Preparation of oxidized carbon black grafted with nanoscale silica and its demulsification performance in water-in-oil emulsion

Colloids and Surfaces A 582 (2019) 123878

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Preparation of oxidized carbon black grafted with nanoscale silica and its demulsification performance in water-in-oil emulsion

T

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Fan Yea, Xia Jianga, Yuanzhu Mia, , Jiazhe Kuanga, Zhiming Huanga,b, Fan Yua, Zejun Zhanga, Huaikui Yuana a b

School of Chemistry & Environmental Engineering, Yangtze University, Jingzhou, 434023, PR China China Oilfield Service Limited, Tianjin, 300450, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon material Interfacial activity Demulsification performance Crude oil emulsion

In order to reduce the cost of demulsifiers and improve the demulsification performance of carbon material demulsifier, the nano-SiO2 was grafted onto the surface of oxidized carbon black (Ox-CB) by sol-gel method in current study. The novel composite demulsifier was analyzed by fourier transform infrared spectroscopy (FT-IR), field-emission scanning electron microscope (FE-SEM), energy dispersive X-ray (EDX) and X-ray diffraction spectra (XRD) which confirmed the partial grafting of SiO2 on the oxidized carbon black (Ox-CB).The influences of temperature, settling time, and concentration of the composite demulsifier on the efficiency for breaking up water-in-oil emulsions were investigated in detail by bottle test. It was found that the best operating conditions to remove more than 90% of the water from the original emulsion were a temperature of 75 ℃ and a demulsifier concentration of 500 mg/L. The wettability, oil-water interfacial tensions (IFT) and interfacial activity of the demulsifier were further measured to explore the demulsification mechanism. Possible demulsification mechanism showed that the composite demulsifier could lead to the breakup of water-in-oil emulsions by bridging, flocculation and coalescence. The high interfacial activity is the main factor affecting the demulsification.

1. Introduction Water cut of crude oil is a common phenomenon in the process of oil and gas field development. It not only brings a series of difficulties for

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oil extraction, but also has an adverse influence on processing and refining [1]. So it is of great importance to remove water completely from the crude oil emulsion before transporting or refining process [2,3]. Chemical demulsification is the most widely used method to

Corresponding author. E-mail address: [email protected] (Y. Mi).

https://doi.org/10.1016/j.colsurfa.2019.123878 Received 22 June 2019; Received in revised form 17 August 2019; Accepted 27 August 2019 Available online 28 August 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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separate the water completely from emulsion in oil field as the chemical additives can accelerate the emulsion breaking process [4,5]. With the nanotechnology development, the design and application of nanomaterials can take advantage of its own capabilities to improve the performance of water separation [6] and have attracted significant attention in the petroleum industry [7]. Nanoparticles such as SiO2 [2,8], TiO2 [6], magnetic nanoparticles [9–12], carbon nanomaterials [13–17] (graphene oxide, carbon nanotubes et al.) have been employed as demulsifiers in previous studies, overcoming some shortfalls of traditional demulsifiers in the process of demulsification. For example, Wang et al. [8] dispersed nano-SiO2 into polyether demulsifier TA1031 to form nanomodified demulsifier using an in situ preparation method. The demulsification efficiency was improved by about 20% and the sedimentation time was also shortened when the ratio of nano-SiO2 and TA1031 was 1:10. Nikkhah et al. [6] reported a commercial demulsifier modified with nano-TiO2 particles under ultrasonic conditions. The demulsifying efficiency is greater than 90% and the settling time was also reduced. Liu et al. [16] also reported a magnetic graphene oxide(M-GO) demulsifier, which could be reused for 6–7 times with 99.98% efficiency. Xu et al. [18] prepared a series of functional fluorinated graphene (FG) and the hydrazine hydrate modified FG (HFG) which had superior demulsification performances in oily wastewater ranging from acidic to alkaline conditions and at different concentrations of NaCl. Wang et al. [14] prepared a series of reduced graphene oxide (rGO). The optimal samples could recover 99.97% oil from the oil-in-water emulsions at ambient temperature for 30 min gravity settling. Liu et al. [17] used a functionalized multiwalled carbon nanotubes (CNTs) to remove the oil from oily wastewater and the demulsification efficiency was 99.8%. As already reported, the carbon materials such as GO and CNTs had excellent demulsification performance. However, the relatively high production cost limited their large-scale application. As is well known, carbon black is a low-cost and non-toxic raw material. Therefore, carbon black-based materials can not only reduce the cost of demulsifier, but also reduce the risk of environmental pollution. In this study, a new carbon-based demulsifier (Ox-CB@SiO2) was prepared used carbon black as start materials, which had relatively low cost and good interfacial activity. The current work demonstrated that Ox-CB@SiO2 had a potential application in the treatment of water-in-oil emulsion.

2.2. Oxidation of carbon black Briefly, 0.5 g of carbon black (CB) was added into a mixed solution of HNO3 (150 mL) and H2SO4 (50 mL) under ultra-sonication for about 30 min. And then, the above suspension was reacted under reflux at 80 °C for 10 h. After cooling to room temperature, it was subsequently centrifuged after washing with deionized water repeatedly until the separation water is neutral. At last, the as-prepared samples (oxidized carbon black, Ox-CB) was dried by vacuum freeze-drying. 2.3. Preparation of Ox-CB@SiO2 Sol-gel method was utilized to prepare the Ox-CB@SiO2. Firstly, a solution containing 50 mL of ethanol, 15 mL of ammonia solution, 75 mL of deionized water and 0.1 g of CB was stirred under ultrasonic for 30 min to ensure complete mixing. Then, the ethanol solution containing 5 mL of tetraethyl orthosilicate was added to the above suspension, and the reaction proceeded at ambient temperature for 10 h. Afterwards, the suspension was transferred to a Teflon-lined stainless steel autoclave and heated at 110 ℃ for about 10 h. After cooled to ambient temperature, the products are subjected to centrifugation and washed with ethanol and deionized water for several times to remove the residual reactants until pH value reached about 7, and then to be vacuum freeze-dried for further using and characterization. In comparison with the above composite materials, SiO2 was prepared by the same method in which no Ox-CB was added. Fig. 1 shows reactions schematic of as-prepared Ox-CB@SiO2. 2.4. Emulsion preparation The water-in-oil emulsion was prepared by mixing the crude oil and deionized water. The crude oil samples was used without further treatment. Briefly, 200 mL of crude oil and 80 mL of deionized water were mixed at 60 ℃ and stirred at 10,000 rpm until the two phases became completely homogeneous by using a homogenizer (FJ-200, Shanghai) to obtain a homogeneous W/O emulsion. The W/O emulsion could keep stable for at least 24 h under ambient temperature without apparent water/oil separation. 2.5. Bottle test

2. Experimental procedure

The bottle test was used to perform the demulsification experiments of Ox-CB@SiO2. Typically, 0.2 wt% of Ox-CB@SiO2 dispersions with different volume was added into 20 mL of W/O emulsion, and one of them was used as blank test without adding demulsifiers. The dosage of Ox-CB@SiO2 in emulsion can be calculated from the mass concentration of Ox-CB@SiO2. Firstly, the bottles were put into a 70 °C water bath for 10 min, and then vigorously shaking for 2 min. Afterwards, the bottles was placed in the settled water bath to achieve the water/oil separation. The demulsification efficiency was determined by the volume of removed water, and it was calculated by the following equation:

2.1. Chemicals Carbon black (99.5%,1 μm) was purchased from Shanghai Macklin biochemical Co. Ltd. Ethanol, tetraethyl orthosilicate (TEOS) and ammonia solution (NH3·H2O) were purchased from Aladdin Chemistry (Shanghai, China). Nitric acid (HNO3) and concentrated sulfuric acid (H2SO4)were supplied by Sinopharm Chemical Reagent limited corporation. The crude oil sample was provided by Jianghan oilfield (Hubei Province, China). Physicochemical properties of crude oil was listed in Table 1. 0# diesel (ρ25°C =0.823 g cm−3; η25 °C = 5.43 mPa s) was provided by Sinopec gas station. All chemicals were used as received without further purification.

DE (wt%) = V/V0×100%

(1)

Where DE is the demulsification efficiency (%),V0 is the water content in original water/oil emulsion, V is the removed water content after demulsification. Table 1 Physicochemical properties of the crude oil samples.

2.6. Characterizations

Density (20℃) (g/cm3)

Viscosity (50℃) (mPa s)

Freezing point (℃)

Asphaltene (%)

Resin (%)

Wax (%)

Water (%)

0.8830

128

34

1.65

25.06

7.81

30

Fourier transform infrared spectroscopy (FT-IR) with a resolution of 4 cm−1 was utilized to determine the chemical groups of the samples (Nicolet 6700, Thermo Fisher Scientific, USA). The samples was uniformly dispersed in KBr by compressing the powder into tablets. The surface morphology of the as-prepared samples was characterized on 2

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Fig. 1. Reaction schematic of as-prepared Ox-CB@SiO2.

MIRA3 field-emission scanning electron microscope (FE-SEM, TESCAN Co., Czech) with an acceleration voltage of 15 kV. And the elemental analysis was detected in energy dispersive X-ray spectroscopy (EDX). Xray diffraction spectra (XRD) was acquired with a diffractometer (D8 Discover) using Cu Kα radiation at a rate of 0.05°/min (Bruker, Germany). The interfacial tensions (IFT) were measured by DSA 30 Process Tensiometer (Kruss. Germany). The temperature was kept constant at 25 ℃. The tensiometer was tested and calibrated before each measurement. The three-phase contact angle (θ) was observed on a DSA 30 contact angle goniometer (Kruss, Germany) by the sessile drop method at ambient temperature. The demulsification mechanism was investigated used an polarizing microscope equipped with a digital camera (Caikang, DM2500 P).

(eC]O). The peaks at about 1620 cm−1 is assigned to the vibration of the adsorbed water and also ascribed to CeC stretching vibrations, which is detected in all spectra. The peaks at 1400 cm−1 is assigned to the wagging vibration of eCH2 and also attributed to the bending vibration of the OeH bend. The peaks at 1100 cm−1 is assigned to unsymmetric stretching vibration of SieOeSi and SieOeC [2,20].The peaks at 955 cm−1 is assigned to the wagging vibration of SieOH, which disappeared in composite products. The peaks at 800 cm−1 and 470 cm−1 is ascribed to symmetric stretching vibration of SieO. As mentioned above, we can confirm that the SiO2 is grafted onto the OxCB.

3. Results and discussion

The surface morphology of CB, Ox-CB and Ox-CB@SiO2 has been investigated by FE-SEM (Fig. 3). As shown in Fig. 3a, CB exhibits irregular granular morphology with an average size of 1 μm. Fig. 3b reveals that Ox-CB is composed of even smaller subunits with an average dimension of 500 nm. The oxidized processing decreased the size of CB particles and increased the structural imperfection. The low-magnificaion image of Ox-CB@SiO2 (Fig. 3c) shows that lots of spherical SiO2 nanoparticles are evenly distributed on the surface of the Ox-CB, which are ascribed to the hydrolysis of tetraethyl orthosilicate. The enlarged image is shown in Fig. 3d, the SiO2 nanoparticles is uniform and smooth which may be attached to Ox-CB by physical adsorption or chemical bonding. In addition, the chemical composition of Ox-CB@SiO2 was determined by energy dispersive X-ray (EDX) analysis. Fig. 3e shows the peaks of C, O and Si and no impurity elements. The mass ratio of C and Si is approximately 2.6 from the EDX analysis, which is close to the theoretical calculation. The experimental results are consistent with the conclusion that the SiO2 is grafted onto the surface of Ox-CB particles. And the surface area of Ox-CB@SiO2 is examined using a Micromeritics instrument, the result shows that the BET surface area values is 37.7970 m2/g.

3.2. FE-SEM and EDX of Ox-CB@SiO2 demulsifier

3.1. FT-IR of Ox-CB@SiO2 demulsifier The FT-IR spectra of CB, Ox-CB, SiO2 and Ox-CB/SiO2 are illustrated in Fig. 2. The broad adsorption band located at 3420 cm−1 can be ascribed to the stretching of the hydroxyl groups. It was obvious that the peaks at 3131 cm−1 is assigned to the asymmetric stretching vibration of OeH. The peaks at 2362 cm-1 is assigned to the background of CO2 [19].Due to the oxidation of CB, adsorption peak located at 1732 cm-1 in Fig. 1d is assigned to stretching vibration of the carboxylic group

3.3. XRD of Ox-CB@SiO2 demulsifier XRD analysis was further used to investigate the crystal information of SiO2, CB, Ox-CB and Ox-CB@SiO2. As is shown in Fig. 4a and b, two diffraction peaks located at 25.3° and 43.3° are attributed to the 002 and 101 plane of graphite hexagonal lattice, respectively. The peak located at 2θ = 25.3° corresponds to the d-spacing average of CB and Ox-CB is 0.351 nm measured by Bragg's law, which is between carbonization and graphitization [21]. The peaks at 78.7° belong to the 110 plane of graphite in Fig. 4b. As shown in Fig. 4c, the diffraction peak located at 23.0° is ascribed to the broad diffraction of amorphous SiO2.

Fig. 2. FT-IR spectra of SiO2(a), CB(b), Ox-CB(c)and Ox-CB@SiO2 (d). 3

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Fig. 3. SEM images of CB (a), Ox-CB (b), Ox-CB@SiO2 (c and d), and EDX spectrum of Ox-CB@SiO2 (e).

The XRD diffraction pattern of Ox-CB@SiO2 is similar to pure SiO2 while the peak at 22.1° is stronger in Fig. 4d. 3.4. Interfacial activity of Ox-CB@SiO2 demulsifier The as-prepared Ox-CB@SiO2 is a composite material with an amphiphilic structure, it expected to exhibit good interfacial activity. For the purpose of analyzing the interfacial activity of Ox-CB@SiO2, four vials containing water and diesel were prepared. One as a blank control, and CB, Ox-CB and Ox-CB@SiO2 of 6 mg were separately added into the other three vials, and then thoroughly shaken for 10 min to form the emulsion. As shown in Fig. 5a (no shaking), it is found that the oil-water interface is clear in all four vials. The CB (Fig. 5a2) and Ox-CB@SiO2 (Fig. 5a4) were accumulated at the diesel/water interface, and Ox-CB (Fig. 5a3) was quickly dispersed into the water phase. After shaken and settled for 10 min, the droplets in blank vial coalesced rapidly and showed milky-white like in Fig. 5b1. The Ox-CB (Fig. 5b3) was quickly dispersed into the water phase while the interface was clear. It

Fig. 4. XRD patterns of CB(a), Ox-CB(b), SiO2(c)and Ox-CB @SiO2(d).

4

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Fig. 5. Interfacial activity of blank (1), CB (2), Ox-CB (3) and Ox-CB@SiO2 (4) in the water/diesel emulsion. (No shaking (a), shaking for10 min and settled for 10 min (b) and 3days (c), respectively). Inset: Top view of the corresponding samples.

confirmed the strong hydrophilicity of Ox-CB. However, the hydrophobic CB (Fig. 5b2) was dispersed into the water phase, too. Maybe there are two reasons for the phenomenon. Firstly, there may be some hydrophilic group on its surface. Secondly, the capillary phenomenon caused by the porous structure changes its hydrophobicity. A detailed study of the phenomenon is underway. The Ox-CB@SiO2 (Fig. 5b4) material was migrated rapidly to the diesel/water interface and stay there. After shaken for 10 min and settled for 3 days, the CB accumulated in the aqueous phase began to shrink, and a small amount of clear water appeared at the bottom (Fig. 5c2) while there was no change for Ox-CB in Fig. 5c3. The Ox-CB@SiO2 materials remained at the diesel /water interface for about 3 days without further diffusion in Fig. 5c4. The results demonstrate that the Ox-CB@SiO2 particles possess the best interfacial activity. Fig. 7. Effect of different concentrations of Ox-CB@SiO2 on IFT.

3.5. IFT of Ox-CB@SiO2 demulsifier

of diesel/water mixture with different concentration of Ox-CB@SiO2 was measured. As shown in Fig. 7, it can be seen that the effect of demulsifier on IFT is significant when the Ox-CB@SiO2 concentrations increased from 100 to 500 mg/L. The IFT of diesel/water mixture gradually decrease from 15.22 mN/M to 13.53 mN/M with increasing concentration of Ox-CB@SiO2. It is indicted that the same structure demulsifiers with higher concentrations can reduce the surface tension more and have better demulsification performance in current study.

The IFT of oil/water interface is another key factor contributing to the demulsification. In order to investigate the relationship of interfacial activity and IFT, the IFT of diesel/water mixture containing CB, Ox-CB, Ox-CB@SiO2 and SiO2 were measured respectively. Fig. 6 is the dynamic IFT of different samples with the concentration of 50 mg/L and the error is about 0.5 mN/M. All samples have lower interfacial tension than the blank water sample (Fig. 6). In these samples, the IFT of SiO2 could be reduced to 15.22 mN/M. It is even lower than that of Ox-CB@ SiO2 with the best interfacial activity. Although many studies believed that lower interfacial tension was beneficial to the improvement of demulsification performance, Berger et al. [22] suggested that demulsification performance was determined by interfacial activity and structure of the demulsifier. In order to investigate the effect of concentration of the Ox-CB@ SiO2 on its interfacial tension and demulsification performance, the IFT

3.6. Wettability of Ox-CB@SiO2 demulsifier Wettability of Ox-CB@SiO2 was detected by measuring the threephase contact angles (θ). A developed powder tablets method was used to investigate the three-phase contact angles (θ) of different samples, the results are shown in Fig. 8. The three-phase contact angle (θ) of SiO2 (Fig. 8a) is 50.0°, indicating the hydrophilic SiO2 surface covered with lots of hydroxyl groups. Besides, the three-phase contact angle (θ) of CB (Fig. 8b) was 144.3°and Ox-CB (Fig. 8c) was 78.2°. The wettability alteration of Ox-CB compared with CB is caused by the oxidation of CB, which indicate that hydrophilic groups such as carboxyl and hydroxyl groups are introduced on the CB. In addition, it shows that θ value of Ox-CB/SiO2 (Fig. 8d) is 90.2°. In comparison with the Ox-CB, the increase in contact angle of Ox-CB/SiO2 can be attributed to the fact that the hydrophilic group of the Ox-CB is partly reacted with SiO2 when it grafted on the Ox-CB. Furthermore, it is reported that the stability of the emulsifier is worst and the demulsifier possess the optimal demulsification efficiency when θ is about 90°, which is consistent with the demulsification result. It attributes to be equally wetted by both the continuous phase and the dispersed phase [9]. The demulsifier strongly accumulate at oil/water interfaces, the hydrophilic end enter the aqueous phase while the hydrophobic end enter the oil phase. It is believed that the distribution and adsorption of demulsifier at the interface is a key factor in the demulsification, and it is consistent with the results of

Fig. 6. Effect of different samples on IFT. (a) blank, (b) CB, (c) Ox-CB (d) Ox-CB@SiO2 and (e) SiO2. 5

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Fig. 8. Three-phase contact angles (θ) of different samples. (a) SiO2, (b) CB, (c) Ox-CB and (d) Ox-CB@SiO2.

water which indicate that the emulsion can keep stable so far as to it is at high temperature. When the demulsifier dosage is 100 mg/L, the water removal rate is only 5.1% after 180 min. It is difficult to separate the water from the emulsion effectively by using a simple gravity settling method. The experiments results indicate that demulsification efficiency increases with increasing demulsifier concentration. When the concentration of Ox-CB@SiO2 demulsifier is 500 mg/L, it has an excellent demulsification performance with 93.2% efficiency. Furthermore, the water phase is very clear and there is no sediment except for a small amount of black matter which attached to the bottle wall. Fig. 10 shows the effect of the sedimentation time and temperature on the demulsification efficiency. The temperature from 55 to 75℃ was carried out to explore the demulsification process while the sedimentation time was increased from 0 to 180 min with the concentration of 500 mg/L. It is found that the sedimentation time has a strong effect on the separation of oil and water. As shown in Fig. 10, the water removal rate increase with increasing sedimentation time, while it became almost constant after 180 min. When the sedimentation time is

the interfacial activity.

3.7. Bottle test of Ox-CB@SiO2 demulsifier Bottle test is a simple and intuitive method to investigate the demulsification efficiency of the demulsifier in oil-water emulsion. Demulsifier concentration, sedimentation time and temperature are key factors affecting the demulsification efficiency. In order to confirm the demulsification performance of Ox-CB@SiO2, a series of experiments were carried out and the error of DE is about 0.2%. Firstly, the demulsification performance of different materials was measured respectively at 70℃ for 180 min (Fig. 9A (inset)). It is found that no water is removed from the crude oil when Ox-CB is used as demulsifier. The demulsification efficiency of Ox-CB@SiO2 can reach 93% while that of SiO2 is only 46%, which is calculated accurately based on amount of water separated from the emulsion. The effect of different Ox-CB@SiO2 concentration on demulsifying efficiency is shown in Fig. 9. The concentration of Ox-CB@SiO2 is from 0 to 500 mg/L at 70 ℃ for 180 min. As shown in Fig. 9B (inset), the blank control do not precipitate any

Fig. 9. Effect of different Ox-CB@SiO2 concentration on the demulsification efficiency. A (Inset): (a) Ox-CB, (b) SiO2, (c) Ox-CB@SiO2. B (Inset): (a) blank, (b) 100 mg/L, (c) 200 mg/L, (d) 300 mg/L, (e) 400 mg/L, (f) 500 mg/L.

Fig. 10. Effect of the sedimentation time and temperature on the demulsification efficiency. Inset: The image of demulsification process. 6

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Fig. 11. Optical micrograph of the crude oil emulsions after settling for 0 min(a),60 min(b) and 120 min (c),water phase(d) and oil phase(e) after settling for 180 min. (500 mg/mL of demulsifier).

increased, there is enough time for droplet sedimentation and phase separation. Besides, it can also be noticed that demulsification efficiency increases with increasing temperature from 55 to 75℃. However, 93.5% water is removed from the crude oil at 75℃, while it is 93.2% at 70℃. Therefore, the optimal demulsification temperature is 70℃. Besides, the demulsification efficiency shows a much sharper decay with decreasing temperatures. The demulsification efficiency is 68% at 65℃ and only 42% at 60℃. On the one hand, it can be explained by the molecular motion of the demulsifier. The increasing temperature can decrease the viscosity of the emulsion, meanwhile the Brownian movement of the droplets can be strengthened. The probability of droplet collision, flocculation and coagulation increased, which is benefits to oil-water separation [23]. The viscosity of oil at high temperature is much lower than the viscosity of oil at low temperature, so it has better demulsification performance at high temperature. On the other hand, it is believed that the water-inoil emulsion is stabilized by a rigid film which is formed by asphaltenes located at the interface [24]. In summary, the viscosity of the emulsion is reduced at high temperature, and the demulsifier is more likely to be migrated to the interface and replace the asphaltenes, thereby reducing the strength of the interface film. In other words, it is much easier to quickly damage the oil/water interface film for Ox-CB@SiO2

demulsifier at high temperature.

3.8. Possible demulsification mechanism For the purpose of investigating the demulsification mechanism of the W/O emulsion driven by the Ox-CB@SiO2, microscopic state of the emulsion at different settling times was observed with a polarizing microscope. In the micrograph, the dark part is the oil phase and the light part is the water phase. The images show that it is a typical waterin-oil emulsion sample. The original crude oil emulsion has a poor light transmittance, in which the water droplets have a diameter of 3–10 μm (Fig. 11a). It can be observed that the size of the water droplets increases slowly with increasing settling time, while its number is gradually decreased. However, the process is quite slow if there is no demulsifier in the emulsion. Once the Ox-CB@SiO2 was added into the water-in-oil emulsion, coalescence of the water droplets was quickly occurred to form new water droplets with a big size in Fig. 11b. After settling for 120 min, coalescence of the big water droplets in the oil-water emulsion to form irregular water aggregation (Fig. 11c), and then the irregular water aggregation could be coagulated and precipitated at the bottom of vial. As shown in Fig. 11d, there is few oil droplets suspending in separated water phase after settling for 180 min at 70℃. Meanwhile, there is very 7

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Fig. 12. Simulation of the demulsification process.

crude oil emulsion and then the low interfacial tension gives it an excellent ability to replace the nature emulsifier. What’s more, the possible demulsification mechanism indicated that the aggregation and distribution of the Ox-CB@SiO2 demulsifier at the interface may successfully destroy the protective film of the emulsion formed by the asphaltenes and it improve the coalescence behavior of the water droplets. The findings in current work demonstrate that the Ox-CB@SiO2 is a simple, low cost and high interfacial active demulsifier to remove the oil from the water-in-oil emulsions, which displays a considerable application prospection in oil industry. Furthermore, the findings in current work also provide a new perspective for the development of new demulsifier in the treatment process of crude oil emulsions.

small amount of water droplets in the oil phase (Fig. 11e). Therefore, it demonstrates that the demulsifier has an excellent coalescence, flocculation and bridging ability. The demulsification process includes two aspects. First, the demulsifier is migrated to the oil-water interface, and then the demulsifier replaces the emulsifier at the interface [25]. It greatly reduces the strength of the interface film, thereby promoting the droplets coalescence. In current work, the demulsifier has a suitable amphiphilic structure with a three-phase contact angle of about 90°. This structure gives it a good interfacial activity and thus it is rapidly migrated to the oil-water interface. Furthermore, the relatively low interfacial tension of the demulsifier gives it an excellent ability to replace the emulsifier at the interface. It is believed that droplets coalescence is a crucial step of demulsification, which can increase the size of partial droplet and promotes droplets rearrangement in the oil/water emulsion, reducing the droplets number, leading to a loose or collapsed structure, and finally inducing demulsification [25]. Fig. 12 shows the process of simulating demulsification. The Ox-CB@SiO2 demulsifier shows the stronger interfacial activity and it is easy to penetrate into the interfacial film at the oil-water interface. Then, the adjacent droplets are bridged and decreased the oil-water interfacial tension. The cracking of the interfacial film could be accelerated and the demulsification efficiency is improved, when the interfacial activity of the demulsifier exceeds the dynamic interfacial tension gradient [26]. In the process of demulsification, two small adjacent droplets are bridged by demulsifier. Two adjacent small droplets are bridged and broken by a demulsifier to form a larger droplet, and then the two big droplets are merged into one larger droplet. Afterwards, it will develop into a channels phase. Generally speaking, coalescence behavior occurs quickly after triggered by coalescence propagation dynamics. Ramalho et al. [27] suggested that the rupture of the adjacent interface film accelerates the formation of the channel and the coalescence between the water droplets. It is similar to the current study.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was supported by the Open Project Program of State Key Laboratory of Petroleum Pollution Control (Grant No. PPC2016006), CNPC Research Institute of Safety and Environmental Technology. References [1] S. Pei, Y. Zhao, Z. Wang, Organosilicone modified styrene-acryliclatex preparation and crude oil dehydration, Tenside Surfactants Deterg. 55 (1) (2018) 71–77. [2] W. Fang-Hui, Z. Hong, The application and research of dispersing in situ nano-SiO2 in polyether demulsifier TA1031, J. Dispers. Sci. Technol. 29 (8) (2008) 1081–1084. [3] J. Peng, Q. Liu, Z. Xu, J. Masliyah, Novel magnetic demulsifier for water removal from diluted bitumen emulsion, Energy Fuels 26 (5) (2012) 2705–2710. [4] Y. Wang, L. Zhang, T. Sun, S. Zhao, J. Yu, A study of interfacial dilational properties of two different structure demulsifiers at oil–water interfaces, J. Colloid Interface Sci. 270 (1) (2004) 163–170. [5] L. Hao, B. Jiang, L. Zhang, H. Yang, Y. Sun, B. Wang, N. Yang, Efficient demulsification of diesel-in-water emulsions by different structural dendrimer-based demulsifiers, Ind. Eng. Chem. Res. 55 (6) (2016) 1748–1759. [6] M. Nikkhah, T. Tohidian, M.R. Rahimpour, A. Jahanmiri, Efficient demulsification of water-in-oil emulsion by a novel nano-titania modified chemical demulsifier, Chem. Eng. Res. Des. 94 (2015) 164–172. [7] M. Khalil, B.M. Jan, C.W. Tong, M.A. Berawi, Advanced nanomaterials in oil and gas industry: design, application and challenges, Appl. Energy 191 (2017) 287–310. [8] F.H. Wang, L.B. Shen, H. Zhu, K.F. Han, The preparation of a polyether demulsifier modified by nano-SiO2 and the effect on asphaltenes and resins, Pet. Sci. Technol. 29 (24) (2011) 2521–2529. [9] J. Liang, H. Li, J. Yan, W. Hou, Demulsification of oleic-acid-coated magnetite nanoparticles for cyclohexane-in-water nanoemulsions, Energy Fuels 28 (9) (2014) 6172–6178. [10] J. Peng, Q. Liu, Z. Xu, J. Masliyah, Synthesis of interfacially active and magnetically

4. Conclusions In current study, a simple and effective in situ sol-gel method was used to prepare a low cost and high interfacial active Ox-CB@SiO2 composite material with high demulsification efficiency. This Ox-CB@ SiO2 is obtained by partial grafting of spherical nano-SiO2 onto oxidized carbon black with irregular granular morphology and the average size of about 500 nm. The water removal rate could reach 93.5% with colourless aqueous phase and clear interface, when the demulsifier concentration was 500 mg/L at 75℃ for 180 min. Its three-phase contact angle is 90.2°. The interfacial activity and wettability of Ox-CB@SiO2 materials allowed it to be rapidly migrated to the oil-water interface of 8

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[11]

[12]

[13]

[14]

[15]

[16]

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