Shale gas fracturing flowback wastewater treatment system and method

Abstract

The invention belongs to the field of oil and gas field wastewater treatment, and particularly relates to a shale gas fracturing flowback wastewater treatment system and method, and the system comprises treatment units such as adjustment homogenization, dissolved air flotation, multi-stage chemical precipitation, coagulation separation, electrochemical catalytic oxidation, membrane treatment and magnetic control evaporative crystallization. The dissolved air flotation unit generates micro-nano bubbles through a venturi-rotational flow coupling device and cooperates with PAC / PAM to realize efficient flotation; softening agents are added into the multi-stage sedimentation tank by stages, closed-loop control is carried out, and soluble barium, boron, fluoride and hardness are removed; the electrochemical oxidation unit strengthens mass transfer through ultrasonic waves and degrades organic matters difficult to treat; the membrane treatment realizes up-to-standard discharge of wastewater through a hollow fiber membrane bioreactor-roll type ultrafiltration membrane-anti-pollution type reverse osmosis membrane integrated system; the magnetic control evaporation crystallizer utilizes the magnetic field gradient and the temperature gradient to cooperatively regulate and control the crystallization process, the membrane separation strong brine is subjected to quality separation treatment, and high-purity salt products are produced. The method provided by the invention can effectively realize up-to-standard discharge treatment of shale gas fracturing flowback wastewater and resource utilization of evaporative crystallization salt, is also suitable for treating shale gas recovery wastewater with similar water quality characteristics, and has good popularization and application prospects.

Description

Technical Field

[0001] The invention relates to the field of oil and gas field wastewater treatment, and in particular to a shale gas fracturing flowback wastewater treatment system and method. Background Art

[0002] Shale gas is a beneficial supplement to conventional natural gas. Sichuan and Chongqing are the main battlefields for the commercial development of shale gas in my country. Efforts are being made to build a national natural gas (shale gas) production base of 100 billion cubic meters in Sichuan and Chongqing to ensure the security of national energy resources. Large-scale hydraulic fracturing is the dominant technology for the successful exploitation of shale gas at home and abroad. Hydraulic fracturing fluid is mainly composed of water and sand (about 98%), containing a variety of organic treatment agents and inorganic salt additives, such as corrosion inhibitors, surfactants, drag reducers, acids, flow improvers, etc., and different geological structures, different fracturing fluid systems and different backflow processes lead to significant temporal and spatial changes in pollutants in the backflow fluid. Fracturing backflow wastewater has the characteristics of large water volume (the proportion of backflow volume to fracturing fluid ranges from 5% to 30% on average), high salinity, many types of pollutants, high concentration of organic pollutants, poor biodegradability, and great difficulty in treatment. In addition, the emission standards are not unified, which brings great challenges to the standard discharge treatment of fracturing backflow wastewater. Most of the technologies for the treatment of shale gas fracturing flowback wastewater that meets the discharge standards are in the indoor research or pilot stage. Currently, except for a very small number of shale gas fields that have built treatment facilities that directly discharge wastewater into natural water bodies (treatment processes include "coagulation and sedimentation + Fenton oxidation + electrolytic oxidation + multi-media filtration + ultrafiltration + reverse osmosis + MVR", "chemical coagulation + electrolytic oxidation + breakpoint chlorination + solid-liquid separation + four-effect evaporation", "flotation + catalytic oxidation coupled chemical softening and hardness removal + ultrafiltration + resin softening + electrodialysis + MBR + reverse osmosis + multi-effect evaporation crystallization", "homogeneous regulation + flocculation flotation + ozone catalytic oxidation + softening + tubular membrane filtration + nanofiltration + reverse osmosis + MVR", etc., the effluent quality mainly complies with the "Comprehensive Sewage Discharge Standard" (GB 8978-1996) first-level emission standard, chloride emission limit ≤ 350mg / L) environmental protection facilities, other shale gas field (block) fracturing return wastewater is mainly transported to surrounding towns or industrial sewage treatment plants by tank trucks after pre-treatment and further treated before discharge, mainly implementing the sewage treatment plant emission standards, which has brought certain pressure to these sewage treatment plants and ecological environment supervision.

[0003] In order to strengthen the management of shale gas fracturing flowback wastewater treatment, Sichuan and Chongqing jointly formulated a unified standard for shale gas mining water pollutant emission standards (mandatory standards). The Sichuan Provincial Standard was released in December 2024, and the Chongqing Municipal Standard is planned to be released in March 2025 and implemented on July 1, 2025. The standard involves 21 indicators such as chemical oxygen demand (COD), fluoride, chloride, total dissolved solids (TDS), boron, and soluble barium, which puts forward new requirements for the treatment of shale gas fracturing flowback wastewater. Therefore, how to achieve large-scale treatment of shale gas fracturing flowback wastewater in a low-carbon, economical and efficient manner and meet the emission standards has become a key problem that needs to be solved urgently. Summary of the invention

[0004] In response to the above-mentioned key technical issues, the present invention provides a shale gas fracturing flowback wastewater treatment system and method, which realizes large-scale low-carbon, economical and efficient standard treatment and discharge of shale gas fracturing flowback wastewater through the coupling of process parameters of each treatment unit and the coordination of material flow / energy flow.

[0005] The purpose of the present invention is achieved by adopting the following technical solutions: A shale gas fracturing flowback wastewater treatment system and method, characterized by: It includes the following treatment units connected in sequence along the wastewater treatment flow: Regulating and equalizing tank (S1): equipped with stratified guide plates, bottom dynamic equalization system and online water quality monitoring system; A dissolved air flotation unit (S2) provided with a dissolved air energy releaser is configured to generate bubbles with a particle size of 10-50 μm through a micro-nano bubble generating device, and to add PAC and ionic PAM for flotation. The micro-nano bubble generating device includes a combined structure of a venturi ejector and a cyclone cutter; Multi-stage series chemical precipitation softening reaction tank (S3), each stage of the reaction tank is independently equipped with a reaction environment online monitoring and control system with a mechanical stirring device, pH sensor, conductivity meter and automatic dosing unit interlocked with the sensor, and sodium sulfate, sodium carbonate and calcium hydroxide are added in stages through a preset program to remove soluble barium, boron and fluoride and hardness; The coagulation and separation composite device (S4) is configured as a gradient dosing system with three-stage dosing ports, and a multi-stage solid-liquid separation structure including an inclined plate sedimentation zone and a tubular centrifuge; An electrochemical catalytic oxidation reactor (S5) is configured as a plate electrode group having staggered titanium-based ruthenium-iridium coated anodes and graphite cathodes, and an auxiliary mass transfer device integrating a fixed ultrasonic vibrator; A membrane treatment integrated module (S6), comprising a submerged PVDF hollow fiber membrane bioreactor, a spiral ultrafiltration membrane assembly, and an anti-fouling reverse osmosis membrane stack with an antiscalant dosing device connected in sequence; A magnetron evaporation crystallizer (S7) configured as a spiral mass transfer channel with a rare earth permanent magnet group arranged in a Halbach array and a falling film vacuum evaporation system; The system couples the process parameters of each unit with the coordination of material flow / energy flow to ensure that the effluent COD is ≤30mg / L, chloride is ≤500mg / L, soluble barium is ≤2mg / L, boron is ≤2mg / L, fluoride is ≤10mg / L, TDS is ≤1000mg / L, total hardness is ≤150mg / L, and benzene is not detected, meeting or exceeding the direct emission limits in Table 1 of the "Sichuan Shale Gas Exploitation Water Pollutant Emission Standard" (DB51 / 3203-2024).

[0006] The layered guide plates of the regulating equalization tank (S1) are multi-layer horizontal guide plates. The distance between the upper guide plate and the bottom of the tank is 1 / 5-1 / 3 of the total height of the tank body. The guide plate is inclined at an angle of 15-30°, and a zigzag flow channel is formed between adjacent guide plates. The aperture of the perforated aeration pipe of the bottom dynamic equalization system is Φ3-5mm, the hole spacing is 50-100mm, and the aeration intensity is controlled by a variable frequency fan to be 0.05-0.15m 3 / (m 2 ·min); the online monitoring module collects parameters such as pH, COD, chloride, total dissolved solids (TDS), flow rate, etc. in real time, and the data collection frequency is not less than once / 30min.

[0007] The ratio of the venturi ejector throat diameter to the water inlet diameter of the dissolved air flotation unit (S2) is 1:3-1:5, the dissolved air-water return ratio (volume ratio) is 30%-50%, the working pressure is 0.35-0.45MPa, the PAC dosage is 20-40mg / L, the PAM dosage is 0.8-1.5mg / L, and the bubble surface ζ potential is -25mV to -35mV (regulated by adding anionic surfactants).

[0008] The linear speed of the blades of the mechanical stirring device of the chemical precipitation softening reaction tank (S3) is controlled at 0.8-1.5m / s; sodium sulfate (concentration 5-15%) is added to the primary reaction tank, and the dosage is controlled by the pH sensor interlock to maintain the pH in the range of 9.0-10.5, and the hydraulic retention time is 30-45min; sodium carbonate (concentration 8-20%) is added to the secondary reaction tank, and the pH is controlled in the range of 10.5-11.5, and the hydraulic retention time is 20-35min; calcium hydroxide (concentration 15-30%) is added to the tertiary reaction tank, and the pH is controlled in the range of 11.5-12.5, and the hydraulic retention time is 15-25min, and an online hardness monitor is set between the secondary and tertiary reaction tanks (forming a closed-loop control with the dosing pump).

[0009] The first dosing port of the coagulation separation composite device (S4) is used to add PAC (concentration 5-10wt%, dosage 50-200mg / L), the second dosing port is used to add ionic PAM (concentration 0.1-0.5wt%, dosage 3-10mg / L), and the third dosing port is used to add pH regulator (concentration 5-15wt%, pH value controlled at 7.5-9.5). The spacing between the dosing ports is 3-5 times the pipe diameter, and a diaphragm pump is equipped to achieve an adjustable dosing rate of 0.5-2.0L / min; the mixing reaction zone adopts a three-stage folded plate mixer, and the folded plate angle 60-75°, the spacing between adjacent folded plates is 100-150mm, and the hydraulic retention time is 2-5 minutes; the inclined plate is made of polypropylene composite material, the plate length is 1.2-1.8m, the inclination angle is 55-60°, the plate spacing is 50-80mm, the surface is treated with a hydrophilic coating, and the rising flow rate is controlled at 0.8-1.2mm / s; the tubular centrifuge is connected to the mud discharge port at the bottom of the sedimentation area through a diversion channel, the centrifuge drum has a length-to-diameter ratio of 15:1-20:1, a rotation speed of 8000-12000rpm, and is equipped with an automatic slag discharge device. The slag discharge cycle is 15-30 minutes / time.

[0010] The thickness of the plate electrode group of the electrochemical catalytic oxidation reactor (S5) is 2±0.1mm, the electrode spacing is 5-10mm, and the current density is 15-25mA / cm 2 The working frequency of the fixed ultrasonic device is 28kHz±5% and the power density is 0.5-1.2W / cm 3 The ORP sensor is used to control the redox potential in the reactor in real time to maintain it in the range of 800-1200mV.

[0011] The PVDF hollow fiber membrane of the membrane treatment integrated module (S6) has an average pore size of 0.1-0.2μm, a porosity of 60-80%, an inner diameter of 0.8-1.2mm, and an outer diameter of 1.5-2.0mm. It adopts an external pressure filtration method; the reverse osmosis membrane uses a seawater desalination grade polyamide composite membrane with a retention rate of >99.5% and a water production rate of >80%. The effective area of ​​a single membrane element is ≥37m 2 , operating pressure 4.0-5.5MPa, temperature tolerance range 5-45℃, pH tolerance range 2-11; set up a membrane pollution early warning system based on transmembrane pressure difference, water production change and data acquisition, the data acquisition frequency is not less than 1 time / minute, when the transmembrane pressure difference increases by 15% or the water production decreases by 20%, the citric acid (concentration 2-5wt%) and sodium hypochlorite (effective chlorine concentration 0.1-0.5wt%) alternating cleaning program is triggered, the number of alternations in the cleaning cycle is set to acid wash: alkali wash = 2:1, and the cleaning flow rate is controlled at a membrane surface flow rate of 1.5-2.0m / s.

[0012] The spiral mass transfer channel of the magnetron evaporation crystallizer (S7) is made of 316L stainless steel, with a pitch of 150-200mm, a channel diameter of Φ300-500mm, a wall thickness of 2.0±0.1mm, and an inner surface electrolytically polished (Ra≤0.8μm); a permanent magnet array with a magnetic field gradient of 0.5-0.7T / cm is set in the channel, and the pole spacing is 20-30mm; the evaporation system maintains a temperature gradient of 70-85℃ (preheating section 70-75℃ (accounting for 30%), evaporation section 78-82℃ (accounting for 50%), crystallization section 83-85℃ (accounting for 20%), temperature control accuracy ±0.5℃), equipped with a cyclone crystallizer with a centrifugal force ≥2000g, the outlet is connected to a vibrating fluidized bed dryer, and the purity of sodium chloride in the crystallized product is ≥95%.

[0013] The beneficial effects of the present invention are: The present invention provides a shale gas fracturing flowback wastewater treatment system and method combining regulating homogenization, dissolved air flotation, multi-stage chemical precipitation, coagulation separation, electrochemical catalytic oxidation, membrane treatment and magnetic control evaporation crystallization. The wastewater is first pre-treated by regulating homogenization, dissolved air flotation, multi-stage chemical precipitation, coagulation separation and electrochemical catalytic oxidation, and then the wastewater is concentrated by integrated membrane treatment (membrane bioreactor + ultrafiltration + reverse osmosis). The concentrated wastewater is then subjected to magnetic control evaporation crystallization, which can achieve the coupling of process parameters of each treatment unit and the coordination of material flow / energy flow, improve the coordinated removal capacity of multiple pollutants, and make the treated effluent water meet the "Sichuan Shale Gas Exploitation Water Pollutant Discharge Standard" (DB51 / 3203-20 24) and greatly reduce energy consumption and direct treatment costs; the online water quality monitoring system of the regulating homogenization tank (S1), the online monitoring and control system of the reaction environment of the chemical precipitation softening reaction tank (S3), the real-time ORP regulation of the electrochemical catalytic oxidation reactor (S5), the membrane pollution early warning system of the membrane treatment integrated module (S6), etc. provided by the present invention realize the intelligent control of the whole process of wastewater treatment and ensure the stability of the system; the present invention provides a magnetically controlled evaporation crystallizer (S7), which can achieve zero discharge of reverse osmosis concentrated wastewater through complete crystallization and produce high-purity crystalline sodium chloride that can be utilized as a resource; the present invention is also suitable for treating shale gas production wastewater with similar water quality characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The present invention is a process flow chart of a shale gas fracturing flowback wastewater treatment system and method. DETAILED DESCRIPTION

[0015] The technical solution of the present invention is: a shale gas fracturing flowback wastewater treatment system and method, including the following treatment units connected in sequence along the wastewater treatment flow direction: Regulating and equalizing tank (S1): equipped with stratified guide plates, bottom dynamic equalization system and online water quality monitoring system; A dissolved air flotation unit (S2) provided with a dissolved air energy releaser is configured to generate bubbles with a particle size of 10-50 μm through a micro-nano bubble generating device, and to add PAC and ionic PAM for flotation. The micro-nano bubble generating device includes a combined structure of a venturi ejector and a cyclone cutter; Multi-stage series chemical precipitation softening reaction tank (S3), each stage of the reaction tank is independently equipped with a reaction environment online monitoring and control system with a mechanical stirring device, pH sensor, conductivity meter and automatic dosing unit interlocked with the sensor, and sodium sulfate, sodium carbonate and calcium hydroxide are added in stages through a preset program to remove soluble barium, boron and fluoride and hardness; The coagulation and separation composite device (S4) is configured as a gradient dosing system with three-stage dosing ports, and a multi-stage solid-liquid separation structure including an inclined plate sedimentation zone and a tubular centrifuge; An electrochemical catalytic oxidation reactor (S5) is configured as a plate electrode group having staggered titanium-based ruthenium-iridium coated anodes and graphite cathodes, and an auxiliary mass transfer device integrating a fixed ultrasonic vibrator; A membrane treatment integrated module (S6), comprising a submerged PVDF hollow fiber membrane bioreactor, a spiral ultrafiltration membrane assembly, and an anti-fouling reverse osmosis membrane stack with an antiscalant dosing device connected in sequence; A magnetron evaporation crystallizer (S7) configured as a spiral mass transfer channel with a rare earth permanent magnet group arranged in a Halbach array and a falling film vacuum evaporation system; The system couples the process parameters of each unit with the coordination of material flow / energy flow to ensure that the effluent COD is ≤30mg / L, chloride is ≤500mg / L, soluble barium is ≤2mg / L, boron is ≤2mg / L, fluoride is ≤10mg / L, TDS is ≤1000mg / L, total hardness is ≤150mg / L, and benzene is not detected, meeting or exceeding the direct emission limits in Table 1 of the "Sichuan Shale Gas Exploitation Water Pollutant Emission Standard" (DB51 / 3203-2024).

[0016] The layered guide plates of the regulating equalization tank (S1) are multi-layer horizontal guide plates. The distance between the upper guide plate and the bottom of the tank is 1 / 5-1 / 3 of the total height of the tank body. The guide plate is inclined at an angle of 15-30°, and a zigzag flow channel is formed between adjacent guide plates. The aperture of the perforated aeration pipe of the bottom dynamic equalization system is Φ3-5mm, the hole spacing is 50-100mm, and the aeration intensity is controlled by a variable frequency fan to be 0.05-0.15m 3 / (m 2 ·min); the online monitoring module collects parameters such as pH, COD, chloride, total dissolved solids (TDS), flow rate, etc. in real time, and the data collection frequency is not less than once / 30min.

[0017] The ratio of the venturi ejector throat diameter to the water inlet diameter of the dissolved air flotation unit (S2) is 1:3-1:5, the dissolved air-water return ratio (volume ratio) is 30%-50%, the working pressure is 0.35-0.45MPa, the PAC dosage is 20-40mg / L, the PAM dosage is 0.8-1.5mg / L, and the bubble surface ζ potential is -25mV to -35mV (regulated by adding anionic surfactants).

[0018] The linear speed of the blades of the mechanical stirring device of the chemical precipitation softening reaction tank (S3) is controlled at 0.8-1.5m / s; sodium sulfate (concentration 5-15%) is added to the primary reaction tank, and the dosage is controlled by the pH sensor interlock to maintain the pH in the range of 9.0-10.5, and the hydraulic retention time is 30-45min; sodium carbonate (concentration 8-20%) is added to the secondary reaction tank, and the pH is controlled in the range of 10.5-11.5, and the hydraulic retention time is 20-35min; calcium hydroxide (concentration 15-30%) is added to the tertiary reaction tank, and the pH is controlled in the range of 11.5-12.5, and the hydraulic retention time is 15-25min, and an online hardness monitor is set between the secondary and tertiary reaction tanks (forming a closed-loop control with the dosing pump).

[0019] The first dosing port of the coagulation separation composite device (S4) is used to add PAC (concentration 5-10wt%, dosage 50-200mg / L), the second dosing port is used to add ionic PAM (concentration 0.1-0.5wt%, dosage 3-10mg / L), and the third dosing port is used to add pH regulator (concentration 5-15wt%, pH value controlled at 7.5-9.5). The spacing between the dosing ports is 3-5 times the pipe diameter, and a diaphragm pump is equipped to achieve an adjustable dosing rate of 0.5-2.0L / min; the mixing reaction zone adopts a three-stage folded plate mixer, and the folded plate angle 60-75°, the spacing between adjacent folded plates is 100-150mm, and the hydraulic retention time is 2-5 minutes; the inclined plate is made of polypropylene composite material, the plate length is 1.2-1.8m, the inclination angle is 55-60°, the plate spacing is 50-80mm, the surface is treated with a hydrophilic coating, and the rising flow rate is controlled at 0.8-1.2mm / s; the tubular centrifuge is connected to the mud discharge port at the bottom of the sedimentation area through a diversion channel, the centrifuge drum has a length-to-diameter ratio of 15:1-20:1, a rotation speed of 8000-12000rpm, and is equipped with an automatic slag discharge device. The slag discharge cycle is 15-30 minutes / time.

[0020] The thickness of the plate electrode group of the electrochemical catalytic oxidation reactor (S5) is 2±0.1mm, the electrode spacing is 5-10mm, and the current density is 15-25mA / cm 2 The working frequency of the fixed ultrasonic device is 28kHz±5% and the power density is 0.5-1.2W / cm 3The ORP sensor is used to control the redox potential in the reactor in real time to maintain it in the range of 800-1200mV.

[0021] The PVDF hollow fiber membrane of the membrane treatment integrated module (S6) has an average pore size of 0.1-0.2μm, a porosity of 60-80%, an inner diameter of 0.8-1.2mm, and an outer diameter of 1.5-2.0mm. It adopts an external pressure filtration method; the reverse osmosis membrane uses a seawater desalination grade polyamide composite membrane with a retention rate of >99.5% and a water production rate of >80%. The effective area of ​​a single membrane element is ≥37m 2 , operating pressure 4.0-5.5MPa, temperature tolerance range 5-45℃, pH tolerance range 2-11; set up a membrane pollution early warning system based on transmembrane pressure difference, water production change and data acquisition, the data acquisition frequency is not less than 1 time / minute, when the transmembrane pressure difference increases by 15% or the water production decreases by 20%, the citric acid (concentration 2-5wt%) and sodium hypochlorite (effective chlorine concentration 0.1-0.5wt%) alternating cleaning program is triggered, the number of alternations in the cleaning cycle is set to acid wash: alkali wash = 2:1, and the cleaning flow rate is controlled at a membrane surface flow rate of 1.5-2.0m / s.

[0022] The spiral mass transfer channel of the magnetron evaporation crystallizer (S7) is made of 316L stainless steel, with a pitch of 150-200mm, a channel diameter of Φ300-500mm, a wall thickness of 2.0±0.1mm, and an inner surface electrolytically polished (Ra≤0.8μm); a permanent magnet array with a magnetic field gradient of 0.5-0.7T / cm is set in the channel, and the pole spacing is 20-30mm; the evaporation system maintains a temperature gradient of 70-85℃ (preheating section 70-75℃ (accounting for 30%), evaporation section 78-82℃ (accounting for 50%), crystallization section 83-85℃ (accounting for 20%), temperature control accuracy ±0.5℃), equipped with a cyclone crystallizer with a centrifugal force ≥2000g, the outlet is connected to a vibrating fluidized bed dryer, and the purity of sodium chloride in the crystallized product is ≥95%.

[0023] The present invention is further described below with reference to examples, but the present invention is not limited to these examples.

[0024] Example 1: This example treats shale gas fracturing flowback wastewater, including the following treatment units connected in sequence along the wastewater treatment flow direction: (1) Adjustment and equalization tank (S1): equipped with stratified guide plates, a dynamic equalization system for the bottom of the pool and an online water quality monitoring system; the stratified guide plates are three-layer horizontal guide plates, the distance between the upper guide plate and the bottom of the pool is 1 / 3 of the total height of the pool body, the guide plate inclination angle is 30° (based on computational fluid dynamics (CFD) simulation optimization, the turbulence intensity is ≥0.15m² / s²), and a zigzag flow channel is formed between adjacent guide plates; the bottom perforated aeration pipe of the dynamic equalization system has a hole diameter of Φ3mm, a hole spacing of 90mm, and aeration intensity of the variable frequency fan is 0.06m 3 / (m 2 ·min) (When less than 0.06m 3 / (m 2 ·min); the online monitoring module collects parameters such as pH, COD, chloride, total dissolved solids (TDS), flow rate, etc. in real time at a frequency of once every 15 minutes.

[0025] (2) A dissolved air flotation unit (S2) equipped with a dissolved air energy releaser is configured to generate bubbles with a particle size of 10-50 μm through a micro-nano bubble generating device, and to add PAC and anionic PAM for flotation. The micro-nano bubble generating device includes a combined structure of a venturi ejector and a cyclone cutter (the outlet of the venturi ejector and the inlet of the cyclone cutter are connected by a flange, the inner diameter of the cyclone cutter is twice the diameter of the ejector throat, the blade inclination angle is 45°, and the cyclone angular velocity is ≥120 rad / s); the ratio of the venturi ejector throat diameter to the water inlet diameter is 1:3, the dissolved air water return ratio is 30%, the working pressure is 0.35 MPa, the PAC dosage is 35 mg / L, the PAM dosage is 1.0 mg / L, and anionic surfactant is added to adjust the bubble surface ζ potential to -25 mV.

[0026] (3) Multi-stage chemical precipitation softening reaction tank (S3) is connected in series. Each reaction tank is independently equipped with an online monitoring and control system for the reaction environment with a mechanical stirring device, a pH sensor, a conductivity meter and an automatic dosing unit interlocked with the sensor. Sodium sulfate, sodium carbonate and calcium hydroxide are added in stages according to a preset program to remove soluble barium, boron and fluoride and hardness. The linear speed of the mechanical stirring device blade is controlled at 1.0 m / s. Sodium sulfate (concentration 8%) is added to the first reaction tank, and the dosing amount is controlled by the pH sensor interlock to maintain the pH at 9.0±0.2, hydraulic retention time is 30min; sodium carbonate (concentration 12%) is added to the secondary reaction tank to control the pH at 10.5±0.2, and the hydraulic retention time is 25min; calcium hydroxide (concentration 15%) is added to the tertiary reaction tank to control the pH at 11.5±0.2, and the hydraulic retention time is 20min (optimized and determined by the reaction kinetic model (quasi-second-order rate constant k=0.15L / (mol·min))), and the outlet water hardness is ensured to be ≤150mg / L by an online hardness monitor.

[0027] (4) The coagulation and separation composite device (S4) is configured as a gradient dosing system with three-stage dosing ports and a multi-stage solid-liquid separation structure including an inclined plate sedimentation zone and a tubular centrifuge; PAC (concentration 8wt%, dosing amount 80mg / L) is added to the first dosing port, PAM (concentration 0.2wt%, dosing amount 2mg / L) is added to the second dosing port, and pH regulator (concentration 10wt%, pH value controlled at 8.0±0.1) is added to the third dosing port. The distance between the dosing ports is 5 times the pipe diameter, and the dosing acceleration rate is 1.0 L / min; the mixing reaction zone adopts a three-stage folded plate mixer with a folded plate angle of 65°, a spacing of 100mm between adjacent folded plates, and a hydraulic retention time of 5min; the inclined plate is made of polypropylene composite material with an inclination angle of 55°, a plate spacing of 60mm, and a surface treated with a hydrophilic coating. The rising flow rate is controlled at 1.0mm / s; the tubular centrifuge is connected to the mud discharge port at the bottom of the sedimentation zone through a diversion channel. The centrifuge drum has a length-to-diameter ratio of 20:1, a rotation speed of 10,000rpm, and is equipped with an automatic slag discharge device with a slag discharge cycle of 30min / time.

[0028] (5) Electrochemical catalytic oxidation reactor (S5), configured as a plate electrode group with staggered titanium-based ruthenium-iridium coated anodes and graphite cathodes, and an integrated fixed ultrasonic vibrator auxiliary mass transfer device; the plate electrode group has a single-pole plate thickness of 2.0 mm, an electrode spacing of 8 mm, and a current density of 20 mA / cm 2 , integrated fixed ultrasonic vibrator with operating frequency of 28kHz and power density of 1.0W / cm 3 The redox potential in the reactor is controlled in real time by the ORP sensor to be maintained at around 1000±50mV.

[0029] (6) Membrane treatment integrated module (S6), including an immersed PVDF hollow fiber membrane bioreactor, a rolled ultrafiltration membrane assembly and an anti-fouling reverse osmosis membrane stack with an antiscalant dosing device connected in sequence; the average pore size of the PVDF hollow fiber membrane is 0.1 μm, the porosity is 70%, the inner diameter of the membrane is 1.2 mm, the outer diameter is 2.0 mm, and an external pressure filtration method is adopted; the reverse osmosis membrane is Dow SW30HRLE-400 seawater desalination grade polyamide composite membrane (test conditions: 25°C, 32000 mg / L NaCl solution, desalination rate ≥99.7%), and the effective area of ​​a single membrane element is 37 m 2 , operating pressure 4.5MPa, water production rate 82%; set up a membrane pollution early warning system. When the transmembrane pressure difference increases by 15% or the water production decreases by 20%, the alternating cleaning program of citric acid (concentration 5wt%) and sodium hypochlorite (effective chlorine concentration 0.5wt%) is triggered. The alternation times of the cleaning cycle are set to acid cleaning: alkaline cleaning = 2:1, and the cleaning flow rate is controlled at a membrane surface flow rate of 2.0m / s.

[0030] (7) Magnetron evaporation crystallizer (S7), configured as a spiral mass transfer channel with a rare earth permanent magnet group arranged in a Halbach array and a falling film vacuum evaporation system; the spiral mass transfer channel is made of 316L stainless steel, with a pitch of 200mm, a channel diameter of Φ500mm, a wall thickness of 2.0mm, and an inner surface treated with electrolytic polishing (Ra≤0.8μm); a permanent magnet array with a magnetic field gradient of 0.7T / cm is set in the channel, and the pole spacing is 30mm; the evaporation system maintains a temperature gradient of 70-85℃ (preheating section 70-75℃ (accounting for 30%), evaporation section 78-82℃ (accounting for 50%), crystallization section 83-85℃ (accounting for 20%), temperature control accuracy ±0.5℃), equipped with a cyclone crystallizer with a centrifugal force ≥2000g, the outlet is connected to a vibrating fluidized bed dryer, and the purity of sodium chloride in the crystallized product is ≥95%.

[0031] Embodiment 2: This embodiment adopts the steps of embodiment 1 and adjusts some parameters: (1) Adjustment of equalization tank (S1): guide plate inclination angle 20°, spacing 1 / 4 of the tank height (CFD simulated turbulence intensity ≥ 0.18 m² / s²), aeration intensity 0.12 m³ / (m²·min), monitoring frequency 1 time / 20 min; (2) Dissolved air flotation unit (S2): The ratio of the diameter of the venturi ejector throat to the diameter of the water inlet pipe is 1:4 (water inlet pipe Φ100 mm → throat pipe Φ25 mm), PAC 35 mg / L, PAM 1.0 mg / L, ζ potential -28 mV (adjusted by adding 0.5 mg / L of sodium dodecyl sulfate); (3) Chemical precipitation softening reaction tank (S3): primary reaction: 10% sodium sulfate, pH controlled at 10.0±0.2, hydraulic retention time 40 min (blade linear speed 1.2 m / s), secondary reaction: 15% sodium carbonate, pH controlled at 11.0±0.2, hydraulic retention time 25 min, tertiary reaction: 25% calcium hydroxide, pH controlled at 12.2±0.2, hydraulic retention time 18 min; (4) Coagulation separation composite device (S4): Gradient addition of PAC 130 mg / L (8 wt%) → PAM 4 mg / L (0.3 wt%) → NaOH adjusted to pH 8.8 ± 0.1, rising flow rate of inclined plate sedimentation zone 1.0 mm / s, tubular centrifuge speed 10000 rpm (length-to-diameter ratio 18:1); (5) Electrochemical catalytic oxidation reactor (S5): electrode spacing 6 mm, current density 20 mA / cm², ultrasonic power density 1.2 W / cm³; (6) Membrane treatment integrated module (S6): reverse osmosis operating pressure 4.8 MPa, scale inhibitor (aminophosphonic acid) dosage 4 mg / L, after the membrane pollution warning is triggered, 3 wt% citric acid (flow rate 1.8 m / s) and 0.3 wt% sodium hypochlorite are used for alternating cleaning; (7) Magnetron evaporation crystallizer (S7): magnetic field gradient 0.6 T / cm, evaporation temperature gradient preheating 73 °C → evaporation 80 °C → crystallization 84 °C, cyclone crystallizer centrifugal force 2500 g, crystallized salt purity 96.8%.

[0032] Embodiment 3: This embodiment adopts the steps of embodiment 1 and adjusts some parameters: (1) Dissolved air flotation unit (S2): The dissolved air-water reflux ratio is increased to 45%, and the working pressure is 0.42 MPa; (2) Chemical precipitation softening reaction tank (S3): The HRT of the tertiary reaction tank is extended to 22 min; (3) Coagulation and separation composite device (S4): PAC and PAM dosages were increased to 200 mg / L (8 wt%) and 7 mg / L (0.3 wt%), respectively; (4) Membrane treatment integrated module (S6): Reverse osmosis adopts two-stage series connection, operating pressure is 5.3MPa, and the scale inhibitor dosage is increased to 6mg / L; (5) Magnetron evaporation crystallizer (S7): The magnetic field gradient of the magnetron evaporation section is enhanced to 0.7 T / cm, and the purity of the crystallized salt is ≥96.5%.

[0033] Comparative Example 1: The magnetron evaporation crystallizer (S7) was cancelled and replaced with conventional multi-effect evaporation (without magnetic field enhancement), and the other steps were carried out according to Example 1. The evaporator scaling period was shortened from 30 days in Example 1 to 9 days (the data was verified by 3 repeated experiments, CV=4.2%), the per ton water vapor consumption increased from 1.8t / m³ to 2.25t / m³, and the purity of the crystallized salt was reduced from 95% to 88.5% (including CaSO4 impurities accounting for more than 8%).

[0034] Comparative Example 2: The gradient dosing order of the chemical precipitation softening reaction tank (S3) was changed to calcium hydroxide → sodium sulfate → sodium carbonate, and the other steps were carried out according to Example 1. The barium ion removal rate was reduced from 99.8% in Example 1 to 72% (it was necessary to extend the HRT to 35 min to recover to 90%), and the amount of sodium sulfate was increased from 10kg / m³ to 17kg / m³ (comparative experimental data SD = ±0.5kg / m³).

[0035] Comparative Example 3: The ultrasonic device was removed from the electrochemical catalytic oxidation reactor (S5), the electrode spacing was expanded to 15 mm, and the other steps were carried out according to Example 1. The COD removal efficiency was reduced from 84% to 58% (under the same residence time), the electrode passivation rate was accelerated, and the cleaning cycle was shortened from 72 hours to 8 hours (comparative experimental data CV = 6.1%) In Examples 1-3, shale gas fracturing flowback wastewater with different water quality characteristics is coupled with the process parameters of each treatment unit and coordinated with the material flow / energy flow, and 21 indicators such as COD, chloride, fluoride, TDS, etc. of the treated water meet or exceed the direct emission limits in Table 1 of the "Sichuan Shale Gas Exploitation Water Pollutant Emission Standard" (DB51 / 3203-2024) (see the table below). Compared with Comparative Examples 1-3, Examples 1-3 achieve multiple goals such as improved treatment efficiency (such as a 5-15% increase in the removal rate of key indicators such as COD and TDS), reduced operating costs (such as a 29-44% reduction in energy consumption per ton of water and a 20-46% saving in reagent costs) and enhanced system stability (such as a 233% extension of the equipment scaling cycle and a 50% increase in the electrode life) through innovative practices such as precise parameter matching (such as the S1 guide plate inclination angle being associated with the turbulence intensity), reaction sequence optimization (such as S3 gradient dosing to inhibit the wrapping effect), and energy field coupling (S5 ultrasonic wave + S7 magnetic field). It gives full play to the synergistic effect of multiple physical fields between the treatment units, solves the low-carbon, economical and efficient treatment problems of high salt, high TDS, difficult-to-degrade organic matter, and complex ions in fracturing flowback wastewater, and provides a scalable and replicable solution for the treatment and discharge of shale gas fracturing flowback wastewater to meet standards.

[0036]

[0037] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention. The scope of protection claimed by the present invention is defined by the attached claims and their equivalents.

Claims

1. A shale gas fracturing flowback wastewater treatment system and method, characterized in that: It includes the following treatment units connected in sequence along the wastewater treatment flow: Regulating and equalizing tank (S1): equipped with stratified guide plates, bottom dynamic equalization system and online water quality monitoring system; A dissolved air flotation unit (S2) provided with a dissolved air energy releaser is configured to generate bubbles with a particle size of 10-50 μm through a micro-nano bubble generating device, and to add PAC and ionic PAM for flotation, wherein the micro-nano bubble generating device comprises a combined structure of a venturi ejector and a cyclone cutter; Multi-stage series chemical precipitation softening reaction tank (S3), each stage of the reaction tank is independently equipped with a reaction environment online monitoring and control system with a mechanical stirring device, pH sensor, conductivity meter and automatic dosing unit interlocked with the sensor, and sodium sulfate, sodium carbonate and calcium hydroxide are added in stages through a preset program to remove soluble barium, boron and fluoride and hardness; The coagulation and separation composite device (S4) is configured as a gradient dosing system with three-stage dosing ports, and a multi-stage solid-liquid separation structure including an inclined plate sedimentation zone and a tubular centrifuge; An electrochemical catalytic oxidation reactor (S5) is configured as a plate electrode group having staggered titanium-based ruthenium-iridium coated anodes and graphite cathodes, and an auxiliary mass transfer device integrating a fixed ultrasonic vibrator; A membrane treatment integrated module (S6), comprising a submerged PVDF hollow fiber membrane bioreactor, a spiral ultrafiltration membrane assembly, and an anti-fouling reverse osmosis membrane stack with an antiscalant dosing device connected in sequence; A magnetron evaporation crystallizer (S7) configured as a spiral mass transfer channel with a rare earth permanent magnet group arranged in a Halbach array and a falling film vacuum evaporation system; The system couples the process parameters of each unit with the coordination of material flow / energy flow to ensure that the effluent COD is ≤30mg / L, chloride is ≤500mg / L, soluble barium is ≤2mg / L, boron is ≤2mg / L, fluoride is ≤10mg / L, TDS is ≤1000mg / L, total hardness is ≤150mg / L, and benzene series are not detected, which meets or exceeds the direct emission limits in Table 1 of the "Sichuan Shale Gas Exploitation Water Pollutant Emission Standard" (DB51 / 3203-2024).

2. A shale gas fracturing flowback wastewater treatment system and method according to claim 1, characterized in that: The layered guide plates of the regulating equalization tank (S1) are multi-layer horizontal guide plates, the distance between the upper guide plate and the bottom of the tank is 1 / 5-1 / 3 of the total height of the tank body, the guide plate is inclined at an angle of 15-30°, and a zigzag flow channel is formed between adjacent guide plates; the aperture of the perforated aeration pipe at the bottom of the tank for the dynamic equalization system is Φ3-5mm, the hole spacing is 50-100mm, and the aeration intensity is controlled by a variable frequency fan to be 0.05-0.15m 3 / (m 2 ·min); the online monitoring module collects parameters such as pH, COD, chloride, total dissolved solids (TDS), flow rate, etc. in real time, and the data collection frequency is not less than once / 30min.

3. A shale gas fracturing flowback wastewater treatment system and method according to claim 1, characterized in that: The ratio of the venturi ejector throat diameter to the water inlet diameter of the dissolved air flotation unit (S2) is 1:3-1:5, the dissolved air-water return ratio (volume ratio) is 30%-50%, the working pressure is 0.35-0.45MPa, the PAC dosage is 20-40mg / L, the PAM dosage is 0.8-1.5mg / L, and the bubble surface ζ potential is -25mV to -35mV (controlled by adding anionic surfactant).

4. A shale gas fracturing flowback wastewater treatment system and method according to claim 1, characterized in that: The mechanical stirring device of the chemical precipitation softening reaction tank (S3) has a blade linear speed controlled at 0.8-1.5 m / s; sodium sulfate (concentration 5-15%) is added to the primary reaction tank, and the dosage is controlled by a pH sensor interlock to maintain the pH in the range of 9.0-10.5, and the hydraulic retention time is 30-45 min; sodium carbonate (concentration 8-20%) is added to the secondary reaction tank, and the pH is controlled in the range of 10.5-11.5, and the hydraulic retention time is 20-35 min; calcium hydroxide (concentration 15-30%) is added to the tertiary reaction tank, and the pH is controlled in the range of 11.5-12.5, and the hydraulic retention time is 15-25 min, and an online hardness monitor is arranged between the secondary and tertiary reaction tanks (forming a closed-loop control with the dosing pump).

5. A shale gas fracturing flowback wastewater treatment system and method according to claim 1, characterized in that: The first dosing port of the coagulation separation composite device (S4) adds PAC (concentration 5-10wt%, dosage 50-200mg / L), the second dosing port adds ionic PAM (concentration 0.1-0.5wt%, dosage 3-10mg / L), and the third dosing port adds pH regulator (concentration 5-15wt%, pH value controlled at 7.5-9.5). The spacing between the dosing ports is 3-5 times the pipe diameter, and a diaphragm pump is equipped to achieve an adjustable dosing rate of 0.5-2.0L / min; the mixing reaction zone adopts a three-stage folding plate mixer, and the folding plate clamp The angle is 60-75°, the spacing between adjacent folded plates is 100-150mm, and the hydraulic retention time is 2-5 minutes; the inclined plate is made of polypropylene composite material, the plate length is 1.2-1.8m, the inclination angle is 55-60°, the plate spacing is 50-80mm, the surface is treated with a hydrophilic coating, and the rising flow rate is controlled at 0.8-1.2mm / s; the tubular centrifuge is connected to the mud discharge port at the bottom of the sedimentation area through a diversion channel, the centrifuge drum has a length-to-diameter ratio of 15:1-20:1, a rotation speed of 8000-12000rpm, and is equipped with an automatic slag discharge device. The slag discharge cycle is 15-30 minutes / time.

6. A shale gas fracturing flowback wastewater treatment system and method according to claim 1, characterized in that: The plate electrode group of the electrochemical catalytic oxidation reactor (S5) has a monopolar plate thickness of 2±0.1 mm, an electrode spacing of 5-10 mm, and a current density of 15-25 mA / cm 2 The working frequency of the fixed ultrasonic device is 28kHz±5% and the power density is 0.5-1.2W / cm 3 The ORP sensor is used to control the redox potential in the reactor in real time to maintain it in the range of 800-1200mV.

7. A shale gas fracturing flowback wastewater treatment system and method according to claim 1, characterized in that: The PVDF hollow fiber membrane of the membrane treatment integrated module (S6) has an average pore size of 0.1-0.2 μm, a porosity of 60-80%, an inner diameter of 0.8-1.2 mm, and an outer diameter of 1.5-2.0 mm, and adopts an external pressure filtration method; the reverse osmosis membrane uses a seawater desalination grade polyamide composite membrane with a rejection rate of more than 99.5% and a water production rate of more than 80%, and the effective area of ​​a single membrane element is ≥37 m 2 , operating pressure 4.0-5.5MPa, temperature tolerance range 5-45℃, pH tolerance range 2-11; set up a membrane pollution early warning system based on transmembrane pressure difference, water production change and data acquisition, the data acquisition frequency is not less than 1 time / minute, when the transmembrane pressure difference increases by 15% or the water production decreases by 20%, the citric acid (concentration 2-5wt%) and sodium hypochlorite (effective chlorine concentration 0.1-0.5wt%) alternating cleaning program is triggered, the number of alternations in the cleaning cycle is set to acid wash: alkali wash = 2:1, and the cleaning flow rate is controlled at a membrane surface flow rate of 1.5-2.0m / s.

8. A shale gas fracturing flowback wastewater treatment system and method according to claim 1, characterized in that: The spiral mass transfer channel of the magnetron evaporation crystallizer (S7) is made of 316L stainless steel, with a pitch of 150-200mm, a channel diameter of Φ300-500mm, a wall thickness of 2.0±0.1mm, and an inner surface subjected to electrolytic polishing (Ra≤0.8μm); a permanent magnet array with a magnetic field gradient of 0.5-0.7T / cm is arranged in the channel, and the magnetic pole spacing is 20-30mm; the evaporation system maintains a temperature gradient of 70-85°C (preheating section 70-75°C (accounting for 30%), evaporation section 78-82°C (accounting for 50%), crystallization section 83-85°C (accounting for 20%), temperature control accuracy ±0.5°C), is equipped with a cyclone crystallizer with a centrifugal force ≥2000g, and the outlet is connected to a vibrating fluidized bed dryer, and the purity of sodium chloride in the crystallized product is ≥95%.

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