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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Demulsification of Oil-in-water (O/W) Emulsion in Bidirectional Pulsed Electric Field Boping Ren, and Yong Kang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01581 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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Demulsification of Oil-in-water (O/W) Emulsion in Bidirectional Pulsed Electric Field Boping Ren, Yong Kang* School of Chemical Engineering and Technology, Tianjin University, Tianjin300350, China

ACS Paragon Plus Environment

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ABSTRACT: The researches about electric demulsification of oil-in-water (O/W) emulsion are not enough at present especially compared with that about electric demulsification of water-in-oil (W/O) emulsion. As an easy and novel method, bidirectional pulsed electric field (BPEF) was investigated to demulsify the O/W emulsion in this work. Here we report that BPEF could actuate O/W emulsion to form rotational flow and drove oil droplets to form oil-droplet chains and to coalescence. In order to interpret the mutual attraction and coalescence of oil drops in BPEF, we put forward the hypothesis that charges on oil drop surface would redistribute in BPEF and built the charge redistribution model according to the adsorption phenomenon of oil droplets. The behavior of oil drops in BPEF could be successfully explained in terms of the hypothesis and the model. The charge redistribution on oil droplet surface could be evaluated by the two parameters we proposed. An amended potential redistribution formula of oil drop surface was also obtained according to the model. The O/W emulsions were successfully demulsified by BPEF in the experiments. It showed that BPEF could be a significant method for electric demulsification of O/W emulsions.

KEYWORDS: Demulsification; O/W emulsion; Bidirectional pulsed electric field; Coalescence


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INTRODUCTION In order to separate oil and water from O/W emulsion, the demulsification researches are widely carried out, among which demulsification using electric field draws much interest and endeavor.1,2 Though electrical demulsification of water-in-oil (W/O) emulsion has been extensively studied and applied for a long time, the demulsification of O/W emulsion is generally considered to be more complex and difficult to achieve by electrical method. Because the continuous aqueous phase is conductive, the current density required to induce forming oil-droplet chains is much larger compared to that required for W/O emulsion.3However, some researchers have applied electric field to demulsify O/W emulsion and the results proved that this method was available and significant. Ichikawa et al4, 5 studied the demulsification of dense oil-in-water emulsion in DC low-voltage electric field and found that it simultaneously took place over the entire space between two plate electrodes. Hosseini et al6 introduced a non-uniform electric field into the benzene-in-water emulsion and about 80% benzene was demulsified from the emulsion in ten minutes. The experimental evidence shows that the mechanism of electrical demulsification for O/W emulsion may be different from that for W/O emulsion. More importantly, it was proved that electrical demulsification of O/W emulsion is doable. O/W emulsion forms in the way that oil droplets are dispersed in water phase, which keeps stable under the combined action of the electrostatic repulsive force, steric hindrance and short-range hydration force. The main coalescence barriers among oil droplets are the electrostatic repulsive force and steric hindrance. The short-range hydration force is much smaller than the other two forces and can be ignored.7-12The steric hindrance works by the surfactants which stop oil-droplets from coalescing by forming a dense polymer film on the oil drop surface.13-16 The


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electrostatic repulsive force, which is caused by the like charges adsorbed on oil droplet surface, baffles the aggregation of oil droplets with each other. The charges generate potential distribution on oil droplet surface. Thus, when two charged oil droplets approach, the overlap of potential distribution area gives rise to the potential energy barrier increasing rapidly and oil drops repulsing forcefully.17-19 Besides, the influence of electrostatic repulsive force on coalescence of oil droplets in O/W emulsion is vital because there are much more charges adsorbed on oil drop surface than that on aqueous droplet surface in W/O emulsion. Consequently, the surface potential barrier of oil droplets should be reduced in demulsification process of O/W emulsion.20, 21 The potential barrier of oil droplet surface can be reduced by applying external electric fields into O/W emulsion. And demulsification of O/W emulsion can take place in AC or DC electric field. 4-6 However, the large electric current generated by AC and DC electric fields would heat the emulsion, which confines a low voltage in O/W demulsification process by AC and DC electric fields. Pulsed electric field is widely applied in electric demulsification for W/O emulsion. The electric current generated by pulsed electric field is small at high voltage and the short circuit caused by water chain formation can be effectively reduced. 22-25 This prominent advantage might provide a new breakthrough for electric demulsification of O/W emulsion. Besides, pulsed electric field has been scarcely applied to demulsify O/W emulsion so far. Therefore, motivated by the need to explore the demulsification of O/W emulsion in pulsed electric field, this paper focused on introducing bidirectional pulsed electric field(BPEF) into O/W emulsion and studying the coalescence of oil droplets and the demulsification effect of O/W emulsion in BPEF. In order to explore the BPEF’ influence on the surface potential of oil droplets, the O/W emulsion without any surfactants was prepared to avoid the steric hindrance.


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After that, the O/W emulsion with surfactant and a crude oil in water emulsion were demulsified in BPEF respectively. EXPERIMENTAL SETUP AND METHODS Preparation of O/W emulsions. The oil used in this work was 0# diesel purchased from Sinopec with the density of 842.7 kg/m3 and the viscosity of 2.83mPa·s at 293K. The crude oil was from Dagang Oilfield of China National Petroleum Corporation(CNPC) with the density of 882.6 kg/m3 at 293K and the kinetic viscosity of 17.3×10-6m2/s at 323K .The conductivity of deionized water (Yongqingyuan Pure Water Production Center, Tianjin, China) was 1~3µs/cm. The surfactant used in the work was polysorbate 80 which was analytically pure (Tianjin Jiang tian Chemical Technology Co., Ltd, China). The O/W emulsion without surfactant was prepared by mixing 0# diesel (5vol.%) and deionized water in a 1L beaker vigorously with a homogenizer (D-8401WZ, Tianjin Huaxing Co., Ltd, China) at 3100rpm for 10mins. The O/W emulsion with the surfactant was prepared by mixing 0# diesel (5vol.%) and the water solution of polysorbate 80(0.1wt.%) in a 1L beaker at 3100rpm for 10mins. The crude oil in water emulsion was prepared by mixing the crude oil(5vol.%) and the water solution of polysorbate 80(0.25wt.%) in a 1L beaker at 3100rpm for 10mins.After being laid for 24h, the oil slick was removed and the rest of O/W mixture was stirred at 3100rpm for 10mins again to get the O/W emulsion for using in experiments. The prepared O/W emulsion could be stored for seven days. The diameter of oil drops maintained in the range of 1µm to15µm measured with the microscope image analysis system (BX43 Olympus Microscope, Japan) to record the oil drops with a frame speed of 2000 frames per second. And the emulsions were stirred at 800rpm during every experiment to keep uniform. In order to be easily observable, Sudan Red III was used to dye the 0# diesel oil.


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Measurements. The oil content of O/W emulsion without surfactant was detected with a UV spectrophotometer (L6S, Shanghai Precision Scientific Instrument Ltd. Co., China). The emulsion turbidity was determined with the scattering photoelectric turbidity meter (WGZ-100, Shanghai 3th Optical Instrument Factory, China). The size distribution of oil droplets was measured with the microscope image analysis system and Image-Pro Plus6.0 (Media Cybernetics, Inc., USA). The bidirectional pulsed electric field was generated with a pulsed power pack (10002DM, Shanghai Suoyi Science &Technology Ltd. Co., China) with the voltage up to 1000V in step of 1V, the frequency up to 10000Hz in step of 1Hz and the duty cycle, which is the ratio of pulse output time to one cycle time, from 0 to 1. The output voltage and current values are displayed and adjusted on the LCD panel of pulsed power supply. Experimental setup and procedure. The schematic diagram of the experimental setup is shown in Fig.1.The experimental setup is constituted with an O/W emulsion tank (a) with a stirrer (b), an influent pump(c), an electro-demulsification tank (d) with multi-electrode assembly (e), a cooling water tank (g) with a pump (f), a pulsed power supply (h), a ruler (i) and a liquid collection tank (j). The emulsion in the O/W emulsion tank is kept homogeneous by stirring. The electro-demulsification tank is a double-deck glass cylinder. The inner cylinder, with the internal diameter 40mm, height 320mm and wall thickness 5mm, is filled with the multi-electrode assembly and the O/W emulsion. It is the place where the electric demulsification took place. The multi-electrode assembly is consisted of 32 rodlike titanium electrodes, with the rod diameter 2mm and length 320mm, which are bound by two round teflon pore plates with the diameter of 39mm and thickness 5mm, as shown in Fig1(e). One of the titanium rods is fixed at the center of the pore plates as the anode. The other titanium rods as the cathodes surrounding the anode are installed at the peripheral pores of the two round plates and they are linked with


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each other by a copper wire. The internal diameter of the outer cylinder of the electro-demulsification tank is 70mm with height 300mm and wall thickness 5mm. The annular gap between the outer and inner cylinders is filled with cooling water pumped from the cooling water tank to keep the O/W emulsion at room temperature.

Figure 1. Schematic diagram of the experimental setup: O/W emulsion tank(a), stirrer(b), influent pump(c), electro-demulsification tank(d), multi-electrode assembly(e), water pump(f), cooling water tank(g), pulsed power supply(h), ruler(i) and liquid collection tank(j).

In the experiments, the electrode assembly was first set up in the inner cylinder of the electro-demulsification tank. Then, the annular gap of the electro-demulsification tank was filled with cooling water. The anode and cathodes of the electrode assembly were connected with the positive and negative outputs of the pulsed power supply, respectively. After that, the O/W emulsions with volume of 300mL were pumped into the inner cylinder from the O/W emulsion tank. Then turned on the pulsed power supply and set the values of its voltage, frequency and duty cycle. During the demulsification process, the thickness of the separated oil layer was recorded by the ruler which was fixed on the outer surface of the cylinder. After the treatment was finished, the clear liquid in the electro-demulsification tank was extracted into the liquid


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collection tank for measuring the oil content and turbidity. The demulsification process of every experiment lasted for two hours. RESULTS AND DISCUSSION Demulsification behavior of O/W emulsion in BPEF. The dynamic behavior of the O/W emulsion without surfactant in BPEF with voltage 900V, frequency 50Hz and duty cycle 0.5 was shown in Fig.2a. Before application of BPEF, the O/W emulsion remained stable and uniform. Once the BPEF was activated, the emulsion slowly started to flow up and down as revealed in Fig.2a (30s). With BPEF continuously taking effect, the flowing intensity of emulsion enhanced and the rotational flow appeared distinctly in the emulsion as seen in Fig.2a (60s). When BPEF lasted for 120s, the red oil layer formed on the emulsion surface. The emulsion flowing slowed down and the oil layer became thicker at 300s. When BPEF took effect for 600s, the emulsion was clear, and electrodes could be seen distinctly as shown in Fig.2a.

Figure 2. (a) Demulsification process of the O/W emulsion in BPEF with its voltage 900V, frequency 50Hz and duty cycle 0.5. (b) Log-Normal distribution of oil droplets during the demulsification process:


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P(d)-the fraction of oil droplets with a certain diameter, d-the diameter of oil droplet. (c) The O/W emulsion treated by BPEF with its voltage 900V, frequency 50Hz and duty cycle 0.5 (left) and the untreated (right).

The size distribution of oil droplets at different demulsification time was measured to investigate the BPEF’s influence on the coalescence of oil droplets as shown in Fig.2b. Before the BPEF was applied, the size range of oil droplets was narrow as shown in Fig2b (0s) and the average diameter of oil droplets was 5.26µm listed in Table 1. When BPEF lasted from 0s to 120s, the standard deviation (SD) of oil droplet diameters increased from 2.25 to 8.48, which indicated that the size distribution of oil droplets became wide. And the average diameter increased quickly from 5.26µm to 16.37µm as seen in Table 1. This was consistent with what Fig.2b presented. Formation of large oil droplets indicated that BPEF drove the coalescence of oil droplets. Then the coalesced oil droplets floated upward, and the aqueous phase moved downward, which gave rise to the rotational flow of O/W emulsion and oil/water separation. When the BPEF lasted from 120s to 600s, the oil layer appeared and its thickness continuously increased. With the large oil drops floating upward to form oil layer, the oil droplets in the emulsion continuously decreased. And the average diameters and the SD values of oil droplets decreased from 16.37µm and 8.48 to 6.85µm and 2.98, respectively, as listed in Table 1. Table 1. Average diameters and SD values of oil droplets during the demulsification process

Demulsification time (s)

Average diameter (µm)

SD(µm)

0

5.26

2.25

30

10.27

5.37

60

13.85

7.36


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120

16.37

8.48

300

8.75

3.81

600

6.85

2.98

Since the electric current in the emulsion caused by BPEF was below 40mA, there were almost no bubbles produced by water electrolysis during demulsification of the emulsion. It is demonstrated that oil droplets movement was certainly caused by BPEF rather than electro-flotation or heating. The comparative experiments that the same O/W emulsion demulsified by DC, AC electric field and BPEF under the same voltage 400V were conducted as shown in Figure S1. It can be seen that the emulsion under BPEF moved distinctly and the oil layer increased faster than that under AC and DC electric field. Therefore, the demulsification effect under BFEF was much better than that under AC or DC electric field. It was shown in Fig.2c that the oil was separated from the emulsion and formed oil layer after demulsification by BPEF for 2 hours. Based on the above, it can be concluded that BPEF could drive oil droplets to coalesce and make the emulsion flow rotationally, which led to the demulsification of O/W emulsion. And the BPEF was better than AC and DC electric field for the demulsification of O/W emulsion. The movement and coalescence of oil droplets under BPEF should be further explored. Movement Behavior of oil drops in BPEF. In order to probe into the demulsification mechanism of O/W emulsion without surfactant by BPEF, the coalescence process of oil droplets in O/W emulsion under BPEF was surveyed with the Olympus BX43 microscope image analysis system. The images of oil droplets under the BPEF with its voltage 900V, frequency 50Hz and duty cycle 0.5 were shown in Fig.3. The direction of electric field E was presented in Fig.3. It can be seen that the oil drops were uniformly dispersed in the aqueous phase before BPEF was


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activated. When BPEF started to take effect, the oil droplets approached with each other and then strung straightly in line, which formed mutually paralleled oil-droplet chains as shown in Fig.3 (10s). This contact of oil droplets indicated that the electrostatic repulsion among oil droplets was observably reduced by BPEF. As the demulsification continuing, the oil-droplet chains slowly aggregated together and formed oil-droplet clusters as presented in Fig.3 (35s). Then oil droplets in a cluster contacted and coalesced with each other in BPEF. When BPEF acted for 80 seconds, the large oil droplets appeared and then grew larger by swallowing up micro oil droplets during the time between 80s and 368s. From 292s to 368s the big oil drops still strung together and small oil drops were absorbed into the big ones in BPEF as shown in Fig.3, which indicated that the surface potential of oil droplets was reduced to the extent that the mutual attraction between oil droplets could overcome their electrostatic repulsion. During the formation of oil-droplet chains, the oil droplets attracted with each other and strung straightly along the BPEF direction E. It was obvious that there was a kind of attractive force between the adjacent oil droplets in BPEF, which could be seen in the dynamic attraction process of oil drops as shown in the Video S1. Because there were no surfactants but charges adsorbed onto the surface of oil drops. Thus, it could be explained that the attractive force came from the mutual action of the charges on oil droplet surface.26-29


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Figure 3. Microscopic images of oil droplets at different demulsification time in the BPEF with its voltage 900V, frequency 50Hz and duty cycle 0.5. X-axis is the horizontal direction. Y-axis is the vertical direction. E is the electric field direction, the angle between E and X-axis is 63°.

Theoretical analysis and Model. Oil droplet was negatively charged in O/W emulsion. On the basis of DLVO theory, positive charges dispersed in the electrical double layer of oil drops to keep electric neutrality of the oil-water system.30-32 Thus it could be assumed that when the BPEF was applied, the mobile negative charges on oil droplet moved towards the anode and the positive charges in the electric double layer of oil drops moved towards the cathode. Because the BPEF direction alternated quickly during one pulse cycle, which lead the charges to move alternately on oil droplet surface corresponding with the change of BPEF direction. However, the adjacent areas of two oil droplets along with the BPFE direction charged oppositely at the same time in BPEF because the negative and positive charges moved oppositely. Thus the adjacent areas of oil droplets always attracted with each other along with the BPFE direction. According to the above, BPEF made the charges move and redistribute on oil drop surface, which resulted in its surface potential redistributing as well. Because the diameters of oil drops were much


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smaller than the size of electrodes. The cylindrical surfaces of the anode and cathode could be taken as the paralleled plates to oil drops between them. Therefore, the hypothesis was put forward that the charges on the oil drop surface redistributed in BPEF. And we built the charge redistribution model that the negative charges moved towards the anode and the positive charges moved towards the cathode as shown in Fig.4(a). The shaded surface areas of oil droplet1 represented electric charges as shown in Fig.4(b).

Figure 4. (a) The model of charge redistribution on the surface of oil droplet in BPEF. (b)Oil droplet1 in the model: O point was the oil droplet center. A was a point on the oil droplet surface. B was the projection point of A on the x-y plane. (x, y, z) was the rectangular coordinate system. The left shaded area of the oil drop surface represented negative charges and the right shaded area of the oil drop surface represented positive charges. The left electrode plate was positive, and the right electrode plate was negative at the moment. (c) The distribution area of negative and positive charges described by θ and ϕ in the model. The positive directions of θ and ϕ are anti-clockwise.

Because there was no any surfactants and electrolytes added into the O/W emulsion prepared in this work, the surface potential ψ0 of an oil droplet is not high in electric double layer. And diameter of oil drops ranged from 1µm to 16.37µm, which is far greater than the size of charges


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in fact. Therefore, based on the theory of electric double layer,33-35 the potential distribution of oil drop surface was shown as following.

a r

ψ =ψ 0 ⋅ ⋅ exp  −κ ( r − a ) 

(1)

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 ∑ ni 0 zi 2 e2  κ =    ε kT 

(2)

Where ψ is the electrostatic potential at the distance of r away from the droplet center. ψ0 is the potential at oil droplet surface. a is the radius of oil droplet. r is the distance away from the droplet center. κ is referred as Debye–Hückel reciprocal length, which is affected by the charges absorbed on oil drop surface and the charge density (Σni0zie) 36-38. And κ-1 can be used directly to evaluate the thickness of electric double layer. ε is the dielectric constant of aqueous phase. e is the electronic charge. k is Boltzmann’s constant. T is the absolute temperature. ni0 is the concentration of a certain ion with its valence zi.39-43 In accordance with the above model, the charges distribution was non-uniform on the whole surface of oil droplet in BPEF so that the charge density(Σni0zie) should be amended. It was assumed that the charge density was a function of the charge location on oil drop surface, then it could be rewritten as Eqn.(3).

∑n

z e =ρ i 0 ( x , y , z )

(3)

i0 i

Where ρi0 (x, y, z) was the surface charge density function of oil droplet. (x, y, z) was the rectangular coordinates of charge location on oil drop surface. The spheroidal coordinates


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(x=asinθcosϕ, y=asinθsinϕ and z=asinθ) and Eqn.(3) were applied to Eqn.(2), which occurred as the following shown. 12

 a ρ (θ , φ ) zi e  κ ′=  i 0  ε kT  

(4)

Where κʹ is the amendment of κ and they have the same physical significance. θ is the zenith angle between the line OA( between any point A on the oil drop surface and droplet center O) and z axis. ϕ is the azimuth angle between the line OB (the projection line of line OA on xy-plane) and x axis as shown in Fig.4(a). Then the amended potential distribution formula of oil droplet surface was presented as follows by introducing Eqn.(4) into Eqn.(1).

a r

ψ =ψ 0 ⋅ ⋅ exp  −κ ′ ( r − a ) 

(5)

When BPEF was applied, the charges redistributed as shown in Fig.4(b). The distribution range of negative charges could be illustrated by ∆θ=|θ1- θ2| and ∆ϕ=|ϕ1- ϕ2|. And the distribution range of positive charges could be illustrated by ∆θʹ=|θ1ʹ- θ2ʹ| and ∆ϕʹ=|ϕ1ʹ- ϕ2ʹ|. The above model gave a qualitative description of the charges distribution on oil drop surface by using the zenith angle θ and azimuth angle ϕ. It could describe the areas of positive and negative charges distribution and be helpful for explaining the mutual attraction and its intensity between oil drops in BPEF. In order to confirm the nonuniformity of charges distribution and the model validity, it was chosen a single large oil droplet (the diameter 87.6µm) to focus on in the microscopic image as shown in Fig.5(a). It was found that small oil drops were adsorbed onto the two poles of the large one along BPEF direction E. The charges distribution could be reflected by the contacted areas of big oil drop and small oil drops, which could be evaluated by the range of azimuth angle ϕ.


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And the range of ϕ was presented by ∆ϕ as shown in Fig5 (b), which was consistent with the model we proposed.

Figure 5. (a) Microscopic image of oil drops in the BPEF with its voltage 900V, frequency 50Hz and duty cycle 0.5: X-axis is the horizontal direction, Y-axis is the vertical direction, E is the electric field direction, the angle between E and the x-axis is 27°. (b) The enlarged image of a large oil droplet (diameter 87.6µm) adsorbing small oil drops cut from the framed zone in Fig. 5 (a): the point O is the center of the oil droplet. ϕ1, the angle between E and OB, which equals to 163.5°. ϕ2, the angle between E and OC, which equals to 238.2°. ∆ϕ=|ϕ1- ϕ2|=74.7°. The positive direction of the angle is anti-clockwise.

According to the above, it was in conformity with the hypothesis that the charges on oil drop surface redistributed in BPEF. The redistribution model of the charges on oil drop surface could explain the mechanism of the oil-droplet chain formation and the mutual attraction among oil drops in BPEF. And it was proved that the BPEF could reduce the surface potential barrier of oil droplets. Then the BPEF actuated the coalescence of oil drops and demulsified the O/W emulsion. The demulsification experiments of O/W emulsion under different BPEF voltages, frequencies and duty cycles were conducted. And the hypothesis and model were used to interpret the demulsification phenomenons and results.


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Effect of BPEF voltage on demulsification of O/W emulsion. The effect of BPEF voltage on demulsification of O/W emulsion was examined by measuring the floated oil thickness as shown in Fig.6(a). The BPEF frequency was set as 50Hz and duty cycle was 0.5. When the voltage was changed below 500V, there was no evident demulsification appearance after the BPEF being applied for 2h. Once the voltage reached to 500V, the demulsification took place apparently and immediately. And the thickness of floated oil layer increased faster with the voltage increasing, which indicated that the increased BPEF voltage accelerated the demulsification process. This could be explained that the increased voltage promotes more positive and negative charges gathered on two poles of oil drop in BPEF according to the model we proposed. The ∆θ, ∆ϕ, ∆θʹ and ∆ϕʹ become smaller with increasing of BPEF voltage, which means that charges would be more like to concentrate together leading to the stronger mutual attraction among oil drops. It enhances the contact and the coalescence among adjacent oil drops. It also could be seen in Fig.6(a) that the oil thickness rose quickly in the first 30 minutes. After that, increasing of oil thickness slowed down and tended to be steady. Because large oil droplets from coalescence of micro ones moved upward to the emulsion surface and formed oil layer, which lead to the oil thickness increased fast in the first 30mins. Then, the quantity of oil drops in the emulsion reduced and the distance between oil drops became larger. It needed more time for micro oil drops to move and contact with each other. Therefore, the coalescence process and the increasing of oil layer thickness slowed down. The oil content and turbidity of the clear liquid after treatment were also tested to evaluate the effect of BPEF voltage on demulsification of O/W emulsion. It was observed in Fig.6(b) that oil content and turbidity reduced quickly when the voltage increasing from 500V to 600V and then


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decreased slowly with the voltage increasing from 600V to 900V. The oil content and turbidity of clear liquid reached to the minimum when the voltage was 900V. The oil content and turbidity of the clear liquid after treatment were also tested to evaluate the effect of BPEF voltage on demulsification of O/W emulsion. It was observed in Fig.6(b) that oil content and turbidity reduced quickly when the voltage increasing from 500V to 600V and then decreased slowly with the voltage increasing from 600V to 900V. The oil content and turbidity of clear liquid reached to the minimum when the voltage was 900V.

Figure 6. Variation of the oil thickness (a) and the oil content and turbidity of clear liquid after treatment (b) under different BPEF voltages with its frequency 50Hz and duty cycle 0.5.

Effect of BPEF frequency on demulsification of O/W emulsion. Changes of the oil thickness under different BPEF pulse frequencies with its voltage of 900V and duty cycle of 0.5 were shown in Fig. 7(a). The oil thickness increased fast during the front 30 minutes and then slowly. Most importantly, the oil thickness decreased with the increasing of BPEF frequency. When the frequency reduced to 25Hz, the oil thickness reached to the maximum and the demulsification effect reached to the top as well. This is because under the same voltage and duty cycle when


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BPEF frequency increases, the duration of one pulse and the pulse output time become shorter. Thus, the positive and negative charges could not have sufficient time to gather on two poles of oil drop surface. At the same time, ∆θ, ∆ϕ, ∆θʹ and ∆ϕʹ are large, which means that charges scatter in a large area of oil droplet surface leading to the weakened mutual attraction among oil drops. Then the coalescence of oil drops is weakened, which led to poor demulsification effect of O/W emulsion.

Figure 7. Variation of the oil thickness(a) and the oil content and turbidity of clear liquid after treatment (b) under different BPEF frequencies with its voltage 900V and duty cycle 0.5.

The oil content and turbidity of clear liquid after demulsification of the O/W emulsion under different BPEF pulse frequency with its voltage 900V and duty cycle 0.5 were shown in Fig. 7(b). It could be seen that oil content and turbidity increased with increasing of BPEF frequency, which indicated that the demulsification effect became worse at a high BPEF frequency.

Effect of BPEF duty cycle on demulsification of O/W emulsion. The demulsification effect of O/W emulsion was illustrated by the variation of oil thickness under different BPEF duty cycles with its voltage 900V and frequency 25Hz as presented in Fig. 8(a). When duty cycle was 0.1, the oil thickness was very thin and increased slowly. It is because that the pulse output time is


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too short that the charges have insufficient time to gather together at the two poles of oil drops and the mutual attraction between oil drops is too small under the same voltage and frequency. With duty cycle increasing from 0.3 to 0.9, the oil thickness increased dramatically during the front 20mins and then kept stable as shown in Fig 8(a). And the final oil thickness decreased with the duty cycle increasing from 0.3 to 0.9. It could be explained that the duration of mutual attraction between two poles of adjacent oil droplets rises with duty cycle increasing, which led to oil drops stick together more tightly. Once the oil drops attracted and sticked together, they would move synchronously. The vibration effect of oil drops in BPEF was badly weakened, which went against the coalescence of oil drops and caused worse demulsification effect.

Figure 8. Variation of the oil thickness (a) and the oil content and turbidity of clear liquid after treatment (b) at different BPEF duty cycles with its voltage 900V and frequency 50Hz.

The oil content and turbidity of clear liquid after demulsification under different BPEF duty cycles were presented in Fig.8(b). It could be seen that oil content and turbidity decreased dramatically with the duty cycle increasing from 0.1 to 0.3 and then increased with duty cycle increasing from 0.5 to 0.9. On the basis of the above results, it was deduced that the optimal duty cycle was 0.3~0.5.


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Demulsification of O/W emulsion added surfactant and the crude oil in water emulsion in BPEF. The O/W emulsion added polysorbate 80 and the crude oil in water emulsion added polysorbate 80 were demulsified respectively by BPEF as shown in Fig.9. Before BPEF being activated, the emulsions were kept uniform and stable as presented in Fig.9 (0s). When BPEF started to take effect, the emulsion moved slowly, and the rotational flow occurred when BPEF lasted for 170s. The intensity of rotational flow increased with BPEF lasting from 170s to 15min, during which oil layer appeared at 600s. The rotational flow existed in the crude oil in water emulsion, which was not distinctly observed because of the soluble components in the crude oil. With application of the BPEF from 15min to 60min, the oil separated from emulsions increased continuously, which indicated that BPEF had obvious demulsification effect on O/W emulsions.

Figure 9. Demulsification process of O/W emulsion added polysorbate 80 (0.1wt.%) (a) and the crude oil in water emulsion with polysorbate 80 (0.25wt.%) (b) in the BPEF with its voltage 400V, frequency 50Hz and duty cycle 0.5.


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After the treatment, the liquid beneath oil layer in the O/W emulsion was extracted and compared with the raw emulsion as revealed in Fig.10. It could be seen that the liquid was much clear as shown in Fig.10a(2) compared with original O/W emulsion with surfactant. As to the crude oil in water emulsion, the treated liquid was not clear but the curde oil layer was obviously separated from emulsion by BPEF as presented in Fig.10c. Therefore, it was indicated that BPEF had distinct demulsification effect on O/W emulsion with surfactant and the crude oil in water emulsion.

Figure 10. Demulsification effect of BPEF on O/W emulsion added surfactant and the crude oil in water emulsion. (a) 1-the raw O/W emulsion added surfactant; 2-the liquid after treatment of O/W emulsion with surfactant. (b) 1-the original crude oil in water emulsion; 2-the liquid after treatment of crude oil in water emulsion. (c) The presentation of the crude oil layer on the surface of crude oil in water emulsion separated by BPEF.


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CONCLUSIONS In this study, the bidirectional pulsed electric field (BPEF) was applied to demulsify the O/W emulsions. It was found that BPEF could actuate O/W emulsions to form rotational flow and drive oil droplets to coalesce. The O/W emulsions were successfully demulsified by BPEF. BPEF is much better than DC or AC electric field on the demulsification of O/W emulsions .The most significant finding was that oil-droplet chains formed when BPEF taking effect and were paralleled to the BPEF direction. Thus, the formation of oil-droplet chains could be achieved in electric field without generating large electric current. Oil drops attracted with each other during oil-droplet chain formation in BPEF, which indicated that the electrostatic repulsion among oil droplets was reduced by BPEF. The hypothesis that charges on oil droplet surface redistributed in BPEF and the redistribution model that the negative charges moved towards the anode and the positive charges moved towards the cathode in BPEF were put forward. Two parameters, namely zenith angle θ and azimuth angle ϕ, could be used to evaluate the charges redistribution on oil droplet surface. The observation of the single large oil droplet in BPEF, which adsorbed small oil drops on its surface, gave the strong support for the hypothesis and model we proposed. The amended potential redistribution formula of the oil droplet surface was obtained according to the model we proposed. Under the experimental conditions, it was also found that the increased BPEF voltage enhanced the demulsification effect. The demulsification effect became worse with the increase of BPEF frequency and firstly improved then decreased with increasing of BPEF duty cycle. Furthermore, the O/W emulsion with surfactant and the crude oil in water emulsion also were successfully demulsified by BPEF.


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ACKNOWLEDGEMENTS This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

AUTHOR INFORMATION Corresponding Author Yong Kang Tel./fax: +86 22 2740 3389 E-mail: [email protected]

Present Addresses School of Chemical Engineering and Technology, Tianjin University, Tianjin300350, China

ABBREVIATIONS BPEF, bidirectional pulsed electric field; O/W emulsion, oil-in-water emulsion; W/O emulsion, water-in-oil emulsion.

ASSOCIATED CONTENT Supporting Information. Video S1: the dynamic attraction process of oil drops in BPEF (AVI). Figure S1: the demulsification process of the same O/W emulsion under AC, DC electric field and BPEF with the same voltage 400V (TIFF).


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