Development and Performance Study of Continuous Oil–Water Separation Device Based on Superhydrophobic/Oleophilic Mesh

1. Introduction

Oily wastewater is widely distributed in the field of oil and gas extraction, which can cause environmental damage and resource waste without further treatment [1]. At present, the existing treatment methods for oily wastewater vary depending on the environmental conditions and oil content; they include gravity separation, flocculation separation, membrane separation, and adsorption separation [2,3,4,5,6]. These technologies have demonstrated excellent oil–water separation performance in practical production, but they also face issues such as low treatment efficiency and high operating costs [7]. As solid waste is generated during the flocculation process, it further increases the complexity of the process [6]. In particular, some traditional oil–water separation materials exhibit the characteristic of simultaneously adsorbing both oil and water, which not only reduces the separation efficiency but also makes post-adsorption material disposal difficult. Based on advanced oleophilic and hydrophobic materials as oil–water separation media, developing a new oil–water separation technologies can effectively improve the separation efficiency of oily wastewater, and enhance oil and gas resource recovery efficiency [8,9]. Due to the superhydrophobicity and superoleophilicity in the air, superhydrophobic materials can be processed into two-dimensional mesh or three-dimensional porous sponge for oil–water separation, making these a research hotspot in oil–water separation technology.

The development of superhydrophobic materials is inspired by biomimetic science, primarily referring to surfaces with a contact angle (CA) greater than 150° and a sliding angle less than 10° [10,11]. According to their study of the unique wettability of lotus leaves, water striders, spiders, and other creatures, Jiang et al. [12,13] identified that ultra-low surface energy and micro–nanostructures are the key factors for constructing superhydrophobic surfaces. After that, various superhydrophobic surfaces were prepared using templates, chemical corrosion, anodization, thermal oxidation, and spraying methods, which effectively drove the advancement of interfacial science. Moreover, based on the special properties of superhydrophobic surfaces, superhydrophobic materials have been applied in numerous fields, including self-cleaning, anti-icing, oil–water separation, anti-corrosion, and granulation [14,15,16,17,18]. In turn, this also promoted the development of industrial production. In 2004, Jiang’s group developed a superhydrophobic/superoleophilic mesh using a simple spraying method, which exhibited excellent oil–water separation performance [19]. Researchers have since used various methods to fabricate two-dimensional (2D) superhydrophobic meshes and three-dimensional (3D) superhydrophobic porous adsorption materials, achieving efficient oil recovery while minimizing the influence of water [20,21].

Even though superhydrophobic/superoleophilic materials have shown great potential in applications for oil–water separation, they also remain largely confined to laboratory research and are difficult to use in industrial production. The main reason is that the wettability of superhydrophobic/superoleophilic materials primarily relies on their unique surface structures, which suffer from poor mechanical stability in practical applications. Additionally, the modified components used in the construction of superhydrophobic coatings are prone to dissolution by oil, resulting in coating damage and high economic costs [10,22,23]. Moreover, in actual production or daily life, the density of oil is usually lower than that of water, resulting in oil floating on the water. When superhydrophobic meshes are used for oil–water separation, the bottom of the mesh can be easily blocked by water, preventing the oil from coming into contact with the mesh and reducing the efficiency of oil–water separation. Furthermore, as the pressure of the water increases, it also can penetrate the superhydrophobic mesh [24,25]. Therefore, the development of continuous oil–water separation devices based on superhydrophobic materials faces challenges. The effective way to solve the problem of oil-water entrainment is to design a reasonable oil–water separation channel, which can increase the contact between the mesh and the oil–water mixture for separation.

In this study, a stable superhydrophobic surface was developed using stainless-steel mesh as the substrate, combined with polytetrafluoroethylene (PTFE)/polyvinylidene fluoride (PVDF) micro–nano powder and epoxy adhesive. Stainless-steel wire mesh is characterized by its corrosion-resistance and low cost, which can effectively decrease the cost of the oil–water separation process. A simple and scalable spray-coating method was employed as the modification technique. The stainless-steel mesh was shaped into V-shaped grooves, and the effects of the geometry and process parameters of the single channel on the oil–water separation efficiency were systematically explored. The optimized parameters were determined through a carefully evaluation. Furthermore, a scaled-up separation device was designed and fabricated based on the optimized parameters. The performance of the device was assessed by performing a long-term, continuous operation in separating oil–water mixtures. The stability of the surface resistant to the oil was also evaluated.

2. Materials and Method

2.1. Materials and Characterization

The epoxy adhesive was purchased from Hunan Brothers New Materials Co., Ltd. (Changsha, China). The micro/nano polytetrafluoroethylene (PTFE) powder and nano-scale polyvinylidene fluoride (PVDF) powder were obtained from Dongguan Huachuangxin Plastics Co., Ltd. (Dongguan, China). The perfluorosilane was obtained from Shanghai Titan Scientific Co., Ltd. (Shanghai, China). The kerosene was of analytical grade and obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). The ethyl acetate was purchased from Shanghai Titan Scientific Co., Ltd. The stainless-steel mesh of 100, 120, and 150 mesh were purchased from Runda Mesh Industry in Hengshui, Hebei Province.

The morphology and composition of the samples were analyzed via scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS, JSM-7610F, JEOL, Tokyo, Japan). A contact angle goniometer (Shanghai Zhongchen Digital Technology Equipment Co., Ltd., Shanghai, China, JC2000C1) was used to evaluate the wettability of the surface. The chemical composition of the surface was measured using Fourier transform infrared spectroscopy (FTIR) (Spectrum Two Li10014, PerkinElmer, Waltham, MA, USA) and X-ray photoelectron spectrometer (XPS) (Escalab250Xi, Thermo Scientific, Waltham, MA, USA) with an Al Kα (E = 1486.6 eV).

2.2. Preparation of Superhydrophobic Surface

The composition of the paint used to prepare the superhydrophobic surface included micro/nano-scale powder, adhesives, and dispersants. The micro/nano-scale powder was composed of 1 g of micron-scale Polytetrafluoroethylene (PTFE) powder, 1.5 g of nano-scale PTFE powder, and 0.5 g of PVDF powder. The adhesives were prepared with 1 g of E44 epoxy resin, and 0.2 g of curing agent. The addition of 0.45 g of perfluorosilane (CAS No.: 83048-65-1) was used to enhance the hydrophobic ability. Then, the above materials were dispersed into 30 g of ethyl acetate. After ultrasonic treatment for 30 min, the superhydrophobic stainless-steel mesh was fabricated using a spray-drying process, with a spraying distance of 15–20 cm. The V-shaped stainless-steel meshes with 100, 120, and 150 mesh sizes were cleaned for 30 min to remove surface contaminants. The prepared coating mixture was then uniformly sprayed onto the surface of the stainless-steel mesh using a spray gun. The paint was applied to the stainless-steel mesh at a density of 0.1–0.2 mL/cm². Finally, the sprayed meshes were placed in an oven at 80 °C and cured for 3 h. The CAs of water and kerosene on the prepared superhydrophobic surface were measured to evaluate the wettability of the coatings.

2.3. Oil–Water Separation Process

In the oil-water separation process, an oil–water mixture with a total mass of 500 g was prepared, where the mass fraction of oil in the mixture ranged from 10 wt% to 30 wt%. The mixture was stirred at a speed of 600–1000 rpm for 5 min using a magnetic stirrer. To maintain the stability of the oil–water mixture, the magnetic stirring was in continuous operation during the oil–water separation process. Then, the prepared oil–water mixture was transferred to the oil–water separation tank using a peristaltic pump. The rotation speed of the peristaltic pump was controlled at 30–100 rpm, with the flow rate calibrated to 21.1 L/h at 100 rpm. The experimental setup for a single-channel device is shown in Figure 1. The opening and tilt angle of the channel could be adjusted using a support frame to control the residence time and contact area of the mixture within the channel. The separated oil and water at the outlet were collected, weighed, and recorded. The separation efficiency (η) was calculated using the following equation:

$η = \frac{m - m_{1}}{m_{0}} \times 100 %$

(1)

where m is the total mass of the oil–water mixture before separation (g), m₀ is the mass of the kerosene before separation (g), and m₁ is the mass of the separated mixture at the outlet (g). The single-channel oil–water separation device consisted of a support, a 15 cm long V-shaped channel, and a polymethyl methacrylate (PMMA) baffle (thickness: 0.8 cm) at the inlet of channel to prevent backflow of the oil–water mixture. The width of the V-shaped channel could be adjusted by modifying the angle of the support, allowing for the evaluation of separation efficiency in channels with different structures. During the separation process, the oil–water mixture was transported to the channel inlet using a peristaltic pump. The oil penetrated the stainless-steel mesh and drips into the oil collection tank, while water flows out from the end of the channel. Prior to the separation process, the device was pre-wetted with pure water and pure oil to reduce errors caused by residual liquid in the device. The separation time was controlled throughout the experiment. Based on the optimized oil–water separation parameters of a single channel, in order to further improve the processing capacity of the equipment for oil–water mixtures, this study also designed and fabricated a multi-channel continuous oil–water separation device. Additionally, a preliminary investigation was carried out on the efficiency of the scaled-up oil–water separation.

4. Conclusions

In this study, a V-shaped channel was prepared with a modified mesh, which was prepared by modifying the surface of a stainless-steel mesh using a spraying method. The paint was uniformly distributed on the wires of the mesh; the modified surface exhibited excellent superhydrophobicity, with a water contact angle of 153.20°. The structural parameters of a V-shaped single-channel used for oil–water separation were optimized. and the effect of process parameters and oil content on the separation efficiency were studied. When the mesh sizes were 100 and 120, a small amount of water penetrated through the pores of the mesh, which reduced the separation efficiency. Taking into account both separation efficiency and separation flux, the optimal mesh pore size was 150 mesh, achieving single-stage and two-stage oil separation efficiencies of 92.79% and 98.96%, respectively. The valley angle of the V-shaped mesh had a significant impact on separation efficiency. A smaller angle resulted in higher separation efficiency, but also reduced the flow rate. To keep the balance between the separation efficiency and flux, the optimal valley angle was determined to be 38.9°. With the increase of channel inclination, the residence time of oil–water mixture in the channel was reduced, which also decreased the separation efficiency. When the inlet flow rate was 352.94 mL/min, the single-stage and two-stage separation efficiencies were 88.36% and 97.31%, respectively. In addition, a multi-channel device was designed and manufactured, and its continuous performance for separating oil and water evaluated. The results showed that the equipment exhibited excellent operational stability. After 36 h of continuous separation, the single-stage and two-stage separation efficiencies for the oil phase were 94.60% and 98.76%, respectively. This study provides a foundation for the design and application of oil–water separation devices based on superhydrophobic materials, offering practical solutions for the application of superhydrophobic materials in industrial production.

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Tianxin Chen www.mdpi.com

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Development and Performance Study of Continuous Oil–Water Separation Device Based on Superhydrophobic/Oleophilic Mesh

1. Introduction

2. Materials and Method

2.1. Materials and Characterization

2.2. Preparation of Superhydrophobic Surface

2.3. Oil–Water Separation Process

4. Conclusions

Greenberg

1. Introduction

2. Materials and Method

2.1. Materials and Characterization

2.2. Preparation of Superhydrophobic Surface

2.3. Oil–Water Separation Process

4. Conclusions

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