Integrated fracturing flowback fluid brine resourceization process

Abstract

The integrated fracturing flowback fluid brine resourceization process belongs to the technical field of fracturing flowback fluid treatment and high-salinity wastewater resourceization, and comprises the following steps: modifying nano-Fe3O4 by a silane coupling agent, and grafting the modified nano-Fe3O4 with polyacrylamide to prepare a composite flocculation material; making fracturing flowback fluid sequentially pass through oil removal by air flotation, advanced oxidation gel breaking, and then adding the flocculation material and an alkali agent for softening and precipitation, and obtaining a non-ultrafiltration-dependent pretreated clear liquid by multi-medium and precision filtration; adjusting the pH value of the clear liquid and adding a scale inhibitor, and then making the clear liquid pass through a multi-stage nanofiltration system to separate salts and intercept impurities, and the permeate is a dilute sodium chloride solution; the dilute liquid is concentrated by reverse osmosis at high pressure, clean water is recovered, and high-concentration sodium chloride brine is sent into a mechanical steam recompression evaporation crystallization system, and industrial-grade sodium chloride products are prepared by centrifugal drying, the present application avoids the membrane fouling problem that easily occurs when an ultrafiltration membrane is used to treat fracturing flowback fluid with high viscosity and high organic matter, and also simplifies the system configuration and improves the operation stability in the pretreatment stage.

Description

Technical Field

[0001] This invention relates to the field of fracturing flowback fluid treatment and high-salt wastewater resource utilization, specifically an integrated fracturing flowback fluid brine resource utilization process. Background Technology

[0002] Fracturing flowback fluid treatment is a crucial link in oil and gas environmental protection and water recycling. The operational status and hydrodynamic stability of the process directly affect the resource conversion and overall treatment effect of the fluid. The treatment system mainly includes pretreatment, membrane separation, and evaporation crystallization units. Impurities are removed and salts are enriched through the flow of fluid between various devices. Optimizing the treatment path of high-salt fluids, improving crystallization purity, and reducing energy consumption have become key research directions in the environmental protection field. Existing technologies mainly rely on conventional flocculation combined with ultrafiltration and direct evaporation processes, which cannot effectively degrade residual polymers and emulsified oils and are difficult to adapt to complex matrix environments with high salt and high organic matter. At the same time, the lack of a precise salt separation and retention mechanism causes a large number of scaling ions to directly reach the evaporator, resulting in severe scaling, and cannot effectively cope with the membrane fouling and heat transfer attenuation caused by drastic fluctuations in water quality.

[0003] Furthermore, traditional processes exhibit strong irreversible fouling characteristics when handling complex fluids. Existing technologies struggle to maintain overall hydrodynamic stability, leading to severe declines in desalination rates and water flux, ultimately resulting in system failure and the generation of hazardous solid waste that is difficult to manage. Therefore, a solution is urgently needed to address the problems existing in current technologies. Summary of the Invention

[0004] The purpose of this invention is to provide an integrated fracturing flowback brine resource recovery process to solve the problems mentioned in the background art.

[0005] The technical solution of the present invention includes: S1, ultrasonically dispersing nano-ferric oxide in deionized water, adding a silane coupling agent for modification, then adding acrylamide monomer and a free radical initiator, and mechanically stirring the reaction in a constant temperature water bath at 60-80°C for 1-2 hours to allow the surface-modified nano-ferric oxide to undergo a chemical grafting reaction with polyacrylamide to form a composite structure, and then centrifuging, washing and vacuum drying at 50-60°C to obtain a modified composite flocculant material;

[0006] S2. The fracturing flowback fluid is sequentially subjected to air flotation for oil removal, advanced oxidation to break up the gel, and softening and settling. During oxidation to break up the gel, hydrogen peroxide and ferrous sulfate are added to adjust the pH to 3.0-4.0. During softening and settling, a combination of sodium hydroxide and sodium carbonate is added to degas the fluid in stages and adjust the pH to 10.0-11.0. Then, the modified composite flocculant is added for flocculation and sedimentation. Subsequently, the fluid is sequentially filtered through multi-media filtration and precision filtration to obtain pretreated clear liquid. The pretreatment process does not include an ultrafiltration step.

[0007] S3. Add scale inhibitor to the pretreated clear liquid and adjust the pH value to 6.5-7.5, then pressurize to 1.5-2.5MPa and pump it into a multi-stage nanofiltration system for separation. The retentate is concentrated water rich in divalent ions, polyvalent ions and residual organic matter, and the permeate is a dilute sodium chloride solution.

[0008] S4. Pressurize the dilute sodium chloride solution to 4.0-8.0 MPa and introduce it into the reverse osmosis system for high-pressure concentration treatment. Concentrate the dilute sodium chloride solution to a total dissolved solids mass concentration of 60-80 g / L. Control the reverse osmosis water recovery rate at 70%-85%. The reverse osmosis permeate is reused or discharged as clean water, and the reverse osmosis retentate is a high-concentration sodium chloride brine.

[0009] S5. The high-concentration sodium chloride brine is fed into a mechanical vapor recompression evaporation crystallization system for gradient temperature evaporation crystallization. The generated secondary condensate undergoes liquid-liquid heat exchange with the high-concentration sodium chloride brine entering the system. After crystallization, the product is dehydrated by centrifugation and dried in a fluidized bed to obtain industrial-grade sodium chloride.

[0010] Optionally, in step S1, the mass ratio of nano-ferric oxide, the silane coupling agent, and the polyacrylamide is 10:1:3 to 10:2:5, the silane coupling agent is 3-aminopropyltriethoxysilane or 3-methacryloyloxypropyltrimethoxysilane, the free radical initiator is potassium persulfate or ammonium persulfate, and its added mass is 1%-5% of the mass of the polyacrylamide. The liquid-solid ratio of deionized water to nano-ferric oxide is 20-30 mL / g.

[0011] Optionally, the advanced oxidative depolymerization in step S2 is performed using Fenton oxidation treatment. The specific process is as follows: hydrogen peroxide and ferrous sulfate are added to the water after dissolved air flotation oil removal, the pH value is adjusted to 3.0-4.0, and the reaction time is 1.5-2.5 hours to degrade the long-chain polymer into small molecules.

[0012] Optionally, in step S2, the total dissolved solids concentration of the fracturing flowback fluid is 10,000-40,000 mg / L; and in the dissolved air flotation oil removal process, a demulsifier is added to remove surface oil and suspended solids.

[0013] Optionally, in step S2, the alkali agent is a combination of sodium hydroxide and sodium carbonate, and the dosage of the modified composite flocculant is 10-30 mg / L; the multi-media filtration uses quartz sand and activated carbon as the filter media, the precision filtration uses a security filter element with a pore size of 3-5 μm, and the turbidity of the pretreated clear liquid is less than 0.5 NTU.

[0014] Optionally, in step S3, the multi-stage nanofiltration system uses a fouling-resistant wide-channel nanofiltration membrane element, and the multi-stage nanofiltration system has a rejection rate of ≥98% for divalent sulfate ions and a permeability of ≥85% for monovalent chloride ions.

[0015] Optionally, in step S4, the reverse osmosis system uses a seawater desalination reverse osmosis membrane or a disc tube reverse osmosis membrane.

[0016] Optionally, in step S5, the temperature of the secondary condensate is 90-95°C, and the high-concentration sodium chloride brine is preheated to 80-85°C after the liquid-liquid heat exchange before entering the mechanical vapor recompression evaporation crystallization system; after separation, dehydration and drying steps, an industrial-grade sodium chloride product with a dry basis mass fraction of ≥97.5% is obtained.

[0017] This invention provides an integrated process for the resource recovery of fracturing flowback brine, which has the following improvements and advantages compared with the prior art:

[0018] 1. This invention significantly improves the removal efficiency of suspended solids and hardness ions in complex fracturing flowback fluids by preparing a modified composite flocculant formed by chemical grafting nano-ferric oxide and polyacrylamide. The process utilizes the synergistic effect of advanced oxidation breaking and efficient flocculation sedimentation to reduce the turbidity of the pretreated solution to below 0.5 NTU, successfully achieving a non-ultrafiltration-dependent process design. This not only avoids the membrane fouling problem that easily occurs when ultrafiltration membranes are used to treat high-viscosity, high-organic fracturing flowback fluids, but also simplifies system configuration and improves the operational stability of the pretreatment stage.

[0019] 2. The process of this invention uses a multi-stage nanofiltration system with a fouling-resistant wide-channel nanofiltration membrane. By precisely controlling the operating pressure and pH value, it achieves a high proportion of retention of divalent and multivalent ions and residual organic matter, while maintaining a high permeability of monovalent chloride ions. This precise separation technology ensures that the dilute sodium chloride solution entering the subsequent stages has extremely high purity, laying the foundation for the final production of high-quality industrial-grade salt products and avoiding the impact of impurity ions on product quality during the concentration and crystallization stage.

[0020] 3. The system integrates reverse osmosis high-pressure concentration and mechanical vapor recompression evaporation crystallization technology, which significantly concentrates sodium chloride brine to 60-80 g / L with a water recovery rate of 70%-85%. By utilizing the secondary condensate generated by the evaporation system to conduct liquid-liquid heat exchange with the feed brine, the liquid to be treated is preheated to 80-85℃, realizing the cascade recovery and effective circulation of internal heat energy. The final industrial-grade sodium chloride product has a dry basis mass fraction of ≥97.5%, realizing the resource utilization transformation of fracturing flowback fluid from high-risk wastewater to high-purity industrial salt and reusable clean water. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0022] Example 1:

[0023] An integrated process for the resource utilization of fracturing flowback brine specifically includes the following steps:

[0024] S1. Nano-iron oxide is ultrasonically dispersed in deionized water, modified with silane coupling agent, and then acrylamide monomer and potassium persulfate are added as free radical initiators. The mixture is mechanically stirred in a constant temperature water bath at 60°C for 1 hour to allow the surface-modified nano-iron oxide to undergo a chemical grafting reaction with polyacrylamide to form a composite structure. After centrifugation and washing and vacuum drying at 50°C, the modified composite flocculant is obtained.

[0025] S2. The fracturing flowback fluid is sequentially subjected to air flotation for oil removal, advanced oxidation degelatinization, and softening precipitation. The advanced oxidation degelatinization is carried out by adding hydrogen peroxide as an oxidant and ferrous sulfate as a catalyst, and adjusting the pH value to 3.5. The chemical softening precipitation treatment is carried out by first adding an alkaline agent composed of sodium hydroxide and sodium carbonate for stepwise neutralization and degassing, and then adjusting the pH value to 10.0. Subsequently, the modified composite flocculant prepared in step S1 is added for flocculation and precipitation. The pretreated clear liquid is obtained by sequentially passing through multi-media filtration and precision filtration. The pretreatment process does not include an ultrafiltration step.

[0026] S3. After adding scale inhibitor to the pretreatment solution and adjusting the pH to 6.5, pressurize it to 1.5MPa with a high-pressure pump and pump it into a multi-stage nanofiltration system for separation. The retentate is concentrated water rich in divalent ions, polyvalent ions and residual organic matter, and the permeate is a dilute sodium chloride solution.

[0027] S4. The dilute sodium chloride solution is pressurized to 4.0 MPa by a high-pressure pump and introduced into the reverse osmosis system for high-pressure concentration treatment. The dilute sodium chloride solution is concentrated to a total dissolved solids mass concentration of 60 g / L. The reverse osmosis water recovery rate is controlled at 70%. The reverse osmosis permeate is reused or discharged as clean water, and the reverse osmosis retentate is high-concentration sodium chloride brine.

[0028] S5. High-concentration sodium chloride brine is fed into a mechanical vapor recompression evaporation crystallization system for gradient temperature evaporation crystallization. During this process, the secondary condensate generated is exchanged with the high-concentration sodium chloride brine entering the system through liquid-liquid heat exchange. After crystallization, the product is obtained by centrifugal dehydration and fluidized bed drying to obtain industrial-grade sodium chloride.

[0029] This embodiment reconstructs the treatment path of high-salinity complex fluids through a physicochemical coupling mechanism; the preparation process of modified composite flocculant introduces ultrasonic cavitation effect to destroy the agglomeration state of nanoparticles; silane coupling agent constructs active reaction sites in situ on the particle surface; and a constant temperature water bath of 60°C is set to ensure that the appropriate stretching of polyacrylamide molecular chains matches the kinetics of the grafting reaction. This temperature index indicates that the chemical grafting reaction has reached the initial activation energy threshold, thus avoiding the thermal degradation of polymer chains caused by high temperature.

[0030] The non-ultrafiltration-dependent pretreatment optimizes the water matrix through the synergistic effect of chemical targeted flocculation and strong oxidative degradation; the pH value is precisely controlled to 10.0 during the softening and sedimentation stage, so that the concentration of scale-forming alkaline earth metal ions exceeds the solubility product constant and precipitates out; the multi-stage nanofiltration desalination utilizes the Donnan effect on the membrane surface to repel polyvalent anions, and sets the influent pressure of 1.5MPa to ensure the hydrodynamic balance between solvent permeation flux and concentration polarization effect. The retentate is enriched with scale precursors, while the permeate is converted into a high-purity monovalent salt system.

[0031] The reverse osmosis volume reduction concentration operates at an operating pressure of 4.0 MPa, which significantly compresses the fluid volume and increases the total dissolved solids mass concentration to 60 g / L. The setting of this concentration gradient greatly reduces the energy consumption base of subsequent thermodynamic phase change. The resource-based evaporation and crystallization stage uses secondary condensate for liquid-liquid heat exchange, realizing the cascade utilization of the internal enthalpy and the high degree of self-consistency of energy flow.

[0032] Example 2:

[0033] In step S1, the mass ratio of nano-ferric oxide, silane coupling agent, and polyacrylamide is 10:1:3, the silane coupling agent is 3-aminopropyltriethoxysilane, and the volume of deionized water is 20 mL / g of the mass of nano-ferric oxide. In step S2, the advanced oxidative depolymerization is carried out using Fenton oxidation treatment. The specific process is as follows: hydrogen peroxide and ferrous sulfate are added to the water after dissolved air flotation oil removal, the pH value is adjusted to 3.0, the reaction time is 1.5 hours, and the long-chain polymer is degraded into small molecules.

[0034] In this embodiment, the stoichiometric ratio of nano-ferric oxide, silane coupling agent, and polyacrylamide of 10:1:3 ensures sufficient coverage of amino functional groups on the surface of the magnetic core, while controlling the thickness of the polymer shell to avoid steric hindrance that could hinder subsequent flocculation and trapping behavior. 3-aminopropyltriethoxysilane, as a bridging molecule, undergoes condensation with the hydroxyl groups on the surface of the iron oxide after alkoxy hydrolysis, while the amino terminus provides a covalent anchoring point for the polymer chain.

[0035] Advanced oxidative depolymerization employs Fenton oxidation treatment, strictly anchoring the system pH at 3.0. This acidic environment is the critical threshold for maintaining a high yield of hydroxyl radicals. After a 1.5-hour chain oxidation reaction, the recalcitrant long-chain polymer carbon skeleton in the fracturing flowback fluid undergoes chain scission, and the macromolecular three-dimensional network disintegrates into low-molecular-weight fragments. This structural remodeling process eliminates the material basis for irreversible organic fouling of subsequent membrane separation components.

[0036] Example 3:

[0037] In step S2, the total dissolved solids concentration of the fracturing flowback fluid is 10000 mg / L; during the dissolved air flotation oil removal process, a demulsifier is added to remove surface floating oil and suspended solids; the alkali agent is a combination of sodium hydroxide and sodium carbonate, and the dosage of the modified composite flocculant is 10 mg / L; multi-media filtration uses quartz sand and activated carbon as filter media, and precision filtration uses a security filter element with a pore size of 3 μm, resulting in a pretreated clear liquid with a turbidity of less than 0.5 NTU;

[0038] In this embodiment, the introduction of a demulsifier in dissolved air flotation for oil removal can disrupt the stability of the double electric layer at the oil-water interface, promoting the aggregation and floating of fine oil droplets. Since the system has just undergone strong acid Fenton oxidation, the use of a step-by-step addition of alkali agent combined with degassing can effectively remove free carbon dioxide from the water. Specifically, 50% of the total amount of alkali agent is added first and degassed at low speed for 10-15 minutes. After the release of bubbles is gradual, the remaining 50% of the alkali agent is added dropwise at a uniform speed, avoiding ineffective consumption of alkali agent and buffering the violent exothermic acid-base reaction. Subsequently, a combination of sodium hydroxide and sodium carbonate is used as the alkali agent to synergistically precipitate calcium and magnesium hardness using the common ion effect.

[0039] The addition of a low dose of 10 mg / L of modified composite flocculant, with its dense magnetic polymer network, generates strong charge neutralization and netting / sweeping effects, rapidly encapsulating tiny precipitates to form dense flocs. Multi-media filtration and precision filtration with 3 μm pore size form the final physical filtration layer, which stabilizes the turbidity of the pretreated effluent below 0.5 NTU. The non-ultrafiltration-dependent pretreatment architecture optimizes the material balance of the pretreatment system while ensuring the quality of the effluent.

[0040] Example 4:

[0041] In step S3, the multi-stage nanofiltration system uses anti-fouling wide-channel nanofiltration membrane elements. The multi-stage nanofiltration system has a rejection rate of ≥98% for divalent sulfate ions and a permeability of ≥85% for monovalent chloride ions. In step S4, the reverse osmosis system uses seawater desalination reverse osmosis membranes. In step S5, the temperature of the secondary condensate is 90℃, and the high-concentration sodium chloride brine is preheated to 80℃ after liquid-liquid heat exchange before entering the mechanical steam recompression evaporation crystallization system. The dry basis mass fraction of the separated industrial-grade sodium chloride product is ≥97.5%.

[0042] The multi-stage nanofiltration system uses anti-fouling wide-channel nanofiltration membrane elements to mitigate the risk of localized supersaturation crystallization on the inorganic salt surface caused by concentration polarization. Its high rejection rate of 98% for divalent sulfate ions and 85% permeability for monovalent chloride ions verify the precise targeted screening capability of the component in complex saline lake fluids, successfully decoupling the mixed salt system. The reverse osmosis system uses high-pressure resistant seawater desalination reverse osmosis membranes to cope with the thermodynamic resistance of the rapid increase in fluid osmotic pressure during the concentration process.

[0043] The 90°C secondary condensate discharged from the mechanical vapor recompression evaporation crystallization system contains a large amount of latent heat. Through liquid-liquid heat exchange, the high-concentration sodium chloride brine is preheated to 80°C, effectively overcoming the thermal hysteresis effect range of cold materials entering the evaporator and reducing the shaft power consumption of the compressor. The final precipitated industrial-grade sodium chloride product meets the dry basis mass fraction standard, avoiding the production of hazardous solid waste and realizing a closed-loop resource utilization.

[0044] Example 5:

[0045] This embodiment focuses on parameter optimization and verification under conventional medium-load conditions. During the preparation of the modified composite flocculant, the constant temperature water bath temperature was increased to 70°C, the vacuum drying temperature was set to 55°C, the mechanical stirring reaction time was extended to 1.5 hours, the mass ratio of nano-ferric oxide, silane coupling agent and polyacrylamide was adjusted to 10:1.5:4, the volume of deionized water was set to 25 mL / g, and the mass of free radical initiator added was set to 4% of the mass of polyacrylamide. This ratio strengthens the bonding strength of the organic-inorganic interface.

[0046] The total dissolved solids (TDS) concentration of the fracturing flowback fluid was set at 35,000 mg / L with moderate salinity; the pH of the advanced oxidation breaker was adjusted to 3.5, and the reaction time was 2.0 hours; the pH of the chemical softening and precipitation treatment was set to 10.5, and the dosage of the modified composite flocculant was increased to 20 mg / L; a 4 μm pore size was used for precision filtration; the pH of the multi-stage nanofiltration desalination feed water was adjusted to 7.0, and the operating pressure was increased to 2.0 MPa to overcome the increased osmotic pressure; the reverse osmosis reduction and concentration operation pressure was set to 6.0 MPa, and the concentration was achieved to a TDS concentration of 70 g / L with a water recovery rate of 75%.

[0047] During the evaporation and crystallization stage, the secondary condensate temperature is 93℃, and the feed is preheated to 83℃. This system has corresponding process adaptability, hydrodynamic stability, and robustness under conventional formulations when dealing with typical medium-concentration fracturing flowback fluids.

[0048] Specifically, compared to the 60°C water bath temperature in Example 1, the 70°C condition in this example increased the grafting rate of the modified composite flocculant by approximately 18% and accelerated the flocculation and sedimentation rate by 25%. The operating pressure of the multi-stage nanofiltration desalination was increased to 2.0 MPa, compared to 1.5 MPa. This effectively maintained the membrane flux despite the increased influent osmotic pressure, while ensuring that the rejection rate of divalent ions remained stable at over 98%, achieving an excellent balance between separation accuracy and water flux.

[0049] Comparative Example 1:

[0050] This comparative example uses the industry's traditional ultrafiltration plus reverse osmosis plus mechanical vapor recompression direct evaporation process to treat fracturing flowback fluid with a total dissolved solids concentration of 35,000 mg / L. In the pretreatment stage, only conventional polyacrylamide is used for flocculation, without advanced oxidation to break up the gel, and then it directly enters the ultrafiltration membrane module. Because the long-chain polymers and emulsified oils remaining in the fracturing fluid are not effectively degraded, the micropores of the ultrafiltration membrane quickly undergo irreversible adsorption and fouling, and the transmembrane pressure difference exceeds the safety threshold within 48 hours of operation.

[0051] Meanwhile, due to the lack of a multi-stage nanofiltration salt separation and retention mechanism, the reverse osmosis concentrate contains a large number of divalent scaling ions and organic impurities that directly enter the evaporator, causing a dense scale layer to form rapidly on the surface of the heat exchange tubes, resulting in a significant decrease in the heat transfer coefficient; the final precipitated crystallization product is a dark mixed salt containing calcium sulfate, magnesium chloride and a large amount of organic matter, which deviates from the original intention of resource utilization.

[0052] Comparative Example 2:

[0053] This comparative example retains the multi-stage nanofiltration desalination and reverse osmosis concentration architecture in the overall process. However, in the non-ultrafiltration-dependent pretreatment stage, the modified composite flocculant specially made in this invention is not used. Instead, a conventional combination of commercially available ordinary polyaluminum chloride and nonionic polyacrylamide is used. The remaining operating parameters are consistent with those in Example 2.

[0054] Test results show that conventional flocculants are unable to form dense flocs with sufficient specific gravity and shear strength when faced with the extremely complex high-salt, high-organic matrix in fracturing flowback fluid. The floc penetration phenomenon in the sedimentation tank effluent is severe, leading to frequent clogging of the precision filtration components. Due to the failure to effectively capture fine suspended solids and colloids in the water, a large number of pollutants penetrate the pretreatment defense line and reach the multi-stage nanofiltration system, causing severe filter cake layer fouling and flow channel blockage on the nanofiltration membrane surface. The system desalination rate and water flux show irreversible decline after two weeks of operation, proving the core anchoring role of the proprietary modified composite flocculant material of this invention in maintaining the overall hydrodynamic stability of the process.

[0055] Verification experiment:

[0056] To comprehensively evaluate the overall performance of the integrated process of this invention, continuous and stable operation verification tests were conducted on the system operation status and product quality of Examples 1 to 5 and Comparative Examples 1 to 2. The tests were conducted on a pilot skid-mounted platform with a daily processing capacity of 50 m³, focusing on the antifouling performance of the core membrane module, the overall thermodynamic and kinetic operating indicators of the system, and the purity of the final crystallized salt.

[0057] Testing standards:

[0058] The purity of the crystallized salt is evaluated according to the secondary standard of sun-dried industrial salt in the national standard "Industrial Salt"; the turbidity of the water is determined according to the standard for the determination of water turbidity; the antifouling performance of the membrane system is quantitatively evaluated by monitoring the standardized permeate flow rate decay rate and the transmembrane pressure difference growth rate; the overall energy consumption of the system is calculated based on the standard coal equivalent of the electrical and thermal energy consumed per ton of raw water treated.

[0059] Specific testing process:

[0060] The parameters set for each embodiment and comparative example were input into the control system of the pilot-scale platform, and each operating condition was continuously run for 30 days. During this period, the turbidity of the pretreated clear liquid, the inlet and outlet water pressure, flow rate, and conductivity data of the nanofiltration and reverse osmosis systems were collected in real time through online instruments. Every 24 hours, manual samples were taken to test the ionic composition and chemical oxygen demand of the multi-stage nanofiltration permeate. The solid salt produced by the evaporation and crystallization system was dried in a fluidized bed, and 500 grams were randomly sampled daily and sent to an independent laboratory for dry basis mass fraction and impurity ion titration analysis. At the same time, the consumption of chemical reagents and the cumulative power consumption of the high-pressure pump and compressor under each operating condition were recorded to calculate the energy consumption and material consumption per ton of water.

[0061] Table 1 Performance test data of Examples 1-5 and Comparative Examples 1-2

[0062] Parameter Indicators Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Turbidity (NTU) of pretreated supernatant 0.42 0.38 0.45 0.40 0.41 3.50 1.85 Average monthly increase in nanofiltration membrane pressure differential (MPa) 0.015 0.018 0.025 0.016 0.021 - 0.150 Reverse osmosis water recovery rate (%) 70.0 75.2 85.0 72.5 80.4 55.0 68.0 Dry basis mass fraction of crystalline salt (%) 97.6 98.2 98.5 97.8 98.3 - 95.1 Comprehensive energy consumption per ton of water treated (kWh / m³) 15.2 16.8 20.5 16.0 18.5 38.5 19.2

[0063] Test data show that Examples 1 to 5 successfully avoided the risk of irreversible fouling of traditional ultrafiltration membranes through a cascade mechanism of non-ultrafiltration-dependent pretreatment and multi-stage nanofiltration for precise salt separation. The monthly average increase in nanofiltration membrane pressure differential was controlled at an extremely low level, demonstrating excellent hydrodynamic stability. Among them, Example 5 still produced industrial-grade sodium chloride with a purity of up to 98.3% when treating 35,000 mg / L medium salinity raw water, achieving efficient removal of complex scaling precursors.

[0064] In Comparative Example 1, due to the lack of pre-treatment strong oxidizing gel breaking and targeted salt separation, the membrane system became fouled, and the resulting mixed salt became hazardous solid waste, with a treatment energy consumption as high as 38.5 kWh / m³. Although Comparative Example 2 had a salt separation structure, the lack of a dedicated modified composite flocculant resulted in severely excessive pretreatment turbidity, leading to a surge in the pressure difference of the subsequent nanofiltration membrane and a sharp deterioration in the system's robustness. This invention, through in-situ construction and synergistic coupling using multi-dimensional technical means, successfully achieved the conversion of high-hazardous wastewater into high-purity crystalline salt, improving system stability.

[0065] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. An integrated process for the resource utilization of fracturing flowback brine, characterized in that, include: S1. Nano-iron oxide is ultrasonically dispersed in deionized water, modified with silane coupling agent, then acrylamide monomer and free radical initiator are added, and the mixture is mechanically stirred in a constant temperature water bath at 60-80℃ for 1-2 hours. After centrifugation and washing, and vacuum drying at 50-60℃, the modified composite flocculant is obtained. S2. The fracturing flowback fluid is sequentially subjected to air flotation for oil removal, advanced oxidation to break up the gel, and softening and settling. During oxidation to break up the gel, hydrogen peroxide and ferrous sulfate are added to adjust the pH to 3.0-4.

0. During softening and settling, a combination of sodium hydroxide and sodium carbonate is added to degas the fluid in stages and adjust the pH to 10.0-11.

0. Then, the modified composite flocculant is added for flocculation and sedimentation. The fluid is then sequentially filtered through multi-media filtration and precision filtration to obtain a pretreated clear liquid. The pretreatment process does not include an ultrafiltration step. S3. After adding scale inhibitor to the pretreated clear liquid and adjusting the pH value to 6.5-7.5, pressurize it to 1.5-2.5MPa and enter the multi-stage nanofiltration system for separation. The retentate is concentrated water rich in divalent ions, polyvalent ions and residual organic matter, and the permeate is a dilute sodium chloride solution. S4. Pressurize the dilute sodium chloride solution to 4.0-8.0 MPa and introduce it into the reverse osmosis system for high-pressure concentration treatment. Concentrate the dilute sodium chloride solution to a total dissolved solids mass concentration of 60-80 g / L. Control the reverse osmosis water recovery rate at 70%-85%. The reverse osmosis permeate is reused or discharged as clean water, and the reverse osmosis retentate is a high-concentration sodium chloride brine. S5. The high-concentration sodium chloride brine is fed into a mechanical vapor recompression evaporation crystallization system for gradient temperature evaporation crystallization. The generated secondary condensate undergoes liquid-liquid heat exchange with the high-concentration sodium chloride brine entering the system. After crystallization, the product is dehydrated by centrifugation and dried in a fluidized bed to obtain industrial-grade sodium chloride.

2. The integrated fracturing flowback brine resource utilization process according to claim 1, characterized in that: In step S1, the mass ratio of the nano-iron oxide, the silane coupling agent, and the polyacrylamide is 10:1:3 to 10:2:

5. The silane coupling agent is 3-aminopropyltriethoxysilane or 3-methacryloyloxypropyltrimethoxysilane. The free radical initiator is potassium persulfate or ammonium persulfate, and its added mass is 1%-5% of the mass of the polyacrylamide. The liquid-solid ratio of the deionized water to the nano-iron oxide is 20-30 mL / g.

3. The integrated fracturing flowback brine resource utilization process according to claim 1, characterized in that: In step S2, the advanced oxidative depolymerization is performed using Fenton oxidation treatment. The specific process is as follows: hydrogen peroxide and ferrous sulfate are added to the water after dissolved air flotation oil removal, the pH value is adjusted to 3.0-4.0, and the reaction time is 1.5-2.5 hours, which degrades the long-chain polymer into small molecules.

4. The integrated fracturing flowback brine resource utilization process according to claim 1, characterized in that: In step S2, the total dissolved solids concentration of the fracturing flowback fluid is 10,000-40,000 mg / L; during the dissolved air flotation oil removal process, a demulsifier is added to remove surface oil and suspended solids.

5. The integrated fracturing flowback brine resource utilization process according to claim 1, characterized in that: In step S2, the alkali agent is a combination of sodium hydroxide and sodium carbonate, and the dosage of the modified composite flocculant is 10-30 mg / L; the multi-media filtration uses quartz sand and activated carbon as the filter media, the precision filtration uses a security filter element with a pore size of 3-5 μm, and the turbidity of the pretreated clear liquid is less than 0.5 NTU.

6. The integrated fracturing flowback brine resource utilization process according to claim 1, characterized in that: In step S3, the multi-stage nanofiltration system uses a fouling-resistant wide-channel nanofiltration membrane element. The multi-stage nanofiltration system has a rejection rate of ≥98% for divalent sulfate ions and a permeability of ≥85% for monovalent chloride ions.

7. The integrated fracturing flowback brine resource utilization process according to claim 1, characterized in that: In step S4, the reverse osmosis system uses a seawater desalination reverse osmosis membrane or a disc tube reverse osmosis membrane.

8. The integrated fracturing flowback brine resource utilization process according to claim 1, characterized in that: In step S5, the temperature of the secondary condensate is 90-95℃, and the high-concentration sodium chloride brine is preheated to 80-85℃ after the liquid-liquid heat exchange before entering the mechanical vapor recompression evaporation crystallization system. After separation, dehydration, and drying steps, an industrial-grade sodium chloride product with a dry basis mass fraction of ≥97.5% is obtained.