Integrated coalescence-selective filtration device and method for treating oily wastewater based on wettability gradient

Abstract

This invention relates to an integrated coalescence-selective filtration device and method for treating oily wastewater based on wettability gradients. The separation device includes a vertical coalescence channel, a horizontal oil channel, and a horizontal water channel, employing a one-inlet, two-outlet three-dimensional flow channel integrated structure. This couples the demulsification and separation processes within a single device, reducing the device volume by 50%. Spontaneous oil-water separation is achieved through a triple coupling effect of gravity, capillary force, and wettability gradient, requiring no external power. The wettability gradient design ensures a system pressure drop benefit of <10 kPa, reducing energy loss. The effluent oil content is <5 mg / L, exceeding the GB31570-2015 standard, with an oil recovery rate >99%. It can treat stubborn emulsions containing surfactants, overcoming the limitations of traditional chemical demulsifiers. This invention proposes an integrated "structure-function" design paradigm, providing a technical path for the development of compact separation equipment through geometric parameter optimization and matching with materials exhibiting extreme wettability.

Description

Technical Field

[0001] This invention relates to the technical field of special wettability oil-water separation devices, specifically to an integrated coalescence-selective filtration device and method for treating oily wastewater based on wettability gradient. Background Technology

[0002] Oily wastewater has a wide range of sources, mainly concentrated in the catering, food processing, petrochemical, metal processing, textile printing and dyeing, and leather industries. Discharging untreated oily wastewater not only causes serious environmental damage but also wastes oil and water resources. Traditional oily wastewater treatment processes mainly include gravity sedimentation, air flotation, membrane separation, adsorption, biochemical methods, and chemical coagulation. These separation methods all have certain drawbacks. In oil-in-water emulsions, oil droplets form a kinetically stable system due to the synergistic effects of surface electrostatic repulsion, steric hindrance, and short-range hydration. Traditional demulsification technologies (such as chemical demulsification and centrifugal separation) are unable to disrupt this stability. Specifically, the oil droplet size is small (<10μm), and the oil droplet aggregation efficiency is low (<60%), especially for stubborn emulsions containing surfactants (such as Span 80 stabilized emulsions), where the effluent oil concentration is generally >20mg / L. Traditional agglomeration materials (such as polypropylene fibers) have limited wettability and cannot break the interfacial stability mechanism of micron-sized oil droplets. Existing technologies rely on separate operations of demulsifiers and separators, resulting in equipment redundancy, extended processes, and multiple energy conversion losses. Specifically, the separate processes require multi-stage pumping, traditional equipment has high pressure drop, requires additional power, and increases operating costs; the equipment occupies a large area, making it difficult to adapt to the needs of compact industrial scenarios.

[0003] For example, the aqueous phase oil removal device disclosed in patent CN202420310945.2 adopts a series process of filter, coalescer (including oleophilic coalescing filter element and partitioned cavity) and separator. It is claimed that it can control the oil content of the treated aqueous phase and the water content of the recovered oil phase to below 15ppm. However, current water-in-oil emulsion treatment technology has significant system limitations: traditional oleophilic coalescing filter elements have low demulsification efficiency for stubborn emulsions containing surfactants (such as Span 80 and SDS), especially failing to break through the interface stabilization mechanism of 10-50μm micron oil droplets, making it difficult to stably reduce the oil content of the effluent to below the discharge / reuse standard of 5ppm; its step-by-step process design leads to equipment redundancy, excessive pressure drop and energy consumption problems, and due to the lack of quantitative design tools to optimize coalescence time and equipment parameters, it is difficult to adapt to compact requirements; the single function of materials leads to easy clogging and short life of filter elements, requiring additional water removal coalescing filter elements and excessive reliance on chemical demulsifiers, increasing maintenance costs and the risk of secondary pollution. The root cause lies in the fact that the interface stabilization mechanism of micron-sized oil droplets has not been effectively overcome, and the lack of quantitative design tools has led to the inability to accurately optimize the coalescence process and equipment, exacerbating the contradiction between equipment redundancy and compactness requirements.

[0004] For example, patent CN202322946362.7 discloses an oil-water separation device. An inlet pipe is connected to the left side of the main tank. The inlet pipe is used in combination with a baffle to prevent the oil-water mixture from impacting the liquid surface when entering the tank. A coalescer is installed inside the main tank. After the oil-water mixture enters the main tank, it first enters the coalescer as the liquid level rises. As it passes through the coalescer from the inside out, the coalescer captures and gathers water droplets mixed in the oil, forming larger droplets. When the oil-water mixture enters the coalescer, because the density of water is greater than that of oil, the water naturally sinks to form a water layer. In this process, water droplets are captured and gathered together, forming larger droplets. Since the density of water is greater than that of oil, it is easier to separate and stratify with the oil. The separated oil overflows into the oil drain tank as the liquid level rises, and then is discharged from the oil drain outlet. The separated water enters the drainage tank and is then discharged from the drainage outlet, effectively separating oil and water. Current coalescing devices suffer from the following technical bottlenecks: their working principle is based on the penetration of oil-water mixtures into the coalescing medium from the inside out. However, this design has three major limitations: 1) Insufficient adaptability of the separation mechanism—gravity separation is more suitable for water-in-oil emulsions (such as crude oil dehydration), but its treatment efficiency for oil-in-water emulsions (such as industrial oily wastewater) is significantly reduced, especially for micron-sized emulsified oil droplets (droplet size <50μm) and stubborn emulsions containing surfactants (such as the Span80 / SDS system), as the oil droplets lack sufficient upward driving force; 2) Lack of theoretical support for structural optimization—the separation efficiency of the equipment is limited by the passive matching of fluid residence time and coalescing medium pore size, requiring the increase of tank volume to compensate for performance defects, resulting in significant redundancy in the design of the equipment, with a footprint exceeding the requirements of compact industrial scenarios by more than 30%; 3) Inherent defects in functional design—the coalescing medium relies only on a single oleophilic surface and does not construct a synergistic mechanism of wettability gradient and flow field enhancement, making it more prone to medium blockage when processing high-viscosity oils, requiring frequent cleaning or replacement.

[0005] Therefore, developing a technology that addresses the challenges of small oil droplet size, high stability, low operating costs, simple equipment, and strong treatment capacity for emulsified oily wastewater is an urgent technical problem to be solved. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides an integrated oily wastewater treatment device and method based on wettability gradient coalescence-selective filtration. The vertical coalescence channel, horizontal oil channel, and horizontal water channel in the treatment device adopt an integrated three-dimensional flow channel structure with one inlet and two outlets. The wetting gradient field of the vertical channel and the selective filtration of the horizontal channel work synergistically to solve the problem of the step-by-step "demulsification-separation" process. By setting specific gradient wettability medium filling ratios and channel geometric parameters, the "wetting gradient force-driven-flow field synergy" model is strengthened. The oil-water separation method defined in this invention, based on a demulsification-separation coupling process driven by wetting gradient force, is suitable for stubborn emulsions containing surfactants.

[0007] This invention successfully solves the problem of emulsion stability by mathematically quantifying the time required for oil droplets to coalesce from their initial size to the target size. This model achieves, for the first time, a precise correlation between separation efficiency η≥99.6% and equipment geometric parameters, replacing the traditional experience-driven trial-and-error design approach. Based on this model, an integrated "coalescing-selective filtration" device was designed: employing a three-dimensional flow channel structure with a single inlet and two independent channels (one inlet, two outlets), integrating the vertical coalescence channel with the horizontal oil and water channels into a single device; driving the directional migration and coalescence of oil droplets through a wetting gradient medium, and achieving simultaneous oil-water separation with superhydrophilic underwater superoleophobic quartz sand (horizontal water channel) and superhydrophobic superoleophilic quartz sand (horizontal oil channel). This device completely breaks through the limitations of traditional step-by-step processes, reducing the equipment volume by more than 50% compared to traditional designs, lowering energy consumption, and significantly improving adaptability to compact industrial scenarios.

[0008] Specifically, the oily wastewater treatment device based on wettability gradient coalescence-selective filtration provided by this invention includes a vertical coalescence channel, a horizontal oil channel, and a horizontal water channel, adopting a one-inlet, two-outlet three-dimensional flow channel integrated structure. It integrates the three-step process of demulsifier, coalescer, and separator into a single device. The coordinated design of the vertical coalescence channel with the horizontal oil and water channels enables the demulsification and separation processes to be completed simultaneously. This invention achieves dynamic coupling between the vertical coalescence channel and the horizontal dual-channel composite flow field design. The triple coupling of gravity, capillary force, and wettability gradient achieves self-sufficiency in energy, realizing separation with zero external power.

[0009] "Zero external power" specifically means that the separation process has no active driving force. The core steps of oil-water separation (demulsification, coalescence, and stratification) rely entirely on the triple coupling effect without the need for additional mechanical or chemical energy input. The density difference drives the gravity that causes oil to float and underwater to settle. The capillary force of extremely wettable materials (such as superhydrophilic underwater superoleophobic quartz sand) selectively adsorbs water through capillary effect. The asymmetric surface energy field (superhydrophilic underwater superoleophobic + superhydrophobic superoleophilic region) generates a directional driving force. The wetting gradient force that promotes the migration and coalescence of oil droplets can achieve oil-water separation through demulsification, coalescence, and stratification.

[0010] The vertical coalescence channel is filled with 40-120 mesh quartz sand; the quartz sand is a gradient of superhydrophilic underwater superoleophobic quartz sand and superhydrophobic superoleophilic quartz sand in a volume ratio of 2:1 to 1:2. An asymmetric surface energy field is constructed, driving the directional migration of oil droplets through wettability gradient forces; this unbalanced force not only enhances the demulsification ability of the coalescer but also reduces head loss, thereby promoting the gradual coalescence and separation of oil droplets. Superhydrophilic underwater superoleophobic quartz sand can disrupt oil droplet stability and promote droplet detachment, reducing clogging, but an excessively high proportion can easily reduce coalescence efficiency; superhydrophobic superoleophilic quartz sand preferentially adsorbs oil droplets, promoting oil droplet adhesion and coalescence, but may increase the risk of coalescence bed clogging. When superhydrophilic underwater superoleophobic quartz sand and superhydrophobic superoleophobic quartz sand are packed in a volume ratio gradient of 2:1 to 1:2, the wetting forces at the interfaces of the two wettability types enhance the probability of oil droplet coalescence. Oil droplets tend to move away from the superhydrophilic interface and closer to the superhydrophobic interface, significantly improving coalescence efficiency. Furthermore, the hybrid design of superhydrophilic underwater superoleophobic quartz sand and superhydrophobic superoleophobic quartz sand plays a crucial role in reducing bed pressure drop. Superhydrophilic underwater superoleophobic quartz sand forms a continuous water film by adsorbing water molecules, while superhydrophobic superoleophobic quartz sand forms a continuous oil film by adsorbing oil droplets. This dual-film structure effectively avoids the pore blockage problem caused by liquid bridging in a single coalescing bed. When the continuous aqueous phase passes through the water film on the surface of the superhydrophilic underwater superoleophobic quartz sand, the flow resistance is significantly reduced, thereby lowering the bed pressure drop.

[0011] Meanwhile, the wetting force generated by the wettability gradient between the superhydrophilic underwater superoleophobic quartz sand and the superhydrophobic superoleophilic quartz sand further promotes the migration of oil droplets to the surface of the superhydrophobic superoleophilic quartz sand, significantly improving the coalescence efficiency of the oil droplets. The wetting force generated by this wettability gradient causes oil droplet particles in the liquid phase to tend to move away from the superhydrophilic interface and closer to the superhydrophobic interface, thereby significantly increasing the probability of oil droplet wetting and coalescence.

[0012] Furthermore, the unbalanced force of the constructed asymmetric surface energy field, combined with the laminar shear force of 0.1~0.5m / h in the vertical coalescence channel, causes the oil droplets to undergo a chain reaction of "adsorption-collision-fusion", increasing the particle size from 1-50μm to >500μm and the demulsification efficiency to >98%.

[0013] The length-to-diameter ratio of the horizontal oil channel and the horizontal water channel is 4:1 to 3:1. The cross-sectional areas of the two channels are equal to ensure balanced flow. Design rationale: When the length-to-diameter ratio is >4:1 to 3:1, the contact time between oil and water in the channel is prolonged; when the length-to-diameter ratio is <4:1 to 3:1, pressure drop accumulates, leading to a decrease in throughput.

[0014] Furthermore, the horizontal oil channel is filled with 40-120 mesh superhydrophobic and superoleophilic quartz sand, forming a low-adhesion oil film with a roll-off angle <5° and an oil recovery rate >99%; the horizontal water channel is filled with 40-120 mesh superhydrophilic underwater superoleophobic quartz sand, which repels oil droplets through the hydration layer, with a contact angle >150° and an oil content in the effluent <5 mg / L. The gradient distribution of the superhydrophobic and superoleophilic quartz sand and the superhydrophilic underwater superoleophobic quartz sand creates a surface energy difference, generating a non-equilibrium force to drive the directional migration of oil droplets, i.e., generating a wetting gradient force.

[0015] Furthermore, the preparation steps for superhydrophilic underwater superoleophobic quartz sand are as follows:

[0016] Step S1: Pour the piranha solution into a beaker containing quartz sand and react for 1.5~2.5h. Wash the quartz sand three times with deionized water and anhydrous ethanol respectively. Dry the washed quartz sand to obtain activated quartz sand.

[0017] Step S2: Add anhydrous ethanol and ammonium hydroxide to tetraethyl orthosilicate, heat and stir for 80-100 min; cool to room temperature, add the activated quartz sand prepared in step S1 to the cooled sol, stir and mix, wash the quartz sand 3 times with deionized water, and dry the washed quartz sand at 75-85℃ for 10-12 h to obtain superhydrophilic underwater superoleophobic quartz sand.

[0018] Further, in step S1: the piranha solution is slowly poured into a beaker containing 190-210g of quartz sand (QS) and reacted for 1.5-2.5h. The quartz sand is washed three times with 500mL of deionized water and 500mL of anhydrous ethanol, each time for 10-20min. The washed quartz sand is then dried at 55-65℃ for 5-7h to obtain activated quartz sand. The piranha solution is an acidic piranha solution, which is prepared by mixing concentrated sulfuric acid with a concentration of 96%-98% and hydrogen peroxide with a volume ratio of 3:1.

[0019] Step S2: Slowly add 90 wt.% anhydrous ethanol and 4 wt.% ammonium hydroxide (NH4OH) to 6 wt.% tetraethyl orthosilicate. Heat and stir in a water bath at 65-75℃ for 80-100 min at a stirring rate of 400-600 rpm to ensure complete hydrolysis of TEOS and prepare a sol. Cool the sol to room temperature and add 190-210 g of activated quartz sand prepared in step S1 to the cooled sol. Stir and mix for 9-11 h. Wash the quartz sand three times with 500 mL of deionized water for 10-20 min each time. Dry the washed quartz sand at 75-85℃ for 10-12 h to remove water and organic matter from the gel and obtain superhydrophilic underwater superoleophobic quartz sand.

[0020] Furthermore, the preparation steps for superhydrophobic and superoleophilic quartz sand are as follows:

[0021] Superhydrophilic underwater superoleophobic quartz sand and silane coupling agent were dispersed in n-heptane and stirred at 75-85℃ for 9-10 h. Then, the solution was centrifuged, the solid was collected and washed three times with 500-600 mL of n-heptane, and finally vacuum dried for 11-12 h to obtain superhydrophilic superoleophobic quartz sand.

[0022] Further, 190-200 g of superhydrophilic underwater superoleophobic quartz sand and 3-5 mL of silane coupling agent were dispersed in 190-210 mL of n-heptane and stirred at 75-85 °C for 9-10 h. Then, the solution was centrifuged, the solid was collected, and washed three times with 500-600 mL of n-heptane for 10-20 min each time. Finally, the solid was vacuum dried at 75-85 °C for 11-12 h to obtain superhydrophilic superoleophobic quartz sand.

[0023] Furthermore, the silane coupling agent is 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS).

[0024] Furthermore, the hydraulic residence time of oily wastewater in the vertical coalescence channel is:

[0025] ;

[0026] in:

[0027] t The hydraulic residence time (s) of oily wastewater in the vertical coalescence channel;

[0028] D 0 The diameter of the oil droplets before coalescence is in meters (m).

[0029] D The diameter of the coalesced oil droplets is in meters (m).

[0030] k This is the coalescence coefficient;

[0031] m The viscosity of the oil-water emulsion is Pa·s;

[0032] c O The surface tension of the oil is mN / m;

[0033] i O The static contact angle of oil on a superhydrophilic, superoleophobic quartz sand surface is expressed in °.

[0034] e Porosity of the quartz sand filling in the horizontal oil channel and horizontal water channel, °;

[0035] dO Let be the equivalent particle size of the quartz sand in the horizontal oil channel, in meters.

[0036] Furthermore, the lengths of the quartz sand filling in the horizontal oil channel and the horizontal water channel of the oil-water separation device are as follows:

[0037] ;

[0038] ;

[0039] in:

[0040] L O The length of the quartz sand filling in the horizontal oil channel, in meters;

[0041] L W The length of the quartz sand filling in the horizontal water channel, in meters;

[0042] c O The surface tension of the oil is mN / m;

[0043] c W The surface tension of water is mN / m;

[0044] i O The static contact angle of oil on a superhydrophilic, superoleophobic quartz sand surface is expressed in °.

[0045] i W The static contact angle of water on the surface of superhydrophobic and superoleophilic quartz sand, in °;

[0046] e Porosity of the quartz sand filling in the horizontal oil channel and horizontal water channel;

[0047] α The shape factor of the quartz sand;

[0048] d O Let be the equivalent particle size of the quartz sand in the horizontal oil channel, in meters.

[0049] d W Let be the equivalent particle size of quartz sand in the horizontal water channel, in meters.

[0050] m O Where is the viscosity of diesel oil, in mPa·s;

[0051] m W Where is the viscosity of water, mPa·s;

[0052] v O The apparent flow velocity of the oil in the horizontal oil channel is m / s;

[0053] v W Let be the apparent velocity of water in the horizontal water channel, in m / s.

[0054] The present invention also provides a method for separating oil and water using an integrated oil-water separation device based on wettability gradient coalescence-selective filtration as described above, comprising the following steps:

[0055] Oily wastewater enters the vertical coalescence channel through the feed inlet. Under the action of the wettability gradient formed by superhydrophilic underwater superoleophobic quartz sand and superhydrophobic superoleophilic quartz sand, oil droplets coalesce and then enter the intermediate chamber.

[0056] After the oily wastewater in the intermediate chamber flows to the horizontal oil channel, the water is intercepted by the capillary force of the superhydrophobic and superoleophilic quartz sand, and the oil flows out from the oil outlet after passing through the horizontal oil channel. The outflowing oil is recycled.

[0057] After the oily wastewater in the intermediate chamber flows into the horizontal water channel, the oil is intercepted by the capillary force of the superhydrophilic underwater superoleophobic quartz sand, and the water flows out from the outlet after passing through the horizontal water channel; the oil content of the effluent is <5mg / L.

[0058] Compared with the prior art, the advantages and positive effects of the present invention are:

[0059] 1. This invention employs a flow field-interface coupling design, specifically a one-inlet, two-outlet three-dimensional flow channel, which, through the synergy of a vertical coalescing channel and horizontal dual channels, couples the demulsification and separation processes into a single device, reducing the device volume by 50%.

[0060] 2. This invention achieves spontaneous oil-water separation through the triple coupling of gravity, capillary force, and wetting gradient, without the need for external power;

[0061] 3. The wetting gradient design ensures that the system pressure drop is less than 10 kPa, significantly reducing energy loss;

[0062] 4. The oil content in the effluent is <5mg / L, which is better than the GB31570-2015 standard; the oil recovery rate is >99%; it can treat stubborn emulsions containing surfactants, such as 0.1% Span80 emulsions, breaking through the limitations of traditional chemical demulsifiers.

[0063] 5. This invention reveals the coupling mechanism between oil droplet coalescence dynamics and selective filtration in a composite flow field, and establishes a theoretical model of "wetting gradient force driven-flow field synergistic enhancement", providing a new theoretical framework for emulsion separation;

[0064] 6. This invention proposes an integrated "structure-function" design paradigm, which provides a technical path for the development of compact separation equipment through geometric parameter optimization and matching with materials with extreme wettability. Attached Figure Description

[0065] Figure 1 This is a schematic diagram of the integrated oil-water separation device based on wettability gradient coalescence-selective filtration in Example 1;

[0066] Figure 2 The effect of the vertical coalescence channel on the droplet size distribution of the water-in-diesel emulsion in Example 2: (a) droplet size before coalescence; (b) droplet size after coalescence;

[0067] Figure 3 (a) Water contact angle and (b) underwater diesel contact angle of the superhydrophilic underwater superoleophobic quartz sand in Example 2;

[0068] Figure 4 The water contact angle and oil contact angle of the superhydrophobic and superoleophilic quartz sand in Example 2 are (a) and (b) respectively.

[0069] In the diagram: 1-Inlet; 2-Vertical coalescence channel; 3-Horizontal oil channel; 4-Oil outlet; 5-Horizontal water channel; 6-Water outlet; 7-Support; 8-Intermediate chamber. Detailed Implementation

[0070] The present invention can be better understood from the following embodiments. However, those skilled in the art will readily understand that the descriptions in the embodiments are for illustrative purposes only and should not, and will not, limit the invention as detailed in the claims.

[0071] The technical solution of the present invention will be further explained below with reference to implementation examples.

[0072] Example 1

[0073] This embodiment provides an integrated oil-water separation device based on wettability gradient, combining coalescence and selective filtration. Figure 1 As shown, it includes a feed inlet 1, a vertical coalescence channel 2, a horizontal oil channel 3, and a horizontal water channel 5. The vertical coalescence channel 2 is filled with 80-mesh quartz sand, and a support 7 is provided below the horizontal oil channel 3 and the horizontal water channel 5. The quartz sand is a gradient filling of superhydrophilic underwater superoleophobic and superhydrophobic superoleophilic quartz sand in a volume ratio of 1:1 to construct an asymmetric surface energy field, which drives the directional migration of oil droplets through wettability gradient force. The wettability gradient force can not only enhance the demulsification ability of the coalescer, but also reduce head loss to promote the gradual coalescence and separation of oil droplets.

[0074] Example 2

[0075] This embodiment provides a method for separating oil and water using an integrated oil-water separation device based on wettability gradient coalescence-selective filtration according to Embodiment 1. The method includes the following steps: a water-in-diesel emulsion with a concentration of 1000 mg / L and an oil droplet size of 1 μm is introduced into a vertical coalescence channel 2 through inlet 1. The apparent flow rate of the vertical coalescence channel 2 is 0.15 m / h. Superhydrophobic and superoleophilic quartz sand in the vertical coalescence channel 2 adsorbs oil droplets and promotes their coalescence. The superhydrophilic sand adsorbs water droplets to form a continuous aqueous phase. The superhydrophilic underwater superhydrophobic... Oil-bearing quartz sand preferentially adsorbs water molecules while repelling oil droplets. Under the action of gravity and capillary pressure, water forms a continuous water flow channel. After passing through the intermediate chamber 8, the water phase is discharged through the outlet 6 of the horizontal water channel. Superhydrophobic and superoleophilic quartz sand preferentially adsorbs oil droplets while repelling water molecules. Under the action of gravity and capillary pressure, oil forms a continuous oil flow channel. The oil phase is discharged through the oil outlet 4 of the horizontal oil channel. According to conventional ultraviolet spectrophotometry, the oil content of the effluent is <5mg / L, which is better than the GB31570-2015 standard.

[0076] The constructed asymmetric surface energy field, combined with the 0.15 m / h laminar shear force of the vertical coalescence channel, causes the oil droplets to undergo a chain reaction of "adsorption-collision-fusion," such as... Figure 2 As shown, after treatment with the vertical coalescence channel, the droplet size distribution of the water-in-diesel emulsion changed significantly: the median droplet size (Dx(50)) of the emulsion before coalescence was 1 μm, while the average droplet size of the emulsion after coalescence increased to 500 μm. This sharp increase in droplet size indicates that the emulsion underwent significant coalescence, causing the system to change from a stable emulsified state (emulsified oil) to an unstable dispersed state (dispersed oil). This phase transition disrupts the stability of the emulsion, significantly reduces the specific surface area of ​​the oil-water interface, and thus weakens the hindering effect of the interfacial film. This change is beneficial for the efficient separation of the oil and water phases by the horizontal oil channel and horizontal water channel in subsequent processes, providing favorable conditions for rapid oil-water stratification and collection.

[0077] Particle size testing method: The oil droplet size distribution in the emulsion was analyzed using a Malvern laser particle size analyzer (MASTERSIZER3000, Malvern, England). The Malvern laser particle size analyzer was preheated and a 300RF lens was selected. The refractive index of the oil phase (1.44-1.48) and the refractive index of the water phase (1.33) were set. After measuring the background with pure water, 50-100 μL of sample was slowly added, the shading was controlled at 8-15%, the circulation pump flow rate was kept at 1000 rpm, and the measurement was repeated 3 times. The median particle size (Dx(50)) was recorded to ensure data stability.

[0078] The preparation steps of superhydrophilic underwater superoleophobic quartz sand are as follows:

[0079] Step S1: Slowly pour 500 mL of piranha solution into a beaker containing 200 g of quartz sand (QS) and react for 2 h. Wash the quartz sand three times with 500 mL of deionized water and 500 mL of anhydrous ethanol, 15 min each time. Dry the washed quartz sand at 60 °C for 6 h to obtain activated quartz sand. The piranha solution is an acidic piranha solution, prepared by mixing 98% concentrated sulfuric acid and 30% hydrogen peroxide in a volume ratio of 3:1.

[0080] Step S2: 90 wt.% anhydrous ethanol and 4 wt.% ammonium hydroxide (NH4OH) were slowly added to 6 wt.% tetraethyl orthosilicate. The mixture was heated in a water bath at 70°C and stirred for 80 min at a stirring rate of 500 rpm to ensure complete hydrolysis of TEOS, thus preparing a sol. The sol was cooled to room temperature, and 200 g of activated quartz sand prepared in step S1 was added to the cooled sol. The mixture was stirred and mixed for 9 h. The quartz sand was washed three times with 500 mL of deionized water for 15 min each time. The washed quartz sand was dried at 80°C for 12 h to remove water and organic matter from the gel, thus obtaining superhydrophilic underwater superoleophobic quartz sand.

[0081] The static contact angles of water and oil were determined using a video optical contact angle meter (OCA25, EastDefy, Germany). The hydrophobic sample was horizontally fixed on the test platform, and 2 μL of ultrapure water or diesel fuel was precisely added using a microsyringe. Real-time images of the droplets were acquired using a high-resolution CCD camera, and the three-phase contact points were fitted using the Young-Laplace equation to calculate the static contact angle value. Measurements were taken at three different locations for each sample, and the results were calculated as the average ± standard deviation. Figure 3 As shown, functionalized quartz sand achieves superoleophobicity (underwater oil contact angle of 150.4°) and superhydrophilicity (water contact angle of 0°) through the rough structure of nano-SiO2.

[0082] The separation efficiency and water flux of the horizontal water channel are calculated as follows:

[0083] ;

[0084] In the formula, For water separation efficiency, and The oil content, in mg / L, is measured in the oily wastewater flowing out of the horizontal water channel and the oily wastewater in the intermediate chamber, respectively. In this embodiment, ， After calculation, .

[0085] ;

[0086] In the formula, It is the water flux, L·m -2 ·h -1 ; It is the volume of water flowing out of the horizontal water channel, in L; It is the cross-sectional area of ​​the horizontal water channel, in meters. 2 ; It is the time, in hours, required for water to completely pass through a horizontal water channel. In this embodiment... , , After calculation, The separation efficiency is 99.6%.

[0087] The preparation steps of superhydrophobic and superoleophilic quartz sand are as follows:

[0088] Silanization treatment: The aforementioned superhydrophilic underwater superoleophobic quartz sand and 4 mL of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) silane coupling agent were dispersed in 200 mL of n-heptane and stirred at 80 °C for 9 h. Then, the solution was centrifuged at 1500 rpm for 5 min, the solid was collected, and washed three times with 500 mL of n-heptane for 15 min each time. Finally, it was vacuum dried at 80 °C for 11 h to obtain superhydrophobic and superoleophilic quartz sand. Functionalized quartz sand is modified through a rough nano-SiO2 structure + silane coupling agent molecular layer, such as... Figure 4 As shown, it achieves superhydrophobicity (water contact angle of 154°), superoleophilicity (oil contact angle of 0°), a separation efficiency of 99.05%, and an oil flux of 1675 Lm. -2 h -1 After 1000 cycles of testing, the water contact angle decreased by 2%.

[0089] The separation efficiency and oil flux of the horizontal oil channel are calculated as follows:

[0090] ;

[0091] In the formula, For oil separation efficiency, and The values ​​are the water content in the oil flowing out of the horizontal oil channel and the water content in the oily wastewater in the intermediate chamber, respectively, in mg / L.

[0092] ;

[0093] In the formula, It is the oil flux, L·m -2 ·h -1 ; It is the volume of oil flowing out of the horizontal oil channel, in liters (L). It is the cross-sectional area of ​​the horizontal oil passage, m 2 ; It is the time, h, required for oil to completely pass through the horizontal oil channel.

[0094] In this embodiment, experimental data, , , ; , , , .

[0095] The contact angle decay is calculated as follows:

[0096] ;

[0097] In the formula, i W The static contact angle of water on a superhydrophobic and superoleophilic silica sand surface. i t It is the static contact angle of water on the surface of superhydrophobic and superoleophilic quartz sand after a certain period of time or under certain conditions.

[0098] The initial water contact angle of the superhydrophobic and superoleophilic quartz sand was 154°, and after 1000 cycles of testing, the water contact angle was 150.9°, with a decay rate of 2%.

[0099] The hydraulic residence time of oily wastewater in the vertical coalescence channel of the oil-water separator is as follows:

[0100] ;

[0101] The lengths of the quartz sand filling in the horizontal oil channel and horizontal water channel of the oil-water separator are as follows:

[0102] ;

[0103] ;

[0104] in:

[0105] t The hydraulic residence time (s) of oily wastewater in the vertical coalescence channel;

[0106] L O The length of the quartz sand filling in the horizontal oil channel, in meters;

[0107] L W The length of the quartz sand filling in the horizontal water channel, in meters;

[0108] D 0 The oil droplet size before coalescence is 1×10⁻⁶. -6 m;

[0109] D The particle size of the coalesced oil droplets is 5 × 10⁻⁶. -4 m;

[0110] k The coalescence coefficient is set to 0.35.

[0111] m The viscosity of the oil-water emulsion is 1.0 mPa·s;

[0112] c O The surface tension of the oil is 30 mN / m;

[0113] c W The surface tension of water is 72 mN / m;

[0114] i O The static contact angle of oil on the superhydrophilic underwater superoleophobic quartz sand surface is taken as 150.4°.

[0115] i W The static contact angle of water on the superhydrophobic and superoleophilic quartz sand surface is taken as 154°.

[0116] e The porosity of the quartz sand filling in the horizontal oil channel and the horizontal water channel is taken as 0.43.

[0117] α The shape factor of the quartz sand is taken as 0.78;

[0118] d O The equivalent particle size of the quartz sand in the horizontal oil channel is taken as 1.8 × 10⁻⁶. -4 m;

[0119] d W The equivalent particle size of the quartz sand in the horizontal water channel is taken as 1.8 × 10⁻⁶. -4 m;

[0120] m O The viscosity of diesel oil is 3 mPa·s;

[0121] m W Let be the viscosity of water, taken as 1 mPa·s;

[0122] v O Let be the apparent flow velocity of the oil in the horizontal oil channel, taken as 4.17 × 10⁻⁶. -5 m / s;

[0123] v W Let be the apparent velocity of water in the horizontal water channel, taken as 4.17 × 10⁻⁶. -5 m / s;

[0124] Substitute the reference parameters:

[0125] ;

[0126] It can be seen that 1μm oil droplets aggregate to 500μm within 30 minutes.

[0127] Substitute the reference parameters:

[0128] ;

[0129] ;

[0130] In summary, this embodiment achieves this through a quantitative model:

[0131] (1) Efficient coalescence: 1μm oil droplets coalesce to 500μm within 30 minutes, solving the problem of kinetic stability of oil-in-water emulsions.

[0132] (2) Compact design: The length of the quartz sand filling in the horizontal oil channel is precisely optimized to 0.12m, and the length of the quartz sand filling in the horizontal water channel is precisely optimized to 0.14m. While ensuring a separation efficiency of 99.6%, the equipment volume is reduced.

[0133] The core value of the formula

[0134] Scientific significance: For the first time, a quantitative dynamic model of oil droplet "directional migration-channel coalescence-two-phase separation" was established through the synergistic control of wettability gradient field and pore flow field, providing a theoretical foundation for the design of high-throughput oil-water separation system;

[0135] Engineering significance: It provides a parametric design tool for the integrated "coalescing-filtration" device, realizing closed-loop optimization from material properties → flow field control → geometric parameters, replacing the experience-based trial and error mode.

[0136] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for separating oily wastewater using an integrated coalescence-selective filtration oily wastewater treatment device based on wettability gradient, characterized in that: The integrated coalescence-selective filtration oily wastewater treatment device based on wettability gradient includes a vertical coalescence channel, a horizontal oil channel, and a horizontal water channel, adopting a one-inlet, two-outlet three-dimensional flow channel integrated structure. The oily wastewater is emulsified oil with an oil droplet size <50μm. The apparent flow velocity of the vertical coalescence channel is 0.1~0.5m / h. The vertical coalescence channel is filled with 40~120 mesh quartz sand, which is a gradient of superhydrophilic underwater superoleophobic quartz sand and superhydrophobic superoleophilic quartz sand in a volume ratio of 2:1~1:

2. The aspect ratio of the horizontal oil channel and the horizontal water channel is 4:1~3:

1. The horizontal oil channel is filled with 40-120 mesh superhydrophobic and superoleophilic quartz sand, and the horizontal water channel is filled with 40-120 mesh superhydrophobic underwater superoleophilic quartz sand. The method for separating oily wastewater using an integrated coalescence-selective filtration oily wastewater treatment device based on wettability gradient includes the following steps: Oily wastewater enters a vertical coalescence channel through the inlet. Under the action of the wettability gradient formed by superhydrophilic underwater superoleophobic quartz sand and superhydrophobic superoleophilic quartz sand, oil droplets coalesce and then enter the intermediate chamber. In the intermediate chamber, the oily wastewater flows to the horizontal oil channel. Under the capillary force of the superhydrophobic superoleophilic quartz sand, the water is intercepted, and the oil flows out through the horizontal oil channel from the oil outlet. The outflowing oil is recycled. Then, in the intermediate chamber, the oily wastewater flows to the horizontal water channel. Under the capillary force of the superhydrophilic underwater superoleophobic quartz sand, the oil is intercepted, and the water flows out through the horizontal water channel from the water outlet. The oil content of the effluent is <5 mg / L. The hydraulic residence time of oily wastewater in the vertical coalescence channel is: ； in: The hydraulic residence time (s) of oily wastewater in the vertical coalescence channel; The diameter of the oil droplets before coalescence is in meters (m). The diameter of the coalesced oil droplets is in meters (m). This is the coalescence coefficient; The viscosity of the oil-water emulsion is Pa. . s; The surface tension of the oil is mN / m; The static contact angle of oil on a superhydrophilic, superoleophobic quartz sand surface is expressed in °. Porosity of the quartz sand filling in the horizontal oil channel and horizontal water channel; Let be the equivalent particle size of the quartz sand in the horizontal oil channel, in meters.

2. The method for separating oily wastewater using the integrated coalescence-selective filtration oily wastewater treatment device based on wettability gradient according to claim 1, characterized in that, The preparation steps of superhydrophilic underwater superoleophobic quartz sand are as follows: Step S1: Pour the piranha solution into a beaker containing quartz sand and react for 1.5~2.5h. Wash the quartz sand three times with deionized water and anhydrous ethanol respectively. Dry the washed quartz sand to obtain activated quartz sand. Step S2: Add anhydrous ethanol and ammonium hydroxide to tetraethyl orthosilicate, heat and stir for 80-100 min; cool to room temperature, add the activated quartz sand prepared in step S1 to the cooled sol, stir and mix, wash the quartz sand 3 times with deionized water, and dry the washed quartz sand at 75-85℃ for 10-12 h to obtain superhydrophilic underwater superoleophobic quartz sand.

3. The method for separating oily wastewater using the integrated coalescence-selective filtration oily wastewater treatment device based on wettability gradient according to claim 1, characterized in that, The preparation steps of superhydrophobic and superoleophilic quartz sand are as follows: Superhydrophilic underwater superoleophobic quartz sand and silane coupling agent were dispersed in n-heptane and stirred at 75-85℃ for 9-10 h. Then, the solution was centrifuged, the solid was collected and washed three times with 500-600 mL of n-heptane, and finally vacuum dried for 11-12 h to obtain superhydrophilic superoleophobic quartz sand.

4. The method for separating oily wastewater using the integrated coalescence-selective filtration oily wastewater treatment device based on wettability gradient according to claim 1, characterized in that, The lengths of the quartz sand filling in the horizontal oil channel and the horizontal water channel are respectively: ； ； in: The length of the quartz sand filling in the horizontal oil channel, in meters; The length of the quartz sand filling in the horizontal water channel, in meters; The surface tension of the oil is mN / m; The surface tension of water is mN / m; The static contact angle of oil on a superhydrophilic, superoleophobic quartz sand surface is expressed in °. The static contact angle of water on the surface of superhydrophobic and superoleophilic quartz sand, in °; Porosity of the quartz sand filling in the horizontal oil channel and horizontal water channel; The shape factor of the quartz sand; Let be the equivalent particle size of the quartz sand in the horizontal oil channel, in meters. Let be the equivalent particle size of quartz sand in the horizontal water channel, in meters. The viscosity of diesel oil is given in mPa. . s; The viscosity of water, in mPa . s; The apparent flow velocity of the oil in the horizontal oil channel is m / s; Let be the apparent velocity of water in the horizontal water channel, in m / s.

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