stage compression, the compression energy consumption of 110 bar operation condition can be reduced by about 15% compared with 120 bar at the same temperature).

[0032] The high-pressure compressor unit and electric heater are equipped with dual over-limit interlocks with a pressure limit of 135 bar and a temperature limit of 70°C, providing ample margin for fluctuations compared to the 120 bar operating window to prevent accidental shutdowns due to minor operating condition fluctuations. The entire skid-mounted system is equipped with emergency pressure relief and venting recovery pipelines, ensuring that overpressure media are uniformly introduced into the flash evaporation system. The emergency pressure relief pipeline first sends the media to a buffer collection tank before sending it to the flash evaporation unit, preventing instantaneous flow impacts that could cause overpressure in the flash evaporation equipment. The buffer collection tank is equipped with a high-level interlocked reflux pipeline; if the liquid level exceeds the limit, it automatically flows back to the flash evaporation system, preventing the risk of media overflow due to overcapacity.

[0033] The complete process of gradient pressurization in this scheme is as follows: First, turn off the vacuum pump and introduce CO2 at a low speed of 0.5~1 bar / min to restore the pressure inside the tank to atmospheric pressure; then, increase the pressure in three stages: the first stage is 5~10 bar and hold for 2 minutes, the second stage is 30~40 bar and hold for 3 minutes, and the third stage is to increase to the target pressure of 100~120 bar. The pressurization rate throughout the process is ≤2 bar / min to avoid instantaneous pressure difference impacting the floating roof and the sealing flange on the top of the tank.

[0034] This solution adopts a collaborative model of "distributed high-pressure nozzle array on the tank top for full-area spraying + explosion-proof robot inside the tank for fixed-point reinforcement": the arrangement of nozzles on the tank top, the spray angle, and the outlet pressure are iteratively optimized through three-dimensional CFD fluid simulation of the storage tank to eliminate cleaning dead zones; a precision pressure reducing valve is installed at the front end of the nozzle, with a working pressure of 80-120 bar; the robot is equipped with a rotating nozzle with a working pressure of 80-100 bar and wireless remote control; the entire cleaning robot inside the tank complies with the GB 3836 series IIB T4 explosion-proof standard and is suitable for intermittent hazardous environments in Zone 2 after inerting; Complete co-solvent operation process: After mechanical pre-cleaning with clean water, stop the water supply. By adjusting the cleaning intensity and reflux ratio, control the residual light gasoline content on the tank wall to 1%–5% as an in-situ co-solvent; no additional addition is required. For diesel storage tanks, after mechanical pre-cleaning and when the oxygen content inside the tank is stable at <8%, [further details on co-solvent operation are needed]. Then, under continuous nitrogen protection, the pressure inside the tank is smoothly switched from a slight negative pressure to a slight positive pressure (the slight positive pressure is only used for safe naphtha filling and is not used as an oxygen content condition for cleaning operations). Then, 0.5%~1% of light naphtha is injected into the tank as an auxiliary co-solvent through a sealed dedicated interface on the top of the tank. The following safety requirements are met throughout the filling process: Filling flow rate control (initial flow rate ≤1m / s, ≤4m / s after the liquid level is 2m above the inlet to prevent static electricity buildup); all filling pipelines, storage tanks, and interfaces are statically connected and reliably grounded, with a grounding resistance ≤10Ω; continuous monitoring of oxygen content and combustible gas concentration inside the tank; if the oxygen content ≥8% or the combustible gas concentration ≥10% LEL, filling is immediately stopped and nitrogen inerting is restored; after filling, the tank is immediately restored to an inerting state with oxygen <8%, and the S4 supercritical cleaning process begins. No external organic solvents are required, enhancing the dissolution capacity of asphaltene and heavy paraffin.

[0035] All high-pressure CO2 delivery pipelines in this solution are covered with an outer heat insulation layer and equipped with low-temperature protective gloves to prevent personnel from being frostbitten by high-pressure low-temperature media spray; the CO2 circulation inside the tank continuously strips away the dense residual oil film.

[0036] S5. Staged pressure relief flash evaporation closed-loop recovery: Supercritical CO2 carrying organic matter is introduced into the flash tank, with optimized stable operating conditions of 40-50 bar and 10-15℃. This temperature is much higher than the triple point of CO2 (-56.6℃ / 5.11 bar). Although the operating conditions are in the liquid phase region of CO2, organic matter is released through subsequent staged pressure reduction and vaporization. The flash tank outlet is equipped with a two-stage pressure reducing valve. The first stage reduces the pressure to 20-25 bar (maintaining the liquid phase), and the second stage reduces the pressure to 5-10 bar (entering the gas-liquid two-phase region). Organic matter is released in stages during this process, while avoiding the direct drop of CO2 below the triple point to generate dry ice. It is equipped with segmented electric heating, two-stage gradient pressure relief buffer, and dry ice filter bypass triple protection. The flash tank in this design is equipped with a stratified settling buffer zone, an automatic heavy oil discharge valve at the bottom, and a high-precision coalescing oil removal filter at the vapor phase outlet to intercept trace oil droplets and prevent heavy oil from being carried into the compressor unit, causing liquid slugging and lubricating oil contamination. After filtration, compression and purification, the gaseous CO2 is circulated in a closed loop: the internal medium recovery rate of the flash evaporation closed unit can reach 95%. Affected by the slight escape loss of the tank top flange and robot interface, the overall circulation recovery rate of the whole system is ≥90%. The small amount of loss is automatically replenished in real time by the industrial liquid CO2 storage tank; the heavy precipitated oil is recovered to the backup storage tank.

[0037] S6. Localized VOCs Compliant Exhaust Gas Treatment: The light component non-condensable gases generated during the flash evaporation process are sequentially condensed through a two-stage main gradient condensation process at -30℃ and -50℃, with an auxiliary low-temperature cold trap at -70℃. Setting notes: -70℃ is higher than the sublimation temperature of dry ice (-78.5℃), which can prevent solid CO2 blockage inside the cold trap and efficiently recover trace amounts of C3-C4 light hydrocarbons; residual exhaust gas is coupled with deep adsorption by activated carbon. After treatment, the NMHC in the exhaust gas is ≤20 g / m³ (Table 1), the leakage at the dynamic and static sealing points of the equipment is ≤500μmol / mol (Clause 5.2), the NMHC at the vent on the top of the tank is ≤4000μmol / mol (Clause 5.3), and the hourly average NMHC at the enterprise boundary is ≤4.0mg / m³ (Clause 5.4). All indicators meet the requirements of the local standard DB11 / 206-2023.

[0038] S7. Inertial Replacement Completion Acceptance: Nitrogen replacement operation is controlled by a gas exchange rate of ≥5 times within the tank. 4For storage tanks with a volume of 5000-100000 m³, the continuous replacement time should be no less than 2 hours. For other storage tanks with a volume of 5000-100000 m³, the total nitrogen supply and replacement time should be adjusted according to the volume ratio. Three indicators should be monitored simultaneously online at multiple points: CO2 volume concentration ≤0.1%, oxygen content 19.5%-23.5%, and combustible gas volume fraction ≤0.2% (approximately 14% LEL). The tank cleanliness acceptance test can only be completed after all indicators meet the standards.

[0039] This plan includes a tiered HSE protection system covering the entire operational process: (1) CO2 concentration monitoring is set with two levels of alarm: Level 1 audible and visual alarm at 5000 ppm, Level 2 whole machine interlock shutdown at about 10000 ppm, synchronous online monitoring of oxygen content below 19.5% triggers alarm; (2) All high-pressure transmission pipelines use API Spec 7K certified high-pressure hoses with a rated working pressure ≥15MPa; before the equipment is put into use, a two-stage airtightness test is performed: 0.6MPa low pressure holding for 30 minutes to check for leaks; the maximum operating pressure of the system is 12MPa, and a static pressure holding test of 18MPa for 10 minutes is performed according to 1.5 times the design pressure, which meets the API Spec 7K hose acceptance specifications. The equipment is put into use only after both tests are passed. (3) The high-pressure CO2 transmission pipeline is wrapped with a heat insulation layer, and the operators are equipped with portable composite gas detectors, positive pressure air respirators, and low-temperature protective gloves; (4) The supercritical skid-mounted equipment area is physically isolated from the manhole operation area of ​​the storage tank, and is equipped with an emergency pressure relief oil and gas closed-loop recovery pipeline to avoid the medium being vented on-site.

[0040] Although the specific embodiments of the invention have been described in detail with reference to the accompanying drawings, this should not be construed as limiting the scope of protection of this patent; various modifications and variations that can be made by a person skilled in the art without inventive effort within the scope described in the claims are still within the scope of protection of this patent.

Claims

1. A method for cleaning refined oil storage tanks with supercritical CO2, characterized in that, Includes the following steps: S1: Empty the oil from the oil storage tank, isolate the process pipelines, seal the blind flanges, cut off all media connection paths, and complete basic safety pretreatment; S2: The inner wall of the oil storage tank is pre-cleaned with low-pressure flexible water jet through a nozzle to remove polar inorganic solid impurities, and the polar inorganic solid impurities are separated by a three-phase separation device; after the pre-cleaning is completed, the amount of residual virgin light oil on the tank wall is controlled, and the virgin light oil is used as an in-situ co-solvent. S3: Continuously introduce high-purity nitrogen into the oil storage tank for inertization until the limiting oxygen concentration and combustible gas concentration are not higher than the preset values; then, vacuum the oil storage tank and maintain the negative pressure inside the oil storage tank to the preset range to completely remove the air and light oil gas inside the oil storage tank. S4: Supercritical CO2 is sprayed all-round on the inner wall of the oil storage tank through the nozzle, and the spraying process adopts gradient pressure increase until the preset pressure is reached. Supercritical CO2, together with in-situ co-solvent, continuously dissolves residual heavy organic oil. S5: Supercritical CO2 mixed fluid containing heavy organic matter is introduced into a flash tank for directional precipitation. The precipitated gaseous CO2 is filtered, compressed and purified for recycling, and the precipitated heavy oil is recovered. S6: Pre-treat the light component non-condensable gases generated during the flash evaporation process until the NMHC concentration is not higher than the preset value before releasing them. S7: After the entire cleaning operation is completed, use nitrogen to replace the CO2 gas in the oil storage tank until the gas composition and safety indicators meet the standards, then the oil storage tank can be opened.

2. The method for cleaning finished oil storage tanks with supercritical CO2 according to claim 1, characterized in that, The nozzle in step S2 is a 180° wide-angle explosion-proof nozzle, which is mounted on an explosion-proof tracked robot.

3. The method for cleaning refined oil storage tanks with supercritical CO2 according to claim 1, characterized in that, In step S2, when the oil storage tank is a gasoline tank, the original light oil is original light gasoline with a content of 1% to 5%; when the oil storage tank is a diesel tank, the original light oil is light naphtha with a content of 0.5% to 1%, and the oxygen content in the tank is stable at <8% before filling, and the filling pipeline and the storage tank are protected by electrostatic grounding.

4. The method for cleaning refined oil storage tanks with supercritical CO2 according to claim 1, characterized in that, In step S3, the preset value for the limiting oxygen concentration is 8%, the preset value for the combustible gas concentration is 10%, and the preset range for the negative pressure inside the oil storage tank is -50 to -100 Pa.

5. The method for cleaning finished oil storage tanks with supercritical CO2 according to claim 1, characterized in that, In step S3, a triple real-time interlock is established for limiting oxygen concentration, combustible gas concentration, and negative pressure. When any one of the limiting oxygen concentration, combustible gas concentration, and negative pressure exceeds the preset value, an alarm is automatically triggered and the operation is stopped.

6. The method for cleaning finished oil storage tanks with supercritical CO2 according to claim 1, characterized in that, The supercritical CO2 in step S4 is obtained by compressing and heating industrial liquid CO2 to 100-120 bar and 50-60°C.

7. The method for cleaning refined oil storage tanks with supercritical CO2 according to claim 1, characterized in that, The method of supercritical CO2 all-round spraying in step S4 is as follows: the inner wall of the oil storage tank is fully covered by a nozzle array fixed inside the oil storage tank, with a working pressure of 80-120 bar; then, the nozzles carried by the explosion-proof tracked robot are used for targeted and enhanced spraying, with a working pressure of 80-100 bar.

8. The method for cleaning finished oil storage tanks with supercritical CO2 according to claim 1, characterized in that, In step S5, the flash tank is equipped with a layered settling buffer zone, an automatic heavy oil discharge valve at the bottom, a coalescing oil removal filter and a two-stage pressure reducing valve at the gas phase outlet. The first stage reduces the pressure to 20~25 bar and the second stage reduces the pressure to 5~10 bar. It is equipped with a segmented electric heating and dry ice filter bypass to prevent CO2 from precipitating out and clogging the pipeline.

9. The method for cleaning finished oil storage tanks with supercritical CO2 according to claim 1, characterized in that, Step S6 specifically includes: the light component non-condensable gas generated during the flash evaporation process is sequentially condensed through two main gradients at -30 ℃ and -50 ℃, and equipped with an auxiliary low-temperature cold trap at -70 ℃, and then deeply adsorbed by activated carbon until the NMHC concentration is no higher than 20 g / m³.

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