A method for treating oily sludge based on supercritical carbon dioxide stripping

Abstract

This invention belongs to the field of oily sludge treatment technology, specifically disclosing a method for treating oily sludge based on supercritical carbon dioxide stripping. First, the oily sludge is conditioned using anhydrous sodium carbonate. Second, acidification treatment under supercritical carbon dioxide conditions creates a porous structure in the sludge, allowing for thorough extraction of oil from its surface. Then, external conditions are altered to convert the supercritical CO2 into a gaseous state, completing the separation from the oil phase and achieving efficient oil stripping from the sludge. This invention is environmentally friendly, produces no secondary pollution, and exhibits excellent oily sludge treatment effect and high efficiency. The oil content of the treated sludge can be reduced to below 2%, meeting the emission standards for oily sludge. This method achieves harmless treatment and resource utilization of oily sludge, providing technical support for the efficient utilization of oily sludge resources. It offers significant comprehensive economic and environmental benefits and has high industrial application value.

Description

Technical Field

[0001] This invention relates to the field of oily sludge treatment technology, specifically to a method for treating oily sludge based on supercritical carbon dioxide stripping. Background Technology

[0002] Oily sludge mainly refers to oily sludge formed during oilfield production, resulting from the mixture of crude oil or other oil products with the solid phase. It is a type of solid waste rich in mineral oil, primarily composed of crude oil, mud, and water. The solid particle size is mostly between 1 and 100 μm. The oil composition depends on the type of crude oil, different refinery processes, and operating conditions, and the composition of the sludge may vary over time. Typically, sludge contains a certain amount of crude oil (concentration 5%–80% by mass), heavy metal ions (such as iron, copper, and sulfide), and inorganic salts (concentration 5%–20% by mass) compounds. The oil phase in this sludge generally contains benzene compounds, phenols, and other substances, accompanied by foul odor and toxicity. Direct contact with the natural environment can poison, acidify, or alkalize the soil, leading to changes in soil structure and texture, hindering plant root growth, and causing significant pollution to water bodies and vegetation. It also results in the waste of petroleum resources. Summary of the Invention

[0003] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a method for treating oily sludge based on supercritical carbon dioxide stripping, so as to solve the problem that benzene series compounds, sludge and other substances in oily sludge cause great pollution to water bodies and vegetation, and at the same time cause waste of petroleum resources.

[0004] To achieve the above objectives, the present invention employs the following technical solution:

[0005] This invention provides a method for treating oily sludge based on supercritical carbon dioxide stripping, comprising:

[0006] Step 1: Conditioning and centrifuging of oily sludge

[0007] Oily sludge is injected into an oily sludge conditioning tank. A conditioning agent is added to the oily sludge conditioning tank for conditioning and centrifugal dewatering. The oily sludge in the oily sludge conditioning tank is then acidified with an acidifying agent. The sludge is then put into a plate and frame filter press for filtration. The wastewater after filtration is sent to a combined wastewater treatment plant for treatment. The filter cake and oily sludge cake are mixed together and used as oily sludge to be treated.

[0008] Step 2: Supercritical stripping

[0009] The oil sludge to be treated is fed into a segmented supercritical fluid extraction device using a screw conveyor. The low-pressure carbon dioxide in the gas storage tank is compressed into supercritical CO2 by a compressor, and the supercritical CO2 is used as a stripping agent to perform supercritical fluid stripping of the oil phase of the oil sludge to be treated, resulting in a mixture of stripped sludge, stripped CO2, and stripped oil phase. The stripped sludge falls into the oil sludge storage tank at the bottom of the segmented supercritical fluid extraction device.

[0010] Step 3: CO2 Separation and Recycling

[0011] The stripped CO2 and the stripped oil phase mixture first enter the first separation unit for preliminary separation, and then enter the second separation unit for deep separation. The stripped CO2 is converted from a supercritical state to a gaseous state and discharged from the top of the first and second separation units, and enters the gas storage tank for recycling. At the same time, a de-CO2 oil phase is generated, which is discharged from the bottom of the first and second separation units.

[0012] Furthermore, the conditioning agent mentioned in step 1 is anhydrous sodium carbonate or anhydrous sodium bicarbonate and cationic polyacrylamide flocculant, and the total amount added is 0.1 to 0.5% of the mass of oily sludge.

[0013] Furthermore, the mass ratio of the anhydrous sodium carbonate or anhydrous sodium bicarbonate to the cationic polyacrylamide flocculant is 1:(1.2 to 1.8).

[0014] Furthermore, the cationic polyacrylamide flocculant is a cationic polyacrylamide with a weight-average molecular weight of 8,000 to 120,000 Daltons.

[0015] Furthermore, the acidifying agent in step 2 is acetic acid, citric acid, malic acid, or other biodegradable organic acids, and the acidification treatment adjusts the pH to 3-4.

[0016] Furthermore, in step 2, the low-pressure carbon dioxide in the storage tank is compressed into supercritical CO2 by a compressor and then heat is exchanged through a heat exchanger. Specifically, the low-pressure carbon dioxide in the storage tank with a pressure below 5 MPa is compressed to 30 MPa by a compressor, and the low-pressure carbon dioxide is compressed into supercritical CO2. During the compression process, the temperature of the carbon dioxide rises to 70-80°C, and the temperature of the supercritical CO2 is reduced to 60°C through a heat exchanger.

[0017] Furthermore, the volume ratio of the stripping agent to the sludge to be treated in step 2 is 1:(1-6).

[0018] Furthermore, in step 2, the reaction temperature for supercritical fluid stripping of the oil phase of the sludge to be treated is 60°C, the supercritical swelling pressure is 18–30 MPa, and the swelling time is 2.5–3 h.

[0019] Furthermore, the oil content of the sludge after stripping in step 2 is 0.8% to 2%.

[0020] Furthermore, in step 3, after the stripped CO2 and stripped oil phase mixture enter the first separation device and the second separation device, the pressure of the first separation device is reduced to 5 MPa and the pressure of the second separation device is reduced to atmospheric pressure.

[0021] The present invention has at least the following beneficial effects:

[0022] This invention first uses anhydrous sodium carbonate to condition oily sludge, then acidifies it under supercritical carbon dioxide conditions to create a porous structure, fully extracting the oil from the sludge surface. Next, external conditions are altered to convert the supercritical CO2 into a gaseous state, separating it from the oil phase and achieving efficient oil removal from the sludge. This invention is environmentally friendly, produces no secondary pollution, and offers excellent oily sludge treatment with high efficiency. The treated sludge retains less than 2% oil, meeting emission standards. It achieves harmless treatment and resource utilization of oily sludge, providing technical support for the efficient utilization of oily sludge resources. The invention offers significant economic and environmental benefits and has high industrial application value. Attached Figure Description

[0023] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0024] In the attached diagram:

[0025] Figure 1 This is a schematic diagram of the process flow for supercritical CO2 stripping treatment of oily sludge according to the present invention.

[0026] Reference numerals in the attached drawings: 1. Sludge conditioning tank; 2. Plate and frame filter press; 3. Screw conveyor; 4. Segmented supercritical fluid extraction device; 5. First separation device; 6. Second separation device; 7. Gas storage tank; 8. Compressor; 9. Heat exchanger. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0028] The following detailed description is exemplary and intended to provide further detailed explanation of the invention. Unless otherwise specified, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this invention is for describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention.

[0029] Supercritical fluid extraction (SFE) is a rapidly developing new green separation technology. Supercritical fluids are fluids whose temperature and pressure are above their critical temperature and pressure. Supercritical fluids are special fluids between gases and liquids, possessing the dual properties of both. Their density is close to that of liquids, their solubility for liquids and solids is close to that of ordinary liquid solvents, while their viscosity and diffusion rate are close to those of ordinary gases. Therefore, supercritical fluids have good permeability and strong solubility, as well as a high mass transfer rate and the ability to quickly reach extraction equilibrium. Using supercritical CO2 to extract oily sludge from oilfields is a highly efficient and environmentally friendly treatment method. Supercritical extraction utilizes these characteristics of supercritical fluids to extract and separate substances. Then, by isothermal depressurization or isobaric heating, the supercritical fluid is converted into an ordinary gas, allowing the extractant to be separated from the substances. This invention selects carbon dioxide as the supercritical extractant, and its critical data are shown in Table 1.

[0030] Table 1 Critical parameters of supercritical fluid extractants

[0031]

[0032] Supercritical CO2 extraction uses CO2 as a solvent. Supercritical CO2 fluid has a high density and dielectric constant, exhibiting strong dissolving power for many substances. The separation rate is much faster than liquid extraction and changes dramatically with pressure and temperature. Therefore, it not only exhibits selectivity for the solubility of certain substances but also facilitates the separation of solvent and extract. Compared with conventional separation methods, it features low extraction temperature, high selectivity, high efficiency, low energy consumption, and no pollution. It is particularly suitable for the extraction of lipid-soluble, high-boiling-point, and heat-sensitive substances, and is also suitable for the fine separation of different components. This invention, considering the properties of oil sludge, selects an extraction pressure of 5–20 MPa.

[0033] Table 2 shows that under supercritical conditions, fluids exhibit dual properties of both gas and liquid phases, possessing both high diffusion coefficients and low viscosity comparable to gases, as well as densities and good dissolving power similar to liquids. Carbon dioxide, as a supercritical fluid, has a critical temperature of 31.1℃ and a critical pressure of 7.39 MPa, and its operating temperature and pressure are relatively easy to achieve. The dissolving power of CO2 depends entirely on temperature and pressure, exhibiting characteristics of supercritical fluids such as density close to that of liquids, viscosity close to that of gases, and diffusion rates. At this point, the mass transfer rate of CO2 is much higher than its solvent extraction rate in the liquid state, and equilibrium is reached quickly. Considering the properties of the oil sludge, a relatively low extraction temperature of 30–80℃ is used.

[0034] Table 2 Comparison of the properties of supercritical fluids with gases and liquids

[0035]

[0036] This invention provides a method for treating oily sludge based on supercritical carbon dioxide stripping. First, the oily sludge is conditioned using a special conditioning agent. Second, it is acidified under supercritical carbon dioxide to make the sludge porous, fully extracting the oil from the surface of the oily sludge. Then, the external conditions are changed to convert the supercritical CO2 after extraction into a gaseous state, completing the separation from the oil phase and achieving the goal of efficiently stripping oil from the oily sludge.

[0037] like Figure 1 As shown, the present invention discloses a method for treating oily sludge based on supercritical carbon dioxide stripping, comprising:

[0038] Step 1: Conditioning and centrifuging of oily sludge

[0039] Oily sludge is pumped into oily sludge conditioning tank 1 by a sludge pump. A conditioning agent is added to oily sludge conditioning tank 1 for conditioning and centrifugal dewatering. The conditioning agent is anhydrous sodium carbonate or anhydrous sodium bicarbonate in a mass ratio of 1:(1.2-1.8) and cationic polyacrylamide flocculant with a weight average molecular weight of 8000-120000 Daltons. The total amount added is 0.1-0.5% of the mass of oily sludge. After standing for 6 hours, the oily sludge in oily sludge conditioning tank 1 is acidified with an acidifying agent, such as acetic acid, citric acid, malic acid or other green, environmentally friendly and biodegradable organic acids, and the pH is adjusted to 3-4. The sludge is then filtered in a plate and frame filter press 2. After filtration, the wastewater is sent to a combined wastewater treatment plant. The filter cake and oily sludge cake are mixed and used as oily sludge to be treated.

[0040] Step 2: Supercritical stripping

[0041] The sludge to be treated is fed into the segmented supercritical fluid extraction device 4 via screw conveyor 3. The low-pressure carbon dioxide in the storage tank 7, with a pressure below 5 MPa, is compressed to 30 MPa by compressor 8, transforming it into supercritical CO2. During compression, the temperature of the carbon dioxide rises to 70-80℃. Heat exchange is then performed through heat exchanger 9, reducing the temperature of the supercritical CO2 to 60℃. Using supercritical CO2 as the stripping agent, the sludge falls from top to bottom due to gravity, while the supercritical carbon dioxide flows upwards and interacts with the sludge. After the reverse mixing reaction of the treated oily sludge, the oil phase of the sludge to be treated is stripped by supercritical fluid. The volume ratio of the stripping agent to the oily sludge to be treated is 1:(1-6), the reaction temperature is 60℃, the supercritical swelling pressure is 18-30MPa, and the swelling time is 2.5-3h. A mixture of stripped sludge, stripped CO2, and stripped oil phase is obtained. The oil content of the stripped sludge is 0.8-2%. The stripped sludge falls into the oily sludge storage tank at the bottom of the segmented supercritical fluid extraction device 4, and the oil content meets the sludge discharge standard requirements.

[0042] Step 3: CO2 recycling

[0043] The stripped CO2 and the stripped oil phase mixture first enter the first separation device 5 for preliminary separation, and then enter the second separation device 6 for deep separation. The pressure of the first separation device 5 is reduced to 5 MPa, and the pressure of the second separation device 6 is reduced to atmospheric pressure. The stripped CO2 is converted from a supercritical state to a gaseous state and discharged from the top of the first separation device 5 and the second separation device 6, and enters the gas storage tank 7 for recycling. At the same time, stripped oil is generated and discharged from the bottom of the first separation device 5 and the second separation device 6.

[0044] Example 1

[0045] Taking the oily sludge generated in the wastewater treatment section of an oilfield in northern Shaanxi as an example, its initial water content is 60%, oil content is 26%, and sludge content is 14%. A method for treating oily sludge based on supercritical carbon dioxide stripping includes:

[0046] Step 1: Conditioning and centrifuging of oily sludge

[0047] Oily sludge was pumped into oily sludge conditioning tank 1, and anhydrous sodium carbonate and cationic polyacrylamide with a weight average molecular weight of 10,000 Daltons were added to the conditioning tank 1 for conditioning and centrifugal dewatering. The mass ratio of anhydrous sodium carbonate to cationic polyacrylamide was 1:1.2, and the total amount added was 0.2% of the mass of oily sludge. After standing for 6 hours, the oily sludge in the conditioning tank 1 was acidified with an acidifying agent to adjust the pH to 3. It was then filtered in a plate and frame filter press 2. The resulting oily sludge cake had a water content of 27%, an oil content of 48%, and a mud content of 25%. The wastewater after filtration was sent to the wastewater treatment plant. The filter cake and oily sludge cake were mixed and used as oily sludge to be treated.

[0048] Step 2: Supercritical stripping

[0049] The sludge to be treated is fed into the segmented supercritical fluid extraction device 4 via a screw conveyor 3. Low-pressure carbon dioxide (below 5 MPa) in the gas storage tank 7 is compressed to 30 MPa by a compressor 8 to obtain supercritical CO2. During compression, the temperature of the carbon dioxide rises to 70°C. The temperature of the supercritical CO2 is then reduced to 60°C via a heat exchanger 9. Using supercritical CO2 as a stripping agent, the sludge falls from top to bottom due to gravity, while the supercritical carbon dioxide flows from bottom to top and mixes counter-currently with the sludge. Afterwards, the oil phase of the sludge to be treated is stripped using supercritical fluid. The volume ratio of the stripping agent to the sludge to be treated is 1:3, the reaction temperature is 60℃, the supercritical swelling pressure is 20MPa, and the swelling time is 2.5h. A mixture of stripped sludge, stripped CO2, and stripped oil phase is obtained. The water content of the stripped sludge is 23%, the oil content is 2%, and the sludge content is 75%. The stripped sludge falls into the oil sludge storage tank at the bottom of the segmented supercritical fluid extraction device 4, and the oil content meets the sludge discharge standard requirements.

[0050] Step 3: CO2 recycling

[0051] The stripped CO2 and the stripped oil phase mixture first enter the first separation device 5 for preliminary separation, and then enter the second separation device 6 for deep separation. The pressure of the first separation device 5 is reduced to 5 MPa, and the pressure of the second separation device 6 is reduced to atmospheric pressure. The stripped CO2 is converted from a supercritical state to a gaseous state and discharged from the top of the first separation device 5 and the second separation device 6, and enters the gas storage tank 7 for recycling. At the same time, stripped oil is generated and discharged from the bottom of the first separation device 5 and the second separation device 6.

[0052] Example 2

[0053] Taking the oily sludge generated in the wastewater treatment section of an oilfield in northern Shaanxi as an example, its initial water content is 60%, oil content is 26%, and sludge content is 14%. A method for treating oily sludge based on supercritical carbon dioxide stripping includes:

[0054] Step 1: Conditioning and centrifuging of oily sludge

[0055] Oily sludge was pumped into oily sludge conditioning tank 1, and anhydrous sodium carbonate and cationic polyacrylamide with a weight average molecular weight of 10,000 Daltons were added to the conditioning tank 1 for conditioning and centrifugal dewatering. The mass ratio of anhydrous sodium carbonate to cationic polyacrylamide was 1:1.2, and the total amount added was 0.2% of the mass of oily sludge. After standing for 6 hours, the oily sludge in the conditioning tank 1 was acidified with an acidifying agent to adjust the pH to 3. It was then filtered in a plate and frame filter press 2. The resulting oily sludge cake had a water content of 27%, an oil content of 48%, and a mud content of 25%. The wastewater after filtration was sent to the wastewater treatment plant. The filter cake and oily sludge cake were mixed and used as oily sludge to be treated.

[0056] Step 2: Supercritical stripping

[0057] The oil sludge to be treated is fed into the segmented supercritical fluid extraction device 4 via a screw conveyor 3. Low-pressure carbon dioxide (pressure below 5 MPa) in the storage tank 7 is compressed to 30 MPa by a compressor 8 to obtain supercritical CO2. During compression, the temperature of the carbon dioxide rises to 70°C. The temperature of the supercritical CO2 is then reduced to 60°C via a heat exchanger 9. Using supercritical CO2 as a stripping agent, the oil sludge falls from top to bottom under gravity, while supercritical carbon dioxide flows from bottom to top and mixes with the oil sludge in a counter-current reaction. The oil phase of the oil sludge is then stripped using supercritical fluid extraction. The volume ratio of the stripping agent to the oil sludge is 1:1, the reaction temperature is 60°C, the supercritical swelling pressure is 20 MPa, and the swelling time is 2.5 h. This yields a mixture of stripped sludge, stripped CO2, and stripped oil phase. The oil content of the stripped sludge is 1.1%. The stripped sludge falls into the oil sludge storage tank at the bottom of the segmented supercritical fluid extraction device 4, and the oil content meets the sludge discharge standards.

[0058] Step 3: CO2 recycling

[0059] The stripped CO2 and the stripped oil phase mixture first enter the first separation device 5 for preliminary separation, and then enter the second separation device 6 for deep separation. The pressure of the first separation device 5 is reduced to 5 MPa, and the pressure of the second separation device 6 is reduced to atmospheric pressure. The stripped CO2 is converted from a supercritical state to a gaseous state and discharged from the top of the first separation device 5 and the second separation device 6, and enters the gas storage tank 7 for recycling. At the same time, stripped oil is generated and discharged from the bottom of the first separation device 5 and the second separation device 6.

[0060] Example 3

[0061] Taking the oily sludge generated in the wastewater treatment section of an oilfield in northern Shaanxi as an example, its initial water content is 60%, oil content is 26%, and sludge content is 14%. A method for treating oily sludge based on supercritical carbon dioxide stripping includes:

[0062] Step 1: Conditioning and centrifuging of oily sludge

[0063] Oily sludge was pumped into oily sludge conditioning tank 1, and anhydrous sodium carbonate and cationic polyacrylamide with a weight average molecular weight of 10,000 Daltons were added to the conditioning tank 1 for conditioning and centrifugal dewatering. The mass ratio of anhydrous sodium carbonate to cationic polyacrylamide was 1:1.2, and the total amount added was 0.2% of the mass of oily sludge. After standing for 6 hours, the oily sludge in the conditioning tank 1 was acidified with an acidifying agent to adjust the pH to 3. It was then filtered in a plate and frame filter press 2. The resulting oily sludge cake had a water content of 27%, an oil content of 48%, and a mud content of 25%. The wastewater after filtration was sent to the wastewater treatment plant. The filter cake and oily sludge cake were mixed and used as oily sludge to be treated.

[0064] Step 2: Supercritical stripping

[0065] The oil sludge to be treated is fed into the segmented supercritical fluid extraction device 4 via a screw conveyor 3. Low-pressure carbon dioxide (pressure below 5 MPa) in the storage tank 7 is compressed to 30 MPa by a compressor 8 to obtain supercritical CO2. During compression, the temperature of the carbon dioxide rises to 70°C. The temperature of the supercritical CO2 is then reduced to 60°C via a heat exchanger 9. Using supercritical CO2 as a stripping agent, the oil sludge falls from top to bottom under gravity, while supercritical carbon dioxide flows from bottom to top and mixes with the oil sludge in a counter-current reaction. The oil phase of the oil sludge is then stripped using supercritical fluid extraction. The volume ratio of the stripping agent to the oil sludge is 1:1, the reaction temperature is 60°C, the supercritical swelling pressure is 18 MPa, and the swelling time is 3 hours. This yields a mixture of stripped sludge, stripped CO2, and stripped oil phase. The oil content of the stripped sludge is 0.8%. The stripped sludge falls into the oil sludge storage tank at the bottom of the segmented supercritical fluid extraction device 4, and the oil content meets the sludge discharge standards.

[0066] Step 3: CO2 recycling

[0067] The stripped CO2 and the stripped oil phase mixture first enter the first separation device 5 for preliminary separation, and then enter the second separation device 6 for deep separation. The pressure of the first separation device 5 is reduced to 5 MPa, and the pressure of the second separation device 6 is reduced to atmospheric pressure. The stripped CO2 is converted from a supercritical state to a gaseous state and discharged from the top of the first separation device 5 and the second separation device 6, and enters the gas storage tank 7 for recycling. At the same time, stripped oil is generated and discharged from the bottom of the first separation device 5 and the second separation device 6.

[0068] Example 4

[0069] Step 1: Conditioning and centrifuging of oily sludge

[0070] Oily sludge is pumped into oily sludge conditioning tank 1 by a sludge pump. Anhydrous sodium carbonate and cationic polyacrylamide with a weight average molecular weight of 8000 Daltons are added to oily sludge conditioning tank 1 for conditioning and centrifugal dewatering. The mass ratio of anhydrous sodium carbonate to cationic polyacrylamide is 1:1.2, and the total amount added is 0.1% of the mass of oily sludge. After standing for 6 hours, the oily sludge in oily sludge conditioning tank 1 is acidified with an acidifying agent to adjust the pH to 3. It is then put into plate and frame filter press 2 for filtration. The wastewater after filtration is sent to the wastewater treatment plant. The filter cake and oily sludge cake are mixed and used as oily sludge to be treated.

[0071] Step 2: Supercritical stripping

[0072] The oil sludge to be treated is fed into the segmented supercritical fluid extraction device 4 via a screw conveyor 3. Low-pressure carbon dioxide (pressure below 5 MPa) in the storage tank 7 is compressed to 30 MPa by a compressor 8 to obtain supercritical CO2. During compression, the temperature of the carbon dioxide rises to 70°C. The temperature of the supercritical CO2 is then reduced to 60°C via a heat exchanger 9. Using supercritical CO2 as a stripping agent, the oil sludge falls from top to bottom under gravity, while supercritical carbon dioxide flows from bottom to top and mixes with the oil sludge in a counter-current reaction. The oil phase of the oil sludge is then stripped using supercritical fluid extraction. The volume ratio of the stripping agent to the oil sludge is 1:1, the reaction temperature is 60°C, the supercritical swelling pressure is 18 MPa, and the swelling time is 2.5 h. This yields a mixture of stripped sludge, stripped CO2, and stripped oil phase. The oil content of the stripped sludge is 0.8%. The stripped sludge falls into the oil sludge storage tank at the bottom of the segmented supercritical fluid extraction device 4, and the oil content meets the sludge discharge standards.

[0073] Step 3: CO2 recycling

[0074] The stripped CO2 and the stripped oil phase mixture first enter the first separation device 5 for preliminary separation, and then enter the second separation device 6 for deep separation. The pressure of the first separation device 5 is reduced to 5 MPa, and the pressure of the second separation device 6 is reduced to atmospheric pressure. The stripped CO2 is converted from a supercritical state to a gaseous state and discharged from the top of the first separation device 5 and the second separation device 6, and enters the gas storage tank 7 for recycling. At the same time, stripped oil is generated and discharged from the bottom of the first separation device 5 and the second separation device 6.

[0075] Example 5

[0076] Step 1: Conditioning and centrifuging of oily sludge

[0077] Oily sludge is pumped into oily sludge conditioning tank 1 by a sludge pump. Anhydrous sodium carbonate and cationic polyacrylamide with a weight average molecular weight of 120,000 Daltons are added to oily sludge conditioning tank 1 for conditioning and centrifugal dewatering. The mass ratio of anhydrous sodium carbonate to cationic polyacrylamide is 1:1.8, and the total amount added is 0.5% of the mass of oily sludge. After standing for 6 hours, the oily sludge in oily sludge conditioning tank 1 is acidified with an acidifying agent to adjust the pH to 4. It is then put into plate and frame filter press 2 for filtration. The wastewater after filtration is sent to the wastewater treatment plant. The filter cake and oily sludge cake are mixed and used as oily sludge to be treated.

[0078] Step 2: Supercritical stripping

[0079] The oil sludge to be treated is fed into the segmented supercritical fluid extraction device 4 via a screw conveyor 3. Low-pressure carbon dioxide (pressure below 5 MPa) in the storage tank 7 is compressed to 30 MPa by a compressor 8 to obtain supercritical CO2. During compression, the temperature of the carbon dioxide rises to 80°C. The temperature of the supercritical CO2 is then reduced to 60°C via a heat exchanger 9. Using supercritical CO2 as a stripping agent, the oil sludge falls from top to bottom due to gravity, while supercritical carbon dioxide flows from bottom to top and mixes with the oil sludge in a counter-current reaction. The oil phase of the oil sludge is then subjected to supercritical fluid stripping. The volume ratio of the stripping agent to the oil sludge is 1:6, the reaction temperature is 60°C, the supercritical swelling pressure is 30 MPa, and the swelling time is 3 hours. This yields a mixture of stripped sludge, stripped CO2, and stripped oil phase. The oil content of the stripped sludge is 2%. The stripped sludge falls into the oil sludge storage tank at the bottom of the segmented supercritical fluid extraction device 4, and the oil content meets the sludge discharge standard requirements.

[0080] Step 3: CO2 recycling

[0081] The stripped CO2 and the stripped oil phase mixture first enter the first separation device 5 for preliminary separation, and then enter the second separation device 6 for deep separation. The pressure of the first separation device 5 is reduced to 5 MPa, and the pressure of the second separation device 6 is reduced to atmospheric pressure. The stripped CO2 is converted from a supercritical state to a gaseous state and discharged from the top of the first separation device 5 and the second separation device 6, and enters the gas storage tank 7 for recycling. At the same time, stripped oil is generated and discharged from the bottom of the first separation device 5 and the second separation device 6.

[0082] As is known from common technical knowledge, this invention can be implemented through other embodiments that do not depart from its spirit or essential characteristics. Therefore, the disclosed embodiments described above are merely illustrative in all respects and are not the only ones. All modifications within the scope of this invention or its equivalents are included in this invention.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for treating oily sludge based on supercritical carbon dioxide stripping, characterized by, Comprise: Step 1: oil sludge conditioning centrifugation The oil sludge is injected into the oil sludge conditioning tank (1), and a conditioning agent is added to the oil sludge conditioning tank (1) for conditioning centrifugal dewatering. The oil sludge in the oil sludge conditioning tank (1) is acidified with an acidifying agent to make the sludge porous. The sludge is then fed into a plate and frame filter press (2) for pressure filtration. The filtered sludge is mixed with the oil sludge cake and then treated as the sludge to be treated. Step 2: supercritical stripping The sludge to be treated is fed into a segmented supercritical fluid extraction device (4) by a screw conveyor (3). The low-pressure carbon dioxide in the gas storage tank (7) is compressed into supercritical CO2 by a compressor (8) and then heated by a heat exchanger (9). The supercritical CO2 is used as a stripping agent to strip the oil phase of the sludge to be treated. The stripped sludge, stripped CO2, and stripped oil phase mixture are obtained. The stripped sludge falls into the sludge storage tank at the bottom of the segmented supercritical fluid extraction device (4). Step 3: CO2 separation and recycling The stripped CO2 and stripped oil phase mixture are first separated in the first separation device (5) and then deeply separated in the second separation device (6). The stripped CO2 is converted from a supercritical state to a gaseous state and discharged from the top of the first and second separation devices (5 and 6) for recycling. The CO2-removed oil phase is discharged from the bottom of the first and second separation devices (5 and 6). In step 2, the acidifying agent is acetic acid, citric acid, malic acid, or other biodegradable organic acids. The acidification process adjusts the pH to 3-4. In step 2, the volume ratio of the stripping agent to the sludge to be treated is 1: (1-6).

2. The method for treating oily sludge based on supercritical carbon dioxide stripping according to claim 1, characterized in that, In step 1, the conditioning agent is anhydrous sodium carbonate or anhydrous sodium bicarbonate and a cationic polyacrylamide flocculant. The total amount of the conditioning agent added is 0.1-0.5% of the mass of the oil sludge.

3. A method for the treatment of oily sludge based on supercritical carbon dioxide stripping according to claim 2, characterized in that, The mass ratio of the anhydrous sodium carbonate or anhydrous sodium bicarbonate to the cationic polyacrylamide flocculant is 1: (1.2-1.8).

4. The method for treating oily sludge based on supercritical carbon dioxide stripping according to claim 2, characterized in that, The cationic polyacrylamide flocculant has a weight average molecular weight of 8000-120000 Daltons.

5. The method for treating oily sludge based on supercritical carbon dioxide stripping according to claim 1, characterized in that, In step 2, the low-pressure carbon dioxide in the gas storage tank (7) is compressed into supercritical CO2 by a compressor (8) and then heated by a heat exchanger (9). Specifically, the low-pressure carbon dioxide with a pressure lower than 5 MPa in the gas storage tank (7) is compressed to 30 MPa by the compressor (8). The temperature of the carbon dioxide rises to 70-80°C during the compression process. The supercritical CO2 is then cooled to 60°C by the heat exchanger (9).

6. The method for treating oily sludge based on supercritical carbon dioxide stripping according to claim 1, characterized in that, In step 2, the reaction temperature for supercritical fluid stripping of the oil phase of the sludge to be treated is 60°C, the supercritical swelling pressure is 18-30 MPa, and the swelling time is 2.5-3 hours.

7. The method for treating oily sludge based on supercritical carbon dioxide stripping according to claim 1, characterized in that, In step 2, the oil content of the stripped sludge is 0.8-2%.

8. The method for treating oily sludge based on supercritical carbon dioxide stripping according to claim 1, characterized in that, After the stripped CO2 and the stripped oil phase mixture described in Step 3 enter the first separation device (5) and the second separation device (6), the pressure of the first separation device (5) is reduced to 5 MPa, and the pressure of the second separation device (6) is reduced to atmospheric pressure.

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